Research On The Effect Of Annealing Treatment On Fatigue Behavior Of As-Extruded ZK60 Magnesium Alloy

As the third major metal engineering material after steel and aluminum alloy,Magnesium alloys are known as the "green engineering material" of the 21st century due to their low density,high specific strength and specific stiffness,good shock absorption performance and electromagnetic shielding performance,which have been widely used in aerospace,transportation,machinery industry,electronic products and other fields.The extruded ZK60 commercial magnesium alloy has a higher static strength than other magnesium alloys.In practical applications,in order to obtain good machinability and plastic elongation,it is usually necessary to anneal the extruded ZK60 magnesium alloy to eliminate residual stress and work hardening caused by the extrusion process.However,the annealing process changes the microstructure of the alloy and affects its properties ultimately,especially fatigue properties.In this paper,commercial extruded ZK60 magnesium alloy profiles were selected as research objects,and the effects of annealing treatment on microstructure evolution and fatigue behavior were studied in detail.The details are as follows:The extruded ZK60 profiles were annealed at 420? for 1 h,4 h and 8 h,respectively.Uniaxial tension cycle load(R=0.1),symmetrical tension and compression cycle load(R=-1)and corrosion fatigue test were carried out on different annealed samples.The effects of annealing on the fatigue behavior of the extruded ZK60 magnesium alloy were investigated by means of scanning electron microscopy and surface observation.The mechanism of fatigue crack initiation and crack propagation was characterized.Based on the response law of fatigue performance to annealing treatment,the optimum annealing time for improving the fatigue properties of the alloy under different service conditions was determined.The results show that the annealing treatment will reduce the fatigue strength of the extruded ZK60 magnesium alloy under the conditions of uniaxial tension cycle load,symmetrical tension and compression cycle load.The fatigue cracks of as-extruded and 420?/1 h ZK60 magnesium alloy are initiated at the grain boundarys(GBs)and mainly propagate in the intergranular manner.420?/4 h and 420?/8 h ZK60 magnesium alloy fatigue cracks initiation at the persistent slip bands(PSBs),the crack propagation mode is a mixed mode of transgranular and intergranular.Under the cyclic load of symmetrical tension and compression,the fatigue cracks of different treated samples are all initiated in the twin boundarys(TBs).At the same time,under different fatigue loading conditions,the fatigue crack propagation patterns of the treated samples did not change.It is found that the coarsening of the grains promotes the initiation and accumulation of twins,increases the probability of fatigue crack initiation,and ultimately reduces the fatigue strength of the alloy under symmetric cyclic loading.Combined with weight loss,hydrogen evolution,electrochemistry and immersion experiments,it is shown that the annealing treatment can improve the corrosion resistance of the extruded ZK60 magnesium alloy,and the second phase and grain size are the main controlling factors affecting the corrosion resistance of the alloy.In comparison,420?/4 h has the highest corrosion resistance.Due to the corrosion fatigue crack initiation of magnesium alloys and its corrosion resistance,420?/4 h sample has the highest corrosion fatigue strength.