Inhibiting plastic tensile instability of non-symmetric thin-walled shell component via increasing regional metal inflow based on heterogeneous pressure-carrying medium

Heterogeneous pressure-carrying medium was employed to establish a differentiated pressure field on sheet metal in flexible die forming process in this work,which aimed at matching the non-symmetric shape of target component and improving metal inflow to avoid local tensile instability.Specifically,metal inflow corresponding to the differentiated pressure field was analytically evaluated.Forming of a typical non-symmetric shell component was experimentally and numerically studied based on the proposed method.Compared with forming processes based on the uniform pressure,difference of metal inflow in two sides of the non-symmetric component increased from 2.16 mm to 3.36 mm and metal inflow in critical region increased by 11.9%when differentiated pressure field(taking heterogeneous elastomer#4–3 for example)was employed.The resultant maximum thinning ratio decreased by 4.2% and the uniformity of shell thickness increased by 16.9%.With the decrease of Shore hardness of elastomer in the formed region,stress path in the ready-to-form region transferred towards the bi-axial tension stress state,i.e.,stress ratio(a)increased.And,stress triaxiality(η)in characteristic regions were regulated appropriately,which decreased the risk of tensile instability.It was attributed to the decreased normal pressure and frictional resistance at sheet/elastomer interface in the formed region.