Influence of rotating magnetic field on solidification microstructure and tensile properties of Sn-Bi lead-free solders

The solidification microstructures and corresponding mechanical properties of Sn-20Bi and Sn-20Bi-0.4Cu alloys were studied with and without rotating magnetic field (RMF). Without RMF, the microstructures of both solders exhibit great numbers of undesirable columnar structure of dendrite primary beta-Sn phase and large Cu6Sn5 phase. The RMF-driven flow has induced columnar-to-equiaxed transition (CET) of dendrite primary beta-Sn phase, which simultaneously results in dendrite fragmentation, and provokes distinct grain and intermetallic compound (IMC) refinement effects due to Lorentz force acting on the melt. The individual contributions of RMF on the lattice strain, crystallite size, stress and energy density were evaluated from X-ray peak broadening using various Williamson-Hall models. RMF can also reduce effectively the lattice strain and crystallite size of both solders, which are likely vital prerequisite for property control on metallic materials. Tensile tests showed that the tensile strength of Sn-20Bi solder was enhanced and ductility of Sn-20Bi-0.4Cu alloy was improved. These effects could increase the elastic compliance and develop the drop impact reliability of bulk solders.