Patent Application
20060157219
The present invention relates generally to metal deposition and, more particularly, to improving the quality of deposited metal used to fabricate or repair structures.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Referring to
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.
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.
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.
Alternative systems and methods may be used to provide and apply ultrasonic energy.
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.
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.
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.
