Page 26 - MetalForming May 2010

Tooling Technology
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementation of other press shop analysis tools. Stuart’s sheetmetal forming experience includes 24 years at National Steel Corporation and
12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA. His most recent project is technical editor of the AHSS Application Guidelines—Version 4.1, which now is available for downloading free from www.worldautosteel.org. Keeler Technologies LLC
P.O. Box 283
Grosse Ile, MI 48138
Fax: 734/671-2271
E-mail: keeltech@comcast.net
Virtual forming (computerized die tryout, computerized metalform- ing, etc.) is revolutionizing the sheetmetal industry. The advantages these programs offer are numerous, including time and cost reductions for die design/tryout and improved stamp- ing quality.
Many years ago, a popular slogan was applied to manufacturing prod- ucts, cuisine and personal lives: GIGO or Garbage In Garbage Out. The impli- cations were well understood and most people reacted accordingly. Then com- puters became popular and the mean- ing of GIGO changed to Garbage In Gospel Out. Since computers cannot lie, the output was taken to be the truth. Unfortunately, virtual forming has become a major victim of GIGO. This month’s column explores some GIGO problems associated with material- property inputs.
Drop-down menus are a common method for telling the computer which metal and grade should be used for the analysis. Click on AKDQ (aluminum- killed draw-quality) or the more current FS (forming steel) or DS (drawing steel) and a set of typical properties are sent to the program. In contrast, many dies actually create stampings from alloys with a formability property range from -3σ to +3σ of the typical value. Forcing the computer to use only the typical property rarely provides a meaningful analysis.
For example, one key material parameter needed for virtual forming is the work-hardening exponent or n- value. For AKDQ steel, this n-value can range from 0.18 to 0.23, yet a change of 0.01 can shift a stamping between make and break. For an accurate forming analysis, the n-value of the intended
production material must be entered— not a typical value. An important advantage of virtual forming for die tryout is running the analysis with four different n-values. These values are the -3σ, typical, +3σ, and production to gain an insight of how n-value influ- ences the final stamping. Changing the n-value requires only a simple keyboard entry. With hard tooling, one must find or order four coils of material with the desired properties and run statistical evaluations of all four coils.
Selecting one high-strength low-alloy (HSLA) steel from the drop-down menu also enters a single n-value. This assumes that the n-value is relatively constant over the full range of defor- mation. The n-value is calculated between the strain values of 10 percent and uniform elongation/maximum load. (This steel is shown as the dashed line in the graph with an approximate n-value of 0.15 within the indicated measurement strain range.)
In contrast, two relatively new advanced high strength steels (AHSS) have n-values that change with strain. One now must compute instantaneous values of n that show the n-value for dif- ferent amounts of strain. The first AHSS is dual phase (DP) steel (solid line in the graph). Note the early increase in n- value to a value of 0.2. This higher n- value at low strains is effective in resist- ing the formation of localized strain gradients. However, this increased n- value is exhausted before reaching 10 percent strain. This means traditional tensile samples of the HSLA and DP steels will have approximately the same n-values and the computer will not rec- ognize the stretchability improvement of the DP steel. The results of a com- puterized die tryout will suffer a GIGO
THE SCIENCE OF FORMING
Material Properties for Virtual Metalforming
Stuart Keeler’s next seminar is “Higher Strength Steels—Solving the Problems” scheduled for June 23 in Grand Rapids, Mi. Check www.metalforming.com for this and other seminars.
24 METALFORMING / MAY 2010
www.metalformingmagazine.com
STUART KEELER