| Automotive lightweight is an important part of energy saving and emission reduction.Aluminum alloy is one of the lightweight materials of automobile.The development of new high strength and.high toughness aluminum alloy is always the research focus of automotive lightweight development,and it is also one of the main means to achieve automotive lightweight.The development of new materials includes the control and optimization of alloy composition and the development of casting process.Squeeze casting,as an excellent production process for automotive aluminum castings,can produce high performance automotive parts with complex shape,compact structure and T6 treatment.However,the commonly used Al-7%Si-0.3%Mg(A356)squeeze casting alloy still has the disadvantage of insufficient mechanical properties.Therefore,in this paper,Al-7%Si-0.3%Mg alloy is used as the base alloy to control the content of Mg and Cu elements,and X-ray diffraction,optical microscopy,scanning electron microscopy,transmission electron microscopy and other analysis and testing means are used.The effects of Mg and Cu content and heat treatment process on the microstructure,mechanical properties and corrosion resistance of squeeze casting Al-Si-Mg-Cu alloy were studied,and a squeeze casting alloy with high strength and toughness and good corrosion resistance was developed.The results show that:When the Mg content in the squeeze casting Al-7%Si-0.3%Mg alloy increases from 0.3wt.%to 0.5wt.%,the phase composition of the alloy does not change,and it is mainly composed of α-Al,eutectic Si,Mg2Si and Fe-containing phase.The increase of Mg content leads to the increase of the quantity density of Mg2Si phase in the as-cast alloy.After solution treatment at 535℃,the fibrous eutectic Si is spheroidized and the Mg2Si phase is solute into the matrix.The hardness and strength of Al-7%Si-0.3/0.5%Mg alloy reach the peak at solution time of 8h.After T6 heat treatment,the hardness and strength of Al-7%Si-0.3%Mg alloy reach the peak at 8h of aging,and the Vickers hardness,yield strength,tensile strength and elongation can reach 111.0HV,268MPa,327MPa and 6.6%,respectively.After T6 heat treatment,the hardness and strength of Al-7%Si-0.5%Mg alloy also reach the peak at 8h of aging time,and the Vickers hardness,yield strength,tensile strength and elongation of Al-7%Si-0.5%Mg alloy can reach 112.3HV,288MPa,346MPa and 6.3%,respectively.Transmission electron microscopy(TEM)analysis shows that β" nano-phase mainly precipitates during the aging process of Al-7%Si0.3/0.5%Mg alloy.When Mg content increases from 0.3%to 0.5%,the quantitative density of β" phase increases from 3.92×109/mm2 to 5.25×109/mm2.The mechanical properties of the alloy are strengthened by the increase of the number density of nanophase.Based on the properties of Al-7%Si-0.5%Mg alloy,the mechanical properties were further strengthened by adding a small amount of Cu element.After T6 heat treatment,the Vickers hardness,yield strength,tensile strength and elongation of Al-7%Si-0.5%Mg0.35%Cu alloy can reach 115.3HV,290MPa,374MPa and 9.1%,respectively.The Vickers hardness,yield strength,tensile strength and elongation of Al-7%Si-0.5%Mg0.65%Cu alloy T6 after heat treatment can reach 121.2HV,302MPa,390MPa and 11.0%,respectively.The optimal T6 heat treatment process for the two alloys is 535℃8h+170℃-10h.The addition of 0.35%and 0.65%Cu increases the density and decreases the size of Q’ and β" nano-phase during aging process.The addition of 0.35%Cu increases the total density of nano-enhanced phase to 6.03×109/mm2,and the addition of 0.65%Cu increases the total density of nano-enhanced phase precipitated by aging to 7.80×109/mm2.The calculation results of the strengthening mechanism show that the increasing of the number density of the strengthening phase can effectively increase the mechanical strengthening effect of the precipitated phase,and the yield strength of the precipitated phase increased from 122MPa of 0.3%Mg alloy to 155MPa of 0.5%Mg0.65%Cu by the cutting mechanism,which is basically consistent with the experimental results.It can be seen that the addition of Mg and Cu elements mainly affects the aging strengthening effect of the alloy,resulting in the improvement of mechanical properties.Then electrochemical test and corrosion resistance analysis were carried out on alloy samples after T6 heat treatment under peak condition.The results show that the impedance value of Al-7%Si-Mg-Cu alloy decreases and the radius of Nyquist capacitor ring decreases with the increase of Mg and Cu elements.Al-7%Si-0.3%Mg,Al-7%Si0.5%Mg,Al-7%Si-0.5%Mg-0.35%Cu,Al-7Si%-0.5%Mg-0.65%Cu corrosion current density were 1.14×10-5,1.18×10-5,1.25×10-5,1.30×10-5 A/cm2.It can be seen that the current corrosion density increases slightly with a small increase of Mg and Cu content.The precipitated nano-phase of Mg and Cu elements in the aging process can accelerate the corrosion of the alloy,and the addition of a small amount of Mg and Cu can effectively improve the mechanical properties of the alloy to produce less corrosion damage.This indicates that the Al-7%Si-0.5%Mg-0.65%Cu alloy can keep close to the corrosion resistance of Al-7%Si-0.3%Mg alloy under the condition of effective strengthening of mechanical properties.Through squeeze casting preparation and heat treatment process control,the alloy design can maintain good corrosion resistance under the premise of effectively improving mechanical properties,so as to meet the development goal of both mechanical and corrosion resistance of automotive aluminum casting. |
