METHOD OF FABRICATING AN INTERFACIAL STRUCTURE AND A FABRICATED INTERFACIAL STRUCTURE

A method of fabricating an interfacial structure, the interfacial structure comprising a substrate and a projection on the substrate, the method comprising the steps of:   a) providing the substrate;   b) creating a number of steps on a surface of the substrate; and   c) fabricating the projection on the substrate by additive manufacturing onto the number of steps, thereby creating a stepped interfacial joint between the substrate and the projection.  A fabricated interfacial structure comprising: a substrate having a number of steps created on a surface of the substrate; a projection fabricated by additive manufacturing onto the number of steps; and a stepped interfacial joint between the substrate and the projection.

This invention relates to a method of fabricating an interfacial structure and a fabricated interfacial structure.

BACKGROUND

Various engineering applications require geometrical modifications to be made to components, including, but not limited to, flanges, ridges and other functional structures and features. The aerospace and automotive industries, in particular, have key applications that utilize fabricated interfacial structures comprising two interfacing metallic solid bodies, such as air-foils and exhaust manifolds. Such fabricated interfacial structures often comprise a substrate and a projection fabricated on the substrate, where the substrate could be a newly fabricated part or an existing part. In view of the known disadvantages of using fasteners and adhesives, in applications like remanufacturing and feature modification in the aerospace and automotive industries, laser metal deposition (LIVID) has instead been used to fabricate projections on substrates. However, this approach embodies intrinsic disadvantages as existing methods of fabricating interfacial structures using LIVID are generally weak at the interfacial location due to poor interfacial bonding between the substrate and the projection built by LIVID on the substrate. There is therefore a demand for a method of fabricating interfacial structures of two or more parts that avoids the disadvantages of poor interfacial strength associated with building or joining parts using existing LIVID techniques to fabricate projections on substrates.

SUMMARY

According to a first aspect, there is provided a method of fabricating an interfacial structure, the interfacial structure comprising a substrate and a projection on the substrate, the method comprising the steps of:a) providing the substrate;b) creating a number of steps on a surface of the substrate; andc) fabricating the projection on the substrate by additive manufacturing onto the number of steps, thereby creating a stepped interfacial joint between the substrate and the projection.

Step b) may comprise creating the number of steps as a recess on the surface of the substrate.

Step b) may comprise creating the number of steps to fully surround the recess.

Step b) may comprise creating the number of steps as a protrusion on the surface of the substrate.

Step b) may comprise creating the number of steps to fully surround the projection.

Step b) may comprise creating the number of steps by subtractive manufacturing.

In step b), the number of steps may be created by metal machining and in step c), the projection may be created by laser metal deposition.

Step a) may comprise fabricating the substrate by additive manufacturing.

Step b) may comprise creating the number of steps during additive manufacturing fabrication of the substrate.

Step a) may comprise creating a fillet between at least one upwards-facing surface and one sideways-facing surface.

Step a) may comprise creating a chamfer between at least one sideways-facing surface and one upwards-facing surface.

Step b) may comprise fabricating a thin-walled solid body of the projection onto the number of steps.

Step b) may comprise fabricating a non-hollow portion of the projection onto the number of steps.

According to a second aspect, there is provided a fabricated interfacial structure comprising: a substrate having a number of steps created on a surface of the substrate; a projection fabricated by additive manufacturing onto the number of steps; and a stepped interfacial joint between the substrate and the projection.

The number of steps may be created as a recess on the surface of the substrate.

The number of steps may fully surround the recess.

The number of steps may be created as a protrusion on the surface of the substrate.

The number of steps may fully surround the protrusion.

The projection may comprise a thin-walled solid body fabricated onto the number of steps.

The projection may comprise a non-hollow solid body fabricated onto the number of steps.

For both aspects, the stepped interfacial joint may comprise a metallurgical bond.

Each of the number of steps may comprise a sideways-facing surface and an upwards-facing surface when the surface of the substrate may be facing up, each sideways-facing surface may be at an angle θ from the vertical and each upwards-facing surface may be at an angle α from the horizontal, and θ and α each may range from 0° to 80°.

DETAILED DESCRIPTION

Exemplary embodiments of a method100of fabricating an interfacial structure200and the fabricated interfacial structure200will be described below with reference toFIGS. 1 to 25. The same reference numerals are used across the figures to refer to the same or similar parts.

