Metal forming process

METAL FORMING PROCESS

 

1. METAL FORMING PROCESS - CONCEPTS

Metal Forming: In the early days, mankind discovered that a heated piece of metal could be more easily hammered into different shapes. Metal Forming is also known as Metal Working. As the name implies, Metal Forming means forming of metals into different shapes by Metal Forming Processes. In this process, no chip is removed from the metal.

Metal Forming Processes: Metal Forming Process is defined as a process in which the desired shape and size of a component are obtained through the plastic deformation of metal. This can be achieved either by heating the metal and applying a relatively small amount of mechanical force or by applying a large amount of force.

Metal forming process changes the shape and dimensions of the raw material, but the volume of the metal remains substantially constant throughout the process. It is an economical method of manufacturing components, because loss of material is too less.

• Raw material for metal forming is a Bar / Billet/ Blank / Ingot / Metal Stock.

• Various metal forming processes are :

Forging, Rolling, Extrusion and Sheet Metal Forming (Drawing).

i. Deformation of Metals

Deformation is the change in form or shape of material under the action of applied forces. There are two types of deformations:

1. Elastic Deformation

Elastic deformation of a material is defined as the ability of the material to return to its original position after the removal of the external force. When a metal crystal is subjected to external force, a change of shape takes place. Why? This is because atoms are displaced. Removal of the force will allow the atoms to return to their normal equilibrium position, so long as the material is elastic. The elastic deformation is thus temporary. Hence, the material comes back to its original shape.:

2. Plastic Deformation

Plastic deformation is a permanent deformation. Plasticity is the property of a metal which enables the formation of permanent deformation without fracture. When a metal is stressed far beyond the elastic range, it will never return to its original form or shape due to plastic deformation.

 

2. CLASSIFICATION OF METAL FORMING PROCESSES

The metal forming processes can be grouped under two broad categories as:

1. HOT WORKING 2. COLD WORKING

1. Hot Working

Metals have crystals or grains separated from each other by grain boundaries. This can be observed in their microstructure, magnified more than 1000 times by viewing through a microscope. The necessary plastic deformation in a metal can be achieved either by heating the metal and then applying a small mechanical force or by the application of large amount of force without heating.

Re-crystallization

If the temperature of the metal is increased to a certain degree (a definite value for each metal), new refined (re-crystallized) grains are formed. The process of formation of these new refined grains is called Re-crystallization.

Re-crystallization does not produce new structures, but produces new crystals of the same structure.

Re-crystallization Temperature

The temperature at which re-crystallization process is completed is called Re-crystallization Temperature.

Hot Working

The hot and cold-working processes can be distinguished by the temperatures at which metal forming processes are carried out. The mechanical working of metals above the re-crystallization temperature resulting in plastic deformation is called Hot Working.

Hot Working Processes

Hot Forging, Hot Rolling, Hot Extrusion, Hot Drawing or Cupping, Roll Piercing.

Products by Hot Working: Automobile connecting rod, crankshaft, etc., are  produced by hot working. The re-crystallization temperature generally lies between one-third to one-half of the melting point of most of the metals. For metals like lead and tin, the re-crystallization temperature is quite low.

2. Cold Working

The mechanical working of metals below the re-crystallization temperature resulting in plastic deformation is called Cold Working.

Cold Working Processes

1. Sheering : Blanking, Piercing, Punching

2. Bending : Angle bending, Plunging, Plate bending, Bending of rods and pipes

3. Drawing : Wire / Rod / Tube drawing, Drawing or Cupping, Deep drawing

4. Squeezing : Coining, Embossing, Riveting

5. Extrusion : Cold extrusion

Products by Cold Working: Bolt heads, rivets, nails, gears, etc., are produced by cold working.

 

3. FORGING

Forging is the working or forming of metal into the required shape by hammering or pressing. It is the oldest of the metal working arts, having its origin with the primitive blacksmith of metalage times. Forging was first used to make jewelry, coins and various implements by hammering metals with tools made of stone.

The development of machinery to replace the arm of the smith occurred early during Industrial Revolution. Today, there is a wide variety of forging machinery which is capable of making parts ranging in size from a bolt to a turbine rotor or an entire airplane wing.

Principles of Forging

Forging: Forging is defined as a process in which the desired size and shape are obtained through the plastic deformation of metal.

