Trimming and Sheared Edge Stretchability of Light Weight Sheet Metal Blanks

Abstract

Abstract In recent years, implementation of aluminum alloys, advanced high strength steels (AHSS) and ultra high strength steels (UHSS) is quickly increasing in automotive components due to their superior structural performance and vehicle weight reduction capabilities. However, these materials are often sensitive to sheared edge cracking if stretching along the sheared edge occurs in such processes as drawing of panels with blanked windows, stretch flanging and stretch hemming of edges of the panel. This study is dedicated to development of experimental techniques necessary to account for all these factors as well as reporting the experimental results and general trends. Analysis of hole punching process indicated that uniformity of the cutting clearance is rather difficult to keep consistent, especially for UHSS material where cutting forces are substantially higher than for mild steels or aluminum alloys, and stiffness of the tool starts playing critical role. Therefore, majority of experimental studies were performed as tensile tests of samples sheared along straight line in a dedicated trim tool where special measures were taken to achieve consistency of the die clearance. Experimental results on sheared edge stretchability of Aluminum alloy 6111-T4 and UHSS steel DP980 are reported. Mechanism of fracture propagation in trimming and hole punching processes is discussed in conjunction with sheared edge stretchability. Propagation of fracture from the upper cutting edge to the lower cutting edge of the trim die is the mechanism which enables higher stretchability of the sheared edge. Mechanism of fracture leading to crack propagation to the free surface of the blank usually results in formation of burrs and lower sheared edge stretchability. Rather unique mechanism of fracture observed for trimming of UHSS DP980 steel leads to burr breaking at the final stage of the shearing process.