Patent Application
20230366060
The technical field of the invention is that of the purification of metal powders for use in an additive manufacturing method.
TECHNOLOGICAL BACKGROUND OF THE INVENTION During the manufacture of metal powders, mainly in methods using plasmas or electric arcs, or during their use in additive manufacturing by known so-called “laser beam melting” or “electron beam melting” techniques, part of the molten metals is superheated and vaporised or ejected in the form of microdroplets or vapour into the surrounding atmosphere of the method in progress. Some of the micro-droplets then deposit onto the surface of the powder grains, forming satellites, involving a degradation in the sphericity of the powder grains. The reduction in the sphericity of the powder grains directly influences their ability to be spread or distributed in the final method.
Although the surrounding atmosphere of the method is generally comprised of mostly neutral gases such as argon, nitrogen or helium, it always contains a residual content of impurities such as oxygen or moisture. Some additive manufacturing machines do not control the oxygen content below 1000 ppm, which is sufficient to oxidise the microdroplets and vapours generated. Oxidised vapour condenses into highly oxidised nano-sized particles which can also be deposited onto the powder grains. Pollution of powder batches by these oxidised particles adversely affects quality of the powders through a significant increase in oxygen content and an increase in hygiene and safety risks during their handling and use. The increase in the oxygen content of the powder batches degrades mechanical characteristics of the parts thus produced by additive manufacturing. Finally, the presence of the particles, even partially oxidised, greatly reduces lower flammability limits of the powder batches and can yield health problems when handling them.
U.S. Pat. No. 7,572,315 provides a method for purifying metal powders in order to reduce the amount of contaminants in powder batches. The method involves suspending metal powder including contaminants in a solution including, for example, alcohol or acetone, followed by separating the contaminants by means of intense ultrasonic vibrations. When the contaminants are suspended in the solution, a recovery step comprising sieving or centrifuging, as well as filtration makes it possible to recover the metal powder. However, no measures are taken to avoid the contaminants redepositing onto the powder grains during the recovery stage, reducing efficiency of the purification method.
The invention provides a solution to the problems previously discussed, by providing a reproducible method for purifying metal powders, whether new or to be recycled, for obtaining a metal powder including a reduced oxygen content and an absence of satellites on the surface of the powder grains.
The invention relates to a method for purifying a powder including grains and contaminants, including:
The contaminants include highly oxidised particles that may have been deposited onto the surface of the powder grains, as well as satellites on the surface of the powder grains.
Mechanical energy applied to the suspension is transmitted to the contaminants and to the grains, allowing some of the contaminants to be lifted off the powder grains. Mechanical energy enables the satellites to be lifted off the powder grains more easily as the adhesion force between grains and satellites is low. Mechanical energy applied to the suspension during the removal step avoids the contaminants being redeposited onto the powder grains. Thus the contaminants are effectively removed from the powder. The oxygen content of the powder thus purified is reduced. The powder grains, being free of surface contaminants, have a high sphericity.
In addition to the characteristics just discussed in the previous paragraphs, the method according to the invention may have one or more of the following additional characteristics, considered individually or according to any technically possible combinations:
The term “sonication” refers to the treatment with ultrasound.
The invention also relates to a device configured to implement the purification method according to the invention, the device further comprising:
The invention and its different applications will be better understood upon reading the following description and examining the accompanying figures.
The figures are set forth by way of illustrating and are in no way limiting purposes of the invention.
The figures are set forth by way of illustrating and are in no way limiting purposes of the invention. Unless otherwise specified, a same element appearing in different figures has a unique reference.
