EP0995027B1 - Method for making a cylinder head with integrated valve seats and cylinder head with integrated valve seats - Google Patents

Description

The present invention relates generally to a method of manufacture of a cylinder head with integrated valve seats, in particular an aluminum alloy cylinder head for an internal combustion engine.

More particularly, the present invention relates to a method manufacturing valve seats integrated into a cylinder head by deposition by transferred arc plasma of a coating alloy layer on seat areas of a raw foundry cylinder head, in particular an aluminum alloy cylinder head.

Current current technology of manufacturing seats valve of a cylinder head of an engine is to insert by hooping in dwellings provided for this purpose in the cylinder head of the attached seats (inserts) in cast or sintered steel. This technique requires machining precise housing for receiving inserts and requires relatively large thicknesses of the wall between the combustion and the cooling circuit in the cylinder head. Moreover, the use of inserts for the realization of the valve seats leaves a layer of air between the insert and the cylinder head which constitutes a barrier thermal interfering with the thermal transfer between the combustion and the cylinder head.

The installation by sintering of an insert induces constraints in the breech, in particular in the area of the cross-seat trigger guard. The thermal cycles specific to the operation of the motors cause significant thermomechanical constraints on the insert-cylinder head couple. These thermomechanical constraints can lead to cracking of the cylinder head in the inter-seat area, trigger guard or at added seat loosening. In addition to the cylinder head, the valve is the element most requested in this configuration because it must evacuate a large amount of heat. Therefore, its manufacture requires advanced techniques such as the use of multimaterials and stelliting.

Therefore, it would be desirable to have a method of manufacturing valve seats from an engine cylinder head that remedies the disadvantages associated with valve seats reported.

It has been proposed in document JP-A-61-76742, to produce integrated valve seats. The approach chosen in this document is laser beam deposition of layers of alloy coatings specific to breech seat areas.

Laser beam deposition is advantageous in that it allows fast cooling rates and energy management method of manufacturing valve seats. This process allows to obtain deposits with reduced dilution and having a typical rapid cooling microstructure.

More particularly, the document JP-A-61-76742 describes a process for manufacturing valve seats integrated in a cylinder head light alloy in which the cylinder head seating area is reinforced with ceramic fibers during the casting of the cylinder head and which consists in forming a layer of an anti-wear material by means of a laser beam.

In practice, the coating material is deposited in the form of a paste on the breech seat areas, then melted using of a laser beam and quickly cooled in air.

The coating materials are very specific alloys, % in weight Exhaust valve seats Intake valve seats Co Complement 10 - - - Cr 10 - - - - W 5 - - - - MB 1 8 5 6 5 V 0.5 - - - - VS 1.5 1 1 0.8 - Fe - Complement Complement - - Or - 2 2 30 - Cu - - - Complement 4.5 al - - - - Complement Yes - - - - 17 having the following compositions:

The process of document JP-A-61-74742 presents several disadvantages.

First, the use of fibrous reinforcement in the alloy of the cylinder head in the seat areas considerably complicates the process. Indeed, it is necessary to introduce into the casting a fibrous preform with the ensuing wettability problems. On the other hand, the integrated seat with a footprint equivalent to the seats reported. Using a laser beam as an energy source requires that the surface of the seat area on which the deposit is homogeneous, that is to say without surface irregularities can scatter the beam randomly to obtain a uniform heating throughout the seating area. A stage of polishing the breech seat area is therefore necessary. Finally, the diameter of the molten bath created by the laser beam is incompatible with a high yield because all the powder in outside the bath does not participate in the formation of the layer of coating.

The present invention therefore relates to a manufacturing process a cylinder head with integrated valve seats which remedies disadvantages of the prior art, and in particular which does not require the use of fibrous reinforcement in the seat areas of the cylinder head.

The present invention also relates to a method of manufacture of a cylinder head with integrated valve seats which does not require not a machining, and in particular a polishing of the seat areas of the cylinder head.

The present invention also relates to a manufacturing process a cylinder head with integrated valve seats which remedies disadvantages of laser beam deposition.

