DE1101916B - Method of making composite metal - Google Patents

Description

Method of Making Composite Metal The invention relates to a method for producing at least two-layer composite metal, in which the metals to be joined are cleaned, pressed together and then heat treated. The term "metal" is used in a general sense and includes all elemental metals or alloys of metal.

Methods for making composite metals without the use of a liquid phase at the joint surface have long been known. In general, all known processes can be classified into two groups: (1) One-step process in which the metals are identical early to a high temperature near the The melting point of one of the metals is heated and closed at the same time either by pressure or successive rolling are deformed, whereby the metals change at elevated temperature pressed together and at the same time for a right time to be held on each other. Typical for a one-step process at high temperature is solid phase welding, where two relatively pure metals together- pressed and heated for a considerable time and kept under pressure to get between both create a bond. (2) Multi-step processes in which some of the mentioned stages carried out one after the other will. For example, there are two-step processes for aluminum plating steel or other base metal by (1) cleaning the aluminum with wire brushes and cleaning the surface of the other metal or not cleaning it at all, and (2) rolling the two metals together under vigorous Deformation is carried out at or slightly above room temperature. These processes were used to produce aluminum clad steel in which the aluminum coating was so thin that the bond strength was no longer measurable at all. This is because very thin layers of soft metals, although they are only weakly bound to themselves, cannot be removed from the substrate without tearing it into pieces. This is similar to sticking damp, thin paper on a hard surface. Even if a good bond is undoubtedly not achieved, the weak paper overlay is difficult to pull off without tearing it. In order to make these aluminum-clad metals deformable, they were annealed, with great care not to damage the bond. In another method of plating steel strips with copper, nickel or their alloys, the steel strips are heated to temperatures between 300 and 500 ° C, after which the heated sheet metal strips together with the cold copper, nickel or the copper-nickel alloy in a roll pass be rolled at a degree of deformation of about 60%. Special care was not taken during the cleaning process to prepare the surfaces for bonding, nor was any subsequent heat treatment planned. In this case too, the platings were very thin and the strength of the bond could not be determined.

Another method of using stainless steel in a hot condition Tying aluminum as a preliminary stage in the manufacture of saucepans consists of Degreasing and chemical etching of stainless steel to get a rough surface to achieve, after which the aluminum surface is roughened and the metals together be brought into contact. The metals are now heated to 316 to 482 ° C and hot-rolled in a single pass, the total thickness by at least 511 / o is decreased. The pressure is enough here to get the soft aluminum into the recesses to flow into the surface of the harder stainless steel, whereby a temporary bond of considerable strength is created. After that Composite metal tied in this way provisionally and if necessary is stretched, it will be up to or near the recrystallization temperature heated by aluminum to improve the bond. It is inexpedient with this Process to bond the two metals at temperatures below 316 ° C.

There is also a cold welding process in which certain areas are pressed together under extremely high pressure at room temperature, so that a large local deformation and welded points arise. Any Attempt to test the weld strength of these individual limited areas by means of a subsequent Improving heat treatment was not done.

It is also known to join two aluminum metals, in which a protective coating is applied to a base metal in such a way that a protective layer and base metal in multiple repetitions alternately cold, in particular through Rolling, deformed and annealed, with a grain fragmentation and during rolling grain growth occurs during annealing. Since only after repeating the processes a firm connection of both layers by growing together of crystals and by diffusion, it is necessary in the case of practical Application of the procedure to take precautions to ensure a separation of the to be connected Metal layers, for example by working in elevations and depressions, To prevent rivets etc. This is particularly necessary because in this process, the hindering the bringing together of the two metal layers Oxide layers are not removed. There are several ways to perform this procedure cumbersome work processes and expensive and bulky tools are required.

In another known method of cladding tubes, rods or profiles made of steel with stainless steel, the cladding takes place in the Way that after plugging the jacket and core plastically by cold drawing deformed and then annealed at plating temperature. Cold drawing and annealing repeated several times.

