Al-Zn-Mg-Cu铝合金厚板搅拌摩擦焊接头搅拌区微观组织

采用搅拌摩擦焊接(Friction stir welding, FSW)技术对20 mm厚Al-Zn-Mg-Cu铝合金板材进行了焊接,随后采用热电偶、电子背散射衍射技术和透射电镜等手段研究了接头的温度分布和搅拌区沿厚度方向微观组织的不均匀性。结果表明,焊接峰值温度沿板材厚度方向逐渐降低。距焊缝中心10 mm处,板材表面焊接峰值温度最高为430 ℃,板材底面峰值温度最低为302 ℃。温度梯度是导致晶粒尺寸沿厚度方向逐渐减小的主要原因,晶粒细化是连续动态再结晶、不连续动态再结晶和几何动态再结晶综合作用的结果。搅拌区晶粒没有明显的择优取向,且该区域的析出相发生溶解再析出,析出相主要为η′相。     

7050铝合金搅拌摩擦焊动态再结晶组织影响因素

进行了不同旋转频率和焊接速度下的7050铝合金搅拌摩擦焊试验,研究了搅拌头旋转频率和焊接速度对焊核区晶粒尺寸的影响．为进一步分析焊接参数影响焊核区晶粒尺寸的机理,进行了不同应变速率和变形温度下的等温压缩试验．分析了变形参数对动态再结晶的影响规律．结果表明,焊核区晶粒尺寸随搅拌头旋转频率的变化不大,随焊接速度的增加而减小．在发生完全动态再结晶的范围之内,再结晶晶粒尺寸随着lnZ值的增大而减小．焊接参数对z参数具有不同的影响规律,进而影响焊核区晶粒尺寸．     

5A05/C11000(T2)的搅拌摩擦焊接头组织结构分析

对厚4mm的铝合金5加5/纯铜C11000(T2)薄板的搅拌摩擦焊接头组织结构进行了研究.结果表明:当焊接前进速度为1～3mm/s,转速为1 100r/min时,在搅拌头轴肩下大约1 mm处出现无序混合的涡流状薄层间混结构.采用扫描电镜、电子探针、金相显微镜等测试方法对接头的显微组织特征进行了分析,焊核区及其两侧的塑性转变区主要由细小的动态再结晶晶粒组成.     

Effect of welding speed on microstructure and mechanical properties of Al−Mg−Mn−Zr−Tialloy sheet during friction stir welding

Effects of welding speed on the microstructure evolution in the stir zone (SZ) and mechanical properties of the friction stir welding (FSW) joints were studied by OM, XRD, SEM, TEM, EBSD and tensile testing. Compared with the base metal (BM), an obviously fine dynamic recrystallization (DRX) microstructure occurs in the SZ and the DRX grain size decreases from 5.6 to 4.4 μm with the increasing of welding speed. Fine DRX microstructure is mainly achieved by continuous dynamic recrystallization (CDRX) mechanism, strain induced boundary migration (SIBM) mechanism and particle stimulated nucleation (PSN) mechanism. Meanwhile, the geometric coalescence and the Burke−Turnbull mechanism are the main DRX grain growth mechanisms. Among all the welding speeds, the joint welded at rotation speed of 1500 r/min and welding speed of 75 mm/min has the greatest tensile properties, i.e. ultimate tensile strength (UTS) of (509±2) MPa, yield strength (YS) of (282±4) MPa, elongation (El) of (23±1)%, and the joint efficiency of 73%.     

搅拌摩擦焊中动态再结晶及硬度分布的数值模拟

使用率相关弹粘塑性本构模型模拟了搅拌摩擦焊接过程,并着重研究了过程参数对搅拌摩擦焊接动态再结晶过程以及搅拌区内材料硬度的影响．结果表明,在搅拌区内焊接构件上、下表面沿垂直于焊缝方向的硬度分布规律不同．焊接构件顶部材料的硬度分布符合实验得到的结果,即焊缝中心线附近材料硬度较低,热力影响区外材料硬度逐渐升高并最终达到母材的硬度;但是在焊接构件下表面并不显示这一硬度分布规律．搅拌区内材料的硬度与搅拌头转速无明显关系,但随焊速的增加而增加．焊接构件中部材料的晶粒尺寸大于焊接构件底部材料的晶粒尺寸,且搅拌区内晶粒尺寸随搅拌头转速的增加趋于均匀．     

Phase-field modelling of dynamic recrystallization process during friction stir welding of aluminium alloys

ABSTRACT

Numerical investigation on dynamic recrystallization in friction stir welds is of great significance to control the microstructure evolution and grains size in joints. The recrystallization and grain growth in the nugget centre of friction stir welded 6061 aluminium alloy are numerically simulated by combining the multiphase-field model with Kocks–Mecking dislocation model and employing the calculated temperature and strain rate variations with time in FSW process. The reliability of the model is verified by electron backscattered diffraction measurement results of grains size distribution at the same position. The specific reasons for different grain sizes under different levels of welding speed are quantitatively analysed. The ratios of recrystallization duration to deformation period and incubation period to deformation time are determined.     

