Page 33 - MetalForming September 2010

0.2
Electrozinc—Steel 2
(A)
0.1
0
D E B A C F G H I J K L MN O P
Lubrication Code
Electrozinc—Steel 3
(B)
0.2
0.1
0
D E B A C F G H I J K L MN O P
Lubrication Code
Fig. 2—DBS data showing (A) electrogalvanized steel 2 is relatively insensitive to type of applied lubricant, while (B) shows high sensitivity of electrogalvanized steel 3.
low COF. This allows blank size and configuration or draw bead height to control material flow. A high COF lubri- cant probably would place the stamping near the edge of the deformation cliff with little opportunity to design control parameters of the blank. It also suggests that when steel 3 is used for different stampings, the same lubricant must also be used to achieve more predictable stampings. Using different lubricants with different COF would create dimen- sional variations similar to large differ- ences in mechanical properties of the incoming metal.
Additional DBS tests (Fig. 3) were conducted at different test speeds of the strip through the draw bead. Tested was steel 4 (hot-dipped galvanized alloy – galvannealed) with a steel draw bead and lubricants A and C from Fig. 2. The results show that using lubricant A at slow forming speeds would create a COF greater than 0.30. A repeat of the test resulted in the strip locking in the bead and tearing without any movement. The computed COF for this failure was 0.35.
In contrast, the low COF lubricant C showed very little increase in COF at very slow forming speeds. Ask most press shop personnel if they ever form sheet- metal at speeds of 0.15 in./min. and almost all will reply, never. Again, stop to analyze the forming process. At the begin- ning of the forming operation the blank must increase from no motion to the forming speed—passing through the 0.15 in./min. speed range (high COF). The reverse happens at the end of the form- ing process. Unfortunately, that is reverse of what the metal should be doing. One really wants a low COF to permit blank motion to start easily. The high COF should come later in the stroke to control metal flow. At the end of the forming process, with the stamping having high strain and near the edge of the deforma- tion cliff, the worst condition is a high COF locking the sheet just when it needs to flow freely to complete the stamping.
Friction can be our friend or enemy. It all depends on understanding how to choose the proper lubricant and then mak- ing it behave the way we need it to. MF
0.3
0.2
0.1
Steel 4
Lube A Lube C
0
0.15 0.86 5102050100
Test Speed (in./min.)—Log Scale
Fig. 3—Lubricant A from Fig. 2 shows a high COF at low forming speeds.
friction forces. Replacing the fixed beads with roller beads (center sketch) allows measurement of only the deformation force without the friction component. Subtracting the two forces (right sketch) provides the frictional force and calcula- tion of the COF. This test allows evalua- tion of different materials, lubricants, draw bead metals, test speeds and all combinations of these four variables.
The August 2010 Science of Forming column described how the COF of elec- trogalvanized steel could vary greatly depending on the characteristics of the production coating line. DBS tests also showed some interesting COF results for these same steels. Fig. 2 presents test results for electrogalvanized steel from
two different steel producers (codes 2 and 3). These two steels were tested with 16 different lubricants ranging from very poor (lubricant A—very thin layer of flex rolling oil) to excellent applied lubricants.
The difference is obvious. Steel 2 has a narrow range of COF values for a wide range of lubricants. Numerous lubricants applied to steel 2 have the same value of COF. This electrogalvanized steel would be characterized as stable or robust.
Steel from producer 3 reacts com- pletely different. The COF is very sensi- tive to the applied lubricant. Changing the lubricant can triple the value of the COF. This leads to several conclusions. Using steel from producer 3 means one must carefully choose a lubricant with a
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DBS Coefficient of Friction
DBS Coefficient of Friction DBS Coefficient of Friction