Metal Alloy Casting Process To Reduce Microsegregation And Macrosegregation In Casting

Abstract

Apparatus for reducing segregation in metal alloy castings. The apparatus is operable to reduce segregate spacing (i.e., dendrite arm spacing) in short range segregation (microsegregation) and the overall magnitude of long range segregation (macrosegregation) in castings by controlling the heat flow process, in an active manner, during solidification, thereby reducing the time lapse between initiation and completion of the such solidification. The apparatus functions to decrease the distance within the material being cast between the liquidus-isotherm and the solidus-isotherm by increasing the temperature of the liquid melt around the region of the liquidus-isotherm.

Description

The invention herein was made in the course of a contract under the sponsorship of the United States Army Materiel Command, Frankford Arsenal.

The present invention relates to metal casting processes. While the concepts herein disclosed have use in connection with general casting techniques, they are of greatest importance and are described with particular reference to continuous casting techniques, the latter being the process by which billets or ingots are formed.

It is known that segregate spacing, that is, dendrite arm spacing, has influence on the properties of castings and on the properties of wrought products made from such castings. The subject, generally, is discussed in a doctoral thesis of A. P. Campagna entitled "Heat Flow and Solidification of Alloys" and a master's thesis of T. N. Caldwell entitled "Experimental Verification of a New Heat Flow Analysis of Solidification", the work which led to both theses having been done at the Massachusetts Institute of Technology under the supervision of the inventors Flemings and Mehrabian herein. A principal object of the present invention is to teach a way of reducing segregate spacing (i.e., dendrite arm spacing) in short range segregation (microsegregation) and the overall magnitude of long range segregation (macrosegregation) in castings through control of heat flow in the casting during solidification thereof.

Further objects are discussed in the description to follow and are particularly pointed out in the appended claims.

By way of summary, the objects of the invention, generally, are attained in apparatus for reducing segregation in a metal alloy casting, said apparatus including a mold to receive the molten or liquid alloy metal of which the casting is made. Cooling means is provided for withdrawing heat from one region of the liquid metal in the mold to effect solidification of the liquid metal alloy and the cooling means acts also to withdraw heat from the casting thereby formed. Means is provided for adding heat in a controlled fashion to the liquid melt in the region of the liquidus-isotherm to retard the velocity of that isotherm while maintaining the velocity of the solidus-isotherm. In this way the width of the liquid-solid zone that exists between the liquidus and solidus isotherms is reduced thereby reducing both short range and long range segregation.

The invention will now be discussed with reference to the accompanying drawing, in which:

FIG. 1 is an elevation section view, schematic in form, to illustrate solidification of a continuous-casting made using prior art techniques;

FIG. 2 is a view, similar to FIG. 1, showing solidification of a similar casting but one made using the herein described teachings and shows a reduction in the physical distance between the liquidus-isotherm and the solidus-isotherm from the separation distance in FIG. 1, and FIG. 2 shows schematically, as well, apparatus operable to effect such reduction;

FIG. 3 is a side elevation view, partially schematic in form and partially in block diagram form, to show apparatus adapted to perform the casting techniques disclosed here and is a modification of the apparatus of FIG. 2;

FIG. 4 shows a modification of the apparatus of FIG. 3;

FIG. 5 shows a further modification of the apparatus of FIG. 3; and

FIG. 6 shows, in plan view, still another modification of the apparatus of FIG. 3.

Before going into a detailed discussion of the invention, a brief overall discussion is given. In general, engineering alloys to which the invention is directed freeze over a range of temperatures which differentiates them from pure materials that solidify at a single temperature designated as their melting point. Therefore, alloy solidification is characterized by the presence of a liquid-solid "mushy" region that extends over a range of temperatures dependent only on the composition of the alloy. The solidification of an alloy against a cold mold is illustrated in FIG. 1, where three distinct regions, a solid region 5', a liquid region 3' and a liquid-solid mushy region 4' of an alloy casting 2' are shown. The liquid-solid mushy region 4' is delineated by two isothermal boundaries: the liquidus isotherm labeled 8' (the solidification front, or start of freezing) and the solidus isotherm labeled 9' (end of freezing). Solidification of the alloy takes place due to extraction of heat through the cold mold wall and both the liquidus and the solidus isotherms move vertically upward in FIG. 1. Since the process discussed herein is a continuous one and since, as shown, the casting formed moves downward as indicated by the arrow labeled A, such "vertical" movement of the isotherms is relative to, say, the solid region 5'; whereas the isotherms are stationary with relation to the mold, for example. The distance between the liquidus and the solidus isotherms is designated as the width of the mushy region, and the speed with which each isotherm moves is designated as the velocity of that isotherm. Finally, the local solidification time is the time that a given location in the casting spends between the temperatures of the liquidus and solidus isotherms, or the time that it takes for both these isotherms to sweep across the given location.

During solidification of such an alloy, the different elements, that are combined to make up the cast alloy, segregate. As outlined earlier, the short range distances over which these different elements segregate are designated as the segregate spacings or dendrite arm spacings. These segregate spacings, which have a profound influence on the homogenization and mechanical properties of the cast alloy, are directly proportional to the `local solidification time` described in the preceding paragraph.

