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少量的合金元素来开发新型的耐应力腐蚀开裂的可焊Al-Zn-Mg合金。Al-Zn-Mg合金的应力腐蚀开裂是由点蚀或腐蚀引起的。在本研究中，采用点蚀电位的测量作为耐蚀性的评价。在Al-4.5%Zn-1.2%Mg合金中分别添加0.2%Cr 0.5%Mn、0.05%Ti和0.2%Fe-0.1%Si，制备出Cr、Zr和Cu含量最高可达1%的Al-4.5%Zn-1.2%Mg合金，以提高合金的耐蚀性。利用阳极极化特性，从冶金和电化学两方面研究了微量合金元素对合金时效特性和耐蚀性的影响。结果总结如下：1。在Al-4.5%Zn-1.2%Mg合金中加入Cu或Cr使点蚀电位升高。而在0.2%Cu和0.2%Cr以上的晶界处则主要发生点蚀。因此，对于应力腐蚀开裂，加入0.2%Cu或0.2%Cr是有效的。0.05%Ti、0.2% fe、0.1% si或0.5%Mn的加入均能有效抑制晶粒生长，但对点蚀电位影响不大。在Al-4.5%Zn-1.2%Mg合金中添加0.2%的Zr对晶粒细化有影响，但添加0.5%以上的Zr会降低腐蚀电位，导致应力腐蚀开裂。在上述结果的基础上，在Al4.5%Zn -1.2%Mg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可有效改善焊缝热影响区的应力腐蚀开裂和耐蚀性。

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.