Assessing the Microstructure of SnBiZn Alloys during Semisolid Treatment

Abstract

The development of new alloys for semisolid processing is a key demand of Industry 4.0, driven by the limited knowledge in this area, particularly for Sn-based alloys. The primary goal is to design alloys suitable for thixocasting and semisolid additive manufacturing. While Zn is known to influence SnBi alloys by affecting their microstructure, wettability, and intermetallic compound formation, its impact on globule size, morphology, and microstructural coarsening/stability over aging remains largely unexplored, despite the potential implications for semisolid processability and final product reliability. In this research, stability in high-temperature experiments is verified for the Sn40Bi and Sn40Bi2Zn alloys (wt%). Five different heat treatment periods of 20, 40, 60, 90, and 120 min are applied to the samples, each subjected to its respective eutectic temperature according to CALPHAD computations. The microstructure of both alloys is characterized by Sn-rich globules for times higher than 90 min, which are noticeably finer and less elongated in the Sn40Bi2Zn alloy compared to the Sn40Bi. This globule refinement in Sn40Bi2Zn alloy can be attributed to the presence of Zn needles at the dendrite boundaries that pin the Sn-rich regions and prevent them from growing freely. Over time, while Bi saturates near its solubility limit in Sn matrix at 60 min and remains stable up to 120 min, the Sn40Bi2Zn alloy exhibits a progressive increase in Bi content between 60 and 120 min, reaching the saturation level at 120 min. The Sn40Bi2Zn alloy exhibits higher globule microhardness in the 90 min samples, primarily due to Zn hardening in solid solution.