【作者】 董鹏；

【导师】 孙大千；

【作者基本信息】 吉林大学 ， 材料加工工程， 2014， 博士

【摘要】 随着高速铁路技术的发展，铝合金在高速动车组列车制造中得到了越来越多的应用。作为轨道车辆生产制造的核心技术，铝合金的焊接技术直接影响车辆的质量、生产成本和行车安全。然而，使用传统的熔化焊方法焊接铝合金存在的主要焊接性问题为：焊缝中的气孔、焊接热裂纹、接头变形及软化等。这些焊接性问题与铝合金的物理性质等因素有关，很难得到根本的解决，这一定程度上制约了铝合金在高速列车制造中的更广泛的应用。搅拌摩擦焊（FSW）是一种新型的固相连接技术，被认为是解决铝合金焊接性问题最为有效的方法之一。因此，开展高速列车用6005A-T6铝合金FSW接头的组织与性能研究，具有重要的理论意义和实用价值。本文首先系统、深入地研究了6005A-T6铝合金FSW接头的组织与性能特点。研究结果表明，6005A-T6铝合金FSW接头可分为焊核区（NZ）、热机械影响区（TMAZ）、热影响区（HAZ）和母材（BM）。BM组织呈纤维状，晶粒内部分布着大量细小的针状β’’相，β’’与-Al基本呈共格关系，在BM晶界上连续地分布着单质Si相和Q相，沿晶界存在较宽的沉淀无析出带（PFZ）。NZ在FSW过程中经历了动态再结晶，同时还有沉淀相的溶解。TMAZ的突出特点为沿一定方向弯曲变形的晶粒，基体中存在着高密度的位错。在NZ和TMAZ的晶界上仅存在极少量的单质Si相。HAZ的微观组织是不均匀的，靠近TMAZ的HAZ（记为HAZ1）中为较均匀的近等轴晶粒，而靠近BM的HAZ（记为HAZ2）晶粒仍具有纤维状特点。HAZ晶粒内部的主要沉淀相为棒/板状β’相、板状Q’相和针状β’’相，β’相和Q’相与-Al基体呈半共格关系。随着距焊缝中心距离减小，HAZ中β’’相减少而β’相和Q’相增多且明显粗化。HAZ晶界上非连续地分布着单质Si和Q相，晶界两侧的PFZ宽度有所减小。6005A-T6铝合金FSW接头的显微硬度分布呈W型。BM具有最高的硬度值，NZ的硬度较TMAZ更高，硬度最小值位于HAZ1内，随着距焊缝中心距离的增加，HAZ硬度显著升高。显微硬度的变化主要与晶内沉淀相的种类、尺寸和密度有关。拉伸性能测试结果表明，FSW接头的抗拉强度可达到母材强度的70%以上，延伸率可达母材的75%以上。FSW接头具有较母材更高的抗晶间腐蚀性能。母材晶界大量的第二相（单质Si相和Q相）与两侧的PFZ易形成原电池，导致其抗晶间腐蚀性能明显降低。NZ晶界上的第二相明显减少，因此显著提高了其抗晶间腐蚀性能。较系统地研究了焊接参数（焊接速度、旋转速度、下压量）和焊后时效处理对6005A-T6铝合金FSW接头微观组织及力学性能的影响规律。焊接速度的增大、搅拌头旋转速度的减小或下压量的降低，将导致焊接热输入减小，NZ和HAZ1晶粒细化，HAZ1沉淀相（β’相和Q’相）有细化的趋势，NZ硬度略有升高；反之则焊接热输入增大，NZ和HAZ1晶粒粗化，HAZ1沉淀相（β’相和Q’相）有粗化的趋势，NZ硬度略有降低。不适当的焊接参数导致产生孔洞、飞边等焊接缺陷，明显影响接头的拉伸性能及焊接质量。焊后自然时效处理对接头组织及硬度具有一定的影响。随着时效时间的延长，NZ和TMAZ发生自然时效现象，由过饱和固溶体→团簇→GP区，宏观表现为硬度的逐渐提高，而HAZ的组织和硬度在自然时效过程中无明显变化。焊后人工时效处理可使基体中固溶的溶质原子以沉淀相的方式析出，因此有利于提高接头的整体硬度水平和抗拉强度。优化焊接参数是提高接头力学性能的有效途径。采用二次回归正交试验方法优化的6005A-T6铝合金FSW参数为：焊接速度370mm/min，搅拌头旋转速度为1381r/min，下压量为0.11mm，FSW接头的抗拉强度计算值为222.4MPa，FSW接头抗拉强度的试验值为212.7MPa，计算值与试验值的误差为4.36%。依据粘着摩擦机制，建立了FSW过程的产热方程，计算表明，在FSW过程中对产热的贡献依次为轴肩凹端面（83.9%）、搅拌针侧面（11.0%）、轴肩外侧面（2.7%）、搅拌针底端面（2.4%）。因此，在温度场的有限元模拟计算时考虑轴肩外侧面的产热作用是必要的。在此基础上，通过ANSYS有限元模拟软件对FSW热过程进行了数值模拟分析。数值模拟得到的FSW接头不同位置的热循环曲线与实测结果基本吻合，并且能够较好的反映出焊接参数对接头温度场的影响规律。根据6005A-T6铝合金FSW接头沉淀相形貌的研究结果，将沉淀相形貌假定为更接近于实际情况的圆柱形，以析出热力学、生长动力学和强化理论为基础，建立了6005A铝合金组织演变模型和强化模型，从而实现了6005A-T6铝合金FSW接头组织的定量描述及接头显微硬度分布的预测，预测结果与试验结果基本一致。更多还原

