Abstract:
The invention provides a method and apparatus for forming a freeform metal structure by wire feed additive manufacturing, wherein a metal wire end is moved along an area of deposition and is melted by a heat source, the melting being deposited on the metal structure as a metallic build-up material, wherein during the cooling process build-up material is subjected to at least one pulsed magnetic field effecting plastic deformation of the build-up material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Patent Application No. EP 15 150 312.5, filed on Jan. 7, 2015, which claims priority to European Patent Application No. EP 14 198 613.3, filed on Dec. 17, 2014, both of which are incorporated herein by reference in their entirety. 
       BACKGROUND 
       [0002]    1. Field of Technology 
         [0003]    The invention relates to a method and an apparatus for forming a freeform metal structure using a combination of wire feed additive manufacturing and electromagnetic forming. 
         [0004]    2. Brief Discussion of Related Art 
         [0005]    It is known in the state of the art to manufacture parts using wire feed additive manufacturing resulting in saving processing time and costs. The method of wire feed additive manufacturing comprises a heat source for providing a melting pool on a metal structure into which a metal wire is subsequently fed and being deposited on a metal structure in a liquid state, which allows exceptional time saving potentials compared to conventional forming techniques such as milling from bulk, forging or casting. On the other hand, wire feed additive manufacturing brings along solidifying and cooling effects shortly after deposition which result in distortion in the parts being manufactured due to shrinking processes in the deposited build-up materials. As a consequence, high level residual stresses might occur leading to the formation of part cracks during the build-up process. If no cracks are detectable, the distortion within the build-up material can still be corrected by carrying out subsequent forming procedures or by adding post-weld heat treatments to the deformed material in the affected areas. This can, however, lead to considerable inaccuracies in the manufacturing process resulting in poor dimensional manufacturing tolerances. 
         [0006]    Furthermore, it is known to combine wire feed additive manufacturing with after-deposition rolling treatment to refine possibly distorted microstructures where a rolling device being able to withstand rolling forces up to 100 kN is required while simultaneously ensuring to provide the desired accuracies when depositing the build-up material using wire feed additive manufacturing. 
       SUMMARY 
       [0007]    Therefore, it is object of the present invention to provide a method allowing use of wire feed additive manufacturing without encountering induced side effects like distortion build-up in the areas which have been built up using wire feed additive manufacturing. 
         [0008]    This object it solved by the features set out in the independent claims. Beneficial embodiments and arrangements of the inventions are subject of the dependent claims. Further advantages of the invention are provided in the following description and the explanations of preferred embodiments which are presented in the drawings. 
         [0009]    The problem is solved by a method for forming a freeform metal structure by wire feed additive manufacturing, wherein a metal wire end is moved along an area of deposition and is melted by a heat source, the melting being deposited on the metal structure as a metallic build-up material, whereby the method is characterized in that during the cooling process the build-up material is subjected to at least one pulsed magnetic field effecting plastic deformation of the build-up material. The main principle used in the problem&#39;s solution is to subject the build-up material to at least one pulsed magnetic field resulting in plastic deformation of the material which has been deposited using wire feed additive manufacturing. This at least one magnetic pulse causes Lorentz forces resulting from an opposite-directed interaction of the magnetic field caused by the magnetic pulse with currents being inducted in the material leading to very high strain rates up to 10 4 . As a result of the Lorentz forces, plastic deformation of the build-up material occurs while noticeably reducing the residual stress within the deformed part of the material. A typical set-up for electromagnetic forming processes consists of the following components:
       capacitor banks as a source of electrical energy for the discharge current and the appropriate charging control units,   a power converter for supplying charging current for the capacitor banks,   a power distribution module,   a low resistance high current switch, designed for extremely short switching times and thus extremely short pulses,   a wiring system providing high current, low inductance, low resistance transport capabilities for delivering discharge current from the capacitor banks to at least one magnetic coil,   at least one magnetic coil designed to evoke the desired geometric form of the electromagnetically deformed build-up material.       
