WO2017017341A1

WO2017017341A1 - Method for forming an ultrafine-grained flat metal object

Info

Publication number: WO2017017341A1
Application number: PCT/FR2016/051847
Authority: WO
Inventors: Yakiv BEYGELZIMER; Laszlo Toth; Jean-Jacques FUNDENBERGER
Original assignee: Université De Lorraine
Priority date: 2015-07-24
Filing date: 2016-07-19
Publication date: 2017-02-02
Also published as: EP3297776A1; FR3039084B1; EP3297776B1; FR3039084A1

Abstract

The invention relates to a method for forming a part in which a metal material undergoes severe plastic deformation. For this purpose, a bar (3) of said material is inserted into a die (1) in an insertion direction (F), the die (1) including an intake area (11) with a cross-section that matches the cross-section of the bar (3) and an output area (13), the output area (13) being arranged in a lateral output direction. The die (1) comprises an extrusion area (14) which has the same direction as the intake area (11) and a smaller size than the intake area (11), and a transition surface (120) downstream from the output area (13). The bar (3) is extruded through the extrusion area (14), the transition surface (120) creating a pressure to push a portion of the material through the outlet area (13) and to extract the transformed material in sheet form (4).

Description

METHOD FOR FORMING METALLIC FLAT OBJECT A

ULTRAFINS GRAINS

Technical area

The present invention relates to a method of forming a flat object such as a sheet with a material having an ultrafine grain structure.

In the description below, references in brackets ([]) refer to the list of references at the end of the text.

State of the art

The ability to produce objects with an ultra-fine grain metal material is well established with severe plastic deformation methods such as equal-section angular extrusion, multi-axis deformation, extrusion-torsion, high-pressure twisting or rolling. multipass. Reducing the size of the grains to sub-micron or nanometric sizes has the effect of giving the metal material a high yield strength, satisfactory ductility, good resistance to fatigue, low temperatures and wear. This has been observed with many metals or metal alloys, whether steels, alloys based on titanium, aluminum, magnesium or lithium.

During the implementation of a severe plastic deformation, microstructural changes are induced by very large plastic deformations imposed on a sample, a factor often greater than 3.

The most common method is rolling [1]. It requires many successive passes, each reducing the total thickness of a sheet. This method is slow to implement or requires expensive and cumbersome installations. The same remarks apply to a related method of cumulative roll bonding (in English) detailed in document [2].

Another technique for working on smaller samples is a constant section angular extrusion technique in which a sample is forced through a matrix having an angled channel, for example at right angles. This technique, corresponding to the preamble of the main claim, requires the implementation of very large forces and makes the design and manufacture of the matrix very delicate. References [3], [4] and [5] are documents that expose this method.

In a variant of this method shown in document [6], the sample is pressed against a rough wall and rotated to a stop wall where it undergoes deformation by being pushed radially.

A continuous shearing process is shown by publications [7] and [8] in which continuous web material is forced by a pair of rolls into a die with a side exit.

In the method of document [9], a continuous strip is repeatedly subjected to corrugated deformations and then to rectifications.

The document [10] proposes a method in which part of an original part is cut on the surface and channeled in a radial space. Such a method does not make it possible to control the static pressure on the material and cracks can be created along sliding planes.

The document [1 1] shows an extrusion process whose output is not symmetrical. Such a method does not achieve high levels of deformation.

Description of the invention

The aim of the invention is to propose a method for producing parts having an ultrafine grain microstructure, obtained by plastic deformation severe, which is quick to implement and can be implemented with moderate efforts.

With these objectives in view, the subject of the invention is a process for forming a part in which a metallic material undergoes severe plastic deformation in order to confer on it an ultra-fine grain structure, into which a bar of said material is introduced into a matrix in an insertion direction, the matrix having an entry area with a cross-section with the section of the bar and an exit zone, the exit zone being oriented in a lateral exit direction with respect to the direction method for extracting the transformed material, the method being characterized in that, the matrix having an extrusion zone having the same direction as the input zone and of smaller size than the input zone in the exit direction in a ratio between 0.75 and 1 excluded, a transition zone comprising a transition face downstream of the exit zone and making the transition between the zone of In the extrusion zone, the extrusion zone is extruded, the transition face creating a pressure to push a portion of the material through the exit zone and extracting the converted material into sheet form.

