The influence of surface topography on the forming friction of automotive aluminum sheet

Interest in utilizing aluminum alloys in automobiles has increased in recent years as a result of the desire to lower automobile weight and, consequently, increase fuel economy. While aluminum alloy use in cast parts has increased, outer body panel applications are still being investigated. One avenue of improving the formability of these alloys may be through patterning of the sheet surface. Surface patterns hold the lubricant during the forming process, with a resulting decrease in the sheet-die surface contact. While it has been speculated that an optimum surface pattern would consist of discrete cavities, detailed investigation into the reduction of forming friction by utilizing discrete patterns is lacking.; A series of discrete patterns were investigated to determine the dependence of the forming friction of automotive aluminum alloys on pattern lubricant carrying capacity and on material strength. Automotive aluminum alloys used in outer body panel applications were rolled on experimental rolls that had been prepared with a variety of discrete patterns. All patterns for each alloy were characterized before and after testing both optically and, to determine pattern lubricant capacity, using three dimensional laser profilometry. A draw bead simulation (DBS) friction tester was designed and fabricated to determine the forming friction of the patterned sheets. Tensile testing and frictionless DBS testing were performed to ascertain the material properties of each sheet. The most striking result of this work was the inversely linear dependence of forming friction on the lubricant carrying capacity of the discrete patterns. Other important observations include the fact that forming friction trends for particular patterns remained the same between alloy sheets of similar thickness, with friction increasing both with material strength and with pattern degradation resulting from roll wear. In addition, post-test surface roughening destroyed patterns 2{dollar}mu{dollar} or less in depth while patterns greater than 2{dollar}mu{dollar} in depth maintained most of their shape. The inversely linear dependence of forming friction on discrete pattern lubricant carrying capacity and the increase of forming friction with material strength were used to develop a constitutive equation for use in describing the forming friction behavior of the discretely patterned surfaces.