Friction Stir Welding of near α and α + β Titanium Alloys: Metallurgical and Mechanical Characterization

Abstract

Butt welds of friction stir welded dissimilar titanium alloys (near α: Ti-6242 standard grain (SG) and α + β ; Ti-54M) were produced for varying processing parameters (rotation speed: rpm and traverse speed; mm·min−1). Microstructures, microhardness, and fractured surfaces were analyzed for three different rpms and mm·min−1 with Ti-6242 SG and Ti-54M kept on the advancing and retreating side, respectively. While constant traverse speed (varying rotation speed) has no significant effect on micrographic patterns in weld nugget, constant rotation speed (with increasing traverse speed) results in an increasing number of streaks with specified spacing (advances per revolution) (consisting of material migrating from retreating side) on the advancing side. Although, hardness variation within streaks (due to lower values of v ω ; where v and ω are traverse and rotation speed) were challenging to evaluate, yet hardness maps imitated the micrographic morphology of the weld nugget. For varying rotation (225–325 rpm) and traverse speed (100–150 mm·min−1), corresponding microstructure evolutions on the advancing and retreating side were related, with variations in evolving temperatures for corresponding welding parameters. Fractured surfaces revealed an appearance of a combination of transcrystalline and intercrystalline fracture for all the processing parameters. Nature of solid state joining has been shown with a distinct boundary between Ti-6242 SG and Ti-54M, demonstrating the interlocking between streaks of different aspect ratios of these two alloys.

Keywords

• friction stir welding
• dissimilar titanium alloys
• microstructure
• microhardness
• fractured surfaces