Selective Laser Re-melting
The additive manufacturing (AM) industry is heavily employed in a wide variety of applications today. Initially, the different processes have been used for concept modelling and rapid prototyping but are now capable of building fully functional parts. Selective laser melting (SLM) is one of the rapidly growing technologies since its inception in early 2000s. It creates parts by melting powder materials in layers using a laser heat source based on the information provided by a three-dimensional computer-aided design model. The parameters of SLM have been continuously optimised while attempting to produce fully dense parts comparable to the traditionally counterparts. This is an ongoing area of research due to the considerable number of variables involved in the process, including but not limited to powder material properties (powder deposition, particle morphology, particle size, particle size distribution and particle porosity), laser parameters (laser power, laser scan speed and laser scan spacing), and build chamber conditions (atmosphere, powder bed temperature and substrate plate preheating).
Selective laser re-melting (SLR) is yet another approach visualised for improving the quality of SLM parts by integrating the laser surface re-melting (LSR) schemes into the SLM process planning. By re-melting every layer of a part, improved mechanical and physical properties can be obtained through decreased porosities. The re-melting process promotes grain refinement with a larger temperature gradient and balling within solidified layers is reduced leading to the reduction of pores and defects. However, the SLR technique further complicates the small SLM process window and requires careful selection of parameters for a successful build. Additionally, past SLR experiments employed laser powers less than 100 W as were made available with the older SLM machines.
This study explores the effects of laser re-melting in SLM with varying energy density settings, establishing the process to structure and the structure to property relationships. Laser powers up to 375 W are used, with appropriate laser scan speed settings, ensuring the minimum energy densities as required for the laser melting of 316L stainless steel powders. Microstructural analyses are performed on the cross-sectional areas of the parts evaluating the formation of melt pools and the structures within. Both mechanical and physical properties including surface roughness of the top and the lateral faces, hardness, tensile strength, and density are the critical responses measured and analysed based on experimental conditions with varying levels of laser re-melting. Other aspects such as the laser scan strategies and the build orientations are also given due considerations in the experimental designs.
All the experiments are conducted on the Renishaw laser melting system. One of the main problems faced is that the re-melting approach led to excessive heating, bubbling and loss of the layer structures when attempted at the same original density levels as required for the first pass. This has led to limiting the energy densities in the repeated passes at either a half or a quarter of the original energy density level. Certain improvements are noticed from the laser re-melting process, though the end results are the combined effects of a number of factors.