Microstructure and mechanical properties of M2 high speed steel produced by electron beam melting

Abstract

M2 high-carbon and high-alloy steel was prepared by electron beam melting (EBM) in this study. The optimal process parameters were explored by varying the beam current, scan speed, and scanning spacing. The microstructure and properties of manufactured samples, including chemical composition, grain morphology, texture, phase composition, the relative density, compressive strength and ductility were examined by means of SEM, TEM, EBSD, as well as compression testing. The results show that the line energy is closely related to the surface morphology and microstructure of the formed samples. High line energy leads to bulging of the sample surface. Decrease of the line energy results in an increase of the cooling rate that contributes to the refinement of grains and carbides of the as-built samples. However, when the line energy is too low, spheroidizing phenomenon takes place at the sample surface, and the density of sample is decreased significantly. The optimal combination of process parameters is achieved with 7.8 mA scanning current, 2.4 m/s scanning speed and 0.1 mm scanning spacing. The relative density of the sample can reach to 99.6%, with 793HV microhardness and 2860 MPa compressive strength. Due to the rapid cooling and gradient “annealing” process during EBM, the microstructure of the single-scan layer presents a “sandwich” structure composed of fine equiaxed crystals, columnar dendrites and large equiaxed crystals in the heat-affected zone.