Formation mechanism of elongated cavities in keyhole plasma arc welding
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摘要: 等离子弧焊接中厚钢板时，常存在焊接工艺窗口窄问题. 在中厚不锈钢板小孔型等离子弧焊(keyhole plasma arc welding，K-PAW )中发现，离子气体流量较弱而使匙孔未打开，形成盲孔时，会在焊缝内产生平行于焊接方向的长条形气孔，形成条形气孔时，等离子弧形态发生明显变化，电弧面积增加且波动更为剧烈，出现向后的反射. 使用高速摄影装置研究了条形气孔的形成过程.结果表明，中厚板等离子弧焊中，条形气孔是在一定的焊接速度和较低的电弧能量下，熔池前壁倾角较大，等离子弧受熔池前壁反射作用冲击熔池后壁，使熔池后壁发生弯曲，并在一定的凝固条件下保留，产生平行长气孔. 对条形气孔的研究有助于为匙孔等离子弧焊接中匙孔形成条件及机制提供新认识.Abstract: Narrow welding process window often impedes plasma arc welding of medium - thick plate. In keyhole plasma arc welding (K-PAW), it is found that if the keyhole is not opened due to the weak ionic gas flow rate and the blind hole is formed, an elongated cavity parallel to the welding direction will be generated in the weld. When the cavity is formed, the morphology of plasma arc changes obviously. The arc area increases and fluctuates more violently, causing backward reflection. The formation process of elongated cavities was studied by high-speed camera. Experimental results show that the formation processs of elongated cavities is as follows: at a certain welding speed and a low arc energy, the dip angle of the front wall of the molten pool is enlarged, and the plasma arc reflected by the front wall will impact the back side, which would be bended and retained under solidification, then an elongated cavity exists. The study of elongated cavities is helpful to provide a new understanding of keyhole formation conditions and mechanism in keyhole plasma arc welding.
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Keywords:
- plasma arc welding /
- elongated cavity /
- keyhole /
- molten pool flow
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图 2 嵌入透明石英玻璃的等离子弧焊高速摄影试验装置
Figure 2. PAW high-speed photographic system with transparent quartz glass
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图 3 焊缝X射线缺陷检测结果. (a) I = 200 A, Q = 2. 5 L/min; (b) I = 200 A, Q = 3. 5 L/min; (c) I = 220 A, Q = 1. 5 L/min; (d) I = 220 A, Q = 2. 5 L/min; (e) I = 220 A, Q = 3. 5 L/min; (f) I = 240 A, Q = 1. 5 L/min; (g) I = 240 A, Q = 2. 5 L/min; (h) I = 240 A, Q = 3. 5 L/min; (i) 图3d局部放大; (j) 图3e局部放大
Figure 3. X-ray inspection results of weld defects. (a) I = 200 A, Q = 2. 5 L/min; (b) I = 200 A, Q = 3. 5 L/min; (c) I = 220 A, Q = 1. 5 L/min; (d) I = 220 A, Q = 2. 5 L/min; (e) I = 220 A, Q = 3. 5 L/min; (f) I = 240 A, Q = 1. 5 L/min; (g) I = 240 A, Q = 2. 5 L/min; (h) I = 240 A, Q = 3. 5 L/min; (i) local enlargment of Fig. 3d; (j) local enlargment of Fig. 3e
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图 4 熔池侧面热成像结果
Figure 4. Thermal imaging results of molten pool. (a) I = 220 A, Q = 1.5 L/min; (b) I = 220 A, Q = 2.5 L/min; (c) I = 220 A, Q = 3.5 L/min
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图 5 正面电弧形态拍摄与识别. (a)经黑白化的电弧照片；(b)二值化后的电弧轮廓，边缘周期性移动；(c)二值化后的电弧轮廓，边缘周期性移动；(d)二值化后的电弧轮廓，边缘周期性移动；(e)二值化后的电弧轮廓，与图5b相对应；(f)二值化后的电弧轮廓，与图 5c相对应；(g)二值化后的电弧轮廓，与图5d相对应
Figure 5. Filming and recognition of PAW arc shape. (a) gray scale image of arc; (b) binarized arc contour with periodic edge movement; (c) binarized arc contour with periodic edge movement; (d) binarized arc contour with periodic edge movement; (e) binarized arc contour, corresponding to Fig. 5b; (f) binarized arc contour, corresponding to Fig. 5c; (g) binarized arc contour, corresponding to Fig. 5d
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图 6 透过透明玻璃获得的高速摄影结果(I = 220 A, Q = 1.5 L/min)
Figure 6. High-speed photography results with transparent glass (I = 220 A, Q = 1.5 L/min)
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图 7 透过透明玻璃获得的高速摄影结果(I = 220 A, Q = 2.5 L/min)
Figure 7. High-speed photography results with transparent glass (I = 220 A, Q = 2.5 L/min)
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图 8 透过透明玻璃获得的高速摄影结果(I = 220 A, Q = 3.5 L/min)
Figure 8. High-speed photography results with transparent glass (I = 220 A, Q = 3.5 L/min)
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图 9 不同离子气体流量下，等离子弧焊在透明玻璃辅助下的高速摄影结果对比
Figure 9. Comparison of high speed photography results of plasma arc welding under different ion gas flow rate assisted by transparent glass. (a) I = 220 A, Q = 1.5 L/min; (b) I = 220 A, Q = 2.5 L/min; (c) I = 220 A, Q = 3.5 L/min
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图 10 中厚板盲孔等离子弧焊接中条形气孔形成过程示意图
Figure 10. Diagram of forming process of elongated cavities in medium stainless steel plate plasma arc welding. (a) keyhole formation process; (b) front wall inclined, plasma flow impacted the rear wall; (c) elongated cavity formed in the solidification process
表 1 8 mm不锈钢等离子弧焊工艺参数
Table 1 8 mm stainless steel PAW technology experiment parameters
试样编号 焊枪高度h/mm 离子气体流量Q/(L·min−1) 焊接电流I / A 焊接速度v /(mm·s−1) 保护气体流量q/(L·min−1) 1 4.5 2.5 200 3.333 12 2 4.5 3.5 200 3.333 12 3 4.5 1.5 220 3.333 12 4 4.5 2.5 220 3.333 12 5 4.5 3.5 220 3.333 12 6 4.5 1.5 240 3.333 12 7 4.5 2.5 240 3.333 12 8 4.5 3.5 240 3.333 12 
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表 2 不同离子气体流量下正面电弧形态统计处理结果
Table 2 Statistic processing results of frontal arc shape under different plasma gas flow rate
焊枪高度
h/mm离子气流量
Q/(L·min−1)焊接电流
I/A电弧面积
S/mm2面积标准差
σ4.5 1.5 220 45.92 1.38 4.5 2.5 220 55.95 2.03 4.5 3.5 220 49.53 0.97
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