【作者】 刘强；

【导师】 王快社；

【作者基本信息】 西安建筑科技大学 ， 材料加工工程， 2024， 博士

【摘要】 随着工业轻量化的快速发展,镁合金厚板焊接结构件的需求迅速增加。搅拌摩擦焊(Friction stirwelding,FSW)作为一种固相焊接技术,在焊接镁合金方面具有巨大潜力。但常规FSW焊接镁合金厚板时温度场分布不均匀,接头厚度方向微观组织差异大,严重影响接头力学性能。为此,本文创新性采用差动双轴肩搅拌摩擦焊接(Differential double-shoulder friction stir welding,DDS-FSW)新技术焊接了镁合金厚板。DDS-FSW具有独立控制的轴肩和搅拌针,可有效避免常规FSW焊接镁合金厚板上下表面温差大、材料流动不均匀和接头组织差异大等问题,有望实现镁合金厚板的高效优质连接。然而,目前关于镁合金厚板DDS-FSW的研究尚处于起步阶段,因此有必要对其接头组织性能开展深入研究。本文采用实验和模拟相结合的方式,系统研究了 AZ31镁合金厚板DDS-FSW温度场和塑性流场分布特征、微观组织演变、拉伸性能和疲劳性能。主要研究内容和结论如下:通过逆运算求解温度与摩擦系数之间的函数关系,建立了 AZ31镁合金厚板DDS-FSW数值模型,揭示了 DDS-FSW过程中温度场和塑性流场的分布特征。结果表明,DDS-FSW实现了镁合金厚板焊接温度场的均匀分布,与FSW相比,DDS-FSW厚度方向上焊接温度均匀性提高了 92%。焊接过程中材料流动速率主要受到搅拌工具转速和焊接温度的影响,增大轴肩或搅拌针的转速可增大其作用区材料流动速率和紊乱程度。基于宏观与微观相结合的检测方法,精准表征了接头晶粒尺寸和晶粒取向的分布特征,阐明了接头织构的形成机制。结果表明,DDS-FSW可有效提高接头晶粒尺寸均匀性,DDS-FSW121接头厚度方向上晶粒尺寸差仅为5μm。与FSW接头相比,晶粒尺寸均匀性提高了 67%。影响接头晶粒尺寸的主要因素为焊接温度和应变速率。此外,DDS-FSW增加了镁合金厚板接头织构组分,提高轴肩或搅拌针转速均会降低其各自作用区的织构强度,接头织构的形成机制为焊接过程中材料的差速流动和剪切变形。评价了接头拉伸性能,基于粘塑性自洽模型分析了拉伸变形过程中接头裂纹萌生位置塑性变形机制的演变规律。结果表明,DDS-FSW同时提高了镁合金厚板接头强塑性,DDS-FSW421接头抗拉强度系数达到了 93.8%。接头裂纹萌生的主要影响因素为“取向微界面”引起的应变在界面处传递受阻,从而诱导了接头裂纹萌生。FSW接头裂纹萌生位置塑性变形过程中主要的变形机制为基面<a>滑移和{1012}拉伸孪生,DDS-FSW121接头在变形初期以基面<a>滑移和{1012}拉伸孪生为主,变形后期以基面<a>滑移和柱面<a>滑移为主,DDS-FSW421接头主要的变形机制为基面<a>滑移和柱面<a>滑移。基于滞回能模型,建立了适用于镁合金厚板FSW接头和DDS-FSW接头的疲劳寿命预测模型,阐明了接头循环变形过程中的塑性变形机制,揭示了接头低周疲劳损伤机制。结果表明,0.3%应变幅下,DDS-FSW421接头疲劳寿命相比于FSW接头提高了 200%。低应变幅下接头主要变形机制为基面滑移和{1012}孪生,在高应变幅下FSW接头更易发生基面滑移,DDS-FSW接头更易发生{1012}孪生。相对于FSW,DDS-FSW降低了镁合金厚板接头搅拌区横截面分布的不对称性、弱化了“取向微界面”强度、提高了孪晶在接头中的比例和分布的均匀性,从而有效改善了接头低周疲劳性能。更多还原