The method 101 includes a step of preparing 120 a suspension 4 including the powder 10 and a solvent 3. Advantageously, the solvent 3 has a significant physico-chemical affinity with the contaminants 2. For example, the solvent 3 may have a high wettability towards the contaminants 2. The solvent 3 can also modify the zeta potential of the grains 1 and the contaminants 2 suspended in the solvent 3. The zeta potential represents the electric charge that a particle acquires by virtue of the ions or molecules that surround it when it is in a solution. The zeta potential can for example be influenced by the pH of the solution. The physico-chemical affinity of the solvent 3 with the contaminants 2 will facilitate separation of the contaminants 2 from the grains 1 and avoid the contaminants 2 being redeposited onto the grains 1 at a later stage. The solvent 3 can for example be an alcohol or an alcoholic solution. The powder:solvent volume ratio within the suspension 4 is between 1:1 and 1:50 and preferably between 1:10 and 1:30. The concentration of the solvent 3 should be equal to or in excess of the powder concentration 10 to allow dispersion of the contaminants 2. At a powder:solvent volume ratio of 1:10, the solvent 3 is sufficiently in excess, allowing proper dispersion of the contaminants 2. Stirring the suspension 4 avoids the formation of agglomerates. However, in order to further limit formation of agglomerates, the powder 10 can advantageously be poured into the solvent 3.
The method 101 is compatible with the use of CO2 in the supercritical phase as the solvent 3 to produce the suspension 4. For this, it is sufficient that the manufacturing device can maintain the supercritical state of CO2, that is a pressure greater than 70 bar and a temperature greater than 35° C. Removal of the solvent 3 and contaminants 2 should be performed by filtration by controlling the pressure gradient by means of a weir.
The method 101 comprises a dispersion step 130 comprising separating the grains 1 and the contaminants 2 by means of mechanical energy 5 applied to the suspension 4 and dispersing them into the solvent 3. Mechanical energy 5 separates the contaminants 2 from each grain 1 and disperses the contaminants 2 homogeneously within the suspension 4. The dispersion step 130 is performed following the preparation step 120. The duration of the dispersion step 130 is adjusted so that the grains 1 and the contaminants 2 are homogeneously dispersed within the suspension 4. The duration of the dispersion step 130 may for example be between 1 min and 10 min. Mechanical energy 5 may be derived from stirring of the solution 4 performed by stirring means. The stirring means may for example comprise motor-driven blades or paddles. Mechanical energy 5 may also be derived from sonification of the suspension 4 performed by means of an ultrasonic assembly. The ultrasonic assembly may, for example, include a sonotrode immersed in the suspension 4, excited by an ultrasonic source. Advantageously, mechanical energy 5 may result from the combined action of stirring and sonication within the solution 4. Mechanical energy 5 is advantageously high so that all the contaminants 2 are separated from the grains 1 and dispersed efficiently in the suspension 4. However, heat dissipated by mechanical energy 5 should preferably not exceed a boundary value beyond which the solvent 3 heats up and can evaporate. For this, the stirring speed and/or the level of sonication are preferably set to the maximum values that do not heat up the solvent 3.
By the term “stirring of the solution”, it is meant mechanical stirring of the solution.
For example, the speed of the stirring means is between 5000 rpm and 20000 rpm. by way of example, the inventors achieve a satisfactory dispersion with stirring performed by a vertical blade rotating at a speed between 13000 rpm and 17000 rpm for 10 min.
Sonification can be performed by means of ultrasound with a wavelength of, for example, between 20 kHz and 1 MHz. However, depending on the type of dispersion desired, a narrower range may be selected.
For example, sonication with a so-called low wavelength, between 20 kHz and 30 kHz, allows formation of large cavitation bubbles. For example, at 25 kHz, bubbles are between 100 μm and 150 μm in size. The formation of these bubbles induces powerful cleaning and thus efficient separation of the contaminants 2.
According to another example, sonication with a so-called medium wavelength, between 40 kHz and 70 kHz, makes it possible to form cavitation bubbles of ten times smaller size. The impact force from breaking up the cavitation bubbles is then lower but the cavitation bubbles are more numerous. In this case, the bubbles rather induce a fine cleaning. In one example of the implementation of the invention, a satisfactory dispersion is obtained for example with a sonication frequency of 45 kHz.
According to another example, sonication with a so-called megasonic wavelength, in the order of 1 MHz, allows very gentle cleaning by virtue of cavitation bubbles of submicrometre size. Megasonic wavelengths are used, for example, in the field of microelectronics to clean substrates. Megasonic sonication also includes a microcurrent phenomenon induced by pressure gradients produced by the standing ultrasonic waves. Microcurrents can appear below the cavitation threshold and occur on a characteristic scale of a few micrometres to a few centimetres.