LF6铝合金薄板搅拌摩擦焊工艺研究

试验研究了LF6铝合金薄板搅拌摩擦焊工艺，优化了工艺参数，分析了焊缝表面成形及焊接接头组织，力学性能，结果表明：LF6铝合金具有良好的搅拌摩擦焊性能，焊缝区发生动态再结晶，焊缝晶粒被细化，优化工艺参数可使焊接接头强度达到或高于母材。     

2024铝合金搅拌摩擦焊过程组织演化分析

采用\"急停快冷\"的方法对2024铝合金搅拌摩擦焊(FSW)的组织演化过程进行了研究,对搅拌头前、后方不同区域进行了EBSD测试,分析了FSW过程中组织演化的规律和机理.结果表明,在FSW前期发生了不连续动态再结晶,在FSW过程中与亚晶界吸收位错而取向差增加相关的连续动态再结晶是其动态形核的主导机制,在随后的冷却过程中晶粒长大成为终态组织,期间发生了部分的静态再结晶.     

基于元胞自动机AZ31镁合金微观组织模型

以AZ31镁合金在热压缩过程中微观组织演变为基础,结合元胞自动机模型(CA),建立了镁合金变形过程中再结晶晶粒尺寸模型和动态再结晶百分数模型。通过对铸态AZ31镁合金在不同变形条件下的热压缩实验,推导出镁合金的位错密度模型、临界位错密度模型、形核率模型和晶粒长大模型。结合元胞自动机具体演变规则,建立元胞自动机模型,并利用应力应变曲线及晶粒大小验证元胞自动机的模拟结果,验证该模型的准确性,结合实验数据和JMAK理论,推导出再结晶晶粒尺寸模型和动态再结晶百分数模型。借助DEFORM-3D分析软件得到镁合金在变形过程中,晶粒尺寸分布的变化情况以及动态再结晶百分数分布的变化情况。     

采用元胞自动机法模拟变温条件下低碳钢静态再结晶过程

基于亚晶异常长大形核机制,采用元胞自动机方法,建立了低碳钢静态再结晶仿真模型,模拟了退火均热温度对再结晶过程的影响。模拟结果表明,再结晶初期存在明显的孕育期,没有再结晶晶核产生;随着温度的升高,再结晶晶核优先在晶界和晶粒内储存能较大处产生;再结晶完成时的晶粒尺寸基本不受退火均热温度的影响;提高退火均热温度可以显著加快再结晶速度。     

焊接热循环对5083铝合金搅拌摩擦焊缝晶粒结构的影响

采用搅拌摩擦焊(friction stir welding, FSW)对2 mm厚的5083铝合金进行对接焊接,并获得了无缺陷的焊接接头. 利用光学显微镜(optical microscope,OM)和电子背散射衍射(electron back scatterdiffraction,EBSD)研究焊接热循环对焊缝晶粒结构的影响. 结果表明,随着搅拌头转速增大,应变率增大,动态再结晶主导形成的初始再结晶晶粒减小;焊接峰值温度从285 ℃升至421 ℃,冷却速度从32.3 ℃/s降至20.2 ℃/s,使焊缝的晶粒长大倾向增大,焊缝中心的平均晶粒尺寸从1.4 μm增加至8.2 μm. 在高热输入条件下,具有Goss型 {110}<001> 再结晶织构的晶粒长大速度较快,导致焊缝中心的剪切织构开始向再结晶织构转变,焊接热循环的变化不会显著改变焊缝的晶粒结构和织构类型,只对最终的晶粒尺寸和大角度晶界的比例有所影响,这种微观结构的稳定性归因于在整个较大的热输入范围内焊缝的晶粒细化由连续动态再结晶机制为主导.     

Properties and structure of friction stir welded joints in 1424 and V-1461 (Al–Li) alloys*

The effect of the friction stir welding (FSW) conditions on the structure of welded joint and mechanical properties of 1424 and V-1461 alloys is investigated. FSW is accompanied by the formation of a recrystallized fine-grain microstructure in the welded joint. It is shown that the increase of the heat input to the welded sheets does not increase the average grain size in the weld zone (the average grain size is 1.5–2.2 μm). The tensile strength of the welded joints depends on the welding conditions for both alloys. Special features of the microstructure formed in the zone of the welded joint are discussed and the effect of the microstructure on the mechanical properties of the welded joints and evolution under the effect of heat treatment after FSW are determined.     

Optimization of deformation parameters of dynamic recrystallization for 7055 aluminum alloy by cellular automaton

In order to simulate the microstructure evolution during hot compressive deformation, models of dynamic recrystallization (DRX) by cellular automaton (CA) method for 7055 aluminum alloy were established. The hot compression tests were conducted to obtain material constants, and models of dislocation density, nucleation rate and recrystallized grain growth were fitted by least square method. The effects of strain, strain rate, deformation temperature and initial grain size on microstructure variation were studied. The results show that the DRX plays a vital role in grain refinement in hot deformation. Large strain, high temperature and small strain rate are beneficial to grain refinement. The stable size of recrystallized grain is not concerned with initial grain size, but depends on strain rate and temperature. Kinetic characteristic of DRX process was analyzed. By comparison of simulated and experimental flow stress–strain curves and metallographs, it is found that the established CA models can accurately predict the microstructure evolution of 7055 aluminum alloy during hot compressive deformation.     

