Fatigue Properties of High Strength Al Alloys under Ultrasonic Fatigue

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The fatigue properties of two high strength Al alloys, Al 6061-T6 and Al 7075-T6, in the life range toward 10^9 cycles were investigated in air and at room temperature under ultrasonic fatigue as well as rotary bending fatigue for the purpose of comparison. Our research shows that(1) Fatigue strength increases in both alloys under ultrasonic fatigue, and the influence of ultrasonic frequency is remarkable in the short life region for Al 7075 and in the long life for Al 6061, respectively, meaning that the characteristic of materials plays an important role in the ultrasonic fatigue study.(2) Fatigue cracks initiate from slip bands and no influence of frequency on the cracking mechanism in the two alloys is recognized. But the crack initiation and its early propagation are suppressed under ultrasonic fatigue. In case of ultrasonic fatigue, crack path changes from the initial nearly vertical direction into approximately 60 degree with respect to the stress axle, and most of fatigue life is spent in the growth of small cracks less than ^-100 micron.(3) Striations are rarely found on the fracture surfaces of ultrasonically fatigued specimens, instead, well developed voids are observed along with brittle cleavages, implying that fracture mode transits from tensile mode to shear mode.Furthermore, ultrasonic fatigue behavior in some high strength alloys such as nickel-base superalloy and marageing steels, is also being investigated in order to better understand the effect of ultrasonic frequency on the strength of metallic materials that have different crystal structures.