It is quite contemplatable to perform sonication at different frequencies in order to disperse different types of contaminants 2.
Purification of the powder advantageously benefits from the combination of several operating parameters as described previously. For example, powder purification is improved when the stirring speed is between 5000 rpm and 20000 rpm and when the suspension has a volume ratio between 1:1 and 1:50, or even between 1:10 and 1:30. The purification of the powder is further improved when sonication is also applied to the suspension with a frequency between 20 kHz and 1 MHz.
Mechanical energy is not derived from centrifugation, however intense. If the suspension is actually moved in the reference frame of the laboratory to carry out the centrifugation. However, it is a question of mimicking the effect of gravity in order to reduce the decantation time. Thus, in the reference frame of the suspension being centrifuged, said suspension is at rest and only under the influence of intense gravity. There is therefore no mechanical energy as such applied to the suspension.
The method 101 comprises a step of removing 140 the contaminants 2 and the solvent 3 from the suspension 4 in order to keep only the powder grains 1. The removal 140 implements a removal means. The removal means implements, for example, filtration, preferably under vacuum. In this case, the removal means comprises a filter configured to allow only particles smaller than the size of the grains 1 to pass through. The removal means may also implement centrifugation. In order to limit the likelihood of contaminants 2 being redeposited onto the grains 1 during removal 140, mechanical energy 5 is kept throughout the removal step 140. Advantageously, the duration of the removal step 140 is also reduced so as to further reduce the likelihood of the contaminants 2 redepositing onto the grains 1 or agglomerating together. Advantageously, the duration of the removal step 140 is less than 10 minutes per 100 grams of powder 10. In the case of filtration, the duration of the removal step 140 can be defined by the filtration flow rate, greater than 0.5 L/min, and preferably greater than 1 L/min.
The grains 1, still wetted by a residue of solvent 3, are recovered and then undergo a drying step 150 during which the rest of solvent 3 evaporates. In order to limit oxidation of the grains 1, the drying step 150 is carried out in a drying means comprising a controlled atmosphere. The controlled atmosphere includes a neutral gas such as argon or nitrogen. The oxygen content in the controlled atmosphere is low, advantageously below 1000 ppm and preferably below 100 ppm. The drying step 150 should preferably not degrade quality of the grains 1. The drying temperature is below the melting point of the grains 1 and preferably below 150° C.
The controlled atmosphere may also include a number of neutral gases including for example nitrogen and/or argon.
The method 102 includes a step of sieving 110 the raw powder 10′ during which macroscopic contaminants 2′ are removed, thus obtaining the powder 10 as defined with reference to
During the removal step 140, it is possible that a first part 21 of the contaminants 2 is not removed with the solvent 3 and to have an unsatisfactory quality control 170. The first part 21 of the contaminants 2 may have redeposited onto the grains 1 during the removal step 140 or may not have separated from the grains 1 during the dispersion step 130. At the end of the removal step 140, only the solvent 3 and a second part 22 of the contaminants 2 have been removed. At the end of the removal step 140, the grains 1 and the first part 21 of the contaminants 2 form a partially purified powder 10″. The efficiency of the method 102 can be improved by performing the preparation step 120, the dispersion step 130 and the removal step 140 consecutively and several times. In
As an alternative to repeatedly performing the preparation 120, dispersion 130 and removal 140 steps, the method 102 may include a quality control step 170, performed following the removal step 140. The quality control step 170 makes it possible to qualitatively determine removal of contaminants 2 following the dispersion 130 and removal 140 steps. As the grains 1 have a shorter settling time than the contaminants 2 and the solvent 3, the quality control step 170 advantageously comprises an analysis of the settling rate of the partially purified powder 10″. The settling rate analysis is performed on a sample of the partially purified powder 10″ mixed with the solvent 3 at a powder:solvent volume ratio of 1:4 so as to form a control suspension. The settling rate of the control suspension is analysed over a settling duration of between 15 min and 30 min. If the settling rate is sufficiently high, that is if the settling height of the grains 1 is sufficiently low at the end of the settling duration, for example less than 30% of the height of the control suspension, an indicator is generated. Otherwise, the preparation 120, dispersion 130 and removal 140 steps are performed again. The quality control step 170 allows the previous steps 120, 130, 140 to be triggered only when necessary, thus reducing the time required to carry out the method 102.