As shown inFIGS. 1 and 25, in the method100of fabricating an interfacial structure200, a substrate20is provided (110) as a recipient for a projection30that is to be fabricated on the substrate20. The projection30is fabricated by additive manufacturing on the substrate20(130) and extends outwardly from a surface29of the substrate20. Throughout the present specification, the projection30may interchangeably be referred to as an interfacial projection30as the projection30interfaces with the substrate20at an interface290to form an interfacial joint210. The interfacial joint210may interchangeably referred to as an interfacial build/joint210since the projection30is simultaneously built up and joined to the substrate20by additive manufacturing on the substrate20at the interfacial joint210. The term “substrate” is used throughout the present specification to refer to any type of part that the projection30is fabricated on. For example, the substrate20may be a newly fabricated part made by any known method including but not limited to additive manufacturing, or the substrate20may be an existing part including but not limited to an existing part having a damage site to be remanufactured.

In the method100, the substrate20is provided (110) and a number of steps22are created on the surface29of the substrate20(120) using any known method such as metal machining, mechanical fabricating, laser treatment or even during additive manufacturing fabrication of the substrate20. In an exemplary embodiment, the substrate20may be fabricated by additive manufacturing while the number of steps22are created by metal machining on the fabricated substrate20. The number of steps22created may range from two to several hundred, depending on the application's requirements and implementation form. As can be seen in all the figures, each of the number of steps22comprises a sideways-facing surface40and an upwards-facing surface50when the surface29of the substrate20is facing up. The distance between adjacent sideways-facing surfaces40defines a width w of each step22and the distance between adjacent upwards-facing surfaces50defines a height h of each step22, as depicted in inFIGS. 1, 4 and 7. A combination of different h and w values can be used within a single instance of a stepped joint210implementation. For example, one of the number of steps22can have a particular step height h value while another of the number of steps22within a same stepped interface290implementation can have a differing h value. These differing h values can be denoted as h−1, h−2, and so on. Similarly, one of the number of steps22can have a particular step width w value while another of the number of steps22within a same stepped interface290implementation can have a differing w value. These differing w values can be denoted as w−1, w−2, and so on.

As indicated inFIGS. 1 and 4, the step height h at the interface290may be optimized by adjusting h to a value ranging between 0.1 mm and 5 mm, depending on the application's requirements and implementation form. Similarly, the step width w at the interface290may be optimized by adjusting w to a value ranging between 1 mm and 300 mm, depending on the application's requirements and implementation form. The step width w is preferably directly related to the step height h and the actual number of steps22created on the substrate20.

Each sideways-facing surface40of the number of steps22is created at an angle θ from the vertical (referred to as the vertical step angle θ) and each upwards-facing surface of the number of steps22is created at an angle α from the horizontal (referred to as the horizontal step angle α), as also depicted in inFIGS. 1, 4 and 7. The vertical step angle θ at the interface290may be optimized by adjusting it to an angle between 0° and 80°. Similarly, the horizontal step angle α at the interface290may be optimized by adjusting it to an angle ranging between 0° and 80°. Both angle selections are dependent on the application's requirements and implementation form. A combination of different “α” and “θ” values can be used within a single instance of stepped joint implementation. For example, one of the number of steps22can have a particular α value while another of the number of steps22within the same stepped interface implementation can have a differing α value. These α values can be denoted as α-1, α-2, and so on. Similarly, one of the number of steps22can have a particular θ value, and another of the number of steps22within the same stepped interface implementation can have a differing θ value. These θ values can be denoted as θ-1, θ-2, and so on.

Furthermore, the number of steps22may have a chamfered configuration as shown inFIG. 7, or a filleted configuration inFIG. 8where a fillet60of radius r is created between adjacent upwards-facing surface50. As indicated inFIG. 8, in the case where a filleted stepped joint210design is picked over a chamfered stepped joint210design, the fillet radius r can be optimized by adjusting it to a value ranging between 0.5 mm and 5 mm. The fillet interfacial build/joint design is defined based on h, and r. As indicated inFIGS. 7 and 8, stepped interfacial build/joint variants in the form of a concave or convex, as well as a chamfer or fillet substrate interface design can be selected based on the geometrical accessibility and availability at the substrate preparation stage of the manufacturing process.

After creating the number of steps22on the substrate20(120), the projection30is then fabricated on the substrate20by additive manufacturing onto the number of steps22(130) such that a stepped interfacial joint210is created between the projection30and the substrate20. Fabricating the projection30comprises building up the projection30layer by layer using additive manufacturing that directly deposits material of the projection30on the number of steps22on the substrate20. The substrate20and the projection30may be made of metal so that the projection30is joined to the substrate20by a stepped interfacial build/joint210that comprises a metallurgical bond, for example, when the additive manufacturing comprises metallic direct energy deposition (DED) such as laser metal deposition (LMD).

