| Abstract Scope |
Introduction:
Stringer-bead gas metal arc welding (GMAW) deposits made with aluminum electrodes and argon shielding at high currents are susceptible to finger-penetration, plasma-jet induced porosity defects, and lack of toe fusion. These problems are a result of the constricted heat field provided by argon arcs and grow with increasing pool size and current. In aluminum plate welding, helium or helium argon mixture are recommended as means to improve fusion and bead shape. Significant cost savings would be realized by welding aluminum with pure argon because it is 4-5x cheaper than helium. Mechanical oscillation can be used to distribute heat and improve bead shape on mechanized welding applications. Rotating electrode (RE) GMAW is a unique method for arc oscillation that rotates the contact tip in lieu of the torch making it more agile for mechanized, high speed, and/or manual welding. This project investigated the effects of inert gas shielding and rotating electrode parameters on aluminum weld quality and bead shape. As expected, helium and helium-argon shielding gases provided better under bead shape, penetration profile, and dilution compared to pure argon. Similar improvements were made by using the RE-GMAW process when using preferred RE diameters and frequencies.
Experimental Approach:
Argon, helium, and helium argon (stringer-bead) bead on plate (BOP) were made to characterize the effects of shielding gas on bead shape characteristics. These results were compared to a series of RE-GMAW BOP tests using different RE diameters and frequencies with pure argon shielding. All weld tests were made using an OTC WB-P500L power supply and an air-cooled SpinArc RE-GMAW torch and a Bug-o welding tractor to control travel speeds. The base material was 16-mm AL5083 and the welding electrode was 1.6-mm (0.062-in,) ER5183. A matrix of constant deposit area (DA) tests was performed to evaluate four different shielding gases, and a range of RE parameters. All BOP tests were made using a normal work and travel angle at a constant 3-mm (1/8-in.)) arc length. The deposit area was held constant by using a constant wire feed speed (WFS) to travel speed (TS) ratio of 20. For each constant DA test, BOP welds were made at high, medium, and low power levels to evaluate the effects of current and travel speed on bead shape. Here the WFS and TS ranged from 5.72 to 9.53 meters per minute and 0.286 and 0.476 mpm, respectively. The current range and voltage ranged from 147 amps to 294 amps and 21.1 to 28.0, respectively. The shielding gas tests compared pure argon, 75% argon – 25% helium, 50% argon – 50% helium and pure helium. These tests were compared to RE BOP tests made using RE torch diameter settings of 2 and 4 where the RE frequency was set to provide 19.69 revolutions per centimeter. This spin frequency was selected to ensure uniform weld ripple on the bead face. Weld cross-section macrographs were used to compare underbead profile, reinforcement profile, penetration, toe lack of fusion, and base metal dilution.
Results and Discussion:
A series of BOP tests were completed with and without arc rotation using pure argon and these tests were compared to BOP tests made with pure argon (Ar), 75% argon – 25% (75/25), helium, 50% argon – 50% helium (50/50) and pure helium (He). As expected, helium and argon-helium shielding gases provided a significant improvement in under bead shape, penetration profile and dilution compared to pure argon. Toe lack of fusion (LOF) ranged from 0.5 to 0.6 mm with Ar to 0.2 to 0.35 with 50/50 to 0.1 to 0.2 mm with He shielding gas.
Similar improvements were made by using the RE-GMAW process when using torch RE diameter settings of 2 and 4. Toe LOF was reduced to 0.15 and 0.05 mm at medium power using RE diameters of 2 and 4, respectively. The RE diameter directly affected the underbead penetration profile where a completely elliptical profile was achieved with RE diameter 4. The RE diameter 2 setting yielded the best overall bead shape characteristics and had minimum toe LOF with smooth underbead and reinforcement shape. Larger RE diameters did result in better underbead fusion profile but the reinforcement was very asymmetric. Excessive RE frequency at large RE diameter promoted a pool wave that formed a very asymmetric bead reinforcement profile. Future work should evaluate additional test matrices to determine the preferred RE frequency and diameter as a function of different deposit sizes and power.
The asymmetric bead reinforcement was also found to offer benefits for out-of-position groove welding. By controlling the RE direction, the asymmetric bead shape is an advantage for horizontal position where the positive reinforcement side of the pool can be used to offset gravity and improve bead shape. The results of this work show that RE-GMAW technology has the potential to improve bead shape and fusion quality of aluminum weldments while also reducing the cost of the welding operation by using Ar versus He shielding.
Conclusions:
Helium and argon-helium shielding gases provided a significant improvement in under-bead shape, penetration profile and dilution compared to pure argon when comparing stringer-bead BOP tests. Preferred RE-GMAW parameters were found to offer the best bead shape with minimum toe LOF. Excessive RE frequency at large RE diameter promoted a pool wave that formed a very asymmetric bead reinforcement profile. By controlling the RE direction, the asymmetric bead shape is an advantage for horizontal position to offset gravity and improve bead shape. RE-GMAW has shown the ability to improve bead penetration profile as well as reduce LOF issues at weld toes. These improvements can be accomplished using pure argon shielding gas which provides significant cost savings compared to welding with argon – helium mixtures.
Key Words:
Rotating electrode, spin arc, aluminum bead shape, aluminum GMAW |