Not applicable.
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This invention relates to additive metal layering techniques using an emitted energy source.
Additive metal deposition is an industrial technique that builds fully-dense structures by melting powdered or wire metal, via a laser or other energy source, into solidifying beads, which are deposited side by side and layer upon layer upon a work piece substrate. It is known to utilize the process to repair and rebuild a worn or damaged component using a laser to build up structure on the component. The process is particularly useful to add features such as bosses or flanges on subcomponents of fabricated structures. The basic process involves adding layers to the component to create a surface feature on the component via the introduction of depositing material (delivered in the form of injected powder or a wire) into a laser beam. The additive process is known by several names including “laser cladding,” “laser metal deposition,” “direct metal deposition” or “additive metal layering.”
Additive metal layering is typically performed by using a computer aided design (“CAD”) to map the geometry of a part and then depositing metal, layer-by-layer, on the part. The CAD mapped geometry is input into a computer controlled (robotic) part handler that can manipulate the part in multiple axes of movement during the deposition process. In all of these techniques a heat source (typically an industrial laser beam) is used to create a melt pool into which a wire or powdered feedstock is fed in order to create beads upon solidification. In practice, the heat source is under computer numerical control and is focused onto a workpiece, producing the melt pool. A small amount of powder or wire metal is introduced into the melt pool, building up the part in a thin layer. The beam follows a previously determined toolpath. The toolpath is generated based on the CAD data that computes the needed part layer by layer. The beads are created by means of relative motion of the melt pool and the substrate, e.g. using an industrial robot arm or an XY-table. A part is then built by depositing the beads side by side and layer upon layer. The most popular approach combines a high power laser heat source with metal powder as the additive material.
Careful tuning of the deposition tool and parameters, such as the powder or wire feed rate, the energy input, and the traverse speed are therefore important in order to obtain layers, which are free from defects such as shape irregularities, lack-of-fusion or cracks. Droplet forming, i.e. globular transfer of the molten metal, is also a common disturbance that affects the geometrical profile of the deposited beads and stability of the additive layers.
Regulating the necessary needed power is critical to system operation and achieving a high-quality layered end product. The currently known laser additive processes attempt to address deposition quality issues in either of two ways. In this respect, the prior laser additive processes use a constant laser power or one regulated by a feedback (a/k/a “closed-loop”) sensor.
The issue with using a constant laser power is that the operator has to optimize the power level for a worst case scenario, typically the start of the process. This results in variations in both geometry and material properties as the melt pool size and temperature gradients vary with the local energy balance conditions around the melt pool. Using a constant energy throughout the deposition process is problematic because the additive process changes the geometry of the built structure during the process. Hence, the chosen constant power level represents a compromise selection. For example and as shown in
Feedback systems represent an attempt to address the deficits of the constant power system. The typical prior art feedback systems attempt to control the deposition process by monitoring the dimensions of the part or the melt pool during the deposition process. Feedback or closed-loop systems are inherently reactionary, and thus can only react to conditions that have already drifted away from nominal. There is thus a need in the art for an improved method of regulating laser power during the additive process.
The instant invention addresses the deficits of the prior art by providing for a method of predicting needed laser power during an additive layer process. The calculated predictive levels can then be input into the laser power controller to regulate laser power at intervals during the additive path deposition process. The inventive method is a power schedule calculation method for an additive deposition process using a beam source that calculates optimum beam power for any point P(s) along an additive path that will be traveled to form a build that has a geometry and is formed from deposited material added to a substrate. The inventive method utilizes a calculated idealized geometry for each point P(s) along the additive path. The idealized geometry for each point P(s) comprises a melt pool, hot zone and bulk portion.
