METHOD FOR FORMING A COATING OF DUCT OF A CYLINDER HEAD AND CYLINDER HEAD THUS OBTAINED

Abstract

The invention relates to a method for forming a lining on the walls of an inner pipe of a cast aluminium-alloy part, including inserting a cathode into the pipe, circulating an electrolyte solution in said pipe between the cathode and the walls of the pipe forming an anode, and applying a potential difference between the anode and the cathode, the method being characterised in that applying the potential difference between the anode and the cathode includes applying a series of DC voltage pulses to the anode. The invention also relates to a cylinder head in which the exhaust pipes are lined with a lining obtained by implementing said method.

Claims

1. A method for forming an aluminium oxide coating on walls of an inner duct of a cast part in aluminium alloy, the method comprising inserting a cathode in the duct, circulating an electrolyte solution in said duct between the cathode and the anode-forming walls of the duct, and applying a potential difference between the anode and the cathode, the method being characterized in that applying the potential difference between the anode and cathode comprises applying a series of DC voltage pulses to the anode.

2. The method for forming according to claim 1, wherein each pulse of the series has a duration of between 0.01 and 0.02 s and two successive pulses are separated by 0.001 to 0.01 s.

3. The method for forming according to claim 1, wherein the voltage applied to the anode varies over the series of pulses and is between 0 and 150 V to maintain a current density of between 10 and 50 A/dm.sup.2 of surface to be treated.

4. The method for forming according to claim 1, wherein the total duration of the series of pulses is between 30 and 300 s as a function of the type of alloy to be treated and the desired oxide thickness.

5. The method for forming according to claim 1, wherein the electrolyte comprises 10 to 20% sulfuric acid and 1 to 5% ferrous sulfate.

6. The method for forming according to claim 1, wherein the electrolyte flow rate in a duct is between 0.5 and 2.0 m.sup.3/h per dm.sup.2 of surface to be treated.

7. The method for forming according to claim 1, wherein the temperature of the electrolyte in a duct is between 10 C. and 0 C.

8. The method for forming according to claim 1, wherein the cathode is shaped to match the shape of the inner duct(s) of the cast part, leaving a mean interstice of between 3 and 15 mm between the cathode and the duct wall.

9. An engine cylinder head in aluminium alloy, wherein, on the walls of at least one inner duct, it comprises a coating in aluminium oxide having a thickness of between 50 and 200 m, adapted to ensure sealing and thermal insulation of the inner duct wall of the cylinder head when exhaust gases flow inside said duct at a temperature higher than 900 C.

10. The engine cylinder head according to claim 9, the cylinder head being obtained by implementing a method forming an aluminium oxide coating on walls of an inner duct of a cast part in aluminium alloy, the method comprising inserting a cathode in the duct, circulating an electrolyte solution in said duct between the cathode and the anode-forming walls of the duct, and applying a potential difference between the anode and the cathode, and the method being characterized in that applying the potential difference between the anode and cathode comprises applying a series of DC voltage pulses to the anode.

11. The engine cylinder head according to claim 9, wherein the inner ducts provided with an oxide coating are exhaust ducts of combustion products.

12. The engine cylinder head according to claim 10, wherein the inner ducts provided with an oxide coating are exhaust ducts of combustion products.

Description

DESCRIPTION OF THE FIGURES

[0037] Other characteristics, objectives and advantages of the invention will become apparent from the following description that is solely illustrative and nonlimiting, and is to be read in connection with the appended drawings in which, in addition to Figure A which illustrates the notion of unitary volume:

[0038] FIG. 1 schematically illustrates a system for implementing a method for forming a coating on a cylinder head conforming to one embodiment of the invention.

[0039] FIG. 2a illustrates inner ducts of a cylinder head, and FIG. 2b illustrates a cylinder head with integrated exhaust gas collector.

[0040] FIG. 3 illustrates a cathode shaped to match the shape of the inner ducts of a cylinder head.

[0041] FIG. 4 illustrates the changes in voltage applied to the cylinder head, and the current density between the anode and cathode when implementing the method for forming an insulating coating.

[0042] FIG. 5 gives an EDS analysis spectrum of the aluminium oxide deposited with the method.

[0043] FIG. 6 is an illustration in cross section of the geometry of a cylinder head inner duct for which the method to form a coating according to the invention is adapted.

[0044] FIG. 7a illustrates an observation section of the thickness of the anodization layer.

