Abstract:
A process for forming metal components of controlled porosity by welding, in which a predetermined amount of metal elements ( 50 ) of oblong shape is introduced into a mold ( 10 ) in which it is distributed isotropically. The metal elements are then subjected to increasing pressure (P) until the component has its final shape. The walls of the mold are then held in position and an electric current (I) flows through the metal elements and welds them together by local melting at the points of contact due to the Joule effect.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the production of components by welding. The invention relates more particularly to a process for welding metal fiber by capacitor discharge in order to produce components of required shape. 
     DESCRIPTION OF THE RELATED ART 
     Existing capacitor discharge welding processes are known (GB 1 508 350). These processes consist in passing a current, generally obtained by discharging a capacitor, through particles of metallic material so as to weld them together. These processes have been applied to powders of spherical particles (P. A. Vityaz et al, “Contact formation during the electric pulse sintering of a titanium alloy powder”, Belorussian Republican Powder Metallurgy Scientific Production Association, translated by Poroshkovaya Metallurgiya, No. 7 (331), pages 20-23, July 1990) or of elongate particles such as fibers (S. T. S. Al Hassani et al, “Preforming using high-voltage electrical discharge”, Powder Metallurgy, No. 1, page 45, 1980). 
     These processes have sometimes also been combined with the application of pressure (R. W. Boesel et al, “Spark sintering tames exotic P/M materials”, Materials Engineering, page 32, October 1969), so as to facilitate the welding and eliminate as far as possible the porosity of the components thus welded. These components are compact and their level of porosity is close to 0 (if Vm is the volume of material and Vc is the volume of the finished component, the degree of porosity τ is defined as τ=1 −(Vm/Vc)). 
     SUMMARY OF THE INVENTION 
     In contrast, the disclosed invention is aimed at the manufacture of porous components. These components may, for example, be supports for an active material, such as the fibrous structures for catalytic converters. 
     It is necessary for these components to have a very high level of porosity, allied with excellent mechanical strength over a wide temperature range. 
     The desired levels of porosity start from 0.60 and typically are in the region of 0.95. The level varies according to the shape and the function of the components to be produced. 
     Finally, the manufacture of these components must also be controlled so as to achieve good reproducibility with precise dimensions. 
     This type of application therefore produces specific problems to which the process of the invention and the apparatus for implementing it prove a solution. 
     The invention is a process for forming metal components of controlled porosity by welding, comprising the known successive steps consisting of: 
     preparing a predetermined amount of metal elements of anistropic geometrical shape, intended for constituting a component; 
     distributing this predetermined amount of metal elements in a mold having at least one movable part; 
     exerting pressure using a movable part of the mold controlled by an external means, which movable part possibly constituting an electrode, in at least one main direction on this predetermined amount of metal elements, said pressure being intended to reinforce and maintain the points of contact between these elements; 
     simultaneously passing an electric current through this predetermined amount of metal elements via a set of two electrodes of opposite polarity in order to join these metal elements together by welding, said two electrodes being placed so that the direction of flow of the current is overall coaxial with said main direction of the pressure exerted on the predetermined amount of metal elements; and 
     removing the component from the mold. 
     The expression “elements of anisotropic geometrical shape” is understood to mean articles having at least one of the three dimensions significantly different from the other or others. 
     The process of the invention is characterized in that: 
     the predetermined amount of metal elements is obtained by weighing a mass of metal elements whose value M is defined as a function of the desired degree of porosity τ, the volume of the component Vc and the density of the metal alloy used ρa by the formula: 
     
       
           M=VC ρa (1 −τ) 
       
