The Influence Of Precipitation On Fatigue Behavior And Fatigue Properties Of Al-4Cu Alloy

Cast aluminium alloys have been widely used in the automotive industry,trains,ships,aerospace and other fields due to their good casting properties.The heat treatable aluminum alloy has been widely used in the industry due to its advantages in strengthening the process.There are many ways to strengthen the heat treatable alloy,such as precipitation strengthening,fine grain strengthening,dispersion strengthening,etc.The effect of precipitation strengthening is the most significant.The heat treatment process of aluminum alloy is accompanied by the evolution of precipitation phase,so the heat treatment strengthening effect of aluminum alloy is closely related to the type of precipitated phase.With the advancement of the industrial level,heat treatment and work hardening have been able to produce the requirements for satisfying the stiffness and strength of aluminum alloys,and instead focus on the service life and safety of components.During the process of using components,it is inevitable to be affected by cyclic stress,and finally fatigue failure occurs,which affects the service life of the components.The fatigue problem of aluminum alloy has become a major issue in the industrial application of aluminum alloys.Therefore,the fatigue properties of aluminum alloys are investigated.The impact is important for predicting component life and safety.As the main strengthening method,precipitation strengthening is of practical significance to explore the influence of precipitation on the fatigue properties of aluminum alloy.Al-Cu binary alloy is used as model alloy.The sequence of precipitation phase is relatively simple,and when the percentage of quality of Cu element in Al-Cu alloy is 4%⁶%,the strengthening of Al-Cu alloy can achieve the best effect.Therefore,Al-4Cu is selected as the research object to explore the influence of precipitation on the fatigue properties of aluminum alloy.In this paper,the effects of heat treatment on the microstructure of Al-4Cu alloy have been studied by means of metallographic microscope and transmission electron microscopy.The effect of precipitation on the fatigue properties of Al-4Cu alloy has been studied by means of fatigue testing machine.The method of sliding trace analysis and DIC analysis have been studied.The deformation mechanism and strain non-uniformity of Al-4Cu alloy under cyclic stress are studied.The theoretical basis for predicting the fatigue life of aluminum alloy and improving the safety of aluminum alloy components is provided.The main conclusions obtained are as follows:?1?A large number of dendrites are formed in the as-cast sample due to the undercooling of the composition,and the eutectic phase composed of?solid solution and a small amount of Al₂Cu is formed at the grain boundary and the dendrite.The primary precipitated phase Al₂Cu,can be seen in the grain.There was no needle-like precipitated phase structure in the alloy.After solid solution treatment,the eutectic phase at the dendrite and grain boundary has been basically dissolved,and the grain boundary has become smooth.There is no obvious eutectic phase in the grain boundary and grain,and the second phase has completely solid solution into the matrix.After peak aging treatment,a large number of needle-like precipitated phase?"phase and a little of??phase and?phase,mainly?"phase can be seen,which are dispersed and distributed in the matrix.?2?The yield strength of cast Al-Cu alloy is only 89.56 MPa,the tensile strength is189.43 MPa,and the elongation is 6.7%.The yield strength of cast Al-4Cu alloy after peak aging treatment is 196.1 MPa.The tensile strength was 307.1 MPa and the elongation was 15%,which increased by 118.96%,62.12%and 123.88%,respectively.With the increase of cycle,the maximum strain amplitude of as-cast and peak aging Al-4Cu alloys decreases at first and then increases,that is,cyclic hardening occurs first,and then cyclic softens until fracture.The fatigue properties of cast Al-4Cu alloy in high stress zone were improved by peak aging treatment.?3?For as-cast Al-4Cu alloy,the grain slip first starts along the slip system with the highest SF value to form a resident slip band,which indicates that the grain orientation has an effect on the fatigue damage mechanism of as-cast Al-4Cu alloy.?4?There are few precipitated phases in the as-cast alloy and little hindrance to slippage.Under cyclic stress,dislocation accumulates in the grain,forming dense resident slip zone,which promotes the initiation and propagation of fatigue cracks.Resulting in poor fatigue life.After peak aging treatment,a large number of dispersed?"phases were formed in the grain,which had strong inhibitory effect on dislocation slip,and there was no obvious resident slip zone in the grain,and the fatigue properties of the alloy were better.?5?The tensile fracture of as-cast aluminum alloy is composed of transgranular fracture and intransgranular fracture,mainly transgranular fracture can be see.The tensile fracture of peak aging aluminum alloy is characterized by transgranular fracture.The fatigue three regions of as-cast Al-4Cu alloy are not obvious,but the peak aging Al-4Cu alloy shows obvious fatigue fracture zone,including crack initiation zone,crack propagation zone and unstable fracture zone.