GB2196281A - Directionally solidified light metal casting - Google Patents

Abstract

A method of casting, e.g. an automobile wheel involves cooling a molten metal charge by abstracting heat from the charge sequentially, location by contiguous location, in the direction in which solidification is to progress. The rate and/or period at which heat is abstracted at each successive location is such that substantially no solidification is present in the metal forward of the location at which heat is currently being abstracted from the charge. A mould (11) has a multiplicity of thin wall heat conductive major parts so supported with respect to one another as to define a charge-receiving space (17) therebetween. The several said mould parts each have barrier members (63) defining a multiplicity of contiguous cooling zones, A, B, C, etc., and a multiplicity of spray means (79) are controlled sequentially as determined by the direction of desired cooling to deliver coolant spray at the mould parts within the cooling zones. Control of coolant supply to the zones may be by thermocouples. <IMAGE>

Description

SPECIFICATION Method and apparatus for producing light alloy castings This invention relates to a method of producing light metal castings, such, for example, as alminium alloy or magnesium alloy castings, to apparatus for use in performing the method, and to castings produced using the method.

Although the method is of especial merit where the alloy composition is such that solidification takes place progressively over a range of falling temperature, it has merit even where the alloy solidifies over a much smaller range of falling temperature, at a nominally constant temperature, that is to say.

Where solidification in a light metal alloy occurs by degrees, over a sensible falling tem- perature range, the volm etric contraction, which accompanies the solidification process, tends to cause, in the solidifying mass, a starvation of molten metal or 'feed'. The starvation is due to the formation in the solidifying mass, as it progresses towards the full solid state, of a 'slurry' of random interlocking crystals in liquid. There is a resulting viscosity increase in the randomly solidifying material the effect of which is to retard the flow of yet molten metal to the points of contraction in the mass. The effect of starving the points of contraction of molten feed is to deny substantial homogeneity in the finally solidified mass of metal, such lack of homogeneity revealing itself in voids, fissures and micropores in the solid.

It is known that slow cooling of solidifying metal gives rise especially at large cross-sections to a porous condition in the solid. Fast cooling alone does not, however, necessarily avoid the problem.

It is known to reduce the magnitude of the problem by promoting directional solidification in light metal charges. One known method of promoting such directional solidification for billet type castings employs a mould constructed with thin steel walls, the heat capacities of which are tdo small to effect solidification in the molten mass. With the mould full of molten metal in a tranquil state, the mould is lowered slowly into water or passed through water sprays. A considerable measure of longitudinal directional solidification may be achieved using the last described method.

Many cast components, automobile wheel castings for example, are safety critical, and there is a continuing pressure to produce castings of improved quality and with greater economy. The known directional cooling methods, whilst effecting substantial improvements in casting quality are still not wholly satisfactory; defects earlier noted are still present albeit to a diminished extent. The reason for this appears to be due to the fact that the latent heat of fusion liberated during solidification is many times greater than the heat released in cooling the liquid metal, per unit per "C. The known casting methods do not resort to radical heat transfer measures, as herein contemplated, in order to bring about directional controlled solidification rapidly, in the metal charge.

According to the invention, a method of producing light metal castings, by directional cooling of molten metal charge contained in a mould, comprises: abstracting heat from the molten charge sequentially location by contiguous location in the direction in which solidification is to progress and at such a rate and/or for such a period at each said location that substantially no solidification is present in the metal forward of the location at which heat is currently being abstracted.

Solidification, so effected, not only ensures a substantially void-free casting, it has other benefits: a finer dispersion of micro-segregations, smaller and better primary crystal structure, ideal eutectic formation. The result is a casting of superior mechanical properties, especially after further simple heat treatment of the casting.

According to the invention, also, an apparatus for use in performing the above-stated method comprises: a mould having a multiplicity of mould parts so supported with respect to one another as to define a charge-receiving space therebetween, at least one of the said mould parts being a thin wall heat conductive part; and means operable, when a molten charge of light metal shall have been supplied to the charge receiving space, for abstracting heat from the molten charge, through the said thin wall part, sequentially location by contiguous location in the direction in which solidification is to progress in the aforestated manner in the metal forward of the location at which heat is currently being abstracted from the metal charge.

