Technical ReportEffect of post-weld heat treatments on microstructure and mechanical properties of friction welded alloy 718 joints R. Damodaram, S. Ganesh Sundara Raman ⇑ , K. Prasad Rao Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India a r t i c l e i n f o Article history: Received 21 March 2013 Accepted 29 July 2013 Available online 6 August 2013 a b s t r a c t The effect of post-weld heat treatments on the microstructure and mechanical properties of friction welded joints of alloy 718 was studied in the present work. Alloy 718 rods were friction welded with two prior heat treatments – solution treatment and solution treatment and aging. Solution treatment was done at 995 °C for 1 h. Aging was done at 720 °C for 8 h followed by furnace cooling to 620 °C and holding at 620 °C for 8 h followed by air cooling. After friction welding, the joint samples were subjected to two types of post-weld heat treatments – direct aging (aging after welding, the same aging treatment mentioned above) and solution treatment and aging. Electron back scattered diffraction technique and transmission electron microscopy were used to study the development of microstructure. Hardness and tensile properties of the weld joints were evaluated. In the as-welded condition, samples welded with prior solution treatment and aging condition exhibited lower hardness at the weld zone and inferior tensile properties compared to the base material due to the dissolution of strengthening precipitates in the weld zone. On the other hand, formation of fine grains due to dynamic recrystallization led to higher hardness at the weld zone compared to the base material welded with prior solution treatment condition. Solution treatment and aging post-weld heat treatment resulted in an abnormal grain growth in the weld zone and thermomechanically affected zone. Owing to the formation of strengthening precipitates, solu- tion treatment and aging post-weld heat treatment resulted in a significant increase in tensile strength of joint samples compared to that of as-welded friction weld joints. However, solution treatment and aging post-weld heat treatment done on friction weld joint samples with prior solution treatment or solution treatment and aging heat treatment condition resulted in inferior tensile properties compared to those of samples subjected to direct aging post-weld heat treatment. This may be attributed to grain coarsening that occurred during the post-weld solution treatment. Therefore, direct aging after welding is the recom- mended post-weld heat treatment for friction welded alloy 718 joints as compared to solution treatment and aging after welding. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Alloy 718 is a c 00 (Ni 3 Nb) strengthened Ni–Fe based superalloy that exhibits excellent corrosion resistance and outstanding strength at elevated temperatures. Alloy 718 is commonly welded using fusion welding techniques such as tungsten inert gas weld- ing, electron beam welding, and laser welding [1]. However, there are problems associated with fusion welding of alloy 718 such as the formation of Laves phase, Niobium segregation, microfissuring, which could occur in the fusion zone or heat affected zone (HAZ), and affect the mechanical properties and service life [2–6]. Appli- cation of a solid state welding process like friction welding could be an alternative joining method to overcome these problems. Friction welding has been commercially used in joining of aero engine components, such as turbine blade – disk (blisk) assemblies, compressor wheel, compressor rotor, and rotor drum [7,8]. During friction welding process, the material at the weld zone, thermome- chanically affected zone (TMAZ) and HAZ undergoes changes in temperature, gradient of strain, strain rate and microstructure. Friction weld zone, in general, consists of very fine grains due to the occurrence of dynamic recrystallization during the process [9]. Wang et al. [10] reported an average grain size of 2–5 lm in friction weld zone of alloy 718. As the temperatures experienced during friction welding are above the solvus temperatures of the strengthening precipitates, dissolution of the precipitates can occur in the weld zone and TMAZ/HAZ. Post-weld heat treatment is hence recommended to regain the mechanical properties of alloy in the weld zone. Wang et al. [11] observed no change in the fine grain microstructure of weld zone after post-weld solution treatment at 1050 °C followed by aging treatment. Daus et al. [12] observed a reduction in hard- ness value in the weld zone compared to base material due to dis- solution of c 00 precipitates and loss in hardness could be recovered 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.07.091 ⇑ Corresponding author. Tel.: +91 44 22574768; fax: +91 44 22570509. E-mail address:
