Additive manufacturing (AM) is a rapidly developing field of manufacturing that is replacing traditional manufacturing processes. Additive manufacturing of components from metals, metal alloys as well as metal matrix composite is an emerging field that offers several salient benefits when compared against traditional manufacturing processes such a casting, forging and machining. Some of the salient benefits of AM processes are significant cost saving by maximising material usage with minimal waste and an increased design flexibility. AM processes do not require expensive tooling or dies and therefore provide flexibility to manufacture complex components with quick turnaround reducing production lead times and dependencies. AM can also be used to manufacture metal alloys and metal matrix composites that have superior physical and mechanical properties compared to pure metal components.

Hascoet, Jy & Parrot, Jerome & Mognol, Pascal & Willmann, Etienne. (2017). Induction heating in a wire additive manufacturing approach. Welding in the World. 62. 10.1007/s40194-017-0533-y.

Hascoet et al. published paper referenced herein, presented an approach to depositing wire material on substrate using an induction coil. A U-shaped coil of single turn was used to melt the wire and heat the substrate or previous layer to an optimal temperature for good fusion. There are several limitations with the approach and as concluded in the paper, the method is difficult to deposit layers side by side. The deposited layers presented are not uniform and the material feed rate of up to lOOmm/min is very low compared against prevalent AM processes. Finally, it discloses a process of fully melting feedstock material and heating the substrate or previous layer which does not address the heat problem described below.

Chinese patent number CN109550947A discloses the use of induction heating of metal wire for additive manufacturing. It describes a process for uniformly heating and melting feedstock material using high frequency current through concentric coils where the skin depth is half of the wire diameter. The process thus intends for full melting of wire material and does not address the heat issue described below. It also limits the wire size based on skin effect depth achievable and consequently does not provide a competitive deposition rate. Finally, the process also intends to apply the induction heating to heat the preceding deposited layer to ensure fusion by placing the coil close to the layer in similar manner to Hascoet et al. The preceding research and work surrounding this patent describes a process that uses secondary direct heating of the substrate with a heat source in the form of laser beam to improve fusion between the layers, again not addressing the heat issues later described. United States patent US10349510B2 - discloses an AM nozzle using plasma gas to heat the source material described as "ink" for deposition. It intends for small scale deposition on surfaces to create surface features and does not intend to use material in the form of wires. The process describes a sheath gas directed to the source material to melt and fuse it to the deposition layer, again not disclosing a partly solid wire source material. Optionally it discloses heating of the plasma gas using inductive coil supplied by radio frequency power source and does not intend on direct heating of source material through the inductive coil.

As most metals melt at high temperature, the heat applied has an adverse effect on mechanical properties introducing problems such as distortion, residual stress as well as oxidation of the materials.

There are two main streams: powder AM and wire AM. Powder bed AM typically uses a high power beam heat source to fuse powder to produce complex components to a very high level of accuracy. Wire AM is akin to welding with a filler material and produces near net shaped components at a much faster rate than powder AM. Both techniques rely on heating the feedstock material beyond its melting temperature creating a local melt pool to fuse material to a substrate or preceding layer and as such the component experiences very high levels of heating.

Furthermore, currently powder bed AM techniques are limited in size to typically less than 1 m3 in volume and therefore the preferred method of additively manufacturing medium to large scale components is through a range of processes commonly referred to as Direct Energy Deposition (DED) where a heat source is used to melt and deposit feedstock material in the form of wire or powder. Most DED processes do not have the same resolution as powder bed AM techniques and therefore the final components are near net shape components that may require some machining to obtain desired final component. Of the various DED processes, arc-based deposition methods typically offer the highest of deposition rates (around lOkg/hr) by feeding material at very high feed rates. Electron Beam Additive manufacturing (EBAM) is also an attractive process as the component is built under vacuum environment but is very expensive compared to arc processes.

Unless performed in vacuum, components can experience some oxidation despite taking measures to reduce oxygen level to a few parts per million. Metals are good conductors of heat and therefore the heat applied during manufacture is absorbed by the preceding layers. This encourages grain growth in the direction of heat which results in anisotropic behaviour of the component and whereby the mechanical property is affected along that direction which is undesirable. Most metals applied for industrial applications have melting points in the region of 1000°C. Refractory metals such as Tantalum, Tungsten have even higher melting points, above than 3000°C.

