Welding Processes
Laser welding of zinc coated steels *
Laser welding of zinc coated steels in an overlap configuration remains a challenge. A simple overlap joint geometry is desirable in these applications but has been shown to be extremely detrimental to laser welding because the zinc vapour formed at the interface between the two sheets expands into the keyhole and disrupts fluid flow in the melt pool, which often leads to metal ejection. Instead, a gap must be left between the sheets to allow zinc vapour to escape. However, macroscopic welding defects can be substantially eliminated in the zero gap configuration with an elongated keyhole, which prevents periodic keyhole collapse. High speed video provides insight into the keyhole dynamics during welding. It appears that the dynamic pressure of zinc vapour can effectively elongate the keyhole with a thick zinc coating and the process can reach a stable state when an elongated keyhole is continuously present. A simple analytical model has been developed to describe the influence of zinc vapour on keyhole elongation. The research suggests that elongating the keyhole with a modified lasr beam configuration should produce a stable weld regardless of the zinc coating thickness.
Elongated keyhole
Unstable keyhole
The Figures show keyholes photographed through the welding head (coaxial with the laser beam), together with the resultant weld appearances. The keyhole oscillates but is stable in the case of the uncoated steel. The keyhole closes intermittently in the case of the 7μm coating, resulting in an unstable weld. The keyhole oscillates but remains open in the case of the 20 μm coating, resulting in a good quality weld.
Uncoated steel
20 μm Zn Coated
7 μm Zn Coated
* Pan, Y and Richardson, I.M. ‘Keyhole Behaviour during Laser Welding of Zinc-coated Steel’ J. Phys. D: Appl. Phys. 44,045502, 2011.
Gas Metal Arc Welding - underwater ⁑
Welding arcs become increasingly unstable with increasing ambient pressure (increasing water depth), making process control difficult. Nevertheless, the gas metal arc welding process has been developed for deep water, diverless operations and has been successfully applied for hot-tap seals and pipeline tie-ins. Successful application is based on control of the dynamic response (d𝐼/dt) of the power source and suppression of plasma jets by means of arc length reduction, to avoid disruption of metal transfer and the ejection of spatter droplets. Arc length self-adjustment requires a change in output slope of the power source to compensate for the increased arc voltage at elevated pressures. Power source control is implemented to ensure that a unit change in arc length (∆𝓁) produces a constant variation in arc current (∆𝐼) independent of ambient pressure. Despite the inevitably unstable arc at high pressures, it has been shown that the welding process can be held stable to water depths of at least 4,000 m and the physics of the process suggest that the depth range can be extended significantly, without loss of process performance.
Illustration of arc jet behaviour and its influence on metal transfer at elevated pressures.
Cross-section of a weld made in 13Cr steel at a pressure equivalent to 1,600 m water depth.
Above: Orbital pipeline welds on 30 mm thick API X65 made at a pressure equivalent to 1,630 m water depth.
Below: Mechanical properties of the weld.
Transient (left) and averaged (right) electrical signals from the GMA welding process operating at 1,630 m water depth.
⁑ Richardson, I.M.; Nixon, J.H.; Nosal, P.; Hart, P. and Billingham, J. ‘Hyperbaric GMA Welding To 2,500m Water Depth’, Proc. Proc. Int. Conf. OMAE, New Orleans, USA, Paper OMAE 2000 - 2160, February 14-17 2000.
See also
Nixon, J.H.; Hart, P.R. and Richardson, I.M. ‘Diverless Underwater Welding: Theory and Operation’, Proc. Int. Conf. ICAWT 2000: Gas Metal Arc Welding For The 21st Century, Orlando, Florida, December 6-8, 2000.
Woodward, N. 'Developments in Diverless Subsea Welding', Welding Journal 85(10):35-39, October 2006.
Richardson, I.M.; Woodward, N.J.; Armstrong, M.A.P. and Berge, J.O. ‘Developments in Dry Hyperbaric Arc Welding - A review of progress over the past ten years’, Proceedings of the International Workshop on the State of the Art, Science and Reliability of Underwater Welding and Inspection Technology, Houston, Texas 17-19 November (2010). Ed. Liu, S; Olson, D.L.; Else; M.; Merritt, J. and Cridland, M., Publ. ABS, ISBN 0-943870-06-2.
Hot-spot formation during resistance upset butt
welding ♰
Resistance upset butt welding (RUW) has been employed for wheel rim manufacturing for many decades. The trend toward improved production methods and the use of high strength materials, accompanied by an increasing demand for knowledge about the RUW process fuelled investigation of process behaviour. One of the phenomena observed is an uneven heat distribution and temperature profile resulting in excessive heating, referred to as hot-spot formation, toward the outer edges of the workpiece. A comparison of experimental observations with predictions from a simple thermal model indicate that small differences in contact resistance and/or contact pressure between the faying surface will always generate uneven heating. Excess resistive heat dissipation (𝐼²R) occurs in regions of higher contact resistance. The associated temperature rise results in an increase in local resistivity and a runaway local temperature rise. Control of the interfacial pressure distribution, interface geometry and material cleanliness is important for optimum process control and the reduction of weld rejection rates.
Above: (a) Welding equipment and (b) a schematic overview of the attachment positions of thermocouples (red crosses) and voltage probes (black dots).
Below: Voltage distribution over the length of the weld sample.
Temperature modelled with uniform current density distribution (leftmost) and with non-uniform current density distribution.
Below: Photo of a solidified hotspot and temperatures measured on the joint centre line.
♰ Kerstens, N.F.H. and Richardson I.M. ‘Heat Distribution in Resistance Upset Butt Welding’, Journal of Materials Processing Technology 209, 2715–2722, 2009.
Measured temperature profile at the joint interface.
