Laser Powder Bed Fusion and Invar 36 Alloys

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Invar 36 is widely used in engineering components such as optical mounting, valves of aircraft control devices and precision instruments. Despite its high strength and good ductility, conventionally wrought Invar 36 alloy is hard to process due to its single austenite phase (Kaladhar, Subbaiah and Rao 2012). The development of laser powder bed fusion (PBF-LB) technology has made it possible to additively manufacture complex Invar 36 components with higher efficiency and accuracy. However, little systematic study on the influences of varying scanning speed on the microstructure and mechanical properties of Invar 36 has been conducted.

This paper presents a systematic study on the effects of scanning speed on the tensile stress-strain curves, microstructure, defects and metallography of PBF-LB Invar 36 block samples. The microstructure observations and mechanical tests, especially fatigue crack propagation experiments, are carried out on the block samples fabricated at Ev = 74.1, 105.8 and 185.3 J/mm3. It is found that the inadequate laser energy density leads to hardly overlapping melting traces, thus generating numerous defects. These defects adversely affect the tensile properties of the Invar 36 alloy, as it fails from the rapid aggregation of defect particles. In contrast, the adequate laser energy density remarkably enlarges the overlapping of melting traces and creates large molten pools with steady boundaries, resulting in favourable microstructures and low porosity. This enables the Invar 36 alloy to have superior and completed fatigue life generated from small crack propagation, long crack propagation and final fracture stages.

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