Lubrication is an important input to almost every sheet metal forming operation. The lubricants have the following interactions with the forming process:
1. Control metal flow from the binder.
2. Redistribute strains over the punch.
3. Maximize/minimize the growth of strain gradients (deformation localization).
4. Reduce surface damage from die wear (galling and scoring).
5. Remove heat from the deformation zone.
6. Change the influence of surface coatings.
All these effects become more important as the strength of the sheet metal increases. Therefore, special attention to lubrication is required when considering AHSS.
Higher strength steels (HSLA and AHSS) have less capacity for stretch with a lower n-value compared to mild steels. Due to their strength, deformation forces to achieve the part geometry can be quite high. Additionally, these higher strength materials are often specified in thinner gages to allow for weight reduction, which makes them more prone to buckling than thicker steels. In order to maintain flatness, higher restraining forces are required [Since restraining force is a function of the coefficient of friction (C.O.F.) times the blankholder force, the restraining force falls and metal flow increases]. This combination results in higher contact pressure between the metal and the die, and higher interface temperatures. In order to counter these conditions, specialized lubricants that have both a lower coefficient of friction, and the ability to maintain chemistry and viscosity at elevated temperatures, are required.
Higher forming energy causes both the part and the die to increase in temperature. A study by Irmco1 measuring the temperatures on stampings produced from 350 MPa and 560 MPa steels clearly show increasing part temperatures with increasing strength levels, as shown in Table 1.
The die temperatures are also significantly higher, and most conventional water-based or oil-based lubricants suffer viscosity reduction, with a corresponding increase in the coefficient of friction.
Ohio State University2 has performed many lubricant studies with AHSS steels; Figure 1 following shows the temperature profile on a DP900 stamping.
The highest temperatures are found on the die opening radius, and exceed 200C, reinforcing that part and die temperatures increase with increasing material strength. Without high-temperature additives, the lubricant effectiveness deteriorates, and heavy scoring and galling may result. As production speed increases (the number of parts per minute), the amount of heat generated increases, with a corresponding increase in sheet metal and die temperature.
One key to managing this heat problem when forming higher strength steels is application of a better lubricant. The chemistries of these better lubricants are less prone to viscosity changes and lubricant breakdown. Water-based lubricants disperse more heat than oil-based lubricants. Some parts may require tunnels drilled inside the tooling for circulating cooling liquids. These tunnels target hot spots (thermal gradients) that tend to localize deformation leading to failures.
Lubricants for AHSS Stampings
Some lubricant companies have developed stable, low C.O.F. lubricants; one example is the dry (barrier) lubricant. These polymer-based lubricants separate the sheet metal from the die. The dry lube C.O.F. for the same sheet metal and die combination can be 3 to 4 times lower compared to a good wet lubricant, and this performance acts to reduce the effective binder restraining force requirements. Since restraining force is a function of the coefficient of friction (C.O.F.) times the blankholder force, the restraining force increases and metal flow decreases. The net effect is a reduction in the amount of punch stretching required to form the part. Ultimately, forming strains and die wear are reduced. Fuchs Lubricants3 provides the guidelines shown in Table 2 for AHSS lubricant selection.
Additives to Extreme Polymer (EP) lubricants create a protective barrier or film between the sheet metal and die. At elevated temperatures, the EP breaks down and deposits a metallic salt layer, which acts as a further temperature insulator (analogous to a surface coating on the die) and allows continued functionality of the lubricant. The complete separation of sheet metal and die by the barrier lubricant also means isolation of any differences in coating characteristics. Finally, a known and constant C.O.F. over the entire stamping greatly improves the accuracy of Computer Forming-Process Development (computerized die tryout).