6061铝合金薄板无针搅拌摩擦焊工艺研究与微观组织分析

发布时间：2018-10-07 18:32

【摘要】：6000系铝合金作为性能优异且使用广泛的轻质合金材料,拥有成为汽车轻量化制造原材料的巨大潜力。然而6000系铝合金使用传统焊接方法难以获得优质的焊缝,铝合金薄板更是如此。因此对于6000系铝合金薄板连接技术的研究十分必要,近年来出现的一种名为无针搅拌摩擦焊的新型焊接技术能获得优质稳定的薄板铝合金焊缝,但目前缺少对于6000系铝合金薄板无针搅拌摩擦焊工艺的系统性研究。本研究选择1mm厚6061-T6铝合金薄板作为焊接材料,使用无针FSW技术,针对影响接头性能的主要焊接参数包括:主轴转速、焊接速度、下压量设计实验方案,获得对焊与搭焊两种形式的焊缝。测试焊缝力学拉伸性能与横截面显微硬度;采用金相显微镜观察焊缝区缺陷;电子背散射衍射技术(EBSD)观察焊缝区域材料晶粒尺寸与形状;扫描电镜(SEM)观察分析拉伸断口形貌。主要结论如下:6061铝合金薄板无针FSW对焊实验所获得的焊缝成形较为美观。焊缝抗拉强度随焊接速度的增加呈现先升后降的趋势,峰值为198MPa,达到母材的73.4%。延伸率随主轴转速的升高而变大,最大值为20%。下压量对焊缝抗拉强度有一定影响,本研究中最优下压量为0.1mm。自然时效能够有效提升焊缝的力学拉伸性能。焊缝横截面显微硬度分布为“W”形或“一”字形,抗拉强度高的焊缝显微硬度最小值更大。拉伸试样断裂位置基本位于焊缝中心处。光学显微观察结果显示焊缝背部没有未焊透缺陷。本研究最佳对焊工艺方案为:主轴转速1500rpm,焊接速度300mm/min,下压量0.1mm,焊后自然时效一周以上。6061铝合金薄板无针FSW搭焊焊缝外观无明显缺陷,焊缝力学拉伸性能随焊接速度升高为先升后降的趋势,最大值为199.5MPa,达到母材的73.9%。焊缝横截面显微硬度测试结果表明,无针FSW搭焊接头上、下板显微硬度为“W”或“一”字形分布;上板硬度下降区域大于下板;力学拉伸性能好的无针FSW搭焊接头显微硬度最小值更大。拉伸试样断裂于上板后退侧热影响区。光学显微观察结果显示,力学拉伸性能好的焊缝上、下板之间的融合区域更宽,高转速的焊缝前进侧容易出现Hook缺陷。最优工艺参数为:主轴转速1500rpm,焊接速度300mm/min,下压量0.1mm。采用EBSD观察性能最好的无针FSW对焊与搭焊焊缝,发现对焊与搭焊焊缝上板焊缝中心位置的组织是经历剧烈塑性变形后的细小等轴晶,热机影响区同时受塑性变形与热循环作用,晶粒大小不均匀,热影响区只受热循环作用,晶粒最大;搭焊焊缝下板都是受热长大的粗大晶粒。从拉伸断口的SEM结果来看,高延伸率对焊焊缝为韧性拉伸断裂,搭焊焊缝都为脆性断裂。
[Abstract]:As a light alloy material with excellent properties and wide use, 6000 series aluminum alloy has great potential as raw material for automobile lightweight manufacturing. However, it is difficult to obtain high quality welding seam for 6000 series aluminum alloy by traditional welding method, especially for aluminum alloy sheet. Therefore, it is necessary to study the joining technology of 6000 series aluminum alloy sheet. In recent years, a new type of welding technology called needle-free friction stir welding can obtain high quality and stable thin aluminum alloy weld. However, there is a lack of systematic research on the non-needle friction stir welding process for 6000 series aluminum alloy sheet. In this study, 1mm thick 6061-T6 aluminum alloy sheet is selected as welding material, and the needle-free FSW technology is used. The main welding parameters that affect the performance of joint include: spindle speed, welding speed, down pressure design experimental scheme. Two types of weld seam, butt welding and lap welding, are obtained. The mechanical tensile properties and cross section microhardness of the weld were tested; the defects in the weld zone were observed by metallographic microscope; the grain size and shape of the weld material were observed by electron backscatter diffraction (EBSD); and the tensile fracture morphology was observed and analyzed by SEM (SEM). The main conclusions are as follows: 1. The weld shape obtained by needle-free FSW butt welding experiment is more beautiful. The tensile strength of the weld increases first and then decreases with the increase of welding speed, and the peak value is 198 MPA, reaching 73.4% of the base metal. The elongation increases with the increase of spindle speed, and the maximum value is 20. The drop pressure has a certain effect on the tensile strength of the weld, and the optimum drop pressure is 0.1 mm. in this study. Natural aging can effectively improve the mechanical tensile properties of the weld. The microhardness distribution of cross section of weld is "W" or "one", and the minimum value of micro-hardness of weld with high tensile strength is larger. The fracture position of tensile specimen is basically located at the center of weld. The results of optical microscopic observation showed that there was no penetration defect in the back of the weld. The optimum butt welding process scheme is as follows: spindle speed 1500 rpm, welding speed 300 mm / min, pressure 0.1 mm, welding appearance no obvious defect in the welding seam of natural aging more than one week after welding, without needle FSW welding of aluminum alloy sheet, and the welding speed is 300mm / min, and the welding speed is 0.1 mm / min. With the increase of welding speed, the mechanical tensile properties of the weld increased first and then decreased, and the maximum value was 199.5MPa, which reached 73.9 of the base metal. The results of microhardness test of weld cross section show that the microhardness of the lower plate is "W" or "one" distribution on the top of the welded joint without needle FSW, and the decreasing area of the hardness of the upper plate is larger than that of the lower plate. The minimum microhardness of needle-less FSW lap welded joints with good tensile properties is higher. The tensile specimen is fractured in the heat affected zone of the receding side of the upper plate. The results of optical microscopic observation show that the fusion area between the lower plates is wider in the weld with good mechanical tensile properties, and the Hook defects are easy to appear in the forward side of the weld at high speed. The optimum technological parameters are as follows: spindle rotation speed 1500 rpm, welding speed 300 mm / min, down pressure 0.1mm. EBSD was used to observe the best non-needle FSW butt welding and lap welding weld. It was found that the microstructure of weld center of butt welding and lap weld was small equiaxed crystal after severe plastic deformation, and the influence zone of heat engine was affected by plastic deformation and thermal cycling simultaneously. The grain size is not uniform, the heat affected zone is only affected by heat cycling, the grain size is the largest, and the bottom plate of lap welding weld is the coarse grain grown by heating. According to the SEM results of tensile fracture, the high elongation butt weld is ductile tensile fracture, while the lap weld is brittle fracture.
【学位授予单位】：重庆交通大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG453.9;TG146.21

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