Influence of Process Parameters on the Forming Quality and Metal Flow Characteristics of the Billet During Hot Extrusion of an Automotive Luggage Rack

Automotive roof racks are important lightweight accessories for vehicles, and their extrusion performance is affected by the coupled effects of material hot deformation behavior, die flow resistance and billet surface layer transport. In this study, Al-0.9Mg-0.6Si alloy samples were subjected to hot compression tests at 350–500 °C and strain rates of 0.01–10 s−1. The corrected true stress–true strain data were used to establish and validate an Arrhenius-type constitutive model, which was then implemented in HyperXtrude to simulate the hot extrusion of an automotive roof rack profile. The hot working map showed that the main rheological instability region was located at high strain rates, and the preferred processing window was 437–500 °C and 0.01–0.6 s−1. EBSD analysis showed that hot compression refined the microstructure relative to the initial average grain size of 173.147 μm, and the most uniform grain size distribution was obtained at 500 °C and 0.1 s−1. The ODF results indicated strengthened {111}<121> and <110>//TD texture components after compression. The finite-element results showed that the standard deviation of outlet velocity (SDV), used here as an index of outlet flow uniformity, increased with ram speed, billet preheating temperature and die preheating temperature, but decreased with increasing container temperature. Finally, grain size and texture measurements from butt discard samples were compared with simulated surface layer flow paths, supporting the predicted difference between simple axial flow and complex recirculating flow near the die.