As such a tremendous amount of heat is required to melt the material. All prevalent DED AM processes rely on melting the feedstock material to deposit the material layer by layer. A lot of energy is therefore required to produce heat necessary to deposit the material and construct the desired final component. The heat applied presents a significant problem that can adversely affect the property of the component.

The heat applied can also introduce distortion depending on the geometry of component and the build sequence. Residual stress and shrinkage are observed due to the thermal cycles which can lead to cracking of the component when not addressed. The heat problem is further exacerbated as the size of component increases and therefore large-scale additive manufacturing will need careful management of heat. Despite these limiting factors, the industry has steadily adopted the AM process for metal, and it is now used in various industry such as aerospace, automotive, railway, tooling.

Another important factor to consider on AM processes is the build rate. Currently arc based processes offer the highest of deposition rate by feeding wire material at high speed of up to lOm/min. Medium to large scale components manufactured by AM for industrial applications demand high deposition rates for it to be provide tangible benefits. Prevalent AM processes typically feed wire material of less than 2mm in diameter. By using larger size wires or rods similar or higher deposition rates may be achieved despite feeding material at fraction of the feed rate.

Notably all of the prior art methods mentioned have to deal with an issue of how to get the source material to fuse with the colder deposition layer. This often requiring additional separate means of heating this layer.

All these processes described above are still using the heating process to fully melt the material and therefore do not solve the heat treatment issue described above. It is one object of the current invention to at least in part begin to address these outlined problems.

The invention aims to address the heat problem by introducing a deposition technique whereby the bulk of feedstock source material is not melted which represents a departure from prevalent AM processes.

It is also an objective of invention to provide an alternative process capable of producing high deposition rates necessary for medium and large-scale component manufacture yet using less energy.

Another objective of the invention is to enable large scale additive manufacture of metal alloy and Metal Matrix Composite (MMC) component(s) using feedstock material comprising potentially multiple materials arranged in layers coaxially. An additive process for MMC and metal alloys is disclosed here in whereby a source material comprising of multiple layers of material arranged such that these layers are heated selectively and partially melted and then deposited for a layer by layer construction. Another object is to manufacture Functionally Graded Material (FGM) components using sequentially fed feedstock material with gradually varying composition.

Furthermore, the invention proposes an integral energising method for improving the ability of the source material to fuse to a preceding deposition layer. Through heating of a shielding gas this energy is transferred to the deposition area improving the fusibility of the source material to the preceding layer.

According to an aspect of the invention, an induction heating process applies directed heat onto a material by passing an alternating electrical current through induction coils. It is a non-contact process and can scale from small fabrication in orders of microns to large scale component heating such as billets in the order of metres. The process uses alternating current passed around the coils to produce changing magnetic and electrical fields, the changing magnetic field induces current on a component placed inside the coil, heating it through generation of eddy currents.

The invention described herein addresses the heat, oxidation and distortion issue described above. The process only melts the outer surface of the feedstock (the source) material by directing the heat to the outer surface of the source material and not heating the bulk inner material. Such a process requires only a fraction of heat to melt the outer layer and therefore consumes less energy. By outer surface this may also be the outer surface of an inner core where a multi-layered source material is used.

The frequency of applied alternating current dictates the type of heating on the component. At very low frequency such as mains frequency, through body or bulk heating is observed. This process is used in industry for processes typically involving large scale components. Processes such as forging, forming, melting, annealing and heat treatment use low frequency induction heating to evenly heat components. Medium frequency induction heating in the range of 1kHz to 50 kHz is applied in industry for similar applications but at reduced scale.

