Metal-coated aluminium alloy particles, process for producing same, and additive process using the coated, aluminium alloy-particles

Abstract

The present invention concerns a metal-coated aluminium alloy particles (1) (= Me-coated Al-alloy particles), comprising, • individual particles (= Al-particles) of the aluminium alloy (= Al-alloy) in the form of powder (2p) or of a wire (2w), with • a continuous coating (3c) of a metal composition (3) (= Me-coating) applied over substantially a whole area of the Al-particles, wherein the metal composition comprises a metal mixture (Me + MeOx) of a metal (Me) selected from titanium or zirconium (i.e., Me = Ti or Zr) and of an oxide of formula, MeOx, of the same metal (i.e., Me + MeOx = Ti + TiOx or Zr + ZrOx), wherein x = 0.01 to 0.5, and oxygen atoms are present in the metal mixture between 0.1 and 50 atom%, preferably between 1 and 33 atom% on an atomic base (= atom.%) of the metal mixture, preferably between 10 and 20 atom.%.

Description

METAL-COATED ALUMINIUM ALLOY PARTICLES, PROCESS FOR PRODUCING SAME, AND ADDITIVE PROCESS USING THE COATED, ALUMINIUM ALLOY-PARTICLESFIELD OF THE INVENTION

[0001] The invention relates to aluminium alloy particles suitable for additive processing with substantially reduced hot cracking. In particular, it concerns aluminium particles in the form of powder or wire coated with a continuous coating of a metal composition comprising a metal mixture (Me + MeOx) of a metal (Me) selected from titanium or zirconium (i.e., Me = Ti or Zr) and of an oxide of formula, MeOx, of the same metal (i.e., Me + MeOx = Ti + TiOx or Zr + ZrOx),

[0002] The use of the metal coated particles of the present invention in additive manufacturing of parts substantially reduces the occurrence of hot cracking and reduces formation of pores caused by solid state trapping of hydrogen.BACKGROUND OF THE INVENTION

[0003] A wide variety of aluminium allpys (= Al-alloy) are available showing a range of different mechanical properties. As shown in Figure 1 , Al-alloys parts are traditionally produced either by casting methods, or by machining wrought Al-alloys. Specific Al-alloy grades are more suitable for casting methods, such as the series Nxx.x, with N = 1 to 5 or 7, and other are more suitable wrought Al-alloys, such as the series Nxxx each x independently varying from 0 to 9. High performance Al-alloy parts are generally produced with wrought Al-alloys.

[0004] Additive manufacturing or additive processing is yet an alternative and attractive production technique illustrated in Figures 5(a)&5(b), 6(a)&6(b), and 7 (discussed in more details in continuation), allowing complex geometry 3D-parts to be produced by feeding and laying Al-alloy particles onto a support to form a layer and continuously locally melting and then allowing to solidify the Al-alloy according to a predefined geometry as it is being laid. The operation is repeated on top of a thus formed solidified Al-alloy layer until forming the 3D-part. The processing conditions in additive manufacturing are close to autogenous welding which, as well known by the skilled person, can be problematic and even impossible for some Al-alloy grades. Additive processing can also be used for repairing a damaged part made of an Al-alloy. Since the Al-alloy particles must be melted, it is clear that only weldable Al-alloy grades are suitable for additive processing. As indicated in Figure 1 , heat treatable alloys are more often used for high strength applications than non-heat treatable alloys, because the heat treatment causes precipitation hardening of the alloy. Casting Al-alloys generally have lower mechanical properties than wrought Al-alloys, the latter heat treatable alloys are preferred for producing advanced engineering parts suitable for aerospace applications and the like. Wrought Al-alloys can also increase their mechanical properties by strain hardening processes, every time such wrought Al-alloys is laminated, forged or otherwise strained during the manufacturing of a part.

[0005] As in welding applications, the biggest challenge with additive processing is hot cracking. Hot cracking happens after the melted Al-alloys is cooled. Depending on the composition and associated phase diagram of the components forming the Al-alloy, upon colling, some alloying components of the Al-alloy migrate into the liquid phase remaining between the growing dendrites and are eventually concentrated at the grain boundaries. The stress of solidification may cause cracking at the grain boundaries. Because this problem has not been satisfactorily solved to date, additive processing with heat treatable wrought Al-alloys is still incipient. Cracking sensitivity curves are available for each possible element added to aluminium. Figure 2 shows examples of such cracking sensitivity curves for Si (e.g., 4xxx grades), Cu (e.g., 2xxx grades), Mg (e.g., 5 xxx grades), and Mg-SI (e.g., 6xxx grades). These curves show the probability of cracking as a function of the amount of the added element. The higher the curve, the higher the probability of having hot cracking. For example, the peaks of the curves for silicon, magnesium, and magnesium silicide cracking curves are at around 1 %, whilst the peak for Cu is at about 3%.

