Abstract:
A cold drawing process for forming titanium blanks into titanium pieces in which the titanium blank is pre-treated with a wet lubricant prior to drawing. The wet lubricant lubricates the interfaces of the titanium blank and the punch to substantially reduce galling and hardening of the surface of the titanium blank from the drawing process, thereby eliminating the need to anneal the formed titanium pieces prior to additional drawing, stamping, punching or other forming processes.

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to a drawing process for forming titanium pieces. Specifically, the present invention is directed to a drawing process in which roughened titanium blanks are pre-treated with a wet lubricant to minimize galling of the titanium during drawing. 
     BACKGROUND OF THE INVENTION 
     Titanium and titanium alloys are often used in casings for implantable medical devices. The high strength to weight ratio of titanium provides a lightweight, yet structurally strong casing. Similarly, the corrosion resistant nature of titanium allows titanium casings to survive the corrosive fluids within the body. Titanium can undergo a passivation process to further improve the corrosion resistance of the titanium. An added advantage is that titanium is non-toxic and biocompatible reducing the likelihood that the patient will suffer complications from the implantation resulting from the casing itself. While titanium has many material characteristics that are advantageous for implantable medical devices, the material characteristics of titanium also make forming titanium into the appropriate shape difficult. 
     The high melting point of titanium makes melting or heating titanium for molding or hot forming titanium into the appropriate shapes impractical for high volume manufacturing. Accordingly, titanium casings are typically made in a cold drawing process where a generally planar blank is shaped into the appropriate cup shape by mechanically deforming a titanium blank. In the drawing process, a recessed die is positioned beneath the blank and a punch is pushed against the blank deforming the center portion of the blank into the shape defined by the recess of the die. The punch typically comprises a metal element that is hydraulically pressed with substantial force against the titanium blank. An inherent drawback of the drawing process is that the high pressure metal on metal contact between the titanium blank and the punch typically results in adhesive wear, or galling, of the surface of the drawn titanium piece. In particular, the deformed edge portions of the drawn titanium piece are particularly susceptible to galling. Galling can weaken the titanium pieces, providing an uneven titanium surface and form areas where corrosion can begin. The drawing process can also harden the titanium piece, making the deformed edge portions brittle and at risk for fracturing or tearing during subsequent drawing or punching processes. As a result, drawn titanium pieces are typically heated with an annealer to smooth the surface of the titanium pieces and soften the titanium prior to additional drawing, punching, or other forming of the titanium pieces. 
     During the annealing process, the titanium pieces must typically be heated to over 1600° F. to induce a material change in the titanium smoothing the galling and softening the titanium. The high annealing temperature for the titanium typically prevents a continuous annealing process as substantial time is required to heat the titanium pieces to the required temperature. Similarly, the high annealing temperature also requires a substantial cooling time in which the titanium pieces are cooled back to a safe handling temperature before being removed from the annealer. Accordingly, a batch annealing process, in which a plurality of drawn titanium pieces are drawn individually before being heated together as a batch, is frequently used to provide some efficiency to the annealing process. However, the batch process is time consuming and labor intensive requiring operators to manually load and unload a plurality of drawn pieces into and out of the annealer creating a substantial bottleneck in the production process. 
     Although titanium has numerous material characteristics that make it a superior material choice for many applications, the same material characteristics present numerous challenges for drawing and other forming processes. In particular, the substantial inefficiencies of conventional titanium drawing processes create a need for streamlining the process of producing titanium casings. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to a cold drawing process for forming titanium blanks in which generally planar titanium blanks are pressed with a punch to form shaped titanium pieces. Specifically, the present invention is directed to a pre-treatment process in which a wet lubricant is applied to the blanks to lubricate the contacting surfaces of the titanium blank and the punch. The wet lubricant reduces galling and work hardening of the portions of the titanium blank deformed during the drawing process, thereby eliminating the need to anneal the formed titanium pieces prior to additional drawing, punching or other forming processes. Removing the annealing step removes a substantial bottleneck in the titanium forming process enabling a continuous or nearly continuous process in which each drawn titanium piece is immediately moved onto the next process step, substantially increasing the production rate of the formed titanium pieces. 
     In certain embodiments, the wet lubricant can comprise a blended graphite and mineral spirits lubricant. In certain embodiments, the graphite comprises between about fifteen and about thirty-five percent of the lubricant blend by weight. In other embodiments, the graphite comprises about twenty percent of the lubricant blend by weight. The graphite and mineral spirits composition can be applied to the titanium blank, wherein the mineral spirit component adheres the graphite component to the surface of the titanium blank to provide a solid lubricant coating on the titanium blank without chemically bonding the graphite to the titanium. During the drawing of the titanium blank, the mineral spirits maintain the graphite on the surface of the titanium blank such that the graphite acts as a solid lubricant for the contacting surfaces of the titanium blank and the punch. Unlike conventional lubricant oils that can be wiped off the surface by the punch during the drawing process, the viscosity of the mineral oil retains the graphite component on the titanium blank through the drawing process. 
     In certain embodiments, the graphite and mineral spirits composition can be stripped from the formed titanium pieces following drawing through a pressurized water wash process without the use of additional chemical solvents. In certain aspects, the pressurized water stream can be applied between 900 psi and 1100 psi. In other aspects, the applied water can be applied at about 1000 psi. This process is particularly advantageous with respect to many medical applications of titanium sheeting, such as titanium casings for implantable devices where any chemical residue from mineral spirits, graphite or chemical solvents can impact the health of the patient. Similarly, any residue on the titanium casings can also disrupt any subsequent passivation processes that the titanium may be subjected to prior to implantation, increasing the risk that the titanium will corrode after implantation. 
