双轴肩搅拌摩擦焊工艺研究

发布时间：2018-03-27 09:05

本文选题：BT-FSW　切入点：夹具及搅拌头设计　出处：《兰州理工大学》2017年硕士论文

【摘要】：双轴肩搅拌摩擦焊是传统搅拌摩擦焊的一种改进,该方法既保留了搅拌摩擦焊半固态连接的优点,又改善了传统搅拌摩擦焊焊接效率较低、焊缝根部缺陷等问题,因此具有很高的研究和应用价值,无论在国内还是国外均受到了极高的关注。本文采用优化设计的双轴肩搅拌头对6mm厚LF21铝合金进行双轴肩搅拌摩擦焊试验,分析了主轴转速、轴肩下压量、焊接速度对焊缝接头的宏观形貌、微观组织、力学性能的影响。并将双轴肩搅拌摩擦焊与常规的单轴肩搅拌摩擦焊和单轴肩双面焊的焊缝接头进行组织、力学性能、耐蚀性能对比,分析了不同种类搅拌头对焊缝成形的影响。焊接试验表明:双轴肩搅拌摩擦焊焊缝接头在一定的工艺区间内成形良好、内外无缺陷的焊缝,焊缝横截面形状呈“马鞍状”,接头上、中、下部分的硬度分布整体均呈“W”形,母材硬度最高、热影响区最低,热机械影响区硬度介于母材与热影响区之间,随着转速的上升,焊缝热影响区和机械热影响区发生软化现象,硬度下降,区间扩大,焊缝断口从混合断裂逐渐变为韧性断裂,抗拉强度呈先上升后下降的趋势。随着焊接速度的增加焊缝单位长度热输入减少,焊缝抗拉强度和延伸率都得到提高,随着焊接速度的提升呈现先上升后下降的趋势。单轴肩搅拌摩擦焊焊缝呈“漏斗形”、双轴肩搅拌摩擦焊焊缝呈“马鞍形”焊缝中心存在一个“汤匙”形状的交汇区域、单轴肩双面焊搅拌摩擦焊焊缝整体形状与双轴肩搅拌摩擦焊形状相似,但焊缝中心处无“汤匙”形状的交汇区域。拉伸试验中三种焊缝的抗拉强度相近,从强度、位移曲线发现焊缝冷作硬化现象T-FSWBT-FSWD-FSW,导致焊缝延伸率D-FSWBT-FSWT-FSW。利用强度、位移曲线进行积分运算,发现双轴肩搅拌摩擦焊在断裂前具有最大的断裂能。三种接头都呈现韧性断裂。电化学腐蚀试验表面双轴肩搅拌摩擦焊的上、下表面耐腐蚀性具最强的,单轴肩搅拌摩擦焊焊缝下表面的耐腐蚀能力最差,与表面腐蚀试验现象一致。利用背散射和能谱对腐蚀表面进行分析发现,焊缝与轴肩接触面具有数量更多、更加弥散的Mg、Si第二相,一方面第二相可以优先形成致密的氧化物防止腐蚀进一步进行,另一方面第二相析出造成基材合金成分减少,抗腐蚀能力提高。
[Abstract]:Double shaft shoulder friction stir welding is an improvement of traditional friction stir welding. This method not only preserves the advantages of semi-solid joint of friction stir welding, but also improves the problems of low efficiency and weld root defects in traditional friction stir welding. Therefore, it has high research and application value, and has been paid great attention to both at home and abroad. In this paper, the double shaft shoulder friction stir welding test of 6mm thick LF21 aluminum alloy is carried out with the optimized design of double shaft shoulder stir head, and the spindle rotation speed is analyzed. The effects of shoulder compression and welding speed on the macroscopic morphology, microstructure and mechanical properties of the weld joint were studied. The effects of different kinds of stir head on weld forming are analyzed. The welding test shows that the weld joint formed well in a certain process range and has no internal and external defects. The shape of cross section of weld is "saddle shape", the hardness distribution of the upper, middle and lower parts of the joint is "W" as a whole, the hardness of the base metal is the highest and the heat affected zone is the lowest. The hardness of the thermo-mechanical affected zone lies between the base metal and the heat-affected zone, and increases with the increase of rotational speed. The weld heat affected zone and mechanical heat affected zone soften, the hardness decreases, the interval expands, and the fracture surface changes from mixed fracture to ductile fracture. With the increase of welding speed and the decrease of heat input per unit length of weld, the tensile strength and elongation of weld increased. With the increase of welding speed, the welding seam of single shaft shoulder friction stir welding is "funnel shape", and that of double shaft shoulder friction stir welding weld is "saddle shape", there is a "tablespoon" shape intersection region in the center of the weld, the welding speed of single shaft shoulder friction stir welding is "funnel-shaped". The overall shape of friction stir welding weld with single shoulder and double side welding is similar to that of friction stir welding with double shaft shoulder, but there is no "spoon" intersection region in the center of weld. The tensile strength of the three kinds of welds in tensile test is similar. The displacement curve found that the cold working hardening phenomenon T-FSWBT-FSWD-FSWW resulted in the weld elongation D-FSWT-FSWT-FSW.Using the strength and displacement curves, the integral calculation was carried out. It was found that the maximum fracture energy was obtained before the fracture of the double axle shoulder friction stir welding. All the three kinds of joints showed ductile fracture. The corrosion resistance of the lower surface was the strongest on the surface of the electrochemical corrosion test. Single shoulder friction stir welding has the worst corrosion resistance, which is consistent with the surface corrosion test. By using backscatter and energy spectrum to analyze the corrosion surface, it is found that there are more contact surfaces between the weld and the shaft shoulder. On the one hand, the second phase preferentially forms dense oxides to prevent further corrosion, on the other hand, the precipitation of the second phase results in the decrease of the composition of the base alloy and the improvement of corrosion resistance.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG453.9

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