New method for position and energy controlled surface mechanical attrition treatment and its effects in 304 stainless steel

Gradient nanograin metals (GNMs) are of great interest due to their strength and retention of ductility. These microstructures are often created using surface mechanical attrition treatment (SMAT), which coldworks a surface through numerous impacts from ball bearings. Recently, the use of short-duration SMAT to form planar heterogeneous GNMs has been proposed as a method to control mechanical properties by applying nonoverlapping impacts on a coarse grained metal, where the degree of surface coverage may be used to dictate the strength-ductility balance. While clear trends between the surface coverage and mechanical properties have been reported, there has been no means to precisely dictate the impact characteristics. To remove randomness from the surface impact process, a position and energy controlled SMAT (PECSMAT) impactor was developed to create planar heterogeneous GNMs with patterned arrays of indents. Computer numerical control and energy measurement allow the indenter to record the deformation energy of each impacted site. This industrially relevant approach enables direct translation into current industrial processing and enables unique research on nanograin formation as a function of plastic deformation energy, impact number, overlap, angle, diameter, and more. The impactor was found to have positional variation of less than 17 μm, while the plastic deformation energy of each impact varies less than 8.9% of the desired set-point value. The utility of this approach is demonstrated in 304 stainless steel, with the strength of the material controlled by varying indent surface coverage, allowing 70% increase in yield strength with only 17% decrease in ductility under the conditions tested.