     
     the predetermined amount of metal elements is distributed isotropically in the mold; 
     the pressure exerted is progressively increased until the component has the required shape, thus giving the component the desired level of porosity; and 
     the movable part of the mold is then held in position and, simultaneously, the electric current flows through the metal elements and welds them together by local melting at the points of contact due to the Joule effect or by creation of a local arc. 
     The expression “local melting at the points of contact” is understood to mean melting relating to only part of each of the cross sections in three dimensions of the metal elements. This melting is such that, on the one hand, the mechanical strength of each metal element in question, although momentarily reduced, remains sufficient for all of these elements to retain the shape acquired during the previous step, thus retaining the isotropic distribution in the mold, and, on the other hand, the mechanical strength of the component is optimal for the use. 
     The elements of anisotropic geometrical shape of the invention preferably have one dimension significantly different from the other two. They are therefore generally oblong and advantageously are in the form of needles, flakes or nonwoven fibers. 
     It is very desirable for easy implementation of the process for the elements to distribute themselves spontaneously in an isotropic manner in the mold. Elements of approximately cubic or spherical shape for example distribute themselves spontaneously in an isotropic manner in a mold. However, these elements are not anisotropic. Their use in the process of the invention does not allow the desired level of porosity to be achieved (τmax =0.5 in the case of tubes and 0.48 in the case of spheres). 
     However, elements having both an anisotropic geometrical shape and the ability to distribute themselves spontaneously in an isotropic manner in a mold do exist. Such elements are obtained in particular by the technique of casting on a wheel. In fact, the elements produced using this technique have, among other characteristics, the particular feature of having surface asperities, mainly on the edges parallel to the significantly different dimension. These asperities prevent the elements from sliding against one another and thus prevent them from being distributed anisotropically under the effect of gravity. 
     Thus, they distribute themselves isotropically in the mold without any further manipulation such as, for example, shaking them. The level of porosity spontaneously obtained may be up to 0.99, which value may be greater than that of the desired level of porosity. To maintain the isotropic character of the fiber distribution in the component, the spontaneous level of porosity must remain close to that desired. For this purpose, the metal elements may be ground or chopped beforehand so as to size them according to the significantly different dimension with a suitable value. 
     It is therefore by applying pressure to the metal elements by means of a movable part of the mold that the required shape is given to the component and, likewise, the desired level of porosity. The applied pressure progressively increases up to the necessary value so that the movable part of the mold reaches the position corresponding to the required shape. There is therefore a balance between the force exerted by the external means and the elastic reaction force from the compressed elements. 
     The movable part is then held in position. It should be understood by this that the movable part of the mold can no longer change position, even if the reaction force exerted by the compressed elements suddenly varies. 
     This is because, when the electric current flows through the metal elements, the local melting causes the force exerted by these elements on the movable part of the mold to suddenly be decreased. If the force from the external means is kept constant and if the movable part is left free in position, this results in strong compression and deformation of the component owing to the imbalance in the forces. 
     With the movable part held in position, it is necessary to deliver an electric current flowing through the metal elements such that it allows local melting, as defined above, but without causing complete melting at the contact points, this melting being defined as melting over the entire cross section of the metal elements. This is because, if the current delivered is too high, complete melting of many elements takes place and, by gravity, this results in deformation of the component. 
     The electric current thus controlled is advantageously delivered by an electrical generator using a capacitor of capacitance C, which constitutes an economic, simple and well-suited means for this type of application. 
     The apparatus according to the invention comprises a set of electrodes, at least one of which is fastened to a movable wall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The process according to the invention will be more clearly understood from the detailed, but non-limiting, description of several methods of implementing it and with the aid of the references to the appended drawings in which: 
     FIG. 1 shows schematically a sectional view of an apparatus having one movable wall according to the invention, implementing the process; 
     FIG. 2 shows schematically a sectional view of another apparatus, having two movable walls, with the component having the required shape; 
     FIG. 3 is a diagram showing the mechanical strength of a particular component obtained by implementing the present invention as a function of the electrical energy dissipated to form this component. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus in FIG. 1 allows the process according to the invention to be implemented. It comprises a mold  10  and an electrical circuit  20 . 
     The mold  10  consists of fixed walls  12  and a movable wall  14 . The fixed walls together form a space open at one end, a predetermined amount of metal elements  50 , for example fibers, being placed inside said space. 