 
         [0016]    The properties of the electromagnetic forming process can be adjusted by varying the discharge parameters of the capacitor banks. The magnitude of the induced Lorentz forces and, thus, the magnitude of the plastic deformation of the material is directly affected by the magnitude of the discharge current sent to the at least one magnetic coil, whereas the strain-rate of the deformation process can be influenced by a variation excitation frequency and of the duration of the discharge current and, thus, by a variation of the magnetic pulse length. The method of electromagnetic forming exhibits a variety of advantages over conventional forming methods, the most important of which being electromagnetic forming being applicable as a contactless procedure and, therefore, causing considerably less wear out effects in the manufacturing equipment. Furthermore, it is possible to carry out the deposition process with a low cost and highly flexible robot arm since only low weight magnetic coils arranged to follow the wire feeder must be additionally mounted to a standard wire feed additive manufacturing apparatus. The magnetic coils being fabricated as low weight components ensure high flexibility in terms of geometric design of the coils leading to numerous possible shapes, the most prominent of which being flat and U-shaped. Conclusively, it is also possible to preserve the so-called grain refining effect occurring during mechanical rolling treatment of deposited build-up material to reduce material waste due to inserting high amounts of energy into the material and therefore spontaneously triggering grain growth in a plurality of locations within the material. Additionally, the grain size can be influenced by a variation of the strain rate of the deformation process using electromagnetic forming techniques. 
         [0017]    The life period of the equipment components used to relief the residual stresses is considerably increased due to the contactless procedure. Furthermore, the magnitude of the plastic deformation can be tuned by the magnitude of the discharge current pulse. And the strain rate of the deforming process can be influenced by the excitation frequency and the current discharge duration. 
         [0018]    According to a first preferred embodiment of the invention, the plastic strain of the build-up material caused by the pulsed magnetic field to be greater than 10% within the area of deposition. This condition has been derived from experimental observations exhibiting stable processing parameters within the specified range of material strain and ensuring substantial relief of distortion within the material build-up due to high plastic strain. 
         [0019]    According to a further preferred embodiment of the invention, the pulse length of the pulsed magnetic field is in a range between 1 μsec and 1 msec, depending on the materials which are subjected to plastic deformation by means of electromagnetic forming. Within the given range of magnetic pulse lengths, the strain rate can be varied to a range of 10000 to 100 1/sec resulting in a considerable reduction of residual stress within the build-up material during deformation. 
         [0020]    In a further aspect of the invention, the temperature range at which the build-up material is subjected to the at least one magnetic pulse is set to a range of 50 to 80% of the melting temperature of the build-up material. This ensures that the build-up material is still warm, but not in a liquid state when being subjected to the at least one magnetic pulse resulting in controllable deformation properties of the material along with being able to achieve the required strain rates as discussed before. Furthermore, unwanted drop outs of liquid build-up material during the deformation process are safely prevented. 
         [0021]    In addition to that, a further aspect of the invention suggests that the build-up material is subjected to a series of subsequent magnetic pulses, whereby the number of subsequent magnetic pulses may be between 2 and 10, preferably 3 and 5. By applying subsequent pulses accordingly, it is possible to achieve a higher degree of plastic deformation without sacrificing mechanical stability due to extensive mechanical strain. 
         [0022]    A beneficial variation of the invention can be obtained by subjecting the build-up material to a plurality of magnetic fields emanating from a plurality of magnetic coils arranged in various positions around the build-up material. This way, more complex geometries can be formed by simultaneously or delayed subjecting the build-up material to various magnetic fields, which can each easily be tuned in terms of pulse magnitude and pulse length to fulfill the exact demands of the geometry desired to be plastically formed. 
