New developments in advanced welding

The A-TIG process

3.3.1 Introduction

The tungsten inert gas (TIG or GTAW) welding process is suited to welding operations requiring considerable precision and high joint quality. However, these advantages are offset by the limited thickness of material that can be welded in a single pass and by the poor productivity of the process. The poor productivity results from a combination of relatively low welding speeds and the high number of passes required to fill the weld joints in thicker material.

A new process variant, known as ‘A-TIG’, uses an activating flux to overcome these limitations by increasing the penetration significantly that can be achieved at a given current (Lucas and Howse, 1996). The concept of using such a flux was first proposed by the E. O. Paton Institute of Electric Welding in the former Soviet Union (Lucas and Howse, 1996; Lucas, 2000; Howse and Lucas, 2000). The first published papers that described their use for welding titanium alloys appeared in the 1960s (Gurevich et al., 1965, Gurevich and Zamkov, 1966). The result is that the penetration depth can be dramatically increased, by between 1.5 to 2.5 times relative to the conventional TIG process, by the simple application of a thin coating of the flux to the surface of the base material before arcing (Lucas et al., 1996). Consequently, A-TIG is expected to bring about large productivity benefits and accordingly intense interest in this process has been shown recently.

3.3.2 Flux and equipment

A-TIG is a simple process variant that does not require any special equipment (Lucas and Howse, 1996). In contrast, attempting to increase the depth of weld penetration by using other processes such as plasma or laser welding would require a substantial investment in new equipment. Plasma welding requires specialised torches and power supplies, while laser welding is dependent on expensive, high power lasers and precision beam and component manipulation (Okazaki and Okaniwa, 2002). In addition, these processes can require a significant commitment to appropriate procedure development for specific applications. In contrast, the A-TIG process just needs conventional TIG equipment - a standard power source and TIG torch with the normal size and type of tungsten electrode. These items would be existing equipment in most workshops, laboratories, factories and plants (Okazaki and Okaniwa, 2002).

The activating flux is provided in the form of a fine powder which is

mixed with acetone or MEK (methyl ethyl ketone) into a paste and painted on the surface of the material to be welded (Lucas and Howse, 1996; Howse and Lucas, 2000; Tanaka 2002). The paste can be applied by a brush or with an aerosol applicator such as a spray (see Fig. 3.6). The A-TIG process can be used in both manual and mechanised welding operations (Lucas, 2000). The flux appears to be equally suitable for increasing the depth of penetration for welds produced with either argon or argon-helium shielding gases (Lucas and Howse, 1996; Anderson and Wiktorowicz, 1996).

Activating fluxes are available commercially from a number of companies in the UK, USA, Japan and so on (Lucas 2000; Tanaka 2002). There are many formulations which have been designed for welding materials such as carbon-manganese steel, low alloy steel, stainless steel, nickel-based alloy and titanium alloy. Although there are no published data formally on chemical compositions of commercial brands, there appears to be range of flux compositions in some literature (Ostrovskii et al., 1977, Lucas and Howse, 1996; Lucas et al., 1996; Ootsuki et al., 2000; Tanaka 2002). The activating fluxes are predominantly composed of the oxides of titanium (TiO2), silicon (SiO2) and chromium (Cr2O3) with the addition of small quantities of halides as minor elements. Examples of included halides are sodium fluoride (NaF), calcium fluoride (CaF2) and aluminium fluoride (AlF3) (Lucas and Howse, 1996). As an example, the following flux composition has been reported for welding carbon-manganese steel and was produced in the former Soviet Union (Ostrovskii et al., 1977): SiO2 57.3%, NaF 6.4%, TiO2 13.6%, Ti 13.6% and Cr2O3 9.1% (permissible deviation +/-2%).

3.3.3 Arc phenomena in the A-TIG process

It is well known that A-TIG shows a visible constriction of the arc compared with the more diffuse conventional TIG at the same current level (Lucas and Howse 1996; Lucas 2000; Howse and Lucas, 2000; Lucas et al., 1996).

Tanaka et al. (2000) have made and compared experimental observations of interactive phenomena between the arc and the weld pool in the A-TIG and conventional TIG processes. They employed pure TiO2 as the flux, since a simple composition aided in the understanding of the phenomenon and this compound was one of the main elements of several fluxes on the market (Ostrovskii et al., 1977; Lucas and Howse, 1996; Lucas et al., 1996; Ootsuki et al., 2000; Tanaka, 2002). Figure 3.7 shows cross-sections of welds made with and without flux at the three different welding currents. The material was an austenitic stainless steel (AISI 304) of 10 mm thickness. The shielding gas was helium, the welding speed was 200mm/min, and the arc gap was 5 mm. It can be seen from the figure that the depth/width ratio of welds with flux was higher than that of welds without flux, independent of welding current. This figure suggests that a satisfactory increase in penetration depth can be expected even with a flux consisting of only TiO2.

The arc in the helium shielded TIG process has a characteristic appearance, both with and without flux. In the case without flux, there is a large, wide region of blue luminous plasma in the lower part of the arc. The blue luminous plasma appears to be mainly composed of metal vapour from the weld pool. In the case of A-TIG, the region of the blue luminous plasma is constricted at the centre in the lower part of the arc and the anode spot can be observed at the centre of the weld pool surface.

