WO2018194481A1

WO2018194481A1 - Additive manufacturing technique including direct resistive heating of a workpiece

Info

Publication number: WO2018194481A1
Application number: PCT/RU2017/000249
Authority: WO
Inventors: Dmitry Leonidovich NESTERENKO
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG; Siemens Corp
Priority date: 2017-04-19
Filing date: 2017-04-19
Publication date: 2018-10-25
Anticipated expiration: 2019-10-19

Abstract

An additive manufacturing technique is presented. A workpiece is positioned on a building platform of a part building module of an additive manufacturing apparatus. Electrical contacts are attached to the workpiece. The electrical contacts are connected to a current source. A layer of powdered metal material is spread on the building platform and a surface of the workpiece positioned on the building platform. Thus the workpiece is embedded in a bed of the powdered metal material. Subsequently, direct resistive heating of the workpiece is performed by providing the workpiece with the electrical current from the current source via the electrical contacts attached to the workpiece. Finally, one or more portions of a surface of the layer of powdered metal material are selectively scanned by an energy beam arrangement, thereby melting or sintering the selectively scanned portions onto the underlying workpiece.

Description

Additive Manufacturing technique including direct resistive heating of a workpiece

The present invention relates to additive manufacturing (AM) and in particular to systems and methods of additive manufacturing having direct resistive heating of a workpiece.

Additive Manufacturing (AM}, also known as Additive Layer Manufacturing (ALM) , 3D printing, rapid prototyping or freeform fabrication, is a group of processes of joining additive materials i.e. plastic, metal or ceramic to make objects from 3D model data, usually building it up layer upon layer.

Additive manufacturing (AM) is a relatively new consolidation process that is able to produce a functional complex part, layer by layer, without moulds or dies. This process uses a powerful heat source such as a laser beam or a welding arc to melt a controlled amount of additive material, for example metal in the form of metallic powder or wire, which is then deposited, initially, on a building platform or a surface of a prefabricated workpiece. Subsequent layers are then built up upon each preceding layer. As opposed to conventional machining processes, this computer-aided manufacturing (CAM) technology builds complete functional parts or, alternatively, builds features on existing components i.e. on a workpiece, by adding material to the workpiece layer by layer rather than by removing it as is done in machining.

Additive manufacturing often starts by slicing a three dimensional representation, for example a CAD model, of a part to be manufactured into very thin layers, thereby creating a two dimensional image of each layer. As aforementioned the part to be manufactured can be a part that is to be built on a workpiece, for example during repairing of a chipped turbine blade the chipped turbine blade is the workpiece and the patch formed to fill or reform the chipped part is the part that is built on the workpiece. The workpiece is positioned on a build platform. To form each layer, popular laser additive manufacturing techniques such as selective laser melting (SLM) and selective laser sintering (SLS) involve mechanical pre-placement of a thin layer of metal powder of precise thickness on a surface of the workpiece and in adjoining horizontal surface above the build platform. Such pre-placement is achieved by using a mechanical wiper or by a powder spreading mechanism to sweep or spread a uniform layer of the powder or to screed the layer, after which an energy beam, such as a laser, is indexed across the powder layer according to the two dimensional pattern of solid material for the respective layer. After the indexing operation is complete for the respective layer, the build platform, and therefore the horizontal plane of deposited material, is lowered and the process is repeated until the three dimensional part is completely built on the workpiece as desired. In order to protect the thin layers of fine metal particles from contaminants and from moisture pickup, the operation is usually performed under an atmosphere of inert gas, such as argon or nitrogen.

Nowadays the AM processes are widely used in aerospace and energy industries, medical applications, jewelry, etc. Selective Laser Melting (SLM) and Selective Laser Sintering (SLS) , such as Direct metal laser sintering (DMLS) , Direct metal laser melting (DMLM) , are AM processes that use energy in the form of a high-power laser beam to create three- dimensional metal parts by fusing, or sintering in case of SLS, fine particles of thin powder layer together.

Although the SLM/SLS technologies become widely used in various applications, the SLM/SLS technologies have some limitations such as surface roughness, part accuracy, and the formation of layered residual stresses, which are reinforced by the high thermal gradients due to melting and solidification in a very short time. To control and vary the part properties and quality, the SLM/SLS technologies processing parameters, including laser power, laser scan speed, layer thickness and preheating, need to be varied and controlled.

During the SLM/SLS processes, high thermal gradients are present inside the parts because of the fast heating and cooling of the material. These thermal gradients lead to thermal stresses, which may cause residual stresses or even micro/macro cracks in the part that is built. To resolve these issues, preheating before the build process can be implemented, and thus during the building up of the part, the temperature differences within the material will be lower, which results in lower thermal gradients . Preheating of the substrate and the powder bed prior to the laser beam exposure provides beneficial effects during the SLM/SLS processes. Setting the temperature of the powder close to its melting point might save the energy induced by the laser and improve the wettability of the solid by the liquid phase i.e. of the underlying surface of the workpiece or of a layer formed in a previous step. In addition, preheating of the powder reduces the thermal gradients and slows down the cooling rates within heat affected zone lowering susceptibility for residual stresses formation and cracking during solidification.

