Study On The Reaction And Surface Damage Of Al-Si Alloy With Different Carbons

When the diesel engine works,the inner wall of the combustion chamber is directly in contact with the high temperature and high pressure gas.The mixing of oil and gas and the combustion work are carried out simultaneously.The atomized diesel is difficult to be fully combusted,and the carbon coating will be generated in the piston ring bank and the piston crown.With the increase of power density,the working temperature of combustion chamber is further increased,carbon deposition may produce chemical reaction with piston aluminum alloy,which leads to the change of piston surface characteristics,and then induces the rapid failure of piston.However,the influence of Al-C reaction at high temperature on the surface characteristics of the alloy is still unclear,which limits the improvement of the power density of diesel engine.Therefore,it is of great engineering value and scientific significance to clarify the influence law and mechanism of Al-C reaction on the surface characteristics of piston aluminum alloy.This subject takes Al-x%Si(x=7,12,18wt.%)as the research object,and uses high-temperature thermal exposure and gas shock tests to study the reaction kinetics conditions of different carbon sources and piston aluminum alloys under steady and unsteady heat sources and their effects on surface characteristics.Uses surface roughness tester and Brinell hardness tester to study the evolution of surface roughness and hardness of the alloy.The microstructure,composition and phase of the alloy were analyzed by optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS)and X-ray diffraction(XRD).The results show that:(1)The surface roughness of heat-treated Al-Si alloy continues to increase with the increase of thermal exposure temperature,which is mainly caused by the cracking of Al/Si interface.With the increase of Si content,the increase of roughness decreases,and the increase of Si content can effectively reduce the tendency of phase boundary cracking of Al-Si alloy,making it difficult for the alloy to have phase boundary cracking at lower temperature.In the as-cast Al-Si alloy,the roughness of Al-12%Si decreases first and then increases with the increase of heat exposure temperature,the roughness of Al-18%Si shows a rising trend,and the roughness of Al-x%Si alloy basically follows RaT6>Raheat exposure>Raas-cast.The change in the surface roughness of the alloy is coordinated by the cracking of the Al/Si phase boundary and the precipitation of the rare earth phase,with the increase of Si content,the high temperature stability of Al-x%Si alloy is enhanced.The surface roughness of Al-12%Si alloy with different gas content decreases first and then increases at high temperature.With the increase of gas content,the change of surface roughness decreases gradually.After 30min degassing,the roughness of Al-12%Si alloy at all temperatures is below RaT6.The results shows that the generation of internal defects will accelerate and induce the Al/Si boundary cracking.Compared with the as-cast Al-Si alloy,the surface roughness change range is significantly reduced when the heat treatment is in the initial state.The heat treatment will cause residual stress on the surface of the alloy,which leads to stress concentration at different positions and promotes the cracking of Al/Si phase boundary.Moreover,the larger surface roughness is more prone to damage under thermal exposure.(2)After thermal exposure,the surface roughness of Al-12%Si alloy coated with carbon changes significantly.The diffusion of carbon powder increases the surface roughness slightly,and the formation of Al4C3 causes a sharp increase in the surface roughness.With the increase of temperature,the Al-C reaction becomes more intense,and the growth range of surface roughness increases significantly.Al-12%Si alloy and 2000 mesh carbon powder will react under certain conditions to form a white needle-like easily hydrolyzable phase Al4C3.With the increase of thermal exposure temperature,the reaction time of Al-C decreases greatly,and the reaction occurs at 550?/8h,500?/32h,450?/64h and 400?/164h,respectively.When the physical and chemical properties of carbon source are changed,the Al-C reaction of Al-12%Si alloy is intensified with the increase of specific surface area.With the increase of Gibbs free energy of carbon source,the Al-C reaction is more likely to occur.Al-12%Si reacts with 1000mesh carbon powder at 550?/16h and with 2000mesh carbon black at 550?/4h,respectively,but does not react with graphite powder even at 550?/96h.(3)In the gas impact environment,when the total loading time is constant,with the decrease of single loading time,within a certain range,the macro morphology of Al-x%Si alloy is weakened,the increase of surface roughness is reduced,the linear ablation rate and mass ablation rate are reduced.With the increase of Si content,the roughness decreases,and the linear ablation rate and mass ablation rate also shows a decreasing trend.With the increase of Si content,the high temperature stability of Al-x%Si alloy is improved.Over a certain range,the damage of Al-Si alloy increases with the decrease of single loading time.The coupling of heat and force causes the damage of the alloy to increase,and the surface roughness increases sharply.The ablation of Al-Si alloy is mainly due to the interaction of precipitation and aggregation of high silicon substances in the alloy and airflow scouring.Gas shock is applied to the resin-coated Al-12%Si alloy,with the decrease of the single loading time,the surface roughness of Al-12%Si alloy increase amplitude decreases,and the linear ablation rate and mass ablation rate both decrease,and the decrease range is gradually weakened,Al/C diffusion will appear when the gas shock is severe,and Al4C3 will appear on the surface of the alloy under the gas shock of a single load of 50s.