Review of selective laser melting: Materials and applications, , , , , , , C. Y. Yap , C. K. Chua , Z. L. Dong , Z. H. Liu , D. Q. Zhang , L. E. Loh , and S. L. Sing Citation: Appl. Phys. Rev. 2, 041101 (2015); doi: 10.1063/1.4935926 View online: http://dx.doi.org/10.1063/1.4935926 View Table of Contents: http://aip.scitation.org/toc/are/2/4 Published by the American Institute of Physics Articles you may be interested in Laser powder bed fusion additive manufacturing of metals; physics, computational, and materials challenges Appl. Phys. Rev. 2, 041304041304 (2015); 10.1063/1.4937809 Mechanical response of TiAl6V4 lattice structures manufactured by selective laser melting in quasistatic and dynamic compression tests Appl. Phys. Rev. 27, S17006S17006 (2014); 10.2351/1.4898835 Heat transfer and material flow during laser assisted multi-layer additive manufacturing Appl. Phys. Rev. 116, 124905124905 (2014); 10.1063/1.4896751 Investigation on reducing distortion by preheating during manufacture of aluminum components using selective laser melting Appl. Phys. Rev. 26, 012004012004 (2013); 10.2351/1.4828755 APPLIED PHYSICS REVIEWS 2, 041101 (2015) APPLIED PHYSICS REVIEWS—FOCUSED REVIEW Review of selective laser melting: Materials and applications C. Y. Yap,1,2,a) C. K. Chua,1,b) Z. L. Dong,3,c) Z. H. Liu,1,d) D. Q. Zhang,1,e) L. E. Loh,1,f) and S. L. Sing1,g) 1 Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Block N3.1 - B2c - 01, Singapore 639798 2 Energy Research Institute @ NTU, Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, Block S2 - B3a - 01, Singapore 639798 3 School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Avenue, Block N4.1, Singapore 639798 (Received 14 July 2015; accepted 23 October 2015; published online 9 December 2015) Selective Laser Melting (SLM) is a particular rapid prototyping, 3D printing, or Additive Manufacturing (AM) technique designed to use high power-density laser to melt and fuse metallic powders. A component is built by selectively melting and fusing powders within and between layers. The SLM technique is also commonly known as direct selective laser sintering, LaserCusing, and direct metal laser sintering, and this technique has been proven to produce near net-shape parts up to 99.9% relative density. This enables the process to build near full density functional parts and has viable economic benefits. Recent developments of fibre optics and high-power laser have also enabled SLM to process different metallic materials, such as copper, aluminium, and tungsten. Similarly, this has also opened up research opportunities in SLM of ceramic and composite materials. The review presents the SLM process and some of the common physical phenomena associated with this AM technology. It then focuses on the following areas: (a) applications of SLM materials and (b) mechanical properties of SLM parts achieved in research publications. The review is not meant to put a ceiling on the capabilities of the SLM process but to enable readers to have an overview on the material properties achieved by the SLM process so far. Trends in research of SLM are also C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4935926] elaborated in the last section. V TABLE OF CONTENTS 2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 C. Inconel and nickel-based alloys . . . . . . . . . . . 10 1. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 10 I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 A. Background on selective laser melting . . . . . 2 D. Other metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 B. Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 12 II. PHYSICAL PHENOMENA IN SLM . . . . . . . . . . . 3 2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 A. Laser and material interaction. . . . . . . . . . . . . 3 d. Micro-hardness . . . . . . . . . . . . . . . . . . . . 13 B. Balling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 IV. CERAMICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C. Thermal fluctuation and its effects. . . . . . . . . 5 A. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 III. METALS IN SLM AND THEIR 1. Medical and dental applications. . . . . . . . . 13 APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 B. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 A. Steel and iron-based alloys . . . . . . . . . . . . . . . 6 1. Relative density . . . . . . . . . . . . . . . . . . . . . . 14 1. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Surface roughness. . . . . . . . . . . . . . . . . . . . . 14 B. Titanium and its alloys. . . . . . . . . . . . . . . . . . . 9 V. COMPOSITES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 9 A. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1. Medical implants . . . . . . . . . . . . . . . . . . . . . 15 a) 2. Aerospace and automotive . . . . . . . . . . . . . 15
[email protected] b) Author to whom correspondence should be addressed. Electronic mail: 3. Other applications . . . . . . . . . . . . . . . . . . . . 15
[email protected]. Ph. +65 6790 5486 B. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 c)
[email protected] 1. Relative density . . . . . . . . . . . . . . . . . . . . . . 15 d)
[email protected] e) 2. Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
[email protected] f)
[email protected] 3. Micro-hardness and wear rate . . . . . . . . . . 15 g)
[email protected] VI. SUMMARY AND REAEARCH TRENDS . . . . . 16 1931-9401/2015/2(4)/041101/21/$30.00 2, 041101-1 C 2015 AIP Publishing LLC V a high processing. processing time. Meiners. with laying a thin layer of metal powder on a substrate plate resulting in parts with high porosity and low strength. with filled with nitrogen gas or argon gas to provide an inert Dr. Concept of SLM process. a next layer of SLM. .2 due to van der Waals forces. energy density laser is used to melt and fuse selected areas usually needed to improve the SLS components. some of the SLM machines are capable of powders. The process is then repeated for successive layers of powder eliminating the need for the above-mentioned downstream until the required components are completely built. is time according to the processed data. It is a powder bed fusion process that uses high in. full melting of powder is achieved by the use of powder is deposited on top and the laser scans a new layer.041101-2 Yap et al. Andres of atmosphere to protect the heated metal parts against oxida- Fraunhofer ILT to produce metal components from metallic tion. 041101 (2015) I. other than the removal of parts and supports from substrate ware. patent was also filed result in poor resolution and build tolerance. Fockele and Dr. (ii) Process is repeats for successive layers. according to computer ranges from 20 to 100 lm. (i) High-power laser melts selective areas of the powder bed. K. After the powder is laid. such as laser power.1 In 2001. ments in product quality. Figure 1 illustrates the concept of the Selective Laser Melting (SLM) is an additive manufac. are adjusted such that a single melt ing reliability compared to binder-based laser sintering AM vector can fuse completely with the neighbouring melt vec. which binds powder mate- scanning of individual layers. The thickness of the layer usually regions of powder. Furthermore. W. This makes SLM a superior Additive Manufacturing overhanging features and to generate slice data for laser (AM) process compared to SLS. Rev. scanning speed. Phys. Before the CAD data are uploaded to melting the powder material. such as Magics. Post- in a building chamber. 2. Dr. the net-shape components without the need for post-processing. D. layer by layer.3 Process processes. STereoLithography (STL) files have to be processed by soft. while smaller by Das and Beaman based on their pioneering works in powders have the tendency to agglomerate together easily direct selective laser sintering (SLS). and layer thickness. Once the laser scanning is consuming and significantly lengthens the process. Background on selective laser melting cess is automated. INTRODUCTION (EDM). Wissenbach. the entire pro- A. loose powders are removed from the building adapted to perform repairs on damaged components. M.4 Powders with larger particulate sizes Office and published in 1998. SLM building process. turing process developed by Dr. and manufactur- ing. This is chosen as a balance aided design (CAD) data. data preparation to removal of fabricated component from Literatures have shown that SLM is capable of fully the building platform. hatch spac. to provide support structures for any plate. Direct Digital Manufacturing Lab at Georgia Tech has devel- strate plate manually or by electrical discharge machining oped Scanning Laser Epitaxy (SLE) for additive repair of FIG. processes. The building process starts rials via solid state sintering or melting of binding agents. tors and the preceding layer. The patent for this technology was between achieving fine resolution and allowing for good first applied in 1997 to the German Patent and Trade Mark powder flowability. resulting in poor powder flow- The SLM process consists of a series of steps from CAD ability and hence poor powder deposition. producing fully dense near the SLM machine for production of components. Once the laser scanning process In addition to direct manufacturing. high-intensity laser and binder materials are not required. and Dr. 1. Appl. such as heat treatment and material infiltration. G. the building platform is lowered. providing pre-heating either to the substrate plate or the tensity laser as an energy source to melt and fuse selective entire building chamber. the building chamber is often Schwarze of F & S Stereolithographietechnik GmbH. Besides the data preparation and removal of fabri- cated component from the building platform. During the SLM process. The current SLM technology provides improve- parameters. In completed. SLM has also been is completed. (iii) Loose powder removed and finished part revealed. The chamber and the component can be separated from the sub. and layer thickness is essential for SLM proc- that these figures do not show the limits of the technology. In order to keep this review Yb:YAG crystal is expected to replace Nd:YAG crystal for focused.8 SLE laser sintering process to Nd:YAG fibre laser (k 1. essing to successfully build near full-density parts. publications in which manufacturing processes high power diode-pumped lasers and other potential applica- involve complete melting of powder bed by laser will be tions. Literatures on simulation and numerical analysis of SLM are beyond the scope of this review.” and “other metals. 