厚板搅拌摩擦焊焊缝金属塑性流动的力学机制研究

In practice, the cone-shaped stir pin and low rotational speed are often used during the friction stir welding for thick plates, and cold welding conditions are often used for some high strength aluminum alloys. But for a certain thickness of aluminum alloy plate, there are not any existed theory to be used for designing the stir tool and welding parameters, the weld defects, such as the aluminum clad penetration and loose structure, often occurs in the weld. For these, the proposer suggested based on the "sucking-extruding" theory for materials flow in the thickness direction, at the developmental stage of the extruded zone around the pin tip, the extruding force that the extruded zone exerts on its surrounding area and the restrain forces that the materials around the extruded zone exert on the extruded zone are extremely important for the weld formation. In this project, by means of using different screw thread on the pin surface, the extruding force is varied because the materials amount flowed into the extruded zone per unit of time is changed; by means of changing the shape and size of the pin, the chemical composition and the heat treatment state of metal and temperature field of the joint, the restrain forces that inhibit the outward extension of extruded zone around the pin tip and upward flow of metals around the pin are changed. And then, the relationship between the weld formation and the restrain forces for the extension of extruded zone will be analyzed, aimed to disclosure the effects of welding parameters, tool shape, the chemical composition and mechanical properties of metal, temperature field, etc. on the weld formation. The results of the project can provide theoretical support for the optimization of weld parameters and the design of tool for friction stir welding of thick plates.

厚板搅拌摩擦焊接中，常采用锥形搅拌针、低搅拌头旋转速度焊接，某些高强铝合金常采用冷规范焊接；但针对具体材料与板厚，搅拌头及焊接工艺参数设计仍缺乏理论指导，焊缝中易于形成包铝层伸入、疏松等缺陷。申请者认为，沿搅拌针表面螺纹流动的金属在搅拌针端部积聚形成的挤压区扩展过程中，挤压区对周边材料的挤压力及所受到的约束力对焊缝成形有显著影响。本项目通过改变搅拌针表面螺纹尺寸，使单位时间内流入挤压区的金属量发生变化，从而改变挤压力的大小；通过改变搅拌针形状与长度、材料成分及热处理状态、焊接温度场，改变邻近搅拌针端部的材料焊接时的力学性能，使挤压区朝周边扩展所受到的阻力和沿搅拌针外侧向上流动形成飞边的阻力相对大小发生变化；研究焊缝成形与挤压区扩展时所受到的约束作用的关系，掌握焊接工艺参数、搅拌针形状、材料化学成分及力学性能等对焊缝成形的影响规律，为厚板搅拌摩擦焊接工艺参数的优化、搅拌头的设计提供理论依据。

采用厚度为20 mm厚的7075、2024、5A06等铝合金板进行搅拌摩擦焊，通过改变搅拌针形貌、锥度、端部形貌，轴肩形貌、焊前预热温度等对焊缝塑性金属流动行为及其组织性能的影响，研究厚板搅拌摩擦焊焊缝金属塑性流动的力学机制。结果表明，厚板焊接时焊缝金属的塑性流动受制于焊缝底部区域的温度及材料的变形抗力。一般情况下，焊缝上表层、底部温度相差较大，可达100℃；同一厚度方向上，焊缝前进边温度比返回边温度高约15℃，导致焊缝中上部力学性能最好，底部性能最差。在焊缝轴肩作用区和焊核之间常出现疏松、孔洞等缺陷，常伴随着焊缝底部焊核体积扩展不足、焊核体积较小，分析认为与焊缝底部金属温度较低、周边金属对挤压区内塑化金属阻力较大、金属流动性较差有关。增加搅拌针表面螺纹头数或提高搅拌针表面螺纹截面积，有助于增加单位时间内流入挤压区的金属量，焊核体积增大、底部区温度提高，进而提高焊核对周围金属的挤压力、降低周围金属的变形抗力，使得在焊核外侧的底部金属朝焊缝上部塑性流动，在轴肩的阻碍下迫使上部金属朝搅拌针中心流动，填充焊缝上部的空腔，形成致密的焊缝。搅拌针端部形状不同，改变其对周围金属的作用力方向，从而影响焊缝底部金属的流动状态，改善焊缝底部成形。材料热处理状态通过摩擦热量和变形抗力影响焊缝金属的流动行为，但情况较为复杂；7075-T6铝合金变形抗力较大，但是7075-T6铝合金与搅拌头的摩擦系数较高，摩擦产热较7075-O铝合金多，焊缝底部温度比7075-O铝合金焊核约高18℃，降低了焊接时焊核周围金属的变形抗力，焊核底部宽度大于7075-O铝合金的焊核宽度；7075-T6铝合金与7075-O铝合金对接时，则焊核多偏于较软的7075-O铝合金一侧。材料成分对焊缝成形的影响同样与摩擦产热和变形抗力有关。在50-300℃范围内加热底板时，加热温度为200℃时焊核底部宽度最大、焊缝成形最好。总体而言，厚板焊接时提高焊缝底部温度，可以优化焊缝及近缝区金属的流动行为，防止焊接缺陷的产生，提高焊接接头的力学性能。