The object of the present invention is a hierarchical material obtainable by an additive manufacturing process, characterized by a lattice structure in which inserts are present inside the porosities, said inserts are made of the same or a different material from that of the structure, are not constrained to said structure and free to move within the space boundaries inside the porosities, independently of one another and of the structure itself.

The invention also relates to tools and other articles comprising or consisting of said hierarchical material.

Prior art

Hierarchical structures contain structural elements which, in turn, have a structure. The hierarchical order of a structure or material can be defined as the number n of levels of scale with recognized structure. n= 0 corresponds to a material viewed as a continuum for the purpose of analysis of its physical properties; n = 1 (first order) could represent a lattice of continuous ribs or the atomic lattice of a crystal.

Known hierarchical materials may be made by traditional methods, namely by removing excess material or melting in moulds, for example by powder metallurgy. In such cases, the variation in the physicochemical performance of the material is obtained by differentially varying the size of the porosities. Said technique does not allow porous structures with inserts to be made easily. Other methods, based on chemical processes and subsequent processing, start from matrices of hierarchical materials, and inserts of a different material are added during the manufacturing process to impart precisely the characteristics of a hierarchical material; using this approach, the resulting material appears monolithic, and does not possess porosities that guarantee a weight reduction.

Examples of hierarchical materials useful for application as bone implants are disclosed in EP 3 137 125 and US 10,849,756.

EP 2 334 836 discloses a hierarchical composite material of a ferrous alloy characterized by an alternating macro-microstructure wherein globular micrometric particles of titanium carbide are surrounded by areas of ferrous alloy with dimensions ranging between 1 and 12 mm. The material, obtained by self-propagating high- temperature synthesis, is useful for machine and tool parts that are subject to wear.

CN106984822 discloses a porous honeycomb structure obtainable by 3D-printing techniques for use as a material for vehicle components.

US 7,871,578 discloses porous structures useful for the manufacture of heat exchangers.

DE 10 2010 063 725 discloses a structure with parallelepipedal cavities, inside which material in the gaseous state is present. The gas can be the air that remains trapped during the additive manufacturing process, or vapors deriving from the material used for the lattice structure. The patent describes a structure designed for vibration damping.

WO 2020/085897 discloses a structure wherein two lattice structures called matrices are assembled together and are movable relative to each other. The structure consists of two or more matrices wherein each point of one matrix occupies a cell of the other matrix. Each matrix moves integrally and rigidly relative to itself and the other matrix, within the limits of the cells wherein its points are located and limited to interference with the other matrix. Some movements of one matrix are therefore hindered by the intersection with the other matrix, and vice versa. There are no inserts in the cells able to move independently of the rest of the structure in any direction, including rotation around themselves or around any axis.

DE 10 2017 208 63 discloses a lattice structure wherein the cells are occupied by a material called a filler, for the purpose of vibration damping. The lattice structure requires a coating process. The filler material is added during a second step of the process occupies the gaps present in the structure, and is therefore geometrically constrained to said structure. It does not allow the production of inserts which are completely independent of one another and of any shape.

EP3210703A1 discloses a lattice structure contained in an outer jacket wherein a filler material in liquid or powder form is added, and then solidified. As it is also a filler, the geometry thereof is constrained to the main structure

WO 2019/226195 discloses various lattice structures. Inserts are not present or are not free to move because they are integrated into the matrix.

Szyniszewski, S., et al., (2020). Non-cuttable material created through local resonance and strain rate effects. Scientific reports, 10(1), 1-24, describe a material having 15% steel density, characterized by a grid of ceramic segments inside a cell matrix having 15% steel density. The material is non-cuttable by an angle grinder and a power drill..

None of the documents cited describe the critical and essential characteristics of the invention, listed below.

Description of the invention

The object of the present invention is hierarchical materials obtainable by additive manufacturing techniques which exhibit improved strength characteristics when subjected to loading actions, with performances similar to or better than those of hierarchical materials obtained by a chemical process.

The materials according to the invention, characterized by inserts inside the porosities of the main load-bearing structure, are lighter than materials having a conventional monolithic structure.

The inserts are free to move inside the porosities in any direction, independently of one another and of the lattice structure and, depending on the stress to which the material is subjected, give it advantageous properties and performances according to the state of stress, such as mechanical or fluid-dynamic loading.

The materials are obtainable by additive manufacturing technologies in a single step.

The invention therefore provides a hierarchical material, characterized by a lattice structure wherein, inside the porosities, inserts are present, which are made of a material which is the same as or different from that of the structure and are not constrained to said structure but free to move within the space in the porosities independently of one another, said material being obtainable by an additive manufacturing process.

Suitable additive manufacturing techniques include, for example, directed energy deposition, powder bed fusion and selective laser melting.

The inserts can be made of the same material as the lattice structure and in different shapes, such as a substantially spherical shape.

The material of the invention can consist of stainless steel, cobalt-chromium alloys, aluminum alloys, nickel alloys or titanium alloys, such as a Ti6A14V alloy, or any other component usable in additive manufacturing techniques.