In contrast to these known processes, at least two-layer composite metal in which the metals to be joined are cleaned, are pressed together and then heat-treated, according to the invention a multi-stage Process developed, the characteristic of which is that the metals to the connecting surfaces by heating surface layers that act as barriers cleaned, if necessary by heating on the to be connected Surface at least the barrier skin of one of the metals in a relatively brittle oxide skin is converted that the metals with a cross section reduction of at least 30% and one below the recrystallization temperature of the metal with the lowest recrystallization temperature lying with the formation of a such a relatively low number of nuclear binding sites, d. H. Put on where the atoms of the metals are in the area of interatomic attraction, are pressed together that the metals only with a low, for the subsequent mechanical handling sufficient bond strength are connected and that finally, the metals, which are so relatively weakly connected, then contribute are heat-treated at a temperature which is below the temperature the liquid phase or metallic interconnections in noticeable quantities form in such a way that the core bonds enlarge and the metals with the desired Bond strength are connected.

That achieved by the first two procedural steps is proportionate small bond strength is therefore of particular importance for carrying out the method Significance because for this provisional bond in contrast to the previous procedure Means such as rollers or presses can be used that generate only a relatively low pressure. This means that in many In some cases, existing facilities can be used and not how with the known methods, relatively heavy presses or roll stands acquired Need to become. But this process is also suitable for setting up new systems therefore advantages, because the acquisition of the device for compressing and to reduce the cross-section of the metals is less costly.

The second process step, pressing together, is therefore advantageous performed because it can be done at cold working temperatures. Under "Cold forming temperatures" are all temperatures from room temperature to to understand the lowest recrystallization temperature of the metal, the lowest Has recrystallization temperature.

Due to the enlargement of the contact surfaces or intermediate surfaces resulting from the compression, either by rolling or by other methods, several effects can be achieved at once: (a) Further untouched surfaces are exposed placed under the deformation pressure in to be brought into touch; (b) certain brittle skins on the surface are pulled apart, which then exposed surfaces are exposed; (c) Shear stresses are caused by the act of surface tension and the Forces that cause the surfaces to compressing; (d) the crystalline structure of the touching Surfaces will be distorted. The consequences of these four effects are explained below.

However, the third stage of the process according to the invention should first will be discussed, which consists in the fact that the compressed and in cross-section reduced metal layers are heat-treated in such a way that the core bond is transformed into a continuous bond. This can be done both at temperatures above and below the recrystallization temperature of one or both partners where the rate of growth of these nuclei is a function of Is time and temperature and increases with high temperature and longer duration. The heat treatment can be carried out in any way, provided that only the contact surfaces be heated evenly. In general, it is advantageous temperatures above the recrystallization temperature of the metal with the lowest recrystallization temperature apply. However, heat treatment takes place below the temperature at which it becomes brittle Form intermediate bonds or liquid phases. By heat treatment on such Temperature not only greatly reduces the time it takes to form the bond, but it the metals are also made softer by the way, but can, as already indicated, the heat treatment below the recrystallization temperature be carried out, e.g. B. when the relevant metal layers in the cold hardened, caused by the deformation state are to be maintained. In this case, the treatment time should be increased accordingly.

Overall, this results in a method that is relatively simple and inexpensive facilities for making high quality composite metals is feasible.

It should be emphasized that the removal of the barrier membranes on the surfaces to be joined are of particular importance for pressing, and straight heating creates surfaces that are exposed to relatively low pressure can be pre-connected in the desired manner.

Obviously the importance of the right kind of surface treatment never been properly understood. Brush off or other previously suggested Purification processes allow the re-formation of a more or less cohesive one Skin of molecular dimensions arise which the atoms of one metal cannot into the area of attraction of the atoms of the other metal, and this Condition is necessary, especially for the union of metals. For example Pure alcohol is generally considered to be a good detergent. If two Surfaces are brought to an absolutely clean state and if one of them then wiped off with alcohol, according to the procedure one union can only then arise when the surface is treated again according to the invention or when the metals are heated so far that those originating from the alcohol film-like residues are removed. Such a surface wiped with alcohol However, it was found to be sufficient precisely for the cleaning processes according to the prior art considered.

Another example of the formation of such skins is the following experiment mentions: Two pieces, consisting of fine silver and silver solder, the latter having a composition of 50% silver, 15.5% copper, 16.5% zinc and had 18% cadmium were very carefully stripped of the molecular skin, as described below. A piece of silver was made with a piece of silver solder in a single pass through a roll stand one minute after the mentioned cleaning rolled. The resulting bond was very good after heat treatment.

Another piece of the purified silver and the purified silver solder were stored in air at room temperature for approximately 1 hour. The relative humidity the air was high. Then these pieces were done in a similar fashion in one stitch deformed the aforementioned roll frame in the same manner as before. These two Pieces, however, did not form a bond and came out of the roll stand ununited because of the molecular film that forms during the hour would have.

