FR2937572A1 - Element's e.g. cylinder head, sealing surface forming method for e.g. diesel engine, involves realizing fusion of metallic alloy on partial zone of sealing surface by heat source e.g. laser beam, to eliminate surface defects on zone - Google Patents

Abstract

The method involves forming a blank of a sealing surface (5) by milling, and forming a coating on the sealing surface by milling, where the sealing surface includes orifices for passage of fluid such as water and oil. Fusion of metallic alloy (7) on a partial zone of the sealing surface is realized by a heat source e.g. laser beam (12) such as carbon di-oxide beam, to eliminate surface defects on the zone, where the fusion of the metallic alloy is formed between formation of the blank and formation of coating. An independent claim is also included for a method for obtaining a cylinder head made of metallic alloy.

Description

METHOD FOR MAKING A SEALING SURFACE OF AN ENGINE ELEMENT, SUCH AS A CYLINDER HEAD

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing a sealing surface of a metal alloy motor element.

The invention relates more particularly to a method of producing a sealing surface of a cylinder head of an internal combustion engine. BACKGROUND Conventionally, an internal combustion engine is composed of a cylinder block, a cylinder head and a cylinder head gasket ensuring the seal between the cylinder block and the cylinder head. The cylinder block and the cylinder head respectively comprise sealing surfaces (also called joint plane), that is surfaces intended to be facing one another and to be in contact with each other. the cylinder head gasket. These sealing surfaces have traditionally a perfect flatness and a low roughness and are made by machining said surface from a cylinder head and cylinder blocks obtained by casting.

Today, breech crudes can be obtained by foundry using the lost pattern method (PMP) or "lost foam". Compared to conventional processes such as gravity casting in metal shells, this technique offers better cost efficiency, better quality while allowing complex, precise parts to be made and generating less pollution.

The main steps of molding a cylinder head by the PMP process consist successively in the manufacture of an exact replica of the expanded polystyrene yoke, an implementation of the replica in a sand mold, a casting of an alloy metal, usually an aluminum alloy, melted in the mold, which destroys the replica polystyrene, and after cooling of the metal alloy, a sand shrinkage and cleaning of the cylinder head obtained. However, during the casting of the metal alloy and the destruction by combustion or sublimation of the polystyrene replica, there is formation of gas which must be evacuated from the mold. It has been found that some of the gas is trapped in the aluminum alloy and form cracks and / or porosities. The machining of the sealing surface makes these cracks and / or porosities appear, which poses problems of sealing surface quality and good cooperation in sealing with the cylinder head gasket, when it is assembled between the cylinder head and the cylinder block.

The invention aims to overcome the disadvantage of the prior art by providing a method of producing a sealing surface of a motor element improving the surface quality of said sealing surface. The invention therefore relates to a method for producing a sealing surface of a metal alloy motor element, such as a cylinder head, comprising the steps of:

15 - Making a roughing of the sealing surface by milling, - Performing a finishing of the sealing surface by milling, characterized in that it comprises a step of performing a melting of the metal alloy at least on a partial area of the sealing surface by a heat source, to eliminate surface defects on said partial area. Advantageously, the melting step is performed between the roughing step and the finishing step of the milled sealing surface. Homogeneous zones of metal alloy are thus formed which, after finishing machining, will have a surface quality sufficient to allow sealing contact.

Advantageously, the melting step is performed following the roughing step and the finishing step of the sealing surface by milling. In this case, the thickness of metal alloy to melt is lower, which limits the power of the heat source.

In addition, the process may individually or in combination have the following characteristics:

The sealing surface having fluid passage orifices, advantageously, the melt zone comprises at least one peripheral zone surrounding the fluid passage orifices. This variant is guided by an optimization of the areas to be treated. Advantageously, the step of melting the metal alloy on the partial zone is carried out by a sweep of the heat source on said partial zone.

Preferably, the heat source is selected from a laser, a torch, an electron beam, an electric arc, a plasma torch, an electrical induction means comprising an induction source corresponding to the partial zone.

The invention also relates to a process for obtaining a metal alloy cylinder head comprising a lost-model casting step and the production of a sealing surface according to the invention.

The invention also relates to a cylinder head having at least a partial zone of its resurfaced sealing surface.

Finally, the invention also relates to an internal combustion engine comprising a cylinder head according to the invention.

Other features and advantages will appear on reading the following description of two particular embodiments of the method according to the invention, not limiting of the invention, with reference to the figures in which:

- Figure 1 is a schematic representation of an internal combustion engine comprising a cylinder head obtained by the method according to the invention. - Figure 2 is a schematic representation of a section of a cylinder head raw output foundry according to the so-called lost model method. - Figure 3 is a schematic representation of a breech section in a first step according to a first embodiment of the method of the invention. - Figure 4 is a schematic representation of a breech section in a second step according to a first embodiment of the method of the invention. - Figure 5 is a schematic front view of a yoke joint plane showing an area of the joint plane to be treated by local melting according to the method of the invention. - Figure 6 is a schematic representation of a breech section in a third step according to a first embodiment of the method of the invention. - Figure 7 is a schematic representation of a breech section during a local melting step at the joint plane of the cylinder head, according to a second embodiment of the method of the invention.

