Method for direct shape forming in cold spray process and system

The term "3D printing" covers a variety of processes in which material is joined or solidified under computer control to create a three-dimensional object, with material being added together (such as liquid molecules or powder grains being fused together), typically layer by layer. In the last years, precision, repeatability and material range have increased to the point that some 3D-printing processes are considered via ble as an industrial-production technology, whereby the term additive manufacturing can be used synonymously. One of the key advantages of 3D printing is the ability to produce very complex shapes or geometries, and a prerequisite for produc ing any 3D printed part is a digital 3D model or a CAD file.

A variety of processes, equipment, and materials are used in the production of a three-dimensional object.

Some of the different types of physical transformations which are used in 3D printing include melt extrusion, light polymerization, continuous liquid interface production and sintering.

The most-commonly used 3D-printing process is a material ex trusion technique called Fused Filament Fabrication (FFF) or Fused Deposition Modeling® (FDM®).±

Cold spraying (CS) on the other hand is a coating deposition method. Solid powders (1 to 50 micrometers in diameter) are accelerated in a supersonic gas jet to velocities up to ca. 1200 m/s. During impact with the substrate, particles undergo plastic deformation and adhere to the surface. To achieve a uniform thickness the spraying nozzle is scanned along the substrate. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be deposited using cold spraying. The kinetic energy of the particles, supplied by the expansion of the gas, is converted to plastic deformation energy during bonding. Unlike thermal spraying techniques, e.g., plasma spraying, arc spraying, flame spraying, or high velocity oxygen fuel (HVOF), the powders are not melted dur ing the spraying process.

Using additive manufacturing processes based on the cold spray deposition technology places several challenges, i. e. the impossibility of arbitrarily starting and stopping the deposition process in order to deposit material with clearly defined shapes. This problem is due to the difficulty of quickly turning on and off the cold spray deposition process which needs a regime condition to work properly and has rela tively long transient phases before turning on and after turning off. The consequence of this problem is that only continuous cold spray deposition trajectories can effectively be performed, strongly limiting the range of parts that can be produced with this technology.

The problem has up to now been solved by depositing a so called "blank" part, which means depositing material in ex cess and then by machining it in order to obtain the desired geometry of the final part. Such solution obviously greatly limits the productivity of this technology since it increases the time needed to reach the desired final shape.

Another solution that is used to obtain clear borders is to use a deposition substrate with plastic inserts in the areas where material should not be deposited. However, such solu tion works only for the first layers of deposited material and cannot be used to control the geometry of the produced part in each of the three dimensions, still limiting the fea sible shapes.

From CN 109226760 is already known a method and device for additive manufacturing that combines two methods that both work with metal material: a laser melting method combined with a cold spray method. The purpose of the invention is to offer a solution to the above described problems.

The problem is solved by a method to produce a structure with the features of claim 1. The problem is also solved by a sys tem with the features of claim 4.

According to the invention, the method to produce a structure by means of additive manufacturing by using at least a first additive manufacturing method using polymer material and a second additive manufacturing method using a metal, particu larly the cold spraying method, whereby the production of the structure of the at least first and second additive manufac turing methods is performed after building a first part of the structure either in polymer or metal on a deposition sub strate, building the structure in layers follows the steps: a)with the first method at least a part of a polymer mold form is produced on the structure and, b)with the second method a metal is applied by cold spray on the polymer mold form in a number of layers to reach a specified metal thickness, and c)go back to step a), if the structure is not finished.

The system to produce a structure by means of additive manu facturing according to the invention, containing

- a first additive manufacturing manipulator operated with a first tool suitable to apply a polymer material by using a first additive manufacturing method, and

- a second additive manufacturing manipulator operated with a second tool suitable to apply a metal material, by using the cold spraying method, where the production of the structure by the at least first and second additive manufacturing tools is performed after build ing a first part of the structure either in polymer or metal on a deposition substrate, operating the tools in an alter nating way and building the structure in layers: a)with the first tool at least a part of a polymer mold form and, b)with the second tool a metal is applied by cold spray in a number of layers to reach a specified metal thickness, and repeating a) and b) until the structure is finished.

Further advantageous embodiments are described in the depend ent claims.

In one advantageous aspect of the invention the method shows a further step: removing the polymer parts of the structure at the end or during the production process either mechanically or chemi cally.

Further another option of the method is to at least once re place step a) by

Step al) a part of polymer preformed mold form is placed on the structure.

In an additional embodiment of the system according to the invention the first and the second tool are mounted on one single manipulator.

In a further advantageous variation of the system the first and the second tool are mounted on a rotating holder.

