7075铝合金搅拌摩擦焊接头强度失配及变形损伤机理研究
发布时间：2018-10-29 18:17
【摘要】：随着列车轻量化和高速化,给铝合金车体的焊接技术及其焊接结构可靠性带来更大的挑战。搅拌摩擦焊(FSW)是铝合金车体最具潜力的材料连接技术。7075铝合金由于比强度、比刚度较高、塑性较好,在载运工具制造以及航空航天等领域应用广泛。本文利用搅拌摩擦焊(FSW)技术,对厚度为6 mm的7075铝合金进行不同工艺平板对焊试验,使用一系列微观/纳观组织表征手段对接头焊核区(WNZ),热机影响区(TMAZ),热影响区(HAZ)及母材(BM)中的沉淀相尺寸及形状进行定量表征,并借助透射电镜(TEM)对不同形状沉淀相的高分辨图像进行观测,通过晶格条纹间距计算,实现对沉淀相种类的精确表征,以探求7075铝合金FSW接头不同微区强度失配机理。在此基础上,对接头进行FSW焊后常规热处理与分级热处理试验,并对热处理后接头微观组织以及显微硬度分布进行表征,进而优化热处理工艺参数。最后,借助拉伸卸载以及原位拉伸试验对7075铝合金FSW接头变形损伤机理进行研究。研究结果表明:(1)7075铝合金FSW接头显微硬度分布呈极其不均匀分布的“W”型,微区显微硬度值与微区中沉淀相种类和尺寸、晶粒尺寸及加工硬化行为有关;(2)接头中沉淀相主要有棒状MgZn2相、椭圆状AlCuMg相以及胶囊状Al2CuMg相三种;强化相种类不同,微区显微硬度值不同,AlCuMg相和Al2CuMg相强化效果好于MgZn2相;WNZ中沉淀相主要是AlCuMg和Al2CuMg,加之细晶强化,显微硬度较高,达到145 HV;相比WNZ,TMAZ中AlCuMg和Al2CuMg相对较少,MgZn2相对较多,显微硬度降低;HAZ中MgZn2相对更多,加工硬化和细晶强化效果也较弱,显微硬度进一步下降,HAZ与TMAZ交界处显微硬度达到整个接头的最低值,约为125 HV;(3)FSW接头中WNZ、TMAZ、HAZ及BM均出现无析出区(PFZ),PFZ的存在使接头在一定程度上发生软化;(4)7075铝合金最佳FSW工艺为搅拌头旋转速度800 r/min,焊接速度300mm/min,FSW接头抗拉强度为514 MPa,达到BM的91%。相比于原始接头,常规热处理与分级热处理后,接头中沉淀相经历了固溶处理+人工时效、沉淀相发生显著细化,显微硬度明显提高并趋于均匀分布,常规热处理与分级热处理后FSW接头平均显微硬度分别达到165 HV、160 HV;最佳热处理工艺为固溶温度470℃、时效时间36 h,最佳分级热处理工艺为一级时效时间12 h、二级时效时间12 h,常规热处理效果好于分级热处理。(5)7075铝合金FSW接头在拉伸过程中,微裂纹首先在WNZ底部形核,接着又在前进侧HAZ与TMAZ交界处形核,最终前进侧HAZ与TMAZ交界处的裂纹发展成主裂纹,主裂纹沿前进侧HAZ与TMAZ的交界处向前扩展导致接头断裂。前进侧HAZ与TMAZ的交界处是整个7075铝合金FSW接头的最薄弱环节。
[Abstract]:With the lightening and high speed of the train, the welding technology of aluminum alloy body and the reliability of welding structure are more challenging. Friction stir welding (FSW) is the most potential material joining technology for aluminum alloy body. 7075 aluminum alloy is widely used in the manufacture of carrier tools and aerospace due to its high specific strength, high specific stiffness and good plasticity. In this paper, friction stir welding (FSW) technique is used to test the plate butt welding of 7075 aluminum alloy with a thickness of 6 mm. A series of microscopic / nano microstructure characterization methods are used to characterize the (WNZ), thermal impact zone (TMAZ),) in the nuke zone of the joint. The size and shape of precipitate phase in heat-affected zone (HAZ) and base metal (BM) were quantitatively characterized. The high-resolution images of precipitated phases with different shapes were observed by transmission electron microscopy (TEM), and the lattice fringe spacing was calculated. The types of precipitated phases were accurately characterized to find out the mismatch mechanism of different microstructures in 7075 aluminum alloy FSW joints. On this basis, the FSW welding routine heat treatment and graded heat treatment tests were carried out, and the microstructure and microhardness distribution of the joints after heat treatment were characterized, and the heat treatment process parameters were optimized. Finally, the deformation damage mechanism of 7075 aluminum alloy FSW joint was studied by tensile unloading and in situ tensile test. The results show that: (1) the microhardness distribution of 7075 aluminum alloy FSW joints is "W" type with extremely uneven distribution, and the microhardness value is related to the type and size of precipitate phase, grain size and work hardening behavior; (2) there are three kinds of precipitated phases in the joint: rod-like MgZn2 phase, ellipsoid AlCuMg phase and capsule Al2CuMg phase, the strengthening effect of AlCuMg phase and Al2CuMg phase is better than that of MgZn2 phase. The precipitation phase in WNZ is mainly composed of AlCuMg and Al2CuMg, and fine grain strengthening. The microhardness of the precipitated phase is higher than that of WNZ,TMAZ. Compared with that of WNZ,TMAZ, there are fewer AlCuMg and Al2CuMg, more MgZn2 and lower microhardness. There are more MgZn2 in HAZ, and the effect of work hardening and fine grain strengthening is also weaker. The microhardness of HAZ / TMAZ joint decreases further. The microhardness at the junction of HAZ and TMAZ reaches the lowest value of the whole joint, about 125 HV;. (3) the absence of (PFZ), PFZ in WNZ,TMAZ,HAZ and BM in FSW joints makes the joints soften to some extent; (4) the optimum FSW process for 7075 aluminum alloy is that the rotating speed of the stirring head is 800rmin, the welding speed is 300mm / min, the tensile strength of the joint is 514 MPa, and the tensile strength of the joint is 91cm of BM. Compared with the original joint, after conventional heat treatment and graded heat treatment, the precipitation phase in the joint experienced the artificial aging of solution treatment, the precipitation phase was refined significantly, and the microhardness increased obviously and tended to uniform distribution. The average microhardness of FSW joints after conventional heat treatment and graded heat treatment is 165 HV,160 HV;, respectively. The optimum heat treatment process is as follows: solution temperature 470 鈩，