接触模型对搅拌摩擦焊接数值模拟的影响

在完全热力耦合搅拌摩擦焊接数值模型中采用两种接触模型--经典的Coulomb接触模型和修正的Coulomb接触模型,模拟了搅拌摩擦焊接过程,以分析不同接触模型对搅拌摩擦焊接过程数值模拟的影响.结果表明,对于低转速的搅拌摩擦焊接,两种模型的预测结果区别不大;但是对于高转速,由于界面摩擦剪切应力没有上限,采用经典的Coulomb接触模型无法模拟,需采用修正的Coulomb接触模型.搅拌头转速的增加不会改变搅拌摩擦焊接技术固态连接的本质.当采用高转速时,焊接构件上、下表面的变形趋于均匀.有利于得到均匀的显微结构.     

静态再结晶过程的元胞自动机模拟(Ⅱ)——晶粒尺寸和边数的变化

根据元胞自动机的模拟结果,研究了静态再结晶过程中晶粒尺寸和晶粒边数的变化情况及分布规律。结果表明：在整个再结晶过程中,单个形变晶粒的消失速度和单个再结晶晶粒的生长速度呈现随机性;但品粒的平均直径随着时间先明显减小后小幅增加;虽然晶粒的尺寸分布不均,但此再结晶过程不会形成超大尺寸的晶粒。再结晶完成时,形变量越大,晶粒平均直径越小;晶粒边数的分布呈现正态分布的特征。本研究进一步加深了对再结晶过程的认识。     

Microstructural analysis of the stir zone of Al alloy produced by friction stir spot welding

The present study applied friction spot joining (FSJ), which was recently developed as a lap joining technique of Al alloys, to two sheets of Al alloy 6061, 1 mm in thickness, and then examined the microstructural feature in the weld. The weld had the nugget-shaped stir zone around the exit hole of the probe, and the stir zone exhibited the equiaxed grain structure having finer grain size than that of the base material. The crystallographic texture analysis using electron backscattered diffraction method suggested that the material movement occurred along the rotating direction of the welding tool in the wide region including the stir zone. In the periphery of the nugget-shaped stir zone, which was characterized as the region having the finer grain size than that of the stir zone interior, any inclusions and precipitates were not found in the SEM scale. The weld was softened around the weld centre. The softening could be explained by dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSJ.     

微合金钢热变形组织与性能演变的CA模拟

建立了预测微合金钢热变形奥氏体动态再结晶组织与性能演变的元胞自动机模型.采用基于位错密度的动态再结晶理论,主要考虑动态再结晶的形核、晶粒长大,实现对动态再结晶过程晶粒形态、体积分数及晶粒尺寸的定量化表征及其演变过程的可视化描述,获得了位错密度及流变应力等参数.模拟得到的动态再结晶组织形貌及基于位错密度变化计算出的流变应力与实验结果吻合较好.     

搅拌摩擦增材成型过程仿真与显微性能预测

搅拌摩擦增材制造(FSAM)是一种新型固态增材技术。逐层重复搅拌摩擦成型,是FSAM的显著特点。在剧烈流动变形、热力耦合作用下,母材晶粒逐层破碎细化,形成新的再结晶组织,最终形成增材成型构件。以AZ31镁合金板件FSAM为研究对象,首先建立多层薄板增材成型的计算流体力学仿真模型,研究转速对材料流变、温度场、应变率的影响规律,并与实验测量值对比验证。其次计算增材区域镁合金材料热变形过程的Zener-Hollomon参数,利用经验公式法关联Z参数与增材区再结晶晶粒尺寸。最终,结合1000 r/min转速工况下的显微硬度实验测量结果,提出FSAM搅拌区再结晶组织显微硬度的快速预测算法。结果表明：随着增材板件层数增加,增材区平均晶粒尺寸减小,平均硬度值增大;随搅拌头转速的增大,增材区材料的应变率、再结晶晶粒尺寸都呈逐渐增大趋势,显微硬度呈下降趋势。     

基于数学模型的焊接过程中热影响区晶粒长大行为分析

在能量和曲率的基础上,建立了晶粒长大的能量-曲率驱动的元胞自动机模型。该模型能够准确建立晶粒生长过程中晶粒尺寸和时间、生长速率和温度、生长速度和曲率之间的关系,还能反映晶粒尺寸分布的时间不变性规律。建立了元胞自动机和实际时间的转变公式,实现了元胞自动机对现实时间中焊接热影响区的晶粒长大进行模拟。结果表明,模拟结果与热影响区晶粒的理论分布规律十分吻合。