Porosity and microstructure of steel tubes spray-formed by close-coupled atomizer

Close-coupled atomizers (CCA) can be used to reduce the porosity of spray-formed deposits compared to free fall atomizers (FFA), possibly due to smaller size and higher velocity of the atomized droplets. Previous studies on spray-formed AISI 52100 (100Cr6) tubular deposits show that CCA leads to less porosity in the inner and outer surface regions of the tubular deposits. However, there are very few investigations on the thermal profiles of the spray-formed deposits and on the influences of process conditions during spray forming via CCA. To provide insight into the thermal profile of the deposits, in this study in-situ measurement of the deposit surface temperature is performed by a two color pyrometer and the substrate temperature is measured by means of thermocouples. The porosity and microstructure of the spray-formed deposits under various process conditions are investigated by means of optical microscopy and image analysis. This investigation shows that the deposit thickness has a higher influence on the deposit surface temperature than the gas to melt flow rate ratio (GMR). Higher deposit temperature results in lower porosity and larger grain size. Moreover, the deposition positions on the deposit length has an influence on the porosity. The porosity in the vicinity of the substrate is higher at the deposit end position than the deposit start position. Finally, the present study suggests an optimal deposit surface temperature range for spray forming of highly dense tubular deposit by CCA.