MXPA99010980A - Sintered mechanical part with abrasionproof surface and method for producing same - Google Patents

Abstract

The invention concerns a mechanical part with abrasionproof surface characterised in that it comprises a sintered metallic body obtained from metallic powders and a laser-deposited cermet coating. The coating has a certain thickness whereof a portion is metallurgically bound with the metallic body. The laser deposit enables the sintered part to be surface-melted under the effect of the laser beam. The surface of the sintered part to be coated is therefore fused over a thickness ranging between 10&mgr;m and 1 mm, which enables the surface pores to be closed, as is characteristic of sintered parts, thereby increasing its resistance to shocks. Moreover, the small surface coated at a given moment by the laser enables the self-hardening of the exposed part, following the beam displacement, by the heat-sink effect of the surrounding metallic volume. The resulting coating also has very low porosity owing to the complete fusion of the powders by laser.

Description

MEC NICA SINTERIZED PIECE WITH ANTIABRASION SURFACE AND METHOD FOR ITS MANUFACTURE FIELD OF THE INVENTION The present invention relates to the treatment of an abrasion-resistant surface by laser of a mechanical part. More particularly, the present invention relates to the surface treatment of a sintered mechanical part obtained by powder metallurgy by laser deposition of a cermet coating. Being the cermet of a composite material formed of ceramic products wrapped in a metallic binder. The present invention also concerns a manufacturing method1 of one of these mechanical parts. DESCRIPTION OF THE PREVIOUS TECHNIQUE The coatings composed of spherical tungsten carbides in a nickel-chromium matrix and deposited by laser on iron ingots or of traditional steel and therefore, not sintered, already exist in the prior art. An example of this type of coating is described by way of example in Canadian patent application No. 2,126,517. The deposit by laser is a technique of coating that allows to deposit thick layers of very hard material on the surface of a metal piece. A continuous C02 laser emits an infrared beam whose energy is used to superficially melt the base metal to be coated as well as the filler metal carried in the form of fine powder. A coaxial tube crossed at its center by a laser beam allows the entry and injection of the powders that form the coating, the latter looks like a weld bead. To date, this type of laser deposit is used only to coat traditional non-sintered metal parts, used more particularly in very abrasive conditions. It is well known in the prior art that the mechanical parts manufactured by powder metallurgy do not possess the physical characteristics to work with tension, abrasion or friction and this is due to the presence of a large number of pores on the surface of these parts. sintered, thus decreasing the crack initiation period compared to a forged or machined piece. Thus, the porosity of the surface of the parts manufactured by the powder metallurgy prevents the production of mechanical parts, which must withstand impacts and / or abrasive wear due to the short period of crack initiation. SUMMARY OF THE INVENTION An object of the present invention is to propose a sintered mechanical part obtained by powder metallurgy and having a very high resistance to impact, abrasion and friction, as well as a good mechanical strength of the body of the piece . More particularly, the present invention considers a mechanical part with an anti-abrasion surface characterized in that it comprises: a sintered metal body obtained by powder metallurgy; and a cermet coating (ceramic-metal) covering the metallic body and having an external surface constituting the anti-abrasion surface, said coating having a certain thickness whose portion is metallurgically joined to the metallic body. The technician will understand that by "metallurgically bonded to the metal body", means that the coating is fused to the surface of the sintered part, the microstructure at the base of the coating is intimately bound to the microstructure of the body of the part. The mechanical part can comprise any part traditionally used in highly abrasive or high tension conditions, for example, the deepening pellets mounted on the arms of the debarker. The present invention thus considers a method for manufacturing the mechanical part described hereinabove. More particularly, the method is characterized in that it comprises the following steps: (a) providing a sintered mechanical part obtained by powder metallurgy; and (b) depositing by a laser method a cermet coating on an external surface of said mechanical part. The laser deposition process preferably comprises the following steps: directing a laser beam on the external surface of the piece, the laser beam releasing a certain temperature and melting a certain thickness of said outer surface; injecting into the laser beam a constant flow of a mixture of ceramic powders and metal powders intended to form the cermet coating, the ceramic powders having a melting temperature higher than the temperature of the laser beam and the metal powders having a melting temperature less elevated than the temperature of the laser beam, so that the laser melts the metallic powders of the powder mixture that is deposited on the external surface of the piece; and moving the laser beam relatively in the mechanical part to thereby sweep the outer surface and form the cermet coating. The mixture of powders can be injected into the laser beam by means of a coaxial tube traversed at its center by the laser beam, the tube allowing the entry of the powder mixture and its injection into the laser beam. The laser beam is preferably fixed and the mechanical part is installed on a moving table that can be moved relative to said laser beam. This coating according to the present invention being deposited by laser allows to melt the sintered part to be coated under the effect of the laser beam superficially. The surface of the sintered part to be coated is fused to a thickness that can leave from 10 μm to 1 mm, which allows the locking of the surface pores, typical of the sintered parts and, consequently, the increase of its impact resistance . In addition, the small area covered at a given instant by the laser allows the auto-tempering of the exposed area, followed by the displacement of the beam, by the effect of heat wells of the surrounding metallic volume. The coating obtained according to the present invention thus has very little porosity because of the complete melting of the powders of the sintered part by the laser. Other objects, features and advantages of the present invention will arise from the following description of a preferred embodiment, made in relation to the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a dehuller arm on which a sintered barking tablet having an anti-abrasion coating according to a preferred embodiment of the present invention is mounted. Figure 2 schematically represents in cross-section a portion of the working surface of the debarking tablet of Figure 1.

