Fatigue Behavior Of Crucial Parts In Extruded AZ80 Magnesium Alloy Automotive Wheel

The main reference of fatigue resistance design for traditional magnesium alloy automotive wheels is numerical simulation analysis and rotary fatigue testing.In order to ensure the reliability and durability of products during the service period,the weak position of the structure is inevitably over-designed,which not only weakens the lightweight advantage of magnesium alloys but also does not provide a solution to the inherent damage mode of the material.In addition,the history of thermo-mechanical forming of magnesium alloy wheels has also led to inhomogeneity distribution of the microstructures,further causing differences in fatigue behavior at various parts.This paper mainly studies the static and fatigue mechanical properties of the magnesium alloy wheel prepared by a novel method,analyzes microstructure in the disc and rim under this forming process,as well as their fatigue behavior and failure mechanism,further provides the theoretical support and data reference for regulating and optimizing its fatigue performance and fatigue life prediction.Microstructure and texture analysis,stress-strain response monitoring,twinningdetwinning behavior evolution,and fatigue fracture morphology analysis have been carried out to investigate the static and fatigue performance of the AZ80 wheel.The link between microstructure and cyclic deformation behavior of the extruded AZ80 wheels was established.The results showed that the rim sample exhibited finer and more homogeneous dynamic recrystallized grains with the average grain size of ~17.2 ?m,while ~30.5 ?m for the disc sample.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.The twinning-detwinning behavior participated in the cyclic deformation above the strain amplitude of 0.4% for both samples,resulting in asymmetric hysteresis loops and residual twins.The initial evolution of the rim sample ex-situ EBSD(?? = 1%)indicated that after the first cycle,the volume fraction of residual twins was about 0.5%.The fatigue crack initiation site was observed with second phase particles and secondary cracks near the surface for all specimens,and in particular,cleavage-like faceted area with twins was identified at strain amplitude of 1%.The effect of precipitation on strain-controlled fatigue performance was investigated via the extruded AZ80 automotive wheel with different peak-aging conditions(T5 and T6).After T5 and T6 treatments,a better precipitation strengthening effect was achieved,i.e.from 68.4 HV to 84.8 HV(T5-36h)and from 64.2 HV to 82.4 HV(T6-24h),respectively.The peak-aging treatments also significantly enhanced the monotonic strengths under tension and compression conditions.As the area fraction of the second phase rises(extruded-14.5%,T6-57.4%,T5-75.6%),the activation of bulk and persistent twinning-detwinning behavior in the early stages at cyclic deformation weakened or even disappeared.For the strain amplitude ??/2<7.0%,the stable hysteresis loops of both aged samples were basically symmetrical,the predominant characteristics at these strain amplitudes were cyclic stabilization.Analysis of crack growth indicates that the pattern of propagation was related to the coalescence of voids in the cyclic plastic zone.Compared to the extruded and T6-treated counterparts,a large number of lamellar phases in T5 sample suppressed the cyclic plastic deformation at the crack tip,resulting the deflection of cracks,further leading to the longer fatigue lifetimes at certain strain amplitudes.At low strain amplitude(??/2?0.3%)and high strain amplitude(??/2?0.8%),the extruded disc sample have shown superior fatigue life,the former was related to the early crack initiation of aged samples,while the other was attributed to the low structural sensitivity of crack propagation under high stress conditions.The fatigue life of the medium strain amplitudes 0.3%<??/2<0.8%)was arranged in descending order at the sequence of T5,extrusion,and T6 samples.The energy-based JV model was suitable for modelling the extruded AZ80 magnesium alloy wheels and their aging condiation,which can provide a reliable reference for the early fatigue design stage of lightweight magnesium alloy wheels.