The method according to the invention makes it possible to obtain a very fine product even starting from a bar of large section. However, the efforts to provide are relatively limited since it is not necessary to deform the entire section of the bar, but only a part. Moreover, the deformations obtained are of a great magnitude, so that the sheet obtained does not need to be worked again: an ultrafine grain structure is obtained directly. The process can be applied to many metallic materials. For example, pure metals or metal alloys, whether steels, alloys based on titanium, aluminum, magnesium or lithium, are used.

According to a variant of the process, further pressure is exerted in the extrusion zone. This back pressure causes a increasing the hydrostatic pressure at the transition zone, which more surely ensures that the material flows towards the exit zone. It constitutes an additional parameter of the process on which one can act to implement the method.

Preferably, the matrix is chosen so that:

with:

Ψ section reduction ratio between the output section and the input section; the angle between the insertion direction and the transition face; t = a / h where 2a is the width of the exit zone and h is the difference in size between the entry zone and the exit zone; and m is the coefficient of friction between the material and the transition face. The process according to the invention does not occur if certain conditions are not met. In particular, it would be possible to obtain a simple extrusion without the material flowing towards the exit zone if the hydrostatic pressure is not sufficient. The conditions described above are sufficient to ensure, even in the absence of back pressure, that the material flows to the exit zone to form the sheet. This relationship shows the conditions are more conducive if the reduction ratio Ψ decreases, if the angle increases or if the ratio t decreases.

In the case of the exercise of a back pressure, the matrix and the back pressure are chosen so that:

with:

Ψ section reduction ratio between the output section and the input section; the angle between the insertion direction and the transition face; t = a / h where 2a is the width of the exit zone and h is the difference in size between the entry zone and the exit zone;

m is the coefficient of friction between the material and the transition face;

p _bp is the back pressure; and k is the elastic limit in shear of the material.

This relationship shows that the exercise of backpressure facilitates the method according to the invention by allowing a wide range of choices for other parameters such as the angle a, the ratio t or the section reduction ratio.

Brief description of the figures

The invention will be better understood and other features and advantages will appear on reading the description which follows, the description referring to the appended drawings among which:

- Figure 1 is a schematic representation of a longitudinal sectional matrix in which the method according to a first embodiment of the invention is implemented;

FIG. 2 is a view similar to FIG. 1 for a second embodiment of the invention: FIG. 3 is a view of a detail of FIG. 1, also common to FIG. 2, without implementation of the method according to the invention;

FIG. 4 is a view similar to FIG. 3 with implementation of the method according to the invention;

FIGS. 5a and 5b are diagrams representing the operating conditions of the method according to the first embodiment;

- Figures 6a and 6b are diagrams similar to those of Figures 5a and 5b according to the operating conditions of the second embodiment of the method;

FIG. 7 is a diagram showing the evolution of a von Mises deformation for the sheet;

- Figure 8 is a side photo of a bar of which the sheet is manufactured.

DETAILED DESCRIPTION The sheet forming method according to the invention is implemented in a tool comprising a die 1 and a first piston 2 for pushing a bar 3 in the die 1.

The bar 3 is made of a metal material with which it is desired to manufacture the sheet 4 after its structural transformation to obtain an ultrafine grain structure.

The matrix 1, shown in FIG. 1 in longitudinal section in an insertion direction, comprises an input zone 1 1 able to receive a bar 3 of said material in the insertion direction F, a transition zone 12, a outlet zone 13 and an extrusion zone 14. The outlet zone 13 is oriented in a lateral outlet direction with respect to the introduction direction F. The extrusion zone 14 is in the extension of the zone of 1 1 entry, but with a dimension, considered as a height, less on the side of the exit zone 13. The respective heights of the extrusion zone 14 and the entry zone 1 1 are in a ratio between 0 , 75 and 1 excluded. In this example, the section is rectangular, the different areas of the matrix 1 having an equal width in the direction perpendicular to the cutting plane.

The transition zone 12 has a transition face 120 downstream of the output zone 13 by making the transition between the input zone 11 and the extrusion zone 14. The transition face 120 is inclined at an angle has compared to the direction of introduction F.