041101 (2015) nickel-based alloys CMSX-4. and layer thickness. Scope adjusted to optimize the process. In SLM. the materials are further divided into “steel and iron. In this section. often results in balling due to lack of wetting of molten pool highest hardness. However. This is due to the cover the surface of the damaged part up to 2 mm in powder higher absorptance of metallic powders to radiation of such thickness. applications of the SLM melted. Under the category of density that is available to heat up and melt the powders.” “titanium and its alloys. adapted from the selective ing. Instead. the laser window. usually a combination of low materials are elaborated. Yb:YAG crystal has damaged component. The advancements in laser technology will continue to included and SLM will be the term used to represent these bring about higher energy efficiency to the SLM process.5. suitable combination of laser power. results in regular porosity in built parts as adjacent melt lines nent have been melted. investigations on these aspects of SLM are presented to shed light on the physics involved in the SLM process. essentially the same as SLM. such as highest relative density. and a lower thermal loading per unit pump power. disrupting the delivery of laser power.” pendent on the material and proportional to the mass to be Under each material group. Together with the absorptance of powders to the laser are organized according to the materials used. are time. such as Hence. Appl. When heating and melting occurs. Furthermore. high laser and low scan- compiled and tabulated for readers to have a quick glance ning speed may result in extensive material evaporation and and for ease of comparison. PHYSICAL PHENOMENA IN SLM SLM involves the heating and melting of powder mate- rial with laser beam and rapid solidification of the melted ma- terial to form the desired component. hatch spacing. processes. The absorptance. a larger absorption bandwidth to reduce thermal manage- Other additive manufacturing processes. compared melt the powder material. Section VI illustrates the trend is the absorption of energy by the powder. and large layer thickness. achieved. of research in SLM and its future outlook. Relative density of SLM parts is the keyhole effect.6 lm). they are meant to demonstrate the progress of One of the integral aspects of laser-material interaction research in the SLM process. high scanning speed. do not fuse together completely. hatch spac- from CO2 laser (k 10. such as ment requirements for diode lasers. poor hatch spacing often often examined to determine if all the powders in the compo.7 and IN100. Sections II–V SLM. tration of these process parameters commonly studied in ena commonly mentioned in SLM literatures. it is to note hatch spacing. Strength and hardness are basic ma.6 Rene 80. enough to and subsequently to Yb: YAG fibre laser.9 However.06 lm) requires a small amount of powder for repair. A. scanning speed. laser power.” “Inconel and nickel. mainly metals. scanning speed. metals. 2. Laser and material interaction SLM was designed with the intent to heat up and melt metallic materials.10 In addition. heat capacity and latent based alloys.041101-3 Yap et al. Moreover. the balling phenomena that disrupt the formation of continuous melts. and the thermal fluctuation experienced by the material during the process that can lead to crack formation and com- ponent failure. Figure 2 provides an illus- This review first presents some of the physical phenom. these parameters affect the volumetric energy ceramics and composite materials. irradiation. are required. heat have to be taken into account. These are heavily de- based alloys. defined as the ratio of the energy flux absorbed by the II. such as the absorptivity of the powder material to laser irradiation. covering cracks and holes in the to the commonly used Nd:YAG crystal. Moreover. The laser systems for SLM progressed FIG. a longer upper-state life- LaserCusing and Direct Metal Laser Sintering (DMLS). Insufficient energy. the best properties laser power. Rev. Phys. scanning speed. and lowest surface roughness. grinding and polishing.11 Surface roughness determines if post-processes. vaporization in terial properties that can be compared to cast parts to SLM often results in condensation of volatilized materials on indicate the suitability of SLM parts for various applications. A high power Nd:YAG laser is then deployed to wavelengths in the infrared region. . SLM process parameters: laser power. highest strength. and layer thickness are the common process parameters B. 2. There are several physical phenomena that are important to the process. are also with the preceding layer. oxygen level at 0. investigated the absorptance of powder materials to of the SLM process based on AA6061 has also been devel- infrared irradiation of CO2 laser (wavelength. balling may ag- gravate in subsequent layers and cause the SLM process and jam the powder coating mechanism with large metallic beads that extend above the powder bed (Figure 4). allows for prediction of the absorp.12 as the laser pulse could be adjusted for different pulse dura- The high absorptance of powder material can be tions. Phys.15 The model. Tolochko spacing in the process.16 Furthermore. Kruth. a uniform beam laser could be used of powder materials to laser irradiation can be drastically dif. Appl. The effects of powder size or size distribution on the processing envelope of differ- ent materials are still unclear as they have yet to be investi- gated independently. powder size and powder substantiated by the data.11. taking into account the geometry and erties. an aluminium alloy.06 lm). such as hardness and strength. powders with narrow range of parti- cle size flow better and generate parts with higher strength and hardness. Physicists have studied the absorptivity of various ent effects of Gaussian beam profile and uniform beam profile materials to irradiation of different wavelengths. shrinkage and material loss due to vaporization of the sorptance of bulk materials with smooth surfaces. Int.20 However. surfaces. Rev. suggests that smaller 316L stainless absorption with respect to the material absorption and to pre. 3423–3434 (2005). it is noted that the scanning speed of this system process. eters.12 In comparison to the ab. J. with experiment data (data points) and prediction by isotropic specular shown that balling could be reduced significantly by keeping reflection model (solid line) and diffuse reflection model (dotted line). Balling Balling is a particular phenomenon in SLM where mol- ten metal forms spheroidal beads due to insufficient wetting of the preceding layer and surface tension. k ¼ 10. with D50 of 38 lm. applying a combination high laser Heat Mass Transfer 48. a simplified modelling et al. Copyright 2005 Elsevier. However. In addition to laser parameters. B. studied this phenomenon with a 2D op. power and low scanning speed or applying re-scanning of . In Li et al.13 Wang et al.041101-4 Yap et al. and the difference between pulsed and continuous lasers.0% density than steel powders particular material.’s study of the balling phenomenon. affects examine the effects of the energy distribution profile of lasers the energy efficiency of the SLM process. which also result in higher optical penetration of the top surface but was detrimental to that of the side depth.6 lm) oped. 2. universality of their effects across different materials is also not established.9 FIG. A Their work provide models to calculate the ratio of the total study by Bourell et al. was limited to 400 mm/s. there are few published works on this. possibly restricted by the highest tance of powder bed with the absorptance of bulk material frequency and longest pulse duration attainable for a contin- and match experimental data with both the isotropic specular uous melt track. Inconel 625 with pulsed laser.06 lm. V. In more severe cases. Their results showed explained by the multiple reflections of the laser beam in the that a ramp-down pulse shape lowered the surface roughness powder bed. Using the pulsed laser allowed the absorptance of titanium bulk material is 30%.19 Another study by Liu et al. Moreover.14 In an attempt to model the SLM Moreover. in SLM. P.1%. 041101 (2015) material to the energy flux incident upon the material.17 materials have significantly higher absorptance regardless of Mumtaz and Hopkinson18 reported a study on SLM of the wavelength of irradiation.15 Reproduced with permission from A. It can even deter. Gusarov and J. were yet to be tested. However. at k ¼ 1. steel powders with D50 of 15 lm and 28 lm require a lower dict the optimal laser parameters with which to process a energy density to achieve 99. These studies only showed some of the possi- ble effects of powder size and size distribution on the required SLM processing parameters. confirmed In addition to the studies on absorptance of irradiation that existence of smaller powders allows high density parts with various materials. In SLM. structure of the powder.13 while them to add another dimension of control to the SLM process that of titanium powder is 77%. Loh mine the feasibility of processing materials with a specific et al. Hence. powder process. reflection model and diffuse reflection model. Effective absorptance of powder bed vs absorptance of bulk mate. and pulse shapes. For instance. However. distribution also have effects on the required process param- tance of powder bed based on that of bulk material Figure 3. Gusarov and Kruth compared the effective absorp. The numerical model takes into account the volumetric and Nd-YAG laser (k ¼ 1. it is rial. was able to achieve a larger melt width for similar amount of laser is irradiated onto a thin powder bed and the absorptivity melt penetration. frequencies. to increase the productivity of SLM by reducing the hatch ferent from their corresponding bulk materials. 