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Article history:Received 21 March 2013Accepted 29 July 2013Available online 6 August 2013

which could occur in the fusion zone or heat affected zone (HAZ),and affect the mechanical properties and service life [26]. Appli-cation of a solid state welding process like friction welding couldbe an alternative joining method to overcome these problems.

Friction welding has been commercially used in joining of aeroengine components, such as turbine blade disk (blisk) assemblies,

ne grains due toring the psize of 25

above the solvus temperatures of the strengthening precipitates,dissolution of the precipitates can occur in the weld zone andTMAZ/HAZ. Post-weld heat treatment is hence recommended toregain the mechanical properties of alloy in the weld zone. Wanget al. [11] observed no change in the ne grain microstructure ofweld zone after post-weld solution treatment at 1050 C followedby 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 c00 precipitates and loss in hardness could be recovered Corresponding author. Tel.: +91 44 22574768; fax: +91 44 22570509.

Materials and Design 53 (2014) 954961

Contents lists availab

an

elsE-mail address: ganesh@iitm.ac.in (S. Ganesh Sundara Raman).the formation of Laves phase, Niobium segregation, microssuring, As the temperatures experienced during friction welding arestrength at elevated temperatures. Alloy 718 is commonly weldedusing fusion welding techniques such as tungsten inert gas weld-ing, electron beam welding, and laser welding [1]. However, thereare problems associated with fusion welding of alloy 718 such as

Friction weld zone, in general, consists of very the occurrence of dynamic recrystallization du[9]. Wang et al. [10] reported an average grainfriction weld zone of alloy 718.0261-3069/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.07.091rocesslm in1. Introduction

Alloy 718 is a c00 (Ni3Nb) strengthened NiFe based superalloythat exhibits excellent corrosion resistance and outstanding

compressor wheel, compressor rotor, and rotor drum [7,8]. Duringfriction welding process, the material at the weld zone, thermome-chanically affected zone (TMAZ) and HAZ undergoes changes intemperature, gradient of strain, strain rate and microstructure.a b s t r a c t

The effect of post-weld heat treatments on the microstructure and mechanical properties of frictionwelded joints of alloy 718 was studied in the present work. Alloy 718 rods were friction welded withtwo prior heat treatments solution treatment and solution treatment and aging. Solution treatmentwas done at 995 C for 1 h. Aging was done at 720 C for 8 h followed by furnace cooling to 620 C andholding at 620 C for 8 h followed by air cooling. After friction welding, the joint samples were subjectedto two types of post-weld heat treatments direct aging (aging after welding, the same aging treatmentmentioned above) and solution treatment and aging. Electron back scattered diffraction technique andtransmission electron microscopy were used to study the development of microstructure. Hardnessand tensile properties of the weld joints were evaluated. In the as-welded condition, samples weldedwith prior solution treatment and aging condition exhibited lower hardness at the weld zone and inferiortensile properties compared to the base material due to the dissolution of strengthening precipitates inthe weld zone. On the other hand, formation of ne grains due to dynamic recrystallization led to higherhardness 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 weldzone and thermomechanically affected zone. Owing to the formation of strengthening precipitates, solu-tion treatment and aging post-weld heat treatment resulted in a signicant increase in tensile strength ofjoint samples compared to that of as-welded friction weld joints. However, solution treatment and agingpost-weld heat treatment done on friction weld joint samples with prior solution treatment or solutiontreatment and aging heat treatment condition resulted in inferior tensile properties compared to those ofsamples subjected to direct aging post-weld heat treatment. This may be attributed to grain coarseningthat 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 treatmentand aging after welding.

2013 Elsevier Ltd. All rights reserved.Technical Report

Effect of post-weld heat treatments on mproperties of friction welded alloy 718 jo

R. Damodaram, S. Ganesh Sundara Raman , K. PrasaDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology M

Materials

journal homepage: www.rostructure and mechanicalts

Raoas, Chennai 600036, India

le at ScienceDirect

d Design

evier .com/locate /matdes

after a post-weld heat treatment (PWHT) due to reprecipitation oc00 and c0 precipitates in RR 1000 (Ni- based super alloy) to Incone718 inertia friction weld zone. Huang et al. [13] reported that precipitate coarsening and reduction in strength depend on holdintime during PWHT of inertia friction welded Alloy 720Li to Incone718. Kim et al. [14] observed improvement in the mechanical properties after PWHT for 8 h on friction welded alloy 718 and SNCrWstainless steel (in wt% 0.2 C, 1.4 Si, 19.8 Cr, 9.5 Ni and 68.3 Fe) duto the formation of c00 strengthening precipitate.

