铝合金超声疲劳行为研究

STUDY ON THE ULTRASONIC FATIGUE BEHAVIOR OF ALUMINUM ALLOYS

研究LY10、LY12、2219、6061和AS5U3GY35五种铝合金的超声疲劳行为。获得的6条S—N曲线显示,铝合金在107～1010疲劳周次区间仍然发生疲劳断裂,N=1010时的疲劳强度显著低于N=107时的疲劳强度。断口分析表明,铝合金对表面完整性极其敏感。与黑色金属和钛合金不同,铝合金试件在极高周次疲劳中,裂纹仍然主要由表面或次表面起裂。铝合金试件的疲劳断口没有发现明显的“鱼眼”现象和黑影区。超声频率下极高的应变速率和位错移动速率增加了脆性质点开裂和沉淀相的溶解现象,引起微观组织局部失效,从而形成疲劳裂纹的萌生和扩展。

Fatigue endurance experiments have been performed on five kinds of Aluminum alloys, such as 2219, 6061, LY10, LY12 and AS5U3G-Y35, using the ultrasonic fatigue testing method at 20 kHz. The S-TV curves show that fatigue failure can occur over 107 cycles, and that fatigue endurance stress continues to decrease with increasing cycles to failure between 107 and 1010 cycles. Fatigue strength at 1010 cycles is obviously lower than that at 107 cycles. Scanning electron microscopy is used to examine the fatigue rupture section of all specimens. It was found that fatigue fracture initiation in duralumin such as 2219, 6061, LY10 and LY12 are mostly at the surface of specimen since the materials are very sensitive to the surface condition and also due to the lack of microstructural defects at the interior. There is no clear fish-eye and optically dark area (ODA) seen in the crack initiation region, different from fatigue initiation characteristics of high strength steel and Titanium alloy tested in high cycle range. High strain rate and dislocation transfer at ultrasonic frequency accelerates the crack of fragile particles and the dissolution of deposit phase in the Aluminum alloys. This results in local failure of the microstructure, and accelerates the fatigue crack initiation and growth.