Step-structured design and ZrC alloying for high-strength molybdenum alloy butt joints

To resolve the “weldability paradox” and intrinsic brittleness of molybdenum (Mo) alloy butt joints, this study proposes a paradigm shift by integrating stepped geometric design with a Ti–ZrC hybrid strengthening framework and thermal field regulation. By re-engineering the joint interface into a stepped configuration, a horizontal brazing-fusion hybrid zone was established, facilitating a strategic load-sharing mechanism between the Mo–Ti solid solution layer and the fusion zone (FZ). The introduction of ZrC particles purified grain boundaries via oxygen scavenging, significantly enhancing the FZ's microhardness and structural integrity. Furthermore, a 400°C preheating treatment optimized the elemental diffusion kinetics, extending the effective brazing length from 439 μm to 737 μm and dramatically increasing the proportion of low-angle grain boundaries (LAGBs). Consequently, the joint achieved a peak tensile strength of 374.2 MPa, yielding a remarkable joint efficiency of 53.5% relative to the 700 MPa base metal, while effectively shifting the primary failure region from the weld centerline to the heat-affected zone (HAZ). This multi-modal approach offers a robust technical pathway for engineering high-reliability refractory metal structures in extreme environments.