Patent Application
20170106477
1. Field
The present disclosure relates to additive manufacturing, more specifically to techniques and systems for additive manufacturing processes (e.g., powder bed fusion and direct energy deposition for building or repairing metal parts).
2. Description of Related Art
Traditionally, some main criteria for selection of powdered alloys for additive manufacturing are weldability, propensity to form a stable weld pool without keyhole porosity, and absence of defects during the solidification process. In addition to these criteria, other very important technological aspects of the fusion process also need to be considered because they can affect the material microstructure evolution.
There are many challenges in the Powder Bed Fusion and Direct Energy Deposition technologies that prevent additively built components from being implemented for demanding applications. For example the desired grain size and morphology is not controllable with traditional techniques. Grain directionality is typically considered as one of the weakest points of additively build material.
Another challenge is that optimal melting/solidification rates are difficult to achieve. Depending on alloy composition and part cross section the solidification rate difference may produce undesirable phases, microstructure defects, and excessive thermal stresses.
In certain cases, precursor material is difficult to segregate from alloying elements. Material chemical composition uniformity may be altered during the fusion process as a result. Additionally, weld pool melt velocities create strong molten metal turbulence that leads to spattering and melt ejections.
Further, molten metal surface tension affects weld pool geometry. For thin walled structures, controlling the width of weld pool is critical. Weld pool consists of several areas such as molten metal, liquid phase, and solid phase sintering. The total width of the additively built walls varies depending on the size of partially melted and satellite particles that are bonded to the solid state sintered area of the weld pool.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for improved additive manufacturing procedures. The present disclosure provides a solution for this need.
A method for additively manufacturing an article includes applying energy to a powder to produce a weld pool of molten powder and applying an electromagnetic field to the weld pool to control one or more characteristics of the weld pool. Applying the electromagnetic field can include applying an electric field and/or a magnetic field to the weld pool.
Applying energy to a powder can include applying a laser beam. The laser beam can be moved along a melt direction to melt the powder in a powder bed or along with the deposition of powder injected into the focused laser beam.
In certain embodiments, applying the magnetic field to the weld pool can include applying the magnetic field such that a magnetic induction vector of the magnetic field is oriented at a certain angle (e.g., perpendicular) to the melt direction at the weld pool. Applying the electromagnetic field to control one or more characteristics of the weld pool can include controlling at least one of molten flow and/or convection, grain growth rate, grain morphology, and/or weld pool geometry.
Controlling weld pool geometry can include reducing a cross-sectional area of the weld pool to reduce wall thickness of an additively manufactured article. In certain embodiments, controlling molten flow can include controlling molten flow rate of the molten flow within the weld pool.
In accordance with at least one aspect of this disclosure, a system for additive manufacturing includes a build platform for additively constructing an article thereon, energy applicator configured to heat and melt a powder on the build platform to create a weld pool of molten powder, and an electromagnetic field system configured to selectively apply an electromagnetic field to the weld pool. The electromagnetic field system can be operatively connected to the energy applicator to activate with activation of the energy applicator.
In certain embodiments, the energy applicator can include a laser. The laser can be configured to move relative to a build platform (e.g., mechanically or via a scanning reflector to move the laser beam). In certain embodiments, the electromagnetic field system can be configured to move with the laser (e.g. mechanically or via computer control of electric power, current and voltage to an arrangement of electromagnets).
In certain embodiments, the electromagnetic field system can include a plurality of magnets disposed in a linear pattern (e.g., parallel) relative to the laser beam motion. In certain embodiments, the electromagnetic field system can include a plurality of electromagnets disposed in a circular manner surrounding the laser beam. The plurality of electromagnets can be configured to be activated to create a magnetic field having an induction vector at a controlled angle relative (e.g. perpendicular) to the direction of motion of the laser.
The system can further include a control system operatively connected to activate/deactivate each electromagnet of the plurality of electromagnets as desired to create a predetermined magnetic field strength and orientation. The control system can be configured to activate two diametrically opposed electromagnets at a time.
In accordance with at least one aspect of this disclosure, a laser sintered article includes at least a portion thereof that was exposed to an electromagnetic field when exposed to a laser during manufacture.
These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings.
So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, embodiments thereof will be described in detail herein below with reference to certain figures, wherein:
Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, an illustrative view of an embodiment of a method in accordance with the disclosure is shown in
Referring to
Applying energy 101 to a powder can include applying a laser beam 203a. The laser beam 203a can be moved in a melt direction 315 along a powder bed (e.g., on build platform 201). It is also contemplated that the laser beam 203a can be moved along with the deposition of powder 209 injected into the laser beam 203a instead of using a powder bed.
