MICROSTRUCTURE AND CRACK SENSITIVITY OF LASER-FUSION ZONES OF TI-46 MOL-PERCENT AL-2 MOL-PERCENT MO ALLOY

摘要

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 alpha(2) + massive gamma + lamellar (alpha(2) + gamma) --> massive alpha(2) + massive gamma --> alpha(2). In laser surface melting, a single-phase structure of alpha(2) was seen when the calculated average cooling rates between 1773 and 1273 K were above approximately 4000 K s(-1). In laser welding, the microstructure of the fusion zones was mainly composed of massive alpha(2) + massive gamma + lamellar. The hardness of the fusion zones increased with increasing cooling rate and the single-phase structure of alpha(2) 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 k s(-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. [References: 9]