Ball Burnishing of Mg Alloy Using a Newly Developed Burnishing Tool with On-Machine Force Control

Abstract

Burnishing is a surface finishing process, in which a very smooth surface finish is obtained by pressing a ball or roller against a machined workpiece. Additionally, owing to the surface plastic deformation caused by the movement of the hardened ball or roller, other surface properties such as hardness, fatigue life, and wear resistance can be improved. Burnishing force is one of the most important factors affecting the surface modification quality. However, methods for precisely monitoring and controlling the burnishing force are rarely investigated. In this research, a novel ball burnishing tool embedded with a load-cell and air-servo system was developed and fabricated for application to a CNC machining center. Using a specially designed control software, the burnishing force in the burnishing process was monitored in real time and controlled constantly and precisely by a force feedback system. Magnesium alloy AZ31 specimens were used to evaluate the performance of the developed system. The experiments were divided into two parts. In the first part, the effects of the processing parameters on the surface roughness were investigated. The results indicated that the surface roughness could be improved from Ra= 1.95 μm to Ra= 0.26 μm. In the second part, the effects of the burnishing parameters on the surface properties were investigated by conducting experiments using Taguchi’s orthogonal array. The results suggested that the burnishing force was the most significant factors affecting the surface hardness and grain size. The Vickers hardness could be increased from HV62 to HV149. The average grain size was reduced after the burnishing process, and a work-hardening layer thickness of 0.75 mm was achieved. X-ray diffraction results indicated that the crystal orientation was modified after burnishing, and the maximum measured compressed residual stress was 186.3 MPa in the tool feed direction and 87.8 MPa in the step over direction.