The resulting fabricated interfacial structure200thus comprises an interfacial build/joint210having a stepped joint interface290between the substrate20and the projection30. By employing an interfacial projection30design in the form of a stepped joint interface290, an improved interfacial bond between the substrate20and the projection30is achieved. A stepped interface290spreads an acting load over a larger area at the stepped interfacial joint210, hence strengthening it.

Exemplary embodiments of interfacial structures200fabricated using the method100can be seen inFIGS. 2, 3, 7(a) and8(a) where the projection30comprises a non-hollow solid body and the number of steps22are created as a recess28on the surface29of the substrate20. For example, the interfacial projection30may have a cuboid, cylindrical or air-foil configuration as shown inFIGS. 2(a), 2(b)and3respectively, and the stepped interface290may have a chamfered or filleted configuration as shown inFIGS. 7(a) and 8(a).

FIGS. 4 and 5show alternative embodiments of interfacial structures200fabricated where the projection30comprises a thin-walled solid body and the number of steps22are created as an annular recess28on the surface29of the substrate20. By “thin-walled solid body”, this is meant that the solid body has an at least partially tubular configuration where a central portion of the solid body projection30is hollow, as can be seen inFIGS. 4 and 5. The stepped joint interface290may have a symmetrical cross-sectional profile as shown inFIG. 4(a)or it may have an asymmetrical cross-sectional profile with an extended trench configuration as shown inFIG. 4(b). For example, the interfacial projection30may have a cuboid or cylindrical thin-walled solid body configuration and the stepped recess28created in the substrate20may correspondingly comprise a rectangular annular recess28or circular annular recess28respectively as shown inFIGS. 5(a), and 5(b).FIG. 6shows another embodiment of a fabricated interfacial structure200comprising a thin-walled solid body projection30having an exhaust manifold configuration that is fabricated by additive manufacturing onto multiple recesses28each comprising a single step22on the surface29of the substrate20.

While the projection30has been depicted as comprising either a fully non-hollow solid body or a fully thin-walled solid body as shown inFIGS. 2 to 8, it should be noted that the interfacial projection design can also be extended to various other free-form geometries as may be desired.

As an alternative to the number of steps22being created as a recess28on the surface29of the substrate20, the number of steps22may instead be created as a protrusion25on the surface29of the substrate20, as shown inFIGS. 7(b) and 8(b).

The strength of the interfacial build/joint210where the projection30interfaces and joins the substrate20is proportional to the net interfacial area of the joint interface290. Prior art interfacial joints typically have a flat joint interface between two joined bodies that result in a smaller interfacial area than a stepped interfacial build/joint design. Advantageously, a stepped interfacial build/joint210would use various step design parameters such as h, w, r, θ and α as described above to define its design, as indicated inFIGS. 1, 4, 7 and 8. These step design parameters maximize the net interfacial build/joint area of the joint interface290.

For a cuboid interfacial build/joint design as shown inFIG. 2 (a), the conventional (prior art) manifestation of an interfacial build/joint feature would be a flat interface area where Area=Length×Breadth. Likewise, for a thin-walled cuboid interfacial build/joint design as shown inFIG. 5 (a), the conventional (prior art) manifestation of an interfacial build/joint feature would be a flat interface area where Area=(Outer Length×Outer Breadth)−(Inner Length×Inner Breadth). For a cylindrical interfacial build/joint design as shown inFIG. 2 (b), the conventional (prior art) manifestation of an interfacial build/joint feature would be a flat interface area where “Area=π×radius2”. Likewise, for a thin-walled cuboid interfacial build/joint design as shown inFIG. 5 (b), the conventional (prior art) manifestation of an interfacial build/joint feature would be a flat interface area where “Area=(π×Outer radius2)−(π×Inner radius2)”. For free-form interfacial joint designs as shown inFIGS. 3 and 6, the conventional (prior art) manifestation would also be that of a flat interface area.

In contrast with the above-described conventional (prior art) manifestations of interfacial build/joint designs that typically have a flat interface area, in the present application, by introducing stepped features comprising a number of steps22at the build/joint interface290, the above-defined parameters of h, w, r, α, θ and number of steps22as shown inFIGS. 1, 4, 7 and 8can be adjusted and optimized to increase the interfacial build/joint area significantly.

