The Study Of Quenching Sensitivity Of7000Series Aluminum Alloys

More lightweight demands for aircraft fuselage, cabin siding, wing beam etc in aerospace field, making the lightweight aluminum alloy components trend to large and overall development. To ensure the uniformity of proerties cross the thickness section, the aluminum alloys are required to have low quenching sensitivity. The study for quenching sensitivity of7000series aluminum alloy will be given the theoretical significance and application value.7000series alloies are strengthened through thermal treatment to get a α-Al supersaturated solid solution and then by the decomposition of the solid solution to precipitate. When the supersaturated solid solution is stable, cross the thickness section the precipitate will have small difference after ageing and little difference in performance even at different quenching rates.In this study, XRD, XAFS, SEM,3DAP and electrical conductivity were used. The influencing regularities of the addition of Zn, Mg, Cu three alloying element and the stability on supersaturated solid solution were systematic investigated. The process of quench-induced precipitate of the7000series alloys was studied. The quenching sensitivity of the7000series aluminum alloys was discussed theoretically.Due to the large complexity on the stability influence factors of7000series Aluminum alloy supersaturated solid solution, the relationship between Zn, Mg, Cu alloying element with binary alloy and ternary alloy were studied firstly, the After solid solution treatment, it was found that the supersaturated solid solution of the Al-xCu binary alloy was not stable with preferential precipitation in comparsion with the Al-xZn and Al-xMg alloys; With the incease in Cu addition, the density and size of the precipitation phases in the matrix of Al-xCu and Al-xZn-yCu alloy become higher and larger.In the binary and ternary alloy systems, the largest change of the lattice constant of the supersaturated solid solution occurred in the alloys with Cu addition. The lattice constant change caused by the addition of alloying elements followed the order of|Δacu|>|Δamg|>|Δazn|; The larger the lattice constant change, the less stable the supersaturated solid solution was.In the as-quenched7000series aluminum alloys, the trend of lattice constant changes due to the same alloying element addition was the same in the binary and ternary alloys. In comparison with the addition of Zn and Mg, the same amount of Cu addition led to the largest change of lattice constant of the supersaturated solid solution; When the (Zn+Mg+Cu) value is about5.8at%-7at%, the change of lattice constant of the supersaturated solid solution is smaller. After solid solution treatment and stay in room temperature for1.5h, alloy7050with larger lattice constant change had the higher cluster density of9.3x1024/m3. The cluster densities of7085were3.5x1024/m3. These clusters normally have less than30atoms with the size less than1nm in the2D HRTEM field. Under the appropriate thermodynamic conditions, the cluster will grow and become coarser and accelerate the process of precipitate.Of the total small clusters composed of10-15atoms, the Cu containing clusters accounted for65%in7085alloy. The average content of Cu atoms in the clusters was about5at%-6at%. The Cu containing clusters accounted for82%of the total clusters in7050alloy. The content of Cu atoms in the clusters was about9at%-10at%, much higher than the Cu atom content of1%in the alloy. With the increase of cluster size, the Cu atom content increased in both alloys. In the clusters with more than30atoms, the percentage of Cu containing clusters increased to95%. It indicates that Cu atoms are quenching sensitive elements in the aluminum alloys. Cu atoms can cause large lattice distortion with high formation energy. They can overcome the nucleation barrier to reduce the nucleation energy and accelerate partial coarse phase precipitation. Therefore, the high Cu containing supersaturated solid solution of7050alloy is least stable. After1.5h stay in room temperature, the density of the precipitation phases with size larger than2nm was0.7/nm3in7050alloy and0.035/nm3in7085alloy. There was no significant precipitation in matrix of7021alloy without Cu alloying element.In the matrix of quenched7000series alloys, the quench-induced precipitate occurs preferentially on the following locations in the order of:grain boundary, sub-grain boundary and Al18Cr2Mg3particles, Al3Zr particles, and excessive Cu atoms. They all can increase the quenching sensitivity of aluminum alloys but with different capabilities of inducing precipitation.The end quenching experiment indicates that in7B04alloy the second phase Al18Cr2Mg3particles are another major factors after the (sub)grain boundaries that can induce and promote the second phase precipitation and growth. The Al18Cr2Mg3particles in the location near the quenched end function as nucleus and induce the formation of a second phase to dramatically increase the quenching sensitivity. In7050alloy, the coherent Al3Zr and the excessive Cu atoms are the second important factors after (sub)grain boundaries that accelerate second phase precipitation and growth. In the matrix of7085, the second phase Al3Zr particles can affect the formation and growth of the second phases. However, in comparison with7050alloy, the major alloying element Cu content is not high, the α-Al supersaturated solid solution is more stable, and quenching sensitivity is lower. This is in agreement with the experiment results of TTP curve, which can be used to characterize the stability of supersaturated solid solutions.