Field of the Invention

The present invention concerns 3-D printing apparatus using extruded metal-wire or powder print material or Weld Additive Manufacturing ("WAM"), as it is known in the art. In particular, the invention relates to a trailing shield for use in the process. The invention also concerns a process for 3-D printing or Weld Additive Manufacturing (“WAM") using extruded metal-wire or powder.

Background to the Invention

The use of 3-D printers is becoming ever more widespread, as the methodology improves and devices become cheaper and easier to utilise. Where the printing material is an organic polymer, often used in prototyping, such devices are almost household items, and can be afforded by relatively small firms. 3-D printers also offer the opportunity to construct articles which would otherwise be almost impossible: for example including cavities, internal channels or other complicated structures.

Printing articles from metal is more difficult due to the higher temperatures required to convert the metal into the liquid state compared to that required for an organic polymeric material. Not only is the process more energy intensive, but it is also potentially more dangerous to the user. Additionally, the higher temperatures used can lead to the metals being oxidised by atmospheric oxygen and care needs to be taken therefore that the printing takes place in an inert atmosphere.

One methodology used in printing metal objects is to cover a flat surface with a metal powder. Sintering, using for example, a laser then provides a layer of the object being printed. A further layer of powder is then overlaid the first layer and sintering repeated. The laser is controlled such that with additional layers, the 3-D object is produced in which each sintered layer is fused to its neighbouring layers to produce that object. The disadvantage of the method is that when the object is finished, the unsintered powder needs to be collected because it is injurious to health, pyrophoric and expensive.

An alternative method, utilised in the hereindescribed invention, utilises an extruded metal- wire or powder to provide the metal from which the article is formed and is known as Weld Additive Manufacturing ("WAM"). One of the difficulties in this type of method is that the apparatus needs to be able to adapt to articles of different sizes and also to adapt to the changing size of the article as it is formed. Also, as the article becomes larger, heat dispersion needs to be made more efficient.

It is an object of the current invention to provide an apparatus to address the above problems. The apparatus and process are described with respect to the metal being in the form of a wire, which apparatus and process can also be applied to extruded powder. It is a further object of the invention to provide a method of manufacture which addresses the above problems.

Summary of the Invention

According to a first aspect of the invention, there is provided a trailing shield for use in a 3- D printing process and in conjunction with a printing surface on which an article is printed, the trailing shield comprising a cover element,

said cover element including support means for supporting a heat source and for supporting a print material,

the cover element having a wall defining a printing chamber, said wall also defining a print aperture, the cover element including one or more inlets fluidly connecting the exterior of the cover with the printing chamber, at least one inlet being fluidly linked to an inert-gas supply, and further including one or more temperature sensors.

The trailing shield enables 3-D metal printing to be carried out more energy efficiently and rapidly and for a smaller footprint to be required for the printer.

Optionally the trailing shield is mounted for movement across a printing surface. Further optionally, the trailing shield comprises wheels on which the trailing shield moves, said wheels being still yet further optionally mounted on rails. This arrangement enables the trailing shield to manoeuvre across a printing surface to reduce the area which needs to be kept oxygen free.

The heat source is conveniently selected from a gas-metal, plasma transfer or gas-tungsten arc to provide the high temperatures required.

Preferably, the support means comprises a channel whose lower end is further preferably directed towards the direction of travel. Removal of the heat source is thereby facilitated.

Preferably, the trailing shield includes one or more openable closure members to alter the size of the aperture, the or each closure member being operably mounted to the cover element.

The trailing shield conveniently includes one or more inlets housing nozzles connected to an inert gas supply such as argon. Further conveniently at least one of the inlets is connected to a cryogenically cooled argon supply to provide temperature control beneath the cover element.

