1 Engineering Materials Chapter 3

2 INTRODUCTION Within the last couple of decades, very rapid development of engineering materials has taken place, resulting in a huge number of commercially available materials with a wide spectrum of properties. choice of material The engineer's choice of material is based not only on the physical, chemical, and mechanical properties but also on the technological properties, which describe the suitability of a material for a particular manufacturing process. IMPORTANT MATERIAL PROPERTIES IN MANUFACTURING It is very difficult to state exactly which properties or, more correctly, which combination of properties a material intended for a given process must possess. But it is often possible to identify certain dominating properties or characteristics which any material must have for it to be processed by a given process or process group. To evaluate these technological properties, many specialized test methods have been developed which describe suitability of a material for the particular process or group of processes. Forming from the Liquid Material State: It includes the following phases: Phase 1: meltingPhase 2: forming (creation of shape)Phase 3: solidification (stabilization of shape)

3 INTRODUCTION Forming from the liquid state requires primarily that the material can be melted, and that furnace to do this is available. This depends on the range of melting points and the requirements of the furnace equipment in producing a complete melt. These requirements depend mainly on the chemical composition of the material. If the melt can be produced, the next question is the availability of a suitable mold or die material for an appropriate solidification.

4 Forming from the Solid Material State Forming from the solid material state can be carried out by mass-conserving processes, mass- reducing processes, or joining processes. Mass-Conserving Processes ductility the ductility of a given material decides the surface creation principle and the information increase obtainable without fracture. In the forming of metals, the primary basic process is mechanical plastic deformation. The suitability of a material to undergo plastic deformation is determined primarily by its ductility (measured by the reduction of area in the tensile test). The amount of plastic deformation necessary to produce the desired component depends on the chosen surface creation principle and the intended increase in shape information. In other words, the ductility of a given material decides the surface creation principle and the information increase obtainable without fracture. Stress-strain curves are the most important information source when evaluating the suitability of a material to undergo plastic deformation. The strain at instability, the percent elongation, and the reduction of area are the most important characteristics. For most forming processes, there is a good correlation between the reduction of area and the "formability" of the material. For most forming processes, there is a good correlation between the reduction of area and the "formability" of the material. The stress-strain curves also reveal the stresses necessary to produce the desired deformation. The stresses and strains and the resulting forces, work, and energy are important for tool or die design and for the choice of process machinery.

5 Forming from the Solid Material State The important parameters are state of stress, strain rate, and temperature. state of stressAs mentioned previously, the conditions under which a given process is carried out can influence "formability" to a great extent. The important parameters are state of stress, strain rate, and temperature. Concerning the state of stress, it can be stated that forming under compressive stresses is generally easier than under tensile stresses, since the tendencies toward instability and tensile fracture are suppressed. strain rate High temperatures Furthermore, a superimposed hydrostatic pressure increases formability (ductility) and is utilized in certain processes. In most processes the state of stress varies throughout the deformation zone; therefore, it can sometimes be difficult to identify the limiting state of stress. As seen in Fig. 2.5, the strain rate also influences the ductility of a metal. Increased strain rate leads to decreased ductility and an increase in the stresses required to produce a certain deformation. The most commonly utilized industrial processes are carried out at room temperature; consequently, the strain rate does not create problems. However, for those processes that are carried out at elevated temperatures, the effects of strain rate must be taken into consideration (see Fig. 2.5). High temperatures can result in a material with a constant flow stress (yield stress) which is independent of the strain. In this state the material is able to undergo very large deformations, as the temperature is above the recrystallization temperature, where new strain-free grains are produced continuously and almost instantly.

6 Classification of some Engineering Materials