The first piston 2 has substantially the same section as the input zone 1 1 and is able to push the bar 3 in the insertion direction F through the die 1. The bar 3 also has a section adjusted to that of the entry zone 1 1.

According to the method of the invention, the bar 3 is introduced through the inlet zone 1 1, the first piston 2 is actuated, the bar 3 is extruded through the extrusion zone 14 at the same time as a sheet 4 by a portion of the material of the bar 3 which flows through the exit zone 13.

By forcing the passage of the bar 3 with a force F, the first piston 2 induces the passage of the initial thickness H ₀ of the bar 3 at a reduced height of H ₀ -h. In the transition zone 12, a large pressure is induced so that the outer portion of the bar 3 flows into the exit zone 13 and forms the sheet 4 with a thickness 2a. In fluent, the material undergoes severe plastic deformation, which gives it the desired ultrafine grain structure properties.

In order to determine the conditions under which the process occurs, according to the theory of the majors, two cases are considered: two fields of kinematically admissible speed, one which simulates the direct extrusion through the matrix 1 (towards the zone of extrusion 14), shown in Figure 3, the other with the formation of the sheet 4, shown in Figure 4.

In the case of FIG. 3, it is established that the pressure applied by the first piston 2 P _a can be expressed by:

with Ψ the section reduction ratio between the output section and the input section; The angle between the insertion direction F and the transition face 120; m is the coefficient of friction between the material and the transition face 120; k is the elastic limit in shear of the material; w _r is a friction power of the bar 3 against the entry zone 1 1; and V ₀ is the speed of movement of the first piston 2.

To evaluate the operating pressure q between the bar 3 and the transition face 120, the equilibrium of the forces exerted on the bar 3 is written, omitting the friction forces:

P _a H ₀ = qh. (2)

Replacing / ^, we get:

q _{= 2} 1η (ΐ - ψ) _| gold tan

k Ψ Ψ

For small section reduction ratios (Ψ <0.2), it is assumed that:

1η (1 - Ψ) ¾ -Ψ

hence: i _{= 2 +} . (4) k Ψ This relation shows that the reduction of the section ratio Ψ induces an increase of the local pressure q in a hyperbolic way.

Thus, a small section reduction induces a high pressure in the transition zone 12.

In order to determine under which conditions the formation of the sheet 4 can take place, the pressure P _b applied by the first piston 2 is expressed for the case of FIG. 4:

The formation of sheet 4 takes place provided that:

P _b <P _a - (6)

By defining :

the formation of the sheet 4 takes place provided that ^ <i, otherwise it is the scenario of Figure 3 that happens.

To calculate the plastic deformation during the formation of the sheet 4, with reference to FIG. 4, a discontinuity of the velocity field is considered along an upstream line AB and a downstream line AC, in the sectional view. longitudinal. The point A is the vertex of the angle between the entry zone 11 and the exit zone 13, the point C is the vertex of the angle between the exit zone 13 and the transition face 120, and the point A is substantially the intersection between the axis of the exit zone 13 and the projection along the insertion direction F of the vertex of the angle between the transition face 120 and the extrusion zone 14. The deformation of Shearing on these lines is defined by the formula:

(8)

V 'n

where AV _T is the differential of the tangential velocity components v _n is the normal velocity component on the discontinuity line. We can deduce the expression of the total deformation during the formation of the sheet 1

(9)

Now using the von equivalent deformation formula

Mises, we get:

3

The evolution of e _u , the equivalent deformation in the von Mises direction, as a function of t is shown in FIG. 7. This figure shows that for thin sheets and sheets, von Mises deformation is characteristic of deformation processes. severe plastic.

The isolines of χ are shown in Figures 5a and 5b, respectively for an angle of 30 ° and 60 °. The domain Da, Db in which the formation of the sheet 4 takes place is represented in gray, above the line χ = \.

According to a second embodiment, shown in FIG. 2, the tooling is completed by a second piston 5 able to exert a counter-pressure on the end of the bar 3 in the extrusion zone 14.

It is thus possible to control the hydrostatic pressure during the formation of the sheet. The force exerted by the second piston is named F _bp . In the same way as before, we establish that:

(1 1)

(12) where p _bp = F _bp / s, and s is the section of the extrusion zone 14. The parameter χ 'using P _b now has the form:

The isolines of χ for P _bp = 2k are shown in Figures 6a and 6b, respectively for an angle of 30 ° and 60 °. The domain Da ', Db' in which the formation of the sheet 4 takes place is shown in gray, above the line χ '= 1.

