Abstract:
The invention relates to an upsetting method for working a metal slug having a determined slenderness, characterized by the fact that the slug is at least partly positioned in the direction of its length, in an upsetting pot in a cylindrical housing provided for this purpose, and pressure is exerted on the slug in the direction of its length by means of a punch, until the slug fills the whole section of the housing, so as to reduce its slenderness, and to obtain a cylindrical slug with a section equal to that of the housing, in preparation for a forging operation. By this method, it is possible to upset slugs with large slenderness.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to an upsetting method for working a metal slug, a method for preparing a slug for a forging operation according to the method and a device for implementing the method.  
         [0003]     2. Discussion of the Background  
         [0004]     Forged metal parts are generally obtained by forging of slugs or billets which are raw blanks of semi-finished metal parts globally in the form of bars, used as basic elements for forming the part to be obtained by forging, their volume corresponding to the volume of the latter, increased by the lost volume during the forging. For example, in a turbojet engine, the fan disks or compressor drums are obtained by forging metal slugs.  
         [0005]     In the aeronautical field, the safety criteria are restricting and impose checks at all the manufacturing steps. The slugs must notably be checked, for example with ultrasound, in order to detect the presence of inclusions or defects in the metal, which are at the origin of cracks during forging and possibly of failures of the finished part. In order to provide a satisfactory check with ultrasound, recent standards impose a relatively small section for the slugs, in the special case of the aeronautical field, of the order of 150 to 330 mm for nickel- or titanium-based metal slugs. If the finished parts of the turbojet engine are of a large volume, the slugs should therefore be of large slenderness, to compensate their small section.  
         [0006]     These slugs, for which the length-over-section-diameter ratio, i.e., the slenderness, may initially be of the order of 12 for 1, should therefore be worked a plurality of times, in order to obtain slugs for which the slenderness in the special case is substantially equal to 3 for 1, a ratio for which it is possible to forge them, without their being contained sideways, without any risks of buckling or creation of defects in the fibers of the metal. The values given here correspond to nickel- or titanium-based metal slugs, with the rheology used for manufacturing parts of a turbojet engine. Working means hot deformation of a metal part in order to obtain an increase in its diameter and a reduction of its length, at an equal volume. Working is here obtained by upsetting, by placing the metal slug under pressure.  
         [0007]     The prior art proposes upsetting devices for working metal slugs comprising two half-shells, each including a frustro-conical shaped housing. A slug is positioned in the lower half-shell, the two half-shells being pressed against each other by a press, in order to provide upsetting of the slug which therefore assumes the shape, here with a hexagonal longitudinal section, corresponding to the housing between both half-shells. Several upsetting operations are required for obtaining the slug which may be used in the forge.  
         [0008]     As the primary slugs, i.e., the slugs as provided before the first upsetting, are of large slenderness, there is a risk of buckling during upsetting. It is therefore necessary to proceed with a large number of upsetting operations, the slenderness being only slightly reduced at each operation, in order to obtain a slug, to reduce the risk of buckling, without however canceling the latter. As the geometry of the shells is fixed, it is necessary to have as many upsetting devices as there are geometries of slugs, both from the point of view of their section and of their length; the number of required devices is therefore very large because of the different geometries and volumes of the parts of a turbojet engine on the one hand, of the necessity of carrying out a large number of upsetting operations on the other hand. Moreover, the dimensions of the devices from the prior art are large, because of the presence of two half-shells. The obtained slugs are already in the form of blanks, since they conform to the shape of both half-shells, which may be a nuisance for their forging; metal flashes formed at the contact between both half-shells should further be suppressed by machining. Finally, the heat losses during the upsetting operation are large, as the slug extends between both half-shells at a distance from one another and therefore is in contact with air.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention aims at overcoming these drawbacks.  
         [0010]     For this purpose, the invention relates to an upsetting method for working a metal slug, having a determined slenderness, characterized by the fact that the slug is at least partly positioned in the direction of its length in an upsetting pot, into a cylindrical housing provided for this purpose, and pressure is exerted on the slug in the direction of its length by means of a punch, until the slug fills the whole section of the housing, in order to reduce its slenderness and to obtain a cylindrical slug with a section equal to that of the housing, in preparation for a forging operation.  