In the context, the expression "thin wall heat conductive part" has the meaning given earlier, that is to say, having a heat capacity which is too small to effect solidification in the molten mass. Clearly the term 'thin' must be construed in relation to the mass of molten metal present at the several locations at which cooling to solidification is to occur sequentially. And the rate of heat abstraction e.g. the temperature at which a coolant is run may affect the permissible thinness of the wall.

Again, whilst stainless steel may be the preferred thin wall material, cupro-nickel, beryllium copper or other heat conductive materials may be employed; and the choice of material may also affect the gauge of the sheet material of the wall.

Preferably, each of the said multiplicity of mould parts is a thin wall heat conductive part; and each has means operable when a molten charge of light metal is present in the mould for abstracting heat from the molten charge through the thin wall part associated therewith in the aforestated manner, the heat being abstracted through the thin wall parts sequentially at opposed positions by contiguous opposed positions of the wall parts.

The means for abstracting heat, in the stated manner, from charge of light metal through a thin wall part may comprise barrier members extending outwardly from the said thin wall part and serving to define therebetween a multiplicity of contiguous zones of the thin wall part and, hence, of a charge when present within the charge receiving space; a multiplicity of coolant delivery means respectively associated with the several contiguous said zones between the said barrier members; and means for supplying coolant to the coolant delivery means in a sequence to effect cooling to solidification in the aforestated manner.

The several said coolant delivery means may each comprise a spray means for directing coolant fluid at the surface portions of the thin wall part within the zone between the barrier members associated with the relevant spray means.

At least one of the barrier members may be a wall of a said zone. There may be apertures communicating between adjacent said contiguous zones through which coolant fluid may pass from zone to zone after contact with the several thin wall surface portions respectively associated with the said zones. The apertures may constitute elements of a coolant recirculating system which also includes at least one outlet leading from the mould and by way of which coolant liquid is conducted.

At least one of the thin wall mould parts may comprise a multiplicity of separable parts.

The foregoing and other features of an apparatus embodying the invention are hereinafter described with reference to the accompanying drawings, in which: Fig. 1 depicts an axial section through a mould for an automobile wheel light metal casting; Fig. 2 is a part sectional plan view of the mould of Fig. 1; Fig. 3 is a detail in the direction of arrow Ill of Fig. 1; and Fig. 4 shows a further detail of Fig. 1.

The apparatus comprises a mould 11 having first and second thin wall heat conductive parts, 13, 15, respectively, so supported with respect to one another as to define a charge receiving space 17 therebetween; and means operable, when the space 17 has been charged with a light alloy mass, for abstracting heat from the molten charge, through the thin wall parts, sequentially location by contiguous location; A, B, C and so on, in the direction in which solidification is to progress and at such a rate and/or for such a period at each such location, that substantially no solidified metal is present forward of the location at which heat is currently being abstracted until completion in the riser. Each of the elements of the apparatus as above broadly stated are considered in greater detail hereafter.The avoidance of presolidification of the poured alloy enables castings of abnormally narrow cross-sections to be produced. In such circm stances relative values of such factors as pre-heat and cooling rate need to be optimally adjusted.

The thin wall mould parts 13, 15, which may be formed by electrodeposition of the wall material on a master, are supported so as to define the charge-receiving space 17 by a frame structure comprising a base plate 17, a central structure 21 and an upper plane 23.

The central structure 21 is an upstanding steel part having a cylinder hub member 27 dividied into upper and lower portions, 27a, 27b, respectively at a transverse parting plane p the enlarged lower end 27b of which is seated in a receptacle 29 formed in the base plate 19 and having its upper end received in a pssage 31 through the upper plate 23. The portion 27a has, towards its lower end, an integral annular structural part 33. The part 33 of the portion 27a has an outer cylindrical portion 35 upstanding from an annular horizontal flange portion 37 integral with the central cylindrical portion 27. The lower portion 27b of the hub member 27 is undercut to provide an annular recess 39. From the lower edge of the outer cylindrical portion 35 there is a skirt extension portion 41 lipped at its edge and constituting part of the thin wall 15 of the male mould part.

The central structure 21 forms part of the metal running system for the suppiy of molten metal to the metal receiving space 17. The cylindrical portion 27 has an axial passage 43 extending into a cylindrical chamber formed at the parting plane P by two opposing cylindrical half chambers, 45a, 45b, in the upper and lower portions 27a, 27b of the hub member 27. A multiplicity of radially extending feeder passages, as 46, communicate between the chamber 45 and the annular recess 39 which leads to the charge space 17. Within the chamber 45 there is an expendible ceramic foam filter 47 to effect filtering of impurities in the metal charge. The ceramic filter 47 also distributes and smooths the metal flow.

