摘要
本文首次提出了随焊旋转挤压(WTRE)控制铝合金薄板焊件残余应力变形及防止热裂纹新方法。通过一特定形状的挤压头对电弧后方的焊缝区金属进行旋转挤压,其所产生的塑性延展能够降低焊缝区的纵向残余拉应力水平,从而起到减小甚至消除焊接残余变形的作用。该方法还能有效抑制铝合金焊接热裂纹的产生和扩展,从而为解决高强铝合金薄板焊件的大变形和高热裂问题提供了一种行之有效的新工艺。
根据随焊旋转挤压工作原理,搭建了WTRE装置设计框图,并研制出能很好满足试验要求的WTRE装置。
采用三维弹塑性有限元法对2A12T4铝合金薄板常规焊件和WTRE焊件的焊接应力和变形进行了模拟,并揭示了随焊旋转挤压控制焊接残余变形的机理:挤压头对焊缝区金属的塑性延展能够减小该区域中的纵向残余拉应力,根据内应力平衡原则,WTRE焊件的纵向平均压应力值也被降低,当其低于板件的临界失稳应力时,就能消除焊接失稳变形。对于尺寸为270mm×130mm×2mm的2A12T4铝合金薄板焊件,采用适当的随焊旋转挤压工艺能够将其最大纵向挠度降至0.3mm以下,只有常规焊状态的4%。
挤压头与焊枪的距离、挤压头对焊件的垂直压力、挤压头的直径、挤压头的转速和其在焊件上的作用位置是影响焊接变形控制效果的重要参数。挤压头与焊枪距离的确定原则为:在保证焊件挤压后冷却过程中不产生会导致失稳变形的残余压应力的前提下,挤压头与焊枪的距离应尽可能小。旋转挤压过程产生的摩擦热从两个相反的方面影响焊接变形的控制效果,少量的摩擦热能够提高挤压头下方金属的塑性,因而有利于焊接变形的控制,但过多的摩擦热会带来不期望的应力应变场,从而不利于焊接变形的控制。保持其它工艺参数不变,挤压头对焊件的垂直压力有一个最佳范围,在此范围内焊接变形的控制效果较好,过大或过小的垂直压力均不利于焊接变形的控制。旋转挤压对焊接变形的有效控制区域是焊缝和近缝区,当焊缝和近缝区金属均能被挤压头充分延展时,才能获得理想的变形控制效果。
根据BTR区间焊缝金属横向应变的产生机制,提出了横向拘束控制焊接热裂纹的方法。该方法控制焊接热裂纹的机理主要包括两个方面,一是通过限制焊件回转变形来减小BTR区间焊缝金属的横向拉伸应变,二是通过增加熔池两侧受热压缩区金属横向不均匀膨胀程度来增大BTR区间焊缝金属的横向压缩应变。
随焊旋转挤压能够有效抑制铝合金焊接热裂纹的产生和扩展,其机理基于对起弧端热裂纹的压合和对焊件回转变形的拘束作用。试验结果表明,挤压头对焊件的垂直压力和杆枪距是决定焊接热裂纹控制效果的两个重要参数,在合适的参数规范下,可获得无热裂纹的2A12T4铝合金焊件。
随焊旋转挤压对铝合金焊缝的表面形貌影响很大,常规焊缝表面比较粗糙,在焊趾部位存在明显应力集中,而随焊旋转挤压焊缝表面由均匀细密排列的圆环纹组成,表面较为平整光滑,降低了几何缺口效应的影响。
拉伸和疲劳试验结果表明,2A12T4铝合金WTRE接头的抗拉强度比常规焊接头平均提高了约40MPa,前者的疲劳寿命也明显高于后者。金相组织和拉伸断口形貌观察表明,随焊旋转挤压能够细化晶粒,减少气孔等焊接缺陷。
In this dissertation, a new method named Welding with Trailing Rotating Extrusion (WTRE) is proposed to control welding stress, deformation and hot cracking of aluminum alloy thin-plate weldments. This method makes use of a extrusion head with given form of end face to exert proper pressure, while the extrusion head is rotating, on metals in and around the weld behind the welding arc to produce plastic elongation, which can reduce the level of longitudinal residual tensile stress in the weld and its neighbouring zone, so mitigate or even eliminate welding residual deformation. WTRE can also restrain effectively the germination and expansion of hot cracks in aluminum alloy weldments and so comes forth to be an innovative technology used for solving the buckling distortion and hot cracking problems of high-strength aluminum alloy thin-plate weldments.
