淬火及超塑性变形对TC4激光焊接头组织演变机制的影响

Effect of quenching and superplastic deformation on microstructure evolution mechanism of TC4 alloy laser welded joint

对经淬火处理后的TC4钛合金激光焊接接头进行超塑性变形试验,通过光学显微镜和扫描电镜观察接头在超塑性变形过程中的组织变化,研究其组织演变机制。结果表明,超塑性变形过程中焊缝和母材组织演变的趋势一致,均是由针状α'马氏体转变为等轴状组织,但母材中的α'相转变速度比焊缝快。变形过程中,焊缝和母材中等轴组织含量在逐渐接近,且存在均匀化现象。采用组织均匀化系数ΔS来表征焊缝和母材的组织均匀化程度,发现ΔS均随着变形量的增大呈现出逐渐增大的趋势:变形量为50%,ΔS为0;当变形量达到350%时,ΔS增至65.8%。变形初期,淬火TC4钛合金激光焊接接头的合金元素发生扩散,针状α'马氏体分解形成α相和β相的片层状组织,此时变形机制是动态再结晶和动态回复;随着变形量的增大,接头的片层状组织由于塞积,相互挤压造成破裂,在晶界滑移的过程中发生球化,晶界滑移是此时接头变形的主要原因。

The superplastic deformation test of TC4 titanium alloy laser welded joint after quenching was carried out.The microstructure changes of the joint during superplastic deformation were observed by optical microscope and scanning electron microscopy,and the microstructure evolution mechanism was studied.The results show that the microstructure evolution trend of the weld and the base metal is the same during superplastic deformation,and both of them change from acicularα'martensite to equiaxed microstructure,but theα'phase transformation rate in the base metal is faster than that in the weld metal.In the process of deformation,the amount of equiaxed microstructure in the weld and the base metal is gradually close to each other,and there is homogenization phenomenon.The microstructure homogenization coefficientΔS is used to characterize the microstructure homogenization degree of the weld and the base metal.It is found that theΔS increases gradually with the increase of deformation.When the deformation is 50%,theΔS is 0,and when the deformation is 350%,theΔS increases to 65.8%.At the initial stage of deformation,the alloy elements of laser welded joint of quenched TC4 titanium alloy are diffused,the acicularα'martensite is decomposed into lamellar structure ofαphase andβphase,and the deformation mechanism is dynamic recrystallizing and dynamic recovery.With the increase of deformation,the lamellar structure of the joint ruptures due to accumulation and mutual extrusion,and spheroidizing occurs in the process of grain boundary slippage.The grain boundary slip is the main reason for the deformation of the joint at this time.