A study on galling initiation in friction coupling stretch bending with advanced high strength hot-dip galvanized sheet

As a form of surface damage arising between sliding solids, galling has become a critical issue in the use of advanced high strength steel (AHSS) sheet and it will reduce tool life and lower part quality. To clarify the dynamic evolution of galling initiation and to identify its initiation quantitatively, this study was conducted to simulate the sliding of hot-dip galvanized AHSS sheet against cold-work DC53 die with the use of a self-developed sheet/strip tribo-tester. Such tester is capable to yield plastic deformation and stretch bending simultaneously. The test allowed the identification of galling initiation via analyzing the step response of recorded friction coefficient which was subsequently corroborated with the observation of surface morphology on die surface. The difference of sheet surface roughness in vertical direction (δR_y) was proposed as a quantitative index for the analysis and identification of galling initiation. Dynamic evolution of galling initiation was investigated with the help of SEM analyses on the surface of both slid sheet and die. Wear mechanisms affecting the friction behavior and galling initiation were subsequently suggested and discussed.