Figure 3 represents schematically and in part a laser refill device for employing the present invention. Figure 4 is a scanning electron microscope (SEM) photograph showing the microstructure of the junction formed between a coating obtained by projection of plasma onto a base metal; and Figure 5 is a scanning electron microscope (SEM) photograph showing the microstructure of the bond formed between a coating according to the present invention obtained by laser deposition on a base metal obtained by powder metallurgy. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Figure 1 shows a debarking arm (2) for dehuller with rotating ring, mounted on this arm is a debarking tablet (4) manufactured according to the present invention. This arm (2) comprises a first end (6) adapted to be attached to the rotating ring of the debarker. The arm (2) comprises a second end (8) that constitutes the working surface of the arm (2) that serves to remove the bark of a tree as it moves longitudinally into the interior of the ring. The pill (4) is operatively fixed to this second end. This second end (8) is the part of the arm that is used to debark the trees and must be able to withstand very abrasive conditions. A debarking tablet according to the present invention can thus be used advantageously, it has a very hard cermet coating able to withstand these working conditions. It should be well understood that, although the preferred embodiment illustrated here represents a barker chip, this is no more than an example of a mechanical part according to the present invention among many others. In fact, any mechanical part traditionally used in highly abrasive or high tension conditions can be manufactured according to the present invention. The following mechanical parts are other examples of parts that can be manufactured according to the present invention: In the mining industry: shredders, bales, crushers, conveyors, and so on. In the ceramic and related industry: scrapers, blades, molds, conveyor screws, valve seats, etc. In the paper industry: plates of refiners, plates of bottoms of pulpadoras, pallets, et cetera. In the metallurgical industry: cylinders, beams, rollers, etc. In the plastics industry: the tips of the threads of extrusion and injection screws; and In the agri-food industry: rollers, rows, deflectors, screws.