[0045] FIG. 7b illustrates another observation section of the thickness of the anodization layer.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

[0046] With reference to FIG. 1, a cast part 10 in aluminium alloy is schematically illustrated. This cast part is of complex geometry and particularly comprises cored inner ducts 11.

[0047] The constituent alloy of this cast part is aluminium-silicon based of hypo-eutectic type comprising less than 12.5 weight % of silicon and may contain alloying elements such as copper and magnesium.

[0048] As a nonlimiting example, the constituent alloy of this part 10 is of type AA319 or an alloy of type AA356.

[0049] As illustrated in FIG. 2, the cast part is advantageously an engine cylinder head 10. In this case, the inner ducts 11 under consideration are advantageously exhaust ducts for combustion products. In this respect, the cylinder head 10 is advantageously a cylinder head comprising an integrated exhaust gas collector, as is the case for example for the cylinder head in FIG. 2b. FIG. 2b also illustrates the combustion chambers 19 of the cylinder head.

[0050] To limit heat exchanges between exhaust gases circulating in the duct 11, the temperature of which may exceed 900 C., and the part 10, a method is implemented to form an insulating coating 13 in aluminium oxide on the inner walls of each duct 11 via anodic oxidation.

[0051] The system 1 used to implement this method is illustrated in FIG. 1.

[0052] It comprises a cathode 3 arranged inside the cylinder head, a circulation circuit 2 of an electrolyte solution between the cathode and the anode-forming walls of the cylinder head, and a circuit 4 controlling the potential difference applied between the anode and cathode, said potential difference generating an oxidation reaction at the anode to form the oxide coating.

Electrolyte Solution Circulation System

[0053] The system 2 for circulating the electrolyte solution in the cylinder head ducts 11 is illustrated in FIG. 1. It advantageously comprises a tank of electrolyte solution 20, a pump 21, and a closed circuit 22 circulating the solution between the tank and the ducts 11 of the cylinder head. The electrolyte solution preferably comprises between 10 and 20% sulfuric acid and from 1 to 5% ferrous sulfate.

[0054] To prevent dissolution of the oxide created by the method to form the coating, this dissolution being catalysed by the heat caused by electrolysis, the solution is advantageously held at a temperature of between 10 C. and 0 C.

[0055] In this respect, the circuit 2 advantageously comprises a member 23 to cool the electrolyte solution. In addition, the pump advantageously has a variable flow rate to modulate the electrolyte flow rate as a function of temperature.

[0056] Advantageously, the pump 21 is sized as a function of the surface area to be coated and thickness of the oxide layer to be grown and is advantageously adapted to circulate a flow of electrolyte solution in the cylinder head at a rate of between 0.5 and 2 m.sup.3 per hour and per square decimetre (/h.Math.dm.sup.2) of surface to be treated.

[0057] The circulation of electrolyte in the ducts at a temperature of between 10 and 0 C. allows a homogeneous coating to be obtained.

Arrangement of the Cathode

[0058] A cathode 3 is positioned inside exhaust ducts 11 of the cylinder head. This cathode is made of a material allowing redox reactions to take place in the electrolyte solution. In particular, the cathode is advantageously in stainless steel of 316L type for example.

[0059] With reference to FIG. 3, the cathode 3 is advantageously shaped to match the shape of the ducts 11 leaving an interstice, preferably a constant interstice, between the cathode and the ducts, allowing circulation of the electrolyte. This makes it possible, when applying a potential difference between the anode and cathode, to set up homogeneous current lines over the entirety of the surface to be coated, and thereby obtain an identical growth rate of the layer on the surface. On completion of the method, this allows a layer of homogeneous thickness to be obtained on all the treated surfaces.

[0060] The mean interstice between the cathode and the wall of a duct is advantageously between 3 and 15 mm. This amounts to a good trade-off regarding the thickness to be maintained between the cathode and the wall of the duct 11, first to promote circulation of electrolyte and the entraining of gases generated by electrolysis, including when the oxide layer starts to be formed, and secondly to maintain sufficient current density to prevent slowing of oxide layer growth.

Anodic Oxidation

[0061] Returning to FIG. 1, the system to implement the method for forming a coating layer on the ducts of the cylinder head 10 further comprises a circuit 4 to control the potential difference between the anode and cathode.

[0062] The circuit 4 comprises a voltage source 40, adapted to deliver a voltage to the anode-forming cylinder head 10, a control unit 41 controlling the voltage source, and one or more sensors (not illustrated) adapted to record the voltages between the anode and cathode, and the current between the anode and cathode, to allow the defined current to be obtained.