Forgeability: Forgeability is defined as the capability of a metal to undergo deformation without cracking.

Simple forging operations can be performed with a heavy hand hammer and an anvil, as was traditionally done by blacksmiths. Forgings require a set of dies and a forging hammer or a press. Metal flow and grain structure can be controlled. Therefore, forged parts have good strength and toughness.

Hot Forging and Cold Forging

Forging done at room temperature is called Cold Forging. Forging done at elevated temperature is called Hot Forging.

Remember! Forging operation should be performed below the melting temperature of the respective metal.

Because of the higher strength of the material, cold forging requires greater forces and the work piece materials must have sufficient ductility at room temperature. Hot forging requires smaller forces, but it produces dimensional accuracy and surface finish that are not so good. Forgings generally require additional finishing operations such as heat treatment to modify the properties and then machining to obtain accurate finished dimensions.

Forging Metals

• Wrought iron, mild steel, carbon steel, alloy steel, non-ferrous metals like brass, bonze and aluminum alloys are the usual raw materials used in forging.

• Cast iron is brittle and hence not forgeable; if heated and beaten, it breaks into pieces.

Forging Applications

• Rivets, bolts, nuts, headed pins, eye bolts, hooks, clamps

• Automobile parts (axles, crankshafts, connecting rods)

• Agricultural implements

• Shafts for turbines Structural components for machinery, aircraft, railroads, etc.

 

4. ROLLING

1. Rolling of Metals

Rolling of Metals is the process of reducing thickness or changing the cross-section of a long work-piece by compressive forces applied through a set of rolls. Rolling may be carried out at room temperature (Cold Rolling) or at elevated temperatures (Hot Rolling).

Applications

Rolling is a major and the most widely used industrial process, because of its higher productivity and low cost. Rolling produces components having constant cross section throughout its length. The industrial utility of this process is distinctly visible by the wide range of rolled products available.

In steel, the metal is cast into a form called Ingot. Rolling is used to deform the metallic ingots into various useful sections. From these ingots, sheets, plates, rounds, etc., are rolled in steel rolling mills. Before rolling, the ingots are heated in a large furnace. The hot ingots are first rolled into slabs, blooms or billets.

Slabs, blooms, billets have rectangular sections and rounded corners as shown in Fig. 4. They are semi-finished shapes.

Slabs are used for rolling sheets and plates. See Fig. 5.

Blooms are used for rolling structural shapes such as channel sections, angles, I-sections, T- sections, etc.

It is also possible to produce special sections such as railway wagon wheel by rolling individual pieces. Billets are used for rolling rounds, squares, wires and other small sections. Width of rolled products may range upto 5 meters and as thin as 0.0025 mm.

2. Principle of Rolling

See Fig. 6. Rolling is the process of forming metal to required shape by passing it through a set of power-driven rotating rollers through a regulated opening. The rolls will have the desired profile on its periphery.

During rolling, plastic deformation of the metal takes  place. The metal is subjected to high compressive stress causing the metal to elongate.

The crystals of the metal are elongated in the direction of rolling. Also the cross-section of the work piece is reduced. When the metal is heated and then rolled, it is called hot rolling as shown. When the metal is rolled in cold condition, it is called cold rolling.

 

5. EXTRUSION

Extrusion is the process by which a block of metal (billet, generally round) is reduced in crosssection by forcing it to flow through a die orifice under high pressure. In general, extrusion is used to produce cylindrical rods or hollow tubes.

Extrusion is the process of confining the metal in a closed cavity and then allowing it to flow from only one opening, so that the metal will take the shape of the opening. The operation is identical to the squeezing of toothpaste out of the toothpaste tube. Almost any solid or hollow cross-section may be produced by extrusion.

The extrusion process consists of forcing metal (confined to a pressure chamber) out through a specially formed die. In this a ram slides inside a container. The ram is operated by hydraulic drive of a press. The die is held in position by a die holder.

The ram is first withdrawn away from the container. A heated round billet is placed into the container. A pressure plate is placed at the end of the ram in contact with the billet. The hot metal is forced out or extruded through the die opening, as an extruded part. The shape and size of the extruded part depends upon the shape and size of the die.