[0006] EP3351322 proposes an additive process of Al-alloys avoiding another problem associated with Al-alloys processed with an additive technique, namely oxidation of the aluminium during melting. The problem is solved by coating the Al-alloy powder with a metallic material, which causes an exothermic reaction with aluminium when melted together. This causes turbulences which prevent the formation of an oxide layer and disperse instead the aluminium oxides thus formed within the bulk of the solidified part. This document does not address the problem of hot cracking.

[0007] WO2020212312 describes a method for functionalizing Al-alloy powder particles with nanoparticles of a second material selected from a long list. In the same field, Tan et al.1describe inoculation of a 7075 Al-alloy powder with 1 wt% Ti submicron particles to reduce hot-cracking occurrence. Coating Al-alloy powder particles with nanoparticles does not solve another major problem of additive manufacturing parts with Al-alloy particles: viz., pores formation due to trapped hydrogen in the meltpool during manufacturing.

[0008] The Al-alloy powder reacts with the moisture of the atmosphere to form aluminium hydroxides at the surface of the particles that are very hard to remove at low temperature. Upon locally melting the particles, the hydrogen concentrates in the meltpool and is trapped in the solidifying material upon rapid cooling forming pores distributed in the material which are detrimental to the mechanical properties thereof. The discontinuous coating formed by the nanoparticles does not prevent atmospheric moisture from reaching and reacting with the Al-alloy particles, thus leading to hydrogen linked issues.

[0009] It can be seen that additive manufacturing with Al-alloy particles is full of challenges not1Tan et al. Materials Science & Engineering A 821 (2021) 141638yet solved. The present invention proposes a solution for allowing 3D-parts to be manufactured with Al-alloy particles generally considered as difficult or even impossible to process by additive methods. These and other advantages are described in more details in the following sections.SUMMARY OF THE INVENTION

[0010] The present invention concerns metal-coated aluminium alloy particles (= Me-coated Al-alloy particles), comprising,• individual particles (= Al-particles) of the aluminium alloy (= Al-alloy) in the form of powder or of a wire, with• a continuous coating of a metal composition (= Me-coating) applied over substantially a whole area of the Al-particles, wherein the metal composition comprises a metal mixture (Me + MeOx) of a metal (Me) selected from titanium or zirconium (i.e., Me = Ti or Zr) and of an oxide of formula, MeOx, of the same metal (i.e., Me + MeOx = Ti + TiOx or Zr + ZrOx), wherein x = 0.01 to 0.5, and oxygen atoms are present in the metal mixture between 0.1 and 50 atom%, preferably between 1 and 33 atom% on an atomic base (= atom.%) of the metal mixture, preferably between 10 and 20 atom.%.

[0011] The Me-coating is present in the Al-alloy powder in an amount comprised between 0.5 and 6 wt.%, preferably between 0.8 and 4 wt.%, more preferably between 1 and 3 wt.% of the total weight of the Me-coated Al-alloy particles and is preferably applied by physical vapour deposition (PVD), preferably by magnetron sputtering.

[0012] In one embodiment, the individual particles are preferably in the form of a powder formed of substantially spherical individual particles having a mean diameter comprised between 20 and 200 pm, preferably between 30 and 120 pm. The Me-coating on the substantially spherical individual particles can have an average thickness comprised between 50 and 300 nm, preferably between 100 and 200 nm, measured according to ISO13320:2020. The measuring method is based on the deflection of a laser beam by a set of particles dispersed in a liquid or air stream. The diffraction or scattering angles are characteristic of the size of the particles.

[0013] In an alternative embodiment, the individual particles can be in the form of a wire having a mean diameter comprised between 500 pm and 10 mm, preferably between 1 and 5 mm. The Me-coating on wires can have an average thickness comprised between 12 and 18 pm, preferably between 13 and 15 pm.

[0014] The aluminium alloy is preferably an aluminium alloy of the series 2000, 6000 or 7000, preferably an aluminium alloy of the type 7075, 2024, or 6061 , as they cannot easily be processed by additive manufacturing techniques.

[0015] The metal (Me) of the metal composition preferably has at least 2N (= 99%) purity,preferably at least 3N (= 99.9%) purity.

[0016] The present invention also concerns a process for producing the Me-coated alloy particles as defined supra, comprising the following steps,• providing the Al-particles formed of individual particles having an outer area,• depositing the metal composition comprising the metal mixture (= Me + MeOx) onto the individual particles until forming the continuous coating over substantially the entire outer area of the individual particles, to form the Me-coated Al-particles,• storing the Me-coated Al- particles ready for use.

[0017] The deposition of the metal composition can be carried out by a dry deposition method, preferably by physical vacuum deposition (PVD), more preferably by magnetron sputtering or, alternatively, or by chemical vapour deposition (CVD), preferably by plasma enhanced chemical vapour deposition (PECVD). A gas composition for forming a plasma for the dry deposition of the metal composition can be controlled such as to ensure that an amount of oxygen atoms present in the metal mixture is preferably comprised between 0.1 and 50 atom%, preferably between 1 and 33 atom.% on an atomic base (= atom.%) of the metal mixture, preferably between 10 and 20 atom.%.

