【作者】 车洪艳；

【导师】 陈剑虹； 朱亮；

【作者基本信息】 兰州理工大学 ， 材料学， 2008， 博士

【摘要】 节能和环保是当今社会发展的主题,汽车轻量化正是满足了这一需求。铝合金的比强度超过钢,但比刚度较差。由于铝合金具有较好的延展性,通过挤压、冲压、压铸、焊接等工艺能得到复杂截面的构件及空间框架结构,与传统的钢制半壳型结构相比,不仅减轻了重量,而且保证这种车身具有最佳的刚度,并在此基础上采用先进的组合框架方法将是未来交通工具发展的方向。本文结合试验和数值计算,研究铝合金部件重点研究复杂截面挤压部件在轴向载荷作用下的变形及损伤行为和T型管焊接部件在平面弯曲载荷作用下的变形和损伤行为。首先对6000系列车用铝合金材料进行力学性能表征。主要对AA6014、6061、6063三种材料进行表征。考虑挤压所造成的材料的各向异性以及应力状态、应变速率的影响,并对6061和6063的断口进行观察。在对母材进行材料表征时要综合考虑以下几个因素：不同应力状态、应变速率、材料各向异性。在选择损伤模型时要考虑到应力状态的影响,在三向应力度较低的情况下Johnson-cook损伤模型能够很好的预测材料的损伤行为,在三向应力度较高的情况下,Gurson模型得到较好的结果。模拟时的损伤参数还与网格尺寸有关,随着网格尺寸的增加,损伤参数减小。宏观试验结果表明：三种材料平板拉伸时的断裂应变大于缺口拉伸时的断裂应变,而平板拉伸时的屈服应力小于缺口拉伸时的屈服应力,在剪切时材料的剪切断裂应变远大于平板拉伸时的断裂应变。对于6063和6061两种材料断口观察发现,随着三向应力度的增大,韧窝断裂机制增加,剪切断裂机制减小。缺口拉伸断口主要以韧窝为主,而剪切断口是以大型剪切韧窝和剪切面为主。平板拉伸试样宏观断口为剪切断裂和正拉伸断裂两种混合方式,最终的断裂以剪切断裂为主,宏观上表现为或者沿宽度方向剪断,或者沿厚度方向剪断；缺口拉伸试样主要以拉伸断裂为主。针对6063平板拉伸试验所遇到的关于断裂路径的问题应用UMAT对断裂路径进行了预测。对复杂截面异厚度的均质部件在轴向载荷作用下的变形及损伤行为进行了研究。以AA6014为研究对象,部件由德国奔驰公司提供。对部件进行轴向压缩试验,并应用前面得到的材料力学性能参数及损伤参数对部件在轴向载荷作用下的变形及损伤行为进行预测。结果表明通过仿真与试验相结合的方法可以得到合适的变形及损伤参数；损伤模型与三向应力度有关,因此要选择合适的损伤模型模拟不同载荷作用下的材料的变形及损伤行为；因为损伤参数与网格尺寸相关,模拟部件时网格尺寸比模拟试样时粗大,因此需要根据网格尺寸调整损伤参数,本文对比了不同网格尺寸模拟的结果。在部件模拟中将试样设置为均质,为得到更精确的仿真结果,与制造工艺过程相结合的仿真应该是未来仿真发展的方向。对铝合金焊接接头局部材料进行了力学性能表征。提出了一种双孔微剪切试验方法,无需制备小试样,在被测试样区域打两个通孔,直接进行剪切试验,测定了焊接接头各区域的局部强度及局部材料的剪切变形和损伤行为。并通过改变两孔之间的距离及孔的半径来实现局部材料在不同应力状态下的变形及损伤行为。应用TIG (Tungsten inert Gas)焊制备铝合金焊接接头,其中对接接头母材为6061和6063,T型板接头及T型管接头母材为6063。焊接对接接头时,通过多次试验发现,焊缝预留间隙对焊缝成形影响较大,焊接薄板时,预留合适间隙0.5-1mm时有利于焊缝双面成形,间隙过大易发生焊缝烧穿,间隙太小需要较大的热输入才能保证焊缝成形,但这样会增加热影响区的宽度从而降低接头的强度。在焊接T型接头角焊缝时,由于板薄焊接电流相对较小,因此角焊缝的熔深也很小,焊缝预留间隙和开坡口对增加焊缝的熔深几乎没有效果,反而会出现焊缝塌陷和形成冷搭接现象。因此文中所有角焊缝的熔深在强度计算时忽略不计,有限元模型中也不考虑角焊缝熔深。金相观察及硬度分析可以发现,焊接接头的焊缝及热影响区与母材在组织结构与力学性能上存在一定的差别,因此要想准确表征焊接接头的性能必须要考虑焊接接头的力学性能不均匀性。针对铝合金对接焊接接头及T型焊接接头进行了双孔微剪切试验,并结合有限元反推法,获得了不同焊接接头各区域的真应力应变曲线及损伤参数。在前面研究的基础上,对平板对接接头进行了拉伸试验及有限元模拟。选用成形良好的焊接接头,主要研究熔化焊对接接头在动载荷及静载荷作用下的变形及损伤行为。并且基于Gurson和Johnson-cook损伤模型结合有限元模拟研究低匹配和高匹配焊接接头及不同匹配情况下焊缝相对宽度对接头性能的影响。同样的焊接工艺下,动态拉伸时,在没有焊接缺陷的影响下,抗拉强度及断裂应变比其在静态拉伸条件下有所提高,且应变率越大,抗拉强度和断裂应变越高；不同材料的动态拉伸试验结果显示：6061的抗拉强度及断裂应变与6063相比要高些,说明6061的材料性能优于6063；同样材料不同的取样方式(垂直于焊缝和沿着焊缝)的动态拉伸试验结果显示：沿着焊缝取样的试样质量优于垂直于焊缝取样的试样,且抗拉强度及断裂应变高于垂直于焊缝取样的试样；以焊缝成形及质量良好的静态平板拉伸试样进行微剪切试验,并结合有限元模拟得到焊接接头各个区域的材料的力学性能参数及损伤参数,并应用得到的参数模拟静态平板拉伸试验得到了很好的结果,说明微剪切试验及建立的有限元模型的正确性；应用有限元仿真的方法研究了不同匹配焊接接头及焊缝相对宽度对接头性能的影响,结果发现：对于低匹配的焊接接头,损伤主要发生在热影响区,且相对宽度越窄,损伤越容易发生,因此建议对于低匹配的焊接接头要选择一个合理的宽度的焊缝；对于高匹配焊接接头,损伤主要发生在母材上,且相对宽度越窄,损伤越不容易发生,因此建议对于高匹配的焊接接头要选择一个尽可能窄的焊缝。T型部件的试验研究及有限元模拟。通过试验和有限元分析相结合,研究了6063铝合金T型焊接结构在平面弯曲载荷作用下的变形及损伤行为。设计的T型试样可以实现角焊缝在拉应力状态下的变形及损伤行为,可以作为表征T型部件的“标准”试样的参考。Gurson和Johnson-cook损伤模型能够预测T型部件的损伤行为,Gurson模型预测结果要好于Johnson-cook模型。试验结果表明,S型焊缝裂纹在熔合线附近产生之后逐渐向热影响区扩展,最后宏观裂纹在热影响区形成。U型焊缝宏观裂纹产生于焊缝,最后向热影响区扩展,有限元仿真也很好的预测了这一过程,验证了模型的正确性。更多还原

【Abstract】 As result of the need to save energy and protect the environment, the use of aluminium alloy space frames are increasing. Aluminium alloy exhibits the property of high specific strength and low specific stiffness compared with steel.Meanwhile Aluminium alloy has good ductility, which being manufactured in complicated cross-section component and space-frame by extrusion, stamping, die-casting and welding. The use of aluminium alloy not only reduces the weight of space-frame but ensure stiffness as well. At the same time, advanced aluminium alloy space-frame body is the development direction of transportation means in the future. This paper studied deformation and damage behaviours of aluminium-alloy component, focus on extrusion-complicated component under axial compression and tubular T-joints component under plane bending combining tests with FEM (finite element method).In this paper, the series of 6000 (AA6014、6061、6063) aluminium-alloy were characterized under tensile and shear loadings including