Trace Sn Of Al - Cu - (mg) Alloy Aging Behavior And The Influence Of The Microstructure

In this paper, we have study the effects of small additions of Sn on Al-3.5Cu, Al-3.5Cu-0.4Mg and Al-2.5Cu-1.5Mg alloy, respectively. Micro Vickers hardness testing, tensile properties testing were used to study the effects on mechanical properties.Optiacal metallographic microscope, Scanning electron microscope, Transmisson electron microscopy and Scanning Calorimetry methed were used study the efftecs on microstructure evolution and precipitation process. The main conclusions of this paper were as follows:1) Trace additions of Sn can fine the grains of Al-3.5Cu, Al-3.5Cu-0.4Mg and Al-2.5Cu-1.5Mg.2) Trace addition of Sn can increase the rate of ageing and enhance the effect of ageing hardening and strengthening. Compared with separate addition of0.3%Sn(mass fraction,%) alloy, the addition of0.15%In and0.15%Sn has more favorable effect on ageing strengthening.The addition of Sn can promote an increase in the number density and the dispersive distribution of θ’phase. During the early stages of ageing, the number of Sn’(In’) particles exceeded that of θr precipitates and it was inferred that formation of the Sn’(In’)preceded nucleation and growth of θ’. The Sn’(In’) particles provide effective heterogeneous nucleation sites for θ’and increase the nucleation rate of θ’. In addition,θ’phase exhibited a low rate of coarsening in Al-3.5Cu-Sn-(In) alloy.3) It’s shown that a higher peak hardness can be obtained in the Sn-containing alloys. Additions of Sn into Al-Cu-Mg alloy stimulate the precipitation of θ’(Al2Cu). Many fine and uniform θ’ can be precipitated in underaged(15h),while the σ phase (Al5Cu6Mg2) and Ω(Al2Cu) have been inhibited in Al-Cu-Mg-Sn alloy. The Sn microalloy effects can be attributed to the formation of insoluble Mg2Sn particles in AQ condition, which remove Mg atom from matrix. The removal of Mg leaves an insufficient of Mg so that precipitation of the σ and Ω is inhibited. It also shifts the alloy matrix composition toward the (α+θ+S) phase field to promote θ’ precipitate. In addition,θ’ and S phase in Sn-containing alloy exhibit a lower rate of coarsening at over-aged condition.4) Trace addition of Sn to Al-2.5Cu-1.5Mg alloy can increase the hardening and strengthening of the second stage of ageing. Addition of0.4%Sn can significantly improve ductility of the Al-2.5Cu-1.5Mg alloy, it’s shows the elongation at peak hardnenss and after exposure1000h of Sn-containing alloy is20.5%and10.5%, respectively. The addition of Sn can promote S phase precipitate and inhibit the S phase coarsen at over-aged.