Porosity formation mechanism and its prevention in laser welding
Porosity formation mechanism and its prevention in laser welding
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DOI：
10.1533/wint.2003.3138
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发表时间：
2003-06
影响因子：
--
通讯作者：
A. Matsunawa;M. Mizutani;S. Katayama;N. Seto
中科院分区：
文献类型：
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作者：
A. Matsunawa;M. Mizutani;S. Katayama;N. Seto
激光焊接过程中，由于聚焦点处功率密度高，熔池表面产生强烈的蒸发，反作用力使熔池表面形成深凹痕；焊接过程进行深孔，通常称为锁孔。这种锁孔的形成使窄焊头宽的深熔焊接成为可能，这是高能量密度电源焊接的特点。液体表面的穿透孔具有固有的不稳定性，其不稳定性随着孔直径与深度的比值（即长径比）的增大而增大。在锁眼激光焊接中，当焊接速度一定时，熔深随激光输出的增加而增加；同时，我们知道，在这个过程中，气孔（也称为空腔气穴）的频繁发生是激光焊接的一个特点。然而，其启动机制尚未确定。传统上认为连续波激光焊接过程中形成的锁孔是相对稳定的，所有关于锁孔焊接现象的数学模型都假定蒸发反作用力、静水压力和表面张力压力（拉普拉斯压力）在准稳态下处于平衡状态。然而，用这种假设来解释孔隙度的形成是极其困难的。通过对激光焊接过程中气孔的产生及抑制的研究，认为脉冲和连续激光焊接过程中气孔的形成是由小孔不稳定现象引起的。因此，尝试直接观察锁孔的不稳定现象。目前，对激光焊接过程中等离子体的行为及其特性进行了大量的研究；然而，还没有一篇论文从解决缺陷发生的机理以及相关发射谱与声谱之间的关系的角度，对等离子体行为与锁孔动力学之间的关系进行全面的研究。下面介绍了使用具有高时间/地点分辨力的设备观察锁眼动力学的结果和孔隙形成机制。
During laser welding intense evaporation develops from the molten pool surface due to the high power density at the focusing point and the molten pool surface is deeply indented by the reaction force; the welding process proceeds with a deep hole, commonly called a keyhole. The formation of this type of keyhole makes feasible deep penetration welding with a narrow bead width and these are the special features of high energy density power source welding. The penetrated hole on the liquid surface is inherently unstable and its instability increases with increasing ratio of the hole diameter to the depth, in other words, the aspect ratio. In keyhole laser welding, the penetration depth increases with increasing laser output when the welding speed is constant; simultaneously, it is known that, during this process, there is a frequent occurrence of porosity (also called as cavity gas pocket) which is a characteristic of laser welding. However, the initiation mechanism of this has not been identified. Conventionally, the keyhole formed during CW laser welding is considered to be relatively stable and all the mathematical models concerning the keyhole welding phenomena have been constructed with the assumption that the evaporative reaction force, hydrostatic pressure and surface tension pressure (Laplace pressure) are in equilibrium under the quasi-steady state. However, it is extremely difficult to explain porosity formation using such an assumption. From the results of research concerning the occurrence of porosity in laser welding and its suppression, the authors have come to believe that the distinctive porosity formation during welding with pulsed and continuous lasers is caused by the unstable phenomena of the keyhole. Consequently, an attempt was made to observe directly unstable phenomena of keyhole. At present, there are numerous examples of investigation regarding the plasma behaviour and its characteristics during laser welding; however, there is no paper describing a comprehensive investigation into the relationship between the plasma behaviour and the keyhole dynamics from the viewpoint of solving the mechanism for the occurrence of defects and the correlation between the related emission spectrum and the acoustic spectrum. The following describes the results of observation of keyhole dynamics using equipment with a high time/ place resolving power and porosity formation mechanisms.