Selective evaporation of some titanium alloys components in the electron beam furnace

Abstract

The use of an electron beam furnace for casting parts in titanium alloys assures high purity products, avoids any undesirable contamination, permits a high overheating of the bath and a direct recycling of scraps. On the other hand there are some disadvantages, and specifically the loss of the most volatile alloy elements due both to the high overheating and the high vacuum degree. Therefore the manufacture of certain critical castings such as some aircraft cast structures, whose composition has to face very strict tolerances, becomes a delicate problem. The purpose of this work was to study the properties of melting, overheating and casting in the electron beam furnace of two types of titanium alloys: Ti-5A1-2.5Sn and Ti-6A1-4V; as a reference the behaviour of commercially pure titanium was also investigated. The evaporation tests were carried out on cylindric specimens in a laboratory type electron beam furnace, working under controlled conditions which may be easily reproduced. After the treatment each test piece was cut into sections, metallographycally examined and analyzed by an electron microprobe, in order to obtain the composition profiles on several planes normal to the base. In this way evaporation rates of the various alloy components as well as vapour phase compositions were determined; by employing a sufficiently large number of samples it was, therefore, possible to set out the evaporation curve of each alloy component. The obtained results, which have been also theoretically discussed, enable useful suggestions for the proper use of an electron beam furnace for industrial production.