GB2156855A - Alloying process - Google Patents

Abstract

An alloying process for producing a solid alloy of special microstructure comprises heating a mixture of alloyable or interwettable powders until the mixture is part-liquid and part-solid, vigorously agitating the mixture to break up any dendrites, and solidifying the alloy. The microstructure is of uniform dispersion of discrete high-melting particles within a matrix of a lower-melting phase. This processing can be performed at low temperatures, e.g. a mere 100 K above the solidus of the resulting alloy. Subsequent thixocasting is also made easier at low temperatures. Alloys particularly referred to include Al-Cu, Al-Cu-Zn-Mg, Al-Si, Al-Sn, Co-Cr, Cu-Mn, Cu-Ni, Cu-Sn, Cu-Zn, Fe-Cu, Fe-Ni, Fe-Si, Mg-Al, Mg-Al-Zn, Mg-Zn, Ni-Ag, Pb-Sn, Zn-Al, Zn-Cu, steels, cast iron and superalloys.

Description

SPECIFICATION Alloying process This invention relates to alloying processes, more particularly, this invention relates to alloying processes utilising powders such as metal or oxide powders; and to solid alloys so produced.

By "metal powder" is meant herein metal in any finely-divided form and includes not only metal particles and swarf prepared by comminution from the bulk but also metal which has been atomised or produced by other conventional metal powder producing technology; for example, reduction of metal oxide powders.

Throughout the last decade the process of stir- or rheo-casting has been developed. In this process an alloy is heated so that at least a part of it is in the liquid phase; vigorously stirred while a part of it is in the solid phase; and cooled to provide a solid alloy of improved morphology. It is usual in stir-casting processes to heat the entire alloy charge above the liquidus prior to cooling.

There are a number of disadvantages attendant on such processes; thus, the need for a stirrer tends, in practice, to entrain impurities into the final, solid alloy and, in the batch, also dictates a form of casting less amenable to further processing, for example by thixocasting.

In our UK Patent Application 8408975 we have described and claimed, in relation to certain alloys, means for overcoming these disadvantages.

This invention seeks to provide an alloying process which, in addition to overcoming the aforementioned disadvantages, also avoids the need for prealloyed charge and is effected without recourse to heating above the liquidus.

According, therefore, to one aspect of this invention there is provided a process for producing a solid alloy having a uniform dispersion of discrete particles of at least one primary solid phase within a matrix of at least one, lower melting secondary solid phase, which process comprises heating a mixture, preferably an intimate mixture, of wettable or alloyable, finely-divided solid powders (usually metals) until the mixture is part liquid and part solid; vigorously agitating the entire mixture to create shear strain which acts to remove dendrite arms and provide further nucleation sites and promotes heat and mass transfer within the forming alloy; and solidifying the alloy so formed. The shear strain preferably is achieved by substantially instantaneous changes in direction of the agitation.

The agitation may continue until most of the alloy has solidified, and preferably continues until the alloy has cooled to below the solidus temperature. Where the powders incrude a non-metal (e.g. oxide) this is preferably near-spherical; examples are ZrO2 in Pt, and ThO2 in Ni; the liquid metal must wet the oxide.

When examined metallographically the alloys prepared by the process of this invention are found to have small equiaxed grains of a primary solid phase surrounded by low melting material, and greater compositional uniformity than is found in traditional gravity die castings. The grain size may be from 10 to 500 microns, e.g. 10 to 100 microns (in the same sample). Typically the or each primary phase is substantially free from interconnecting dendrites comprising instead discrete degenerate dendrites with little branched structure and approaching a spherical configuration, a characteristic which readily distinguishes them from conventional (that is, gravity cast) alloys of the same composition which are found to have a dendritic or cellular structure.