The porosities size can range within wide limits, but typically ranges between 10 mm ³ and 15 mm ³ , while the insert size ranges between 0.3 mm ³ and 0.6 mm ³ .

The elemental cell, with the insert included, can be repeated periodically or non- periodically in the space. In non-periodical structures the same elemental cells will be present, but with variable dimensions. The elemental cell of the main structure can have any geometrical shape, but the actual structure must be designed to leave at least one cavity wherein the insert will be inserted. The geometry of the inserts can be either the same for all the cavities in the structure, or have a variable shape and size in some or all cavities. Moreover, not all the cavities must necessarily contain inserts, depending on the final application. In any event, the geometry of the insert must be large enough not to allow the insert to exit from its cavity; in other words the cavity left in the lattice must be small enough to embed the insert, while still leaving it free to move. Moreover, multiple inserts with the same or different geometrical shapes and sizes can be present in a single cavity.

The materials according to the invention can be advantageously used for the production of tools, devices or structures resistant to cutting wear (for example anti-theft devices for bicycles and motorcycles, safety structures in workplaces such as cutting protection cages, toy components, etc.) or as heat-exchange accelerators.

The inserts can play different roles within the structure, depending on the application of the invention. In the case of structures resistant to mechanical cutting, the inserts oscillate during cutting, generating an unstable cut and accelerating tool wear. The role of the inserts is to generate an action that hinders cutting. The movement of the inserts induced by the cutting action causes deterioration of the tool, as the inserts impart a mechanical action to the tool which tends to increase cutting vibration and tool temperature. Said two aspects combine to increase the rate of wear on the blade and damage to the tool, making cutting ineffective. In the case of cutting with hand cutters, the presence of inserts during cutting compacts the structure, making it similar to the behavior of the monolithic material and therefore increasing cutting resistance. In the case of fluid-dynamic applications for heat exchangers, the presence of inserts that are free to oscillate in the cavity contributes to increasing the turbulence of the flow, and therefore the heat exchange.

Description of figures

Figure 1 illustrates an example of the structure wherein the main structure is a modified octet-truss lattice wherein the inserts have a spherical shape.

Figure 2 illustrates the successive steps in the preparation process of a configuration of the material comprising the elemental cells shown in Figure 1, repeated periodically and showing the final size of the cylindrical structure.

Figure 3 illustrates one of the geometrical shapes of the material according to the invention.

Figure 4 illustrates an example of a Ti6A14V structure produced by laser powder bed fusion technology.

Figure 5 shows some details of the structure in stereomicroscopic images.

Figure 6 shows a three-dimensional reconstruction of the structure using computerized tomography, and a cross-section of the component wherein the inserts not connected to said structure are visible. Figures 7 and 8 show the results obtained during cutting of a cylindrical portion of a material according to the invention made of titanium alloy Ti6A14V.

Hereinafter disclosed, by way of example but not limitation, is an embodiment of the invention relating to a structure with high resistance to cutting by mechanical tools.

The cylindrical component was made of titanium alloy Ti6A14V using laser powder bed fusion additive technology. Various prototypes were constructed by varying the dimensions of the elemental cell, but maintaining the final size of the cylindrical component. Figure 4 shows the prototype with the size of the elemental cell greater in the as-built (by the machine) condition on the left, and the corresponding CAD model inserted in the production machine on the right.

Disclosed below, again by way of example but not limitation, are the stages of the preparation process of a cylindrical material according to the invention with reference to Figures 2a-2e. The same process can also be used for the construction of other materials with different shapes.

Starting with the construction of the latticed unit cell, said cell is periodically repeated to form a square of 5x5 cells (fig. 2a). The spheres are also repeated, in order, to fill the gap between the struts. The 5x5 matrix is then cut to create a cylinder with a diameter of 13 mm (fig. 2b). All the cut parts are eliminated (Fig. 2c), and replaced with new struts to recreate the circular shape (Fig. 2d).

Finally, the element obtained in the last step is repeated along the z axis until a height of 30 mm is reached (Fig. 2e).

The various structures were cut with a Bosch MetalMax AIZ32AT instrument with reinforced carbide teeth. To evaluate the efficacy of the material in counteracting the mechanical cut, the teeth of the tool were measured before and after cutting. Figure 7 shows a portion of the tool after cutting superimposed on the corresponding area of the virgin tool. Cutting was not completed because of damage to the blade, and breakage of over 50% of the teeth of the tool (Figure 8). The teeth of the tool proved to be substantially worn. The inserts present in the structure, set in motion during cutting, cause a high vibration and increase the temperature of the tool, giving rise to extensive, significant wear throughout the tool surface (Figure 8). By analogy with the experiments conducted on the hierarchical material, Figure 8 shows the results obtained during cutting of a cylindrical portion of a component made of titanium alloy Ti6A14V, using the same additive manufacturing technology. The wear observed on the cutting blade, and the temperature, are lower than with the corresponding hierarchical structure, demonstrating the efficacy of the structure in terms of performance.