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 con taining a partially ionized metal vapor is formed throughout the depth of the material. A full description of flow conditions inside this ho le 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, t he dominant force keeping the keyhole open against surface tension is the fluid mechanical pressure in the plasma rather than the ablation p ressure. This has a bearing on the problem of the formation of pores i n the interior of the welded material. It is clearly observable on fil ms made of the surface of the weld piece acid of cross-sections throug h it that the keyhole has some of the characteristics of instability. It is shown that under some circumstances the balance of pressure agai nst the forces of surface tension can be sufficiently great for caviti es within the molten material to be formed. Estimates can then be made for the spacing of these cavities.