Roughness and microstructure characterization of AISI 316L laser-powder bed fusion specimens after applying a vibration-assisted ball burnishing process

Abstract

The objective of this study is to analyze the effect of adding an ultrasonic vibration assistance within a ball burnishing (VABB) process onto cylindrical AISI 316L laser powder bed fusion (LPBF) specimens. The purpose of the specimens is to improve surface integrity, in terms of roughness reduction and microstructure reconstruction, to optimize the fatigue performance for testing samples according to ISO 1099:2017, reducing surface imperfections with burnishing. All printed specimens were manufactured in the Z direction using a laser power of 195W, a scanning speed of 900 mm/s, a hatch distance of 0.1 mm, a layer thickness of 40 μm, and a scan angle of 67°. The preliminary experimental campaign is based on a full factorial design of 3 variables (burnishing force, number of passes, and BB/VABB) with 2 levels for each. It was found that the estimated depth of affectation is 4-5 μm and 5-7 μm, for BB and VABB respectively, thus demonstrating the positive effect of the vibration assistance. The softening effect while deforming, caused by the vibration assistance, helped to reconstruct the surface, obtaining higher depths of affectation. In terms of topology, the surface average roughness for all specimens is within the range of 0.2-0.4 μm, showing an improvement of 74% in the best case compared to the machined surface. The material distribution parameters, Skewness and Kurtosis, show an almost perfect Gaussian distribution for all the specimens analyzed.