The Influence of Heat Treatments of Sustainable Zinc on Microstructure and Mechanical Properties

Abstract

Zinc is important in many applications due to its resistance to corrosion under different conditions; it is used in galvanization. This article presents a study of zinc from remelting waste zinc (waste collected from manufacturing scrap ; galvanizing zinc) and recycled by sand casting, then heat treated under different conditions. Impact and hardness testing ascertained the mechanical properties of the zinc casting, while optical microscopy characterized its microstructure. The study found that the grain growth increased after natural aging and reached a maximum of approximately 41 % of the treatment at 350 °C, for 3 h followed by water quenching, which exhibited the largest grain size, indicating significant grain growth. In contrast, the treatment at 350 °C, for 5 h followed by water quenching refined the grain structure and reduced the precipitation fraction, suggesting a more stable microstructure. The as-cast sample exhibited the highest hardness (approximately 94 HV), whereas heat treatment initially reduced hardness because of microstructural contrast dissolution during solution treatment. Among the heat-treated specimens, the treatment at 350 °C, for 5 h followed by water quenching, produced the highest hardness (approximately 82 HV), indicating an effective balance between precipitation strengthening and grain refinement. Specimens showed intermediate hardness values in the heat treatment at 370 °C for 3 h and 5 h, then aged at 180 °C for 90 min, which are attributed to grain growth during aging. The results demonstrate that heat treatment significantly affects the precipitation behavior, grain evolution, and hardness of high-purity zinc, with the treatment at 370 °C, for 5 h followed by water quenching providing the most favorable combination of microstructural stability and mechanical performance. The impact strength was strongly influenced by the heat treatment condition, with the heat treatment at 370 °C for 5 h, then aged at 180 °C for 90 min achieving the highest value of 5 J/cm², comparable to commercial ZP3 (Zamak 3) and higher than ZP5 (Zamak 5).