In contrast to previous knowledge, it is absolutely necessary create a condition with no barriers, barriers, or barrier films are present that have so far been neglected because their importance is not was recognized. The term "barrier skins" is understood to mean films or layers, which are not necessarily continuous and which have molecular dimensions in thickness to have. Such skins either cannot be removed by the known methods or they will form again after cleaning.

Long chains of polar molecules are typical of these barrier membranes, natural oxides and composite skins, both of which are usually hydrate compounds represent, also chemically absorbed layers and absorbed liquid and gas layers.

The natural oxy dhs, namely oxy dhs, made by simple Exposure of the surfaces to air at room temperature will result in hydration are, d. e., the water in various forms, such as. B. hydrates, absorbed layers and possibly otherwise bound water, are required for the process very harmful.

On the other hand, very thin, barely visible, dehydrated oxide membranes, namely, skins that have been created by heating do not have a uniform blocking effect. If these dehydrated films are thicker, they can mechanically unite disturb and then show the same effect as a strong pollution. In damp Atmospheres or oily air become the dehydrated oxides, especially at low temperatures, to be effective within minutes, if not seconds Barrier layers.

According to the method according to the invention, the barrier skin removal, z. B. also by bright annealing, after the usual cleaning or pickling with subsequent Heating to remove the skin formed by the immersion.

This heating must reach such a high temperature that either the barrier skin created by this immersion decomposes or into a harmless one Skin is transformed, this heating must be so low at the same time that Oxidation is avoided unless oxidation is prevented by other means. For example, a furnace with protective gas can be used for such heating but it is also possible to use a jet of hot air or another gas jet, use direct flames, infrared heat, or high frequency induction heating.

However, mechanical abrasion is particularly useful, with Sandblasting, rubbing with sanding belts or brushes are the last method priority is given since no embedded particles remain here.

In addition to the cutting effect of the wire brush, there is a very considerable one Surface heat is generated that is used to prevent mechanical abrasion Disintegrate and repel barrier membranes. The heat generated by vigorous rubbing can be surprisingly large, even so large that the surface layers melt of metals with a high melting point is possible with long enough abrasion. on the other hand it is possible to achieve such surface heating by choosing the exposure time to achieve how they are necessary for the decomposition of the not yet removed barrier membranes is without, however, the harmful oxide skins newly formed on the metal surface will.

However, due to the heat caused by the abrasion, the temperature of the metal are not increased so much that harmful oxvd skins in the meantime between purification and union, if namely those treated Surfaces come into contact with air. When using neutral or protective gas atmospheres can the energy consumed during rubbing can be increased without risk, since harmful oxides cannot form.

As mentioned earlier, those formed by the heat are dehydrated Oxide layers when the metals are united are only slightly damaging when they are very are thin.

The dehydrated oxide layers can be regarded as brittle, so that when the metals are deformed they break apart in such a way that then completely pure metal surfaces come to light. It can be assumed that the natural oxides or the not immediately dehydrated oxides either tough enough are to stretch with the expanding surface, or that they immediately have any bridge the resulting gaps.

Serve as an example for rendering the hydrated oxide skin harmless the case of the natural oxide that forms on aluminum in normal air and that for the connection by the method according to the invention, a barrier layer represents.

However, this can turn into a dry, harmless skin by heating be transformed when it is thin enough. Therefore the process concludes as a surface treatment step the production of a new surface on metal, so that surface treatment for the purposes of the invention the manufacture of a new surface for the to be joined Metals means regardless of whether the new surface is removed by removing an existing one Barrier skin originates or whether the existing skin in one without a barrier skin effect is transformed.

When using the surface treatment on steel strip or sheet steel it was found that rotating steel wire brushes from 304.8 to 203.2 mm in diameter can be used at a speed of 2000 to 4000 rpm. These brushes are adjusted so that there is a significant heat output, that of the speed of passage of the strip, its thickness and its type is dependent. Hard metals demand more power than soft metals.

In this context, the residual heat of the strips is important, while these are being transported by the brushes to the compound facilities. It is known that absorption layers can be destroyed by heat and that relatively low temperatures lead to the formation of absorption layers Surfaces that have been freed from such skins can delay. In this way can, apart from the short time intervals between brushing and deformation, the absorption-delaying effect of temperatures above room temperature, preferably above the boiling point of water. If the The rise in temperature is so great that it favors the formation of harmful oxide layers protective gas must be used until the moment of assembly and deformation be used. If protective gas atmospheres are used and up to this point in time can be maintained, the low viscosity and the high diffusion rate some types of reducing atmospheres, especially hydrogen, as more favorable Factor for preventing small bubbles and irregularities on the contact surface be exploited.

