RU2792335C1 - Method for direct laser synthesis of superelastic endodontic instruments from titanium nickelide - Google Patents

Abstract

FIELD: additive manufacturing; 3D printing.

SUBSTANCE: invention relates to the field of additive manufacturing, in particular to a method for obtaining self-adapting files by 3D printing. It can be used for the manufacture of dental instruments with superelastic properties, in particular for cleaning and/or shaping and/or canal widening. To produce a superelastic self-adaptive file, the initial titanium nickelide powder is prepared, a thin-walled 3D model of the self-adaptive file is prepared using CAD, then converted into an STL model, and the model is separated into separate layers using software. An executable file containing the coordinates of the scan vectors for the laser in each layer is prepared using the STL model with a single surface, the normal vectors of which are facing the construction axis, and loaded into the selective laser melting unit (SLM). Titanium nickelide powder is loaded into the batcher hopper of the installation and layer-by-layer synthesis of a self-adapting file is carried out by the SLM method on a titanium nickelide substrate. Each layer is formed by single laser passes until a self-adapting file is obtained. The resulting file contains a cylindrical base and a mesh structure, between which additional bridges are made. All structural elements of the self-adapting file are made with angles between the overhanging surfaces and the substrate plane of more than 35°.

EFFECT: increase in the resolution of the SLM method for the synthesis of micro-objects from titanium nickelide is provided.

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Description

Technical field

The invention relates to the fields of additive manufacturing and dental tools, in particular to a tool for cleaning and/or shaping and/or expanding a channel existing inside or passing through a solid object, and in particular to a method for obtaining self-adapting files by 3D printing from nickelide powder titanium, which have the properties of superelasticity.

State of the art

In recent decades, the number of endodontic operations has increased significantly due to the predisposition of patients to endodontic treatment of teeth, in particular root canals, rather than prosthetics in the event of severe pulpitis. Classical endodontic treatment consists of tooth canal preparation, its mechanical cleaning and irrigation, followed by obturation to prevent the penetration of bacteria into the canal and infection of the tooth in the apical hard-to-reach part (see [1] N.A. Yudina, Modern standards of endodontic treatment, Part 1, Problem articles and reviews, 2012, pp. 5-9).

At the first stage, the endodontist prepares the tooth for access to the root canal to a depth as close as possible to the apical region at the dentin-cement border. At the second stage, a thorough cleaning and disinfection of the root canal is carried out. The tool that is used in this step is called a file. With the help of this tool, an abrasive effect is exerted on the inside of the canal when the file moves up and down at a high frequency.

Irregular forms of the root canal significantly complicate the treatment (see [2] N.A. Yudina, Modern standards of endodontic treatment, Part 2, Irrigation and obturation of root canals, 2012). Before the advent of the superelastic Self-Adjusting File (SAF), endodontists had to significantly increase the width of the canal to perform the operation with traditional instruments, which significantly reduced the likelihood of successful dental treatment (see [3] K. Bansal, SAF: Paving A Way to Minimal Invasive Endodontics, 3, 2015, 144-149). However, with the help of self-adapting files, it has become possible to process highly curvilinear or even C-shaped root canals in a minimally invasive approach, thereby significantly reducing dentin removal.

During the operation, the above tool undergoes significant deformation. It is important that all deformations are reversible (elastic) to prevent tool breakage. The intermetallic phase of nickel-titanium alloy has unique superelastic properties, high corrosion resistance, biocompatibility and low Young's modulus. These properties make endodontic instruments made of this material flexible, versatile and indispensable for the operations described above with irregularly shaped root canals.

Self-adapting files are currently produced by subtractive methods, namely by laser cutting hollow tubes, which leads to the formation of a large amount of waste (up to 70% waste). Wastes from such production significantly increase the cost of products, as a result of which endodontic treatment of severe forms of pulpitis becomes less accessible. Another disadvantage of the method is the limitation on the shape of the tool, namely, the mesh structure cannot deviate from the shape of the tube and does not vary in thickness, which leads to uneven distribution of the load during operation, as well as to the appearance of stress concentrators. Finally, in the case of laser cutting, an additional technological operation is required to increase the surface roughness of the tools. This patent considers the possibility of using additive technologies (AT) for the manufacture of these tools, namely the high-resolution Selective Laser Melting (SLM) method. The material during SLM is used by 95% due to the possibility of recycling the used powder up to 12-16 times. Also, this AT method has no restrictions on the shape of the product, and the surface roughness inherent in products made by the SLM method can be used for abrasive cleaning.

