In recent years, automobile parts are increasingly made of aluminum alloy, high-grade and ultra-high-strength steel, which have excellent structural properties and lightweight characteristics. However, compared with commonly used low carbon steels, the formability of these materials is more restricted.
When designing sheet metal drawing processes for automobile die design, they must have accurate information about the distribution of restraining forces, which is usually provided by a series of drawing beads arranged along the periphery of the mold cavity. Pull bead has become the most effective method to provide binding force, which can pull out complex-shaped parts from the sheet without wrinkles and cracks.
The binding force produced by the pull bead is the result of bending/bending the sheet under a certain degree of stretching and friction. When stretched beads are used, the adhesive force required to achieve the necessary binding force is significantly lower.
According to the experimental data of H. Nine, the clamping force is about 80% to 90% of the binding force of the fixed beads. For flat adhesives, when the binding force is generated only by friction, the adhesive force needs to be five to seven times greater according to the coefficient of friction (COF). If there is no pull bead, a larger press size is required to provide sufficient binding force.
Researchers at the University of Auckland Center for Advanced Manufacturing and Materials (CAMM) analyzed how to adjust the binding force by changing the penetration force of the magnetic beads. The combination of experimental work and numerical simulation proves that commercial simulation software can be used to predict the adjustment of the required material flow in the stamping die. The experimentally measured binding force can help determine the weld penetration required in the mold design to achieve the assigned line weld binding force in the simulation.
The drawbead simulator used in this experimental study is constructed based on the concept of sheet metal drawing die: the sheet metal flows between the convex and concave parts of the drawbead with a certain gap, so that the sheet metal flows to the rolling Outside the region (see Figure 1). In this study, the radius of the male and female beads is 4 mm. The widths are 9 mm and 13 mm respectively. The gap is adjusted by an equalizer block, which includes a set of calibration shims that separate the flange of the tool. The hydraulic cylinder clamps the parts of the simulator together. The beading simulator is built on a rigid mold base system with a clamping force of 34.2kN.
Researchers use the following experimental steps:
1. Install the drawbead simulator in a 50 kN tensile frame, and the weight of the tool is supported by a steel table bolted to the lower plate of the tensile frame.
2. Install the tension rib insert into the tension rib simulator, and achieve the target clearance and compression rib penetration force through the calibration gasket.
3. Check the gap between the blades with a feeler gauge.
4. Lubricate a piece of 6111-T4 aluminum alloy plate with a thickness of 0.9 mm and a width of 50.8 mm, clamp the upper end with the upper clamp of the tensile testing machine, and clamp the lower end between the drawbead inserts with a hydraulic cylinder to Leave enough material to be pulled through the beads.
5. Pull the 130 mm sample through the pull bead at a speed of 200 mm/min.
Numerical simulation method
Using simulation software, the researchers developed a numerical model of sheet flow through drawbeads. The comparison of numerical and experimental results shows that the calibrated simulation model can be used to guide the experimental process of linking the geometry of drawbeads with splitting, wrinkling and springback.
The material deformation in the stretched bead is simulated in two steps:
1. Simulate strip deformation when turning off the magnetic beads.
2. Simulate the process of material flowing through the beads.
In the model, after the steel belt is formed by stretching beads in step 1, in step 2, the same stretching beads move to the left in the horizontal direction, while the plate is restricted from one side. The result of the in-plane force is used to evaluate the corresponding restraint force on the restrained side of the strip.
Experimental results and numerical simulation
The experimental study was carried out with adjustable stretch bead inserts. The advancement of the bead and the gap between the flanges of the convex and concave parts of the tool can be changed independently. The convex bead is made as a separate insert, and its position is adjusted by the first set of shims; the gap between the flanges of the drawbead insert is adjusted by the second set of shims acting together with the equalizing block.
When the penetration of different convex beads and the gap between the flanges of the convex bead and the concave bead is 0.99 mm (the sheet metal thickness gap between the plate and the insert is 10%), the result of the restraint force of the pull bead shows that the restraint force Can vary by about a quarter. This provides mold designers with sufficient flexibility to perform virtual mold trials and to achieve the required binding force distribution around the perimeter of the blank by selecting the corresponding weld penetration force. When the penetration depth of the magnetic beads is small, the material may not completely conform to the shape of the magnetic beads, and the wrapping angle of the frictional force on the binding force changes significantly with the penetration depth of the magnetic beads. Therefore, it is necessary to analyze each weld geometry.
The numerical simulation of the software aims to define the COF that relates the numerical binding force to its experimental value. The results show that 0.14 provides the best correlation for all penetration depths. The experimental and simulation results of different penetration depths of magnetic beads from 1 mm to 10 mm are shown.
The contact pressure changes at different positions on the surface of the drawbead. The positive and negative peaks indicate whether pressure is applied to the mother bead or the male bead. The pressure distribution depends on the geometry of the magnetic beads and is therefore not uniform.
Experimental research and numerical simulation show that by adjusting the penetration force of the bead, the restraining force of the drawn bead can be significantly changed. The results show that if the COF is selected based on the correlation with the experimental data, the numerical simulation can be successfully used to design the distribution of constraint forces.
A simple set of tests performed with a bead drawing tool can be used as a transition between the selection of the necessary binding force and the mold design.
These experiments and numerical studies show a simple method to define the COF in the small radius area of the stamping die with significant friction.
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