Preferably the trailing shield includes one or more further sensors to provide additional information to the operator of the trailing shield. Further preferably, the or each further sensors comprises one or more oxygen sensors, to allow monitoring of the penetration of atmospheric air to the area on which printing is taking place. Yet further preferably, the or each oxygen sensor is coupled to a control means controlling the flow of inert gas to ensure that oxygen is excluded from the printing area. Preferably the or each temperature sensor is an optical sensor, and further preferably, a dual waveband pyrometer. Optionally at least one temperature sensor is housed in the front portion of the cover element.

Preferably, the trailing shield includes a measurement device to determine the height of the printed material from a printing surface, said measurement device further preferably comprising a laser measurement device. Optionally, the trailing shield includes shielding members, movably mounted to minimise damage to the cover element. The shielding members further optionally are in the form of plates, yet further optionally of 2 -4 mm thick stainless steel.

Preferably, the shielding members are movable automatically.

Preferably, the cover element includes one or more flexible skirts to minimise spatter outside the trailing shield and to reduce oxygen ingress beneath the trailing shield, which flexible skirts are further preferably formed of a silicone rubber material. Preferably a mesh material is interposed between a printing surface and the or each sensor element and the or each inlet nozzle to provide protection and also aid in dispersing the inert gas beneath the cover element.

Brief Description of the Drawings

The invention is now described with reference to the accompanying drawings which show by way of example only, one embodiment of apparatus for conducting 3-D printing. In the drawings:

Figure 1 illustrates a first embodiment of trailing shield;

Figure 2 illustrates a second embodiment of trailing shield;

Figure 3 illustrates a section through a trailing shield;

Figure 4 is a sectional illustration of the side of a third embodiment of trailing shield; Figure 5 is a sectional illustration of the front of the trailing shield of Figure 4;

Figures 6a and 6b are, respectively, a top view and an underside top view of the trailing shield of Figure 4;

Figure 7 a is a top view of a fourth embodiment of trailing shield;

Figure 7b is an underside view of the fourth embodiment of trailing shield;

Figure 8 is a top view of a fifth embodiment of trailing shield; and

Figure 9 is a top view of a sixth embodiment of trailing shield.

Detailed Description of the Invention

The use of 3-D printing of prototype articles or in producing articles which would be otherwise impossible to manufacture is becoming more widespread. Although 3-D printing using organic polymeric materials is now almost routine and within the scope of small companies or even households to utilise, printing objects in metal is still expensive, not least because of the energy usage and increased safety requirements, along with the difficulties, when carrying out wire printing, with objects having unusual or widely varying width profiles.

In a typical metal wire or filament 3-D printing or WAM apparatus, the object under consideration is built up under a cowl or body portion which is often a dome-shaped piece of metal. The cowl is capable of movement across a surface to move with the heat source and the metal source as these build up the article constructed. Polypropylene enclosures have been tried, but they are usually difficult to put in place and use, and do not consistently give a good result. The volume beneath the cowl is filled with a gas which is inert under the high temperature printing conditions, and is usually a noble gas such as argon, although others such as carbon dioxide can be used. The gas flow is maintained, using a positive flow pump. The cowl has functionality to allow a heat source and a metal source to be brought together to allow the metal printing to take place. Referring to Figure 1, this illustrates in a general manner the features of a trailing shield 10 in accordance with the invention. The trailing shield 10 defines a volume, or printing chamber, within which a product is formed. The volume defined is open along one face in the form of an aperture to allow the product to be deposited and built up, or printed, on the surface of a platen or the like. The trailing shield 10 is retained for movement by a motive means (not illustrated). The trailing shield 10 can include wheels or the like to aid movement and can be supported on rails, along which the trailing shield 10 can move and which rails can themselves move to aid and support movement.

The trailing shield 10 as shown has an annular cover element or body portion 11 which houses in-use a heated element. It should be noted that the absolute diameter or shape of the body portion 11 is not critical. Important is that the body portion provide sufficient coverage of the area being welded, and is of sufficient height to allow construction of the desired object.

