Method for producing a structure by additive manufacturing with a cementation step

Abstract

The invention relates to a method for producing a structure by additive manufacturing, said method being carried out in a manufacturing chamber, said structure being made of a metallic material, said method comprising the following steps: providing a powder comprising particles of said metallic material, depositing at least one layer of said powder on a solid substrate or on an underlying layer of said powder, locally melting said layer of powder deposited by scanning with a laser beam, so as to form a molten bath, and cooling the molten bath so as to solidify it, a gas mixture comprising less than 5% by volume of a hydrocarbon being used so as to increase the carbon content of the material for certain layers of the structure. Abstract figure: Fig. 1

Description

Title of the invention: Method for producing a structure by additive manufacturing with a cementation step

[0001] The present invention relates to a method for producing a structure by additive manufacturing, and more particularly a production method comprising a cementation step.

[0002] A method for hardening the surface of a steel by carburizing, also called cementation, is known from the prior art. This method consists of a thermochemical treatment carried out using a furnace. This method consists of a succession of steps carried out after the manufacture of the structure. The structure, made of steel and to be hardened, therefore undergoes, after its manufacture, a heat treatment at, for example, at least 900 °C. During this heat treatment, an active gas is introduced into the atmosphere. The active gas is, for example, a hydrocarbon such as methane, propane, ethylene or acetylene. This step lasts a few hours, for example between 2 and 4 hours, and allows carbon to diffuse to a certain depth in the structure. Thus, the material is enriched with carbon over a certain depth, for example over a few millimeters. Then the structure undergoes a quenching process, called martensitic quenching, in order to transform the carbon-enriched layer into martensite.This quenching is carried out in a furnace, for example at 180°C. This quenching also lasts for example a few hours. Finally, a low temperature tempering step is carried out, for example this tempering can be carried out at 240°C in the furnace. This last step is carried out for a few hours and allows the stresses in the material to be released.

[0003] This hardening process applies to structures resulting from different manufacturing processes such as machining, a foundry process or even forging. However, this hardening process is restrictive because it requires additional means and steps after the structure has been manufactured, so several hours of post-treatment are necessary.

[0004] The present invention aims to remedy the aforementioned drawbacks by proposing a method for producing a structure by additive manufacturing with a hardening step which is less restrictive and which is less time-consuming to implement.

[0005] The invention then relates to a method for producing a structure by additive manufacturing, said method being carried out in a manufacturing chamber, said structure being made of a metallic material, said method comprising a first sequence comprising the following steps:

[0006] a) providing a powder comprising particles of said metallic material,

[0007] b) depositing at least a first layer of said powder on a solid substrate or on a first underlying layer of said powder,

[0008] c) locally melting said first layer of deposited powder by means of a laser source emitting a laser beam scanning said first layer, so as to form a first molten bath,

[0009] d) cooling the first molten bath so as to solidify it,

[0010] steps b) to d) being repeated until a first thickness of said solidified metallic material having a first carbon content is obtained, said method comprising a second sequence comprising the following steps:

[0011] e) depositing at least a second layer of said powder on said first thickness of said solidified metallic material,

[0012] f) locally melting said second layer of deposited powder by means of said laser source emitting said laser beam scanning said second layer, so as to form a second molten bath,

[0013] g) cooling said second molten bath so as to solidify it,

[0014] steps e) to g) being carried out with the introduction of a gas mixture into the manufacturing chamber during at least said step e), said gas mixture comprising argon, nitrogen and / or helium and at least one hydrocarbon, said steps e) to g) being repeated until a second thickness of solidified metallic material having a second carbon content greater than the first carbon content is obtained,

[0015] said method being characterized in that: - the said material is steel, - said gas mixture contains less than 10% by volume of said at least one hydrocarbon, preferably between 1% and 5% by volume of said at least one hydrocarbon.

[0016] The invention will be better understood upon reading the following description and examining the accompanying figure. This figure is given only as an illustration but in no way limits the invention.

[0017] [Fig-1] is a schematic representation of an embodiment of the method according to the invention;

[0018] With reference to [Fig.l], the method of producing a structure by additive manufacturing is carried out in a manufacturing chamber 111, in particular for implementing the additive manufacturing steps. The structure is built using these steps, layer by layer, and in metallic material. More particularly, the material used is a low-alloy steel, and for example the material is a steel with a carbon content of less than 0.4%.

[0019] The method comprises a first sequence of manufacturing steps.

[0020] A step a) of the method consists of providing a powder comprising the particles of the metallic material or else providing a wire of the metallic material.

[0021] A following step b) consists of depositing at least a first layer of the powder on a solid substrate 3. During additive manufacturing, this step consists of depositing at least one layer 5 of the supplied powder on an underlying layer 4. The powder is thus deposited layer by layer. In one embodiment, the first layer 5 of powder has a thickness of less than 100 qm. In one embodiment the layer of powder has a thickness of between 10 qm and 100 qm. In one embodiment, the layer has a thickness of around 70 qm.

[0022] A following step c) consists of locally melting said layer of powder 5 deposited by scanning a laser beam 11, so as to form a first molten bath. A laser source 1 emits this laser beam 11. The laser beam 11 is oriented towards the layer 5 to be melted using a head 2 comprising an orientable mirror system.

[0023] A next step d) consists of cooling the molten bath so as to solidify it. This cooling is carried out by moving the laser away.

[0024] The structure thus formed is composed of the chosen metallic material. The metallic material then has a first carbon content.

[0025] Steps b) and c) also apply to a DED (Direct Energy Deposition) additive manufacturing process using a wire of material rather than a powder. The wire is supplied into the chamber 111, then this wire is melted by the laser beam 11. This wire thus forms a bead deposited layer by layer.

