Fatigue characteristics of the extruded AZ80 automotive wheel

Without action to make vehicles more efficient, carbon emissions and environmental degradation would rise significantly. So far, various approaches have been proposed; part of which include light-weighting. Light-weighting has been presented as one of the pivotal methods in vehicles to improve fuel efficiency and reduce carbon dioxide emissions. This approach entails adoption of light-weight alloys that possess a high strength-to-weight ratio for the manufacture of various parts of the vehicle. Specifically, magnesium (Mg) alloys, which suit the description in addition credit to the fact that they possess excellent vibration absorption, good fatigue response, satisfactory castability, are ductile and have excellent machinability. As such, Mg alloys are recognized as the lightest commercially available metal with great development potential in automotive industry. A review of existing literature on Mg alloys reveals that almost 90% of the alloy’s non-load bearing products are manufactured by casting process. This process is not devoid of defects whose elimination is a crucial problem that must be addressed if there are to be broader applications of Mg alloys in safety-critical structural automotive components. In comparison to as-cast Mg alloys, wrought alloys have shown higher strength, better ductility and superior fatigue response.

Consequently, to date, a great number of studies have been conducted in order to understand the monotonic property and fatigue behavior of wrought Mg alloys, including rolled and extruded AZ, AM, and ZK-series. In particular, the AZ80 alloys have been investigated for their static and fatigue properties of cast-forged, rolled and extruded through both stress and strain-controlled fatigue tests. Nonetheless, further investigations are still necessary, particularly concerning the effect of the extrusion process for AZ80. In this view, researchers from the North University of China: Professor Xi Zhao, Pengcheng Gao (MSc), Professor. Zhimin Zhang, Professor Qiang Wang and Fafa Yan (MSc.) investigated the effects of the new extrusion process on the mechanical properties; specifically, the fatigue behavior, using specimens taken from the extruded AZ80 automobile wheels via fully-reversed strain-controlled fatigue tests. Their work is currently published in the International Journal of Fatigue.

With their goal being to provide a reliable theoretical foundation for fatigue resistant design of the magnesium alloy automobile wheel, the researchers took specimens machined from rim and disc of the extruded AZ80 wheel and subjected them to strain controlled fatigue tests. Overall, the samples were characterized via microstructure and texture analysis, stress-strain response monitoring, twinning/detwinning behavior evolution and fatigue fracture morphology analysis.

The authors reported that based on their micro structure analysis, the rim sample exhibited finer and more homogeneous dynamic recrystallized grains with the average grain size of ~17.2μm, while ~30.5μm for the disc sample. Further, they recorded that the rim specimen achieved the ultimate tensile strength of ~339 MPa with a noticeable elongation to fracture of ~14.6%. In addition, the rim sample also showed superior fatigue properties as compared with the disc sample, which was related to the smaller grains and the weaker texture intensity.

In summary, the presented study by North University of China scientists looked carefully at the monotonic and fatigue behavior of AZ80 magnesium alloy wheel prepared by a new extrusion process. Remarkably, the authors were able to establish the link between microstructure 、mechanical behaviors and fatigue behavior of the extruded AZ80 wheels. Further, SEM fracture surface characterization was utilized to describe the mechanism underlying fatigue crack initiation and failure. In a statement to Advances in Engineering, Professor Xi Zhao highlighted that in addition to their remarkable observation concerning the AZ80 alloy, their work presented an indispensable foundation for future studies.