DETAILED DESCRIPTION FIG. 1 shows an internal combustion engine 1 comprising a cylinder head 2, a cylinder head gasket 3 and a cylinder block 4. The cylinder head gasket 3 is arranged tight between the cylinder head 3 and the cylinder block 4. The engine 1 can indifferently be a gasoline engine type or diesel type.

Figure 2 shows a schematic sectional view of a cylinder head 2 of metal alloy 7, such as an aluminum alloy, foundry blank output according to the lost pattern method (PMP) or "lost foam".

Conventionally, the main steps of molding a cylinder head by the PMP process consists successively in the manufacture of an exact replica of the expanded polystyrene yoke, an implementation of the replica in a sand mold, a casting of a metal alloy, usually an aluminum alloy, melted in the mold, which destroys the polystyrene replica, and after cooling of the metal alloy, sand shrinkage and cleaning of the cylinder head 2 obtained.

The cylinder head 2 comprises in particular a sealing surface 5, commonly known as the joint plane, intended to come opposite the cylinder head gasket 3 when they are assembled with the cylinder block 4. The cylinder head 2 may have withdrawals with respect to the joint plane 5 corresponding to combustion chambers 6.

During the combustion or sublimation destruction of the polystyrene replica, there is formation of gas that must escape from the mold. It has been found that part of the gas is trapped in the aluminum alloy of the cylinder head 2. The aluminum alloy 7 then has porosities of the cracks 8 and / or porosities 9, (symbolized in clear on the Figure 2). These porosities 8 and / or these cracks 9 are not necessarily visible on the surface of the cylinder head 5 at its raw casting output.

The joint plane 5 is traditionally subjected to grinding by passages of a milling cutter to obtain perfect flatness and low roughness.

Figures 3 to 6 illustrate and detail the steps of a first embodiment of the method according to the invention.

FIG. 3 shows the yoke 2 during a first step of the method of producing the joint plane 5 according to the invention. This first production step is a so-called milling step in roughing of the joint plane 5. Indeed, a milling cutter 10 sweeps the joint plane 5, in a direction given as an indication by the arrow 11 in FIG. during this first step to a removal of material of the order of a few millimeters, as an indication of the order of 3 millimeters, essentially to ensure the flatness of said joint plane 5.

However, as shown in FIG. 3, this first step of grinding the sealing surface 6 causes the cracks 8 and / or the porosities 9 to appear at the surface of said joint plane 5, which will pose problems of good cooperation. of the cylinder head gasket 3 in terms of sealing, when it will subsequently be assembled between the cylinder head 2 and the cylinder block 4.

FIG. 4 shows the yoke 2 during a second step of the method of producing the joint plane 5 according to the invention. According to the invention, this second production step is a local melting step of the metal alloy 7. During this second step, a heat source capable of providing a sufficient energy density to cause the local melting of a metal, such as a laser beam 12, of the CO 2 laser or YAG laser type focused substantially at the level of the joint plane 5, sweeps said joint plane 5, in a direction given as an indication by the arrow 13 in the figure 4, and conducts a local melting of the metal alloy 7 at the surface of the joint plane 5. This local fusion also reaches the metal alloy immediately below the surface of the joint plane. The heat source is then adjusted to effect local melting at a determined depth, preferably less than or equal to 1 mm. The liquid metal alloy thus drowns the surface defects that represent the cracks 8 and / or the porosities 9. After cooling, homogeneous zones 14 of metal alloy are finally obtained.

FIG. 5 is a diagrammatic front view of the joint plane 5 of the cylinder head 2. The cylinder head 2 comprises orifices for cooling water passages of the engine 1, orifices for the passage of oil 16 for lubricating the engine 1 as well as openings 17 corresponding to the combustion chambers 6 crossing the joint plane 5. The yoke 2 furthermore comprises bores 18 passing through the joint plane 5 for the passage of fastening means of the cylinder head 2 to the cylinder block 4.

Advantageously, to reduce the time required for the treatment of the joint plane 5 of the second melting step and avoid heating of the joint plane 5 which could be responsible for thermal deformation, the laser beam 12 scans a partial area 19 of the joint plane 5 and proceeds to the local melting of the metal alloy, to treat the surface defects of the joint plane 5 only on said partial area 19. In order to reduce the time required for the treatment of the joint plane 5, the sweeping the heat source constituted by the laser is effected over a determined width L, preferably between 3 and 8 mm. According to the focusing of the laser 12, the local melting of the metal alloy can be made on the surface of the joint plane, but the local melting can also be carried out under the surface of the joint plane 5, just like the techniques so-called transparency welding.