[0026] Steps b) to d) are repeated until a first thickness of structure is obtained in this metallic material characterized by the first carbon content.

[0027] The method comprises a second sequence of manufacturing steps, steps e) to g) described below.

[0028] Once this first thickness is reached, the second sequence is started and a particular gas mixture 6 will be introduced into the chamber 111 of the additive manufacturing process. The gas mixture 6 is a mixture of argon, nitrogen and / or helium with a hydrocarbon. In one embodiment, the hydrocarbon represents less than 10% by volume of the gas mixture 6. In one embodiment, the hydrocarbon represents less than 5% by volume of the gas mixture 6. In one embodiment, the gas mixture 6 contains between 1% and 5% by volume of said hydrocarbon. The hydrocarbon is at least one of: methane, propane, ethylene, acetylene.

[0029] A following step e) then consists of depositing at least a second layer 5 of the powder provided on said first thickness of said solidified metallic material. The powder is thus deposited layer by layer. In one embodiment, the second layer 5 of powder has a thickness of less than 100 qm. In one embodiment the powder layer has a thickness of between 10 μm and 100 μm. In one embodiment, the layer has a thickness of about 70 μm.

[0030] A following step f) consists of locally melting said second layer of powder 5 deposited by scanning a laser beam 11, so as to form a second molten bath. A laser source 1 emits this laser beam 11. The laser beam 11 is oriented towards the layer 5 to be melted using a head 2 comprising an orientable mirror system.

[0031] The gas mixture 6 is introduced at least during step f) so that the gas mixture 6 is above the powder layer 5 before melting.

[0032] This gas mixture 6 is introduced into said manufacturing chamber in the entire volume of the manufacturing chamber. In one embodiment, this gas mixture 6 is introduced by means of a nozzle to introduce it into a precise volume above the powder layer 5, in particular for DED (Direct Energy Deposition) additive manufacturing processes and / or using a metal wire rather than powder.

[0033] During step f), that is to say during the melting of the second layer 5. The laser beam 11 will pass through the gas mixture 6. However, the temperature obtained in the vicinity of the laser impact zone is high, of the order of 1000°C to 1500°C depending on the power delivered by the laser. This temperature will allow the cracking by the laser beam 11 of the hydrocarbon molecule, the release of the carbon and its absorption in the molten metal. The material thus obtained therefore has a second carbon content which is higher than the first carbon content. The absorption of carbon in the molten pool is much faster than during the traditional carburizing process described.

[0034] A next step g) consists of cooling the molten bath so as to solidify it. This cooling is carried out by moving the laser away. It has been observed that this cooling has the same characteristics as quenching, which allows direct metallurgical transformation into martensite. It is therefore no longer necessary to carry out the steps of carburizing after additive manufacturing. It is therefore no longer necessary to carry out a heat treatment to promote the enrichment of the material in carbon.

[0035] In one embodiment, the gas mixture 6 is introduced during step e). In one embodiment, the gas mixture 6 is introduced during step e) and f). In one embodiment, the gas mixture 6 is introduced during step e) and f) and g)-

[0036] Steps e) to g) are repeated until a second thickness is obtained so that said structure is made of said carbon-enriched material in said second thickness. The structure thus has in this second thickness a material with a second carbon content higher than the first carbon content. In one embodiment, this second thickness is produced on the last layers of the structure. The gas mixture 6 can also be introduced from step b), the material being enriched with carbon throughout the manufacture of the structure. For example, for a powder thickness of 60 μm, obtaining a cemented layer of 0.6 mm will require the production of 10 carbon-enriched layers. In one embodiment, this second thickness is covered by a third thickness produced by repeating steps b) to d), therefore without the addition of the gas mixture 6.

[0037] In one embodiment, the laser radiation has a frequency between 800 nm and 1100 nm and a focal spot with a diameter of less than 200 pm. In one embodiment, the power of the laser 1 is between 100 W and 2000 W. In one embodiment, the power of the laser 1 is less than 1000 W. In one embodiment, the power of the laser is between 100 W and 500 W.

Claims

[Claim 1] Claims A method of producing a structure by additive manufacturing, said method being carried out in a manufacturing chamber (111), said structure being made of a metallic material, said method comprising a first sequence comprising the following steps: a) providing a powder comprising particles of said metallic material, b) depositing at least a first layer (5) of said powder on a solid substrate (3) or on a first underlying layer (4) of said powder, c) locally melting said first layer (5) of deposited powder by means of a laser source (1) emitting a laser beam (11) scanning said first layer (5), so as to form a first molten bath, d) cooling the first molten bath so as to solidify it, steps b) to d) being repeated until a first thickness of said solidified metallic material having a first carbon content is obtained,said method comprising a second sequence comprising the following steps:, e) depositing at least a second layer (5) of said powder on said first thickness of said solidified metallic material, f) locally melting said second layer (5) of deposited powder by means of said laser source (1) emitting said laser beam (11) scanning said second layer (5), so as to form a second molten bath, g) cooling said second molten bath so as to solidify it, steps e) to g) being carried out with the introduction of a gas mixture (6) into the manufacturing chamber (111) during at least said step e), said gas mixture (6) comprising argon, nitrogen and / or helium and at least one hydrocarbon, said steps e) to g) being repeated until a second thickness of solidified metallic material having a second carbon content greater than the first carbon content is obtained, said method being characterized in that: - the said material is steel, - said gas mixture (6) contains less than 10% by volume of said at least one hydrocarbon, preferably between 1% and 5% in volume of said at least one hydrocarbon.

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