Mechanisms of fold formation during flange upsetting of tubular parts

In the present paper a two-stage upsetting process preventing buckling at critical upsetting heights of tubular parts of the aluminum alloy EN AW 6082 O1 is analyzed. Numerical investigations have been conducted to determine influencing parameters on the annular fold formation and numerous experimental investigations have been conducted for their validation. It has been shown, that using a two-stage upsetting process buckling at critical upsetting height ratios can be prevented by optimized pre-upsetting shapes obtained by using numerical simulation previously. Although preventing buckling, folding in the annular flange occurs always for investigated geometries within second upsetting stage in experimental investigations while numerical simulations predict sound part quality. Using micro hardness measurements, REM analysis and an alternative process route, further mechanisms and influence factors have been found. These lead to annular folding apart from buckling of the tubular part and are presented in this paper. Furthermore observed folding without buckling in experimental analysis recently cannot be predicted numerically by using common extrapolation of flow curves for high effective strain values, which is also discussed in this contribution.