简介概要

Microstructural evolution,tensile property and dynamic compressive property of FSWed Ti-6Al-4V alloy

来源期刊：Rare Metals2020年第2期

论文作者：Jia-Wei Bao Su-Yuan Yang Ting Yang

文章页码：169 - 175

摘要：Friction stir welding was applied to Ti-6 A1-4 V plates with 5 mm in thickness.The microstructure and mechanical properties were investigated.A full lamellar microstructure was developed near the top surface,and the size of prior β grain gradually decreases as the distance from the top surface increases.The microstructure of the bottom is fine equiaxed a grains,and the mean size is2 μm.A mixture microstructure consisting of primary a,lamellar α+β and fine equiaxed α is discovered in thermomechanically affected zone（TMAZ）.Results of transverse tensile test show that the tensile strength of the joint reaches 98% that of the base material（BM）.Quasi-static compression test shows that the joint exhibits larger compressive strength and failure strain than the BM.Dynamic compressive strength of the joint is close to that of the BM;furthermore,the strain at the peak stress and energy absorption of the joint are larger than those of the BM.

详情信息展示

稀有金属(英文版) 2020,39(02),169-175

Microstructural evolution,tensile property and dynamic compressive property of FSWed Ti-6Al-4V alloy

Jia-Wei Bao Su-Yuan Yang Ting Yang

作者简介：*Su-Yuan Yang,e-mail:yangsuyuan@bit.edu.cn;

收稿日期：4 September 2017

基金：financially supported by the National Natural Science Foundation of China (No.51571031);

Microstructural evolution,tensile property and dynamic compressive property of FSWed Ti-6Al-4V alloy

Jia-Wei Bao Su-Yuan Yang Ting Yang

School of Materials Science and Engineering,Beijing Institute of Technology

Abstract：

Friction stir welding was applied to Ti-6 A1-4 V plates with 5 mm in thickness.The microstructure and mechanical properties were investigated.A full lamellar microstructure was developed near the top surface,and the size of prior β grain gradually decreases as the distance from the top surface increases.The microstructure of the bottom is fine equiaxed a grains,and the mean size is2 μm.A mixture microstructure consisting of primary a,lamellar α+β and fine equiaxed α is discovered in thermomechanically affected zone(TMAZ).Results of transverse tensile test show that the tensile strength of the joint reaches 98% that of the base material(BM).Quasi-static compression test shows that the joint exhibits larger compressive strength and failure strain than the BM.Dynamic compressive strength of the joint is close to that of the BM;furthermore,the strain at the peak stress and energy absorption of the joint are larger than those of the BM.

Keyword：

Friction stir welding; Ti-6Al-4V; Microstructure; Tensile property; Dynamic compressive property;

Received： 4 September 2017

1 Introduction

As a very effective joining technique,friction stir welding(FSW) has attracted considerable attention.It has been successfully applied to soft materials such as aluminum,magnesium and copper [ 1, 2] .FSW of high-strength and high-temperature materials is still being developed,such as titanium.Ti alloys have high specific strengths and good erosion resistance,and it has been applied widely in the aerospace,automotive field and biological field.However,Ti alloys also have a high-softening temperature and low thermal conductivity,making it difficult to be welded by FSW.During the past decade,FSW of Ti alloys was gradually becoming a hot spot.Researches on microstructure,mechanical properties,corrosion behaviors and tool wear mechanism have been reported.Pilchak et al. [ 3, 4] investigated the microstructural evolution of FSW of mill-annealed and fully lamellar Ti-6Al-4V and the evolution of an initially strain-free fully lamellar microstructure to equiaxed,and bimodal and lamellar microstructure was characterized.Esmaily et al. [ 5] obtained bimodal microstructure with a peak temperature belowβ-transus temperature,andα'martensite phase was observed in FSWed specimens.Fuji et al. [ 6] and Kitamura et al. [ 7] controlled the microstructure and mechanical properties of FSW Ti-6Al-4V joints by controlling temperature during FSW.And the stir zone consisting of the fully equiaxed primaryαphase was obtained when the welding was performed belowβ-transus temperature.Jiang et al. [ 8] accomplished the welding of titanium by stationary shoulder FSW.A unique method was developed to differentiate the heat-affected zone and the thermomechanically affected zone (TMAZ).Yoon et al. [ 9, 10, 11] investigated the effect of the initial microstructure and the rotation rate on microstructure of FSWed Ti-6Al-4V plates.Su et al. [ 12] reported the microstructure and mechanical properties of a friction stir-processed Ti-6Al-4V under different processing parameters.Wang et al. [ 13] performed FSW of Ti-6Al-4V using two different tools.Room-temperature and high-temperature tensile test was used to characterize the mechanical property of the material.Zhang et al. [ 14] reported the FSW of Ti-6Al-4V under different rotational speeds,and the effect of rotational speed on microstructure and properties was investigated.Wu and Yang [ 15] analyzed effect of pin geometry and rotation speed on the migration interface of overlap joints.Edwards et al. [ 16, 17, 18, 19, 20, 21, 22, 23, 24] made a lot of achievements,and studies on weld parameters,microstructure and mechanical properties have been widely quoted.Because of the patent protection,these papers did not disclose the specific parameters of the weld tools.