The principle behind this aspect of the invention is explained by electromagnetic induction. The alternating current produces changing magnetic and electric fields around the coil, the rate of change of such fields is governed by the current frequency. When an electrically conductive material is placed between the coil, the change in magnetic field induces electrical current in the material which heats the material through Joule heating. The frequency of the alternating current applied dictates the type of heating and at very high frequency the eddy current induced is concentrated on a thin outer layer, a phenomenon referred to as 'skin effect'. The thin layer is known as 'skin depth' or 'depth of penetration' and is the depth from the outer surface beyond which the magnitude of current drops exponentially. It is governed by the equation

Skin depth

Where, p = resistivity of material p = permeability of material u) = angular frequency of current

The skin depth (6) is inversely proportional to the frequency and therefore decreases with increasing frequency. At low frequencies (less than 1kHz) skin depth of most metals is in the region of few mm, increasing the frequency to several kHz, the skin depth is typically less than a 1 mm. It must be noted that skin depth is not the depth of heated layer.

Using induction heating through the application of high frequency alternating current will concentrate the heat on the outer surface, rapidly increasing the temperature of the component up to its melting point within a fraction of a second.

By using alternating current of more than 1MHz, and preferably between IMhz to 50 Mhz, the skin depth for most conductive materials is less than a tenth of millimetre (0.1mm). By concentrating the heat energy over a thin layer, significantly high energy densities can be obtained which heats the region rapidly to the melting temperature of the material. The bulk of inner core material is not heated by the induction coil and will only experience heat through conduction from the heated outer layer. Some ferro material such as iron will experience heating through hysteresis due to change in magnetic flux however the heat produced is fraction of heat produced by eddy current and not concentrated. By providing suitable heat energy, the melt pool on the outer surface can be controlled to desired level necessary to wet the surface where the material is being deposited. By wet the surface it is meant that the local melt pool is formed comprised of source material but also a small layer of the deposition layer.

To achieve a good fusion between the melted feedstock and the layer it is being deposited on this deposition layer can also be heated. For the feedstock material to fuse on the substrate or preceding layer, the molten layer must wet the surface of the deposition layer. The wetting depends on several factors, most of which are associated with the characteristics of the surface being deposited upon. Most metals typically have a thin oxide layer which hinders wetting, further the application of a melt pool on a relatively cold surface compared to molten layer is also not conducive to wetting. Another issue with the process is that the feedstock is only heated inside the coil, so it will start to lose heat after passing through the coil, which can make it difficult to control the volume and geometry of the deposition. Consequently, such a process may be prone to incomplete fusion between layers. Further the shape of deposited layer will be challenging to maintain without the application of a secondary heat source to manipulate the molten deposit. Therefore a means of heating or energising the deposition surface is preferred to ensure good fusion between the layers of the AM build using induction coils. Known DED processes apply heat to melt the feed material as well as the surface of deposition, creating a local melt pool thereby facilitating fusion between the layers.

The invention herein employs the use of alternating electrical current above 1MHz which can energise gases passing through the coil separated by a dielectric layer. The energised gas can therefore be used to heat the substrate or preceding layer and aid wetting of the partially melted deposit.

The intense selective induction heating through skin effect can also be applied to additively manufacture components and structures from complex alloys, metal matrix composite (MMC) or functional graded material (FGM) by using feedstock material which is in the form of either coaxial or cored wire/rods. MMC is a composite material where reinforcement material is embedded in a metal matrix. MMC comprises of at least two constituent parts: a) a matrix of for example metal or alloy such as Al, Ti, Cu, Co-Ni, Steel etc and b) reinforcement made from another metal or metal alloy or ceramic material such as Alumina, SiC, Zirconia, TiN, CBN etc in the form of either particulate, powder, whisker or fibres. MMC components can be fabricated by various methods but the commonly used method is either stir casting or infiltration method whereby the reinforcement is fed onto a molten metal matrix. To additively manufacture the MMC component using the selective induction heating process, a cored or coaxial wire or rod is used where the reinforcement material is surrounded by the metal matrix. The reinforcement may be coated or mixed with wetting agents to aid wetting with the metal matrix. By design the matrix material has a lower melting temperature compared to the reinforcement. Upon application of induction heating, the metal melts and infuses with the reinforcing material and is deposited on surface by wetting and fusing the molten metal layer. Thus, a complex MMC structure can be produced from feedstock comprising of several layers of matrix and reinforcements.

