Page 46 - MetalForming March 2020

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Don’t Forget About the Surface
Picture this scenario: A metal stamper goes months without any
radii and tool surface finish back to nominal conditions when stamping problems occur.
Sheet Surface
Stampers often overlook the influence of the sheet metal surface on material flow. When viewed with the naked eye, the surface often looks flat and
smooth. Higher magnification tells a different story, revealing peaks and val- leys (Fig. 1).
We can characterize sheet metal sur- faces with several parameters, with aver- age surface roughness (Ra) and peak count (PC) as the most common. To simplify, think of Ra as the height of the trees in a forest and PC as the number of trees. Changes in Ra and PC lead to changes in the interaction between the workpiece, lubricant and tool surface.
As one of the last steps in the rolling mill, coils of sheet metal pass through large work rolls having engineered sur- face profiles. In addition to incremen- tally reducing the sheet thickness, the work-roll texture presses into the sheet surface.
Roll surfaces continually wear. Rolling mills sequentially process orders requiring a restricted surface profile, in order to achieve the proper surface. They process other orders when they can schedule appropriate matches for grade, thickness and width. The rolling campaign continues until reaching the mill control limit, typically
26 μm
0.55 mm
0.55 mm
issues, but then experiences a 20-percent scrap rate on a handful of shipments. Stamp-
ing conditions have not changed, as the shop has con-
tinued to use the same recipe
for die setting, gauging and ton-
nage. The YTE properties (yield and tensile strength, and elongation) on the metal certifications remain in com- pliance with the ordered specification, leaving the stamping company no clos- er to resolving the issue.
Often, a metal stamper will rely only yon the yield strength, tensile strength and elongation listed on the metal certs— a common mistake since the sample used to generate the certified properties was obtained from the end of a mile- long, 20-ton coil. Depending on the ordered requirements, the steel mill may pull a tensile test only once for a 300- ton heat, from which 15 coils were pro- duced, all of which processed at separate times under different conditions.
I recommend that stampers send out a sample coupon, taken from the prob- lem area, for testing at an accredited
Dr. Danny Schaeffler, with 30 years of materials and applications experi- ence, is president of Engineering Quality Solu- tions (EQS) and Chief Content Officer of 4M Partners. 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 is the Metallurgy and Forming Technical Editor of the next release of the AHSS Applications Guidelines available from WorldAutoSteel.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
44 MetalForming/March 2020
www.metalformingmagazine.com
Fig. 1—Peaks and valleys on sheet metal surfaces.
lab experienced in testing sheet metal. When experiencing splits, test for more than YTEs—also look at uniform elon- gation and n-value, obtained during a standard test and determined from the stress-strain curve. R-value testing, which may incur an additional cost, can prove worthwhile if the part looks like a deep-drawn cup and you have a ref- erence for what you typically receive.
Tensile properties represent only one component of the forming system that affects stamping success. Friction characterizes the ease of material flow and quantifies the local interaction between the sheet metal, tool surface and lubricant. A different lubricant type, quantity or distribution will change material flow. When quality issues arise, and assuming that the stamper has not changed the lubricant, dilution or dis- persion method, check for plugged or misdirected nozzles. In most cases, sloshing on more lube only creates a mess and wastes time and money.
Tool surfaces wear over time. Work- ing with higher-strength sheet metal increases the risk of altering the die- surface topog-
raphy. Geomet-
rical changes of
the tooling can
occur if the tool
hardness is not
optimized for
the sheet metal
characteristics.
Bring the local
ABC
High Friction High Friction Low Friction
Fig. 2—Sheet metal and tool surface roughness and peak count should be different to minimize friction.