     The movable wall  14  closes this space, holding the metal fibers  50 , but can slide parallel to itself in the closed space by an external means (not shown) so as to be able to apply to the fibers the pressure P needed to obtain the desired level of porosity. When this level is reached, the component has the required shape and the movable wall is then stopped. The external means employed may, for example, be an actuator servocontrolled in terms of force and then of position. 
     The electric circuit  20  comprises a switch  28 , a capacitor  30  and a set of electrodes  22 ,  24 , assumed to have no thickness. There are complementary means for controlling the intensity of the current I and the circuit for charging the capacitor, which defines the voltage V across the terminals of the capacitor, but these have not been shown. 
     Each of the opposed movable wall  14  and fixed wall  12  is equipped with an electrode,  24  and  22  respectively, which is connected to one of the terminals of the capacitor  30 , one of which is connected via the switch  28 . 
     A component is produced with fibers, obtained by a process for casting them on a wheel, in the following manner. 
     The required component has the shape of a cylinder with a circular base 7.5 cm in diameter, a height of 10 cm and a level of porosity of 0.95. The metal alloy used has a density of 7.1 g/cm3. 
     The fibers have a crescent-shaped cross section falling within an approximately 100 μm by 500 μm rectangle and have a length of about 5 cm. 
     The predetermined amount of fibers has a mass M=0.157 kg. The mold  10  has a fixed wall  12  consisting of an end wall supporting a circular electrode, having an inside diameter of 7.5 cm, and a cylindrical shell, having an inside diameter also of 7.5 cm and a length of more than 10 cm. The amount of fibers is introduced into the mold  10 . The fibers distribute themselves spontaneously in an isotropic manner in the mold, with a level of porosity greater than 0.95. The movable wall  14 , supporting a circular electrode  24  having a diameter very close to 7.5 cm, is then introduced into the cylindrical shell and, under the action of the external means, compresses the fibers until the distance between the movable wall  14  and the opposite fixed wall  12  becomes 10 cm. The movable wall  14  is then held in this position. The component has the required shape and the desired level of porosity. The switch  28  is then closed, causing the electric current to flow through the fibers  50 . The capacitor, precharged by a voltage of 19 kV, has a capacitance of 106 μF. The energy thus used for the welding is 20 kJ. The mold is then opened by retracting the movable wall  14  and the component is removed from the mold. 
     None of these operations requires preheating the fibers to a particular temperature or the presence of a particular gaseous atmosphere, although, in a known manner, the presence of an inert gas such as argon is favorable. The process is therefore simple and rapidly implemented, the time needed to recharge the capacitor being carried out in parallel with the step of removing the component, the step of distributing the fibers in the mold and the compression step. 
     FIG. 2 shows an alternative embodiment in which the electrodes are supported by two opposed movable walls  14 . The main benefit of this apparatus resides in the easier handling of the component  100  after welding. Each movable wall  14  closes one end of the open space bounded by the fixed wall  12 , holding the metal fibers  50  in place, but can slide parallel to itself in the closed space by an external means (not shown), so as to be able to apply to the fibers the pressure P needed to obtain the desired level of porosity. The external means used for each movable wall may, for example, be an actuator servocontrolled in terms of force and then of position. 
     The preferred method of implementing the process according to the invention may be optimized by taking into account the results described below. In this part of the description, the parameter used is expressed as energy per unit area (kJ/cm2). The area involved is the cross section of the component in a plane perpendicular to the direction of flow of the current. For a given apparatus, this parameter is a function of the current employed on discharging the capacitor, even if some of the energy delivered is consumed outside the component to be welded. 
     It has been shown that, in order to obtain components  100  consisting of metal fibers whose porosity is approximately 95%, it is necessary to dissipate a minimum energy of 0.1 kJ/cm2. Below this value, there is insufficient welding together of the fibers. 
     This result was obtained by subjecting control fibrous components  100  (average diameter 23 mm) to discharges from the capacitor  30  of increasing energy. Measurement of the quality of the weld, and therefore of the mechanical strength of the components  100 , was carried out by tensile tests. Attached heads made of curable resin were fitted onto each end of these components  100  in order to allow them to be gripped in the jaws. FIG. 3 shows the variation in mechanical strength in daN as a function of the energy per unit area (kJ/cm2). It may be seen that the mechanical strength increases with increasing energy per unit area, but tends to flatten out above 0.1 kJ/cm2. Experiments have shown that above 0.5 kJ/cm2, for a porosity of about 95%, there is excessive melting of the fibers resulting in excess energy. 
     Moreover, the energy E stored in a capacitor  30  is given by the expression E=½ CV2, where E is in joules, the capacitance C of the capacitor is in farads and the voltage V applied to the capacitor  30  is in volts. A given energy level may therefore be obtained by varying the capacitance or the voltage. 
     Fibrous components  100  (diameter 75 mm, length 100 mm, cross section 44 cm2, porosity 95%) were welded with a constant energy of 20 kJ (0.45 kJ/cm2) for two capacitances, 74 μF (23 kV) and 106 μF (19 kV). Measurement of the quality of the weld, and therefore of the mechanical strength of the components, was carried out, as previously, by tensile tests. 
     The results are given in table I below, in which it may be seen that the maximum force, expressed in daN, is obtained for the higher capacitance (106 μF), and therefore the lower voltage (19 kV). Three tests were carried out per condition. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE I 
               
