Abstract:
A method for depositing clad material ( 24 ) onto a substrate ( 10 ) by melting a layer of powdered material ( 16 ) using an energy beam ( 20 ), and also applying vibratory mechanical energy ( 27, 29  and/or  31 ). The vibratory mechanical energy may be applied before, during or after the melting and solidification of the powdered material in order to preheat the powder, to distribute powder over a top surface ( 18 ) of the substrate, to control the formation of dendrites in the clad material as the melt pool ( 22 ) solidifies, to remove slag, and/or to perform stress relief. Simultaneous application of beam energy and vibratory mechanical energy facilitates the continuous deposition of the clad material, including directionally solidified material.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of materials technology, and more particularly to processes for depositing a cladding material by melting a powdered material on a substrate surface with an energy beam. 
       BACKGROUND OF THE INVENTION 
       [0002]    Selective laser melting (SLM) and electron beam melting (EBM) are known additive manufacturing processes whereby a powdered feed material is melted and fused into a homogeneous mass by the application of an energy beam in a layer-by-layer process for forming a three dimensional object. These processes are useful for creating intricate shapes by melting small filler material particles (e.g. 20-100 microns) with a small diameter beam at focus (e.g. 50 microns) with precise computer controlled movement of the beam. However, these processes tend to be slow and expensive, and they produce only small grain sized equiaxed and polycrystalline microstructures. Moreover, they are limited to depositing material onto a top surface of a component where the component does not project above the processing plane, since the powder is typically applied to the processing plane by a wiper action which spreads the feed material across the processing plane. Accordingly, improved powder deposition processes are needed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The invention is explained in the following description in view of the sole drawing that shows an embodiment of an improved selective laser melting process. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0004]    The present inventors have developed an improved process for depositing a powdered feed material onto a substrate surface which overcomes many of the limitations of prior art SLM and EBM processes. In addition to the application of heat energy with an energy beam as is provided in known processes, the present invention advantageously incorporates the use of vibratory mechanical energy. The vibratory mechanical energy may be applied to the powder and/or to the substrate before, during and/or after the application of the beam energy in various embodiments of the invention, as described more fully below. 
         [0005]    The sole FIGURE illustrates aspects of the invention. A substrate material  10  is supported in a bed of powdered material  12  within a container  14 . A layer  16  of the powdered material is distributed over a surface  18  of the substrate  10 , and is being melted by an energy beam  20  being traversed over the surface  18  in the direction of the arrow. The melted powder forms a traveling melt pool  22  which then cools and solidifies to form a layer of clad material  24  on the substrate  10 . The energy beam may be a light beam, a laser beam, a particle beam, a charged-particle beam, an electron beam, a molecular beam, etc. The powdered material is typically a metal alloy, but may include ceramic, flux, plastic, glass, composite, and/or other powdered ingredients and mixtures thereof. 
         [0006]    In one embodiment, the layer  16  of the powdered material is distributed over the surface  18  by vibratory mechanical energy applied to the bed of powdered material  12 . The vibratory mechanical energy  27  may be imparted to the bed of powdered material  12  by an electro-mechanical transducer  26  in contact with a surface the container  14 . Alternatively, a similar transducer  28  may be used in direct contact with the substrate  10  to apply the energy  29 , or a pencil head shaker  30  may be submerged into the bed of powdered material  12  to apply the energy  31 . The vibratory mechanical energy functions to loosen or “fluidize” the bed of powdered material  12  and to cause it to form a horizontally level upper surface  32 . The height of the support structure  34  for the substrate  10  may be adjusted or the quantity of the powdered material within the container  14  may be controlled such that the level upper surface  32  of the bed of powdered material  12  forms a desired thickness for layer  16  over the substrate surface  18 . 
         [0007]    The vibratory mechanical energy may be applied at a single frequency or over a range of frequencies, including from low frequencies (e.g. 50 Hz or less) to ultrasonic frequencies (above 18 kHz). The use of vibratory mechanical energy to move powder onto the processing plane advantageously eliminates the need for a wiper arm as is commonly provided in prior art SLM machines. This makes it possible to apply clad material  24  onto a substrate  10  having a portion  36  which extends above the working plane of the surface  18  being coated. 
