S.L. Semiatin

Thermomechanical processing of beta titanium alloys—an overview

Thermomechanical processing (TMP) is associated with two major requirements: (i) to produce usable shapes through primary working (ingot breakdown) and secondary mill operations (hot rolling or forging) and (ii) to optimize mechanical properties through microstructure control during the different stages of the thermomechanical process. This paper reviews the thermomechanical processing of beta titanium alloys in general and high temperature deformation mechanisms, microstructure control during TMP, and final mechanical properties in particular.

Modeling microstructural development during the forging of Waspaloy

A model for predicting the evolution of microstructure in Waspaloy during thermomechanical proc-essing was developed in terms of dynamic recrystallization (DRX), metadynamic recrystallization, and grain growth phenomena. Three sets of experiments were conducted to develop the model: (1) preheating tests to model grain growth prior to hot deformation; (2) compression tests in a Gleeble testing machine with different deformation and cooling conditions to model DRX, metadynamic recrystallization, and short time grain growth during the post deformation dwell period and cooling; and (3) pancake and closed die forging tests conducted in a manufacturing environment to verify and refine the model. The microstructural model was combined with finite element modeling (FEM) to predict microstructure development during forging of Waspaloy. Model predictions showed good agreement with microstructures obtained in actual isothermal and hammer forgings carried out at a forging shop.

Workability of a gamma titanium aluminide alloy during equal channel angular extrusion

Canned performs of the titanium aluminide Ti-45.5Al-2Cr-2Nb were hot worked via equal channel angular extrusion (ECAE). The following conclusions are drawn regarding the effects of extrusion temperature and microstructural condition on workability controlled by shear localization: (1) The tendency for nonuniform deformation during ECAE increases rapidly as the preheat temperature decrease. The trend is most pronounced for material in a cast + HIP`ed condition as compared to that in a wrought condition. The nonuniform flow may develop into well defined shear bands and shear cracks in the cast + HIP`ed titanium aluminide. (2) The occurrence of shear bands and the severity of flow localization within the shear bands can be correlated at least on a first-order basis to material flow behavior as quantified by the alpha parameter, the ratio of the normalized flow softening rate to the strain rate sensitivity exponent. (3) Multi-pass ECAE sequences to breakdown and refine the structure of near-gamma titanium aluminide ingot can be designed through proper consideration of the effect of temperature and material condition on flow localization tendencies. However, can design to minimize die chilling may play an important role in industrial implementation of the ECAE process for this alloy system.

Design of metal-matrix composite consolidation practices based on the foil/fiber/foil approach

Abstract The main issues that influence the processing of continuous fiber MMCs based on the foil/fiber/foil approach were examined in order to develop a quantitative method for optimizing such processes. Because of the form of the describing equations, simple graphical means to obtain design data were developed and applied for Ti-6A1-4V/SCS-6 fiber composites. For this particular system, a range of processing parameters that can produce fully dense composites with optimal reaction zone thickness, subject to certain temperature and time constraints, was identified.

Grain growth in a conventional titanium alloy during rapid, continuous heat treatment

The objective of the present work was to analyze the kinetics of beta grain growth during rapid, continuous heating of a conventional alpha-beta titanium alloy. The analysis was based on approximate, closed-form theoretical expressions derived by Bourell and Kaysser and Soper and Semiatin as well as a fully numerical, computer-based approach. The problem and approach discussed here differs from previous investigations of grain growth during continuous heating and cooling, most of which have been for austenite grain growth in the heat-affected zone during welding of steels. In this regard, the main features of the present work are the very high heating rates involved, the avoidance of the application of complex numerical integration schemes, and the avoidance of using isothermal grain growth kinetic data to fit continuous heating results.

Modeling the hot consolidation of ceramic and metal powders

Modeling of the consolidation of ceramic and metal powders by sintering, hot pressing, and hotisostatic pressing (HIP) was conducted using a continuum yield function and associated-flow rule modified to incorporate microstructure effects such as grain growth, pore size, and pore geometry. It was shown that consolidation behavior can be described over the entire range of densities through two parameters, the stress intensification factor and Poisson’s ratio, which are readily measured using uniaxial upset tests. Both parameters are functions of relative density, whose exact dependence varies from one material to another. Furthermore, it was demonstrated that in sinter forging of ceramics, an “apparent” Poisson’s ratio depending on stress level (relative to the sintering stress) gives a quantitative measure of the compctition between sintering and creep deformation. The accuracy of the microstructure-sensitive yield function was established through finite-element modeling (FEM) simulations of the isothermal sintering of a soda-lime glass, sinter forging of alumina, and die pressing of an alpha-two titanium aluminide alloy.