The principle method of deposition is as follows. An alternating electrical current of suitable frequencies and power are supplied to an induction coil (From lOOKHz up to and including 100MHz) heating the feedstock material (source material) through induction skin effect heating. The feedstock material is fed through the torch (torch and nozzle are used inter-changeably through-out and apply to the same feature) by rollers driven by a motor controlled through the control unit and is insulated from the coil by a dielectric tube. As the material passes through the induction coil, changes in magnetic flux induce current on the material that concentrate on a thin layer on the outer surface of material referred to as skin depth. The thickness of the layer or depth of penetration depends on the frequency of current as well as resistivity and permeability of the source material. The thin layer on the outer surface is rapidly heated through Joule heating from the induced current. The current intensity drops exponentially beyond the skin depth and therefore the inner bulk material does not experience heating from induction coil and only experiences heat through conduction from the heated outer layer. Under application of a suitable current, the thin layer on the outer surface of material is melted. The molten layer will grow inward upon continued application of heat which can be controlled by adjusting the feed rate and current so that the bulk material is not melted. The feedstock material with the molten outer layer is then deposited onto the substrate or component by wetting the surface with the molten material creating a fused layer. A shielding gas may be fed through the torch (nozzle) to prevent oxidation of the molten layer. The shielding gas is typically is an inert gas such as Argon, Helium but can also be mixture of gas with traces of active elements like Hydrogen etc. The shielding gas can be energised by the electric and magnetic field of the coil(s) as well as the induced fields on the material. The energised gas can also aid wetting of the molten layer on the deposition surface by supplying additional heat to the deposition surface. A component can thus be produced by depositing the partially melted material layer by layer. It will be appreciated by the skilled reader that by varying the induction frequency used to heat the source material and the induction frequency used to heat the shielding gas the invention also encompasses full melt additive manufacturing processes. The method of deposition using cored wire or coaxial wire comprising of more than one material is similar in that it relies on the outer molten layer to wet and fuse on the depositing surface. The heating regime is however more complex, and the process relies on melting at least one part of one constituent material. By design, the feedstock is made from materials grouped coaxially such that the outer layer comprises a material with lower melting temperature than the inner material. The feedstock can also be configured to have multiple layers of materials such that the outer and selective inner layers undergo partial melting. The heating regime will be dictated by the material properties, only freely conductive materials will experience heating from the induction coil directly and the heating rate will differ based on the material properties. Taking an example of a feedstock comprised of two conductive materials arranged with the lower melting material on the outside, when the feedstock is passed through the coil carrying the current then both materials will experience inductive heating whereby the thin outer layer is heated. The material on the outside with the lower melting temperature is heated from two fronts and will melt before the inner material thereby providing a molten layer for deposition. The process described is generalised to aid understanding, it is expected that the two materials will mix at the interface and form eutectic system with varying phases.

In another embodiment a secondary source of heat is applied in the form of gas plasma produced by an induction coupled torch (nozzle) which also relies on an alternating current in the MHz range to generate the plasma. The plasma is used to energise the surface for deposition and improve wetting of the molten material. The heat from the plasma can also be used to sustain the molten layer and prevent the feedstock material from freezing before deposition. A further torch (nozzle) can be applied for scenarios where the primary torch is not able to produce sufficient energised gas to energise sufficiently the deposition layer.

In another embodiment, the feedstock material is in a form of a rod stored in magazine configured to feed them sequentially. The rods are arranged in the order of sequence and selected such that the composition of material gradually varies between each rod. When the rods are deposited sequentially the final component will have a varying composition over its volume, producing a functionally graded structure.

In another embodiment the torch is configured with two sets of coils operating at different frequencies.

