Low-cost homopolar generator promises to revolutionise welding in infrastructure repairs
Crumbling infrastructure is an issue in many countries. Last year, Italy saw the catastrophic collapse of the Morandi Bridge near Genoa. In the US, many bridges and railways are in urgent need of repair. And here in the UK, bridge closures still regularly cause long-term traffic chaos. Often, the long time needed for repairs is a major reason for works to be delayed: having essential transport links out of action increases delivery times and ramps up costs for industry and agriculture. And part of the reason for this is that it just takes a long time to weld together big bits of metal on site.
Now, researchers at the Cockrell School of Engineering at the University of Texas at Austin claim that they may have made a breakthrough that could reduce the welding time for infrastructure components to mere seconds. The team, at the Cockrell School's Centre for Electromechanics, is working on a technique known as homopolar welding, which has been a subject of interest to University of Texas engineers since the 1990s.
Homopolar welding uses very high electrical currents to join together pieces of metal. Its crucial equipment is the homopolar generator (HPG) which produces these currents in the mega-amp scale. First discovered over 150 years ago, HPGs already have applications in industry, most commonly in forging, but for a variety of reasons have not been widely used for welding.
The most useful characteristic of an HPG is its ability to store energy at low power for a long time in a rotating disc, and then deliver very high power over a short time. It therefore is subject to all the drawbacks of rotating electrical machinery in terms of components such as bearings and bushings, which are subject to wear and need to be regularly maintained. This has meant that HPGs are in general large and expensive machines, not suited to be used on construction sites.
But the potential advantages of homopolar welding are huge, commented Scott Pish, research engineer at the Centre for Electromechanics. “Because no filler material is used and the energy [heat] is delivered to the weld zone as an impulse, less total electrical energy is required to complete the weld,” he said in a statement. “In just a few seconds, we can now weld bridge parts that would otherwise have taken hours to complete.” Moreover, because the energy is delivered over a very short time, it does not change the chemical characteristics of the materials being welded and strength is maintained.
In research sponsored by the US Department of Transport, the Austin team has used components such as ceramic rolling element bearings to replace more expensive hydrostatic bearings, commercial electric brush mechanisms to replace bespoke brushes, variable speed low-inertia induction motors to spin the generator rather than a high pressure hydraulic motor, special-purpose bus bars with cross sections designed to resist high forces, and completely eliminated high-pressure hydraulic auxiliary systems. The result, they claim, is a compact HPG some 60 per cent cheaper than previous versions, which is suited to on-site civil engineering applications.
The range of applications to which the technique might be suited is large, the team claims. The main task it is targeting is butt-splicing together steel plates made by rolling mills to produce the longer plates needed for bridges. Currently, the techniques used to do this have changed little since the 1950s and are slow. Homopolar welding could do it in seconds, they claim. It could also be used to weld together sections of train track. Outside transport, it could also be used to repair oil and gas pipelines, particularly those used in fracking which is a hugely important process in the US and in some parts of Europe. According to welding experts at The Welding Institute (TWI) in Cambridge, pipe welding was one of the first applications explored by the University of Texas team.
Another application area could be in processing nuclear waste. Spent fuel rods must be first welded inside very robust containments before they can be reprocessed. The compact HPG could be part of a welding robot to accomplish this task without exposing humans to radiation. "The HPG has also been used to sinter metal powders,” The Austin team said. “X-ray targets in CT scanners are made from sintered molybdenum and tungsten. The ability to sinter these metals has already been demonstrated with pulsed compaction. An inexpensive generator could have a large role to play in this medical field.”
TWI told The Engineer that producing a compact HPG is only part of the solution to the problem. "The difficulty with using HPGs in welding is in the rotating disc aspect of the machinery," a spokesman explained. "You need to magnetically couple the energy pulse to the piece you are trying to weld, and because the pulse is generated by rotating disc it is circular. You could certainly deliver the very high power needed in a very short time to weld large pieces of steel, but coupling the power into something shaped like a flat plate or a train track rail is another matter.”
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