The metal running system also includes a feeder or riser 49 extending around the whole or part of the upper flange of the mould. Remote feeders or shrink 'bob' are contained by permanent ceramic inserts, as 49. Metal supply is from a crucible and tundish (not shown) seating on a ceramic fabric seal 51 on the upper plate 23. Arrangements are provided permitting the escape of air from the space 17. These arrangements may be constituted by vents incorporated in joints or in special inserts as 53.

The thin wall mould parts, 13, 15, which, typically, are of 1 to 2 mm stainless steel or cupro-nickel, are composites fabricated of machined cast and thin gauge formings/weldments. As mentioned earlier, however, the mould parts may be formed by electrodeposition of an appropriate metal onto a master.

The thin wall mould part 13 is a female mould part; the part 15 the male part. Further the female mould part 13 is constituted by a multiplicity of separable parts, in the present case five separable parts.

More specifically, the mould part 13 comprises a wheel web or disc thin wall mould portion 13a and four quadrant shaped thin wall mould portions, as 13b, 13c, which together constitute a wheel wall, rim or flange mould part.

The male mould part 15 is a deeply dished unitary member the bottom of which forms the second wheel web or disc thin wall mould portion 15a, whilst its uplifted rim 15b forms the second wheel wall rim flange mould portion.

It will be seen that the bottom of the male mould part 15 has a large central aperture and that the edge 55 of the thin wall around the said aperture is beneath the lipped edge of the skin portion constituted by the skirt portion 41 of the hub member 27, and that the upper rim of the mould part 15 contacts the upper plate 23. A multiplicity of circmferentially distributed core inserts, as 57, serve to support the male part 15 and, as will be further described hereinafter, provide drains for coolant employed. Further the core inserts 57 serve to form in the eventual casting the several holes through the web or disc portion thereof the sections between the said holes thereby simulating wheel 'spokes'.

The web or disc thin wall mould portion 13a of the female mould part 13 also has a large central aperture the rim of the mould portion 13a around the latter aperture rests upon the lower surface of the annular recess 39, the outer rim of the portion 13a resting upon the base plate 19.

The four quadrant shaped thin wall mould portions as 13b, 13c are supported with their lower ends in contact with the base plate 19.

The thin wall mould portions, as 13b, are elements of a quadrantal part-annular closed chest 59 the other elements of which are, for each chest member, an outer casing member 61, a multiplicity of spaced horizontal barrier members 63, bridging the space between the outer casing member 61 and the thin wall portion as 13b, a bottom quadrantal part-annular plate 65 provided with a passage 67 therethrough, and an upper quadrantal part-annular plate 69 having a register portion 71 a and two plates 72 (Fig. 2) respectively closing the ends of the chests.

Between the plates 69 and the upper plate 23 of the frame structure, there is an annular plate 73 attached to the plate 23 and having a register portion 71 b complementary to that of the plates 69.

The several said elements, of which the chest parts are constructed, define quadrantal part-annular zonal compartments, A, B, C and so on. The compartments have, as their inner walls, the thin wall mould portions, 13b. Each of the zonal compartments of the chest parts have apertures 75 through their barrier members 63.

A quadrantal pipe 79 extends between the end plates of each zonal compartment, being held with the ends of the pipe in aligned apertures in the end plates and being so formed at the said ends that, when the chest parts are brought together to make a fluid tight seal at the mating ends of the pipes of annularly adjacent compartments of the several chest parts. In an alternative construction (not illustrated) the pipes 79 are separate pipes one in each quadrantal sector.

The pipes 79, which are for the distribution of coolant liquid, have in each pipe a distribution of slots, as 81 (Fig. 4), the slots being formed in that surface of each pipe which faces the thin wall portion 13b, of the relevant chest part.

The zonal compartments associated with the thin wall web portion 13a of the female mould part 13, are, in principle, similar to those of the quadrantal zonal chest portion. In the case of the zonal compartments of the portion 13a, each compartment is a horizontal full annular compartment; and the coolant distribution tubes 79 are substantially full circular tubes.

