Metal Forming | Mechanical Engineering SSC JE (Technical) PDF Download

 METAL FORMING  

Four Important techniques are:

• Rolling : The process of plastically deforming metal by passing it between rollers.
• Forging : The workpiece is compressed between two opposing dies so that the die shapes are imparted to the work.
• Extrusion : The work material is forced to flow through a die opening taking its shape
• 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)

Terminology Semi-finished product 

• Ingot : is the first solid form of steel.
• Bloom : is the product of first breakdown of ingot has square corss section 6 x 6 in or larger.
• Billet : is hot rolled from a bloom and is square, 1.5 in on a side or larger.
• Slab : is the hot rolled ingot or bloom rectangular cross section to in or more wide and 1.5 in or more thick.

Plastic Deformation

• Deformation beyond elastic limits.
• Due to slip, grain fragmenttion, movement of atoms and lattice distortion.

Strain Hardening

• When metal is formed in cold state, there is no recrystailization of grains and thus recovery from grain distortion or fragmentation does not take place
• As grain deformation proceeds, greater resistance to this action results in increased hardness and strength i.e., strain hardening.

Recrystallisation Temperature (Rx temp.)

• The minimum temperature at which the complete recrystallisation of a cold worked metal occurs within a specified period of approximately one hour.
• Rx temp. decreases strength and increases ductility
• If working above Rx temp., hot-working process whereas working below are cold-working process.
• Rx temp. varies between 1/3 to 1/2 melting paint.
• For Alloy Rx temp. = 0.5 x Melting temp. (Kelvin ).
• Rx temp. of lead and Tin is below room temp.
• Rx temp. of Cadmium and Zinc is room temp.

Malleability & Ductility

• Malleability is the property of a material whereby it can be shaped into sheet when cold hammering or rolling is done.
• Ductility is the ability of material to undergo permanent defornmation in the form of wire.

Cold Working 

Advantages of Cold Working

• Better accuracy, closer tolerances
• Better surface finish
• Strain hardening increases strenfth and hardness
• Grain flow during deformation can cause desirable directional propoerties in product
• No heating of work required (less total energy)

Disadvantage of Cold Working

• Equipment of higher forces and power required.
• Surfaces of starting work piece must be free of scale and dirt.
• Ductility and strain hardening limit the amount of forming that can be done.
• In some operations, metal must be annealed to allow further deformation.
• Some metals are simply not ductile enough to be cold worked.

Hot Working 

Advantages of Hot Working 

• The porosity of the metal is largely eliminated.
• The grain structure of the metal is refined.
• The impurities like slag are squeezed into fibers and distributed throughout the metal.
• The mechanical properties such as toughness, percentage elongation, percentage reduction in area, and resistance to shock and vibration are improved due to the refinement of grains.

Dis-advantages of Hot Working

• It requires expensive tools.
• It produces poor surface finish, due to the rapid oxidation and scale formation on the metal surface.
• Due to the poor surface finish, close tolerance cannot be maintained.

Rolling

• Definition: The process of plastically deforming metal by passing it between rollers.
• Most widely used, high production and close tolerance.
• Friction between the rolls and the metal surface produces high compressive stress.

Hot Rolling 

• Done above the recrystallization temp.
• Results fine grained structure.

Cold Rolling 

•  Done below the recrystallization temp.

Ring Rolling 

• Ring rolls are used for tube rolling, ring rolling.
• As the rolls squeeze and rotate, the wall thickness is reduced and the diameter of the ring increases.

Pack rolling

• Pack rolling involves hot rolling multiple sheets of material at once, such as aluminium foil.
• Aluminum sheets (aluminum foil) Thread rolling
• Used to produce threads in substantial quantities.
• This is a cold forming process in which the threads are formed by rolling a thread blank between hardened dies that cause the metal to flow radially into the desired shape.
• No metal is removed, greater scrength, smoother, harder.