Aspects and advantages of the invention will be set forth in part in the following descriptions or may be obvious from the description, or may be learned through practice of the inven tion.

The invention solves the problem by proposing a cold spray deposition system which is based on two manipulators (for ex ample industrial robots), the first having a cold spray gun as tool and the second having a deposition tool which can ex trude a polymer. The first manipulator performs the cold spray deposition pro cess as in standard cold spray applications while the second one deposits a polymeric material in the areas where the dep osition process has to be blocked.

A full and enabling disclosure of the present invention in cluding the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which Fig. 1 shows an overall view of the proposed system,

Fig. 2 exemplifies the process: the profile of the first lay er to be built on the left and on the right the deposited layer,

Fig. 3 shows the operation of the second manipulator,

Fig. 4 illustrates all steps of the production process, and Fig. 5 discloses an alternative solution of the system mount ed on a single holder.

Figure 1 shows an overall view of the proposed system. The two manipulators 1, 2 with mounted tools 11, 21 work on the same deposition substrate 3 which is the base on top of which a part is built.

In the following, the operation scheme of the proposed system is described. The operation of the two manipulators alone is firstly detailed and then the overall working method of the system is explained.

Manipulator 2 with a second tool, the cold spray gun 21, per forms the actual deposition process additively manufacturing the metal part 33 layer by layer. In order to do so, the cold spray gun 21 activates the cold spray flow 22 and is then moved by the manipulator along the deposition trajectory.

Figure 2 exemplifies the process: on the left, the profile 31 of the first layer to be built using the deposition process is shown as well as the trajectory to be executed 32 by the cold spray gun. On the right the deposited layer 33 is shown. This production process is well suited to create components with continuous shapes and with few or no isolated features.

Figure 3 shows manipulator 1 equipped with a second tool 11, a polymer extruder.

Such extruders can be based on state-of-the-art technologies. Examples of implementation of the extruder include using an extrusion screw, feeding wheels or a pump to extrude polymers in the form of e. g. pellets, filament or liquid. The goal of this device is to deposit a material that prevents the opera tion of the cold spray process. Such material is used simi larly to a mold in casting or masks in electronic manufactur ing, this means that it is shaped leaving voids where the metal should be deposited. Since the polymer can be extruded to any desired shape, any shape can be cold sprayed by fill ing the voids left by the polymer body. As already mentioned, the cold spray process works well in regime condition and has relatively long transient phases which make it hard or impos sible to obtain clearly defined shapes and detailed features without further operations. The use of the polymer gives therefore greater freedom and precision to the cold spray process.

Figure 3 shows the operation of the second manipulator 1. The part 33 shown in Figure 2 is used as example. The block of polymer 34 is deposited on the surface of a layer of the part being manufactured creating a negative shape of the features that have to be created. The next layer of metal can then be deposited using the cold spray method.

Figure 4 shows a combined operation of the two manipulators 1, 2 with all the steps of the production process. The two manipulators work in an alternated fashion depositing metal 33 and polymer 34 as needed. Steps 1, top left, and 2, top in the middle, show the deposition of the first layer of metal with the cold spray process. In the steps 3, top right, and 4, down left, the polymeric material 34 is placed on the sur face of the previously fabricated layer 33. The subsequent layer of metal is deposited in the steps 5, down in the mid dle, and 6, down right, exploiting the polymer block 34 to produce the desired features.

Even though a planar example is shown, any working direction can be used, allowing to freely create part features. The polymer is at least partly eroded when it is hit by the metal powder flow. In an advantageous embodiment of the invention remaining polymer can be removed at the end or during the production process either mechanically or chemically.

The proposed invention reduces the time and cost needed to create cold-sprayed metal parts with clearly defined shapes and features. Such features cannot be directly produced using the cold spray method and therefore in the state of the art a cold-sprayed material blank is usually machined to obtain the desired geometries and precision.

The advantage in terms of production time and cost reduction is generated by the use of polymeric material to prevent the build-up of material where desired and consequently obtain the final product shape.

Moreover, the use of a manipulator with a polymer extruder makes it possible to freely place the polymer as desired on the surface of the part being produced with the cold spray method.

An alternative solution of the proposed method would be to use preformed polymer profiles or sheets instead of deposit ing the polymer with an extruder directly on the part being printed. Such method would however significantly reduce the flexibility of the proposed technology since a polymer ex truder can easier create a desired shape in any needed posi tion. As further alternative implementation of the proposed system, a single manipulator 4 can be used either with a tool changer to manage the two tools 41 and 42 or with both tools mounted on a holder as shown in Figure 5. In a further arrangement, the holder of the tools has the ability to rotate. This ar rangement saves space in the production zone.