3. Their investigation found that a uniform beam laser few of these works are done with powder materials. used AA6061. forming rough and bead-shaped surfaces.21 It obstructs the formation of continuous melt lines. to examine the differ- laser. the effects of pulse shape on other prop- tical ray tracing model. there are also research works that to be built with lower laser energy intensity and better sur- face finish. This paper gave three main methods to reducing them fluctuation during the SLM process. introduced “sectorial mechanism can lead to crack formation and delamination of scanning” as a scanning strategy in SLM.9 Das and Kruth et al. and heating of powder bed to 160 C resulted in a 40% reduc- The materials experience varying degrees of thermal tion. laser.24 the absence of a chemical reaction.25 It was found that heat treatment of such ide films are ways to minimise the occurrence of balling.11. with the temperature gradient mechanism. Int. Li et al. 1025–1035 (2011). 2. It was found . explained that an oxide film on Ti-6Al-4V during SLM and it shows that thermal gradient is the preceding layer impedes interlayer bonding and leads to more pronounced in Ti-6Al-4V due to the difference in ther- balling as liquid metals generally do not wet oxide films in mal properties. as explained by Kempen various research works with regards to the SLM process. Thermal fluctuation and its effects as the SLM forming process reduced residual stress by 55%. J.041101-5 Yap et al. Phys. Copyright 2011 Springer. 4. et al. re-scanning of laser with the same parameters C. Hence.9 Reproduced with permission from R. Yasa et al. addressed the issue of residual stresses and thermal stresses. Rev. poor interlayer bonding also leads delami. Technol.26 This strategy parts as shown in Figure 5.. Appl. phosphate powders.22 In combination with Shiomi et al. 041101 (2015) FIG. Adv.23 This More recently. A recent study has also compared breaks down a layer into small square grids and neighbouring the temperature profile in SLM of 316 L stainless steel and grids are scanned perpendicular to one another. 59. SLM components at 600 C–700 C for 1 h reduced residual stress by 70%. methods of reducing them in SLM components made of nation. This causes residual and these methods have since been repeatedly examined in stress on the components built. Aggravation of balling that jams powder coating mechanism in SLM. Manuf. lowering the oxygen level during the process chrome molybdenum steel mixed with nickel and copper and introducing repeat exposure of laser to break up the ox. selected due to their widespread application and their H13 tool steel. 5. it was not until 2010 that Tolosa et al. 041101 (2015) FIG. The main ent during the process. and tungsten.27 Reproduced with permission from Y. Secondly. and post.29 However. they used a eutectic mix- to compensate for steel’s relatively high density. ytterbium fibre laser with a wavelength tuation experienced by the SLM part during the process of 1065 nm was used to process the 316L stainless steel pow- and allowed the finished parts to be crack-free. 5. Phys. Phys. Abe et al. Hagedorn et al. have ware. such as Autofab or SLM Build Processor and is known also been tested for SLM and they are discussed in Section as chessboard strategy. achieved 99. III D. Appl. This can the right scanning strategy and parameters. scanning speed. The problem of crack formation and delamination is especially prevalent in ceramic materials.9% relative density with SLM III.- C. Steel and iron-based alloys require very high melting temperatures and have very low From 2003 to 2013. et al. In their study. ture of alumina and zirconia so that the required melting Most of the research publications on SLM of ferrous temperature was lowered to 1860 C. These approaches directly lowered the temperature fluc. Proc. copper. selecting material and resulted in high porosity components. and crack formation scanning speed caused incomplete melting of the powder can be mitigated by combination of preheating. As research progressed. by Fraunhofer ILT. These metals were first reported investigation of M2 high speed steel (tool steel). many other metals. decreasing the process heat treatment. Copyright 2013 The University of Texas at Austin. This enables product developers to harness the toughness order to tackle the problem. resulting in very high thermal gradi- tions on the SLM of steel or ferrous alloys alone. The inher. der.. Solid Freeform Fabrication Symposium Proceedings (2013). Ceramics A. Combined with its low fracture advantage of processing metals with SLM lies with the flexi- toughness. ran pre- ary laser was used to preheat the surface powder to tem. and 314S stainless steel in 2004. residual stress.. a second. Rev. The experiment showed that low laser power and high ent issues of balling. 587–594 (2010). liminary tests on this material in 2001 but it was not success- peratures above 1600 C as the primary laser melted the ful due to balling effects. metals are based on 316L stainless steel. This strategy is now available in processing soft. 2. significantly. In ity. be improved by increasing the laser power. there are more than 100 publica- thermal conductivity. such as aluminium. Kempen et al. titanium.041101-6 Yap et al. and nickel. Copyright 2010 Elsevier. that “sectorial scanning” is able to reduce the residual stress material cost.27 Firstly. 6.28 The first paper that reported selected area according to CAD data as shown in Figure successful SLM of 316L stainless steel was made by Jandin 6. Temperature gradient mecha- nism (top) leading to crack formation and delamination (bottom). Hagedorn’s research group of steel and add considerable weight-saving to their design made two approaches. METALS IN SLM AND THEIR APPLICATION of 316L stainless steel powder.30 Most of the SLM research revolves around three types Research group led by Childs from University of Leeds of metals: iron. ceramics present a difficult challenge for bility of SLM to fabricate components with structured poros- researchers to produce a crack-free part via SLM.23 Reproduced with permission from K. Preheating mechanism with CO2 laser. magnesium. or both. They FIG. . such as (ITER). it is to note that a CO2 laser.5 mm/s. and small parts with conformal cooling channels. cooling channels using SLM. As do most AM technologies. Appl. such as Fe-Al. Chen et al. instead The feasibility of using SLM steel parts for biomedical of the Nd:YAG laser commonly used now. mandibular canal ling31 and experiments32 and showed that a combination of segment.48 further investigated the effectiveness of complex shape heat exchangers manufactured via SLM and 1. and support structures or tubular bone made high laser power and low scanning speed produces stable melt from SLM 316L stainless steel.37 published a biocompatible metal frame.33 Later.43 The paper mum power of 200 W was used in their experiments and presented four different case studies on successful surgical scanning speeds were as low as 0. tion of the diverter thimbles in the cooling system of the ing steel. Following this. 041101 (2015) examined the melt track in the SLM process through model. excellent resistance to oxidation. Li et al. by exploiting the flexibility of the SLM However. with a maxi. Furthermore. Wong et al. there has not been any published study on implants process. in jet engines. tures. stainless steel and 17-4 PH stainless steel powder mixture. The structure contains repeated three- is that the products from SLM can be customized to suit indi. Their works demonstrated that pensive compared to most conventional manufacturing different cooling rates can be achieved in the mould using methods. Yang et al. Garcia et al. Wang et al.44 display- Different variants of steel have also been investigated for ing the ability of SLM to create small and complex parts processing by SLM. Brandner et al. etc. The of orthodontic products using a self-developed SLM group went on to examine single layer scanning of H13 machine and they were able to achieve the required sur- by a raster-scanning strategy. with Inox904L steel. weight structures was reported by Santorinaios et al. according to needs. Fe-Ni-Cu-P. The ability of SLM to create sented SLM as a “promising direction to solve challenging complex lattice structures has also led to research in cellular medical problems. Heat exchangers. Fe-Ni-Cr. Jerrard et al. These structures enable engineers vidual needs at virtually no customization cost and fully to easily alter the density and strength of the overall product functional parts can be built directly. have investigated the SLM of 316L made from SLM steel parts. radiation. and lightweight structures for the aerospace and automo.041101-7 Yap et al. such as furnace fixtures.”36 The main benefit of SLM in this field lightweight structures. As the current SLM process is ex. has also to produce metallic parts with complex geometries been studied for such application. Phys.50 also fabricated mould inserts with conformal and biocompatibility. provides the freedom of design to make conformal cooling sitional ratio of the contents. Beal et al.41 investi- track were mapped out for these materials and the maxi. turbine blades tive industries. Bertrand and Smurov40 separately reported SLM denture nents. and tool steel X110CrMoVAl 8-2.52 on the work for dental prostheses and Wehmoller et al. Badrossamay and face quality and mechanical properties. these research works cally improved. Both their experimental with gradient porosity where the dense portion is designed and simulation results showed that thermal history of the for strength and the porous portion is designed to enhance process affects the amount of powder melted under laser tissue growth in biocompatible implants. iron-based alloys and intermetallics. 