Process of manufacturing aluminum-lead bearing material

[57] ABSTRACT

A process for the manufacture of aluminum-lead materials with lead proportions from 3 to 26 percent by weight, especially for use as journal or bearing materials wherein atomized overheated molten particles of the material are very rapidly solidified to form granules. A mass of the granules may be mechanically worked to produce a semi-finished article.

Claims, 4 Drawing Figures

<img file="US3905083A_D0001.tif" />

PATENTEIjSEP 1 6Ϊ975

3,905,083

SHEET 1 OF 2

<img file="US3905083A_D0002.tif" />

Fig. 1

<img file="US3905083A_D0003.tif" />

Fig. 2

PATENTEOSEP I 61975

3.905,083

SHEET 2 Of 2

<img file="US3905083A_D0004.tif" />

<img file="US3905083A_D0005.tif" />

3,905

PROCESS OF MANUFACTURING ALUMINUM-LEAD BEARING MATERIAL

BACKGROUND OF THE INVENTION

Bearing materials based on aluminum containing high proportions of tin are useful in practice. Similar to the development of other bearing materials, wherein tin was replaced by the more economical lead, the attempt has been made to utilize aluminum-lead alloys with proportions of 3-26 percent by weight of lead as bearing materials (German Offenlegungsschrift No. 1,913,168). Such bearing materials however do not represent genuine alloys.

Rather, the lead is found in the form of a precipitate in the aluminum matrix. If extremely fine and evenly distributed lead precipitates could be achieved throughout the aluminum matrix, then one might expect outstanding frictionally low gliding properties. Also, such materials would be appreciably more economical than the previously used aluminum-tin alloys. However, no appropriate process was previously known for manufacturing on an industrial scale aluminum-lead alloys suitable as bearing materials.

It is true that the proposal already was advanced to ²⁵separately melt aluminum and lead and to cast both melts together in free fall and to let them drop upon a cooled surface (German Offenlegungsschrift No. 1,533,254). However, as shown in practice, one may only achieve relative coarse distribution of the lead ³θ through the aluminum structure. Because of the known miscibility gap of aluminum and lead in the liquid state, the separate melts brought together in this process will have to be heated much beyond their melting points in order to mix at all when meeting. This fact increases ³⁵the technical difficulties, especially with respect of the high affinity of aluminum for the most varied other elements, especially oxygen, which occurs at temperatures above the melting point. Therefore this process so far could not be carried in industrial practice. ⁴⁰

Now a proposal also was advanced to the effect that an aluminum-lead melt first be intensively mixed and then be cooled in controlled manner, and that it be subjected during cooling, especially when passing through the zone of the miscibility gap, to external forces, for ⁴⁵instance gravitational or centrifugal ones (U.S. Pat. No. 3,410,331). Such a procedure is extraordinarily difficult and foremost fails to achieve an even distribution, rather it leads to precipitation which can only be controlled by the external forces during the controlled ⁵⁰cooling, but not eliminated.

Furthermore the suggestion already has been made with respect to manufacturing aluminum-lead alloys that the aluminum and lead melts be separately sprayed upon a steel plate in a nitrogen atmosphere (French Pat. No. 2,095,384). The spraying occurs by atomizing with nitrogen. The melts are divided into extremely fine droplets on account of the gas flow and are cooled so rapidly as to be nearly solidified when impinging upon the steel plate. Therefore one fails to achieve sufficient bonding to the substrate and there will be a porous structure. Droplets of the required fineness (diameters are of the order of magnitude of microns) cannot be centrifugally deposited by means of this process. They ₆₅are either wholly or largely solidified already before impinging. Therefore the incorporation of lead particles of the size of microns in an aluminum matrix may not ,083 ' 2 be achieved in this manner. Furthermore, highly pure or inert gases are required as protective atmospheres and drive means. This however will eliminate the economic advantage of lead with respect to tin.

