Materials Forming

Materials Forming
Materials forming of solid state (deformation working or plastic deformation or mechanical forming) can be defined as a process in which the desired size and shape are obtained through the deformation of materials plastically under the action of externally applied forces without removal of materials and concerned volume and mass.
Materials forming processes like spinning, extrusion, rolling, forging, drawing etc. are used widely lately. It is due to the fact that material forming is the waste less process which is highly economical, they give high dimensional accuracy, easy formability for complex shapes and good surface finish with desired properties. The material forming is based upon the plastic deformation of article. The forming with solid state is performed by two methods (hot working and cold working);
Hot working is done at a temperature above recrystallization but below its melting point and Cold working is done at temperature below recrystallization temperature. So, no appreciable recovery can take place during deformation.
Hot Working
Properties
1. Hot working is done at a temperature above recrystallization but below its melting point.
2. Hardening due to plastic deformation is completely eliminated by recovery and recrystallization.
3. Improvement of mechanical properties such as elongation, reduction of area and impact values.
4. No effect on ultimate tensile strength, yield point, fatigue strength and hardness.
5. Poor surface finish due to oxidation.
6. Refinement of crystals occurs.
7. Due to hot working cracks and blowholes are welded up.
8. No internal or residual stress developed.
9. Force required for deformation is less.
10. Light equipment is used in hot working.
11. Difficult to handle a hot worked metal.
12. Hot working processes are hot forging, hot rolling, hot spinning, hot extrusion, hot drawing, and hot piercing, pipe welding.
Advantages of Hot Working
1. Porosity in the metal is largely eliminated. Most ingots contain many small blow holes. These are pressed together and eliminated.
2. Impurities in the form of inclusions are broken up.
3. Coarse or columnar grains are refined. Since this hot work is in the recrystalline temperature range, it should be continued until the low limit is reached to provide a tine grain structure.
4. Physical properties are generally improved owing principally to grain refinement. Ductility and resistance to impact are improved, strength is increased, and greater homogeneity is developed in the metal. The greatest strength of rolled steel exists in the direction of metal flow.
5. The amount of energy necessary to change the shape of steel in the plastic state is far less than that required when the steel is cold.
Disadvantages/Limitations of Hot Working
1. Because of the high temperature of the metal, there is rapid oxidation or scaling of the surface with accompanying poor surface finish.
2. Difficult to achieve close tolerances due to scaling.
3. Some metals cannot be hot worked because of their brittleness at high temperatures.
4. Hot working equipment and maintenance costs are high.
Cold Working
Properties
l. Cold working is done at temperature below recrystallization temperature. So, no appreciable recovery can take place during deformation.
2. Hardening is not eliminated since working is done below recrystallization temperature.
3. Decreases elongation, reduction of area etc.
4. Increase in ultimate tensile strength, yield point and hardness.
5. Good surface finish is obtained.
6. Crystallization does not occur. Grains are only elongated.
7. Possibility of crack formation and growth is great.
8. Internal and residual stresses are developed in the metal.
9. Force required for deformation is high.
10. Heavy and powerful equipment is used for cold working.
11. Easier to handle cold parts.
12. Cold working processes are cold rolling, cold extrusion, press work (drawing, squeezing, bending, and shearing).
Advantages of Cold Working
1. Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. Further, there is no possibility of decarburisation of the surface.
2. Since the working is done in cold state, hence no oxide formation on the surface and consequently, good surface finish is obtained.
3. Greater dimensional accuracy is achieved.
4. Easier to handle cold parts and also economical for small sizes.
5. Better mechanical properties are achieved.
Disadvantages/Limitations of Cold Working
1. Only small sized components can be easily worked as greater forces are required for large sections. Due to large deforming forces, heavy and expensive capital equipment is required.
2. The grain structure is not refined and residual stresses have harmful effects on certain properties of metals.
3. Many of the metals have less ductility e.g., carbon steel and certain alloy steels, cannot be cold worked at room temperature. It is therefore, limited to ductile metals and the range of shapes produced is not as wide as can be obtained by machining.
4. Tooling costs are high and as such it is used when large quantities of similar components are required.