Method for producing a multilayer element having a protective coating

Abstract

Process for producing an element comprising a multilayer architecture, the layers of which comprise primary channels on their upper faces, said process comprising the following successive steps: (a) producing secondary channels on the lower faces of each layer, each secondary channel being intended to be facing a primary channel of the neighboring lower layer within the architecture, (b) depositing a coating that protects against oxidation at a temperature of between 500 C. and 1000 C. and against corrosion over all of the lower and upper surfaces of the layers, (c) sanding or mechanical cleaning of the surfaces intended to be assembled, and (d) assembling via superposition of the various layers so that each secondary channel of a lower face of an upper layer is facing and is centered on a primary channel of the neighboring lower layer,
the width of each secondary channel being greater than the width of the primary channel which it is facing within the architecture.

Claims

1. A process for producing a heat exchanger comprising a multilayer architecture, the layers of which comprise primary channels on upper faces thereof, said process comprising the following successive steps: for each layer, producing secondary channels on a lower face of thereof, each secondary channel of a layer being intended to be facing a primary channel of an adjacently lower layer within the architecture; depositing a coating over all of the lower and upper surfaces of the layers, the coating protecting against oxidation at temperatures between 500 C. and 1000 C. and also protecting against corrosion; for each layer, sanding or mechanically cleaning portions of the faces that that, during assembly of the multilayer architecture, are intended to be diffusion welded to sanded or mechanically cleaned portions of adjacent layers; superposing each of the sanded or mechanically cleaned layers so that each secondary channel is facing and is centered on a primary channel of an adjacently lower layer within the architecture; and diffusion welding the superposed layers, wherein a width of each secondary channel is greater than a width of the primary channel which it is facing within the architecture.

2. The process of claim 1, wherein the coating is formed from a mixture comprising an activating agent powder, an Ni.sub.2Al.sub.3 metal powder and a solvent Al.sub.2O.sub.3.

3. The process of claim 2, wherein the process further comprises, after said diffusion welding: burying the heat exchanger in the mixture of powders; and heating the heat exchanger under vacuum or under Ar at a temperature between 950 and 1000 C. for a duration of between 8 and 10 hours.

4. The process of claim 1, wherein the coating is formed from a mixture comprising an activating agent powder, an Al metal powder and a solvent Al.sub.2O.sub.3.

5. The process of claim 4, wherein the process further comprises, after said diffusion welding step: burying the heat exchanger in the mixture of powders; and heating the buried heat exchanger at a temperature of about 600 C. for a duration of between 8 and 10 hours so as to forma first layer of NiAl.sub.3; and after said heating step, annealing the heat exchanger at a temperature of between 1000 C. and 1100 C. for a duration of between 4 and 8 hours so as to convert the NiAl.sub.3 layer into a NiAl layer.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a cross-sectional schematic view of the inventive multilayer architecture.

(2) FIG. 2 is a flow chart showing the main steps of an embodiment of the inventive process.

DETAILED DESCRIPTION OF THE INVENTION

(3) The expression centered on is understood to mean centering with a margin of error of less than 0.15 mm.

(4) The expression secondary channels is understood to mean additional channels located on the opposite face of the layers having primary channels at the surface.

(5) The process according to the invention makes it possible to avoid the production of masking in zones having a complex architecture, i.e. in the channels, which is difficult to carry out and which may generate a contamination of the coating or of the surfaces to be assembled.

(6) It should be noted that the secondary channels have the objective, after deposition of the coating and assembly of the various layers, of providing a complete and homogeneous protection of the whole of the surface of the channels, without local lack of coating that may generate a preferred site of corrosion.

(7) The channels will preferably have a semicircular cross section and the counter-channels will preferably have a cross section of half-rectangle shape, when considering a rectangle cut lengthwise.

(8) Within the context of the invention, the coating may be formed by pack cementation by carrying out a low-activity aluminization starting from a mixture of a metal (Ni.sub.2Al.sub.3) powder, a diluent (Al.sub.2O.sub.3) powder and also a powder of an activating agent (such as NH.sub.4F, NH.sub.4Cl, CrCl.sub.3).

(9) In this case, the process may comprise, downstream of the assembly step:

(10) (i) a step of heating, under vacuum or under Ar, the element buried in the mixture of powders at a temperature of between 950 C. and 1000 C. for a duration of between 8 and 10 h. This process makes it possible to directly form the desired NiAl coating.

(11) Another possibility is to choose to form a coating by pack cementation by carrying out a high-activity aluminization starting from a mixture comprising an Al metal powder, a diluent (Al.sub.2O.sub.3) powder and a powder of an activating agent (such as NH.sub.4F, NH.sub.4Cl, CrCl.sub.3).

(12) In this case, said process comprises, downstream of the assembly step:

(13) (i) a first step of heating the element buried in the mixture of powders at a temperature of 600 C. for a duration of between 8 and 10 h so as to form a first layer of NiAl.sub.3; and

(14) (ii) a second step of annealing the element resulting from step (i) at a temperature of between 1000 C. and 1100 C. for a duration of between 4 and 8 h so as to convert this layer of (brittle) NiAl.sub.3 into NiAl (desired coating).

(15) The step of producing the secondary channels may comprise mechanical machining or chemical milling.

(16) The assembly step may be carried out in the following manner: by diffusion welding, a technique that consists, in principle, in obtaining from two separate elements a single homogeneous block by diffusion of material in the solid state by applying a constant pressure during a heating cycle in a vacuum furnace (press furnace).

(17) It should be noted that the element in question here is preferably an element made of metal alloy and the coating is preferably an anti-corrosion coating.

(18) FIG. 2 schematically shows the main steps of the process according to the invention:

(19) Step (a): production of secondary channels on the lower faces of each layer, each secondary channel being intended to be facing a primary channel of the neighboring lower layer within the architecture. These secondary channels will have to be centered on the primary channels of the opposite face and have a width greater than the width of the primary channels in order to ensure a protection of the whole of the surface of the channel after assembly, including in the case of a slight error in positioning the parts on one another during the assembly.

(20) Step (b): deposition of a protective coating on all of the lower and upper surfaces of the layers. In the present case, masking is completely sidestepped.

(21) Step (c): mechanical grinding of the surfaces intended to be assembled. By virtue of this technique (to be explained), only the surfaces of the primary and secondary channels retain the coating, the other surfaces being bared in order to be more easily assembled.

(22) Step (d): assembling via superposition of the various layers so that each secondary channel of a lower face of an upper layer is facing and is centered on a primary channel of the neighboring lower layer. This results, after assembly, in an assembled part having channels that are coated homogeneously over the whole of their surface.

(23) Another subject of the present invention is a metallic heat exchanger comprising a multilayer architecture, each layer comprising primary channels on its upper face, characterized in that: each lower face of the layers comprises secondary channels centered on the channels of the neighboring lower layer within the architecture and having a width greater than the width of the primary channels, and a coating that protects against oxidation at a temperature of between 500 C. and 1000 C. and against corrosion, and the thickness variation of which is less than 10 m over all of the surfaces of the primary and secondary channels.

(24) Preferably, the heat exchanger may have one or more of the following features: the thickness of the coating is between 50 and 100 m, the channels are millimeter-sized channels, the layers of the architecture have a thickness of between 1.6 and 2 mm.

(25) Preferably, the heat exchanger according to the invention will be used for the production of hydrogen.

(26) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(27) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(28) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

(29) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(30) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(31) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(32) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.