| Porosity is an inherent problem that associates with discrete droplet processes involving the consolidation of numerous solidifying droplets, such as in spray forming and plasma spraying. A variety of deleterious effects may result from the presence of porosity in the deposited materials, including the degradation of mechanical properties, such as bond strength, hardness and low cycle fatigue life, and corrosion resistance; and the degradation of the quality of deposited materials, which largely limits the applicability of these processes as near net shape processes. The primary sources of porosity in deposited materials generally include gas porosity, porosity from solidification shrinkage and interstitial porosity. In view of the inherent characteristics of discrete deposition processes, interstitial porosity has been suggested to play a critical role in porosity formation. The formation of porosity depends on numerous factors, such as physical properties of materials, physical properties of gases, and a variety of processing parameters. However, these factors have been rarely investigated. Consequently, in the present study, both experimental and numerical approaches were employed with the objectives of providing insight into the mechanisms of porosity evolution during discrete droplet processes and finding ways to control or minimize the amount of porosity in deposited materials. Correspondingly, in the experimental approach, two parallel studies were completed. First, porosity evolution during low pressure plasma spraying of W was systematically studied. Five mechanisms of porosity formation were identified and discussed. Second, a novel spray forming technique, namely low spray forming was developed. By using this technique, the amount of porosity in as-deposited 2024 Al alloy was significantly reduced in comparison with that obtained by using conventional spray forming. In the numerical approach, a porosity model was established on the basis of particle packing theory, fluid mechanics and droplet thermal history. By using this model, the amount of porosity can be readily estimated on the basis of the average fraction of solid in the incident spray and the packing density of solidified particles. The effects of processing parameters, i.e., melt superheat, melt flow rate, atomization gas pressure and deposition distance on porosity were studied. The influences of atomization gas chemistry and alloy composition on porosity were also investigated on the selected atomization gases (N{dollar}sb2{dollar} and Ar) and alloy systems (Al-Cu, Cu-Ti, Fe-Ti, Ni-Cr and Sn-Pb). |
