强制冷却对铝合金焊接接头组织性能的影响

发布时间：2018-09-10 14:16

【摘要】：6系铝合金的高强度、优良焊接性能等特点使其广泛应用于轨道车辆的车体制造中,由于其导热性好,焊接时需要大的热输入,这就导致焊接接头软化现象严重,力学性能明显降低,采用小的热输入容易导致未熔合等缺陷,因此如何在保证接头熔合良好前提下改善软化问题对6系铝合金的广泛应用有重要意义。本文对8 mm厚6系铝合金板材进行脉冲MIG焊接,焊接过程中焊缝底部垫板内部通过循环水进行冷却(水冷),将水冷接头与相同参数、自然冷却状态下焊接试板进行对比,通过微观组织和力学性能试验结果分析快速冷却对焊接接头组织性能的影响。与自然冷却相比,焊接过程中水冷明显降低了焊接峰值温度,提高了冷却速度,减小了高温停留时间;焊缝两侧的纵向残余应力有所降低;金相组织结果显示焊缝中心等轴晶尺寸有所减小,焊缝边缘处柱状晶区域变长,填充和盖面焊道部分熔化区晶粒重熔区域分别减小约40μm和80μm,热影响区晶粒长大程度明显减小;TEM试验观察到水冷接头析出相尺寸明显较小;显微硬度结果显示水冷接头软化区缩小了约6 mm,打底和盖面焊道焊缝处硬度提高约4 HV,软化区硬度值整体有所提高;拉伸结果表明断裂位置发生在软化区,水冷焊接接头抗拉强度提高约12 MPa,名义屈服强度提高约7 MPa,延伸率和断面收缩率与自然冷却条件接头无明显变化;断口扫描结果观察到大量等轴韧窝,说明断裂型式为塑性断裂,韧窝形态与自然冷却条件下接头断口无明显差别,说明两者塑性性能相差不大。试验结果表明焊接过程中快速冷却有利于提高软化区的强度,改善焊接接头的组织,提高焊接接头整体的力学性能。
[Abstract]:Due to its high strength and excellent welding properties, the 6 series aluminum alloy is widely used in the car body manufacture of rail vehicles. Due to its good thermal conductivity and large heat input during welding, the softening phenomenon of welded joints is serious. The mechanical properties are obviously reduced and the small heat input is easy to lead to defects such as non-fusion. So how to improve the softening problem on the premise of good fusion of the joints is of great significance to the wide application of 6-series aluminum alloys. In this paper, pulse MIG welding of 8 mm thick 6 series aluminum alloy sheet is carried out. In the process of welding, the bottom gasket of the weld is cooled (water-cooled) through circulating water, and the water-cooled joint is compared with the welding test plate under the same parameters and natural cooling condition. The effect of rapid cooling on the microstructure and properties of welded joints was analyzed by the results of microstructure and mechanical properties tests. Compared with natural cooling, water cooling decreases welding peak temperature, increases cooling rate and decreases residence time at high temperature, and the longitudinal residual stress on both sides of weld decreases. The results of metallographic structure show that the size of equiaxed grain in the center of the weld decreases, and the columnar region at the edge of the weld becomes longer. The grain remelting area in the partially melted zone of the filler and the cover welding pass was reduced by 40 渭 m and 80 渭 m, respectively, and the grain length of the heat-affected zone was significantly reduced by TEM test. The size of precipitated phase in the water-cooled joint was obviously smaller than that in the heat-affected zone. The results of microhardness show that the softening zone of water-cooled joint decreases about 6 mm, and the hardness of weld seam increases about 4 HV, and the tensile results show that the fracture occurs in the softening zone. Tensile strength of water-cooled welded joints increased about 12 MPa, nominal yield strength increased about 7 MPa, elongation and cross section shrinkage did not change obviously with natural cooling conditions and a large number of equiaxed dimples were observed by fracture scanning. It shows that the fracture type is plastic fracture and the dimple morphology has no obvious difference with the fracture surface under natural cooling condition, which indicates that the plastic properties of the two joints are not different from each other. The experimental results show that rapid cooling during welding process is beneficial to increase the strength of softening zone, improve the microstructure of welded joints and improve the mechanical properties of welded joints as a whole.
【学位授予单位】：河北科技大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG407

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