Detecting galling onset in aluminum alloy stamping, Part III

Editor’s Note: 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 automotive structural parts. This research is presented in three parts. Part I reported results for D6510 and S0050A die materials with no surface treatments. Part II presented a study of bare and DLC-coated D2 tool steel.

The researchers set out to determine galling onset of nitrided and hard-chromed D6510 and S0050A inserts in contact with 2.5-mm-thick aluminum sheet 5754 and to quantify the influence of lubrication, contact force, and initial roughness of the die surface. The design of the tool is described in detail in “Measuring friction for stamping of UHSS.” The experimental methodology was the same as in Part I.

They compared three lubrication conditions: Ferrocote 61A-US, Drycote 2-90 (DC2-90), and no lubricant applied.

Results

Galling onset was determined by examining the inserts and taking profile measurements of the galling area using a Bruker profilometer. For the D6510 hard-chromed inserts, Figure 1 shows galling with DC2-90 lubricant, and Figure 2 shows galling with no lubricant applied. In both figures, the profile of the deposit is shown on the right side; the red outline on the profile is the area of deposit. Dry conditions resulted in severe galling on the inserts.

Scratching was another sign of wear on the D6510 hard-chromed inserts. Figure 3 shows scratching with no lubricant applied. Figure 4 shows tests with DC2-90 lubricant and the resulting rapid rise in the coefficient of friction (COF) curve for 100-kN clamping force, which corresponds to 57 MPa of average contact pressure. Some scratching of the sheet was observed at lower clamping forces; at 60 kN, the COF curve shows less significant growth, even though undesirable effects are still seen.

The D6510 nitrided insert with DC2-90 lubricant showed great performance in terms of the galling threshold, with deposits appearing only at 200-kN holding force, which corresponded to 113 MPa. The surface of this insert is shown in Figure 5 along with a profile of deposit, with the area of deposit outlined in red. Figure 6 shows that the COF curves were stable, with a minor rise at 200 kN, which corresponds to galling initiation at 113 MPa of average contact pressure.

The average contact pressures at which galling initiated in this series of experiments are shown in Figure 7. The inserts’ pre-test average roughness is indicated by dots corresponding to each group of inserts. Initial pre-test roughness was 348 nm for D6510 nitrided, 179 nm for S0050A nitrided, 149 nm for D6510 chromed, and 142 nm for S0050A chromed.

For the D6510 nitrided inserts, galling began at 28 MPa of average contact pressure with 50 mg/ft.² of 61-AUS, at 113 MPa with DC2-90, and at 7 MPa with no lubricant applied.

For the S0050A nitrided inserts, galling started at 23 MPa of average contact pressure with 50 mg/ft.² of 61-AUS, at 34 MPa with DC2-90, and at 7 MPa with no lubricant applied.

For the D6510 hard-chromed inserts, galling began at 17 MPa of average contact pressure with 50 mg/ft.² of 61-AUS, at 57 MPa with DC2-90, and at 7 MPa with no lubricant applied.

FIGURE 1. The area of deposit on a D6510 hard-chromed insert tested with DC2-90 lubricant is outlined in red and profiled at right.

For the S0050A hard-chromed inserts, galling started at 11 MPa of average contact pressure with 50 mg/ft.² of 61-AUS, at 11 MPa with DC2-90, and at 7 MPa with no lubricant applied.

As shown, the highest galling threshold was achieved with nitrided inserts with DC2-90 lubricant. These inserts had the highest roughness before the test, which might have helped to trap lubricant on the surface and increase the galling threshold.

Conclusions

• Galling thresholds were highest with DC2-90 lubricant and lowest with no lubricant. Dry spots are certainly very harmful from a galling perspective.
• For both D6510 and S0050A, the highest galling threshold and lowest COF were achieved with nitrided inserts with DC2-90 lubricant.

This research project was funded in part by the United States Council for Automotive Research with contributions from Novelis Corp., which provided 5754 aluminum alloy coil; Teikuro Corp., which performed hard-chrome coatings of tested inserts; Sun Steel Treating, which performed nitriding of tool inserts; and Quaker-Houghton, which provided lubricants and technical recommendations for lubricant application.

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