One of the existing approaches for preheating the substrate and powder bed during SLM/SLS processes is based on heating building platform by installing a heating element underneath the building platform. In this approach the building platform, and the all the powder stacked on top of it are be heated up. In order to prevent the excessive heating of the surrounding structure, for example, walls surrounding the building platform, passive cooling (use of insulation) as well as active cooling are applied. The temperature of the base plate is constantly monitored by a thermocouple probe. The current preheating system can achieve temperatures up to 500°C for long runs.

Another approach is to heat up surface of the powder bed by using Infra-red heating. Infra-red heaters are generally placed above the building platform to maintain a desired temperature (up to 900°C) of the powder bed and the powder in the feed cartridge.

Yet another approach is to use Laser-beam heating. In AM systems equipped with this technique, each powder bed layer is scanned in two stages, the preheating stage and the melting stage. In preheating stage, a high current laser beam with a high scanning speed is used to preheat the powder layer (up to 0.4 - 0.6 of melting temperature) in multiple passes .

Yet another approach is to use Induction heating, in which the powder bed along with a workpiece is placed inside an Induction coil surrounding the powder bed and the workpiece placed therein, and thus resulting into heating of the powder bed and the workpiece .

However, none of the aforementioned techniques provide directed and specific heating i.e. they do not primarily heat up the workpiece surface or a region of spread powder layer that is required to be heated, on the contrary they heat up the entire powder bed or feed cartridge or building platform. It will be beneficial to provide a technique that ensures primarily heating of the workpiece and of a region of the spread powder layer that is limited on top of the workpiece or that is limited on top of a previously added layer formed on the workpiece, before the AM process is carried out for the spread layer.

Furthermore, existing approaches for preheating the substrate or workpiece and spread powder layer during SLM/SLS processes have limited abilities in terms of maximum preheating temperature obtainable and uniformity of produced temperature field. Hence, there is a requirement for a technique for preheating, primarily, of the workpiece and of a region of the spread powder layer that is limited on top of the workpiece or that is limited on top of a previously added layer formed on the workpiece .

Thus an object of the present invention is to provide an additive manufacturing technique, in particular an additive manufacturing apparatus and an additive manufacturing method for preheating, primarily, of the workpiece and of a region of the spread powder layer that is limited on top of the workpiece or that is limited on top of a previously added layer formed on the workpiece.

The above object is achieved by an additive manufacturing apparatus according to claim 1 of the present technique, and an additive manufacturing method according to claim 6. Advantageous embodiments of the present technique are provided in dependent claims.

In an aspect of the present technique, an additive manufacturing apparatus is presented. The additive manufacturing apparatus, hereinafter also referred to as the AM apparatus or the apparatus, includes a part building module, a direct resistive heating arrangement and an energy beam arrangement. The part building module bounds a bed of powdered metal material and includes a building platform. The building platform receives and supports the bed of powdered metal material and a workpiece embedded within the bed of powdered metal material. The direct resistive heating arrangement provides electrical current to the workpiece. The direct resistive heating arrangement includes a current source and electrical contacts. The electrical contacts are attached to the workpiece and provide electrical current to the workpiece when so attached. The energy beam arrangement selectively scans portions of a surface of the bed of powdered metal material to melt or sinter the selectively scanned portions onto the workpiece. The electrical^' current provided to the workpiece by the direct resistive heating arrangement flows through the workpiece and causes heating of the workpiece as a result of electrical resistance of the workpiece (resistive heating) , and since the electrical current is directly provided to the workpiece it is referred to as direct resistive heating.

The electrical contacts may be removably clipped on the workpiece. Furthermore the current source may include mechanism to modulate characteristics of the electrical current provided to the workpiece, such as amount, voltage, and/or frequency of the electrical current.

The direct resistive heating heats up the workpiece directly, i.e. the heating is not effected by heating up the entire powder bed or the building platform, as is done in the aforementioned conventional additive manufacturing techniques. By increasing or decreasing the electrical current, the amount of heating of the workpiece can be controlled. Lesser amount of electrical current provided to the workpiece results in lesser heating, and vice versa. As a result of heating of the workpiece the powder of the powder bed that is directly in contact and closely surrounding the workpiece also heats up. This helps in heating up a region of the powder layer that is limited on top surface of the workpiece and which subsequently is selectively scanned to be added to the workpiece .