         [0023]    The problem is furthermore solved by an apparatus for performing the above mentioned method, including a wire feeding means for feeding wire to a melting spot where the wire is melted for forming a build-up material along the moving direction, a heating means for heating the wire at the melting spot, at least one magnetic coil means arranged to generate a magnetic field around and in said build-up material in moving direction behind the melting spot, and an electric energy supply means for supplying pulse shaped electric energy to the said coil means. The apparatus exhibiting the properties as said is equipped with means for performing methods of wire feeding additive manufacturing and electromagnetic forming, preferably with a variable arrangement of the at least one coil means in order to maximize the apparatus&#39; areas of possible applications. Preferrably, the magnetic coil means comprises a so-called pancake coil of disk shape which usually has one or two layers of spirally formed electric lines. This enables to exert forces on flat metallic substrates. 
         [0024]    Preferably, the holding structure of said apparatus is arranged onto a handling arm allowing the holding structure to be movable into at least three translatory degrees of freedom. This proposed arrangement can be achieved e.g. by mounting the holding structure onto a handling arm of a roboting device being able to integrate said apparatus into a highly automated production line to ensure constant reproducibility of the parts being manufactured and subjected to plastic deformation by said apparatus. This is made possible, because the novel means incorporate basically only a light weight magnetic coil in addition to the wire feeder and melting device. 
         [0025]    In a further extension, the apparatus&#39; coil means is suggested to exhibit a planar, preferably disc-like shape. Using that kind of geometric design, magnetic fields strongly adjusted into one direction can be created allowing exact controllability of the field strength and the pulse length. This geometric coil design is especially recommended if there is only one direction in which plastic deformation of the build-up material is desired. 
         [0026]    As a variation of the preceding invention&#39;s extent, it is proposed for the coil means to feature a U-shaped cross section with a central portion and 2 adjacent side portions which surround the build-up material from the top and both sides. With this type of coil means, magnetic fields emanating from various spatial directions can be applied to the build-up material resulting in the apparatus being able to create plastic deformation of the build-up material from various directions. This way, more geometrically complex geometries can be formed using said combination of wire feed additive manufacturing and electromagnetic forming by subjecting the build-up material to at least one magnetic pulse emanating from only one magnetic coil means with said shape. 
         [0027]    As a final extent of the invention, the apparatus is proposed to feature two coil means arranged to exert Lorentz forces from two distinct directions. This way, the apparatus is equipped to simultaneously apply independently controllable Lorentz forces within the build-up material resulting in being able to create more sophisticated and complex geometric designs by plastic deformation of the build-up material than it would be possible with just one magnetic coil means. This means, however, that each of the two magnetic coil means must be equipped with the necessary supplemental means to provide the necessary coils&#39; high current feeds via capacitor bank means being fed by an electrical supply means and being inter-connected by a wiring system being low on resistance and conductance. 
         [0028]    Further advantages, properties and distinctions of the invention derive from the following description in which, if necessary referring to the figures, at least one example is described in detail. Equal, similar and/or analog parts are marked with the equal references. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    In the drawings: 
           [0030]      FIG. 1  is a schematic diagram of an apparatus for performing a method according to the invention in a side-view perspective; and 
           [0031]      FIG. 2  is a schematic diagram of an apparatus for performing a method according to the invention in an isometric view. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows in a schematic diagram the basic principle of an apparatus for performing a method according to the invention in a side-view perspective. An electric energy supply means  10  is connected to a capacitor bank means  12  by a wire system designed to handle high currents and to be low on inductance and resistance. The capacitor bank means  12  is preferably designed to consist of a variety of single capacitor devices being lagged to maximize the resulting capacity of the capacitor bank means  12 . A typical capacitor device has a capacity of 50 up to 400 μF. The typical discharge energy is between 200 and 2000 J. The typical initial capacitor voltage is between 5 and 25 kV. 
         [0033]    While the electric energy supply means  10  is arranged to charge the capacitor bank means  12 , the capacitor bank means  12  can rapidly be discharged by activating a switch means  14  being designed to emit extremely short current pulses to at least one magnetic coil means  22  which is mounted close to a metal structure  26 . Typical pulses are in the range of 1 μsec-1 msec. 