2. Hence. H20 tool steel. there .46 incorporated Fe-Ni. dimensional single cells. heat exchange piping. Light weight structures. The first paper on SLM of steel light- Kruth et al. ductility. Medical and dental applications. SLM the SLM component can be adjusted by varying the compo.49 built functionally graded injection mould and investi- There are many applications of SLM steel materials. However.45 In a similar manner. such as medicine. c. International Thermonuclear Experimental Reactor Besides steels. The research contributed to the fabrica- AISI Maraging 300 steel.47 showed that SLM can pro- mainly examined the suitable processing parameters to duce micro-scale devices with wall thickness of about achieve fully dense components with these materials and their 100 lm which enables the fabrication of micro-scale struc- resultant microstructures.35 Their work showed that hardness and magnetic adherence of b. The process envelopes for a stable continuous melt frameworks with the same material. Applications the effects on the heat exchange and fluid pressure.42 studied Childs34 continued their studies on SLM to include M2 the possibility of making SLM 316L stainless steel parts tool steel and 316L stainless steel. such as strength. and it is often used in high temperature systems. spiral conformal cooling channels for injection moulding have also been tested with SLM although publications of such and found that both cycle time and part quality were dramati- materials are much fewer. Yadroitsev et al. applications has been confirmed by Bibb et al. and Fe-Cr-Al alloys. wear and where conformal cooling channels are required for tooling corrosion. gated the heat conductivity and heat capacity of the parts.39 and tracks beneficial for the production of fully dense compo. a. guides in different maxillofacial (jaw and face) surgeries. industries. gated the optimization of building accuracy and density mum achievable scanning speed was just 15 mm/s. Rev. These include Inox904L stainless steel. ing. Thus far.38 crush behaviour of such structures. reported body implants of cortical bone.51 Fe-Al has high strength directly also allows researchers to look into applications to weight ratio. pre. The unique ability of SLM SLM of ferrous intermetallics. proposed applications are in high value-added cooling channels. new grades of steel can channels and this is especially applicable to injection mould- be produced via SLM to tailor properties to specific func. Fe-Al. Smurov’s research group demonstrated a mini pump die tions and needs. depending on the properties. 15-5 and 17-4 precipitation harden. Appl. Reported surface roughness of steel and International material data sheet for the materials’ den. One of the limitations of the SLM indicator of the quality of the SLM parts. Highest reported relative density of SLM steel and iron-based static and blast response. For instance.00 65 of a sandwich structure which makes it lightweight and Fe þ 0.00 73 investigated the SLM steel honeycomb structure with nega. Similar results were also achieved with NH4HCO3 microstructure throughout the part. H20 tool steel 99. Other applications.54 shock alloys. honeycomb-like struc. where achieved by adding 0. where the material is required to show M2 high speed steel 97.00 76 sion.77 This phenom- d. reported relative densities above 90%. tooling purposes. Strength.00 69 lished.58 examined its drop-weight impact 304L stainless steel 92. 316L stainless steel 99.95 (laser remelting) 72 tures have also been studied. such as sand- retical density of the bulk material. Rehme and Emmelmann59 H13 tool steel 90. 041101 (2015) have been more research works that investigated the quasi. Usually.00 67 Fe3Al 99.8% C 93.50 70 and Mines et al. Shen’s group managed to create less malleable compared to their cast counterparts. is listed on the right col- porous stainless steel filter elements by SLM was studied by umn for readers to have a point of reference. .99 75 Ultra high carbon steel 92. iron-based alloy components made via SLM is shown in sities. The theoretical density blasting. Only Shen et al. For alloys. However. the segrega- as additives. b. 2. Relative density. This results in a more uniform additives.90 30 Besides cellular lattice structures. Most of the SLM steel or iron-based materials have Table III.50 68 structure with SLM steel lattice core have been pub- Fe-Ni 98. TABLE I. This is due to the limita- tions of laser spot diameter and the resultant melt profile cre. % References tures.55 failure mechanism56 of the steel lattice struc- Material Highest relative density. some research groups may not follow the ASTM E8 standard Their studies showed that SLM was able to produce a part for tensile test of metallic materials.62 made a case study The table below shows the highest reported ultimate ten- for AM of tyre tread ring segment mould with H20 tool steel. In other SLM applications of steel enon is common for SLM materials. Table IV shows the highest reported micro-hardness of the various SLM steels and ferrous alloys. The reported Yadroitsev et al. The annealed material UTS. This structure can be applied as crash impact absorbers in bullet proof vest and artificial intervertebral discs. along with their Feuerhahn et al.00 74 high compressibility and minimum resistance to compres. Rev. Milovanovic et al. resulting in higher strength that cast parts.00 71 behaviour. shot-peening. Chemical composition is more uniform throughout the part.63 studied the SLM of a high hardness tool respective yield strength (YS) and elongation. One may also refer to the ASM 316L stainless steel. a.57 studied the compressive behav- Fe-Ni-Cr 99. tion of alloying elements takes place in a much smaller scale. or manual grinding is often needed to can be calculated from the atomic weight and crystal achieve a smooth and shiny surface with the exception for structure of the material. often reported that SLM steel components are stronger but ated by the laser. Post processing. Production of fine-structured available from ASM International. Surface roughness. sile strength (UTS) for each bulk material. Relative density process is the surface roughness and it is common to achieve is the ratio of the density attained with SLM and the theo. Relative density is often used as an c.60 This was generally attributed to the nature of the SLM process.00 66 strong. the engineering strength of SLM steel parts is of par. The pores very small amount of material is melted at a time and rapid and walls were created as a result of the gas generated by the solidification takes place.50 62 tive Poisson’s ratio. or iron-based alloys. However.61 The drawback is that the pores and wall struc. AISI Marage 300 steel 99. Phys. few investigations of the sandwich Fe-Al intermetallics 98.1 wt. roughness of about 20 lm. Guan with very high hardness with fine structure at the same time. In most of these works. 316L stainless steel was used. 316L stainless steel 99. It is comparison.041101-8 Yap et al.78 used Chinese GB/T 228–2002 standard for their test This cannot be achieved by conventional methods. Properties to applications. Steel is often used because of its strength. et al.64 strengths of lattice/cellular structures are not in Table II as they are dependent on design and can be tailored according 2. There are many different tests available for hardness and the Hence. Micro-hardness is important for materials. This is micro-scale pores with walls thinner than 1 lm. % of H3BO3 and KBF4. Micro-hardness. spray forming or forging. results were converted to the HV scale in this review for amount importance for their respective applications. The cellular lattice structure can be deployed as the core Fe 99. response. tures cannot be controlled. such as on SLM 304 stainless steel. the wall or strut thicknesses of these lattice structures is in the range of 100 lm. It is noted that steel X110CrMoVAl 8-2 for applications in micro-tooling.53 compressive properties. Table I summarizes the reported maximum density of the various SLM steel d.50 69 iour and failure mechanisms of the sandwich structure Fe-Ni-Cu-P 97. Vandenbroucke and tive and sensitive to oxygen.00 (laser re-melting) 72 crowns using SLM cpTi. Lowest reported surface roughness values of SLM steel and 1. and carbon. Highest micro-hardness of SLM steel and iron-based alloys.100 also examined 316L stainless steel 272 (from 104 HRB) 82 cultured cell growth of Staphylocuccus aureus on Ti-6Al- H20 tool steel 336 (from 34 HRC) 62 7Nb scaffold and have demonstrated the potential of this tita- M2 high speed steel 900 86 nium alloy in this application. hydrogen.95 to flush out atmospheric air and provide a layer of protective examined the bone in-growth potential of trabecular-like gas. such as cast.88 Mullen’s group investigated a H20 tool steel 3. Ti-24Nb-4Zr-8Sn. and showed that SLM Ti64 porous scaffolds allow total 4 V alloy (Ti64). carbon. 