In an embodiment of the AM apparatus , the current source is an alternating current source or an alternating current generator. In a related embodiment of the AM apparatus, the current source is a high frequency source. Alternating current (AC) sources/generators are well known and readily available, and thus make the AM apparatus easy to construct. AC current is known to have skin effect and the same happens in the workpiece, and thus AC current is beneficial in heating up the surface of the workpiece and the region of the powder layer adjacent to the surface of the workpiece. High frequency AC source/generator is especially advantageous due to increased skin effect. In another embodiment of the AM apparatus, the current source is a direct current source or a direct current generator. Direct current (DC) sources/generators are well known and readily available, and thus make the AM apparatus easy to construct . In another embodiment of the AM apparatus, at least one of the electrical contacts is configured to be movable along a side surface of the workpiece while maintaining contact with the workpiece such that the electrical contact remains at fixed relative position with respect to the surface of the bed of powdered metal material. Thus as a result of lowering of the build platform, when the workpiece lowers along with the build platform, the electrical contact allows the workpiece to slide along a contact point, i.e. where the electrical contact is physically contacting the workpiece, and thus retaining the relative position of the contact point with respect to the surface of the bed in the build chamber. Thus, if initially one of the electrical contacts, for example pins, connectors, is attached to a lower part of the workpiece, while another electrical contact is attached to an upper part of the workpiece, and subsequently, following adding of a previous layer onto the workpiece, when the build platform along with the workpiece is lowered, the movable electrical contact does not get lowered along with the workpiece. The movable contact in fact remains fixed in its position and allows the workpiece to slide down while still maintaining contact with the workpiece. Thus the need to re- contact the electrical contacts, especially the one that is attached to the upper part of the workpiece, after building of each successive additively manufactured layer onto the workpiece is obviated.

In another aspect of the present technique an additive manufacturing method, hereinafter also referred to as the AM method or simply as the method, is presented. In the method a workpiece is positioned on a building platform of a part building module of an additive manufacturing apparatus. Electrical contacts are attached to the workpiece. The electrical contacts are connected to a current source. Thereafter, a layer of powdered metal material is spread on the building platform and a surface of the workpiece positioned on the building platform. Thus the workpiece is embedded in a bed of the powdered metal material . Subsequently, direct resistive heating of the workpiece is performed by providing the workpiece with the electrical current from the current source via the electrical contacts attached to the workpiece. Finally, one or more portions of a surface of the layer of powdered metal material are selectively scanned by an energy beam arrangement, thereby melting or sintering the selectively scanned portions onto the underlying workpiece . The electrical current provided to the workpiece flows in the workpiece and causes heating of the workpiece as a result of electrical resistance of the workpiece.

The electrical contacts may be removably clipped on the workpiece. The direct resistive heating heats up the workpiece directly, i.e. the heating is not effected by heating up the entire powder bed or the building platform, as is done in the aforementioned conventional additive manufacturing methods . The electrical current provided to the workpiece may be modulated, i.e. an amount, a voltage, and/or a frequency of the electrical current may be changed or adjusted. By increasing or decreasing the electrical current, the amount of heating of the workpiece is controlled. As a result of heating of the workpiece the powder of the powder bed that is adjacent to the workpiece also heats up. This helps in heating up a region of the powder layer that is limited on top surface of the workpiece and which subsequently is selectively scanned and added to the workpiece . In an embodiment of the method, the electrical current provided to the workpiece in performing direct resistive heating of the workpiece is alternating current. In a related embodiment of the method, the alternating current is high frequency alternating current. Alternating current (AC) sources/generators are well known and readily available. AC current is known to have skin effect and the same occurs in the workpiece, and thus AC current is beneficial in heating up the surface of the workpiece and the region of the powder layer adjacent to the surface of the workpiece. High frequency AC current is especially advantageous due to increased skin effect. In another embodiment of the method, the electrical current provided to the workpiece in performing direct resistive heating of the workpiece is direct current. Direct current (DC) sources/generators are well known and readily available. In another embodiment the method, after a layer is previously formed on the workpiece by melting or sintering the selectively scanned portions onto the underlying workpiece as aforementioned, the building platform is lowered along with the workpiece and an existing bed of powdered metal material. The lowering of the building platform provides space on top of the existing bed of powdered metal material to accommodate a new layer of powdered metal material to be subsequently spread. The workpiece at this stage of the method includes the previously formed layer. Thereafter, the new layer of powdered metal material is spread on the existing bed of powdered metal material, i.e. supported on the building platform, and on a surface of the previously formed layer of the workpiece positioned on the building platform. Therefore, the new layer of powdered metal material spreads continuously over the existing bed of powdered metal material and the surface of the previously formed layer of the workpiece. Subsequently, direct resistive heating of the workpiece is performed by providing the workpiece with the electrical current from the current source via the electrical contacts attached to the workpiece. Finally, in the method- of the present technique, one or more portions of a surface of the new layer of powdered metal material are selectively scanned by the energy beam arrangement to melt or sinter the selectively scanned portions onto the workpiece. Thus the method can be repeated in a looped manner and each new layer added to the powder bed may be heated up using the method of the present technique before being melted or sintered, and consequently added, onto the workpiece. U2017/000249

In another embodiment of the method, the electrical current provided in performing direct resistive heating of the workpiece that includes the previously formed layer is alternating current. In a related embodiment of the method, the alternating current is high frequency alternating current .

