Microstructure and Cracking Mechanism of Pure Tungsten and W-Nb Alloys Fabricated by Selective Electron Beam Melting

Pure tungsten and solid solution strengthened W-Nb alloy were fabricated by selective electron beam melting (SEBM). The microstructure and crack defects of two tungsten based materials were analyzed. The results show that columnar crystal structure along the building direction of SEBM forms both in the pure tungsten and W-Nb alloy. The addition of Nb reduces the average size of columnar crystals from 109.78 gm to 25.10 gm. No significant cracks form in the pure tungsten, but microcracks along the grain boundaries are found in W-Nb alloys. The rapid solidification, rapid cooling and holding at elevated temperature process in SEBM cause recovery and recrystallization of W and W-Nb alloys, so that the accumulated stress during the forming process is released. The cracking of W-Nb alloy is mainly formed during solidification, that is, because the liquid metal cannot feed the dendrite in a very short time during the solidification process, nanopores form and gather at the grain boundaries, which causes cracks along the grain boundary under the action of small stress.