Formability and microstructure evolution for hot gas forming of laser-welded TA15 titanium alloy tubes

A laser-welded TA15 titanium alloy tube with outer diameter of 40 mm and thickness of 2 mm was fabricated successfully by hot U-O forming process combining with laser beam welding. The formability of the laser-welded tube was tested at 800 degrees C by hot gas free bulging on the self-designed high pressure gas forming platform. Both the microstructure evolutions of the weld seam and basal material during the hot gas free bulging were investigated by optical microscope and electron backscattered diffraction. It was found that the bulging limit of the laser-welded TA15 titanium alloy tube was 73.9% and the uniform bulging ratio was close to 43.8% taking into account shape deviation and uniformity of wall thickness in this article. Due to the lower forming temperature and shorter forming time, the microstructure of the basal material was obviously refined after the hot gas bulging, which is quite different from the traditional gas forming process like superplastic forming. The main microstructure evolution of the laser-welded TA15 titanium alloy tube during hot gas free bulging was dynamic recrystallization under these conditions, which is also the leading reason for grain refining. (C) 2015 Elsevier Ltd. All rights reserved.