HIGH-STRENGTH ALUMINIUM ALLOYS FOR ADDITIVE MANUFACTURING OF THREE-DIMENSIONAL OBJECTS

Abstract

The present invention relates to aluminium alloys in powder form having a content of at least two elements M from the group comprising Cr, Fe, Ni and Co and at least one element N from the group comprising Ti, Y and Ce, the alloy having a total amount of elements M in the range of 1 to 16 wt %, a total amount of elements N in the range of 0.5 to 5 wt % if the aluminium alloy contains Ti or Ce, and 1 to 10 wt %, if the aluminium alloy contains Y. Such aluminium alloys can be used in additive manufacturing processes, such as selective laser melting, to produce high-strength three-dimensional objects which can be used, for example, in engines for automobiles. The present invention further relates to processes and apparatuses for manufacturing three-dimensional objects from such aluminium alloys, processes for manufacturing such aluminium alloys in powder form, three-dimensional objects manufactured from such aluminium alloys in powder form, and specific aluminium alloys.

Claims

1. An aluminium alloy in powder form having a content of at least two elements M from the group comprising Cr, Fe, Ni and Co and at least one element N from the group comprising Ti, Y and Ce, wherein the alloy has a total amount of elements M in the range of 1 to 16 wt %, a total amount of elements N in the range of 0.5 to 5 wt %, if the aluminium alloy contains Ti or Ce, and 1 to 10 wt %, if the aluminium alloy contains Y.

2. The aluminium alloy in powder form according to claim 1, wherein the aluminum alloy contains at least 0.05 wt % oxygen.

3. The aluminium alloy in powder form according to claim 1 having a content of at least 0.5 or at most 8 wt % Fe, at least 0.5 or at most 4.0 wt % Cr and at least 0.5 or at most 4.0 wt % Ti.

4. The aluminium alloy in powder form according to claim 3 having a content of at least 3 or at most 7 wt %, Fe, at least 2 or at most 4 wt % Cr, at least 1 or at most 4 wt %Ti and at least 80 or at most 93 wt % aluminium.

5. The aluminium alloy in powder form according to claim 1 having a content of at least 1 or at most 7.5 wt % Ni, at least 1 or at most 5.5 wt % Co and at least 2 or at most 10 wt % Y.

6. The aluminium alloy in powder form according to claim 1 having a content of at least 2 or at most 10 wt % Ni, at least 0.5 or at most 6 wt % Fe, and at least 0.5 or at most 5 wt % Ce.

7. The aluminium alloy in powder form according to claim 1, wherein the aluminum alloy has a mean particle size D50 in the range from 0.1 to 500 μm.

8. The aluminium alloy in powder form according to claim 1, wherein the aluminium alloy on which the powder is based has a strength, determined as yield strength, of >300 MPa determined at 23° C., or a hot yield strength of >200 MPa determined at 250° C., or a short time creep strength, determined as stress at a creep strain of 0.5% at 260° C. and a holding time of 6 min, of at least 200 MPa.

9. The aluminium alloy in powder form according to claim 1, obtainable by atomisation of a liquid alloy at a temperature of >850° C., or by mechanical alloying.

10. A process of manufacturing a three-dimensional object, wherein the object is produced by applying a build material layer upon layer and selectively solidifying the build material, by supplying radiation energy, at locations in each layer which are associated with the cross-section of the object in that layer, by scanning the locations with at least one radiation exposure area of an energy radiation beam, wherein the build material comprises an aluminium alloy in powder form according to claim 1.

11. The process according to claim 10, wherein the aluminium alloy in powder form is preheated, preferably to a temperature of at least 130° C.

12. A process of manufacturing an aluminium alloy in powder form wherein a molten aluminium alloy having a composition as specified in claim 1 is atomised in a suitable apparatus, or an aluminium alloy having said composition is produced by mechanical alloying.