The settling duration can advantageously be reduced by resorting to a centrifugation of the control suspension. The settling rate analysis may also be supplemented by a measurement of the relative turbidity of the control suspension. The relative turbidity measurement can be carried out on the principle of static light scattering. For this purpose, the control suspension is poured into a standard cylindrical transparent flask, through which a measurement of the transmitted and backscattered light intensity is carried out. Measurement of the light intensity is carried out over the entire height of the flask in order to detect and quantify sediment heights of the constituents of the control suspension. The relative turbidity of the control suspension directly depends on the concentration of the contaminants 2 separated from the grains 1 and dispersed in the control suspension. At the end of the settling duration, if the average value of the light intensity transmitted is greater than 70%, and preferably greater than 85%, the indicator is generated.
The reactor 300 includes an inlet 340 at the top for introducing the powder 10 and the solvent 3 to form the suspension 4. The reactor 300 includes a mechanical energy source 310 for supplying mechanical energy 5 to the suspension 4. In the embodiment of
At the end of the removal step 140, the grains 1 are placed on the filter 410 at the bottom of the reactor 300, ready for recovery. The contaminants 2 and the solvent 3 are recovered in a recovery flask 430. The recovery flask 430 may include an outlet 440 allowing it to be emptied at the end of the purification method 101. The device 200 includes a vacuum pump 460, connected to the recovery flask 430, for lowering the pressure on one side of the filter 410, in the recovery flask 430. The vacuum pump 460 thus enables the removal step 140 to be performed by vacuum filtration. An overflow flask 450 may be connected between the vacuum pump 450 and the recovery flask 430 so that the solvent 3 cannot reach the vacuum pump 460. Advantageously, the vacuum pump 460 has a discharge 470 for discharging air present in the recovery flask 430 and the overflow flask 450.
According to one implementation mode of the purification method, the step of preparing the suspension including the powder and a solvent is carried out by fluidising the powder in a liquid medium. Fluidising corresponds to injecting a fluid (in liquid and/or gas phase) through a bed of solid particles. According to this embodiment of the method, a bed formed by the powder to be purified is fluidised by means of the solvent. For example, the solvent is injected under the bed of powder to be purified so that the solvent circulates up the bed of powder. The suspension is thus formed by the powder to be purified being fluidised by the solvent.
Fluidisation applies mechanical energy to the suspension, by creating circulation and turbulence, especially creating shear at the powder grains. Shearing peels the contaminants off the grains and disperses the powder grains and contaminants in the solvent.
Fluidisation of the powder also allows continuous removal of contaminants, for example by overflow. The contaminant-soiled solvent is thus pushed over the powder bed by the solvent injected under the powder bed and can thus be easily withdrawn. Fluidisation thus keeps application of mechanical energy during contaminant and solvent removal. Increasing the flow rate of the solvent injected increases mechanical energy applied to the suspension. On the other hand, it reduces the residence time of the solvent in the powder bed.
Mechanical energy from fluidisation can be increased by admixing a gas to the fluidised suspension. The gas is injected under the powder bed, for example, taking the same circuit as the solvent. The addition of the gas makes it possible to increase turbulence of the suspension and thus shear at the powder grains. This improves the dispersion of contaminants. In addition, the addition of the gas also causes contact between the powder grains creating additional shear, which can resemble attrition of the powder grains. This attrition thus enables contaminants to be removed from the powder grains more effectively.
According to one alternative, the soiled solvent can be recycled and freed of contaminants in order to be re-injected under the powder bed. For example, contaminants may be aggregated by flocculation or coagulation for subsequent liquid dispersion.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009916 | Sep 2020 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/076493 | 9/27/2021 | WO |
