EP0995027A1

EP0995027A1 - Method for making a cylinder head with integrated valve seats and cylinder head with integrated valve seats

Info

Publication number: EP0995027A1
Application number: EP98930847A
Authority: EP
Inventors: Adel Ben Abdallah; Philippe Cachot
Original assignee: Renault SAS
Current assignee: Renault SAS
Priority date: 1997-07-10
Filing date: 1998-06-12
Publication date: 2000-04-26
Anticipated expiration: 2018-06-12
Also published as: ES2194329T3; FR2765915B1; EP0995027B1; DE69812101T2; FR2765915A1; WO1999002839A1; DE69812101D1

Abstract

The invention concerns a method comprising the following steps: obtaining a cylinder head made of a light unrefined foundry alloy comprising valve seat zones; carrying out a transferred plasma arc deposition on the seat zones of a coating layer of an alloy with the following composition, in w.t. %: Ni 13-20, Mo 2-8, Co 0-10, Fe 2-8, Si 2-4, B 1-3, Cu the remainder; machining the coat to obtain the desired shape and surface texture for the integrated valve seats: The invention is applicable to internal combustion engine aluminium cylinder heads.

Description

Method for manufacturing a cylinder head with integrated valve seats and cylinder head with integrated valve seats

The present invention relates generally to a method of manufacturing 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 for manufacturing valve seats integrated in a cylinder head by plasma arc deposition transferred from a coating alloy layer onto seat areas of a raw foundry cylinder head, in particular an aluminum alloy cylinder head.

Current current technology for manufacturing the valve seats of a cylinder head of an engine consists in inserting by hooping into housings provided for this purpose in the cylinder head of the insert seats (inserts) of cast or sintered steel. This technique requires precise machining of the housing for receiving the inserts and requires relatively large thicknesses of the wall between the combustion chamber and the cooling circuit in the cylinder head. In addition, the use of inserts for the realization of the valve seats leaves an air layer between the insert and the cylinder head which constitutes a thermal barrier hindering the thermal transfer between the combustion chamber and the cylinder head. The installation by sintering of an insert induces stresses in the cylinder head, in particular in the area of the trigger guard. The thermal cylinders specific to the operation of the engines cause significant thermomechanical stresses on the insert-cylinder head torque. These thermomechanical stresses can lead to cracking of the cylinder head in the inter-seat area, of the trigger guard or to loosening of the attached seat. Besides the cylinder head, the valve is the most stressed element 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 stellitage.

Therefore, it would be desirable to have a method of manufacturing the valve seats of an engine cylinder head which 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 the laser beam deposition of layers of specific alloy coatings on seat zones of the cylinder head.

Deposition by laser beam is advantageous in that it allows rapid cooling rates and energy management of the process for manufacturing the valve seats. This process provides deposits with reduced dilution and having a typical rapid cooling microstructure.

More particularly, the document JP-A-61-76742 describes a method for manufacturing valve seats integrated in a light alloy cylinder head in which the seat area of the cylinder head is reinforced by 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 seat areas of the cylinder head, then melted by means of a laser beam and rapidly 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 - - - -

C 1.5 1 1 0.8 -

Fe - Complement Complement - -

Ni - 2 2 30 -

Cu - - - Complement 4.5

Al - - - - Supplement

If - - - - 17

having the following compositions The process of document JP-A-61-74742 has several drawbacks.

First of all, the use of a 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 problems of wettability which result therefrom. On the other hand, the integrated seat produced has a space equivalent to the attached seats. The use of a laser beam as an energy source requires that the surface of the seat area on which the deposition will be carried out is homogeneous, that is to say without surface irregularities which can scatter the beam randomly so as to obtain uniform heating throughout the seating area. A step of polishing the seat area of the cylinder head 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 outside the bath does not participate in the formation of the coating layer.

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

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

The present invention also relates to a method of manufacturing a cylinder head with integrated valve seats which overcomes the drawbacks of deposition by laser beam. 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 in 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:

Ni 13 - 20

MB 2 - 8

Co 0 - 10

Fe 2 - 8

If 2 - 4

B 1 - 3

Cu Complement; and

- the machining of the coating layer to obtain the desired geometry and surface condition for the integrated valve seats.

A preferred alloy according to the invention is the alloy having the following composition, in percent by weight:

Ni 18

MB 6

Co 6

Fe 6

If 3

B 1

Cu Complement

The method of the invention may further comprise, prior to ^"the step of depositing the coating layer valve seat, a cleaning of the seat areas of the cylinder head by means of an etchant, for example a stripper brazing on aluminum such as Castolin® C 190 stripper in the case of an aluminum alloy cylinder head This stripping step improves the metallurgical bond between the coating layer and the seat areas of the cylinder head and allows elimination impurities such as residual oxides and fats.

