The effect of ultrasonic excitation on the microstructure of selective laser melted 304 L stainless steel

Selective laser melting is an additive manufacturing technology capable of creating near-net-shape products using a variety of materials. Parts made by selective laser melting have shortcomings in material behavior such as anisotropy, residual stresses, low ductility, and low fatigue limit. Such behavior is largely a result of the columnar grain structure found within the printed part. Fundamentally, the highly directional thermal gradient involved in the selective laser melting process, hampers the nucleation and promotes growth in one direction, and likely responsible for the observed columnar structure. The current study aims to modify such unfavorable nucleation conditions by introducing ultrasonic excitation into the selective laser melting process. The experiment was conducted on multiple layers of 304 L austenitic stainless steel as a model material on a specially built selective laser melting platform. Ultrasonic excitation has clearly shown to have an effect on as-printed microstructure leading to more desirable grain structure with a 46 to 54% decrease in aspect ratio as a result of the change in nucleation conditions. The results of this study suggest that ultrasonic excitation is a viable means to altering the selective laser melting grain structure. Moreover, it is highly likely that further optimization, and possibly a certain level of control on the resulting microstructure, can be achieved with the ultrasonic parameters.