Additive manufacturing process for a part by melting a titanium alloy on a powder bed, particularly for aeronautical applications

Abstract

This disclosure relates to a process for manufacturing a part comprising a titanium alloy by selective laser melting of metal powder layers, wherein the titanium alloy comprises: a mass percentage of aluminum greater than or equal to 4.7% and less than or equal to 6.0%, a mass percentage of vanadium greater than or equal to 6.5% and less than or equal to 8.0%, a mass percentage of silicon greater than or equal to 0.15% and less than or equal to 0.6%, a mass percentage of iron less than or equal to 0.3%, a mass percentage of oxygen greater than or equal to 0.15% and less than or equal to 0.23%, the remaining mass percentage comprising titanium and possibly impurities and / or additives, and wherein: a laser power (P) of the laser applied to the metal powder is greater than or equal to 306 W and less than or equal to 378 W, a laser scan rate (V) is greater than or equal to 1305 mm.s-1 and less than or equal to 1420 mm.s-1, and a vector gap (h) defined as a distance between centers, according to a thickness of the layers in the powder bed, of two successive solidified cords between two successive passes of the laser is between 0.080 mm and 0.090 mm. Abbreviated figure: Fig. 1.

Description

Title of the invention: Additive manufacturing process for a part by powder bed fusion of a titanium alloy, particularly for aeronautical applications. Technical field

[0001] This disclosure relates to the additive manufacturing of parts by laser melting of a metal powder. More specifically, it relates to the use of such processes on the Ti575 titanium alloy, particularly for the manufacture of parts intended for the aerospace industry. STATE OF THE ART

[0002] For several years, additive manufacturing processes have been commonly used to manufacture aeronautical metal parts. Such processes are notably used for manufacturing titanium parts, as this metal offers several advantages that make it a particularly suitable material for aeronautical applications, such as low density, high mechanical strength, very good corrosion resistance, and strong compatibility with carbon fiber composites, another material commonly used in the aeronautical industry. For example, titanium alloys are used to manufacture aircraft turbomachinery parts located in a cold zone of the turbomachine, such as fan discs or blades, or low-pressure compressor blades.Titanium alloys can also be used to manufacture aircraft parts that are not part of the engine, such as nacelles or landing gear.

[0003] Compared with subtractive manufacturing processes, which consist of removing material from a block or billet of metal, additive manufacturing makes it possible to form parts with greater precision, and some additive manufacturing processes make this possible without compromising the mechanical properties of the part.

[0004] One of the most suitable titanium alloys for producing parts by additive manufacturing for the aerospace industry is TA6V, which allows for the production of some of the strongest parts that can be produced by additive manufacturing using titanium alloy powder. TA6V exhibits approximately the mechanical properties listed in Table 1 below:

[0005] [Tables 1] Mechanical strength (MPa) 896 MPa Conventional yield strength R0j2 (MPa) 790 MPa Elongation at break 10%

[0006] where the conventional yield strength R0>2 is defined as a stress value leaving a residual plastic deformation of approximately 0.2% and the elongation at break is defined at the end of a tensile test by the equation, where Lu is the length of the specimen at its break and Lo is its length before initiation of the trial.

[0007] With improved engine performance, titanium alloy parts Materials that can be produced by additive manufacturing processes are subjected to increasingly high stresses, such that the mechanical strength of TA6V becomes inadequate with regard to the forces exerted on the part. Furthermore, it is necessary to increase the part's strength without compromising its ductility (i.e., without reducing its elongation at break). EXPOSED

[0008] There is therefore a need for a process enabling the manufacture of titanium alloy parts with precision and mechanical properties adapted to the requirements of the modern aeronautical industry, in particular turbomachine parts aircraft, nacelles or landing gear.

