The Effect Of Si Element Content On Fatigue Performance Of The Aluminum Piston Alloys

Al-Si multi-alloys have the advantages of high specific strength,low density,and good casting properties,therefore,it have been widely used in the area of engine pistons.In such an environment,fatigue failure often occurs due to the high temperature and high pressure.With the increases of the engine's speed and power,the piston's thermal load becomes more severe,This imposes more stringent requirements on the high temperature fatigue properties of aluminum alloys as piston materials.The commonly used piston alloy is a near-eutectic Al-Si alloy.The increase of Si content can lower the thermal expansion coefficient and improve the casting properties of the alloy.The presence of the silicon phase is beneficial to promote the diffusion of Cu atoms and form a dispersed strengthening phase.At the same time,more hard and brittle bulk silicon phase will be formed,which will easily cause stress concentration at the sharp corners of the silicon phase and reduce fatigue performance.The thermal exposure process will improves the morphology of the silicon phase and improves fatigue performance.In order to explore the influence of micro-variation for Si content in near-eutectic point Si on the fatigue life of piston alloy,and to improve the fatigue performance of piston alloy without reducing the coefficient of thermal expansion,this project designed relevant experiments to propose a theoretical reference for the safe use of the engine.In this paper,Al-11.5Si-4Cu-Mg-2Ni multi-alloy was used as the test material.Under the temperature of 350?,the high temperature mechanical properties of the alloy after T6(500?×6h+Quenching+230?×2.5h),T6+350?×50h thermal exposure and T6+350?×100h thermal exposure were tested.For the copper-rich phase precipitated in the alloy,an Al-Si-Cu/Al-Si diffusion couple with different Si contents was prepared for diffusion test,the effect of Si content on the diffusion of Cu in alloy microstructure was studied.The high temperature fatigue properties of the alloys after heattreatment of T6+350?×100h thermal exposure were tested at 350?,the content of Si were 11.5wt.%,12.5wt.%,and 13.5wt.%,respectively.The microstructure of the alloy was observed by optical microscope(OM).The microstructure and fracture characteristics were analyzed by scanning electron microscope(SEM).The distribution of Cu content in the diffusion couple was detected by energy dispersive spectrometer(EDS).The tensile performance results,the fatigue crack initiation and propagation laws,and the diffusion behavior of Cu were deeply analyzed.The influence of the small change of silicon content on the microscopic structure and fatigue behavior of Al-Si piston alloy materials was revealed.The fatigue fracture mechanism of piston materials was expounded in detail.At 350?,the tensile strength of the alloy decreased by 46% and the elongation increased by 123.7% after 50 h thermal exposure treatment compared with the T6 treatment.Moreover,the tensile strength was further lowered with time,and the elongation was further increased.The cleavage platform and dimple characteristics can observed simultaneously in tensile fracture of T6 treatment state alloy.after thermal exposure treatment,the cleavage platform in the fracture was reduced,the dimples were increased with the heat exposure time was prolonged,and the dimples became smaller and deeper.The bulk primary silicon in Al-Si piston alloy was passivated and the eutectic silicon was granulated after T6+350?×100h thermal exposure treatment,and the change of the silicon phase more significantly with thermal exposure time prolongs.At the same time,the copper-rich phase precipitates in the alloy matrix after thermal exposure treatment,and surrounds the silicon phase.With the silicon content increasing in the alloy,the primary silicon's passivation was intensified,and the precipitation of the copper-rich phase increased.During long-term heat preservation in a high temperature environment,Cu diffused into the Al matrix due to the chemical potential,and formed a fine dispersed copper-rich phase.Comparing the height of the sweep energy peak on both sides of the interface,the silicon content increased from 7wt.% to 24 wt.%,and the energy peak height of Cu increased by 50%,indicated that the diffusion amount of Cu increased with the Si content increased.Since the rounding of the silicon phase was aggravated at a high temperature for a long time,the increased content of the silicon maked the interface between the silicon phase and the matrix rise,which provided more channels for the diffusion of copper atoms in the alloy,so the diffusion coefficient of Cu increased in the matrix.and more Copper-rich phase were formed.The median fatigue strength of the alloys with Si mass fraction of 11.5wt.%,12.5wt.%,and 13.5wt.% after heat exposure treatment were 40 MPa,40.5 MPa,and 42.5 MPa,respectively.And at the same stress level,as the Si content increases,the high temperature fatigue life increases.This was because the silicon phase promoted the diffusion of Cu,formed a diffused copper-rich phase,and improved high temperature fatigue performance.Observing thefatigue fracture,the fatigue source was basically in the casting defect and the large silicon phase.With the increase of Si content,the whole macro-fracture was flattened,the area of the expansion zone was increased,the distance between the fatigue strips was reduced,and the cracking of the silicon phase in the instantaneous break zone was reduced.The evolution of the fracture structure was analyzed.When the Si content was 11.5wt.%,the crack propagation directly broke through all the phases.Increasing the Si content,the silicon phase was rounded up after thermal exposure treatment,the fatigue crack of the alloy of 12.5wt.% Si bypasses the silicon phase with less sharp angle and the intermetallic compound with high strength,but expanded through the brittle rich iron.When the Si content reached 13.5wt.%,most of the alloy cracks were deflected by the edge of each phase grain boundary to break along the crystal,and only a very small number of layers passed through the brittle phase.