         [0011]     By means of the invention, the applicant has observed that it was possible to obtain a working rate, i.e., the ratio of the length of the slug before working, over the length of the slug after working, larger than 30%, the worked slug exhibiting neither buckling nor fiber anomalies. It is thereby possible to reduce the number of upsetting operations. Moreover, the obtained slug is of a cylindrical shape, easier to be forged into any shape subsequently. As the punch may be of smaller dimensions than those of a half-shell of the prior art, the bulkiness of the device may be reduced. As the slug at least partially extends into the cylindrical housing, heat losses are reduced and the flashes requiring machining are eliminated, if the slug wholly extends in its housing at the end of upsetting. Further, the method may be arranged so that the upsetting operation is stopped when a certain stress of the punch is attained, which provides better control over the operation as compared with the prior art where the operation was stopped when both half-shells were in contact, independently of the slug which they contained. Moreover, in such an upsetting method, which is performed in a closed volume (i.e., only air but not metal may escape from the enclosure formed by the housing and the punch), at the end of the method a slug with a cylindrical shape is obtained, the fibers of which are all substantially parallel to the axis of the cylinder; this feature is advantageous for aeronautical applications.  
         [0012]     Advantageously, the housing has a diameter up to 1.35 times the diameter of the section of the slug.  
         [0013]     The invention also relates to a method for preparing a slug, for which the initial slenderness is larger than 12 for 1, for a forging operation, wherein the slug is upset a plurality of times according to the method shown below, until a slenderness substantially equal to 3 for 1 is obtained.  
         [0014]     Finally the invention relates to a device for applying the above method, characterized by the fact that it includes an upsetting pot, with a cylindrical housing for receiving the slug, and a punch for putting the slug under pressure.  
         [0015]     Advantageously, the punch is driven by a press table.  
         [0016]     Preferably, the depth of the housing is adjustable according to the dimensions of the slug.  
         [0017]     In this case, it is possible to only use one device for slugs of different slendernesses, which reduces the number of required devices and therefore production costs and bulkiness. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The invention will be better understood with the help of the following description of the preferred embodiment of the invention, with reference to the single appended FIGURE, which illustrates a schematic sectional profile view of the device of the invention, with the punch in the high position, for the left half of the FIGURE, and the punch in the low position for the right half of the FIGURE. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The upsetting device  1  includes an upsetting pot  2 , in this special case with a cylindrical shape, in steel. The pot  2  includes a housing  3 , or sheath  3 , with a cylindrical shape, central here relatively to the section of the pot  2 , and which includes a bottom  4 . The pot  2  is positioned on a supporting table  5 , also in steel, which includes a flange  6  for centering the pot  2 . The table  5  is supported by a lower press table  7 , fixed, in steel, which includes a flange  8  for centering the table  5  supporting the pot  2 .  
         [0020]     Here, the bottom  4  of the housing  3  is pierced with a bore  4 ′ for letting through a cylinder  9 , which also extends through bores  5 ′,  7 ′ provided in the table  5  supporting the pot  2  and the lower press table  7 , respectively. During an upsetting operation, the upper surface of the cylinder  9 , perfectly fitted into the bore  4 ′ of the bottom  4 , may here provide the function of bottom of the housing  3 .  
         [0021]     The upsetting device  1  includes, above the pot  2 , a punch  10 , with a cylindrical shape, supported at its upper end by a punch plate  11 , itself permanently attached to an upper press table  12 , which includes a flange  13  for centering the punch plate  11 . These elements are in steel here. The upper press table  12  is mobile translationally and vertically, along the axis of the punch  10 . The section of the punch  10  corresponds to the section of the housing  3  of the pot  2 .  
         [0022]     The housing  3  is arranged in order to receive a metal slug  14 . It is arranged in order to be able to also receive, on its bottom  4 , in the special case, on the upper surface of the cylinder  9 , blocks  15 ,  16 , in steel here. They are two in number in the FIGURE, and positioned over each other. These blocks  15 ,  16 , the section of which corresponds to the section of the housing  3  of the pot  2 , allow the depth of the housing  3  to be adjusted according to the length of the slug  14  to be upset. It is therefore the upper surface of the upper block  16  which acts as bottom for the slug  14 .  
         [0023]     Here, the bottom of the housing  3 , whether it is the upper surface of the cylinder  9  or the upper surface of a block  16 , includes an impression  17  for centering and pre-forming the slug  14 . In this special case, this impression  17  includes a small flange at its circumference, forming a shoulder, the distance of which to the axis of the cylinder, formed by the housing  3 , is equal to the radius of the slug  14  before working. Thus, when the slug  14  is placed into the housing  3 , it is centred by the shoulder of the impression  17 . Moreover, this impression  17  may be arranged so as to pre-form the end of the slug  14  according to the shape which will be given to the final part by forging the slug  14 , as soon as the upsetting operations are completed.  
         [0024]     The lower surface of the punch  10 , intended to come into contact with the upper surface of the slug  14 , for pressing it and providing its upsetting, also includes an impression  18  for centering and pre-forming the slug  14 , conformed according to the same criteria as the impression  17  of the bottom of the housing  3 .  
         [0025]     An upsetting operation or method for working the slug  14  will now be explained in more details.  
