Modern Control Theory Applied to Remelting of Superalloys

Over the past several years we have worked to develop tools to improve the quality of superalloy ingots produced by vacuum arc remelting (VAR) and electroslag remelting (ESR). Part of this work has focused on developing model-based process controllers that employ predictive, dynamic, low-order electrode melting and ingot solidification models to estimate important process variables. These estimated variables (some of which are not subject to measurement) are used for feedback and to evaluate the health of the processes. Modern controllers are capable of detecting and flagging various process upsets and sensor failures, and can take remedial action under some circumstances. Model-based variable estimates are continuously compared with measurements when available, and the residuals are used to correct the next generation of estimates. This technology has led to improved VAR and ESR melt rate controllers and is currently being used to develop a VAR ingot solidification controller. A first generation ingot pool depth controller has been tested on a laboratory VAR furnace and the results are very encouraging. In this test, a 152 mm diameter Alloy 718 electrode was remelted into a 216 mm diameter ingot, but the technology is easily scaled to industrial sizes. Successful development of this technology could allow for melting at higher powers without the formation of channel segregates (freckles) by stabilizing the ingot solidification zone. It may also allow for the production of larger diameter VAR superalloy ingots than is possible to produce with the current generation of VAR controllers for the same reason.