Effect Of Alloying Elements And Heat-treatment On The High Temperature Strength Of Eutectic Al-Si Alloys

With the rapid development of the automotive industry,the demand for engine performance is rising.So there is a need for piston materials which adapt to the development of a higher power engine.The eutectic Al-Si based casting alloys are widely used in piston production due to their good castability,high strength,light weight,good wear resistance and low thermal expansion.There is an urgent need to upgrade the high temperature performance of existed piston material in order to adapt to the higher power of the engine.The evolution of heat-resistant phases in piston alloy and the effect of alloying elements and heat-treatment on the high temperature strength of eutectic Al-Si alloys were systematically studied by field emission scanning electron microscopy?FESEM?,energy dispersive spectrometer?EDS?,X-ray diffraction?XRD?and tensile testing machine,etc.The results show the main intermetallics are similar in tested specimens at both different heat treatment states,including Al₃CuNi,Al₇Cu₄Ni,Al₄Cu₂Mg₈Si₇,and AlFeSi.The addition of Cr leads to the transformation from needle-like?-AlFeSi phases to fishbone-like?-Al?Fe,Cr?Si phases.For the high temperature?HT?strengths?350??,the ultimate tensile strengths?UTSs?at 350?are reduced with the increase of Cr content in Al-Si alloys,and elongation are increasing accordingly.?-AlFeSi offers better HT strengthening effect as a harder phase than?-Al?Fe,Cr?Si due to its higher Si content.For the ductility,?-Al?Fe,Cr?Si has a more positive effect ascribed to its more rounded appearance compared with needle-like?-AlFeSi.The UTSs at 350?are reduced with the increasing of Er content in Al-Si eutectic alloys.This is because Al₃?Er,Fe,Cu??Ni,Si?₂ phase consumes a lot of Fe,Cu,Ni elements which may form Al₃CuNi and Al₇Cu₄Ni,ect heat-resistant phases originally.Therefore,the reduction in the number of the heat-resistant phases causes the decreasing of UTSs.The addition of Mg element increases the UTSs of Al-Si alloys at350?.There are two reasons,the one is the increasing of volum fraction of Al₄Cu₂Mg₈Si₇,Al₈FeMg₃Si₆ and Mg₂Si phase contributes to improving Al-Si alloy's HT strength.The other one is that eutectic Si network is refined with the addition of Mg.The heat-resistant phases,Al₃CuNi,Al₇Cu₄Ni,etc in combination with 3-D eutectic Si networks form a new strongly interconnected 3D-structure which could enhance the UTSs of eutectic Al-Si alloy.It is found the UTSs at 350?and the aspect ratio of eutectic Si sharply decrease after T6 solution treatment.The interconnected 3-D networks of eutectic Si act as the main strengthening phase at high temperature,and the solution treatment would weaken this interconnected architecture of eutectic Si,thus leading to the decrease of the HT strength.And the plasticity of specimens at T6 state is better than that at T5 state,because the spheroidized eutectic Si could reduce the stress concentration after solution treatment.With the extension of thermal exposure time,the UTSs of the Al-Si alloys decrease,and HT mechanical properties of Al-Si alloys decrease more rapidly within 10h of thermal exposure.Furthermore,the thermal exposure time continues to increase to 100h,the decreasing of UTSs is relatively slow.