[0009] To this end, a method is proposed for manufacturing a part comprising a titanium alloy by selective laser melting of layers of metal powder, wherein the titanium alloy comprises: a mass percentage of aluminium greater than or equal to 4.7% and less than or equal to 6.0%, a vanadium mass percentage greater than or equal to 6.5% and less than or equal to 8.0%, a mass percentage of silicon greater than or equal to 0.15% and less or equal to 0.6%, a mass percentage of iron less than or equal to 0.3% a mass percentage of oxygen greater than or equal to 0.15% and less than or equal to 0.23%,

[0010] the remaining mass percentage comprising titanium and possibly impurities and / or additives,

[0011] and in which: a power (P) of a laser radiation from the laser applied to the metal powder is greater than or equal to 306 W and less than or equal to 378 W, a laser scanning speed (V) greater than or equal to 1305 mm.s 1 and less than or equal to 1420 mm. s', and - a vector gap (h) defined as a distance between centers, according to a thickness of the layers in the powder bed, of two successive solidified cords between two successive passes of the laser is between 0.080 mm and 0.090 mm.

[0012] The use of a titanium alloy as defined above to form a part by additive manufacturing, in combination with the specified parameters, makes it possible to obtain parts with increased mechanical strength and elongation at break compared to titanium parts obtainable with known additive manufacturing processes. This is particularly advantageous for producing highly loaded parts for the aerospace industry.

[0013] According to one embodiment, the ratio between the mass percentage of aluminium and the mass percentage of vanadium is between approximately 0.65 and 0.8.

[0014] According to one embodiment, the power of the laser radiation is greater than or equal to 306 W and less than or equal to 330 W.

[0015] According to one embodiment, the laser scanning speed is greater than or equal to 1330 mm.s 1 and less than or equal to 1390 mm.s '.

[0016] According to one embodiment, the vector gap between two successive passes of the laser is between 0.082 and 0.088 mm.

[0017] According to one embodiment, the power of the laser radiation, the laser scanning speed and the vector gap between two successive passes of the laser are chosen so that a volumetric energy density is greater than or equal to 44 J.mm3 and less than or equal to 52 J.mm3, preferably greater than or equal to 44 J.mm3 and less than or equal to 48 J.mm3, more preferably equal to 45 J.mm3.

[0018] According to one embodiment, the thickness of each layer of powder is greater than or equal to 50 pm and less than or equal to 70 pm, preferably greater than or equal to 55 pm and less than or equal to 65 pm, more preferably equal to 60 pm.

[0019] This disclosure further relates to a mechanical part for an aircraft obtained by means of the manufacturing process defined above, the part being chosen in particular from a part of a landing gear, a part of an aircraft turbomachine, a part of a nacelle or a helicopter part. DESCRIPTION OF THE FIGURES

[0020] Figure 1 shows tensile test results on specimens from parts produced by conventional additive manufacturing on TA6V, as well as on specimens from parts obtained by implementing the proposed process. DETAILED DESCRIPTION OF THE EMBODIMENT METHOD

[0021] The proposed process uses the titanium alloy marketed under the name "TIMETAL® 575" or Ti-575. The process employs selective laser melting, which consists of successively heating layers of powder using laser radiation, with the metal melting at the point where the laser is applied. After the laser radiation passes over a given position in a layer, a "bead" is formed, corresponding to a treated area of ​​that layer. All the solidified beads of a layer thus form a section, which will constitute part of the thickness of the part once cooled. Each section has a thickness that depends on the initial thickness of the layer from which it originates, but which may differ from it due to the melting and solidification processes that allow the section to form.

[0022] The Ti-575 titanium alloy used for implementing the process has the properties listed in Table 2 below.

[0023] [Tables2] Chemical element Mass percentage Al 4.7 - 6.0% V 6.5 - 8.0% Si 0.15-0.6% Fe Up to 0.3% O 0.15-0.23%

[0024] The remaining mass percentage consists essentially of titanium, but may include impurities and / or additives that do not significantly affect the mechanical properties of the alloy.

[0025] According to some embodiments, the Al / V ratio of the alloy, defined as the concentration of aluminium divided by the concentration of vanadium by mass percentage, is between approximately 0.65 and 0.8.

