Abstract:
Systems and methods are disclosed to effectively fracture dendrite arms and/or reduce grain size in a solidifying metal melt pool. Ultrasonic energy is applied to the solidifying metal in the liquid metal pool directly or via a substrate on which the metal is provided. In another embodiment, ultrasonic energy is applied over a range of frequencies and/or tuned to the resonant frequency of solidifying dendrite arms. Advantageously, the present invention prevents or hinders the growth of large columnar dendrites and instead allows for the formation of a high density of randomly oriented grains with a reduction in grain size, thereby enhancing the quality of the deposited metal and therefore improving the mechanical properties of the fabricated or repaired structure.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to metal deposition and, more particularly, to improving the quality of deposited metal used to fabricate or repair structures.  
       BACKGROUND  
       [0002]     There are several metal deposition systems driven by computer aided design (CAD) that build metal alloy structures by an additive layered process on a substrate. Such systems include, but are not limited to, Laser Engineered Net Shaping (LENS) available from Optomec, Inc. of Albuquerque, N. Mex., Direct Metal Deposition (DMD) available from POM Group, Inc. of Auburn Hills, Mich. and from Aeromet Corporation of Eden Prairie, Minn., and Electron Beam Melting (EBM) available from Arcam AB of Molndal, Sweden.  
         [0003]     In each of these systems, liquid metal is provided on the substrate where the liquid metal solidifies. The solidifying metal has a tendency for development of large grain size with highly directional columnar dendrites, which are crystals that branch into two or more parts during growth. In many alloys, this solidification structure is undesirable due to formation of hot tearing cracks resulting from inadequate liquid metal percolation down the columnar dendrite interfaces and anisotropy due to closely aligned grain crystallography. Even in the absence of hot tearing, the large and highly directional grains can have detrimental effects on mechanical properties of the fabricated structure.  
         [0004]     As a result, there is a need for metal deposition methods and systems that eliminate or reduce the large grain size and dendrite formation and instead develop randomly oriented, equiaxed, small grains, thereby improving mechanical properties of the deposited metal.  
       SUMMARY  
       [0005]     Systems and methods are disclosed herein to effectively fracture dendrite arms shortly after their nucleation and growth in a resolidifying metal melt pool. Advantageously, the present invention prevents or hinders the growth of large columnar dendrites and instead allows for the formation of a high density of randomly oriented grains by nucleation and growth on the fractured dendrite arms which are dispersed in the liquid metal, thereby reducing grain size, enhancing the quality of the deposited metal, and therefore improving the mechanical properties of the fabricated structure.  
         [0006]     In accordance with one embodiment of the present invention, a system for enhancing the quality of deposited metal is provided, the system including a metal deposition apparatus that provides liquid metal on a substrate, and an ultrasonic energy source operably coupled to the metal deposition apparatus such that ultrasonic energy is applied to the solidifying metal at the interface with the liquid metal.  
         [0007]     In accordance with another embodiment of the present invention, a method of enhancing the quality of deposited metal is provided, the method including providing a liquid metal on a substrate, applying ultrasonic energy to the solidifying metal in the liquid metal, and solidifying the liquid metal with reduced grain size.  
         [0008]     In accordance with another embodiment of the present invention, another method of enhancing the quality of deposited metal is provided, the method including calculating a dendrite arm fracture length and calculating a resonant frequency applicable for the dendrite arm fracture length. The method further includes providing a liquid metal on a substrate, and applying a tuned ultrasonic energy to the solidifying metal in the liquid metal to decrease grain size as the liquid metal solidifies.  
         [0009]     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows a flowchart illustrating a method of metal deposition in accordance with an embodiment of the present invention.  
         [0011]      FIG. 2  shows a flowchart illustrating another method of metal deposition in accordance with an embodiment of the present invention.  
         [0012]      FIG. 3  shows a block diagram of a metal deposition system in accordance with an embodiment of the present invention.  
         [0013]      FIG. 4  shows a block diagram of another metal deposition system in accordance with an embodiment of the present invention.  
