Method for producing directionally solidified cast alloy articles and apparatus therefor

Abstract

A method of producing directionally solidified cast alloy articles wherein a porous shell mold having a bottom fill cavity is positioned within an exothermic mass and over a chamber located over a chill plate, molten metal is poured into the chamber whereby the mold cavity is filled and the exothermic material is ignited from beneath the exothermic mass and the mold cavity to cause the mold to be heated to a temperature above the melting temperature of the cast metal, heat is then extracted predominantly from one end of the cast metal to establish an improved temperature gradient along the length of the metal in the cavity and the metal is directionally solidified.

Description

This invention relates to a method of casting high temperature alloys and more particularly to a method for casting directionally solidified alloy articles such as gas turbine engine blades and vanes, and apparatus therefor.

Gas turbine blades and vanes are subjected in use to high temperatures and stresses and to extreme thermal cycling. Recent studies have shown that blades and vanes having a directionally oriented columnar grain structure exhibit improved high temperature properties over blades and vanes composed of many equi-axed grains, particularly in fracture resistance and ductility under preloading conditions. In forming directionally solidified columnar grain castings by current methods, a mold having a highly heat conductive chill plate secured to the base thereof is heated to establish a uni-directional temperature gradient along its longitudinal axis and is then filled with molten metal. For example, when casting alloys having a melting point of about 2400.degree.F, the mold is heated to establish a temperature near the chill plate of about 2000.degree.F while in the upper portions of the mold remote from the chill plate the temperature is about 2700.degree.F. As a result of the uni-directional temperature gradient and the chill at the base of the mold, crystals growing in the melt form with their preferred crystallographic orientation, the (100) orientation for body centered and face centered cubic systems, for example, substantially parallel to the direction of the thermal gradient and grow in a direction away from the chill plate, resulting in columnar grain growth from one end of the mold to the other.

One method of heating the mold to establish a temperature gradient involves packing a frusto-conical mass of exothermic material about the mold with the minimum mass end of the frusto-cone being near the chill plate and the largest mass end of the frusto-cone being at the opposite end of the mold. In casting directionally solidified articles by this method the exothermic material is first ignited to heat the mold to a temperature in excess of the melting temperature of the molten metal to be cast therein. After the exothermic material is burned to a hot clinker, it is inserted into a vacuum chamber wherein a suitable high temperature metal is melted and poured into the mold cavity. Thereafter, the mold is removed from the vacuum furnace along with the chill plate and permitted to cool. As is known in the art, to achieve a directionally solidified columnar grain structure, it is necessary to establish a temperature gradient of sufficient magnitude along the length of the article being solidified to achieve directional solidification along the length of the article. If the temperature gradient falls below a critical value for the linear solidification rate used, the metal will solidify in the traditional equi-axed form.

It is an object of this invention to provide a method and apparatus for casting directionally solidified articles which have improved means for establishing a satisfactory temperature gradient to form columnar grains and for controlling the temperature gradient throughout the casting cycle.

Another object of this invention is to simplify the casting of directionally solidified articles such as gas turbine blades and vanes.

These and other objects are accomplished by the provision of a mold apparatus comprising an article forming mold having an elongated cavity in the shape of the article to be cast and a bottom inlet sprue. The apparatus includes a chill plate disposed below the said mold and the mold is provided with downwardly extending projections which are operative to support the mold portion upon the chill plate in spaced relation to a chamber formed between the sprue and the chill plate with said chamber being in communication with said sprue. A vertical downgate associated with the mold is provided in communication with the chamber. Exothermic material exposed to said chamber adjacent the chill plate is packed about the mold portion preferably in the form of a frusto-cone so that a large mass of exotherm is provided at the end of the mold portion remote from the chill plate.

The method involves placing the aforesaid mold apparatus and chill plate assembly into preferably a vacuum furnace and casting molten metal into a downgate whereby the chamber below the mold is filled and the exotherm mass is simultaneously ignited to heat the mold. Since the mass of exotherm is greater at the end of the mold remote from the chill plate and is ignited at a point nearest the chill plate, the mold is heated to a higher temperature at the end remote from the chill plate after the exotherm material has burned, and a temperature gradient is established across the longitudinal axis of the mold. The mold is removed from the vacuum chamber with the chill plate and permitted to cool to produce a directionally solidified article.

Other objects and advantages will be apparent from the following detailed description reference being had to the drawings in which:

FIG. 1 is a cross-sectional view of a mold assembly in accordance with the invention.

