On the evolution of porosity in spray-deposited tool steels
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I. INTRODUCTION
ONE of the most important characteristics of spraydeposited materials is the presence of discontinuities or pores in the materials. Because spray-forming processing essentially involves the deposition of discrete liquid, semiliquid, and solid droplets on a deposition surface, such defects may occur during solidification as a result of one or a combination of the following mechanisms: gas entrapment, interstitial porosity, and solidification shrinkage.[1–5] Although porosity is not always deleterious in spray-deposited materials, it is generally undesirable, especially for applications where strength and ductility are critical. Porosity should be reduced to the lowest possible value by optimizing the spray-deposition conditions or by thermomechanical processing. Regardless of the significant importance of porosity on the performance of spray-deposited materials, porosity in spraydeposited materials remains a critical factor in many applications, and the underlying mechanisms remain far from being understood. Gas-related porosity is anticipated as a result of the limited solid solubility of inert gases in most structural materials. It has been suggested that the formation of gas pores is related to the presence of an excessive proportion of liquid during deposition. For the same material, porosity is relatively high when the liquid fraction is high.[6] On the other hand, the solubility and reactivity of gases used in spray deposition may also affect the formation of gas pores. Experimental results indicate that the materials produced using Ar as an atomization gas consistently exhibit a higher amount of porosity than those produced using N2 as the atomization HAIMING HU, Postdoctoral Associate, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California at Irvine, Irvine, CA 92697. ZIN H. LEE, Professor, is with the Department of Materials Science and Engineering, RASOM, KAIST, Taejon, 305-701, Korea. DAWN R. WHITE, Principal Engineering Specialist, is with the Materials Systems Reliability Department, Ford Research Laboratory, Dearborn, MI 48121. Manuscript submitted April 20, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
gas.[7] It has been argued that nitrogen may react to form nitrides with alloying elements, thereby reducing the partial pressure of N2 in the pores.[6–9] However, in view of the irregular morphology of pores, it is highly improbable that, in all cases, a large proportion of the porosity originates from the rejection of entrapped gases, since gas porosity generally exhibits a spherical morphology. The formation of shrinkage porosity is due to a lack of feeding liquid in the dendritic mushy zone or from a riser, such as those present during ingot casting. Available experimental evidence suggests that a large proportion of the porosity that is observed in spray-deposited materials may be attributed to interstitial porosity. This type of porosity is generally a result of the incomplete filling
