Advanced Welding Processes: Technologies and Process Control

High-energy density processes 143
Nickel alloys. Plasma keyhole welding has been used successfully on a
wide range of nickel alloys (including alloy 200, 400, 600, Hastelloy C,
Inconel 718) and is particularly useful in the thickness range from 2.5 to
7.5 mm. Argon/5% hydrogen has been used for both plasma and shielding
gases for the alloy series 200–600. Typical travel speeds vary from 0.5 m
min –1 at 160 A with 3 mm thick material to 0.22 m min –1 at 310 A with 8.25
mm thick plate.
Titanium. Steady and pulsed keyhole plasma welding techniques have
been applied to titanium and its alloys [156, 157] and, providing adequate
provision is made for gas shielding, high-integrity welds may be made. The
shielding requirement may be met by carrying out the welding operation in
a glove-box, which is vacuum purged and backfilled with argon, or using a
trailing shield of the type shown in Fig. 8.6. The major problem experienced
with this material is undercut; this may be alleviated by:
∑ careful selection of welding parameters on thicknesses up to 3 mm;
∑ pulsed operation with controlled current decay on each pulse;
∑ cosmetic runs with plasma or GTAW and filler;
∑ magnetic arc oscillation along the axis of the weld.
Welding parameters for plasma keyhole welding of titanium are given in
Appendix 7.
Aluminium. Plasma keyhole welding of aluminium is possible if electrode
positive polarity is used as discussed above. It is also possible to use advanced
Backing
gas
Torch gas
8.6 A trailing shield.
Torch motion
Trailing shield gas
Workpiece