Modeling of porosity during spray forming: Part II. Effects of atomization gas chemistry and alloy compositions
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I.
INTRODUCTION
The presence of some amount of porosity in as-deposited materials is inevitable during spray forming, which, for most applications, is disadvantangous. 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 and/or by exploring alternative approaches. As one of various experimental and numerical efforts toward this goal, a porosity model was formulated, and the effects of various processing parameters were studied in Part I of this article. According to the developed model, the amount of porosity in as-deposited materials can be estimated on the basis of the average fraction of solid in the incident spray and the solid particle packing density. The values of porosity vary with these processing parameters by following a V-shaped behavior. From these results, the optimal processing parameters or processing maps for achieving low porosity in the deposited materials can be readily predicted. In the present article, by using this porosity model, the effects of atomization gas chemistry and alloy composition on porosity were investigated. To accomplish this objective, the influences of N2 and Ar on porosity formation were studied. In addition to the Al-4Cu alloy that was used in Part I of this article, four other alloy systems were selected for study, i.e., Cu-6Ti, Sn-15Pb, Ni-20Cr, and Fe-8Ti (wt pct).
ease of discussion, the formulation of porosity is summarized as follows. (1) The porosity present in an as-deposited material can be described as the sum of porosity from the gas porosity, interstitial porosity, and porosity from solidification shrinkage. Since it is difficult to discern between gas porosity and solidification shrinkage,[1---4] both types of porosity are generally treated as solidification porosity. Moreover, it is assumed that the formation of interstitial porosity and solidification shrinkage may be treated as two mutually exclusive phenomena. Consequently, when interstitial porosity is predominant, solidification porosity is neglected. Conversely, when solidification porosity is predominant, interstitial porosity is neglected. It is assumed that the deposited material forms by a two-stage mechanism (as detailed in Part I of this article). As a result, porosity can be estimated using the following equation: p5
$
F≥0
FG
FG b FG 2 (1 2 b )
[1]
F,0
where b is the solidification contraction (b 5 1 2 rm/rs), and F is the porosity coefficent, defined as F5
~ ! ~ ! 12G 1 2 fs 2 G fs
(1 2 b )
[2]
(2) The average fraction of solid in the impinging spray can be determined as follows: II.
FORMULATION
A detailed description of the porosity model has been provided in Part I of this article and, hence, will not be repeated here. However, for purposes of completeness and
WEIDONG CAI, Research Associate, formerly with the Department of Chemical and Biochemical Engineering and Materials Science, is with the Department of
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