Study of the Curling of Tensile Test Specimens from the 6xxx Aluminum Alloy Sheet

The 6xxx Al-Mg-Si heat-treatable alloy is lightweight, has good formability before baking-hardening, and enhanced strength after baking. The aluminum alloy is to replace steel in many automotive stamping sheet metal parts. However, under the normal hot rolling process, 6xxx aluminum alloy is prone to surface roping defects, owing to the uneven texture in the thickness direction. Aluminum with severe roping can also produce curling of the specimen in a uniaxial tensile test. The literature and related phenomena infer that the material thickness direction does not have a uniform r-value. The r-value measured by the test machine does not represent the actual whole r-value of the material. Therefore, in this paper, the actual r-values of the top and bottom surfaces of the material are derived from the geometry of the curl of the specimen. An alkali etching method is developed to layer the aluminum sheet's thickness. The layer is used for tensile testing to verify the aluminum sheet's actual r-value and derive the difference in r-value. Then, experiments and numerical simulations are used to show that the variation of thickness direction r is the leading cause of curl in the tensile specimen. Furthermore, we use numerical simulation to evaluate the difference in r-value of an aluminum sheet in each thickness layer and the effect of thickness layer on curl, and then make a reasonable explanation and correction of the difference between tensile test and simulation. Using this method, we can obtain a more accurate r-value as the material parameter for each thickness direction of an aluminum sheet, which certainly improves the accuracy of numerical simulation and analysis for other forming processes.