Effect of hot isostatic pressing on porosity of wire-arc directed energy deposited TZM/NbZr1 bimetallic structure

This study evaluates the effectiveness of hot isostatic pressing (HIP) in mitigating porosity in TZM-NbZr1 bimetallic structures fabricated via wire-arc directed energy deposition (DED). Two distinct HIP conditions and a combined HIP and heat treatment (HT) were investigated. The pore area fraction decreased from 4 ± 0.05% in the as-built condition to 3.05 ± 0.02% and 1 ± 0.01% after treatment at 1200°C and 1500°C, respectively. Porosity was further reduced to 0.01 ± 0.005% with HIP at 1800°C followed by HT. Plastic deformation strongly influenced pore size and shape at elevated temperatures, particularly above the recrystallisation temperatures of both alloys. At lower treatment temperature, disparity in material flow around the pores resulted in interconnected micro-cracks or micro-void formation. However, the combined HIP and HT process effectively eliminated both cracks and porosities from the interface altering the pore recovery mechanism from micro-void formation to dynamic recrystallisation. Additionally, enhanced Mo diffusion from TZM to NbZr1 was observed at higher temperature.