Effect Of Alloying Elements And Heat Treatment Process On The Thermal Expansion Coefficient Of Al-Si Alloy

Al-Si casting alloys are commonly used materials for piston alloys,and the change of its thermal expansion coefficient has a very important effect on the piston.The piston works under high temperature and high pressure.The high temperature working environment causes thermal stress and expansion inside the piston.Excessive expansion affects the fitting gap between the piston and the cylinder liner,and affects the lubrication of the piston,thereby increasing the resistance of the piston and accelerating each The wear of parts affects the service life of the internal combustion engine.Therefore,it is necessary to deeply study the thermal expansion characteristics of the piston material during the temperature change,obtain the thermal expansion coefficient at different temperatures,improve the dimensional accuracy and qualification rate of the finished product,extend the service life of the material,and improve the overall economic benefits.In this paper,through XRD phase analysis,metallurgical microscope,scanning electron microscope,transmission electron microscope,energy spectrum analysis and other means,several different Al-Si series casting alloys are studied.They are Al-Si binary alloys with different Si content,Cu The content is different,the Si content is 12%Al-Si-Cu ternary alloy,and there are two Al-Si-Cu-Mg-Ni piston alloys with different Cu content and Ni content.The thermal expansion coefficient changes in different states,The influence of alloying elements and heat treatment process on the thermal expansion coefficient is discussed.The specific research results are as follows:1)For as-cast Al-Si binary alloys,the thermal expansion coefficient of the alloy is mainly related to the excess Si phase content in the alloy.The higher the Si content,the more excess Si phases in the structure and the smaller the thermal expansion coefficient of the alloy.The thermal expansion coefficient of the Al-Si binary alloy decreases with the increase of temperature.For Al-Si alloys after solution aging treatment,due to the Si phase rounding passivation,the solidified Si phase becomes less,and the fine and dispersed Si phases are also precipitated on the ?-Al matrix.The Si phase rounding passivation will make the alloy The coefficient of thermal expansion increases,and the finely dispersed Si phase will reduce the coefficient of thermal expansion of the alloy.The thermal expansion coefficient of hypoeutectic Al-Si alloy decreases after solution aging heat treatment,and the influence of the fine and dispersed Si precipitated on the ?-Al matrix on the relative thermal expansion coefficient plays a leading role;the Al-12Si alloy undergoes solution aging treatment,the excess Si phase Rounding passivation has a dominant effect on the thermal expansion coefficient,resulting in a lower thermal expansion coefficient of the alloy after heat treatment;and for different aging temperatures,the more Si phases precipitated during aging,the smaller the thermal expansion coefficient of the material;hypereutectic Al-Si alloys The thermal expansion coefficient does not change much after the solution and aging heat treatment,and the effects of the rounding of the solidified Si phase and the relative thermal expansion coefficient of the passivation and aging Si phase cancel each other out.2)For the Al-Si-Cu ternary alloy,when the Si content is 12%and the Cu content is 2%?6%,the addition of Cu will cause solidification and precipitation of Al2Cu phase,which makes the thermal expansion coefficient smaller;Cu also reduces the coefficient of thermal expansion.After heat treatment in the as-cast alloy,the Al2Cu phase changes less and the residual stress is released,making the thermal expansion coefficient of the alloy larger.After Al-Si-Cu alloy undergoes solid solution aging treatment,the solidified Al2Cu phase becomes less,and at the same time there will be aging precipitated Al2Cu phase in the matrix,the thermal expansion coefficient of the alloy becomes larger;as the solid solution temperature increases,the thermal expansion coefficient Decrease first and then increase,the Al2Cu phase precipitated at the maximum solid solution temperature is the most,and the thermal expansion coefficient is the lowest.The effect of the Al2Cu precipitated by the aging relative to the thermal expansion coefficient is dominant3)In Al-Si-Cu-Mg-Ni alloy,with the addition of Cu and Ni,y-Al7Cu4Ni and ?-Al3CuNi phases with better stability and lower thermal expansion coefficient will be formed in the alloy,which reduces the thermal expansion coefficient of the alloy.Small and more stable.The long-term stabilization treatment can reduce the thermal expansion coefficient of the alloy to a certain extent.Basically,the longer the stabilization treatment time,the more stable the thermal expansion coefficient of the alloy.Aging treatment will precipitate fine Al2Cu phase,which can effectively reduce the thermal expansion coefficient of the alloy.