EP1154026B1 - Process for preparing composite tin-tungsten shot of small dimension - Google Patents

Abstract

A method for the fabrication of composite tin-tungsten elements having a thickness between 1 and 6 mm from a mixture of tin and tungsten powders consists of directly extruding the mixture of powders in the solid state to form a wire with a thickness of 1 to 6 mm, with a drawing speed of less than or equal to 80 mm/s. The wire produced is then cut into sections that are then stamped to obtain the desired shape. An Independent claim is included for the production of a composite tin-tungsten wire from a mixture of tin and tungsten powders.

Description

The present invention is essentially in the field of hunting and fishing and more specifically relates to a new process for manufacturing thin-tungsten composite elements of small thickness, including the manufacture of spherical shot for hunting cartridges or for fishing sinkers.

It is known in the state of the art to use such a non-toxic shot to replace the lead shot.

US Pat. No. 5,877,437, for example, discloses spherical tin-tungsten composite shot for shot cartridges in the form of a malleable tin matrix containing tungsten powder as a filler in a uniformly distributed manner.

No. 5,950,064 discloses a process for obtaining lead-free composite shot by bonding the components in a liquid phase.

According to a first method, the shot may be obtained by dispersing the tungsten powder in the molten tin and then forming drops through calibrated openings at the top of a tower. The drops are then caused to fall into the air or water, which makes it possible to obtain, by cooling, the spherical shot.

According to a second method, the shot may be obtained by molding the dispersion of the tungsten powder in molten tin, but such a method is very expensive for obtaining shot and is more suitable for obtaining projectiles or larger objects.

Furthermore, these fusion processes are difficult to implement because it is very difficult to obtain a homogeneous distribution of powdered tungsten in the tin matrix, especially since the tungsten is not "wet" by the tin melted.

They also have the disadvantage of causing a decrease in the malleability of the tin matrix and the appearance of solder of the tungsten powder, which leads to an undesirable increase in the hardness of the material.

According to a third method, a mixture of tin and tungsten powders is compacted at high pressure in a mold at a temperature below the melting point of the tin.

It is thus possible to achieve projectiles of a few grams (bullets) suitable for striped weapons shoulder or hand, but the process is too expensive to obtain thin elements such as shot.

US Pat. No. 5,399,187 furthermore describes obtaining composite bales that may consist of a tin matrix loaded with tungsten powder.

For ductile metal matrices such as tin, the balls can be obtained by mixing the metal powders, followed by compaction into bars or billets which are then extruded into yarns. The balls are then obtained by forging the wires using punches.

It turns out, however, that if one seeks, according to a standard technique of stretching tin son, to draw a strand of diameter 10-20 mm tin-tungsten composite to reduce the diameter and obtain a wire diameter between 1 and 6 mm, there are many tears at the tin / tungsten interface and the breakage of the wire.

The skilled person is therefore looking for a simple and inexpensive method of manufacturing thin-tungsten composite elements (approximately 1 to 6 mm) such as spherical shot for shot cartridges or shotguns fishing.

The present invention relates to such a method.

Unexpectedly, it has been discovered that by simple extrusion of a solid state mixture of tin and tungsten powders can be obtained directly, without an intermediate drawing and / or drawing step. obtaining bars or billets, a wire whose thickness is between 1 mm and 6 mm having a satisfactory mechanical strength, without tearing at the tin / tungsten interface, provided that the spinning speed is less than or equal to at 80 mm / s.

To obtain the desired composite elements, the wire obtained can be cut into sections which are then stamped to the desired shape, using machines well known to those skilled in the art to perform such a function.

Such a method is particularly simple and inexpensive.

It also makes it possible, especially because of the absence of a tin melting step, to preserve the malleability of the tin matrix and to obtain a homogeneous distribution of the tungsten powder in the matrix.

According to the invention, the mixture of the tin powder and the tungsten powder is directly extruded, in the solid state, into a wire whose thickness is between 1 mm and 6 mm, preferably between 2 mm and 4 mm, limits included, especially when the section of the wire is circular, which is also preferred.

The section of the wire may however be arbitrary, in particular elliptical, square, triangular, rectangular or polygonal.

By "directly" it should be understood that the passage of the mixture of powders to the wire is done without intermediate step, in particular without intermediate obtaining of billets or strands of high diameter, higher by example to 8 mm, and without intermediate drawing operation.

By "extruded", it is conventionally understood that the mixture of powders is pushed through a die. The term "spinning" is synonymous with "extrusion".

By "solid state" it should be understood that the mixture of tin and tungsten powders is extruded at a temperature below the melting temperature of the tin.

Preferably, the mixture is extruded at a temperature between 170 ° C and 225 ° C, more preferably between 190 ° C and 220 ° C inclusive.

According to the process which is the subject of the present invention, the mixture required is first mixed, in the proportions necessary to achieve the desired density, with the aid of a suitable mixer, tungsten powder and tin powder. particle size of the powders being preferably in the range 1 μm-200 μm, more preferably in the range 10 μm-50 μm.

