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Metal forming process (introduction)
- 1. Presentation by: Kiran Vinu MSE(M.TECH) 1MS19MSE03 Topic : Effect of speed of deformation, deformation zone geometry, friction and residual stress on metal forming
- 2. Metal forming Factors affecting metal forming Speed of deformation and its effect Deformation zone geometry and its effect Friction and its effect Residual stress and its effect
- 3. What is metalforming process ? Large set of manufacturing processes in which the material is deformed plastically to take the shape of the die geometry. The tools used for such deformation are die, punch etc. depending on the type of process. Examples of metal forming processes: • Rolling • Forging • Sheet metal working • Extrusion
- 4. Temperature Metallurgicalstructure Speed of deformation Deformation zone geometry Friction Residual stress
- 5. During deformation,the speed of the operation is usually measured by strain rate. Strain rate : Ratio of deformation speed to the instantaneous height of the work metal 𝜀 = 𝑣 ℎ
- 6. Effect of speedof deformation High flow stress. Increased temperature of the work piece. If the speed of deformation is too high : Metal cracking is possible Causes plastic instability in cold working Causes hot shortness in hot working
- 7. Refers tothe zone or region where deformation of the metal takes place when the forming tool comes in contact with it. The homogeneity, the tendency to crack, the pattern of residual stress and porosity are affected. It also affects the properties of formed metal. It is defined as the ratio of thickness or diameter of work to the contact length between the work and die. ∆ = ℎ 𝐿
- 8. Effect of deformationzone geometry As the value of ∆ increases, the yield pressure required for deformation also increases. For large values of ∆, internal cracks may develop in the work material as a result of secondary tensile stresses.
- 9. Higher contactpressure between work and tool results in higher friction and subsequently higher deformation resistance. The coefficient of friction 𝜇 can be given by the relation 𝜇 = 𝐹 𝑁 𝜇 – Coefficient of friction F – Shear force N – Normal force
- 10. The valueof 𝜇 depends on Work metal used Tool material Surface roughness of work and tool Speed of deformation Temperature Type of lubrication used
- 11. Effect of friction It increases the amount of force or power required to carry out an operation. Friction reduces metal flow, hence induces inhomogeneity in the formed part leading to certain defects. Friction increases heat at the interface leading to wearing of the tools. Friction between the work and tool gives rise to shearing stresses along the contact surfaces.
- 12. Consider aspecimen marked with vertical and horizontal grids, subjected to compressive load and deformed with and without friction. The deformation pattern is observed after the test. The effect of friction on metal deformation pattern can be clearly seen from the figure.
- 13. When thereis no friction, the deformation will be uniform. The vertical and horizontal grids will remain uniform and straight. When there is friction, the vertical grids and horizontal grids are bent. Free ends are not affected whereas the contact surfaces are held back causing bent pattern in the metal flow.
- 14. Residual stressesare locked up stresses existing within a body in the absence of external loading or thermal gradient. Residual stresses can occur through a variety of mechanisms including plastic deformations, temperature gradients or structural changes during phase transformations. The maximum value which a residual stress can reach is the yield stress of the material. Metals containing residual stresses can be stress relieved by heating to a temperature where the yield strength of the material is the same or lower than the value of the residual stress such that the material can deform and release stress.
- 15. However slowcooling is required otherwise residual stress can again develop during cooling. It can also be relieved by cryogenic cooling but it is uncommon.
- 16. Effect of residualstress It can cause the component to be visibly distorted and deform it while being machined or worked. It can cause warping of the worked part even after the deformation process is completed. It reduces ductility, increases hardness and thereby reduces the formability of the component.