         [0014]      FIG. 5  shows a block diagram of yet another metal deposition system in accordance with an embodiment of the present invention. 
     
    
       [0015]     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.  
       DETAILED DESCRIPTION  
       [0016]      FIG. 1  shows a flowchart illustrating a method for improving metal deposition in accordance with an embodiment of the present invention. In operation  102 , liquid metal is provided on a substrate via various deposition systems and methods, including but not limited to, Laser Engineered Net Shaping (LENS), Direct Metal Deposition (DMD), and Electron Beam Melting (EBM). Then in operation  104 , ultrasonic energy is applied to the metal growing/re-solidifying in the liquid metal pool in accordance with the present invention. In one embodiment, ultrasonic energy is provided to a substrate on which the metal is deposited and/or directly to the liquid metal pool. In another embodiment, ultrasonic energy is applied over a range of frequencies and/or tuned to the resonant frequency of solidifying dendrite arms of a specific, desired length. In yet another embodiment, ultrasonic energy may be swept through a range of frequencies back and forth. In operation  106 , the metal is allowed to cool and solidify on the substrate.  
         [0017]      FIG. 2  shows a flowchart illustrating another method of metal deposition in accordance with an embodiment of the present invention. In this embodiment, ultrasonic energy is tuned to the resonant frequency of solidifying dendrite arms of a specific, desired length, for example on the order of about [0.1]-[1.0] microns. The resulting resonant vibration of the dendrite arms will rapidly build in amplitude until the dendrite arms are fractured. The fractured dendrite arms then migrate randomly in the liquid metal pool ahead of the original solidification front. As the liquid metal pool continues to cool, a high density of fragmented, randomly oriented, dendrite arm segments act as new solidification nuclei. These nuclei promote growth of randomly oriented, equiaxed, small grains on the order of about [10]-[100] microns. Continued application of the ultrasonic vibration prevents growth of dendrite arms beyond the critical resonant frequency length.  
         [0018]     Referring to  FIG. 2 , in operation  202 , a desired dendrite arm fracture length may be calculated based on the solidification behavior of each alloy. In operation  204 , the resonant frequency of the specific dendrite arm length may be calculated by known physical relationships with the solid elastic moduli at the solidification temperature, and the effect of the viscous liquid metal surrounding the dendrite arms. Natural frequency also depends on geometric information other than length of dendrite arms, such as the shape of the arms (e.g., variation in cross-section area, existence of branches). Other factors, such as impact with vibrating fragments of broken dendrites suspended in the liquid, may also be involved.  
         [0019]     In operation  206 , similar to operation  102  noted above, liquid metal is provided on a substrate via various deposition systems and methods, including but not limited to, LENS, DMD, and EBM. Then in operation  208 , the calculated resonant frequency may be used to apply tuned ultrasonic energy to the metal melt pool in accordance with an embodiment of the present invention. Because many factors are involved in determining resonant frequency to fragment the various dendrite arms and large grain sizes, and accordingly resonant frequency may vary, a range of frequencies centered around the calculated resonant frequency may be utilized to fragment large grain size and dendrite arms in accordance with another embodiment of the present invention. In a further example, ultrasonic energy may be swept through the range of frequencies centered about the calculated resonant frequency for a plurality of cycles. In operation  210 , the metal is then allowed to cool and solidify on the substrate.  
         [0020]      FIGS. 3-5  show block diagrams of different embodiments of a metal deposition system in accordance with the present invention.  
