New developments in advanced welding

The keyhole GTAW process

3.4.1 Introduction

Normally GTA welding of plates of more than a few millimetres in thickness calls for careful edge preparations and multiple passes. One route to achieving deeper penetration has been through the use of active fluxes, as described in the preceding section. This A-TIG process has advantages of simplicity and application across a very broad range of materials. An older and more direct approach has been to weld with much higher currents, as in the ‘high current’ GTAW process (Liptak 1965; Adonyi-Bucurdiu, 1989; Adonyi et al., 1992). With this approach the increased arc forces push aside the liquid weld metal, allowing the arc to access regions well below the plate surface (Fig. 3.15). In practice, however, the degree of penetration is difficult to control, and with rising current the pool becomes increasingly unstable with respect to

3.76 Schematic of conventional melt-in mode gas tungsten arc welding: side (a) and front (b) views.

fluctuations in arc pressure over its surface. Hollow-tipped tungsten electrodes have been developed as one means of reducing arc pressure and improving stability (Yamauchi et al., 1981).

Another variant, ‘keyhole GTAW’ is now attracting industrial attention. Keyhole GTAW differs from other modes in forcing an opening all the way through the joint. Despite this it still completes the weld without the need of a backing bar. This difference is illustrated schematically in Fig. 3.16 and 3.17. The novelty of the process arises both through the peculiar choice of operating conditions and in the use of a torch designed to deliver high axial arc pressures under very stable and reproducible conditions. The process can be implemented using ‘off-the-shelf’ GTAW power sources of suitable rating (a 600 A supply would be suitable for most applications). Enhancements designed to pinch or otherwise constrict the arc are not used.

The process was first introduced to commercial applications in Australia in the late 1990s, but is now finding use around the globe. It is applicable to a wide range of lower conductivity metals and alloys, (e. g. steels, stainless steels, titanium and nickel alloys). Applications requiring long, full-penetration butt welds such as spiral and seam welded pipe would seem to be ideal candidates for the process. In its present state of development it is not suited to highly conductive metals such as copper and aluminium.

3.4.2 Process performance

Keyhole GTAW is easy to implement and can be used within broad operating windows. Its primary attraction is that it is a fast single pass process, providing, for example, full penetration of stainless steel plates from 3 mm to about 12 mm thick - and to about 16 mm for titanium alloys. This is achieved using only minimal edge preparation and filler material because joints are presented in closed square-butt configuration. Such performance represents a significant advantage over GMAW and conventional GTAW for many applications. Similar performance may be obtained with plasma arc welding, but implementation costs are greater and process operation is more complex.

Control over the process is exercised through variations to the electrode geometry, voltage, current, travel speed and shielding gas composition. In general use, however, most parameters are fixed, with subsequent variation of only travel speed and current being sufficient to access most of the operating window. Furthermore, keyhole operation is readily confirmed through observation of the efflux plasma emerging from the root face and this has been used as a simple but effective control strategy.

Keyhole gas tungsten arc welds are not unlike plasma keyhole welds in appearance. Typically, the width of the weld crown is slightly greater than the thickness of the plate, providing an aspect ratio (depth to width) between 0.5 and 1.0. The root bead width tends to be between 2 and 4 mm. When sectioned, the fusion boundary is found to be slightly concave rather than straight. The fusion zones also display often pronounced caps or ‘nail-heads’ giving the impression of two or more passes having been applied. In fact the nail-head results from some additional fusion that occurs in the tail of the weld pool due to the accumulation of superheated weld metal. This can be deduced through inspection of the crater of an abruptly terminated keyhole weld. There it can be seen that the width of the weld increases in the trailing portion of the weld pool.