Homogenization of near-gamma titanium aluminides: Analysis of kinetics and process scaleup feasibility

The homogenization of ingot metallurgy near-gamma titanium aluminides was investigated with respect to the kinetics of dissolution of single-phase gamma grains during heat treatment in the alpha phase field as well the determination of a processing window for use in scaleup of such heat treatments for large production billets. Initially, homogenization treatments were conducted using small samples heated in a highly controllable, low thermal inertia heating device. Post heat-treatment metallography and microprobe analysis quantified the kinetics of dissolution and suggested that a reaction at the interface between the remnant gamma grains and the surrounding alpha matrix controlled homogenization. A reasonable fit to the measured kinetics was obtained by assuming a reaction rate proportional to the square of the difference between (1) the maximum possible aluminum concentration in the alpha phase in equilibrium with the gamma phase and (2) the actual aluminum concentration in the alpha phase during the dissolution process. Scaleup feasibility was established using both an approximate closed-form analytical solution and finite difference method calculations for the heating of round cylinders of various sizes and height-to-diameter ratios. The surface-to-center temperature differences derived from such calculations were used in conjunction with the measured homogenization kinetics to establish heating cycles that would produce homogenization of the entire billet while minimizing grain growth and exposure time at high temperature.

Validation of computer models for the consolidation of metal-matrix composites

Abstract Finite element method (FEM) simulation results were compared to experimental observations to establish the suitability of advanced modeling techniques for the prediction of the consolidation behavior of continuous fiber, metal—matrix composites. Two consolidation techniques were examined: hot isostatic pressing (HIP) of foil—fiber—foil layups and HIP of tapecast monotapes. In both cases, the matrix was the alpha-two titanium aluminide alloy Ti—24Al—11Nb (a/o), and the fibers were silicon carbide. Model predictions and accompanying experimental measurements revealed the important effect of the interface friction—shear factor on consolidation time for foil—fiber—foil layups. In addition, the predicted consolidation times for the foil—fiber—foil method were found to be sensitive to small variations in HIP temperature and material flow properties such as the strain-rate sensitivity, especially for low consolidation temperatures. By contrast, predicted consolidation times for tapecast monotape layups were relatively insensitive to the magnitude of the interface friction—shear factor. The kinetics of densification of the tapecast monotapes were well described using an FEM model incorporating a material-sensitive yield function and associated flow rule.

Fiber fracture during processing of continuous fiber, metal-matrix composites using the foil/fiber/foil technique

The fracture of continuous fibers during processing of foil/fiber/foil (F/F/F) metal-matrix composites (MMCs) has been investigated both experimentally and theoretically. Experimental observations revealed that fiber fracture occurs during the heat-up portion of the consolidation cycle primarily in a bending mode in regions where cross-weave wires are present. Based on these observations, a general model that describes fiber fracture as a function of the processing stress and fiber mat geometry was developed. Model results showed that fiber stresses and, hence, the propensity for fracture are very sensitive to the distance between cross-weave wires in adjacent fiber mats; analytical expressions that allow the definition of a critical distance between such cross-weave wires were derived. The model relations demonstrated that fiber fracture is more likely in areas of a composite in which the fibers are arranged in a rectangular, rather than a triangular, pattern. The experimental and theoretical results were used to develop guide-lines for the design of F/F/F layups to avoid fiber fracture during processing.

A novel process for breakdown forging of coarse-grain intermetallic alloys

The objective of the present work was to develop a novel hot forging process for breakdown of high-temperature intermetallic alloys which exhibit dynamic recrystallization during hot working. During typical forging processes in hydraulic processes, be they based on isothermal or conventional approaches, the ram speed (or sometimes the effective strain rate) is held constant during the forging stroke. In the method introduced here, the ram speed is increased substantially during the forging stroke as the material recrystallizes to a finer-grained structure and its hot workability increases. By this means, fracture is avoided, grain size is reduced, and processing time is decreased, thus improving material quality and reducing cost. The material used to develop and demonstrate the novel forging process was the single phase gamma titanium aluminide, Ti-51Al-2Mn.