Effect Of Si On Microstructures And Mechanical Properties Of Heat-treatable Strengthening Aluminum Alloy

Nowadays,aluminum alloys containing Si get more and more popular and are widely used in many fields.And aluminum alloys containing Si will acquire remarkable aging strengthening ablity by adding appropriate Mg.6xxx series aluminum alloy(which Si is usually<2%)and eutectic Al-Si alloy(which Si approximates 12%)are two common heat-treatable strengthening aluminum alloys with low silicon content and high Si content.The former one has been fully studied for years,and has already received a wide range of applications,and recently it has been found that the later one also has the overall mechanical properties.However,studies on microstructure and properties of heat-treatable strengthening aluminum alloys whose silicon content range from 2%to 12%are still less.This article aims at studying on effect of Si ranging from 1%to 12%on microstructure and mechanical properties of Al-xSi-0.7Mg alloys,revealing the effect rule of Si,and also studying the effect of adding 0.8%Mn.The main results are shown as follows:(1)Macro grain size of 60mm thick Al-xSi-0.7Mg(-0.8Mn)semi-continuous casting alloys does not vary monotonically with Si content,when Si content is about 5%,the grain size is relatively smaller;With the increase of Si content,the secondary dendrite arm spacing of alloy in as-cast organization decreases obviously:in addition,secondary dendrite arm spacing is smaller in the alloy with 0.8%Mn alloy.(2)The as-cast microstructure of Al-xSi-0.7Mg(-0.8Mn)semi-continuous casting alloys consists of dendritic a-Al,eutectic Si,Mg2Si and iron-rich phases;iron-rich phases in Al-xSi-0.7Mg alloy contain two kinds,one is AlFeSi type and the other one is AlFeMgSi type;While quantity of iron-rich phase in Al-xSi-0.7Mg-0.8Mn increase and iron-rich phase is mainly AlFeMnSi type;in addition,when Si content is 11.1%or higher,there will be primary Si in the alloy which is about 10?50?m in size.(3)Over burnt temperature of the Al-xSi-0.7Mg alloys used in this experiment has no remarkable change with the increase of Si content,and is around 556?557 ?;With the addition of 0.8%Mn,the over burnt temperature does not change except the alloy with 1.26%Si.(4)Dendritic ?-Al and primary Si in 60mm thick Al-xSi-0.7Mg(-0.8Mn)semi-continuous casting alloys have no obvious change after heating at 480 ? for 2h,while eutectic Si coarsen and spheroidize obviously,almost all needle eutectic Si granulate,and exsit in the form of sphere or rod;Crystalline phases in the ingot fragment in different degree after hot rolling by 78%deformation;With the increase of Si content,Si particles distribute more uniform,when Si content is 8.81%,Si particles basically distribute uniformly in the matrix,besides with the increase of Si content,there's more rod eutectic Si in the alloy.(5)After solution treatment,quantity of Mg2Si phase in the Al-xSi-0.7Mg(-0.8Mn)hot-rolled plates decreases obviously,eutectic Si coarsen and spheroidize obviously;Iron-rich phase of AlMgFeSi type obviously spheroidize,while AlFeMnSi type slightly spheroidize;in addition,there has been dispersion phases containing Mn ranging from dozens of nanometers to hundred of nanometers and in the form of sphere,bulk,or rod after adding 0.8%Mn in the alloy.(6)Heat treatments of solution treatment at 536? for 2h,quenching in water and aging at 170? are carried on 12mm thick Al-xSi-0.7Mg(-0.8Mn)hot rolled plates.With the increase of aging time,strength of the Al-xSi-0.7Mg(-0.8Mn)hot rolled plates increase,plasticity decrease,when aging for 8h,the strength of plates have been higher,continuting to extend the aging time,strength slightly decrease;With the increase of Si content in the alloy,conductivity of the hot-rolled plates after heat treatment decrease,strength increase,but when Si is more than eutectic content,tensile strength decrease.(7)After aging at 170 ? for a short time(about 1.5h),the Al-xSi-0.7Mg hot-rolled plate whose Si content is around 2.5%acquires the elongation over 31%,and also the tensile strength over 269MPa;When adding 0.8%Mn in the Al-xSi-0.7Mg alloys,the strength of the alloy plates slightly increase,in addition,the plasticity of Al-xSi-0.7Mg-0.8Mn whose Si content is less than 5%will slightly increase after aging at 170? for a long time.