Incorporation Of Dispersoids In Directionally Solidified Castings

Abstract

Inert particles are uniformly dispersed within a molten alloy while it is being directionally solidified in a mold to cause a uniform dispersion within the casting, the dispersion being produced by ultrasonic waves within the molten alloy during solidification.

Description

BACKGROUND OF THE INVENTION

Metals and alloys have been strengthened by a dispersion of inert particles within the alloy, one example of which is a nickel alloy within which thoria particles have been dispersed. Several methods have been used without complete success in obtaining uniformity of particle dispersion particularly when such particles are not wettable by the alloy in which they are embedded. This is particularly true in the plane front mode of solidification or in the cellular front mode.

STATEMENT OF THE INVENTION

One feature of the invention is a process for mixing the particles uniformly in the alloy and for maintaining the uniformed dispersion of the particles during the directional solidification. Another feature is the use of ultrasonic waves generated in the liquid phase of the alloy to maintain a uniform dispersion and to force the particles against the solidification front to incorporate such particles within the solid phase.

Another feature of the invention is a mold apparatus by which the inert particles may be kept in a substantially uniform dispersion while the alloy in the mold is being directionally solidified.

According to the invention the particles which may be non-wettable by the alloy are mixed with the alloy as by a flow of an inert gas stream carrying the particles. The particles are caused to be uniformly dispersed by ultrasonic waves generated in the molten alloy and are kept mixed during directional solidification of the alloy toward the source of the waves, this source being at the end of the mold opposite to the chill plate from which the directional solidification starts and progresses toward the ultrasonic probe which is positioned in the molten alloy and is the source of the waves. These waves force the particles and into the solid liquid interface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a mold apparatus for casting the alloy.

FIG. 2 is a diagrammatic showing of the system.

Referring first to FIG. 1, the mold that may be used in the incorporation of dispersed particles in the casting includes a growth portion 2 resting on a chill plate 4 and communicating at its upper end with a root portion 8 of the mold. The mold shown is used in casting a turbine blade and the mold provides as well as the root forcing portion 8 a platform portion 10, an airfoil shaped blade portion 12, and a shroud forming portion 14 at the upper end of the blade portion. Above the shroud is the open upper end 16 of the mold which permits filling of the mold in the casting process.

The mold is positioned within a susceptor 18 surrounded in turn by a heating coil 20. The latter may be an induction coil preferably tapped midway of its ends as at 22 in order to provide a separate temperature control for the top and bottom portions of the mold. The susceptor rests on the chill plate, being insulated therefrom by a suitable insulating ring 26.

Positioned in the upper end of the mold during the casting of the article therein is a probe 28 by which ultrasonic waves may be generated within the molten alloy during the solidification process. Obviously the probe 28 is in such a position that it may be removed from the liquid alloy before the solidification front reaches a point to encompass the probe. Also shown, but in dotted lines, is a tube 30 which is projected downwardly into the mold and is used for the distribution of the particles into the poured alloy before solidification of the alloy begins. This tube is removed after the particles have been dispersed within the molten alloy so that it is not a part of the mold and does not remain therein during the solidification.

In the casting process, a suitable alloy which has been melted and raised to a super heat of about 50.degree. F above the melting point is poured into the mold after the latter has in turn been heated by the surrounding heating coils to a temperature above the melting point of the alloy also preferably at least 50.degree. F above the melting point. Before solidification begins, the tube 30 is inserted in the molten alloy and the particles are forced into the molten alloy as for example by a stream of inert gas under pressure in which the particles are carried in suspension. During the supply of the particles to the molten alloy and after the particles have been mixed with the alloy, the ultrasonic waves established by the probe 28 serve to mix the particles uniformly throughout the molten alloy in the mold. As the solidification of the alloy begins and progresses upwardly through the article forming portion of the mold the ultrasonic waves serve to keep the particles uniformly dispersed within the molten alloy and also serve to force the particles against and into the liquid solid interface as the interface progresses upwardly from the chill plate toward the top of the mold.

