孔挤压对7050合金耳片组织及疲劳性能的影响

Effect of hole extrusion on microstructure and fatigue properties of 7050 alloy lugs

采用芯棒直接挤压的方法对不同尺寸的7050-T7451铝合金斜耳片进行孔挤压强化,并采用光学显微镜、扫描电镜、透射电镜和疲劳试验等研究孔挤压前后耳片的组织和疲劳性能。结果表明:经孔挤压强化后7050-T7451铝合金斜耳片的显微组织产生了大量的高密度位错,并缠结成位错胞状结构,其疲劳性能也得到了显著地提高,尤其对小尺寸耳片的提高效果更为明显,提高了83.86%,而大尺寸耳片在特定的峰值应力下其疲劳寿命可提高68.92%。这是因为耳孔内壁表层的金属受到芯棒挤压产生了剧烈的塑性变形,形成了组织强化层和残余压应力层,使得疲劳裂纹源的位置和数量发生了改变且疲劳辉纹宽度变窄,这些均降低了孔壁边缘的应力集中系数,减缓了裂纹的萌生和扩展速率,从而提高了耳片的疲劳性能。

By using the method of mandrel direct extrusion, the hole extrusion strengthening of 7050-T7451 aluminum alloy oblique lugs with different sizes was carried out, and microstructure and fatigue properties of the lugs before and after hole extrusion were studied by means of optical microscope, scanning electron microscopy, transmission electron microscopy and fatigue test. The results show that a large number of high density dislocations are observed in the microstructure of the 7050-T7451 aluminum alloy oblique lugs after hole extrusion strengthening, and they are entangled into dislocation cell structure, and the fatigue properties of the 7050-T7451 alloy lugs are significantly improved. Especially for small lug, an increase of 83.86% in the fatigue life may occur, and the fatigue life of large lug can be increased by 68.92% under specific peak stress. This is because the severe plastic deformation of the metal on the inner wall surface of the lug hole after mandrel direct extrusion happens, and leads to the formation of microstructure strengthening layer and residual compressive stress layer, which changes the position and quantity of fatigue crack source and narrows the width of fatigue striation. All of these reduce the stress concentration coefficient at the edge of the hole wall, slow down the initiation and propagation rates of the crack, and thus improve the fatigue properties of the lugs.