strain-rate dependence, anisotropy effects and processing influences. At the same time, stress states also considered when choosing the damage model for the damage behaviour of the components depended strongly on the stress triaxiality. Fracture strain decreased remarkably with increasing triaxiality and damage mechanisms could be quite different at high and low triaxiality. Therefore, it was necessary to characterize the influence of stress triaxiality on damage beahviour with relevant experiments then to simulate it with a verified damage model. The results showed:at low stress triaxiality, Johnson-cook damage model gave good result, while high stress triaxiality, Gurson model gave good result. In addition, damage parameters depended on size of mesh, damage parameters reduced with mesh size increasing. Macroscopic test showed that fracture strain of plate tension was larger than that of notched plate tension; yield stress of plate tension was lesser than that of notched plate tension. Furthermore, fracture strain of pure shear was much larger than that of plate tension. Moreover, the fracture graphic analysis suggested that the fracture appearance in 6061 and 6063:dimple mechanism was increasing and shear mechanism was decreasing with stress triaxiality enhancing. Fracture mechanism of notched plate tension was dimple, but large shear dimple and shear facet were observed in shear test,but shear fracture mechanism and tension fracture mechanism mixed for plate tension test, shear fracture occurred on width or thickness in microscopically. Tension fracture occurred in notch tension test.Fellowed that fracture path of 6063 tension test has been implemented in FORTRAN and introduced in ABAQUS by user subroutine.An extensive experimental research project has been carried out to study the crash behaviour of a complicated cross-section with different thicknesses of aluminium extrusions subjected to axial loading. Test specimens were’made of the aluminium alloy AA6014. Tempers T7 and provided by Daimler Chrysler. Prediction of the deformation and damage behaviour was carried out with the material properties, which were acquired in above. To sum up, deformation damage parameters received combining the test with simulation. Because damage model dependented on stress triaxiality, the appropriate damage model should be chose to simulate deformation and damage behaviour under different type loadings, the finer the element-size, the better the calculation and simulation result, however the calculation will cost a lot of time. Therefore, appropriate element-size not only ensured the precision of results, but saved time as well. Damage parameters depended on element-size, and the element size is coarser in components than in specimens. Thus to the component, it is necessary to adjust the damage parameters according to the element size. The effects of the inhomogeneous material properties on the component behavior depended on loading situation. In future work, a link between extrusion simulation, microstructure simulation and crash simulation will be an important step to crashworthiness assessment.Characterization of aluminium alloy welded joint was carried out. A test method called double-hole micro-shearing was proposed without small specimens. Two through holes beside the measured zone were acquired. Micro-shearing test was performed to get local intensity, deformation, and damage behaviour of local material. Then radius of holes and distance of bridge were changed to study deformation and damage behaviour of local material under different stress states. TIG (Tungsten inert Gas) welding was applied in 6061 and 6063 aluminium alloy joint, then inhomogeneous of organization and mechanical