The inventive method predicts energy needs along the additive path based upon an improved model of the dynamic geometry and thermodynamics of the build during the additive process. The inventive laser power prediction method can be generally described as using four essential components to calculate needed power during the deposition process to create a thermodynamic model of the deposition system. The four components include:
The inventive model is used to predict an appropriate input laser power at definable intervals along the laser path. It accomplishes this by using the path and geometric representation of the part being produced to create an idealized geometry that allows for tenable calculations. As shown in
The model's advantage over a more conventional Eulerian finite element method (“FEM”) mode is demonstrated in
The present invention also includes a system for fabricating a part on a substrate using a deposition beam source that follows an additive path and that is controlled in accordance with the described laser power prediction method. The system includes a computer-aided design database including a description of the part to be fabricated and a database describing the additive path to be traveled as the part is formed. The additive path is composed of a plurality of points. The system further includes a substrate support for supporting the substrate and manipulating it through space, a metal stock delivery system, (e.g., powdered metal injector/sprayer or wire feed) and a controllable beam source capable of emitting a beam onto the substrate and adapted to form a melt pool thereon and a controller adapted to control power to the beam source. The controller is programmed to regulate energy of the produced beam for any point on the additive path in accordance with the required power predicted for that point by the described calculation methods.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The present invention is directed to a laser power prediction method. The method utilizes the following items to create an improved thermodynamic model of the work piece to compute needed laser power at Intervals during the additive process. The method does so using: a) an additive path describing the path of the laser through space; b) a geometric representation of the geometry that the additive path is intended to create; c) a description of the thermodynamic characteristics of the manufacturing environment; and d) computed thermophysical characteristics of the materials involved. This model is used to predict an appropriate input laser power at definable intervals along the laser path. It accomplishes this by using the path and geometric representation of the part being produced to create an idealized geometry that allows for tenable calculations.
As shown in
The laser power input is calculated by performing an energy balance calculation at intervals along the laser's path with the following considerations:
10. Mass of the hot zone; and
11. Area for conduction between the hot zone and bulk structure.
Relevant to describing and demonstrating the inventive method are the following elements and symbols, which have the meanings indicated.
Material Properties
An exemplary calculation method thus proceeds as follows. First, before beginning the formal calculations representing the in-process thermodynamic environment, initial values are input based upon the starting temperature environment
Initialize the energy content of the part:
Hbuild=CpTambmsub (1)
Initialize the mass of the part:
mbuild=msub (2)
Initialize the temperature of the part:
Tbuild=Tamb (3)
Initialize the hot zone temperature:
Thot=Tbuild (4)
Set the path to begin at the beginning:
s=0 (5)
Useful constants should be then pre-calculated.
Maximum volume of the hot zone:
Vmax=2/3r3hotπ (6)
Maximum, area of the hot zone:
Amax=2r2hotπ (7)
Approximate surface area of the melt pool:
Amelt=1/4w2π (8)
Mass per unit length of deposition
mul=wdlρ (9)
Time for the laser to traverse one calculation interval:
Δt=ip/ƒ (10)
With the above initial pre-calculation steps undertaken, one can proceed to calculate needed laser power along a given deposition path according to the following method.
A. Calculate the geometry index for the current position,
Vhot=G∩Z(rhot,P(s),t′(s)) (11)
Ahot=VhotAmax/Vmax (12)
B. Compute the energy balance at the melt pool.
C. Compute the laser power.
D. Update the mass of the build.
mbuild=mbuild+mulip (20)
E. Calculate the energy losses at the build.
F. Update enthalpy of the build.
Hbuild=Hbuild+QlaserΔt−Hmach−HhotHenv (26)
G. Update the build temperature.
Tbuild=Hbuild/Cp (27)
H. Update the hot zone temperature estimate.
Thot=(Tbuild+Ttarget)/2 (28)
I. Update the position along the deposition path.
s=s+ip (29)
J. If the path is not complete, return to step 1.
These calculations are carried out in sequence with each result being an input for the next step in the process. For example, during the calculation of a laser power schedule for a 56.5 mm long×3.3 mm wide×190 mm tail rectangular structure (approximately 500 layers, ˜20 minutes of real machine time), the laser power prediction algorithm calculated the appropriate laser power for 14502 discrete points along the path.
With the foregoing explanation, it will be appreciated that in one embodiment the inventive method is a power schedule calculation method for an additive deposition process using a beam source that calculates optimum beam power for any point P(s) along an additive path that will be traveled to form a build, the build having a geometry and being formed from deposited material added to a structure, the point P(s) along the additive path having associated with it an idealized geometry comprising a melt pool, hot zone and bulk portion.
As shown in
With step 111 completed, the geometry factor computation 104 is completed and the calculation proceeds to compute laser power at step 105.
