Page 40 - MetalForming July 2015

1
n = 0.55 (0.46) m=0
70-30 Brass
2
n = 0.36 (0.22) m = -0.01
n=0.23
m = 0.012 AKDQ Steel
5182-0 Alum
3
n=0.16 m = 0.005
HSLA Steel
n=0.05 m=0.06
4
5
Zn-Ti Alloy
0 25 50 Engineering Strain, %
The Science of Forming
different material. The curves are shown vertically according to uniform elongation, rather than the tradi- tional yield-strength sequence to make them easier to understand. Based on previous information, evaluate the shape of each curve and try to estimate high or low val- ues of n and m.
Curve 1 (70-30 brass): The n-value region, extremely long, indicates a high n-value. The very-short m-value region indicates a zero or negative value. Measured val- ues: n-value = 0.55 initial, 0.46 average for the entire curve. This alloy has n-values in the same range as high- ly stretchable austenitic stainless steel. The m-value is zero.
Curve 2 (aluminum-killed draw-quality (AKDQ) steel: The n-value (0.23) and m-value (0.012) are positive and high.
Fig. 3—The relationship between n-value (the solid line) and m-value (the dashed) during a simple tensile test can reveal much about mate- rial formability. Graphs from A.H. Ghosh, Transaction ASME 1977.
Curve 3 (5182-O aluminum): This material has an n-value of 0.36 (0.22), approximately the same as for AKDQ steel, and a zero or negative m-value (-0.01 here).
Curve 4 (high-strength low-alloy steel: These steels are expected to have lower n- and m-values because they decline as the steel’s yield strength increases. Here, n-value is 0.16, m- value is 0.005.
Curve 5 (Zn-Ti alloy): The curve shape indicates a low n- value (0.05) and a very high positive m-value (0.06).
Like curve 5, a sample of superplastic aluminum has an
extremely high 0.5 m-value. A tensile sample with a 0.505-in.- dia. round cross-section was heated and slowly elongated in a tensile-test machine. The test ended when the sample had elongated 3000 percent and was reduced to the diameter of a human hair. After researchers completed their measure- ments, additional elongation was obtained. A material with a high positive m-value is excellent, while a zero or negative m-value can be disastrous. Many companies continue to search for ways to convert m-values from negative to positive. For example, warm forming has proven successful with cer- tain aluminum grades. MF
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Engineering Stress, psi