An endodontic tool for cleaning/shaping the root canal of a tooth (see [4] EP3071139B1, IPC A61 C5/42, publ. 09/01/2021) containing an elongated rod consisting of a porous material is known from the prior art. The rod has a proximal end part, a distal end and a conical working part, and the working part extends from the proximal end part to the distal end, while the material is made of titanium nickelide. A method for manufacturing an endodontic instrument includes obtaining a porous material with a porosity of 15% to 90% and forming the product by grinding, additive manufacturing, 3D printing, etching, and/or combinations thereof. The disadvantages of this analogue are the non-optimized form of the product for AT, and also the specific technology of additive manufacturing and the way to achieve the necessary resolution for the production of the tool are not considered.

Also known from the prior art is a self-adapting spiral-shaped tool (see [5] US2009130638A1, IPC A61C 3/02, publ. 05/21/2009) in the manufacture of which titanium nickelide and its superelastic properties are used. This analogue only mentions the possibility of using AT for manufacturing, however, the AT method is not specified, and a specific method is not considered how to provide the necessary resolution for tool production.

Also known from the prior art is a method for manufacturing thin-walled parts using selective laser melting technology (see [6] CN109622963A, IPC B22F 3/105, publ. 04/16/2019) containing the following steps: initially a three-dimensional model consists of two parts, in which one a part is a three-dimensional solid object created by a conventional modeling method, and the other is a thin-walled part; further, the executable file for the first part of the 3D model is created in the traditional way, and for the second part, the model is equated to supporting structures, due to which single laser passes are generated along the outer boundary of the model, after which the executable files are combined for further synthesis using selective laser melting technology and the final product formed layer by layer. In this analog, the manufacturing method is not directed to a specific application, in particular to self-adapting files, however, a specific method is considered to increase the resolution of the SLP. The disadvantages of this analogue are: multi-stage, and as a result, the complexity of preparing an executable file for printing, while there is a need for manual processing of parts of the model where high resolution is needed, as well as the inability to control the distance of the laser passage from the outer surface of the 3D model.

Since at the moment the prior art does not present specific works on the production of files from titanium nickelide by the SLM method, but only mentions of the possibility of using AT, the prior art provides analogues for the production of other microobjects from titanium nickelide by the SLM method, namely coronary stents. Coronary stents are also objects with an optimized mesh structure to evenly distribute the load inside the cavity, however, the scope is significantly different.

In the prior art, a method for manufacturing an intravascular stent from titanium nickelide based on automatic powder feeding technology combined with laser processing is known, which is a prototype of selective laser melting technology (see [7] CN105033252A, IPC B22F 3/105, publ. 11.11.2015) . The method includes the following steps: preparing an executable file containing the trajectory of the laser, in accordance with the three-dimensional model of the stent; nickel and titanium powders are added to the dosing system in a certain proportion, after which a layer is formed from a mixture of powders in the construction zone. Next, the powder layer is scanned by a laser beam in accordance with the prepared executive file, and this operation is repeated layer by layer until the final formation of the intravascular stent. At the next stage, electrochemical polishing is carried out to the required surface roughness. The disadvantages of this analog are: the use of additional multi-stage processing to obtain the desired surface roughness of the stent; the use of a mechanical mixture of powders in the required proportion, rather than an atomized alloy, which cannot provide a uniform chemical composition, as a result, the same temperature of the martensitic phase transition throughout the volume of the product.

Also known from the prior art is a method for selective laser melting of a nickel-titanium alloy with a high nickel content (see [8] CN113134627A, IPC B22F 10/28, publ. 07/20/2021), including the formation of a layer of powder, with a nickel content of 53-57% , laser layer processing, in accordance with the parameters of the printing process. Repeat the above steps until the nickel-titanium alloy part is formed. In the laser melting process, the laser power is 80-150W, the laser scanning speed is 150-450mm/s, and the layer thickness is 30-120μm. The disadvantage of this analogue is the lack of guarantee of complete transformation of the mechanical mixture of Nickel and titanium in the intermetallic phase - Nickel titanium. Residual pure nickel is a toxic substance for living organisms (tissues). An increase in the nickel content in the chemical composition, on the one hand, lowers the temperature of the martensitic phase transformation, which ensures the achievement of superelastic properties, but, on the other hand, increases the likelihood of the appearance of metastable phases that worsen the mechanical characteristics.