The weight ratio tin / tungsten is preferably between 0.5 and 2.0, more preferably between 0.7 and 1.5. This produces composite elements, especially shot, having a density of approximately 9 to 12.5.

The extrusion of the powder mixture can then be conducted either continuously by regularly feeding an extruder suitable for this type of operation, or, what is preferred, discontinuously.

According to this preferred discontinuous variant, the mixture of the powders is introduced into the extrusion container (press pot) of an extruder adapted to this type of operation and also comprising, in the usual way, one or more calibrated outlet nozzles (die ), a piston whose geometry is adapted to that of the container for exerting a thrust of the powder mixture through the spinning die, and a heating system of the extrusion container.

The size of the nozzle or nozzles corresponds to the desired section for the wire.

According to a particularly preferred variant of the invention, the mixture of tin powders and tungsten is subjected to a partial vacuum before being extruded. For this, a reduced pressure is imposed in the extrusion container containing the mixture, preferably less than 100 mm Hg, using appropriate pumping means and well known to those skilled in the art.

This results in son of better quality, very little porous, in particular with no inclusion of air.

As mentioned previously, the spinning speed, measured at the nozzle, or nozzles, at the exit of the die, must be ≤ 80 mm / s.

Otherwise, there is a poor surface condition of the wire produced, tears at the tin / tungsten interface, and even the breakage of the wire and / or the appearance of melted particles.

Preferably, the spinning speed is between 1 mm / s and 80 mm / s, better still between 5 mm / s and 60 mm / s.

This limitation of the spinning speed corresponds to a limitation of the supply of energy during the spinning. Preferably, the spinning power developed by the piston of the press is less than 150 W, for example between 10 W and 100 W, better still between 10 W and 70 W, by extrusion nozzle.

According to another preferred variant of the invention, the spinning pressure is between 100 MPa and 300 MPa.

It is also preferred to operate with a ratio between the section of the piston and the total section of the nozzles, called spinning ratio, between 80 and 250.

The speed of the press piston is generally less than 0.6 mm / s, preferably between 0.05 mm / s and 0.5 mm / s.

According to another preferred variant of the invention, the die has no convergent (flat die) and its range (thickness corresponding to the length of the hole of the nozzle) is preferably between 5 mm and 15 mm.

To obtain the desired composite elements, in particular spherical shot, the wire obtained is cut into sections, more particularly into rolls preferably having a length equal to the diameter of the wire, or close to it, and then these elements are then stamped, preferably at room temperature, to obtain the desired form.

Preferably, and in particular according to the preferred variant for which the wire sections are cylinders whose length is close to or equal to the diameter of the wire, this stamping makes it possible to obtain spheres, or pseudospheres which are then ground to the state of spheres whose diameter is very close to or identical to that of the wire.

Such spherical elements are particularly suitable for producing hunting cartridges or lead shots for fishing in fixed, fly or whip, including split pellets.

According to another preferred embodiment, it is also possible to obtain, by stamping, from cylindrical sections for example, elements having an ovoid shape and a thickness close to the thickness or diameter of the wire.

Such ovoid elements are particularly suitable for use as ballasting olives for trolling or casting, including split olives.

All these cutting and stamping operations can be carried out continuously, using machines well known to those skilled in the art and in particular used for the production of balls for ball bearings.

The thickness of the elements obtained is generally identical or very close to that of the wire.

Depending on the thickness of the wire produced, it is possible to obtain composite elements having a thickness that can vary from about 1 mm to about 6 mm.

The subject of the present invention is also the abovementioned method for obtaining the yarn, namely a process for obtaining a tin-tungsten composite yarn from a mixture of tin and tungsten powders, characterized in that the powder mixture is directly extruded, in the solid state, into a wire whose thickness is between 1 mm and 6 mm and in that the spinning speed is less than or equal to 80 mm / s.

The above-mentioned thin-tungsten composite elements may be used for other purposes than the production of hunting cartridges or fishing pellets, in particular and, for example, to produce balancing masses or objects capable of shielding radiation. ionizing.

Such screens and masses can also be obtained from the wire itself, without it being cut beforehand.

The following nonlimiting examples illustrate the invention and the advantages it provides.

Examples 1 to 6 : manufacture of tin-tungsten spherical composite shot according to the invention, of various densities and granulometries

The tungsten powder and tin powder used have a median diameter of about 30 μm .

The tin and tungsten powders are mixed in the proportions mentioned later for each example using a suitable Forberg type mixer, at room temperature (approximately 20 ° C.), for approximately 10 minutes, so that obtain a homogeneous mixture.

The homogeneous mixture of the powders is then poured into the container of an extrusion press. The container, cylindrical in shape, has an internal piston of diameter equal to the inner diameter of the container, with the tolerance of adjustment, so as to ensure the thrust of the mixture to the die.

For these examples, 2 different piston-container diameters (30 mm and 40 mm) were used. This diameter is specified later for each example.

The displacement stroke of the piston (useful length of the container) is 70 mm.

The speed of movement of the spinning piston, driven by a suitable device well known to those skilled in the art, is specified for each example.