Referring now to FIG. 2 the apparatus by which the invention may be carried out is shown as applied to a mold 32 for making an ingot although obviously an article mold as in FIG. 1 may as readily be used. This mold is positioned on a chill plate 34 and is surrounded by the susceptor 36 which in turn is heated by axially aligned induction coils 38 and 40. The induction coils are separately heated as shown for controlling the cooling of the mold.

The particles to be mixed with the molten metal are supplied from a hopper 42 through an aspirator 44 such that argon or other inert gas supplied through a pipe 46 having a valve 48 therein will pick up the individual particles and deliver them through a refractory tube 48 into the molten metal within the ingot mold 32. The tube is shown as projecting nearly to the bottom of the ingot and it is apparent that the effect of the inert gas carrying the particles is to assure a mixing to some extent of the particles within the molten metal.

After the particles are dispersed within the molten metal the tube 48 is withdrawn and an ultrasonic vibration is set up by a refractory probe 50 immersed in the molten alloy. It has been found that approximately a 20 kilocycle frequency is acceptable. Higher power levels are used during the mixing than are used during the solidification after the mixing is completed.

The probe may be supported with its tip in the molten alloy from a retracting support structure 52 providing for removal of the probe as the liquid-solid interface approached the tip of the probe during solidification. The probe may be energized from a variable frequency and power generator and controller 54.

After sufficient time is allowed for uniform distribution of the particles through the molten alloy, directional solidification is initiated within the ingot by reducing or terminating the energy supply to the lower heating coil and allowing the effect of the water-cooled chill plate to establish columnar grain growth upwardly from the plate. As solidification proceeds the ultrasonic waves force the particles against the liquid-solid interface while maintaining a uniform distribution of the particles within the bulk of the liquid phase of the alloy. Although it is not established why there is a force acting on the particles at the interface the existence of this force has been demonstrated and the existence of the force has also be measured experimentally. Obviously as solidification proceeds, the probe is kept above the liquid-solid interface so that it does not become a part of the casting. Furthermore the tube through which the particles are introduced to the molten alloy will have been removed prior to the start of solidification.

Claims

We claim

1. In the dispersion of inert particles in a casting during solidification of the alloy of the casting the steps which involve

mixing the particles into the molten alloy,

generating ultrasonic waves in the molten alloy to obtain a uniform dispersion of the particles in the molten alloy, and thereafter

maintaining a uniform dispersion of the particles by generating ultrasonic waves at a lower level in the molten portion of the alloy during solidification.

2. In the dispersion of inert particles in a directionally solidified casting during solidification of the alloy of the casting within the mold the steps of

adding the inert particles to the molten alloy,

generating ultrasonic waves in the molten portion of the alloy to obtain and to maintain a uniform dispersion of the particles in the alloy, and

causing a directional solidification of the alloy from one end of the mold to the other to obtain an oriented grain structure in the cast article.

3. The process of claim 2 with the added steps of

causing the directional solidification to occur upwardly by the use of a chill plate at the bottom end of the mold, and

positioning the source of the ultrasonic waves in the molten alloy remote from the chill plate.

4. The process of casting a directionally solidified alloy with inert particles therein including the steps of

melting the alloy,

pouring the alloy into the mold,

introducing inert particles into the alloy and mixing the particles in the molten alloy,

generating ultrasonic waves in the molten alloy at one end of the mold as the alloy is solidified, and

simultaneously causing a directional solidification of the alloy from the opposite end in a direction toward the source of the generated waves.

5. The process as in claim 4 in which the directional solidification of the alloy is caused by heat removal from the alloy by a chill plate located at one end of the mold and with the source of the ultrasonic waves located at the end of the mold remote from the chill plate and external to the part of the casting used as the cast article.

6. The process as in claim 1 including the step of solidifying the alloy within the mold in a direction toward the source of the waves to force the inert particles in the molten alloy against the solidification front.

7. The process as in claim 3 including the step of causing the solidification front to move from the chill plate toward the wave generator such that the particles in the molten alloy are forced against the solidification front to be uniformly embedded within the solidified alloy.