According to basic principle of WTRE, the design block diagram of WTRE device was set up and a workable WTRE device that meets the experimental requirement well was developed.
Three-dimensional elastic-plasticity FEM was adopted to simulate stress and deformation of conventional weldment and WTRE weldment in 2A12T4 aluminum alloy. The mechanism of controlling welding residual deformation by WTRE was disclosed that the plastic elongation of metals in and around the weld due to the action of extrusion head can reduce longitudinal residual tensile stress in the weld and its neighboring zone, and as a result decrease the level of average longitudinal residual compressive stress in the whole weldment. The buckling distortion of thin-plate weldments will disappear if only the average longitudinal residual compressive stress can be decreased to below of critical buckling stress by WTRE treatment. As for thin-plate weldments made of 2A12T4 aluminum alloy sheets of 2mm thickness, 130mm width and 270mm length, WTRE treatment can decrease their maximum longitudinal deflection to below 0.3mm, only 4% of conventional as-welded.
The control effect of welding residual deformation depends on some technical parameters, such as the distance between extrusion head and welding gun, the vertical pressure exerted on workpieces, the diameter of extrusion head, the rotary speed of extrusion head, and the force position of extrusion head. The distance between extrusion head and welding gun is determined by the principle that, on the premise of guaranteeing the weldments not to generate overmuch compressive stress that may result in buckling distortion after the rotating extrusion treatment, the distance between extrusion head and welding gun should be as near as possible. The friction heat produced in the process of rotating extrusion influences the control effect of welding deformation from two opposite aspects. A small quantity of friction heat avail the deformation control due to the contribution of heat to the enhancement of metal plasticity, but too much of it will bring about an unexpected stress-strain field, which goes against the mitigation of welding residual deformation. Under the condition of keeping other technical parameters unchanged, the vertical pressure exerted on workpieces has an optimal range. Either excessive pressure or deficient one goes against the control effect of welding residual deformation. The effective force area for WTRE treatment is the weld and its neighbouring zone. Only when metals both in and around the weld are elongated by extrusion head can an ideal control effect of welding residual deformation be attained.
On the basis of the generation mechanism of transverse strain in weld metal which temperature is in BTR, a new method that exerts transverse restraint on weldments to reduce the tendency of welding hot cracking is proposed in this dissertation. The mechanism of transverse restraint method includes two aspects: one is to decrease transverse tensile strain in BTR weld metal through restricting externally-expanded deformation of weldments; another is to increase transverse compressive strain in BTR weld metal through heightening the extent of non-uniform heat expansion in metal of heat compressional zone beside molten pool.
WTRE can inhibit the germination and expansion of welding hot cracks based on the mechanism of merging the inceptive cracks in the arc-starting end and restricting externally-expanded deformation of weldments. Experimental results showed that the vertical pressure exerted on weldments and the distance between extrusion head and welding gun are important technological parameters that have close relationship with control effect of welding hot cracking. When appropriate technological parameters are adopted, 2A12T4 aluminum alloy welds treated with WTRE have no hot cracks.
WTRE has positive effect on surface quality of aluminum alloy welds. The surface of conventional welds is rough and there exists sharp stress concentration at the weld toes, while the surface of WTRE welds is flat and composed of regularly distributed ring stripes, which decreases disadvantageous influence of geometric notch effect.
The results of tensile test showed that the tensile strength of 2A12T4 alum-inum alloy WTRE welded joints is about 40MPa more than that of conventional welded joints. The fatigue test revealed that WTRE can enhance fatigue life of aluminum alloy welded joints evidently. The observation of metallurgical structures and tensile fractures confirmed the efficiency of WTRE in refining crystal grains as well as reducing air holes and other welding defects.
引文
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