The heating element can be of a type known in the art such as a pulsed Mig torch, for example a Fronius CMT. Other types of heat source can be used including arc and lasers, but the invention is suited for use with gas tungsten-, plasma transfer-, or gas metal- arc welding methodologies. The element is held within a throughchannel 12 whose inner side wall 13 is, in the illustrated embodiment, at an angle of 10° to the vertical and orientated downwardly in the direction of travel indicated by the arrow A. Other cross-sectional shapes of throughchannel are suitable depending on the type of heating (such as plasma transfer arc welding (PTAW) which is being applied hence the element which is being held by the shield. The element of Figure 1 is therefore directed in the usual direction of travel of the trailing shield 10. However, the angle made by the element with the direction of travel is not critical and can be perpendicular to the direction of travel. The nose 14 of the trailing shield 10 extends from the body portion 1 1 in the same direction as the usual direction of travel A, and is of length around 5 cm. The nose 14 protects the surrounding areas from spatter by molten metal. The tail 15 of the trailing shield 10 is of sufficient length to protect the weld being formed until the weld has cooled down to below 540C when oxidation will not occur to a damaging extent once the weld is, again, in an oxygen containing atmosphere.

The trailing shield 10 is provided with sensors and connection means enabling the shield to be fluidly connected with gas supplies as are now detailed. The sensors provide information to the user on conditions beneath the shield and at the weld point and ensure that the correct conditions for welding or producing an article are maintained.

In Figure 1, the sensors and inlets are provided as follows, although other distributions of the features within the body can be utilised without departing from the broad concept of the invention. Distributed on the body portion 1 1, nose 14 and tail 15 is a plurality of inlets 16 enabling inert gas, usually argon, to flow into the trailing atmosphere around the metal weld. Although a single inlet can be used for the inert gas, such an arrangement can lead to turbulence in and around the shield, and the use of the plurality of small inlets 16 minimises this risk. Optionally the cross-section of an inlet 16 is adjustable to change the flow rate of inert gas into the body portion.

As indicated above, the purpose of the inert gas flow is to minimise the presence of atmospheric oxygen around the weld site, which presence around the weld would lead to the formation of metal oxides and weakening of the material formed. Oxygen sensors 17 enable the oxygen level beneath the trailing shield 10 to be monitored and appropriate action to be taken, either to increase inert gas flow to reduce oxygen levels or to decrease inert gas flow to reduce excess inert gas usage. The oxygen sensors 17 can be linked to the inert gas flow controls to automatically adjust the levels, which is more efficient and faster reacting. Sensors for other elements or compounds can be included if required. It is important that the temperature of the article under construction be monitored to ensure that the quality of the material formed and of the interlayer bond be sufficiently good. The particular temperature is chosen depending on the construction material. Several features on the shield 10 enable this monitoring to be achieved and also measures to be taken to adjust the temperature to within acceptable values.

Set within the nose 14 of the shield 10 is a first temperature sensor 20. The temperature sensor 20 in the exemplified embodiment is a pyrometer, more specifically a dual wave band pyrometer which measures the interpass temperature of the material being welded. The sensor is therefore a non-contact temperature sensor such as an optical sensor. The temperature sensor 20 is linked to a processor in which data belonging to upper and lower acceptable temperatures are stored. A further set of temperature sensors 21 is provided in the tail 15 of the shield 10, which measure the temperature behind the welded material, and allow the rate of cooling of the depositing metal to be monitored, which information can then be used to check that the process is proceeding as required.

In the event that the temperature beneath the shield 10 in one or more regions becomes too high, then a coolant gas, for example cryogenic argon can be introduced into the volume beneath the body portion via the inlet 16a. The volume of the gas is controlled by a processor which utilises the information provided by the sensors 20 and 21. The coolant gas can be routed around the inside perimeter of the shield 10 prior to its exit from the shield 10 to protect the shield's sensors.

