Process for manufacturing thin tin/tungsten composite elements

Details Process for manufacturing thin tin/tungsten composite elements Process for manufacturing thin tin/tungsten composite elements

2001-04-19

2003-05-27

Mai, Ngoclan (Department: 1742)

Powder metallurgy processes

Forming articles by uniting randomly associated metal particles

Consolidation of powders

Reexamination Certificate

active

06569381

ABSTRACT:

The present invention essentially relates to the field of hunting and fishing and more specifically the subject of the invention is a novel process for manufacturing thin tin/tungsten composite elements, especially for the manufacture of spherical shot for hunting cartridges or for fishing sinkers.
It is known, in the prior art, to use such non-toxic shot as a replacement for lead shot.
For example, U.S. Pat. No. 5,877,437 describes tin/tungsten composite spherical shot for hunting cartridges, in the form of a malleable tin matrix containing, uniformly distributed within it, tungsten powder as filler.
According to a first process, the shot can be obtained by dispersing tungsten powder in molten tin, and then by forming drops through calibrated openings at the top of a tower. Next, these drops fall through the air or water, making it possible to obtain, by cooling, the spherical shot.
According to a second process, the shot can be obtained by moulding the dispersion of tungsten powder in molten tin, but such a process is very expensive for obtaining shot and is more suitable for obtaining more voluminous projectiles or objects.
Moreover, these melt processes are tricky to implement as it is very difficult to obtain a homogeneous distribution of tungsten powder in the tin matrix, especially because tungsten is not “wetted” by molten tin.
They also have the drawback of resulting in a reduction in the maleability of the tin matrix and the appearance of braising of the tungsten powder, which leads to an undesirable increase in the hardness of the material.
According to a third process, a tin/tungsten powder blend is compacted at high pressure in a mould at a temperature below the melting point of tin.
It is thus possible to produce projectiles weighing several grams (bullets) suitable for rifled, shoulder and hand weapons, but the process is too expensive for obtaining thin elements such as shot.
Moreover, U.S. Pat. No. 5,399,187 describes the production of composite bullets that may consist of a tin matrix filled with tungsten powder.
For matrices made of ductile metal such as tin, the bullets may be obtained by blending metal powders, then by compacting them into bars or billets which are then extruded into wires. The bullets are then obtained by forging wires using punches.
However, it proves to be the case that if it is desired, using a standard technique of drawing tin wires, to wire-draw a strand 10-20 mm in diameter made of tin/tungsten composite in order to reduce the diameter and obtain a wire having a diameter of between 1 and 6 mm, considerable tearing at the tin/tungsten interface and breakage of the wire are observed.
A person skilled in the art therefore seeks a simple and inexpensive process for manufacturing tin/tungsten composite elements of small thickness (1 to 6 mm approximately) such as spherical shot for hunting cartridges or fishing sinkers.
Such a process is the subject of the present invention.
Unexpectedly, it has been discovered that it is possible, by simply extruding a blend of tin and tungsten powders in the solid state, to obtain, directly, without any intermediate wire-drawing and/or bar- or billet-forming step, a wire whose thickness is between 1 mm and 6 mm having satisfactory mechanical integrity, without any tearing at the tin/tungsten interface, provided that the extrusion rate is less than or equal to 80 mm/s.
To obtain the desired composite elements, the wire formed may be cut into portions which are then forged to the desired shape using machines well known to those skilled in the art for providing such a function.
Such a process is particularly simple and inexpensive.
In addition, especially because there is no step of melting the tin, it is possible to retain the maleability of the tin matrix and to obtain a homogeneous distribution of the tungsten powder in the matrix.
According to the invention, the blend of tin powder and tungsten powder is extruded, in the solid state, directly into a wire whose thickness is between 1 mm and 6 mm, preferably between 2 mm and 4 mm, limits inclusive, especially when the cross section of the wire is circular, which is also preferred.
However, the wire may have any cross section, especially an elliptical, square, triangular, rectangular or polygonal cross section.
The term “directly” should be understood to mean that the transition from powder blend to wire takes place without an intermediate step, especially no intermediate step for obtaining billets or strands having a high diameter, for example greater than 8 mm, and without an intermediate wire-drawing operation.
The term “extruded” should be understood to mean, conventionally, that the powder blend is pushed through a die.
The term “solid state” should be understood to mean that the tin/tungsten powder blend is extruded at a temperature below the melting point of tin.
Preferably, the blend is extruded at a temperature of between 170° C. and 225° C., and better still between 190° C. and 220° C., limits inclusive.
According to the process forming the subject-matter of the present invention, tungsten powder and tin powder, the particle size of the powders preferably being in the 1 &mgr;m-200 &mgr;m range, and better still within the 10 &mgr;m-50 &mgr;m range, is firstly blended, in the proportions necessary for achieving the desired density, using a suitable blender.
The tin/tungsten weight ratio is preferably between 0.5 and 2.0, better still between 0.7 and 1.5. Composite elements, especially shot, having a density of between 9 and 12.5 approximately, are obtained.
Next, the powder blend may be extruded either continuously, by uniformly feeding an extruder suitable for this type of operation, or, which is preferable, in a discontinuous manner.
According to this preferred discontinuous version, the powder blend is introduced into the extrusion container (compression chamber) of an extruder which is suitable for this type of operation and which also has, conventionally, one or more calibrated exit (die) nozzles, a piston, the geometry of which is tailored to that of the container, making it possible to push the powder blend through the extrusion die, and a system for heating the extrusion container.
The gauge of the nozzle or nozzles corresponds to the desired cross section of the wire.
According to a particularly preferred version of the invention, the tin/tungsten powder blend is subjected to a partial vacuum before being extruded. To do this, a reduced pressure is created in the extrusion container containing the blend, preferably of less than 100 mmHg using suitable pumping means well known to those skilled in the art.
Thus, better quality wires having a very low porosity, and especially having no air inclusions, are obtained.
As mentioned above, the extrusion rate, measured at the die exit nozzle or nozzles, must be ≦80 mm/s.
If this is not so, a poor surface finish of the wire produced, tearing at the tin/tungsten interface and even wire breakage and/or the appearance of melted particles are observed.
Preferably the extrusion rate is between 1 mm/s and 80 mm/s, better still between 5 mm/s and 60 mm/s.
This limitation of the extrusion rate corresponds to a limitation of the energy supplied during the extrusion. Preferably, the extrusion 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, per extrusion nozzle.
According to another preferred version of the invention, the extrusion pressure is between 100 MPa and 300 MPa.
It is also preferable to operate with a ratio of the cross section of the piston to the total cross section of the nozzles, called extrusion ratio, of between 80 and 250.
The speed of the compressing piston is generally less than 0.6 mm/s, preferably between 0.05 mm/s and 0.5 mm/s.
According to another preferred version of the invention, the die does not have a convergent section (flat die) and its land (thickness corresponding to the length of the hole of the nozzle) is preferably between 5 mm and 15 mm.
In order to obtain the desired composite elem

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