Alloys which can be produced by the process of this invention include aluminium alloys, cobalt alloys, copper alloys, iron alloys, lead alloys, magnesium alloys, nickel alloys, refractory-metal alloys, tin alloys or zinc alloys. The process of this invention may also be used to produce alloys of the aforementioned copending application. Examples of such alloys include Al-Cu, Al-Cu-Zn-Mg, Al-Si, Al-Sn, Co-Cr, Cu-Mn, Cu-Ni, Cu-Sn, Cu-Zn, steels, such as tool steels, high speed steels and stainless steels, cast irons, superalloys, Fe-Cu, Fe-Ni, Fe-Si, Mg-Al, Mg-AI-Zn, Mg-Zn, Ni-Ag, Ni-Cr, Pb-Sn, Zn-Al or Zn-Cu alloy.

Sometimes oxide presence is beneficial in slowing down the process so that agitation can modify the structure, but if not, the process may be carried out in appropriate non-oxidising atmospheres, as is especially advantageous with aluminium.

The intimate mixture may be formed by any conventional process for mixing finely-divided solids; minor or local compositional fluctuations may also be ironed out during the vigorous agitation effected in accordance with the process of this invention. Surprisingly, it is found that gravity-induced segregation of the mixture of finely-divided solid metals, if any, due to density differences does not affect the homogeneity of the alloy of the invention.

One or more of the finely-divided solid metals may itself comprise prealloyed components; in general, however, prealloyed finely-divided solid metals of given particle size distribution are less readily available than elemental such metals.

The finely-divided solid metals preferably comprise particles; for example, as may be produced by conventional powder metallurgy processes. The average particle size and particle size distribution of the powders may vary over a broad range: for example, from an average particle size of < 5 y to < 1000,z, for example < 50 IL or less to < 300 IL or more but mixing is promoted if the powders have substantially the same size or size distribution, and sizes below 60 microns are generally advantageous. In accordance with a particularly desirable feature of the present invention the powder comprising the metal which will be the main component of the primary solid phase may be selected with an average particle size, and volume fraction, desired for that phase in the solid alloy so formed.It is found that their average particle size does not vary by more than about 20% in alloying.

By "primary solid phase" is meant herein the or each phase which originates in the individual particles of the or each finely-divided solid metal which remains during the process of the invention suspended, incompletely melted, in a liquid matrix of the remainder of the metal, which matrix becomes the secondary solid phase on complete solidification. This can comprise from 5 to 95 vol % of the resulting alloy.

Another attractive feature of the process of this invention is the reduced heat investment (both in terms of temperatures attained and also duration of vigorous agitation while at elevated temperature) required compared with conventional stir-casting processes. Thus, it is possible with the present invention to accomplish a successful alloying with as little as 5 vol % of the mixture being liquid, though this can be as much as 50 vol % or higher; for example, from 10 to 40 vol %.This enables processing to be effected at much lower temperatures than would be anticipated from stircasting: for example, the temperature may be no more than 200"K, preferably no more than 100'K, above the solidus of the resulting alloy, and preferably lower than half-way between solidus and liquidus, preferably onesixth to one-third of the way up from solidus to liquidus, such as one-quarter. This is of particular advantage to the formation of high melting and/or highly reactive alloys such as those including tungsten. It is further possible with the process of the present invention (depending on alloy and atmosphere) to agitate the mixture at elevated temperature for a very short period of time, typically in the case of Ti-base alloys in argon (e.g. 500 Torr) no more than 20 minutes; for example, no more than 15 minutes.Indeed, it is often found that a satisfactory structure can be obtained in 5 minutes.

By "vigorous agitation" is meant herein agitation which is sufficient to enhance mass and heat transfer and to prevent the formation of interconnected dendritic networks and/or substantially to eliminate or reduce any dendritic branches already formed in the primary solid phase. The vigorous agitation may be effected by at least one driven stirrer inserted into the alloy mixture. The or each stirrer may be formed in several configurations: for example, as a rotating blade or screw or splined auger provided only that they are operable to create shear in the alloy mixture.

The or each stirrer may be driven, for example, by a variable speed electric motor assembly, at an angular speed of from 300 to 30 rpm; for example, from 1 50 to 50 rpm such as about 100 rpm. The vigorous agitation may be maintained for a period typically from 3 to 20 minutes, although it is preferred to agitate for as short a time as is possible: from 5 to 10 minutes is, in general, a satisfactory agitation time.