Referring to
In certain embodiments, controlling molten flow can include controlling molten flow rate of the molten flow within the weld pool 313. Electric current coupled with the magnetic field can create Lorentz force opposing the direction of the weld melt flow direction 315. The applied magnetic field is able to influence the natural convection motion within an electrically neutral molten metal fluid using the phenomenon is known as Hartmann effect, which helps control the melt flow to form more uniform weld pool geometry. The electromagnetic fields can control the flow of the electrically conducting (e.g., metal or metal alloy) weld pool 313 to resist the thermal buoyancy and surface tension driven convection motion and to reduce flow instabilities.
Referring specifically to
The system 200 can also include an electromagnetic field system 205 configured to selectively apply an electromagnetic field to the weld pool 313. In certain embodiments, the electromagnetic field system 205 can be operatively connected to the energy applicator 203 to activate with activation of the energy applicator 203. It is contemplated that the electromagnetic field system 205 can be configured to move with the laser beam 203a or energy applicator 203, or otherwise modify the electromagnetic field to follow the laser beam 203a while still applying a desired electromagnetic field to the weld pool 313.
As shown in
The system 200 can further include a control system 211 operatively connected to activate/deactivate each electromagnet 205a, 205b of the plurality of electromagnets as desired to create a predetermined magnetic field. As shown, the control system 211 can be configured to activate two diametrically opposed electromagnets 205a at a time. Which electromagnets that are active electromagnets 205a and/or the polarity thereof can be controlled to create a predetermined magnetic field relative to the melt direction 315.
For example, as shown in
The directions and/or intensities of the applied fields (e.g., electric and/or magnetic) can be optimized to minimize secondary motion and provide the most uniform temperature gradient possible in the weld pool 313. If the melt direction 315 changes, a different pair of electromagnets can activate simultaneously with the change of the melt direction 315 as shown. The scheme of selective activation of magnetic field can depend on any other suitable manufacturing process parameters (e.g., laser scan direction, speed and power to optimally compensate melt convection).
Referring to
In certain embodiments, it is contemplated that any suitable permanent magnet or electromagnet can be utilized. Also, it is contemplated that each one or more of the electromagnets and/or permanent magnets can be enhanced with the use of magnetic shells as known in the art. For example, a general property of magnetic fields is that they decay with the distance from their magnetic source. However, as is appreciated by those having ordinary skill in the art, surrounding a magnetic source with a magnetic shell can enhance the field as it moves away from the source. Further, a second magnetic shell can concentrate the captured magnetic energy into a small region, allowing magnetic energy to be transferred and concentrated to the melt pool.
Using the above methods and systems, a laser sintered article (or any other additively manufactured article) can include at least a portion thereof that was exposed to an electromagnetic field when exposed to a laser during manufacture, which can impart advantageous properties to such an article. New properties can be created by manipulation of the phase stability through the application of a strong magnetic field combined with thermal treatment. A strong magnetic field can provide a body force that resists motion in the melt, for example. In addition, an oscillating magnetic field can produce induction heating that which can be used control the melt temperature by localized heating. In the case of the formation of superalloys, the magnetic field can be used to control annealing in order to affect solute formation which can result in in improved creep strength.
Melting and re-solidification of metals and metal alloys can be controlled in direct metal laser melting (DMLM) processes. However, the technique is not limited to metals as it can be applied to any similar process involving conducting melts. Other applications could involve controlled solidification of aluminum melts, molten steel, electropolymers, and electronics materials. Magnetic controlled solidification techniques may also be used to control the physical properties of fiber reinforced composites by controlled the pattern, spacing and orientation of microfibers during curing of the slurry.
An advantage to the herein disclosed methods and systems include thinner/controllable weld pool widths and more uniform weld pool geometry, which can be important for producing thin walled structures and components with high resolution features. Some other advantages include reduced thermal stresses, improved surface finish due to a lowered influence of the Marangoni stresses at the upper surface which minimizes the irregular melt pool cross-section shape; improved grain morphology by formation of the grain structures similar to an equiaxed grain structure; and reduced micro segregation by lowering the melt velocities to prevent strong spattering and melt ejections driven by the dynamics in the weld pool.
The methods and systems of the present disclosure, as described above and shown in the drawings, provide for additive manufacturing techniques and systems which can provide articles with superior material properties and higher resolution. While the apparatus and methods of the subject disclosure have been shown and described with reference to embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject disclosure.
