1. Industrial Engineering Department
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. Stresses in Metal Forming
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. Stresses in Metal Forming
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. Material Properties in Metal Forming
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. Independent Variables in Metal Forming
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. Dependent Variables in Metal Forming
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. Independent-Dependent Variables Relationships
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. Metal Forming Classification
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. Bulk Deformation Processes
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. Bulk Deformation Processes
Manufacturing Processes
Bulk Deformation Processes
2- Forging. Deformation process in which work is compressed between two dies
Spring 2005

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

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

17. Material Behavior in Metal Forming
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. Yf = flow stress, that is, the yield strength as a function of strain
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. Average Flow Stress Manufacturing Processes
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. Typical Values of K and n
Manufacturing Processes
Typical Values of K and n
Spring 2005

21. Temperature in Metal Forming
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. Temperature Range Category
Manufacturing Processes
Temperature Ranges Metal Forming
There are three temperature ranges in metal forming processes:
Temperature Range
Category
 0.3 Tm
Cold Working
0.3 Tm – 0.5 Tm
Warm Working
0.5 Tm – 0.75 Tm
Hot Working
Where Tm is the melting point of the metal
Spring 2005

23. These operations are near net shape or net shape processes
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. Advantages of Cold Working
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. Disadvantages of Cold Working
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. 0.3Tm, where Tm = melting point for metal
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. Advantages of Warm Working
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. Deformation at temperatures above recrystallization temperature
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. Advantages of Hot Working
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. Disadvantages of Hot Working
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. Strain Rate Manufacturing Processes
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.
Strain rate 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. Friction in Metal Forming
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. Lubrication in Metal Forming
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. Lubrication in Metal Forming
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