Linear friction welding of Ti60 near-α titanium alloy: Investigating phase transformations and dynamic recrystallization mechanisms

Abstract

Linear friction welding (LFW) is a solid-state welding process used primarily in manufacturing of aeroengine blisks. Ti60 near-α titanium alloy was developed for blisks operating at 600 °C, and shows excellent thermal stability and high-temperature oxidation resistance. In order to promote the wider use of such blisks, the mechanisms of microstructure evolution including phase transformation and dynamic recrystallization of welded Ti60 joint were studied. The effects of phase transformation and dynamic recrystallization on microhardness and tensile strength were investigated as well. Results show that the microstructure of the thermo-mechanically affected zone (TMAZ) deforms severely along the friction direction, forming {0001}[11 2 ¯ 0] α texture. β → metastable β then → α + β transformations occur, leading to lamellar α precipitating from the deformed β. Sufficient continuous dynamic recrystallization makes the WZ almost composed of fine equiaxed grains of 10 μm size. In addition, α → β with the subsequent αˊ transformation occurs in the weld zone (WZ), producing extensive distribution of acicular αˊmartensite inside the recrystallized grains. Under the effects of strain hardening, precipitation strengthening and fine grain strengthening, the hardness in WZ increases significantly (i.e., 381 HV) compared to base metal (BM) (335 HV), with the tensile strength of the joint (943 MPa) also increasing over that of the BM (914 MPa). However, because of inhomogeneous deformation, the elongation of the joint (13.3%) is slightly lower than that of BM (14.1%). • β → metastable β then → α + β transformations occur in TMAZ, leading to lamellar α precipitating from deformed β; α → β with the subsequent αˊ transformation occurs in WZ, producing extensive distribution of acicular αˊmartensite. • Adequate continuous dynamic recrystallization occurs in weld zone, accompanying by violent dislocation motion and formation of many subgrains and low-angle grain boundaries. • The tensile strength of the joint (943 MPa) increases to that of the base metal (914 MPa), but the elongation of the joint (13.3%) is slightly lower than that of the base metal (14.1%) because of inhomogeneous deformation.