As illustrated in Figure 2, the pellet of the debarker (4) with anti-abrasion surface or any other mechanical part according to the present invention, comprises a sintered metal body (10) obtained by the powder metallurgy and a coating (12) of cermet that covers the metallic body (10). The external surface (14) of the coating constitutes the anti-abrasion surface of the piece. The coating (12) has a certain thickness, a portion of which is metallurgically attached to the metal body (10), as can be seen in Figure 5. This portion preferably has 10 μm to 1 mm. The cermet coating (12) is preferably based on tungsten carbides (16), titanium carbides or boron carbides, spheroidal in a metal matrix (18). The metallic matrix (18) is preferably based on at least one of the metals chosen from the group consisting of nickel, chromium and cobalt, more particularly comprising nickel, chromium and cobalt. Advantageously, Ni-9% Cr-Co is used. The coating (12) preferably comprises 65% by weight of tungsten carbides (16) and is substantially free of porosities. The coating (12) for sintered part according to the present invention is obtained by laser deposit. As illustrated in Figure 3, a coaxial tube (20), which is mounted at the outlet of a laser beam C02 of 6 W, injects in the laser beam (22) a constant flow of powders (24) of the material to be deposited. The laser beam (22) fuses the powders (24) and welds them to the base metal (4) in the form of a cord that constitutes the coating (12). By sweeping the surface of the piece (4), a coating is formed at the desired sites. The laser coating (12) is composed of particles of tungsten carbide (16) of very high hardness in a chromium-nickel matrix (18) and has an excellent resistance to abrasion and erosion wear, as well as a very good resistance to the corrosion. Figure 5 shows the microstructure of a coating (26) comprising the carbides (28) obtained by the plasma projection while Figure 5 shows the microstructure of a laser coating (12) on a sintered part (4). As can be seen, the tungsten carbide particles (16) found in the coating (12) by laser deposit are spheroidal, while the carbides (28) obtained by the plasma spray coating (26) rather they tend to be angular in shape. It is also noted that there was fusion of the surface of the sintered piece (4) with the metal part (18) of the coating (12). This fusion allowed to close the pores present in the surface of the sintered metal (4). The laser (22) that is fixed, a table (30) of numerical command with four axes on which the pieces (4) to be coated rest allows to make the deposits precise and uniform by means of relative displacements of the pieces (4) with respect to the laser beam (22). It is possible to make coatings of less than 1 millimeter or more than 10 millimeters thick, by successive passes of the laser (22). The materials that go into the manufacture of coatings by laser deposit are generally mixtures of powders of tungsten carbide, titanium carbide or boron carbide of high purity and alloys of high hardness, depending on the applications, with metallic powders based of nickel, chromium or cobalt. After the deposition process, the metal powders are fused by the laser (22) when the tungsten carbide powders remain solid, thus preserving their very high hardness. These cermet-like materials give the coatings (12) excellent wear resistance due to abrasion and erosion, as well as a very good resistance to corrosion. Many characteristics of the laser deposit are made in such a way that the coatings (12) produced by this technique possess exceptional properties. First, the deposits made by laser are metallurgically bonded to the base metal (10) and are perfectly dense (absence of porosity). The adhesion obtained between the part (10) and the lining (12) is thus excellent. On the contrary, the coatings produced by heat spraying have a high porosity and a special preparation of the treated surfaces to ensure a good adhesion. A more precise control of the energy input on the base metal makes it possible to obtain dilutions of the base metal in the tank of less than 1% and to minimize, even eliminate, any deformation. In addition, the deposit by laser allows to produce fine metallurgical microstructures thanks to the rapidity of re-cooling in the course of the treatment, which thus allows to increase the hardness of the metallic matrix (16) (2400 to 3600 HV). Finally, the use of programs and CNC controllers lead to perfectly reproducible deposits over time and whose final thickness is perfectly controlled. Large series of pieces can be treated in this way. INDUSTRIAL APPLICATIONS OF THE PRESENT INVENTION A mechanical part manufactured by means of powder metallurgy but which does not comprise a coating according to the present invention possesses the following physical and economic characteristics: Presence of a large number of pores on the surface; little resistance to impacts; mechanical capacity generally less than a forged piece; very low density; noise absorption; possibility of using non-miscible alloys by liquid route; possibility of using self-tempered alloys; Little production cost for a series of parts. These characteristics define the penetration power of the market of the technique of production of pieces by powder metallurgy, but it also shows its limits. The porosity of the surface prevents the production of mechanical parts that must withstand impacts and / or abrasive wear due to the short duration of the crack initiation compared to a forged or machined part. This is the reason why the mechanical parts obtained by powder metallurgy are not traditionally used in very abrasive or high tension conditions. It is thus that the mechanical parts according to the present invention, more particularly the coating of WC by laser deposit, raise a revolutionary concept for this sector of the industry. By way of indication, the laser deposit of a coating formed with 65% of spherical WC particles taken in a Ni-9% Cr-Co matrix, allows the following surface improvements of the parts made by sintering of metallic powders: The surface of the piece is fused on a thickness that has from 10 μm to 1 mm. This allows the closing of the pores of the surface of the piece and, consequently, the increase of the resistance to the impacts; the little surface covered at a given moment by the laser beam allows the auto-tempering of the exposed area, followed by the displacement of the beam, by I effect of heat wells of surrounding metallic volume; a very pocoi; porosity of the coating, less than 1%, because of T of the complete fusion of Ni-9% Cr powders by laser.

This does not allow for other procedures, such as the plasma or acetylene torch, due to the very large flow of heat projected on the piece when the temperature necessary for the fusion of the pieces is used. projected powders. The tempering of the piece is then destroyed; and excellent adherence of the coating on the part due to the welding zone. In addition, the coating obtained according to the present invention, which comprises spherical carbides, has the advantages I, as follows: Very large resistance to impacts due to the lesser propensity to initiate cracks in comparison. with a carbide with angular geometry; friction wear due to the lower coefficient of friction I of the spherical carbides comparatively to carbides with angular geometry; and pure and simple limitation of wear I of the surface of the parts due to the hardness of the carbides. In addition, a Ni-9% Cr matrix, such as that described hereinabove, exhibits excellent tenacity, superior to steel. In summary, a sintered part having a coating according to the present invention has the following advantages: Excellent adhesion of the coating due to metallurgical bonds between the coating and the base metal; contrary to the techniques of deposits by plasma projection, absence of porosity and fissures that results in a good resistance to impacts; thickness from 0.5 mm up to several millimeters (extra parts possible); and the carbide particles remain solid during the deposition process, thus preserving their high hardness. The applications of the present invention can be found in a multitude of fields. More particularly, the debarking chips mounted on the arm of the debarker can advantageously be manufactured according to the present invention as well as each of the pieces mentioned hereinabove.