To get the best possible surface with a wire brush treatment To ensure it is necessary to use the brushes and those with the surfaces of the Strip to get in touch guide means clean. These parts must be cleaned in such a way that there is no trace of lubricant, oils, fats or the like. Like., included. A degreasing of the brushes with solvents in steam or liquid form is insufficient. The best results are obtained when brushing first degreased and then heated in air. The parts that come into contact Guides etc. can generally be cleaned sufficiently by rubbing them off.

After the surface treatment, loose dust may need to be removed the surface of the metals must be removed in a dust extractor, and also must the redeposition of dust on the surfaces concerned up to the bonding process be prevented.

When joining metals of different hardness, it is known that less hard metal compared to the harder metal by adding hardening ones Treat additives before joining so that when joining these two metals no technical difficulties arise because one side of the metal is softer than the other and therefore tends to flow. In a preferred embodiment according to the invention it is provided that the less hard metal is cold-hardened in this way becomes that its hardness corresponds approximately to that of the harder metal. In this way the various elongations of these metals can be reduced. This is with soft Metals such as aluminum or silver are important and especially when they are soft Metal is not much thinner than the hard metal. Moreover, it enables the larger one Hardness of such a cold-worked metal, greater pressure and greater stress to allow in the deformation stage at a certain reduction, which then in turn, more core bonds are created.

To increase the contact surfaces of the metals to be connected to achieve, it is useful if the cross-section reduction occurring during compression of the metals to be joined is at least 501 / o.

Before referring to the invention with reference to the drawings and exemplary embodiments will be given some theoretical explanations, but that are not to be regarded as binding.

The classical theory of the manufacture of composite metals teaches that the solid structure of metals is given by the fact that interatomic forces are present surrounding the metal atoms. The resultant of these forces practices only a weak force of attraction at a distance greater than the ten- is up to twenty times the diameter of the atom. The resultant of these forces reached peaks at a considerably smaller distance and then falls to zero in a distance called the equilibrium position. If the metal atoms are compressed closer than this equilibrium position, so form between repulsive forces to them.

To create a perfect bond between the metallic surfaces. try to produce the atoms on the surface of one metal down to Bring the equilibrium distance between the atoms on the surface of the partner.

Now it becomes clear why certain things that cannot be determined by the microscope Skins that are only a few atomic diameters thick are already bound when they are bound act to a large extent disturbing. By increasing the area according to the method according to the invention everyone will still lagged film, in case it is brittle, pulled apart, revealing new clean surfaces. Elastic skins are thinned even further and can then be pierced more easily will.

When two surfaces are pressed against each other, they necessarily can only come into contact at individual specific points, as there is no perfect surface is smooth when the atoms themselves are taken into account. These few points of contact on which the core bonds are formed must now be the full one applied from the outside Withstand stress and are therefore exposed to tremendous pressure. If therefore the compressive strength of the material is exceeded at these points, so occurs a deformation occurs, and there is an increase in the size and number of these myriads of contact surfaces. These are pure metallic contact surfaces.

The course of the process is shown schematically in the drawing. 1 shows a schematic side view of a device on which the invention can be carried out, Fig.2 a partial cross-section of a composite metal with core bonds on a greatly enlarged scale, FIG. 3 is a plan view of FIG. 2, FIG. 4 is a view similar to Fig. 2, which shows the final product after heat treatment. In Fig. 1, which illustrates the first two stages of the process, wherein the compression done by rolling, 1 and 3 mean metal strips that are in solid phase with each other should be connected. The opposing surfaces of the two Tapes that are to be connected to one another are denoted by 5 and 7. at When they are transported to the roll stand 9, the strips go through a surface treatment device 11 to remove the barriers from surfaces 5 and 7 as described above. The tapes then run through a cleaning chamber 13 where the loose dust is removed from the Surfaces 5 and 7, for example by blowing the dust off the surface and vacuuming the blown dust away.

