Surface quality of aluminum extrusions can be hampered by undesirable features, such as pick-up. The key area for pick-up formation is the die bearing, where a sticking zone and a slipping zone can be distinguished. The length of the slipping zone can be calculated based on the pressure distribution and the frictional behavior in the bearing area. Within the slipping zone, material from the extrudate can be transferred to the tool surface. Based on a pick-up formation mechanism, a model describing surface quality of aluminum extrusion products has been developed. The model calculates initiation, growth, as well as detachment of the lumps on the bearing surface. Important parameters are the micro-geometry of the bearing, the constitutive behavior of the aluminum alloy, the pressure distribution in the bearing area, and extrusion parameters such as exit speed and extrudate surface temperature. Results from AA6063 lab scale tests show good agreement with the model results. The calculations can be presented in terms of surface quality diagrams, where contour lines of the (normalized) calculated number of pick-ups are presented. Combined with conventional limit diagrams, these surface quality diagrams provide an enhanced view of the true operational process window.
展开▼