The process outlined herein is configured to not melt the bulk of the feedstock material, as such the final deposited material will be at significantly lower temperature when compared to prevalent wire and powder additive manufacturing systems. This methodology offers a fundamentally different mode of depositing material compared to all prevalent methods. There are several salient advantages and benefits offered by this methodology. By far the biggest improvement offered is a significant reduction in heat input and overall heat cycles experienced by the component. Current systems that apply heat to melt and fuse material necessitates the deposited material to be above the material melting point. As most metals have high melting temperature in comparison to polymers the heat applied is greater by orders of magnitude. The heat experienced by the component during the build unless managed has detrimental effect on the material property. The heat cycles can distort the component, affecting overall form and geometry. Depending on the geometry residual stresses can be introduced in the component. Unless performed in vacuum or complete inert atmosphere the component will experience oxidation to some degree. These issues are more severe for refractory metals which have higher melting temperatures, limiting the range of materials. The proposed methodology offers an alternative process which addresses the heat issue and can be applied to conductive materials with very high melting temperatures.

The proposed methodology also provides a relatively good deposition rate which is an important factor for AM process as it is related to build time. Arc based wire additive manufacturing currently offers the highest of deposition rate of all AM processes and has been the presumptive choice for large scale component manufacturing not possible with powder bed processes. However, wire arc processes typically use wires that are less than 2mm in diameter. Whilst the feed rate of the proposed methodology may not be as high as wire AM, the ability to deposit larger diameter wire will improve the deposition rate. The method proposed herein can be configured to apply feedstock in the form of rods with diameters in excess of 20mm.

According to a first aspect of the invention there is a nozzle (5) for additive manufacturing comprising: a passage for permitting source materials (4) to pass through the nozzle (5); the nozzle being configured to receive a shielding gas (9) an induction heating coil (12) for heating the source materials (4); Characterised in that the nozzle further comprises a further heating means (13) for heating the shielding gas (9) into a plasma to heat a deposition layer (3). Beneficially this further heating of the shielding gas allows for a more efficient fusion of the source material to the deposition layer and allows the induction heating of the source material to be limited to a peripheral skin layer and yet still achieve good fusion of the source material to the deposition layer. This has the advantage that the heat experienced by the source material is significantly limited to the skin layer i.e. the nozzle is configured to concentrate the heating to a relatively thin skin layer. This requires only a fraction of heat to melt the outer layer of the source material and therefore consumes significantly less energy than known processes and also reduces the unwanted effects discussed with the known deposition techniques.

Optionally the further heating means (13) comprises an induction heating coil (13) to provide inductive heating of the shielding gas (9). Beneficially this unifies the heating methods used for the source material and the shielding gas. Using induction heating to energise the shielding gas uses less energy than other standard methods.

This configuration provides for an induction plasma torch which can be powered by same power supply as nozzle (5) and can be tailored to provide energy to heat as well as manipulate the deposited layer in order to produce desired geometry and surface finish.

The skilled reader will appreciate in this claim reference 13 is used for both the further heating means and the induction coil, purely because in this non-limiting example, the further heating means in the Figures is an induction heating coil.

Optionally the induction heating coil (12) and the induction heating coil (13) are operable at independent frequencies from each other. This allows for precise application of the most optimal frequency for the source material and for the shielding gas.

Optionally the induction heating coil (12) is capable of operation under an alternating electrical current in the frequency range of lOOKHz to 100MHz. This range allows the heating effect of the induction coil to be limited to only a very minor skin layer of the source material which avoids unnecessary and unwanted material changes found with other though body heating methods.

Optionally the induction heating coil (13) is capable of operation under an alternating electrical current in the frequency range of 1MHz to 100GHz. This range is optimal for the energisation of the shielding gas.

Optionally the nozzle (5) comprises a plurality of discrete induction heating coils (12). Beneficially this allows for graded and progressive heating of the source material.

Optionally the plurality of induction heating coils (12) are configurable to be operable at different frequencies. This allows for a graded heating effect to be achieved. According to a second aspect of the invention there is proposed an apparatus for additive manufacturing comprising:

- At least one nozzle (5) according to any of claims 1-7;

- A power source for supplying power;

- A feed unit (6) for suppling a source material (4) to the nozzle (5);

- A reservoir for supplying a shielding gas (9) to the nozzle (5) and to a deposition layer (3);

- A motor (8) for controllably moving the nozzle (5) relative to the deposition layer (3);

- A control unit (10) for controlling at least one of: the rate of induction heating, the rate of movement of the source material (4) through the passage, the flow rate of the shielding gas (9) and / or and the relative movement of the nozzle (5) in relation to the deposition layer (3).