本文编号：2298435
[Abstract]:With the lightening and high speed of the train, the welding technology of aluminum alloy body and the reliability of welding structure are more challenging. Friction stir welding (FSW) is the most potential material joining technology for aluminum alloy body. 7075 aluminum alloy is widely used in the manufacture of carrier tools and aerospace due to its high specific strength, high specific stiffness and good plasticity. In this paper, friction stir welding (FSW) technique is used to test the plate butt welding of 7075 aluminum alloy with a thickness of 6 mm. A series of microscopic / nano microstructure characterization methods are used to characterize the (WNZ), thermal impact zone (TMAZ),) in the nuke zone of the joint. The size and shape of precipitate phase in heat-affected zone (HAZ) and base metal (BM) were quantitatively characterized. The high-resolution images of precipitated phases with different shapes were observed by transmission electron microscopy (TEM), and the lattice fringe spacing was calculated. The types of precipitated phases were accurately characterized to find out the mismatch mechanism of different microstructures in 7075 aluminum alloy FSW joints. On this basis, the FSW welding routine heat treatment and graded heat treatment tests were carried out, and the microstructure and microhardness distribution of the joints after heat treatment were characterized, and the heat treatment process parameters were optimized. Finally, the deformation damage mechanism of 7075 aluminum alloy FSW joint was studied by tensile unloading and in situ tensile test. The results show that: (1) the microhardness distribution of 7075 aluminum alloy FSW joints is "W" type with extremely uneven distribution, and the microhardness value is related to the type and size of precipitate phase, grain size and work hardening behavior; (2) there are three kinds of precipitated phases in the joint: rod-like MgZn2 phase, ellipsoid AlCuMg phase and capsule Al2CuMg phase, the strengthening effect of AlCuMg phase and Al2CuMg phase is better than that of MgZn2 phase. The precipitation phase in WNZ is mainly composed of AlCuMg and Al2CuMg, and fine grain strengthening. The microhardness of the precipitated phase is higher than that of WNZ,TMAZ. Compared with that of WNZ,TMAZ, there are fewer AlCuMg and Al2CuMg, more MgZn2 and lower microhardness. There are more MgZn2 in HAZ, and the effect of work hardening and fine grain strengthening is also weaker. The microhardness of HAZ / TMAZ joint decreases further. The microhardness at the junction of HAZ and TMAZ reaches the lowest value of the whole joint, about 125 HV;. (3) the absence of (PFZ), PFZ in WNZ,TMAZ,HAZ and BM in FSW joints makes the joints soften to some extent; (4) the optimum FSW process for 7075 aluminum alloy is that the rotating speed of the stirring head is 800rmin, the welding speed is 300mm / min, the tensile strength of the joint is 514 MPa, and the tensile strength of the joint is 91cm of BM. Compared with the original joint, after conventional heat treatment and graded heat treatment, the precipitation phase in the joint experienced the artificial aging of solution treatment, the precipitation phase was refined significantly, and the microhardness increased obviously and tended to uniform distribution. The average microhardness of FSW joints after conventional heat treatment and graded heat treatment is 165 HV,160 HV;, respectively. The optimum heat treatment process is as follows: solution temperature 470 鈩，
本文编号：2298435
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