[email protected] (S. Ganesh Sundara Raman). Materials and Design 53 (2014) 954–961 Contents lists available at ScienceDirect Materials and Design j our nal homepage: www. el sevi er . com/ l ocat e/ mat des after a post-weld heat treatment (PWHT) due to reprecipitation of c 00 and c 0 precipitates in RR 1000 (Ni- based super alloy) to Inconel 718 inertia friction weld zone. Huang et al. [13] reported that pre- cipitate coarsening and reduction in strength depend on holding time during PWHT of inertia friction welded Alloy 720Li to Inconel 718. Kim et al. [14] observed improvement in the mechanical prop- erties after PWHT for 8 h on friction welded alloy 718 and SNCrW stainless steel (in wt% 0.2 C, 1.4 Si, 19.8 Cr, 9.5 Ni and 68.3 Fe) due to the formation of c 00 strengthening precipitate. In our previous work [15], we reported the microstructure and mechanical properties of friction welded alloy 718 joints in the as- welded and post-weld direct aged conditions. Samples welded in prior solution treatment followed by aging (STA) condition exhib- ited lower hardness at the weld interface compared to the base material in STA condition due to the dissolution of strengthening precipitates. After post-weld aging, a significant increase in hard- ness was observed in the weld zone for samples welded in STA and solution treatment conditions. The present work was under- taken to study the effect of post-weld STA treatment on micro- structure and mechanical properties of friction welded alloy 718 joints. The results were compared with those of welds subjected to direct aging treatment. 2. Experimental details The chemical composition (in wt%) of base material alloy 718 used in the present study was 51.6 Ni, 18.2 Cr, 5.1 Nb, 3.28 Mo, 1.06 Ti, 0.56 Al, 0.33 V, 0.09 Mn, 0.01 S, 0.004 C, 0.003 B and 19.763 Fe. Alloy 718 rods of 13 mm diameter were subjected to two different heat treatment conditions prior to welding: (i) solu- tion treatment (ST) and (ii) STA. Solution treatment was carried out at 995 °C for 1 h and aging was done at 720 °C for 8 h followed by furnace cooling to 620 °C and then aging at 620 °C for 8 h followed by air cooling to room temperature. A furnace with a programma- ble controller was used for the heat treatment. The temperature was measured in the furnace chamber. The difference between the measured temperature and the actual sample temperature was within ±3 °C. The average cooling rate achieved in the furnace was 2 °C/min. A continuous drive rotary friction welding machine was used for welding. Weld parameters used were as follows: friction pres- sure of 300 MPa, upset pressure of 600 MPa, burn off length of 4 mm and speed of 1500 rpm. These parameters were chosen after a number of trial experiments were done with parameters varying in the following range – friction pressure between 200 and Table 1 Sample codes used to represent samples in different conditions. Sl. no. Sample condition Code 1 Base material in ST condition B1 2 Base material in STA condition B2 3 Weld joint with ST as the pre-weld heat treatment W1 4 Weld joint with STA as the pre-weld heat treatment W2 5 Weld joint with ST as the pre-weld heat treatment and DA as the post-weld heat treatment W3 6 Weld joint with STA as the pre-weld heat treatment and DA as the post-weld heat treatment W4 7 Weld joint with ST as the pre-weld heat treatment and STA as the post-weld heat treatment W5 8 Weld joint with STA as the pre-weld heat treatment and STA as the post-weld heat treatment W6 Fig. 1. Macrostructure of a friction welded joint in the as – welded condition (W2 sample). Fig. 2. (a) EBSD (IPF + grain boundary) map of a base metal sample in STA condition (B2 sample), and (b) grain boundary misorientation distribution map. R. Damodaram et al. / Materials and Design 53 (2014) 954–961 955 300 MPa, upset pressure between 300 and 600 MPa and burn off length between 2 and 4 mm. The parameters, which produced a relatively uniform and narrow weld region, were used to produce weld joint samples for the present study. Friction weld joint spec- imens were subjected to two different PWHTs – (i) direct aging (DA) (the same aging treatment mentioned above) and (ii) STA. Fig. 3. EBSD (IPF + grain boundary) map of a W2 sample showing weld zone and TMAZ. Fig. 4. (a) EBSD (IPF + grain boundary) map of weld zone of a W2 sample, and (b) grain boundary misorientation distribution map. Fig. 5. (a) EBSD (IPF + grain boundary) map of TMAZ of a W2 sample, and (b) grain boundary misorientation distribution map. Fig. 6. (a) EBSD (IPF + grain boundary) map of HAZ of a W2 sample, and (b) grain boundary misorientation distribution map. Fig. 7. Transmission electron micrograph of weld zone of a W2 sample showing fine grains. 