         [0026]     The slug  14  may be a primary slug, which has not yet been upset, or a slug having already undergone one or more upsetting operations. It has a section of a certain diameter, preferably between 150 and 500 mm here. The upsetting device  1  is selected according to the diameter of the slug  14 ; the diameter of the housing  3  of the device  1  is larger than the diameter of the slug  14 , preferably up to 1.35 times the diameter of the slug  14 , in this case, equal to 1.3 times the diameter of the slug  14 . According to the length of the slug  14 , blocks  15 ,  16  may be placed beforehand on the bottom of the housing  3 , in order to adjust the depth.  
         [0027]     The slug  14  is heated beforehand, for example between 985 and 1100° C. if it is nickel-based metal, between 920 and 950° C. if it is titanium-based metal. It is put under standard rheological conditions for upsetting. The pot  2 , in steel, as for it, is also heated beforehand, for example between 400 and 500° C., so that the steel is not located in its brittle strength range. The pot  2  was massively dimensioned, in this special case, more massive than required by mechanical criteria, so that heat is better retained.  
         [0028]     The slug  14  is positioned in the housing  3 , wherein it is centred by the impression  17  of the upper block  16 . In order to provide optimum centering of the slug  14  and to avoid that its walls touch those of the housing  3 , which might subsequently generate forging defects, an operator or a gripper of a machine, may also maintain the slug  14  centred, by its sidewalls, near its upper end. The slug  14  may be coated by vitrification, with an enamel layer, with which lubrication of the device  1  may be provided. This lubrication may be obtained by greasing.  
         [0029]     The upper press table  12 , initially in the high position as on the left half of the FIGURE, is then lowered, by a standard press hydraulic mechanism, driving the punch  10  towards the upper end of the slug  14 . Upsetting then takes place, the punch  10  exerting, by action of the upper press table  12 , pressure on the slug  14 , which is upset, as it is contained in this special case initially for about three quarters of its height, in the housing  3 . The punch  10  is lowered here, during the upsetting operation, at a rate from 10 to 20 mm/sec. Upsetting of the slug  14  is expressed by a reduction of its length and an increase in its section.  
         [0030]     The upsetting operation is stopped when a certain stress is reached on the slug  14 . The slug  14  then substantially fills the whole section of the housing, its section having increased by 30% here, its length having decreased accordingly, as there is no change in volume. In this situation, the punch is the low position, as is seen on the right half of the FIGURE. Slug  14  has actually been worked by upsetting.  
         [0031]     The punch  10  is then moved back up out of the housing  3 . The cylinder  9 , driven by an ad hoc device, is actuated upwards and pushes back the blocks  15 ,  16  and the slug  14  in this direction, in order to have the slug  14  emerge from the housing  3 . The slug  14  may therefore be removed and, either again upset, or, if the ratio of its length over its diameter has reached an acceptable value, in this special case 3 for 1, forged in order to manufacture the final part, a fan disc or a compressor drum of a turbojet engine here.  
         [0032]     The cylinder  9  may be again lowered. The blocks  15 ,  16  may be removed or replaced by lifting the pot  2 , which has the effect of freeing them and making them accessible.  
         [0033]     Thus, it is possible to prepare a slug, for which the initial slenderness is larger than 12 for 1, for a forging operation, by upsetting the slug a plurality of times according to the method which has just been presented, until slenderness is obtained substantially equal to 3 for 1.  
         [0034]     By means of the upsetting device  1  of the invention, it is possible to upset a slug  14  with large slenderness, typically with a ratio of more than 12 for 1, without any risk of buckling, as the slug  14  is either entirely or partially contained, in the cylindrical housing  3 . Moreover, a cylindrical worked slug  14  is obtained at the end of the method, easy to forge according to any shape, the forging may be facilitated by pre-forming the ends of the slug  14  by the impressions  17 ,  18  of the bottom of the housing  3  and of the lower surface of the punch  10 .  
         [0035]     Because of retention of the slug  14  in the housing  3 , the heat losses at its periphery are low, which improves the efficiency of the upsetting operation. Moreover, in the case of an incident generating a loss of time during the upsetting operation, it is not inevitably necessary to put the slug  14  and the pot  2  back into an oven to heat them up again, as the heat losses are low. In order to further reduce the heat losses, a heating device, for example resistors embedded in steel, may be provided in the walls of the housing  3 , providing a constant and/or adjustable temperature of the housing  3 .  
         [0036]     By the possibility of putting blocks  15 ,  16  in the bottom of the housing  3 , a same upsetting pot  3  may provide upsetting operations for slugs  14  of different lengths, which reduces the number of upsetting devices  1  required in a factory for producing metal parts obtained by forging slugs  14 .