Effect of pre-welding heat treatments on welding a two-phase Ni3Al alloy

Autogenous gas-tungsten are welding was performed on a two-phase Ni3Al alloy. Plates 3 mm thick were cut from a cast ingot and were subjected to three pre-welding heat treatments (PWHT), namely as-cast, annealed at 1100 °C for 23 h (HT1) and HT1 plus 1120 °C/30 min/air cooled (HT2). The as-cast alloy was characterized by dendrites with many eutectic cells located at dendrite boundaries (DBs). The eutectic cells were destroyed by HT1. However, HT2 made no further changes to the microstructure. Analysis by energy-dispersive X-ray spectroscopy (EDX) revealed that the DBs of as-cast alloy were enriched with Zr. The Zr enrichment at DBs was increased further by HT1 and HT2. No crack visible to the unaided eye was found in either the fusion zone (FZ) or the heat-affected zone (HAZ) due to welding. The FZ consisted of columnar grains with a eutectic structure along the boundaries. The eutectic structure was enriched with Zr and contained many microcracks and some Zr-rich phases. Micro-solidification cracks were occasionally found along boundaries in FZ. The alloy was sensitive to liquidation cracks that occurred along DBs in the HAZ. While PWHT had no effect on the characteristics of the FZ, PWHT had a notable effect on the severity of liquation cracking. The average liquation crack numbers per 5 cm weldment were 12 for as-cast, 14 for HT1 and 17 for HT2. The role of Zr-inducing solidification cracks in FZ and liquation cracks in HAZ is discussed.