EP0429367B1 - Method for making articles having a cavity by compression - Google Patents

Description

The invention relates to a method of manufacturing parts having a cavity by pressing.

A common process (called hot isostatic pressing) for manufacturing parts of non-moldable material consists in pressing a closed and deformable sheath filled with powder of this material by hydrostatic pressure. The pressure causes, in conjunction with an accompanying heating, a sintering of the powder by tamping or densification. The sheath is then split and rejected, and the part can be dimensioned by finishing machining. Isostatic pressing without heating also exists, see JP-A-62110899.

In the case of parts presenting a cavity and of which a typical example is the crucible, one sometimes chooses to press a solid part, that is to say whose sheath follows the external contour but not the contour of the cavity, which is formed by machining after pressing. This solution is in principle quite unsatisfactory because the materials used often work very poorly because of their brittleness and their hardness. It is difficult to obtain a satisfactory surface condition, both by cutting tool and by abrasive tool and the consumption of cutting tools is very high. It even frequently happens that mechanical machining constraints break the parts.

Another solution consists in placing in the sheath, before filling it, a non-deformable core which delimits the cavity. The cores used in foundries have a similar role, but the problems posed by the pressing are different because significant mechanical stresses develop in the sheath, the core as well as in the part as soon as the powder takes consistency.

Some of these constraints are due to the differences in thermal expansion between the part and the sheath on the one hand, the core on the other hand; it is possible, at least in certain cases, to avoid this difficulty by choosing materials having similar expansion coefficients. The sheath and the core are then favorably covered with a non-stick coating which facilitates demolding. It is also possible to reduce the stresses in the room by a judicious choice of temperature and pressure cycles. This avoids cracks and breaks before the drawing.

There is however a phenomenon which it is impossible to compensate for: it is the contraction of the nucleus during densification and its expansion upon return to atmospheric pressure and especially after stripping. The stresses produced tend to expand the part, especially when the sheath has been removed, which made it possible to apply an opposite compression stress. If the material of the part is sufficiently ductile, the part deforms but its rupture by bursting is inevitable in the case of a brittle material.

In carrying out the invention, attempts have been made to overcome this problem and thereby make it possible to press powder of fragile materials in sheaths to directly obtain parts having a cavity.

The method is characterized in that the core is initially of volume greater than the volume of the cavity and undergoes a partial extrusion out of the cavity during pressing. In other words, the core also undergoes plastic deformations and must be constructed from a more ductile material than that of the part.

Depending on the case, the core and the sheath can be in one piece or separate; moreover, the sheath or the core may or may not be covered with a non-stick layer which facilitates demolding. Indeed, the process can be perfectly applied to parts where the core or the sheath are an integral part of the finished product.

The parts can be in particular ceramic. Mention may be made of the oxides (Al₂O₃, CeO₂, ZrO₂), borides (TiB₂), nitrides (TiN, TaN), carbides (TaC, NbC), silicides (Si₃N₄, SiC), mixtures of such ceramics to make granular compounds, composites with ceramic matrix and fiber reinforcement. The invention can also be applied in particular to composites with a metallic matrix and with ceramic or metallic reinforcement as well as to metals and alloys which are not very ductile such as tungsten, cast iron, the alloy of nickel and aluminum in particular.

The cores can be made, for example, of titanium, niobium or tantalum when high temperatures are to be reached. It is also possible to use pure silica glass or silicon enriched with boron oxide. One such body is sold under the VYCOR brand by Corning Corp. Other materials such as low-melting metals and glasses can be used when pressing is carried out at lower temperatures.

The cavity can take various forms. It can be a cylindrical cavity, or a conical undercut when demolding is necessary. One can however envisage cavities almost without communication with the outside, even if it is then necessary to remove the material from the core, which is then eliminated by a chemical attack.

The material of the part to be densified is frequently powder but can also be a cold pre-compacted or pre-sintered body.

• les figures 1A et 1B illustrent un premier exemple ;
• la figure 2 est un diagramme montrant le cycle de température et de pression utilisé pour ce premier exemple ; et
• les figures 3A à 3D illustrent un second exemple.

The following figures illustrate, without limitation, some examples of implementation of the invention:

• Figures 1A and 1B illustrate a first example;
• Figure 2 is a diagram showing the temperature and pressure cycle used for this first example; and
• Figures 3A to 3D illustrate a second example.