In our previous work [15], we reported the microstructure anmechanical properties of friction welded alloy 718 joints in the aswelded and post-weld direct aged conditions. Samples welded iprior solution treatment followed by aging (STA) condition exhibited lower hardness at the weld interface compared to the basmaterial in STA condition due to the dissolution of strengtheninprecipitates. After post-weld aging, a signicant increase in hardness was observed in the weld zone for samples welded in STand solution treatment conditions. The present work was undertaken to study the effect of post-weld STA treatment on microstructure and mechanical properties of friction welded alloy 71

joints. The results were compared with those of welds subjectedto direct aging treatment.

2. Experimental details

The chemical composition (in wt%) of base material alloy 718used 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 and19.763 Fe. Alloy 718 rods of 13 mm diameter were subjected totwo different heat treatment conditions prior to welding: (i) solu-tion treatment (ST) and (ii) STA. Solution treatment was carried outat 995 C for 1 h and aging was done at 720 C for 8 h followed byfurnace cooling to 620 C and then aging at 620 C for 8 h followedby air cooling to room temperature. A furnace with a programma-ble controller was used for the heat treatment. The temperaturewas measured in the furnace chamber. The difference betweenthe measured temperature and the actual sample temperaturewas within 3 C. The average cooling rate achieved in the furnacewas 2 C/min.

A continuous drive rotary friction welding machine was usedfor welding. Weld parameters used were as follows: friction pres-sure of 300 MPa, upset pressure of 600 MPa, burn off length of

TabSam

Fig. 1. Macrostructure of a friction welded joint in the as welded condition (W2sample).

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.

ls anfl-gl-

e

d-n-eg-A-le 1ple codes used to represent samples in different conditions.

Sl.no.

Sample condition Code

1 Base material in ST condition B12 Base material in STA condition B23 Weld joint with ST as the pre-weld heat treatment W14 Weld joint with STA as the pre-weld heat treatment W25 Weld joint with ST as the pre-weld heat treatment and DA as

the post-weld heat treatmentW3

6 Weld joint with STA as the pre-weld heat treatment and DA asthe post-weld heat treatment

W4

7 Weld joint with ST as the pre-weld heat treatment and STA asthe post-weld heat treatment

W5

8 Weld joint with STA as the pre-weld heat treatment and STA asthe post-weld heat treatment

W6

R. Damodaram et al. /Materia-8d Design 53 (2014) 954961 9554 mm and speed of 1500 rpm. These parameters were chosen aftera number of trial experiments were done with parameters varyingin the following range friction pressure between 200 and

Fig. 3. EBSD (IPF + grain boundary) map of a W

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) grainboundary misorientation distribution map.

956 R. Damodaram et al. /Materials and Design 53 (2014) 954961300 MPa, upset pressure between 300 and 600 MPa and burn offlength between 2 and 4 mm. The parameters, which produced arelatively uniform and narrow weld region, were used to produceweld 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.

2 sample showing weld zone and TMAZ.

Fig. 6. (a) EBSD (IPF + grain boundary) map of HAZ of a W2 sample, and (b) grainboundary misorientation distribution map.

Fig. 7. Transmission electron micrograph of weld zone of a W2 sample showingne grains.

along with 32% low angle grain boundaries (angle 15) asshown in Fig 2(b).

Fig. 3 shows weld zone and TMAZ of a W2 sample. Weld zonemicrostructure of a W2 sample consists of new equi-axed recrys-tallized grains with an average grain size of 45 lm (Fig. 4(a)).Weld zone consists of around 68% high angle grain boundaries

d Design 53 (2014) 954961 957intermediate stacking fault energy (copper, nickel, stainless steel)have very slow recovery and dynamic recrystallization occurswhen critical dislocation density and critical strain are achieved

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) grainboundary misorientation distribution map.

