Y. Zhang, New York University College of Dentistry, New York, NY; Y. S. Sato, H. Kokawa, Tohoku University, Sendai, Japan; S. H. C. Park, S. Hirano, Hitachi, Ltd., Hitachi, Japan
The present study applied friction stir welding (FSW) to Ti-15V-3Al-3Cr-3Sn (Ti-15-3-3-3) alloy plate, 3 mm in thickness, and the defect-free weld was successfully produced. The microstructure and mechanical properties of the weld were examined in the as-welded and aged states. The base material consisted of the equiaxed beta grains with a relatively high density of dislocations. An average grain size of the base material was larger than 100 mm. The cross section of the weld could be classified into four regions, such as the stir zone, the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and the unaffected base material region, like a typical friction stir weld. The heavily deformed grains were observed in the TMAZ, and no evident grain growth could be detected in the HAZ through optical microscopic examination. The stir zone was characterized by a refined grain structure with an average size less than 20 mm. The refined grain structure was homogenously distributed throughout the stir zone, whose grain size lay between 14.6 and 18.1 microns. Vickers hardness measurements showed that the hardness in the stir zone (280 HV) was lower than that in base material (286 HV), although a refined grain structure was found in the stir zone. The hardness distribution would be qualitatively explained by the dislocation density. Aging treatment at 933K for 3.6ks was employed for the weld. The alpha phases precipitated in the beta grain interiors, which raised the hardness in the weld. In the aged weld, the stir zone exhibited the lower hardness than the base material. The present study systematically clarified the microstructural factors governing the hardness profiles of the as-welded and aged welds of the Ti-15-3-3-3 alloy.
Summary: The present study applied FSW to Ti-15V-3Al-3Cr-3Sn alloy plate, 3 mm in thickness, and the defect-free weld was successfully produced. The microstructure and mechanical properties were examined in the as-welded and aged welds, and the microstructural factors governing the hardness profiles of the welds were clarified.