Having explained the method mathematically, the inventive method can be verbally summarized and described In summary description, the method comprises creating a geometric description representing the geometry of the build during the additive process and creating a path description that represents the path of the beam source through space during the additive process. The method further includes calculating the idealized geometry for a point P(s) on the additive path based upon the geometric description and path description. In addition, the method includes calculating an energy balance at the melt pool (EBmelt pool) for the point P(s) on the additive path, the energy balance calculation being based upon the following calculations;
a) a calculation of energy radiated from the melt pool (Hrad);
b) a calculation of energy conducted from the melt pool to the hot zone (Hcond), the calculation of Hcond being based upon the calculated idealized geometry and
c) a calculation of energy lost due to convection at the melt pool (Hconv);
The method further includes calculating total energy (Htotal) needed at the point P(s) on the additive path according to the following equation
Htotal=Hdeposited material+Hremelt+EBmelt pool
wherein Hdeposited material represents energy required to melt the deposited material and Hremelt represents energy required to remelt existing material; and
The last step of the first embodiment of the inventive method includes calculating optimum beam source power Qsource for the point P(s) according to the following equation
Qsource=min(Qmax,Htotal/αΔt)
wherein Qmax represent maximimi laser power, α represents a beam absorption coefficient and Δt represents a calculation interval.
The first embodiment inventive method can be refined further by enhancing the calculation of the idealized geometry by calculating a geometry index hot zone volume (Vhot) for the point P(s) on the additive path according to the formula
Vhot=G∩Z(rhot,P(s),t′(s))
wherein G represents deposition geometry, Z represents a hemisphere of a certain radius (r) and having a circular surface centered at a point P(s) on the additive path with a normal direction ({circumflex over (n)}), rhot represents the radius of the hot zone and t′(s) represents tool axis direction at a distance s along the additive path; and
calculating a geometry index hot zone area (Ahot) for the point P(s) on the additive path based upon the calculated geometry Index hot zone volume (Vhot) according to the formula
Ahot=VhotAmax/Vmax
wherein Amax represents the maximum area of the hot zone and Vmax represents the maximum volume of the hot zone
The first embodiment inventive method can be refined further by calculating EBmelt pool according to the formula
EBmelt pool=Hrad+Hcond+Hconv
In another embodiment, the invention is directed to a power schedule calculation method for an additive deposition process using a beam source that calculates optimum beam power for any point P(s) along an additive path that will be traveled to form a build, the build having a geometry and being formed from deposited material added to a structure, the point P(s) along the additive path having associated with it an idealized geometry comprising a melt pool, hot zone and bulk portion. This method comprises creating a build geometry description based upon data representing the geometry of the build during the additive process and creating a path description based upon data, representing the path of the beam source through space during the additive process. The method also involves calculating the mass of the build at a point P(s) during the additive process that accounts for accretion of the build during the additive process and calculating the idealized geometry at point P(s) during the additive process, the idealized geometry calculation calculates the size of the melt pool, hot zone and bulk portion. Additionally, the method involves calculating the temperature of the build at point P(s) during the additive process, the calculation of the temperature of the build includes a calculation of hot zone temperature based upon the idealized geometry.
The second embodiment method further comprises calculating an energy loss of the build at point P(s) during the additive process, the calculation of energy loss being based upon the calculation of temperature and including a calculation of energy conducted from the melt pool to the hot zone; calculating an enthalpy of the build at point P(s) in time during the additive process; calculating total energy needed at the point P(s) based upon the calculated energy loss and enthalpy of the build at point P(s) on the additive path; and calculating an optimum beam source power based upon the calculation of total energy needed.
Using the calculations above, the laser energy delivered during an additive path deposition process can be controlled for each point along the path by regulating power to the laser at each point in accordance with the required power predicted by the above calculation methods. As such, in another embodiment of the invention the method can be programmed into a computer-controlled laser metal deposition system to improve the fabrication of component of almost any geometry that can be produced from a computer database. This system for fabricating a part on a substrate using a deposition beam source that follows an additive path is depicted in
The process described herein can also be used in connection with other traditional welding techniques, such as tungsten inert gas (“TIG”) welding, gas metal are welding (“GMAW”), plasma transferred are (“PTA”) welding and electron beam (“EB”) welding. While the embodiments of the method and system of the present invention have been described herein, numerous modifications, alterations and changes to the described embodiments are possible without departing from the scope of the invention. The embodiments described herein are not intended to be limiting.
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