Referring to FIG. 1, the apparatus of this invention comprises a chill plate 10 on which is mounted a mold assembly including a tapered or partially frusto-conical container 12 including the cover 13 each formed of a suitable ceramic material such as Fiberfax, a porous ceramic mold 14 and the exothermic material 24 packed about the mold within the container 12 and cover 13.

The mold 14 includes the article forming portions 16 having the cavities 15, the sprues 17 and the vents 19; a bottom support portion or plate 18 extending substantially across and engaging the container 12 and having an annular dependent skirt or flange 21 and the depending pedestals or supports 22 formed integrally therewith.

The pedestals or supports 22 support the mold 14 above the chill plate 10 to form a chamber 28 therebetween in fluid flow relation with the downgate 20. The chill plate 10 is provided with suitable cooling coils (not shown) as is well known in the art to cool the plate and extract heat as required in the casting cycle. The plate 18 contains a plurality of openings 30 in fluid flow relation with the chamber 28. It may readily be seen that metal poured into the downgate 20 fills the chamber 28 which acts as a runner to the sprues 17 of the article forming cavities 15 and the openings 30 provide molten metal access to the exotherm material 24. If desired the mold may be designed to provide individual runners between the downgate 20 and the sprues 17 to insure more rapid filling of the mold cavities 15. In consequence, molten metal poured into the downgate 20 enters the sprue 17 of the mold portion 16 and also the openings 30 of the exotherm material 24. The openings 30 are preferably equally distributed in the base plate 18 of the mold to provide for uniform ignition of the exotherm mass 24 across the base thereof.

The porous ceramic mold 14 is preferably a shell mold made in accordance with the well known lost wax process. For purposes of illustration the mold in FIG. 1 is shown to have two article forming portions 16. It will be obvious to those skilled in the art that the mold may comprise three, four or more article forming portions formed integrally with the base plate 18 as a cluster about the downgate 20. Preferably, each mold forming portion is supported by the pedestals 22 attached to the plate 18 for supporting the mold forming portions and the exotherm material 24 packed over the plate 18 about the mold forming portions. The risers 19 receive any particles washed out of the shell mold during casting which travel through the mold cavities thus improving the quality of the cast articles.

In casting articles in accordance with the invention the porous ceramic shell mold 14 is made and placed in the tapered refractory flask or container 12. The flask 12 is preferably lined with heat resistant material (not shown) such as asbestos.

The mold 14 within the flask 12 is placed upon the chill plate 10 and the exotherm material 24 is packed about the article forming portion 16 and the downgate 20 and on top the base plate 18 of the mold. Preferably, the exotherm material 24 is in the form of pebbles formed of aluminum and iron base materials with various fillers and binders. A specific example of suitable pebble sizes include pebbles 11/4 inch long, 7/8 inch wide and 5/8 inch thick. The pebbles are merely packed or arranged about the mold. No mixing, ramming or braking of the material is required in the preparation of the mold. The cover 13 is then placed over the exotherm material 24 as shown in FIG. 1. The exotherm material may, of course, be in the form of a powder if desired.

The mold assembly together with the chill plate 10 is then placed in a vacuum chamber wherein a vacuum melted alloy is poured into the downgate 20. The molten metal flows into the chamber 28 and upward through the sprue 17 into the article forming portions 16 of the mold. Simultaneously, the molten metal enters the openings 30 in the base plate 18 and comes in contact with the exothermic material 24. The molten metal ignites the exotherm material from the bottom thereof and proceeds upward. In due course the entire mass of exotherm material 24 is ignited and forms a hot clinker. The mold assembly is then removed from the vacuum chamber and permitted to cool with the chill plate 10 being suitably cooled to produce a temperature gradient along the longitudinal axis of the mold forming portion 16 which will promote directional solidification of the metal.

It is noted that the mold is packed in an exotherm material 24 of identical properties. The mold is preferably packed in a frusto-conical or tapered container so that there is a progressively greater amount of the exothermic material along the longitudinal axis of the cavity containing portion 16 in a direction away from the chill plate 10. However, a vertical walled container may be successfully used. The heat flow path from the casting to the chill plate becomes progressively poorer as the distance from the chill plate increases because of a large mass of metal that the heat must pass through. The amount of exothermic material along the longitudinal axis is increased as the distance from the chill plate increases to compensate for the poorer heat transmission resulting from the greater mass of metal through which the heat must pass to the chill plate.

