TC4钛合金背面加热辅助FSW的接头成形与性能研究

发布时间：2018-09-04 12:32

【摘要】：钛合金因具有密度小、耐热性好、强度高和抗腐蚀性好等优点而被广泛应用于现代航空制造工业中。然而,传统熔焊方法会造成如气孔、热裂纹、高残余应力和大变形等缺陷,严重降低钛合金接头的性能。搅拌摩擦焊(Friction stir welding,FSW)是一种固相连接技术,其焊接过程中的温度峰值低于材料的熔点,因此可避免诸多熔化焊缺陷。近年来,随着FSW技术在低熔点材料上的应用日趋成熟,国内外学者将更多的精力用于钛合金等高熔点材料的焊接过程中,以期获得良好的焊接接头。众所周知,FSW过程中搅拌头轴肩与工件之间的产热远大于搅拌针且TC4钛合金的热导率很低,因此焊接过程中沿焊缝垂直方向会存在较大的温度梯度,进而在焊缝底部出现撕裂缺陷。该撕裂缺陷的存在使得钛合金FSW的工艺参数范围较窄,不利于在钛合金结构件中的实际应用与推广。为解决此问题,本文提出一种背部加热辅助的FSW工艺。首先,研究常规工艺下TC4钛合金接头成形规律和搅拌头的磨损。当搅拌头转速为200rpm时,焊缝表面氧化现象严重且飞边较大,焊缝内的撕裂缺陷几乎贯穿整个焊缝底部。随着搅拌头转速的降低,接头表面成形趋于良好,内部撕裂缺陷逐渐减小。当转速为100rpm时,FSW接头内的缺陷消失。针对搅拌头的镦粗、粘结、机械磨损和氧化磨损等磨损形式,本文主要采取了低焊速、气体保护和预置小孔等措施。其次,研究背部加热工艺下TC4钛合金接头成形规律。与常规工艺相似,接头表面同样存在飞边、凹陷和氧化等缺陷,但背部加热可明显扩宽工艺参数范围。当搅拌头转速为200rpm时,接头内部仍存在撕裂缺陷,但其尺寸相对于常规工艺下明显减小。当转速降低至150rpm时,撕裂缺陷消失。随着搅拌头转速继续降低,接头内部无缺陷且表面成形良好,原因是背部加热工艺减小了沿焊缝垂直方向的温度梯度。另外,背部加热工艺可增强TC4钛合金材料的流动性,减小搅拌头的磨损。由于背部加热工艺减小了温度梯度,接头中的组织更加均匀,接头中整个焊核区均为片层状组织。最后,对常规工艺和背部加热工艺下接头的力学性能进行测试。当搅拌头转速为100rpm时,常规工艺下的接头抗拉强度为1014MPa,达到母材的98.9%;延伸率为6.8%,为母材的54.4%,接头断裂在焊核区边缘。背部加热工艺下接头的抗拉强度为958.45MPa,比常规工艺略低,但是仍可达到母材的93.6%。两种工艺下接头焊核区硬度均高于母材。
[Abstract]:Titanium alloys are widely used in modern aviation manufacturing industry due to their advantages of low density, good heat resistance, high strength and good corrosion resistance. However, traditional welding methods can cause defects such as porosity, hot cracks, high residual stress and large deformation, which seriously reduce the properties of titanium alloy joints. Friction stir welding (Friction stir welding,FSW) is a solid-state bonding technique, in which the peak temperature in the welding process is lower than the melting point of the material, so many defects can be avoided. In recent years, with the application of FSW technology in low melting point materials, scholars at home and abroad will devote more attention to the welding process of high melting point materials, such as titanium alloys, in order to obtain good welding joints. It is well known that the heat production between the shaft shoulder of the mixing head and the workpiece is much higher than that of the stirring needle and the thermal conductivity of the TC4 titanium alloy is very low during the welding process, so there will be a large temperature gradient along the vertical direction of the weld during the welding process. A tear defect appears at the bottom of the weld. Due to the existence of the tear defect, the range of process parameters of titanium alloy FSW is narrow, which is not conducive to the practical application and popularization of titanium alloy structural parts. In order to solve this problem, a back heating assisted FSW process is proposed in this paper. Firstly, the forming law of TC4 titanium alloy joint and wear of stirring head were studied. When the speed of the stirring head is 200rpm, the surface oxidation of the weld is serious and the flange is large, and the tear defect in the weld almost runs through the bottom of the weld. With the decrease of the rotating speed of the mixing head, the surface forming of the joint tends to be good, and the internal tear defect decreases gradually. When the speed is 100rpm, the defects in the joint disappear. Aiming at the wear forms such as upsetting, bonding, mechanical wear and oxidation wear of agitator, this paper mainly adopts some measures such as low welding speed, gas protection and preset small holes. Secondly, the forming law of TC4 titanium alloy joint under back heating is studied. Similar to the conventional process, the surface of the joint also has defects such as flange, depression and oxidation, but the back heating can obviously widen the range of process parameters. When the speed of the mixing head is 200rpm, there is still tear defect inside the joint, but its size is obviously smaller than that in the conventional process. When the rotational speed is reduced to 150rpm, the tear defect disappears. As the rotating speed of the mixing head continues to decrease, there is no defect inside the joint and the surface is well formed. The reason is that the back heating process reduces the temperature gradient along the vertical direction of the weld. In addition, the back heating process can enhance the fluidity of TC4 titanium alloy material and reduce the wear of the stirring head. Due to the decrease of the temperature gradient in the back heating process, the microstructure of the joint is more uniform, and the whole nugget zone in the joint is lamellar. Finally, the mechanical properties of joints under conventional and back heating processes are tested. When the speed of the stirring head is 100rpm, the tensile strength of the joint is 1014 MPA, which reaches 98.9 of the base metal, and the elongation is 6.8, which is 54.4% of the base metal, and the joint breaks at the edge of the nugget. The tensile strength of the joint under the back heating process is 958.45 MPA, which is slightly lower than that of the conventional process, but it can still reach 93.6% of the base metal. The hardness of joint nugget zone in both processes is higher than that of base metal.
【学位授予单位】：沈阳航空航天大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG407

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