Page 94 - MetalForming October 2014

The Science of Forming
By Stuart Keeler
Galvanized Coating Changes Steel Formability—Part 2
Last month we reviewed the draw-bead simulator (DBS) that provides coefficient of friction (COF) data when pairing different lubricant types with bare steel and dif- ferent types of galvanized steel. The tests ranged from dry (cleaned with no lubricant added) to one of the newer dry- film barrier lubricants. Results ranged from a wide variation in pull loads along a single sample during the dry test, to extreme stick-slip (pre-galling) conditions for an electro- galvanized lubricated sample. COF ranged from 0.08 to 0.35.
This month we discuss the correlation between sheetmetal surface roughness and lubricant quality.
Rough or Smooth?
During metalforming, and in particular forming across draw beads, the surfaces of the stamped part and the die form a key partnership. If the workpiece and draw-bead surfaces are rough (high micro-in. finish), the asperities of the two sur- faces will interlock to restrict motion and result in a high COF. Likewise, when both surfaces are highly polished, almost all lubricant is forced from the interface, causing stick/slip and other interactions that also lead to high COF.
The ideal combination: one rough surface that can carry lubricant into the deformation zone, and one smooth surface over which the asperities can slide. Workpiece surfaces tra- ditionally are rough and match well with smooth, highly polished die surfaces. However, during forming of highly pol- ished sheetmetal, a rough die may be required to maintain a lubricant film within the deformation zone.
A temper mill performs the finishing pass on many cold- rolled steels, sending the steel through rolls that have been ground and then shot blasted to create a micro-in. roughness on the roll surface and peak count within a specified range. The rolling process transfers that surface pattern to the steel surface. In the case of hot-dipped galvanized steel, made by running a hot coil of steel through a vat containing primarily melted zinc, the material often receives the same temper process. Therefore, hot-dipped galvanized steel and
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementa- tion of other press-shop analysis tools. Keeler’s metal- 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 to the seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
bare steel should have similar COF values. And, since COF represents a surface-to-surface interaction, the thickness of the zinc coating does not affect the COF, as proven by test data.
Conversely, electrogalvanized steel is processed differently than hot-dipped galvanized steel, and results in a much less repeatable surface morphology and COF. The process applies zinc to the steel as it passes through a bath of electrolyte and anodes. The design of the bath, anode placement and other process variables contribute to the nature of the galvanized surface.
Lubricant Testing
COF also depends on the characteristics of the lubricant being used. In one study, 17 lubricants were tested against two bare, two hot-dipped and three electrogalvanized-steel sam- ples. The ranges of COF for all lubricants (except dry-film bar- rier lubricant 1) were:
• Bare steel—0.08-0.15
• Hot-dipped galvanized steel—0.11-0.17
• Electrogalvanized steel—0.08-0.24.
The dry-film barrier lubricant had a COF range of less than
0.2
0.1
0.0
0.2
0.1
0.0
2 8 7 17 3 6 14 9 13 15 4 11 5 16 10 12 Lubricant Code
Electrozinc—Steel C
Electrozinc—Steel D
2 8 7 17 3 6 14 9 13 15 4 11 5 16 10 12 Lubricant Code
92 MetalForming/October 2014
www.metalformingmagazine.com
Bar graph shows electrogalvanized steel C immune to different lubricants, while a different electrogalvanized steel D is very sen- sitive to the lubricant characteristics.
COF COF