Also known from the prior art is a method for manufacturing an intravascular stent using 3D printing technology (see [9] CN104224412A, IPC A61F 2/90, publ. 12/24/2014), which includes several stages. First, a 3D model of the intravascular stent is created; set technological parameters and prepare an executable file for a 3D printer. Next, a powder material is prepared, consisting of a nickel-titanium powder and a binder based on stearic acid, to form an intravascular stent blank. Next, layer-by-layer 3D printing is performed until a workpiece is obtained from titanium nickelide and a binder. At the next stage, the removal of the binder, vacuum sintering and cooling of the workpiece are sequentially performed to obtain an intravascular stent. The disadvantage of this analogue is the low resolution technology of powder sintering with a binder and further homogenizing annealing of the product. In addition, uneven distribution of stearic acid can lead to uncontrolled porosity in products.

Also known from the prior art is a method for manufacturing a stent from a nickel-titanium alloy by the additive manufacturing method (see [10] CN112427654A, IPC A61L 31/02; B22F10/28, publ. 03/02/2021). The method includes the following steps: at the first step, the size and shape of the stent is designed in accordance with the 3D data of a particular vessel, at the second step, the designed stent is divided into layers using software, and at the third step, 3D printing of the nickel-titanium alloy powder is performed in according to the stent model; Ni content in nickel-titanium alloy powder 50-52 at. % Ni. 3D printing process parameters: power 50-4000 W, scanning speed 200-3000 mm/s, scanning distance 50-200 microns. The disadvantages of this analog are the low resolution of this approach, there is also no information on the limitations and optimality of the geometric shape of the stent for manufacturing using the SLM technology.

Also known from the prior art is a method for 3D printing with a triple alloy based on nickel, titanium and zirconium (see [11] CN111842888A, IPC B22F 3/105, publ. 10/30/2020), characterized in that the method uses selective laser melting technology for printing shape memory parts. The addition of zirconium to the system makes it possible to increase the temperature of the martensitic phase transition, and the variation of technological parameters makes it possible to locally control this temperature. The NiTiZr alloy starting material is a powder with a particle size of 15-53 µm. In this case, the printing of parts occurs layer by layer according to the specified zones. The disadvantage of this analogue is the inability to obtain superelastic properties at room temperature due to the addition of zirconium to the alloy, which limits the number of potential applications of the method.

From the above analogues, we can conclude that at the moment there are no patents for the production of self-adapting files using specific additive methods. However, there are patents for the manufacture of stents from titanium nickelide using the SLM technology, which contain several stages. Initially, the size and shape of the product is designed in accordance with the end use of the medical device, then the file model is divided into layers using computer software. According to the corresponding cross-sectional profiles, laser scanning paths are formed. The data is loaded into the appropriate interface of the SLM setup, the laser processing parameters are set: laser power, laser scanning speed, layer thickness, overlap parameter (if applicable). Nickel-titanium alloy powder is prepared and placed in the dosing bin. In the synthesis stage, the powder is sent from the dosing hopper to the build area by the powder feeder, and the material is evenly distributed on the substrate by the squeegee,

after which the laser scans the powder layer, the cycle is repeated layer by layer, until the final shape of the product is obtained.

However, despite all the advantages of the additive approach, SLM is a complex physical and chemical metallurgical process. Achieving successful fusion of materials such as nitinol requires a deep understanding and fine-tuning of the process due to the many possible defects. In addition, there are several technological difficulties when printing endodontic instruments. The first problem relates to the insufficient resolution of the classical SLM method for printing at such scales. The diameter of the product is only 2 mm, and the size of the jumper in the grid reaches 100 microns. The second difficulty relates to the need to optimize the technological parameters of the SLM in order, on the one hand, to achieve a minimum melt pool, and on the other hand, to maintain the mechanical properties of the product.

The essence of the invention

The technical challenge facing the invention is a special technology of high-resolution selective laser melting for the manufacture of self-adapting files from titanium nickelide, including original solutions to increase the resolution of the SLM installation and the manufacture of files with a minimum characteristic size factor of 100 μm.

The technical result of the claimed invention is to increase the resolution of the SLM for the synthesis of micro-products (self-adapting files) from titanium nickelide.

The technical problem is solved, and the technical result is achieved through a method for manufacturing superelastic self-adapting files, including: analysis and preparation of the initial nickel-titanium powder material; preparation of a thin-walled 3D model of a self-adaptive file using CAD and conversion to an STL model; division of the model into separate layers using specialized software and preparation of an executable file containing the coordinates of the scan vectors for the laser in each layer; loading the resulting executable file into the SLP unit, loading the powder into the unit's batcher bunker; layer-by-layer synthesis of a self-adaptive file by the SLM method until a finished product is obtained, while preparing the executable file for the selective laser melting installation, an STL model with a single surface is used, the normal vectors of which are directed to the construction axis; when synthesizing a self-adapting file, a substrate of titanium nickelide is used in the SLP setup; each layer consists of single laser passes to create the final self-adapting file layer by layer.