Claims (19)

1. NOVELTY OF THE INVENTION Having described the foregoing invention, it is considered as a novelty and, therefore, claimed as property contained in the following CLAIMS 1. A mechanical part with anti-abrasion surface characterized in that it comprises: a sintered metal body obtained by powder metallurgy; and a cermet lining which covers the metal body and which has an external surface constituting the anti-abrasion surface, said lining has a certain thickness whose portion is metallurgically joined to the metal body.
2. 2. The mechanical part with anti-abrasion surface, according to claim 1, characterized in that the cermet coating is a coating obtained by laser deposit on the metal body.
3. 3. The mechanical part with anti-abrasion surface, according to claim 1, characterized in that the cermet coating is based on carbides of the group consisting of tungsten carbides, titanium carbides and carbides of boron in a metallic matrix.
4. 4. The mechanical part with anti-abrasion surface, in accordance with claim 3, characterized in that the carbides are spherical tungsten carbides.
5. 5. The mechanical part with anti-abrasion surface, in accordance with claim 1, characterized in that said portion of the coating metallurgically bonded to the metal body has a thickness of 10 μm to 1 mm.
6. 6. The mechanical part with anti-abrasion surface, in accordance with claim 3, characterized in that the metal matrix is based on at least one of the metals chosen from the group consisting of nickel, chromium and cobalt.
7. 7. The mechanical part with an anti-abrasion surface, in accordance with claim 3, characterized in that the metallic matrix comprises nickel, chromium and cobalt.
8. 8. The mechanical part with anti-abrasion surface, in accordance with claim 3, characterized in that the metal matrix is a Ni-9% Cr-Co matrix.
9. 9. The mechanical part with anti-abrasion surface, in accordance with claim 3, characterized in that the coating comprises 65% by weight of tungsten carbides.
10. 10. The mechanical part with anti-abrasion surface, in accordance with claim 1, characterized in that the coating is substantially free of porosity.
11. 11. A method of manufacturing a sintered mechanical part with an anti-abrasion surface, the method being characterized in that it comprises the following steps: (a) providing a sintered mechanical part obtained by powder metallurgy; and (b) depositing by a laser method a cermet coating on an external surface of said mechanical part.
12. 12. The method according to claim 11, characterized in that the laser process comprises the following steps: direct a laser beam on the external surface of the piece, the laser beam releases a certain temperature and melts a certain thickness of said external surface; injecting into the laser beam a constant flow of a mixture of ceramic powders and metal powders intended to form the cermet coating, the ceramic powders having a melting temperature higher than the temperature of the laser beam and the metal powders having a melting temperature less elevated than the temperature of the laser beam, so that the laser melts the metallic powders of the powder mixture that is deposited on the external surface of the piece; and moving the laser beam relatively in the mechanical part to thereby sweep the outer surface of the metallic body and form the cermet coating.
13. 13. The procedure in accordance with the claim in claim 12, characterized in that: the mixture of powders is injected into the laser beam by means of a coaxial tube traversed at its center by the laser beam, the tube allows the powder mixture to enter and its injection into the laser beam.
14. 14. The method according to claim 13, characterized in that: the laser beam is fixed and the mechanical part is installed on a moving table that can be moved relative to said laser beam.
15. 15. The method according to claim as claimed in any of claims 10 to 14, characterized in that the cermet coating comprises tungsten carbides in a metal matrix.
16. The method according to claim 12, characterized in that the ceramic powder is a powder of tungsten carbides and that the metal powder is a powder comprising at least one of the elements of the group constituted by nickel, chromium and cobalt.
17. 17. The method according to claim 16, characterized in that the metallic powder is a powder of Ni-9% Cr-Co.
18. 18. A debarking tablet comprising a metallic body having a lower face adapted to be mounted on the end of a barking arm and an anti-abrasion working surface, the tablet being characterized in that: the metallic body is a sintered metal body obtained by powder metallurgy; and the anti-abrasion working surface consists of a cermet lining that covers the metal body, the lining has a certain thickness in a portion that is metallurgically bonded to the metal body.
19. 19. A bar removing tablet according to claim 18, characterized in that the cermet coating is a coating obtained by laser deposit on the metal body.