Metal Working

by

G V R Murty

General Manager

Mishra Dhatu Nigam Ltd.

Hyderabad.

• Introduction

• The nature and purpose of metal working.

• Principles of metal working.

• Types of Metal working Processes.

• Metallurgical factors in Metal Working.

• Defects in Metal Working.

Introduction

• All metal objects , except castings have to undergo at least one mechanical working operation.

• Knowledge of these processes can assist in efficient utilization of resources.

• The basic principles and processes of metal working will be presented here.

Traditional Metal Working Processes

Casting

Bulk Metal Working

Sheet metal processing

Cutting

Joining

Powder- and Ceramics Processing

Plastics processing

Surface treatment

Nature and Purpose of Metal Working

• If cast products had feasibility of desirable mechanical properties in the wide variety of sizes and shapes, mechanical working will become unnecessary.

• This is far from technically possible.

• Homogeneinity ,Isotropy are some of the concerns.

• Metal working greatly improves the mechanical properties .

Principles of metal working

In all metal working operations , the work piece is permanently deformed.

The work piece undergoes deformation under the influence of external force which is derived from the deformation characteristics of the particular material.

These loads are calculated using calculable stress systems.

• The tool, usually called a die, applies stresses that exceed yield strength of metal

• The metal takes a shape determined by the geometry of the die

Principles of metal working

• Desirable material properties:

– Low yield strength and high ductility

• These properties are affected by temperature:

– Ductility increases and yield strength decreases when work temperature is raised

• Other factors:

– Strain rate and friction and lubrication.

Principles of metal working

• Stresses in Metal Forming

• Stresses to plastically deform the metal are usually compressive

– Examples: rolling, forging, extrusion

• However, some forming processes

– Stretch the metal (tensile stresses)

– Others bend the metal (tensile and compressive)

– Still others apply shear stresses

Material Behavior in Metal Forming

• Plastic region of stress-strain curve is primary interest because material is plastically deformed

• In plastic region, metal's behavior is expressed by the flow curve:

where K = strength coefficient; and n = strain hardening exponent

• Stress and strain in flow curve are true stress and true strain

Terminologies in Metal Working

• Flow stress = instantaneous value of stress required to continue deforming the material.

• Recrystallisation Temperature= The temperature at which a fully strained material becomes strain free in one hour.

• Strain= Ratio of input

Cold working is metal forming performed at room temperature.

Advantages: better accuracy, better surface finish, high strength and hardness of the part, no

heating is required.

Disadvantages: higher forces and power, limitations to the amount of forming, additional

annealing for some material is required, and some material are not capable of cold working.

Warm working is metal forming at temperatures above the room temperature but bellow the recrystallization one.

Advantages: lower forces and power, more complex part shapes, no annealing is required.

Disadvantages: some investment in furnaces is needed.

Hot working involves deformation of preheated material at temperatures above the recrystallization temperature.

Advantages: big amount of forming is possible, lower forces and power are required, forming

of materials with low ductility, no work hardening and therefore, no additional annealing is required.

Disadvantages: lower accuracy and surface finish, higher production cost, and shorter tool life.

Bulk Metal Working Processes

Forging: The workpiece is compressed between two opposing dies so that the shapes are imparted to the work.

Types: Open Die forging, Closed Die forging, Isothermal forging,

Rolling: Compressive deformation process in which the thickness of a plate is reduced by squeezing it through rotating cylindrical rolls.

Types: Hot rolling, Cold rolling, Powder rolling

Extrusion: The work material is forced to flow through a die opening taking its shape.

Types; Direct Extrusion,Indirect Extrusion

Drawing: The diameter of a wire or bar is reduced by pulling it through a die opening (bar drawing) or a series of die openings (wire drawing)

Forging

Definition

Forging is a BulkDeformation Process inwhich the work iscompressed between twodies.

Forging

Stages in Impression-die (Closed-Die) Forging

Flash less forging

Rolling

Definition

Rolling is a Bulk Deformation Process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls

Rolling

Hot Rolling

• Initial breakdown of an ingot or a continuously cast slab is achieved by hot rolling. Mechanicalstrength is improved and porosity is reduced.

A material is pushed or drawn through a die of the desired cross-section .Any solid or hollow cross-section may be produced by extrusion, which can create essentially semi-finished parts. The metal can force through a die in the same direction or opposite direction.

Basic bulk deformation processes: (c) extrusion

• Direct extrusion: A metal billet is located into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container.

• Indirect extrusion: The die is mounted to the ram rather than at the opposite end of the container. One advantage of the indirect extrusion process is that there is no friction, during the process, between the billet and the container

liner.

COLD ROLLING

Cold rolling occurs with the metal below its recrystallization temperature(usually at room temperature).

It also improves the surface finish and holds tighter tolerances.

Due to smaller size of the work pieces and their greater strength than hot rolled stock, four high or cluster mills are used.

Commonly cold rolled products include sheets, strips and rods; products smaller than the same products that are hot rolled.

WHAT is DRAWING?

Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die.

The principle of this procedure consist of reducing the thickness of a pointed ,tapered wire by drawing it through a conical opening in a tool made of a hard material.The wire will take shape of the hole.

Basic bulk deformation processes: (d) drawing

Drawing

Commonly used to make wires from round bars

stock (bar)

F (pulling force)

wirediestock (bar)

F (pulling force)

wiredie

Similar to extrusion, except: pulling force is applied

Friction in Metal Forming

• In most metal forming processes, friction is undesirable:

– Metal flow is retarded

– Forces and power are increased

– Wears tooling faster

• Friction and tool wear are more severe in hot working

Lubrication in Metal Forming

• Metalworking lubricants are applied to tool-workinterface in many forming operations to reduce harmful effects of friction

• Benefits:

– Reduced sticking, forces, power, tool wear

– Better surface finish

– Removes heat from the tooling

Considerations in Choosing a Lubricant

• Type of forming process (rolling, forging, sheet metal drawing, etc.)

• Hot working or cold working

• Work material

• Chemical reactivity with tool and work metals

• Ease of application

• Cost

Metallurgical Factors in Metal Working

• Composition of Material.

• Melting or casting process.

• Reheating temperatures and type o furnaces.

• Furnace environment conditions.

• End product properties.(strength, surface finish etc)

Defects in Metal Working

• Inhomogeneous deformation .

• Hot & cold shortness

• Surface cracks and Center bursts.

• Internal blowholes, lapses.

Numerical Methods in Metal Working

• Flow patterns together with associated strain rate and temperature distributions, metal spread and elongation are more important than actual loads.

• Generally only portions of the process requires full loads.

• Today several tools are available to configure the operational loads and predict the ductility, grain size and other features o the product.