METAL FORMING & POWDER METALLURGY
| Lecture 1. | FUNDAMENTALS OF METAL FORMING |
| Lecture 2. | FORGING |
| Lecture 3. | SWAGING |
| Lecture 4. | FORMABILITY OF SHEET METAL |
| Lecture 5. | SHEARING |
| Lecture 6. | SHEET METAL PROCESSES |
| Lecture 7. | PRESSES FOR SHEET METAL WORKING |
| Lecture 8. | DIE AND PUNCH |
| Lecture 9. | HIGH ENERGY RATE FORMING PROCESSES |
| Lecture 10. | POWDER METALLURGY |
| Lecture 11. | METAL POWDERS FOR PM |
| Lecture 12. | POWDER METALLURGY |
FORMABILITY OF SHEET METAL
Formability may be defined as the ease with which material may be forced into a permanent change of shape. The formability of a material depends on several factors. The important one concerns the properties of material like yield strength, strain hardening rate, and ductility. These are greatly temperature dependent. As the temperature of material is increased, the yield strength and rate of strain hardening progressively reduce and ductility increases. The hot working of metal, therefore, permits relatively very large amount of deformation before cracking. There are several methods of predicting formability. A brief description of some important methods follows.Cup or Radial Drawing:
Cup drawing test uses a circular blank from the metal to be tested. It is inserted in a die, and the severity of the draw it is able to withstand without tearing called the drawing ratio, is noted. The drawing ratio is the ratio of the cup diameter to the blank diameter.
Where Rd = drawing ratio
D = blank diameter
d = punch diameter
A drawing ratio of 50 % is considered excellent. As shown in Fig 4.1(a), either a flat bottom punch with lubricated blank may be used to draw the cup, or as shown in Fig 4.1(b) a blank may be drawn by a lubricated hemi – spherical punch. In the first case, the action is principally that of drawing in which cylindrical stretching of material takes place. In the second case, there will be bi – axial stretching of the material. For drawing, the clamping force is just sufficient to prevent buckling of the material at the draw radius as it enters the die. The deformation takes place in the flange and over the draw radius.
Fukui Conical – Cup Test:
It utilizes a hemispherical, smoothly polished punch. No blank holder is required. In each test, a drawing ratio which will result in a broken cup is determined. Formation of wrinkles is avoided by using a fixed ratio between the thickness of the sheet, the size of the blank, and the punch and die diameters. Under these conditions, the test produces a known amount of stretching, drawing, and bending under tension.
Normal Anisotropy Coefficient:
The material is subjected to uniaxial tensile test. The anisotropy coefficient is derived from the ratio of the plastic width strain W to the thickness strain t . A material with a high plastic anisotropy also has a greater “thinning resistance.” In general, the higher the anisotropy coefficient the better the material deforms in drawing operations.
StrainHardening Coefficient:
Strain hardening refers to the fact that as a metal deforms in some area, dislocations occur in the microstructure. As these dislocations pile up, they tend to strengthen the metal against further deformation in that area. Thus the strain is spread throughout the sheet. However, at some point in the deformations, the strain suddenly localizes and necking, or localized thinning, develops. When
this occurs, little further overall deformation of the sheet can be obtained without it fracturing in the necked region. The strain – hardening coefficient therefore reflects how well the metal distributes the strain throughout the sheet, avoiding or delaying localized necking. The higher the strain – hardening coefficient, the move the material will harden as it is being stretched and the greater will be the resistance to localized necking. Necks in the metal harm surface appearance and affect structural integrity. For many stamping operations, stretching of the metal is the critical factor and is dependent on the strain – hardening coefficient. Therefore, stampings that need much drawing should be made from metal having high average strain – hardening coefficients. Yield strength should be low to avoid wrinkles or buckling.\
Forming Limit Curve:The forming – limit curve is a good index of determining the formability of sheet metal. Essentially, it requires to draw a curve that shows a boundary line between acceptable strain levels in forming and those that may cause failure, Fig 4.2.
The curve indicates the relation between major and minor strains that are perpendicular to the plane of the sheet. To determine these strains, a grid of circles is marked on the sheet metal, say by an electrolytic stencil – etching process. After the metal is deformed, the circles are measured to obtain the major strain 1 and the minor strain 2 , as shown in Fig 4.2 Typically, ten to fifteen data points
are obtained from a test specimen in the region of fracture. Ellipses lying both in the failed region and just outside of it are measured.
The forming – limit curve is then drawn to fall below the strains in the necked and fractured zones, and above the strains found just outside these zones (Fig 4.3)
With controlled variation in specimen size it is possible to plot an entire forming – limit curve from one test setup. A reasonably accurate forming limit curve may be obtained with four specimens while a precision curve may be obtained with eight specimens. In may be noted that “local” ductility varies for different metals, so no universal forming – limit curve can be developed. For example, two metals may have peak local ductilities of 20% and 50% at a given minor strain. The metal with the 20 % local ductility (high strain – hardening coefficient) may turn out to be the best choice because the strain will then have a better distribution throughout, allowing the entire sheet to be stretched 20%. If the other sheet showed little strain hardening, it might stretch by 50% in local area, but leave the rest of the sheet relatively unstrained. Through the use of formability – prediction techniques. Designers and fabricators are able to make a wiser choice of metals and obtain date quickly on newer metals. The essential data can be obtained before the die is designed. Also metal suppliers will be able to establish whether a material possesses required formability before it is shipped from the plant.
