Influence of cold spray parameters on the microstructures and residual stress of Zn coatings sprayed on mild steel

Galvanizing is an industrially recognized process for surface heat treatment of steel. However, since the technology is rapidly changing worldwide, newer technologies such as cold spraying have been developed. The aim of the present study was to determine the effect of spraying parameters on the resulting coating mechanical properties, microstructural properties, and residual stresses associated with cold spray deposition of Zn powder on mild steel substrate. The spraying parameters used for these coated samples were varied in terms of temperature, pressure, and stand-off distance in order to determine and evaluate conditions that will give the best deposition efficiency results. The coatings were characterized by field emission scanning electron microscopy/energy dispersive X-ray analysis (FE-SEM/EDX). The X-ray diffraction sin²ψ technique with Cu-Kα radiation was used to identify the different phases, and measure residual stresses. The micro-hardness was determined by using Vickers hardness testing machine. The experimental result revealed that the microstructure of the cold sprayed coating showed evidence of plastic deformation on the particles due to particle flattening, whilst the substrate-coating bonding was improved at higher temperatures and pressures. An increase in pressure and temperature did not have any significant effect on hardness, whilst an increase in stand-off distance resulted with a decrease in the thickness of coatings. The best results in terms of temperature, pressure, and stand-off distance were found at a 525 °C, 8 bars, and 15 mm respectively. In addition, the influence of spraying parameters on the residual stress was investigated and discussed in details.