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| CSIRO | SOLVE | Issue 5 Nov 05 | |
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ARTICLE
LIGHT METALS: The Tip of an Arc Welder
By Jason Major
Powerful magnetic fields such as those found in aluminium, copper and magnesium smelters play havoc with any work involving arc welding. They can make welding difficult and frustrating, often causing slag and molten metal to fly anywhere but where it should go. This makes good welds very difficult to achieve. To find a way to overcome the problem, scientists undertook a study of what actually happens to a welding arc under the influence of a strong magnetic field. A CSIRO research team used high-speed video and computer modelling to expose some long-held secrets about the arc and how it behaves in magnetic fields. The response they came up with was a two-pronged approach: a compact enhancement to the welding torch and a novel power supply. Together they have created a superior welding system that should also improve worker safety and save time, energy and money. Aluminium smelters generate a magnetic field largely because of the high electrical current in the electrolytic cells. The earth’s magnetic field means humans are regularly exposed to about 0.5 gauss (a unit used to measure static magnetic fields). A smelter can generate magnetic fields that reach 1000 gauss. In arc welding, an electric arc between an electrode and the metal to be joined produces the intense heat needed to melt the metal and form a joint. Metal droplets form at the tip of the electrode and are incorporated into the weld puddle – the molten pool of metal that forms at the joint. However, a magnetic field deflects the arc and the molten metal away from the joint because of the current flowing through the electrode and the weld puddle. This makes it hard to maintain and control the arc and achieve a quality weld. At the moment, welders enclose the welding area in bulky metal shields, but these give the arc only limited protection and need a two-person team – one to do the welding and one to continually shift the shield. Sometimes a welding job has to wait until the strength of the magnetic field subsides or there is downtime in the plant operation. Comalco’s senior research engineer, Bill Paul, says the welding system is a useful improvement that will provide options for maintenance and infrastructure expansion that are unavailable at the moment. CSIRO initiated the research 18 months ago through its Light Metals Flagship. Comalco provided its aluminium smelter to test the technologies, and also supplied market feedback and practical input to the research. Before any smelter trials could start, some smart science was needed. The high-speed video could record up to 36,000 frames per second. This gave the team a unique insight into the behaviour of the arc, droplet formation and molten metal in a magnetic field. “The video revealed a droplet’s extreme distortion in strong magnetic fields and with different power sources,” says Dr Voytek Mazur, project leader with CSIRO Manufacturing and Infrastructure Technology. “This became crucial evidence when modelling the torch enhancements.” CSIRO’s novel enhanced torch turns the job into a one-person operation and works in magnetic fields that, at more than 400 gauss, are double the strength that industry can weld in now. CSIRO has initiated patent protection. With the power sources, it was discovered that none of the existing sources used for arc welding had actually been designed to operate in a magnetic field. Dr Mazur’s team tested different power sources in magnetic fields and found the power supply configuration that worked best. The CSIRO configuration produces a strong or ‘stiff’ arc that is more resistant to deflection from a magnetic field. “In combination with the torch enhancements we built a superior welding system, and one that is flexible, easy to use and inexpensive,” Dr Mazur says. There is some fine-tuning to do, such as testing the system in industrial conditions, but if the trials go well they expect a commercial system to be available within a year.
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