Mechanical Working Of Metals

MECHANICAL WORKING OF METALS Hot Working Cold Working BY:Recrystallization Recovery Alloys of Soldering and Brazing

Abhay Thakur 07333 Siddharth Mahajan07334 Adheesh Gupta 07335 Neelam Kumari07337 Ashish Singh

INTRODUCTION The Mechanical working of metal is defined as the plastic defomation of metals under action of externally applied forces.The mechanical working of metals is decribed as hot working and cold working depending upon whether the metal is worked above or below the recrystallization temp .The metal is subjected to mechanical working for the following purpose:q To reduce the original block or ingot into desired shape. q To refine grain size.

Hot Working The working of metals above the recrystallization temperature is called hot working. Recrystallization temperature is the temp. at which new grain are formed in the metal. Hot working of the metals has following advantages and disadvantages . Advantages:q The porosity of metal is largely eliminated,thus producing strong and uniform structure. q The grain structure of the metal is refined. q The impurities like slag are squeezed into fibres and are uniformly distributed throughout the metal. q The deformation of metal is easy, with a small

Disadvantage:1.It requires expensive tools. 2.lt produces poor surface finish, due to rapid oxidation and scale formation on the metal surface. 3.Due to the poor surface finish, close tolerances cannot be maintained. 4.The correct temp. range for working is difficult to maintain. HOT WORKING PROCESSES ARE:1. Hot rolling 4. Hot Extrusion 2. Hot forging 5. Hot Drawing

Hot work tool steel Hot work tool steel are of three types-: q chromium base, type H11H16(serviceable up to 600 F) q tungsten base , type H20H26(serviceable up to 1000 F) q molybdenum base,type H41-H43 where H11-H43 are designation given by AISI system.

Hot Rolling The hot rolling is the most rapid method of converting large sections into desired shapes. The forming of bars, plates, sheets, rails, angles, I-beams and other structural sections are made by hot Rolling. The operation consists of passing the hot ingot through at least two rolls rotating in opposite directions at the same speed.the roll squeeze the passing ingot to reduce its cross-section and increase its length. The first operation to the ingot is carried out at blooming mill where it is rolled to blooms. The blooms are cut up in lengths for subsequent reducing process into billets. The commonly hot rolled are aluminium,

Hot Forging In this process metal is heated to plastic state and then pressure is applied to form it into desired shapes and sizes. The pressure may be applied by hand hammers, power hammers or by forging machines. The different forging process are-: Die forging, press forging, upset forging, roll forging and swaging.

It consists of heating the metal to forging temp and then forming it into desired shape on a spinning lathe. The shaping is done with a blunt tool which contacts the surface of the rotating part and causes the metal to flow to some desired form. This method is generally used for thicker plates and sheets.

Hot Extrusion It consists of pressing metal inside a chamber to force it out by high pressure through an orifice which is shaped to provide the desired form of the finished part. Most metals and their alloys such as copper, aluminium and nickel are directly extruded at elevated temperature. The various methods of extrusion are:Direct Extrusion-In this method, the heated round billet is placed into the die chamber and the dummy block and ram is placed into position. Indirect Extrusion-It is similar to direct

Hot Drawing It is mostly used for thick-walled seamless cylinder. It is done in two stages. The first stage consists of drawing a cup out of a hot circular plates with the help of a die and punch . In second stage reheating the drawn cup and drawing it further to the desired length having the required wall thickness. The second drawing operation is performed through a number of dies, which are arranged in desending order of their diameters to get the gradual

Hot Piercing This process is used for the manufacture of seamless tubes. In this, the heated cylindrical billet of steel are passed between two conical shaped rolls operating in the same direction. A mandrel is provided between these rolls which assists in the piercing and controls the size of the hole as the billet is forced over it. This process use piercing rolls, plug rolling mill, reelers and sizing rolls.

Cold working The working of metals below their recrystallization temperature is known as cold working . Most of cold working process are performed at room temperature . The cold working distorts the grain structure and does not provide an appreciable reduction in size. The extent to which a metal can be cold worked depends upon its ductility . The higher the ductility of the metal , the more it can be cold worked . During cold

Effects of cold working q The stresses are set up in the metal which remain in the metal, unless they are removed by subsequent heat treatment . q A distortion of the grain structure is created. q The strength and hardness of the metal are increased with a corresponding loss in ductility . q The recrystalline temperature for steel is increased .

Cold working process 1. 2. 3. 4. 5. 6.

Cold Cold Cold Cold Cold Cold

rolling forging spinning extrusion drawing bending

Cold Rolling It is generally employed for bars of all shapes , rods, sheets and strips, in order to provide a smooth and bright surface finish. It is also used to finish the hot rolled components to close tolerences and improve their toughness and improve their toughness and hardness. The hot rolled articles are first immersed in a weak solution of sulphuric acid to remove the scale and washed in water, and dried . This process of cleaning the articles is known as pickling.