2.041101-9 Yap et al. is used frameworks. Furthermore. nitrogen. an inert gas. and Ti. gery. In Brazil.00 80 1317 304 stainless steel 717 570 42. Ti-13Zr-Nb. a.91 studied the structure and me- chanical properties of a Ti64 cellular inter-body fusion cage. Appl.80 78 579 316L stainless steel 760 650 30.78 (shot-peening) 62 unit-cell approach89 and various scanning strategies90 to Ultra high carbon steel 18.99 on cranio- 316L stainless steel 279 85 maxillo-facial implants. Moreover.53 79 1317 15-5 PH steel 1470 1100 15. Rev. planning with correct geometrical and anatomical details. Highest reported tensile strengths of SLM steel and iron-based alloys. Biemond et al. Shiomi’s group has built bone and dentals 316L stainless steel 2. they have high specific strength and elastic 304L stainless steel 25. overgrowth of osteoblasts (bone cells). Material UTS (MPa) YS (MPa) Elongation (%) References Material UTS (MPa) Fe 411.98 on endosseous implant and Dybala et al. Medical and dental applications.87 316L stainless steel 5. Almost all of these papers applications and Warnke et al. Further Fe-Ni-Cu-P 230 70 studies on this novel titanium alloy were done by Marcu 304L stainless steel 217 71 et al. Jardini such as Ti-6Al-7Nb. Phys. have also been investigated for the SLM cate titanium implant for a right cranio-maxillo-facial sur- process.5 305.00 (sand blasting) 19 eral groups. Titanium and its alloys Murr et al. the number of publications of SLM of and mechanical properties of SLM Ti64 for biomedical titanium is second to that of steel.96 reported the use of EOSINT M270 system to fabri- 13Nb-13Zr. Fe-Al intermetallic 800 83 Chlebus et al. Lin et al. small localized heating and rapid cooling in implant surfaces produced by SLM of Ti64 in goats and con- SLM reduce the pick-up of interstitial elements. Titanium in the liquid state is highly reac. Moreover.0 76 make porous biocompatible constructs for orthopaedic appli- cations using cpTi. Other titanium based alloys. there were recent studies that seek to Material Micro-hardness (HV) References replace cpTi and Ti64 with other titanium based alloys. B. sional accuracy of the SLM process for fabrication of dental ing.7 71 moduli closer to bone than Co-Cr alloys and stainless steel. Szymczyk et al. such as cluded that the SLM produced parts showed good bone in- hydrogen. and oxygen.93 conducted cell experiments are based on commercially pure titanium (cpTi) or Ti-6Al. Kruth94 and Chen et al.3 … 65 225 Fe-Ni 600 … … 69 … Fe-Ni-Cr 1100 … … 69 … Fe-Ni-Cu-P 505 425 … 70 … 15-5 PH steel 1450 1297 12. such as argon.82 82 Research into titanium body implants has been done by sev- 316L stainless steel 5. In the SLM process. growth characteristics after 15 weeks. This would result in a reduced difference of moduli between implant and . Most research on tita- Material Surface roughness. et al. SLM of Ti-24Nb-4Zr-8Sn has Maraging steel 412 (from 42 HRC) 81 X110CrMoVAl 8-2 tool steel 493 63 been examined by Zhang and Sercombe101 as an improve- Ultra high carbon steel 475 76 ment over Ti64 as a low modulus alloy. Ra (lm) References nium and its alloys is driven by its potential application as body prostheses or dental implants due to their biocompati- Fe-Ni 10 69 Fe-Ni-Cr 10 69 bility.39 separately examined the dimen- making it difficult for conventional processes.97 studied the possibility of using Ti-6Al-7Nb Fe3Al 353 68 for medical implants as it replaces vanadium with niobium Fe-Ni 232 (from 220 HB) 69 in its chemical composition and this alloy has higher corro- Fe-Ni-Cr 450 84 sion resistance and bio-tolerance compared to Ti64. The custom-built implant fitted the patient well and allowed the surgery duration to be reduced due to pre-op TABLE IV. In some of the latest studies. Applications iron-based alloys. 041101 (2015) TABLE II.30 81 1930 TABLE III.92 focused their attention on the microstructure In the last decade.00 19 558 Maraging steel 1290 1214 13. Light weight structures and other applications. a family of nickel-based et al. micro-hardness of 613 HV corresponded to relative densities Besides applications in medical implants and light of 95. SLM also enabled the fabrication of component. Appl. Cooper of these publications are on Inconel. SLM of titanium and titanium-based A particularly interesting nickel based material studied alloys has been very positive in terms of the relative density for the SLM process is the temperature-sensitive shape mem- attained by various researchers. 263.98 94 Ti-6Al-7Nb 99. ory alloy (SMA) NiTi. where the UTS is about 1006 MPa. Ti-6Al-4V 99. also published his work on SLM NiTi SMA and his results showed that the SLM component b. There were few reported results on struction of cell structure with SLM. 041101 (2015) surrounding bone. IN738LC.111 Caiazzo et al. Chromel. Besides Ti-24Nb-4Zr-8Sn 99. Nickel-based alloys studied for the SLM process include ined how freedom of design would enable both component Inconel 625.5% or higher. Hasan et al. % References 13Zr-Nb as an alloy with better biocompatibility and longer lifetime for load-bearing implants. Inconel 718. 2. Rev.8%. thus preventing bone resorption.113 reported that Red Bull Technology used laser AM superalloys typically used in high-temperature applications. Titanium parts fabricated by SLM display had high cyclic stability121 but also slightly lower fracture similar or superior strengths to their cast counter-parts. In motorsports.105 proposed an octahedral cell design for the con- c. At early stages of research. Many of these publications focus on the process parameters for formation of stable melt 2.50 117 allowing individualized medical implants to be built with low customization cost. Table VII shows the highest reported ied the behaviour of SLM Ti64 micro-struts and proposed a micro-hardness of the commercial pure titanium and heat treatment process to improve the properties of these titanium-based alloys fabricated by SLM.2% and 95. cpTi 99. Sun et al.103 also stud.112 analysed the potential of SLM Ti64 in the production of aircraft engine turbine blade with After steel and titanium materials. Most ing methods cannot achieve easily. a. rous structure by using a blend of TiH2 and cpTi powders. samples with a higher rela- weight structures. and respective elongation of the SLM titanium parts are tabulated in Table VI. Xiao et al. Mines et al. causes implant loosening. Phys. However. Nimonic weight reduction and fluid flow improvement.95 97 b. The strength and fracture strain compared to conventional UTS.50 115 ied another low modulus titanium alloy in Ti-13Nb-13Zr for cpTi 99. small amount of material that results in a more uniform has been studied for high temperature applications in aircraft chemical composition and microstructure throughout the engines as swirlers in combustion chambers. SLM of Ti64 was first studied for fabrica- tive density of 97. Wang et al.114 Subsequent research works on titanium and with a relatively gradual phase transition. Relative density.104 fabricated cpTi po- SLM is able to produce components with higher UTS. Strength. examined the compressive strength of high porosity (70%) lattice structures made of SLM titanium.041101-10 Yap et al. The SLM NiTi titanium-based materials were able to achieve relative den- component exhibited a martensitic transition between 32 C sities of 99. technology to build thin wall hydraulic channels and exam. the most studied group of metals for the SLM process.106 charac- surface roughness of SLM titanium parts. Table V summarizes the reported and 59 C and an austenitic transition between 59 C and maximum density of the various SLM titanium materials. in the SLM of Ti-6Al-4V.4 Santos’s group already achieved relative densities of 98% and they have managed to produce two-way trained NiTi for cpTi. Micro-hardness. Applications alloys have higher or equal strength compared to their cast counterparts. UTS of the cast parts is also NiTi. Hastelloy X.108 SLM cpTi. nickel-based alloy is internal channels or cavities which conventional manufactur. Meier et al. Later. 90 C.118 aeronautical applications. It was first reported by Clare et al. 117 reported surface roughness of 5 lm for load behaviour of SLM Ti64 porous structures.96 lm for the top surface of laser AM et al. Properties track since it is a pre-requisite of forming fully dense 3D components. YS.90 (under vacuum) 116 medical applications. Zielinski et al. Speirs et al. This is due to the rapid solidification of very SLM of nickel-based superalloys.107 separately studied the uniaxial loading and bending Ti64 and Ref.58 tested the drop weight impact high micro-hardness may not correspond to high relative behaviour of sandwich structures with micro-lattice cores density. which TABLE V. Highest reported relative density of SLM titanium.109 stud- d. Reference 114 terized the deformation and failure behaviour and Brenne reported a Ra of 3.102 presented Ti- Material Highest relative density. For instance. It is noted that structures. repair . and MAR-M 247. Gorny et al. Surface roughness.122 placed in the table for ease of comparison. the highest made via SLM of Ti64.120 sile110 and SLM of titanium has been investigated for fabrication of waveguide filters for radio frequency applica- C. SLM titanium 1. Inconel and nickel-based alloys tions. Comparing Ti64 parts manufactured by Electron lightweight titanium scaffold structures for both medical and Beam Melting (EBM). such as Inconel 718.6% only gave a micro-hardness of 515 tion of guidance section housing for the Sidewinder mis- HW. For instance. in this review. besides the processing pa- rameters.. and turbocharger rotors. They display higher UTS compared to their cast counterparts. and good weldability125 have allowed it to be applied in these c. there were early works on pure and Chomel as shown in Table VIII. Other metals advantages of SLM. such as freedom of design and high re- Besides steel. high temperatures.40 97 1000 Ti-24Nb-4Zr-8Sn 665 563 13. There have been very few reports areas.140 attempted to process copper and H13 tool steel to make a multi-component mould tool with b. TABLE VII. powder morphology and content. they are grouped to- as die models for bevel gear130 and porous filtration media. Micro-hardness. a common casting alloy. respectively. and Sanz and Navas as shown in Table X. Rev. Properties content gave a higher relative density in the SLM component than larger and irregularly-shaped powders. titanium.126 it was highlighted that However.70 128 . engine components can be made with complex in- aluminium. Highest reported relative density of SLM Inconel and nickel- based alloys.75 136 Ti-24Nb-4Zr-8Sn 225 117 Nimonic 263 99. % References Material Micro-hardness (HV) References Inconel 625 95. a patent for manufacturing of a hollow turbine 4 lm for the top surface and Wang et al. Appl. of aging treatment. Combined with the D. 041101 (2015) TABLE VI. Other literatures a. USA.8 117 … patches. high-temperature strength127 and has possible applications in only a few reports on the micro-hardness can be found. In SLM nickel-based components. on the surface roughness of SLM nickel-based alloys. However. Phys. It is aircraft engines. copper. increased cooling rate. This can