[0018] The present invention also concerns a process for producing a solid part made of Al-alloy by additive manufacturing comprising the following steps,• providing a Me-coated Al-alloy particles (as defined supra,• feeding the Me-coated Al-alloy particles,• locally melting the Me-coated Al-alloy particles with an energy source according to a predefined geometry to form a melted pool which forms a solid part layer upon cooling, wherein the energy source is selected among a laser and an electric arc,• repeating a number of times the former two operations on top of the solid part layer thus formed to form successive solid part layers of desired geometries, until forming the solid part.

[0019] The additive manufacturing can be selected among the group of laser powder bed fusion (L-PBF), arc additive manufacturing (WAAM), or direct energy deposition (DED). The solid part (11s) is preferably exposed to a heat treatment, for increasing mechanical properties of the solid part.

[0020] This process can be used to produce new Al-alloy parts or, alternatively, to repair an existing damaged or worn part.SHORT DESCRIPTION OF THE DRAWINGS

[0021] These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings in which:Figure 1 shows various grades of Al-alloys classified between cast Al-alloys and wrought Al-alloys, and for each of these two classes between heat treatable and non-heat treatable grades.Figure 2 shows cracking sensitivity curves for Al-alloys comprising Si, Cu, Mg, and Mg-si (= magnesium silicide).Figure 3 schematically illustrates the principle of PVD.Figures 4(a) & 4(b) show idealized particles according to the present invention, (a) a partially cut spherical powder particle and (b) a cylindrical wire particle.Figure 5(a) represents an apparatus for producing or repairing parts by laser powder bed fusion (L-PBF).Figure 5(b) shows a detail of the apparatus of Figure 5(a), showing the controlled fusion and cooling of the Al-alloy powder for producing or repairing parts of complex geometries.Figures 6(a) & 6(b) show a detail of apparatuses for producing or repairing parts by direct energy deposition (DED) using (a) an Al-alloy wire and (b) Al-alloy powder.Figure 7 represents an apparatus for producing or repairing parts by wire arc additive manufacturing (WAAM).Figure 8 represents an example of welding two aluminium alloy parts with an aluminium alloy according to the present invention.Figures 9(b) & 9(c) show (b) yield strength (cry), and (c) ultimate strength (cru), of samples produced by L-PBF with coating of powder with Ti + TiOx according to the present invention, “INV” = Al-alloy 7075 out of a 3D-printer, “INV + T6” = “as built” with a thermal treatment ageing of the samples, and “INV+SHT + T6” = Al alloy7075 ” with a solution heat treatment and a thermal treatment ageing of the samples.Figure 10 shows the typical regions in a yield stress vs ultimate strain plot occupied by different grades of printable alloys, including the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0022] As illustrated in Figures 4(a) and 4(b), the present invention concerns metal-coated aluminium alloy particles (1) (= Me-coated Al-alloy particles), comprising individual particles of the aluminium alloy (= Al-alloy) and a continuous coating (3c) of a metal composition (3) (= Me-coating). The individual particles of the Al-alloy can be in the form of powder (2p) (cf. Figure 4(a)) or of a wire (2w) (cf. Figure 4(b)). The continuous Me-coating is applied over substantially a whole area of the Al-particles. The metal composition comprises a metal mixture (Me + MeOx) of a metal (Me) selected from titanium or zirconium (i.e., Me = Ti or Zr) and of an oxide of formula, MeOx, of the same metal (i.e., Me + MeOx = Ti + TiOx or Zr + ZrOx), wherein x = 0.01 to 0.5. The metal oxide (MeOx) is present in amounts such that oxygen atoms are present in the metal mixture between 0.1 and 50 atom%, preferably between 1 and 33 atom% on an atomic base (= atom.%) of the metal mixture, more preferably between 10 and 20 atom.%.

[0023] The Me-coated Al-alloy particles of the present invention allow the production orthe repair of Al-alloy parts by additive manufacturing with substantially less hot cracking issues than with state of the art particles. A corresponding micrograph showing a sample “INV +SHT+ T6” according to the present invention is not shown, as it would appear substantially blank, with substantially no hot crack appearing. Figure 9(b) illustrates the tensile yield strength measured on parts made by additive manufacturing with particles with the Me-coated Al-alloy particles of the present invention with no further treatment after printing (INV = “as build”), the same followed by an ageing T6-treatment (= INV+ T6”), and parts produced with Me-coated Al-alloy particles of the present invention followed by an ageing T6-treatment (=“INV +SHT+ T6”). The tensile properties of the parts produced with the Me-coated Al-alloy particles of the present invention have a yield strength about 18% higher than the “INV + T6” samples. A corresponding micrograph of a sample “INV+SHT + T6” is not shown, because it comprises substantially no crack, and appear substantially blankMe-COATED AI-ALLOY PARTICLES