Fig. 15. Microhardness proles obtained across the weld joints in differentconditions.

958 R. Damodaram et al. /Materials and Design 53 (2014) 954961

ples

ls anTable 2Average values of grain size and diagonal size of the indentations in case of two sam

R. Damodaram et al. /Materia[20]. TMAZ undergoes relatively lower amount of strain and tem-perature compared to weld zone. Serrated, deformed grain bound-aries and ne grains around the original deformed grains wereobserved in TMAZ of a W2 sample (Fig. 5(a)). TMAZ shows around

Sample Weld zone TMAZ

Grain size (lm) Diagonal size (lm) Grain size (lm)

W2 4.5 51.2 50W6 27.0 45.7 41

Table 3Room temperature tensile properties of alloy 718 base material and friction weld joint sa

Sl. no. Sample Yield strength (MPa) Ultimate tensile strength (MPa) El

1a B1 499 996 592a B2 1230 1539 223a W1 570 1003 464a W2 870 1081 155a W3 1266 1532 236a W4 1271 1520 217 W5 1254 1480 178 W6 1221 1434 17

a Data taken from Ref. [15].

Fig. 16. Appearance of fracture surface of tensile tested samples: (a) W5 sample,and (b) W6 sample.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 recrystallizedgrains at and above 1050 C for a strain of 0.7 in hot compressiontesting done at different temperatures between 950 C and1100 C and different strain rates between 0.1 and 5 103 s1.Nucleation of new grains occurs along the deformed old grainboundaries, resulting in the formation of necklace like microstruc-ture [20]. HAZ of a W2 sample showed coarser grains compared tobase material as shown in Fig. 6(a). HAZ shows 46% low angleboundaries 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 andbase material in friction welded joint (W2 sample). Transmissionelectron micrograph of weld zone of a W2 sample (Fig. 7) conrmsthat the weld zone is composed of ne equiaxed grains. Transmis-

(W2 and W6).

Base material

Diagonal size (lm) Grain size (lm) Diagonal size (lm)

55.1 29.0 48.148.0 33.0 46.8

mples in different conditions.

ongation in 24 mm gage length (%) Reduction in area (%) Failure location

54 41 58 Base material42 Interface45 Base material38 Base material42 Base material41 Base material

d Design 53 (2014) 954961 959sion electron micrograph of TMAZ of a W2 sample (Fig. 8) clearlyshows elongated grains, necklaces that form along the prior grainboundaries and dislocation cells/substructure. These specic fea-tures of dynamic recrystallisation mentioned by Humphreys andHatherly [20] are clearly seen. Transmission electron micrographof weld zone after post-weld STA treatment (W6 sample) showsthe formation of strengthening precipitates in the weld zone(Fig. 9).

The post-weld STA heat treated samples (W6) have an averagegrain size of 33 lm in base material portion of the weld joint(Fig. 10(a)). More than 90% high angle boundaries and twins wereobserved (Fig. 10(a and b)). Fig. 11 shows EBSD map of weld zoneand TMAZ after STA PWHT (W6 sample). Signicant changes in theaverage grain size, and TMAZ were observed. Drastic grain growthfrom 4 lm to 27 lm (as shown in Fig. 12(a)) was observed at theweld zone with around 88% high angle boundaries and twins(Fig. 12(b)). Similar observations have been made by Fukumotoet al. [22] in AZ31B magnesium alloy friction weld joints and Huet al. [23] in friction stir welded 2024 aluminum alloy. Grain sizedistribution in the weld zone in as-welded condition (W2) andSTA PWHT condition (W6) is shown Fig. 13. Recrystallization oc-curs in the deformed material due to the formation and migrationof high angle grain boundaries, driven by the stored energy ofdeformation [24]. Driving force for the abnormal grain growtharises by decreased stored energy. Important factors that promote

ls anabnormal grain growth are temperature, solutes and particles,specimen size and texture [20]. In the as-welded condition, TMAZshowed necklace like microstructure (W2 sample). However afterPWHT, grains in the TMAZ became coarse and equi-axed with anaverage grain size of 41 lm (as shown in Fig. 14(a)) with around86% high angle boundaries and twins (as shown Fig. 14(b)). Onlyfew grains were scanned due to coarse grain microstructure. Overall, the post-weld solution treatment was found to have a signi-cant effect on weld zone, TMAZ and base material microstructures.