This beneficially provides for an apparatus to achieve a more efficient and larger scale additive manufacturing process than is otherwise known in the art.

Optionally the apparatus may further comprise a means (14) for moving the further heating means (13) relative to the deposition layer (3) and / or the nozzle (5). Beneficially this allows for more adaptability in the amount of heating of the deposition layer is applied.

Optionally the apparatus is characterised in that it comprises a plurality of nozzles (5) and in that the dimensions of the source material (4) may be different for each nozzle (5). Beneficially this allows for a more refined, smother surface build of the end component. The skilled reader will appreciate that by stating different dimensions of the of the source material in this claim it relates to the form in which the material is expelled from the nozzle, i.e. they can be of a similar form of differing overall cross sectional area taken at plane transverse to the plane of the material passing through the nozzle or indeed they can have a different cross sectional form altogether.

Optionally the apparatus may comprise a low atmosphere or vacuum chamber (15) for substantially enclosing the nozzle (5) and the deposition layer (3). Beneficially this allows for a significant reduction in the risk of oxidation for the deposition layer.

According to a third aspect of the invention there is proposed a method of additive manufacturing comprising at least the steps of:

- Inductively heating a shielding gas (9) into a plasma to heat a deposition layer (3), - Inductively heating a source material sufficient to allow it to be fused onto the deposition layer (3),

Beneficially the heating of the shielding gas allows for a more efficient fusion of the source material to the deposition layer and allows the induction heating of the source material to be limited to a peripheral skin layer through judicious selection of the excitation frequencies.

This has the advantage that the heat experienced by the source material is significantly limited to the skin layer and requires only a fraction of heat to melt the outer layer of the source material and therefore consumes significantly less energy than known processes and also reduces the unwanted effects discussed with the known deposition techniques. It is to be appreciated that through choice of the frequencies of used for the heating processes both of the source material and the shielding gas that the full scope of embodiments covers also a full melt process of the source material.

Optionally the method of additive manufacturing further comprises at least the steps of:

- Melting only a limited peripheral portion of an at least part electrically conductive source materials (4),

- Fusing one layer of the source material (4) to another by depositing the partially melted source material (4) upon a preceding layer of source material (4) to build a component.

Beneficially this allows the heating required in order to melt the source material to be limited to a peripheral (skin) portion of the source material. Meaning that the material characteristics of the rest of the source material are unaffected by the heating.

Optionally the method is characterised in that the at least part electrically conductive source material (4) is constructed at least in part from a metallic constituent. Metals are commonly available conductive materials and offer great strength benefit for additive manufacturing. It also beneficially allows for a less aggressive heating regime to be used for fusing metallic source materials. Metals form an ideal material choice for this method of additive manufacturing using induction skin heating to only melt a thin layer of the source material.

Optionally the method is characterised in that the melting only a limited peripheral portion is achieved through the application of induction heating of the at least part electrically conductive source material (4) using an electrical alternating current of frequencies of lOOKHz to 100MHz. Beneficially these high frequencies ensure the heating of the source material is confined to a peripheral skin layer of the source material and requires less energy than other known processes. This also means that it overcomes the issues of heat penetrating the rest of the source material changing its overall material properties. Optionally the induction heating of the shielding gas is powered by an electrical alternating current of frequencies of 1MHz to 100GHz. This is an ideal frequency range to excite the general common forms of shielding gas used in the field.

Optionally the at least part electrically conductive source materials (4) comprise a metal matrix composite formed of cored wire, coaxial wires and / or rods. Having a conductive element greatly increases the inductive heating effect. By conductive it is meant that this element has free electrons and is easily able to pass an electrical current without undue burden.

Optionally the at least part electrically conductive source material (4) comprises at least two constituent materials formed of; a metal matrix composite and; a reinforcement material arranged coaxially metal matrix composite.

This allows for a unique method of producing components using MMC. The reinforcement material can provide beneficial component properties of the final product.