A further preferable feature of any 3-D printing process as described herein is that the thickness of a layer produced is constant across the layer. Variations in a layer's thickness can lead to differences in the cooling/bonding between layers and an uneven structure, and so variations in either form or strength of the article formed. In order to monitor the layer's thickness a measurement device 22 is provided in the nose 14 of the shield 10. In the embodiment shown the measurement device 22 comprises a laser emitter and a receiver of a type known in the art to give an accurate measurement of the height of the article produced and/or its distance from the shield 10.

In order to aid remote operation of the shield 10 a camera can be provided to feed images to a viewing station. The viewing station can also be used to display the data supplied by the sensors and allow control over the shield to be exercised.

Turning to Figure 2, this illustrates a trailing shield 30 which has a body is rectangular- cuboidal in shape. The body portion 30a of the trailing shield 30 is shown around a body 31 of material already deposited.

As the body 31 grows there can be a tendency, for example where the material is a grade 5 titanium (Ti-6Al-4-v), for the outside edges of the body to acquire a bluish tinge due to oxidation, particularly as the body 31 gets taller. In order to reduce this occurrence, additional shielding members are introduced beneath the trailing shield 30. The shielding members take the form of plates 32, optionally made of a consumable material which are housed on brackets 33, secured to the body portion 30a of the trailing shield 30. The plates 32 are movable towards and away from the body 31 of material being formed during the body's formation and act to protect the outside edges of the material from which the body 31 is formed. This can be a manual operation with the operator adjusting a plate after each pass. However, an automatic adjustment means can be incorporated utilising a linear slide and laser/ultrasonic position sensor. Usually the gap between the edges of the plates 32 and the side of the body 31 is set at around 5 mm. Typically, the plates 32 are formed of a thin, 2 - 4mm thickness, stainless steel material. Usually, over time, the plates 32 become covered in weld spatter and so need to be replaced. A tape, for example, a High Temperature silicone rubber strip, can be used to produce a seal on the initial weld/print onto a flat plate. The tape is fixed as a skirt to the sides of the shield 10 to trap gas within the shield and prevent air from the surrounding atmosphere reaching the shielded area.

The distance of the plates 32 from the side walls of the body portion 30a of the trailing shield 30 should be set at around 10-12 mm. This distance is itself adjustable to suit the process being undertaken. In one embodiment, the plates 32 are mounted on runners to aid their movement.

Referring to Figure 3, this illustrates diagrammatically the operation of the cold metal transfer (CMT) process in building up the object being constructed. The object is constructed on a platen 40 formed of an inert substrate, preferably formed of the same material as that from which the object is being constructed. The torch 41 is held above the platen 40 and such that the end of the gas shroud 42 of the torch 41 is initially at around 14 mm above the surface of the platen 40. Surrounding the torch 41 is the body portion 43a of the trailing shield 43. Said trailing shield 43 has an adjustable bottom edge 44 operably secured to the body portion 43a which is set at a height around 2 mm above that of the end of the gas shroud 42 and around 14 mm from the platen 40. A consumable spatter shield 45 is located on the tail 15 side of the gas shroud 42, with a bottom edge around 6 mm below the end of the gas shroud 42, and a shield 45 acts to protect the tail 15 and the sensors and other features incorporated therein. Typical dimensions for the gas shroud are 25 mm x 10 mm. The spatter shield 45 can be formed of a steel or ceramics material. The metal wire or filament 46 which supplies material to form the object under construction is shown. In order to form an object, the platen can be fixed in position with the shield 43 moving across the platen 40 to form the object. Alternatively, the shield 43 can be held fixed and the platen 40 moved relative to the shield 43. In a further alternative method, both the shield 43 and the platen 40 can be moved at the same time, the movement being co-ordinated to enable the object to be produced.