         [0021]      FIG. 3  shows a metal deposition system  300  including an ultrasonic energy source  302  that is operably coupled to a metal deposition apparatus  304 . Metal deposition apparatus  304  may include various deposition systems, including but not limited to LENS, DMD, and EBM systems. Ultrasonic energy source  302  is coupled such that ultrasonic energy from ultrasonic energy source  302  may be applied directly to a metal melt pool or a metal substrate on which a metal melt pool is provided. p  FIG. 4  shows a metal deposition system  400  including an ultrasonic energy source  401  that provides ultrasonic energy to a solid substrate  402  on which a liquid metal pool is provided. Advantageously, ultrasonic energy is efficiently transferred from the substrate to the solidifying metal to vibrate the growing dendrites and thereby fracture the dendrite tips. The liquid metal may be considered a hindrance to the effectiveness of the fracturing of dendrite tips by the ultrasonic vibrations, since the liquid metal acts as an energy absorbing medium. However, the present invention is effective despite the viscous, energy absorbing nature of the liquid metal because the liquid metal pool is sufficiently small (typically about 1 mm radius) in these metal deposition processes. In one embodiment, ultrasonic energy may be supplied by an electromechanical transducer  404  coupled to the solid metal substrate below the liquid metal pool. In other embodiments, ultrasonic energy may be supplied by a pulsed laser  406  and/or an acoustic speaker  408 . Other ultrasonic energy sources may be applicable.  
         [0022]     With the above system for injecting ultrasonic energy directly to the dendrite tips via the solid metal substrate, an embodiment of the present invention includes directing a primary energy beam (e.g., a laser, an electron beam, or plasma) to develop a very small liquid metal pool on the substrate surface. A secondary, pulsed laser beam is also directed on to the substrate surface in such a way as to impinge the surface close to the edge of the liquid metal pool. The secondary laser beam may be pulsed at a very high frequency to inject ultrasonic vibrations into the solid substrate. The secondary pulsed laser is ideally directed to impinge the solid substrate surface very close to the trailing edge of the moving liquid metal pool. Alternatively, the secondary pulsed laser can trace a ring constantly encircling the edge of the moving liquid metal pool. This will have the effect of injecting high frequency ultrasonic energy into the solid metal substrate surface with most of the energy being directed towards the metal dendrites growing in the liquid metal pool.  
         [0023]     Advantageously, the present invention provides for continuous, real-time fine adjustments of: 1) position and movement relative to the primary energy beam and liquid metal pool; 2) ultrasonic energy level and frequency; and 3) relatively simple system hardware modifications.  
         [0024]     Alternative systems and methods may be used to provide and apply ultrasonic energy.  FIG. 5  shows a metal deposition system  500  including an ultrasonic energy source  501  that provides ultrasonic energy directly to the liquid metal pool  502  in accordance with an embodiment of the present invention. In one example, the ultrasonic energy may be generated and injected to the liquid metal pool by a pulsed laser  506  with a very high peak energy which is pulsed at the desired resonant frequency or range of frequencies. This pulsed laser may be the same laser that is used to melt the substrate and powder pool or it may be a secondary, pulsed laser to the primary melting energy beam. In a second example, ultrasonic energy may be applied with an electromechanical transducer  504  that moves with and touches the solid substrate close to the liquid metal pool. In another example, vibration may be transmitted through a filler wire  510  feeding directly to the metal pool. In yet another example, acoustic vibration through a speaker  508  and/or induction through magnet stirring may be used to apply ultrasonic energy.  
         [0025]     The present invention may be utilized in a variety of applications, including but not limited to manufacturing high performance parts, adding features to existing components, and precision repairing existing high value components. Furthermore, as noted above, the present invention may be used in conjunction with several metal deposition systems and methods driven by computer aided design (CAD), such as LENS, DMD, or EBM.  
         [0026]     Numerous modifications and variations are possible in accordance with the present invention. The liquid metal may be comprised of a variety of metals, including but not limited to nickel, cobalt and iron-based superalloys, steels, copper, aluminum, titanium, niobium, tungsten, molybdenum, rhenium, and alloys thereof. The liquid metal may be provided from powder, wire, or foil feedstock. The various metal deposition systems may also incorporate the necessary and applicable structures to provide the liquid metal on the substrate, such as mirrors, lenses, carrier gases, movable tables, and controlled-environment chambers to name a few. Any laser power used may vary greatly as well, from a few hundred watts to 20 kW or more, depending on the particular material, feed-rate, and other parameters.  
         [0027]     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.