Induced stresses on the joint interface290is such that “Stress=Force÷Area”. Hence, the strength of any interface is proportional to its respective interfacial area. By introducing stepped features in the form of a number of steps created on the substrate20and thereby increasing the interfacial area, the interfacial build/joint210can be strengthened significantly by spreading any acting load over a larger area. Joint strength properties such as 3D stresses against tensile, shear, bending stresses, and impact strength can thus be strengthened.

For instance, for a cuboid interfacial build/joint feature with dimensions “L×B=50 mm×50 mm”, the conventional (prior art) flat interfacial build/joint has a net interfacial area of 2500 mm2. By comparison, the same cuboid interfacial build/joint feature with an added stepped build/joint interface290comprising five steps22where α=0°, θ=0°, w=5 mm and h=3 mm for each step, the net interfacial area is 4300 mm2. Since any acting load on the interfacial build/joint feature is spread over a larger interfacial build/joint area for a similar interfacial build/joint feature with a stepped interfacial build/joint design, the interfacial strength can hence be improved proportionally by 1.5 to 2 times.

Exemplary Application—Repair of Damaged Spur Gear

In an exemplary application of the present invention, a spur gear90(FIG. 9(a)) having a gear tooth91that has been chipped off may be remanufactured using the above described method100. The damage site20of the gear90(FIG. 9(b)) where the chipped off gear tooth91used to be located may be considered the substrate20on which a stepped recess22,28is created using subtractive manufacturing, as shown inFIG. 9(c), to create a stepped recess22,28on the gear90at the damage site20. A remanufactured “new” gear tooth30may then be fabricated as the projection30by additive manufacturing on the stepped recess22,28on the damage site20, so that the new tooth30is joined to the gear20via a stepped interfacial joint210that comprises a metallurgical bond. To do so, the damage site20is first inspected for its degree of wear and damage, as well as any other form of defects, like cracks or plastic deformation. Non-destructive inspection techniques like ultrasonic measurements can be used to detect any cracks that have propagated from the initial chipped area. After diagnosing the degree of damage, a suitable stepped joint interface290that in this example comprises a stepped recess22,28is devised to ensure that the subtractive process removes any defects within the damage site20. The stepped interface290is created in computer aided drawing (CAD) and computer aided manufacturing (CAM) software and produced using subtractive manufacturing techniques on the damage site20with a hybrid machine, for example a milling machine, as seen inFIG. 9(c). The gear tooth30to be built up from the interfacial joint feature210is created in CAD and CAM software and is additively manufactured using LIVID from the same hybrid machine, as can be seen inFIG. 9(d). Lastly, subtractive manufacturing may be used to produce the surface finishing required of the restored gear tooth30.

Investigation into the Mechanical Performance of Three Different Interfacial Structures

A study was conducted to investigate the mechanical performance of three different interfacial joints: flat interfacial joint (prior art), v-shaped interfacial joint (prior art), and stepped interfacial joint210(present disclosure). The flat interfacial joint design is the conventional interfacial design for additively manufactured fabricated interfacial structures. The v-shaped interfacial joint design and the stepped interfacial joint210design are two variants whose mechanical performance are compared to the conventional flat interfacial joint design in this study. The sample fabrication and test sequence are shown inFIG. 10.

In the experiments conducted, a projection30comprising a Stainless Steel 316L cuboid of 170 mm×15 mm×37 mm was built by LIVID over a Stainless Steel 316L substrate20designed with each interfacial joint type being studied. The substrate20design and dimensions for the three different interfacial joints210: flat interfacial joint (prior art), v-shaped interfacial joint (prior art), and stepped interfacial joint (present disclosure) are detailed inFIGS. 11, 12 and 13respectively. The projection30built up by LIVID over the substrate20for each interfacial joint type is illustrated inFIGS. 14, 15 and 16. The deposition sequence of the LIVID to form the projection30is illustrated inFIG. 17. Dimensions of the projection30fabricated by LIVID were selected based on the build volume required to extract six Charpy samples, where the notch is located at the middle of the interfacial structure200. An illustration of the Charpy sample extraction locations from an interfacial structure200comprising the substrate20and projection30fabricated by LIVID on the substrate20is shown inFIG. 18. For each of the Charpy samples obtained, half of its volume was in the LIVID projection30region, and the other half was in the substrate20region, as shown inFIG. 19. Two variants for the Charpy sample for each type of interfacial joint210was used. The two variants differed in where the notch99is located for each Charpy sample type. The Charpy sample for each interfacial joint design type and the location of the notch99for each Charpy notch variant are shown inFIGS. 19(a)-(f). Three Charpy samples were extracted and tested for each notch variant type. The objective of using two notch variants is to investigate the effects of the directionality of the impact on the mechanical performance of the interfacial joint210.