[0024] As shown in Figure 4(a), the Me-coated Al-alloy particles of the present invention can be in the form of a powder. Alternatively, as shown in Figure 4(b), the Me-coated Al-alloy particles of the present invention can be in the form of wires. In both cases, the Me-coating (3c) can be present in the Al-alloy powder in an amount comprised between 0.5 and 6 wt.%, preferably between 0.8 and 4 wt.%, more preferably between 1 and 3 wt.% of the total weight of the Me-coated Al-alloy particles (1). As discussed in continuation, the Me-coating (3c) is preferably applied by physical vapour deposition (PVD), preferably by magnetron sputtering.

[0025] The cores of the Me-coated Al-alloy powder particles are formed by the individual powder particles (2p) of the Al-alloy. They have a mean diameter (Dp) which can be comprised between 20 and 300 pm, preferably between 30 and 250 pm, preferably between 160 and 200 pm or preferably between 50 and 150 pm. The Me-coating (3c) can have an average thickness (tp) comprised between 50 and 300 nm, preferably between 100 and 200 nm.

[0026] The cores of the Me-coated Al-alloy wires are formed by the individual wire particles (2w) of the Al-alloy. They have a mean diameter (Dw) which can be comprised between 500 pm and 10 mm, preferably between 1 and 5 mm. The Me-coating (3c) can have an average thickness (tw)comprised between 12 and 18 pm, preferably between 13 and 15 pm.

[0027] The individual particles (2p, 2w) must be made of Al-alloys which are heat treatable. A list of such Al-alloys is shown in Figure 1 , including the wrought Al-alloy grades 2xxx, 6xxx, and 7xxx, as well as the cast Al-alloy grades 2xx.x, 3xx.x, and 7xx.x. Preferably, the Al-alloys are wrought Al-alloys of the grade series 2xxx, 6xxx, or 7xxx. For example, the present invention is very suitable with Al-alloy grades 7075, 2024, or 6061 , which are used in the aerospace industry for their good mechanical properties, but which are very difficult to weld, requiring preheating the substrates at high temperature before welding, to reduce the occurrence of hot cracking.

[0028] The metal (Me) in the metal composition of the coating (3c) is selected between titanium (Ti) and zirconium (Zr). The metal (Me) of the metal composition (3) preferably has at least 2N (= 99%) purity, preferably at least 3N (= 99.9%) purity. The corresponding oxide (MeOx) selected from TiOx and ZrOx can be present such as to yield an amount of oxygen atoms present in the metal mixture comprised betweenO.1 and 50 atom%, preferably between 1 and 33 atom% of the metal mixture, preferably between 2 and 30 atom.%, more preferably between 5 and 25 atom.%, more preferably between 10 and 20 atom.% or 15 +.2 atom.%. The amount of oxygen in the coating can be controlled during deposition of the coating by a PVD process by controlling the composition of the gas (5g) injected into and forming the plasma (5p) in the chamber (6) (cf. Figure 3).

[0029] The coating (3c) is continuous and is applied over substantially a whole area of the Al-particles. This has the advantage of substantially reducing hydrogen induced blistering. As explained in the Background Art section supra, air moisture reacts with the Al-particles forming aluminium hydroxides at the surface of the particles, which are very difficult to remove. When the particles are melted for additive manufacturing, hydrogen concentrates in the meltpool (1 m) and is trapped therein because of the rapid cooling, forming a porosity favouring the formation and propagation of defects.

[0030] The use of Ti or Zr as a continuous coating offers the possibility to lower the baking temperature of the hydroxide to around 200°C within the range of preheating of most additive manufacturing machines. The continuous coating therefore protects the Al-particles from direct contact with moisture and prevents the formation of hard to remove aluminium hydroxides, replacing them with easier to remove Me-hydroxides. This simplifies powder storing conditions. This advantage comes on top of the effect of inhibiting hot cracking in generally considered as non-printable aluminium alloys.PROCESS FOR COATING THE INDIVIDUAL PARTICLES (2p, 2w) OF Al ALLOY

[0031] The Me-coated alloy particles (1) of the present invention can be produced by a process comprising the following steps, providing the Al-particles (2p, 2w) formed of individual particles having an outer area,depositing the metal composition (3) comprising the metal mixture (= Me + MeOx) onto the individual particles (2p, 2w) until forming the continuous coating (3c) over substantially the entire outer area of the individual particles, to form the Me-coated Al-particles (1),• storing the Me-coated Al- particles ready for use.

[0032] The deposition of the metal composition can be carried out by dry or wet deposition techniques. Preferably, deposition is carried out by a dry deposition method, such as physical vacuum deposition (PVD) or by chemical vapour deposition (CVD).

