Flow and friction behaviors of 6061 aluminum alloy at elevated temperatures and hot stamping of a B-pillar

The flow and friction behaviors of 6061 aluminum alloy (Al6061) at elevated temperatures were investigated to form a B-pillar by hot stamping with Al6061 sheets. The modified Arrhenius and Cowper–Symonds model were developed based on hot tensile tests at temperatures of 350–500 °C and strain rates of 0.01–1 s−1. The coefficients of friction (COF) at temperatures of 300–500 °C were also measured. The tribological test results show that the COF ranged from 0.95 to 1.05 with a loading force of 1 N and from 1.2 to 1.53 with a loading force of 10 N. Finally, hot stamping of a B-pillar using Al6061 sheets with different solution heat treatment temperatures and lubricants was conducted. Stamping experiments show that the B-pillar wrinkled badly and cracked in cold stamping with or without lubricants. The B-pillar also cracked in hot stamping without lubricants, but B-pillars without cracks could be obtained under lubricated conditions. Further finite element simulations with the implementation of Cowper–Symonds constitutive equation and the tested COF values (as references) were used to investigate the crack mechanisms that arise in hot stamping. Simulation results show that the crack was due to adverse friction and large temperature difference between the sidewall and rounded corners caused by non-uniform cooling. Lubrication not only reduces the friction to promote metal flow but also weakens the temperature difference. Thus, parts without cracks can be obtained.