Effect Of Cr Addition On The Microstructure,Mechanical Properties And Thermal Stability Of Al-Si-Cu-Mg Alloy

Commercial Al-Si-Mg-(Cu)casting alloys suffered with declination in their mechanical properties during service at elevated temperatures.This is due to rapidly coarsening of their strengthening precipitates at the temperatures and dramatically losing the strengthening effect.Previous research reported precipitation of refined α-AlCrSi dispersoids in a high number density in Al-Si-Mg alloys with Cr addition during solution treatment at 540℃.The dispersoids potentially has good coarsening-resistance at 300℃ and may be able to strengthen Al-Si casting alloys for elevated temperature applications.The purpose of this study is to improve the structure and properties of the alloy by adding low diffusion element Cr,to solve the problem that the properties of Al-Si-Cu-Mg casting alloy limit its application,so that the casting alloy material can be applied to new engine cylinder head and other parts.In order to explore the strengthening potential of Cr addition in an Al-Si-Cu-Mg casting alloy,we investigated Al-Si-Cu-Mg casting alloy with/without chromium(Cr)addition processed under different solution treatments.Their mechanical properties were evaluated by hardness test and tensile test.Their microstructures were characterized by using scanning electron microscopy and transmission electron microscopy to reveal the effects of Cr addition on the microstructure,mechanical properties and thermal stability of the Al-Si-Cu-Mg alloy.In addition,thermo-calc was employed to understand the effect of Cr and Si contents on volume fraction of a-AlCrSi phases in the Al alloy.The major conclusions of this research are as follow:1.Thermodynamic calculation was performed to predict the influence of Cr and Si content on the volume fraction of α-AlCrSi dispersoid in the Al alloys.It is worth noting that with the increase of Si content,the number of α-AlCrSi dispersoid used for strengthening decreases It unveils that there is a dilemma between castability and dispersoid hardening.2.Systematic characterizations have been conducted on microstructures and mechanical properties of the Cr-free and Cr-containing alloys in different states.The stronger hardening response to solution treatment of the Cr-containing alloy is ascribed to the precipitation of α-AlCrSi dispersoid.In addition,although α-AlCrSi dispersoids have strengthening effects on the solution-treated Al matrix,strengthening precipitates that nucleated on the dispersoid during quenching rapidly coarsened during the heat treatment at 300℃,and dropped the dispersoid strengthening.3.This study for the first time reported a preferential orientation relationship between α-AlCrSi dispersoid and Al matrix and validated a partial-coherent interphase between dispersoid and the Al matrix based on the Edge-to-Edge Model.Though the excellent thermal stability ofα-AlCrSi dispersoid at 300℃ has been validated,the dispersoid is hardly applied to improve elevated-temperature strength for Al-Si casing alloys due to a fact that it strongly interacts with age strengthening precipitates during quenching.