In certain aerospace structures the joining of dissimilar titanium alloys may be necessary. Fusion welding of these alloys together results in the formation of large beta grains and transformed-beta microstructures that can be deleterious to mechanical performance. Friction-stir welding (FSW) was proposed due to the reported microstructural advantages afforded by the process. The purpose of this study was to friction-stir weld dissimilar titanium alloys (Ti-6Al-4V and Timetal 21S, both 1.27 mm in thickness) together and to investigate how macroscopic flow in the stir zone and the resulting weld microstructure affect mechanical properties.
Welds were produced using a refractory tool with travel speeds from 50 to 100 mm/min and tool rotation speeds of 2000 to 3500 revolutions per minute (RPM). Basketweave and colony alpha and beta phase in the prior-beta grains formed on the Ti-6Al-4V side of the stir zone and near-HAZ. The Timetal 21S region of the stir zone consisted of refined (approximately 18 μm in diameter) metastable-beta grains compared to 30 μm diameter grains in the Timetal 21S base material. Metallurgical mixing between the two alloys resulted in a unique alpha-beta microstructure in the stir zone with high hardness (450 Vicker’s Hardness Number (VHN)). The hardness increase was attributed to a fine distribution of alpha and beta phase. The highest tensile strength (1.1 GPa, 158 ksi) and elongation (8%) for as-welded specimens occurred when lower rotation speeds (3000 RPM) and highest travel speeds (100 mm/min) were used. Placement of the Timetal 21S on the retreating side resulted in the failure of tensile samples in the Timetal 21S base material. If the Ti-6Al-4V was placed on the retreating side, the failure occurred in the SZ/TMAZ region on the Timetal 21S side of the weld.
Placement of the Ti-6Al-4V alloy on the retreating side increased the amount of metallurgical mixing between the two alloys by 40% compared to when the Timetal 21S was placed on the retreating side. Electron back-scatter diffraction (EBSD) clearly showed the presence of a TMAZ adjacent to the stir zone on the Timetal 21S side of the weld. This was confirmed by the large number of low angle subgrains within the deformed metastable-beta matrix. A series of aging and solution treatment plus aging heat treatments was given to select as-welded samples. The peak hardness for all regions was obtained for the 500°C-8 hour heat treatment, while the 600°C-8 hour heat treatment effectively equalized the hardness values across the entire weld. The heat treatment response of the metallurgically-mixed alpha-beta microstructure was dependent on alpha stabilizer content.
In terms of material flow, the material directly ahead of the tool shoulder and pin is swept into the stir zone as high-temperature beta phase, so the transport of material in the stir zone during friction-stir welding was, therefore, found to be dependent on the flow stress-dependent viscosity of the alloys. In addition, subgrain formation leads to harder microstructures, so the flow would be hindered in the TMAZ, and subsequently the SZ, upon stirring.