On the relation between fluid dynamic pressure and the formation of pores in laser keyhole welding

In the laser welding of metals with a continuous CO2 laser, a hole containing a partially ionized metal vapor is formed throughout the depth of the material. A full description of flow conditions inside this hole is needed for a complete understanding of the process, but much can be learned from a simpler analysis of this aspect of the problem. The balance of forces that keeps the keyhole open can be investigated in this way. Such a model shows that over most of the keyhole's length, the dominant force keeping the keyhole open against surface tension is the fluid mechanical pressure in the plasma rather than the ablation pressure. This has a bearing on the problem of the formation of pores in the interior of the welded material. It is clearly observable on films made of the surface of the weld piece and of cross-sections through it that the keyhole has some of the characteristics of instability. It is shown that under some circumstances the balance of pressure against the forces of surface tension can be sufficiently great for cavities within the molten material to be formed. Estimates can then be made for the spacing of these cavities.