Abstract:
A method for producing metal sheets from strand-shaped profiles having a low thickness made of magnesium or magnesium alloys, wherein an open or a closed extruded profile is produced in a preceding method step, wherein the extruded profile exiting the extrusion die of an extrusion press is shaped to form a planar metal sheet by the contactless action of electromagnetic forces.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method, and to a system or apparatus for producing metal sheets from strand-shaped profiles having a low thickness (i.e., which are thin) which are produced in particular from magnesium or magnesium alloys, by way of extrusion. 
         [0002]    It is generally known to produce metal sheets by way of rolling in conventional roll stands, wherein corresponding material blocks are reworked to form metal sheets by rolling operations so as to produce metal sheets. Due to the large number of reduction passes from the heavy plate to the thin sheet, this method is very cost-intensive. However, processing magnesium blocks to form corresponding metal sheets is very complex, and the inherent brittleness of magnesium often results in cracking. The high number of reduction passes also adversely affects the structural conditions, and thus the mechanical properties. 
         [0003]    DE 101 50 021 B4 discloses a method and a device for producing profiles or sheet metal parts from magnesium or magnesium alloys by way of forming under compressive conditions using extrusion, rolling, forging or casting, wherein the liquid melt is introduced into a continuous casting or extrusion unit to produce a semi-finished product, and immediately thereafter this semi-finished product is given the net shape thereof by way of forming in the warm state, wherein the temperature of the material after solidification from the melt is maintained in a range of 250° C. to 350° during the entire manufacturing operation, and the manufacturing process from casting to cooling of the formed parts is carried out as a whole in an inert atmosphere or under vacuum. 
         [0004]    The device for carrying out these method steps is characterized in that the system is composed of a chain of a melting furnace, a continuous casting or extrusion unit, with or without roll stand, a cutting unit, one or more presses, and a cooling unit, the collectivity or parts of the aforementioned units being disposed in a protective gas chamber or vacuum chamber. 
         [0005]    Moreover a method for producing formed sheet metal parts from magnesium is described in DE 103 17 080 B4, in which a formed sheet metal part can be produced immediately following a rolling process by way of forming using at least one press in a temperature range of &gt;350° C. to 450° C. The device described for carrying out this method, which is composed of a chain of a melting furnace or crucible, a continuous casting unit, one or more roll stands, a cutting unit, one or more presses, and a cooling unit and is operated in a protective gas chamber or vacuum chamber, is characterized in that a stamping unit, which can be used to introduce dimensionally and cross-sectionally stable stamped holes, and/or formed holes, into blanks coming from the cutting device, is provided between the cutting unit and the press designed as a forming press. 
         [0006]    DE 102 47 129 A1 describes another method for producing profiles or formed sheet metal parts from magnesium or magnesium alloys, in which a semi-finished product in the form of a metal sheet is given the net shape thereof by way of forming, preferably by way of compression molding, wherein the surface is freed from impurities in a method step immediately prior to the forming operation by way of chip removing, and preferably by way of shaving. 
         [0007]    The disadvantage that remains with this method is that it is only possible to produce parts having a limited width, since larger parts require significantly more effort for the working pressures that are to be controlled. The tool and the machine frame must withstand the extrusion pressure that is present during the manufacture of the semi-finished products or the parts, together with a corresponding counter-pressure, and therefore must necessarily be dimensioned considerably larger. 
         [0008]    DE 43 33 500 C2 discloses a method for producing a metal sheet that is stepped in the cross-section and has a solid profile and different wall thicknesses, in which first a semi-finished product is produced, the cross-section of which is similar to the cross-section of the metal sheet in the thickness direction, and in which the semi-finished product is roiled to obtain a metal sheet, wherein, for the production of the semi-finished product, a hollow profile having a wall thickness progression that is distributed over the periphery and corresponds to the desired wall thickness progression of the semi-finished product is extruded, and the hollow profile is severed along a peripheral surface line and formed to obtain the semi-finished product. In addition, two complementary profiles are laid one upon the other, wherein at least one of the profiled contact sides of the complementary profiles is provided with a parting agent, and the two complementary profiles are rolled out simultaneously using cylindrical, which is to say non-stepped, rolls. Prior to rolling, the two complementary profiles are severed on two opposing peripheral surface lines. 
         [0009]    This method is used to produce two parts, respectively. The manufacturing process is discontinuous, and only relatively narrow parts can be produced. Other disadvantages are that only relatively narrow parts can be fabricated and the fact that the manufacturing process is relatively complex due to the manufacture of the semi-finished product that is implemented with two different wall thicknesses and a stepped roll system. 
