Melting, casting and forging problems in titanium alloys

Titanium alloys exhibit a number of problems at the billet evaluation stage which are connected to the melting and casting processes used in their preparation. Some of these, such as the alpha I and HDI defects originate principally in the raw materials used; others such as alpha II and beta fleck are connected with the melting practice. The defect-forming mechanisms will be briefly reviewed and the present incidences discussed. The importance of the defect incidence in relation to final product quality depends on a complex interaction of property requirements, inspection techniques and alloy structure. This interaction is specific to the alloy concerned and is discussed in three cases; premium quality 6/4 and Ti-17, CPTi and airframe quality 10/2/3. These three cases illustrate the importance of the interaction of inspectability, forging practice and the defect incidence obtainable by the various melting methods. It is concluded that conventional melting by double or triple VAR gives a product whose reliability in respect of these defects is uncomfortably close to the limits imposed by probabilistic design and fracture mechanics analysis. Efforts to improve this situation should centre around the increased use of electron beam techniques and also around the use of forging sequences designed to increase inspectability of the billet product. (C) 1998 Elsevier Science S.A. All rights reserved.