Deformation and Forging

21 flashcards · Other · by Liam Kehoe

1. Defects in extrusion

   Surface cracking (tearing) Occurs when extrusion temp, friction or speed is too high Surface temp, rise significantly Extrusion defect Draw surface oxides and impurities toward the centre Reduced by modifying the pattern

2. Disadvantages of cold forming

   Higher forces and power Limitation to the amount of forming Additional annealing is required Limitation of available materials

3. Rolling

   Bulk deformation process of reducing the thickness or changing the cross section of a long workpiece by compressive force applied through a set of rolls

4. Simple Rolling equations

5. Reducing Roll Force

   Can cause significant deflection and flattening of the rolls Use small diameter rolls to reduce contact area Talking smaller reduction per pass Rolling at elevated temp Applying back and front tensions to the strip Reducing friction

6. Defects in Rolled Products

   Surface defects caused by inclusions and impurities in the original cast material by preparations and rolling operations Wavy edges Zipper cracks in the center of strip Edge cracks Alligatoring

7. Optimum die angle for extruding

   Ideal work of deformation is independent of the die angle Frictional work decreases with increasing die angle Redundant work caused by inhomogeneous deformation increase with die angle

8. Bulk deformation process

   Input: bulk materials in a form of cylindrical bars and billets, rectangular billets and slabs or elementary shapes Process: large plastic deformation - rolling, forging, extrusion and wire and bar drawing under cold, warm and hot conditions Output: work materials for subsequent processes or final products (net shaping)

9. Why use forming?

   Geometrical Components having significantly different dimensions are difficult to cast Microstructure In forming it is easier to control microstructure than casting The deformation process alleviates compositional gradients and inhomogeneity of grain size Refractory Materials Materials which have melting point cannot be processed by casting

10. Disadvantages of hot forming

    Lower accuracy and surface finish High production cost Possible warping during cooling

11. Die angle and orifice shape

    Low die angle - surface area is large leading to increased friction at die-billet interface which results in larger ram force Large die angle - More turbulence in metal flow during reduction, which increases ram force required Shape of die orifice affects ram pressure Simplest shape = circular die orifice As cross section becomes more complex, higher pressure and greater force are required

12. Advantages of hot forming

    Big amount of forming is possible Lower forces and power Forming with low ductility materials No additional annealing

13. Disadvantages of warm forming

    Greater forming press loads Strict temperature control Energy costs associated with heating

14. Advantages of warm forming

    Lower forces and power More complex part shapes No annealing is required

15. Types of bulk deformation processes

    Forging - plastic deformation of workpiece by compressive force Rolling - reducing thickness by changing cross-section of a long workpiece by compressive force Extrusion - production of long length of solid or hollow products with constant cross-section by pushing workpiece Drawing - production of long rod, wire, tubing by pulling workpieces

16. Advantages of Cold Forming

    Better accuracy Better surface finish High strength High hardness No heating is required

17. Forging Defects

18. Drawing

    The cross section of a long rod or wire is reduced or changed by pulling it through a die called a draw die The difference between drawing and extrusion The material is pushed through a die (extrusion) whereas it pulled through the die (drawing)

19. What is forming and shaping

    Permanent (plastic) deformation of the metal work piece occurs under tension compression, shear or combinations

20. Chevron Cracking (internal defects)

    Due to hydrostatic tensile stress at the center line of the deformation zone Similar to necked region in the tensile test

21. Forging

    A process of shaping materials into components using solid-state methods The stresses needed to make solids flow are very high and require a lot of energy and strong tooling Because of costs involved forging is only viable for large production volumes