MATERIALS, PROPERTIES AND SELECTION
Students taking the MANT module are expected to read this overview and all of its sub pages prior to attending the module
From the second half of 20th century onward, great strides have been made in the advancement of material science and the techniques for processing materials. As a result a wide range of materials are now available for any imaginable applications. The development of modern transport, particularly the airplane, is associated with a vast increase in varieties of materials and making full use of their properties. More recently the development of nuclear energy has stimulated the need for entirely new metals and alloys and new composite materials exhibiting properties that could not be achieved by ordinary metals. Furthermore, new plastic materials are finding an ever increasing use in all branches of industry.
Materials make up the basic elements, which all manufacturing processes have to work with. Manufacturing high quality products at a low cost requires a detailed knowledge of complex interactions among a large number of factors including product design requirements, materials and their properties and manufacturing processes that convert these materials into required forms. Today there is a wide range of materials and processes available and the task of selecting the best possible material while minimizing the costs of manufacturing is a major challenge. Meeting such a challenge requires a thorough understanding of the characteristics of materials and processes and the associated manufacturing technology.
TYPES OF ENGINEERING MATERIALS
There are a wide variety of materials available, each with its own properties, advantages, limitations and applications. They can be broadly classified as follows:
1) Metallic Materials
2) Polymeric Materials
5) Others (e.g. glass, wood, semiconductors, ...)
A more detailed summary of some of the materials in these categories is given here.
PROPERTIES OF MATERIALS
When selecting a material for a given application the material properties must satisfy the function and the operating conditions of the component or the structure being designed. The properties, which directly influence the choice of material, can be summarized under the following categories:
Mechanical Properties: e.g. stiffness, strength, ductility, hardness, toughness, etc.
Physical Properties: e.g. density, electrical conductivity, thermal conductivity, etc.
Chemical Properties: e.g. corrosion resistance in various environments.
Manufacturing Properties: e.g. formability, machinability, ease of joining, etc.
The functional requirements of a product are directly determined by the mechanical, physical, chemical properties. However, for the product to be technically manufacturable, the material must have the right manufacturing properties. For example, a forged component requires a material with sufficient flowability without cracking during forging, a cast component requires a material that flows readily in the molten state and fills the mould and on solidification does not produce undesirable pores and cracks.
No two materials have the same properties and the choice is usually decided by the best possible combination of material properties and economical factors which necessitates an optimum solution. Material selection task requires a through and scientific approach and the following major aspects need to be satisfied:
a) Functional properties: The correct combination of mechanical, physical and chemical properties to meet the function and operating conditions of the component.
b) Manufacturing properties: Processing or fabrication properties of the material for the conversion processes needed to convert the material into required shape, such as ease of casting, forming, joining, and machining.
c) Economics: Cost of the material as well as cost of processing the material into required shape. As part of overall economics, both availability and recycling aspect should also be taken into account.
A table containing Relative cost data for common materials is given here.
Material selection involves a complex interaction between component function, material, process, component shape and costs. Those who select materials should at least have a broad and basic understanding of properties of materials and their processing characteristics. The function of the component must be clearly defined in order that the required mechanical properties may be identified. An ideal list of requirements may be easy to arrive at, but a material and process to satisfy all of these requirements is unlikely to exist and an appropriate compromise must be found. It is, therefore, important to distinguish essential properties from desirable properties, those that can be compromised in order to achieve the essential properties. Material properties are often quoted independent of shape but in some circumstances geometry can influence the response of a component with respect to stiffness and strength, to a considerable degree.
An additional factor in selecting a material for a particular component is consideration of the manufacturing process. A process must be found that is capable of making the component shape, with the correct accuracy, and with an acceptable cost. In reality, both material and process selection must be considered simultaneously since not all materials are compatible with every process. For example, steel, nickel and titanium cannot be die cast, ceramic materials cannot be machined using conventional techniques and the complexity of component shape limits the process choice further.
It is also important that both the material and processes used must be controlled during manufacture. For example, an incoming stock of raw material, which shows variations in composition and microstructure, cannot be heat treated and machined easily. A sheet metal showing variations in its cold worked condition will exhibit differences in ‘spring back’ characteristics during forming. A cast component may show inclusions and porosity unless melting operation, mould filling and solidification of the casting are controlled. The final functional or mechanical properties of a component, to a large extent, depend on the degree of control it receives during its processing.
Material property data is widely available in various published form including material handbooks, reference books, publications of many technical societies, etc., however, a speedy access to right information may not always be possible. Tools have been developed to assist in identifying the best material choice for a given set of requirements. These include for example, material comparison charts, which contain plots of one property against another