As before, apertures as 75 communicate between the barrier members 63. The base plate 19 constitutes a wall of each compartment of the female mould part 13a.

The annular zonal compartments of the web portion 13a lead to a coolant outlet pipe 85.

The zonal compartments of the quadrantal part-annular chest parts lead to outlet pipes, as 87.

The male mould part has a corresponding arrangement of zonal compartments associated with the thin wall 15. The construction of such compartments, being essentially similar to those associated with the female mould part 13, their description is unnecessary.

It should be observed that the core inserts 57 have a passage 67 communicating between the zonal compartments of the male and female mould parts and, hence, with the outlet pipe 85 from the female mould part. In like manner drain holes communicate between the space between the cylindrical part 35 and the central upstanding part 27.

The several coolant distribution pipes, or pipe assemblies are coupled to an inlet manifold (not shown).

Cooling of a metal charge within the space 17 is by sprayquench from the coolant distribution pipes. The coolant returns to the coolant tank (not shown) by way of the outlet pipes 85, 87 which lead to an outlet manifold (not shown). 8pray-quench cooling is conducted at such a spray velocity as to induce substantial turbulence; it is a fact that heat transfer is at a maximm in such a circmstance.

To further improve heat transfer, the outer surfaces of the then walled mould may be roughened, as by metal, e.g. pure copper, spray deposits.

The coolant employed conveniently is water, or a water polymer mixture. Other coolant fluids, both gaseous and liquid may be chosen, of course, depending on the application.

The drawing shows, to a first approximation, the positioning of the coolant sprays for an automobile wheel casting. The optimal spacing is determined by the optimal progress of the-solidifying contour in the metal charge as the solidus advances from the bottom to the top and from the periphery to the casting centre.

To arrive at the optimal configuration trials using expendible thermocouples, as 88, may be conducted, a cooling curve being plotted for each thermocouple position. The arrival at a near solidus temperature, marginally above 577"C, say 600 C, is the signal for quench at that position and, progressively, at further positions along the directional solidification route through the charge.

Accordingly with a charge at 700"C, say 100" to 1500C superheat, coolant is first directed under control of a rotary or solenoid operated coolant sequencing valve (not shown) to the coolant distribution tubes associated with zonal compartments A, B, C; then, after a short period, to compartment D; and thereafter to compartments E, F, G and H in succession.

As each coolant is directed to each compartment, the coolant supply to the compartment or compartments currently being supplied may be terminated, it being unnecessary to continue to cool the already solid alloy. The termination of supply to each compartment after solidification has occurred at that zonal position is desirable in order to achieve a metal to mould temperature equilibrium within a small range of solidus temperature around, say, 450"C, in order to minimise contraction stresses and also to ease demould action-as well as to speed up the production of castings.

Assistance in these effects would come from natural retardation by air gap formation due to linear or solid contract of the part.

A control feature would be permanent thermocouples 95 in the mould wall to ensure correct initial temperature-the coolant jets can be used to administer some cooling after casting retraction, if necessary.

As well as setting up the operation by thermal analysis the efficiency of the solidification would also be appraised by micro-analysis and measurements of the mean secondary crystal branch spacings which vary inversely as the rate of cooling.

Preparation of the mould surface to receive the molten metal and avoid casting interface defects is necessary. A variety of means are available from common practise such as spray-on refractory wash, dry powder applied electrostatically, carbon, flame deposit. The innovation lends itself to improvements-texturing the surface permanently by electrolytic or abrasive techniques. In addition, a light deposit of dry lubricant, such as colloidal graphite would be used to ease withdrawal of the mould sections.

Whilst one metal running system has been referred to, others, for examples, bottom filling by a low pressure pm p or filling under vaccuum, or centrifugal force may be employed.

The casting having been formed the mould is automatically opened. As shown, the quadrantal part-annular chest parts are connected to radially movable members 89; and the male mould part 15 is vertically movable by means of vertically actuable member 91; and the casting itself may be freed from the female mould part 13a by vertically actuable members 93 which project into the space 17 holding the charge and which are operated automatically after operation of the members 89 and 91.

The invention hereinbefore described has been with reference to the production of light metal castings in a heat conductive thin wall mould. And whilst for many purposes the mould is entirely of that form the invention may find application where a thin wall mould as aforesaid constitutes part of a larger mould the remainder or parts of the remainder of which is or are of conventional construction.