In another embodiment of the method, the electrical current provided in performing direct resistive heating of the workpiece that includes the previously formed layer is direct current .

In another embodiment of the method, the electrical current from the current source via the electrical contacts is supplied to the workpiece continuously from the performing of direct resistive heating of the workpiece to the performing of direct resistive heating of the workpiece that includes the previously formed layer. Thus the rate of cooling of the previously formed layer may be controlled, as the cooling rate will depend on the amount of electrical current provided.

The present technique is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawing, in which:

FIG 1 schematically illustrates a conventionally known additive manufacturing system;

FIG 2 schematically illustrates an exemplary embodiment of an additive manufacturing apparatus having a direct resistive heating arrangement, in accordance with aspects of the present technique;

FIG 3 schematically illustrates exemplary embodiment of positioning of a workpiece and of electrical contacts of the direct resistive heating arrangement during performance of the direct resistive heating in accordance with aspects of the present technique; schematically illustrates exemplary embodiment of a subsequent positioning, compared to the positioning depicted in FIG 3, of the workpiece and of the electrical contacts of the direct resistive heating arrangement during further performance .of the direct resistive heating in accordance with aspects of the present technique; presents a flow chart representing an additive manufacturing method of the present technique; schematically illustrates an exemplary embodiment of positioning of electrical contacts of the direct resistive heating arrangement where one of the electrical contacts is movable; schematically illustrates an exemplary embodiment of the movable electrical contact of the direct resistive heating arrangement; schematically illustrates another exemplary embodiment of the movable electrical contact of the direct resistive heating arrangement; schematically illustrates yet another exemplary embodiment of the movable electrical contact of the direct resistive heating arrangement; and schematically illustrates a further exemplary embodiment of the movable electrical contact of the direct resistive heating arrangement; in accordance with aspects of the present technique. Hereinafter, above-mentioned and other features of the present technique are described in details. Various embodiments are described with reference to the drawing, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be noted that the illustrated embodiments are intended to explain, and not to limit the invention. It may be evident that such embodiments may be practiced without these specific details.

It may be noted that in the present disclosure, the terms "first", "second", etc. are used herein only to facilitate discussion, and carry no particular temporal or chronological significance unless otherwise indicated.

The basic idea of the present technique is to heat a workpiece, onto which layers are to be added by using AM techniques, directly by using the electrical resistance of the workpiece. When electrical current is directly passed through the workpiece it results into heating of the workpiece depending on the characteristics of the electrical current provided and the electrical resistance of the workpiece. Therefore heating of the workpiece is direct, in the sense that it does not come from heating of the building platform or of the surrounding powder bed. Furthermore, direct heating of the workpiece causes indirect heating of the powder adjoining the workpiece as a result of conduction of the heat from the workpiece to the adjoining powder, additionally the powder particles that are in direct contact with the workpiece surface also experience direct resistive heating. FIG 1 schematically represents a conventionally known additive manufacturing system 2. The system 2 generally includes a part building module 10, also known as the build chamber 10, in which a part is build by additive manufacturing (AM) for example by SLM or SLS processes. The part building module 10, hereinafter also referred to as the module 10, is a container for example a box shaped or barrel shaped container and having a top side of the container open. FIG 1 represents such a container having side walls 11, 12 and a bottom surface 15. The side walls 11, 12 and the bottom surface 15 together define a space in which the part is built. The space receives a workpiece 5 when the part is built onto the workpiece 5 for example as a part or integral addition to the workpiece 5. The workpiece 5 is an object that is supposed to be worked on by the AM system and built upon by addition of layer after layer by a suitable AM process by adding layer after layer of powdered metal material 7. The powder metal material 7 is provided by a powder storage module 20, also known as the feed cartridge 20, that stores the powdered metal material 7, hereinafter also referred to as the powder 7. The powder 7 in the feed cartridge 20 is stored in an open top container having side walls 21, 22 and a bottom 26. The bottom 26 is placed on top of a piston 28 that makes the bottom 26 slide or move in Z direction, as represented by the co-ordinate system shown in FIG 1.

When the piston 28 moves upwards in the Z direction, i.e. in a direction 29, the powder 7 from the container 20 is raised above and outside the container 20. The powder 7 is then spread as top surface 9 of a bed 8 of the powder 7 in the module 10 by using a powder spreading mechanism 30 which evenly spreads a thin layer of the powder 7 on the module 10. Usually the layer spread has a thickness of few micrometers, for example between 20 μηι and 100 μπι.