3.2. Microhardness

The microhardness proles across the weld interface in frictionweld 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 basematerial due to the dissolution of strengthening precipitates inthe weld zone. On the other hand, in case of samples welded inST condition (W1 samples), formation of ne grains due to dy-namic recrystallization led to higher hardness at the weld zonecompared 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 duringfriction welding and formation of strengthening precipitates dur-ing PWHT. Post-weld solution treatment and aging produced lowerhardness values in the weld zone and TMAZ (W5 and W6 samples)when compared to post-weld direct aging treatment (W3 and W4samples). This is due to abnormal grain growth that occurred in theweld zone and TMAZ. There is no signicant change in the basematerial hardness values. Friction weld samples with prior ST orSTA heat treatment conditions exhibited similar hardness proleafter post-weld STA treatment.

In case of W2 samples (weld joint with STA as the pre-weld heattreatment), ner grains formed at the weld zone due to dynamicrecrystallization. The grain size of the base material portion ofW2 sample is 29 lm, which is coarser than the grain size of theweld zone (4.5 lm). However, due to the dissolution of strengthen-ing precipitates, the hardness of weld zone decreased. As the basematerial 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 tothat in the weld zone see Table 2). In case of W6 samples, thereis not much difference between the grain size of the weld zone andthat 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 hardnessvalues of weld zone and base material.

3.3. Tensile properties

The weld joint tensile samples in PWHT condition failed in thebase material far away from the weld zone. Post-weld STA treat-ment (W5 and W6 samples) resulted in a signicant increase inthe yield strength and ultimate tensile strength compared withthe as-welded condition (W1 and W2 samples) see Table 3. Thiscould be due to the formation of strengthening precipitates andgrain renement. However, post-weld STA samples with prior STor STA heat treatment conditions (W5 and W6 samples) exhibitedinferior tensile strength and % elongation compared to base mate-rial in STA condition (B2 samples) and samples subjected to post-

960 R. Damodaram et al. /Materiaweld direct aging (W3 and W4 samples). This may be attributedto 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 basematerial 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 strengtheningprecipitates 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 examinationof 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 onthe effect of post-weld heat treatments on the microstructureand 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 priorsolution treatment and aging condition) exhibited newequi-axed recrystallized grains with an average grain sizeof 45 lm. Fine grains around the original deformed grainswere observed in thermomechanically affected zone, whichexperienced 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 weldzone. Thermomechanically affected zone became coarse andequi-axed with an average grain size of 41 lm. The averagegrain size of the base material portion of the joint after thepost weld solution treatment and aging treatment was33 lm.

(3) Solution treatment and aging post-weld heat treatmentresulted in an abnormal grain growth in the weld zone andthermomechanically affected zone. Owing to the formationof strengthening precipitates, solution treatment and agingpost-weld heat treatment resulted in a signicant increasein tensile strength of joint samples compared to that of as-welded friction weld joints. However, solution treatmentand aging post-weld heat treatment done on friction weldjoint samples with prior solution treatment or solutiontreatment and aging heat treatment condition resulted ininferior tensile properties compared to those of samples sub-jected to direct aging post-weld heat treatment. This may beattributed to grain coarsening that occurred during the post-weld solution treatment.

(4) Compared to post-weld solution treatment and aging, post-weld direct aging treatment is recommended to restoremicrostructure and mechanical properties.

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R. Damodaram et al. /Materials and Design 53 (2014) 954961 961

Effect of post-weld heat treatments on microstructure and mechanical properties of friction welded alloy 718 joints1 Introduction2 Experimental details3 Results and discussion3.1 Characterization3.2 Microhardness3.3 Tensile properties

4 ConclusionsReferences