New developments in advanced welding

GMAW hybrid processes and other developments

This section reviews miscellaneous GMAW-related improved processes such as hybrid laser/GMAW, tandem GMAW welding, narrow groove welding and digital power source networking.

1.5.1 Hybrid welding processes: GMAW/LBW

This combination of high penetration laser beam welding (LBW) and good gap bridgeability (GMAW) processes builds on the intelligent combination of the advantages of each process. The resulting welds (Staufer et al., 2003) can be made at high speeds, have good penetration and are less sensitive to gap variations. The GMAW arc stability and droplet transfer are also improved by the intense metal vaporisation caused by LBW. Apparently, the greater amount of ionised metal and electrons in the LBW plasma reduces the need for high ionisation potential and exceeding the electrode work function in the GMAW arc thus provides better arc stability. Disadvantages include: high capital cost and the need for automation and precise beam/arc alignment. Typical GMAW/LBW heads are expensive and complex, as shown in Fig. 1.16.

1.5.2 GMAW brazing

Using low-melting point electrode wire consumables such as Cu-Si, Cu-Ag, Cu-Al alloys allow for low current GMAW-P deposition without melting of the base metal (i. e. electric brazing). This significant development reduces the width of HAZs and damage to Zn coatings in the automotive sheet and produces minimal distortions (Himmelbauer, 2003). The roof panel joint does not require any post weld processing. Additionally, arc-brazing is also being accomplished using traditional and STTtm forms of GMAW-S. GMAW with a CuAl wire electrode also makes possible joining of dissimilar materials with very different melting points such as steel and aluminium.

One disadvantage of GMAW brazing is the low joint strength that can be compensated by using lap joint design. Additional problems have been associated with zinc pickup in the silicon bronze weld and the result is transverse cracking of the weld deposit. This occurs in welds of those members where there is a gap. The gap, via capillary action picks up zinc from both surfaces of the plated base material. Finally, the presence of Cu in the recycled car bodies lowers the quality of the scrap and increases cost because of the difficulty of removing Cu which is very detrimental in steel making (solidification cracking susceptibility).

1.5.3 GMAW tandem welding

As the name implies, two wire electrodes are used in tandem to produce welds. The two wire electrodes are insulated from each other in tandem welding (Himmelbauer, 2003), thus the droplet transfer mode can be adjusted independently, in contrast to double-wire welding. Typically, one electrode can work in continuous arc (synergic CV or synergic CC) and the other in pulsed arc mode (also known as ‘master’ and ‘slave’ wires or ‘lead’ and ‘trail’ wire). Accordingly, the modified process allows for great flexibility in addition to increased travel speed, higher deposition rates, as well as lower spatter. Disadvantages include equipment complexity, as well as the need for automation. Seam tracking may or may not be required (Fig. 1.17). The system employs two power sources, two wire drives, and a control. It is

adapted for either repetitive side-beam type applications or is employed with a welding robot. This variant of the gas metal arc welding process is capable of higher travel speeds, 1.5-2.0 times the speed of a single electrode. Some travel speeds may exceed 150in/min (3.81m/min). Deposition rates of 42 pounds/h (19.1 kg/h) are achievable for heavier plate welding (Nadzam, 2003).

The modes of metal transfer used for the tandem GMAW are axial spray metal transfer or pulsed spray metal transfer. The combinations of the modes that are popularly employed include:

• Spray + pulse: Axial spray transfer on the lead arc followed by pulsed spray transfer on the trail arc.

• Pulse + pulse: Pulsed spray transfer on both the lead and the trail arc.

• Spray + spray: Axial spray transfer on both the lead and the trail arc.

The higher energy spray + spray configuration is used for special heavy plate welding where deeper penetration is required. Pulse + pulse allows for heavy welding or high-speed sheet metal welding.

Central to the successful operation of tandem GMAW is proper understanding of the set-up of the special tandem GMAW welding torch. In most cases, the central axis of the torch should be normal to the weld joint. The lead arc has a built in 6 degree lagging electrode angle, and the trail has a built in 6 degree leading electrode angle.

The contact tip to work distance (CTWD) for higher speed sheet metal type applications should be set at 0.625 in (16mm). The electrode spacing is critical and the shorter CTWD establishes the correct spacing. When the CTWD is held at this position the two arcs become more distinct from one another and shorter arc lengths are used to provide higher travel speeds. Use of tandem GMAW for heavy plate fabrication requires a longer CTWD,
1.0 in (25.4 mm). The longer CTWD provides the correct spacing between the two arcs, and in this scenario, the arcs tend to move very closely together. When held at the longer CTWD the arcs lend themselves for use with much higher wire feed speeds.

1.5.4 Narrow groove GMAW welding

An excellent application of GMAW is for low heat input welding of thick plate, the resulting welds have often been plagued by occasional lack of sidewall fusion. Wire electrode bending and rotating (twisted wire) have been used in the past to overcome this problem. Korean researchers used electromagnetic arc oscillation to alleviate the same problem in the narrow groove (Khang and Na, 2003).

1.5.5 Use of the World Wide Web and digital networks

Digital power sources and the associated data acquisition systems make possible linking them into local area networks (LAN) (Fig. 1.18). Depending on the availability of Internet connections at remote locations, qualified parameters can be directed to individual welding machines. On the other hand, welder procedure qualifications can be sent electronically to one central location.

1.18 Schematic representation of data interchange between GMAW power sources and a CP using internet protocol via Ethernet (Himmelbauer, 2003).