The deposition of a coating layer by plasma transfer arc projection is a coating technique known in itself. Briefly, a transferred arc plasma torch is used, by example a Castolin® torch type GAP-E52.

The cladding gas and the carrier gas are generally helium, while 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. An arc ignition phase, a deposition phase of a coating layer on the seat area, and an arc extinction phase with anti-crater effect. The duration of the deposition cycle will obviously depend on the thickness desired for the deposition, the composition of the powder and the conditions for obtaining the plasma. In general, the complete cycle takes approximately 20 seconds to obtain a coating layer having a thickness of 0.5 to 1.2 mm.

During the ignition phase after the opening of the gases, the pilot arc is started between the cathode and the torch nozzle, then that of the main arc between the cathode and the cylinder head. The powder of the coating alloy is then injected and the displacement of the torch is initiated over the seat area to be coated with a radial oscillating movement of the latter. The deposition phase mainly consists in continuing the displacement of the torch over the seat area to be coated while maintaining the conditions established in the priming phase until complete deposition of the coating layer is obtained. During this phase, a decreasing arc intensity profile is applied all along this phase.

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

During this plasma arc deposition step, the alloy powder injected at the foot of the arc forms a molten bath on the surface of the breech seat area. Due to the high thermal conductivity of the material constituting the cylinder head, for example a light alloy, in particular an aluminum alloy such as alloy AS 5U3, there is rapid cooling of the coating layer / cylinder head assembly. A very fine microstructure is thus obtained for the coating layer, which promotes the mechanical and chemical resistance of the coating layer.

There is shown schematically in Figure 1, before machining, a coating layer deposited on a cylinder head seat area by the method of the invention.

As can be seen in FIG. 1, there is an interface 3 between the coating layer 2 and the cylinder head 1 which constitutes a metallurgical connection between the alloy of the coating layer 2 and the alloy of the cylinder head 1. This inferface , which consists of a diffusion layer of the coating alloy 2 in that of the cylinder head 1, guarantees the holding of the coating layer forming the seat on the cylinder head 1, in particular by controlling the 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 5% by volume. The coating layers according to the invention have a particular composite microstructure developed in situ during deposition on the cylinder head. 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 FIG. 1, the deposition by transferred arc plasma generates in the alloy of the cylinder head 1 a thermally affected zone 4 of a depth of about 0.5 to 1 mm in which the micro structure 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 light alloys and very particularly aluminum alloys. Thus, for the aluminum alloy AS5U3, a hardness HVQ 5 of 120 to 150 was measured in the region thermally affected, while the parts which are not thermally affected by the process of the invention have a hardness HVQ 5 about 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 HVQ 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} at a temperature from 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 wear by erosion, abrasion and adhesion, to chemical and thermal corrosion and a high thermal stability, in particular with respect to aluminum alloys. As previously indicated, the coating layer is machined to obtain the desired geometry and surface finish for the valve seat. This machining step can be done during the machining of the valve guide or the housing of the valve guide.

The process of the invention has many advantages over the prior art.

It eliminates the use of inserts and eliminates the machining operations of the seat zones and the hooping of the cylinder head.

It reduces the size of the cylinder head.

Thus, it is possible to redefine the foundry mold to remove material in the seat areas. By reducing the size of the seat, it is possible to decrease, for equal power, the size of the motor or increase its power for the same size by increasing the useful diameter of the seats. It is also possible to reduce the thickness of the wall of the combustion chamber / cooling circuit, which will promote heat exchanges between the combustion chamber and the cooling circuit. By increasing the heat transfer to the cylinder head, the overall temperature of the valve is reduced as well as the thermal gradients usually encountered between the seat and the rod. This homogenization of the overall temperature of the chamber with the Removing hot spots helps reduce engine fuel consumption, especially at high revs. The reduction in thermomechanical stresses on the valve can allow a simplification of the machining of the latter. The method of the invention also provides a reinforcement of the area of the cross-seat trigger guard by reducing the thermomechanical stresses compared to those induced by hooping and the difference in coefficient of expansion between the insert and the cylinder head. It would also be possible to remove the bridge reinforcement insert.

Finally, the metallurgical link and the materials used to make the integrated seats are compatible with an engine powered by liquid petroleum gas (LPG).

