See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/225589930 Friction Stir Processing Technology: A Review Article in Metallurgical and Materials Transactions A · March 2008 Impact Factor: 1.73 · DOI: 10.1007/s11661-007-9459-0 CITATIONS READS 289 392 1 author: Z.Y. Ma Chinese Academy of Sciences 279 PUBLICATIONS 7,624 CITATIONS SEE PROFILE Available from: Z.Y. Ma Retrieved on: 19 April 2016 [3–12] Currently. resulting in significant microstructural refinement. 2007. MA. FSP is a short-route. The FSP technique has been successfully used for producing the fine-grained structure and surface composite. friction stir processing (FSP). Third. Professor is with the Institute of Metal Research. FSP has distinct advantages. and noise. First. a finegrained microstructure for high-strain-rate superplasticity was obtained in the commercial 7075Al alloy through FSP.[17. FSP parameters. P. and homogeneity of the processed zone.[17. The heating is achieved by the friction between the tool and workpieces and by the plastic deformation of the material.Friction Stir Processing Technology: A Review Z. The FSP technique is emerging as a very effective solid-state processing technique that can provide localized modification and control of microstructures in the near-surface layers of processed metallic components. Quite a few successful applications of FSW have been demonstrated. applications of the FSW technique have significantly outpaced the fundamental understanding of microstructural evolution and microstructure-property relationships. metal matrix composites. Fourth. Thus. FSP does not change the shape and size of the processed components. densification. and cast aluminum alloys. the current state of the understanding and development of FSP is addressed. and thermal exposure. it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld with conventional fusion welding techniques. Chinese Academy of Sciences.19] Furthermore.R.cn Manuscript submitted August 16. a new processing technique. was developed by Mishra et al. In this case. with the depth being between several hundred micrometers and tens of millimeters. 2008 642—VOLUME 39A. to be joined and then traversed along the line of joint when the shoulder touches the plates (Figure 1). Shenyang. China. a rotating tool with Z. modifying the microstructure of materials. the depth of the processed zone can be optionally adjusted by changing the length of the tool pin. for localized microstructural modification for specific property enhancement.[20] and the homogenization of powder metallurgy (PM) aluminum alloys. environment friendliness. In the relatively short duration after its invention. and synthesizing the composite and intermetallic compound in situ. A nonconsumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of plates.Y. which means FSP is a green and energy-efficient technique without deleterious gas.Y. Localized heating softens the material around the pin and the combination of tool rotation and translation results in the movement of material from the front to the back of the pin. Contact e-mail: zyma@imr. In this review article. MA Friction stir processing (FSP). a solid-state joining process originally developed for aluminum alloys. Fifth. and versatility. material mixing. MARCH 2008 pin and shoulder is inserted in a single piece of material. due to its energy efficiency. solid-state processing technique with onestep processing that achieves microstructural refinement.18] for microstructural modification based on the basic principles of FSW.1007/s11661-007-9459-0 The Minerals. the microstructure and mechanical properties of the processed zone can be accurately controlled by optimizing the tool design. The tool heats the workpieces and moves the material to produce the joint. densification.ac. the FSP technique has been used for the fabrication of a surface composite on aluminum substrate. FSP is a versatile technique with a comprehensive function for the fabrication. is an emerging metalworking technique that can provide localized modification and control of microstructures in near-surface layers of processed metallic components. Article published online February 1. Second. and synthesis of materials. eradiation. processing. In particular.[13–16] Recently. it is difficult to achieve an optionally adjusted processed depth using other metalworking techniques. the heat input during FSP comes from friction and plastic deformation. METALLURGICAL AND MATERIALS TRANSACTIONS A . United Kingdom) in 1991.[1. For example. and active cooling/ heating. DOI: 10. The FSW process is considered to be the most significant development in metal joining in the past decades and is a ‘‘green’’ technique. Sixth. processing. and homogeneity. increasing applications are being found for FSP in the fabrication.[21–23] Compared to other metalworking techniques.[17–35] In this article. INTRODUCTION FRICTION stir welding (FSW) is a new solid-state joining technique invented at The Welding Institute (TWI) (Cambridge. and synthesis of metallic materials. developed based on the basic principles of friction stir welding (FSW).. a welded joint is produced in solid state. the current state of understanding and development of FSP are reviewed. The FSP causes intense plastic deformation.2] The basic concept of FSW is remarkably simple. Metals & Materials Society and ASM International 2008 I. By overlapping the processing passes. It has been demonstrated that the FSP parameters. FRICTION-STIR FINE-GRAINED STRUCTURE The use of FSP generates significant frictional heating and intense plastic deformation.[57.7 lm was obtained. On the other hand.[60] and accumulative roll bonding (ARB)[61] have been used to produce the fine-grained materials for superplasticity.4 to 0. A fine-grained microstructure of ~7.[42. The use of the FSP technique resulted in the generation of significant superplasticity in a number of aluminum and magnesium alloys.45] The variation trend for YS and hardness follows the Hall– Petch relationship.46–50] significant superplasticity was illustrated by using full-sized specimens along the FSP direction.