Beginning wear detection in aluminum alloy stamping, part I

        Editor’s note: This study is divided into three parts. Part II will present results for D2 tool steel inserts and two surface coatings. Part 3 will discuss the results for D6510 and S0050A nitrided and hard chromium inserts.
        Brushing is the cold welding of sheet metal particles to the surface of a die. This permanent deposit usually occurs when metal surfaces touch and slide against each other.
        Researchers at the University of Auckland’s Center for Advanced Manufacturing and Materials (CAMM) recently conducted a study to determine which combination of mold materials, mold surface treatments and lubricants is most useful for preventing wear when stamping aluminum structural parts. They used D6510 ductile iron, S0050A cast steel and D2 tool steel for mold materials, along with inserts made from the three materials tested, with lubrication levels and surface finishes typically used in stamping.
        The researchers chose a simple flat-on-flat test design to estimate the average contact pressure corresponding to the onset of wear when stamping 2.5mm thick AA5754 aluminum plates. Two flat blades of the same material, the same roughness and lubrication, with a contact surface of 42 x 42 mm, are clamped together with a controlled clamping force in a traction device simulator (see Figure 1). The test strip is 600 mm long and 50.8 mm wide. In this configuration, the strips are wider than the insert being tested. Since the insert edge radius is 1.5 mm, the influence of the contact edge between the strip and the mold insert is less noticeable.
        The test strip is pulled between two clamped inserts at a speed of 1000 mm/min. for all declared tests. The holding force started with the minimum possible holding force of 13 kN for the draft board simulator and then increased to the clamping force level at which wear was observed. If sheet material is deposited on the surface of the blade, the researchers remove the deposits with 1200-grit sandpaper and then use acetone to remove dirt and particles.
        Three lubrication conditions were compared: 61AUS milling oil (50 mg/ft²), Drycote 2-90 (DC 2-90) and dry oil without lubrication. The NG2 sensor measures the thickness of the lubricant at six locations on each side of the test area.
        Strain damage usually precedes the appearance of scratches on the surface of a metal plate, and general scratches and deformations can be detected by observing the tensile/slip curve on a testing machine. If the pulling force remains practically constant during the experiment, this means that the metal plate being tested does not provide additional resistance.
        When reporting tensile force results and detecting changes in the contact surface, researchers present the results as an average coefficient of friction (COF), which is determined based on Coulomb’s law of friction. Friction forces act on both sides of a strip stretched between two opposing flat blades, and the coefficient of friction is calculated by the formula µ = F₁/2F2 (see Figure 2).
        The researchers determined the presence of wear by examining the blades and measuring the worn area using a Bruker profilometer. In Fig. Figure 3 shows two D6510 inserts when determining the wear of DC2-90 grease. Deposits are shown in red on your profile. Note that deposits of billet aluminum on the surface of the mold insert are more common near the edges of the insert. Although the strip is wider than the insert, it may be easier to squeeze out the grease from the contact surface near the edge of the insert.
        The researchers calculated the coefficient of friction using a formula using the traction force measured by Instron load cells and the clamping force calculated from the hydraulic cylinder pressure of the towbar simulator. The COF curve of the D6510 insert using DC2-90 lubricant is shown in Figure 4. Overall, the coefficient of friction is very low when using DC2-90, so complex shapes and deeper patterns can be created. A moderate increase in COF usually indicates scratches on the insert surface, while a rapid increase indicates wear. In Figure 4, the curve begins to rise at a force of 70 kN.
       Figure 1. Two flat inserts with a contact surface of 42 x 42 mm are fastened together in a traction simulator.
        Scratches usually occur at the same time as deposits appear on the blade, but on the D6510 with DC2-90, scratches occurred at 40 kN of clamping force before wear appeared on the blade (see Figure 5). As the clamping force increases, more scratches appear until wear is observed at 70 kN.
        For the S0050A insert with a clamping force of 50 mg/ft2 61AUS, wear begins at a clamping force of 45 kN. For DC2-90, wear starts at 50 kN. Without lubrication, wear begins at a clamping force of 13 kN.
        For the 50 mg/ft2 61AUS insert D6510, wear begins at 40 kN. For the same insert using DC2-90, it began to wear at 70 kN (scratches were observed at 40 kN), and without applying lubrication – at a clamping force of 13 kN.
        In Fig. Figure 6 shows the average contact pressures corresponding to wear, taking into account the roughness of the blade surface before testing. The initial blade surface roughness before testing was 606 nm for S0050A and 165 nm for D6510. For the S0050A insert with 50 mg/ft2 61AUS, wear begins at an average contact pressure of 26 MPa. For the same blade with DC2-90, wear begins at 28 MPa, and dry wear of the blade begins at 7 MPa.
        For the D6510 insert with 61AUS 50 mg/ft2, wear begins at an average contact pressure of 23 MPa. For DK2-90, wear began at 40 MPa (although scratches were observed on the sample at 23 MPa). In the absence of lubrication, the starting pressure is 7 MPa.
        The wear threshold is highest when DC2-90 grease is applied to S0050A and D6510, and lowest when no grease is used. In terms of irritation, dry patches can be very harmful.
        The research project was funded in part by the Automotive Research Council with contributions from Novelis Inc., which provided the 5754 aluminum alloy coil; Ionbond LLC, which coated the test knives; and Quaker-Houghton, which provided the 5754 aluminum alloy spool. Provides technical advice on lubricants and their applications. Dr. Dajun Zhou from Stellantis provided very helpful comments and actively participated in the discussion of the project’s outcome.
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Post time: Oct-23-2023