[0026] The inventors realized that the use of successive layers of a powder of this metal to form parts by selective laser melting made it possible, under certain conditions of implementation of additive manufacturing defined below, to obtain parts with more advantageous mechanical properties than those obtained for other titanium alloys, such as for example TA6V.

[0027] According to the proposed process, the power P of the laser radiation applied to successive layers of Ti-575 powder is between 306 and 378 W. The laser scanning speed V, corresponding to the speed at which it moves across the powder layer, is between 1305 and 1420 mm / s. Finally, the gap vector h is between 0.080 and 0.090 mm, the gap vector h being defined as the distance between the centers, depending on the thickness of the layers in the powder bed, of two successive solidified cords.

[0028] These three parameters define a parameterity which makes it possible to obtain a Ti-575 part exhibiting advantageous mechanical properties.

[0029] Together, these three parameters define a volumetric energy density of the process, in J.mm3, according to the relation:

[0030] V Ad

[0031] where d represents the layer thickness.

[0032] The volumetric energy density Ev of the process corresponds to the energy imparted by the laser to a volume of powder, and is considered in the field of selective laser melting as characterizing the implementation conditions of a melting process. Indeed, many properties of the part obtained by these processes have been proven to depend directly on the volumetric energy density—examples include the grain size in the metal part, its porosity, and the roughness of its surface.

[0033] Preferably, the parameterization is chosen as defined above for the three parameters considered, and such that the volume energy density Ev is substantially greater than or equal to 44 J.mm3 and substantially less than or equal to 52 J.mm3. Preferably, the volume energy density Ev is substantially greater than or equal to 44 J.mm3 and substantially less than or equal to 48 J.mm3. Even more preferably, the volume energy density Ev is substantially 45 J.mm3.

[0034] Compared to parts obtained by conventional selective laser melting processes on TA6V powder, as well as by a selective laser melting process on Ti-575 powder outside the proposed parameterization, the parts obtained by melting Ti-575 powder according to several examples corresponding to this parameterization are of better quality. This corresponds firstly to an improvement in the mechanical properties of the parts, the parts produced by the proposed process exhibiting: - A mechanical resistance greater than 1050 MPa, - A conventional yield strength of R0>2 > 1000 MPa, - An elongation at break greater than 12%, and therefore a ductility greater than that of parts obtained by melting TA6V.

[0035] Compared with the values ​​obtained for equivalent parts obtained by additive manufacturing from TA6V powder, listed in Table 1, the mechanical properties of the parts obtained by the proposed process are therefore superior.

[0036] These properties are obtained subsequently in heat treatment steps following additive manufacturing. The heat treatment steps may include solution treatment of the part followed by aging of the part, under conditions conventional for those skilled in the art of working with titanium parts produced by additive manufacturing. For example, the heat treatment steps may successively include: - a solution treatment carried out at 910°C for one hour - forced air cooling, - a treatment at 500°C for eight hours, and - cooling in open air.

[0037] The improvement in the quality of the parts obtained, thanks to the proposed process, is also demonstrated by the following quantities: - a reduction in linear and spherical markings, these markings corresponding to defects highlighted in the part, - a reduction in the porosity rate in the part, corresponding to a ratio between a total volume of pores present in the part and an overall volume of the part.

[0038] The indications are signals detected by non-destructive testing methods and / or by micrography, relating to anomalies (for example, cracks, lack of fusion, oxides, porosities, inclusions, pitting, etc.).

[0039] Spherical indications are indications which appear, in control, with rounded shapes (i.e. without sharp angles) and whose length / width ratio is less than or equal to 2.

[0040] Linear (or non-spherical) indications correspond to all indications that cannot be considered as spherical indications as defined in the preceding paragraph.

[0041] These properties for several examples of parts, obtained by additive manufacturing from Ti-575 powder (examples no. 1 to 4) are listed in Table 3.

