孔挤压强化对2A97铝锂合金耳片疲劳性能的影响

Hole Expansion Strengthening on Fatigue Properties of 2A97 Al-Li Alloy Lugs

对2A97-T84铝锂合金不同尺寸的直耳片进行了孔挤压强化,研究孔挤压前后耳片的疲劳性能。结果表明:孔挤压强化工艺能有效提高2A97-T84耳片的疲劳性能,且小尺寸耳片的强化效果优于大尺寸耳片,其小、大尺寸耳片疲劳寿命提高幅度最大可达80.6%和62.5%;当峰值应力相同时,孔挤压强化后小尺寸耳片的疲劳寿命最高为大尺寸耳片的7.6倍。对比断口形貌可知,孔挤压强化后耳片的疲劳裂纹源位置、数量等均发生了变化:其位置由耳孔内表面转移至耳孔内壁近表面处且数量明显减少,疲劳辉纹宽度变窄,这说明经孔挤压强化后,孔内壁表层金属发生了强烈的塑性变形,耳孔周围产生了残余压应力,组织发生了位错塞积和缠结,形成了位错胞状结构,这些均降低了孔壁边缘的应力集中系数,减缓了疲劳裂纹源的萌生,使得疲劳裂纹的扩展速率降低,从而提高了耳片的疲劳性能。

With the development of aerospace,the demand for lightweight alloys is increasing rapidly. 2A97 Al-Li alloy is a new type of aluminum alloy developed independently by China,which meets the demand for the metal engineering materials with low density,high strength and good toughness. It is well known that the lug is an important part which can connect various structural parts together.Most of the aviation structural parts are assembled by installing fasteners in the holes of the lugs,and the stress concentration area is produced near the hole during hole extrusion. However,the hole is easy to break under the action of the cyclic load during service condition. Many scholars had conducted a lot of research,and the results showed that in the fatigue failure of aircraft structures,about80% of the cracks were occurred from the holes of the connecting parts. It was the main factor which restricted the improvement of the overall fatigue life of the aircraft obviously. Therefore,the research on the strengthening technology of the lug had become a key way to improve the safety performance of the aircraft. Hole extrusion strengthening technology was one of the methods,which adopted the local strengthening process to improve the service life of the structure. Because of better strengthening effect and simpler process operation,it had been widely used in engineering. In order to improve the fatigue properties of 2A97 Al-Li alloy lugs,the straight lugs with different sizes were subjected to hole extrusion strengthening,then the fatigue properties and fatigue behaviors of the different straight lugs before and after hole extrusion were investigated. The experimental material of 2A97 Al-Li alloy rolled sheet with 10 mm was firstly treated with 520 ℃/1.5 h solution plus water quenching,then undergone 4% pre-tensile deformation along the rolling direction of the material,and then followed by artificial aging of 180 ℃/18 h,air cooling(T84). Subsequently,the lug hole extrusion test was carried out using the mandrel direct extrusion method in WDW2010 electronic universal testing machine. Then the fatigue test of the straight lugs with different sizes before and after hole extrusion was carried out with Zwick Amsler 150 HFP5100 fatigue testing machine. The results showed that the hole extrusion could effectively improve the fatigue properties of the 2A97-T84 Al-Li alloy straight lugs,and the strengthening effect of the small straight lugs was better than that of the large straight lugs. The increase of fatigue life of small lugs and large lugs was 80.6% and 62.5%,respectively. When the peak stress was the same,the fatigue life of the small straight lugs after hole extrusion strengthening could be up to 7.6 times than that of the large straight lugs after hole extrusion strengthening.Compared with the fracture morphologies before and after hole extrusion,the location and number of fatigue crack sources had been changed. Their locations were changed from the inner surface of the auricle to the near surface of the inner wall,the number of fatigue crack sources was decreased significantly,and the width of fatigue striation was narrower than that of the unextruded reinforcement lugs. All these above could decrease the stress concentration factor on the edge of the hole wall,slow down the initiation and propagation of the fatigue cracks. Moreover,after hole extrusion,the residual compressive stress was produced around the ear hole of 2A97-T84 Al-Li alloy,which reduced the peak tensile stress of the applied alternating load and the average stress,enhanced the ability of the material to resist fatigue fracture and the time of fatigue crack initiation and propagation;the metal on the inner wall of the lugs was undergone larger degree plastic deformation. During hole extrusion,the microstructure of the lugs was changed significantly. The spherical δ’ phase and needle T1phase had higher stacking fault energy,which could promote coplanar slip,lead to dislocation multiplication,cause crack segregation,promote crack closure,and improve the resistance of crack propagation. At the same time,the boundaries were slipped and twisted,and a large number of dislocations were generated and entangled inside the crystal,which formed dislocation cell structure finally. And the dislocation cell structure could hinder the movement of dislocations,make plastic deformation difficult,increase the resistance of crack tip,and decrease the rate of crack growth. Therefore,the fatigue properties of2A97-T84 Al-Li alloy straight lugs were improved significantly after hole extrusion. Compared to the large straight lugs,the small straight lugs had smaller surface area and fewer crack sources,so the time from crack initiation and propagation to the critical size was longer relatively. Therefore,the effect of hole extrusion strengthening for the small straight lugs was better than that of the large lugs.These results provided reliable technological parameters and theoretical basis for the size design and the strengthening technology selection of the straight lugs for 2A97-T84 Al-Li alloy.