Extrusion Ratio: It is defined as the ratio of cross-sectional area of the billet to that of the extruded section. The typical values lie between 20 and 50.

Uses: Rods, tubes, structural shapes, lead-covered cables, railings for sliding doors, structural and architectural shape, door and window frames are typical products of extrusion.

 

6. SHEET METAL FORMING

Many of the consumer goods enjoyed today by the modern man owe their low cost to the press tools. Products made by sheet metal forming are all around us – they include metal desks, beverage cans, car bodies, pipes, elbows, boxes, funnels, kitchen appliances, hoppers, aircraft parts, etc. But for the cheap way of making these sheet metal components, we possibly could not have even thought of having typewriters, file cabinets, mechanical toys, etc., at such low cost. The press tool operation is by far one of the cheapest and fastest ways of the complete manufacture of a component.

Note: Sheet metal is a metal plate/sheet with thickness less than 5 mm. Sheets are specified in thickness by standard gauge numbers. Each gauge number designates a definite thickness. Sheet thickness is inversely proportional to gauge number, i.e., larger the gauge number, lesser is the thickness.

Press Working Operations

Press Working Operations refer to Sheet Metal Work.

Spring-Back: One of the principle concerns in a sheet metal operation is the spring-back of the metal. When the metal is deformed, it is first elastically deformed and then plastically. When the applied load is removed, the plastic component of the deformation remains permanently, but the elastic part springs-back to its original shape.

In sheet metal work, Low Carbon Steel, Galvanized Iron (G.I.), Aluminum, Stainless Steel, Copper and Tin are used.

Types of Press Working Operations are Shearing, Bending, Drawing and Squeezing.

1. Shearing

Before a sheet metal part is made, a blank of suitable dimension is first removed from a large sheet by Shearing. Shearing is the operation of cutting-off a sheet metal between a punch and a die. Types of shearing operations are Blanking and Piercing.

a. Blanking:

See Fig. 8. Blanking is the operation of cuttingoff a flat sheet of desired shape. As the punch descends upon the plate, high pressure is exerted by the punch. This causes the metal plate to be deformed plastically in the die.

Finally the plate is cut off by the punch. The cut portion of the plate drops down through the die opening. To make the blanked out plate drop down freely, die clearance is provided as shown. The sheet metal left on the die is scrap.

Blanking is the first step in a series of operations. The blank is further processed, i.e., bending or drawing is done on the blank.

b. Piercing:

See Fig. 9. Piercing is the operation of producing a hole of any shape in a sheet metal using a punch and die. The punch size will be exactly the same as the size of the hole to be pierced.

The sheet metal or plate is introduced between the punch and the die through the stripper plate. The punch pierces the hole in the metal when it moves down. The pierced out metal, called scrap, drops down through the die opening. After piercing, the punch moves up.

The sheet metal on the die may stick with the punch surface. The stripper plate strips-off the sheet metal from the punch. Clearance is allowed on the die.

2. Bending

Bending is the operation of straining a metal both in tension (Outer Fiber) and compression (Inner Fiber) without appreciable change in its thickness. As the metal is pressed beyond the elastic limit, deformation takes place. When the load is removed, the metal retains the bent shape given by the die.

a. Angle Bending

See Fig. 10. Angle bending is the operation of bending a sheet metal to a sharp acute angle. The punch and the die are shaped to the desired angle, taking into account the effect of spring back of the metal.

Spring Back: In the bending operation, the metal retains the bent shape, even after the punch goes up. However, there is some elastic recovery, resulting in a slight decrease in the bent angle, this known as spring back.

The metal will spring back to a small amount. Hence metal is bent slightly more than the required angle.

b. Forming

See Fig. 11. Forming is the operation of bending a sheet metal to the required form. The sheet metal is placed between the punch and the die and pressed.

The punch and the die have the same shape of the required form in the work piece.

3. Drawing or Cupping

FIG. 11 Many parts made of sheet metal are cylindrical or box-shaped. Examples: Pots and pans, Containers for food and beverages, Kitchen sinks, Automotive fuel tanks, etc. Such parts are made by a process in which a punch forces a flat sheet-metal blank into a die cavity.

See Fig. 12. Drawing or Cupping is the operation of producing cup shaped parts from flat sheet metal blanks. Examples: Bathtubs, automobile bumpers, kitchen appliances such as pressure pans, kettles, bowls, etc.