[0042] [Tables3] Example 1 Example 2 Example 3 Example 4 Laser power P (W) 270 298 318 378 Scanning speed V (mm.s) 1475 1505 1375 1405 Vector spacing h (mm) 0.088 0.094 0.086 0.088 Volumetric energy density Ed (J.mm3) 31.7 37.6 44.6 50.8 Maximum spherical indication (pm) 148 83 92 90 Maximum linear indication (pm) 645 137 64 57 Porosity rate (%) 1.578 0.769 0.234 0.321

[0043] According to one embodiment, the power of the laser radiation P is between 306 and 330 W, preferably about 318 W.

[0044] According to one embodiment, the scanning speed of the laser V is between 1330 and 1390 mm.s', preferably about 1375 mm.s'.

[0045] According to one embodiment, the vector deviation h is between 0.082 and 0.088, preferably about 0.086 mm.

[0046] Selective melting implemented in these narrower ranges unexpectedly corresponds to a different volumetric energy density than that used for conventional additive manufacturing processes on TA6V powder. Indeed, the volumetric energy density for these narrower ranges is between approximately 44 and 52 J.mm³, and is therefore higher than that of conventional TA6V selective melting processes (approximately 32 to 43 J.mm³).

[0047] Figure 1 shows the conventional yield strength R0>2 (in MPa) and the elongation at break (in %) obtained during tensile tests on specimens for a Ta6V part produced by a conventional additive manufacturing process, and for two Ti-575 parts produced using the proposed process. These tensile tests were carried out according to ASTM E8 test standards (European equivalent EN 2002-001). The specimens used have a length of 53 mm, a working area diameter of 4 mm, and a head diameter of 9 mm.

Claims

Demands

1. A method for manufacturing a part comprising a titanium alloy by selective laser melting of metal powder layers, wherein the titanium alloy comprises: a mass percentage of aluminum greater than or equal to 4.7% and less than or equal to 6.0%, a mass percentage of vanadium greater than or equal to 6.5% and less than or equal to 8.0%, a mass percentage of silicon greater than or equal to 0.15% and less than or equal to 0.6%, a mass percentage of iron less than or equal to 0.3%, a mass percentage of oxygen greater than or equal to 0.15% and less than or equal to 0.23%, the remaining mass percentage comprising titanium and optionally impurities and / or additives, and wherein: a laser power (P) of the laser applied to the metal powder is greater than or equal to 306 W and less than or equal to 378 W, a laser scan speed (V) is greater than or equal to 1305 mm. s 1 and less than or equal to 1420 mm.s', and a vector gap (h) defined as a distance between centers, according to a thickness of the layers in the powder bed, of two successive solidified cords between two successive passes of the laser is between 0.080 mm and 0.090 mm.

2. A method according to claim 1, wherein a ratio between the mass percentage of aluminium and the mass percentage of vanadium is between about 0.65 and 0.

8.

3. A method according to any one of claims 1 and 2, wherein the power of the laser radiation (P) is greater than or equal to 306 W and less than or equal to 330 W.

4. A method according to any one of claims 1 to 3, wherein the laser scanning speed (V) is greater than or equal to 1330 mm. s 1 and less than or equal to 1390 mm. s '.

5. A method according to any one of claims 1 to 4, wherein the vector gap (h) between two successive passes of the laser is between 0.082 and 0.088 mm.

6. A method according to any one of claims 1 to 5, wherein the power (P) of the laser radiation, the scanning speed (V) of the laser and the vector gap (h) between two successive passes of the laser are chosen so that a volume energy density (Ev) is greater than or equal to 44 J.mm3 and less than or equal to 52 J.mm3, preferably greater than or equal to 44 J.mm3 and less than or equal to 48 J.mm3, more preferably equal to 45 J.mm3.

7. A method according to any one of claims 1 to 6, wherein a thickness of each layer of powder is greater than or equal to 50 pm and less than or equal to 70 pm, preferably greater than or equal to 55 pm and less than or equal to 65 pm, again preferably equal to 60 pm.

8. Mechanical part for an aircraft obtained by means of the process according to any one of claims 1 to 7, the part being in particular selected from a part of a landing gear, a part of an aircraft turbomachine, a part of a nacelle or a helicopter part.