Author: R Smith
What Is Vacuum Arc Remelting?
Remelting processes are secondary melting processes used when cleanliness and homogeneity requirements are beyond the capability of conventional production and casting processes. One such approach is vacuum arc remelting (VAR), where a starting electrode, cast from a prior production process, is gradually remelted under vacuum with heat supplied via an electric arc. The bottom of the electrode gradually melts and drips down to a molten pool, which continually solidifies, forming the final ingot. Figure 1 depicts a rough diagram and an image of VAR furnaces. There are three main parts to a VAR process: start-up, steady state, and hot top.
Figure 1: Simplified diagram of VAR process and VAR furnaces at Carpenter Technology.
Carpenter Technology's 2D axisymmetric VAR model
As computing power has increased and models become more advanced, computer modeling of processes such as VAR has seen a substantial increase in applications. Carpenter Technology utilizes a 2D axisymmetric VAR model that can include start-up and hot top conditions. The model takes inputs such as melt rate, current, and
alloy properties, and its outputs include conditions during melting, such as temperature distribution and liquid pool shape.
Prediction of liquid pool shape can be used to determine process conditions that result in desired pool shapes and depths. Excessive pool depths, for instance, can lead to formation of serious defects such as freckles. Another use of pool shape prediction is to determine the final solidification point at the end of the hot top part of the process, which can be used to limit the amount of the ingot affected by the hot top.
2D graphs of model output display pool shape and liquid fraction applied to optimizing the hot top part of the process and the steady-state melt rate. The hot top illustrated in Figure 3 represents a series of snapshots of the liquid pool at the end of the process. This can be used to visibly determine the location where the final solidification occurs and thereby compare the effects of different hot top schedules.
Figure 3. Cross-section x-y plots showing final solidification of a VAR ingot.
A comparison of the effect of different melt rates on pool shape is shown in Figure 4. Increasing melt rate has a visible effect on the pool's depth and shape, and these results can be used to choose a melt rate to meet desired pool conditions.
Figure 4. Effect of increasing melt rate on pool depth and shape
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Reference:
K.M. Kelkar, S.V. Patankar, A. Mitchell, O. Kanou, N. Fukada, K. Suzuki, “Computational Modeling of the Vacuum Arc Remelting (VAR) Process Used for the Production of Ingots of Titanium Alloys”, LMPC 2007: Proceedings of the International Symposium on Liquid Metal Processing and Casting, Nancy, France, September 2-5, 2007.