• l'obtention d'une culasse en alliage léger brute de fonderie comportant des zones de siège de soupape;
• le dépôt par plasma à arc transféré sur les zones de siège d'une couche de revêtement d'un alliage ayant la composition suivante, en pourcent en poids : Ni 13 - 20 Mo 2 - 8 Co 0 - 10 Fe 2 - 8 Si 2 - 4 B 1 - 3 Cu Complément ; et
• l'usinage de la couche de revêtement pour obtenir la géométrie et l'état de surface voulus pour les sièges de soupape intégrés.

The above objectives are achieved according to the invention by a method of manufacturing a light alloy cylinder head, preferably of aluminum alloy, comprising integrated valve seats, which comprises:

• obtaining a cylinder head made of foundry crude light alloy comprising valve seat zones;
• the deposition by arc plasma transferred to the seat areas of a coating layer of an alloy having the following composition, in percent by weight: Or 13 - 20 MB 2 - 8 Co 0 - 10 Fe 2 - 8 Yes 2 - 4 B 1 - 3 Cu Complement; and
• machining of the covering layer to obtain the desired geometry and surface finish for the integrated valve seats.

A preferred alloy according to the invention is the alloy having the following composition, in percent by weight: Or 18 MB 6 Co 6 Fe 6 Yes 3 B 1 Cu Complement.

The method of the invention can also comprise, beforehand during the deposition step of the covering layer forming the seat of valve, cleaning the cylinder head seat areas by means of a stripper, for example a stripper for brazing on aluminum such as the Castolin® C 190 stripper in the case of an alloy cylinder head aluminum. This stripping step improves the bond metallurgical between the coating layer and the seat areas of the breech and allows the elimination of impurities such as oxides and residual fats.

The deposition of a coating layer by plasma spraying at transferred arc is a coating technique known in itself.

Briefly, a plasma torch with transferred arc is used, by example a Castolin® torch type GAP-E52.

The cladding gas and the carrier gas are generally helium, however, the plasma gas is generally argon.

The powder having the desired composition for the coating is injected by the torch at the foot of the arch.

The deposit cycle has three phases. A priming phase of the arc, a phase of depositing a coating layer on the area of the seat, and an arc extinction phase with anti-crater effect. The duration of the deposition cycle will obviously depend on the thickness desired for deposition, powder composition and conditions obtaining plasma. In general, the complete cycle lasts around 20 seconds to obtain a coating layer having a thickness from 0.5 to 1.2 mm.

During the priming phase after the opening of the gases, we proceed when the pilot arc strikes between the cathode and the nozzle of the torch, then to that of the main arc between the cathode and the cylinder head. We inject then the powder of the coating alloy and the displacement is initiated of the torch on the seat area to be coated with an oscillating movement radial of it.

The deposit phase mainly consists in continuing the displacement of the torch on the seat area to be covered while keeping the conditions established in the priming phase until the complete deposition of the coating layer. During this phase, we apply a decreasing arc intensity profile along this phase.

The last phase of the cycle is an extinction phase in which we proceed to the fading of the arc, then we cut the arrival of alloy powder and the movement of the torch is stopped. Finally, the gases are cut last. The purpose of this extinction phase is avoid the formation of a crater in the coating layer filed.

During this stage of deposition by transferred arc plasma, the alloy powder injected at the foot of the arch forms a molten bath on the surface of the breech seat area. Due to conductivity high thermal of the material constituting the cylinder head, for example a light alloy, in particular an aluminum alloy such as the AS alloy 5U3, there is rapid cooling of the entire layer of coating / yoke. This gives a very fine microstructure for the coating layer, which promotes mechanical strength and chemical layer coating.

There is shown diagrammatically in FIG. 1, before machining, a coating layer deposited on a breech seat area by the method of the invention.

As seen in Figure 1, there is an interface 3 between the coating layer 2 and the cylinder head 1 which constitutes a connection metallurgical between the alloy of the coating layer 2 and the alloy of the cylinder head 1. This inferface, which consists of a layer of diffusion of the alloy of the coating 2 into that of the cylinder head 1, guarantees the holding of the covering layer forming the seat on the cylinder head 1, in particular by the control of intermetallic compounds (nature, volume and distribution). In general, this interface will have a thickness of the order of 100 μm and the dilution rate of the alloy of the coating layer in the alloy of the cylinder head in this interface is maintained at less than 10% and even less than 5% by volume.

The coating layers according to the invention have a special composite microstructure developed in situ when deposited on the breech. These layers consist of a matrix 5 consisting of a solid solution whose exact composition depends on the constituents of the coating in which solid particles are dispersed 6.

