The eroded condition of the U.S. infrastructure is a hot topic in the news these days. Many roads and bridges are in need of long overdue repairs. The University of Texas at Austin Center for Electromechanics (UT-CEM) has recently developed a new lower-cost Homopolar Generator design that could help. Welding large steel cross sections, like those found on bridge and overpass girders, can be a very labor intensive and thus expensive process for manufacturers of large structures and assemblies. Highly skilled welders and/or expensive automated welding machines are required to complete multiple passes on large cross section welds to form the proper joint. The cross section required for these large welds can be several inches thick and several tens of inches in width. The present method for joining these large steel plates is submerged arc welding or SAW. SAW is an automated process that is used to lay down multiple passes of weld material to join the plates, but this is a very time consuming process and can take up to two days to complete a weld joint in these large steel plates.
To address this issue, the U.S. Department of Transportation (DOT) issued a request for proposals under the U.S. Small Business Innovation Research (SBIR) program seeking less expensive and faster methods of welding these large steel plates for bridge girders. UT-CEM, in partnership with Koo and Associates Inc. (KAI), were recently awarded SBIR Phase 1 funding to develop homopolar generator (HPG) driven pulsed electrical welding as a less expensive and faster solution for welding bridge girders. HPG Pulse Welding of large cross sections can be completed in hours rather than days and also offers improved mechanical properties in the weld zone.
Unlike traditional weld processes that keep the weld zone hot for extended periods of time, Pulse Welding locally heats the weld zone to forging temperature in seconds without heating the parent metal immediately adjacent to the weld, thus the heat energy can quickly dissipate. The shorter time-at-temperature seen in the weld zone prevents degradation of the metal mechanical properties; thus resulting in a high strength weld with almost identical properties as the parent metal. The DOT SBIR project is multi-phased, with the following objectives:
- Design a new lower-cost HPG power supply
- Manufacture a subscale prototype
- Design and fabricate a welding fixture for producing subscale steel weld samples
- Provide pulse welded steel samples for mechanical properites testing
During Phase 1 of this project, the UT-CEM and KAI team leveraged a HPG welding method originally developed and demonstrated for a variety of materials and applications approximately 30 years ago at UT-CEM. Unfortunately, the initial capital cost of the power supply prevented it from being commercialized. The team recently completed the Phase 1 of this project, completing the first two objectives, and successfully demonstrating the prototype generator performance.
To lower the overall cost of the HPG, the new machine design utilized developments in ceramic rolling element bearings, off the shelf brush hardware, and a more simplistic machine topology. The project team estimates that the cost of the new design is approximately 1/3 of the cost of early designs. The research team has successfully tested the HPG generator to full speed, 4200 rpm, and full excitation field. However, the inherent low output voltage of this type of generator has always limited the possible output current due to the impedance of the output load. The peak output demonstrated to date is approximately 200,000 amps, which is one-fifth of the peak current of one million amperes required to weld 12 square inch steel samples for mechanical testing. The research team is presently working on the proposal to the DOT for the next phase of the program which is to design and build a welding fixture and provide weld samples for testing. The welding fixture will be designed with extremely low impedance to allow the low voltage HPG to produce the pulsed output currents required for welding.
As part of the HPG preliminary testing and commissioning, the output of the HPG was connected to an aluminum block clamping fixture which holds a one inch diameter steel bar that is three inches long between the clamps. As a demonstration of the effectiveness of energy transfer from the stored kinetic energy in the HPG rotor to a pulsed electrical output from the generator, the HPG was discharged from 2000 rpm (half design speed) and full field current to pulse heat the steel bar. The steel bar was heated to 1100 C (~ 2000 F) in approximately 10 seconds.