ENM208 METAL FORMING

1. ENM208 METAL FORMING ANADOLU U N I V E R S I T Y Industrial Engineering Department 2006 Saleh AMAITIK

2. Manufacturing Processes Fundamentals of Metal Forming Metal Forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces. Deformation results from the use of a tool, usually a die in metal forming, which applies stresses that exceed the yield strength of the metal. The metal deforms to take a shape determined by the geometry of the die. Spring 2005

3. Manufacturing Processes Stresses in Metal Forming Stresses applied to plastically deform the metal are usually classified into five groups: Compressive Stress. Deformation of a solid body is achieved through Uni-axial or Multi-axial compressive loading Tensile Stress. A plastic deformed shape is achieved through either Uni-axial or Multi-axial deformation. Combined tensile and compressive stresses. A combination of tension and compression to achieve plastic deformation. Spring 2005

4. Manufacturing Processes Stresses in Metal Forming Bending Stress. Permanent deformation is achieved by a bending load. Shearing Stress A solid body in which plastic deformed state has been achieved by a shearing load. Spring 2005

5. Manufacturing Processes Material Properties in Metal Forming To be successfully formed, a metal must possess certain properties. Desirable material properties: • Low yield strength and high ductility These properties are affected by temperature: • Ductility increases and yield strength decreases when work temperature is raised Strain rate and friction are additional factors that affect performance in metal forming. Spring 2005

6. Manufacturing Processes Independent Variables in Metal Forming Independent variables are those aspects of the process over which the engineer has direct control , and they are generally selected or specified when setting up a process. Some of independent variables in a typical forming process • Starting material. • Starting geometry of the workpiece. • Tool or die geometry. • Lubrication. • Starting temperature. • Speed of operation. • Amount of deformation Spring 2005

7. Manufacturing Processes Dependent Variables in Metal Forming Dependent variables, these are the consequences of the independent variable selection. Example of dependent variables include: • Force or power requirements. • Material properties of the product. • Exit (or final) temperature. • Surface finish and dimensional precision. • Nature of the material flow. Spring 2005

8. Manufacturing Processes Independent-Dependent Variables Relationships The link between independent and dependent variables is truly the most important area of knowledge for a person in Metal Forming Spring 2005

9. Manufacturing Processes Metal Forming Classification Metal forming processes can be classified as: 1- Bulk Deformation Processes - Characterized by significant deformations and massive shape changes - "Bulk" refers to workparts with relatively low surface area‑to‑volume ratios - Starting work shapes include cylindrical billets and rectangular bars 2- Sheet Metal Working Processes - Forming and related operations performed on metal sheets, strips, and coils • High surface area‑to‑volume ratio of starting metal, which distinguishes • these from bulk deformation - Often called pressworking because presses perform these operations - Parts produced in sheet-metal operations are called Stampings - Tools used 1- The punch is the positive portion. 2- The die is the negative portion. Spring 2005

10. Manufacturing Processes Bulk Deformation Processes The basic operations in bulk deformation are illustrated in the following figures. 1- Rolling.Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls Spring 2005

11. Manufacturing Processes Bulk Deformation Processes 2- Forging. Deformation process in which work is compressed between two dies Spring 2005

12. Manufacturing Processes Bulk Deformation Processes 3- Extrusion.Compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross‑sectional shape Spring 2005

13. Manufacturing Processes Bulk Deformation Processes 4- Drawing. Cross‑section of a bar, rod, or wire is reduced by pulling it through a die opening Spring 2005

14. Manufacturing Processes Sheet Metal Working Processes 1- Bending. Straining sheet metal around a straight axis to take a permanent bend Spring 2005

15. Manufacturing Processes Sheet Metal Working Processes 2- Drawing. Forming a flat metal sheet into a hollow or concave shape, such as cup, by stretching the metal. Spring 2005

16. Manufacturing Processes Sheet Metal Working Processes 3- Shearing. A shearing operation cuts the work using a punch and die. Spring 2005

17. Manufacturing Processes Material Behavior in Metal Forming The typical stress strain curve for most metals is divided into an elastic region and a plastic region Plastic region of stress-strain curve is primary interest because material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve: where K= strength coefficient (MPa); and n = strain hardening exponent Stress and strain in flow curve are true stress and true strain Spring 2005

18. Manufacturing Processes Flow Stress For most metals at room temperature, strength increases when deformed due to strain hardening. The stress required to continue deformation must be increased to match this increase in strength. Flow stress. Is defined as the instantaneous value of stress required to continue deforming the material – to keep the metal “flowing”. • where • Yf = flow stress, that is, the yield strength as a function of strain Spring 2005

