Failure analysis of hot extrusion die based on dimensional metrology,micro-characterization and numerical simulation — A case study of Ti alloy parts

From an actual production campaign, the hot extrusion process of titanium alloy in a combined die cavity was considered to understand the failure modes of extrusion die. The macro failures and damage states were qualitatively analysed, and the geometric deviation of die cavity was quantified by measuring the local geometric contour. The microstructure of the maximal damage zone of the scrapped die was examined by using optical microscopy (OM) and scanning electron microscopy (SEM). The microhardness profiles from the die surface layer to the matrix was also measured. Combined with a thermal mechanical finite element method (FEM), the squashing and extruding process were modeled to evaluate the thermal and mechanical loadings on the die cavity. The results show that the prime damage zones under different damage states are located at the local positions of the die transition fillets. At the bearing channel and the transition fillets of die cavity, aside from the inevitable wear, the local micro-plastic deformation and thermal mechanical fatigue peeling are the main reasons for the die failure. This investigation aims to guide the design and manufacturing of dies, to select the appropriate material of the dies, and to plan the heat treatment regime and working conditions. Meanwhile, it is also the prerequisite to develop a reasonable die strengthening or repairing method through surface engineering method.