Formula in Rolling 

Geometry of Rolling Process

Draft

• Total reduction or "draft" taken in rolling. Dh = h₀ – h_f = 2(R – R cos a) = D(1 – cosa)

For Unaided entry m³ tana

Maximum Draft Possible 

Dh_max =m²R

Minimum Possible Thickness (h_{f min}) 
h₀– h_fmin =m²R
Forging 

• Forging process is a metal forming process by which metals or alloys are plastically deformed to the desired shapes by a compressive force applied with the help of a pair of dies.

Open and Closed die forging

• In open die forging, the metal is compressed by repeated blows of a mechanical hammer and shape is manipulated manually.
• In closed die forging, the desired configuration is obtained by squeezing the workpiece between two shaped and closed dies.

Advantages of Forging

• Mechanical properties and reliability of the materials increases due to improve in crystal structure.
• Forging reduces the grain size of the metal, which increases strength and toughness.
• Fatigue and creep strength increased.

Disadvantages of Forging 

• Costly
• Poor dimensional accuracy and surface finish.

Draft 

• The draft provided on the sides for withdrawal of the forging.
• Adequate draft should be provided at least 3o for aluminum and 5 to 7o for steel.
• Internal surfaces require more draft than external surfaces.

Flash

• The excess metal added to the stock to ensure complete filling of the die cavity in the finishing impression is called Flash.

Gutter 

• In addition to the flash, provision should be made in the die for additional space so that any excess metal can flow and help in the complete closing of the die. This is called gutter.

• Without a gutter, a flash may become excessively thick, not allowing the dies to close completely.
• Gutter depth and width should be sufficient to accomodate the extra, material.

Drop Forging 

• The drop forging die consists of two halves. The lower half of the die is fixed to the anvil of the machine, while the upper half is fixed to the ram. The heated stock is kept is the lower die while the ram delivers four to five blows on the metal, in quick succession so that the metal spreads and completely fills the die cavity. When the two die halves close, the complete cavity is formed.
• Drop forging is used to produce small components.

Press Forging 

• Metal is squeezed gradually by a hydraulic or mechanical press and component is produced in a single closing of die, hence the dimensional accuracy is much better than drop forging.

Upset Forging

• Increasing the diameter of a material by compressing its length.
• Employs split dies that contain multiple positions or cavities.

Skew Rolling

• Skew rolling produces metal ball

Smith Forging 

• Blacksmith uses this forging method.
• Not used in industry.

Forging Defects

• Scale Pits : Irregular depressions into the surface due to improper cleaning of the stock.
• Die Shift : Due to Misalignment of the two die halves or making the two halves of the forging to be of improper shape.
• Flakes : Internal ruptures caused by the improper cooling.
• Improper Grain Flow : This is caused by the improper design of the die, which makes the flow of metal not flowing the final intended directions.

Forging Defects

• Hot tears and thermal cracking : These are surface cracks occurring due to non-uniform cooling from the forging stage or during heat treatment.
• irregular contours occur at right angles to the direction of metal flow.

Extrusion & Drawing Extrusion

• The extrusion process is like squeezing toothpaste out of a tube.

•  Metal is compressed and forced to flow through a suitably shaped die to form a product with reduced but constant cross section.
• Metal will undergo tri-axial compression.
• Hot extrusion is commonly employed.
• Lead, copper, aluminum, magnesium, adn alloys of these metals are commonly extruded.
• Steel, strainless steels, and nickel-based alloys are difficult to extrude, (high yeild strengths, weldign with wall.) Use phosphate-based and molten glass lubricants.

Advantages of Extrusion

• Any corss-sectional shape can be extruded from the nonferrous metals.
• Many shapes (than rolling)
• No draft
• Huge reduction in cross section.

Limitation of Extrusion

• Cross section must be uniform for the entire length of the product.

Hot Extrusion Process  

• The temperature range for hot extrusion of aluminum is 430-480oC.
• Used to produce curtain rods made of aluminum.

Direct Extrusion

• A solid ram drives the entire billet to and through a stationary die and must provide additional power to overcome the frictional resistance between the surface of the moving billet and the confining chamber.

Indirect Extrusion 

• A hollow ram drives the die back through a stationary, confined billet. Ÿ since no relative motion, friction between the billet and the chamber is eliminated.

