Effect of material structure on trimming and sheared edge stretchability of 6xxx aluminum alloys

Wide variety of 6xxx Aluminum alloys is currently being used for manufacturing of lightweight automotive components. Very often each car manufacturer gets its own variation of the 6xxx alloy. Majority of these alloys have very similar yield and tensile strength as well as the stress-strain curve. Therefore, selection of the best alloy for each technical application might be challenging. In this paper, a comparison of performance of three 6xxx alloys in trimming process and sheared edge stretchability for wide variety of cutting clearances is being studied. This performance is compared with postnecking behavior of the same sheets in a standard tensile test. The ability of material to form visible diffuse neck and the cutting tool indentation prior to initiation of fracture shows some correlation for three studied 6xxx Aluminum alloys. The material with more pronounced diffuse neck shows a stronger tendency to form burrs at larger cutting clearances. However, other parameters of the standard tensile, such as yield stress, tensile strength, uniform elongation and even total elongation of these three alloys are very similar. Sheared edge stretchability measured via tensile testing of sheared samples showed that the materials with larger diffuse necking in the tensile test have higher sensitivity of edge stretchability to the cutting clearance. For large cutting clearances, the crack usually propagates almost vertically and leads to a burr formation. It appeared that the grain aspect ratio also influences the trajectory of the crack: for the materials with substantially elongated grains, the crack typically propagates more vertically. For the materials with nearly equiaxial grains, the cracks show stronger tendency to turn towards the lower cutting edge. The reason for such a behaviour is in crack propagation via grain boundaries. The experimental study of nanohardness measured at grains and grain boundaries for one of the studied alloys indicated that the grain boundaries have about 30% lower nanohardness than the material inside the grains.