IT202100011450A1 - Method and equipment for the production of objects by three-dimensional printing and sintering - Google Patents

Description

DESCRIPTION OF THE INDUSTRIAL INVENTION ENtitled: "Method and apparatus for the production of objects by three-dimensional printing and sintering"

DESCRIPTION

Field of application

The present invention generally relates to the additive production of three-dimensional objects (3D printing) starting from materials in powder form.

State of art

Several processes are known for the manufacture of completely dense solid objects by 3D printing obtained by deposition of successive layers of powder material.

Some techniques are based on the application of a liquid binder which selectively fixes the deposited powder layer by layer to create the three-dimensional structure. Following the removal of the unfixed powder, the object obtained is heated in order to remove the binder and sintered to obtain the consolidation of the powders.

Selective Laser Sintering (SLS), on the other hand, uses a powerful laser beam as an energy source to selectively sinter the powdered material deposited layer by layer to create a three-dimensional structure. Selective Laser Melting (SLM) ? a comparable technique that applies localized melting of the material instead of sintering it. The primary process? generally followed by various post-processing treatments that aim to improve the properties? mechanics, precision and appearance of the parts obtained with these techniques. Mandatory post-processing steps include dust removal which is not ? been sintered or cast and support structures. Also, a heat treatment ? commonly always necessary to relieve residual stress and improve the properties? mechanics of the manufactured component. Finally, a CNC machining can? be used to improve characteristics dimensionally in crucial areas (such as holes or wires).

Am I otherwise? known processes which make it possible to create three-dimensional objects of complex shape by compaction by cold pressing of powders of a deformable material (typically a metal) placed in a mold whose cavity is presents the negative shape of the object to be created. The powders what? compacted are then removed from the mold and subjected to a sintering process at high temperature in a suitable atmosphere.

An object of the present invention ? that of making available an alternative method for creating three-dimensional objects starting from the deposition of powders.

Summary of the invention

In view of the aforementioned object, the invention relates to a method for producing an object with a complex shape, comprising

deposit a plurality? of two-dimensional layers one on top of the other inside a mould, wherein in each of said two-dimensional layers a first powder material and a second powder material are deposited in an incoherent form according to a predetermined pattern, in which complementary portions of each single two-dimensional layer are occupied alternatively by the first material or by the second material, said first material and second material having different mechanical characteristics and sintering characteristics,

put pressure on the plurality? of two-dimensional layers to obtain a compacted product consisting of the first material, e

remove the second material in uncompacted form.

Furthermore, the object of the invention is an apparatus for implementing a method according to one of the preceding claims, comprising a platform,

a platform supported die, said die comprising a die body and a base punch, which is sliding in the mold body along a vertical direction and provides a deposition plane adapted to receive said plurality? of two-dimensional layers, e

a powder dispensing system comprising at least two distributors configured to be arranged above said deposition plane and adapted to respectively dispense said at least one first powder material and the second powder material,

wherein one of said platform and powder delivery system ? movable in a motorized manner along two mutually orthogonal horizontal directions, to control the distribution of said at least one first powder material and said second powder material in each single two-dimensional layer of said plurality of two-dimensional layers, e

in which said base punch ? controllable to lower the base punch level each time each single two-dimensional layer of said plurality is completed of two-dimensional layers.

Thanks to the invention ? Is it possible to manufacture a three-dimensional object in conformity? with the data relating to a 3D digital or CAD model, by means of controlled deposition of powders without the aid of binders or localized sintering or fusion systems or molds with a shape corresponding to the geometry, and subsequent sintering using in particular Spark Plasma Sintering technology ( SPS) or conventional sintering.

3D printing techniques have experienced a rapid diffusion in recent years due to the speed? and efficiency with which they allow to obtain objects of metallic and ceramic materials of complex shape avoiding the conventional processing techniques.

In this, as in most additive technologies, the production of the object is performed through the creation of single two-dimensional (2D) layers in a specific sequence, progressively stacking one layer on top of the other in a direction defined by the Z axis. Specific feature of this invention ? constituted by the controlled deposition in each layer of two types of powders which have very different mechanical and sintering characteristics. A typical example can be made up of a metal powder, easily sintered and easily cold compactable, and a refractory powder, which has a very high sintering temperature and little or no deformability? cold. The part of the layer where the metal powder is deposited will go? to constitute the final object, while the part occupied by the refractory powder will constitute? a sort of container for the object itself. By way of example, the metal part can? be made up of aluminum, copper or steel powder, while the ceramic part is made up of alumina or graphite powder. The deposition of the layers takes place directly inside a mold which contains the sequence of layers of powder, prevents them from collapsing or deforming and allows for the application of uniaxial pressing.

