Modeling and Simulation of Porosity in Spray Deposition
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SPRAY deposition is a promising process which can produce a variety of materials with excellent properties.[1–3] During spray deposition, the molten metal is atomized into droplets by the high velocity gas and then the droplets are deposited onto a substrate. As a result, a fine and homogeneous microstructure with uniform dispersions and free of macroscopic segregation could be obtained due to the rapid solidification.[4,5] In addition, spray deposition provides the capability to manufacture near-net shape products, such as billets, rings, tubes, and flats.[6] Porosity is the main defect in spray-formed materials. In fact, fully dense material could hardly be produced by spray deposition.[7] A range of porosity are present in sprayed preforms leading to the deterioration of mechanical properties. Additional work, such as forging, rolling and hot isostatic pressing (HIPing), is required to eliminate the porosity, which causes excess consumption. Many studies have been performed to better understand the formation mechanism of porosity in spray deposition.[8–10] Mechanisms of porosity formation are classified into three types: gas porosity,
MINGXIANG LIU, ZHENSHAN CUI, and YANGQI LI are with the Institute of Forming Technology and Equipment, School of Materials Science and Engineering, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China; Contact mail: [email protected] Manuscript submitted December 17, 2018. Article published online May 31, 2019. 1908—VOLUME 50B, AUGUST 2019
solidification shrinkage, and interstitial porosity. It has been reported that the liquid fraction of droplets plays a decisive role in the formation of porosity. If the liquid fraction is too high, gas porosity and solidification shrinkage may be generated. For a low liquid fraction, interstitial porosity is formed due to the incomplete filling.[11] Few models have been established to describe porosity formation during spray deposition due to the complexity of the process. The porosity model proposed by Cai et al.[12–14] is commonly used. The model is based on the relative volume comparison between the liquid fraction of the droplets and the voids in the particle packing structure formed by the solid droplets. Although this model can predict the porosity content in spray deposited materials, it lacks physical mechanisms, and cannot describe the local distribution and volume fraction of the porosity in the deposit during spray deposition. However, the prediction of porosity distribution and evolution is beneficial to spray deposition process. At present, this type of model could hardly be found in the literature. In this paper, porosity models were developed, which can predict the amount and distribution of interstitial and gas porosity in a deposit. The model of interstitial porosity was based on liquid feeding in the mushy zone of the deposit during solidification. The model of gas porosity was derived from the diffusion kinetics of gas pore formation. Numerical simulations were carried out by coupling the porosity model with t
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