956 R. Damodaram et al. / Materials and Design 53 (2014) 954–961 Post-weld solution treatment was carried out at 995 °C due to the following reasons. It is the solvus temperature of d phase, to relieve residual stress, to get homogenized microstructure and to improve the mechanical properties [16]. Table 1 shows the codes used to represent samples in different pre- and post-weld heat treatment conditions. Samples for electron back scattered diffraction (EBSD) analysis were polished as per standard metallographic practices followed by fine polishing with colloidal silica. EBSD measurement was car- ried out by using FEI Quanta 200 scanning electron microscope (SEM) equipped with TSL–OIM software at a step size of 0.6 lm. For transmission electron microscopic observations, specimens were polished to a thickness of 100 lm and subjected to twin jet electro polishing with an electrolyte of 10% perchloric acid in methanol at À30 °C. Vickers microhardness measurements were made across weld line with a load of 0.5 kg and dwell time of 15 s. Room temperature tensile tests were carried out as per ASTM: E8/E8M-11 standard specimen configuration. Fracture surfaces of tensile tested samples were observed using an SEM. 3. Results and discussion 3.1. Characterization The macrostructure of the friction welded alloy 718 joint (with STA as the pre-weld heat treatment) in the as-welded condition (W2 sample) is shown in Fig. 1. The weld zone produced was rela- tively uniform and narrow throughout the entire cross section. To understand the microstructural changes during friction welding, EBSD technique provides quantitative measurement of the fraction of high and low angle grain boundaries and average grain size. The EBSD (Inverse Pole Figure (IPF) + grain boundaries) map of the base material in STA condition (B2 sample) is shown in Fig. 2(a). It shows equi-axed grains and annealing twin boundaries with an average grain size of 29 lm. More than 90% of the grain boundaries were high angle grain boundaries (misorientation angle >15°) as shown in Fig 2(b). Fig. 3 shows weld zone and TMAZ of a W2 sample. Weld zone microstructure of a W2 sample consists of new equi-axed recrys- tallized grains with an average grain size of 4–5 lm (Fig. 4(a)). Weld zone consists of around 68% high angle grain boundaries along with 32% low angle grain boundaries (angle <15° misorienta- tion) (Fig. 4(b)). The temperature at the weld zone during friction welding was measured by an infrared thermometer and the peak temperature was 1118 °C [15]. Yang et al. [17] modeled inertia welding of IN 718 and calculated the strain rate to be 250 s À1 for an upset pressure of 250 MPa at 1260 °C. Srinivasan and Prasad [18] developed processing maps for hot working of IN 718. They re- ported the presence of two domains of dynamic recrystallization – one occurring at 950 °C and 0.001 s À1 and the other at 1200 °C and 0.1 s À1 . They suggested that the dynamic recrystallization in the former domain is nucleated by d (Ni 3 Nb) precipitates leading to fine grained microstructure. In the second domain, the availability of interstitial carbon atoms (due to dissolution of carbides in the matrix) increases the rate of dislocation generation and so dynamic recrystallization occurs. In a study on hot compression behavior of alloy 718, Wang et al. [19] reported the occurrence of dynamic recrystallization during deformation of parent grain boundaries, which occurs due to subgrain rotation or by formation of twinning. The sub grain rotation leads to grain boundary shearing resulting in the formation of local orientation and strain gradient, which leads to the dynamic recrystallized grains. Metals with low or intermediate stacking fault energy (copper, nickel, stainless steel) have very slow recovery and dynamic recrystallization occurs when critical dislocation density and critical strain are achieved Fig. 8. Transmission electron micrograph of TMAZ of a W2 sample showing formation of fine grains along the deformed grain boundaries. Fig. 9. Bright field transmission electron micrograph showing c 00 precipitates in weld zone of a W6 sample. Fig. 10. (a) EBSD (IPF + grain boundary) map of base material portion of a W6 sample, and (b) grain boundary misorientation distribution map. R. Damodaram et al. / Materials and Design 53 (2014) 954–961 957 Fig. 11. EBSD (IPF + grain boundary) map of a W6 sample showing weld zone and TMAZ. Fig. 12. (a) EBSD (IPF + grain boundary) map of a W6 sample showing weld zone, and (b) grain boundary misorientation distribution map. Fig. 13. Grain size distribution in weld zone of W2 and W6 samples. Fig. 14. (a) EBSD (IPF + grain boundary) map of TMAZ in a W6 sample, and (b) grain boundary misorientation distribution map. Fig. 15. Microhardness profiles obtained across the weld joints in different conditions. 