GB2209528A - Mould material - Google Patents

Abstract

A shell mould for the production of precision cast components has a face coat of an inorganic or ceramic material comprising a backing of silicon nitride bonded with a silica-based binder medium. The face coat may also comprise silicon nitride, alumina or zircon sand and the silica-based binder may contain yttria.

Description

MOULD MATERIAL The present invention relates to an improved mould material and method of manufacture thereof for use in the casting of metals and particularly, though not exclusively for the production of precision cast components by the Lost wax technique of casting.

The technique of vacuum precision casting by the lost wax process has become the principal means of producing components such as, for exampLe, blades and nozzle guide vanes for gas turbine engines.

The metals cast are generally alloys and may be iron-based, nickel-based or cobalt-based.

Until recently most castings made by this method have been below about 5 Kg in weight and have been of equiaxed grain microstructure.

In the interests of improving specific fuel consumptions in gas turbine engines other metallurgical structures for components such as blades and nozzle guide vanes are now being produced and used commercially.

These other metallurgical structures are known as "directionally solidified" (DS), having a columnar grain structure and "single" or "mono crystal" (SC) wherein the components comprise a single metal crystal.

Such components have improved properties including creep-resistance, for example, and allow the engine to operate at higher temperatures than is the case with equiaxed components.

The method of manufacture of DS and SC components requires the mould shell to be largely unsupported and to remain rigid at temperatures up to and exceeding 15000C, often for many hours.

It has also been more widely recognised recently that the near net shape advantages of material conservation and post-casting machining savings offered by the vacuum precision casting process can be applied with advantage to very large components such as annular casings having thin sections and asymmetric features such as lugs, bosses and struts which are difficult and costly to produce by the conventional route of machining from grossly oversize wrought stock.

Such components may have weights up to 50 Kg and it has been found that as a consequence of mould plasticity at casting temperature the metallostatic pressures exerted on the mould by metal weights of this order causes dimensional deformation of the mould to an unacceptable degree.

Similarly with larger DS and SC now being contemplated the degree of mould plasticity and strength of the present silica bonded zirconlalumino silicate-based shell moulds at the increased temperatures needed for the manufacture of these components are unacceptable.

It is an object of the present invention to provide a shell mould material which does not have the plasticity and strength limitations referred to above.

According to the present invention a shell mould comprises a face coat of an inorganic or ceramic material having a backing comprising silicon nitride bonded with a silica-based binder medium.

The silica-based binder medium may be silica.

In one embodiment of the invention the silica-based binder may further comprise yttria to the end of further increasing high temperature strength properties.

The face coat may be chosen to be compatible with the metal being cast. For nickel-based superalloys the face coat may comprise alumina or zircon sand, for example. Silicon nitride has been found to be suitable as a face coat where aluminium alloys are to be cast.

The shell mould may be formed in known manner by the use of slurry coats followed by dry powder stucco coats until a desired thickness of shell mould has been achieved. The slurry coats may generally comprise particles of the shell material in colloidal silica. Where yttria is also used the yttria may be present in the slurry as yttria powder particles or as colloidal yttria.

During drying and firing of the mould shell the colloidal silica forms a phase which binds the particles together.

Where yttria is present part of the silica and part of the yttria may react to form an inherently stronger binder phase. The binder phase may contain from 1 to 15 wt% of yttria. The actual content of yttria per se will decrease during firing and mould use as more reacts with the silica.

In order that the invention may be more fully understood examples will now be described by way of illustration only with reference to the accompanying figures, of which: Figure 1 is a graph showing the Modulus of Rupture (MOR) ranges of conventional silica-bonded alumino silicate shell mould materials at various testing temperatures 0 up to 1200 C; and Figure 2 which shows the MOR strengths of shell materials according to the present invention tested at 12000C and 15000 C.

Example 1 Shell mould materials were prepared by a 6 layer plus 1 dip seal procedure.

A wax plate pattern was dipped in slurry of colloidal silica containing -325 mesh alumina powder to form a face coat. The coated pattern was then dusted with a -80 mesh alumina prime stucco coat and dried in air in a constant humidity room. After drying the facecoated pattern was then dipped in a slurry comprising colloidal silica and -325 mesh silicon nitride powder after which it was then dusted with silicon nitride particles of -28 to +48 mesh sizes and again dried.

This latter sequence was repeated four further times.

The shell mould was then dipped in a final seal coat of silicon nitride slurry after which the resulting shell mould was dried for 24 hours in a constant humidity cabinet. The dried shell was then de-waxed as is known in the art and fired for 30 minutes at 925 0C.

100 x 20 mm MOR test samples were then cut from the shell moulds so produced.

Example 2 Further shell moulds were produced using an essentially identical process and materials to those of Example 1 except that the colloidal silica used in the slurry contained yttria of about 10 micrometres average particle size. The level of yttria addition was such as to produce a concentration of 9 wt in the dried shell.

Samples of the yttria-containing shell moulds were heated in air for 10, 30 and 60 minutes at 15000C prior to the MOR tests.

Shell moulds of conventional alumino silicate materials were also manufactured using the same manufacturing cycle of dip, dust, dry with a final firing of 30 minutes at 925 0C.

Figure 1 shows the results of MOR tests at temperatures ranging from 8000C to 12000C of conventional silica bonded zircon/alumino silicate shell mould materials.

It will be seen that at 12000C the spread of results ranges from about 6 MPa to about 12 MPa.

Inspection of Figure 2 shows that the MOR of silicabonded silicon nitride has an average of about 18 MPa whilst the material having additions of yttria has an MOR strength approaching 20 MPa at 12000C.

Referring again to Figure 2 which also shows the test results of material tested at 15000C with various soak times at 15000 C ranging from 0 to 60 minutes.

It may be seen that the stength of the material rises from about 6 MPa to about 8 MPa.

The non-yttria containing mould material when tested 0 at 1500 C did not break but deflected plastically giving a load at maximum deflection of about 4 MPa.

At 15000C conventional mould materials do not give an MOR value as they merely deform plastically.

It may be seen that silicon nitride shell material and yttria in the binder phase give very large improvements at 15000 C and allows increased precision and size of components in the casting process.

Claims (10)

1. A shell mould, the mould having a face coat of an inorganic or ceramic material comprising a backing of silicon nitride bonded with a silica-based binder medium.

2. A shell mould according to Claim 1 wherein the binder medium is silica.

3. A shell mould according to either Claim 1 or Claim 2 wherein the silica-based binder further includes yttria.

4. A shell mould according to Claim.3 wherein the bender contains from 1 to 15 wt% of yttria.

5. A shell mould according to any one preceding claim wherein the face coat comprises alumina or zircon sand.

6. A shell mould according to any one of Claims 1 to 4 wherein the face coat comprises silicon nitride.

7. A shell mould according to any one preceding claim wherein it comorises a multiplicity of layers.

8. A shell mould according to any one preceding claim wherein the silicon nitride is in the form of particles of -325 BS mesh and -28 to +48 BS mesh size.

9. A shell mould according to Claim 4 and comprising about 9 wt% of yttria in the binder phase.

10. A shell mould substantially as hereinbefore described with reference to the accompanying specification and either Example 1 or Example 2.