Experimental Study of Slide Burnishing Process of the Pre-milled AA7075 Plate: Surface Topography and Hardening Aspects

Non-rotary flat slide ball burnishing (NRFSBB) was developed as a surface finishing process. Due to the first-time utilization of non-rotary sliding contact in the ball burnishing process, a comprehensive investigation of the polishing and hardening aspects after NRFSBB was conducted. The influence of NRFSBB parameters including vertical depth, step-over, and number of passes on surface roughness and microhardness was examined. NRFSBB experiments were performed on a face-milled AA7075-T651 plate. Compared to the face-milled surface, longitudinal roughness, transverse roughness and microhardness were improved by up to 90.4%, 88.5% and 44%, respectively. Optimal surface properties were achieved using the vertical depth of 0.15 mm, step-over of 0.07 mm and three passes. Topography analysis showed that lower vertical depth improved the area roughness parameter of Sa to 90.3%. Also, a 12.3% higher improvement in Sa was provided by NRFSBB with a step-over of 0.07 mm compared to 0.23 mm. Through-thickness microhardness distribution determined a hardened depth of 700 μm, obtained with a vertical depth of 0.21 mm. SEM–EDS analysis on the burnished cross-sectional sample revealed a high-probable formation of strengthening intermetallics, such as MgZn₂. XRD analysis indicated that the ultra-fine grain (UFG) microstructure was obtained with a vertical depth of 0.09 mm. Meanwhile, higher vertical depths provided nanostructure (NS). Severe deformation of NRFSBB led to a 100-fold increase in dislocation density compared to the face-milled surface. The formation of strengthener intermetallics, grain refinement and increase in dislocation density resulted in a significant enhancement in microhardness at the burnished surface and subsurface.