The module 10 or the build chamber 10 binds the bed 8 of powdered metal material 7 limiting the bed 8 by the side walls 11, 12 and the bottom surface 15. The module 10 also includes a building platform 16, and the bottom surface of the container of the module 10 is formed by the building platform 16, also known as the build platform 16 or simply as the platform 16. The platform 16 receives and supports the bed 8 of powdered metal material 7 and also the workpiece 5 that is positioned on the platform 16 embedded within the bed 8. The platform 16 is placed on top of a piston 18 that makes the platform 16 slide or move in Z direction, as represented by the co-ordinate system shown in FIG 1. When the piston 18 moves downward in the Z direction, i.e. in a direction 19, the bed 8, along with the workpiece 5, is lowered thereby creating a space at surface 9 of the container of the module 10 to accommodate the layer that is spread by the spreading mechanism 30. The layer so spread by the spreading mechanism forms the surface 9 of the bed 8 and also covers a surface 55 of the workpiece 5.

It may be noted that although in FIG 1 only one feed cartridge 20 and associated powder spreading mechanism 30 have been depicted, in most of the AM systems there are generally two such feed cartridges 20 and associated powder spreading mechanisms 30, one on each side of the module 10.

The system 2 also includes an energy beam arrangement 40. The energy beam arrangement 40 generally has an energy source 41 from which an energy beam 42 such as a Laser beam 42 or an electron beam 42 is generated, and a scanning mechanism 44 that directs the beam 42 to specific selected parts of the surface 9 of the powder bed 8 to melt or sinter the selectively scanned portions onto the workpiece 5. The specific portions of the surface 9 to which the beam 42 is directed are referred to as scanned. The selections of portions that are to be scanned by the beam 42 by action of scanning mechanism 44 are based on a 3D model, for example a CAD model, of the part that has to built on the workpiece 5.

Since the part is being built on the workpiece 5, the portions of the surface 9 that are selectively scanned by the beam 42 are generally limited on top of the surface 55 and thus are able to melt or sinter and be added to the surface 55 of the workpiece 5. Once added to the workpiece 5, the portions of the layer so added form part of the workpiece 5.

The build chamber 10, the feed cartridge 20, the spreading mechanism 30, and the energy beam arrangement 40 are well known in the art of additive manufacturing thus not described herein in further details for sake of brevity.

FIG 2 schematically represents an additive manufacturing apparatus 1 according to the present technique. The additive manufacturing apparatus 1, hereinafter also referred to as the AM apparatus 1 or simply as the apparatus 1, includes the build chamber 10, the feed cartridge 20, the powder spreading mechanism 30, and the energy beam arrangement 40 as explained in reference to FIG 1. Additionally, the apparatus 1 includes a direct resistive heating arrangement 60. The direct resistive heating arrangement 60, hereinafter also referred to as the heating arrangement 60 or simply as the arrangement 60, is configured to provide electrical current to the workpiece 5 and thus performing direct resistive heating of the workpiece 5. The heating arrangement 60 includes a current source 62, electric connectors 64 and electrical contacts 66. The electrical contacts 66 are configured to be attached to the workpiece 5. The electric contacts 66, hereinafter also referred to as the contacts 66, are able to be removably attached, i.e. the contacts 66 may be attached and then detached without causing any change in shape or configuration of the workpiece 5. One example of contacts 66 may be an electrical clip, like a binder clip, that is able to be clipped on to or attached to the workpiece 5 and then easily removed when desired by opening the clip. The electric connectors 64, for example electrical wire, connect the current source 62 to the contacts 66 and thus allow flow of electrical current from the current source 62 to the contacts 66 and thereafter to the workpiece 5, when the contacts 66 are connected onto the workpiece 5. The electrical connections 64 along with the contacts 66 may resemble one end of a jumper cable that is used to jump start a car by connecting it to a dead car battery, which resembles in this case the workpiece 5 , from a healthy car battery which resembles in this case the current source 62 that supplies the electrical current. The electrical current provided to the workpiece 5 by the heating arrangement 60 flows in the workpiece 5 and consequently causes heating of the workpiece 5 as a result of electrical resistance of the workpiece 5. Furthermore the heating arrangement 60 or the current source 62 of the heating arrangement 60 may include mechanism to modulate characteristics of the electrical current provided to the workpiece 5, such as amount, voltage, and/or frequency of the electrical current. The mechanism to modulate characteristics of the electrical current may be, but not limited to, a frequency changer or a frequency convertor, a voltage regulator, a voltage convertor, and so on and so forth. By modulating the electric current i.e. by increasing or decreasing frequency and/or voltage of the electric current an amount of heating of the workpiece 5 is controlled.

The current source 62 may be an alternating current source or an alternating current generator. In a related embodiment of the AM apparatus 1, the current source 62 is a high frequency source, which can be achieved by an AC current source 62 coupled with a frequency changer. High frequency as used herein includes a frequency greater than 1kHz .

In another embodiment of the AM apparatus 1, the current source 62 is a direct current source or a direct current generator, for example a battery.

The present technique also presents an additive manufacturing method 100, hereinafter also referred to as the AM method 100 or simply as the method 100, as depicted in the flow chart of FIG 5. The method 100 is implemented by the AM apparatus 1 as explained hereinabove with respect to FIGs 1 and 2. The method 100 of the flow chart of FIG 5 is explained hereinafter in combination with FIGs 3 and . In explaining the method 100 references have been made to the AM apparatus 1 and its components such as the building platform 16 that may be understood to be same as that explained in reference to FIGs 1 and 2.