[5] it is generally believed that the grain refinement is due to dynamic recrystallization.[19. fine and equiaxed recrystallized grains of quite uniform size were produced in the SZ. thereby resulting in an increase in both yield strength (YS) and hardness.[44.[19] Figure 8 shows the effect of grain size on the superplasticity of FSP 7075Al alloys as a function of initial strain rate. 2—Optical micrograph showing fine and equiaxed grains in FSP 7075Al-T651. the ultra-fine-grained microstructure of 0.5 lm was produced at a tool rotation rate of 400 rpm and a traverse speed of 102 mm/min. The microhardness (HV) of FSP AZ31 increases as the grain size decreases.[38] reported the generation of 100-nm recrystallized grains in FSP 7075Al-T651 by means of rapid cooling behind the tool. and active cooling/heating. It is noted that the grain size of FSP samples can be tailor made by adjusting the FSP parameters Fig. as shown in Figure 6.58] torsion under compression. It was reported that the microstructural refinement via FSP resulted in significantly enhanced superplasticity. METALLURGICAL AND MATERIALS TRANSACTIONS A and tool geometries. the microstructural refinement in the light alloys via FSP induced the occurrence of structural superplasticity.41] This ratio is significantly higher than that obtained in conventional thermo mechanical processed (TMP) aluminum alloys with a typical ratio of 50 to 65 pct.36–39] Therefore. using a mixture of water. vertical pressure. workpiece temperature.[2] However.[59] multiaxial alternative forging (MAF). as shown in Figure 5. material chemistry. vertical pressure.[37] Therefore.[2] Figure 3 shows transmission electron microscopy (TEM) micrographs of FSP Al-4Mg-1Zr samples under different FSP conditions. in particular. at processing parameters of 400 rpm and 102 mm/min. tool geometry.[2–4. as a result.Fig. In this case. recrystallized grains 25 to 100 nm in size obtained in FSP 7075Al-T651 by using the ‘‘plunge and extract’’ technique and rapid cooling indicated that the observed micrometric grains in the SZ appeared because of the growth of the nanosized recrystallized grains. MARCH 2008—643 . and a shift to higher optimum strain rates and lower temperatures.[39] In addition to the fine and equiaxed grains. the factors influencing the nucleation and growth of the dynamic recrystallization will determine the resultant grain microstructure in the SZ. the microstructure in FSP aluminum alloys is characterized by a high fraction of high-angle boundaries. it is likely to achieve the nanostructured aluminum alloys via FSP with active cooling.40. In the SZ of FSW/FSP aluminum samples. various processing techniques such as thermomechanical treatment (TMT).[51–54] In the past few years.[2] II. the micrometric grains were usually observed. VOLUME 39A. tool design.56] equal channel angular pressing (ECAP). methanol. it is likely that the mechanical properties of a metallic material can be tailor made via FSP. Although most superplastic data were obtained by using minitension specimens taken from the traverse section of the FSP samples. The fraction of high-angle boundaries is as high as 85 to 95 pct. Table I summarizes the superplastic properties of various FSP alloys. bulk ultra-fine-grained aluminum alloy could be produced via FSP.36. 1—Schematic drawing of FSW. reduced flow stress. The grain size of aluminum and magnesium alloys was continuously reduced by adjusting the FSP conditions. and dry ice (Figure 4). Although there is still a controversy about the grain-refinement mechanism in the SZ. at high strain rates or low temperatures.41.[55.43] Because the grain microstructure can be controlled by changing the FSP parameters. and active cooling exert a significant effect on the size of the recrystallized grains in the SZ.[18. Figure 2 shows a typical microstructure of FSP 7075AlT651. thereby resulting in the occurrence of dynamic recrystallization in the stirred zone (SZ). A typical plot for the Hall–Petch relationship for FSP ZA31 is shown in Figure 7. Su et al. with grain size and optimum strain rate and temperature. 63] Depending on the length of the tool pin. Mahoney et al. and (c) improved tool (12 mm. 350 rpm. and a recrystallization treatment. and 25 mm/min.[62. have illustrated uniform elongations (El. pin). multiple-pass warm rolling with intermittent reheating. For ECAP. it is possible to achieve the superplastic forming of thick plates.[36] Fig.[54] For TMT. a large METALLURGICAL AND MATERIALS TRANSACTIONS A .[38] 644—VOLUME 39A. shoulder and 8 mm. (b) improved tool (12 mm. TMT involves solution treatment. 300 rpm.) up to 500 pct in a 12-mm-thick 7475Al plate prepared via FSP. FSP does not reduce the thickness of the processed plates. at least 4 to 6 passes are required to achieve microstructural refinement. First. FSP is a relatively simple processing technique with a onestep processing that produces a fine-grained microstructure. Other processing techniques are relatively complex and time-consuming and lead to increased material cost. and 203 mm/min. 3—TEM micrograph showing grain structure in FSP Al-4Mg-1Zr. and 203 mm/min. pin).[57. the fine-grained microstructure can be achieved in the plates with a thickness of up to tens of millimeters via FSP. 4—Nanosized grain structure in FSP 7075Al with corresponding selected area diffraction (SAD) pattern. 600 rpm.58] Second. shoulder and 4 mm. MARCH 2008 The advantages of the FSP technique compared to these other processing techniques are summarized. For example. overaging. (a) Standard tool (18 mm. pin). therefore. shoulder and 4 mm. In unpublished research.Fig. A concept of selective superplastic forming has been suggested and demonstrated by means of FSP. and (d) pass 2 as indicated by the regions 1 through 3 in (a). as a function of strain rate. 5—(a) Macroimage of FSP 7075Al after four passes. Fig. produces the superplastic plates having a thickness of less than 3 mm. 8—Effect of grain size on superplastic El. 6—Grain-boundary misorientation distribution in FSP 2024Al. (c) overlap between passes 1 and 2.