Heat Treatment Process Optimization And Fatigue Strength Study Of Semi-continuous Cast Mg-Gd-Zn-Zr Alloy

With the development of technology,Magnesium alloy is expected to be used in vehicle bearing parts,and components such as wheels,engine cylinder block,etc.These parts in the process of serving under cyclic loading test,and the fatigue property of magnesium alloys is becoming increasingly important.How to improve the fatigue strength of magnesium alloy,how to predict fatigue strength simply and effectively magnesium alloy by using the conventional mechanics performance,is gradually becoming the focus of attention of scientific and technological workers.This paper is mainly to study the influence of heat treatment on room temperature tensile properties and fatigue strength of cast Mg_96.32Gd_2.5Zn₁Zr_0.18?wt.%?alloy produced by semi-continuous casting.The following conclusions can be made:?1?It was found that different solution temperatures lead to formation of different phases.Besides?-Mg matrix and Zn–Zr compounds,the secondary phases along the grain boundaries change with solution temperature.At 480°C and 500°C,long period stacking ordered?LPSO?structured X phases and residual eutectic compounds?Mg,Zn?3Gd are observed,while at 520°C,only residual eutectic compounds?Mg,Zn?3Gd exist.Higher solution temperature results in higher aging response and better mechanical properties.The alloy solution treated at 520°C for 8 h and aged at 200°C for 64 h shows the best tensile properties at room temperature:ultimate tensile strength of 405 MPa,yield strength of292 MPa and elongation of 5.3%.The influences of existing forms of Gd and Zn elements on tensile properties of Mg–Gd–Zn alloy were discussed,which indicates that the existing of Gd and Zn elements as precipitates leads to better strengthening effect than that in the form of LPSO structured X phase.?2?Different from room temperature tensile properties,compare with under and peak ageing,the over aging alloy has higher rotating bending fatigue strength at room temperature.The fatigue strength of Mg_96.32Gd_2.5Zn₁Zr_0.18?wt.%?alloy after 520?×12h+200?×256h over ageing treatment is 195MPa,compare with NZ30K-T6 alloy,GW103K-Extruded-T6 ally and AZ91D-T6 alloy,the fatigue improved93.1%,77.3%and 143.8%,respectively.Under different ageing time,Mg_96.32Gd_2.5Zn₁Zr_0.18?wt.%?alloy has a different method of fatigue cumulative damage:Under ageing,intracrystalline persistent slip bands cracking is the main damage way,twin for the secondary way;Peak aging,intracrystalline persistent slip bands cracking is the main damage way,both with grain boundary and second phase cracking;Over aging,crack initiation area does not exist persistent slip bands,the cracking of grain boundary and second phase is the main way of fatigue damage.?3?The fatigue strength of Mg_96.32Gd_2.5Zn₁Zr_0.18?wt.%?alloy can be predicted by conventional tensile/compression performance,the mathematical relational expression is as follows:?_f=0.05637?_bt·?_t^1/2+124.13?R²=0.9578??_f=0.08653?_0.2c·?^1/3+113.09?R²=0.9651??_f=0.03521(?_bt·?_t^1/2+?_0.2c·?_c^1/3)+118.39?R²=0.9908?The combination of fatigue strength of alloy and yield strength and elongation of tensile and compression properties has good mathematical relationship,which can forecast and estimate the fatigue strength of magnesium alloys.Meanwhile we know,improve the yield strength and elongation can improve the fatigue strength of materials.