13. A three-dimensional object, produced using an aluminium alloy in powder form wherein the aluminium alloy in powder form is an aluminium alloy as specified in claim 1, and wherein the three-dimensional object comprises of such an aluminium alloy.

14. A manufacturing apparatus for carrying out a process, wherein the apparatus comprises a laser sintering or laser melting device, a process chamber including an open container with a container wall, a carrier located in the process chamber, wherein the process chamber and the carrier are movable relative to each other in the vertical direction, a storage container and a coater movable in the horizontal direction, and wherein the storage container is at least partially filled with an aluminium alloy in powder form according to claim 1.

15. An aluminium alloy having a content of 2 to 8 wt % Fe, 0.5 to 4.0 wt % Cr and 0.5 to 4.0 wt % Ti and up to 3.0 wt % Si or up to 1 wt % Zr or up to 1 wt % Ce, characterised in that wherein the total amount of Fe, Cr and Ti in the alloy is at least 10 or at most 16.

Description

[0057] Other features and embodiments of the invention can be found in the description of an exemplary embodiment with the aid of the accompanying drawings.

[0058] FIG. 1 shows a schematic illustration, partially reproduced as a cross-section, of an apparatus for the layer-by-layer construction of a three-dimensional object according to an embodiment of the present invention.

[0059] FIG. 2 shows a surface comparison of an impeller prepared by selective laser melting from an aluminium alloy in powder form according to the invention.

[0060] FIG. 3 shows the determination of the short time creep strength of a test body made of an aluminium alloy in powder form according to the invention.

[0061] The apparatus shown in FIG. 1 is a laser sintering or laser melting apparatus a1 known per se. For the construction of an object a2 it contains a process chamber a3 with a chamber wall a4. In the process chamber a3, an upwardly open construction container a5 with a wall a6 is arranged. A working plane a7 is defined by the upper opening of the construction container a5, wherein the area of the working plane a7 lying within the opening, which can be used to build the object a2, is referred to as the construction site a8. A carrier a10, which is movable in a vertical direction V, is arranged in the container a5, to which a base plate all is attached, which terminates the construction container a5 at the bottom and thus forms its base. The base plate all may be a plate formed separately from the carrier a10, that is attached to the carrier a10, or it may be formed integrally with the carrier a10. Depending on the powder and process used, the base plate all may also have a build platform a12 on which the object a2 is built. However, the object a2 can also be built on the base plate all itself, which then serves as the building platform. In FIG. 1, the object a2 to be formed in the building container a5 on the building platform a12 is shown below the working plane a7 in an intermediate state with several solidified layers surrounded by building material a13 that has remained unsolidified. The laser sintering device al further comprises a storage container a14 for a powdery build material a15 which can be solidified by electromagnetic radiation and a coater a16 movable in a horizontal direction H for applying the build material a15 to the construction site a8. The laser sintering device al further comprises an exposure device a20 with a laser a21 which generates a laser beam a22 as an energy radiation beam which is deflected via a deflection device a23 and focused onto the working plane a7 by a focusing device a24 via a coupling window a25 which is provided on the upper side of the process chamber a3 in its wall a4.