[19] rolling reduction. MARCH 2008—645 . a local processing can be achieved via FSP. (b) through (d ) TEM micrographs showing grain structures: (b) pass 1.[44] METALLURGICAL AND MATERIALS TRANSACTIONS A FRICTION-STIR SURFACE/BULK COMPOSITE The use of the FSP technique results in the intense plastic deformation and mixing of material in the VOLUME 39A.[41] Fig.[64] By comparison. it is possible to produce a local fine-grained microstructure in a region that will undergo superplastic deformation. Third. other techniques cannot produce microstructural refinement on a selective basis. the fine and equiaxed grains with high ratio of high-angle boundaries in FSP aluminum alloys resulted in the generation of high-strain-rate superplasticity and lowtemperature superplasticity. indicating the formation of a large fraction of high-angle grain boundaries. 7—Hall–Petch relationship for FSP AZ31 magnesium alloy. as shown in Table I. of FSP 7075Al-T651. In this case. Fourth.Fig. III.[39] Fig. typically. required for obtaining the fine-grained microstructure. 8 3. the true strain during FSP was estimated to be as high as 40. The aluminum plates with a preplaced SiC particle layer were subjected to FSP. L-orientation: The gage length of full-sized specimens is within the SZ along the FSP direction.Table I. was generated on the substrates of 5083Al and A356 aluminum alloys (Figure 9). and was then applied to the surface of the plates. incorporating 15 vol pct SiC into the surface layer of A356 via FSP increased the HV from 98 to 171. Fig. With the optimized tool design and processing parameters. Based on the original study of Mishra et al. 48 36 49 50 51 52 53 54 *T-orientation: The gage length (1. 5 to 27 vol pct of the SiC particles could be incorporated into the aluminum matrix. The SiC powder was added into a small amount of methanol and mixed. to form the surface composites. Materials A Summary of Superplastic Properties of FSP Aluminum and Magnesium Alloys Grain Size (lm) 7075 2024 5083 Al-4Mg-1Zr Al-4Mg-1Zr Al-8. MARCH 2008 Fig. By adjusting the FSP parameters.3 mm) of the minitension specimens is at the center of the SZ on the traverse section of the FSP samples.[20] reported the first result on the fabrication of a SiCp-Al surface composite via FSP. and increased as the SiC particle volume fraction was increased for the FSP SiCp/5083Al surface composite (Figure 10).7 ~3.[65] In this case. it is possible to incorporate the ceramic particles into the metallic substrate plate. Mishra et al.09Sc A356 AZ91 AM60B 7075 7475 3. 646—VOLUME 39A.9Zn-2. The HV of the surface layer of aluminum substrates was significantly enhanced with the incorporation of SiC particles.[20] additional research efforts were dedicated to fabricating the surface/bulk composites via FSP with improved METALLURGICAL AND MATERIALS TRANSACTIONS A .0 4 4 — — Temperature (C) 480 430 530 525 175 310 530 300 250 460 460 Strain Rate (s-1) 1 1 3 1 1 3 1 5 1 1 2 · · · · · · · · · · · -2 10 10-2 10-3 10-1 10-4 10-2 10-3 10-4 10-4 10-3 10-4 Specimen Orientation/Geometry* El. to form a uniform thin SiC particle layer.6Mg-0. (Pct) Reference T/mini size T/mini size T/mini size T/mini size T/mini size T/mini size T/mini size L/full size L/full size L/full size L/full size 1250 525 590 1280 240 1165 650 ~1200 ~970 ~800 670 19 41 46 47. 9—Optical micrograph showing surface composite layers fabricated by FSP in (a) A356 and (b) 5083Al substrates with SiC particle volume fractions of ~15 and ~13 vol pct. Similarly.9 6 1.. with welldistributed particles and a good bonding with the aluminum substrate.5 0. respectively. 10—Variation in hardness with SiC particle volume fraction in SiCp/5083Al surface composite layers fabricated by FSP.7 0.[66] processed zone. a composite layer of ~100 lm. and the SiC particles and multiwalled carbon nanotubes were filled into the groove. FRICTION-STIR MICROSTRUCTURAL MODIFICATION During FSP.68] In their studies. MARCH 2008—647 . FSP can be developed as a generic tool for modifying the microstructure of heterogeneous metallic materials such as cast alloys. Dixit et al.[67] More recently. The bulk composites fabricated via FSP showed enhanced hardness and strengths (Table II).72] Eutectic modifiers and high-temperature VOLUME 39A.26] reported the fabrication of SiC particles and multiwalled carbon-nanotubes-reinforced AZ31 surface composites via FSP. Hv YS (MPa) (MPa) (Pct) AZ61 billet 60 FSP AZ61 72 FSP SiO2/AZ61 105 AZ31 billet 50 FSP AZ31 69 FSP ZrO2/AZ31 98 140 147 225 100 120 143 190 242 251 160 204 232 13 11 4 9 18 6 the NiTi-particle-reinforced metal matrix composites. the rotating pin with a threaded design produces an intense breaking and mixing effect in the processed zone.[72] The as-cast structure of Al-Si-Mg (Cu) alloys is characterized by porosity.[27] successfully dispersed and embedded nitinol particles in 1100Al matrix via FSP. toughness and fatigue resistance.[71.[24] particle predeposition methods.Fig. It is important to note that the fabrication of the surface/bulk composites is achieved under a solid-state condition. The interfacial reaction has been identified to be a key issue for fabricating METALLURGICAL AND MATERIALS TRANSACTIONS A Table II.[24. These microstructural features limit the mechanical properties of cast alloys.9 mm below the surface of the 1100Al plate to load the NiTi powders. in which nanosized SiO2 or ZrO2 were filled to the desired amount. and drilled ~0. and densified structure.67. and coarse primary aluminum dendrites (Figures 12(a) and 13(a)). in particular. metal matrix composites.[69. The FSP was conducted along the groove and produced a thick surface composite layer with remarkably refined grains. 