7050 アルミボルトの機械的特性に及ぼす製造工程の影響

抄録

Aluminum bolts used for fastening aluminum structural members must have high resistance to stress corrosion cracking (SCC) leading to thread damage while maintaining high 0.2 % proof stress. Generally, the bolts of 7050-aluminum alloy are manufactured in the order of Ⅰ: machining, Ⅱ: rolling, Ⅲ: solution heat treatment, and Ⅳ: aging treatment, which was labeled No.1 in this study. This paper presented the effects of manufacturing process order on the mechanical properties and the SCC damage morphologies with the aim of satisfying the required specifications for bolts. Two types of bolts were manufactured in order of Ⅰ→Ⅲ→Ⅱ→Ⅳ, and Ⅲ→Ⅳ→Ⅰ→Ⅱ, which were labeled No.2 and No.3, respectively. The mechanical properties and the damage morphologies were evaluated via tensile, Vickers hardness and salt spray tests, respectively. The values of tensile strength and 0.2 % proof stress of No.1 and No.2 were almost the same, however that of No.3 was lower than No.1 and No.2. The degree of SCC damage in No. 2 was smaller than that of No. 1 and No. 3. Finally, the results were discussed via XRD and EBSD analysis. MgZn₂ and Al₂CuMg of typical compounds and the randomly crystal orientation were detected in the bolts, which is indicated that the manufacturing process order does not affect the production of compounds and the crystal orientation. It is presumed that the mechanical properties and the degree of SCC damage are mainly dominated by the effect of the rolling process after solution heat treatment.