         [0010]    A method for producing formed sheet metal parts and a device for carrying out the method are known from DE 10 2008 048 A1. The method comprises the steps of—extruding or continuously casting a tubular body,—cutting open the tubular body in the longitudinal direction of the same,—expanding the tubular body to form a planar body, and—finishing the planar body to obtain a component in correspondence with the drawing by way of manufacturing technologies that are known per se. The device is essentially composed of a chain of a melting unit, a continuous casting or extrusion unit, a longitudinal cutting unit, a roll stand, and one or more forming units. 
         [0011]    A method for producing metal sheets or sheet metal parts from lightweight metal, preferable magnesium, is known from DE 10 2007 002 322 A1, wherein in one or more preceding method steps an extruded profile having an open structure, or a closed structure with subsequent cutting to form an open structure, is produced, and the same is subsequently subjected in one or more steps to a roller straightening process and a roller bending process across multiple rolling and bending stages. 
       SUMMARY OF THE INVENTION 
       [0012]    It is the object of the invention to provide a method and a system or apparatus for producing metal sheets from strand-shaped profiles have a low thickness, in particular from magnesium or magnesium alloys, wherein the open or closed extruded profiles exiting an extrusion die can be continuously formed to obtain a planar metal sheet. 
         [0013]    According to the invention, this object is achieved by shaping the extruded profile exiting the extrusion die of an extrusion press to form a planar metal sheet by the contactless action of electromagnetic forces and then subjecting the metal sheet to a smoothing process using a rolling or sizing unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]    The drawing shows a representative illustration of a system for producing strand-shaped profiles by way of an extrusion press which are subsequently formed to obtain a planar metal sheet. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The system is essentially composed of an extrusion press  1  for producing an extruded profile  8 , a forming unit composed of a work coil  2  and a counter bearing  3 , and a sizing unit  4 . 
         [0016]    Using the extrusion press  1 , a round billet is formed from a magnesium alloy to obtain an extruded “profile”  8 , i.e., an elongated strand having a “profiled” widthwise cross-section, i.e., a cross-section which includes curvature, for example a profile  6  having three sinusoidal arcs in the cross-section. Thereafter, the profile  6  is positioned above the work coil  2  and formed under the action of a force of a pulsed magnetic field having very high intensity, wherein the profile  6  is formed against the counter bearing  3  to obtain a planar metal sheet  7 . 
         [0017]    A magnetic field that changes over time induces eddy currents in the electrically conducting profile  8 . The magnetic field exerts forces on these currents. The intensity of the forces is dependent on the spatial gradients of the magnetic flux density and the magnitude of the induced currents. The profile  8  is subjected to forces directed toward a lower flow density. The magnetic fields necessary for forming the profiles  8  to obtain planar metal sheets  7  are generated by discharging charged capacitors over the course of a few microseconds via a coil that is adapted to the profile geometry. This creates a very high magnetic pulse on the profile surface, as a result of which a current flows in the profile  8 , which is directed against the coil current, wherein the profile  8  is moved in a predetermined direction toward lower fields i.e., against a planar surface of a body acting as a counter bearing, the planar surface being substantially orthogonal to the aforementioned predetermined direction. 
         [0018]    The intensity of the induced currents and the attendant action of a force on the profile  8  depend on the electrical conductivity. Since magnesium or the magnesium alloy have relatively good electrical conductivity, high pressures act on the surface of the profile  8 . These can amount to several thousand megapascals. This pressure only acts over a short time period, this being in the range of a few microseconds, for the duration of the discharge of the capacitors. During this time, the profile  6  takes up the required forming energy in the form of pulses. After an acceleration phase, the material of the profile  8  moves very quickly, due to the low mass thereof. It is possible for speeds of up to 300 m/s to be achieved. The stresses occurring in the profile  6  become so high that yielding occurs, within the meaning of metal forming technology, and the profile  6  is formed to obtain a planar metal sheet  7 . 
         [0019]    Afterwards the metal sheet  7  passes through a sizing unit for smoothing and is wound to form a coil  5 . Alternatively, it is possible to replace the winding to form the coil  5 , with a stamping or cutting unit, with the aid of which components are stamped from the metal sheet  7  coming from the sizing unit, or the metal sheet  7  is cut into panels or strips. 
         [0020]    So as to form closed profiles  8  to obtain a metal sheet, these are either provided, by the configuration of the die, for example, with a predetermined breaking point along the peripheral surface line during production of the profile, or severed along the peripheral surface line using a cutting unit. Profiles  6  comprising an introduced predetermined breaking point are severed by the action of magnetic forces and formed to obtain a metal sheet  7 . 
         [0021]    Particular advantages of the method according to the invention are that the magnetic fields and the magnetic forces act unimpaired by the material, whereby the magnetoforming process can also be employed under vacuum or in a protective gas atmosphere, and additionally that magnetoforming systems do not require any mechanical contact with the workpiece, whereby surface contamination and tool impressions are avoided. The short process times for the forming operation to obtain the metal sheet  7  are also advantageous, being less than 0.1 s.