Influence of ultrasonic excitation on the mechanical characteristics of SLM 304L stainless steel

Selective laser melting (SLM) is a layer by layer powder bed additive manufacturing process that can be used to create high complexity parts. Stainless steel 304L is a material which can be used in a wide range of applications including medical, aerospace, and automotive industries due to its high corrosion resistance. These same industries are also the first users of SLM and research into the combined use of the two can provide benefits for both. Ultrasonic excitation in casting has shown to decrease grain size due to the effect of the oscillating pressure waves on the nucleation conditions. While ultrasonic excitation has been used the solidifying of metals made through casting, it has not been used before in the SLM process. An investigation on the effect of ultrasonic excitation was conducted on the mechanical properties of the material. The characteristics compared were nanohardness, Young's Modulus, and the anisotropic behavior of said properties. Given that the nucleation conditions were altered, the microstructure was affected by the pressure waves and offset columnar grains development which are formed due to the axial heatsink into the substrate, and thus ensuring a more homogenous development of grains. Given enough power, ultrasonic vibrations should ensure a more even distribution of grains and a reduction of columnar grains. These in term resulted in a reduction of anisotropic behavior, and an increase in nanohardness. The study compared samples made both with and without the ultrasonic excitation applied, as well as with different orientations for anisotropy analysis. A potential improvement in mechanical properties under the influence of ultrasonic vibrations could lead to wider acceptance of the SLM process. Further improvements, such as eliminating the need for thermal post processing could further increase the usability of the technology and 304L as a cheap material for SLM. (C) 2017 The Authors. Published by Elsevier Ltd.