11—SEM micrographs showing the SiO2 particle dispersion in the SiO2/AZ61 composite prepared by FSP (two grooves and four passes).68] Substrates AZ61 AZ31 Materials UTS El.70] IV. No interfacial reaction occurred between the reinforcing particles and the metallic matrix. because they have good castability[71] and can be strengthened by artificial aging. one or two grooves 6 mm in depth and 1.5 to 3 lm (Figure 11). Thus. Furthermore. thereby creating a fine. a high-strain-rate superplasticity of >400 pct was obtained in a AZ61 composite reinforced by nanosized SiO2 particles at high rates of 1 · 10-2 to 3 · 10-1 s-1.6 mm in diameter and 76 mm in length. thereby creating a uniform bulk composite with a certain number of particle clusters of 0. Hardness and Tensile Properties of 10 Volume Percent Nanosized Particles Reinforced Magnesium Matrix Composites Fabricated by FSP[24. uniform.25 mm in width was cut on the AZ61 plate. and good interfacial bonding was observed.[25. Therefore. A multiple-pass FSP was conducted along the length direction of the grooves. A groove 2 mm in depth and 1 mm in width was cut on the AZ31 plate. A.26] With deeper grooves being cut.[25. Cast Aluminum Alloys Alloys of Al-Si-Mg (Cu) are widely used to cast highstrength components in the aerospace and automobile industries. Subjecting the powder-filled plate to FSP produced a uniform NiTi/1100Al composite with directional residual stresses and improved mechanical properties. and nanophase aluminum alloys prepared through the PM technique. Lee et al. They used four small holes 1. the hardness and thermal stability of the surface layer were improved. coarse acicular Si particles. using this particle predeposition method. Morisada et al. One of the methods is to cut one or two grooves along the FSP direction to predeposit the particles inside. demonstrated a successful fabrication of bulk composites via FSP. and the uniform distribution of broken Si particles in the aluminum matrix (Figure 12(b)). due to the intensified stirring effect (Table III).020 648—VOLUME 39A. as summarized in a recent review article. METALLURGICAL AND MATERIALS TRANSACTIONS A . 203 mm/min. the closure of casting porosities. the breakup of coarse acicular Si particles reduced the Si particle cracking under low stress and. 12—Optical micrographs showing morphology and distribution of Si particles in A356 samples: (a) as-cast and (b) FSP at 900 rpm and 203 mm/min. 900 rpm. MARCH 2008 SZ with the refined grains has been widely observed in FSP/FSW wrought aluminum alloys. Their results indicated that FSP resulted in the significant breakup of coarse acicular Si particles and coarse primary aluminum dendrites. consequently. and of multipass FSP A319. the room-temperature natural aging of the supersaturated aluminum solid solution produced by FSP resulted in an increase in the strength of the FSP samples.30 0. The grain size in the FSP samples is much smaller than that in the as-cast structures.70 5.087 2. Ma et al. A356.[76] showed that FSP destroyed the coarse and heterogeneous cast structure of A319 and A356 sand castings and created a uniform distribution of broken second-phase particles (Figure 13(b)). most of the solutes were retained in solution. due to rapid cooling from the FSP temperature. the fatigue resistance of the Al-Si alloys was enhanced by FSP. 900 rpm. thereby improving the ductility and increasing the strength.[73–75] However. Fourth. Increasing the rotation rate and number of FSP passes resulted in a decrease in the size and aspect ratio of the Si particles and the porosity level. minimized the possibility of the void initiation associated with damaged Si particles. indicating the dissolution of most of the Mg2Si precipitates during FSP.Fig. and ADC12 by using fullsized specimens.[29] Second.29] conducted FSP on sand-cast A356 plates under wide FSP parameters.43 1. Similarly.[78] After FSP. indicating the occurrence of dynamic recrystallization during FSP. thereby improving the strength and ductility of Al-Si alloys.032 2. 203 mm/min FSP. in the ductility—is achieved by FSP.50 1.75 2. the ultra-fine Si particles produced by FSP exerted additional strengthening effects on the aluminum matrix through dislocation/particle interaction. Furthermore. TEM examinations revealed that the coarse Mg2Si precipitates in the as-cast A356 sample disappeared after FSP. to enhance the mechanical properties of the castings.[29] heat treatment are widely used to refine the microstructure of cast Al-Si alloys.86 0.99 0. the fundamental elimination of porosity reduced the possibility of void initiation at such porosities. the microstructural refinement of the aluminum matrix produced by FSP (Figure 14) increased the strength of the FSP samples through Hall–Petch strengthening. The TEM observations revealed the generation of a fine-grained structure of 5 to 8 lm in FSP A356[29] and 2 to 3 lm in FSP ADC12. Size and Aspect Ratio of Si Particles and Porosity Volume Fraction in FSP and As-Cast A356[29] Materials As-cast FSP. Santella et al.92 2. none of these approaches can heal the casting porosity effectively and redistribute the Si particles uniformly into the aluminum matrix. The Table III.[29. Third. 300 rpm. Further. First.[2] Furthermore. thereby forming a supersaturated aluminum solid solution.[29] The accelerated dissolution of the Mg2Si precipitates during a short period of the FSW cycle was attributed to significantly accelerated diffusion rates and shortened diffusion distances of the solutes resulting from intense plastic deformation and material mixing.76] The improvement in the mechanical properties of FSP Al-Si alloys is attributed to the following factors. An improvement in the tensile properties of Al-Si alloys—in particular. 2 pass Porosity Volume Particle Size Aspect (lm) Ratio Fraction (Pct) 16.[23.[77] Figure 14 shows the fine-grained structure in the FSP ADC12 alloy. Tables IV and V show the tensile properties of singlepass FSP A356 by using minitension specimens. 51 mm/min FSP.95 0. 102 mm/min).8 further decrease in the porosity level and Si particle size and an increase in the natural aging-strengthening resulting from the additional dissolution of the Mg2Si precipitates produced by two-pass FSP. Aged two-pass FSP samples exhibited a very high strength with a good ductility. due to the dissolution of more precipitates during FSP. (Pct) 169 202 232 255 132 137 140 162 3 30 38 34 153 212 275 304 138 153 204 236 2 26 30 25 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A. 300 rpm. These property changes are attributed to a Table IV.