Refined W-3.5Nb alloy fabricated by electron beam melting via doped carbon

Electron beam melting (EBM) is a promising powder bed fusion additive manufacturing (AM) technology for the preparation of complex refractory components due to the significant advantages of vacuum environment and high level in energy utilization, scanning speed and temperature in the build chamber. However, the fabrication of crack-free and refined W-based alloy remains a challenge. This study aims to address this problem by introducing 0.7 wt% carbon in EBM W-3.5Nb alloy. Results revealed that the as built W-3.5Nb alloy had a typical columnar structure with obvious solidification cracks along the grain boundary. Doping 0.7 wt% carbon generated refined microstructure, which was composed of the initial dendrite of tungsten and the eutectic phase in the dendrites gap. Due to the low distribution coefficient of carbon, the nucleation at the bottom of the molten pool was caused by constitutional supercooling in the front of liquid solidification in a single molten pool, and velocity of the solidification front was reduced by carbon concentration leading increased thermal supercooling. Supercooling (thermal supercooling together with constitutional supercooling) refined the microstructure of W-3.5Nb alloy effectively. Meanwhile, due to the formation of eutectic phase with low melting point, the fluidity of liquid phase at the terminal solidification was improved and the solidification cracks along the grain boundary were reduced. The carbon addition also played a significant role in alloy strengthening. The microhardness of the alloy increased from 382 HV to 696 HV, and the compressive strength increased from 962.89 MPa to 1892.56 MPa. These findings offer a new perspective on the AM fabrication of high-quality W-based alloys.