             
             
               
                   
                   
               
               
                   
                 Capacitance 
                   
               
             
          
           
               
                   
                 E = 20 kJ 
                 74 μF 
                 106 μF 
               
               
                   
                   
               
               
                   
                 Voltage (kV) 
                 23 
                 19 
               
               
                   
                 Maximum 
                 24 
                 52 
               
               
                   
                 tensile force 
                 29 
                 57 
               
               
                   
                 (daN) 
                 25 
                 58 
               
               
                   
                   
               
             
          
         
       
     
     For the 106 μF capacitance, the increase in the energy stored in the capacitor  30 , and therefore dissipated in the components  100  upon discharge, increases up to 70 kJ (36 kV, 1.6 kJ/cm2). 
     The degree of melting of the fibers  50  was seen to increase, becoming very significant at 70 kJ and, to some extent, impairing the initial fibrous structure. The tensile tests on the components  100  obtained (table II below) no longer show an increase in the mechanical strength. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
             
             
               
                   
                   
               
               
                   
                 Energy (kJ) 
                   
               
             
          
           
               
                   
                 20 
                 50 
                 60 
                 70 
               
               
                   
                   
               
             
          
           
               
                   
                 Energy per unit area (kJ/cm2) 
                 0.45 
                 1.14 
                 1.36 
                 1.59 
               
               
                   
                 Voltage (kV) 
                 19 
                 31 
                 34 
                 36 
               
               
                   
                 Maximum 
                 52 
                 42 
                 44 
               
               
                   
                 tensile 
                 57 
                 48 
                 49 
                 46 
               
               
                   
                 force (daN) 
                 58 
                 57 
                 59 
               
               
                   
                   
               
             
          
         
       
     
     These results show that too high an energy causes excessive melting of the fibers  50  at their points of contact. This excessive melting occurs over a large part of the cross section of the fiber at the point of contact. Such melting provides the treated component with sufficient integrity so that it does not deform under gravity, however the strength of the component decreases. 
     During the tests carried out to obtain these results, electric arcs were observed between the electrodes when the voltage was raised in order to increase the energy stored in the capacitor. These electric arcs do not contribute to welding the fibers  50  together. This welding is in fact carried out by the flow of a current I through the metal fibers  50  with the points of contact melting simply owing to the Joule effect or by creation of a local arc. Consequently, the available energy is distributed between energy useful for the welding and energy lost by direct discharge in the gas between the electrodes  22 ,  24 . 
     For an industrial machine, it is therefore preferable to have a capacitor of high capacitance, charged using a moderate voltage, so as to prevent the loss of energy by direct discharge in the gas between the electrodes  22 ,  24 . In addition, this is in the direction of greater safety in an industrial environment in which high voltages are not desirable. 
     The components obtained by this process may be of varied shape, for example they may be parallelepipeds. 
     These varied shapes may require the use of several pairs of electrodes of opposite polarity, supported by the opposing walls of the mold, whether they be fixed or movable. 
     If the porosity of the components  100  is lower (for example 80%), the points of contact are more numerous and the energy needed to produce the welds is higher and may reach several kJ/cm2. 
     The above description mentions only one discharge capacitor  30 . However, it is obvious to a person skilled in the art that a bank of several capacitors  30  may be used to implement the process according to the invention.