958 R. Damodaram et al. / Materials and Design 53 (2014) 954–961 [20]. TMAZ undergoes relatively lower amount of strain and tem- perature compared to weld zone. Serrated, deformed grain bound- aries and fine grains around the original deformed grains were observed in TMAZ of a W2 sample (Fig. 5(a)). TMAZ shows around 57% low angle grain boundaries and 43% high angle grain bound- aries (Fig. 5(b)). Zhou and Baker [21] observed necklace like micro- structure at temperatures below 1050 °C and fully recrystallized grains at and above 1050 °C for a strain of 0.7 in hot compression testing done at different temperatures between 950 °C and 1100 °C and different strain rates between 0.1 and 5 Â 10 À3 s À1 . Nucleation of new grains occurs along the deformed old grain boundaries, resulting in the formation of necklace like microstruc- ture [20]. HAZ of a W2 sample showed coarser grains compared to base material as shown in Fig. 6(a). HAZ shows 46% low angle boundaries and 54% high angle boundaries (Fig. 6(b)). Transmission electron microscopic studies were done to sub- stantiate the results got from EBSD. Samples for transmission elec- tron microscopic observations were extracted from weld zone and base material in friction welded joint (W2 sample). Transmission electron micrograph of weld zone of a W2 sample (Fig. 7) confirms that the weld zone is composed of fine equiaxed grains. Transmis- sion electron micrograph of TMAZ of a W2 sample (Fig. 8) clearly shows elongated grains, necklaces that form along the prior grain boundaries and dislocation cells/substructure. These specific fea- tures of dynamic recrystallisation mentioned by Humphreys and Hatherly [20] are clearly seen. Transmission electron micrograph of weld zone after post-weld STA treatment (W6 sample) shows the formation of strengthening precipitates in the weld zone (Fig. 9). The post-weld STA heat treated samples (W6) have an average grain size of 33 lm in base material portion of the weld joint (Fig. 10(a)). More than 90% high angle boundaries and twins were observed (Fig. 10(a and b)). Fig. 11 shows EBSD map of weld zone and TMAZ after STA PWHT (W6 sample). Significant changes in the average grain size, and TMAZ were observed. Drastic grain growth from 4 lm to 27 lm (as shown in Fig. 12(a)) was observed at the weld zone with around 88% high angle boundaries and twins (Fig. 12(b)). Similar observations have been made by Fukumoto et al. [22] in AZ31B magnesium alloy friction weld joints and Hu et al. [23] in friction stir welded 2024 aluminum alloy. Grain size distribution in the weld zone in as-welded condition (W2) and STA PWHT condition (W6) is shown Fig. 13. Recrystallization oc- curs in the deformed material due to the formation and migration of high angle grain boundaries, driven by the stored energy of deformation [24]. Driving force for the abnormal grain growth arises by decreased stored energy. Important factors that promote Table 2 Average values of grain size and diagonal size of the indentations in case of two samples (W2 and W6). Sample Weld zone TMAZ Base material Grain size (lm) Diagonal size (lm) Grain size (lm) Diagonal size (lm) Grain size (lm) Diagonal size (lm) W2 4.5 51.2 50 55.1 29.0 48.1 W6 27.0 45.7 41 48.0 33.0 46.8 Table 3 Room temperature tensile properties of alloy 718 base material and friction weld joint samples in different conditions. Sl. no. Sample Yield strength (MPa) Ultimate tensile strength (MPa) Elongation in 24 mm gage length (%) Reduction in area (%) Failure location 1 a B1 499 996 59 54 – 2 a B2 1230 1539 22 41 – 3 a W1 570 1003 46 58 Base material 4 a W2 870 1081 15 42 Interface 5 a W3 1266 1532 23 45 Base material 6 a W4 1271 1520 21 38 Base material 7 W5 1254 1480 17 42 Base material 8 W6 1221 1434 17 41 Base material a Data taken from Ref. [15]. Fig. 16. Appearance of fracture surface of tensile tested samples: (a) W5 sample, and (b) W6 sample. R. Damodaram et al. / Materials and Design 53 (2014) 954–961 959 abnormal grain growth are temperature, solutes and particles, specimen size and texture [20]. In the as-welded condition, TMAZ showed necklace like microstructure (W2 sample). However after PWHT, grains in the TMAZ became coarse and equi-axed with an average grain size of 41 lm (as shown in Fig. 14(a)) with around 86% high angle boundaries and twins (as shown Fig. 14(b)). Only few grains were scanned due to coarse grain microstructure. Over all, the post-weld solution treatment was found to have a signifi- cant effect on weld zone, TMAZ and base material microstructures. 