The use of selective laser melting (SLM) to fabricate 18Ni-300 maraging steel components for aerospace and mould making applications has attracted significant interest from both industry and academia. In particular, mould makers have utilised SLM to fabricate steel moulds with integrated conformal cooling channels, which provided enhanced cooling efficiency and reduced injection moulding cycle time compared to their conventional counterparts. However, its widespread application was hindered due to issues such as microstructural heterogeneity, mechanical anisotropy, and manufacturing defects. In the present thesis, the process-microstructure-properties relationships in SLM 18Ni-300 maraging steel were investigated to address these issues and facilitate the fabrication of additively manufactured steel moulds with mechanical properties comparable to their conventionally made counterparts. An extensive literature review was conducted to understand how SLM process parameters (or in-process parameters) influence microstructure evolution and mechanical properties in SLM 18Ni-300 maraging steel and four other steel mould materials (i. e. H13, P20, AISI 420 stainless steel, and S136). This facilitated a novel comparative analysis of laser-powder interactions, rapid solidification, and intrinsic heat treatment across the five steel mould materials, and addressed the lack of comparative analysis specific to this topic. Complex laser-powder interactions such as the scanning motion of the laser during SLM process resulted in mechanical anisotropy and manufacturing defects, which may be mitigated via statistical process optimisation. Rapid solidification resulted in microstructural heterogeneity (i. e. variations in grain size and crystallographic texture), which was subsequently altered during post-processing heat treatment. Intrinsic heat treatment resulted in in situ precipitation hardening in SLM 18Ni-300 maraging steel and martensite tempering in the other four steel mould materials. Experimental investigations were conducted to elucidate the process-microstructure-properties relationships in SLM 18Ni-300 maraging steel. Experiments on additively manufactured individual scan tracks and fully built samples revealed that increasing laser power (P), reducing the scanning speed (v), and reducing the hatch spacing (h) resulted in increased laser energy input (Eₗinear and Eᵥolumetric) and reduced manufacturing defects. Following that, a combined statistical optimisation methodology featuring Taguchi’s methods and grey relational analysis (GRA) was implemented to determine optimal SLM in-process parameters for the multi-response optimisation of mechanical properties. Statistical analysis indicated both P and v had an approximately equal influence on the mechanical properties, while the influence of h was less significant. The mechanical properties of samples fabricated using the optimal SLM in-process parameters have high relative density (i. e. > 99 %) and were comparable with their conventionally made counterparts. Macroscale plastic strain localisation phenomena including propagation of Lüders bands and necking leading to ductile fracture in the fabricated samples were captured using optical imaging-based digital image correlation (optical-DIC). The influence of microstructural heterogeneity (i. e. variations in grain size and crystallographic texture) on the microscale plastic strain localisation in SLM 18Ni-300 maraging steel was investigated using electron backscatter diffraction (EBSD), in situ uniaxial tensile experiments, and scanning electron microscope-based digital image correlation (SEM-DIC). Sub-micron sized speckle patterns were created via magnetron sputtering, which facilitated high-resolution strain measurements via SEM-DIC and addressed the lack of established speckle creation methodology for SLM 18Ni-300 maraging steel. Custom MATLAB scripts were employed to digitally align and overlay the EBSD and SEM-DIC datasets. This methodology facilitated a novel grain-to-grain comparison of the microscale plastic strain localisation in relation to the microstructural heterogeneity, provided grain-level insights into the role of internal misorientations on slip activity in individual grains, as well as addressed the lack of SEM-DIC investigations specific to SLM 18Ni-300 maraging steel. Experimental findings revealed that the microscale plastic strain localisation in SLM 18Ni-300 maraging steel was driven by the interplay between microstructural heterogeneity, Ni-based intermetallics, and the impediment of dislocation motion. The densely distributed equiaxed grains contributed to grain boundary strengthening in the as-built (AB) sample. After post-processing heat treatment, the precipitation of densely distributed Ni-based intermetallics contributed to strain hardening in the solution-aging treatment (SAT) sample. The main deformation mechanism was identified as dislocation slip. Slip preferentially occurred in grains with increased internal misorientation, and intersected regions that exhibited increased kernel average misorientation (KAM). Complex slip behaviour including discrete slip, diffuse slip, and possible cross-slip (or overlapping slip) was identified in the investigated grains of AB and SAT samples. The combined kinematics of the active slip systems were reflected in the in-plane deformation behaviour of the investigated grains. The findings of the present thesis provide fundamental insights into tailoring the microstructure of SLM 18Ni-300 maraging steel for optimised industrial performance. Finally, potential future research directions for SLM 18Ni-300 maraging steel were suggested, including numerical modelling of microstructure evolution and investigation of microscale plastic strain localisation under complex loading conditions such as multiaxial loading and fatigue.
Law Wee King (Sat,) studied this question.