[77] Increasing the tool rotation rate from 300 to 900 rpm resulted in increased strength in the FSP A356 samples (Table IV). This is attributed to a decrease in the porosity level and the Si particle size and an increase in natural aging-strengthening resulting from additional dissolution of the Mg2Si precipitates at a higher tool rotation rate. due to the precipitation of more solutes. For higher rotation rates or two-pass FSP.3 mm and a Gage Width of 1 mm[29] Aged (155 C/4 h) As-Cast or As-FSP Materials As-cast FSP.[76] Table V. (Pct) UTS (MPa) YS (MPa) El.0 2.4 173.6 296. (Pct) — T — T — T L 160.0 2.0 13. A post-FSP artificial aging resulted in a further increase in strength and a reduction in ductility.Fig. 900 rpm.4 162.[29] This indicates that maximum strengths can be achieved in post-FSP aged A356 samples by controlling the FSP parameters. Fig. the artificial aging-strengthening effect was significantly enhanced. 500 mm/min). Tensile Properties of As-Cast and FSP A356 Samples by Using Minitension Specimen with a Gage Length of 1. 51 mm/min FSP. MARCH 2008—649 .6 8. Tensile Properties of As-Cast and Multipass FSP Aluminum Alloys along the Directions Perpendicular (T Direction) or Parallel (L Direction) to the FSP Direction[76. 2 pass UTS (MPa) YS (MPa) El. 13—Optical micrographs showing (a) as-cast microstructure and (b) SZ of A319 (1000 rpm. comparable to the T6-treatment properties.[29] Two-pass FSP with 100 pct overlap resulted in an increase in strength and a decrease in ductility.9 100.77] Materials Cast 319 FSP 319 Cast A356 FSP A356 Cast ADC12 FSP ADC12 FSP ADC12 Tension Direction UTS (MPa) YS (MPa) El.0 190 314 330 154. 203 mm/min FSP.7 7.3 133. 900 rpm. 203 mm/min. and that achieving these strengths does not require a separate T6 solution treatment.4 87. 14—TEM micrographs showing fine-grained structure in FSP ADC12 (1250 rpm.2 7.1 — — — 0. Mg3Zn3Y2. and therefore exhibit poor mechanical properties. 16—SEM micrographs of thixomolded and FSP Mg-6Al-3Ca-0. the Al2Ca network in thixomolded Mg-6Al-3Ca-0.5 Re-0. It is necessary to modify the morphology and the distribution of the coarse second-phase particles in order to enhance the mechanical properties of the castings.2Mn samples showing (a) networklike Al2Ca phase and (b) finegrained structure (1500 rpm. such as a networklike b-Mg17Al12 phase in AZ91. such a procedure is time consuming. along the grain boundaries. It was reported that the multipass ECAP of AZ91. However. Mg3Zn6Y. a solution treatment at ~415 C for up to 40 hours was used to dissolve the eutectic b-Mg17Al12 network in the Mg-Al-Zn alloys. MARCH 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A . homogenized at 413 C for 18 to 19 hours. and Mg5(Gd.[80–83] While the coarse eutectic b-Mg17Al12 network in cast AZ91 was mostly dissolved into the magnesium matrix during FSP (Figure 15). Alternately.2Mn was broken into the fine Al2Ca particles with a size of 20 to 100 nm (Figure 16). The as-cast alloys are characterized by coarse a-Mg dendrites and coarse heterogeneously distributed second-phase particles. and then an aging is conducted to generate the fine precipitates. resulting in not only increased material cost but also surface oxidation and grain coarsening. 15—SEM micrographs showing dendrites and networklike b-Mg17Al12 phase in (a) as-cast AZ91 and fine-grained structure in (b) two-pass FSP AZ91 (400 rpm.Y) can be used at higher temperatures.B. For the heat-resistant Fig. Conventionally. 100 mm/min). a solution treatment at a higher temperature for a long time is required to dissolve the second-phase particles into the magnesium matrix.[79] However.5Re-0. For example. resulted in both significant grain refinement and a change in the b-Mg17Al12 morphology from a long rod-like shape to smaller particles. 1500 mm/min). Cast Magnesium Alloys Alloys of Mg-Al-Zn with a higher aluminum content were widely used as diecast or wrought alloys such as AZ91 and AZ80. Use of the FSP technique resulted in the significant breakup/dissolution of the coarse second-phase particles and grain refinement. the plasticdeformation route was used. Mg14Nd2Y.[32] Fig. at least 4 to 6 passes of ECAP were required to achieve microstructural refinement and homogenization.[82] 650—VOLUME 39A. Heat-resistant magnesium alloys with thermally stable second-phase particles such as Al2Ca. 5Re-0. Grain Size and Microhardness of As-Cast and FSP Magnesium Alloys[32. due to the refinement of both grains and particles.[84] The duration for the conventional thermal cycle is much longer than that associated with the FSP thermal cycle. a relatively large recrystallized grain of ~15 lm was obtained.5Y-4. MARCH 2008—651 . slow cooling rates result in the generation of coarse microstructures with surface and subsurface defects. the FSP sample exhibited significantly enhanced tensile properties. 17—Tensile properties of AZ91D magnesium alloy. due to the extensive continuous precipitation of fine b-Mg17Al12 particles.[29] However. For the AZ91 alloy. Second. After aging at 168 C for 16 hours. in such applications. and nonsparking behavior. While aging did not improve the tensile properties of the cast sample. strength. because of Hall–Petch slopes. the solid-solution strengthening resulting from the supersaturated aluminum also contributed to the improvement in the strength of the FSP sample. k. the significant breakup and dissolution of the coarse b-Mg17Al12 phase reduced the possibility of the cracking and debonding of the b-Mg17Al12 phase under lower stress. First.