[0033] Physical Vapour Deposition (PVD) is a vacuum coating process in which the layer of coating material is deposited atom by atom on the Al-particle substrate.

[0034] PVD is in some respects similar to CVD, except that in PVD the precursors, i.e., the material to be deposited onto the Al-particles surface, start out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in gaseous form. In PVD, the metal composition forming the coating (3c) transitions from a condensed phase forming a target (3t) to a vapour phase in a plasma (5p); and then back to a thin film condensed phase forming the Me-coating (3c). Various types of PVD-techniques, generally varying how the coating material is to be evaporated from the target (3t). They include cathodic arc deposition, electron-beam physical vapor deposition, Evaporative deposition, pulsed laser deposition, sputter deposition (or sputtering), such as magnetron sputtering (e.g., direct current (DC), alternative current (AC), or High Power Magnetron Sputtering (HilPMS)), and the like.

[0035] In sputtering, a highly energetic ions ablates coating material (3) from the target (3t) generating a plasma stream (5p) under nonequilibrium conditions. As shown in Figure 3, a vacuum pump (9) reduces the pressure in the chamber (6), and an inlet allows sputtering gas (5g) to be introduced into the chamber. The atoms (3) of the coating material form a target (3t) enclosed in the chamber (6). The atoms are ejected from the target (3t) by the highly energetic ions and have a wide energy distribution, typically few eV. A fraction of the ejected particles is ionized (about 1 %) forming ionized particles (3i) which can reach the Al-particles substrate from the target (3t), either flying in straight lines or after a random walk, depending on the gas pressure in the chamber (6). The ionized and neutral particles (3i) condense on the surfaces of the Al-particles (2p, 2w) substrate.

[0036] The sputtering gas (5g) is often an inert gas such as argon. Reactive gases can also be used to sputter compounds. This allows controlling the amount of oxygen atoms present in the Me-coating (3c) by controlling the 02-content in the sputtering gas (5g) injected into the chamber (6) through an inlet. The sputtering gas (5g) composition forming a plasma (5p) is therefore preferably controlled such as to ensure that an amount of oxygen atoms present in the metal mixture is comprised between 0.1 and 50 atom%, preferably between 1 and 33 atom.% on an atomic base (= atom.%) of the metal mixture, preferably between 10 and 20 atom.%.

[0037] Magnetron sputtering is a high-rate PVD-vacuum coating technique characterised by the application of a magnetic field and use of a negatively charged cathode to trap electrons near the target materials.

[0038] Alternatively, deposition can be carried out by CVD. In typical CVD, the Al-particle substrate is exposed to one or more volatile precursors, which react and / or decompose on the substrate surface to produce the desired deposit. Plasma enhanced chemical vapour deposition (PECVD) is a deposition technology to deposit thin films using plasma technology. Compared to other deposition technologies such as PVD and Thermal CVD, PECVD can deposit thin films with high uniformity over the Al-particles substrate at relatively low temperature (less than 350°C).

[0039] A major advantage of the present invention is that since the Me-coating (3c) must comprise metal oxides (MeOx), when the prior art teaches the opposite (cf. e.g., EP3351322), the Me-coated Al-alloy particles (1) need not be stored protected from ambient air and can be handled more easily.PROCESS FOR PRODUCING A 3D-PART BY ADDITIVE MANUFACTURING

[0040] The Me-coated Al-alloy particles of the present invention can be used in additive manufacturing processes, either for forming a part, or for repairing an existing part which requires fixing. Additive manufacturing allows complex geometry 3D-parts to be produced by locally melting and allowing to solidify Me-coated Al-alloy particles laid on a substrate along a predefined pattern to form a layer (11sL) of solid Al-alloy. The operation is repeated with successively deposited solid layers (11sL) on top of one another as illustrated in Figures 5(b), 6(a), 6(b), and 7. As discussed earlier, depending on the compositions of the Al-alloys used and depending on the cracking sensitivity of the components forming the Al-alloys, as shown in Figure 2, solid parts (11s) produced by additive processing can be weakened by hot cracking phenomena, unacceptable for aerospace applications and the like.

[0041] Al alloy parts can be produced by additive manufacturing using Me coated Al alloy particles (1) according to the present invention as follows. The Me coated Al alloy particles are fed and laid onto a support (19p) or onto a previously laid solid layer (1 1 sL), or onto an existing Al-part to be repaired. The Me coated Al alloy particles (1) are locally melted with an energy source (13L, 13a) according to a predefined geometry to form a melted pool (1 m). Upon cooling the melted pool solidifies forming a solid part layer (11sL). The former operations are repeated a number of times on top of the solid part layer (11sL) thus formed to form successive solid part layers (11sL) of desired geometries, until forming the solid part (11s). The energy source used for locally melting the Al particles (1) can be selected among a laser (13L) and an electric arc (13a).

