Engine Cylinder Liners Based on Aluminum Alloys and Intermetallic Compounds, and Methods of Obtaining Them

The present invention relates to cylinder liners for internal combustion engines containing a matrix based on aluminum alloys of high mechanical strength, obtained by powder metallurgy. It also concerns some of the methods of producing these cylinder liners.

Engine cylinder liners on aluminum are not new, but their use has always posed problems of compatibility between their working surface and the engine components, such as pistons, which are in contact with them. Attempts have been made to deal with the difficulties encountered in different ways, such as by providing a steel lining, or by coating the internal wall of the cylinder with harder metals such as iron or chromium, but it has not been possible to completely eliminate the difficulties.

Manufacturers then turned to alloys with better mechanical strength, such as hypereutectic aluminum-silicon alloys. However, the primary silicon crystals which appeared when the liner was molded were found to have an annoying tendency to score the surface of the pistons, due to their relatively large size and angular shape, and the surface of the pistons therefore had to be protected by a coating.

At that stage, since there was still a desire to enjoy some advantages provided by hypereutectic A-S alloys, attempts were made to change the structure of the alloys, particularly with respect of the grains of silicon, to try to give them the necessary compatibility without requiring subsequent surface treatments of the pistons. Among the attempts made the following should be mentioned:

Those methods consisting of changing the casting structure, such as those described in French Pat. No. 1 441 860, where an acid attack is made on the aluminummatrix so as to bring the silicon grains into relief. The grains are then polished.

Those methods designed to give a new casting structure. This is the case in French Pat. No. 2 235 534, where the liner is molded under cooling conditions so that it does not have any primary silicon phase, but instead has fibrous or spheroidized particles with dimensions of less than 10 .mu.m.

More recently, in their French Pat. No. 2 343 895, Applicants also turned to a new hypereutectic A-S structure but replaced the casting process with a process of extruding powders obtained by atomization. A method of this type in fact has the advantage of using powders formed at a very high rate of cooling, in which the primary silicon grains are relatively small and in any event, smaller than the size resulting from conventional casting. The size is not affected by the extrusion process and a new structure is thus obtained, with fine, well distributed silicon particles, and this considerably improves the compatibility of the liner with the piston.

However, in the course of tests carried out under particularly severe temperature conditions, the resultant liner is nevertheless found to deteriorate.

After carefully studying this phenomenon, Applicants discovered cause of the low resistance to be local adhesion between the liner and the piston.

They extended their research and reconsidered the criteria for good compatibility, namely:

suitable hardness of the liner material to give it sufficient wear resistance while avoiding the appearance of scores;

a relatively low coefficient of friction to facilitate movement of the components relative to one another; and

a tendency of the materials not to stick together, in order to prevent any surface deterioration.

On studying the various prior art solutions, Applicants concluded that, although the hardness function had been achieved fairly well with the primary silicon particles, and although the friction function had been achieved by adding certain lubricants such as graphite, a satisfactory solution to the non-sticking function had not yet been found among the means hitherto used.

For this reason Applicants paid particular attention to the problem of finding an appropriate non-sticking agent.

Applicants have now succeeded in developing engine liners containing a matrix based on aluminum alloys with high mechanical strength obtained by powder metallurgy, and possibly other elements. The liners are characterized in that they contain a dispersion of grains of at least one added intermetallic compound having a melting point of more than 700.degree. C., said compound being apart from other dispersed intermetallic compounds which may exist as part of the alloy.

The present invention comprises a liner of a composite material, in which grains of intermetallic compound, that is to say, of a chemical compound of two or more metals with its own crystalline system, is dispersed in a matrix based on aluminum alloys of high mechanical strength.

It should be emphasized that the dispersion is different from the point of view of structure and/or composition from that which might be present in the matrix of aluminum alloy. It is in fact possible for the aluminum alloy matrix to contain in and of itself some elements which may together form intermetallic compounds while the matrix is being produced by powder metallurgy. However, these compounds form part of the actual structure of the alloy and thus have nothing to do with the compounds of the invention. Applicants have in fact found that the presence of an added compound according to the present invention in the structure of a liner with a matrix of aluminum alloy has the property of greatly reducing, if not eliminating, the tendency of the liner to become welded locally onto the piston when certain temperature limits are exceeded.