SUMMARY OF THE INVENTION

On the other hand, the invention addresses the task of providing a process for the manufacture of aluminum-lead alloys with lead proportions of 3-26 percent by weight, especially for use in bearing materials, which may be carried out without any difficulties on an industrial scale and which will allow fine and even distribution of the lead precipitates throughout the aluminum matrix. Further, the economic advantage of lead with respect to tin shall be maintained.

This problem is solved by the invention in that the melt of the alloy constituents is raised to a temperature above the miscibility gap and is divided into small melt droplets by evacuation and centrifugation means known per se, said droplets being converted into granule form by very rapid solidification.

The invention is based on the insight and discovery that obtaining an aluminum-lead alloy with fine and even distribution of the lead throughout the aluminum matrix requires more than mere exclusion of gravitational precipitation. All previously known experiments and proposals for obtaining aluminum-lead alloys by cooling the alloy melt failed on account of neglecting additional considerations beyond mere elimination of gravity precipitation. The invention however recognizes the very rapid occurrence of coagulation of the molten lead within the molten aluminum in the form of Ostwald-aging in view of the very high diffusion rate of molten lead and aluminum within the miscibility gap, said aging being independent of mechanical influences or of forces. Only the very rapid solidification to be carried out in accordance with the invention will allow to freeze the melt so rapidly that precipitation and coagulation will be incapable of causing formation of coarse lead particles.

A process known per se for dividing the melt into small melt droplets and for the very rapid solidification of latter into granule form (see German Auslegeschrift No. 1,285,098; Offenlegungsschrift No. I,923j658) will be used in the process of the invention for the manufacture of other aluminum alloys. When making use of this known method, as regards now the manufacture of aluminum-lead alloys, there will be the additional difficulty with regard to previously known applications that the common melt of the alloy constituents must be raised to a temperature beyond the miscibility gap. Surprisingly, it was found that despite the very high temperatures occurring in the melt, the granule particles being obtained will be free from any significant oxidation, furthermore that no hydrogen embrittlement occurred even when casting off in centrifugal manner the melt droplets at those high temperatures into the ordinary atmosphere containing water vapor, provided such embrittlement was prevented at the melt surface. Even more and totally surprising, it was found that in the melt droplets so obtained, even when exceeding a millimeter in diameter, there was no lead gravitational precipitation in the solidified granule.

Thus the process described above allows controllable and continuous operation. The continuously accumulating granulate may be directly processed further into plates or may be extruded into semi-finished products.

3,905,083

In another embodiment of the invention, the melt drops in free flight are very rapidly solidified into elongated-round shaped granules. Said granule, which was obtained in air, has a metallically shiny surface and may be most easily rolled directly into plates or sheets or may be compacted into extrusion billets and be subsequently extruded. Because of the relatively low ratio of surface to volume of the particles and because of the surprisingly minute oxide skin, the proportion of A1₂O₃will not interfere. If one attempted forming appreciably smaller melt droplets by atomizing, then the appreciably larger particle surface to volume ratio would also lead to a considerably higher and interfering proportion of A1₂O₃.

In a second embodiment of the invention, the impinging melt droplets fall upon cooled surfaces on which they spread into thin, flake-like granule particles and where they solidify rapidly. These rapidly solidified flakes so obtained are characterized by extremely finely and evenly distributed lead precipitates in the aluminum matrix. Even when operating in humid air, the surface quality will be outstanding. The flake-like granule may easily be rolled directly into sheets or plates or be compacted and extruded.

Following rolling or extrusion, heat treatment may follow either embodiment and one may obtain in such manner a certain coarsening of the extremely fine lead precipitates in order to set optimum sliding properties of the bearing materials.

It is of special advantage within the scope of the invention that a protective salt cover the hightemperature melt in order to avoid lead losses due to evaporation and in order to prevent hydrogen embrittlement. They may be done at once within the scope of the invention, because the siphon already proposed for implementing the process known per se (German Auslegeschrift No. 1,285,098 or Offenlegungsschrift Nos. 1,923,658 and 2,049,458) may be easily equipped with a sufficiently long suction tube passing through the protective salt layer and sucking the melt from the lower ranges.

