FR2641274A1 - IMPROVEMENTS IN SHELL MOLDS - Google Patents

Abstract

Refractory shell molds are used in the lost wax casting of blade superalloys for gas turbine engines, for example. Changes in superalloys which have been prompted by the need for gas turbine engines to run at higher temperatures have resulted in superalloys having higher melting temperatures than can be supported by refractory shells. known. 100% silica shells have been tried, but have low expansion rates and low "green resistance". The invention provides a mixture of zirconium silicate / alumina, sprinkled with alumina, which has the desired characteristics.

Description

This invention relates to molds for use in the process of

molding

lost wax.

  The lost wax molding process is well known. Roughly speaking, the process consists in forming a precise wax model of the part to be made, in dipping the model in a mud of refractory material so as to form a mold, in melting the wax

  and then pour molten material into the resulting vacuum in the mold.

  Complications have arisen in that the metals which are cast have changed in nature so that gas turbine engines in which parts made by the process described herein are used can operate at higher temperatures. Thus the metals must now be of a type such that they have a higher melting temperature than hitherto. This in turn imposed greater constraints on the molds which were found to be unable to support

constantly such constraints.

  The present invention aims to provide an improved mold for use in

  lost wax molding of superalloys at high temperatures.

  According to the present invention, a mud from which a lost wax molding mold is made, comprises a mixture of zirconium silicate and a first powdered alumina bound by a hydrolysed ethyl silicate and covered by another alumina in powder which is scattered and has a coarser particle size

  than that of said first alumina powder.

  The invention will now be described by way of an example.

  A mud in which a wax model must be immersed in order to form around it a hard shell mold, is prepared by mixing powdered zirconium silicate and powdered alumina with a suitable binder. The binder must

  be such that it will allow the mixture to self-support around the wax model, that is to say

  say when the mixture is in the "green" state according to an expression specific to the field of technology. In addition, the binder must be such that it vaporizes when the mud is heated.

  to be dried and liquefied. One such binder is a hydrolyzed ethyl silicate.

  After having immersed the wax model in the mixture, another powdered alumina, the particle size of which is much coarser than that of the mixed alumina, is sprinkled on the mixture which now has the form of a green coating on the model

waxen.

  The process of alternately immersing the model and strewing the mixture can be carried out as many times as desired in order to achieve a thickness of mud

  around the suitable wax model.

  The preferred particle size of the powdered alumina which is used to form part of the mixture nominally corresponds to the sieve number 200 while the

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  preferred granulometry of the powdered alumina which is strewn, corresponds nominally to the number of sieve 28. In other words, a granulometry, as was said above, which is coarse compared to the sieve number 200 of the alumina in

powder mentioned first.

  The respective amounts of zirconium silicate and alumina powder with nominal number of sieve 200 are as follows: Material Percentage by weight Zirconium silicate 40-70 Alumina with nominal number of sieve 200 70-40 The amount of alumina scattered with nominal number of sieve 28 is such that it is sufficient to cover the green mixture of zirconium silicate / alumina with nominal number of sieve 200 which surrounds a wax model after the operation or each

  operation consisting of immersing it in the mixture.

  Once the diving and strewing process is complete, the green shell

  is cooked in a known manner tafmin to drive out the binder and to melt the structure into powder.

  The resulting structure is such that it has sufficient strength to withstand the high temperature stresses opposed to it when metal of molten superalloys is poured into the space created by the evacuation of the molten wax. . In addition, its coefficient of expansion is the same as that of the shells of the prior art. And so no change needs to be made to

  the tools from which the wax models are made.

  Additional improvement in shell strength can be achieved

  by adding 0.25% to 4% by weight of titanium dioxide to the mixture described above

  above. Such an addition improves the strength characteristics of the shell approximately by a factor of four compared to the strength of the shells of the prior art. However, not all modern superalloys lead to the need

of this addition.

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Claims (4)

Claims:   1.- Goat with which a lost wax molding mold is made, characterized in that it comprises a mixture of zirconium silicate and a first powdered alumina bound by a hydrolized ethyl silicate and in that, when said mud covers a wax model, said mixture is covered by another powdered alumina, the particle size of which is coarser than that of said first alumina in powder.   2. Mud according to claim 1, characterized in that the mixture comprises in percentages by weight: Material Percentage by weight Zirconium silicate 40-70 First powdered alumina 70-40 3.- Mud according to claim I or claim 2 , characterized in that   said first powdered alumina has a nominal sieve number of 200.   4.- Sludge according to any one of claims 1 to 3, characterized in that   said other powdered alumina has a nominal sieve number of 28.   5. Mud according to any one of the preceding claims, characterized in   that said mixture includes an additive comprising 0.25 to 4% by weight of titanium dioxide.