Optionally the at least part electrically conductive source materials (4) comprises a functional gradient material formed of coaxial rods with gradually varying composition and which are stacked sequentially in a magazine and fed in the manner of the sequence to deposit and produce a gradually varying material composition over the volume of the final component or structure. This beneficially allows a greater control of the attributes of the final product.

Optionally the source material (4) for use in the method may comprise a plurality of discrete material layers. Beneficially this allows differing material properties within each of the layers. The outer layer can be chosen to have a lower melting temperature than the inner or vice versa for example.

Optionally the discrete material layers may comprise material in the form of rods, tubes, powder, whiskers, fibre, particulates, or paste.

Optionally the discrete material layers are configured to be formed of a plurality of material types comprising metal, metal alloy, ceramic, or semiconductor material. Figure 1 shows a schematic of the system for additively manufacturing by addition of a deposition layer 3 creating component 2, on a work surface (substrate) 1 which as illustrated can be configured to move in three direction or rotate around the two axes. The system comprises at least one torch (nozzle) 5 to deposit the source material 4 and feed unit 6 to deliver the source material 4, both mounted on a motion system or robot 8 capable of movement across six axes, three linear and three rotational. A central control unit 10 controls, the movement of motion systems 1 & 8; control process parameters such as input power to torch (nozzle) 5 from supply 11, wire feed rate from feed unit 6, shielding gas flow through flow control units 9.

The feedstock source material 4 is illustrated as wire but can be in the form of solid wire, cored wire, coaxial wire or rods. The material normally has at least in part a conductive constituent e.g. a conductive coating. The material may be stored in a wire spool or a magazine for rods. The material in the form of wire limits the diameter size as it becomes less flexible with the increase in size and therefore for diameter above 5mm, the feedstock material in the form of rods is preferred. When the feedstock material comprises at least two constituent material, optionally the arrangement is for the conducting material with lower melting temperature to be on the outer layer. The other material may be of conducting or non-conducting nature such as ceramics and in the form of solid, particulate, powder, paste, whisker or fibres. A multitude of stacking sequences are possible whereby selective layers undergo heating and melting. The material may also be coated or mixed with wetting agents to ensure wetting and or infusion of lower melting materials. The multi material feedstock material can be in the form of wire or rods. The feedstock is fed from a feed unit 6 attached to the torch (nozzle) 5 and comprises a suitable arrangement of rollers driven by motors to feed the source material 4. The feed unit stores the feedstock source material in either reel for wire as illustrated or in a magazine for rods. The magazine unit for storing rods is capable of feeding the rods sequentially to a prescribed order.

The torch 5 also houses the induction coil 12 which is water cooled and separated from the feedstock source material 4 by a dielectric material for example quartz tube to enable energising of shielding gas fed through the torch as well prevent short circuit The induction coil is powered by an alternating electrical supply 11 that provides alternating current in the frequency range of lOOKHz up to 100MHz. The control unit 10 can be programmed to deliver desired current to the induction coil 12 as necessary. The size of the torch 5 and the tube is dictated by the size and type of feedstock source material 4. Larger diameter feedstock source material 4 would require a bigger torch 5. Shielding gas 9 is supplied to the torch through a suitable reservoir with mass flow control unit connected to control unit to regulate flow rates. The shielding gas 9 typically is an inert gas such as Argon or Helium but a gaseous mixture containing low volume of active elements such as hydrogen can also be used.

In another embodiment Figure 2, a secondary heat source in the form of induction plasma torch 13 connected to a motion control system 8 capable of movement in 6 axes controlled via control unit 10. The plasma torch is powered by either same power source or a separate power source 11. Plasma gas 14 is fed to the torch to produce the plasma flame which is used to energise the surface of component being deposited and also prevents the molten layer of material from freezing before deposition.

Both torches can be setup in various configuration with respect to the substrate. The torches can be parallel to the substrate/ previous layer i.e. 0°/180° or any angle between 0 to 180°.

In another embodiment Figure 3, the plasma torch 13 is connected to the torch unit (nozzle) 5 and driven by the same motion system.

In another embodiment Figure 4, multiple torches that are independent of each other is illustrated where one torch unit is connected to a plasma torch. This enables higher deposition rate. This configuration also allows to use different size of feed material and torches to improve surface finish of final component.