The strips 1 and 3 run from the cleaning chamber 13 to the roll stand 9. A new deposit of dust on the surfaces 5 and 7 must be prevented, for example in that the strips after their exit from the cleaning chamber up to the roll stand are held in contact with one another, as indicated at 15. The reels of the Roll stand exert such a pressure on the strips that a considerable extension of the contact surfaces occurs while the metals are exposed to this surface pressure are. So that the enlargement of the contact areas is big enough, it is expedient, at least the entire thickness of the strips running through the roll stand to be reduced by 30 per cent.

In connection with the second stage of the procedure, the following should also be noted mentioned: The deformation stage must, as already stated, the metals in the way deform so that the surfaces that come together are considerably enlarged.

There are several means to achieve this deformation, using which tapes can be connected to metal panels by the process. Suitable are apart from a pair of rollers also wire and pipe drawing tools, where a pipe on a Rod or applied to another tube, extrusion, on which two Metals are pressed together through an opening, transfer presses on which through Pressure formed a multi-layer cup or tube becomes, and finally upsetting devices on which two metal rods in axial Direction are compressed to a larger area than the rod cross-section to create. The basic knowledge on which the invention is based and the basic process steps are the same for all of these deforming agents.

With a11 of these deformations, the expansion of the previously used by the Barrier skin freed the surface so that the atoms of one metal down to the Field of atomic attraction of the other metal at numerous separate ones Places come in such a way that the metals are connected, albeit proportionately weak. After all, the bond is so good that the surfaces are kept in contact and now the bond will be transformed into a complete union can. These separate bonds are called core bonds because these provide the nuclei for later growing into a substantially uninterrupted one Represent binding, as can be seen from Fig. 2 and 3, where the bands 1 and 3 are united by the core sites 43. It must be emphasized, however, that this Core points are in no way different in type from the final bond; rather, they only extend over a large number of extremely small ones Surfaces, whereas the final bond extends essentially over the entire contact surface extends.

In one embodiment of the invention, panels and strips united. In this case a roll stand is used to make the necessary deformations to achieve, with the metals then pressed together by the rollers and at the same time the areas of the surfaces lying on top of one another are enlarged.

In the rolling process, even if a single sheet of metal is rolled, the upper part of the metal has a tendency to move upwards on the exit side to roll up, and similarly the lower part of the rolled 1, metal, to curl down. This tendency is well known and is often referred to as the "gaping effect" designated.

The core points that are formed during deformation under pressure, must now be strong enough to destroy the bond through this gaping to prevent. The binding must also be such that the tape can be processed further can.

A sufficient strength of the core bond can be achieved by that reducing the thickness of the strip in relation to the diameter of the Rolling in accordance with the regulations laid down here by precise definition the thickness of the strip before rolling and the cold stretching characteristics of the relevant Metals is carefully selected. Other possible folding doors are the tension at the ends of the belt and the type of lubrication between the rollers and the surface of the belt.

Around the core bonding areas is an extremely thin open Room 45 (Fig. 2 and 3) is available. Due to the large surface deformation that the contact surfaces were subjected, and by the nature of the surface of the contacting Surfaces, this space is thermodynamically unstable and tries to achieve a lower state Ingesting energy. This is prevented by Atoms of solid metal are not very mobile at relatively low temperatures. When the metals are heated to an elevated temperature, the atoms vibrate within large distances, have and are considerably more mobile able to quickly restore thermodynamic equilibrium. Through this there is a flow of metal atoms in the open space between the original ones Core bonds, causing the bond surfaces to grow at the expense of open space 45 is caused, with this growth ideally spreading over the entire interface extends. Fig.4 shows the growth of the binding area, with the original Core bonds 43 now form a coherent area, as indicated at 43a, have grown together. The original open space 45 is now to a greater extent interrupted and reduced in its area, as can be seen at 45a.

In general it can be said that with a large thickness the to be united Metals and a greater tendency to cold stretch, the degree of deformation accordingly must be larger in order to have a sufficiently strong core bond that prevents the gaping to accomplish. On the other hand, the reduction required to turn off the The better the lubrication and the greater the pull on, the less the gaping effect Band is.

In the case of other deformation processes, the separating forces can be adjusted accordingly the deformation devices used may be different.

