Evolution of Microstructure, Microtexture and Mechanical Properties in Linear Friction Welded Titanium Alloys

Two titanium alloys were welded using linear friction welding (LFW) techniques. The two alloys consisted of one alpha + beta alloy, Ti-6Al-4V, and one near-beta alloy, Ti-5553. The welding conditions were varied in order to assess the effect of each parameter on the mechanical properties, microstructure, and crystallographic texture of the materials. Axial pressures from 50 MPa to 150 MPa, oscillation frequencies from 30 Hz to 110 Hz, and oscillation amplitudes from 1.5 mm to 3 mm were employed.;The maximum strains and strain rates experienced by the material during LFW for each set of welding parameters were estimated based on the process parameters and the measured time of oscillation. A heat input equation was developed in order to estimate the temperature at different points in the joint and temperature measurements were made during welding to corroborate the calculated temperatures. The strains, strain rates and temperatures measured and calculated for the welding conditions employed were found to be sufficient to initiate dynamic recrystallization in both alloys. This finding is in agreement with the microstructures and textures observed in the weld centres.;The near-beta Ti-5553 alloy was examined not only in the as-welded state but also in two post-weld-heat-treated (PWHT) conditions. The TMAZ and weld centre of this alloy were weakened by welding due to the reduction of the alpha phase volume fraction during rapid cooling from super-transus temperatures in and near the weld. With the restoration via PWHT plus aging of the alpha phase fraction, the UTS's for the welded samples were restored to the literature values for the heat-treated condition of this alloy.;In addition to the two titanium alloys comprising the main focus of the study, a number of other materials were examined in the context of linear friction welding. These were: IMI-834, a near-alpha titanium alloy (nominally Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.3Si); CMSX-4, a single crystal Ni-based superalloy (Ni.9.5Co-6.4Cr-6.4Ta-6.4W-5.6Al-2.9Re-1Ti-0.6 Mo-0.1Hf wt %); stainless steel 316L; mild steel A42; as well as dissimilar pairs aluminum alloy 6063 to commercial purity Cu and stainless steel 316L to Zr alloy Zr702. All of these materials were successfully linear friction welded after some refinement of the welding parameters.;The linear friction welded (LFWed) samples of Ti-6Al-4V and Ti-5553 were examined using electron backscatter diffraction techniques (EBSD) to relate the texture and phase changes to the thermomechanical conditions. Characterization of the welds included analysis of the microstructural features of the weld region and thermomechanically affected zone (TMAZ) in relation to the parent material. Mechanical properties were evaluated using tensile tests and microhardness measurements.