Joining mechanism and element distribution in laser micro-welding of NiTi-Cu dissimilar alloys
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摘要:
为实现NiTi形状记忆合金电致热驱动的功能特性，采用激光微连接技术对NiTi丝和铜板异种材料进行焊接，并利用光学显微镜、能谱分析仪等手段分析连接界面的微观结构和元素分布. 基于ANSYS Fluent软件建立NiTi-Cu异种材料激光微连接三维计算流体力学仿真模型，分析了NiTi-Cu激光微连接过程中的温度场、流场演变和元素传输规律. 结果表明，NiTi-Cu激光微连接过程主要分为激光在NiTi丝中的“钻孔”过程、对铜板的预热过程和在铜板中的“钻孔”过程. Ni，Ti，Cu元素混合主要发生于激光对铜板的“钻孔”过程中，元素在金属蒸气反冲压力和Marangoni涡流的驱动下相互混合，Cu元素进入熔池易形成低硬度的Cu-Ti金属间化合物，降低了脆性的Ni-Ti金属间化合物形成的可能性. 试验和仿真结果吻合较好，为优化NiTi-Cu异种材料的激光微连接工艺提供了重要的理论支撑.
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关键词:
- NiTi形状记忆合金 /
- 激光微链接 /
- 异种金属 /
- 计算流体力学 /
- 元素分布
Abstract:To satisfy the thermo-driven requirements of NiTi shape memory alloys in different industrial applications, NiTi wire and Cu plate were welded by laser micro-welding technology. The microstructure and element distribution of the weld were experimentally studied using optical microscopy (OM) and energy dispersive spectrometer (EDS). Based on ANSYS Fluent software, a three-dimensional (3D) computational fluid dynamics (CFD) model for laser micro-welding of NiTi-Cu dissimilar alloys was established to analyze the evolutions of temperature and fluid flow fields and the element transport mechanism. The results indicate that the process of laser micro-welding can be mainly divided into 3 procedures, namely the "drilling" procedure of laser in NiTi wire, the preheating procedure of laser energy on Cu plate, and the "drilling" procedure of laser in Cu plate. The mixing of Ni, Ti, and Cu elements mainly occurs in the drilling process of laser in Cu plate, where the elements are mixing well under the driven of the metal vapor recoil pressure and Marangoni vortex. The Cu content in the molten pool is helpful to form low hardness Cu-Ti intermetallic compounds (IMCs), reducing the possibility of the formation of brittle Ni-Ti IMCs. The experimental and simulation results are in good agreement. The research provides important theoretical support for optimization of the laser micro-welding of NiTi-Cu dissimilar alloys.
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图 1 NiTi-Cu激光微连接示意图(mm)
Figure 1. Diagram of micro laser welding of NiTi-Cu. (a) micro laser welding system; (b) diagram of micro laser welding
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图 3 激光热源模型及能量分布
Figure 3. Laser heat source model and energy distribution. (a) laser heat source model; (b) energy distribution
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图 4 试验与仿真焊缝横截面轮廓对比
Figure 4. Comparison of the experimental and simulated welds on the cross-section. (a) the experimental; (b) simulated welds
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图 6 时间区间1内的熔池动态
Figure 6. Molten pool dynamics within the time interval 1. (a) t = 0.7 ms; (b) t = 2.7 ms
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图 7 时间区间2内的熔池动态
Figure 7. Molten pool dynamics within the time interval 2. (a) t = 6.2 ms; (b) t = 13.3 ms
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图 8 时间区间3内的熔池动态
Figure 8. Molten pool dynamics within the time interval 3. (a) t = 13.9 ms; (b) t = 20.3 ms
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图 9 熔池横截面Cu元素分布
Figure 9. Cu element distribution on the cross-section of molten pool. (a) t = 2.7 ms; (b) t = 6.2 ms; (c) t = 13.3 ms; (d) t = 13.9 ms
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图 10 试验与仿真焊缝中Cu元素分布对比
Figure 10. Comparison of Cu element distribution in experimental and simulated weld beads.(a) enlarged image of region A in Fig. 4a; (b) simulated distribution of Cu element at 20.3 ms; (c) enlarged image of region B in the simulation; (d) comparison of the element content
表 1 NiTi和Cu热物理参数
Table 1 Thermophysical parameters of NiTi and Cu
材料 密度
ρ/(kg·m−3)固相热导率
λ1/(W·m−1·K−1)液相热导率
λ2/ (W·m−1·K−1)固相比热容
C1/( J·kg−1·K−1)液相比热容
C2/( J·kg−1·K−1)NiTi 6 450 13 21.5 489 841 Cu 8 960 385 157.02 481 531 材料 动力粘度
μ/( kg·m−1·s−1)固相温度
T1/ K液相温度
T2/ K蒸发温度
T3/ K熔化潜热
L /(105J·kg−1)NiTi 0.005 74 1 553 1 583 3 033 2.42 Cu 0.004 03 1 356 1 357 2 853 2.05 
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