19. Manufacturing Processes Average Flow Stress The average flow stress (also called the Mean flow stress) is the average value of stress over the stress-strain curve from the beginning of strain to the final (maximum) value that occurs during deformation Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest. Where = average flow stress; and  = maximum strain during deformation process Spring 2005

20. Manufacturing Processes Typical Values of K and n Spring 2005

21. Manufacturing Processes Temperature in Metal Forming For any metal, the values of K and n in the flow curve depend on temperature • Both strength and strain hardening are reduced at • higher temperatures • - In addition, ductility is increased at higher temperatures These property changes are important because; Any deformation operation can be accomplished with lower forces and power at elevated temperature Spring 2005

22. Manufacturing Processes Temperature Ranges Metal Forming There are three temperature ranges in metal forming processes: Where Tm is the melting point of the metal Spring 2005

23. Manufacturing Processes Cold Working Performed at room temperature or slightly above Many cold forming processes are important mass production operations Minimum or no machining usually required These operations are near net shape or net shape processes Spring 2005

24. Manufacturing Processes Advantages of Cold Working Significant advantages of cold forming compared to hot working • Better accuracy, meaning closer tolerances • Better surface finish • Strain hardening increases strength and hardness • Contamination problems are minimized • No heating of work required Spring 2005

25. Manufacturing Processes Disadvantages of Cold Working There are certain disadvantages or limitations associated with cold working • Higher forces are required to initiate and complete the deformation • Heavier and more powerful equipment and stronger tooling are required. • Surfaces of starting workpiece must be free of scale and dirt. • Ductility and strain hardening limit the amount of forming that can be done • In some operations, metal must be annealed to allow • further deformation • In other cases, metal is simply not ductile enough to • be cold worked Spring 2005

26. Manufacturing Processes Warm Working Performed at temperatures above room temperature but below recrystallization temperature. • Dividing line between cold working and warm working often expressed in terms of melting point: • 0.3Tm, where Tm = melting point for metal Spring 2005

27. Manufacturing Processes Advantages of Warm Working The lower strength and strain hardening as well as higher ductility of the metal at the intermediate temperatures provide warm working the following advantages over cold working • Lower forces and power than in cold working • More intricate work geometries possible • Need for annealing may be reduced or eliminated Spring 2005

28. Manufacturing Processes Hot Working Deformation at temperatures above recrystallization temperature Recrystallization temperature = about one‑half of melting point In practice, hot working usually performed somewhat above 0.5Tm • Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level Capability for substantial plastic deformation of the metal ‑ far more than possible with cold working or warm working Spring 2005

29. Manufacturing Processes Advantages of Hot Working • Workpart shape can be significantly altered • Lower forces and power required • Metals that usually fracture in cold working can be hot formed • Strength properties of product are generally isotropic • No strengthening of part occurs from work hardening • Advantageous in cases when part is to be subsequently processed by cold forming Spring 2005

30. Manufacturing Processes Disadvantages of Hot Working • Lower dimensional accuracy • Higher total energy required (due to the thermal energy to heat the workpiece) • Work surface oxidation (scale), poorer surface finish • Shorter tool life Spring 2005

31. Manufacturing Processes Strain Rate The rate at which the metal is strained in a forming process is directly related to the speed of deformation, v In many forming operations, deformation speed is equal to the velocity of the ram or other moving element of the equipment. Strainrate is defined: where = true strain rate (m/s/m) or simply (S-1); and h = instantaneous height of workpiece being deformed • In most practical operations, valuation of strain rate is complicated by • - Work part geometry • - Variations in strain rate in different regions of the part Spring 2005

32. Manufacturing Processes Friction in Metal Forming Friction in metal forming arises because of the close contact between the tool and work surfaces and the high pressures that drive the surfaces together in these operations. In most metal forming processes, friction is undesirable for the following reasons: • Metal flow in the work is retarded. • The forces and power to perform the operation are increased. • Rapid wear of the tool occurs Friction and tool wear are more severe in hot working If the coefficient of friction becomes large enough, a condition known as sticking occurs. Sticking in metal working is the tendency for the two surfaces in relative motion to adhere to each other rather than slide Spring 2005

33. Manufacturing Processes Lubrication in Metal Forming Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction. Benefits obtained from the application of lubricants are: • Reduced sticking, forces, power, tool wear • Better surface finish • Removes heat from the tooling Spring 2005

34. Manufacturing Processes Lubrication in Metal Forming Consideration in choosing an appropriate metalworking lubricant include: • Type of forming process (rolling, forging, sheet metal drawing, etc.) • Hot working or cold working • Work material • Chemical reactivity with tool and work metals • Ease of application • Cost Spring 2005