| As a solid-state joining technique,friction stir welding(FSW)can avoid the inherent imperfections associated with traditional fusion welding techniques and has realized the reliable joining of aluminum alloys and other low melting point metals.However,the FSW of titanium alloy is rather constrained by the narrow process window and severe tool wear behavior.In addition,TC4 titanium alloy was usually employed as base material in previous studies and researches on the other brands of titanium alloys remain to be carried out.For example,α titanium alloy is provided with better seawater corrosion resistance and has been employed in the manufacture of structural components in ship.However,α titanium alloy is more difficult to be FSWed and related work is few,which seriously restricts the development and application of the FSW of titanium alloys.In this study,TA5 titanium alloy,a kind of α titanium alloy used in marine environment,was employed as base material and a low-cost Co-based alloy was used to fabricate FSW tools.The conventional FSW(CFSW)of TA5 titanium alloy was studied.Accordingly,new FSW methods were developed to widen the process window and reduce the negative effects of tool wear.Furthermore,the effectiveness of new methods and resultant joint microstructure and properties were systematically studied.The research on CFSW was firstly conducted,mainly including welding process characteristics,tool wear behavior and its effects on joint microstructure and properties.Results reveal that the process range of CFSW is extremely narrow and cavity defect,which reduces the bearing capacity and ductility of stir zone(SZ),can be always detected above the pin tip affected zone at SZ bottom on advancing side.At a rotation speed of 250 rpm,defect-free joint was obtained when welding speed was decreased to 30 mm/min.Meanwhile,the Co-based alloy tools were seriously worn.The inter diffusion of elements occurred between tool and the base material adhered to the tool surface,forming the interlayer rich in W and Cr.Adhesive wear is the main mechanism of tool wear and increasing heat input can effectively reduce the rate of adhesive wear.The material loss around pin tip leads to lack of penetration at weld root,reducing the fatigue performance of joint.After being introduced into SZ,the β-stable tool elements were dissolved into base material,significantly reducing local transus temperature to much lower than welding peak temperature.Therefore,transformed microstructures,such as acicular α and retained β phases,which are provided with far higher hardness than base material and have significant phase transformation strengthening effects on joint,were obtained in alloyed zone(AZ).The un-alloyed zone(UAZ),where dynamic recrystallization(DRX)occurred and resulted in refined equiaxed α grains with slightly higher hardness than base material,has a fine-grain strengthening effect on joint.The strengthening effect of SZ can effectively reduce the negative impact of cavity and lack of welding on static load strength.As a result,all joints were fractured at weld edges on retreating sides at a strength level close to base material.Based on the formation mechanism of cavity,new FSW methods were developed to eliminate defect and improve welding speed.Tool compositions were employed as tracer material to put an insight into the behavior of material flow.It’s found that cavity defect is actually the residual part of temporary cavity left by the moving tool during welding,and the key to eliminate cavity defect is reducing the temperature gradient along thickness.The simulation results of welding temperature field indicated that reducing heat dissipation at weld back could effectively reduce the temperature gradient along thickness.Titanium alloy supporting friction stir welding(TSFSW)was put forward from the aspect of reducing the thermal conduction property of backing plate.Although the welding speed was slightly increased,the backing plate was softened and deformed during welding due to the poor high temperature resistance of titanium alloy.From the perspective of reducing heat dissipation area from weld back,the conventional large backing plate was replaced by a small-sized supporting column,which was stationary with the rotating tool along welding direction,i.e.,dynamic supporting friction stir welding(DSFSW).A special welding platform was designed and manufactured,and the reliable joining of TA5 titanium alloy was successfully achieved.DSFSW enabled cavity-free joint at a significantly increased welding speed of 55 mm/min(83%higher than CFSW).The behavior of tool wear was also greatly weakened.Combining the existence of supporting column affected zone at weld root that is beneficial the coordinated deformation of j oint during tensile test with the smaller brittle CZ,the DSFSW joint was characterized by better ductility than CFSW joint.In order to further minimize the negative impact of tool wear,the dynamic supporting and tilting-pin penetrating friction stir welding(DSTPFSW)of TA5 titanium alloy was achieved.DSTPFSW experiments were conducted on the same welding platform as DSFSW,but the tool was fabricated with a pin longer than the thickness of parent plate and a blind hole is machined at the top center of supporting column.Compared with CFSW,DSTPFSW can completely avoid lack of penetration and significantly reduce the equivalent heat transfer coefficient at weld back,thereby greatly reducing the temperature gradient along thickness and increasing the welding speed to obtain defect-free joint by 100%,and the tensile strength of joint reached 99.2%of BM.DSTPFSW is capable of significantly reducing the rate of adhesive wear,thereby greatly reducing the material loss of tool.The cavity defect in CFSW joint gradually enlarged with the increase of welding length,seriously downgrading joint quality and reducing the stability of welding process.In contrast,DSTPFSW enables defect-free formation of weld in a length of no less than 0.68 m at a welding speed twice as CFSW using one single tool.Consequently,it’s inferred that DSTPFSW has good application prospects in the FSW of titanium alloys.In order to reveal the FSW mechanism of TA5 titanium alloy,microstructure evolution mechanisms of the joint were studied.It’s found that continuous dynamic recrystallization(CDRX)is the main recrystallization mechanism.Affected by the original microstructural state of base material,twinning deformation is constrained in TMAZ,<a>type and<c+a>type dislocation slips are the main deformation mechanisms.For UAZ,{1012} twinning and {1011} twinning are also important plastic deformation mechanisms.For the UAZ in CFSW joint,the material undergone deformation only in HCP-type a phase zone,both<a>and<c+a>dislocations were operated,prismatic<a>slip was the main mechanism of dislocation slip and the strength of<c+a>dislocation slip around weld bottom was rather low.DSTPFSW can increase the welding peak temperature to above α-β transus point in the upper and middle parts of UAZ,thus the material was deformed in BCC-type β phase zone with better plasticity.At weld bottom,DSTPFSW greatly improves the effective heat input,thereby significantly enhancing<c+a>slip and improving the coordinated deformation ability of material. |
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