Figures 4 and 5 further illustrate a trailing shield in accordance with the present invention. In Figure 4 the body portion 50a of a trailing shield, generally referenced 50, has front and rear walls 51, 52 and a top wall 53. The body portion 50a of the trailing shield 50 houses a torch 54 which provides the heat source to melt the material from which an article is to be constructed. The material is provided in the form of a metal wire or filament 56 extending from the torch 54 or alternatively from a separate support to bring the wire 56 into the region of the CMT or other heat source. A wire mesh 57 acts to prevent damage to the sensors and gas conduits of the trailing shield 50 but also allows the inert gas atmosphere to pass through to the region in which the material forms the article. The mesh also acts to disperse the inert gas over the weld area and heated areas adjacent thereto. A mesh is optionally formed of a stainless-steel material.

In order to monitor the temperature of the material being formed, temperature sensors are provided, one of which is illustrated at 58, and a further at 59. The inlets for the inert gas are shown at 60 and 61 and a further inlet 62 admits cryogenically cooled inert gas to additionally aid temperature control.

As can be seen from Figure 5, the body portion 50a of the trailing shield 50 has an adjustable bottom edge 63 (not shown in Figure 4) which can be moved in the directions B, C as required as the trailing shield 50 moves back and forth along the constructed object and as the height of the object increases. To assist the build process the height of the platen supporting the object can be adjustable, being progressively lowered as the article is formed.

The embodiment of trailing shield of Figures 4 - 6 typically require a flow rate of inert gas of around 200 litres/minute in order to ensure that the metal weld material does not come into contact with oxygen gas and also does not reach too high a temperature providing sufficient energy transfer to cool down the weld down before, for example, it is no longer beneath the trailing shield and hence in contact with an oxygen-containing atmosphere. Such large flow rates of the inert gas can cause problems, particularly where a large number of devices are housed close to each other and operate simultaneously. For example, there is a cost attached to each litre of gas used, which cannot easily be reclaimed. Moreover, the logistical difficulties of providing a constant supply increase as well as the hazards of ensuring that the gas does not escape to other areas where personnel are working.

The embodiment of trailing shield of Figures 7a and 7b requires a lower rate of inert gas flow, typically around 42 litres/minute, to operate effectively. The body portion 70a of the trailing shield 70 has a generally rectangular shape when viewed from above. To achieve the lower flow rate, the shield does not have a plurality of inert gas inlets, distributed across the body portion 70a of the shield 70 - as in the first embodiment - which provide generally an overall inert atmosphere beneath the trailing shield, but instead is provided with an array of inert gas inlets 71, supported within the volume defined by the body portion 70a and directed towards the region around, and especially immediately above the weld point of the metal to minimise any oxidation of the metal during its cooling and solidification. The inlets 71 are housed on brackets 72. The inlets 71 are supplied with inert gas by a single inlet supply tube passing through the body portion 70a, which then splits in order to feed each individual gas inlet 71 in the array. In the embodiment shown the trailing shield comprises 6 directional inlets 71 in two arrays each of three inlets 71. The inlets 71 are generally tubular but with an angled section removed from the end region of the inlets 71 from which the gas exits.

Housed towards the in-use rearward region of the body portion 70a, additional inlets are provided for a cryogenically cooled gas to be introduced as and when required.

In a further embodiment, not illustrated, a different array of directional inlets, optionally 4 - 8 in number can be utilised to direct inert gas flow around the weld region.

It can be envisaged that with suitable control of the weld process the use of cryogenic inert gas is obviated and so such inlets are then not required. This would simplify the supply process and tubing required around the shield.

The embodiment of trailing shield of Figure 8 illustrates a fifth embodiment of trailing shield 80 which has a generally cuboidal in shape body portion 80a and so has a square shape when viewed from above. In this embodiment, there is no provision of inlet for cryogenic inert material. The temperature of the weld material during formation is controlled by the flow of inert gas issuing from the directional inlets.

The embodiment of trailing shield of Figure 9 illustrates a sixth embodiment of trailing shield 90 which has a body portion 90a which is octagonal in shape when viewed from above. The shape of a body portion can be selected to maximise space usage where multiple welds by separate devices are taking place in close proximity to each other.