be shown by comparing the works ties and high oxidation resistance.64 gether in this section. weight and bring about fuel savings for the airlines.131 studied the SLM of NiTi Most of the works on SLM of aluminium alloys are for potential applications in micro-electro-mechanical sys. and high surface quality. Hence. Mumtaz and Hopkinson138 reported microstructure. Material UTS (MPa) YS (MPa) Elongation (%) References Material UTS (MPa) cpTi 654 522 17. Al6061. These turbine blades had dense 10 lm.128 Rickenbacher et al. and AlMg. other metals. In a process tems (MEMS) and smart carrier material for human mesen.4 and Habijan et al. and on SLM of aluminium and its alloys also include pure alu- Nimonic 263 have achieved near 100% relative density minium. such as solution. also recommended IN738LC as a suitable material for heavy-duty gas turbine components.. ternal structures to increase cooling efficiency or to reduce and gold have also been studied for the SLM process. based on AlSi10Mg. tungsten. plex turbine blades by Lu et al.139 it was chymal stem cells (hMSC) to enhance bone growth and found that the quality of SLM component is dependent on healing for critical bone defects. wear resistance. cobalt-chrome. SLM Inconel 718. magnesium. high definition.00 132–134 cpTi 308 116 Inconel 718 99. Table IX TABLE VIII. optimization study conducted by Kempen et al. In a review of manufacturing technologies for com. shown that the micro-hardness can be improved by application ature structural applications and it has good fatigue proper. YS.98 135 Ti-6Al-4V 613 120 Chomel 88. and nickel. Material Highest relative density. Small and spherical powders with higher silicon 2. However. while there are room for improvement for SLM Inconel 625 In the SLM of copper. Hastelloy X.124 shows the UTS. 2. Strength.00 130 Ti-6Al-7Nb 464 97 Hastelloy X 99. Relative density. Nickel-based alloys fabricated by SLM fol. AlSi12. In addition to high-temperature related applications. Surface roughness. Pogson et al.0 119 655 Ti-6Al-4V 1250 1125 6. there was difficulty in low a similar trend to titanium-based alloys. Highest reported tensile strengths of SLM titanium. that surface roughness of SLM Inconel 625 can be as low as Moreover. fatigue resistance. publications on each of these metals are signifi- SLM of nickel-based alloys has been studied for applications cantly fewer. Hastelloy X has high oxidation resistance and good d. copper. Clare et al.123 gas turbine blades. the reported figures have shown that these SLM Fraunhofer ILT and the MCP company had used SLM to parts have good surface roughness which is smaller than produce turbine blades.041101-11 Yap et al.00 94 1055 Ti-6Al-7Nb 1515 1440 1. Highest micro-hardness of SLM titanium.127 reported Ra of blade by SLM was applied for on 31 March 2011 in the 6 lm with contour scanning. and corresponding elongation of the Inconel 718’s excellent corrosion resistance and strength at SLM nickel-based alloys. Nimonic 263 is suitable for medium temper.129 of Wang et al. Bioactive and biodegradable magnesium orthopaedic high melting point of 3422 C and low wettability presented a implants by SLM have also been examined.12 the energy consumption of Conventional methods have limited the shape of metallic the SLM process increased when fabricating copper parts.0 137 1000 Inconel 718 1148 907 25. have also been reported in SLM literatures.111 Vilaro et al. about tool inserts with internal cooling channels made from Other metals.40 129 1096 Hastelloy X 930. such as However.041101-12 Yap et al. and et al. However. K220 copper alloy by SLM technology.146 recorded results of 7000 dental elements produced C18400. making it difficult to machine into complex b. concluded that SLM Co-Cr is suitable for dental applica- Tungsten has also been studied for the SLM process. 30 wt. has many desirable characteristics. such as gold. Inconel 718 365 125 Tungsten’s high density.155 used SLM to build the TABLE X. % Cu10Sn.157 The ability of SLM to manufacture micro-scale . They approached this problem by building a SLM sys. and mass balance in aircrafts. Biomedical and dental applications. 2. over a span of 6 months. Oyague et al. Material UTS (MPa) YS (MPa) Elongation (%) References Material UTS (MPa) Inconel 625 1030 800 10. even at low concentrations. glass to rods or ribbons. has also been published. The additive a. Metallic glass even with scanning speed as high as 400 mm/s. high melting temperature. SLM and reached different conclusions about the suitability However. such as near-theoretical per’s relatively low absorptivity to both Nd:YAG laser (63%) strength. 041101 (2015) TABLE IX. SLM of other copper alloys. and cobalt. SLM to create medium size parts. Researchers have chromium alloy. their laser system had a maximum power of 28 W of SLM technology in producing dental prostheses. and strength. Manfredi et al. One of the common applications for shapes.9 125 1000 IN738LC 1184 933 8.145 presented SLM as a feasible process. also tested SLM aluminium for heat transfer devices. c. and CO2 laser (32%) irradiation. This is due to powder metallurgy is also costly as it requires expensive and copper’s high thermal conductivity of 401 W/(m K). copper’s high reactivity to atmospheric oxygen. % 1.151 unique challenge for the SLM process. elastic modulus. and high elasticity. Aluminium alloys have been ing technique for the production of bulk metallic glass due to tested for applications in waveguide filters alongside with SLM titanium. Gu and Shen141 addressed the issue of oxida- tion by using a copper-based powder mixture with 60 wt. Becker153 published an online article examined. tantalum.5 814 35.134 robotics applications and built a first finger exoskeleton.149 sepa- tem containing a shielding box with circulating argon gas to rately evaluated the fit of dental prostheses produced by lower the partial pressure oxygen level to less than 2 bars. Pogson dedicated tools. and 10 wt. Phys.143 Fabricating tungsten alloys through conventional SLM copper is in the area of heat exchangers. whereas SLM will be able to produce Moreover. % Cu8. et al. silver. In terms and they were only able to achieve single line melt tracks at of hardness. Heat exchangers. radiation shielding. Rev. SLM of pure micro heat-exchangers where wall thickness of about tungsten and tungsten-nickel144 alloys has also been 100 lm is required.148 and Kim et al. Faure per. the water-pump Material Micro-hardness (HV) References has yet to be tested in a race car. K220. Pauly et al. Averyanova et al. good repeatability and these parts made by SLM meet the bility.147 studied the First paper on SLM of pure magnesium was published fabrication of 99% dense Co-Cr dental crown and bridges in 2010 by Ng’s group142 from The Hong Kong Polytechnic University. Applications copper.0 136 785 Nimonic 263 1085 818 24.156 examined the possibility of creating lightweight structural parts for Inconel 625 163 (from HRA) 133.150 a low scanning speed of 20 mm/s. journal publications on each of these metals are few heat sink48 and arterial wick heat pipes.4P. Cobalt-chrome has phosphorus acted as a fluxing agent against oxidation of cop- been studied by various groups for dental applications. Due to cop. and Inconel 718 470 (after aging treatment) 135 Chomel 740 130 good corrosion resistance made it a suitable material for ki- Nimonic 263 370 128 netic energy projectiles. Highest micro-hardness of SLM Inconel and nickel-based water-pump of a race car to demonstrate the capability of alloys. Ayyildiz et al. The challenge of processing magnesium with over a period of 14 months and concluded that SLM has SLM comes from its high reactivity to oxygen and flamma. Its tions. such as CuNi10.0 128 940 preventing the embrittlement of the H13 tool steel by copper the high cooling rate inherent in the process. SLM provides a feasible alternative to proc. Other applications. Appl.152 suggested that SLM of copper can be applied to essing of tungsten and widens its applications. the SLM of copper was also a challenge due to bulk metallic glass and widen its field of applications. Tungsten is very hard and brittle. traditions standards. low Young’s modulus. Highest reported tensile strengths of SLM Inconel and nickel-based alloys.154 and they are mostly focused on process optimization for these materials. Properties CoCr 482 150 a. and silicon monox- parts. Medical and dental applications 24 Carat gold 89. Highest micro-hardness of SLM aluminium. cobalt-chrome. ceramics than that of metals. there are more challenges in the SLM of 20 lm for SLM magnesium parts171 and Sustarsic et al. Relative density. Applications Mg þ 9 wt. to be deposited on to the building platform. tri-calcium-phos- information about the UTS of other metals. In most of the experiments.74 166 K220 99. and cobalt-chrome.00 164 ceramic materials. were used due to the higher absorptivity with ceramic mate- This is evident in the results reported by Sustarsic et al. They have attrib- to 95% for other metals. IV. for each material processed by SLM. creat- Cu þ Cu10Sn þ Cu8. Mapar et al. Nd:YAG fibre laser.. However. Hence. During the SLM process. % Al 75 168 24 Carat gold 29. Ceramics are generally less achieved Ra of 9 lm for CuNi15 alloy.161 found that hardness of powder mixture by spray-drying and this allowed a thin layer TABLE XI. yttria-stabilized zirconia (YSZ). Furthermore. Strength. Nevertheless. there is still room for uted this to the formation of AlCu2 during the SLM process improvement in achieving near-full density parts for these when these powders are mixed. magne- magnetic resonance compatible multi-pinhole collimator. which does not allow a thin layer of ceramic powders d.3 159 2. alumina (Al2O3).06 lm) lasers alloy with 92.20 165 group overcame this problem by lowering the melting temper- CuNi15 92. where Nd:YAG or Yb:YAG (k 1. % References a high energy input is required to increase the temperature to Al6061 96. and cobalt-chrome. The surface roughness improved to 2. ceramic powders have poor flow- ability. This is probably phate (TCP).60 169 CoCr 99. shot-peening.00 163 ing high thermal stresses. As the laser sweeps across the powder bed. cracks can form.172 c.35 lm for SLM ing of SiO2 resulted in glass-like rods with white SiO2 pow- AlSi10Mg. such as gold and silver. Jerrard et al. silicon carbide. Wilkes . sium. it helps to Al6061 þ 30 wt.173 improved the flowability of alumina-zirconia magnesium.00% relative density.4P þ Ni 95. Savalani et al. Phys. Hence.00 69 ature and providing areal heating with a secondary CO2 laser. Table XII shows the highest metals. the AlSi10Mg 99.94 135 Wissenbach’s group has been working on SLM of ce- ramic materials for medical and dental applications. who rials. magnesium. Appl. copper. 