Characterisation of Additive Manufactured Metallic Materials

Abstract

Selective laser melting (SLM) is an additive manufacturing (AM) process with great promise for the production of customised, complex geometry end-use components for metallic materials. The main concern in the industrial application of SLM is ensuring that the resulting microstructural features and mechanical properties of the materials are comparable to or superior to those of conventionally manufactured ones. Hence, the process-structure-property relationship must be fully characterised in great depth to improve SLM products’ quality. Most SLM products go through extensive post-processing, including heat treatment, to tailor their microstructure and performance. Adjusting microstructural constituents via post-strengthening treatments can produce parts with desired and optimum properties. 17-4 precipitation-hardened stainless steel (PHSS) is used in applications requiring high strength and moderate levels of corrosion resistance; it is commonly used in AM, and was the main material studied in this research. The titanium-aluminium-vanadium alloy Ti6Al4Va, which has sophisticated characteristics and diverse applications, was also studied. The effect of the SLM process and subsequent post-heat treatments on the mechanical properties of 17-4 PHSS were studied as follows. Firstly, the microstructural and mechanical properties of SLMed 17-4 PHSS were characterised thoroughly to find the relationship between the SLM process and the intended mechanical properties of the resulting parts. The SLM process consisted of sophisticated induced thermal histories and reheating cycles, which can be assumed as complex applied heat treatment inherently. Secondly, the deformation mechanism and microstructural evolution of the SLM parts were investigated during in situ tensile loading. Thirdly, the SLM parts’ properties were compared with those of conventionally manufactured ones. To enhance the SLM parts' mechanical properties and make them comparable to conventional ones, post-heat treatments were applied. The effect of a variety of industry-standard and non-standard heat treatment procedures on the microstructure and mechanical properties of SLM 17-4 PHSS were investigated comprehensively. The evolution of microstructure and properties were studied using various techniques. Hardness, tensile and wear tests were applied to evaluate mechanical properties. The microstructures were investigated by optical microscopy, scanning electron microscopy, and predominantly the electron backscatter diffraction technique. The SLM 17-4 PHSS's tensile strength was inferior to that of the wrought parts. The total elongation to failure of the SLM part was slightly lower than that of the wrought one, although a higher retained austenite fraction (%γ) was detected, related to induced flaws during SLM fabrication. Due to the lower %C and higher retained %γ, a transformation‐induced‐plasticity effect and consequently high strain hardening capacity was observed for the SLM part under tensile loading. The in situ tensile test showed that low-angle grain boundaries density and their corresponding misorientation increased as loading progressed and evolved into high-angle grain boundaries, leading to grain subdivision and consequent strengthening. A solutionised-aged procedure enhanced the SLM 17-4 PHSS’s elastic and yielding properties, due to the precipitation hardening mechanism, compared to the direct-aged process without solutionising. The elimination of the metastable γ during solutionising led to a lessening of strain hardening, and elongation to failure. When solutionising was not applied in direct-aged conditions, the precipitation hardening mechanism did not occur, attributed to the high solubility of Cu element in γ. Despite the SLM parts’ low hardness compared to the wrought equivalents, SLM wear loss was minor. The wear loss of the solutionised aged parts was lower than that of the direct-aged parts, and was correlated to their higher hardness. In a separate sub-study, the mechanical behaviour and the microstructural evolution of SLM Ti6Al4Va parts were compared to those of the wrought parts. The built orientation effect (i.e. vertical and horizontal built parts) of the SLM process was also studied. Necessity in enhancing ductility at the expense of strength, was concluded since SLM Ti6Al4Va showed superior strength with inferior ductility compared to wrought parts. The research demonstrates that, by implementing the proper post-heat treatment procedures, SLM 17-4 PH SS and SLM Ti6Al4Va parts can be given useful properties that enable them to have much wider application. As the central part of this study, the heat treatment effect highlights the post-treatments' efficiency and applicability for practical cases.