Effect of fusion ratio on microstructure and properties of medium-entropy alloy filled welds

Medium-entropy alloys (MEAs) exhibit excellent properties, making them suitable as filler metals for welds to enhance or improve the performance of welded joints. However, the fusion of base metal (BM) during welding may significantly affect the microstructure and properties of the weld. In this work, low-carbon steel was used as the BM and CoCrNi medium-entropy alloy (MEA) as the filler metal. Different groove types were designed, and welded joints with five different fusion ratios were prepared using filler wire tungsten inert gas (TIG) welding. The effects of the BM fusion ratio on the microstructure and mechanical properties of the weld were studied by element analysis, electron backscatter diffraction (EBSD), hardness, and impact tests. It was found that when the fusion ratio is less than 35%, the phase structure of the weld microstructure is consistent with the single-phase FCC structure of CoCrNi medium-entropy alloy. When the fusion ratio is more than 35%, the BCC phase begins to appear in the weld, forming a FCC+BCC dual-phase structure. When the fusion ratio is more than 80%, the weld structure becomes the single-phase BCC structure. The hardness of the weld increases significantly with the appearance of the BCC phase. The impact toughness of the weld is related to several factors, including the fusion into the BM, grain shape, and the phase composition of the weld zone (WZ).