properties of welded joint were studied. To butt welded joint, base material is 6061 and 6063, while T welded joint is 6063. A lot of experiments proved that reservation clearance affected molding of weld in large degree. Weld shape was good when the right clearance was selected. The results showed that the lager one resulted in destroy of weld, on the contrary, the lesser one, more quantity of heat, which would lead the wider of HAZ that deduced the intensity of weld joint. To the T welded joint, because of thin plate need less welding current, therefore weld penetration depth, which was hardly, influenced by clearance and slope, on the contrary which resulted in weld sag and cold joining. So weld penetration depth was ignored when considered strength calculation in this paper. In order to characterize the properties of welded joint correctly, inhomogeneous of organization and mechanical property which would be indicated by metallographic and hardness test must be considered in detail. Combining simulation with micro-shearing test was applied in T welded joint too, true strain and true stress of different zones on welded joint were acquired. Plate tension tests and simulations of butt joint were performed based on the above result. Deformation and damage behaviour of butt joints with melt welding under static and dynamic loadings were studied. Then influence on properties of joint in over-match and under-match and width of weld under different match based on Gurson and Johnson-cook model were investigated. Fracture intensity and strain were higher under dynamic tension loading than that under static tension loading, the two parameters increased with increasing of strain ratio. The samples in the same weld techniques and no lacuna, the dynamic tension results showed that fracture intensity and strain of 6061 were higher than that of 6063. It certificated that mechanical of 6061 was better than that of 6063, Different sample method of same material (including vertical and along weld direction) test results showed, fracture intensity and strain of samples which along weld direction were better than that of vertical weld direction. Combining Double-hole micro-shearing experiment with simulation were performed on welded joint to get the material mechanical and damage parameters. Then the parameters were used to simulate static plate tension tests. Good results mean correct model and precision test; In addition, based on the correct model influence of mechanical mismatching on the failure of welded joints were studied. The results showed the damage of under-matched joints mainly occurred in HAZ, the narrower the joint, the lower the damage resistance of the specimen, based on this result, a reasonably wide weld should be chosen for under-matched welded joints. While, to the over-matched joints, the damage mainly occurred in the base metal area adjacent to the joint, the narrower the joint, the higher the damage resistance of the joint. A reasonably narrow weld should be chosen for over-matched welded joints.Tests and simulations of T welded joint were performed. Deformation and damage behavior of 6063 T welded joint under plane bend were researched combining tests with simulations. Designed T welded joint samples, which can be used to study deformation, and damage behaviour of corner weld under different stress states. Two damage models (Gurson& Johnson-cook) predicted the damage behaviour, the former give better result than the later. To the S type weld, crack began in melt line then enlarged to HAZ, the marco-crack formed in HAZ in the end. To the U type, weld crack origin nated in weld material, then enlarged to HAZ, simulation gave good prediction for this process.更多还原