Investigation into the effect of subsequent layer addition on the microstructure and properties of previous layers during direct laser deposition of 316L stainless steel

Abstract

Additive manufacturing (AM) is a new and emerging technology in which further research is needed in order to make the most of its potential and unique benefits compared to conventional manufacturing. This current work helps to develop an understanding of AM processes by investigating the effect of subsequent layer addition on previous layers during direct laser deposition. As subsequent layers are added to a part, heat is transferred into previous layers, which may change the microstructure and properties of the pervious layers.There is no current research into the effect of these reheating cycles. In this work a new and innovative approach is used in which samples of different numbers of layers are produced and analysed, allowing the effect of reheating due to subsequent layer addition to be individually investigated. A LENS 450 system is used to produce samples of various numbers of layers (2 layer, 3 layer, 4 layer etc.) and the microstructure and hardness of the same layer in different samples is compared. Before the main research was performed the processing parameters were optimised for the LENS 450 system. It was determined that a powder feed rate of 5rpm, scanning speed of 18ipm and laser power of 300W produced the best component for this system, with a minimum amount of porosity. The optimum sample had only 0.34% porosity and a hardness of 203HV ± 23HV. The overall outcome of the work was that a generation of new knowledge about direct laser deposition processes was obtained, which will thus help in future development and control of AM processes. It was concluded that there was no significant change in properties with the subsequent layer addition due to the austenitic stainless steel exhibiting no phase changes on heating or cooling, which is a property unique to the austenitic stainless steels. END ii Acknowledgement