In Figures 1A and 1B, there is shown respectively the shape of a titanium sheath and its content before and after pressing. In the initial state of FIG. 1A, the sheath 1 has the shape of a cylinder 104 mm high and 39.6 mm in diameter. It contains a titanium core 2 composed of a cylindrical base 3 of 39.6 mm in diameter and 9 mm in height placed on the bottom of the sheath 1 and surmounted by a truncated cone 4 of 35 mm in height and gradually tapering towards the top of the sheath 1 from 22 to 20 mm in diameter. The interior of the sheath 1 is also occupied, opposite the bottom, by a graphite shim 5 of 39.6 mm in diameter and 20 mm in height. The rest of the sheath is filled with tantalum carbide powder TaC intended to form the cavity part. The internal face of the sheath 1 and the surface of the core 2 are covered with non-stick product 9 in sheet form.

After a cycle, represented in FIG. 2 where the temperature and pressure curves T and P have been indicated as a function of time in hours with a common scale in bars and in degrees Celsius, the shape shown has been obtained Figure 1B: the sheath has deformed and in particular contracted radially around the part to be obtained (it is now referenced 1 ') and the core (2') has also changed shape: there remains a 4 'cone of smaller volume as the original cone 4, the material thereof having undergone an overall downward displacement which appears in the form of a bulge 6 in the form of a substantially hemisphere 13 mm in height below the base 3 The 4 'cone is approximately 25 mm high and has a diameter varying between 18 and 16.7 mm.

A crucible can be obtained by cutting slightly above the base 3 along line 7, by demolding the cone 4 ', by demolding the sheath 1, after having split it and while removing the shim 5, and by machining around the part in its part contiguous to the shim 5, which has an annular bulge, as indicated by lines 8.

Wedges such as wedge 5 are often encountered in this technical field, but they are not always useful and their absence is therefore perfectly compatible with a correct embodiment of the invention.

It will be noted that the invention allows the appearance of two favorable phenomena: first, a deformable core guarantees that the pressure is identical at all points inside the sheath, which allows a more uniform densification of the part and is not true when a non-deformable core is used, near which the pressure is more important than near the sheath. Then, the bulge of the core downwards limits the pinching of the sheath at the junction of the bottom and the cylindrical wall, and therefore the risk of seeing it broken above the base 3.

Another exemplary embodiment is shown in FIGS. 3A to 3D. The sheath 10 is here substantially thicker, and in one piece with a cylindrical core 11. Its outer shape is still cylindrical. The assembly is made of titanium and the interior is filled with precompacted tantalum carbide. Instead of a thick sheath, we can have a thin sheath with an inner layer of titanium pre-sintered.

For the sake of accuracy, the bead 12 obtained is shown by hermetically crushing the filler neck of the sheath 10.

The initial state of the system is shown in Figure 3A. FIG. 3B represents the final state after hot isostatic pressing, and we observe, as in the previous example, a bulge 13 at the bottom of the sheath 10, and which comes from the partial extrusion of the core 11 for form a smaller cylindrical core 11 '. The sheath, now referenced 10 ', is contracted radially around the tantalum carbide while substantially retaining a cylindrical shape at this location.

FIG. 3C shows that a composite cylinder 14 can be obtained by dressing the two end faces of the assembly, which in particular makes the cord 12 and the bulge 13 disappear so as to leave only a thick titanium envelope substantially uniform around the tantalum carbide. Finally, FIG. 3D shows that a composite crucible 15 can be obtained by continuing the dressing of the bottom of the composite cylinder 14 until reaching the tantalum carbide, then removing the core 11 ′ by appropriate mechanical or chemical machining. Tantalum carbide is surrounded by a layer of titanium on its outer faces only.

Finally, it should be noted that the method can also be applied to ductile materials for which the methods of the prior art can be envisaged in principle. Such an application of the process according to the invention is particularly useful when the stresses to which the ductile materials would be subjected by previous processes are close to the breaking limit.

Claims (3)

1. Process for the production of parts having a cavity by pressing the material to be densified previously poured into a sealed sheath (1) and containing the core (2) for defining the cavity, characterized in that the core (2) is more voluminous than the cavity prior to pressing and undergoes a partial extrusion outside the cavity during pressing.
2. Process for the production of parts by pressing according to claim 1, characterized in that the sheath (10) and the core (11) are in one piece.
3. Process for the production of parts by pressing according to claim 1, characterized in that at least some of the interfaces between the part, the sheath and the core are covered by an anti-adhesive coating (9).

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