[0042] Typical additive manufacturing processes which can be used in the present invention include laser powder bed fusion (L-PBF), direct energy deposition (DED), and wire arc additive manufacturing (WAAM),Laser powder bed fusion (L-PBF)

[0043] An example of laser powder bed fusion (L-PBF) apparatus is illustrated in Figures 5(a) and 5(b). A load of Me-coated Al-alloy particles according to the present invention are stored in a storing compartment. A feeding system is provided configured for transferring the Me-coated Al-alloy particles from the storing compartment to a manufacturing portion of the apparatus. Typically, a floor (19f) of the storing compartment is mobile and can be raised, e.g., with a feeding piston (17f), to bring a top surface of the Me-coated Al-alloy particles stored in the storing compartment flush with or slightly higher than a top edge of a weir (14) separating the storing compartment from the manufacturing portion. A top layer of Me-coated Al-alloy particles can be raked with a coater blade (15) or a roller from the storing compartment, over the weir (14), to the manufacturing portion, thus forming a layer of loose Me-coated Al-alloy particles sitting on a support (19p). A laser (13L) locally melts the Me-coated Al-alloy particles according to a predefined pattern, thus forming a meltpool (1 m). Upon cooling, the melted Me-coated Al-alloy particles solidify forming a first solid part layer (11 sL) laying on the support (19p). When the first solid part layer (11sL) is formed, the support (19p) is lowered (e.g., with a part piston (17p) to bring the top surface of the first layer flush with the top edge of the weir (14). The floor (19f) of the storing compartment is raised of a predefined height and Me-coated Al-alloy particles are transferred over the weir (14) with the coater blade (15) or roller, over the first solid part layer (11sL). A next solid part layer (11sL) is formed on top of the previously formed solid part layer (11sL) by locally melting the Me-coated Al-alloy particles with the laser (13L). These operations are repeated until the solid part is completed. Parts with extremely intricate 3D-geometries can be produced with this method, which could not be achieved, or at a higher cost, by casting or machining wrought Al-alloys.

[0044] The thickness of each solid part layer (11sL) can be controlled by the amount of Me-coated Al-alloy particles transferred from the storing compartment over the weir (14) and to the manufacturing portion. This can be precisely controlled by controlling the position of the floor (19f) of the storing compartment relative to the top edge of the weir (14). In other words, by controlling how much above the top edge of the weir (14) is the top surface of the of the Me-coated Al-alloy particles stored in the storing compartment.Direct energy deposition (DED)

[0045] As illustrated in Figures 6(a) and 6(b), direct energy deposition (DED) comprises a beam (laser or electron beam) that is moved by a robot and a feeding system (powder (cf. Figure 6(b)) or wire (cf. Figure 6(a))) feeding material on the spot were melting occurs to form the meltpool (1 m) and to build the part upon cooling. The operation is repeated several times to form successive solid part layers (1 1sL) to form the solid part (1 1s). This technique can be used also for local repairs.Wire arc additive manufacturing (WAAM)

[0046] As shown in Figure 7, WAAM uses a wire as feeding material for forming the desired solid part (11s). WAAM uses an electric welding arc (13a) as energy source to locally melt the Al-alloy wire (1). A conventional welding robot can be used to apply WAAM, so that the equipment invenstment is quite reduced. Solid parts (11s) of large sizes can be produced by WAAM with complex geometries. Like DED, hybrid manufacturing, combining different materials is possible with WAAM, which is also suitable for locally repairing damaged or worn parts.Welding

[0047] The Me-coated Al-alloy particles of the present invention can be used forwelding together two parts made of an aluminium alloy. This can be very advantageous, since the same Al-grade can be used as welding material and as parts to be joined, thus increasing compatibility, joint strength, and reducing corrosion issues. An example of welding two Al-alloy parts with Me-coated Al-alloy particles according to the present invention is illustrated in Figure 8.Heat treatments

[0048] Heat treatments are used to improve mechanical properties of alloys and remove the inhomogenous history of material processing prior to this step. Heat treatments enhance homogeneity of the structure yielding higher mechanical properties in alloys by activating metallurgical processes like precipitation hardening. AMS2770 from the American material society defines temperature profiles and conditions for solution heat treatments (SHT) and ageing for aluminium alloys depending on the aluminium grades.

