CH632790A5 - Modification of metal surfaces - Google Patents

Description

The present invention relates to the modification of the surface of reactive metallic parts with low melting point, finished or semi-finished, in particular aluminum or magnesium in the form

elementary or in alloyed form, and more particularly the production of modified physical or chemical properties on metallic surfaces, for example tempered.

There are many methods known and practiced for a long time to improve the resistance of the surfaces of finished or semi-finished metal parts (whether they are elementary metal, alloy or metal compounds) to wear, scuffing , deformation, corrosion, heating and / or erosion; there is in particular a method of melting and alloying, using a laser, a coating with a low melting point or not much higher, with a substrate with a higher melting point to produce resistant surfaces.

U.S. patents Nos. 3952180 and 4015100 of the licensee describe plating and surface alloying processes respectively which solve certain problems, and we will now describe an improvement applicable to the improvement of the surface properties of reactive low-melting metal substrates, by mixing a coating with these surfaces and / or melting the substrate.

It is an important objective of the present invention to provide an improvement in the field of the protection of the resistance to wear of metals, and in related fields, as regards the extension of the methods usable in these same fields and / or obtaining improved products, and more particularly the mixing of a surface coating with a high melting point with a substrate with a lower melting point so as to produce a surface modified at a large volume percentage, that is to say ie comprising more than 50% of the coating material with a high melting point.

It is another objective of the invention to provide very dense and not very porous modified surface layers.

It is yet another object of the invention to provide a method for treating surface layers which accommodates difficult geometries, including reentrant and distant surface regions.

It is yet another objective of the invention to provide a method of forming a surface layer, independently of the electric or magnetic field conditions which may exist in the region or the surface to be treated, or which may appear during treatment.

Still other objectives of the invention are to use basic materials or inexpensive parts, as regards the initial choice and quantity and as regards the limitation of the quantity used; to reduce to a minimum the costs of labor, raw materials and / or time necessary for auxiliary machining and / or heating relating to the formation of surface layer; to give flexibility to the control of the process; minimize accidental effects on the substrate below the surface layer; providing a surface layer with selectively coarse or fine microstructure; to allow working time and a corresponding time of preparation of the substrate and post-treatment reduced to the minimum.

The method according to the invention is characterized as described in claim 1.

The invention can be implemented by coating the substrate with high melting point reinforcing ingredients to reinforce the substrate in a surface layer thereof. In this case, the coating and a surface layer of the substrate are melted over a predetermined depth of the latter, by application of a concentrated beam of radiant energy on the limited surface regions, with an area of the order of 0.0065 to 4.5 cm2, and the relative movement of the beam of radiant energy and of the surface is carried out so as to melt and resolidify the surface sequentially to a practically constant depth and width under practically uniform conditions during linear scanning, so as to define a desired pattern of surface modification. The part is carefully protected to prevent oxidation of the molten surface layer. The conditions of the melting are adjusted so as to induce mixing and forced convection flow of the molten coating material and the molten material of the substrate. Any region so treated is kept in the molten state for less

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2 s, and preferably for less than 1 s, and the substrate forms a very important cold source for the molten region or regions by ensuring rapid solidification from the incident energy beam. The high cooling rates during quenching of the laser melted coating, comparable to those which could only be obtained hitherto by spray cooling techniques, are reported in more detail in the article by Elliot et al., "Rapid Cooling by Laser Melt Quenching", in "Applied Physics Letters", vol. 21, N ° 1, pp. 23-25, July 1972. However, the state of the art supposes limits to any quantitative treatment of the part as a result of phenomena such as the porosity artificially generated by vaporization of the low boiling point constituents of a metal d alloy, as the disc used for example on page 123 of Gagliano et al., "Lasers in Industry", vol. 57, IEEE Proceedings, N ° 2, 1969, pp. 114-117. According to the invention, crystalline microstructures are obtained, while spray cooling allows only amorphous microstructures to be obtained.

The process is preferably carried out at atmospheric pressure or at a pressure higher than atmospheric pressure (to suppress volatilization of the ingredients of the mixture and to avoid the time necessary for immobilization, cleaning and preparation in the treatment. under vacuum), and under an inert protective gas.