[32] C. the NAB components are usually not heat treatable.9 72 1 METALLURGICAL AND MATERIALS TRANSACTIONS A 73 Fig. due to their combination of corrosion resistance. thereby considerably improving the ductility and strength of the FSP sample. due to the low diffusion rate of aluminum in magnesium matrix.80. the post-FSP aging produced extensive continuous precipitation within the grains.3Zn Conditions Grain size Hardness (lm) (Hv) as-cast 100 to 200 two-pass FSP 15 two-pass FSP + 15 aging as-cast 6 70 75 98 single-pass FSP as-cast single-pass FSP 85 60 98 0. a very fine grain of ~1 lm was produced. This is attributed to two competitive factors influencing the hardness. the hardness of the FSP samples is much higher than that of the base metals.82] Materials AZ91 Mg-6Al-3 Ca-0. several investigations have been conducted to evaluate the effect of FSP on the microstructural involution and mechanical properties of the VOLUME 39A.. the remarkable grain refinement increases the YS of the AZ91D alloy significantly.[32] because of the significant breakup and dissolution of the b-Mg17Al12 phase and remarkable grain refinement.[32] The continuous or discontinuous precipitation was previously observed in the AZ91 supersaturated solid solution aged at 168 C for 16 to 24 hours. the hardness of the nugget zone increased considerably. FSP represents an alternative means of selectively strengthening the surfaces of such components.[90] The static strength of thick as-cast sections usually drops well below the minimum specification values. with the sample exhibiting a typical ductile dimplefracture. The as-cast AZ91D sample exhibited lower YS. however. and reduced El. and lower El. that are larger in the magnesium alloy than in the aluminum alloy. due to the significantly accelerated diffusion rate and shortened diffusion distance.[78] For the FSP AZ91 sample with a high supersaturation of aluminum resulting from FSP thermal cycles.[32] presence of the coarse b-Mg17Al12 network at the grain boundaries. UTS. with a strain rate of 100 to 103 s-1[44.. The room-temperature tensile properties of the cast and FSP AZ91D samples are summarized in Figure 17. for the AZ91 alloy.[88] Table VI shows the variation in the HV of the cast magnesium alloys before and after FSP. fracture toughness. due to the extensive precipitation of fine b-Mg17Al12 particles. In this case. the hardness of the as-FSP sample is only slightly higher than that of the base metal. ultimate tensile strengths (UTS). Furthermore. friction coefficients. Thus.85–87] and a strain of up to ~40. due to the Table VI. By comparison.[89] For cast thick-section NAB components. thereby reducing the physical and mechanical properties. Cast NiAl Bronzes Cast nickel-aluminum bronzes (NAB) are widely used for marine components.magnesium alloys.[65] facilitates the dissolution of b-Mg17Al12 phase. which tends to crack or debond from the magnesium matrix early. severe plastic deformation in the nugget zone.[32] For Mg-Al alloys. for FSW/FSP. under lower stress during tensile deformation. the as-FSP sample exhibited a hardness level equivalent to that of the base metal. due to the absence of the pinning particles (Table VI). The dissolution of the b-Mg17Al12 phase in the FSP AZ91 samples was verified by energy-dispersive spectroscopy (EDS) and differential scanning calorimetry (DSC) analyses. In the past few years. it takes up to ~40 hours at ~415 C to achieve the complete dissolution of the b-Mg17Al12 phase. While the breakup and dissolution of the eutectic b-Mg17Al12 phase tend to decrease the hardness of the AZ91D sample. Third. because the fine.2Mn Mg-5. the aged FSP sample exhibited much enhanced YS. the significant grain refinement increases the hardness of the sample.[32] For heat-resistant magnesium alloys. thermally stable particles can prevent grain growth during the FSP thermal cycle. the linear raster obviously reduced the ductility of the NiAl bronze.5-mm prior b grains delineated by grain-boundary a and containing a coarse lamellar a + b colony structure (Figure 19(a)). it was noted that. for multipass FSP covering large surface areas.101] It was reported that the FSP samples in the a/b and b FSP conditions had fatigue lives over an order of magnitude longer than the as-cast sample. and fatigue life. complex. FSP resulted in a 140 to 172 pct increase in YS and a 40 to 57 pct increase in tensile strength. the coarse as-cast or as-cast and hot isostatically pressed (HIP) components do not have the fatigue crack initiation resistance that can be realized in wrought products. 152 mm/min). (Pct) YS (MPa) UTS (MPa) El.97] Furthermore.4 METALLURGICAL AND MATERIALS TRANSACTIONS A . Table VII shows the tensile properties of the multipass FSP NiAl bronze with linear and rectangular spiral rastering patterns. The replacement of mechanically fastened assembled parts by large.[98] It is expected that the fatigue life of titanium castings can be improved by converting the as-cast microstructure to a finegrained wrought microstructure through FSP.8 14. This was attributed to the uplifted grains with columnar morphology in the FSP NiAl bronze with the linear raster pattern. The HIP sample consisted of 1. Cast Titanium Alloys There is an increasing interest in the applications of the near-net-shape structural castings of titanium alloys in the aerospace industries. This indicates that the HIP cycle did not change the as-cast structure.Fig. the microstructure of the FSP Ti-6Al-4V samples was characterized by equiaxed a grains 1 to 2 lm in size with a fine lamellar structure between some of the primary a grains that formed through the decomposition of the b phase after rapid cooling from the maximum temperature (Figure 19(b)). However.[100.4 21. Clearly..3 17. which cause strain localization. MARCH 2008 Rectangular Spiral Raster YS (MPa) UTS (MPa) El. irrespective of FSP rastering patterns. 18—Optical micrographs showing (a) as-cast microstructure of Cu-9Al-5Ni-4Fe under a near-equilibrium condition and (b) uniform microstructure of the center in SZ of FSP sample (800 rpm. as shown in Figure 18. The improvement in the properties of the FSP Ti-6Al-4V samples is attributed to the decrease Tensile Properties of FSP UNS95800 NiAl Bronze[97] Linear Raster FSP Parameters: rpm. However.