In the method 100, in a step 110 a workpiece 5 is positioned on the building platform 16 of the build chamber 10 of an AM apparatus, for example the AM apparatus 1 of FIG 2. In a following step 120, the electrical contacts 66 are attached to the workpiece 5. The electrical contacts 66 are connected to the current source 62.

FIG 3 depicts a workpiece positioned on the platform 16 and to which the contacts 66 are attached. Thereafter, in a step 130 a layer 70, as shown in FIG 3, of powdered metal material 7 is spread on the platform 16 and the surface 55 of the workpiece 5. The workpiece 5 along with the surface 55 is completely embedded in a bed 8 of the powdered metal material 7 after the layer 70 has been spread. Subsequently, in a step 140 direct resistive heating of the workpiece 5 is performed by providing the workpiece 5 with the electrical current from the current source 62 via the electrical contacts 66 attached to the workpiece 5. Finally, in a step 150 one or more portions of the surface 9 of the layer 70 of the powder 7 are selectively scanned by the energy beam arrangement 40, thereby melting or sintering the selectively scanned portions onto the underlying workpiece 5, or more particularly to the surface 55 of the workpiece 5.

In the method 100, the electrical current provided to the workpiece 5 in the step 140 may be modulated, i.e. an amount, a voltage, and/or a frequency of the electrical current may be changed or adjusted during the step 140. As a result of heating of the workpiece 5, the surface 55 of the workpiece 5 also heats up, the powder 7 of the powder bed 8 that is adjacent to the workpiece 5 also heats up for example the powder 7 in a region 77 of the layer 70. The region 77 is the portion of the layer 70 that is limited on top of the surface 55 of the workpiece 5. Furthermore, since the selective scanning performed in the step 150 is also generally limited within the region 77, because the sintered or melted material had to be built onto the workpiece 5 and not separate from the workpiece 5, the direct resistive heating performed in the step 140 causes a pre-heating of the powder 7 in the region 77 of the layer 70 i.e. the powder 7 in the region 77 of the layer 70 is heated before the selective scanning is performed in the step 150.

The electrical current provided to the workpiece 5^' in the step 140 is alternating current. In a related embodiment of the method 100, the alternating current is high frequency- alternating current, for example AC in frequency greater than 1kHz . In an alternate embodiment of the method 100, the electrical current provided to the workpiece 5 in the step 140 is direct current.

After conclusion of the aforementioned steps 110 to 150 of the method 100, the workpiece 5 now has an additional layer or an added layer 75, as shown in FIG 4, which is hereinafter referred to as the previously added layer 75 or previously formed layer 75 for further steps of the method 100. It may be noted that besides the portions of the layer 70 that were sintered or melted in the step 150 of the method 100, the remaining portions of the layer 70 are present in adjoining area of the sintered or melted portions and now form part of the powder bed 8, now also referred to as the existing powder bed 8.

Optionally, in addition to the aforementioned steps, the method 100 may be continued further as follows:

In a step 160, following the step 150, the platform 16 is lowered in the direction 19 (shown in FIGs 1 and 2) along with the workpiece 5 and the existing bed 8 of powdered metal material 7. As a result of the step 160, a space on top of the existing bed 8 is generated. The space so generated is same as the thickness of the next layer that is to be spread on the powder bed 8. Thereafter a new layer 80, as shown in FIG 4, is spread in a step 170 by using the spreading mechanism 30 and the powder 7 provided by the feed cartridge 20. The space created in the step 160 accommodates the new layer 80 of powdered metal material 7 which now forms the surface 9 of the powder bed 8. As shown in FIG 4, the new layer 80 also spreads continuously over the previously formed layer 75 of the workpiece 5. The workpiece 5 at this stage has a surface 56, which includes a surface of the previously formed layer 75.

Subsequently, in a step 180 direct resistive heating of the workpiece 5 having the previously added layer 75 is performed, by providing the workpiece 5 with the electrical current from the current source 62 via the electrical contacts 66 attached to the workpiece 5. As a result of the step 180, the workpiece 5 including the surface 56 of the workpiece 5 heats up. Finally, in the method 100 of the present technique, in a step 190 one or more portions of the surface 9 of the new layer 80 of powdered metal material 7 are selectively scanned by the energy beam arrangement 40 to melt or sinter the selectively scanned portions onto the workpiece 5. In the method 100, the electrical current provided to the workpiece 5 in the step 180 may be modulated, i.e. an amount, a voltage, and/or a frequency of the electrical current may be changed or adjusted during the step 180. As a result of heating of the workpiece 5, the surface 56 of the workpiece 5 also heats up, the powder 7 of the powder bed 8 that is adjacent to the workpiece 5 also heats up for example the powder 7 in a region 87 of the layer 70, as shown in FIG 4. The region 87 is the portion of the layer 80 that is limited on top of the surface 56 of the workpiece 5. Furthermore, since the selective scanning performed in the step 190 is also generally limited within the region 87, because the sintered or melted material had to be built onto the workpiece 5 and not separate from the workpiece 5, the direct resistive heating performed in the step 180 causes a pre- heating of the powder 7 in the region 87 of the layer 80 i.e. the powder 7 in the region 87 of the layer 80 is heated before the selective scanning is performed in the step 190. The electrical current provided to the workpiece 5 in the step 180 is alternating current. In a related embodiment of the method 100, the alternating current is high frequency- alternating current, for example having same frequency range as the AC provided in the step 140. In an alternate embodiment of the method 100, the electrical current provided to the workpiece 5 in the step 180 is direct current.

