Porosity in spray-formed materials

Porosity in spray-formed materials is an important issue, but the formation of porosity is not completely understood. Many experimental results and some theoretical models have been presented in the past. Nevertheless, the prediction of porosity in a deposit is still not possible today. The paper will give some examples picked from literature, which show some general correlations between process parameters and porosity. These correlations can be helpful to form a basic understanding of the process. Finally it is necessary to know more about the conditions of the droplets and the deposit at the point of impingement. These impacting conditions have to be correlated to the porosity to improve the understanding of the process and to make a prediction possible. Determining the impact conditions is a challenge because usually they are not constant with time and some values are difficult to measure. Our experiments show a strong correlation between the surface temperature of the deposit and the porosity. For IN718 and U720 as-sprayed porosities below 1 vol.% were achieved if the deposit surface temperature is app. 1250 degrees C. The average impact angle weighted by the local particle mass flux is also an important parameter. The probability of low as-sprayed porosity is high if the average weighted impact angle is below 25 degrees but decreases dramatically for higher impact angles.