However, the presence of an internal stirrer is found to entrain voids, oxides and other impurities into the cast ingot. Moreover, the ingot from a batch process prepared in the presence of an internal stirrer is typically paraboloidal or irregular in shape and difficult further to process. In accordance therefore, with a preferred aspect of this invention there is provided a modification to the aforementioned process of the invention by which it is possible to dispense with internal stirring. This modification to the process comprises imparting to a mould containing the alloy mixture angular velocity sufficient vigorously to agitate the entire alloy mixture to create shear strain therein.

The angular velocity may be continuously of the same magnitude and/or sense; preferably, however, the sense varies periodically to provide a reciprocating motion preferably of 10-2 to 106 Hz (typically 10-1 to 102 Hz) with a waveform displaying sharp cut-offs such as a square wave, and corresponding reciprocating shear in the entire alloy mixture. Neither the magnitude nor the duration of the angular velocity in a given sense need be the same as that in the opposite sense.

In order to minimise porosity in the solid alloy formed in accordance with this invention, the mixture may be mechanically or isostatically pressed at ambient temperature and/or it may be agitated below the solidus of the resulting alloy or individual powder components, for example to an approximation to its final shape.

In an important aspect of this invention, the present process may be substituted into an existing liquid phase sintering process after the powder consolidation step. The process may be followed by thixocasting, for example into a mould cavity which is rammed towards a stationary support for the alloy.

This invention also provides an alloy wherever prepared by a process of this invention.

The invention further provides a casting, which may be a thixocasting, wherever prepared by a process of this invention.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure I represents a schematic axial crosssection of the reciprocating shear rheocasting apparatus using in the present invention; and Figure 2 represents a schematic view, partially axially cross-sectioned, of the thixocaster used in the present invention.

In the drawings, the reciprocating shear rheocasting apparatus comprises a crucible 1 of cast iron, stainless steel, graphite or refractory oxide, nitride or carbide; for example of Awl203, ZrO2, SiO2, SisN4 or TiC, depending on the alloy, having an internal diameter of 20 mm. This is axially connected from below to a stepping motor (not shown) controlled by a microprocessor. Specifically, the stepping motor is a mini-angle stepper driven by a drive card which, in turn, is controlled by a dedicated computer.Both speed (constant or variable) and direction (clockwise or counterclockwise) are parameters precisely controlled ble by this microprocessor arrangement to provide an oscillation range from 0.1 Hz to 2 Hz through an angular range from 1.8 to 50 . The crucible is surrounded by a coaxially-wound, relatively moveable RF heating coil 2. The apparatus is enclosed in an evacuatable, transparent quartz shroud (not shown) which has entry and exit ports for vacuum/ dry argon line.

In use in accordance with the present invention, a charge 3 comprising a mixture of alloyable, finely-divided solid metals (viz 90 wt %, Cu, smaller than 50 microns, plus balance tin, smaller than 53 microns) is fed into the crucible which is 25 mm in diameter, and pressed at 29 MPa. The apparatus is evacuated to about 10-5 Torr and dry argon is then admitted to 500 Torr.

The RF heating coil is energised and when the mixture is part liquid and part solid (quarter of the way from solidus to liquidus, that is, 925"C) the stepping motor is activated to provide, for approximately 3 to 5 minutes, a rWciprocating shear action at 3 Hz with an angular range of 29 . Thereafter the heating coil is switched off and the alloy permitted to solidify while the agitation is continued. When the solid alloy of the invention so produced has reached ambient temperature the crucible is removed from the apparatus and the ingot broken free.A loose powder mixture (150-250 microns) of Fe + 30 wt % Cu agitated for 1 5 minutes at 1120"C gave a density of 5.2 Mg/m3; a 60 microns mixture gave the same result but continued to densify to 7.0 Mg/m3 after 3 hours. Thus, Fe + 30 wt % Cu and Fe + 10 wt % Cu, which both show a liquidus-solidus gap of over 300C, are suitable for use in the invention, especially with particles of under 50 microns for each metal.