The technical result is also achieved due to the fact that an ytterbium fiber laser with a spot diameter of 30-55 μm is used in the selective laser melting installation.

The technical result is also achieved due to the fact that the nickel-titanium powder material is obtained by alloy atomization, while the powder has a median particle diameter of 15-25 microns.

The technical result is also achieved due to the optimized shape of the self-adapting file obtained by the claimed method, including a cylindrical base and a mesh structure, moreover, there are additional jumpers between the mesh structure and the cylindrical base, and all structural elements of the self-adapting file are made with angles between the overhanging surfaces and the substrate plane of more 35°, which makes the self-adapting file design elements self-supporting.

Brief description of the drawings

On FIG. 1 - Optimized form of a self-adapting file for SLP technology. a) isometric view; b) side view; c) frontal view; d) development of a thin-walled file, pos. (1) - additional jumpers between the mesh structure and the cylindrical base.

On FIG. 2 - Powder layer hatching strategy. a) section A-A on the file development; b) section of the 3D model by the plane A-A, pos. (1) - generated laser movement trajectories by single passes, pos. (2) - a single surface of the STL model.

On FIG. 3 - Synthesized self-adapting files using the SLM method. a) the appearance of the files on the substrate, pos. (1) - synthesized files, pos. (2) - titanium nickelide sheet, pos. (3) - substrate for installation of SLS; b) panoramic SEM image of the entire file; c) the top of the file; d) file tip; e) jumper in the file structure.

Implementation of the invention

The method for manufacturing superelastic self-adapting files consists of several stages: analysis and preparation of the initial nickel-titanium powder material; preparation of a thin-walled 3D model of a self-adaptive file using a computer-aided design system (CAD) and conversion to an STL model; division of the model into separate layers using specialized software and preparation of an executable file containing the coordinates of the scan vectors for the laser in each layer; loading the resulting executable file into the SLM unit, loading the powder into the batcher bunker of the unit and layer-by-layer synthesis of a self-adapting file by the SLM method until the finished product is obtained. The method is implemented as follows.

The first stage of the invention includes the analysis and preparation of the starting powder material. The recommended method for producing nickel-titanium powder is gas atomization of titanium nickelide alloy. It should be noted that it is impossible to use a mechanical mixture of Ni and Ti powders due to the formation of various metastable phases in the course of SLM synthesis. The powder material should have the following chemical composition: 55.5-55.8 wt. % Ni, 44.5-44.2 wt. % Ti, with an oxygen content of not more than 0.05 wt. %. The chemical composition in the powder must be homogeneous in particle volume; sphericity coefficient of 0.85 and above; particle size distribution with a median particle diameter of 15-25 microns and with a Gaussian particle size distribution; fluidity of 100 g of powder material is not more than 35 s through a certified Hall funnel with a hole diameter of 4 mm.

The second stage includes designing a thin-walled 3D model of a self-adapting file using CAD and converting it into an STL model. The recommended version of the model is shown in Fig. 1a)-c), there is also a scan of a thin-walled file (see fig. 1d)), where additional jumpers in the structure are indicated (see pos. (1) fig. 1d)), which gives additional rigidity to the structure and avoids the use of supporting structures. It should be noted that all structural elements of the self-adapting file are made with angles between the overhanging surfaces and the substrate plane of more than 35°, which makes the structural elements of the self-adaptive file self-supporting. An important circumstance is the dependence of the critical angle on the scale of the part. Experimentally, it was found that the smallest angle for this particular micro-object is 35°, with an increase in detail, this angle will tend to 45°.

At the third stage, the model is divided into separate layers using specialized software and an executable file is prepared containing the coordinates of the scan vectors for the laser in each layer. The final number of layers in the product directly depends on the optimal layer thickness, which is determined based on the granulometric composition of the initial powder material. With a median particle diameter of 25 µm, a layer thickness of 30 µm was used, so the self-adapting file model with a height of 18 mm has 600 layers. A distinctive feature of the method is the technique of scanning by unit vectors. To implement this technique, surfaces with normals facing the inside of the product are removed from the STL model (see pos. (2) fig.2b)). To maintain the exact dimensions of the product, it is necessary to assign zero values to the parameters "indentation from the model boundary" and "indentation that compensates for the width of the melt bath". Next, a special algorithm is used that generates unit vectors (laser movement trajectories) in each layer (see Fig. 2a), pos. (1) fig.2b)) and not zones with contours and internal hatching, as in the prior art. Such an algorithm of operation is not typical for SLS installations, and commercial solutions do not support such features at the moment. When generating zones with closed contours using the standard algorithm, the layer is overfilled (and then the powder is over-fused by the laser), which reduces the resolution of the SLM.