At the end of this deposition phase, the compaction of the metal powders deposited inside the mold can be achieved following two different paths.

1) In the first case, the powder bed is cold pressed. For this purpose at both ends? Punches are inserted into the mold and the entire mold is placed in a hydraulic press and subjected to uniaxial cold pressure. This treatment leads to the compaction of the metal fractions of the powder bed, while the ceramic powders, not being deformable, remain uncompacted and transfer the pressure exerted by the punches onto the metal fraction of the powder bed. The entire content of the mold is then extracted, the non-compacted ceramic powder is removed and the remaining object, consisting of the compacted metal powder, is subjected to a subsequent sintering treatment in a high temperature oven in order to obtain complete densification of the mould. object.

2) In the second case in the two extremities? of the mold, what ? in this case preferably made of high density graphite, two punches are inserted, also made of high density graphite. The entire mold is then placed inside a SPS/FAST apparatus to proceed with sintering in the presence of a pressure applied uniaxially. The ceramic fraction of the powder bed is not sintered during this treatment, but serves to transmit the pressure pseudo-isostatically to the metal fraction of the powder bed, which is ? more easily sintered. At the end of the process the mold is removed from the SPS/FAST chamber, the powder bed is removed from the mold, the ceramic powder, which is not sintered, is removed and the metal part, which ? densified constitutes the final object.

Brief description of the drawings

Characteristics and advantages of the process according to the invention will become more? clear with the following detailed description, made with reference to the attached drawings, provided for illustrative and non-limiting purposes only, in which:

- figure 1 ? a schematic representation of a three-dimensional printing apparatus which uses the powder deposition method inside a mould;

- figure 2 represents a detail of the system of figure 1;

- figure 3 shows a detail of a powder delivery system;

- figures 4A-4E show the phases of deposition of the materials inside the mould;

- figure 5A represents the sintering steps according to a first embodiment of the invention; And

- figure 5B represents the sintering step according to a second embodiment of the invention.

With reference to Figures 1 to 3, an additive manufacturing equipment which can be used in the method according to the invention is now described.

The apparatus comprises a motorized platform 1 movable along two mutually orthogonal horizontal directions, indicated by X and Y in the figures. Platform 1? controlled by a? unit? of control 13, through linear actuators 11a and 11b connected to the unit? of control 13 and respectively associated to the two directions of movement X and Y. The movement system can? be made in various forms and therefore not ? to be understood as limited to the configuration described here.

The platform 1 supports a die, comprising a die body 2 and a base punch 8. The base punch 8 is contained within the mold body 2 so as to slide along a vertical axis, indicated with Z in the figures. The base punch 8 provides a deposition plane 8a adapted to receive the powders in the way it will be? described below. How can you? see in figure 2, the platform 1 further comprises a linear actuator 9 which can be associated with the base punch 8, and connected to the unit? of control 13 for controlling the movement of the base punch 13 along the Z axis. In the illustrated example, ? provided is an extractable block 7 integral with the platform 1, on which the mold 2, 8 is mounted and in which ? arranged the linear actuator 9.

The apparatus also comprises a powder dispensing system comprising two distributors, indicated by 4 and 5 in the figures, which are constrained to a fixed support 6. One of these, indicated by 4, allows the powder of a refractory material to be distributed (indicated with a light color in the figures), which does not manifest n? deformability cold, no? sintering at medium temperatures (< 1500?C); the other, indicated with 5, allows you to distribute the powder of the material that will come? sintered to obtain the final object (indicated with a dark color in the figures). According to one embodiment there may be more? distributors to dispense the sinterable material. In one embodiment, this expedient allows to produce more? objects starting from a single powder bed. In one embodiment can? be expected that pi? distributors can be expected to deliver more? sintered materials, in the event that, for example, the? object to be made is composed of more? materials.

The powder delivery system can? use different technologies, chosen on the basis of the chemical-physical characteristics of the powders and the need? to have a uniform and constant dosage. For example, dispensers and related dispensing nozzles can be made using auger, piezoelectric or mechanical vibration systems.

Distributors 4 and 5 are kept in a fixed position throughout the object generation phase and support 6 ? adjustable in height only for the purpose of the initial calibration. The use of fixed distributors prevents the vibrations of the positioning system from disturbing the deposition of the material. Isn't this condition? in any case strictly necessary if positioning systems without vibrating parts are used.

