M.C. de Malherbe

The use of hardness in the study of compaction behaviour and die loading

Abstract An experimental relationship between density, hardness and compacting pressure is obtained for the isostatic compaction of Alcoa grade 1202 atomised aluminum powder. These results are used to evaluate the use of hardness measurement in the determination of density contours within compacts and pressure distributions at the compact—die interface. Density contours and pressure distributions are presented for closed die compacts; the results are in general agreement with those reported in the literature for more complex techniques. The technique is shown to be suitable for use in many situations.

A Hot Compression Testing Apparatus for the Study of Isothermal Forging

An apparatus for uniaxial compression testing has been developed to simulate isothermal forging conditions. The system is capable of applying 100-kN loads at temperatures up to 1200 ± 3°C in a controlled environment and at constant true strain rates between 10−5/s and 1/s. The apparatus is described and its performance discussed. To illustrate its use, results on the flow behavior of nickelbase superalloy compacts and composites are presented that demonstrate the importance of proper control of the testing parameters. The difference in flow strength of the two materials under identical testing conditions is discussed.

Defects in cold-hydrostatically-extruded aluminium, iron and nickel-base powder compacts

Abstract Powder metallurgy fabrication techniques have been developed because of their many advantages and for securing an increased homogeneity of resulting metallurgical structure. Various consolidation and forming processes have been employed for producing shaped parts from metal powders. In the present paper, several physical defects created during the cold-hydrostatic extrusion of iron and aluminium powders, initially compacted by cold isostatic pressing, and of nickel-base superalloy powders, initially hot-isostatically compacted, are reported and described. Comparisons are made with well-known similar defects developed during the extrusion of solid metals. Diagrams are proposed which define — in terms of extrusion pressure and reduction — ranges over which sound or defective compacts are produced.

Closed-Die Forging of Rectangular Billets with Diamond-Shaped Dies

Summary The forging of billets in diamond-shaped closed dies has been investigated. Experiments were conducted at room temperature using Plasticine to simulate the forging of hot-steel. Billets of rectangular cross-section of various height/width ratios were forged between diamond-shaped closed dies of different angles. The effects of die and workpiece geometries and lubrication on die-filling capacity and deformation characteristics, were examined. To observe the metal flow during the process, grid lines were printed on one of the end faces of the billet and flow patterns were determined from a series of photographs of the deformed lines taken at increments of die-stroke. From these deformed patterns deformation modes were able to be suggested. A theoretical study of the metal flow in the initial stages of forging was also conducted. Slip-line field solutions are given to predict the incipient modes of deformation and these are compared with the observed flow patterns.

The Fracture Behavior of Tungsten Wire Reinforced Superalloy Composites during Isothermal Forging

The isothermal forging behavior of a wire reinforced superalloy powder composite has been examined. The material consisted of a Mar-M200 matrix containing 40 pct by volume of tungsten wire and was prepared by hot isostatic pressing. Specimens were deformed by uniaxial compression at constant temperature in the range 1050 °C to 1180 °C, and at constant true strain rates between 10-5 s-1 and 10-1 s-1. Loading was normal to the direction of wire alignment. Microstructural defects existing in the as-pressed composites are compared with defects in the forged materials. An upper bound forming limit occurs when fibers come into contact. However, microstructural damage occurs at lower strains which depends on temperature and strain rate. Observed and calculated values of peak flow stress are used to define practical forming conditions for the material which should avoid the formation of internal damage at low strains.

Spread and fracture patterns in forging superalloy fibre-reinforced composites

Abstract Fibre-reinforced superalloy-matrix composite materials combine superior properties of oxidation resistance, high strength coupled with ductility and toughness at elevated temperatures. However, they are extremely difficult to machine or form by conventional metal working processes due to the strongly anisotropic nature of their properties. Isothermal forging, where the billet and dies are maintained at the forging temperature during deformation, is one of the most important processes evolved in recent years and by combining this process with powder fabrication, highly alloyed casting-type materials can be processed into homogeneous products which are extensively used in the aerospace industry. In the present investigation, the open-die isothermal forging of square-section billets of fibre-reinforced superalloy composites (hot isostatically pressed nickel-base superalloy compacts reinforced with 40% volume fraction of tungsten wires) at constant strain-rates was considered, with the aim of providing useful information concerning the formability of the composite material and, therefore, its industrial applicability, i.e. in the shaping of complex forms. The mode of deformation, the macroscopic fracture behaviour of the composite material and the spread encountered for the various forging parameters has been given. It is concluded that deformation in composites proceeds mainly under conditions of plane-strain, that formability is limited at low strains and that a forging limit criterion of geometric nature is to be expected.

A note on force plane diagrams for elastic and elastic-plastically stressed thick-walled cylinders

Abstract Force plane diagrams, drawn from force equilibrium considerations, have been obtained for elastic, elastic-plastic and plastically stressed thick-walled cylinders subjected to internal pressure deforming under conditions of plane strain. The force on any plane at right angles to the plane of deformation can easily be found by using the diagrams. Force plane diagrams for non-fully plastic cases have not previously been given.

The Influence of Microstructure upon the Creep and Fatigue Crack Growth Behaviour in Inconel 718

The influence of microstructural variations upon the creep and fatigue crack growth rates in Inconel 718 at 650°C have been studied through a fracture mechanics approach. Crack growth rates have been measured for four different heat treatment conditions, one of which was designed to create a serrated grain boundary structure.

Integrated model building in ecology

The application of the modelling process in ecology is outlined. Aspects of modelling as structure‐function, systemic attributes and simulation language are examined. The different levels of model evaluation, its ramifications and implications are traced. The model depends on the process concerned and the sources of the most common errors are mentioned.

Managerial programming for batch processes

The paper describes a method of managerial programming for hatch processes. It is based on a model consisting of throe sections: a production function comprising linear mathematical equations, a managerial function composed of computer decision making elements, and an external managerial function. These functions are linked by a closed-loop model. Productivity and profit are optimized at each iterative step of the programming process. An optimized scheduling of the shop floor is also provided by the computer.