ＴｉＡｌ金属間化合物のレーザ溶融加工における溶融凝固部の組織変化と機械的特性

Abstract

Surface melting and fusion welding of a Ti-46 mol%Al-2 mol%Mo intermetallic compound were performed using a 2.5kW CO2 laser. Microstructures of the laser fusion zones were changed depending upon cooling rate. When the cooling rate at 1390K was above 3000K/s in the laser surface melting, α→γ transformation was suppressed and the fusion zone structure resulted in single phase of α2. In the cooling rates less than 3000K/s, massive γ phase was seen together with α2 phase. In the laser welding, whose cooling rates were approximately 80K/s to 1800K/s, the fusion zone microstructure consisted of massive α2, massive γ and lamellar (α2+γ) phases. Full lamellar structure was formed in the cooling rates less than 20K/s. The hardness of the fusion zones increased from 315Hv of the base metal hardness to more than 500Hv with increasing cooling rate. While all of the laser surface melted zones included cracking, in the laser welding, crack-free welds could be obtained at traverse speeds below 50.0mm/s and pre-heating temperatures above 673K. In the tensile test, the laser welded specimens without cracking were fractured in the base metal. However, the elongation of the weld metal itself was 1/10 times that of the base metal. The elongation of the weld metal was increased threefold by a heat treatment at 1173K for 0.6ks.