Peter R. Sahm

Nickel aluminides: a step toward industrial application

Abstract Intermetallics based on nickel aluminide are subjects of an ongoing development of new high-temperature materials for application as heat shields for combustion chambers and as first-row vanes in industrial gas turbines. Due to the complex geometry of these parts and the complicated machinability of NiAl intermetallics, these components have to be manufactured by near-net-shape casting. This casting technique required an adaptation of the investment casting process to the special properties of NiAl alloys. For this purpose, a special ceramic mold system was developed to prevent casting defects and excessive mold–metal reactions. Suitable casting parameters were established for the production of grain-refined, crack-free heat shields. Accompanying numerical simulations were performed to optimize the investment casting process for the NiAl-alloy helped to save development time and to reduce the number of casting trials noticeably. These modeling results for various casting conditions (mold and pouring temperature, cooling rate) were compared with outcomes of casting trials in aspects of porosity, mold filling and temperature-field development during the casting process.

Influence of convection and grain movement on globular equiaxed solidification

Abstract A two-phase volume averaging model was used to study convection and grain movement, and their influence on the globular equiaxed solidification. Both liquid and solid phases were treated as separate interpenetrating continua. The mass, momentum, species and enthalpy conservation equations for each phase and a grain transport equation were coupled. An ingot casting (Al–4 wt.% Cu) with near globular solidification morphology was simulated. Case studies with different modeling assumptions such as with and without grain movement, and with slip and non-slip boundary conditions for solid phase were presented and compared. Understanding of grain evolution and macrosegregation formation in globular equiaxed solidification was improved.

Primary spacing in directional solidification

A new analytical model is developed to explain the variation in primary spacing λ with growth velocity V. In this model, dendrite growth is resolved into two parts: the growth of the center core and that of the side arms, which are separately treated. In contrast to the assumption in the current models, it is only the dendrite core, not the entire dendrite, whose curvature radius at the tip is directly related to dendrite tip radius R. The primary spacing is considered to be the sum of core diameter and twice the sidearm length. As long as the growth of side arms is suppressed, it becomes cellular growth. As a result, this model gives a reasonable dependence of cell and dendrite spacing on the process parameters. The proposed model has been applied to several alloys to compare its predictions both with experimental data and with the analytical expression of the Hunt-Lu model.

Rapid-Slug-Cooling-Technology (RSCT): a new approach for the production of thixocasting prematerial billets

Prematerial slugs with a globular microstructure serve as feedstock for the thixocasting process. This microstructure is usually produced by inductive stirring of the melt in a modified continuous casting process. Extensive studies at the Foundry-Institute in Aachen show that, for many alloys, the main parameter for the production of suitable prematerial billets is the cooling rate. Therefore the new RSCT-Process (Rapid-Slug-Cooling-Technology) has been developed. With this process a fine and homogeneous microstructure of globular α-phase dendrites can be achieved. It enables, not only a rapid and efficient production of prematerial billets from aluminum and magnesium alloys, but also their in-house recycling. The produced RSCT-Slugs were metallographically analysed with the use of state of the art color etching techniques, which enable a precise measurement of the grain size and the examination of its morphology. In further investigations, the RSCT-Billets were reheated into the semi-solid state and pressed into test dies to examine the mechanical properties.

Numerical study of porosity in titanium dental castings

A commercial software package, MAGMASOFT (MAGMA Giessereitechnologie GmbH, Aachen, Germany), was used to study shrinkage and gas porosity in titanium dental castings. A geometrical model for two simplified tooth crowns connected by a connector bar was created. Both mold filling and solidification of this casting model were numerically simulated. Shrinkage porosity was quantitatively predicted by means of a built-in feeding criterion. The risk of gas pore formation was investigated using the numerical filling and solidification results. The results of the numerical simulations were compared with experiments, which were carried out on a centrifugal casting machine with an investment block mold. The block mold was made of SiO2 based slurry with a 1 mm thick Zr2 face coat to reduce metal–mold reactions. Both melting and casting were carried out under protective argon (40 kPa). The finished castings were sectioned and the shrinkage porosity determined. The experimentally determined shrinkage porosity coincided with the predicted numerical simulation results. No apparent gas porosity was found in these model castings. Several running and gating systems for the above model casting were numerically simulated. An optimized running and gating system design was then experimentally cast, which resulted in porosity-free castings.

