下轴肩冷却对6061-T6铝合金双轴肩搅拌摩擦焊接头性能的影响

发布时间：2018-09-08 15:50

【摘要】：双轴肩搅拌摩擦焊(BTFSW)是由常规搅拌摩擦焊(FSW)进一步发展而来,利用BTFSW有两个轴肩的特点,将两个轴肩所受的作用力相互抵消,有效的减小了主轴负载,突破了焊机对刚性的限制、并能改善焊缝的根部缺陷。但在BTFSW焊接过程中,上轴肩与焊机夹持部分接触,散热条件好,下轴肩体积小,散热条件差,导致接头上下表面存在温度差,从而影响焊缝成形质量。如何在保证焊接质量的前提下,降低双轴肩搅拌头下轴肩的温度,平衡焊件上下表面热量,减小上下表面温度差成为BTFSW的急需解决的难题。本课题将针对原双轴肩搅拌头的缺点,设计新型的双轴肩搅拌头以及配套的冷却装置。将冷却装置安装在双轴肩搅拌头的下轴肩底部,使用螺纹配合与搅拌针连接。焊接时,冷却装置随着双轴肩搅拌头一同旋转,带动冷却液流过冷却装置,带走大量焊接热,最终实现循环冷却。冷却装置主要对搅拌头下轴肩进行冷却,在带走多余焊接热量的同时,平衡接头的上下表面温度。通过研究下轴肩冷却对6061铝合金双轴肩搅拌摩擦焊接头性能影响,能更好的分析冷却条件对于焊接热循环的调节作用,同时在研究相变与焊接热循环以及与接头组织性能的关系基础上,可以有效的对焊接参数进行优化,提高接头性能,为6061-T6铝合金的双轴肩搅拌摩擦焊接奠定理论和工艺基础。试验结果表明:在旋转速度为810 r/min,焊接速度为160 mm/min条件下,对下轴肩冷却可以使工件焊接时得到焊缝表面美观,且无明显孔洞隧道等缺陷。通过对有冷却装置的BTFSW进行温度场研究,发现使用冷却装置可以降低接头区域整体的温度,其中对接头的下表面冷却作用最为显著,同时缩小了被焊工件上下表面的温度差,保证焊缝的上下表面受热均匀。使用冷却装置,还可以使工件在各温度区间的持续时间减少,高温持续时间的缩短抑制了晶粒生长,细化了晶粒组织,提高了接头强度;使用冷却装置后,接头区域的整体范围变窄,接头宽度减小了约4mm,焊核区晶粒粒度变小,排列更为均匀,硬度普遍提高,硬度最小值从55HV提高到60HV;热机影响区范围缩小,强化相析出量增多,同时晶粒长大收受抑制,平均硬度提高,强度增加;热影响区范围减小,前进侧硬度有所降低,这是由于前进侧温度过低所致。
[Abstract]:(BTFSW) is further developed from conventional friction stir welding (FSW). By using the characteristics of BTFSW with two shafts, the interaction between the two shafts is counteracted, and the spindle load is effectively reduced. It breaks through the rigidity limitation of welding machine and can improve the root defect of welding seam. However, in the process of BTFSW welding, the upper shaft shoulder is in contact with the welding machine partly, the heat dissipation condition is good, the lower shaft shoulder volume is small, and the heat dissipation condition is poor, which leads to the temperature difference between the upper and lower surfaces of the joint, thus affecting the weld forming quality. On the premise of ensuring welding quality, how to reduce the temperature of the lower shaft shoulder of the double shaft shoulder stirring head, balance the upper and lower surface heat of the welding piece, and reduce the temperature difference between the upper and lower surface are the urgent problems for BTFSW to solve. In view of the shortcomings of the original double-shaft shoulder mixing head, a new type of double-shaft shoulder mixing head and its supporting cooling device will be designed in this paper. Install the cooling device at the bottom of the lower shaft shoulder of the double shaft shoulder mixing head, and connect with the stirring needle with the thread fit. During welding, the cooling device rotates with the double shaft shoulder mixing head together, drives the coolant to flow through the cooling device, takes away a large amount of welding heat, and finally realizes the circulation cooling. The cooling device is mainly used to cool the lower shaft shoulder of the mixing head and balance the upper and lower surface temperature of the joint while taking away the excess welding heat. By studying the effect of lower shaft shoulder cooling on the performance of 6061 aluminum alloy double shaft shoulder friction stir welding joint, the effect of cooling condition on welding thermal cycle can be better analyzed. At the same time, on the basis of studying the relationship between phase transformation and welding thermal cycle, as well as the microstructure and properties of the joint, the welding parameters can be effectively optimized, and the properties of the joint can be improved, thus laying a theoretical and technological foundation for the 6061-T6 aluminum alloy double-axle shoulder friction stir welding. The experimental results show that under the condition of rotating speed of 810 r / min and welding speed of 160 mm/min, cooling of the lower axle shoulder can make the weld surface beautiful when welding, and there are no obvious defects such as hole tunnel and so on. Through the research on the temperature field of BTFSW with cooling device, it is found that using the cooling device can reduce the whole temperature of the joint area, and the cooling effect on the lower surface of the joint is the most significant, and the temperature difference between the upper and lower surfaces of the welded workpiece is reduced at the same time. Ensure that the upper and lower surfaces of the weld are evenly heated. The use of the cooling device can also reduce the duration of the workpiece in each temperature range, the shortening of the high temperature duration inhibits the grain growth, refines the grain structure, and improves the strength of the joint. The overall area of the joint becomes narrower, the width of the joint decreases by about 4 mm, the grain size of the nugget becomes smaller, the arrangement is more uniform, the hardness is generally increased, and the minimum hardness is increased from 55HV to 60 HV.The area affected by the heat engine decreases, and the amount of enhanced phase precipitation increases. At the same time, the average hardness increases and the strength increases with the inhibition of grain growth, while the range of the heat affected zone decreases and the hardness of the forward side decreases, which is due to the low temperature of the front side.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG457.14

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