[0063] With reference to FIG. 4, to form the oxide layer 13 on the walls of the ducts 11, the control unit 41 drives the voltage source 40 to deliver a series of DC voltage pulses to the anode.

[0064] The frequency of the voltage pulses is advantageously higher than 10 Hz, preferably between 10 and 50 Hz.

[0065] More specifically, each voltage pulse has a duration of less than 0.1 second, and preferably of between 0.01 and 0.02 second, during which time the value of the applied voltage is constant. Each pulse is also separated from the following pulse by a nonzero time interval of less than 0.1 second, preferably less than 0.01 second, and advantageously between 0.001 and 0.01 second. During this time interval, the voltage applied to the anode is therefore zero.

[0066] The application of such a series of voltage pulses allows a reduction in the time needed to implement the method by promoting evacuation of Joules losses and gases.

By way of comparison, the obtaining of an oxide layer having a thickness of between 50 and 200 m requires a treatment time in the order of 70 seconds, whilst the time required in the prior art is in the order of several minutes.

[0067] In addition, the values of the voltage of each pulse change progressively as and when the oxide layer is formed. Indeed, on account of its insulating nature, the oxide layer opposes the setting up of a current between the anode and cathode.

[0068] In particular, the guiding of the voltage source 40 by the control unit 41, is determined by the value of the current density between the anode and cathode. Measurement of the current by the sensors enables the control unit 41 to calculate the current density and, as a function of the result, to drive the value of the voltage delivered by the voltage source 40.

[0069] To maintain sufficient current density for continued growth of the layer, the voltage globally increases over the series of pulses. The desired current density is advantageously between 5 and 50 A/dm.sup.2 of surface to be treated.

[0070] Therefore, the value of the voltage of each pulse is between 0 and 150 V, advantageously between 0 and 120 V, the pulses occurring in the first seconds e.g. the first 5 or 10 first seconds of the method having a voltage of between 0 and du 50 V, and the following pulses advantageously having an increasing voltage up until sufficient voltage to maintain a current density that is advantageously higher than 5 A/dm.sup.2, preferably higher than 10 A/dm.sup.2. This maximum voltage is advantageously between 70 and 150 V, preferably between 70 and 120 V.

[0071] This series of DC voltage pulses at the anode is performed for a time of between 30 and 300 s as a function of the type of alloy to be treated and the thickness of the oxide layer it is desired to obtain.

[0072] Therefore, the application of a potential to the anode generates a potential difference between the cylinder head and the cathode and causes chemical reactions which, on the aluminium of the cylinder head, produce aluminium oxide on the walls of the exhaust ducts 11.

[0073] FIG. 5 illustrates an EDS analysis spectrum (Energy Dispersive Spectroscopy) performed on the aluminium oxide obtained. The relative heights of the peaks of this spectrum indicate an oxide composition having close stoichiometry to that of alumina Al.sub.2O.sub.3, the other components being pollutants derived from the electrolyte composition.

[0074] So that the oxide layer 13 can ensure insulation of the cylinder head when in operation i.e. when gases having a temperature of 950 C. flow inside the inner ducts, the oxide layer formed on each inner duct advantageously has a thickness of between 50 and 200 m. This thickness varies chiefly as a function of the silicon and copper concentration of the treated aluminium alloy. However, it remains sufficiently thin so as not to alter the dimensional characteristics of the product within a tolerance margin of 0.5 mm.

[0075] It has been evidenced that application of type T7 heat treatment i.e. comprising solution treatment at a temperature of between 490 and 540 C. (depending on the aluminium alloy used), quenching in water or air and annealing at a temperature of 200 C. or higher, allows more homogeneous coating layers to be obtained in terms of thickness and density.

[0076] As a nonlimiting illustration, FIGS. 7a and 7b give cross-sectional views of an oxide coating on a cylinder head obtained after treatment following the method of the invention. In these illustrations, the oxide layer is between 34.92 m and 70.32 m and has maximum porosity of 15%. By porosity is meant an overall void percentage within the oxide layer.

[0077] Good layer density is therefore obtained as well as a narrow thickness. It is therefore no longer necessary to carry out post-sealing treatment, re-machining or finishing. In addition, the described method leads to cycle times that are compatible with mass production in the automotive sector (i.e. 5 to 6 min).

[0078] The proposed method, within short time, therefore allows an insulating coating to be obtained of homogeneous thickness on inner ducts of parts in aluminium alloy such as engine cylinder heads.