The unique evolution of HfC during laser powder bed fusion manufacturing W-HfC alloys and its influence on the microstructure and mechanical properties

Relatively low density, columnar crystal structure and cracks are always the inevitable hurdles in achieving excellent mechanical performance for tungsten (W) or tungsten alloys manufactured by laser powder bed fusion (LPBF) process. In this work, HfC particles were introduced to tungsten (W) to investigate their influence on cracking inhibition and mechanical properties of W alloys. Particularly, possible reactions and evolution of HfC particles during the LPBF process were analyzed thermodynamically. It was found that Hf and C, decomposed from HfC, would enter into W lattice to form substitution solid solution and reacted with W to form tungsten carbides during the cooling process, acting as the wall of cellular structure. Meanwhile, the added micron-sized HfC particles went through melting, re-solidification to regenerate during the LPBF process. Microstructure examination indicated that the crystallographic orientation relationship between re-generated nano-sized HfC nanoparticles and W matrix was (200)_HfC // (110)_W and [0$\bar{1}$1]_HfC // [001]_W due to the small lattice mismatch and low interface energy between them. Mitigated thermal stress and uniformly dispersed carbide particles not only suppressed cracking but also refined the grains from 47.0 μm to 34.6 μm and altered columnar crystal structure with weakened <111>// BD texture. Together with solid solution strengthening, the maximum compressive strength of W-HfC alloys reached 1832 MPa. The elucidation about the evolution of carbide particles during the LPBF process may provide an applicable strategy for a reasonable carbide strengthening phase for W alloys based on lattice matching during laser additive manufacturing process.