[0049] Contrary to wrought Al-alloys, 3D-printed Al-alloy parts cannot benefit of strain hardening, since a neat-geometry part is produced requiring no further handling. This additional strain hardening mechanism gives generally higher mechanical properties to wrought Al-alloy parts than to 3D-printed Al-alloy parts. On the other hand, wrought Al-alloy parts are restricted to fairly simple and basic geometries, thus limiting their applications, whilst 3D-printing has no limit in geometry complexity.EXAMPLE

[0050] Powder particles (2p) of Al-alloy of grade 7075 of mean diameter 45 pm, measured according to ISO13320:2020, were coated by PVD with a layer of Me-coating comprising titanium and TiOx, of mean thickness, tp = 150 nm to form Me-coated Al-alloy particles (1) according to the present invention. Tensile dog bone samples were produced in a printing machine by L-PBF with the powder particles (1 , 2p). The dog bone samples according to the present invention were divided in three batches, and the tensile properties thereof were measured,3D" are the dog bone samples produced with the first batch ) as they came out of theprinting machine• “3D + Age" are the “3D” samples further exposed to an ageing heat treatment at a temperature of 121 °C during 23 h according to T6 as defined in AMS 2770H-11 ,• “INV +SHT+ Age" are the dog bone samples ”3D + Age” exposed prior to ageing to a solution heat treatment (SHT) at a temperature of 466"C during 50 min as defined in AMS 2770H-11.

[0051] As discussed supra, heat treatments are used to improve mechanical properties of alloys. The samples have been treated according to AMS2770 standard for the 7075 aluminium grade. The samples were tested in tensile mode. The samples 3D + Age and 3D + SHT + Age were heat treated to be comparable with other printable aluminium grades. The results of the tensile tests are listed in Table 1 and illustrated in Figures 9(a) to 9(c). Figure 10 compares the mechanical properties obtained with Me-coated Al-alloy particles according to the present invention with the mechanical properties of a number of existing Al-alloy grades suitable for additive manufacturing.Table 1: tensile properties measured on the various samples.

[0052] Figure 9(a) shows a typical stress-strain curve measured on Al-alloy samples, indicating the positions of the Young modulus (E), yield strength (cry), ultimate strength (cru), and ultimate yield (su). Figures 9(b) and 9(c) show respectively, the yield strength (cry), and ultimate strength (cru). for the various samples.

[0053] It can be seen from the results listed in Table 1 and plotted in Figures 9(b) and 9(c), that the “3D” samples showed good elongation with a suboptimal value of the yield strength. As expected, ageing and SHT both increase the yield strength of Al-alloys.

[0054] Figure 10 compares the mechanical properties of various printable Al-alloys in a typical cry vs EU. Cast alloys, labelled “Cast” in Figure 10 are the Al-alloys represented in Figure 1 with grades 1xx.x to 7xx.x. They generally show moderate mechanical properties. Special grades comprising nanoparticles, labelled “nanoP” in Figure 10 show excellent strength values with, however, moderate ultimate strain values. They are quite expensive. Similarly, Al-alloys comprising scandium, labelled “Sc” in Figure 10 show very good mechanical properties, but are quite expensive. New Al-alloys were specifically developed for additive manufacturing and are labelled as “New Al” in Figure 10, including e.g., AIMgty® alloys from Fehrmann or Aheadd® CP1alloys from Constellium. They show intermediate yield strengths between cast alloys and the nanoP and Sc-alloy types, with generally high ductility (i.e., high values of EU.

[0055] The Me-coated Al-alloy particles of the present invention are at the high end of the yield strengths achieved with the “new Al” (i.e., high cry-values) but at the low end of ductility (low Eu-values). A main advantage of the Me-coated Al-alloy particles of the present invention compared with the “new Al” alloys, is that the Al-alloy particles used in the present invention are standard Al-alloys, most of them already qualified for aerospace, medical, and automotive applications, rendering the qualification of the Me-coated Al-alloy particles of the present invention substantially easier and faster to obtain than the new materials indicated under “New Al”. In addition, the present invention opens the possibility to use recycled non printable material (with sufficient basic quality) in a new high performance manufacturing 3D printing technology by the addition of the Me Coating.CONCLUDING COMMENTS

[0056] The Me-coated Al-alloy particles (1) of the present invention represent a break-through in the field of additive manufacturing of Al-alloys, as it allows Al-alloys grades generally considered as not processable by additive manufacturing because of their high sensitivity to hot-cracking to be used in conventional additive manufacturing processes without any particular precautions, such as protection from contact with ambient air, and the like. The Me-coating forming a continuous layer over substantially the whole area of the Al-particles (2p, 2w) offers a protection against hydrogen induced cracking, as the hydroxides formed by reaction of the air moisture with Ti or Zr can be removed at temperatures compatible with the temperatures commonly used in additive manufacturing for preheating the Me-coated Al-alloy particles (i.e., not more than about 200°C).

[0057] The Me-coating (3c) can be applied onto the Al-particles (2p, 2w) using conventional PVD- and CVD-techniques in an efficient and reproducible manner. Once coated, the Me-coated Al-alloy particles can be stored without any particular precaution.

[0058] Almost any additive manufacturing technique, including welding techniques, can be used with the Me-coated Al-alloy particles of the present invention for forming solid parts (11s) which can be of large size and have complex and intricate 3D-geometries, with Al-alloy grades not seriously contemplated to date for use in additive manufacturing techniques.