[91–97] This type of microstructural evolution in the FSP NiAl bronze significantly improved the tensile properties and fatigue strength.0 25. a typical microstructure obtained with slow cooling from above the b transus. and fastener holes. mm/min 800. due to the elimination of grain-boundary a of the as-cast microstructure. 76 1200.8 24. 51 As-cast 652—VOLUME 39A. The microstructural refinement of Ti-6Al-4V resulted in increased YS. (Pct) 504 522 518 192 765 769 805 530 12.[101] Furthermore. By comparison.[92] cast NAB.8 460 507 472 743 761 743 30.[97] D. it was indicated that rastering procedures produced an obvious effect on the mechanical properties of the FSP NiAl bronze—in particular. while the rectangular spiral raster resulted in an improvement in tensile El. the tensile ductility and stress corrosion resistance of the FSP samples were enhanced. due to the elimination of fasteners Table VII. 102 1000. crack initiation resistance. one-piece castings reduces the structural weight and improves fatigue life.[91–97] It was indicated that FSP resulted in significant microstructural homogenization and refinements in the as-cast NAB and converted the microstructures from a cast to a wrought condition.[91. on ductility. Figure 19 shows the effect of FSP on the microstructure of a HIP investment-cast Ti-6Al-4V sample. and disperse the particles into the aluminum matrix. (b) as-extruded. 102 mm/min).[99] as shown in Figure 19. even under a large extrusion ratio or rolling reduction. Fig.Fig. and (c) as-FSP. MARCH 2008—653 . However.[103–107] Figure 20 shows SEM micrographs of PM Al-10 pct Ti-2 pct Cu nanophase aluminum alloys under as-HIP. 20—SEM micrographs of PM-processed Al-10 pct Ti-2 pct Cu nanophase aluminum alloys under (a) as-HIP. It is well established that slip length has a predominant effect on the mechanical properties of Ti-6Al-4V. such as extrusion and rolling.[99] in the effective slip length.[21] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A. the extrusion and rolling usually cannot achieve a complete homogenization of the PM alloys and composites. a subsequent hot-working route. close the porosity.[102] E. PM Aluminum-Based Alloys and Composites For aluminum-based alloys or composites prepared through the PM technique. due to significant microstructural refinement by FSP. 19—Backscattered SEM images of (a) HIP investment-cast Ti-6Al-4V sample with fully lamellar a + b structure and (b) FSP Ti-6Al-4V sample with fine-grained structure (100 rpm. is necessary to break down the prior-particle boundaries. For a fixed matrix powder size. respectively. and as-FSP conditions. and the accelerated diffusion of elements. a significantly enhanced ductility was achieved. 21—Optical micrographs showing SiC particle distribution in 25vol pct SiCp/6061Al composites: (a) as-extruded condition and (b) FSP condition.4 13. it is difficult to fabricate the composites with uniformly distributed particles using the current PM technique. (Pct) Al-10 pct as-HIP Ti-2 pct Cu as-extruded as-FSP 25 vol pct as-extruded SiCp/6061Al as-FSP 266 740 703 244 264 — 660 645 208 224 0 1. 654—VOLUME 39A. and the microstructure was still heterogeneous. followed by a subsequent extrusion at an extrusion ratio Table VIII. and rewelding) of powder particles.22] Materials Condition UTS (MPa) YS (MPa) El.2 of 25:1. when very small reinforcements are used. due to the intense plastic deformation and material mixing. Tensile Properties of As-Extruded/As-HIP and FSP PM Aluminum Alloy and Composite[21.as-extruded. Fig. V. fracturing.[78] Biallas et al. both the UTS and YS of the alloy increased significantly. it is likely to achieve in-situ reaction between elements via FSP. The substantially improved distribution uniformity of the SiC particles increases the strength and ductility of the FSP composite significantly (Table VIII).[65] resulting in the significant breakup of coarse secondary phase and material mixing.[109] Therefore. have suggested that the material flow around the pin during FSW is somewhat similar to that of regular milling of the metal. Figure 21 shows optical micrographs of as-extruded and FSP 25 vol pct SiCp/6061Al composites. Clearly. The FSP technique. This indicates that the extrusion did not produce a uniform distribution of the SiC particles when a large initial Al/SiC particle size ratio of 8. FRICTION-STIR IN-SITU SYNTHESIS As discussed earlier.3 4. consisting of the repeated deformation (welding. Tensile tests indicate that the as-HIP Al-10 pct Ti-2 pct Cu nanophase aluminum alloy exhibited very low tensile strength without ductility (Table VIII). with effective mixing and dispersion ability. By comparison. FSP is a very effective approach to improving the mechanical properties of PM nanophase aluminum alloys. After the extrusion. FSP produces thermal exposure and intense plastic deformation with a strain rate of 100 to 103 s-1[44. with the UTS and YS being slightly reduced. The as-HIP structure is quite heterogeneous and is characterized by nonbroken.[108] Mechanical alloying. This is attributed to the insufficient and directional deformation of the extrusion process. with coarse Al3Ti particles being aligned along the extrusion direction (Figure 20(b)). After FSP. with Al and SiC particle sizes of 108. and the SiC particles exhibited a relatively random distribution without preferential orientation (Figure 21(b)). FSP created quite a uniform microstructure. could potentially enable the fabrication of the composites with uniformly distributed fine reinforcing particles. It is expected that this type of uniform structure will exhibit enhanced mechanical properties. but the ductility was still very low.1 was adopted.4 8. After FSP.4 lm. cryomilled powders and aluminum-rich regions in between (Figure 20(a)). the particle distribution was quite heterogeneous. both the cryomilled powders and the aluminum-rich regions were elongated along the extrusion direction. in which the cryomilledpowder boundaries and aluminum-rich regions were completely eliminated and fine Al3Ti particles were uniformly distributed into the aluminum matrix (Figure 20(c)). has been widely used as a means of