Although considerable studies about FSW of Ti alloys have been published,most studies focused on the microstructure of the welded joint,and limited literature was available on the mechanical properties of FSWed titanium.Furthermore,the FSW of titanium is affected by welding tools,processing parameters (rotation speed,traverse speed) and welding surroundings.The purpose of the present study is to accomplish the FSW of Ti-6Al-4V by self-designed weld tools under appropriate processing parameters.The microstructure and mechanical properties were studied.

2 Experimental

The material used in this investigation was 5-mm-thick mill-annealed Ti-6Al-4V titanium alloy sheet.Representative micrograph is shown in Fig.1.The FSW was accomplished at Beijing FSW Technology Co,Ltd.The welding was performed on a tungsten alloy backing plate and a W-Re alloy tool.The tool shoulder diameter was25 mm.The pin was conical shape with 4.4 mm in length,and the pin was tapered from 8.5 mm at the shoulder to5 mm at the pin tip.A hole with 7.0 mm in diameter and4.5 mm in depth was made before welding,in order to reduce the wear of the tool.Weld was made along the longitudinal direction of the sheet (perpendicular to the rolling direction of the sheet) with rotation speed of100 r·min^-1 and 50 mm-min^-1.The tool tilt angle was2.5°,and the plunged depth was 0.3 mm.During welding,argon gas flowed around the tool to prevent the joints from being oxidized.

Fig.1 SEM image of representative Ti-6Al-4V base material microstructure

After welding,FSW samples were cross-sectioned transverse to the welding direction for metallographic analysis.Specimens were mechanically polished and chemically etched using etching reagent (2 ml hydrofluoric acid,8 ml nitric acid and 90 ml water).Microstructural characteristics of the base material and the transverse weld cross sections were examined by optical microscope (OM,Axiovert-200 MAT) and scan electron microscope (SEM,Hitachi S4800).

Tensile test samples were cut perpendicular to the welding direction.The dimensions of the specimens are shown in Fig.2.The thickness of the samples was 1 mm.The quasi-static compressive strength was tested on universal testing machine (WDW-E100D) under the strain rate of 1×10^-3 s^-1.The samples were cut from the advancing side to the retreating side with 4 mm in diameter and 4 mm in length.Dynamic compression tests were performed at room temperature using the split Hopkinson pressure bar(SHPB) technique,and the strain rates were set as 1000,1500 and 2000 s^-1.The samples were cut in the center of the weld joint,and the dimension of the samples was the same as that of the quasi-static samples.The calculation was based on the theory of one-dimensional elastic-wave propagation within the pressure loading bars [ 25, 26, 27] .

3 Results and discussion

3.1 Microstructure of stir zone

Figure 3 presents microstructure of Ti-6Al-4V alloys at the center of stir zone after FSW.The microstructure at the center of the stir zone exhibits inhomogeneous distribution along the plate thickness.Fully lamellar micros truc ture develops near the top surface.The size of priorβgrain gradually decreases as the distance from the top surface increases.Figure 3c exhibits a mixture of the equiaxedαgrains withα+βlamellar structure.Figure 3d shows the microstructure at the bottom of the stir zone,which has an equiaxedαgrain structure with an average grain size of about 2μm.

Fig.2 Mechanical property test diagram:a dimensions of tensile test samples (mm) and b sampling diagram of quasi-static compression test (AS,advancing side;RS,retreating side)

Fig.3 SEM images of stir zone with distances from weld surface:a d=0.5 mm,b d=1.5 mm,c d=2.5 mm and d d=4.0 mm

Owing to the low thermal conductivity of titanium alloy(about 1/5 that of iron and 1/14 that of aluminum at room temperature) and the decrease in the heat input through the thickness as the pin tapers down in size,a temperature gradient along the thickness direction exists during FSW.The top surface was rubbed directly with the shoulder that created considerable heat in this region.The maximum temperature exceeds theβ-transus temperature,and the residence time is long.Allαgrains transfer toβphase,and the priorβgrains have grew under high temperature.During the cooling process,the priorβgrains transform toα+βlamellar microstructure.With the distance from the surface increases,the size of the prior grains gradually decreases.In the middle thickness of the weld,the temperature at this location exceedsβ-transus temperature,but the maintaining time is not long enough to make the base material (BM) completely transfer toβphase.During the cooling process,after the tool has passed,theβsingle phase transforms toαphase to form the microstructure with priorαgrains.At the bottom of the weld,the size of pin tapers is small,resulting in that the heat input is less than that of the top surface of the weld.Since the thermal conductivity of titanium is low,the heat coming from the surface is limited,and these two reasons lead to the relatively low temperature compared with the top surface.The temperature near the bottom of the stir zone does not exceedβ-transus temperature.Compared with BM,the micro structure is significantly refined.It implies that dynamic recrystallization occurs in the bottom of the weld belowβ-transus temperature [ 10, 28, 29] .

