New developments in advanced welding

Formation of a root bead

The stability of the root bead can also be attributed to surface tension. For example, it is noted that the radius of curvature across the root bead (rw) must be greater than half the width of the root bead, w (i. e. rw > w/z). Typically, w/z is between 1 and 2 mm. On the other hand, the radius of curvature along the solidifying bead (ra) can be very large at high welding speeds, implying that the maximum pressure that can be sustained at high welding speeds is approximately 2g/ w. This pressure can be balanced against that due to the head of liquid metal, pgh. Inserting realistic values for w and g indicates that those keyhole welds in AISI 304 stainless steel thicker than
about 7 mm will not be supported at high welding speeds. However, if the welding speed, and hence ra, is reduced, stability might be restored. Of course there is a limit to the reduction in ra - it certainly cannot take values below w/2, for example. The implication is that the process operates to a limiting plate thickness and that the welding speed must be reduced as this limit is approached. The process can handle 12 mm thick austenitic stainless steel but only over a narrow range in welding speed that certainly does not exceed 400mm/minute.

The surface tension also appears to limit the minimum thickness of plate that can be welded. In this case the limit is established by geometric considerations associated with the width of the weld pool and is not directly dependent on material properties. Failure results in cutting of the plate. In practice, the process is very difficult to operate with materials less than 3 mm thick. However, such thicknesses are readily accommodated with more conventional welding modes, leaving little incentive to develop practical solutions.

3.4.4 Arc-keyhole relations

As outlined earlier (Section 3.2.2) high current welding arcs exert noticeable forces on the weld pools and tend to push the molten metal aside. An expression relating the total arc force to the anode and cathode radii (ra and re) developed by Converti (1981) is

[3.24]

The ratio ra/re is sometimes referred to as the arc expansion ratio.

The principal consideration in generating the conditions necessary for keyhole GTAW is the production of a high peak arc pressure and hence the minimisation of the cathode emission radius re.

In keyhole GTAW the cathode emission region is confined to the tip of the tungsten electrode. The electrons emitted from this region maintain the current in the arc plasma. There are various mechanisms by which electrons can pass from an electrode into a surrounding gas or plasma. In this case the mechanism is thermal; the electrode tip is so hot that electrons can ‘evaporate’ into the surrounding gas from where the arc voltage drags them through the plasma to the weld pool. Consequently, the area of this region is sensitive to the heat flow within the electrode, and this in turn affects the current at which a keyhole can form (this is referred to as the threshold current). Thus changing the electrode stick-out, taper, diameter or composition are all means of altering the point of transition to keyhole mode. Choosing large diameter electrodes is one of several simple strategies for reducing the area of emission and therefore the threshold current.

The liberation of electrons from the tip requires a great deal of energy, and so the process cools the emission region. As a result the region tends towards a constant temperature, and its area is strongly correlated with the arc current. However, the peak arc pressure is dependent on the cross-sectional radius of this region and not on its area. The implication from this is that the peak arc pressure can be increased significantly by reducing the included angle of the electrode tip. This is because the cross-sectional radius of the emission region, re is a function of both the emission area, Ae and the included angle, q (see Fig. 3.22):

, . J

Ae sin 2

p

In practice the included angle is usually kept between 45° and 60°.

Helium-rich arcs are not desirable for keyhole welding because, apparently, the high viscosity of helium dampens the action of the arc core. However, they are very effective in transferring thermal energy to the weld pool and this can be advantageous. Fortunately there are means of obtaining high conductivity without reducing the arc pressure. In particular, diatomic gases absorb substantial amounts of energy in dissociation and this provides a highly efficient energy transport mechanism known as ‘reactive thermal conductivity’. The two most likely candidate gases are hydrogen and nitrogen.

These can be added to argon to give significantly better keyholing potential than argon alone. Hydrogen additions of up to about 10% can be used with austenitic stainless steels, while similar concentrations of nitrogen have been used with duplex stainless steels. Naturally, such additions cannot be used indiscriminately, as they can have seriously detrimental effects on many metals and alloys.

The final arc parameter of interest is arc length. Arc length has a significant effect on keyhole behaviour when operation is near the limits of the process envelope. Also, keyholes tend not to form when the electrode tip is submerged. In one set of trials the electrode was incrementally raised from a submerged position. At first a keyhole could not be formed. Keyholing only became possible when the tip was raised to be approximately level with the plate surface, but the required current was high. However, on continued increase in arc length the threshold current exhibited a rapid transition to a significantly lower value. This lower level then remained constant on continued raising of the electrode (see Fig. 3.23). The implication was that operation with too short an arc length can give inconsistent performance, with the threshold current either becoming random between the two levels, or the process failing completely. At the other extreme of arc length the weld pool may also broaden, this time due to expansion of the arc. Thinner (e. g. 6 mm) plate does not appear to be sensitive to this but thicker sections have shown quite well-defined upper as well as lower limits to arc length, and by association, arc voltage.

76 New developments in advanced welding

3.4.5 Summary

Keyhole mode gas tungsten arc welding is a new process variant with considerable potential. It is able to capture some of the best features of the GTAW process (cleanliness, controllability and versatility), while adding much sought-after productivity gains. Laboratory and industrial experience suggests that this is an attractive option for applications involving automated flat position welding of materials above 3 mm thickness.

However, the process has not been widely studied and although it appears that there is a basic understanding of the mechanics and operational details, there is scope for considerably more analysis and development. For example, issues relating to highly distorted free surfaces are central to understanding the keyhole, but are not well-understood from a welding perspective. Given the perceived potential for this process it is hoped that it will receive more attention in the future.