Selective laser melting of AISI 4140 steel

This thesis focused on investigating the microstructure and mechanical properties of Selective Laser Melting (SLM) manufactured AISI 4140 steel under a range of in-situ heat loading conditions. In the SLM process, the process parameters determine the basic performance and microstructure of an as-built object. In this study, single tracks of 4140 steel powder were produced initially.  This initial experimental study informed the effect of laser power, scanning speed, and track energy density on the melt pool formation, surface morphology, roughness, and continuity of the scan tracks for constant powder layer thickness. The results showed that the surface quality, roughness, and continuity were significantly affected by the track energy density. <br><br> Multiple layer cubes were then fabricated using two groups of process parameters with the track energy being determined from the initial scan track experiments. They were integrated to cover a range of SLM conditions by varying the laser power, scan speed, scan spacing, and layer thickness. Volumetric energy density was a crucial parameter in controlling the quality of the cubes in terms of the defects present (porosity and lack of fusion). Based on the X-ray CT scanning analysis and optical microscopy results of each sample's density, the most suitable manufacturing parameters were identified. The optimised parameters were then used to observe the effects of the volumetric energy density on the microstructure and mechanical properties of the SLM-fabricated parts. The results showed that the predominant phases present in the samples were ferrite/martensite, where martensite can decompose into ferrite and carbides under different energy density parameters. Differences in the fraction of carbides produced at different energy density parameters entailed distinct mechanical properties. <br><br>  To understand the effect that different in-situ heat loading levels have on part microstructure and their mechanical properties, different powder recoating times were applied under a constant volumetric energy density. The results showed that different recoating times led to different fractions of carbides in the samples which resulted in different mechanical properties. Such a complex thermal cycle led to an inhomogeneous microstructure of the samples with a greater fraction of carbides observed in the sample closest to the build platform.<br><br> Scanning electron microscopy and X-ray diffraction were performed to investigate the microstructural inhomogeneity and phase distribution in the SLM-fabricated 4140 steel cylindrical samples of 12 mm in diameter and 100 mm in height. Electron backscatter diffraction (EBSD) analysis was conducted to measure the area fraction of carbides. The EBSD results showed a gradual decrease in the fraction of the carbides from the bottom (considered the part which is attached with support to the build platform) to the top of the samples. Transmission electron microscopy (TEM) was performed to detect the different carbides present by using selected area electron diffraction (SAED) analysis. The TEM results showed two different sizes of carbides (8-50 nm, 50-100 nm) with the coarser particles contributing to the lower strength of the samples. Tensile testing and hardness measurements were carried out to measure the mechanical properties of the in-situ heat treated samples.  The results showed that in as-manufactured samples a longer recoating time led to higher tensile strength (1034.42 ± 4) MPa and lower tensile ductility (12.37 ± 2.5) % whereas a shorter recoating time led to lower tensile strength (804.39 ± 4) MPa and higher tensile ductility (23.13 ± 2) %. Such operating envelopes were established for producing AISI 4140 deposits with acceptable quality, equivalent or similar to wrought product and the possibility of in-situ control of properties was demonstrated. Based on the mechanical property tests it was concluded that by in-situ heat treatment, it was possible to customise the mechanical properties of parts in terms of their tensile strength and tensile ductility without any post heat treatment.<br><br> The research findings from this project contribute to an improved understanding of the microstructure and mechanical properties of SLM-manufactured 4140 steel under different in-situ heat loading conditions.