In practice, the laser beam 12 can be embedded on a digitally programmable multiaxis displacement unit, not shown. It is then possible to program a predetermined path of the partial zone 19 to be treated.

Advantageously, the partial zone 19 comprises at least one peripheral zone 20 surrounding the fluid passage orifices, that is to say in FIG. 5: the water cooling orifices 15 of the engine 1, the orifices of passage of oil 16 lubricating the engine 1 and the openings 17 corresponding to the combustion chambers 6 and may constitute a source of gas leakage of said combustion chambers.

The peripheral zones 20 may also be adjusted to coincide with the contact areas of the cylinder head gasket 3, when it will subsequently be assembled between the cylinder head 2 and the cylinder block 4. This ensures a leakproof insulation of the fluids passing through the water passage orifices 15, the oil passage holes 16 and the openings 17 corresponding to the combustion chambers 6.

Acceptable heat sources for performing the melting of the metal alloy at the joint plane 5 are not limited to the laser. Indeed, the laser is a particular source of heat among other heat sources capable of achieving a local melting of metal on the surface that are, for example, a torch, an electron beam, the arc welding means , a plasma torch. All these sources cause a local temperature rise and need to be moved to treat a specific surface.

Another heat source that is also acceptable, not shown, and does not require sweeping, is an electric induction heating means with induction sources disposed on the joint plane 5 and corresponding to the partial zone 19.

FIG. 6 shows the yoke 2 during a third step of producing the joint plane 5, according to the invention. This third production step is a so-called finishing milling step of the joint plane 5. In fact, during this third step, a finishing cutter 21 scans the joint plane 5, in a direction given as an indication by the arrow 22 in FIG. 6, and carries out a removal of a small thickness of material, indicatively less than one millimeter, for example of the order of 0.5 millimeter, to finalize the rectification of the joint plane 5 and to guarantee in particular its surface state and in particular its roughness.

In a second advantageous embodiment, the method for producing the sealing surface 5 of the yoke 2, the local melting step of the metal alloy is preferably carried out following the step of finishing the joint plane. 5 by milling.

For this second embodiment of the method according to the invention, the following succession of steps is then carried out:

- A first step of roughing the sealing surface 5 by milling, with a removal of material of the order of a few millimeters, essentially to ensure the flatness of said joint plane 5. - A second step of finishing the surface sealing 5 by milling, with a material removal of small material thickness, as an indication less than a millimeter, for example of the order of 0.5 millimeter, to finalize the grinding of the joint plane 5. - A third step, illustrated by FIG. 7, of melting of the metal alloy 7 at the joint plane 5, during which a heat source, such as a laser beam 12, of the CO 2 laser or YAG laser type, is focused substantially at the level of the joint plane 5, sweeps said joint plane 5, in a direction given as an indication by the arrow 23 in Figure 7, and proceeds to a local melting of the metal alloy 7.

This second embodiment has the additional advantage over the first embodiment of the method according to the invention of requiring a laser 12, of lesser power, since the thickness of the metal alloy 7 to be melted is, for second mode realization, low (as an indication, a few tenths of a millimeter) so as not to degrade the quality of the joint plane 5 obtained during the second finishing milling step.

Obviously, the application of the method of the invention to a cylinder head sealing surface for an engine is in no way limiting. Indeed, the method of the invention can be applied to any other part of an engine comprising a sealing surface, for example a cylinder block.

Claims (9)

Revendications1. A method of producing a sealing surface (5) of a metal alloy motor element (1) (7), such as a cylinder head (2), comprising the steps of: - producing a blank of the sealing surface (5) by milling, - performing a finishing of the sealing surface (5) by milling, characterized in that it comprises a step consisting in making a melting of the metal alloy (7) at least on a partial area (19) of the sealing surface (5) by a heat source, to eliminate surface defects on said partial area (19). 2. Method according to claim 1 characterized in that the melting step is performed between the roughing step and the finishing step of the sealing surface (5) by milling. 3. Method according to claim 1 characterized in that the melting step is performed following the roughing step and the finishing step of the sealing surface (5) by milling. 4. Method according to any one of the preceding claims, the sealing surface (5) having orifices (15, 16, 17) for fluid passage, characterized in that the fused partial zone (19) comprises at least one peripheral zone (20) surrounding the orifices (15, 16, 17) for fluid passage. 5. Method according to any one of the preceding claims, characterized in that the melting step of the metal alloy on the partial zone (19) is performed by a sweep of the heat source on said partial zone (19). ). 6. Method according to any one of the preceding claims, characterized in that the heat source is selected from a laser (12), a torch, an electron beam, an electric arc, a plasma torch, a means of electrical induction comprising an induction source corresponding to the partial area (19). 7. Process for obtaining a metal alloy yoke (2) (7) comprising a lost-model casting step and producing a sealing surface (5) according to one of the preceding claims. 8. Cylinder head having at least a partial area (19) of its sealing surface (5) remelted. 9. Engine comprising a cylinder head according to claim 8.