041101 (2015) structure also allowed Deprez et al. Micro-hardness. Wissenbach’s research Cu10Sn þ Cu8. Moreover. % Al 82.170 reported Ra of 14.158 to study tungsten as a TABLE XII. ceramics have a combination of high melt- ing temperatures and low ductility.90 167 A. tungsten.162 There is little (SiO2).92 158 1. silica 220 MPa and a breaking elongation of 11%.159 It allows the making of jewellery and ornaments with complex designs otherwise not Al6061 50 161 feasible with conventional methods.160 Mg þ 9 wt.69 dense than metals. Rev.4P powder 84. The highest tensile strength reported for AlSi10Mg was 400 MPa. the fig- ures attained for aluminium alloys and cobalt-chrome are the SLM part increased greatly when copper powder was above 96%. Highest reported relative density of SLM aluminium. dental porcelain. This is different from most experiments with metallic obtained relatively low strength of 400 MPa for CuNi15 powders. Surface roughness.5 lm by ders adhering to the sides. Table XI shows the highest reported relative density reported micro-hardness of the various SLM metals. High porosity usually means compromised internal ide. There are very few reports on the have shown that full melting of SiO2 was still possible with surface roughness of these metals produced by SLM. Combined with the low ductility of Cu based powder 95. Material Highest relative density. been reported for some aluminium alloys. CERAMICS b.50 161 melting point. is of Material Micro-hardness (HV) References interest to the jewellery industry. gold. with a corresponding YS of Ceramic materials studied for the SLM process included Li2O-Al2O3-SiO2 (LAS) glass.00 168 Tungsten 89.6 lm) structure. C18400 96. % Cu 200 161 AlSi10Mg 149 162 reduce the cost of production by out-phasing the use of ex- CuNi15 116 (from 110 HB) 69 pensive moulds and dies and by allowing unused powder to K220 192 167 be reused for the next batch of production. and gold. %. Yap et al. 2.041101-13 Yap et al. CO2 lasers (k 10.69 were used. reported surface roughness of Generally. because researchers have yet to produce near full-dense alumina-silica mixture.50 162 joint parts will experience high temperature fluctuation. leading to a much lower strength and ductility. In terms of relative density. Micro-hardness of SLM parts has to be spread evenly on the building platform. alumina-zirconia mixtures. gold. Although ceramic materials generally have a higher coupling efficiency with CO2 lasers. Their experiments on single track melt- Calignano et al. copper. the relative densities range from 82% added to the Al6061 powder at 30 wt. SLM of precious metals. with one vided a shorter and simpler alternative to conventional of the powder acting as the matrix material and the other as mould making and shortened the time required of the sand the reinforcing particle. 181 also reported a compressive strength Al2O3-SiO2 120 (FS) 180 of 15. SLM processing of composite materials involves rapid casting process. the speci- men was shown to be of high density but contained numer. 2. V. A. carried out SLM for a powder mixture containing tita- lation or wear resistant coating. During the SLM process. Gu alumina-zirconia can be applied to electrical or thermal insu- et al.176 Hagedorn et al. enabled by direct laser sintering. usually a ceramic. Other applications. This allowed a lower Al2O3-ZrO2 100 (close to) 27 melting temperature and kept TCP as the dominant phase in Silicate/hydroxyapatite 72 176 the part. The surface roughness. Wang’s group studied the laser General interests in SLM of composite materials have melting of silica sand for applications in metal casting caught on in the past few years. Strength (4PBS) or flexural strength Material (FS) (MPa) References There is no report on the UTS or stress-strain behaviour of SLM ceramics.178 investigated the potential of SLM on steel to create SLM composite materials. Phys. and nozzle.041101-14 Yap et al. Liu176 used silica solution-hydroxyapatite (HA) a secondary laser to solve the problem of crack formation slurry to build bone scaffold for tissue engineering and eval. ZrO2-Y2O3 56 178 ous cracks.175 ture and had potential for tissue engineering. % References implant with TCP. the part was highly porous and had a low compressive strength of 8 MPa. who deployed a sec- ondary laser as a source of pre-heating. However. than 150 lm. depending on the properties of the composite and the XIII.175 or melting of ceramics is 19 lm.1 (FS) 174 achieved 0. Four-point bending strength 2. Dental porcelain 31 (4PBS) 183 Furthermore. and boron nitride and aluminium oxide coated in titanium. nium carbide (TiC) has been used with titanium and stainless Bertrand et al. an in-situ reac- B. Surface roughness bridge were reported by Hagedorn et al. Four-point bending strength and flexural strength of SLM ce- temperature fluctuation of the SLM process. Rev. tita- casting process. tion with zirconia and a bioresorbable bone substitute Material Highest relative density. peratures and brittle nature of ceramics lead to crack formation when the parts undergo high temperature fluctua. . and achieve high relative density. Appl. It was reported that the most cases.174 published papers on a SLM-produced dental restora. pro- melting a mixture of two or more type of powders.179 nium powder and silicon nitride (Si3N4) with a composition molar ratio of 9:1. achieving high relative den- There are many applications of SLM composite materi- sity with ceramic materials is difficult as shown in Table als. High melting tem. and post-process method to improve the density. However. were able to achieve near full relative density by using a secondary laser for heating to reduce the TABLE XIV.7 (4PBS) 176 SLM ceramics and these results are tabulated in Table XIV. cobalt braze alloy Amdry 788. Relative density TiN. COMPOSITES a. 182 Al2O3-ZrO2 538. However. For instance. Under light optical microscopy. Their works involved nickel rough surfaces. However. strength. Applications tion during the process. Highest reported relative density of SLM ceramic materials.184 In surface roughness of the moulds. there are pioneering moulds. the part The lowest reported figure for Ra of direct laser sintering built had poor surface finish with Rz of 150 lm. TABLE XIII.79 (4PBS) 182 point bending strength (4PBS) and flexural strengths (FS) of Al2O3-ZrO2 500 (4PBS) 27 Silicate and hydroxyapatite 4. Works on dental restoration 3. However. Properties tion took place: 9Ti þ Si3N4 ¼ 4TiN þ Ti5Si3. there are reports on the four. The reported Rz values were no lower results concluded that the scaffold was suitable for cell cul. Hagedorn et al.185 Their work displayed another possibility of the SLM The inherent properties of ceramics make them chal. yttria-zirconia by building thin wall structures. process where the processed components had different com- lenging materials to fabricate with SLM. part of tur- There were also research works that involved in-situ for- bine blade. ZrO2 9.5 MPa for direct laser sintered silica and Ref. deployed scaffolds. silicate infiltration was used as a superalloy Mar-M-247.27 ramic materials. Microstructural analysis showed a Ti5Si3 matrix reinforced by granular 1. TCP bone substitute implant was made by using Al2O3-SiO2 95 180 alkaline borosilicate glass as binder. it also led high uated its bioactivity with microculture tetrazolium test. pounds compared to the original powder material. Ref. Hence.177 The laser-melted parts had high porosity and works in this area by Das et al. It was also suggested that SLM of mation of reinforcement particles during the SLM process. 041101 (2015) et al.181 Liu achieved Ra of SLM also provides the capability to fabricate ceramic 25 lm with silicate-HA mixture. Hence.8 MPa for TCP. 187 Lindner et al.30 198 intermediate temperature aerospace and power generation WC/Ni 90. Appl. Hao et al. This implant exploits the strength of steel and the ability of HA to stimu.188 of zirconia from 0% to 10% and this would reduce the thermal coefficient mismatch between the substrate material and the 3.197 TiC/Ti5Si3 96.191 and current SLM strength have also been reported for some SLM composite technology will be able to produce metallic and composite materials.041101-15 Yap et al. arcing resist- load-bearing bone implants. and a unique shape mem. Das et al used nickel based Gu and Shen attained UTS of 270 MPa for tungsten carbide- super alloy cermet to study the SLM production of abrasive cobalt reinforced copper MMC. inherent properties of the reinforcement materials. the type of strength sion resistance. % References added to titanium silicide (Ti5Si3) to improve its fracture TM toughness for high temperature structural applications. 