CHOICE OF MATERIALS 

 

The three fundamental criteria, which determine economic production, are: -

1. Functional design of the part or assembly with maximum simplicity consistent with appropriate aesthetic Quality.
2. Choice of a material from a compromise of physical properties appearance, cost and ease of processing.
3. Choice of the correct process to produce the component so that it is produced no more accurately than necessary for it to fulfil its design requirements, i.e. the process employed should give the lowest unit cost.

 

The object of this part of the course is to look into the way engineering materials are used and their properties, which make them suitable for specific applications.

ENGINEERING MATERIALS

In selecting a suitable material the designer must take into consideration the widely differing characteristics in physical properties, machining properties methods of forming and possible service 1ife, also the manufacturing process that wil1 provide the lowest unit cost.

Materials are of two basic types i.e. metallic or non-metallic.
The attached diagram briefly outlines some of the more important materials from the vast range of materials that are used in engineering.
Few of the materials exist as natural elements in nature particularly the metals, which have to undergo a refining process before they can be used for manufacturing purposes. Once separated they must have an atomic structure which is stableat the required operating temperature over a prolonged period of time.

Commercial forms of Supply

 

After the metal refining stages the material is cast into ingots, which are then processed according to the requirements of particular industries. The supply of material in a form suitable for a specific manufacturing process is an essential requirement in economic manufacture.

Most common metals are either cast or hot worked to achieve the desired shape. The process used for this stage of manufacture is known as primary processes and includes casting, rolling, forging, extrusion, drawing etc.

In many cases, particularly with steel, hot working is used for the primary processes. As well as producing the desired form of the material other advantages are obtained of which the following are the most important: -

1. The material is above its recrystallization temperature and less energy is required to change its shape.
2. Physical properties are generally improved due to the refinement of the grain structure.

Typical forms of supply can be summarised as follows:-

Consideration of mechanical properties in metal

All engineering materials have certain mechanical properties. The most important of these are as follows:-

(1)          Ductility
This is the ability of the material to be drawn out into a smaller cross section by a tensile force without rupturing. Wire drawing depends upon a materials ductility for its successful operation. A ductile material must be both strong and plastic, for example, lead wire is difficult to draw because the strength of lead is low.

(2)           Malleability
This is a materials ability to be hammered, pressed or rolled into a plate without breaking. It requires that the metal shall be plastic but not so dependent on strength, for example, lead is a very malleable material.

(3)           E1asticity
The elasticity of a material is its power return to its original shape after deformation by a force. A material maybe stretched, compressed, or its volume changed by pressure on all sides (e.g. immersion in a liquid). Many materials behave to some extent like powerful elastic, and within limits, will recover their original shape when the load on them is removed. The' elastic limit' of a material is the limit of the elasticity of a material and is expressed in Newtons/mm2

(4)           Toughness
This is the capacity of a material to withstand shock loads without breaking.

(5)           Plasticity
This is a rather similar property, to malleability, and involves permanent deformation without rupture. It is the extreme opposite to elasticity, as may be shown by comparing the behaviour of a piece of elastic rubber and a piece of plasticine under a restraining force. Plasticity is necessary for forging and metals may be rendered plastic by heating them, e.g. steel is plastic when at bright red heat.

(6)           Brittleness
This is the opposite to plasticity and denotes a lack of elasticity. A brittle metal will break when a force is applied. Some forms of cast iron and high carbon steels (files) are examples of brittle metals. Copper becomes brittle near its melting point, but most metals become less brittle when heat is applied. High carbon steel is an example, when cold it is extremely brittle, but can easily be bent and worked when red hot. Brittle metals require care when welding them, due to the lack of elasticity.

(7)           Strength.
This is the ability of the material to withstand large forces without breaking. The force may be either tensile, compressive or shear.

(8)           Hardness
This is a property or a material to resist wear, penetration, scratching or indentation. It can be measured on various scales, viz. Brinell or Rockwell. Hardness decreases with the rise in temperature. The addition of carbon to steel greatly increases its hardness and the operations of rolling, drawing, pressing and hammering greatly effect it.

Alloys-

 

Generally an alloy has a base, this is a single metal which forms more than 50% of the total mass, and one or more alloying elements which may be present in large or extremely small amounts. Obviously the cost of the base metal will have a big effect on the ultimate cost of the alloy, therefore cheap base materials are used wherever possible. Steel is an example of this, iron being the base of steel, and is quite cheap, but expensive alloying elements such as molybdenum and vanadium, may be present in quite small quantities. These small additions can have a decisive influence on certain desirable properties.

Generally speaking, if two or more metals are fused together and allowed to solidify an alloy is obtained.