[0062] Further, the laser sintering apparatus al includes a control unit a29 through which the individual components of the apparatus al are controlled in a coordinated manner to perform the building process. The control unit a29 may include a CPU whose operation is controlled by a computer program (software). The computer program may be stored separately from the apparatus on a storage medium from which it can be loaded into the device, in particular into the control unit. In operation, to apply a powder layer, the carrier a10 is first lowered by a height corresponding to the desired layer thickness. By moving the coater a16 over the working plane a7, a layer of the powdered build material a15 is then applied. For safety, the coater a16 pushes a slightly larger amount of build material a15 in front of it than is required to build up the layer. The coater a16 pushes the systematic excess of build material a15 into an overflow container a18. An overflow container a18 is arranged on both sides of the construction container a5. The build material in powder form a15 is applied at least over the entire cross-section of the object a2 to be produced, preferably over the entire construction site a8, i.e. the area of the working plane a7, which can be lowered by a vertical movement of the carrier a10. Subsequently, the cross-section of the object a2 to be produced is scanned by the laser beam a22 with a radiation exposure area (not shown), which schematically represents an intersection of the energy radiation beam with the working plane a7. As a result, the build material in powder form a15 is solidified at locations corresponding to the cross-section of the object a2 to be produced. These steps are repeated until the object a2 is completed and can be removed from the construction container a5. For generating a preferably laminar process gas stream a34 in the process chamber a3, the laser sintering device al further comprises a gas supply channel a32, a gas inlet nozzle a30, a gas outlet opening a31 and a gas discharge channel a33. The process gas stream a34 moves horizontally across the construction site a8. The gas supply and discharge may also be controlled by the control unit a29 (not shown). The gas extracted from the process chamber a3 may be fed to a filter device (not shown), and the filtered gas may be fed back to the process chamber a3 via the gas feed duct a32, forming a recirculation system with a closed gas loop. Instead of only one gas inlet nozzle a30 and one gas outlet opening a31, several nozzles or openings can be provided in each case.

[0063] In the apparatus according to the invention, the storage container a14 is at least partially filled with an aluminium alloy in powder form a15 as indicated above.

[0064] Finally, another aspect of the present invention relates to an aluminium alloy having a content of 2 to 8 wt % Fe, 0.5 to 4.0 wt % Cr and 0.5 to 4.0 wt % Ti, and optionally up to 3.0 wt % Si and/or up to 1 wt % Zr and/or up to 1 wt % Ce, wherein the total amount of Fe, Cr and Ti in the alloy is at least 10 and/or at most 16 and preferably at least 11 and/or at most 13 wt %. A particularly preferred aluminium alloy contains 5.1±1 wt % Fe, 3.5±1 wt % Cr and 2.5±1 wt % Ti, and a total amount of Si, Mn, Mg and 0 of 0.05 to 1 wt % and in particular 0.1 to 0.6 wt % can be stated as further preferred.

[0065] The present invention is further illustrated by a number of examples which, however, should not be construed as in any way determining the scope of protection of this application.

[0066] The aluminium alloys and three-dimensional objects below were characterised using the methods described below:

[0067] The mean particle size D50 was determined according to ISO 13320 using a HELOS device from Symphatex GmbH.

[0068] The bulk density was determined according to ISO 3923/1 with a Hall flowmeter.

[0069] The flowability was determined according to ISO 4490 with a Hall flowmeter, 2.5 mm.

[0070] Densities are determined using the Archimedes principle according to ISO 3369: “Undurchlassige Sintermetallwerkstoffe and Hartmetalle-Bestimmung der Dichte” for three-dimensional objects produced as density cubes by selective laser sintering or selective laser melting. In this density measurement method, the mass of a sample is measured in both air and water and the measured mass difference between the two measurements is then used to estimate the sample volume based on the known density of water. The measured weight and volume of the sample can then be used to calculate its density. For the tests, all sides of the density cube samples are manually sanded with Struers SiC#320 sandpaper using a Struers Labo-Pol-5 sample preparation system to reduce surface roughness and thus the possibility of falsifying the test result due to trapped air bubbles on the sample surfaces. Ion-exchanged water is used for weighing when immersed in water, and a small amount of dishwashing liquid is added to the water to reduce its surface tension.

[0071] The procedure is carried out with a laboratory scale (Kern PLT 650-3M) using a built-in density calculation programme. For the automatic calculation, the water temperature is measured before the tests. The measurements are repeated five times for each sample, switching the sample between each measurement, and the samples are thoroughly dried before each new measurement. The results shown below are the averaged values of the five repetitions.

