ME 612 Metal Forming and Theory of Plasticity. 1. Introduction

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1 Metal Forming and Theory of Plasticity Yrd.Doç. e mail: azsenalp@gyte.edu.tr Makine Mühendisliği Bölümü Gebze Yüksek Teknoloji Enstitüsü

2 In general, it is possible to evaluate metal forming operations as part of the manufacturing processes. Manufacturing processes in general can be classified as follows: 1) Primary Shaping, such as Casting (sand and die), melt extrusion, metal powder pressing. No initial shape > well defined final shape. 2) Metal Forming: Rolling, Extrusion, Cold and Hot Forging, Bending, Deep Drawing, Rod and Tube Drawing,... Material is formed by Plastic Deformation. 3) Metal Cutting: Sawing, Turning, Milling, Broaching, where a new shape is made by removing of material. 4) Metal Treatment: Heat Treating, Anodizing, Surface Hardening. No change in shape, but changes in properties or appearance. 5) Joining: a) Metallurgical: Welding, Brazing, Diffusion Bonding. b) Mechanical: Riveting, Shrink fitting, Mechanical Assembly (bolts, screws,...). 2

3 In metal forming operations shape change is obtained by plastic deformation. It is preferred as there is no material removal and improvement of material properties is achieved. 3

4 1.1. Variables, Classification and Description of Metal Forming Processes In Metal Forming a simple geometry is transformed into a complex one. The tools (dies) "store" the desired geometry and impart pressure on the material through tool/material interface. The physical phenomena describing a forming operation are difficult to formulate for large number of process variables, e.g., for design of a forming operation the followings should be considered: 1) The kinematics: metal flow, i.e. shape, velocities, strain, strain rate. 2) The limits of formability, i.e. under what condition the material fails internally or on the surface. 3) The forces and stresses required, so, the capacity and type of presses can be determined, etc. 4

5 1.1. Variables, Classification and Description of Metal Forming Processes Metal forming operations can be performed cold or hot. Hot forming operations are the ones where the metal is heated above the recrystallization temperature. If classified according to dimensions of the material; Bulk (volume) or sheet metal forming If classified according to forces applied: Compresive forces (Ex: forging, extrusion,... ) Tensile forces (Ex: Wire drawing, deep drawing,strech forming... ) Moment in cross secttinal area (Ex: folding, strech folding,... ) Shear forces (Ex: Piercing, ) 5

6 1.2. Metal Forming as a System A Metal forming system is composed of 1) Work piece {Material, Geometry} 2) Tooling (dies) {Material, Geometry} 3) Interface condition, i.e. frictional characteristics of the two surfaces 4) Mechanics of plastic deformation 5) Equipment used 6) Plant environment 7) Characteristics of the final product {Geometry, Mechanical Properties, Metallurgical Properties)} 6

7 1.2. Metal Forming as a System Figure 1.1. Factors effecting workpiece in transferring to final product Metal flow determines the characteristics of the final product and formation of defects (e.g. cracks, folds, wrinkles). 7

8 1.3. Advantages of Metal Forming As a Manufacturing Process 1) Almost no scrap material. 2) Obtaining final product in short time. 3) Obtaining better mechanical and metallurgical properties. (strength, toughness, grain size,... ) 8

9 1.4. Structure of Metals, Deformation Mechanism of Crystals Structure of Metals: Metals and alloys are crystalline in solid state. It is not yet possible to relate the behavior of individual atoms in and around its lattice to Macroscopic properties of the metal. Unit Cell: (Lattice) the specific arrangement of atoms in a crystal. The crystal of the metalisformed bythe 3 D repetition of unit cells. Crystal (Grain): is the 3D array of Unit Cells. 9

10 1.4. Structure of Metals, Deformation Mechanism of Crystals Most metal's structure fall within 3 groups. BCC, Body Centered Cubic, such as: V, Cr, Fe, Fe FCC, Face Centered Cubic, such as: Cu, Al, Fe gibi HCP, Hexagonal Closed Packed, such as: Zn, Ti, Mg gibi Figure 1.2. (a) BCC, (b) FCC,(c) HCP. 10

11 1.4. Structure of Metals, Deformation Mechanism of Crystals Crystal structures of some metals: BCC FCC HCP Chrome Aluminium Beryllium Iron Demir Magnesium Molybdenum Copper Zinc Tungsten Gold Cobalt Vanadium Lead Titanyum Tantalum Nickel Zirconium Titanium Silver Zirconium Cobalt 11

12 1.4. Structure of Metals, Deformation Mechanism of Crystals Deformation Mechanism of Crystals: 1) The deformation of metals occurs by sliding of blocks of crystal over one another along definite crystallographic planes, called slip planes. Slip planes are the most populated planes and directions in the crystal. Sliding of blocks of material is produced by shear stresses in the slip plane. If the crystal was perfect, the shearing stress required to initiate plastic deformation would be ~ 100 times greater than realistic values; so > dislocations {edge, screw} 12

13 1.4. Structure of Metals, Deformation Mechanism of Crystals Figure 1.3. The deformation of metals by sliding of blocks 13

14 1.4. Structure of Metals, Deformation Mechanism of Crystals 2) Deformation by Twinning {Mechanical (Shock Loading (large )) and Thermal} The second mechanism of deformation in metals. It results when a portion of the crystal takes up an orientation which has mirror symmetry to the untwinned lattice. 14