The container is provided on the one hand with a device (heating envelope) for heating the mixture of powders that it contains, and on the other hand with a vacuum connection associated with a sinter which makes it possible to establish a reduced pressure in the container filled with the mixture of powders before extrusion.

The extrusion press also comprises a cylindrical flat die (without converging) having a span (thickness) of 10 mm and provided with a single cylindrical hole located in central position with a diameter of 2.6 mm or 3.2 mm is specified for each example.

The drilling of the die is initially obstructed by an aluminum seal so as to achieve a partial vacuum in the container.

The container is then heated and evacuated to a reduced pressure of less than 80 mm Hg.

After heating to a temperature mentioned later for each test, and maintaining the partial vacuum, for about 45 minutes, the powder mixture is extruded. The seal closing the drilling of the die bursts due to the pressure exerted.

The following table specifies and summarizes the operating conditions specific to each example.

For all these examples, a yarn having the following physical and mechanical properties is obtained:

Density measured by weighing and measurement of dimensions

Example 1 9.43 Example 2 9.69 Example 3 9.84 Examples 4 to 6: not measured.

The densities measured are close to the theoretical density. The densification is ≥ 95%.

Metallographic analyzes

They show, for all the examples, an orientation of the elements in the direction of the length of the wire. There is a good homogeneity of distribution of tungsten and tin, and no or very few inclusions of gas.

The surface state of the wire, for all the examples, is very satisfactory. However, it is less good for example 6, while remaining satisfactory.

Ductility of the thread

It is encrypted by the bending angle of the wire (angle swept by the two parts of the wire at the moment of breakage) Example 1 80 ° Example 2 60 ° Example 3 60 ° Examples 4 to 6: not measured.

This ductility is satisfactory.

Vickers hardness

It is practiced either on a section of the wire (hardness through), or on a section of the wire in the direction of the length (hardness long direction) Example 1 13.52 (long) and 13.03 (across) Example 2 15.07 (long) and 14.27 (across) Example 3 14.87 (long) and 15.23 (through) Examples 4 to 6: not measured.

This hardness is satisfactory.

The yarns obtained for these 6 examples were then cut into cylinders whose length is equal to the diameter of the yarn using a machine well known to those skilled in the art to perform this operation.

The rolls were then stamped, at room temperature (about 20 ° C.), into spheres having a diameter approximately identical to that of the rolls and the wire, using a machine that is also well known to those skilled in the art. to carry out this operation, particularly in the field of ball bearings.

Thus, for Examples 1, 4, 5 and 6 of the spherical shot of diameter about 3.2 mm, and for Examples 2 and 3 of the spherical shot diameter of about 2.6 mm.

Comparative Examples A and B

These comparative examples do not form part of the invention. They were made for the sole purpose of showing that selecting a certain speed domain The spinning method according to the invention is not arbitrary, but necessary to provide the desired technical effect, namely to obtain a yarn having a satisfactory surface condition.

Examples 4 to 6 according to the invention have been reproduced, all things being identical, by modifying the speed of the spinning piston and therefore the expected spinning speed.

For Comparative Example A, a spinning piston speed of 1 mm / sec was imposed. The measured spinning pressure is 144 MPa.

The spinning power is 181 W.

The expected speed of spinning is 156 mm / s, but it has been found that the wire is coming out in pieces, which is undesirable.

For comparative example B, a speed of the spinning piston of 10 mm / s was imposed. The measured spinning pressure is 255 MPa. The spinning power is 3200 W.

The expected speed of spinning is 1560 mm / s, but only melted particles appear.

Claims (10)

1. Method for producing tin-tungsten composite elements having a thickness of between 1 mm and 6 mm from a mixture of tin and tungsten powders, characterized in that :

   - the mixture of powders is directly extruded in the solid state in a wire of which the thickness lies between 1 mm and 6 mm,

   - the extrusion rate is less than or equal to 80 mm/s,

   - the wire obtained is cut up into lengths that are then forged so as to obtain the desired shape.
2. Method according to claim 1, characterized in that the wire has a circular cross section and in that its diameter lies between 2 mm and 4 mm.
3. Method according to claim 1, characterized in that the mixture of tin and tungsten powders is extruded at a temperature of between 170°C and 225°C.
4. Method according to claim 1, characterized in that the mixture of tin and tungsten powders is subjected to a partial vacuum before being extruded.
5. Method according to claim 1, characterized in that the extrusion pressure lies between 100 MPa and 300 MPa.
6. Method according to claim 1, characterized in that the extrusion ratio lies between 80 and 250.
7. Method according to claim 1, characterized in that the extrusion power lies between 10 W and 100 W.
8. Method according to claim 1, characterized in that the composite elements have a density of between 9 and 12.5.
9. Method according to either of claims 1 or 2, characterized in that the composite elements constitute spherical shot for sporting cartridges or for sinkers for fishing.
10. Method according to claim 1 for obtaining a tin-tungsten composite wire from a mixture of tin and tungsten powders, characterized in that :

    - the mixture of powders is directly extruded in the solid state in a wire of which the thickness lies between 1 mm and 6 mm,

    - the extrusion rate is less than or equal to 80 mm/s.