【Abstract】 With the rapid development of high-speed railway technology, aluminum alloys havegained wide application in manufacturing of high-speed trains. As the key technology for thehigh-speed train manufacturing, the welding of aluminum alloys strongly influences on thequality and cost of trains and traffic safety. However, the conventional fusion welding foraluminum alloys often brings some defects such as porosities, hot cracks, distortion andsoftening. It is very difficult to solve these weldability problems because they are relatedwith physical properties of aluminum alloys. These problems limit more widespreadapplications of aluminium alloys in manufacturing of high-speed trains. As a solid-statejoining technique, friction stir welding (FSW) was considered to be the more desired methodfor joining aluminium alloys. Therefore, it has great theoretical significance and practicalvalues to carry out the study on microstructures and properties of friction stir welding jointsof6005A-T6aluminum alloy used in high-speed trains.Firstly, the characteristics of microstructures and properties of6005A-T6aluminumalloy FSW joints were studied systematacially. The results showed the FSW joint of6005A-T6aluminum alloy was composed of nugget zone (NZ), thermo-mechanicallyaffected zone (TMAZ), heat-affected zone (HAZ) and base metal (BM). The BM exhibitedfibrous microstructure, and a large number of fine needle-shaped β’’ precipitates, which arebasically coherent with-Al matrix, were observed within BM grains. Elemental Si and Qprecipitates continuously distributed at grain boundaries of the BM. Additionally, wideprecipitate-free zones (PFZ) were also found along grain boundaries of the BM. NZunderwent dynamic recrystallization and precipitates dissolution during FSW. TMAZ ischaracterized by elongated grains with a high density of dislocations. A few elemental Si wasfound at grain boundaries of NZ and TMAZ. The microstructure of HAZ was non-uniform.HAZ close to the TMAZ (named HAZ1) exhibited equiaxed grains, while HAZ close to the BM (named HAZ2) had fibrous grain morphology similar to BM. The predominantprecipitates in HAZ were rod/lath-shaped β’, lath-shaped Q’ and needle-shaped β’’ phases.With the distance decreasing away the weld center, the density of β’’ phases within HAZdecreased while β’ and Q’ phases within HAZ increased and coarsed. It should be noted thatβ’ and Q’ phases are semi-coherent with-Al matrix. Elemental Si and Q precipitatesdiscontinuously distributed at grain boundaries of HAZ. In addition, the PFZ width of HAZwas smaller than that of BM. The hardness distribution of FSW joint approximates to theW-type. BM has the highest hardness value, and NZ has higher hardness than TMAZ. Theminimum hardness is located at the HAZ1. The hardness of HAZ increased with increasingthe distance away the weld center. The hardness changes are associated with theintragranular precipitate evolution, i.e. the kind, size and density of the intragranularprecipitates. The results of tensile tests showed that the tensile strength and elongation ofFSW joints are higher than70%and75%of those of BM, respectively. Moreover, FSW jointexhibited stronger intergranular corrosion (IGC) resistance than BM. The presence ofcontinuous cathodic precipitates (Si and Q phases) at grain boundaries and the PFZ alonggrain boundaries of the BM resulted in the severe IGC susceptibility. The significantelimination of IGC in NZ is related to the low volume fraction of intergranular precipitates.Secondly, effects of welding parameters (welding speed, rotational speed and plungedepth) and postweld treatments on microstructures and mechanical properties of FSW jointsof6005A-T6aluminum alloy were studied. The results indicated that the increasing ofwelding speed, decreasing of rotational speed or decreasing of plunge depth resulted in thedecreasing of heat input, grain refining for NZ and HAZ1, precipitates (β’ and Q’) refining forHAZ1and slight increasing in hardness of NZ. Inadequate welding parameters should causedefect formation in the joint such as holes and flash, which deteriorate the tensile properties.Postweld natural aging (PWNA) treatment affected microstructures and hardness distributionof FSW joints. The microstructure evolution in NZ and TMAZ during PWNA wassupersaturated solid solution (SSS)→clusters→GP zones. As a result, the hardness increasedwith increasing PWNA time. However, the microstructure and hardness of HAZ wereunaffected by PWNA. Postweld artificial aging (PWAA) led the solution atoms to form β’’ precipitates which are benefit for increasing the hardness level and tensile strength of joints.Furthermore, the welding parameters were optimized by means of quadratic regressionorthogonal combination test, and the optimal parameters for6005A-T6aluminum alloy FSWare the welding speed of370mm/min, rotational speed of1381r/min, and plunge depth of0.11mm. The calculated tensile strength of FSW joint for the optimal parameters was222.4MPa and the experimental one was212.7MPa, the error between the calculated value andexperimental result being4.36%.Furthermore, an analytical model for the heat generation in FSW was established, basedon the hypothesis of sticking condition between tool and workpiece. Results showed that thecontributions for heat generation of shoulder concave, pin side, shoulder side and pin tip are83.9%,11.0%,2.7%and2.4%, respectively. This implies that it is neccesary to consider theeffect of shoulder side on heat generation in condition of numerical modelling for FSWthermal process. Hence, the temperature fields of joint during FSW were analyzed by meansof numerical simulation with the ANSYS software. Results showed excellent concordance inthermal cycles of different locations between simulated ones and experimentalmeasurements. The effects of welding parameters on the temperature distribution could beclarified by the thermal model.Finally, based on the microstructural investigation for6005A aluminum alloy joints,considering cylinder-shaped morphology for precipitates is closer to the real situation. Then,mirostructural evolution model and hardening model of6005A aluminium alloy wereestablished, based on the precipitation thermodynamics, growth kinetics and hardeningtheory. The quantitative description of the microstructural evolution during FSW and theprediction of mechanical properties for the FSW joints were realized by means of themirostructural evolution model and hardening model combined with the thermal model. Thecalculated results of hardness distribution of FSW joints showed concordance withexperimental measurements.更多还原