As shown in Figure 1, the plasma transfer arc deposit generates in the alloy of the cylinder head 1 a thermally affected area 4 of a depth of about 0.5 to 1 mm in which the microstructure of the alloy of the cylinder head is refined with respect to the rest of the cylinder head 1. This is due to the generally high thermal conductivity of the cylinder head alloys, in particular of the light alloys and more particularly of the aluminum alloys. Thus, for the aluminum alloy AS5U3, a hardness HV _0.5 of 120 to 150 was measured in the thermally affected zone, while the parts which are not thermally affected by the process of the invention have a hardness HV _0.5 of around 80.

The coatings forming the valve seats according to the invention generally have a thickness of 0.5 to 1.2 mm before machining, which allows them to be self-supporting with respect to the cylinder head in order to withstand mechanical stresses. They have very high mechanical and thermal characteristics, such as a hardness HV _0.5 ranging from 200 to 500, a thermal conductivity greater than 30 W / mK and a coefficient of thermal expansion of approximately 18.10 ^-6 K ^-1 to a temperature of 400 ° C to 600 ° C (which makes them compatible with cylinder head alloys, in particular aluminum alloys such as the AS5U3 alloy).

In addition, they have a high resistance to erosion wear, abrasion and adhesion, chemical and thermal corrosion and high thermal stability, in particular against alloys aluminum.

As previously indicated, the coating layer is machined to obtain the desired geometry and surface finish for the seat of valve. This machining step can be done during machining of the guide valve housing or valve guide housing.

The process of the invention has many advantages by compared to the prior art.

It removes the use of inserts and removes the machining operations of the seat and hooping areas of the cylinder head.

It reduces the size of the cylinder head.

Thus, it is possible to redefine the foundry mold to remove material from the seat areas. Decreasing the size of the seat, we can reduce, for equal power, the size of the engine or increase its power for the same size in increasing the useful diameter of the seats. We can still reduce the wall thickness of the combustion chamber / circuit cooling, which will promote heat exchange between the combustion chamber and the cooling circuit. Increasing heat transfer to the cylinder head, the temperature is lowered overall of the valve as well as the thermal gradients usually encountered between the litter and the stem. This homogenization of the overall temperature of the chamber with the removing hot spots reduces consumption by engine fuel, especially at high revs. Reduction thermomechanical stresses on the valve can allow a simplification of the machining thereof.

The method of the invention also provides a strengthening of the cross-seat trigger guard area by reducing constraints thermomechanical compared to those induced by hooping and difference in coefficient of expansion between the insert and the cylinder head. he would also be possible to remove the reinforcement insert from the trigger guard.

Finally, the metallurgical bond and the materials used for realize the integrated seats are compatible with a motorization running on liquid petroleum gas (LPG).

The present invention also relates to a cylinder head, in particular an aluminum alloy cylinder head, comprising seats of integrated valves consisting of a coating layer of a alloy having the compositions indicated above in the context of the manufacturing process.

For example, the deposition of a layer of coating of the Ni18-Mo6-Co6-Fe6-Si3-B1-Cu alloy on areas AS5U3 aluminum alloy cylinder head seat raw foundry.

The seat areas can be initially stripped with a solution of an aluminum stripper (Castolin® C 190) applied on the seat areas.

Phase 1 - Amorçage et transfert d'arc.

Gaz plasmagène : Argon 4 à 6 l/minute

Gaz de gainage : Hélium 20 à 40 l/minute

Gaz porteur : Hélium 6 à 10 l/minute.

The alloy coating layer is then deposited on the seat areas by plasma transfer arc projection with a Castolin® torch type GAP-E52, under the following conditions:

Phase 1 - Arc initiation and transfer.

Plasma gas: Argon 4 to 6 l / minute

Cladding gas: Helium 20 to 40 l / minute

Carrier gas: Helium 6 to 10 l / minute.

After opening the throttles, the pilot arc is started (cathode / nozzle) then transfer to establish the main arc (Cathode / yoke). The intensity of the main arc is 70 Amps about when it started. The alloy powder is injected and the moving the torch on the workpiece with an oscillating movement radial of the torch.