Cold Extrusion

• Used with low-strength metals such as lead, tin, zinc, and aluminum to produce collapsible tubes for toopthpaste, medications, and other creams; small "cans" for shielding electronic components and larger cans for food and beverages.

Backward cold extrusion

• The metal is extruded through the gap between cthe punch and die opposite to the punch movement.
• For softer materials such as aluminium and its alloys.
• Used for making collapsible tubes, cans for liquids and similar articles.

Impact Extrusion

The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch.

Hydrostatic Extrusion

• Another type of cold extrusion process.
• It is forward extrusion, but the fluid pressure surrounding the billet prevents upsetting.

Extrusion Defects

• Surface crack due to high temperature, high speed, high friction etc.
• Defects at low temperature due to sticking of metals in die land.
• Centre Burst or Chevron defect are attributed to a state of hydrostatic tensile stress at the centreline in the deformation zone in the die. Tendency increases with increasing die angle and amount of impurities. Tendency decrease with increasig extrusion ratio and friction.

Wire Drawing 

• A cold working process to obtain wires from rods of bigger diameters through a die.
• Same process as bar drawing except that it involves smaller-diameter material.

Wire Drawing Die

• Die Materials : tool steels or tungsten carbides or polycrystalline diamage.

Rod and Tube Drawing

Drawing 

• Drawing is a plastic deformation process in which a flat sheet or plate is formed into a three-dimensional part with a depth more than several times the thickness of the metal.
• As a punch descends into a mating die, the metal assumes the desired configuration.

Defects in Drawing - Wrinkle 

• An insufficient blank holder pressure causes wrinkles to develop on the flange, which may also extend to the wall of the cup.

Defects in Drawing- Fracture

• Further, too much of a blank holder pressure and friction may cause a thinning of the walls and a fracture at the flange, bottom, and the corners (if any).

Defects in Drawing - earing

• While drawing a rolled stock, ears or lobes tend to occur because of the anisotropy induced by the rolling operation.

Coining

• Coining is essentially a cold-forging operation except for the fact that the flow of the metal occurs only at the top layers and not the entire volume.
• Coining is used for making coins, medals and similar  articles.

Bending 

•  After basic shearing operation, we can bend a part to give it some shape.
• Bending parts depends upon material properties at the locaton of the bend.
• At bend, bi-axial compression and bi-axial tension is there.

Power Metallurgy

• Powder metallurgy is the name given to the process by which fine powered materials are blended, pressed into a desired shape (compacted), and then heated (sintered) in a controlled atmosphere to bond the contacting surfaces of the particles and establish the desired properties.

Manufacturing of Powder 

Atomization using a gas stream 

• Molten metal is forced through a small orifice and is disintegrated by a jet of compressed air, inert gas or water jet. It is used for low melting point materials, brass, bronze, Zn, Tn, Al, Pb etc.

Manufacturing of Powder
 Reduction

• Meal oxides are turned to pure powder when exposed to below melting gases results in a product of cake of sponge metal.
• Used for iron, Cu, tungsten, molybdenum, Ni and Cobalt.

Manufacturing of Powder
 Grinding

• This metallic powder is nothing but the unburnt tiny chips formed during the process of grinding.

Electrolytic Deposition 

• Used for iron, copper, silver
• Process is similar to electroplating.
• Granulations - as metals are cooled they are stirred rapidly
• Machining - coarse powders such as magnesium
• Milling - crushers and rollers to break down metals. Used for brittle materials.
• Shooting - drops of molten metal are dropped in water, used for low melting point materials.
• Condensation - Metals are boiled to produce metal vapours and then condensed to obtain metal powders. Used for Zn, Mg, Cd.

Blending

• Blending or mixing operations can be done either dry or wet.
• Lubricants such as graphite or stearic acid improve the flow characteristics and compressibility at the expense of reduced strength.

Sintering

• Controlled atmosphere : no oxygen.
• Heat to 0.75* T melt
• Particles bind together, diffusion, recrystaliztion and grain growth takes place.
• Part shrinks in size.
• Density increases, up to 95%.
• Strength increases, Brittleness reduces, Porosity decreases. Toughness increases.