At the final fourth stage, the resulting executable file is loaded into the SLM installation, the powder is loaded into the batcher hopper of the installation, and the layer-by-layer synthesis of the self-adapting file by the SLM method takes place until the finished product is obtained. The setup uses a CW ytterbium fiber laser with a nominal power of 200 W, with a Gaussian power density distribution (TEM00), with a wavelength of 1070 nm, with a laser spot diameter at a focal length of 30-55 µm. To ensure the fusion of titanium nickelide powder, a substrate is used (see pos. (3) fig. 3a)) on which a sheet of titanium nickelide is fixed (see pos. (2) fig. 3a)), moreover, with a difference in chemical composition from powder material not more than 0.2 wt. % Ni. An inert argon atmosphere with an oxygen content of no more than 100 ppm is provided in the installation chamber. To ensure the necessary compaction of the powder layer, a silicone squeegee is used.

The resolution of the SLM technology, in addition to the fraction of the initial powder material, the laser spot diameter, and the synthesis strategy, also depends on the synthesis modes (laser power, laser speed, beam diameter, and layer thickness). To ensure high resolution of SLM technology, it is necessary, on the one hand, to achieve the smallest size of the melt bath, and on the other hand, to preserve the mechanical properties of the product by optimizing the technological parameters of SLM. Possible technological parameters of the SLM process for nickel-titanium powder are: laser power in the range of 50-200 W; scanning speed in the range of 100-1500 mm/s; and the thickness of the powder layer in the range of 20-30 microns. The optimal parameters for the synthesis of a self-adaptive file are: laser power of 70 W, scanning speed of 800 mm/s, with a powder layer thickness of 30 µm. These modes were optimized by measuring the width and depth of the melt pool on single laser passes, as well as mechanical tensile tests of samples cut from thin walls, which were synthesized using the considered strategy of single laser passes. The combination of unit vector synthesis and the application of modes that provide the smallest size of the melt pool while achieving the required mechanical properties, guarantee the maximum resolution of the technology, which allowed the printing of self-adapting files (see pos. (1) fig.3a), fig. 3b)-e)).

The technical result is achieved through the use of an original strategy for shading the powder layer with a laser; using a laser with a smaller spot diameter at a focal length of 30-55 microns; using nickel titanium powder fine

fractions, namely with a median particle diameter of 15-25 microns; using an optimized form of a self-adapting file for the SLM method, in which all structural elements are made with angles between overhanging surfaces and the substrate plane of more than 35°, which makes the structural elements of a self-adaptive file self-supporting; application of optimized technological parameters: laser power of 70 W, scanning speed of 800 mm/s with a powder layer thickness of 30 µm.

Claims (4)

1. A method for manufacturing superelastic self-adaptive files from titanium nickelide, including analysis and preparation of the initial powder of titanium nickelide, preparation of a thin-walled 3D model of a self-adaptive file using CAD and conversion to an STL model, separation of the model into separate layers using specialized software and preparation of an executable file containing coordinates of the scan vectors for the laser in each layer, loading the resulting executable file into the selective laser melting (SLM) installation, loading titanium nickelide powder into the hopper of the installation dispenser, layer-by-layer synthesis of the self-adapting file by the SLM method until the finished product is obtained, characterized in that when preparing the executable file for the installation of selective laser melting, an STL model with a single surface is used, the normal vectors of which are directed to the construction axis; laser passes to create the final self-adapting file layer by layer. 2. The method according to claim 1, characterized in that an ytterbium fiber laser with a spot diameter of 30-55 μm is used in the selective laser melting installation. 3. The method according to claim 1, characterized in that titanium nickelide powder is obtained by alloy atomization, while the powder has a median particle diameter of 15-25 microns. 4. A self-adapting file obtained by the method according to claim 1, containing a cylindrical base and a mesh structure, characterized in that there are additional jumpers between the mesh structure and the cylindrical base, and all structural elements of the self-adapting file are made with angles between the overhanging surfaces and the substrate plane of more than 35 °, which makes the self-adapting file design elements self-supporting.

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