The Science of Forming
Sheetmetal Properties-- n, m, FLD and r--New or Old?
Metal producers and suppliers—especially those that provide steels—as well as many leading-edge metalforming companies are familiar with the properties n, m, FLD and r. Many call them new properties because they’ve learned about them within the last few years. Others consider them old properties, in use for decades.
In truth, these four properties are old—very old. The n-value, about 70 years old, is the work-hardening exponent that describes the rate of work hardening as a workpiece deforms. Jevon, in his 1940 book The Metallurgy of Deep Drawing and Pressing, detailed the importance of knowing the rate of work hardening as a function of strain. A high n-value is the major contributor to increased stretchability. Without work hardening, a majority of products today could not be made in their current form.
The strain-rate hardening exponent (m-value) represents the change in yield strength as forming speed (or crash velocity) increases. A positive m-value increases the strength of high-deformation areas and reduces strain localization in a high-stress area. A negative m-value decreases the strength for the same deformation and increases strain localization. Jevon also discussed the concept of strain-rate hardening; a positive m-value proves beneficial for increased stretchability.
The FLD (forming limit diagram) is a more complex material property. Introduced as a press-shop analysis tool 40 years ago, the FLD defines the maximum allowable stretch for different combinations of deformation. Strains at different locations in the stamping can be measured by circle grids and then plotted on the FLD to determine deformation severity and the safety margin relative to failure. Tracking strains on the FLD allows troubleshooters to immediately assess the value of any changes made to the forming-system inputs—die components, lubricant, etc.
The r-value (anisotropy ratio), first published in 1959, is important in cup drawing. The mean-r measures the resistance to deformation in the thickness direction. Greater thinning resistance translates into deeper single-draw cups. Delta-r correlates to the amount and direction of earing in a deep-drawn cup. For steel, r-value is important only for cold-rolled, low-strength alloys.
Why Such Resistance?
These four properties have been discussed many times since this column started in the January 2000 issue of MetalForming. Why has their acceptance taken so long, or worse, been ignored by many metalformers? One barrier has been the source of the primary research. Over the past half century or more, most metalforming research has been conducted by universities or steel mills. The basic concepts are discovered, but two factors are missing.
First, the findings are published in very technical journals, such as Metallurgical Transactions, that are read primarily only by other researchers. Second, it may take a decade or more before the research results can be translated into recommendations that can be implemented in the press shop.
Steel companies use these four properties to evaluate the production quality of their alloys. Until the early 1990s, these companies would guarantee the ability to make a part, and would select for the metalformers the proper type of steel and its properties to accomplish the task. Many sheetmetal buyers still function this way.
In contrast, today’s leading-edge press shops study their stampings, determine for themselves the required steel type and properties, and order steel based on that data. This switch has transferred the knowledge base from researcher to end user. In conjunction with some steel suppliers, mill process codes are being created for specific, difficult stampings to ensure the same steel type and properties are made and shipped for repeat orders.
Know Your Steel
During die tryout, knowing the properties of the tryout coil should be mandatory. Does the coil have mean values or are they very high or low? Why spend so much time and money finishing a die to material properties you may never see again?
Typical test results provide the yield strength, tensile strength and total elongation. While useful information, the required properties directly related to formability of steel are n, FLD and r. What about m-value? Research shows that for steel, m-value is positive and proportional to n-value. Therefore, n-value and m-value act together for increasing stretchability as n-value increases. For some aluminum alloys and other nonferrous alloys, m-value can be negative and negate some of the benefit of the n-value.
With the properties of the tryout coil known, apparent problems with a production coil can be checked. Some larger press shops have a tensile-test-specimen cutter and a simple tensile-test machine. They send samples from the coil to the lab for evaluation, and quickly learn whether or not they must pull the coil because the steel properties are out of range, or perhaps find that the properties are in-range but the production equipment has shifted out of process range.The virtual press shop (computerized die tryout) validates the importance of n, m, FLD and r. The virtual stamping requires the exact values of n, m and r found in the production coil before it can predict the strain magnitudes and distributions found in the physical stamping. The FLD is used for all virtual stampings to predict the onset of failure found in the physical stamping. Using typical or mean values for the virtual stamping does not correlate with the physical stamping. MF
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