[0072] The determination of tensile strength, yield strength, elongation at break and E-modulus was carried out according to the tensile test in accordance with the standard DIN EN ISO 6892-1: 2016 “Metallische Werkstoffe-Zugversuch-Teil 1: Prüfverfahren bei Raumtemperatur”. Three-dimensional objects produced by selective laser sintering or selective laser melting as tensile test pieces (specimens) are used for tensile tests. The cross-sectional diameter of each specimen is reduced with a lathe so that it reaches its smallest value, about 5.0 mm, in the middle of the specimens. This diameter is checked with a micrometer.

[0073] The ends of the specimens are threaded for attachment. The test is carried out e.g. with the universal testing machine inspekt table 50 kn (Hegewald & Peschke Mess-und Prüftechnik GmbH). The tensile force is increased by 10 MPa/s during the elastic phase of the material behaviour and reduced to 0.375 MPa/s at the start of the plastic deformation phase.

[0074] During the tests, the maximum load, the yield strength (Rp0.2 limit), the tensile strength, the E-modulus and the elongation at break of the specimens are recorded and then the reduction in cross-sectional area at the point of break is measured with a caliper.

[0075] The properties of hot tensile strength, E-modulus, hot yield strength and elongation at break at 250° C. were determined according to DIN EN ISO 6892-2:2011 A113.

[0076] The hardness testing of the three-dimensional objects produced as samples by selective laser sintering or selective laser melting is performed using the Brinell method according to the standard DIN EN ISO 6506-1: 2015 “Metallische Werkstoffe-Härteprüfung nach Brinell-Teil 1: Prüfverfahren”. Samples of density cubes are used for testing.

[0077] The tests are performed three times for each sample and the measured values are given with an accuracy of 1 HBW. The numerical data given below indicate the sphere diameter of the test sphere used in the determination (e.g. 2.5 mm) and the test load (e.g. 62.5 kp).

[0078] The thermal conductivity was determined according to the formula λ=a.Math.cp.Math.ρ from the measured thermal diffusivity a LFA (Laser Flash method measuring device 427/company Netzsch Ar atmosphere 100 ml/min, two built samples each: discs with a diameter of 12.6 mm and a thickness of 3 to 3.5 mm, plane parallel faces, temperature range 21 to 250° C.), the specific heat capacity cp and the temperature-dependent density ρ, taking into account the measured thermal expansion αtechn. The Laser Flash measuring method is a measuring method for the direct determination of the thermal diffusivity. Here, a sample is heated for a short moment by means of a laser. To be able to carry out a measurement, the sample is first placed in a sample holder and covered with a graphite layer that absorbs thermal radiation. Then the sample holder together with the sample is placed in the system, where it is brought to the desired measuring temperature by an oven. Once the temperature is reached, a defined amount of heat is introduced into the sample with an excitation pulse. A detection laser is then used to determine the heat reflection of the sample on the other side of the sample holder. This usually shows an increase in the sample temperature after the heat input and then a slow drop, which can be steeper or flatter depending on the thermal diffusivity of the sample. From this data, the thermal conductivity is calculated directly by means of a mathematical model.

[0079] The specific heat capacity cp was determined using a Setaram high temperature calorimeter, at a measurement interval of 80 to 250° C., 5 K/min heating rate, He atmosphere, continuous comparison method, two built samples each: cylinders with 4.9 mm diameter and 16 mm length, plane parallel faces.

[0080] The thermal expansion αtechn was determined using a DIL 402 C dilatometer, measuring range 20 to 250° C., 5 K/min heating rate in He atmosphere, specimens: two built specimens each: cylinder with 4 mm diameter and 25 mm length, plane parallel faces.

[0081] The values given for the specific heat capacity and thermal expansion are mean values of the measured samples.

