1 Plastic Deformation of Metals and Related Properties 1.1INTRODUCTION Metal forming is the backbone of modern manufacturing industry besides being a major industry in itself. Throughout the world hundreds of million tons of metals go through metal forming processes every year. As much as 15-20% of GDP of industrialized nations comes from metal forming industry. Besides, it fulfils a social cause by providing job opportunities to millions of workers. Metal forming industry, in general, is a bulk producer of semi-finished and finished goods and this is one reason that it is viable to undertake large scale research and development projects because even a small saving per ton adds up to huge sums. In metal forming processes, the product shapes are produced by plastic deformation. Hence it is important to know the plastic flow properties of metals and alloys for optimizing the processes. Also the resulting component properties depend upon the intensity and the conditions of plastic deformation during forming. Many forming processes produce raw materials for other processes which in turn produce finished or semi-finished products. For example, steel plants produce sheet metal which is used by automobile industry to manufacture components of automobiles and their bodies. In fact sheet metal is used by a number of manufacturers for producing a large variety of household and industrial products. Similarly billets produced by steel plants are used by re-rolling mills for rolling into products like angles, channels, bars etc. Bars may be further used for manufacturing forgings, wires, bright bars and machined products. Similarly the manufacturers of rivets, screws, bolts and nuts buy wire from wire manufacturers and process them further. Therefore, the producers of semi-finished materials such as sheet metal, bar stock and wires, etc. have to consider that they produce such properties in their products which are required by down stream industry engaged in further processing of these products. For example, deep drawability of sheet metal increases with increase in anisotropy ratio (see Section1.9), therefore, rolling parameters such as finishing temperature, cold reduction etc, are adjusted to produce higher anisotropy ratio in the sheet metal which is to be used for deep drawing. The properties of metals and alloys are highly influenced by their microstructure which may be modified or altered by alloying elements, by heating or heat treatment or by plastic deformation. For example, metals and alloys may be hardened by plastic deformation. It would, therefore, be helpful if we look at metals at the micro level. 1
2Fundamentals of Metal Forming Processes 1.2BASIC STRUCTURE OF METALS AND ALLOYS 1.2.1Grain Structure The microstructure of solid metallic bodies consists of grains. Grains consist of unit cells in which atoms are arranged in a particular order. The cell structure repeats itself throughout the volume of the grain (Fig. 1.1). That is why the grains are also called crystallites. The structure is called lattice in which atoms are placed at lattice points. In metals, generally there is only one atom at a lattice point. There are many types of structures of unit cells for different materials, however, metals generally possess one of the following three cell structures. (i)Body centered cubic structure (BCC). (ii)Face centered cubic structure (FCC). (iii)Hexagonal closed packed structure (HCP). The three cell structures are illustrated in Fig. 1.2. Some metals such as iron (Fe), cobalt (Co), titanium (Ti), etc. change their cell structure at different temperatures. Grain boundaries Atoms Fig. 1.1.Grain structure Body Centered Cubic Structure:The structure consists of one atom at each of the eight corners of a cubical element and one atom atthecenter of the cube in Fig. 1.2(a). Metals with this structure are chromium (Cr), hafnium (Hf) at temperatures greater than 1975o C, iron (Fe) except at temperatures 911oC to 1392o C, molybdenum(Mo), niobium (Nb), tantalum (Ta), titanium (Ti) at temperatures greater than 882°C and tungsten (W), etc. (a) BCC(b) FCC(c) HCP Fig. 1.2.Cell structures of metals
Plastic Deformation of Metals and Related Properties3 (c) Grain boundary (b) Edge dislocation(a) VacancyGrain boundary Face Centered Cubic Structure:In this structure there is an atom at each of the eight corners of the cubical element and there is an atom in the middle of each face of the cube as shown in Fig. 1.2(b). Metals with this structure are aluminum (Al), cobalt at temperature greater than 1120°C, copper (Cu), gold (Au), iron (Fe) between temperatures 911°C-1392°C, lead (Pb), nickel (Ni), silver (Ag), platinum (Pt), etc. Hexagonal Closed Packed Structure:In this cell structure there is an atom at each corner of a hexagonal prismatic element, besides, there are three atoms symmetrically placed between the two end faces as shown in Fig. 1.2(c) and one atom each at center of the flat end faces. Metals with this structure are beryllium (Be), cobalt at temperatures less than 1120°C, magnesium (Mg), zinc (Zn), titanium (Ti) at temperatures less than 882°C, etc. 1.2.2Lattice Defects Ideally there should not be any defect in lattice structure, however, imperfections and defects may occur due to alloying elements, plastic deformation, grain boundaries, etc. The lattice structure generally contains following types of defects which are illustrated in Fig. 1.3. (i)Point defects or imperfections. (ii)Line defects which are also called dislocations. (iii) Surface defects—grain boundaries. Point defects are caused by various reasons such as (i) absence of an atom from a lattice point,(ii)an atom getting to a site which is not a lattice point, (iii) an atom of a different element (alloy) substituting an atom of parent metal, etc. Point defects disturb the natural arrangement of atoms in its vicinity and consequently atoms surrounding the point defect are either stretched apart or are pushed too close. This gives rise to additional pull or push among the atoms. Fig. 1.3.Imperfections in lattice structure The type and concentration of these imperfections or defects greatly influence the properties of metals and alloys. The defects may also be induced or controlled by alloying, heat treatment or plastic deformation in order to obtain a change in the mechanical properties. Line defects or dislocations are important for plastic deformations. The following two types of dislocations are observed.