The Science of Forming


 

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Sheetmetal Deformaton--Review of Key Points

By: Stuart Keeler

Friday, January 01, 2010
 
The last seven columns (June through December 2009) described the different stages of deformation as the material transforms from a blank to the completed stamping. This transformation may be successful or a horrendous failure in terms of stamping shape, thinning or ability to hold water. Different types of deformation are related to specific mechanical properties of the material. Understanding these relationships assist in part design, troubleshooting production problems and transitioning from one alloy to another.

While the sequence of the seven columns coincided with increase in deformation, the explanations were rather extensive. To complete the series, this month lists the key points from each column. This list can be used several ways: a refresher about each column, an index for going back to learn more about a specific topic, or knowledge checklist.

Understanding the Behavior of Sheetmetal—June 2009

Place a blank in the die and lower the punch to the blank. Then force the punch into the blank by a small amount. The blank will stretch and/or compress elastically.

• Increasing or decreasing the inter-atomic spacing causes elastic deformation.

• This elastic bond between atoms is strong, causing a rapid increase in stress for a small amount of strain. The rate of stress increase is called Young’s modulus.

• Approximate Young’s modulus: 30 million for steel, 10 million for aluminum.

• Unloading the blank releases the elastic stresses and causes the blank to spring back to its original shape.

• If a permanent geometry was created by the deformation, some elastic stresses may be trapped and unable to go back to zero. These stresses are residual stresses that can change during subsequent operations on the stamping.

More About Springs and Springback—July 2009 (Available online only)

• Two types of springback are mean (average) and variable.

• Mean springback usually can be corrected by tool design, such as preforming, over-bending, over-crowning, etc.

• Variable springback requires improvement towards constant process control.

• Three ways to minimize springback:

1) Change the bad elastic stresses into good elastic stresses.

2) Minimize the magnitude of the elastic stresses.

3) Mechanically lock in the elastic stresses.

What is Yield Strength?—July/August 2009

• At some value of elastic stress called the yield strength, a new mode of deformation—slip systems—become activated. Slip systems are discontinuities in the atomic structure that can move through the sheetmetal and cause plastic (permanent) deformation.

• Various methods can be used to raise material yield strength, such as smaller grain size, replacement of atoms with larger or smaller atoms, and inclusion of extra atoms. This thermal-mechanical strengthening creates interference to the movement of the slip systems.

• The amount of elastic stresses and the accompanying degree of springback increase as yield strength increases.

Why Work Hardening is So Important—September 2009 (Available online only)

• Most alloys become stronger as they deform. This strengthening is called work hardening.

• Without work hardening, all deformation in the blank would be concentrated in very narrow bands and fail without creating any useable shape.

• Work hardening strengthens the initial areas being deformed and causes deformation to be more uniformly distributed throughout the stamping.

• The most common measure of work hardening is the n-value obtained from the tensile test stress-strain curve equation

σ = KЄn, where σ is the true stress, K is a material constant, Є is the true strain and n is the work hardening exponent.

• Sharper geometrical features in a stamping naturally tend to localize deformation and generate localized thinning. These stamping designs require higher n-values to prevent early forming failures.

More Understanding About Work Hardening—October 2009

• The n-value is the slope of the stress-strain curve between the yield and tensile strength. A steeper slope means more work hardening and a higher n-value.

• Unfortunately, the mechanisms used to strengthen the material are the same ones that decrease the n-value.

• Application of higher-strength steels usually require part or tool designs that avoid localization of deformation.

• The n-values also control the maximum allowable amount of stretch defined by the forming limit curve discussed in the December 2009 column.

Sheetmetal Failures: Fracture or Necking —November/December 2009

• The usual mode of fracture is a ductile fracture—not a brittle fracture.

• As the yield strength or the amount of cold work increases, the fracture remains a ductile fracture.

• The actual fracture of sheetmetal is not the termination of useful deformation. Instead, a local or thickness neck usually occurs prior to the onset of fracture and terminates all useful deformation outside of the neck.

Determining Maximum Allowable Stretch —December 2009 (Available online only)

• For steel and many other metal alloys, the maximum allowable stretch is defined by the onset of the local or thickness neck in the stamping.

• For a sheet of metal (a single set of properties), the maximum allowable stretch relates to the minor stretch value at each location in the stamping. Since minor stretch depends on part and tooling design, a large range of minor stretch values can be found in any stamping. This means a large range of maximum allowable stretch values also exist.

• The forming limit curve translates this multiplicity of stretching limits into a useful press shop tool.

Numerous mechanical properties can be obtained for each alloy. Understanding the interaction of these properties and their influence on the transformation of a blank into a useful shape is important for any successful press shop—both physical and virtual. MF

 


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