Claims

CLAIMS1 . Metal-coated aluminium alloy particles (1) (= Me-coated Al-alloy particles), comprising,• individual particles (= Al-particles) of the aluminium alloy (= Al-alloy) in the form of powder (2p) or of a wire (2w), with• a continuous coating (3c) of a metal composition (3) (= Me-coating) applied over substantially a whole area of the Al-particles, wherein the metal composition comprises a metal mixture (Me + MeOx) of a metal (Me) selected from titanium or zirconium (i.e., Me = Ti or Zr) and of an oxide of formula, MeOx, of the same metal (i.e., Me + MeOx = Ti + TiOx or Zr + ZrOx), wherein x = 0.01 to 0.5, and oxygen atoms are present in the metal mixture between 0.1 and 50 atom%, preferably between 1 and 33 atom% on an atomic base (= atom.%) of the metal mixture, preferably between 10 and 20 atom.% wherein the Me-coating (3c) is present in the Al-alloy powder in an amount comprised between 0.5 and 6 wt.% of the total weight of the Me-coated Al-alloy particles (1).

2. Me-coated Al-alloy particles according to claim 1 , wherein the Me-coating (3c) is present in the Al-alloy powder in an amount comprised between 0.8 and 4 wt.%, preferably between 1 and 3 wt.% of the total weight of the Me-coated Al-alloy particles (1), and is preferably applied by physical vapour deposition (PVD), preferably by magnetron sputtering.

3. Me-coated Al-alloy particles according to claim 2, wherein the individual particles (2p) are in the form of a powder formed of substantially spherical individual particles having a mean diameter (Dp) comprised between 20 and 200 pm, preferably between 30 and 120 pm, measured according to ISO13320:2020.

4. Me-coated Al-alloy particles according to claim 3, wherein the Me-coating (3c) has an average thickness (tp) comprised between 50 and 300 nm, preferably between 100 and 200 nm.

5. Me-coated Al-alloy particles according to claim 2, wherein the individual particles (2w) are in the form of a wire having a mean diameter (Dw) comprised between 500 pm and 10 mm, preferably between 1 and 5 mm.

6. Me-coated Al-alloy particles according to claim 5, wherein the Me-coating (3c) has an average thickness (tw) comprised between 12 and 18 pm, preferably between 13 and 15 pm.

7. Me-coated Al-alloy particles according to any one of the preceding claims, wherein the aluminium alloy is an aluminium alloy of the series 2000, 6000 or 7000, preferably an aluminium alloy of the type 7075, 2024, or 6061 .

8. Me-coated Al-alloy particles according to any one of the preceding claims, wherein the metal (Me) of the metal composition (3) has at least 2N (= 99%) purity, preferably at least 3N (= 99.9%) purity.

9. Process for producing Me-coated alloy particles (1) according to any one of the preceding claims, comprising the following steps,• providing the Al-particles (2p, 2w) formed of individual particles having an outer area,• depositing the metal composition (3) comprising the metal mixture (= Me + MeOx) onto the individual particles (2p, 2w) until forming the continuous coating (3c) over substantially the entire outer area of the individual particles, to form the Me-coated Al-particles (1),• storing the Me-coated Al- particles ready for use.

10. Process according to claim 9, wherein deposition of the metal composition is carried out by a dry deposition method, preferably by physical vacuum deposition (PVD), more preferably by magnetron sputtering or, alternatively, or by chemical vapour deposition (CVD), preferably by plasma enhanced chemical vapour deposition (PECVD).

11. Process according to claim 10, wherein a gas composition (5g) for forming a plasma (5p) for the dry deposition of the metal composition (3) is controlled such as to ensure that an amount of oxygen atoms present in the metal mixture is comprised between 0.1 and 50 atom%, preferably between 1 and 33 atom.% on an atomic base (= atom.%) of the metal mixture, preferably between 10 and 20 atom.%.

12. Process for producing a solid part (11s) made of Al-alloy by additive manufacturing comprising the following steps,• providing a Me-coated Al-alloy particles (1) according to any one of claims 1 to 8,• feeding the Me-coated Al-alloy particles,• locally melting the Me-coated Al-alloy particles (1) with an energy source according to a predefined geometry to form a melted pool (1 m) which forms a solid part layer (11sL) upon cooling, wherein the energy source is selected among a laser (13L) and an electric arc (13a),• repeating a number of times the former two operations on top of the solid part layer (11sL) thus formed to form successive solid part layers (11sL) of desired geometries , until forming the solid part (1 1s).

13. Process according to the preceding claim, wherein the additive manufacturing is selected among the group of laser powder bed fusion (L-PBF), arc additive manufacturing (WAAM), or direct energy deposition (DED).

14. Process according to claim 12 or 13, wherein the solid part (11s) is exposed to a heat treatment, for increasing mechanical properties of the solid part.

15. Process according to anyone of claims 12 to 14, either,• for the production of an Al-alloy part, or• for repairing an existing damaged part.

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