RU2621324C2 - Procedure for assessment of ability of flats to stamping - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: sheet metal blank of the test metal in the form of a plate with a previously applied coordinate grid is placed on a matrix with an elliptical working cavity in the plane, is loaded before destruction by a universal elastic punch with a hemispherical shaping surface and according to the grid the limiting intensity of the logarithmic deformations accumulated by the moment of destruction is determined. The stampability of the tested metal and its suitability for the manufacture of any part are judged by the result of comparison with the calculated or experimentally established value of the accumulated intensity of the logarithmic deformation for this part.

EFFECT: increased sheet metal stampability estimation accuracy.

1 dwg

Description

The invention relates to the processing of metals by pressure and can be used to assess the stampability of sheet metal in mechanical engineering, automotive, aircraft and other industries.

A known method for evaluating the stampability of sheet metal, namely, that the preform in the form of a plate is mounted on a matrix, loaded with a metal punch with a spherical working surface until it breaks, the fracture moment is determined by cracks in the bulging part of the preform, the depth of the formed hole is recorded, and the data obtained stampability of the tested metal [1]. The disadvantage of this method is the inability to conduct tests under conditions of uniform biaxial tension due to the uneven loading conditions of the workpiece.

Closest to the proposed technical solution is a method for evaluating the stampability of sheet metal, presented in [2].

In this method, a sheet blank of the test metal, with the coordinate grid pre-applied, is mounted on a matrix having a working cavity ellipse in plan and formed by a metal punch until it is destroyed. As a result of the molding, an elliptical domed product is obtained in plan with a ratio of the lengths of the minor and major axes equal to 0.4-1.0. The ultimate strains are established on the coordinate grid, their magnitude is compared with the accumulated strains for any part, and the suitability of the tested metal for the manufacture of this part is evaluated by their ratio.

Known technical solution has the following disadvantages:

- low accuracy due to a significant effect on the test results of friction between the workpiece and the punch, as a result of which the central part of the workpiece is deformed under conditions of inhomogeneous biaxial tension;

- the great complexity and significant consumption of material associated with the need to manufacture individual punches for various ratios of the lengths of the small and large axes of the elliptical matrix.

The claimed technical solution is aimed at improving the accuracy of the method for evaluating the stampability of sheet metal, reducing its complexity and reducing material consumption.

This is achieved by the fact that in the method for evaluating the formability of sheet metal according to the invention in order to ensure uniform biaxial tension, to reduce the complexity of the test and to reduce the consumption of material, the workpiece is loaded with a universal elastic punch with a hemispherical forming surface until it breaks.

The use of a universal elastic punch with a hemispherical forming surface during testing allows you to initially localize metal deformation in the central zone of the workpiece, create deliberate thinning on it and ensure the destruction of the workpiece in its central zone. In this case, from the side of the elastic medium, a hydrostatic (uniform) pressure is created on the workpiece, the amount of friction restraining forces between the workpiece and the punch is significantly reduced, and their influence on the distribution of deformations in the central part of the workpiece is minimized. As a result, the central part of the workpiece is deformed under loading under conditions of uniform biaxial tension.

In addition, the use of a universal flexible punch with a hemispherical forming surface during molding eliminates the need to manufacture individual metal punches for different length ratios of the small and large axes of the elliptical matrix and thereby reduce the complexity of the test and reduce material consumption.

The drawing shows a diagram of the method: to the left of the axis of symmetry - the initial position of the workpiece before molding, to the right is the position of the workpiece after molding.

The method is as follows. A blank in the form of a plate is cut from a sheet of the test metal. A coordinate grid is applied to the workpiece by the photocontact method. Then the workpiece 1 is placed on the mirror of the matrix 2 with an ellipse in terms of the working cavity. A press force is applied to the container 3, in which the universal flexible punch 4 with a hemispherical forming surface is placed, located coaxially with the die, and the workpiece is loaded until it is destroyed. As a result of the molding, an elliptical domed product is obtained in plan with a ratio of the lengths of the minor and major axes equal to 0.4-1.0.

After molding, the workpiece is removed from the matrix, using the grid method, its deformed state is established and the ultimate intensity of logarithmic deformations ε _0pr accumulated at the time of fracture is calculated.

Comparing the values of ε _0pr with the calculated or experimentally established value of the cumulative intensity of logarithmic strains ε ₀ for any part, the degree of proximity of the deformed state in the problematic zone from the point of view of possible destruction to the limiting part is determined, the plasticity margin of the tested metal is determined and its suitability for manufacturing this part.

