Date

2023-1-16

Author

Çelik, Gökhan

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Laser Powder Bed Fusion (LPBF) process is one of the most well-known additive manufacturing methods for the production of complex and functional parts from metal powder material. Residual stresses cause a major setback in the LPBF process and restrict the serviceability of the parts, particularly in advanced technology applications. Process parameters have a crucial impact on residual stress formation and residual stresses alter the reliability of material properties. Therefore, the influence of process parameters on residual stresses needed to be investigated to provide more accurate mechanical properties for design and production optimization for the LPBF process. 17-4 PH stainless steel is desirable among the stainless steels for various fields, including aerospace, aviation, energy, and chemical industries, due to their superior high strength, hardness, and corrosion properties. Moreover, 17-4 PH stainless steels are easily machinable at solution heat treated condition. Their high weldability with high corrosion resistance properties makes them very attractive for industries. The process parameters’ effect on surface residual stresses could be performed by XRD residual stress measurement method which is very sensitive to surface and subsurface residual stresses. Residual stress specimens produced by LPBF process in as built condition had more than fifty times surface residual stresses compared to the conventionally produced bar specimens which indicated the importance of the investigation of surface residual stresses of LPBF parts. In this thesis; the influence of scanning strategies, laser power, exposure time, volumetric energy density, hatch distance, point distance, layer thickness, preheating of the baseplate, and separation from the base plate were analyzed for 17-4 Precipitation Hardened (PH) stainless steel parts manufactured by LPBF additive manufacturing system in terms of residual stress accumulation on the surface and close to the very surface region by X-Ray Diffraction (XRD) method. The analyses show that, volumetric energy density (VED), laser power, and exposure time optimization played a crucial role in residual stress minimization. VED parameter optimization was crucial for overall penetration and surface residual stress formation on material. High laser power and less exposure time within the defined parameter range and specified VED value could be the optimum process parameter in terms of residual stress accumulation on the surface of a material. On the other hand, point distance and hatch distance had a slight influence when compared with the effect of volumetric energy density, laser power, and exposure time within the defined ranges. The as built parts’ residual stresses could be reduced by up to ~67%. However, the tensile strength and yield strength of a material could also be decreased by 14% and 12%, respectively. Therefore, from an engineering standpoint, a trade-off between lower surface residual stress values and mechanical strength shall be taken into account.

Subject Keywords

Residual Stress, Additive Manufacturing, Laser Powder Bed Fusion, 17-4 PH Stainless Steel

URI

https://hdl.handle.net/11511/102003

Collections

Graduate School of Natural and Applied Sciences, Thesis