**4. Friction measurement tests for forming processes**

There are several tests that were developed to measure the friction coefficient and friction factor in order to both understand how each process variable influences it and also to be loaded in finite element software for simulation of part manufacturing. The most common test still used today is the pin-to-disk test. **Figure 6** schematically shows how these tests work. This test consists of placing a specimen in the form of a ring or disc in a testing machine and forcing a pin against the specimen, making a rotary movement along the axis of the specimen. During the test, the tangential force and the normal force are measured in the contact area. The test can be done with a flat or spherical pin, depending on the condition being analyzed. The analysis is quite versatile, as friction can be determined for different sliding speeds, lubrication conditions, pressure forces and even working temperatures, depending on the complexity of the machine. Another advantage of this test is that some machines are marketed solely and are already adapted according to the most used standards. However, there are some disadvantages of this test that are related to the working pressures, which cannot be very large and in relation to the manufacturing processes, not being applied to some cases such as forging and sheet bending.

In processes where the contact pressure reaches values high enough to get close to the maximum shear stress of the work material, there is another test called the ring upsetting test, shown in **Figure 7**. This test has good applicability, reproducibility, and reduced cost. Therefore, it is often cited in the literature to determine both the friction coefficient and the friction factor for different lubrication states, both hot and cold. The sample is produced in a cylindrical ring shape with a hole in

the center (**Figure 7** in the upper left corner). The sample is pressed axially between two parallel plane dies and after a specific strain, the geometry is measured and compared to the initial geometry. Thus, depending on the relationship between the start and end diameters, there will be a different friction on the part. If there is too much friction, the inside diameter gets smaller and the outside diameter larger. In case of little friction, the internal diameter becomes larger. To determine the friction values in this test, calibration curves are needed (**Figure 7** on the right), which can be obtained in different ways, but the most common is with the help of numerical simulations. The graph in **Figure 7** on the right shows the results of the ring upsetting test with different lubrication states. It should be noted that the friction coefficients determined are not absolute values, but average values over the entire contact surface between the specimen and the tool that were also determined along the entire path traveled.

For sheet metal forming process, the most used test in the literature today is the Strip Drawing Test or Bending Under Tension (BUT) Test shown in **Figure 8**. The test consists of bending a sheet metal strip through a pin of predetermined radius and on this pin to make the sheet slide. For this, a force is applied at one of the sheet ends so that there is relative movement between the sheet and the pin. At the other end, a force is applied against the movement in order to tension the sheet and to be able to vary the contact pressure incident on the pin. The force that generates the movement is F1 and the force that is applied in the opposite direction is F2. The radius pin r has the function of simulating the friction in the passage of the stamping die radius, since it is in this region that the tensions are greater.

In this test, there are two forces required to make the sheet slide on the pin, one is the friction force between the contact surfaces and the other is the force required to bend and unbend the sheet. As the purpose of the test is to know the friction force between the contact surfaces, it is carried out in two steps. In the first one, the pin through which the sheet passes can freely rotate through its axis, so that there is no relative movement in the pin/sheet interface. This creates a condition of minimal friction, as the force required to make the sheet move is due solely to the sheet's bending and unbending force. In the second step, this same pin is fixed on its axis,

#### **Figure 7.**

*Specimen geometry in ring upsetting and a graphic calculated via FEM simulation for the evaluation of ring upsetting tests. Source: Klocke [5].*

*The Role of Friction on Metal Forming Processes DOI: http://dx.doi.org/10.5772/intechopen.101387*

**Figure 8.** *Bending under tension test. Source: Folle and Schaeffer [11].*

preventing any movement. The force required to make the sheet move is then made up of the bending force plus the friction force. Thus, the bending force, measured in the first step, can be deducted from the second, and only the friction force is obtained as a result.

The BUT test described above was conceived from the idea of knowing the friction in the passage of the die radius and is the traditional way of performing this test. However, some authors [12–15] have proposed some variations on this test in order to facilitate its construction or generate results closer to the deep drawing process. This is primarily due to the fact that to run the BUT test, it is necessary to build specific equipment for this, which is often difficult to perform. Thus, authors [13–16] have proposed some systems in which it is possible to adapt to a universal testing machine. In other works [17, 18], a measurement is proposed in which a torque is obtained on the pin through which the sheet passes. The idea of this variant of the BUT test aims to eliminate the test carried out with the free pin (which can rotate on its axis), since the torque measured on the pin is generated solely by the friction force, which is the purpose of the test. This test has the advantage of being able to represent the most critical region of sheet metal forming processes, which is in the die radius, however, it is only valid for sheet metal, not applying to bulk forming. Another disadvantage is that there is still no marketable machine that can be purchased for this purpose, all available models were built in research institutes, requiring a project for their implementation.
