Research On Microstructure And Properties Of Lightweight And High-Strength Of Cast Al-Li Alloy

Aluminum-lithium alloy is widely used in the aerospace field because of its low density,high specific stiffness and specific strength,and good performance at low temperature.From the perspective of cast aluminum-lithium alloy,this paper mainly studies the structure and properties of cast aluminum-lithium alloy with orthogonal composition design on the content of Mg,Li,Cu elements and the Mg/Li ratio to obtain the alloy with good properties of light weight and strong toughness,which has important reference value for the development of light weight and high strength cast Al-Li alloys.The study of alloy phases shows that the second phase of the alloy mainly includes Al Li（δ）,Al₂Mg Li（T）,and Al₂Cu Mg（S）.The second phase is distributed in bands at the grain boundaries,but in the crystal grain is granular or short-rod.the Al-6Mg-2.5Li-0.5Cu-0.1Zr alloy has the highest percentage of the second phase,which is 12.67%.When the ratio of Mg/Li increases,the percentage of the second phase increases first and then decreases at the condition of 0.2%Cu.When Mg/Li=3,the percentage of the second phase is the largest,which is 9.64%.Among the alloys of orthogonal design,in the as-cast state,the hardness of Al-6Mg-2.5Li-0.5Cu-0.1Zr alloy is the highest,which is 120HV;after heat treatment,the hardness of Al-5Mg-2.5Li-0.2Cu-0.1Zr is the highest,which is136HV.Through orthogonal analysis,the main factor that affects the hardness of the alloy is the content of Li,followed by the content of Mg and Cu.After the significance test,the change of the content of Li has a significant effect on the hardness of the alloy.The hardness test of alloys with different Mg/Li ratios shows that the hardness of the alloys first increases and then decreases with the increase of Mg/Li ratio at the condition of 0.2%Cu.In the test of mechanical properties,the tensile strength of Al-5Mg-2.5Li-0.2Cu-0.1Zr alloy in the as-cast state is the highest,which is 321MPa,the yield strength is 146MPa,and the elongation rate is 9.0%.Its strength matches well with its shape.In the heat-treated state,Al-4Mg-2.5Li-0.8Cu-0.1Zr and Al-5Mg-2.5Li-0.2Cu-0.1Zr alloys have higher strengths,the tensile strength is respectively 341MPa and 342MPa,the yield strength is 231MPa and 200MPa,the elongation is 2.2%and 7.4%respectively,however,Al-4Mg-2.5Li-0.8Cu-0.1Zr alloy has poor plasticity,and Al-5Mg-2.5Li-0.2Cu-0.1Zr alloy has relatively better plastic toughness.Through orthogonal analysis,the optimal composition ratio of the alloy is:Al-5Mg-2.5Li-0.8Cu-0.1Zr,and the measured density is 2.397g/cm³.At the condition of 0.2%Cu,the tensile strength and yield strength of the as-cast alloy increase first and then decrease with the increase of the Mg/Li ratio,and reach the maximum when Mg/Li=2.5,which is 329MPa.The elongation continues to increase as the Mg/Li ratio increases.In the heat treatment state,the change of the alloy tensile strength with the Mg/Li ratio is the same as that in the cast state.The tensile strength and elongation are the largest when Mg/Li=2.5,which are 367MPa and13.5%respectively.In the orthogonal heat treatment process design,the order of the factors affecting the tensile strength,yield strength,and elongation are:T_s>T_t>t_s>t_t,T_t>T_s>t_s>t_t,T_t>t_s>t_t>T_s.The optimal heat treatment process obtained by comprehensive analysis is:solid solution at 465℃for 12h,then aging at 175℃for 19h.The tensile properties of the alloy after the optimal heat treatment process are:the tensile strength is 357MPa,the yield strength is 226MPa,and the elongation is 6.2%.