The present invention also relates to a cylinder head, in particular an aluminum alloy cylinder head, comprising integrated valve seats constituted by a coating layer of an alloy having the compositions indicated above within the framework of the manufacturing process.

For example, a coating layer of the Nil8-Mo6-Co6-Fe6-Si3-B l-Cu alloy has been deposited on the seat areas of an AS5U3 aluminum alloy cylinder head. foundry gross.

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

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 1 / minute Cladding gas: Helium 20 to 40 1 / minute Carrier gas: Helium 6 to 10 1 / minute.

After opening the gases, we start the pilot arc (cathode / nozzle) then transfer to establish the main arc (cathode / cylinder head). The intensity of the main arc is around 70 Amps when it starts. The alloy powder is injected and the displacement of the torch on the workpiece is initiated with a radial oscillating movement of the torch. In the present example, the cylinder head is fixed and the torch is mounted on a 5-axis robot. The torch follows a circular trajectory conforming to the seat area associated with an oscillating movement perpendicular to its main movement. Finally, the torch turns on itself in order to preserve the configuration of the powder injector relative to the displacement. The circular displacement speed of the torch is between 200 and 450 rnrn / rninute, while the oscillation takes place at a frequency of 2 to 3 Hz over a width of approximately 3 mm. Alternatively, one can use a configuration in which the cylinder head is rotated (rotation relative to the axis of the seat) and a torch driven only by oscillating movements.

Phase 2 - Main deposit cycle.

The coating layer is deposited while keeping the kinematic parameters of phase 1. However, the intensity of the main arc is decreased throughout this phase, for example from 70 to 60 amperes, in order to maintain identical conditions around the entire perimeter of the seat.

The duration of this deposit 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 off the gas supply. During processing, the cylinder head is at room temperature. The rise in temperature of the aluminum is localized to an area close to the surface (under the arch foot, 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 for large displacement engines where perfect alignment between the seat and the valve guide is sought. The cutting conditions are quite conventional because the coating material has very good machinability. The seat obtained has a special microstructure which gives it its mechanical, thermal and chemical resistance properties. The dense structure and without porosity of the coating allows obtaining after machining a seat having the required geometry and surface condition. The metallurgical connection between the covering bead and the cylinder head participates in the thermal transfer to the cylinder head. The thermodynamic stability of the coating cord-aluminum couple guarantees resistance to thermomechanical fatigue.

Claims

1. A method of manufacturing a light alloy cylinder head comprising integrated valve seats, characterized in that it comprises:

- Obtaining a cylinder head in 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:

Ni 13 - 20

MB 2 - 8

Co 0 - 10

Fe 2 - 8

If 2 - 4

B 1 - 3

Cu Complement; and - machining the coating layer to obtain the desired geometry and surface finish for the integrated valve seats.

2. Method according to claim 1, characterized in that the alloy of the coating layer has the following composition, in percent by weight:

Ni 18

MB 6

Co 6

Fe 6

If 3

B 1

Cu Complement.

3. Method according to any one of claims 1 or 2, characterized in that it further comprises, prior to the step of depositing the coating layer, a step of pickling the areas of seat.

4. Method according to any one of claims 1 to 3, characterized in that the step of plasma deposition with transferred arc comprises a phase of striking the arc, a phase of deposition and a phase of extinction of the arc.

5. Method according to claim 4, characterized in that the deposition phase is implemented with an arc of decreasing intensity.

6. Method according to claim 4 or 5, characterized in that the arc extinction phase is a phase with anti-crater effect.

7. Method according to any one of the preceding claims, characterized 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, characterized in that the cylinder head is made of aluminum alloy.

9. Light alloy cylinder head with integrated valve seats, characterized in that the integrated valve seats consist of a coating layer of an alloy of composition, in percent by weight:

Ni 13 - 20 MB 2 - 8

Co 0 - 10

Fe 2 - 8

If 2 - 4

B 1 - 3 Cu Complement.

10. Light alloy cylinder head according to claim 9, characterized in that the alloy of the coating layer has the composition, in percent by weight:

Ni 18

MB 6

Co 6

Fe 6

If 3

B 1 Cu Complement.

1 1. Light alloy cylinder head according to claim 10, characterized in that the alloy of the coating layer has a hardness HVQ 5 of 200 to 500, a thermal conductivity greater than 30 W / mK and a coefficient of thermal expansion a a temperature of 400 to 600 ° C of 18.10- ⁶ K ^_1 .

12. Light alloy cylinder head according to any one of claims 9 to 11, characterized in that the light alloy is an aluminum alloy.