The transient energy application zone for melting may be oscillated locally at 100-1000 Hz, to further promote the mixing of the ingredients. Such an oscillation possibly includes a local scanning of a beam of radiant energy and / or a modification of the contour of the beam, for example by alternation between a rectangular shape and a round shape of the beam.

The alloy jacket is optionally rebalanced with a continuous wave laser beam at a higher speed than in the first scan so as to produce an alloy jacket with refined grains (the refining factor being at least equal to 10) . Optionally, the refining of the grains is applied to the substrate itself.

The process of the present invention uses the continuous wave laser equipment which is described in the U.S. patents. Nos 3702972, 3721915.3810043.3713030.3848104 and 3952180 of the licensee.

Thanks to the process which is the subject of the present invention, it is possible to manufacture a part from a base metal chosen as a function of its cost price and / or of its chemical properties, and it is possible to modify its working surface so as to obtain the characteristics required for a particular application, for example hardness, mechanical strength or ductility at high temperature, wear resistance, or corrosion resistance.

On the attached drawing boards:

fig. 1 is a diagram showing a surface treated before and after modification in accordance with a preferred embodiment of the invention;

fig. 2 and 3 are photomicrographs, magnified 100 times, of sections of a part treated as indicated in fig. 1, and fig. 6 is a section, magnified 2000 times, of FIG. 2;

fig. 4 is a diagram giving the volume percentage of silicon particles (on the left), on the one hand, and the weight percentage of silicon (on the right), on the other hand, as a function of the depth of the part (in millimeters) ;

fig. 5 is a diagram giving the Knoop hardness (500 g) in kilograms per square millimeter (on the left), on the one hand, and the Rockwell hardness (on the right), on the other hand, as a function of the depth of the jacket d alloy, in millimeters, and figs. 7 and 8 are photomicrographs, magnified 500 times, representing another aspect of the applications of the invention.

Fig. 1 shows a basic metal substrate S, for example a valve seat or a ball bearing cage, or a similar part, made of aluminum or magnesium (elementary or alloyed) with a PC powder coating which is fluid or bonded by a volatile or semi-sintered binder, or else applied by plasma or torch spraying or by painting with a volatile binder, and preliminary heating to dry the coating of a material with a high melting point. This coating is optionally applied in spots or in strips in regular geometric shapes or as an irregular pattern, depending on the requirements of the final application. In an example application, the PC powder coating strip was made of flowing powder and was 6 mm wide and 1.5 mm high and had a porosity of about 50%. This strip was scanned lengthwise with a laser beam having a diameter D smaller than the width of the test strip, to melt this strip as well as a limited depth (of less than 50% of the thickness of the strip of the substrate. The molten metals re-solidified as the laser beam swept them, by transmission of heat to the highly conductive part of the substrate forming the cold source.

A complex resolidified jacket C was formed, the height of which, above the original surface of the part (WP—), was greater than the height of the original powder coating, and the thickness of which, at the - below the original surface of the part (WP +), was less than half of WP—. A zone Z of refined grain substrate material, of thickness less than the average thickness of this jacket, appears near the jacket.

When the coating is subjected to a preliminary sintering or a prior agglomeration and that it is made to adhere mechanically or that it is joined in another way to the substrate, the final shape of the jacket C conforms (with a slight shrinkage) to the original form of the PC tape.

Within the jacket are large particles of silicon, at a rate of approximately 70% by volume, in a silicon-aluminum eutectic matrix representing approximately 30% by volume, on average, with a higher concentration of silicon in the WP region - of the shirt only in its WP + region.

Figs. 2 and 3 are sectional photomicrographs, magnified 100 times, previous examples of actual treatment of a coating of elementary silicon on an aluminum alloy substrate (AA390) (in the form of a 12.7 mm d cast plate thickness), by scanning with an f / 21 laser beam of 5 mm in diameter and 4.3 kW of power at a processing speed of 51 cm / min. The photomicrographs of figs. 2 and 3 were taken at the locations indicated in fig. 1. Fig. 2 actually consists of two of these photomicrographs, superimposed to reveal a greater depth.

