Published: Jan 1978
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A new approach, the application of microstructure control to products possessing an optimized alloy chemistry, has been taken to develop an improved combination of properties in 7000 series wrought aluminum products. The technique developed is an ingot (intermediate) thermal-mechanical treatment (ITMT) process which optimizes the combination of static strength, fracture toughness, fatigue strength, and stress corrosion resistance of commercial aluminum alloy die forgings. Previously, high fracture toughness had been achieved in wrought products by sacrificing tensile strength or stress corrosion properties. The ITMT processes produce either a recrystallized grain structure by thermal treatments prior to working at lower-than-conventional (warm) working temperatures or an altered grain structure by hot working the previously recrystallized structure. In both cases, the resulting grain morphology is desirable for good fracture toughness. Results of testing forging material show that ITMT on a high purity 7475 alloy produces advantageous fatigue crack propagation rates in 3.5 percent salt solution in comparison to 7075-T73, and stress corrosion resistance equivalent to 7075-T73 in addition to attractive tensile properties and high fracture toughness.
corrosion fatigue, thermal-mechanical heat treatment, crack propagation, stress corrosion resistance, forging, microstructure
Senior stress engineer, Boeing Vertol Company, Philadelphia, Pa