         [0008]    The vibratory mechanical energy may be provided intermittently or continuously as desired to achieve a desired distribution of the powdered material over the surface  18 . Continuous powder delivery has the potential for significantly increasing the speed of the deposition process by allowing the powder delivery and melting to proceed concurrently. Moreover, directional solidification of the deposited clad material  24  is now possible by continuously feeding and melting material over a broad area, such as by applying heat energy with a diode laser or by rapidly scanning a high power laser beam. The application of vibratory mechanical energy to the bed of powdered material  12  will result in some preheating of the powder, in particular if ultrasonic energy is used, thereby reducing an amount of heat that must be applied via the beam  20  in order to achieve melting. A chill plate  38  may be utilized to influence the direction of heat transfer from the melt pool  22  in order to facilitate the directional solidification of the clad material  24 , including the deposition of single crystal material. The chill plate may further incorporate sides (e.g. of zirconia) extending above its surface that laterally insulate the deposit made thereon. By discouraging lateral heat conduction, these insulating features would further enhance uniaxial heat extraction and ensure directional solidification. 
         [0009]    The vibratory mechanical energy may be applied as the weld pool  22  material solidifies in a manner effective to break up dendrites that may be forming during the solidification, thereby providing grain refinement and improved mechanical properties to the deposited clad material  24 . Effective vibration frequencies may vary depending upon the alloy of the substrate  10 . For example, magnesium alloys have been cited to benefit from frequencies up to about 16 Hz while steels have been cited to benefit from frequencies up to about 400 Hz. Furthermore, a component may benefit from application of resonant frequencies that are dependent on its specific geometry. To this end, a vibration sensor(s)  40  may be useful in detecting such resonances and thereby providing feedback to adjust vibrator speeds that optimize vibrational effect. 
         [0010]    The vibratory mechanical energy may be applied after the clad material  24  is solidified in a manner effective to introduce stress relief. For example, large amplitudes of vibration that induce stresses approaching the fatigue limit of the material being processed can effect significant relief of residual stresses. 
         [0011]    In embodiments where the clad material  24  includes a difficult to weld superalloy material, the layer of powdered material  16  may include a powdered flux material, as described in commonly owned United States patent application publication number US 2013/0136868 A1, incorporated by reference herein. The melted flux material will form an uppermost layer of slag material as part of the deposited clad material  24 , and the slag material must most normally be removed prior to the deposition of the next layer of clad material. In such embodiments, vibratory mechanical energy may be applied in a manner effective to release the slag from the substrate  10  by mechanically breaking the layer of slag and loosening it from the underlying deposited superalloy material. Frequencies effective in achieving such detachment are likely similar to those common in mechanical tools such as chipping hammers and needle guns (e.g. up to hundreds on Hz) but may beneficially extend up to up to the kilohertz range. 
         [0012]    While the vibratory mechanical energy may be applied to the powder and/or to the substrate before, during and/or after the application of the beam energy, it need not be applied in the same manner, at the same frequency, or from the same location during these different phases of the deposition process. For example, preheating of the powder may be accomplished with ultrasonic energy applied by the pencil head shaker  30  until a desired powder temperature is achieved, then movement of the powder may be further stimulated by the application of vibratory mechanical energy at a lower frequency applied by the transducer  26 , and then still lower frequency vibratory mechanical energy may be applied by transducer  28  during and/or after the melting and solidification steps. Standing or moving waves may be induced in the bed of powdered material  12  or the substrate  10  in order to accomplish movement of powder, control of dendrite formation, slag removal and/or stress relief. The process and powder bed may or may not be further assisted by fluidizing gas, inert cover gas or general process space vacuum. 
         [0013]    In a further embodiment, powdered material  12  may include at least two distinct types of particles, such as different size particles, different shaped particles, different density particles, etc. The vibratory mechanical energy may be controlled to preferentially react with one type of particle in favor of another type of particle, such as using different frequencies of vibratory mechanical energy to preferentially heat particles of metal alloy more than particles of flux material. In another example, metallic alloy and ceramic particles may be moved onto substrate surface  18  by using multiple frequencies of mechanical vibratory energy simultaneously; then the different types of particles may be segregated into respective layers on the surface  18  by further vibratory mechanical energy of a single frequency which promotes the “floating” of the ceramic above the metallic alloy. Similarly, relatively smaller particles may be induced to migrate into cracks or openings in the surface  18  while relatively larger particles are retained on the surface  18 . 
         [0014]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.