Fig. 6 is a photomicrography, magnified 2000 times, taken from the interior of a region with a high density of silicon from the photomicrography of FIG. 2.

Figs. 4 and 5 show the gradients of silicon composition, and of resulting hardness, going down from the surface of the jacket through the depth of the jacket, each distinct gradient (including a progressive modification of more than 20% in the WP + region of the depth of the jacket) being in contradiction with the homogeneous nature of the alloy layers obtained until now using a laser.

Generally, in the practice of the invention, the surface to be modified is scanned with a laser beam of 1 to 20 kW, concentrated on a circle of 0.5 to 17.8 mm in diameter, or on an equivalent area but of another shape (for example on a square or a rectangle of the same area), at a speed of 12 cm at 1.2 m / min, the conditions being adjusted on average so that the power density a little stronger (about 20%) than to combine a coating with a low melting point in the same substrate with a significant dilution (more than 50% by weight) of the coating material, and a significantly higher power density significant (about 40%) than that which would be used to apply a coating to the substrate without significant modification of the composition of the coating. Typical melt residence times for any given region of the surface layer are 0.1 to 1.0 s, and cooling time for the molten region to 50% or less of the solidus temperature applicable for the alloy composition, is substantially equal to the heating time-

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is lying. During the melting, the only thermal gradients induce a significant degree of mixing of the coating ingredients with the melted surface layer part. In addition, it is believed that a pressure wave is induced by the large energy supply, and that this pressure wave further promotes vigorous mixing, substantially in a convection recirculation of what is estimated to be 50 to 200 times around a given point while the material is in the molten or semi-molten state. As the large particles of silicon (or other phase with a high melting point) precipitate, recirculation by convection continues in the suspension thus formed, until the aluminum-silicon matrix freezes. Meanwhile, the silicon particles which precipitated the first ones reach a large dimension, perhaps by coalescence (width 40 to 100; j. And aspect ratio from 2 to 5 x).

The radiant energy laser beam which has been described above is obtained and possibly manipulated, using a continuous wave laser apparatus of the type described in the patents of the USA. Nos 3721915, 3702973,3577096 and 3713030 of the licensee.

An energy absorbing undercoat is optionally used under the PC powder coating, as described in the above-mentioned patents.

A protective gas is used to prevent oxidation of the molten coating. A helium current of 0.28 m3 / h will generally be used (for a laser scanning speed of 51 cm / min), followed by an argon current also protective at the rate of 0.28 m3 / h, in the surface treatment area.

A method of surface modification and a resulting product have been described which correspond to the above-mentioned objectives. The processing time is very short, and the constraints of space, material and cost are low. The disturbance of the properties of the underlying substrate below its thin, fine-grained zone is minimal. Complex alloys or jackets are formed by passing minor constituents of the substrate through a reinforcing material, so that the finished jacket in a very dense matrix (99.9%) with 0.1 to 10.0% in weight of secondary particles of the original coating material in the matrix of the material of the refined grain substrate.

Other materials with a high melting point (above 1000 ° C.) which can be used as coatings on aluminum or magnesium substrates in accordance with the invention include the elementary or alloyed forms of the metals Mo, W, Cr, V, Hf, Zr, Fe, B, Be, Ni, Co, Ta, Nb, Ti, Pd, Th, Rh, Re, Os, Ir, Pt, Cu, Au and Mn, as well as ceramic materials and refractory. The coating can be cast or forged, or it can be a very dense consolidated structure, for example a wire or a sheet, instead of powdered forms or other porous or spongy forms.

Thanks to the invention, the substrates can be of lower quality, and therefore be less costly, in cases where a given requirement - for example workability and / or higher density - is fixed for a surface such that that of a valve seat in an aluminum cylinder head, or segment grooves in aluminum pistons for internal combustion engines.