As previously noted, an important consideration in the directional solidification of articles is that the temperature gradient at the solidification front must be at least above a critical value, as is determined by the solidification rate employed in order that the directional solidification of crystals proceed. If the temperature gradient falls below this figure the metal cools to form equi-axed crystals rather than the longitudinal directional crystals.

An important advantage of this invention is that the molten metal ignites the exothermic material from the bottom at a point relatively close to the chill plate. Since the mass of exothermic material also increases away from the chill plate it is readily apparent that the method of this invention will develop a higher temperature differential between the chill plate and the end of the casting remote from the chill plate than if the exothermic material was ignited at the top of the mold at a point farthest from the chill plate as in prior art methods. The molten metal filling the chamber 28 also provides a good means of heat transmission from the casting to the chill plate since the sprues 17 are in full communication with the cavities 15 and the chamber 28 and the chamber 28 is in direct contact with the chill plate 10 over a large area. In the method of this invention the taper of the container 12 and the temperature of the chill plate 10 are calculated to provide the necessary temperature differential to establish the required temperature gradient for proper directional solidification.

Illustrative of the method is the casting of Mar-M-246 alloy having the nominal composition by weight of 0.15% carbon; 0.1% manganese; 0.05% silicon, 9.0% chromium; 10.0% cobalt; 2.5% molybdenum; 10.0% tungsten; 1.5% titanium; 5.5% aluminum; 0.015% boron; 0.05% zirconium; 0.15% maximum iron; balance nickel. The molten alloy is preferably vacuum melted and poured at a temperature of about 2900.degree.F. Pouring of the metal ignites the exothermic material which causes the mold temperature to rise to about 3100.degree.. At this time the mold is removed from the furnace with the chill plate still intact. Within a few minutes the temperature at the base of the mold drops to about 2900.degree.F thereby establishing a 200.degree. temperature gradient along the length of the alloy within the mold cavity. Further cooling of the mold causes directional solidification of the alloy upwardly from the chill plate.

Although the invention has been described in terms of a specific embodiment, it will be recognized that various modifications and other forms may be adopted within the scope of the invention.

Claims

What is claimed is:

1. A method for casting directionally solidified articles comprising:

providing a mold assembly including an article forming mold portion having a cavity in the shape of the article to be cast and a bottom inlet sprue, a chill plate below said mold portion, support means associated with said mold portion operative to support said mold portion on said chill plate in spaced relation to provide a chamber between said sprue and said chill plate with said sprue being in communication with said chamber, a vertical downgate communicating with said chamber, an exothermic composition about said mold portion exposed to said chamber,

pouring a molten metal into said downgate, said molten metal filling said chamber and said mold cavity and simultaneously contacting and igniting said exothermic material only at said chamber, said burning exothermic material initially heating said mold above the melting temperature of said molten metal within said mold portion without contacting said molten metal,

withdrawing heat by means of said chill plate from the metal in said chamber and progressively from the metal in said sprue and said cavity whereby the metal in said cavity is directionally solidified.

2. Mold assembly for casting directionally solidified articles including

an article forming shell mold portion having a cavity in the shape of the article to be cast, a bottom inlet sprue, and a horizontally disposed support layer at the base of said bottom inlet sprue,

a chill plate adapted for positioning below said support layer,

support means associated with said support layer operative to support said support layer on said chill plate in spaced relation to provide a chamber between said support layer and said chill plate with said sprue being in communication with said chamber,

a vertical downgate communicating with said chamber at said support layer,

a refractory flask surrounding said mold and in coextensive engagement with said support layer,

an exothermic composition within said flask supported by said support layer and disposed about said mold portion,

said support layer having an opening therethrough exposing said exothermic composition to said chamber.

3. Mold assembly for casting directionally solidified metal articles comprising

an integrally formed refractory shell mold incuding a plurality of article forming mold portions having cavities in the shape of the article to be cast and having bottom inlet sprues arranged about a vertically disposed downgate,

a support layer beneath said mold portions attached to the base of said downgate and to said sprues supporting said mold portions and said downgate,

a refractory flask surrounding said mold and in coextensive engagement with said support layer,

a chill plate positioned below said support layer in spaced relation thereto,

pedestal support means associated with said support layer operative to support said support layer on said chill plate in said spaced relation to provide a chamber between said support layer, said chill plate and said flask with said sprues and said downgate being in communication with said chamber,

an exothermic composition packed within said flask and supported by said support layer and disposed about said mold portions,

said support layer having a plurality of openings therethrough providing communication between said chamber and said exothermic material.