Patent Application
20090081066
This invention relates to a method of forming an article.
The laser sintering/melting of powdered materials to form solid parts is now well known, though the forming of nominally 100% dense metal parts by a practical process is relatively new.
In particular there are commercial powder bed ‘Selective Laser Sintering’/‘Selective Laser Melting’ (SLS/SLM) machines such as the M270 produced by Electron Optical Systems (EOS) M270 and the Realizer II marketed by MCP. These take metal powders under an inert atmosphere and direct a CO2 or fibre laser onto a layer of powder.
There are other similar powder bed machines and somewhat related processes such as ‘Direct Laser Fabrication’ (blown powder). Generically a point source of energy such as a laser or electron beam is selectively applied to a layer of powder and a 3D part built up layer wise from a ‘slicing’ of a design file. The heat from the point source melts or sinters the particulates to form a solid that may be further processed to fully solidify or render useful by various means not relevant to this disclosure.
The term Direct Layer Deposition (DLD) can be used to describe all such processes and we will here describe only the powder bed SLS/SLM process. In the powder bed systems, powder is fused at a single plane and the object thus built is formed from a cycle of selectively sintered/melted powder at the upper surface of the powder bed, stepping the powder bed and part built object down by one layer and recoating powder across the surface of the bed and part-built object. This way objects are built up in layers and the built part is surrounded by the powder bed. Extremely accurate building is possible with good surface finishes.
Powders are typically 10's to 100's of microns in size and the layers are typically 10's to 100's of microns thick.
The point source of energy penetrates into the surface to a depth greater than the nominal layer thickness and typically to a depth of 1˜1.5 times a layer thickness such that if 20 micron layers are being build, the penetration of the energy source is 20 to 30 microns. This is clearly visible from a micrograph of a cross section of a built part and evidences that energy penetrates into the previously built surface causing fusion of the previously built surface to the new powder applied.
Support structures are required (as is familiar to those skilled in this art) for certain geometries and structures. Broadly, any surface that is at less than approximately 30 degrees from horizontal will require a support. These supports provide a ‘foundation’ upon which melted powder can wet and solidify (rather than simply balling up as ‘splatter’) and/or provides mechanical tethering to keep geometric accuracy of the structure as it is built particularly as thermal stress may be created by the build process.
By their very nature these supports are not part of the desired geometry and therefore need to be designed, built and removed. This is both time consuming and requires skill thereby adding delay and cost.
A further problem is that the geometry of the part may be such that it is difficult or impossible to locate support structures that can be removed—thereby limiting the geometries that can be made by the DLD process.
Two or more parts may be joined together e.g. in a welding process where a point source of energy and materials is applied to the place of conjoining. It is also known to ‘build up’ material by e.g. welding and then further work this deposited material by e.g. filing, cutting or machining such that a repair can be effected upon a part that is broken or damaged.
Whilst useful this process is time consuming and the second part made by welding is not (near) net shape and subsequent working such as cutting and gauging is required.
It is also known to mould a second material onto inserts but in these cases the mechanical linkage of the two parts and the building method of the second part are different. There are therefore 3 layers and two interfaces in this system being the two parts and the bonding system and thereby it is more prone to failure. Typically the insert is of a higher temperature material as the moulding temperature must not damage or distort the insert. Also there are geometric limits to moulding processes and materials limitations dictated by the moulding process.
It is known to clad one material to a part by applying a powder of the second material to the first part and applying e.g. heat and/or pressure to fuse the second material and the first to the second.
Hot Isostatic Pressure (HIP) of powdered material to build net shape parts or to join parts is known. HiPing is also known to create a surface layer of at least 0.2 mm of one material upon a blank made of another material e.g. see U.S. Pat. No. 6,015,627 where an aluminium drum is placed in a HiPing can and hard powdered material is HIPed onto the surface to form a ‘blank’ which is then removed from the can and forged into a magnetic head drum for use in helical scan magnetic recording apparatus.
From one aspect the invention consists in a method of making an article including:
(a) determining the greatest dimension of an article along a selected axis;
(b) manufacturing a first part of the article between an end and a face lying in the laterally extending plane containing the dimension;
(c) placing the article on the support of direct layer deposition (DLD) apparatus with the face upwards; and
(d) forming the remaining part of the article using the DLD apparatus.
The part may be clamped to the support.
The DLD apparatus may be either a selective layer sintering apparatus or a selective laser melt apparatus.
It is particularly preferred that the step (d) joins the forming remaining part to the first part.
The parts may be of different materials.
From another aspect the invention is a method (or any object made by such a method) of making complex solid objects by joining an already built part to a further part by using the same method for joining as to build that further part.