【Abstract】 With the development of industrial lightweight,the demand for the welding structural components of magnesium(Mg)alloy thick plates has rapidly increased.Friction stir welding(FSW),as a solid-state welding technology,shows great potential for welding Mg alloys.However,the Mg alloy thick plates welded by conventional FSW exhibit uneven temperature distribution and significant differences in microstructure along the thickness direction,which severely affects the mechanical properties.Therefore,a differential double-shoulder friction stir welding(DDS-FSW)was first proposed to weld Mg alloy thick plates in this work.DDS-FSW,with independently controlled shoulder and pin,can effectively avoid drawbacks caused by conventional FSW,and is expected to achieve high-quality joints for Mg alloy thick plates.However,the current research on DDS-FSW of Mg alloy thick plates is still at the initial stage,and it is necessary to further study the microstructure and properties of the joint.In this paper,the distribution characteristics of the temperature field and plastic flow field,microstructure evolution,tensile and fatigue properties of DDS-FSW joints were systematically studied by the combination of experiment and simulation.The main contents and conclusions of this study are as follows:The function relationship between temperature and friction coefficient was determined by inverse calculation,and the numerical model of AZ31 Mg alloy thick plates during DDS-FSW was established,and the distribution characteristics of temperature and plastic flow field during DDS-FSW were revealed.The results indicated that DDS-FSW could make the welding temperature field in Mg alloy thick plates distribute evenly.Compared to FSW joints,the temperature uniformity of the DDSFSW141 joint along the thickness direction is increased by 92%.The material flow rate during welding was mainly influenced by the rotation speed of the stirring tool.Increasing the rotation speed of the shoulder or the pin could enhance the flow rate and turbulence of the material in the action zones.The grain orientation and grain size were accurately characterized based on the combination of macroscopic and microscopic detection methods,and the formation mechanism of the texture was clarified.The results indicated that DDS-FSW could effectively improve the grain uniformity of the joint.The grain size difference along the thickness direction of the DDS-FSW121 joint is only 5 μm,exhibiting a remarkable enhancement in grain size uniformity by 67%compared to the FSW joint.The grain size was mainly affected by the welding temperature and strain rate.In addition,texture components of Mg alloy thick plates joint were increased by DDS-FSW.Increasing the rotation speed of the shoulder or pin could decrease the texture strength in the action zones.The formation mechanism of the texture involves differential material flow and shear deformation during the welding.The tensile properties of the joints were evaluated,and the evolution of the plastic deformation mechanism at the crack initiation site during tensile deformation was analyzed based on the visco-plastic self-consistent model.The results indicated that DDS-FSW could simultaneously improve the strength and plasticity of Mg alloy thick plates joints,and the tensile strength coefficient of 93.8%could be achieved for DDSFSW421 joint.The main factor influencing crack initiation was the strain transfer obstruction caused by the "oriented micro-interface".During the plastic deformation,the primary deformation mechanisms at the crack initiation site of FSW joints involved basal<a>slip and {1012} twinning.At the early stages of deformation,the DDSFSW121 joint was primarily characterized by basal<a>slip and {1012} twinning,while in the later stages of deformation,it was dominated by basal<a>slip and prismatic<a>slip.The main deformation mechanisms for the DDS-FSW421 joint were basal<a>slip and prismatic<a>slip.A fatigue life prediction model was established for Mg alloy thick plates FSW and DDS-FSW joints based on the hysteresis energy model.The plastic deformation mechanism during cyclic deformation of joint was explained,and the low-cycle fatigue damage mechanism was investigated.The results indicated that at the 0.3%strain amplitude,the fatigue life of the DDS-FSW421 joint was increased by 200%compared to the FSW joint.The main deformation mechanisms of the two joints were basal slip and {1012} twinning at the low strain amplitude.At high strain amplitude,basal slip was more likely to occur in the FSW joint while {1012} twinning occurred in the DDSFSW joint.Compared with FSW,DDS-FSW could reduce the asymmetric distribution of the stir zone,weaken the strength of "orientated micro-interface",and increased the proportion and distribution uniformity of twins,thus effectively improving the lowcycle fatigue properties of the joints.更多还原