Microstructure and crack sensitivity of laser-fusion zones of Ti−46 mol% Al−2 mol% Mo alloy

Laser surface melting and laser welding were performed on Ti-46 mol % Al-2 mol % Mo using a 2.5 kW CO2 laser. Microstructures of the fusion zones were changed in the following way as the cooling rate increased: massive α2+massive γ+lamellar(α2+γ)→ massive α2+massive γ→α2. In laser surface melting, a single-phase structure of α2 was seen when the calculated average cooling rates between 1773 and 1273 K were above approximately 4000 Ks−1. In laser welding, the microstructure of the fusion zones was mainly composed of massive α2+massive γ+lamellar. The hardness of the fusion zones increased with increasing cooling rate and the single-phase structure of α2 showed hardness above 500 Hv. While all of the laser surface-melted zones included cracking, in laser welding, crack-free welds could be obtained at traverse speeds below 50.0 mm s−1 and pre-heating temperatures above 573 K. As a result, cracking was prevented by selecting optimum welding parameters which result in calculated cooling rates between 1073 and 873 K below approximately 30 Ks−1 and the hardness of the fusion zones below approximately 400 Hv. In tensile tests, the laser-welded specimens without weld cracking fractured in the base metal. Thus, laser welding can be applied to the joining of Ti-46 mol % Al-2 mol % Mo.