Thus the remainder of an intermediate portion of the entire mould may be constituted by a mould portion or portions defining a space for a relatively thick molten light metal charge. Or, again, the invention may find application in increasing the value of a riser as, for example, by shortening its cooling period following completion of feeding.

The invention finds application in compound moulding system, that is where expendible moulds-sand core, for example, are unavoidable. A typical example is in the casting of light metal cylinder heads.

It will be appreciated that the design of the several parts to be brought together in the fabrication of the mould are related to the overall geometry of the casting and in particular the need to 'ieave' smoothly; convenience of mould construction is also an important factor in the design of its several parts; and this is of especial significance where the casting is of a particularly complex three dimensional form and not (as in the example described) a symmetrical casting having no highly convoluted or re-entrant portions.

The thin wall mould has the further merit of portability. This is of value in transfer in a production loop, use in a rotational mode, and in a space environment.

Whilst the invention has been described with reference to the accompanying drawings and certain generalisations asserted as to the nature and scope of the invention the invention also embraces or comprises any novel subject matter or combination including novel subject matter as herein disclosed or contemplated.

Claims (15)

1. A method of producing light metal castings, by directional cooling of molten metal charge contained in a mould which comprises: abstracting heat from the molten charge sequentially, location by contiguous location, in the direction in which solidification is to progress and at such a rate and/or for such a period at each location that substantially no solidification is present in the metal forward of the location at which heat is currently being abstracted.

2. A method of producing light metal castings as claimed in claim 1 in which heat is abstracted from the molten charge by sprayquench action.

3. An apparatus for use in performing the method claimed in claim 1 which comprises: a mould having a multiplicity of separable parts so supported with respect to one another as to define a charge-receiving space therebetween, at least one of the said mould parts being a thin wall heat conductive part; and means operable, when a molten charge of light metal shall have been supplied to the charge-receiving space, for abstracting heat, in the aforestated manner, from the molten charge through the said at least one thin wall part, sequentially, location by contiguous location, in the direction in which solidification is to progress.

4. The apparatus as claimed in claim 3 in which each of the said mould parts is a thin wall heat conductive part; and each of the said parts has means operable, when a molten charge of light metal is present in the mould for abstracting heat from the molten charge through the thin wall part associated therewith, in the aforestated manner, the heat being abstracted through the thin wall parts, sequentially at opposed positions by contiguous opposed positions of the said thin wall parts.

5. The apparatus as claimed in claim 3 or 4 in which the said means for abstracting heat from a charge of light metal through a thin wall part comprises a multiplicity of barrier members extending outwardly from the said thin wall part, and serving to define therebetween a multiplicity of contiguous zones of the said thin wall part and, hence, of the charge when present within the charge-receiving space; a multiplicity of coolant delivery means respectively associated with the several contiguous said zones between the said barrier members; and means for supplying coolant to the said coolant delivery means in a sequence to effect cooling of a charge to solidification, in the aforestated manner.

6. The apparatus as claimed in claim 5 in which the said coolant deivery means comprises spray means for directing coolant fluid at the thin wall heat conductive part, within the zone between the barrier members associated with the relevant spray means.

7. The apparatus as claimed in claim 5 or 6 in which at least one of the said barrier members is a wall of a said zone.

8. The apparatus as claimed in claim 5, 6 or 7 in which at least some of the said barrier members have apertures communicating between adjacent said contiguous zones through which coolant fluid may pass from zone to zone after contact with the said several thin wall surface portions respectively associated with the said zones.

9. The apparatus as claimed in claim 8 in which the said apertures constitute elements of a coolant recirculatory system which also includes at least one outlet leading from the said mould and by way of which coolant liquid is conducted.

10. The apparatus as claimed in any of claims 3 to 9 in which at least one of the mould parts comprises a multiplicity of separable parts.

11. A light metal casting produced in accordance with the method claimed in claim 1.

12. A light metal casting as claimed in claim 1 1 produced using the apparatus of any of claims 3 to 10.

13. A method of producing light metal castings substantially as hereinbefore described with reference to the accompanying drawings.

14. The apparatus for producing light metal castings substantially as hereinbefore described with reference to the accompanying drawings.

15. A light metal casting substantially as hereinbefore described produced in accordance with the method and employing the apparatus substantially as hereinbefore described.