Wurong Wang

Microstructural evolution of Al-Si coating and its influence on high temperature tribological behavior of ultra-high strength steel against H13 steel

Al-Si coated ultra-high strength steel (UHSS) has been commonly applied in hot stamping process. The influence of austenitizing temperature on microstructure of Al-Si coating of UHSS during hot stamping process and its tribological behavior against H13 steel under elevated temperature were simulatively investigated. The austenitizing temperature of Al-Si coated UHSS and its microstructual evolution were confirmed and analyzed by differential scanning calorimetry and scanning electron microscopy. A novel approach to tribological testing by replicating hot stamping process temperature history was presented. Results show that the hard and stable phases Fe2Al5 + FeAl2 formed on Al-Si coating surface after exposure to 930 °C for 5 min, which was found to be correlated to the tribological behavior of coating. The friction coefficient of coated steel was more stable and higher than that of uncoated one. The main wear mechanism of Al-Si coated UHSS was adhesion wear, while abrasive wear was dominant for the uncoated UHSS.

Effect of Heat Treatment Conditions on Mechanical Properties and Precipitates in Sheet Metal Hot Stamping of 7075 Aluminum Alloy

Due to the low formability at room temperature, the application of 7XXX aluminum alloy sheet is limited. The hot forming-quenching process is an effective technology for the forming of some complex-shaped aluminum alloy parts. In this study, heat treatment was integrated with sheet metal stamping to improve formability and strength of 7075 aluminum alloy sheet. The effect of different heat treatment parameters on the strength of 7075 aluminum alloy sheet was investigated under stamping and heat treatment integrated process, based on which the optimal heat treatment combination was determined. Furthermore, the evolution of precipitates with the variation of aging temperature and time was studied by transmission electron microscope. In addition, quenching under water-cooled dies was conducted for 7075 aluminum alloy sheet after solution heat treatment, in which the hardness under die quenching was compared with that of underwater quenching and air quenching. Finally, the tensile test illustrates that, after die quenching and successive artificial aging, the tensile strength of 7075 aluminum alloy sheet was achieved at around 548xa0MPa. Shear fracture with long strip step-shaped pattern was observed in aging state.

An investigation on the resultant-based failure criterion for resistance spot welding joint in crush test

ABSTRACT The failure of resistance spot welding (RSW) joint will severely affect the deformation behaviours of sheet metal parts and it is crucial for vehicle safety to predict whether and when the RSW joints will fail in crash loading. The objective of this paper is to investigate the resultant-based failure criterion for RSW joint and to verify its applicability in crush simulation of structural components. The testing methods of the failure criterion were proposed and implemented for single-spot RSW joint. The failure parameters of tension, shear, bending and torsion were resolved from the testing result of single-spot RSW joint. Validation of the failure criterion was conducted through a crush test of a component consisting of multiple RSW joints. It was concluded that the proposed testing methods and the resultant-based failure criterion could reflect the spot welding performance and predict the failure of RSW joints in the crush simulation.