Study On Microstructure And Mechanical Properties Of Al-Cu-Mg-Ag Alloys With Various Cu Contents

Al-Cu-Mg-Ag alloys have become an ideal material for the skins and internal stress structures of aerospace vehicle due to the higher specific strength,good heat resistance,excellent fatigue crack propagation resistance and easy processing.In recent years,with the continuous increasing of the flight speed and distance of aircraft,the requirements for the fatigue performance and high temperature creep performance of materials have also become higher and higher.Therefore,developing an aluminum alloy material with excellent toughness,fatigue resistance and high temperature creep resistance by optimizing the content of the main alloying elements of the Al-Cu-Mg-Ag alloy has important engineering application significance.In this study,three Al-x Cu-Mg-Ag（x=5.30,5.65,6.00wt.%）alloys with Cu content around its solid solubility limit in Al（5.65wt.%）were prepared.The microstructural evolution of the experimental alloys was observed by the optical microscopy（OM）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The tensile properties,fatigue properties and high temperature creep properties of the experimental alloys were investigated by tensile test,fatigue crack propagation rates test,creep performance test.The results showed:（1）The change of Cu content has little effect on the grain structure but has a greater effect on the precipitates of the three Al-Cu-Mg-Ag alloys.The grain size of the as-cast structure of the three alloys is about 100μm,and the extruded structure is all intermittent fibrous.A large number of dispersedΩphases of a small lattice mismatch and a good coherent relationship with the matrix are precipitated in the under-aged alloys.The alloy with 6.00wt.%Cu has the highest number density ofΩphase.（2）The Al-Cu-Mg-Ag alloy with 6.00wt.%Cu possesses the best tensile properties.The tensile strength and yield strength under the condition of peak-aged（180℃/12h）are 507±12.5 MPa and 407±8.4 MPa,respectively.Compared with the5.30wt.%Cu alloy,it increases by 5.8%and 7.1%,respectively.Compared with the5.65wt.%Cu alloy,it increases by 12.4%and 9.7%respectively.（3）The Al-Cu-Mg-Ag alloy with 6.00wt.%Cu has the best fatigue crack propagation resistance.When the stress ratio（R）is 0.1,the fatigue life of the alloy is1559520 cycles,which is 1.2 times of the alloy with 5.30wt.%Cu and 2.7 times of the alloy with 5.65wt.%,respectively.When the stress intensity range（ΔK）is 30MPa·m^1/2,the fatigue crack growth rate of the alloy is 9.12×10^-5mm/cycle,which is 29%of the alloy with 5.30wt.%Cu and 14%of the alloy with 5.65wt.%Cu,respectively.Observation of the fatigue fracture shows that the gap of adjacent fatigue striation of the alloys with 6.00wt.%Cu is 0.24μm,which is significantly smaller than the other two alloys.The enhanced fatigue crack propagation resistance of this alloy is mainly due to its high number density ofΩphase,which increases the reversibility of dislocation slip and promotes the closure and deflection of fatigue cracks.（4）TheΩphase with good thermal stability significantly enhances the high temperature creep properties of Al-Cu-Mg-Ag alloy.The alloy with 6.00wt.%Cu has the best creep resistance,under the condition of 240℃/150MPa,the steady creep rate of the alloy is 5.23×10^-7s^-1,which is 44%of the alloy with 5.30wt.%Cu and 20%of the alloy with 5.65wt.%Cu,respectively.under the condition of 220℃/250MPa,the creep life of the alloy is 28.0h,which is 1.8 times of the alloy with 5.30wt.%Cu and2.6 times of the alloy with 5.65wt.%Cu,respectively.The creep mechanism of Al-Cu-Mg-Ag alloy can be attributed to the dislocation climb with the existence of threshold stress.