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Note for Manufacturing Science 1 - MS1 By UPTU Risers

  • Manufacturing Science 1 - MS1
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  • uttar pradesh technical university - uptu
  • Mechanical Engineering
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MANUFACTURING SCIENCE-I UNIT-1 METAL FORMING PROCESS Forming can be defined as the process in which desired shape and size are obtained through the plastic deformation of the material the stresses include during the process are greater than the yield strength of loading applied may be tensile, compressive, bending or shearing or the combination of the above. Forming is very economical since there is no loss of material. ELASTIC DEFORMATION: If an external load is applied to a metallic piece it is deformed elastically displacing individual atoms from their equilibrium position. Tensile stresses developed try to increase the inter atomic spacing while compressive stresses try to decrease on the removal of load the loading does not exceed the elastic limit original position of the atoms are regained elastic behaviour is governed by Hooke’s law. PLASTIC DEFORMATION: It is the permanent deformation which present even after the removal of external loads in crystalline materials at temp. Level greater than 0.4 T m. the permanent deformation is called plastic deformation. It is the function of stress temp. rate of straining and the stress may be tensile compressive or shear. This deformation takes place by the process slip and twining the plastic deformation makes the forming process possible like rolling and forging etc. PLASTIC DEFORMATION AND YIELD CRITERIA: The plastic deformation takes place when applied stress level exceed. The surface level defined the yield stress, following are the two criterion. 1. TRESCA’S MAX SHEAR STRESS CRITERIAN: According to Tresca the plastic flow initiates when the max shear stress reaches a limiting value defined as shear yield stress (K) If the principle stresses at a point in the material are σ 1 σ 2 and σ 3 (σ 1 ≥ σ 2 ≥ σ 3 ) then max shear stress τ max is given by τ max = ½ (σ 1 – σ 3 ) Plastic deformation occurs when τ max = K Hence Tresca criteria become ½ (σ 1 – σ 3 ) = K ---------- (1) It is evident from the equation that yielding is independent of intermediate principle stress σ 2. 2. VAN MISES MAX DISTORTION ENERGY CRITERIAN: According to van mises the plastic flow occurs when the shear strain energy reaches the critical value. The shear strain energy per unit volume (ε) can be express in terms of three principle stress as Ε= 1/6G [(σ 1 – σ 2 )2 + (σ 2 – σ 3 )2 + (σ 3 – σ 1 )2] By: ANIMESH PAL By: ANIMESH PAL Email: animesh14@gmail.com Email: animesh14@gmail.com Page 1

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G= shear modulus of the material According to this criterion plastic flow occurs when the RHS of the above equation reaches a critical valve say ‘A’ hence the criteria becomes (σ 1 – σ 2 )2 + (σ 2 – σ 3 )2 + (σ 3 – σ 1 )2 = 6AG = C ---------- (2) Where C is constant In this criterion the initiation of plastic deformation of flow depends on all three principle stresses. RELATIONSHIP BETWEEN TENSILE AND SHEAR YIELD STRESSES: To applied the yield criteria it is necessary to know the right hand side of equation (2) Material properties are determined from uniaxial tensile test which give the value of tensile yield stress (σ y ) which can be used to determine the shear yield stress (K) When yielding occurs under Uniaxial tensile loading σ 1 = σ y, σ2 = σ3 = 0 Hence the constant from equation (2) will be C = 2 σ y2 Considering yielding under pure torsion the state of stress in a material for a two dimensional situation is shown with the help of Mohr’s circle K K σ1 σ3 σ3 σ1 σ1 = K σ2 = 0 σ 3 = -K It is clear from Mohr’s Circle that σ 1 = K, σ 2 = 0, σ3 = -K Substituting these values in equation (2) we get C = 6K2 Since the constant C is independent of type of loading the relation between K and σ y can be obtain by equating the two values of C K = σ y /√3 Applying Tresca’s yield criterion to these two pure loading conditions. The relation between K and σ y is K = σ y /2 By: ANIMESH PAL By: ANIMESH PAL Email: animesh14@gmail.com Email: animesh14@gmail.com Page 2

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COLD AND HOT WORKING OF METALS 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. In comparison with hot working, the advantages of cold working are 1. 2. 3. 4. 5. 6. 7. 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 interchangeablity. Better strength, fatigue, and wear properties of material. Directional properties can be imparted. Contamination problems are almost negligible. Some disadvantages associated with cold-working processes are: 1. 2. 3. 4. 5. 6. Higher forces are required for deformation. Heavier and more powerful equipment is required. Less ductility is available. Metal surfaces must be clean and scale-free. Strain hardening occurs (may require intermediate annealing). Undesirable residual stresses may be produced Cold forming processes, in general, are better suited to large-scale production of parts because of the cost of the required equipment and tooling. 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. In comparison with cold working, the advantages of hot working are 1. 2. 3. 4. 5. 6. No strain hardening Lesser forces are required for deformation Greater ductility of material is available, and therefore more deformation is possible. Favourable grain size is obtained leading to better mechanical properties of material Equipment of lesser power is needed No residual stresses in the material. Some disadvantages associated in the hot-working of metals are: 1. Heat energy is needed 2. Poor surface finish of material due to scaling of surface 3. Poor accuracy and dimensional control of parts 4. Poor reproducibility and interchangeability of parts By: ANIMESH PAL Email: animesh14@gmail.com By: ANIMESH PAL Email: animesh14@gmail.com Page 3

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5. 6. Handling and maintaining of hot metal is difficult and troublesome Lower life of tooling and equipment. 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. Forging is an effective method of producing many useful shapes. The process is generally used to produce discrete parts. Typical forged parts include rivets, bolts, crane hooks, connecting rods, gears, turbine shafts, hand tools, railroads, and a variety of structural components used to manufacture machinery. The forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications. A variety of forging processes have been developed that can be used for either producing a single piece or mass – produce hundreds of identical parts. Some common forging processes are: 1. 2. 3. 4. 5. 6. Open –forging(Smith Forging) Drop forging(Closed die forging0 Press Forging Upset Forging Swaging Roll forging 1. Open forging(Smith 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. Open die forging does not confine the flow of metal; the operator obtains the desired shape of forging by manipulating the work material between blows. 2. 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. 3. Press Forging: It 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. Dies are generally heated during press forging to reduce heat loss, promote more By: ANIMESH PAL By: ANIMESH PAL Email: animesh14@gmail.com Email: animesh14@gmail.com Page 4

Lecture Notes