In this example, the bolt is fixed and the torch is mounted on a 5-axis robot. The torch follows a circular path conforms to the seat area associated with an oscillating movement perpendicular to its main displacement. Finally, the torch turns on itself in order to keep the configuration of the injector powder versus displacement. Travel speed torch circular is between 200 and 450 mm / minute, however, the oscillation takes place at a frequency of 2 to 3 Hz on a width of about 3 mm. Alternatively, a configuration in which the cylinder head is rotated (rotation by relative to the axis of the seat) and a torch animated only by oscillating movements.

Phase 2 - Main deposit cycle.

The coating layer is deposited while retaining the kinematic parameters of phase 1. However, we decrease, throughout this phase, the intensity of the main arc, for example from 70 to 60 Amps, in order to maintain identical conditions on the entire perimeter of the seat.

The duration of this filing phase is generally of the order of 15 to 20 seconds.

Phase 3 - fainting of the arc.

We proceed to the fading of the arc, we cut the arrival of the alloy powder, and we stop the movement. Finally, we cut the finish gases.

During processing, the cylinder head is at room temperature. The aluminum temperature rise is localized to a nearby area from the surface (under the arch, melting depth less than 1 mm), because the thermal conductivity and the mass of the cylinder head are high.

The valve seats are then machined.

This step is already part of the machining range of large displacement where a perfect alignment between the seat is sought and the valve guide. The cutting conditions are quite because the coating material has very good machinability.

The seat obtained has a particular microstructure which gives its mechanical, thermal and chemical resistance properties. The dense and porous structure of the coating allows obtaining after machining a seat with the required geometry and surface condition. The metallurgical connection between the covering bead and the cylinder head participates in heat transfer to the cylinder head. Stability thermodynamics of the coating cord-aluminum couple guarantees resistance to thermomechanical fatigue.

Claims (12)

1. Method of manufacture of a cylinder head in light alloy comprising integrated valve seats, characterised in that it comprises

   obtaining a rough cast cylinder head in light alloy comprising valve seat areas;

   deposition onto the seat areas by transferred plasma arc of a coating layer of an alloy having the following composition, in percentages by weight: Ni 13-20 Mo 2-8 Co 0-10 Fe 2-8 Si 2-4 B 1-3 Cu complement; and

   machining of the coating layer in order to obtain the desired geometry and surface state for the integrated valve seats.
2. Method according to claim 1, characterised in that the alloy used for the coating layer has the following composition, in percentages by weight: Ni 18 Mo 6 Co 6 Fe 6 Si 3 B 1 Cu complement.
3. Method according to any one of claims 1 or 2, characterised in that it further comprises, prior to the step of deposition of the coating layer, a step of cleaning of the seat areas.
4. Method according to any one of claims 1 to 3, characterised in that the step of deposition by transferred plasma arc comprises a phase of igniting the arc, a deposition phase and a phase of extinguishing the arc.
5. Method according to claim 4, characterised in that the deposition phase is implemented using an arc with decaying intensity.
6. Method according to claim 4 or 5, characterised in that the phase of extinguishing the arc involves proceeding, to obtain an anti-cratering effect, by fading the arc, then cutting off the supply of coating alloy powder, then stopping relative movement of a plasma deposition torch with respect to the valve seat areas, and lastly the cutting off the gases supplied for the plasma deposition.
7. Method according to any one of the preceding claims, characterised in that the coating layer has a thickness of 0.5 to 1.2 mm.
8. Method according to any one of the preceding claims, characterised in that the cylinder head is in aluminium alloy.
9. Cylinder head in light alloy with integrated valve seats, characterised in that the integrated valve seats are constituted by a coating layer of an alloy composed, in percentages by weight, of Ni 13-20 Mo 2 - 8 Co 0-10 Fe 2-8 Si 2 - 4 B 1 -3 Cu complement.
10. Cylinder head in light alloy according to claim 9, characterised in that the coating layer alloy has the following composition, in percentage by weight: Ni 18 Mo 6 Co 6 Fe 6 Si 3 B 1 Cu complement.
11. Cylinder head in light alloy according to claim 10, characterised in that the coating layer alloy has a hardness of HV_0.5 between 200 to 500, thermal conductivity greater than 30 Wm K, and a thermal expansion coefficient at a temperature of 400 to 600°C of 18.10^-6 K^-1.
12. Cylinder head in light alloy according to one of claims 9 to 11, characterised in that the light alloy is an aluminium alloy.

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