Galling is cold welding of sheet metal particles to the surface of the stamping die. This permanent deposit often occurs when metal surfaces are in contact and sliding against each other.
Researchers at the Oakland University Center of Advanced Manufacturing and Materials (CAMM) recently conducted a study to determine which combination of die material, die surface treatment, and lubricant was most favorable for preventing galling when stamping aluminum structural parts. They used as die materials D6510 ductile iron, S0050A cast steel, and D2 tool steel, along with inserts of three tested materials, with lubricant amounts and surface treatments typically used in stamping.
The researchers selected a simple flat-to-flat test configuration for estimating the average contact pressure corresponding to the galling onset when stamping 2.5-mm-thick AA5754 aluminum sheet. Two flat inserts of identical material and similar roughness and lubrication amount with 42- by 42-mm contact surfaces were clamped together in the draw bead simulator with a controllable amount of clamping force (see Figure 1). The tested strips were 600 mm long and 50.8 mm wide. In this configuration, the strips were wider than the tested inserts. Because the insert had a 1.5-mm radius on the edge, the effect of the edge of the contact between the strip and the die insert was less pronounced.
Tested strips were pulled between two clamped inserts at 1,000 mm/min. for all reported tests. Holding force began at 13 kN, the minimum possible holding force of the draw bead simulator, and increased to the clamping force level at which galling was observed. If sheet material deposited on the insert surface, researchers removed the deposit with 1,200-grit sandpaper, then used acetone to remove dirt and particles.
Three lubrication conditions were compared: 61AUS mill oil (50 mg/ft.²), Drycote 2-90 (DC 2-90), and dry with no lubricant. An NG2 sensor measured lubrication thickness in six locations on each side of the strip testing area.
Galling often is preceded by scratches on the sheet metal surface, and overall scratching and galling can be detected by monitoring the pulling force/sliding displacement curve on the testing machine. If the pulling force remains almost constant during the experiment, it means that no additional resistance from the tested sheet metal occurred.
While reporting the results on the pulling force and detecting changes on the contact surface, the researchers presented the results in the form of average coefficient of friction (COF), determined based on Coulomb friction law. Friction forces act on both sides of a strip drawn between two opposing flat inserts, and COF is calculated using the formula µ = F₁/2F₂ (see Figure 2).
The researchers determined galling onset by examining the inserts and measuring the galling area using a Bruker profilometer. Figure 3 shows both D6510 inserts when galling was determined for DC2-90 lubricant. Deposits on the profile are shown in red. Note that the deposits of aluminum blank on the die insert surface were more common closer to the edge of the insert. Even though the strip was wider than the insert, squeezing lubricant from the contact surface might be easier in the area close to the edge of the insert.
The researchers calculated COF according to the formula using the pulling force measured by an Instron load cell and clamping force calculated from the pressure in the draw bead simulator hydraulic cylinder. The COF curves for the D6510 inserts with DC2-90 lubricant are presented in Figure 4. Overall, the COF was very low with DC2-90, which allows forming of complicated shapes and deeper draws. A moderate increase in COF usually indicates scratches on the sheet surface, while a quick rise indicates galling initiation. In Figure 4, the curve begins to rise at 70 kN of force.