On the other hand, these elements also segregate over large distances (macrosegregation), and this type of segregation is dependent on the size of the liquid-solid mushy zone and the rate of solidification (i.e., the speed with which liquidus and solidus isotherms sweep by a given location in the casting).

In conventional methods of continuous casting, a molten metal alloy is introduced directly to an axially short, water-cooled, chill mold. The cooling capacity of the chill mold is such that a solid layer forms in the mold capable of supporting the V-shaped pool of liquid in the center of the casting. The ingot is further cooled by the direct application of a coolant as it exits from the bottom of the mold. The coolant (i.e., water) is applied in such a quantity that the transversely solidifying ingot becomes completely solid at a predetermined distance below the bottom of the mold. The foregoing is shown schematically in FIG. 1 of the drawing where three regions in the formation of casting 2', i.e., liquid, liquid-plus-solid, and solid, during solidification of continuously cast billet are labeled 3', 4' and 5', respectively, as mentioned; since this is a continuous casting process, the casting 2' moves downward so that the isotherms appear stationary in space. There is movement, however, relative to the casting, as before mentioned. In the next paragraph there is discussed the formation of a casting 2 by employing the teachings of the present invention.

Turning now to FIG. 2, apparatus is shown generally at 100 for reducing segregation in a metal alloy casting in a continuous casting process. The apparatus 100 comprises a mold 1 to receive the molten or liquid metal alloy of which the casting is made; reference may be made to U.S. Patent No. 3,477,494 (Burkhart et al.) for details of molds, liquid metal alloy feed devices and related equipment used to provide a continuously-cast product, the product there (and here) of most interest being made of aluminum alloys. The mold 1 usually is a water cooled metal chill mold, as shown in the patent; there is an annular, square or otherwise-shaped central opening in the mold to receive the molten metal and to initiate formation of a casting 2. The cooling capacity of this chill mold is such that a solid layer 11' forms in the mold before the casting exits from the bottom of the mold at 12'. A water spray 10 directed upon the casting 2 serves to effect further withdrawal of heat from the casting to cause complete solidification of the casting. In the embodiment of FIG. 2, the liquid metal alloy introduced is superheated and convection in the liquid melt within the mold, which is labeled 3, is retarded by a transverse magnetic field of the order of 2,000 gauss provided by magnetic pole pieces 6 and 7; see U.S. Patent No. 3,464,812 (Flemings et al.). In this way superheat in the liquid metal alloy 3 is preserved and this superheat manifests itself as additional heat in the vicinity of the liquidus-isotherm labeled 8 to retard the velocity of that isotherm while maintaining the velocity of the solidus isotherm labeled 9, thereby reducing the size or width of the liquid-solid mushy zone 4 (from the width of the zone 4') that exists between the liquidus and solidus isotherms. In this way local solidification time, which is used herein to denote the time difference between initiation and completion of solidification at any particular location in the casting, is reduced and the cast structure 2 formed is refined over the structure 2' formed in the apparatus shown in FIG. 1. It should be noted here that "refined," as used in the present specification refers to reduction in segregate spacing in microsegregation (i.e., dendrite arm spacing), hence, is closely related to local solidification time; on the other hand, macrosegregation (long range segregation) is a function of the width of the liquid-solid zone 4 and the reducing (or shortening) of that zone reduces or eliminates macrosegregation. Thus, the effect of reducing the width of the liquid-solid zone during casting affects both microsegregation and macrosegregation, but the mechanism in one case differs from that in the other. The beneficial effect of reducing the width of the mushy zone 4 by the addition of 150.degree. C superheat to a binary alloy of aluminum and copper (4.5% aluminum) in the absence of convection in the melt zone 3, has been investigated and recognized in the thesis of T. W. Caldwell, aforementioned, the contents of which is incorporated herein by reference.

The magnetic field provided by the pole pieces 6 and 7 is directed transversely to the axis of the casting 2 in FIG. 2 and is a d-c or uni-directional field of substantially constant magnitude. This magnitude of the field in the liquid melt 3 must be sufficient to create magnetic forces (i.e., forces between eddy currents, induced when the liquid metal moves relative to the field, and the magnetic inducing field) within the melt which are larger than the forces of convection. As mentioned, a 2,000 gauss field, when casting aluminum alloys, has been found adequate. It will be appreciated that the mold 1 is a fairly large piece of equipment in operable apparatus, and the pole pieces 6 and 7 to provide the field uniformity and magnitude in the gap occupied by the mold will also be quite large. The d-c magnetic field can also be provided by a solenoidal coil 11 wound about the mold 1, as shown in FIG. 3 the mold being disposed within the central opening of the coil 11. The electric current in the coil 11 will be quite large; to provide such current the coil 11 may be water-cooled copper tubing, or superconducting coils can be used. Electric power to the coil 11 is provided by an electric power source 14. In FIG. 3 there are also shown a source of molten metal 13 and a molten-metal feed device 12.