In the drawings, the thixocaster comprises a horizontally-aligned, axially-fed cylindrical mould 4 comprising a mould cavity 5 communicating with an axial shot chamber 6 within a ceramic investment 7. The investment is surrounded by a carbon susceptor sleeve 8 for an RF heating coil 9 which is coaxially wound about a quartz shroud 10 having entry and exit ports 11 and 1 2 for a vacuum/dry argon line. A piston 1 3 co-operates with the shot chamber and is driven by a pneumatic activator system indicated generally at 14. The ram face 1 5 of the piston is equipped with a thermocouple 1 6 and strain gauge transducer 17.

In use in accordance with this invention, a slug of the solid alloy of the invention, weighing about 20 grams, is inserted into the shot chamber. The thixocaster is evacuated to about 10-5 Torr and dry argon is then admitted to 500 Torr or as a continuous argon gas flow. The RF heating coil is energised with the ram face of the piston seated against the slug and when the temperature and softness of the slug are determined to be appropriate the piston is actuated and the alloy is rammed into the mould cavity. Alternatively, the alloy slug may be compressed between a stationary ceramic support and a ceramic mould cavity advanced by the ram. Thereafter the heating coil is switched off and the alloy permitted completely to solidify to form the casting.

Claims (16)

1. A process for producing a solid alloy having a uniform dispersion of discrete particles of at least one primary solid phase within a matrix of at least one, lower melting secondary solid phase, which process comprises heating a mixture of wettable or alloys ble, finely-divided solid powders until the mixture is part liquid and part solid; vigorously agitating the entire mixture to create shear strain within the forming alloy; and solidifying the alloy so formed.

2. A process according to Claim 1 wherein the solid alloy comprises an aluminium alloy, a cobalt alloy, a copper alloy, an iron alloy, a lead alloy, a magnesium alloy, a nickel alloy or a zinc alloy.

3. A process according to Claim 2 wherein the solid alloy comprises one of a Al-Cu, Al Cu-Zn-Mg, Al-Si, Al-Sn, Co-Cr, Cu-Mn, Cu-Ni, Cu-Sn, Cu-Zn, steel, cast iron, superalloy, Fe Cu, Fe-Ni, Fe-Si, Mg-Al, Mg-Al-An, Mg-Zn, Ni Ag, Pb-Sn, Zn-Al and Zn-Cu alloy.

4. A process according to any preceding claim wherein at least one of the finely-divided solid metals comprises prealloyed components.

5. A process according to any preceding claim wherein at least one of the finely-divided solid metals has an average particle size from 5 to 1000 microns.

6. A process according to Claim 5, wherein said size is from 50 to 300 microns.

7. A process according to any preceding claim wherein the mixture is not heated above the liquidus temperature of the resulting alloy.

8. A process according to Claim 7 wherein the mixture is heated no more than 100on above the solidus temperature of the resulting alloy.

9. A process according to Claim 7 or 8 wherein at least 5 vol % of the mixture is liquid.

10. A process according to Claim 9, wherein from 10 to 40 vol % of the mixture is liquid.

11. A process according to any preceding claim wherein a mould containing the alloy mixture is given an angular velocity sufficient vigorously to agitate the entire alloy mixture to create shear strain therein.

12. A process according to Claim 11, wherein the angular velocity has a reciprocation at 10- ' to 102 Hz with a waveform displaying sharp cut-offs.

1 3. A process according to Claim 12, wherein the waveform is a square wave.

14. A process according to any preceding claim wherein the resulting alloy comprises from 5 vol % to 95 vol % of primary solid phase.

1 5. A process according to any preceding claim wherein the resulting alloy is subsequently thixocast.

16. A process according to Claim 15, wherein the alloy is thixocast into a mould cavity which is rammed towards a stationary support for the alloy.

1 7. A solid alloy produced or cast by a process according to any preceding claim.