Cold Forging 

The cold forging is also called swaging. During this method , the metal is allowed to flow in some pre-determined shape according to design of dies , by a compressive force or impact. It is widely used in forming ductile metals. Following are three, commonly used , cold process:-

Contd….. 1. Sizing:-It is the simplest form of forging . It is the operation of slightly compressing a forging , casting or steel assembly to obtain close tolerance and a flat surface . The metal is confined only in a vertical direction. 2. Cold Heading:- This process is extensively used for making bolts , rivets and other similar headed parts. This is usually done on a cold header machine. 3. Rotary Swaging:- This method is used for reducing the diameters of round bars and tubes by rotating dies which open and close rapidly on the work. The end of the rod is tapered or reduced in size by a combination of pressure

Cold Spinning 

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Metal spinning, also known as spin forming or spinning, is a metal working process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric part. Spinning can be performed by automated lathe such as CNC lathe. It is done at room temperature. It is applicable to soft metals like aluminium , brass , copper , magnesium and their

COLD EXTRUSION 

Extrusion is the process by which long straight metal parts can be produced. Extrusion is done by squeezing metal in a closed cavity through a tool, known as a die using either a mechanical or hydraulic press.

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Cold extrusion is the extrusion process done at room temperature . This process can be used for materials that can withstand the stresses created by extrusion. Examples of the metals that can be extruded are lead, tin, aluminum alloys, copper, titanium,

TUNGSTEN CARBIDE COLD EXTRUSION DIE

COLD DRAWING  

It is employed for rods , bars , wires , tubes. Various cold drawing processes are: 1]Bar drawing :Bars or rods that are drawn cannot be coiled therefore straight-pull draw benches are used. Chain drives are used to draw work pieces up to 30 m (98 ft). Hydraulic cylinders are used for shorter length work pieces .

CONTINUED…. 2] Wire drawing: This technique has long been used to produce flexible metal wire by drawing the material through a series of dies of decreasing size. These dies are manufactured from a number of materials, the most common being tungsten carbide and diamond. Wire drawing process are of two types: a]single draft process b]continuous process

CONTINUED…. 3]Tube drawing : Tube drawing is very similar to bar drawing, except the beginning stock is a tube. It is used to decrease the diameter, improve surface finish and improve dimensional accuracy.

COLD BENDING § In this process bars , rods , wires , tubes and metal sheets are bent in cold conditions generally at ambient temperature. § Materials commonly employed for cold bending are copper , carbon steel , bronze, stainless steel , brass , zinc and aluminium. § Roll bending: metal plates and strips are

PLATE BENDING MACHINE

Recrystallization : q The term recrystallization may be defined as the process of forming strain free new grains, in metal, by heating it to a temperature known as recrystallization temperature. q As the upper temperature of the recovery range is : reached, minute new crystals appear in the microstructure These new crystals have the same composition and lattice structure as the original under formed grains and are not elongated but approximately uniform in the dimensions. The new crystals generally appear at the most drastically deformed portions of the grain, usually the grain boundaries and slip planes. The cluster of the atoms from which the new grain boundaries are formed is called a nucleus. Recrystallization takes place by the combination of nucleation of strain free grains and the growth of these nuclei to absorb the entire cold-worked material.

Initially, there is an incubation period in which energy is developed in order to start the process going. Here, the incubation period is to allow the strain-free nuclei to reach a visible microscopic size. It is important to realize that the growth of recrystallized embryos is irreversible. During the crystallization, solidification from the liquid would start when a group of atoms had reached a critical size to form a stable cluster. Embryos i.e. the cluster of less than critical size , would redissolve or disappear. However, since there is no simple way to recreate the distorted, dislocation filled structure, the recrystallization embryo cannot redissolve. Therefore, these embryos merely wait for additional energy to attract the more atoms into their lattice structure. Eventually the critical size exceeded, and visible recrystallization begins. The incubation period corresponds to the irreversible growth of the embryos.

Recrystallization may be obtained by examining it in terms of the lattice. During plastic deformation, slip planes and the grain boundaries are localized points of high internal energy as the result of the pile up of the dislocations. Because of the nature of strain hardening, it is not possible for the dislocations or the atoms to move back to form a strain free lattice from the distorted lattice. In a simplified analogy, considering some atoms, at the grain boundaries or slip planes, have been pushed upon energy hill to a value of E1 equal to the internal energy due to deformation above the internal energy of undeformed lattice. The

The atoms cannot reach the energy of the strain free crystal. The energy difference is supplied by heat. When the temperature is reached at which these localized areas have energy equal to E2, they give up the part of energy as heat of recrysatllisation and form nuclei of new strain free grains. Part of this heat of recrystallization is absorbed by surrounding atoms so that they have sufficient energy to overcome the rigidity of the distorted lattice and be attracted into the lattice structure of the strain free grains, initiating grain growth. The number and energy content of these high energy points depend to a large extent on the amount of

Recrystallization temperature q The recrystallization temperature does not refer to a definite temperature below which recrystallization will not occur but refers to the approximate temperature at which a highly cold worked material completely recrystallizes in 1 hr. very pure metals seems to have low crystallization temperature as compared with impure metal and alloys. Increasing the annealing time, decreases the time of recrystallization temperature. q The recrystalline temperature is far more sensitive to changes in temperature than to variation in time at constant temperature. q Recrystallization is indicated by the sharp drop in tensile strength. q For equal amounts of the cold working, more strain hardening is introduced into initially fine grained metal than coarse grained metal. Therefore, the finer the initial grain sizes the

