Formability of sheet metal is the ability of sheet metal to be drawn into cups or formed into the desired shapes without any defect. However, in order to have a measure for sake of comparison, the limit drawing ratio (LDR) is taken as a measure of drawability.
LDR is defined as below-
For most of the sheet materials which are generally deep drawn, this ratio varies from 1.8 to 2.4. With this value of LDR only small height of cylindrical cups may be obtained. For greater height one has to carry out more than one draw.
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With higher drawability the number of draws can be reduced thus making the product cheap. After certain value of blank diameter the failure occurs at the punch profile radius. The draw ability of sheet metal is affected by following factors.
Factors which Affect Drawability:
The following factors affect draw ability:
1. Tool geometry of deep drawing set up.
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2. Lubrication.
3. Blank holding pressure.
4. Regulation of die and punch temperature.
5. Sheet metal properties.
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Affect of Tool Geometry and Lubrication on Drawability:
Optimizing tool geometry involves providing proper punch profile radius and die profile radius. Besides, the friction on punch profile radius is crucial because it restricts the movement of sheet on punch profile radius and thus restrains the neck formation and failure of sheet.
As regards the value of optimum radius on die and punch the extensive work done by Albin at IIT Bombay in 1965 is worth mentioning here. He made a large number of dies and punches for this purpose. His work shows that there is an optimum value of punch profile radius that results in higher drawability. He did work with 1mm sheet and the Fig. 11.23 due to him shows that the optimum value of punch profile is nearly six times the sheet thickness.
The die profile radius was varied from 3mm to 12 mm and Fig. 11.24 shows that drawability continuously increases with increase in die profile radius. The diagram shows that higher values of die profile radius should have been tried to get the optimum value though this range adequately covers range generally used in practice. However at very high value of die profile radius there is danger that wrinkles may form.
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Also the punch profile radius should, be made rough to the extent that it does not leave marks on the component surface and need not be lubricated. The increase in friction on punch profile radius will restrain the neck formation and will give higher drawability. At the same time the die surface should be ground and polished and well lubricated to decrease the co-efficient of friction, so that the drawing stresses are low.
Blank Holding Pressure v/s Drawability:
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The purpose of providing blank holding pressure is to keep the sheet flat and to prevent its buckling under the action of induced circumferential compression so that the product is wrinkle free.
Excessive blank holding pressure can lead to increased frictional stresses in flange drawing which ultimately results in higher stresses in cup wall and at the punch profile radius where due to bending under tension, the thinning and necking may take place.
In fact as the drawing proceeds the blank holding force required for the process decreases. Some die designs in which springs are used to impart blank holding pressure actually do just the reverse. In these designs the blank holding force increases as the drawing proceeds because the spring is stretched more with movement of press ram. Such designs decrease the drawability of sheet.
Drawability v/s Temperature Control of Die and Punch:
During drawing, heating of flange can reduce the flange drawing stress and hence increase the drawability. However, the material near the punch profile radius should remain strong. Naturally this portion of material should not be heated.
To achieve these objectives the heating elements are embedded in the die while the punch bottom is refrigerated from -10 to -15°C so that the sheet near the punch profile remain cool while the flange in contact with die gets heated. Moon et al. have designed such an apparatus and carried out experiments on aluminum.
Heating elements may also be provided in the blank holding plate. However, the experiments of Moon et al. show that if the flange material gets over heated (> 200°C) there is possibility of flange failure.
The authors carried out experiment at a punch speeds of 1, 4, 8, 12 mm/sec. The beneficial results are observed even without refrigeration of punch. The authors find optimum conditions, i.e. die temperature of 200°C and punch temperature of -10°C.
Sheet Metal Properties and Drawability:
The important properties of sheet metal which affect drawability are as below:
1. Value of anisotropy ratio which in turn depends upon composition as well as on processing parameters.
2. Value of strain hardening exponent n.
3. Yield strength and pre-straining.
4. Processing variable during manufacture of sheet metal such as total cold reduction, hot finishing temperature, coiling temperature at the end of hot rolling, slab reheat temperature, etc. All these variables, in fact, influence anisotropy ratio.
Anisotropy in sheet metal can be useful in deep drawing and other forming processes.
During rolling the sheet develops anisotropy both in the plane of the sheet, called planer anisotropy as well in the thickness of sheet called anisotropy ratio. The planer anisotropy is responsible for development of ears on the cup rim while anisotropy ratio affects the thinning at the most stressed portion of metal in deep drawing, that is on the punch profile radius and hence it affects the drawability of the sheet.
Value of r varies if the test specimen is cut in different directions with respect to rolling direction. So the average value of r is determined. The average of the values of r for the test specimen cut at 0°, 45° and 90° with the rolling direction is found as-
The normal anisotropy enhances the limiting drawing ratio (LDR). Figure 11.27 illustrates it for some steels.
The anisotropy ratio r̅ is a function of composition of steel as well as the sheet processing parameters. An excellent review by Hoile has highlighted following points with regards to IF steels.
1. Carbon, nitrogen and sulfur contribute to decreasing of anisotropy ratio through various mechanisms. The bad effect of sulfur may be offset by adequate addition of Mn so that MnS is formed and micro-structure is refined. The influence of carbon on ratio r̅ is illustrated in Fig. 11.28.
2. If nitrogen increases from 0.002 to 0.006 % by weight the anisotropy falls by 0.15.
3. Titanium and Niobium are very effective in removing traces of nitrogen, sulfur and carbon and alloying them with IF steel improves the r̅ values (Fig. 11.29). The addition of Si, Mn and P decrease the r̅ values as illustrated in Fig. 11.30.
4. The trace impurities like copper decreases the average value of r̅. An impurity level of 0.2 % by weight may decrease r̅ by 0.3. Chromium has similar effect. Both these effects are highly dependant on slab reheat temperature. The above effects are for slab reheat temperature of 1200°C. If the slab reheat temperature is reduced to 1050°C the trends gets reversed and both Cu and Cr enhance the r̅ value.
5. Niobium is also used in IF steel to scavenge traces of C and N. IF steels with Ti have cold work embrittlement transition temperature of-30°C while the alloy with Ti-Nb and with only Nb have cold work embrittlement temperature of-50°C.
6. Coiling temperature of more than 700°C, in sheet rolling is beneficial for r̅ values.
7. Rapid cooling after final hot rolling pass gives small grain size and value of r may improve by as much as 0.2. The slow cooling on the other side gives grain growth resulting in poor r values.
8. Cold reduction between 80-90 %, in sheet rolling, gives highest value of r̅.
9. Several authors have carried out work on effect of coating like Zn and Tin and plastic coating on the drawability. The drawability would increase if the coating decreases the co-efficient of friction, and, reverse will happen if the friction increases.
10. The process of coating if it involves heating would affect the value of r̅. Hot dip Zn coating for instance decreases r̅ by as much as 0.2. The drop on r̅ value is proportional to the r̅ value of uncoated steel.
11. The coefficient of friction depends upon the type of coating.