Figure 3.8 shows results of spectroscopic measurements of TIG welding arc plasmas with and without flux. Three line intensities, He I (438.793 nm), Cr I (425.435nm) and Fe I (430.79nm), were measured. Typical line spectra are shown in Fig. 3.9. Further line intensities of neutral metal atoms and metal ions could be detected, but line intensities of titanium (Ti I and Ti II) and oxygen (O I ) could not be detected within the visible range. The intensity of each measured line is indicated by the grey scale in Fig. 3.8. In the case

z

without flux, the intense regions of Cr I and Fe I were considerably expanded in the lower part of the arc. However, in the case of A-TIG, both regions were only observed at the centre in the lower part of the arc. Therefore, it can be supposed that the blue luminous region is plasma mainly composed of metal vapour from the weld pool, namely the metal plasma. However, the He I region remained unchanged, i. e. it was independent of flux. This means that the arc constriction in the A-TIG process was associated with a change of metal vapour from the weld pool. It also appears that vapours from flux only weakly affect the arc constriction, as the line intensities of titanium and oxygen were very difficult to detect in the arc plasma.

It is well known that the metal vapour concentration depends on the surface temperature of the weld pool (Block-bolten and Eagar, 1984). Accordingly, pyrometric measurements of surface temperatures were included in the study by Tanaka et al. (2000). Figure 3.10 shows the radial temperature distributions on the weld pool surface with and without flux. Without flux, the surface temperature decreased gradually from the centre to the edge of the weld pool. With flux, however, the surface temperature at the centre of the weld pool was higher, at approximately 2350 K, while it became lower than the temperature without flux at an outer radius of about 1.5 mm. This means that the surface temperature gradient in A-TIG is much higher than in the conventional TIG process. It may be noted that a second peak in surface temperature appeared at about 2.5 mm radius for the A-TIG process (Fig.

3.10) . However, this peak temperature was the surface temperature of flux heated directly by the arc on the unmelted base metal and so not related to the weld pool.

Tanaka et al. (2000) also measured the relationship between the surface tension and coating density of the TiO2 flux by using the technique of weld pool oscillation (Xiao and den Ouden, 1990). It was found that at first surface tension decreased sharply with coating density but became approximately constant at densities greater than about 1 mg/cm2 (see Fig.

3.11) . This constant value of surface tension of about 1 N/m is quite similar to a value Ogino et al. (1983) measured by the sessile drop method at 1873 K. They studied the effects of oxygen and sulphur on the surface tension of molten iron and found that surface tension decreased sharply with oxygen content to a value of about 1 N/m at 300 ppm. Furthermore, they also showed that surface tension decreased equally sharply with sulphur content.

Tanaka et al. (2000) also investigated the relationship between the penetration depth and coating density of the TiO2 flux, as shown in Fig. 3.12. They found that penetration depth increased sharply with the coating density before becoming approximately constant at densities greater than about 1 mg/ cm2. Using a standard technique, such as manual application by brush, the coating density of flux is approximately 15mg/cm2. The change in penetration depth in Fig. 3.12 correlates with the change in surface tension in Fig. 3.11. From these results, it may be concluded that surface tension is an important element in the mechanism of the A-TIG process.

Recently, Lu et al. (2002) have studied the effects of various oxide-based fluxes on the penetration depth in the A-TIG process. They selected five single component activating fluxes: pure Cu2O, NiO, Cr2O3, SiO2 and TiO2,

and investigated the relationships between the flux type, the depth/width ratio of the weld penetration and the oxygen content in the weld metal. Each flux gave a different depth/width ratio for each coating density, this being principally dependent on their relative chemical stability. However, Lu et al. (2002) found that the depth/width ratio increased by 1.5 to 2.0 times as the oxygen content in the weld metal passed through the range of 70-300ppm, independent of the flux composition. Too low or too high oxygen content in the weld did not increase the depth/width ratio. They concluded that the oxygen from the decomposition of the flux in the weld pool altered the temperature coefficient of surface tension of the weld pool, which in turn changed the depth/width ratio of the weld penetration by inverting the direction of marangoni convective fluid flow. This is consistent with the results of Taimatsu et al. (1992) who showed that oxygen was an active element in pure liquid iron in the range of 150-350ppm. They found that in this range the temperature coefficient of the surface tension of the Fe-O alloy was positive, while out of the range, the temperature coefficient was negative or nearly zero. Therefore Lu et al. (2002) clearly demonstrated that the oxygen from the decomposition of the flux in the weld pool was a key to the understanding of the A-TIG phenomena.

The presence of surface active impurities, such as oxygen, sulphur, etc., in the base material is known to affect the geometry of the weld bead (Makara et al., 1977). These surface active elements also increase the penetration depth (Makara et al., 1977). Katayama et al. (2001) directly observed the phenomenon of convective fluid flow in the weld pool by using a micro­focused X-ray transmission method during a TIG welding process. They confirmed that different sulphur contents inverted the direction of convective flow in the weld pool of the same type 304 stainless steel. An outward fluid flow was observed for low sulphur content (40 ppm), whereas an inward fluid flow was observed when the sulphur content was high (110 ppm). It was confirmed that the difference in flow direction in the weld pool dramatically changed the geometry or depth/width ratio of weld penetration. In view of the above, it is concluded that the mechanism for the effect of the flux and that of surface active impurities in the base material are the same.