In the embodiment of FIG. 4, heat to the liquid melt 3 in the vicinity of the liquidus isotherm is provided by a high electrical resistance heating element 15 in thermal contact with the melt 3 but located at a region removed or remote from the liquidus isotherm. Means is provided for convecting the liquid melt to convey the added heat to the liquidus isotherm. The convecting means in the apparatus of FIG. 4 includes magnetic pole pieces 17, 17', 18, and 18' energized by windings, designated 19, 19', etc. in the figure, which in turn are connected to an electrical power source 20. The power source 20 provides a-c electric currents of about 1 to 60 hertz, depending on the parameters of the system (including mold size, volume of melt 3, material being cast, etc.); the magnitude of the a-c field, thus produced, is again the order of at least 2,000 gauss; but here as well the magnitude will depend on the parameters. The poles 17-18, 17'-18' act to induce electric currents in the liquid metal surface and as the electric currents in the windings 19, 19', etc. alternate (and with appropriate phasing) the field appears to move, thereby to cause the melt 3 to oscillate as indicated by the arrows A. It will be appreciated that additional poles can be employed to control the convection of the melt and that a polyphase electric system may be used to create a rotary field.

In the casting system of FIG. 5 heat is added to the melt 3 at the remote region by two electrodes 21 and 22 disposed above the liquid metal 3. A source of high-voltage electric energy 23 is connected across the electrodes 21 and 22 to to provide an arc therebetween, heat to the melt 3 being transmitted by radiation from the arc and being convected to the liquidus isotherm by virtue of eddy currents induced in the melt 3 by the eddy-current fields of coils 24, 24', 25 and 25' which are flat coils and which are energized by an electric power source 26. The coils 24, 24' etc. are oriented and wound to induce convection in the desired direction. Again, the magnitude and frequency of the eddy-current inducing field will depend upon the material of a melt, mold size etc., but frequencies of the order of 60 hertz to 1 kilohertz can ordinarily be used. Field magnitudes of the order of say 1000 to 2000 gauss will suffice for must uses.

In the casting apparatus of FIG. 6, heat is added at a region of the melt 3 remote from the liquidus isotherm by passing high magnitudes of electric current between two electrodes 27 and 28 which are connected to and energized by an electric power source 29. A magnetic field, oriented in the x direction in the figure between magnetic pole pieces 30 and 31 and thus oriented orthogonally to the generally z-directed electric current between the electrodes 27 and 28, serves, through interaction with the electric current, to convect the added heat to the liquidus isotherm. The pole pieces 30 and 31 can provide a uni-directional or d-c magnetic field and the power source 29 can be an a-c or a d-c source, depending upon what is required in a particular piece of apparatus, or the field between the poles 30 and 31 can be a-c (i.e., 60 cycle or some other power frequency) properly phased with the current between the electrodes 27 and 28. It should be kept in mind, as well, that the depth of the electric current in the y direction in the melt 3 can and should be controlled to maximize heat transfer by convection and to otherwise control the transfer.

Further modifications of the invention will occur to persons skilled in the art and all such modifications are deemed to be within the spirit and scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. A method of reducing segregation in metal alloy castings in a continuous-casting process, that comprises, introducing a molten liquid metal of said alloy into a mold, withdrawing heat from one region of the liquid metal in the mold to effect solidification thereof, simultaneously adding heat in a controlled manner to the liquid metal, and internally controlling flow of said heat through the liquid metal to retard the velocity of the liquidus isotherm while maintaining substantially the velocity of the solidus isotherm, thereby reducing the width of liquid-solid mushy zone that exists between the liquidus and solidus isotherms and hence reducing both short range and long range segregation, the liquid metal being heated by superheating the molten metal prior to introduction to the mold to provide the necessary added heat and said flow of the heat being controlled by retarding convection of the liquid metal within the mold.

2. A method of reducing segregation in metal alloy castings in a continuous-casting process, that comprises, introducing a molten liquid metal of said alloy into a mold, withdrawing heat from one region of the liquid metal in the mold to effect solidification thereof, simultaneously adding heat in a controlled manner to the liquid metal, and internally controlling flow of said heat through the liquid metal to retard the velocity of the liquidus isotherm while maintaining substantially the velocity of the solidus isotherm, thereby reducing the width of liquid-solid mushy zone that exists between the liquidus and solidus isotherms and hence reducing both short range and long range segregation, a uni-directional magnet field of substantially constant magnitude throughout the region occupied by the liquid metal in the mold being introduced to control the flow of heat by retarding convection within the liquid metal during the solidification period and thereby raise the temperature of the melt in the vicinity of the liquidus isotherm.

3. In a method of casting a body of metal within which a liquid-solid mushy zone is established during solidification between solidus and liquidus isotherms defining a solid and a liquid zone, the steps of: superheating the liquid metal introduced into the liquid zone; and internally controlling the flow of heat through said liquid zone to maximize the heating of liquid metal adjacent to the liquid isotherm causing approach thereof toward the solidus isotherm, said flow of heat being controlled by magnetically retarding convection within the liquid zone.