Role of heat balance on the microstructure evolution of cold spray coated AZ31B with AA7075

Abstract

A promising solid-state coating mechanism based on the cold spray technique provides highly advantageous conditions on thermal-sensitive magnesium alloys. To study the effect of heat balance in cold spray coating on microstructure, experiments were designed to successfully coat AA7075 on AZ31B with two different heat balance conditions to yield a coated sample with tensile residual stress and a sample with compressive residual stress in both coating and substrate. The effects of coating temperature on the microstructure of magnesium alloy and the interfaces of coated samples were then analyzed by SEM, EBSD, TEM in high- and low-heat input coating conditions. The interface of the AA7075 coating and magnesium alloy substrate under both conditions consists of a narrow-band layer with very fine grains, followed by columnar grains of magnesium that have grown perpendicular to the interface. At higher temperatures, this layer became wider. No intermetallic phase was detected at the interface under either condition. It is shown that the microstructure of the substrate was affected by coating temperature, leading to stress relief, dynamic recrystallization and even dynamic grain growth of magnesium under high temperature. Reducing the heat input and increasing the heat transfer decreased microstructural changes in the substrate.