With reference to figure 2, a bed 3 formed by the two powders is deposited inside the mold 2, 8, which grows in successive layers on the surface 8a of the base punch 8 which represents the deposition plane. The production of the object is carried out by creating single two-dimensional layers 3a in a specific sequence, progressively stacking one layer on top of the other in a direction defined by the Z axis. The dispensing of the two types of powder is always performed keeping the the sinterable powder distributors 5 and that of the refractory powder 4 are of the same height. The base punch 8 is progressively lowered at the end of each layer so as to leave the distance between the distributors 4,5 and the upper surface of the powder bed unchanged .

Once the deposition of each layer (metal and inert powder) has finished, the base punch 8 is lowered by the linear actuator 9, freeing up space for the deposition of a new layer and allowing the distributors 4 and 5 to fill the space made uniformly available.

This deposition geometry allows the distributors 4, 5 to reach the edges of the cavity with precision as well. inside of the mold 2, 8.

The inventors used a piezoelectric system as actuation device for the powder distributors, controlled by an external signal generator capable of producing vibrations in the frequency range of a few kHz.

With reference to figure 3, each vending machine 4.5 ? managed by an electronics 12 which automatically interacts with the? unit? control 13 of the equipment.

? therefore it is possible to manage the entire printing process using the software and hardware of a common commercial 3D printer. The system requires only the typical firmware configuration of the various commercial printers.

The control electronics 12 activates the single vending machine at the appropriate moment, but will it be? the electronics combined with that particular dispenser which takes care of the constant dispensing over time.

Each distributor 4.5 ? connected to its own tank 10 by means of a powder conveyor 11.

With reference to figures 4A-4E, the process of deposition of the materials inside the mold is now described.

At the beginning of the process, the element 8, which constitutes the lower punch of the mould, is positioned, dust-free, a few tens of microns below the upper limit of the mold body 2. In this way the tip of the distributors 4, 5 is at the correct distance from the deposition plane 8a (figure 4A).

The motorized platform 1 provides for the movement of the mold 2, 8 while the first distributor 5 is activated so as to obtain a controlled flow of the powder of the sinterizable material (figure 4B).

After the controlled deposition of the first state of the powder of the sinterable material, the electronics activates the second distributor 4 to distribute, in the free parts, the powder of the refractory material, which will occupy? the spaces in which the powder of the sinterable material had not been deposited. Also in this case the distribution of the powder will be? achieved by movements of the motorized platform 1 (figure 4C). The sequence of deposition of the dust pu? be different from the one described above.

After the previous phase, the whole first layer of printing? been completed. The control system moves the lower punch 8 of the mold downwards by an amount? correspond to the thickness of a layer of dust. The extent of the displacement depends on the type of powders used and the configuration of the delivery systems (figure 4D).

The operations of depositing the powders and moving the lower punch 8 downwards (steps from 4B to 4D) continue cyclically until the end of the printing.

In figure 4And the printing process? business suit; inside the mold ? present a bed of powders 3, formed by a plurality? of two-dimensional layers and made up partly of the powders of the sinterable material (in dark colour), which will constitute the final object, and partly of the refractory powder (in light colour). In the upper part of the mold 2, 8 is a second punch 8 inserted? and the whole mould, made up of elements 2, 3, 8 and 8?, can? what? be removed from the dust deposition equipment without the dust being able to move or mix.

The densification of the part of the powder bed 3 constituted by the powders of the sinterable material is at this point using techniques per se? notes to the person skilled in the art.

With reference to Figure 5A, a first technique is now described.

The mold is placed in a uniaxial press 16 and subjected to cold pressing until the fraction of the content consisting of deformable powders is compacted. The uniaxial compressive force, indicated by the arrows 15 in figure 5a, is transmitted to the powder bed 3 thanks to the punches 8, 8? which are slidably arranged within the mold body 2. Once the pressing operation has been completed, the contents of the mold are removed. The fraction made up of refractory powders, which will not result? compacted, is removed with appropriate tools thus revealing? the compacted object, indicated with 19. This is then transferred to a furnace 18 where the sintering process takes place in a suitable atmosphere. This involves a heat treatment at a temperature between 2/3 and 3/4 of the melting temperature of the material for a few hours. At the end of this treatment the object will be completed.

Since the mold is not transferred to the oven in the process described above, it can be made of material that does not need to be resistant to sintering temperatures, e.g. steel.

With reference to Figure 5B, a second technique is now described.

