Microstructure and mechanical properties of molybdenum-titanium-zirconium-carbon alloy TZM processed via laser powder-bed fusion

Abstract

Molybdenum, processed by laser powder-bed fusion (LPBF), is susceptible to hot cracking because segregated oxygen impurities significantly weaken grain boundaries through the formation of MoO2. The present study reports on the LPBF processing of the most important molybdenum alloy TZM, whose alloying elements—titanium, zirconium, and carbon—lead to particle and solid solution strengthening. Results of investigations into the resulting microstructure and mechanical properties when processing TZM by LPBF are presented. The alloying elements suppress the segregation of oxygen to the grain boundaries so that crack-free samples with a density of 99.7 ± 0.3% may be produced. The microstructure shows grains that are columnar due to epitaxial grain growth and a weak 〈111〉 fiber texture parallel to the building direction. Mo2C and ZrO2 particles with a size of <50 nm are precipitated in the grain interior. Oxygen is not only bound by ZrO2. Research has shown that the ternary molybdenum‑titanium carbide, which can be found in LPBF – TZM – as is the case with pressed/sintered TZM—can dissolve oxygen. While the bending strength is 591 ± 26 MPa for samples in which only pores with a diameter of <50 μm could be detected on the fracture surface, the bending strength drops to 267 ± 50 MPa for samples with defects of 400 μm. In both cases, the fracture mode is transgranular brittle.