Apart from the non-sticking properties tests have been shown that these compounds enable hard points to be formed in the liner, in the manner of grains of silicon. This increases the wear resistance of the liner, so that the presence of primary silicon can be dispensed with.

These compounds have additionally been found to have a good coefficient of friction; they can thus act as a lubricant either by themselves or in association with graphite, in which case they enhance this type of property.

Of the intermetallic compounds studied, Ni.sub.3 Sn, wherein three nickel atoms are combined with one tin atom to form crystals of the hexagonal type, have been found to perform particularly well in its functions as a "non-sticking" agent, a lubricant and a wear resistant product.

The intermetallic compounds of the present invention are distributed evenly in the mass of the liner in granular form. However, if they are to be fully effective, the grains are preferably calibrated, that is to say, they correspond to the narrowest possible grain size curve, where the dimensions are in any case from 5 to 50 .mu.m. This excludes both excessively fine grains, which lead to seizing of the liner manufacturing equipment, and excessively large grains which cause an increase in the coefficient of friction.

To obtain a favorable compromise between the advantages provided by each constituent of the liner, it has been found that the proportion of grains of intermetallic compound should be from 5 to 15% of the liner mass.

The grains may vary in appearance according to how they are obtained. Thus, one can utilize not only grains prepared by grinding but also grains manufactured by spraying the compound in the liquid state; such grains consequently have a more rounded contour.

As far as the matrix of aluminum alloy is concerned, an alloy of high mechanical strength is used, preferably either of the A--S.sub.12 type containing elements like copper and magnesium, such as A--S.sub.12 UG where the copper may range from 1 to 5% by weight, or from the 7,000 series such as A--ZGU with 1 to 12% by weight of silicon added to them. Some examples are A--S.sub.12 U.sub.4 G or A--S.sub.12 Z.sub.5 GU.

In cases where the lubricating properties of the liner must be improved, the liner structure may include appropriate materials such as graphite in particular, the content of which is then from 3 to 10% by weight.

The present invention also concerns some of the methods of producing the aforementioned liners.

The methods described have a common element, which comprises converting the matrix of high strength aluminum alloy from the liquid state into a powder. This is done by any existing process, such as centrifugal spraying, atomization, etc. The powder is then screened to eliminate particles of dimensions not within the range from 60 to 400 .mu.m, then mixed with grains of intermetallic compound of a grain size from 5 to 50 .mu.m and in a quantity such that they represent from 5 to 15% by weight of the mass of the liner; 3 to 10% by weight of graphite or any other element adapted to improve the lubricating properties may also be added. After appropriate homogenization the mixture is then treated in one of two different ways, either by sintering or by extrusion.

In the case of sintering, the powder mixture is shaped by compressing it in a vertical or isostatic press while cold, and is then sintered in controlled atmosphere. The resulting liner is machined to the appropriate dimensions.

In the case of extrusion, the mixture is compressed while cold in billet form or charged directly into the pot of a press, then extruded in tubular form, possibly after preheating, shielded from the atmosphere.

The extrusion equipment used is well known in the art. It may be either bridge machinery or a flat die/floating needle unit. The tube thus obtained at the outlet from the press is trimmed and cut to the length of the liners, and these liners are then machined.

It is possible to carry out quenching directly on the tube leaving the die, and then to proceed to a thermal annealing treatment, in order to improve the mechanical properties of the liner produced.

The mixture may also be compressed into the form of blanks which are backward extruded to form cups, the bottom and opposed edge of which are then cut up to obtain liners which are subsequently machined. It is equally possible to carry out direct quenching of the cups after extrusion.

The liners according to the invention may be used particularly in the automobile industry and in any kind of industry where liner-piston units of good compatibility must be obtained from aluminum alloy.