Study On The Microstructure And Elevated Temperature Mechanical Properties Of The Piston Aluninium Alloy Treated By Thermal Exposure

Pistons are key components that affect the performance indicators and service life of diesel engines,which require high temperature strength,good wear resistance,corrosion resistance and small coefficient of thermal expansion.At present,alloying,fine grain strengthening and modification treatment are adopted to improve the microstructure and properties of the aluminum piston alloy which meets the requirements of engine working conditions.However,in order to adapt to more severe working conditions,the higher high temperature strength and fatigue property is needed to satisfy the working requirements of high-power diesel engines with the increase of power consumption.Therefore,in this paper,the thermal exposure treatment process was applied to improve the fatigue life of the piston alloy,the change of alloy microstructure and properties under the actual operating temperature of the piston was simulated to provide reliable theoretical basis and design parameters for the piston.In this paper,high temperature tensile properties and hardness of Al-12.5Si-4Cu-Mg-2.6Ni new piston alloy at T6 treatment and T6+200°C,250°C,300°C,350°C,385°C and 425°C heat exposure were studied.Three states,T6 treatment,T6+350°C/100h and T6+425°C/100h heat exposure treatments,were selected and high cycle bending fatigue were tested at the condition of 350°C,stress 50MPa,frequency 80Hz and 35Hz,sine wave loading.Compared to Al-12.5Si-3Cu-1Mg-2.0Ni present piston alloy,the new piston microstructure change,fatigue crack initiation mechanism and fatigue fracture mechanism were investigated by Nicon300optical microscope,Tescan VEGA·IIXMU scanning electron microscope and JEM-2010transmission electron microscope.The conclusions are as follows:?1?After 100h heat exposure of the new piston alloy at T6 and T6+200°C,250°C,300°C,350°C,385°C and 425°C,the high temperature tensile test was carried out.The results show that the tensile strength Rm and the elongation of the alloy treated by T6 process is 93.54MPa and 3.76%,respectively;Rm has a little change at thermal exposure temperature below 200°C and starts to decrease at temperature above 200°C,while the elongation gradually increases.The Rm and elongation of T6+350?/100h treatment alloy is 61.04MPa and 8.55%.Compared with the alloy of T6 treatment,Rm is reduced by 32.5MPa while the elongation is more than doubled.The Rm of T6+425?/100h treatment alloy is 73.4MPa and elongation is 9.48%,they increased by 12.36MPa and 0.93%respectively compared with the T6+350?/100h treatment.The hardness also satisfies the variation law of high temperature tensile strength.After the heat exposure treatment,the long needle-like eutectic silicon change into particles,the primary Si is rounded,and the second phase is passivated,which reduces the splitting effect of the stress concentration on the substrate,and thus the plasticity of material is improved.?2?The cycles of the new piston alloy?frequency 80Hz?in the three states T6,T6+350?/100h and T6+425?/100h in order:8811677,38211384 and 39271075 times.After T6+350?/100h and T6+425?/100h treatment,the fatigue life of piston alloy is more than 4times higher than that of T6 treatment.The fatigue life of piston alloy after T6+425?/100h treatment is higher than that of T6+350?/100h treatment.In the three states,the fatigue life of the piston alloy at the loading frequency 80Hz is about twice as long as that at 35Hz.?3?In the transient breaking zone,the T6 treatment is mainly quasi-cleavage fracture,and the silicon phase is severely broken;after T6+350?/100h and T6+425?/100h treatment,the quasi-cleavage and dimple-fusing fractures are dominant,and the silicon phase is less broken.The number of dimples increases.The fatigue crack of T6 treatment is easy to form stress concentration on the large silicon phase,and the surface appears to be fragmented or there are more pore initiation at the interface with the matrix.The cracks nucleate,aggregate and prematurely expand to form long cracks in the vicinity of the pores,and break when reaching a certain size.After T6+350?/100h and T6+425?/100h treatment,the eutectic silicon is granulated into particles,and the crack is prefer to round it,which changes the direction of crack propagation.And requires more stress to continue to expand,which resulted in more difficult to crack propagation,thereby material fatigue life is improved.?4?After the thermal exposure treatment of the new piston,a large amount of scattered white spotted tissue is precipitated in the matrix and near the silicon phase,and the number increases greatly with the change of fatigue cycle.The Al₂Cu phase with different crystal face index can be determined by transmission analysis and diffraction pattern.It is observed that the Al₃CuNi phase almost disappears,and the Al₇Cu₄Ni phase is mostly granulated,which reduces the stress concentration in the matrix and improves the plasticity of the material.In the T6 state,the number of precipitated phases during the fatigue test are smaller,the original precipitated?phase is coarsened,the obstacle of the dislocations bypassed is small,and a large number of dislocations agglomeration form a splitting effect on the matrix,resulting in the final failure of the material formation crack.After the heat exposure treatment,a large number of fine?phases are re-dispersed in the matrix,and the dislocation entanglement hinders the dislocation motion in the?phase,forming a large number of dislocation clusters,thereby improving the fatigue life of the material.