In a roll stand, the strips are pressed against one another by the rollers, producing a relatively large increase in surface area, and the core bonds are formed without a liquid phase appearing on the contacting surfaces. In general, in this cold rolling operation, the increase in the contact area can be easily calculated from the decrease in the thickness of the composite metal produced by the rolling. For the sake of simplicity, the increase in area should therefore be expressed by the reduction in thickness when combining by rolling. The following tables list various exemplary embodiments for the various types of composite metals. These tables also give convenient times and temperatures for the heating stage essential for the bond to grow. Rolled frame heating Metal and Thickness (mm) Reduk- Roll- Starting- No. tion through- speed- temperature- temperature- surface- "knife speed rature time atmosphere preparation ABC o / 'mm m / min. ° C Minutes 1 Ta brass - 74 73 approximately 760 to 10 air exhaust 1.270 90:10 4.15 816 second loops 3,175 den 1) 2 Stainless Cu Stainless 77 133 9.14 704 to 2 Shielding gas exhaust Steel 18: 8 3.175 Steel 18: 8 816 grind 0.305 0.305 3 N Ti Mo - 80 73 approx. 760 to 15 protective gas exhaust Grind 1.575 0.178 4.15 816 4 Invar Everdur - 68 73 about 593 to 2 protective gas exhaust 0.737 98.5 Cu 4.15 649 grind 1.5 Si "0.991 5 12 ct.Ni - 66 73 9.14 537 to 2 shielding gas exhaust Yellow gold 0.254 593 grinding 0.635 6 Fe Ag - 72 178 8.38 593 to 2 shielding gas exhaust Grind 3.175 0.254 649 7 2 S A18) Cu - 67 133 9.14 399 15 Air abrasion 2.032 0.254 with wire brush 2) 8 low Ag - 68 133 9.14 537 to 2 shielding gas exhaust carbonized 0.381 593. grinding 3) stole 3.175 9 Variation Ag - 55 133 12.19 649 50 Inert gas exhaust Monel 0.254 grind 45 Ni with wire 55 Cu brush4) 2.286 10 Invar »F« metal - 54 152 10.67 649 50 shielding gas wire 36 Ni 98 Cu 2 Ag brush) Balance Fe 1.727 1.016 The following example was carried out at an elevated cold forming temperature under a protective gas atmosphere up to the merging point. The protective gas atmosphere consisted of dissociated ammonia and extended up to a range of 38.1 mm from the In summary, the invention consists in carrying out the following process steps, which have never before been combined: 1. Treatment of the surfaces of the metals to be joined by removing the barrier membranes, as described.

2. Deformation of the metals, while their surfaces in the 1 treated condition in such a way that they are compressed and their opposite surfaces with this deformation of at least 30% can be enlarged considerably so that extremely numerous core bonds are formed.

3. Heating of the metals held together by the core bonds be to their growth and spread to increase the overall strength of the To achieve bond. Nip point of the rollers. Inert gas jets were also on the strips near the reels are directed to give the strip an extra Protection on its way of 38.1 mm length between the inert gas chamber and the roller clamping point to grant.

Claims (4)

PATENT CLAIMS: 1. Process for producing at least two-layer Composite metal, in which the metals to be joined are cleaned, pressed together and are then heat-treated, characterized in that the metals to be joined Surfaces cleaned by heating surface layers that act as barriers be, where appropriate by heating on the surface to be joined at least the barrier skin of one of the metals is converted into a relatively brittle oxide skin is that the metals with a cross section reduction of at least 30% and one below the recrystallization temperature of the lowest metal Recrystallization temperature lying temperature with the formation of such a relatively low number of nuclear binding sites, d. H. Places where the atoms of the Metals are in the area of interatomic attraction, are compressed, that the metals with only a small, for the subsequent mechanical handling sufficient bond strength are connected and that ultimately the so proportionate weakly bonded metals are then heat-treated at a temperature which is below the temperature at which the liquid phase or metallic Form intermediate compounds in a noticeable amount, such that the aforementioned Enlarge core bonds and bond the metals with the desired bond strength will. 2. Process in which the surfaces to be joined by the metals Brushing to be cleaned, according to claim 1, characterized in that by intensive Brush off these metal surfaces so that the barrier skins are at least heated be partially eliminated. 3. Method of joining metals of different Hardness by solidifying the less hard metal before joining after one of the preceding claims, characterized in that the less hard metal is so cold-hardened that its hardness r ° twa corresponds to that of the harder metal. 4. The method according to any one of the preceding claims, characterized in that the cross-section reduction is at least 50%. Considered publications: German Patent Nos. 609 276, 670 748, 749695, 749771; Zeitschrift für Metallkunde, 1937, pp. 1 and 2; Willi Machu, Metallische Überzüge, 3rd edition, 1948, Akademische Verlagsgesellschaft Leipzig, p. 34.