Surface roughening of an aluminum 6016 alloy during bending and hemming

Hang-on parts of modern passenger cars such as doors and hoods are mainly manufactured by a two-step hemming process during assembly, whereby the edge of the already deep-drawn, blanked and flanged outer panel is further bent to a hem. This hem ties together outer and inner panel of hang-on part. Owing to the bending processes during hemming, large amounts of strain occur on the outer side of the hemming rope, causing local roughening of the formed sheet surface, which may become critical in terms of the quality of the coated component. In this study, the surface roughening during bending and hemming of a typical aluminum sheet material for hang-on parts is examined. In a first step, the roughening process is determined as a function of the bending angle and plastic strain during bending and hemming. Then hemming was further investigated to describe surface roughening as a function of the flanging radius and pre-strain level. The results of the plate bending test and hemming test at identical bending angles and plastic strain levels showed different levels of surface roughening, caused by the non-congruent size and local position of the forming zone in both bending methods.