Recovery, recrystalline and grain growth and variation in chief mechanical properties

RECOVERY: The term ‘recovery’ may be defined as the process of removing internal stresses due to cold working, in the metal, by heating it to a relatively low temperature. The property changes produced do not cause appreciable change in the microstructure. At a given temperature, the rate of decrease in residual strain hardening is the fastest at the beginning and drops off at longer times. Also, the amount of reduction in residual stress that occurs in a practical time increases with

When the load which has caused plastic deformation in a polycrystalline material is released, all the elastic deformation does not disappear. This is due to the different orientation of the crystals, which will not allow some of them to move back when the load is released. As the temperature is increased, there is some spring back of these elastically displaced atoms which relieves most of the internal stress. In some cases there may be slight amount of plastic flow, which may result in hardness and strength. Electrical

Since the mechanical properties of the metal are essentially unchanged, the principal application of heating in the recovery range is in stress relieving cold worked alloys to prevent stress-corrosion cracking or to minimize the distortion produced by residual stresses. Commercially, this low temperature treatment in the recovery range is known

SOLDERING ALLOYS

DEFINITION OF SOLDERING Solderingis a process in which two or more metals are joined by means of another metal that has a melting point below 10000F (5400C) but lower than that of the metal to be joined. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by

SOLDERING ALLOYS Ø Soldering alloys are mainly consist of lead and tin Ø They have melting point below 1000 0F (540 0C) Ø These alloys are usually known as soft alloys. Ø High –tin alloys possess high tensile and shear strength and less ductility as compared to highlead alloys. Ø There are some soldering alloys which neither contain lead nor tin. Such solders are used for high temperature conditions.

SOFT SOLDERING ALLOYS

Tin-Lead constitutional diagram

Continuous maximum safe load on Single Lap Joint for two solders at 70 0F

Flux qFlux is a chemical agent used during soldering for cleansing action. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. q It must be of such a character that the residue is non – corrosive , non hygroscopic , non conducting for electrical work. qIt should not volatilize, decompose or carbonize during soldering.

Common fluxes are: § Rosin for soldering electrical connections. § Salt or acid type fluxes composed of chlorides of ammonia , zinc , aluminum etc. are good fluxes as they do not carbonize. § Some fluxes containing organic acids such as stearic ,oleic , benzoic are also used but these are corrosive .

Functions of Flux  

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To clean the surface To prevent oxidation of the surface when hot To promote alloying of metal surface with the solder To promote wetting of the surface by the molten solder

BRAZING ALLOYS

Definition Of Brazing Brazing is a process in which two or more metals are joined by means of another metal that has a melting point above 10000F (5400C) but lower than that of the metal to be joined. Common brazements are about 1⁄3 as strong as the parent material, to create high-strength brazes, a brazement can be annealed to homogenize the grain structure and composition

TYPES OF BRAZING ALLOYS q Copper and Copper base alloys q Silver Brazing Alloys q Aluminum Alloys

Copper and Copper base alloys 

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Pure copper is used for brazing of Ferrous parts at a temperature of about 2150 F Copper – zinc alloys constitute the largest portion of copper base brazing alloys with melting range from 1400F- 1700F

Copper base Brazing Alloys

Aluminum Brazing Alloys 

Aluminum brazing alloys consist principally of aluminum to which other elements are added to decrease the melting point range substantially below the melting point of aluminium.

Silver base Brazing Alloys 

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Generally known as "hard solders“ or “silver solders” The silver solders are developed from copper- zinc alloys to which silver and other elements like cadmium, phosphorus and tin are added to lower the melting point. Their melting point ranges from 1100F1600F. These alloys permits brazing at low temperature which is necessary to prevent damage to base metal due to structural

Silver base Brazing Alloys

Brazing Techniques Torch Brazing 

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In this type of brazing oxyfuel gas is used as heat source . In this joint is 1st heated with the torch & then depositing the filler metal. Part thickness ranges from 0.25-6 mm

Furnace Brazing  In the furnace brazing, as name suggest furnace is used as the heat source. The joining parts are cleaned & loaded with filler metal and then put in the furnace.  In furnace part thickness is high. Induction Brazing  in this type high frequency a.c. current is used as heat source. Parts to be joined are loaded with filler metal & placed near induction coils for rapid heating.  Part thickness is less than 3 mm.

Brazing Techniques…….. Resistance Brazing 

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In this type source of heat is through the electric resistance of the components to be brazed. In this parts are either preloaded or filler metal supplied during brazing. Part thickness in this case is of order 0.1-12 mm.

Dip Brazing • •

This is carried out by dipping the assemblies to be brazed in the molten solution of filler metal , which serves as heat source. Thus all the parts are coated with filler metal. Part thickness is less than 5 mm.

High Energy Beams • •

In this type of electron beams & laser beams are used as heat source for heating the assemblies. This is used for the high precision applications .

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