Page 35 - MetalForming November 2013

One primary source of data is experimentation—con- ducted within press shops and in research laboratories and universities. Some experiments produce equations that pre- dict how a steel grade (or other metal alloy) will workharden. Many lower-strength mild steels follow a power law equation, such as:
production, the dies and the stampings heat up from the deformation energy. Friction and lubrication changes due to heat can play a major role in how the stamping forms. Also, the increased temperature can change die dimensions, clear- ances and other geometrical relationships.
Unfortunately, even university research can generate incorrect data, when researchers fail to understand a process. Assume a study is conducted to measure the maximum amount of stretch deformation that could be tolerated by butt welding two sheets of different thicknesses. The test sequence (shown in the accompanying illustration) produces data showing that welding two sheets of different thicknesses severely reduces the amount of stretch deformation at fail- ure. This is a false conclusion based on the data. The offset welded pieces were compared to the original uncut and unwelded sheet. After cutting and rewelding the original sheet without any thickness offset, the greatest reduction in stretch (0.7) was observed. The weld was the major culprit, not the thickness offset. MF
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σ = k€n, where n-value is a constant.
However, for the new advanced high-strength steels, the n-value changes as the steel deforms, and instantaneous n-values are required to describe deformation behavior.
In the press shop, new dies undergo a series of tryout hits. Some press shops use as-received steel that produces little data. Other shops circle-grid the blanks to obtain addition- al information for the three-step analysis described above. Even these trials can produce incorrect data.
The usual procedure is to work a die, put a blank in the die and make a hit. The formed stamping then is examined, with or without circle grids. Even here, an experimental error is present. The die is at room temperature, or cold. In
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MetalForming/November 2013 33
The Science of Forming
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