The mold is removed from platform 1 for insertion into a Spark Plasma Sintering (SPS) or Field Assisted Sintering (FAST) system, denoted 17. To be used in such a system, mold parts 2, 8, 8? they are generally made of high-density graphite. The system 17 performs three functions: i) it maintains a controlled atmosphere during the sintering process, ii) it applies a uniaxial pressure on the mould, iii) it supplies the electric power necessary to obtain the heating of the mould.

The system 17 comprises three main parts: 1) a vacuum chamber 14 which allows the mold and the bed of powders contained therein to be maintained in a low vacuum (minimum pressure for example about 102 Pa), or in an inert atmosphere consisting from gases such as nitrogen or argon. The use of vacuum or inert atmosphere has the primary purpose of avoiding contact between the oxygen present in the atmosphere and the graphite of the mold used for the densification of the sample. Does chamber 17 have two compression pistons 17? placed axially and able to slide vertically, which have the purpose of transferring the uniaxial compression and the electric current necessary for the realization of the process to the mould. The pistons 17? are made of good electrically conductive material, such as copper.

2) A hydraulic system (not shown) for applying and controlling uniaxial compression. Does the hydraulic system allow the pistons 17 to be controlled? to allow the application of a controlled load, which can? be held constant throughout the process to compensate for any variations in sample and mold size that may occur during the densification process.

3) A 17 power system?? capable of supplying and controlling the electric power required to heat the mold and the powder bed it contains.

In system 17, the mold is subjected to a rapid thermal cycle in the presence of controlled uniaxial pressure. In this phase the only material to densify will be? the fraction of sinterable powders present in the mould. Will the ceramic powder remain? unchanged and will provide? to transfer the uniaxial pressure quasiisostatically to the sinterable fraction. The thermal cycle typically requires a few minutes (5-10) at a temperature between 1/2 and 2/3 of the melting temperature of the material to be sintered. At the end of the thermal cycle, the content of the mold is removed, the fraction made up of the refractory powders, which will not result? compacted, will it? removed with appropriate tools revealing cos? the compacted object.

Claims (10)

A method of producing a three-dimensional shaped object, comprising deposit a plurality? of two-dimensional layers (3a) one above the other inside a mold (2, 8), wherein in each of said two-dimensional layers at least one first powder material and one second powder material are deposited in an incoherent form according to a predetermined pattern, in which complementary portions of each single two-dimensional layer are occupied alternatively by said at least one first material or by the second material, said at least one first material and second material having different mechanical characteristics and sintering characteristics, put pressure on the plurality? of two-dimensional layers (3a) to obtain a compacted product consisting of said at least one first material, e remove the second material in uncompacted form. 2. A method according to claim 1, wherein pressuring the plurality? of two-dimensional layers (3a) comprises apply a uniaxial compression on the plurality? of two-dimensional layers (3a) in the mold (2, 8, 8?). 3. A method according to claim 2, wherein pressuring the plurality? of two-dimensional layers (3a) further comprises heat the plurality? of two-dimensional layers (3a) in such a way that the compacted product obtained is a sintered compacted product. 4. A method according to claim 3, wherein subjecting the plurality of? of two-dimensional layers (3a) comprises heat the plurality? of two-dimensional layers (3a) by applying electric current through the mold (10). The method according to claim 2, further comprising subjecting the compacted product to heat to obtain a sintered compacted product. The method according to claim 5, wherein subjecting the compacted product to heat comprises heat the compacted product in an oven (18). 7. Method according to one of the preceding claims, wherein said at least one first material ? sintered at lower temperatures than the sintering temperatures of the second material. 8. A method according to claim 7, wherein said at least one first material is a cold deformable material and the second material? a substantially non-cold deformable material. 9. A method according to claim 7 or 8, wherein said at least one first material ? a metallic material and the second material ? a refractory material. 10. Apparatus for implementing a method according to one of the preceding claims, comprising a platform (7), a mold (2, 8) supported by the platform (7), said mold comprising a mold body (2) and a base punch (8), which is sliding in the mold body (2) along a vertical direction (Z) and provides a deposition plane (8a) adapted to receive said plurality? of two-dimensional layers (3a), e a powder dispensing system comprising at least two distributors (5, 4) configured to be arranged above said deposition plane and adapted to respectively dispense said at least one first powder material and the second powder material, wherein one of said platform and powder delivery system ? movable in a motorized manner along two mutually orthogonal horizontal directions (X, Y), to control the distribution of said at least one first powder material and said second powder material in each single two-dimensional layer of said plurality of two-dimensional layers, e in which said base punch ? controllable to lower the base punch (8) each time each single two-dimensional layer of said plurality is completed of two-dimensional layers (3a).