Figure 4 shows the micros truc ture across the width of the weld,from the advancing side,through the center to the retreating side.The grains are fairly similar in size and structure,and there are no obvious differences from the advancing side to the retreating side.It can be inferred that the temperature from the advancing side to the retreating side had no obvious difference.

3.2 Microstructure of TMAZ

Figure 5 shows OM images of TMAZ at advancing side.Figure 5b exhibits the micros truc ture of Location 1.The micro structure consists of primaryα,lamellarα+βand fine equiaxedα.Figure 5c presents the microstructure of Location 2.The microstructure along the border between stir zone and BM consists of equiaxedαgrains with 1.5μm in mean grain size.Those images imply that the recrystallization and the phase transformation play important roles in TMAZ.Figure 5d shows the microstructure of Location 3.The microstructure near the stir zone is similar to that of BM.It should be noted that researchers have different understandings about the region between stir zone and BM.Some researchers found that only heat affected zone (HAZ) existed between stir zone and BM [ 14, 30, 31] .And some other researchers considered that TMAZ was also existed in FSW [ 8, 32] .According to the observations in this study,a narrow TMAZ with obvious deformation characteristics in FSW is observed and the grains that grow significantly are hardly found.It indicates that the heataffected zone hardly exists in this study.

Fig.4 SEM images of microstructure of Ti-6Al-4V alloys:a advancing side,b center of stir zone and c retreating side

Fig.5 Micro structure of TMAZ at advancing side:a OM image of thermomechanical zone and enlarged views of b Location 1,c Location 2 and d Location 3 in a

3.3 Transverse tensile test

Table 1 presents the tensile properties of weld joints.The tensile samples were cut from the surface (Sample 1) and the location from the weld surface of 2 mm (Sample 2).The results show that the tensile strength of the joint is1000 MPa and it reaches 98%that of the BM.The elongation of the weld joint is 12.8%,which is higher than the former results [ 7, 18, 31, 33] .Figure 6 shows the morphology of the fracture surface.Fractographs of the BM and welded materials show ductile fracture with dimple features.The fractograph of BM presents deeper dimples compared with the welded materials.It indicates that BM has better plastic deformation ability than welded material.

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Table 1 Tensile test results

Fig.6 SEM images of transverse tensile fracture surface morphology:a BM and b weld

Fig.7 Strain-stress curves of quasi-static compression tests

3.4 Quasi-static compressive property

The quasi-static compressive properties are presented in Fig.7.The compressive strength of BM is 1277 MPa,and the failure strain is 0.372.All samples across the joint have larger compressive strength and failure strain than BM.Samples 1 and 4 have similar compressive strength and failure strain,and the values were 1335 MPa and 0.400,respectively.Samples 2 and 3 which are in the center of the weld joint have the largest compressive strength and failure strain,and the values are 1357 MPa and 0.418,respectively.The compressive results indicate that the quasistatic compressive property of the weld joint is better than that of BM.

3.5 Dynamic compressive property

Figure 8 shows true stress-strain curves for BM and weld joint materials under dynamic compression,respectively.Table 2 shows the mean values of the dynamic compressive test under different strain rates.When samples fractured at 2000 s^-1,the dynamic compressive strength is almost the same as that of BM and weld joint,but the strain at the peak stress of the weld joint is 60%larger than that of BM,and the energy absorption of the weld joint is 50%higher than that of BM.The stress-strain curves show that the strain hardening effect of the BM samples is not obvious;however,the weld joint samples show apparent strain hardening effect,which may cause larger strain at the peak stress of the weld joint.Combined with the microstructure of the parent and weld joint,it can be concluded that the fine lamellar/equiaxed mixed microstructure possesses better dynamic compressive property than the original microstructure.

Fig.8 Dynamic compressive property for different samples:a BM and b weld joint

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Table 2 Dynamic compressive results

4 Conclusion

In the present study,the microstructural characteristics and mechanical properties of Ti-6Al-4V friction stir-welded joints were investigated.The microstructure of the welds varies with thickness.Full lamellar structure is observed near the top of the weld,and the mean size of priorβphase decreases with the increase in distance away from the top surface.Fine equiaxedαis discovered at the bottom of the weld.The TMAZ exists obvious deformation characteristics.Priorα,lamellarα+βand fine equiaxedαare observed in TMAZ.Friction stir weld in Ti-6Al-4V alloy can obtain similar tensile strength with the BM.However,elongation to failure in the welded samples reduces.The welded materials exhibit a larger compressive strength and failure strain compared with the BM in quasi-static compression test.The dynamic compressive strength of the weld joint is almost the same as that of the BM.Besides,the strain at the peak stress and energy absorption of the joint is larger than that of the BM.