Description

The setup is similar to that used in Blanking except that the punch and the die are provided with the necessary rounding at the corners to allow for the smooth flow of metal during Drawing. The sheet metal blank is placed over the die. .

Pressure Plate holds the blank on the die. It presses the blank against the die to prevent wrinkling. Sufficient clearance is given between the punch and die.

Working: When the punch moves down, the sheet metal blank is pushed into the die opening. Plastic deformation takes place in the blank. The metal is drawn over the edges of the die opening to form the sides of the cup.

4. Squeezing

The squeezing operation is the most severe of all cold press operations. Tremendous amount of pressure is required to squeeze a metal which is made to flow in a cold state, within the cavity of the die and punch to attain the desired shape. Hydraulic press is used for squeezing. Squeezing can be done only on ductile materials having good plasticity.

Squeezing Operations are: 1. Coining 2. Embossing 3. Cold Extruding 4. Riveting

1. Coining

See Fig. 13. As the name implies, this process is widely used in producing coins and ornamental parts by squeezing the metal between a punch and die.

A blank of correct size is placed within the punch and die. Both the punch and die are engraved to have the required pattern and shape. Tremendous pressure is applied on the blank from both the ends.

The movement of the die and punch is guided by a guide plate. The blank metal flows in the cold state and fills up the cavity of the punch and die.

The coin produced gets a sharp impression on its two slides, corresponding to the engravings on the punch and the die.

2. Embossing

See Fig. 14. Embossing is the operation of giving impression of letters, figures or designs in sheet metal blanks. Blank is placed between the punch and the die.

Both the punch and the die have the engravings required on the blank. The die will have projections and the punch will have the corresponding depressions. The punch presses down the blank on the die. The blank metal flows in the cold state and have the engravings marked on it by squeezing. This operation is mostly used on name plates, tin containers, etc.

 

7. HOT WORKING: ADVANTAGES AND DISADVANTAGES

Advantages

1. During hot working operations, the metal remains in plastic state. Therefore, force required to change the shape is less when compared to cold working.

2. The grains of the metal after hot working are refined. Therefore, mechanical properties such as toughness, ductility, elongation, etc., can be improved. The resistance to impact is increased.

3. Porosity in the metal is mostly eliminated.

4. Impurities are broken up and distributed throughout the metal. Hence, internal structure of the metal gets refined.

5. Power required to finish the product from the ingot is reduced.

6. Metal, labor and time are saved.

7. It is a quick and economical process.

8. This process is suitable for nearly all the commercial metals.

Disadvantages

1. Hot metal surface oxidizes rapidly due to high temperature of the metal. This results in poor surface finish and material loss.

2. Close tolerances cannot be obtained.

3. High temperature heating devices are required, which increases the investment cost.

4. Life of the tools used is reduced as the tools have to work at high temperatures.

5. Automation is difficult, because of high working temperatures.

6. The metal is worked within a particular range of temperatures. If worked below, the metal gets cracked or distorted. If it is heated above the required range, the metal may burn. Hence, too much care is needed while maintaining the optimum temperature.

7. Thin gauge sheets cannot be produced.

 

8. COLD WORKING : ADVANTAGES AND LIMITATIONS

Advantages

1. Accurate dimensional control can be obtained by cold working process. Hence no machining allowance is required.

2. It increases strength, elasticity and hardness of the metal parts worked.

3. Because the deformation occurs at low temperatures, no surface oxidation or scaling occurs in the process.

4. Surface finish obtained is excellent, as no surface oxidation results during the process.

5. For metals that do not respond to heat treatment, cold working is a possible method to increase hardness.

6. Thin gauge sheets can be produced by cold working.

7. Automation is possible because of low working temperatures.

Disadvantages

1. As the strength of the metal is very high at low temperatures, large forces are required for deformation, calling for high capacity machines which are costly.

2. Only small sized components can be easily cold worked, as for the larger sections, greater forces are required. Sections of more than 25mm diameter are rarely cold rolled.

3. All metals and alloys cannot be cold formed. It is limited to ductile metals only.

4. The grain structure is not refined. Distortion of grain structure is created.

5. Because of very high forces involved, the tooling must be specially designed. Hence the tool cost is high.