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UNIT 1

INTRODUCTION OF MANUFACTURING SCIENCE I

By:Zoha NasirAssistant ProfessorJIT, Barabanki

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IntroductionProduction or manufacturing can be simply defined as value

addition processes by which raw materials of low utility and value due to its inadequate material properties and poor or irregular size, shape and finish are converted into high utility and valued products with definite dimensions, forms and finish imparting some functional ability.

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Manufacturing ProcessesThis refers to science and technology of manufacturing products effectively, efficiently, economically and environment-friendly through:

Application of any existing manufacturing process and system Proper selection of input materials, tools, machines and

environments. Improvement of the existing materials and processes Development of new materials, systems, processes and

techniques

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Manufacturing Science and technology are growing exponentially to meet the growing demands for:

Increase and maintenance of productivity, quality and economy specially in respect of liberalization and global competitiveness.

Making micro and ultra precision components for the modern electronics, computers and medical applications.

Processing exotic materials, coming up with rapid and vast advent of science and technology like aerospace and nuclear engineering.

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Classification Of Manufacturing ProcessAll such manufacturing processes can be broadly classified in four major groups as follows: (a) Shaping or forming Manufacturing a solid product of definite size and shape from a given material taken in three possible states: in solid state – e.g., forging rolling, extrusion, drawing etc. in liquid or semi-liquid state – e.g., casting, injection moulding etc.. . in powder form – e.g., powder metallurgical process. (b) Joining process Welding, brazing, soldering etc. (c) Removal process Machining (Traditional or Non-traditional), Grinding etc. (d) Regenerative manufacturing Production of solid products in layer by layer from raw materials in different form: • liquid – e.g., stereo lithography • powder – e.g., selective sintering • sheet – e.g., LOM (laminated object manufacturing) • wire – e.g., FDM. (Fused Deposition Modelling)

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Continued… There are four basic production processes for producing desired shape of a

product. These are casting, machining, joining (welding, mechanical fasteners, epoxy, etc.), and deformation processes.

Casting process exploit the fluidity of a metal in liquid state as it takes shape and solidifies in a mold.

Machining processes provide desired shape with good accuracy and precision but tend to waste material in the generation of removed portions.

Joining processes permit complex shapes to be constructed from simpler components and have a wide domain of applications.

 Deformation processes exploit a remarkable property of metals, which is their ability to flow plastically in the solid state without deterioration of their properties.

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Elastic And Plastic Deformation

Deformation is the change in dimensions or form under the action of applied forces .

Deformation is caused either by mechanical action of force or by various physical processes.

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Continued…

Elastic deformation is defined as the deformation which completely disappears as soon as action of external forces ceases.

When the stresses exceeds the elastic limit, plastic deformation starts.

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Yield criteria

It is a hypothesis concerning the limit of elasticity under any possible combination of stresses.

The yield criteria is obtained from stress strain curve either as a characteristic value or as yield strength.

The yield criteria is also called the plasticity condition.

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Tresca’s Yield Criteria

According to tresca’s theory, “the plastic flow initiates when the maximum shear stress reaches a critical value”.

This limiting value is defined as shear yield stress K.

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Von- Mises Yield Criteria

This criteria is based on the distortion energy theory.

According to this criteria, “yielding begins when the shear strain energy reaches a critical value” for a particular material.

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Hot Working V/S Cold Working To deform the metal permanently, the stress must exceed the elastic limit.

At room temperature, the metal is in a more rigid state than when at higher temperature. Thus, to deform the metal greater pressures are needed when it is in cold state than when in hot state.

The amount of deformation that a metal can undergo at room temperature depends on its ductility.

The higher the ductility of a metal, the more the deformation it can undergo.

Pure metals can withstand greater amount of deformation than metals having alloying elements, since alloying increases the tendency and rapidity of strain hardening.

Metals having large grains are more ductile than those having smaller grains.

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Cold Working

Plastic deformation of metals below the recrystallization temperature is known as cold working.

It is generally performed at room temperature. In some cases, slightly elevated temperatures may be used to provide increased ductility and reduced strength.

Cold working offers a number of distinct advantages, and for this reason various cold-working processes have become extremely important.

Significant advances in recent years have extended the use of cold forming, and the trend appears likely to continue.

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Advantages  In comparison with hot working, the advantages of cold working are:

No heating is required. Better surface finish is obtained. Better dimensional control is achieved; therefore no secondary

machining is generally needed. Products possess better reproducibility and interchangeability. Better strength, fatigue, and wear properties of material. Directional properties can be imparted. Contamination problems are almost negligible.

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Warm Working

Metal deformation carried out at temperatures intermediate to hot and cold forming is called Warm Forming . Compared to cold forming, warm forming offers several advantages. These include:

Lesser loads on tooling and equipment Greater metal ductility Fewer number of annealing operation ( because of less strain hardening )

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Hot Working

Plastic deformation of metal carried out at temperature above the recrystallization temperature, is called hot working.

Under the action of heat and force, when the atoms of metal reach a certain higher energy level, the new crystals start forming. This is called recrystallization.

When this happens, the old grain structure deformed by previously carried out mechanical working no longer exist, instead new crystals which are strain-free are formed.

In hot working, the temperature at which the working is completed is critical since any extra heat left in the material after working will promote grain growth, leading to poor mechanical properties of material.

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Advantages  In comparison with cold working, the advantages of hot working are:

No strain hardening. Lesser forces are required for deformation. Greater ductility of material is available, and therefore more deformation is

possible. Favorable grain size is obtained leading to better mechanical properties of

material. Equipment of lesser power is needed. No residual stresses in the material.

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FORGING

Forging is a process in which material is shaped by the application of localized compressive forces exerted manually or with power hammers, presses or special forging machines.

The process may be carried out on materials in either hot or cold state. When forging is done cold, processes are given special names.

Therefore, the term forging usually implies hot forging carried out at temperatures which are above the recrystallization temperature of the material.

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Some common forging processes are:

Open – die hammer forging Impression – die drop forging Press Forging Upset Forging Swaging Rotary Forging Roll forging

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Open – Die Hummer Forging.

It is the simplest forging process which is quite flexible but not suitable for large scale production.

It is a slow process. The resulting size and shape of the forging are dependent on

the skill of the operator.

                   

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Impression – Die Drop Forging (Closed – Die Forging)

The process uses shaped dies to control the flow of metal. The heated metal is positioned in the lower cavity and on it one or more blows are struck by the upper die.

This hammering makes the metal to flow and fill the die cavity completely. Excess metal is squeezed out around the periphery of the cavity to form flash.

On completion of forging, the flash is trimmed off with the help of a trimming die.

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Forging Force Requirement

The forging force, F, required to forge material by impression – die forging operation can be determined by the relation:

F = k . s f . A where k is a constant.

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Press Forging

Press forging, which is mostly used for forging of large sections of metal, uses hydraulic press to obtain slow and squeezing action instead of a series of blows as in drop forging.

The continuous action of the hydraulic press helps to obtain uniform deformation throughout the entire depth of the work piece.

Therefore, the impressions obtained in press forging are more clean.

        

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Upset Forging

Upset forging involves increasing the cross – section of a material at the expense of its corresponding length.

Upset forging was initially developed for making bolt heads in a continuous manner, but presently it is the most widely used of all forging processes.