Page 41 - MetalForming June 2014

The Science of Forming
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–m +m
Strain
Fig. 3—Onset of diffuse necking ends tensile elonga- tion with a negative m-value, but positive m-values resist necking for extended elongation.
the cross-sectional area of the tensile specimen relative to the non-necking areas. A neg- ative m-value accelerates shrinkage at that location and fracture or total elongation rapidly occurs. A positive m- value strengthens the materi- al at that location and allows elongation of the tensile sam- ple to continue. Comparing the two results, the negative m-value material can have a total elongation of 23 percent compared to 45 percent for the positive m-value material.
of edge cracks created during hole punching; and
• Any other areas undergoing posi- tive elongation.
Aluminum’s Positive m-Value
Researchers are studying ways to change the m-value of some aluminum alloys from negative to positive, through warm forming, increasing total elongation by a factor of five. The ulti- mate demonstration of what a posi- tive m-value can do occurred with superplastic aluminum, in tests con- ducted in the late 1950s at the Massa- chusetts Institute of Technology.
Researchers placed a round tensile- test sample (0.5 m-value) with a stan- dard diameter of 0.505 in. in a tensile machine and elongated it at a very slow speed. They stopped the test when the specimen had reached an elongation of 3000 percent. This reduced the speci- men to the diameter of a human hair, and yet it remained capable of further elongation. MF
to control the gradient and the gradient grows (solid line). Both conditions are affected identically by press speed.
Gradients are not the only forming problem affected by negative m-values. The stress-strain curves from a tensile test (Fig. 3) illustrate differences in total deformation. At the point of maximum load (ultimate tensile strength), a diffuse (width) neck begins to form. This reduces
Other metalforming applications affected adversely by negative m-values include:
• Bending, where outer fibers act like a tensile sample;
• Edge stretching that turns small edge cracks into major gradient cracks; • Stretch flanging that creates a pos-
itive stretch;
• Hole expansion, limited by growth
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MetalForming/June 2014 39
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Stress