J.-C. Gebelin

Modelling of the investment casting process

Abstract The aim of this paper is to present the modelling work carried for the FOCAST project in order to acquire a better understanding of the physical phenomena which control the investment casting processes. The paper will first present the general approach used by the authors. Then, for each process stage studied, the models used and their hypotheses are presented and discussed. The parameters needed for the modelling are also listed. From these models, and using simulation packages, numerical simulations have been carried out. The results obtained are presented, with a special focus on the comparison with experimental results where available.

Analysis of the mechanical deformation arising from investment casting of directionally solidified nickel-based superalloys

Abstract To provide insight into the factors causing recrystallisation of nickel-based single crystal superalloys, analysis of the thermal–mechanical deformation caused by investment casting of these components is presented. Three-dimensional thermal–mechanical finite element analysis is first used to demonstrate that the reaction of the casting and mould—at least in the aerofoil section—can be approximated as one-dimensional. One-dimensional models are then built based upon static equilibrium for plasticity on the microscale caused by differential thermal contraction of metal, mould and core, using temperature dependent material properties. The models take various forms to study the mechanical response under different situations relevant to practical applications. The results indicate that the plastic strain causing recrystallisation is likely to be induced during cooling at temperatures above 1000°C. The relative importance of thicker and stiffer ceramic shells is studied. Our analysis indicates that it is important to account for creep deformation for such applications.

Linear friction welding of Ti6Al4V: experiments and modelling

Abstract Linear friction welding of the Ti6Al4V alloy is studied. A new definition of the energy input rate is proposed, based on an integration over time of the in-plane force and velocity; a strong correlation with the upset rate is then found. The effective friction coefficient is estimated to be 0·5±0·1 for varying frequencies and amplitudes, with only a weak dependence on the processing conditions displayed. A model is proposed that accounts for both the conditioning and equilibrium stages of the process, which is shown to be in good agreement with the experimental data. The model is used to study the mechanism by which the flash is formed. A criterion is proposed by which the rippled nature of its morphology can be predicted.

Modelling of inertia welding of IN718 superalloy

Abstract A simple model for the inertia welding of a nickel based superalloy is proposed. The heat flow occurring in the vicinity of the joint is considered, assuming it to be one-dimensional, and this is coupled to a treatment of the stress state expected there using Hill’s general method, so that the upset can be estimated. A state variable constitutive model is included, for the IN718 alloy. It is demonstrated that many of the important characteristics of the process are predicted correctly. It is shown that the shear stress developed at the last stage of the process must be accounted if the upset is to be correctly predicted. The results are compared with those from a 2½D finite element model of the process, and the differences rationalised.

Effect of spiral shape on grain selection during casting of single crystal turbine blades

Abstract In the development of turbine blades, solidification structures have progressed from equiaxed to directionally solidified (DS) and then to single crystal (SX). The transition from DS to SX was achieved by introducing a grain selector which consists of two parts: a starter block referring to the grain orientation optimisation and a spiral part to ensure that only one grain can eventually survive and grow into the blade. With emphasis on the spiral selector, the microstructure evolution and grain competitive growth is visualised using a coupled macroscale ProCAST and mesoscale cellular automaton finite element (CAFE) model in this study. To improve the efficiency of the spiral grain selector and to save cost in casting, the effects of spiral geometries on the grain selection are investigated. Simulation results reveal that the spiral becomes more efficient in grain number selection with a smaller spiral thickness (d T) and a larger spiral diameter (d S).

Validation of a Model of Linear Friction Welding of Ti6Al4V by Considering Welds of Different Sizes

A model for the linear friction welding of the alloy Ti6Al4V was tested experimentally. Instrumented welds were carried out on rectilinear geometries of various dimensions, and the thermal profiles, upset rates, in-plane forces and subsequent micro hardness were measured for comparison. In particular the effects of weld size perpendicular and parallel to the oscillation were investigated, including a case in which the two sides of the weld had different sizes. The predictions of the model were found to be in good agreement with the experimental results, which provides confirmation that the model is useful for the purposes of design.

Hydrogen Transport and Rationalization of Porosity Formation during Welding of Titanium Alloys

The transport of hydrogen during fusion welding of the titanium alloy Ti-6Al4V is analyzed. A coupled thermodynamic/kinetic treatment is proposed for the mass transport within and around the weld pool. The modeling indicates that hydrogen accumulates in the weld pool as a consequence of the thermodynamic driving forces that arise; a region of hydrogen depletion exists in cooler, surrounding regions in the heat-affected zone and beyond. Coupling with a hydrogen diffusion-controlled bubble growth model is used to simulate bubble growth in the melt and, thus, to make predictions of the hydrogen concentration barrier needed for pore formation. The effects of surface tension of liquid metal and the radius of preexisting microbubble size on the barrier are discussed. The work provides insights into the mechanism of porosity formation in titanium alloys.

Coupled thermodynamic/kinetic model for hydrogen transport during electron beam welding of titanium alloy

Abstract Hydrogen transport during the welding of titanium alloy Ti–6Al–4V is analysed. A coupled thermodynamic/kinetic treatment is proposed in which the driving force for hydrogen migration is its chemical potential gradient, which is in turn calculated using the Thermo-Calc software package. The model is applied to the case of the electron beam welding of Ti–6Al–4V, for which a simple process model is presented for the temperature evolution expected. There is a thermodynamic driving force for accumulation of hydrogen in the weld pool. However, agreement with the limited amount of experimental data in the literature for the hydrogen field caused by welding indicates that account needs to be taken of the hydrogen degassing from the weld pool.

Effect of moisture upon mechanical properties of ceramic moulds during high pressure steam dewaxing

Abstract Investment casting research is being carried out by the University of Birmingham, sponsored by EPSRC and a consortium of industrial companies. The programme is aimed at developing a fundamental understanding of the process, with a view to routinely producing sound, net shape castings. A key stage within the investment process is that involving the removal of wax from the unfired ceramic shell. This important process is carried out within the confines of a sealed pressure vessel, more commonly referred to as a Boilerclave (trademark, Leeds and Bradford Boiler Co. Ltd, UK) with external pressure gauges as the only indication of what is actually happening inside the cavity. The dewaxstage is a key stage in the process as wax needs to be removed from a weakceramic shell system without cracking or dimensional alteration, which would be reflected in outoftolerance casting scrap. Owing to the nature of the process, wax removal is probably the least understood or controlled aspect of the whole investment sequence. The following paper is the third in a series which contain results obtained at the University of Birmingham using a specially instrumented stream autoclave which allows visual data capture, thermal and steam pressure profiles within the chamber and thermal instrumentation of waxes and shells to be obtained. The results of simple conductivity calculations and experimentation to determine the effect of moisture upon the strength of ceramic shells will be presented and the strengthening role of wax penetration into the primary coat of the ceramic shell during dewax will be investigated.

Implementing CFD Modelling to Address Defect Formation in Core Injection Moulding

Ceramic cores are used in casting processes to create complex internal shapes within the final component. The work presented here uses Computational Fluid Dynamics (CFD) analysis to predict the filling and solidification behaviour of ceramic core material during the Ceramic Injection Moulding (CIM) process in the large, complex geometries now typical of modern cores. The aim of the study is to develop a predictive capability to identify key defects in the core that might otherwise only be observed after a number of expensive manufacturing processes. Manufacturing trials using short shots have been carried out in order to validate the transient flow patterns of the paste; this has highlighted the occurrence of jetting, weld lines and flow defects that are highly dependent on the injection parameters and runner designs employed. Analysis of the modelled solidification, shear rates, stagnation points and phase migration has driven die and process optimisation in a production environment.