Assessment of reused powder and powder loss in laser powder bed fusion process for production development

Abstract

Laser powder bed fusion (LPBF) is the most popular metal additive manufacturing (AM) technique for producing complex and high-quality parts. However, to make production economically viable, metal powder feedstock needs to be reused. In addition, estimating lost powder during the process is essential to estimate the total cost of manufactured parts. This thesis aimed to assess these key areas in case company’s AM production unit utilizing an LPBF printer, in which only virgin powder is loaded into the first build chamber and used powder is loaded into the second build chamber. The powder material is 316L stainless steel, and it is always sieved when loaded into the chamber. Using a combination of literature review and experimental analyses, key topics were evaluated, and, based on the results obtained, recommendations for production development, focusing on powder handling, were proposed.

Differences between virgin and used 316L powder were estimated by analyzing particle size distribution (PSD) using laser diffraction, morphology and chemical composition using SEM-EDS, and flowability using the Hall flow test. Parts made from these powders were analyzed with an optical microscope to measure porosity, macrostructure, and microstructure, and tensile tests were conducted to compare mechanical properties. Samples were taken in the middle of production, and therefore, powder lots were not tracked, introducing an element of randomness to the experiments. However, these tests effectively showcased the properties of the used powder in a real production unit.

Powder loss measurements were made during production by conducting three runs and weighing the powder from each output. Simultaneously giving information about powder loss differences between build chambers and powder distribution across different outputs. Powder loss is expressed as a percentage of the printed part mass, and the previous value was 40 %. As the literature review suggested, even though the used powder showed degradation, especially chemically, when compared to virgin powder, some properties of the used powder and parts made from used powder had better or similar properties compared to virgin powder. Powder loss measurements showed less waste in the chamber with virgin powder compared to the second chamber. However, just a single loss value was suggested to make production more straightforward. In that case, powder loss percentage came down to 34 %. Powder distribution measurements showed that more than half of the powder from each run ended up in used powder storage making the storage fill up overtime. Therefore, mixing some used powder with virgin powder, or running the first chamber less, was suggested.

Since used powder was declared to be adequate for the process with just sieving, no further powder treatment equipment is necessary. However, equipment like a powder load tool would make the production environment safer, protect powder from environmental contamination and decrease processing time. A powder mixer would be useful if virgin and used powder are desired to be mixed or if used powder is desired to be homogenized. An external sieving unit could be beneficial if the printer's internal sieve is under too much load. An automated depowdering cabinet would decrease manual labor by the operators. When evaluating equipment acquisitions, it should be noted that equipment should be compatible to enable powder circulation between equipment in an inert atmosphere. When fully automating the production line is a timely topic, rail or robot-based conveyor systems should be considered.