EXAMPLE 1

[0082] Various aluminium alloys in powder form were produced with the compositions and properties given in Table 1 below:

TABLE-US-00001 TABLE 1 alloy number 1 2 3 Fe in wt % 5.8 5.6 4.5 Cr in wt % 3.5 3.5 3.5 Ti in wt % 3.2 3.1 3 O in wt % 0.09 0.23 0.16 Si in wt % 0.17 0.16 Mn in wt % 0.06 0.04 Mg in wt % 0.03 0.02 Al in wt % balance to balance to balance to 100% 100% 100% Properties Mean particle 35.8 31.9 36.5 size D50 in μm Flowability (FG 67 27.5 61 Hall) in s bulk density in 1.45 1.60 1.51 g/cm.sup.3

[0083] The smaller particle size of aluminium alloy 2 provided improved surface quality and reduced crack sensitivity in the manufacture of three-dimensional objects compared to aluminium alloy 1. Aluminium alloy 2 has a higher bulk density and also showed better flowability, which is probably due to the higher oxygen content leading to a reduction of the forces between the particles. Alloy 3 combines the advantageous properties of alloys 1 and 2.

[0084] The powders consisted of coarse and mainly spherical particles. While aluminium alloy 1 contained few particles with a size of less than 10 μm, aluminium alloy 2 contained a substantial amount of fine particles in the powder. Powder 3 was characterised by a smaller amount of fines compared to powder 2. With these powders, layer thicknesses of 20 to 60 μm could be reliably produced.

EXAMPLE 2

[0085] Three-dimensional test objects were produced with an EOS M290 (EOSPrint version 2.x, laser power 270 W, line speed 850 mm/s, hatch distance 0.1 mm, layer thickness 0.05 mm), using the aluminium alloy 3.

[0086] For this purpose, a preheating temperature of 195° C. was set in the sample chamber. With the aluminium alloys, densities of >99% for the manufactured objects could be achieved. The objects made of aluminium alloy 1 showed a slightly higher sensitivity to brittle cracks.

[0087] Complex test objects could be produced with the aluminium alloys. A manufactured impeller with the dimensions showed maximum deviations from the specification of ±0.15 mm (see FIG. 2).

[0088] The following properties were determined for samples made of aluminium alloy 3 with a density of 2.9 g/cm.sup.3:

TABLE-US-00002 TABLE 2 tensile test at room test cube test cube temperature at 250° C. at 20° C. at 250° C. treatment as prepared as prepared stabilised* stabilised* tensile strength 450 310 in MPa - vertical building direction tensile strength 450 310 in MPa - horizontal building direction yield strength in 340 270 MPa - vertical building direction yield strength in 360 290 MPa - horizontal building direction elongation at 4 6 break in % E-modulus in GPa 85 75 hardness in HBW 154 2.5/62.5 thermal 61-70 77-81 conductivity in W/(m .Math. K) heat capacity in 0.844 0.928 kJ/(kg .Math. K) thermal 19.7 21.1 expansion in 10.sup.−6 K.sup.−1 =tempering at 350° C. for 10 h

[0089] In addition, galvanic corrosion studies were carried out, wherein the samples made of aluminium alloy 1 were compared with corresponding samples made of Al 99.5. A saturated calomel electrode was used as the reference electrode. The measurements were carried out in 0.01 M NaCl solution at 25° C. with a platinum sheet as counter electrode. This showed a significantly lower negative potential for the aluminium alloy according to the invention compared to the sample made of Al 99.5.

EXAMPLE 3

Determination of the Short Time Creep Strength of Aluminium Alloy 1

[0090] The short time creep strength of aluminium alloy 1 was determined according to DIN EN ISO 6892-2:2011-05 A. For this purpose, samples were brought to different stress levels at 260° C. and then kept under constant stress. The permanent elongation resulting after 6 min is recorded as a measured value. The stress at which 0.5% elongation results is used as the reference value for the comparison.

[0091] The results of these tests are shown in FIG. 3. For the aluminium alloy 1, a short time creep strength, determined as stress at a creep strain of 0.5% at 260° C. and a holding time of 6 min, of about 260 MPa could be determined, which is significantly higher than the short time creep strength described for other aluminium alloys (in the range of 9 to 170 MPa). For additively manufactured Al-MMC, a short time creep strength of 170 MPa was determined (not shown).