Effect of Sn Content on Solidification Microstructure and Cracking in Cu-Sn Brazed Joints

Solidification cracking in brazed joints is significantly influenced by solidification conditions and alloy composition, making it a critical factor for the reliability of parts and products. Numerous studies have been conducted to predict solidification cracking in processes like casting, welding, and joining, leading to the development of an index to assess crack susceptibility based on alloy composition and solidification behavior. This study investigates the microstructure and solidification cracking of Cu-Sn brazed T-joints for Custainless steel tubes, and evaluates the effectiveness of the solidification index. As the Sn content in the Cu-Sn binary alloy was varied from 5 to 10 wt%, the liquidus temperature and viscosity decreased, therefore allowing deeper penetration into the joint. The cracks in the joint were identified as solidification cracks, based on the dendrite shape, and their size decreased slightly as the Sn content was increased from 5 to 10 wt%. Various solidification crack indices have predicted that Sn content in the range of 1–2 wt% is most sensitive to solidification cracking. Specifically, the improved Crack Susceptibility Coefficient (CSC) index that accounts for stress evolution during final solidification, demonstrated that 10Sn solder had a higher resistance to solidification cracking compared to 5Sn solder. Thermal expansion and contraction of the joint during brazing were identified as the major causes of stress-induced cracking. Furthermore, higher Sn content in the 10Sn solder produced a larger interdendritic area and finer dendritic structure while maintaining equiaxed dendrites. The growth factor (Q; a product of compositional undercooling and partitioning coefficient), derived from the Scheil diagram, was strongly associated with dendrite refinement.