Implementation of the proposed metal-saving method will allow, in comparison with the known technical solution, to increase the accuracy and reliability of assessing the ability of the sheet metal to be molded for molding operations with an uneven biaxial tension scheme, reduce the complexity of the test and reduce the consumption of material.

An example of a specific implementation of the method

The implementation of the method was carried out on a standard testing machine TsD-40. Three flat blanks of rectangular shape with dimensions in the plan of 100 × 180 mm and a thickness of 1.5 mm, made of aluminum alloy D16AM, were subjected to molding. The workpieces were loaded to failure in an experimental draft stamp. Prior to testing, a replaceable adapter ring with an ellipse hole having axial sizes of 35 and 70 mm was installed on a round matrix mirror with a diameter of 80 mm. Molding was carried out with an elastic punch with a hemispherical forming surface made of polyurethane grade SKU-7L. Before testing, an antifriction coating based on fluoroplast-4 was applied to the forming surface of the elastic punch. To reduce the effect of friction forces, a polyethylene film 0.1 mm thick was placed between the sample and the matrix.

To establish the maximum intensity of logarithmic strains ε _0pr accumulated by the moment of fracture and to verify that the conditions for homogeneous biaxial uneven stretching in the central part of the workpiece are tested, strains were determined using a coordinate grid previously applied to the specimen by a photocontact method from a system of intersecting circles with a diameter of d = 2.6 mm . For this purpose, using the UIM-22 instrumental microscope, the largest a and smallest b cell sizes of the deformed coordinate grid were measured with an accuracy of ± 0.001 mm on both sides of the crack formed (normal to it). Measurements began with the whole cell closest to the crack adjacent to the cell dissected by the crack.

The largest ε ₁ and lowest ε ₂ main logarithmic strains were calculated taking into account the curvature of the surface of the workpiece according to the formulas

where R ₁ , R ₂ are the radii of curvature of the surface of the workpiece in the directions corresponding to the strains ε ₁ , ε ₂ .

From the calculated strain values, the parameter of the form of the deformed state α = ε ₁ / ε _{2 was} determined. In almost the entire central part of each of the tested billets, the parameter α remained constant and equal to 0.26, which indicated that this zone of the billet during its molding was deformed under conditions of a uniform deformed state. The destruction of all three tested billets occurred near their center, this also confirmed that during molding conditions of uniform deformation are realized.

To establish the limit values of the principal strains ε _1pr and ε _2pr , corresponding to the destruction of the workpiece material, we plotted the changes in strains ε ₁ , ε ₂ depending on the distance from the crack. By extrapolating the obtained dependences to the edge of the crack, the limiting values of the main logarithmic strains ε _1pr , ε _{2pr were set} .

The ultimate intensity of logarithmic deformation ε _0pr accumulated at the time of fracture was calculated by the formula

The value ε _0pr , averaged over the results of molding three blanks, turned out to be equal to 0.34.

The stampability of the tested metal and its suitability for the manufacture of any part can be judged by the result of comparing ε _0pr with the calculated or experimentally established value of the cumulative intensity of logarithmic deformations ε ₀ for this part.

Thus, the presented experimental data allow us to conclude that it is possible to implement, with a sufficient degree of accuracy, the proposed method for evaluating the stampability of sheet metal.

Using the proposed method will allow to evaluate the stampability of sheet metals used in various industries by testing in mechanical laboratories of industrial enterprises and research institutes.

Information sources

1. Avdeev B.A. Technique for determining the mechanical properties of materials. M .: Engineering. 1965.S. 391-397.

2. A.S. USSR 1618483, class B21D 22/20, G01N 3/28, 01/07/91. BI No. 1.

Claims (1)

A method for evaluating the formability of sheet metal, including applying a coordinate grid to a sheet of test metal, installing it on a matrix having an ellipse-shaped working cavity, forming the billet before breaking to obtain a domed ellipse-shaped product in plan with a ratio of the lengths of the minor and major axes equal to 0.4-1.0, and comparison of the ultimate and accumulated deformations determined by the method of coordinate grids, according to the results of which metal stampability is judged, characterized in that the workpiece is molded to p zrusheniya carried out in homogeneous conditions by deformation of the elastic universal punch with a hemispherical forming surface.

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