Aluminum has a thermal conductivity of 0.53 cal / cm-s- ° C, magnesium a thermal conductivity of 0.36 cal / cm-s- ° C, and their alloys have a conductivity of the same order. Preferably, the present invention is applied to substrates whose thermal conductivity exceeds 0.25 cal / cm-s-'C and whose melting point (or liquidus, in the case of an alloy) is from 400 to 800 ° C, and is sufficient to ensure sufficient conductivity to prevent melting beyond a predetermined depth of the substrate, and to guarantee limited growth of the grains of the highest melting point phase, which therefore reprecipitates the first (for example, silicon in the case of a silicon-aluminum alloy jacket).

The jacket of the resulting product is unique by its high density, its great adhesion, its microstructure and its metallurgical diffusion bond, compared to the product obtained, according to the prior art, by spraying with a torch or plasma.

The coating material has a melting point or a liquidus of

1000 ° C, or at least 200 ° C higher, and preferably much higher than that of the substrate.

According to another aspect of the process of the invention, a depth profile of microscopic hardness, taken in very close sections, would reveal spectacularly different hardnesses, that is to say numerous alternating peaks and troughs, the primary particles. silicon or other coating material having a higher hardness (being on a higher Rockwell scale) than the intermediate zones of the matrix.

The invention can also be used for refining the grains of the substrate, apart from the formation of an applied coating jacket.

Castings and forgings normally have inclusions, for example intermetallic compounds, oxides and sulfides, in addition to the pores they contain. If these defects are found near the surface, they may affect the fatigue strength, corrosion resistance and wear resistance of a part. Consequently, it is sought to obtain a desired grain structure and a more homogeneous chemical composition at desired locations on the surface. For example, if a small part of a large part is subjected to heavy wear and / or strong corrosion, and to fatigue, the best would be to have a fine grain structure and uniformly dispersed alloy inclusions. .

In the current state of the art, it is possible to achieve a refining of the grains by a suitable thermomechanical treatment, which is of course carried out in the solid state. Therefore, the process takes several hours, and it takes a large amount of thermal energy to accomplish it. On the other hand, with the known treatment, it is the grains of the whole part which are refined.

Scanning the surface with a laser beam emitting a continuous wave provides a refined structure. The beam is adjusted so as to obtain rapid fusion, followed by rapid solidification. As this process is carried out in the liquid state, it is significantly faster.

It is possible to locally melt the surface of an inexpensive part to a predetermined depth with a laser beam. It is thus possible to bring the surface temperature from 200 to 400 ° C above the melting point. The superheated molten liquid on the surface will dissolve the inclusions while becoming chemically homogeneous, while, more importantly, the entire part will be at room temperature. Consequently, due to the rapid extraction of heat from the molten liquid (total heating and cooling to 50% of the melting point taking place in a maximum of 2 s), the crystallization of the liquid in solid form takes place quickly. As the crystals can reach a large size, all the liquid is completely frozen. This gives a fine dendritic structure consisting of fine inclusions. On the other hand, the chemical composition of this structure is more uniform than before the treatment.

For example, an aluminum alloy AA 390 containing large inclusions of primary silicon was treated by melting the surface to a depth of about 1 tnm by applying a laser beam having a power density of 27.8 kW. / cm2 at a processing speed of 51 cm / min. The beam stop time was 0.4 to 0.5 s. The cooling rate near the freezing point of the alloy is estimated to be 103-104 ° C / s based on the branch spacing of the dendrites which was approximately 5 µm. Since the surface of aluminum has a very high reflectivity for a laser beam of 10.6 µm wavelength, it has been coated with an energy absorbing agent by treating it with about 10% of a sodium hydroxide solution for about 10 min to reveal a black coating of oxide or hydroxide. The surface thus coated was rinsed with cold water and dried before being treated with the laser.

