Effect of geometric scaling on the microstructural development of selective laser melted metal parts

Abstract

Process parameters play a huge role in affecting materials' microstructure and mechanical properties in selective laser melting (SLM). This study discusses the effect of geometric scaling, delay time (i.e., the time it takes for the laser beam to jump from one scan vector to the adjacent scan vector, which is controlled by the jump speed), gas flow, and defocusing distance on microstructural development and mechanical properties of selective laser melted 316L stainless steel. 316L stainless steel has been used in various industries such as aerospace and petrochemical industries due to its superior properties such as very good corrosion resistance. Several characterisation techniques are conducted to study the influence of process parameters on the microstructure and mechanical properties. Optical microscopy and electron backscatter diffraction (EBSD) was conducted to study the melt pool morphology and crystallographic texture, respectively, which made up the microstructure. Density and tensile tests were carried out to analyse the mechanical properties like ultimate tensile strength (UTS), yield strength (YS) and elongation of 316L stainless steel. For melt pool analysis, it was found that an increased delay (slower jump speed) allows more time for the vapour plume to be removed. Thus, the laser can penetrate deeper. This allows narrower and deeper melt pools to form. Meanwhile, an increased defocusing distance allows shallower melt pools to form. For EBSD analysis, as delay increases, the grains are orientated more randomly, and the presence of centreline grains is more prominent. For density test, increasing the delay time could produce samples of varying densities. As the gas flow rate increases, the density of the samples becomes lower. As defocusing distance increases, samples of slightly larger densities can be produced. For the tensile test, the introduction of a delay or adjusting the laser’s jump speed does not significantly affect the UTS and strain at UTS for both produced and heat-treated parts. However, the YS increases when the delay increases. After heat treatment was conducted to release all the residual stresses, the samples’ YS fell to a similar level to the samples with no delay. An increase in delay can lead to a slight increase in strain at fracture. The UTS and YS are lower at a lower gas flow rate, and strain at fracture is higher. As for the defocusing distance, there is no significant effect on the UTS, strain at UTS and strain at fracture, except for the YS, which increased slightly when the defocusing distance was reduced. As this study only considers the above-mentioned process parameters, moving forward, the effect of other process parameters such as printing orientation and delay time on microstructure and mechanical properties could be explored. Furthermore, to gain more practical sensing of the material’s durability for real-world application, the SLM parts’ microstructure and mechanical properties can be analysed again after subjecting them to weathering conditions using the accelerated weathering machine.