1. Field of the Invention
The present invention relates to a method for the assembly of and the interconnection by diffusion of bodies of metal alloys. More particularly the invention relates to methods of the kind comprising a sequence of activation of parts to be joined and a sequence of applying pressure when hot which gives rise to interconnection by interdiffusion of the solids.
2. Summary of the Prior Art
Metal alloys of which interconnection can be effected by these methods are alloys of the kind which are resistant to heat or to corrosion comprising, having in proportions by weight, at least 50% of a metal of the group nickel, cobalt and iron or an alloy of at least two of these metals. Certain of these alloys are designated by the term "super-alloys" and in such alloys the remainder is constituted by elements such as chromium, aluminum, molybdenum, titanium, tungsten, niobium, etc., which form a solid solution and form intermetallic compounds or alternatively form with carbon dispersed phases providing the required functional characteristics (mechanical strength and resistance to corrosion) at elevated temperatures. These alloys are used, for example, in the construction of turbo machines.
By "body", there is to be understood herein and in that which follows, simultaneously, and indifferently, either relatively large parts of predetermined shape and dimensions, for example intended to form a welded assembly, or powder grains intended for the production of parts by sintering and by compaction when hot. In practice, the interconnections, junctions or joints envisaged by the invention concern simultaneously, either two relatively large parts, or such a part and a powder, or powders, and, for the interconnection or joining of two relatively large parts, the invention is applicable whether or not an external deposit element is incorporated and which is constituted, for example, by a brazing compound. The assembly method and interconnection concerned thus encompasses different assembly and connection methods either by welding/diffusion, or by brazing/diffusion or by sintering or by compaction when hot. Briefly, the principles of these methods generally known to the man skilled in the art are will now be summarized.
The method of welding/diffusion involves welding the solid phase in which the parts are kept in contact under a given pressure and brought to a predetermined temperature, lower than the initiation of the melting temperature, over a predetermined time period. These operational conditions lead to local plastic deformations of the contact surfaces, which, in turn, give rise to an intimate contact of the latter thus rendering possible the migration of atoms between elements and/or recyrstallisations at the interface. In the ideal case, once the operation is completed no presently available technique enables the initial contact interface to be distinguished, whether this is considered from the microphotographic, chemical or mechanical aspects.
As for brazing/diffusion, it consists in the insertion between the two parts to be assembled of a thin layer (foil or powder) of an alloy particularly with a nickel base comprising additions (fluxes) such that the temperature of the liquids of the alloy will be less than the temperature of the initiation of melting of the superalloy. Heating causes initially the fusion of this layer and its connection with the superficial contact layers of the parts, then the reduction in the local proportion of the or each flux by migration into adjacent zones of the parts. In fact, it does not relate to a welding method by solids interdiffusion but the application of a known brazing technique then the diffusion of the fluxes of the brazing material in order to elevate the temperature of fusion of the connecting layer.
Sintering is likewise shown to be of interest for the production of parts of super-alloys, given the difficulties encountered during the use of conventional methods such as machining or forging. Sintering enables the production of complex shaped parts starting from a superalloy powder which, after having been shaped, is submitted to the operation of sintering itself. The latter consists either in heating the powders to a predetermined temperature after compaction when cold in a mould or matrix, or to place the powders under pressure at a predetermined temperature, lower than the temperature of commencement of melting fusion or the latter enables the production of a consolidation of the part by a phenomenon analagous to welding by diffusion at the zone of each grain. The physical and mechanical properties of the sintered parts clearly depend upon the quality of the junctions or connections formed between the powdered grains.
The assembly of the bodies can be carried out either between similar materials, or between different materials by their composition or their nature: a dense material and a powdered material for example.
The superior mechanical characteristics of the alloy used, are accompanied on the negative side by difficulties of fabrication of the parts or of the assemblies, especially by sintering or diffusion welding.
It is in practice apparent during tests of the mechanical strength of assemblies of parts of alloys welded by diffusion, and micrographic examination, that for certain grades of super-alloys or for certain special alloys of the non-oxidizing type, the connection produced after welding by diffusion is of mediocre quality; similarly the strength of sintered parts produced by super-alloys is poor, the cohesion of the assembly being less than values conventionally obtained. In certain cases no joint or connection is possible.
Observations based on electron microscope pictures, and the study of constituents by microanalysis have enabled evidence to be produced that these defects were due to the formation in the superficial contact layers of segregations or precipitates which may harm and even prevent diffusion. These segregations or precipitations result from the migration of certain constituents during the increase in temperature preceding the operation which effects the joint or connection.
This phenomenon of the formation of a barrier to diffusion is more particularly apparent for super-alloys containing, in addition to a non-negligeable amount of carbon, a relatively high quantity of titanium, for example upwards of 0.15% of carbon and 1.5% of titanium. It has been shown in this case, after diffusion welding, for example on one hand the presence in the interface of a quasi-continuous boundary formed by segregation of titanium compounds comprising especially carbon and titanium and, on the other hand, the absence of recrystallisation. FIG. 1a shows a microphotograph, obtained at an enlargement of 2500 times, at one example of an assembly of parts in which the phenomena hereinbefore noted may be observed. Mechanical tests confirm that the joint or connection thus produced is defective.
In French Patent Specification No. 2 380 354, with the object of resolving these difficulties, a preparatory treatment before the connection operation is proposed, consisting in a roughening treatment by heating in an enclosure where a hydro-halogen atmosphere is circulated composed of a mixture of hydrogen and of a hydrogen halide. French Patent Specification Nos. 1 170 557 and 1 243 415 also set out the conditions for carrying out the treatment in a fluoride atmosphere produced from chromium fluoride and ammonium fluoride in the form of a cement in the treatment enclosure obtained under a reducing atmosphere, for example of hydrogen. The use of the atmosphere of hydrogen can, however, lead to certain difficulties and constraints particularly the way of putting the technique into practice.