The electrical current may be provided intermittently i.e. only before the performance of the subsequent selective scanning steps such as the step 190. Alternatively, electrical current may be provided or supplied continuously for example in another embodiment of the method 100, the electrical current is supplied to the workpiece 5 continuously from the step 140 to the step 180. The electrical current when supplied continuously may be modulated to control the heating rate or cooling rate of the workpiece 5.

Hereinafter FIGs 6 to 10 are used to explain different type of the electrical contacts 66 that may be used in the AM apparatus 1 of FIG 2 and/or the AM method 100 of FIG 5. As shown in FIG 6, the AM apparatus 1 may include at least one electrical contact 66 that is configured to be movable along a side surface 57 of the workpiece 5 while maintaining contact with the workpiece 5, and more particularly while maintaining contact with the side surface 57 of the workpiece 5. Such an electrical contact 66 of the AM apparatus 1 is hereinafter referred to as the movable contact 67. The AM apparatus 1 may include at least one movable electrical contact 67 that moves relative to the workpiece 5, along the side surface 57 of the workpiece 5, while maintaining contact with the workpiece 5 such that the movable contact 67 remains at fixed relative position with respect to the surface 9 of the bed 8 of powdered metal material 7. Different embodiments of the movable connector 67 that may be used in the AM apparatus 1 of FIG 2 and 6 are depicted in FIGs 7 to 10. The movable connector 67 includes a contact head 92, a spring- loaded element 94 and a contact base 96. The contact head 92 established direct physical contact with the workpiece 5. The contact base 96 is fixed to the or within the side wall 11 of the build chamber 10. The contact base 96 supports the spring- loaded element 94 that extends out from the contact base 96 towards the workpiece 5, when present. The spring-loaded element 94 is present between the contact head 92. The spring-loaded element 94 is compressed when the workpiece 5 is positioned in the build chamber 10 and in the compressed position of the spring-loaded element 94 the contact head 92 pushes, albeit gently, against the side surface 57. The contact head 92, for example may be, but not limited to, a gear-shaped pin as shown in FIG 7 or a ball-shaped pin as shown in FIGs 8 to 10.

When the platform 16 is lowered the contact head 92 brushes along the side surface 57 of the workpiece 5 and allows the workpiece 5 to slide down, without causing bending or angular motion of the contact head 92. While the workpiece 5 moves down, the side surface 57 of the workpiece 5 brushing against the contact head 92, the contact head 92 remains pressed onto the side surface 57 due to the compressed state, of the spring-loaded element 94.

The contact base 96 may be present on the surface of the wall 11 or present embedded in the wall 11. The spring- loaded element 94 may extend between the contact head 92 and the contact base 96 as shown in FIG 7 and 8. Alternatively a part of the spring-loaded element 94 or the entire spring- loaded element 94 may be present confined within the contact base 96 as shown in FIGs 9 and 10. While the present technique has been described in detail with reference to certain embodiments, it should be appreciated that the present technique is not limited to those precise embodiments. Rather, in view of the present disclosure which describes exemplary modes for practicing the invention, many modifications and variations would present themselves, to those skilled in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.

List of Reference Characters

1 AM apparatus

2 conventionally known system

5 work piece/substrate

7 powdered metal material

8 bed of powdered material

9 surface of the bed

10 part building module

11 wall

12 wall

15 surface of the building platform

16 building platform

18 piston

19 direction of movement of the piston

20 powder feed module or feed cartridge

21 wall

22 wall

26 powder platform

28 piston

29 direction of movement of the piston 30 powder spreading mechanism

39 direction of powder spreading

40 energy beam arrangement

41 energy source

42 power beam

44 scanning mechanism

55 surface of the work piece

56 surface of the workpiece

57 side surface of the workpiece

60 direct resistive heating arrangement

62 current source

64 electrical connections

66 electrical contacts 67 movable electrical contact

70 layer of powdered metal material

75 previously formed layer of the workpiece

77 region of the layer of powdered metal material

80 new layer of powdered metal material

87 region of the new layer of powdered metal material

92 contact head

94 spring- loaded element

96 contact base

100 AM method

110 positioning the workpiece on the building platform

120 attaching the electrical contacts to the workpiece

130 spreading a layer of powdered metal material

140 performing direct resistive heating of the workpiece 150 selectively scanning portions of a surface of the layer

