Investigation On Mechanical Properties And Damage Mechanisms Of Cast Eutectic Al-Si Piston Alloy

During the past decades,the Al-Si alloys have been used increasingly in the automotive industry,because of their high strength-to-weight,low thermal expansion coefficient and excellent formability.In a start-stop cycle,a high temperature gradient can be created between the piston head and piston skirt.And the maximum operating temperature and stress may exceed 400℃ and 20 MPa respectively at piston head.Based on the working conditions of the diesel engine piston,several investigations have been carried out:1)Tensile strength evolution and damage mechanisms at different temperatures.The Al-Si alloy involving large alloy additions often leads to multiple solidification phases with complex three-dimensional network.In addition to the primary phase,the nano-precipitation phase is another important factor influencing the behaviors.The tensile properties show two-stage tendencies:the former stage(25℃-280℃)is determined by easily broken phases with inherent brittleness(such as primary Si),and the fracture behavior presents rapid brittle fracture after reaching the critical stress(about 430 MPa,based on in-situ technique and the elastic stress field model).The later one(280℃-425℃)is dominated by particles debonding and θ phase coarsening.The plastic deformation behavior,dynamic recovery and flow process become more significant on account of thermal activation.The Considere criterion h=K indicates that the transition of damage behaviors from insufficient local strength to insufficient matrix strength and the corresponding failure model shifts from brittle to ductile fracture.Based on the damage mechanisms,the elastic field model and thermal activation relation model have been established to characterize the strength at different temperature ranges.2)Low-cycle fatigue properties and life prediction at different temperatures and strain rates.With increasing temperature,the fatigue life increases at first and then reduces,which is derived from the deformation mechanism.At the lower temperatures,the primary Si cracking induced by piling-up of dislocations dominates fatigue damage behaviors.Both of increasing temperature and reducing the strain-rate can restrain the phase cracking,thus increase fatigue life.However,with further increasing temperature,the micro-scale void caused by dislocation annihilation dominates the phase debonding behavior,leading to the decrease of fatigue life.In order to evaluate the fatigue life,a hysteresis energy model has been proposed.The fatigue life can be controlled by two parameters,i.e.,the intrinsic fatigue toughness W0(the resistance to crack propagation)and the fatigue cracking exponent β(the resistance to fatigue cracking),which dominate the LCF damage mechanisms(from fatigue-induced particle cracking to rapid fatigue crack growth).3)Thermo-mechanical fatigue behavior and life prediction in the different temperature ranges and constraint factors.For TMF damage behavior,the cracks mainly initiate from the broken primary silicon in the temperature of 120-350℃ range,and commonly nucleate from the boundary between primary Si and matrix in the temperature of 120～425℃ range.Taking the phase angles into consideration,the mean tensile stress for out of phase TMF(OP-TMF,η<0)and mean compressive stress for in-phase TMF(IP-TMF,η>0)can be found.The fatigue life of IP-TMF is longer than that of OP-TMF except for higher constraint factor,and the life of TMF decreases with the increasing absolute value of η.There are two different crack initiation mechanisms under the constrain TMF:primary phases broken mainly for OP-TMF and matrix deboned mainly for IP-TMF.The oxidation may have only little influence on the crack behavior and TMF life,considering the obvious damage by deboned or broken primary phases.The tensile stress is key factor for fatigue crack initiation and growth in primary phases under cyclic deformation.A new energy-based model for LCF and TMF life prediction was proposed based on the hysteresis energy with strain rate modification,considering both fatigue and creep damages.In addition,another strain energy method under different constrain factors provided a simple way to assess the lifetime of components subject to thermo-mechanical fatigue with an acceptable accuracy.4)Damage behavior under different loading conditions and property optimization by ultrasonic melt treatment.Upon the tensile loading,the micro-cracks initiates from the primary Si near the surface when the tensile stress close to the tensile strength,and then the micro-cracks grow rapidly through eutectic Si and intermetallic.For the LCF,the micro-cracks initiate from the shrinkage pores regularly due to the localization of fatigue damage in the GC alloy.The lamellar A13CuNi provides a preferred path for the crack propagation after fatigue crack initiation.The ultrasonic melt treatment(UT)was used to optimize the microstructures and reduce casting defects(shrinkage pores),resulting in the improvement of tensile and LCF properties.For the UT alloy(pore-free),the micro-crack initiates from some coarser primary Si due to the accumulation of micro-scale plastic deformation rather than from the shrinkage pores.Furthermore,the higher resistance of crack propagation can also be realized with refinement of microstructures in UT alloy.In this way,the remarkable improvement of the high temperature fatigue and TMF properties have also been performed.