Embodiments in Fig 1 to 4 can be housed in vacuum or inert atmosphere chamber 15 as illustrated in Figure 5 for system in Fig 1.

Figure 6 is an illustration of the torch showing the induction coil 12 powered by the power supply 11 and the feed source material 4 enclosed inside a dielectric material and shielding gas 9 fed to the torch (nozzle 5).

Figures 7 and 8 illustrate various combinations of feedstock source material 4 possible for deposition. Figure 7 show cored wire / rod where both materials can be solid metal or metallic alloy or the inner material is in the form of powder, whisker, fibre or paste made of either ceramic, dielectric, semiconductor or metals. The configuration will facilitate manufacture of a metal alloys as well as metal matrix alloy. Figure 8 shows a coaxial configuration of three materials. The possible stacking sequence are that all three layers comprise conductive metal or metal alloys, or the middle layer may constitute a nonconducting material in various forms described earlier sandwiched between the metal layers which may be of differing composition or same.

Figure 9 shows a configuration using source material in the form of rods which may be of single material or comprised of plurality of materials arranged coaxially. The rods are stored and arranged sequentially in the magazine and is fed onto the nozzle sequentially by a driving mechanism (not shown) like typical drive unit used on wires. The figure illustrates various size (diameter) rods which may be of same or differing composition that are fed in a sequence which will enable manufacture of functionally graded material.

Using the various embodiments of the systems above the method of deposition is described below:

An alternating current of suitable frequency in the regions of 100kHz to 100MHz and power is supplied to the induction coil 12 to heat the feedstock source material 4. Feedstock source material 4 is driven through motors in the feed unit 6 controlled by the control unit 10, as the material passes through the torch (nozzle) 5 changes in magnetic flux induces current on the source material 4 such that they are concentrated in the outer layer of the source material 4 referred to as skin depth. This region of the source material 4 is heated rapidly through Joule heating beyond the melting temperature of the source material 4. Shielding gas 9 supplied to the torch (nozzle) 5 prevents the molten region from oxidation and can also optionally energise an area of the deposition layer. The feed rate is selected to prevent complete melting of the source material 4 through transfer of heat on conduction. The partially melted source material 4 is then deposited on the component surface 2 or substrate 1, at a suitable angle using the molten outer layer to wet and fuse on the deposition area. It is to be understood that by deposition layer (3) this refers either to the initial substrate layer or a preceding layer of fused source material (4).

The intense selective induction heating through skin effect can also be applied to additively manufacture components and structures from complex alloys, metal matrix composite (MMC) or functional graded material (FGM) by using feedstock material is in the form of either coaxial or cored wire/rods. MMC is a composite material where reinforcement materials are embedded in a metal matrix. MMC comprises of at least two constituent parts: a) matrix of either metal or alloy such as Al, Ti, Cu, Co-Ni, Steel etc and b) reinforcement made from another metal or metal alloy or ceramic material such as Alumina, SiC, Zirconia, TiN, CBN etc in the form of either particulate, powder, whisker or fibres. MMC components can be fabricated by various methods but the commonly used method is either stir casting or infiltration method whereby the reinforcement is fed onto a molten metal matrix.

To additively manufacture the MMC component using the selective induction heating process, a cored or coaxial wire or rod is used where the reinforcement material is surrounded by the metal matrix. The reinforcement may be coated or mixed with wetting agents to aid wetting with the metal matrix. By design the matrix material has lower melting temperature compared to the reinforcement. Upon application of induction heating, the metal melts and infuses with the reinforcing material and is deposited on surface by wetting and fusing the molten metal layer. Thus, a complex MMC structure can be produced from feedstock comprising of several layers of matrix and reinforcements.

Functional graded materials (FGM) are advanced composite materials with gradual variations in their compositions and structure over the volume and therefore properties varying locally. A novel approach to FGM components is also proposed using the coaxial rods stacked in a magazine and fed to the torch sequentially. The rods can be made from gradually differing composition of materials, thereby depositing material with gradual change in composition and properties