It is clear from this that the application of backpressure extends the area Da ', Db' in which the formation of the sheet 4 takes place in comparison with the case without back pressure.

Example 1

By extruding a commercially pure aluminum bar 3 by the invented process, the following results were obtained:

Material: Al 1050

Initial hardness: 44 HV

Initial grain size: 200 μιτι

The extrusion was conducted at room temperature according to the method of the invention as described above, using a back pressure. A sheet having the following characteristics was thus obtained:

Thickness of the sheet produced: 0.7 mm

Hardness of the sheet: 108 HV

Estimated plastic deformation in the sheet: γ = 8.1

Size of grain in the sheet: 610 nm.

Example 2

The extrusion of commercially pure aluminum was carried out at room temperature according to the process of the invention as described above, with a pressure of 300 kN using a counter pressure of 100 kN. There was thus obtained a sheet thickness of 1.5 mm, as illustrated by the photo of Figure 8.

List of references

R. Song, D. Ponge, D. Raabe, J. G. Speer, D.K. Matlock, Conventional Rolling, Materials Science and Engineering A 441 (2006) 1-17, Overview of processing, microstructure and mechanical properties of ultra fine grained steels.

Saito Y. et al. Ultra-Fine Grained Bulk Steel Production by Accumulative Roll-Bonding (ARB) Process, Scripta Materialia, 40 (1999) 795-800.

US 7,191,630 B2

US 5,850,755 A.

L. S. Toth, R. Lapovok, A. Hasani, CF. Gu, Non-equal channel angular pressing of aluminum alloy, Scripta Materialia 61 (2009) 1 121-1 124.

US 7,152,448 B2, published December 26, 2006

Y. Saito, H. Utsunomiya, H. Suzuki, Sakai T., Improvement in the R-value of Aluminum Strip by a Continuous Shear Deformation Process, Scripta Materialia 42 (2000) 1 139-1 144. C.Y. Nam, J.H. Han, Y.H. Chung, M.C. Shin, Effect of Precipitates on Microstructural Evolution of 7050 Alloy Sheet During Equal Channel Angular Rolling, Materials Science and Engineering 347 (2003) 253-257.

US 6,197,129 B1.

US 7,617,750 B2.

US 2013/0055783 A1

Claims

1. Process for forming a part in which a metallic material undergoes severe plastic deformation in order to confer on it an ultra-fine grain structure, into which a bar (3) of said material is introduced into a matrix (1) in a direction of introduction (F), the die (1) having an entry zone (1 1) with a cross-section with the section of the bar (3) and an exit zone (13), the exit zone (13) being oriented in a lateral exit direction with respect to the insertion direction (F) for extracting the transformed material, the method being characterized in that the die (1) having an extrusion zone (14) having the same direction the input zone (1 1) and smaller than the input zone (1 1) in the output direction in a ratio between 0.75 and 1 excluded, a transition zone (12) comprising a transition face (120) downstream of the exit zone (13) and performing the t transition between the inlet zone (1 1) and the extrusion zone (14), the bar (3) is extruded through the extrusion zone (14), the transition face (120) creating a pressure for pushing a portion of the material through the exit zone (13) and extracting the converted material into sheet form (4).

2. Method according to claim 1, wherein there is further a back pressure in the extrusion zone (14).

3. Method according to claim 1, wherein the matrix (1) is chosen so that

X 1

with:

Ψ section reduction ratio between the output section and the input section; at the angle between the insertion direction (F) and the transition face (120); t = a / h where 2a is the width of the exit zone (13) and h is the difference in size between the entry zone (1 1) and the exit zone (13); and m is the coefficient of friction between the material and the transition face (120).

4. Method according to claim 2, wherein the matrix (1) and the back pressure are chosen so that:

sin a J k with: Ψ the section reduction ratio between the output section and the input section; at the angle between the insertion direction (F) and the transition face (120); t = a / h where 2a is the width of the exit zone (13) and h is the difference in size between the entry zone (1 1) and the exit zone (13);

m is the coefficient of friction between the material and the transition face (120);