The invention achieves an extremely even distribution of very fine lead precipitates throughout the aluminum matrix on account of ensuring the necessary high cooling rates of the high temperature aluminum-lead melt, whereas, as regards the known processes and especially those relating to the castingrolling process (German Offenlegungsschrift Nos. 1,533,254 and 1,913,168), cooling rates of the order of magnitude of 10³°C/sec, and as regards atomizing the melt, about lO^C/sec, can be achieved, compared with orders of magnitude of 10®°C/sec for the process of the invention, such rates being used.

The drawing illustrates the invention.

FIG. 1 shows a temperature-mixing diagram for aluminum and lead;

FIG. 2 is a pictorial representation of an embodiment of the invention;

FIG. 3 is a pictorial representation of a second embodiment of the process of the invention, in side view; and

FIG. 4 is a pictorial representation of the embodiment of the process of FIG. 3 in top view.

DESCRIPTION

As shown by FIG. 1, aluminum will be solid up to the temperature of aluminum-side eutectic mixture, i.e.,

658.5° weight-percent of lead content, aluminum and lead will be present in two separate liquid phases and as far as the dissociation curve shown in FIG. 1 by the broken line at the upper diagram edge. The dissociation curve is shown in FIG. 1 in dashed lines above 1,000° for the aluminum side and above 1,100° for the lead side, on account of its precise scientific determination as yet being lacking. The alloy is miscible above the dissociation curve and therefore represents a genuine mixture of aluminum and lead atoms.

In conformity with the invention, the state of the alloy melt in FIG. 1 above the dissociation curve should be frozen as much as possible. The larger the temperature interval that is being traversed, and wherein aluminum and lead will be present in separate liquid phases, and the slower the transition through this interval, the larger, that is, the coarser, the precipitated lead particles that are sealed in the aluminum matrix, in view of the diffusion processes that will take place.

As further shown by FIG. 1, the temperature at which aluminum and lead will become miscible will strongly increase with increasing lead content along the aluminum-side branch of the dissociation curve. This means that the temperature interval which is to be traversed when freezing the mixing state will also increase appreciably with increasing lead content. Furthermore, the affinity of aluminum for other elements, especially oxygen, hydrogen and the likes, will naturally increase appreciably with increasing temperature.

FIG. 2 shows in schematic form an embodiment of the invention. A centrifugal lifter 2 is immersed from the top into an aluminum-lead melt 1 in the genuine alloying or mixing state, the lifter being made to rotate rapidly by motor 3. As shown, melt 1 is covered by a protective layer 4 consisting of molten salts. Suction lifter 2 by means of its analoguous shank passes through this protective layer 4 into melt 1 and casts off the evacuated melt from its upper part as melt droplets 5 in a circular path symmetrical to its rotational axis, approximately obliquely upwards and radially outwards, as indicated by the paths shown in FIG. 2. Melt droplets 5 will very rapidly cool during their free flight, typically at an average rate of up to 10⁵°C/sec. Melt droplets 5 will solidify into elongated and round granule particles which, as shown, will be collected in peripheral gutters 6 and will be drained off for further processing.

The example of FIG. 3 and 4 is based on the same design in equipment, though with the difference of there being peripherally mounted cooling surfaces 7 in lieu of the collecting and draining gutters of FIG. 1. Melt droplets 5 will impinge upon these cooling surfaces which are oblique to the flight path and therefore will be very rapidly cooled and thus they will form like flakes or flocks along the cooling surfaces, resulting in a flaking granule 8, which is also collected and removed for further processing. The cooling rates so achieved are higher than for the case of FIG. 1, and may reach 10⁷°C/sec. As shown in FIG. 4 by the dashed and solid lines, the feasibility is provided for adjusting cooling surfaces 7 in such angular position with respect to the radii from suction lifter 2 so as to be optimum for the particular application. The optimum value may be determined empirically.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are

3,905,083 therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are 5 therefore intended to be embraced therein.