1.192 Mumtaz and Hopkinson186 explored forced iron matrix. highest of all composite materials TM functionally graded zirconia reinforced Waspaloy .30 202 ern nickel based superalloys.00 204 3. Dadbakhsh et al.210 Song et al. high specific strength. SiC/Fe 99. a nickel reported thus far.199 for applications. have studied titanium-based composites for in. Micro-hardness and wear rate thermal coating. TiC ZrO2/Waspaloy 99. phate/poly(D. Some research has been motivated by specific Tungsten reinforced copper was explored by Li et al. requirement varies according to the intended application.L)-lactide composite intended for non-load ally graded composite had layer-wise increasing concentration bearing or low-load bearing implants. or heat resistant coating in engines. Most of the applications are in carbide (WC) reinforced Ni2W4C(M6C) for applications in high value-added industries. biodegradable composite material from shown in Table XV. Rev.70 203 forced by TiC was also examined to increase its wear b-tricalcium phosphate/poly(D. achieved For applications in the aerospace and the automotive UTS of 95 MPa for HA/stainless steel composite.L)-lactide reaching four- point bending strength of 23 MPa.00 188 resistance for aerospace applications. as essed a high density. b-tri-calcium phosphate and poly(D.40 201 applications as the alloy matrix is lighter compared to mod.186 applications in heavy duty electrical contacts. Kim and Creasy obtained a fracture counterparts. high temperature structural materials. Highest reported relative density of SLM composite materials. WC-Co/Cu 94.208 Hon and Gill achieved UTS of 31 MPa for poly- 2. fracture toughness and four-point bending have been examined by Salmoria et al. such as heat resistant coating in engines. ory effect. termediate and high temperature applications. The function. Hao et al. Relative density late bone and tissue re-growth at the interface of the implant Reported relative densities of SLM composites range and bone tissue in the human body.20 207 nickel-graphite powder mixture to attain in situ tungsten .193 focused on aluminium based metal matrix composites for aerospace and automotive Composite materials have improved hardness due to the applications to exploit the light-weight. Gu et al. 2.66% with an average of about 95%. This material has the potential to function as porous implants or Strength of SLM composite materials has been reported drug delivery system as NiTi has high strength.70 206 Gu also worked with Meiners198 on SLM of tungsten. 189) reinforcements were Material density. proc- from 82% to 99.66 186 TiC/Invar36 90. examined the SLM processing of HA and B. Aerospace and automotive amide/SiC composite by using 50 wt. TABLE XV. biocompatibility.190 Polymer drug delivery systems built by SLS Besides the UTS.30 205 TiN/Ti5Si3 97.187 ance and light-weight structures.189 ages of microelectronic devices. reported UTS turbine blade tips to minimize gas leakage and improve effi. 041101 (2015) demands of the applications. The reinforced copper was reported to have superior thermal management properties 1. Phys. WC/Cu 95. However. For instance. adequately industries. Medical implants and higher microwave absorption capacity. research has focused on high temperature perform- strong enough for loading bearing bone implant functions.188 ance electrodes. high corro. Strength experimented with HA and they processed it with NiTi.207 Lindner et al.90 189 FeO3/AlSi10Mg 82.188 Shishkovskii et al. of 764 MPa and YS of 302 MPa with silicon carbide rein- ciency of gas turbines. Other applications TiC/Ti 98. toughness of 95 MPa for Nylon 6 reinforced with clay nano- particles. For tooling and thermal conductivity of aluminium.187 biodegradable implants. Properties stainless steel for load bearing bone implants. reported a maximum four- based superalloy for high temperature applications.196 SLM of pure titanium rein.04 J/mm (Refs. and medicine. and blocking material for microwave pack- and light-weight. 2 and 209). such as aerospace. machining tools and moulds with improved micro-hardness. for a few early researches.L)-lactide 98.00 200 reinforced titanium-aluminium matrix was also examined for WC/Ni2W4C(M6C) 96.194 Titanium Highest relative nitride (TiN)195 and TiC (Ref. automotive. also 2. thermal point bending strength of 23 MPa for b-tricalcium phos- structures.% SiC and laser energy density of 0. Lowest reported wear rates of SLM composite materials. such as aerospace and medicine. with the exception of 2008. Vaezi et al. TABLE XVII. Figure 7 In their review. Research Material Wear rate (mm3/Nm) References interests in SLM of titanium rose sharply from 2010 as SLM titanium was found to be suitable for medical applications. As most of the TiC/Ti5Si3 980. Phys. The popularity of steel and titanium in the SLM process.3 189 research works are on metallic materials. such as NiTi.211 Table Ti6Al4V due to its vanadium composition. highlighted the potential of SLM shows the compositional breakdown of the materials as a powder bed fusion for multi-material additive FIG. desired. While ceramics and composite materials account for about Material Micro-hardness (HV) References 17% combined. Rev. % La2O3 401. high micro-hardness and low wear rates are years.145 Besides SLM of individual materials.84 105 206 Interests in high-temperature super-alloys.041101-16 Yap et al. where the composition can be easily modified by blending of differ- This literature survey has shown that research in SLM ent powders and smart materials. Research publications on SLM of various materials. Hydroxyapatite/stainless steel 241. have sparked the interests in SLM of and titanium accounting for over half of all the publications bulk metallic glass. there are also research works on SLM of multiple materials. research on SLM FeO3/AlSi10Mg 165 204 of metallic materials follows a similar trend.8 212 tor as published by Wohlers Report 2015.8 107 197 such as load-bearing implants or light-weight prostheses. Research on novel materials has increased in recent applications. Steel and titanium account for about 58% materials. such as Inconel SiC/Al 0. of all the research publications on SLM from 1999 to 2014.97 Ti24Nb4Zr8Sn XVI and Table XVII show the micro-hardness and wear rates (Ref. 041101 (2015) TABLE XVI.42 104 189 TiN/Ti5Si3 6. VI. 117) and Ti13Nb13Zr (Ref. new titanium alloys. TiC/Ti 1. . TiC/Ti5Si3 1. Highest reported micro-hardness of SLM composite researched upon. such as Ti6Al7Nb. provide titanium alloys with low moduli to reduce moduli mismatch between the implant and surrounding bone tissue. Gu and Zhang have worked on titanium based SLM have been examined to possibly replace the controversial composites to reduce the wear rate of the materials. Data are based on research publications on SLM. 7. research in SLM of steel and iron-based alloys started early and has been the leading material in this field.77 This could be WC/Cu 417. with steel recently. and DMLS indexed by Web of Science and ScienceDirect. Most has been mostly focused on metallic materials.6 203 SiC/Fe 480 213 caused by the 2008 global economic crisis.65 104 214 and other metals. based materials is due to their applications in high value. have also increased in recent years as shown by Figure 8(c). For instance. Pauly et al.9 198 2014. The dip corresponds with WC/Ni 1292 201 the decrease in revenues from various aspects of the AM sec- WC-Co/Cu þ 1 wt. there is a general trend of con- TiC/Invar36 380 200 tinual increase in the number of publications from 1999 to WC/Ni2W4C(M6C) 1870. 103) have been studied to of the various SLM composite materials reported.4 187 On a year-by-year basis. Figures 8(a) and 8(b) illustrate the trend of research for SLM and on the different types of materials. respectively. Appl. taking advantage of the high cooling rate from 1999 to 2014. 2. TiC/Ti 577 205 there are slight increases in the number of research publica- TiN/Ti5Si3 1358 206 tions on SLM of ceramics and composite materials. added industries. In recent years. SUMMARY AND REAEARCH TRENDS There are also interests in metal matrix composites. LaserCusing. Within metallic materials. it is common recent research works that focus on large powder-bed based to have components designed with simple geometries to ease AM equipment218 and post-processing of SLM parts. interests in direct part manufacturing for at the interface between the two dissimilar metals. Hussein et al. examined almost 50% in 2014 to USD 48. LaserCusing. It is shown by the increase in global In SLM. shot-peening. and composite materials. support structures are required for overhanging revenue for AM products and services. and that of AM metals that increased sink to conduct excessive heat away. or polishing and these methods have been proven the interfacial characteristics. use of the design of support structures with Ti6Al4V so as to reduce AM for direct part manufacturing takes up 42. In addition to material development. used SLM to join 316L ous types of heat treatment. It would also lower the cost of single consolidated parts to be built. . Liu et al.216 They achieved this by creat. As of 2014. 041101 (2015) FIG. improving ductility and reducing surface roughness clever design of the material coating system to solve the to meet the requirements of applications. of simple parts to be joint together. 2. (b) Research publications on SLM of metals. there are also In conventional manufacturing processes. ceramics. which quadrupled in parts.217 This would significantly reduce the amount form of direct metal part production that is tool-less and ca- of materials needed as support structure. This results in reduction in inventory and allows required for support structure. and laser scanning time cal features. The built part was found to have good metallurgical bonding In recent years. Rev. Data are based on research publications on SLM.77 SLM offers a SLM process. Most manufacturing. metals have increased.6% of total the required volume fraction for overhanging features during products and services revenues from AM. and DMLS indexed by Web of Science and ScienceDirect. (c) Research publications on SLM between different metallic materials.215 However.7 million. They include vari- problem of contamination. Phys. manufacturing.041101-17 Yap et al. stainless steel and copper (C18400) together and examined grinding. reduce the time pable of producing extremely complex shapes and geometri- required for support removal. facturing processes could only achieved by making a number quent batches of production. reducing residual stresses. Appl. (a) Research publications on SLM from 1999 to 2014. With the geometric freedom offered by SLM. age-hardening. 8. that traditional manu- the SLM process as more powders can be reused for subse. 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