         [0034]    The metal structure  26  is mounted onto a holding structure  28  being able to move the metal structure  26  into at least one direction. The metal structure  26  itself comprises an area of deposition  24 , where a metal wire  20  is being fed from a wire feeding means  18  and heated by a heating means  16 , both of them also being able to move into the at least one direction. The heating means  16  is preferably designed to be a laser source specified to locally generate temperatures sufficient to melt common types of metals such as iron, steel or aluminum, while the wire feeding means  18  is typically arranged to be a roll-shaped device mounted onto a supporting structure. The metal wire  20  has typically an elliptical or prismatic cross section with the semimajor of 0.25 to 4 mm. The dimension of the semiminor is typically between 0.25 and 4 mm, too. 
         [0035]    When the metal wire  20  is fed to the area of deposition  24  on the metal structure  26 , it is immediately melted by the heating means  16  resulting in growth of build-up material on the metal structure  26 . Shortly after deposition, the build-up material showing temperatures of 50 to 80% of its melting temperature in Kelvin is subjected to at least one pulsed magnetic field effecting plastic deformation caused by Lorentz forces within the build-up material as a response to the applied pulsed magnetic field and, thus, forcing the build-up material to alter shape according to a desired geometry. Preferrably, a pulse series of 2 to 10, more preferably of 3 to 5 subsequent pulses is applied. 
         [0036]    The plastic strain of the material is preferably chosen to be at least 10% within the area of deposition  24  in order to achieve substantial relief of mechanical stress and, thus, a significant reduction of distortion within the build-up material once being fed to the metal structure  26 . The at least one pulsed magnetic field preferably exhibits a pulse length of 1 μsec to 1 msec resulting in strain rates of 10000 to 100 1/s within the build-up material. These very high strain rates as compared to those of mechanical rolling (in the order of 1/s) will result in an increase in formability and a reduction in wrinkling. 
         [0037]    In a basic form of the invention, the magnetic coil means  22  is designed to be a planar-shaped, preferably disc-shaped, magnetic coil emitting pulsed magnetic fields into one preferred direction. In order to withstand high currents, the magnetic coil means&#39;  22  wires must be low on resistance. In order to be able to emit highly directed pulsed magnetic fields, the magnetic coils means  22  also is required to be low on inductance. 
         [0038]    In a further embodiment of the invention, the magnetic coil means  22  is designed to consist of two coil means arranged around the build-up material and thus being able to exhibit Lorentz forces from two distinct directions allowing the build-up material to be exhibited to spatially more complex plastic deformation. As a variation of the invention, it is also possible to design the magnetic coil means  22  to feature a U-shaped cross section with a central portion and 2 adjacent side portions, as shown in  FIG. 2  and hinted in  FIG. 1 . The magnetic coil means  22  being designed in such way so as to exert Lorentz forces onto the build-up material from three distinct directions, allowing to force the build-up material into even more complex geometric forms. 
         [0039]    In  FIG. 2 , the said apparatus to perform a method according to the invention is shown in an isometric view. In this figure, the holding structure  28  is mounted onto a handling arm  30  of a roboting device allowing the holding structure  26  to move into at least three translatory degrees of freedom and, thus, enabling the apparatus to create even more geometrically complex parts using the described combination of wire feed additive manufacturing and electromagnetic forming. 
       LIST OF REFERENCES 
       [0000]    
       
           10 —Electric energy supply means; 
           12 —Capacitor bank means; 
           14 —Switch means; 
           16 —Heat source/heating means; 
           18 —Wire feeding means; 
           20 —Metal Wire; 
           22 —Magnetic coil means; 
           24 —Area of deposition; 
           26 —Metal structure; 
           28 —Holding structure; and 
           30 —Handling arm.