The fracture surface topology of the Charpy samples were measured using a Zeiss Smart Zoom 5 with the 3D depth-of-focus microscopy method.

Charpy tests were conducted using a Zwick Roell, Amsler RKP 450 equipped with a 300 J pendulum hammer. Images of the Charpy tester and the Charpy sample mounting is shown inFIGS. 20 (a) and 20(b)respectively. Photographs of the post-test Charpy samples are shown inFIG. 21. Results for the Charpy test are shown inFIG. 22, and main effects plot for the different interfacial joints and notch variants are shown inFIG. 23. The V-shaped and stepped interfacial joint210designs produced a 9% to 119% improvement in toughness compared to the conventional flat interfacial joint design. The stepped joint interface210with a rotated notch produced the greatest improvement in toughness. This indicates that the stepped interfacial joint210created using the presently disclosed method100has a stronger mechanical performance in one direction over the other.

The main effects plot fromFIG. 22show that both the interfacial joint type and the directionality of the impact (as determined from the different notch variants) play an important role in the mechanical performance of the joint. Fracture surface topology images of the Charpy samples as shown inFIG. 24were taken using a Zeiss Smart Zoom 5 using a 3D depth of focus reconstruction method, with 34 times magnification, 30 μm Z-axis resolution. The fracture surface topology microscopy images show that the crack propagation occurs along the joint interface as indicated by the two white arrows in each figure, a contributing factor to the difference in mechanical performance for each interfacial joint design type.

Using the above described method100, no fasteners or adhesives are needed to join the projection30to the substrate20as the projection30and the substrate20are joined by a stepped interfacial joint210comprising a metallurgical bond arising from the use of additive manufacturing to fabricate the projection30on the number of steps22created on the substrate20. The present method100also addresses the problem of poor bonding found at conventional flat interfacial joints that arise from fabricating projections on substrates using current LIVID methods. Unlike current LIVID methods that build on flat or grooved substrates the presently disclosed method introduces stepped interfacial features that provide a mechanically stronger joint than the conventional flat interfacial joint. The stepped interfacial joint210thus created is shown through the experiments described above to have superior toughness over conventional flat interfacial joints as well as V-shaped interfacial joints. The disclosed method100and resulting stepped interfacial joint210therefore avoid the problems of conventional fastener and adhesive joints and also provide superior joint toughness over existing flat interfacial joints, making them particularly suitable for aerospace and automotive applications to build and repair metal engine structures such as air-foils and exhaust manifolds, for example.

In an exemplary embodiment, by combining subtractive manufacturing in creating the number of steps on the substrate (120) with additive manufacturing in fabricating the projection30on the number of steps on the substrate (130), the presently disclosed method100allows structures with complex transition geometries at joint interfaces to be fabricated with mechanical interlocking interfaces that are metallurgically bonded. This allows for structures with unique geometries to be fabricated, thereby enabling development of products and parts that were once too costly to fabricate or could not feasibly be fabricated at all. The subtractive and additive manufacturing steps may even be combined in a single machine in hybrid manufacturing which is an emergent technology within the additive manufacturing sphere that aims to streamline and simplify the additive manufacturing process into conventional subtractive manufacturing lines. In this way, the incorporation of additive manufacturing into a manufacturing line is greatly simplified and hybrid manufacturing can be used to create the stepped interfacial features as disclosed in the present application, where subtractive manufacturing is first used to create the interfacial steps prior to using additive manufacturing to build up the intended feature as a projection. In a hybrid manufacturing implementation of the present method, additive manufacturing may even be initially used to fabricate the substrate prior to using subtractive manufacturing to create the number of steps on the surface of the substrate and followed by fabricating the projection by additive manufacturing on the number of steps. In this way, inherent weakness in the single-layer joint between the projection and the substrate of a structure that is fabricated entirely by additive manufacturing alone is avoided as the present method creates a stepped interface between the substrate and the projection, thereby increasing bonding area and accordingly bonding and joint strength between the substrate and the projection.

Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations and combination in details of design, construction and/or operation may be made without departing from the present invention. For example, the shapes and dimensions of the substrates and projections that may be used and/or created in various embodiments of the presently disclosed method and fabricated interfacial structure are not limited to those described above with reference to the accompanying figures.