Either method may include the application of a point source of energy such as a laser or electron beam that can both soften or melt the surface of the already built part and also the materials used to build the further part to form the object. It is understood that the recipe of the method may have variations through time such as applied power, pressure, dwell time and build depth whilst still being the same method.
In particular the method to build the further part is a selective layering process where material is added e.g. by the application of material and a point source of energy to cause it to fuse together.
More particularly the invention is the process of making an object by a contiguous joining and net shape or near net shape building process of one part such as by a powder bed SLS/SLM process to an already built part of that object.
It should be understood that the materials of the two parts may be the same or different and in particular the material of the second part may have a higher melting point that the material of the first part. For example the first part may be of a Stainless Steel and the second part Cobalt Chrome or other high temperature alloy such as a Nickel super alloy.
To clarify, the co-joined parts are not made by a single, interrupted but otherwise continuous process and in particular the invention is in objects that cannot be made by a single (perhaps interrupted) process. A mechanical translation and/or materials change or the like is carried between the two processes.
The invention is also any solid object wherein the object is in at least two co-joined parts wherein at least one part is net shape or near net shape and the joining is contiguous with that one part and not the other part.
This is counter intuitive for two reasons. In this invention the joining is contiguous with the forming of the second part. It is expected to commence with already formed parts and bring them together to be joined (e.g. by welding) or to create a mass of second material upon a first part e.g. by welding and then subsequently form the second part e.g. through cutting, filing and the like.
Further, the powder bed process is considered a method a building a single part to replace assemblies, so it is not obvious to turn it into a joining process. So the invention may be seen as a method of using a selective layer deposition process to join two parts together.
For example the invention enables a complex part to be built with a pre-built part as foundation for a powder bed SLS/SLM process and in particular a part (preferably but not necessarily made by a powder bed process) is located such that a powder bed process can commence at a surface of the already built part. This requirement includes the pre-part is located;
a) Firmly, to avoid intolerable movement induced by the thermal stress of the SLS/SLM process,
b) Vertically, with a powder source aligned with respect to the layer height to be built upon the building surface of the pre-built part, in particular the upper surface of a powder bed (e.g. within about 4 times the diameter of the average particulate size),
c) Laterally such that the building process (induced by the energy source) and the pre-built part are correctly aligned with respect to each other.
A method of achieving this is to form an attachment part for the first part at the same time as the first part and by the same means as the first part whereby the first part, once removed can be relocated and clamped to this attachment part to provide accurate location during the building of the second part.
The invention will be described by reference to the figures, in which:
Diagrams a˜d of
a and b show intermediate stages of the powder bed SLS/SLM process. The start point is a base plate 1 and powder 4. A thin layer of powder is placed by various means familiar to those familiar with these machines across the base plate—e.g. by pushing powder from a powder source bin with a re-coater blade 8. A point source of energy 6 such as a laser beam is directed at the powder causing it to fuse to form a solid 2 being part of a desired 3 dimensional solid object. The base plate is indexed downwards and a fresh layer of powder created across the base plate and the solid 2 in build. The ambient 5 is an inert gas such as argon or is de-oxygenated air. Typically the base plate 2 is warmed to above ambient to enable easy thermal control. At 3 a support structure is being built. This is not a desired part of the solid object but is required by a subsequent part of the building process. Such supports can be custom designed or generated automatically by “Magics” software from Materialise, Leuven, Belgium.
At FIG. 1.b is a later intermediate step immediately prior to the commencement of the building of 2a, a part of the solid 2 shown at FIG. 1.c. This part 2a is impossible to build without support 3 beneath it. If the point source of energy 6 is directed at the powder bed 4 then the powder simply fuses into discrete lumps. The support 3 provides a surface for wetting and to provide mechanical stability and enable a continuous build of part 2a and 2.
At FIG. 1.d is shown the completed solid 2 in powder bed 4.
In this simplistic example it can still be seen that removal of support 3 is non trivial and time consuming. It should be appreciated that there are practical real-world examples where supports are required inside objects or in other hard to reach places rendering the object either costly or unmanufacturable by this otherwise suitable process.
In
The intermediate solid object 2i is now removed from the base plate 1 e.g. by wire Electro Discharge Machine or saw, inverted and placed back on the base plate as shown at
The building process continues until the object 2 is completed as shown at
As can be appreciated there may be additional heat treatments and stress relieving steps without affecting the generality of the invention.
Further, intermediate object 2i may be made from a different material or indeed by a different process and could for example by a machined, forged or cast part. By such means a multi-material construct may be formed without discrete joining processes.
| Number | Date | Country | |
|---|---|---|---|
| 60975260 | Sep 2007 | US |
