Aluminum-zinc-magnesium strong weldable alloy
The role of alloying elements and impurity elements
With the increase of Zn and Mg content, the tensile strength and heat treatment of Zn and Mg:Al-Zn-Mg alloys generally increase. The stress corrosion tendency of the alloy is related to the sum of Zn and Mg contents. For alloys with high Mg low Zn or high Zn low Mg, the alloy has quite good stress corrosion resistance as long as the sum of Zn and Mg mass fraction is not more than 7%. The weld cracking tendency of the alloy decreases as the Mg content increases.
The trace alloying elements in the Al-Zn-Mg-based alloy are Mn, Cr, Cu, Zr, Ti, etc., and Fe and Si are main impurity elements.
Mn and Cr: Adding Mn and Cr improves the corrosion resistance of the alloy. The effect is obvious when containing 0.2% Mn-0.4% Mn. The effect of adding Cr is more. If Mn and Cr are added at the same time, the effect of reducing stress corrosion tendency More strongly, the addition amount of Cr is preferably 0.1% - 0.2%.
Zr: The weldability of the Al-Zn-Mg alloy was remarkably improved. When 0.2% Zr was added to the AlZn5MgCu0.35Cr0.35 alloy, the tendency of weld cracking was greatly reduced. Zr also increases the recrystallization final temperature of the alloy, adding >0.2% Zr to the AlZn4.5Mg1.8Mn0.6 alloy, and the recrystallization temperature of the alloy rises above 500 °C. Therefore, the material retains the deformed structure after solution treatment. . The addition of 0.1% Zr - 0.2% Zr to the Mn-containing Al-Zn-Mg alloy can also improve the stress corrosion resistance, but the effect of Zr is not as good as that of Cr.
Ti: The addition of Ti to the Al-Zn-Mg-based alloy refines the cast structure and improves the weldability of the alloy, but the effect is lower than that of Zr. If Ti and Zr are added at the same time, the effect is better. The addition of >0.15% Zr to the AlZn5Mg3Cu0.3 alloy containing 0.12% Ti has better solderability and a relatively high elongation, and the same effect as when +0.2% Zr is added alone can be obtained. Ti also increases the recrystallization temperature of the alloy.
Cu: Adding a small amount of Cu to the Al-Zn-Mg-based alloy improves the stress corrosion resistance and tensile strength of the alloy, but the weldability of the alloy is lowered.
Fe: It is an impurity of Al-Zn-Mg alloy, which reduces the corrosion resistance and mechanical properties of the alloy, especially for alloys with Mn content, so the Fe content should be as low as possible and should be limited to <0.3%.
Si: It is also an inherent impurity of the alloy, which reduces the corrosion resistance and mechanical properties of the alloy and increases the tendency of the alloy to crack. The limit should be limited to <0.3%.
The large solubility of Mg in Al is 17.4% (450 ° C) and 1.0% at room temperature. The solubility of Zn is higher, 31.6% at the eutectoid temperature (275 ° C), 12.6% at 200 ° C, and ≥ 2% at room temperature. Therefore, Zn, Mg and Al can form a high concentration ternary solid solution. According to the ternary phase diagram of Al-Zn-Mg system, in addition to the α, β, η and γ phases, the alloy also has a ternary compound T (Al2Mg3Zn3), and the T phase can also be used in the concentration range (AlZn) 49Mg32. Said. The composition of the industrial Al-Zn-Mg alloy is mostly in the hatching range shown by M in the figure, and the main strengthening phase is T and η, so the industrial alloy is called an α+T type alloy. The η phase and the T phase not only have a great solubility in Al, but also have a considerable solubility change, so that they have a strong age hardening effect. The molecular formula of the β phase is -Al3Mg2, T-Al2Mg3Zn3, η-MgZn2, γ-MgZn5. The Zn/Mg ratio of the T phase is about 2.71, but the concentration of Zn and Mg in the T phase varies widely. The main strengthening phase of the alloy with a Zn/Mg ratio of 1-4 is T, and only Zn/Mg>4 The alloy has an η phase, and the alloy with Zn/Mg=6-7 is completely composed of the η phase.