Page 42 - MetalForming August 2019

The Science of Forming By Daniel J. Schaeffler, Ph.D.
A Successful Stamped-Product Launch: Know What Goes Into Your Tooling
Your part-development process results in quality stampings if your production process remains in control and the sheet metal resides within ordered tolerances. Metal stam- pers should know their processes well enough to control them. Accounting for the wide range in potential metal properties when using only one or two shipments during tryout, though chal- lenging, must be addressed prior to tooling buyoff in order to produce high- quality, low-cost stamped parts in needed volumes. Strain analysis pro- vides this guidance,
ed a production supplier, or the mate- rial is purchased from a service center that buys material on the open market. The risk of using different suppliers: While they all should meet the specified property limits, each supplier might take up a different portion of the allow- able tolerance. Tuning your process to a limited range associated with one supplier makes it challenging to accommodate product from another supplier, even if that product also falls within allowed limits.
A Certification of Material Properties document should
change relative to the rolling or grain direction, so have tests performed in the longitudinal (0-deg.), transverse (90-deg.) and diagonal (45-deg.) ori- entations. Be sure to ask for more than the YTEs, or yield strength, tensile strength and total elongation. Uniform elongation, the plastic anisotropy ratio (r-value) and the strain hardening exponent (n-value) all help characterize different aspects of metal flow. The more details available, the more accu- rate the forming simulation.
Specify Important Properties When Ordering
Sheet metal orders include require- ments such as thickness and grade. The properties to be satisfied are a function of the specification against which the order is placed. SAE/AISI 1008 steel is not the same as ordering steel to ASTM A1008. Either one will provide a steel with chemistry com- plying with defined limits for carbon, manganese and other elements. How- ever, unless ordering a high-strength steel, there are no required tensile properties that must be satisfied prior to shipment from the steel mill.
If you think that it’s good enough to order based on chemistry only, think about two paperclips. Bend one of them a few times. Though the chem- istry remains identical, one has more inherent formability remaining after this cold working operation. This formability loss is seen in coils unin- tentionally produced with surface-flat- ness issues such as waves or buckles. Strength increases and ductility decreases during reprocessing to elim- inate these surface defects. The sheet mill can ship this coil to satisfy any order calling out only chemistry requirements. In the example of low- carbon steels, ordering to SAE Speci- fication J2329 ensures a sheet product
but first, you must know what initial materials shipments contain.
“The way you specify sheet thickness influences the product received.”
come with each ship- ment received. These metal certs list at least the chemistry and per- haps also the tensile properties of the differ- ent coils that make up
Ideally, the materi-
al received for tryout
will come from the
same sheet metal production mill to be used when a part undergoes full production, thus minimizing a poten- tial source of variation. This is not always possible, since either the pur- chasing department has not yet select-
the shipment. Testing requirements for some orders at the sheet metal pro- duction mill may be such that only one tensile test is needed for a 300-ton heat, even though the coils produced from this melt are each processed differently and, therefore, will have different ten- sile properties. At best, a tensile test was performed on the coil applied to your order, but even here the informa- tion shown on the cert sheet comes from a sample taken from either the head or tail end of a mile-long coil. The development of a new product takes multiple years and hundreds of thousands of dollars. Minimize the risk of being misled by the certs and spend a few hundred dollars to confirm the properties of the metal fed into your simulation and placed into your dies.
Sending a sample to an accredited testing lab provides valuable informa- tion about the sheet metal, and also generates the stress-strain curves need- ed to accurately simulate metal flow during stamping. Metal properties
Danny Schaeffler, with 30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
40 MetalForming/August 2019
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