Modeling of porosity during spray forming: Part I. Effects of processing parameters
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I.
INTRODUCTION
Spray-forming processes have attracted considerable attention due to a combination of advantages such as near– net shape manufacturing;[1,2] elimination of macrosegregation;[1,2,3] elimination of multiple processing steps as necessitated by powder metallurgy processing;[1,4] refined microstructure;[1,2,4,5] extended solid solubility;[1,5] and formation of nonequilibrium phases that in some cases enhance mechanical properties.[1] However, the presence of some porosity in the as-deposited materials is inevitable. For instance, porosity has been reported to vary from 1 to 10 pct for a variety of Al alloys.[6] The presence of porosity in deposited materials brings about at least two challenges. First, the material properties, especially those at elevated temperatures, may degrade due to the presence of porosity.[1,7,8] Second, secondary working, such as extrusion, rolling, forging and hot isostatic pressing, is required to achieve full or near-full density, which in turn limits the applicability of spray forming as a flexible near–net shape manufacturing process. As a consequence, research efforts have been initiated to better understand the formation mechanisms of porosity and to find ways to control and/or minimize the porosity level, either by manipulating the processing parameters or by exploring alternative approaches. As a result of these efforts during the past decade, significant progress has been achieved in understanding sprayforming processes and three likely mechanisms have been proposed: gas porosity, interstitial porosity, and solidification shrinkage.[1–4,9] Among these three mechanisms, interstitial porosity has been widely reported to play a critical role in porosity formation, particularly when spray forming sheet or strip geometries.[2–4,9–11] The formation of interstitial porosity can be envisaged as follows. Upon impingement, solidified individual droplets overlap each other, forming interstices. If the fraction of liquid is too low, these interWEIDONG CAI, Research Associate, formerly with the Department of Chemical and Biochemical Engineering and Materials Science, is with the Department of Electrical and Computer Engineering, University of California, Irvine. ENRIQUE J. LAVERNIA, Professor, is with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, Irvine, CA 92697. Manuscript submitted June 25, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
stices will not be completely filled by liquid and will lead to porosity formation.[4] More recently, the results of an experimental study provided further support to the notion that porosity is strongly dependent on the fraction of liquid in the incident spray.[6] In this study, it was shown that a low level of porosity may be achieved by maintaining an optimal fraction of solid under a controlled pressure environment.[6] The formation of porosity during spray forming is influenced by many factors, such as processing parameters, thermodynamic properties of the materials, and t
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