Undercooling of superalloy melts : basis of a new manufacturing technique for single-crystal turbine blades

The solidification of an undercooled superalloy melt has been investigated to advance a new technique for single-crystal (SC) turbine blade manufacture, the so-called autonomous idrectional solidification (ADS) technique. A special SiO2-free ceramic shell coated inside with a nucleation-inhibiting amorphous layer was used for the mould material. A small longitudinal temperature gradient ensured the alignment of the dendritic structure. A maximum undercooling of about 80 K could be achieved for the superalloy CMSX-6. It was found that above a critical undercooling of about 30 K the dendritic structure changed into a fine equiaxed grain structure. The size of the equiaxed grains was comparable with the secondary arm spacing of the dendrites. Within specimens with a large longitudinal temperature gradient, a transition between the fine-grained and the dendritic structure was observed. In these regions equiaxed grains are present side by side with primary dendrite trunks. Thus the temperature distribution within the undercooled superalloy melt has to be controlled carefully in order to produce an SC microstructure. Nevertheless, it could be shown that it is possible to produce SC superalloy turbine blades by the ADS technique.

Numerical simulation of the casting process of titanium tooth crowns and bridges

The objectives of this paper were to simulate the casting process of titanium tooth crowns and bridges; to predict and control porosity defect. A casting simulation software, MAGMASOFT, was used. The geometry of the crowns with fine details of the occlusal surface were digitized by means of laser measuring technique, then converted and read in the simulation software. Both mold filling and solidification were simulated, the shrinkage porosity was predicted by a “feeding criterion”, and the gas pore sensitivity was studied based on the mold filling and solidification simulations. Two types of dental prostheses (a single-crown casting and a three-unit-bridge) with various sprue designs were numerically “poured”, and only one optimal design for each prosthesis was recommended for real casting trial. With the numerically optimized design, real titanium dental prostheses (five replicas for each) were made on a centrifugal casting machine. All the castings endured radiographic examination, and no porosity was detected in the cast prostheses. It indicates that the numerical simulation is an efficient tool for dental casting design and porosity control.

Porosity formation in axi-symmetric castings produced by counter-pressure casting method

We propose a method for simultaneous treatment of heat and mass transfer processes and porosity formation of castings produced by Counter Pressure Casting (CPC) method. The method enables us to account for the influence of the CPC parameters on the mechanical properties of the casting. Numerical results and comparison with experimental data are given for an axis-symmetric casting (hemisphere with constant wall thickness).

Coupled modelling of the solidification process predicting temperatures, stresses and microstructures

Abstract A coupled modelling of various phenomena which characterize the solidification of castings is presented. Thermal and mechanical calculations are linked by taking into account local and transient heat transfer coefficients at the casting mould interface dependent on the contact pressure or the air gap width. A new approach was developed to model this air gap-contact problem. A sophisticated microstructure model of dendritic solidification is introduced to determine the release of latent heat which influences the temperature evolution. Such a comprehensive simulation predicting temperatures, stresses and microstructure leads to a more accurate description of the casting process in order to optimize process parameters especially with regard to mechanical properties of the cast part.

Numerical simulation of the casting process of titanium removable partial denture frameworks

The objective of this work was to study the filling incompleteness and porosity defects in titanium removal partial denture frameworks by means of numerical simulation. Two frameworks, one for lower jaw and one for upper jaw, were chosen according to dentists’ recommendation to be simulated. Geometry of the frameworks were laser-digitized and converted into a simulation software (MAGMASOFT). Both mold filling and solidification of the castings with different sprue designs (e.g. tree, ball, and runner-bar) were numerically calculated. The shrinkage porosity was quantitatively predicted by a feeding criterion, the potential filling defect and gas pore sensitivity were estimated based on the filling and solidification results. A satisfactory sprue design with process parameters was finally recommended for real casting trials (four replica for each frameworks). All the frameworks were successfully cast. Through X-ray radiographic inspections it was found that all the castings were acceptably sound except for only one case in which gas bubbles were detected in the grasp region of the frame. It is concluded that numerical simulation aids to achieve understanding of the casting process and defect formation in titanium frameworks, hence to minimize the risk of producing defect casting by improving the sprue design and process parameters.