Microstructure characteristics and formation mechanism of linear friction welded TC4-DT titanium alloy joint

Abstract

The microstructure characteristics and formation mechanism of linear friction welded (LFW) TC4-DT damage-tolerant titanium alloy joint were investigated . The microstructure of each zone of the joint was analyzed in detail by using an optical microscopy and a scanning electron microscopy; the microhardness distribution of the joint was tested by means of a microhardness tester. The results show that dynamic recrystallization occurs in weld zone (WZ); the WZ temperature exceeds the β-transus temperature during welding, and both β→α′ and β→α phase transformation occur in WZ under rapid cooling after welding, resulting in a large number of α′ martensite and secondary lamellar α are formed. Due to the high deformation resistance of TC4-DT titanium alloy, the thermo-mechanically affected zone (TMAZ) of this joint is relatively narrow. The structure of the TMAZ is elongated, deformed and broken seriously under the strong thermo-mechanically coupling effect. Moreover, a few α′ martensite and a large number of secondary lamellar α are formed in TMAZ under the condition of rapid cooling after welding. The microstructure characteristics of α colony with different orientation of the base metal (BM) is basically preserved in the heat affected zone (HAZ). However, due to the influence of heat, the mutual diffusion of elements occurs at the α/β phase boundary in α colony, the interlayer β is consumed, and the primary α grows up. The refined crystalline strengthening and second phase strengthening of the WZ microstructure, the strain strengthening and second phase strengthening of the TMAZ microstructure, and the growth of α phase in the HAZ make the microhardness of above zones higher than that of the BM.

Keywords

• tc4-dt
• linear friction welding
• microstructure
• formation mechanism