構造用Al-Zn-Mg合金の応力腐食割れにおよぼす冷間加工の影響について

抄録

Experiments were conducted to investigate the effects of cold working on mechanical properties and stress corrosion resistance of structural Al-Zn-Mg alloys, and the relation between the stress corrosion cracking and shear cracks was discussed.
The results obtrained were summarized as follows.
(1) Tensile strength, yield strength, and hardness of the high temperature aged material were higher than those of the low temperature aged one. The above values were increased with the increase of cold reduction ratio in the both materials. However, the rates of increase in these values by cold reduction were more remakable in the low temperature aged material than in the high temperature aged one. Then, at the cold reduction ratio of 20%, these values of the low temperature aged material were higher than those of the high temperature aged one.
(2) Stress corrosion resistance in the short transverse direction was higher in the low temperature aged material than in the high temperature aged one. The resistance was decreased with the increase of cold reduction ratio in the both materials. However, the rate of decrease in the resistance by cold reduction was more remarkable in the low temperature aged material than in the high temperature aged one. Then, at the cold reduction ratio of 20%, the reistance of the low temperature aged material under higher applied stresses was lower than that of the high temperature aged one.
(3) The life of shear cracks was longer in the high temperature aged material than in the low temperature aged one.
(4) Shear cracks are supposed to be stress corrosion cracking due to the residual stress caused by shearing. The reasons for higher stress corrosion resistance of the low temperature aged material and higher shear crack resistance of the high temperature aged one would be as follows.
(a) The reduction ratio of cold working in the sheared part is about 20%.
(b) Assuming that the residual stress caused by shearing is equivalent to the yield strength of the sheared part, the stress of the low temperature aged material is higher than that of the high temperature aged one.
(c) In the low temperature aged material, stress corrosion resistance is higher but at cold reduction ratio of 20%, the resistance under higher applied stresses is lower. Accordingly, shear cracks are more likely to occur in the low temperature aged material than in the high temperature aged one.