synthesizing nanostructured material. and the SiC cluster was distinctly visible (Figure 21(a)).6 and 13. The composite was prepared via the PM route. MARCH 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A . the SiC particles tended to align along the extrusion direction. After the extrusion. the distribution uniformity of the SiC particles improved significantly.85–87] and a strain of up to ~40. For the as-extruded composite. [33] Fig. and nanosized Al3Ti particles formed in situ were uniformly dispersed in the ultra-finegrained aluminum matrix. showing (a) coarse Al2Cu/Cu particles under as-sintered condition and (b) fine and uniformly distributed Al2Cu particles after a subsequent two-pass FSP. reported the first result on the fabrication of in-situ ultra-fine-grained Al-Al2Cu composite. as shown in Figure 23(b).Fig. only a small amount of Al-Ti reaction occurred around the Ti particles (Figure 23(a)). a lower sintering temperature of 500 C for 20 minutes could not achieve the reaction between aluminum and copper. However. With an increase in the Ti content. The FSP accelerated the in-situ reaction significantly. the resultant Al2Cu particles are coarse and heterogeneously distributed. creating an ultra-fine-grained Al-Al2Cu composite (Figure 22(b)). Recently. pct Ti sample sintered at 610 C and (b) TEM bright-field image showing uniformly distributed nanosized Al3Ti particles in four-pass FSP Al-10 at. 23—(a) BEI showing coarse unreacted Ti particles in Al-15 at. Hsu et al. This indicates that. but the ductility decreases. pct Ti sample. A twopass FSP on the billets sintered at both 500 C and 530 C resulted in a thorough Al-Cu reaction and a uniform distribution of the resultant Al2Cu particles METALLURGICAL AND MATERIALS TRANSACTIONS A in the aluminum matrix. even though even the reaction between aluminum and copper can be thoroughly completed under higher sintering temperatures for a longer time.[35] investigated the effect of FSP on the in-situ reaction between aluminum and titanium in an Al-Ti sintered billet. 22—Backscattered electron imaging (BEI) of Al-15 at. Increasing the sintering temperature to 530 C enhanced the reaction significantly. pct Cu samples sintered at 530 C. Metal Matrix Composites Hsu et al. It was indicated that even at a higher sintering temperature of 610 C.[35] A. After a four-pass FSP. Tensile tests indicate that the Al-Al3Ti composites prepared by FSP exhibit high strength and modulus (Table IX). This ultra-finegrained Al-Al2Cu composite exhibited higher hardness and compressive strength. both the strength and modulus increase. VOLUME 39A. MARCH 2008—655 . the initial coarse Cu or resultant Al2Cu particles were heterogeneously distributed in the aluminum matrix (Figure 22(a)). the Al-Ti reaction completed fundamentally.[33] For the Al-Cu sintered billet. 5 system. Furthermore.[34] second-phase particles. with the desired portions of Mg.5Al25Zn37. Figure 25 shows the hardness profiles along the cross sections of FSP samples with various ratios of Mg. the thin foils of AZ31. pct Ti 82 95 108 277 383 471 313 435 518 18 14 1 The composite of Al-10 at. some other nanosized ternary or binary phases were also detected.2. 0. Al. 25—Hardness profiles along the transverse cross-sectional plane of Mg60Al15Zn25. B. the formation of Mg3Al2Zn3 phase and other phases increased the hardness of the material significantly. and Zn. thereby creating a fine. as shown in Figure 24(a).5 system after three passes without liquid N2 cooling. the refinement of matrix grains. pct Ti Al-15 at. FSP offers very attractive possibilities for commercial success. Multipass FSP was conducted with or without liquid N2 cooling. and versatile. Fig. the closure of porosity. the maximum hardness approaches nearly 400 Hv.[34] reported the first result on the fabrication of multi-elemental Mg-Al-Zn intermetallic alloys via FSP. with a thickness of 1. Although a number of challenges still exist.125 mm. Mg50Al20Zn30. Intermetallic Compounds Chuang et al. environment friendly. were stacked like a sandwich and held vertically. SUMMARY AND FUTURE OUTLOOK Intense plastic deformation and thermal exposure during FSP causes the breakup of coarse dendrites and Fig. pct Ti exhibits a combination of high strength/modulus and good ductility. and synthesizing in-situ the composite/intermetallic compound have demonstrated that FSP is energy efficient. The successful applications of FSP in producing the fine-grained structure and surface composite. Clearly. and can be developed to be a generic metalworking technique that can provide the localized modification and control of microstructures in the near-surface layers of processed metallic components. and Mg37. and pore-free wrought structure. at a tool rotation rate of 1500 rpm and a traverse speed of 20 mm/min. (Pct) Al-5 at. MARCH 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A . and the dissolution of precipitates. an increasing number of applications will be found for FSP in the fabrication.5Al25Zn37. and Zn. It was revealed that FSP resulted in the generation of a Mg3Al2Zn3 phase and significant grain refinement (Figure 24(b)). VI. after three passes without liquid N2 cooling. Al. For the Mg37. respectively. In their study.125-mm thick in lighter contrast) and (b) SEM/BEI micrograph showing the phase dispersion in the Mg37. 24—(a) Schematic illustration of the stacked AZ31 Mg alloy sheets (1-mm thick in dark contrast) with pure Al and pure Zn foils (0.Table IX. and 0.5Al25Zn37.5.and 0. modifying the microstructure of nonhomogeneous materials. processing. uniform. Tensile Properties of Al-Al3Ti Composites Prepared by Four-Pass FSP[35] Materials E (GPa) YS (MPa) UTS (MPa) El.2. With further research efforts and an increased understanding of the FSP process. Al. and Zn. The hardness of the FSP samples increased with the increase in the volume fraction of the Mg3Al2Zn3 phase.[34] 656—VOLUME 39A. pct Ti Al-10 at. and synthesis of metallic materials. R. G. J. Ding. Rhodes.B. pp. Needham. Z. 66. 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