Composition Control in Hearth Melting Processes

摘要

The present techniques of both electron beam and plasma hearth melting both introduce the possibility of significant axial composition change in the ingots of titanium alloy produced. This aspect of the processes has been a strong component of the logic leading to the specified production methods of hearth melting plus VAR. The disadvantages of this specification are not only economic, but also technical. The VAR step may introduce defects which were not present in the hearth melted product. Electrode preparation for VAR can introduce alpha-1 problems; the VAR ingot thermal regime can lead to beta fleck and to alpha-2 defects. Hearth melting has been shown to remove the alpha-1 causes very effectively It has a very different thermal regime to that found in VAR in which the temperature gradients in equivalent geometry ingots melted at the same rate are less than half those in VAR. This situation leads to an enhanced structure in which beta fleck segregation is not usually found. The implications of this for near-beta alloys and for low cost alloys using significant additions of iron are important. In this work we present the results of computations and experiment to demonstrate the extent to which composition may be controlled in the hearth processes. We consider the evaporative process in EB relative to the temperature regime and metal flow; the effect of non-uniform feedstock; and the potential for axial segregation in the ingot. In addition we present techniques and strategies for on-line composition control in the processes, by X-ray and other methods, with considerations as to the validity and industrial viability of the techniques. We conclude that in an industrially-viable control regime, the composition uniformity of the ingot produced should fall within that expected for most products hence removing the requirement for a further VAR homogenisation step.