The photomicrographs of figs. 7 and 8 show the state of the surface of the part (a layer of approximately 1 mm of average thickness) before and after treatment. Before treatment, angular particles of primary silicon can be observed reaching up to 60 p.m,

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as well as the aluminum-silicon eutectic; the surface of the part has a certain roughness. After treatment, the structure has fine angular particles of silicon, from 1 to 4 μm in size approximately, dispersed uniformly in the matrix of the aluminum alloy; the roughness of the wafer has been completely removed. 5

In general, the treatment can be applied to surface layers 0.125 to 3 mm thick, preferably approximately 1 mm, using a beam stop time of less than 1 s, preferably approximately 1 'S. Stopping time does not mean that the beam is immobilized, but corresponds to the speed of a beam which moves continuously and to the diameter of this beam which strikes the surface to be treated producing 5 to 100 kW-s / cm2 of surface treated. We adapt the energy supply to each metal and to the desired depth of fusion. A protective gas is provided, as described above with respect to the use of a high melting point coating.

It is possible to apply the refining of grains in a second step, after the first step consisting in forming a jacket as described above in relation to FIGS. 1 to 6. Re-scanning by a laser beam involves heating and cooling even faster than for the formation of the original jacket to obtain in the finished product a grain refinement by a factor at least equal to 10. This implies preferably a higher scanning speed and / or a lower power density in the second time compared to the first time, which is compensated by the better heat transmission through the metallurgical bond of the jacket to the substrate, compared to the interfacial heat transmission conditions which exist in the first instance.

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Claims (12)

632,790 CLAIMS 1. A method of forming an alloy jacket in a surface layer, characterized in that a predetermined area of a metal substrate is coated with a layer of metal, the thickness of the substrate being such that the substrate constitutes a cold source sufficient to ensure, at any point not directly heated, rapid resolidification of any part of the coating previously brought to its melting point, or liquidus, the latter being at least greater 200 C to that of the substrate, then sweep the said area with a laser beam at a speed and with a power density which, in conjunction with the thermal conductivity of the material chosen for the substrate, ensure rapid heating, implying the melting of the coating and of a predetermined thickness of the substrate corresponding to the part of the alloy jacket which it represents, in less than 2 s, and rapid cooling and resolidification of the molten coating so as to produce an alloy jacket, having a density of 99%, with the particles of the coating material in a matrix of a eutectic of coating materials and of substrate bonded by metallurgical diffusion to the substrate, said particles of the coating material being concentrated upwards within said jacket . 2. Method according to claim 1, characterized in that the conditions are adjusted so as to obtain a layer of substrate with refined grains near said jacket. 3. Method according to claim 1, characterized in that the substrate is a metal or an alloy whose melting point or the liquidus temperature is 400 to 800 ° C, and the coating is a metal or an alloy whose melting point or the liquidus temperature is more than 1000 ° C. 4. Method according to claim 3, characterized in that the substrate is aluminum or magnesium or an alloy of one of these two metals. 5. Method according to claim 3, characterized in that the coating material is silicon, iron, nickel, cobalt, molybdenum, tungsten, chromium, vanadium, zirconium, hafnium, tantalum, niobium, titanium, boron, beryllium, palladium, rhodium, rhenium, iridium, platinum, copper, gold, manganese or osmium, in elemental or alloyed form. 6. Method according to claim 1, characterized in that the coating material is silicon, iron, nickel, cobalt, molybdenum, tungsten, chromium, vanadium, zirconium, hafnium, tantalum, niobium, titanium, boron, beryllium, palladium, rhodium, rhenium, iridium, platinum, copper, gold, manganese or osmium, in elemental or alloyed form. 7. Method according to claim 6, characterized in that the substrate comprises aluminum and the coating comprises silicon. 8. Method according to claim 1, characterized in that the coating has a thickness of 0.25 to 2.5 mm. 9. Method according to claim 1, characterized in that the coating is in consolidated form. 10. Method according to claim 1, characterized in that the coating is in the form of a fluid powder. 11. Method according to one of claims 1 to 5, characterized in that one proceeds to a subsequent rapid re-scanning of said jacket with the laser beam to redesign and resolidify the jacket even faster, thereby reducing the size said particles by a factor at least equal to 10. 12. Method according to claim 11, characterized in that the rescan is carried out by means of a laser emitting a continuous wave and whose beam has a power of more than 20 kW / cm2.