Analysis of the effect of surface mechanical attrition treatment on the mechanical properties of 17-4 PH stainless steel obtained by material extrusion

In this study, the influence of the surface mechanical attrition treatment (SMAT) on a 17-4PH stainless steel made by material extrusion additive manufacturing is investigated under mechanical loading. The evolutions of the deformations at the local scale have been performed during in-situ tensile tests up to failure around 4kN. The strain maps are obtained with an original process based on the use of nanogauges displacement from the recorded of scanning electron microscope images. These maps allow to analyze the deformation mechanisms of as-fabricated and mechanically treated samples. The porosities evolutions at the surface are especially investigated for the two types of samples. The crack propagation in the as-fabricated samples is strongly influenced by porosities/defects related to the additive manufacturing process. Moreover, the SMATed samples present slip bands at the surface during the deformation. This deformation mechanism is similar to the one commonly observed in metallic materials obtained with traditional processes. Even if the application of SMAT does not show huge modifications of tensile properties with similar ultimate tensile strength around 650 MPa, it allows to improve the material surface quality by drastically reducing the surface roughness. SMAT treatment also allows a reduction of porosities in few microns from the sample surface. These improvements present a limited impact on tensile properties, but leads to its possible use for industrial applications when applied as an innovative post-treatment on metal part obtained by additive manufacturing.