3.2. Microhardness The microhardness profiles across the weld interface in friction weld joints in different conditions are shown in Fig. 15. In the as- welded condition, samples welded in STA condition (W2 samples) exhibited lower hardness at the weld zone compared to the base material due to the dissolution of strengthening precipitates in the weld zone. On the other hand, in case of samples welded in ST condition (W1 samples), formation of fine grains due to dy- namic recrystallization led to higher hardness at the weld zone compared to the base material. After post-weld STA treatment, friction weld joint specimens (W5 and W6 samples) showed an in- crease in the hardness in all zones compared to that in the as- welded condition (W1 and W2 samples). The increase in the hard- ness at the weld zone is due to dynamic recrystallization during friction welding and formation of strengthening precipitates dur- ing PWHT. Post-weld solution treatment and aging produced lower hardness values in the weld zone and TMAZ (W5 and W6 samples) when compared to post-weld direct aging treatment (W3 and W4 samples). This is due to abnormal grain growth that occurred in the weld zone and TMAZ. There is no significant change in the base material hardness values. Friction weld samples with prior ST or STA heat treatment conditions exhibited similar hardness profile after post-weld STA treatment. In case of W2 samples (weld joint with STA as the pre-weld heat treatment), finer grains formed at the weld zone due to dynamic recrystallization. The grain size of the base material portion of W2 sample is 29 lm, which is coarser than the grain size of the weld zone (4.5 lm). However, due to the dissolution of strengthen- ing precipitates, the hardness of weld zone decreased. As the base material portion did not experience higher temperature as experi- enced by the weld zone, there was no dissolution of the strength- ening precipitates and so the hardness was much higher (indentation size was smaller in the base material compared to that in the weld zone – see Table 2). In case of W6 samples, there is not much difference between the grain size of the weld zone and that of the base material. It may be noted that strengthening pre- cipitates formed in the weld zone during the post-weld STA treat- ment. So there was not much difference between the hardness values of weld zone and base material. 3.3. Tensile properties The weld joint tensile samples in PWHT condition failed in the base material far away from the weld zone. Post-weld STA treat- ment (W5 and W6 samples) resulted in a significant increase in the yield strength and ultimate tensile strength compared with the as-welded condition (W1 and W2 samples) – see Table 3. This could be due to the formation of strengthening precipitates and grain refinement. However, post-weld STA samples with prior ST or STA heat treatment conditions (W5 and W6 samples) exhibited inferior tensile strength and % elongation compared to base mate- rial in STA condition (B2 samples) and samples subjected to post- weld direct aging (W3 and W4 samples). This may be attributed to grain coarsening that occurred in the post-weld solution treat- ment. Cao et al. [25] observed that post-weld STA treatment re- sulted in a slight decrease in ultimate tensile strength of base material due to grain coarsening in laser welding of Inconel 718. The base material in ST condition (B1 samples) exhibited the high- est % elongation. This may be due to dissolution of strengthening precipitates in the matrix. After post-weld STA treatment, samples (W5 and W6 samples) showed reduction in ductility due to forma- tion of strengthening precipitates in the matrix. SEM examination of fracture surfaces of tensile tested samples showed dimples indi- cating ductile mode of failure (Fig. 16). 4. Conclusions Based on the results obtained in the present investigation on the effect of post-weld heat treatments on the microstructure and mechanical properties of friction welded joints of alloy 718, the following conclusions are drawn. (1) The as-welded friction weld zone of alloy 718 (with prior solution treatment and aging condition) exhibited new equi-axed recrystallized grains with an average grain size of 4–5 lm. Fine grains around the original deformed grains were observed in thermomechanically affected zone, which experienced relatively lower strain and temperature com- pared to weld zone. (2) Post-weld solution treatment and aging resulted in a signif- icant increase in grain size from 4 lm to 27 lm at the weld zone. Thermomechanically affected zone became coarse and equi-axed with an average grain size of 41 lm. The average grain size of the base material portion of the joint after the post – weld solution treatment and aging treatment was 33 lm. (3) Solution treatment and aging post-weld heat treatment resulted in an abnormal grain growth in the weld zone and thermomechanically affected zone. Owing to the formation of strengthening precipitates, solution treatment and aging post-weld heat treatment resulted in a significant increase in tensile strength of joint samples compared to that of as- welded friction weld joints. 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