Effect Of Cu Content And Heat Treatment On Microstructures And Properties Of Al-Si-Mg-Mn-xCu Cast Aluminum Alloys

Al-Si cast aluminum alloys have become the most widely used cast aluminum alloy materials because of their excellent castability,good corrosion resistance and cutting performance.They are often used in the manufacture of cylinder block,cylinder head and piston of automobile engine.However,with the continuous development of automotive industry,more and more high requirements are put forward for the strength,ductility and other properties of Al-Si cast aluminum alloys.Therefore,it is necessary to carry out research to improve the properties of Al-Si cast aluminum alloys.At present,the common and effective approachs to control the microstructures and properties of alloys are microalloying and heat treatment.In this study,four groups of Al-Si-Mg-Mn-xCu cast aluminum alloys with different Cu contents were prepared by vacuum die casting process;Three-dimensional X-ray microscope(CT),optical microscope(OM),scanning electron microscope(SEM),energy spectrometer(EDS),transmission electron microscope(TEM),differential scanning calorimetry(DSC),Phase diagram calculation and micro-hardness test and tensile test were used to systematically study:(1)Effect of the addition of different amounts of Cu on the as-cast microstructures and properties of Al-Si-Mg-Mn-xCu cast aluminum alloys;(2)Effect of solution treatment on the microstructures and mechanical properties of Al-Si-Mg-Mn-xCu cast aluminum alloys,and optimizing the process parameters of heat treatment;(3)Effect of artificial aging treatment on the microstructures and mechanical properties of Al-Si-Mg-Mn-xCu cast aluminum alloys.The main conclusions are shown as below:(1)A few small gas porosities were found in the as-cast microstructure of the four aluminum alloys,and no obvious large defects.The number and size of the gas porosities in the alloys increased with the increase of Cu content.The as-cast microstructures of the four casting alloys generally consist of the primary?-Al matrix,eutectic Si and?-Al(Fe,Mn)Si,Mg₂Si,Q-Al₅Cu₂Mg₈Si₆,?-Al₂Cu phases.Among them,the?-Al matrix has a petal-like or dendritic morphologies,and the eutectic Si has a fibrous and short rod-like morphologies.But?-Al₂Cu phases were not found in Al alloy and different intermetallic phases exist in different morphologies.The area fraction of?-Al₂Cu phases in the alloy increased with the levels of Cu element.(2)The micro-hardness of the four groups of as-cast alloys increased with the level of Cu element.The eutectic Si particles have been fragmented,spheroidized and partially dissolved after solution treatment at 530? for 4 h.And with a further increases of the solution time,which can be attributed to the silicon particles became substantially coarser.The Q-Al₅Cu₂Mg₈Si₆,?-Al₂Cu and Mg₂Si phases were completely dissolved in the Al matrix during the solution heat treatment,but the?-Al(Fe,Mn)Si phases underwent only a small amount of dissolution,which show the?-Al(Fe,Mn)Si phases have excellent high temperature stability.(3)The nanoscale precipitates are mainly Q'and?'phases in the peak-aged alloys;Among them,the main Q'precipitates were found in A1 alloy,and the main Q'and?'precipitates were found in A2,A3 and A4 alloys.The Q'and?'precipitates increased with the increase of Cu content in the alloys.The Q'and?'precipitates significantly coarsen after over aging treatment,which reduced the precipitation strengthening effect of the alloys.(4)The yield strength(YS)and ultimate strength(UTS)of the four groups of as-cast and peak-aged alloys increased with the increase of Cu content,while the elongation of the alloys gradually decreased with the levels of Cu element.Compared with A1 alloy,the ultimate strength(UTS)and yield strength(YS)of the as-cast A4alloys were increased by 17.15%and 18.42%,respectively;but the elongation was decreased by 25.78%.The strength and elongation of the alloys increased after the peak aging treatment.The fracture characteristics of the four groups of as-cast aluminum alloys are all mixed fractures,but are mainly in the form of brittle fracture.After the peak aging treatment,the amount of the dimples on the fracture surface of the four alloys increased significantly,and the dimples became more uniform and deeper,and the fracture mode in the form of ductile fracture.