On the evolution of porosity in spray-deposited tool steels

In this article, porosity, as defined by the distribution and amount of pores in spray-deposited A-2 tool steel, was investigated. Ceramic and copper substrates were used in order to understand the influence of substrate material on the formation of porosity. Moreover, the relationship between porosity and spray distance was studied using three different spray distances together with a ceramic substrate. Distinct porosity zones were identified in the deposited material. Interstitial and gas-related porosity were both present in the deposits. Experimental results show that the optimum parameters correspond to a 178-mm deposition distance, in the case of a ceramic substrate. The experimental results suggested that the pore formation mechanism in spray-deposited materials is closely dependent on the thickness of the mushy layer in the upper surface of the deposit during the spray-forming process. The thickness of the mushy zone is determined by the solid fraction contained in the impinging droplets and the thermal-transfer conditions. Moreover, the deposition angle appears to closely influence the pore morphology, as well as the thickness of the surface porosity band of the deposits. Finally, the present results suggest that residual internal stress may be an important factor in influencing crack formation in the spray-deposited tool steel.