Homopolar Pulsed Welding for Offshore Applications

As a low impedance energy storage device, the pulsed homopolar generator (HPG) is capable of delivering a multimegawatt, megampere-current pulse into a resistive or inductive load with high efficiency. Such HPGs have been used for over ten years as power supplies for research in high energy, high-rate processing of metal alloy components and systems. Most of these processes rely on resistive heating during the current pulse to rapidly heat the material as required for a particular process. One such application is homopolar pulsed welding (HPW), a solid state, forged welding process in which heat generation is concentrated at the interface between workpieces as homopolar current is conducted between them. Because of the very high peak current and power (from 8 to 20 kA/cm2 and 50 to 100 kWfcm2), weld time is very short, reducing time-at-temperature exposure and related microstructural changes. Welding is accomplished in air, no flux or filler is used, and the interface disappears completely in a good weld. This paper reports on recent and ongoing research into the weldability by HPW of various alloys applicable to offshore systems, including stainless steel and titanium alloys, but primarily focusing on various grades of high strength steel pipe. The research includes an investigation of weld parameters, sensitivity of the process to the weld parameters, metallurgical and mechanical evaluation of weld quality, and the development of a real time quality assurance capability that can certify nondestructively an HPW weld before it is removed from the welding fixture. The research presented here is sponsored by the National Science Foundation (NSF) and various private industry companies.