Development of New Weldable Al-Zn-Mg Alloy with High Resistance to Stress Corrosion Cracking.

本研究的目的是通过添加微量合金元素来开发新型的抗应力腐蚀开裂的可焊接Al-Zn-Mg合金。在Al-4.5%Zn-1.2%镁合金中分别加入0.2%Cr0.5%Mn0.05%Ti0.2%Fe-0.1%Si，然后添加不同含量的铬的Al-4.5%Zn-1.2%镁合金，为了提高合金的耐蚀性，制备了含量不超过1%的合金元素ZR和Cu。利用阳极极化特性，通过金相和电化学方法研究了微量合金元素对合金时效特性和耐蚀性的影响。研究结果总结如下：1.研究结果。在Al-4.5%锌-1.2%镁合金中添加铜或铬会导致点蚀电位的增加。但点蚀主要发生在0.2wt%以上的晶界。因此，对于应力腐蚀开裂而言，添加0.2%的铜或0.2%的铬是有效的。添加0.05%的钛、0.2%的Fe-0.1%的Si或0.5%的Mn可以有效地抑制晶粒长大，但对点蚀电位影响不大。在Al-4.5%Zn-1.2%镁合金中添加0.2%的Zr影响合金的晶粒细化，而添加0.5%以上的Zr会降低合金的腐蚀电位，导致应力腐蚀开裂。在此基础上，在Al4.5%Zn-1.2%镁合金中添加0.05%Ti、0.2%Cr、0.5%Mn、0.2%Fe-0.1%Si、0.2%Zr和0.2%Cu，有效地改善了焊接热影响区的应力腐蚀开裂和耐蚀性能。

The purpose of this investigation is to present the development of new weldable Al-Zn-Mg alloy with high resistance to stress corrosion cracking by means of additions of minor alloying elements.The stress corrosion cracking of Al-Zn-Mg alloy occurs by the pitting or corrosion. In this investigation, the measurement of pitting potential was carried out as the evaluation of corrosion resistance.As the new alloy, Al-4.5%Zn-1.2%Mg alloys separately added 0.2%Cr 0.5%Mn,0.05%Ti and 0.2%Fe-0.1%Si, and then Al-4.5%Zn-1.2%Mg alloy with various contents of Cr,Zr and Cu up to 1 % in order to improve the corrosion resistance were prepared.The effect of the minor alloying elements on aging characteristics and corrosion resistance was studied by means of metallurgy and electrochemistry by anodic polarization characteristics. The results were summarised as follows.1. The addition of Cu or Cr to the Al-4.5%Zn-1.2%Mg alloy causes the increase of the pitting potential. But the pitting occurs predominantly at the grain boundaries above 0.2%Cu or 0.2%Cr. Therefore,addition of 0.2%Cu or 0.2%Cr is effective ,concerning the stress corrosion cracking.2. The addition of 0.05%Ti,0.2%Fe-0.1%Si or 0.5%Mn is effective to inhibit the grain growth, but hardly affect the pitting potential.3. The Zr addition up to 0.2% to the Al-4.5%Zn-1.2%Mg alloy affects the refining of the grain size,but the Zr addition above 0.5% causes the reduction of corrosion potential and causes the stress corrosion cracking.4. On the basis of the results above mentioned, the additions of 0.05%Ti,0.2%Cr,0.5%Mn,0.2%Fe-0.1%Si,0.2%Zr and 0.2%Cu to the Al4.5%Zn -1.2%Mg alloy are effective to improve the stress corrosion cracking and corrosion resistance of the weld heat affected zone.