160 lowering the building platform

170 spreading a new layer of powdered metal material

180 performing direct resistive heating of the workpiece

190 selectively scanning portions of a surface of the new layer

Claims

Patent Claims :

1. An additive manufacturing apparatus (1) comprising:

- a part building module (10) configured to bound a bed (8) of powdered metal material (7) and comprising a building platform (16) configured to receive and to support the bed (8) of powdered metal material (7) and a workpiece (5) embedded in the bed (8) of powdered metal material (7) ;

- a direct resistive heating arrangement (60) configured to provide electrical current to the workpiece (5) , wherein the direct resistive heating arrangement (60) comprises a current source (62) and electrical contacts (66) and wherein the electrical contacts (66) are configured to be attached to the workpiece (5) and to provide electrical current to the workpiece (5) ; and

an energy beam arrangement (40) configured to selectively scan portions of a surface (9) of the bed (8) of powdered metal material (7) to melt or sinter the selectively scanned portions onto the workpiece (5) .

2. The additive manufacturing apparatus (1) according to claim 1, wherein the current source (62) is an alternating current source or an alternating current generator.

3. The additive manufacturing apparatus (1) according to claim 2, wherein the current source (62) is a high frequency source .

4. The additive manufacturing apparatus (1) according to claim 1, wherein the current source (62) is a direct current source or a direct current generator.

5. The additive manufacturing apparatus (1) according to any of claims 1 to 4 , wherein at least one of the electrical contacts (66) is configured to be movable along a side surface (57) of the workpiece (5) while maintaining contact with the workpiece (5) such that the electrical contact (66) remains at fixed relative position with respect to the surface (9) of the bed (8) of powdered metal material (7) .

6. An additive manufacturing method (100) comprising:

- positioning (110) a workpiece (5) on a building platform (16) of a part building module (10) of an additive manufacturing apparatus (1) ; attaching (120) electrical contacts (66) to the workpiece (5) , wherein the electrical contacts (66) are connected to a current source (62) ;

spreading (130) a layer (70) of powdered metal material (7) on the building platform (16) and a surface (55) of the workpiece (5) positioned on the building platform (16) ;

- performing (140) direct resistive heating of the workpiece (5) by providing the workpiece (5) with an electrical current from the current source (62) via the electrical contacts (66) attached to the workpiece (5) ; and selectively scanning (150) , by an energy beam arrangement (40) , portions of a surface (9) of the layer (70) of powdered metal material (7) to melt or sinter the selectively scanned portions onto the workpiece (5) . .

7. The additive manufacturing method (100) according to claim 6, wherein the electrical current provided to the workpiece

(5) in performing (140) direct resistive heating of the workpiece (5) is alternating current.

8. The additive manufacturing method (100) according to claim 7, wherein the alternating current is high frequency alternating current.

9. The additive manufacturing method (100) according to claim 6, wherein the electrical current provided to the workpiece (5) in performing (140) direct resistive heating of the workpiece (5) is direct current.

10. The additive manufacturing method (100) according to any of claims 6 to 9 , wherein the electrical current provided to the workpiece (5) in performing (140) direct resistive heating of the workpiece (5) is modulated.

11. The additive manufacturing method (100) according to any of claims 6 to 10, further comprising:

- lowering (160) the building platform (16) along with the workpiece (5) and an existing bed (8) of powdered metal material (7) to accommodate a new layer (80) of powdered metal material (7) , wherein the workpiece (5) comprises a previously formed layer (75) resulting from the method (100) of claim 6;

- spreading (170) the new layer (80) of powdered metal material (7) on the existing bed (8) of powdered metal material (7) and a surface (56) of the previously formed layer (75) of the workpiece (5) ;

- performing (180) direct resistive heating of the workpiece (5) by providing the workpiece (5) with the electrical current from the current source (62) via the electrical contacts (66) attached to the workpiece (5) ; and selectively scanning (190) , by the energy beam arrangement (40) , portions of a surface of the new layer (80) of powdered metal material (7) to melt or sinter the selectively scanned portions onto the workpiece (5) .

12. The additive manufacturing method (100) according to claim 11, wherein the electrical current provided to the workpiece (5) in performing (180) direct resistive heating of the workpiece (5) is alternating current.

13. The additive manufacturing method (100) according to claim 12, wherein the alternating current is high frequency alternating current.

14. The additive manufacturing method (100) according to claim 11, wherein the electrical current provided to the workpiece (5) in performing (180) direct resistive heating of the workpiece (5) is direct current.

15. The additive manufacturing method (100) according to claims 11 wherein the electrical current from the current source (62) via the electrical contacts (66) is supplied to the workpiece (5) continuously from the performing (140) of direct resistive heating of the workpiece (5) to the performing (180) of direct resistive heating of the workpiece (5) having the previously formed layer (75) .