     In certain embodiments, the surface of the titanium blank can be roughened prior to the application of the wet lubricant. The roughening of the titanium surface further improves the adhesion of the graphite component to the titanium surface without chemically bonding the graphite component to the titanium surface. The roughened surface better retains the wet lubricant during the drawing process as the punch is pressed against the titanium surface. In certain aspects, the titanium surface can have a roughness of between about 20 Ra to about 40 Ra. In other aspects, the titanium surface can have a roughness of between about 10 Ra to about 50 Ra. If the titanium surface is too smooth the lubricant can be wiped off the titanium surface during drawing. Conversely, if the titanium surface is too rough, the wet lubricant can be difficult to strip from the crevices formed in the roughened surface during the cleaning process resulting in pockets of the lubricant being retained on the titanium surface. 
     A method of forming titanium pieces from generally planar titanium blanks, according to an embodiment of the present invention, can comprise providing a first die defining a first recessed cavity and a first punch movable along a first linear path intersecting the first recessed cavity. According to certain embodiments, the first die can be movable along the first linear path, while the first punch remains fixed in position. The method can also comprises providing a generally planar titanium blank having at least one roughened face. The method can also comprise applying a wet lubricant to the roughened face of the titanium blank. The method can further comprise positioning the titanium blank on the first die such that a portion of the titanium blank is aligned with the first recessed cavity of the first die. Finally, the method can also comprise moving the first punch along the first linear path to engage the lubricated roughened face of the titanium blank and deform the aligned portion of the titanium blank into the first recessed cavity of the first die. 
     In certain embodiments, the method can further comprise providing a second die defining a second recessed cavity and a second punch movable along a second linear path intersecting the second recessed cavity, wherein the second recessed cavity is deeper than the first recessed cavity. According to certain embodiments, the second die can be movable along the second linear path, while the first punch remains fixed in position. The method can also comprise applying additional wet lubricant to the roughened face of the formed titanium piece to supplement or replace the wet lubricant on the titanium blank. In certain aspects, the amount of additional wet lubricant added can comprise about 40% of the original amount of wet lubricant added at the first drawn station. The method can further comprise positioning the formed titanium piece on the second die such that deformed portion of the titanium piece is received within the second recessed cavity of the second die. The method can also comprise moving the second punch along the second linear path to engage the lubricated roughened face of the titanium piece and further elongate the deformed portion of the titanium piece. The two stage lubricating and drawing approach reduces the likelihood that the titanium blank will fracture then if the entire drawing process was done in a single draw station. 
     A method of producing a titanium casing having a base portion and at least one wall portion angled relative to the base portion, can comprise providing a first die defining a first recessed cavity and a first punch movable along a first linear path intersecting the first recessed cavity. The method can also comprise providing a generally planar titanium blank having at least one roughened face. The method can also comprise applying a wet lubricant to the roughened face of the titanium blank. The method can further comprise positioning the titanium blank on the first die such that a portion of the titanium blank is aligned with the first recessed cavity of the first die. Finally, the method can also comprise moving the first punch along the first linear path to engage the lubricated roughened face of the titanium blank and press a planar base portion of the titanium blank into first recessed portion and deforming the titanium blank against the first die to form at least one wall portion angled relative to the base portion. The method can further comprise rinsing the formed titanium piece with a high pressure water stream to strip the wet lubricant from the formed titanium piece. Finally, the method can comprise a punching process in which the titanium blank is trimmed with a punching die to remove any excess portions of the formed titanium piece. In certain aspects, the method can also comprise a passivation process in which the titanium piece is treated to form an anti-corrosion layer on the titanium piece. 
     The above summary of the various representative embodiments of the invention is not intended to describe each illustrated embodiment or every implementation of the invention. Rather, the embodiments are chosen and described so that others skilled in the art can appreciate and understand the principles and practices of the invention. The figures in the detailed description that follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a system for forming a titanium blank into a formed titanium piece according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional side view of a drawing station, according to an embodiment of the present invention. 
         FIG. 3  is a detailed cross-sectional side view of the drawing station depicted in  FIG. 2 . 
         FIG. 4  is a perspective view of a titanium blank for use with the present invention. 
         FIG. 5  is a perspective view of a formed titanium piece formed from the titanium blank depicted in  FIG. 4  according to an embodiment of the present invention. 
         FIG. 6  is a perspective view of the titanium piece depicted in  FIG. 5  after punching to remove excess titanium. 
         FIG. 7  is a flow chart of a system for forming a titanium piece according to an embodiment of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     As depicted in  FIGS. 1-3 , a drawing system  10  for forming titanium blanks into formed titanium pieces, according to an embodiment of the present invention, generally comprises a first drawing station  12  and a first transfer assembly  14 . For the purposes of this disclosure, a titanium blank refers to a generally planar titanium element that has not been drawn. Similarly, a titanium piece refers to a titanium element that has been drawn at least once. The first drawing assembly  12  comprises a first die  16  and a first punch  18  movable along a first linear axis a-a. In certain aspects, the first drawing assembly  12  further comprises a locking press  20  for engaging the edge portions of the titanium blank to secure the titanium blank to the first die  16 . The first die  16  defines a first recess  22  corresponding to the intermediate or final shape of the formed titanium piece. In one aspect, the first recess  22  comprises a generally tubular shape having a generally horizontal bottom and vertical walls defining a first recess depth d 1 . In operation, the first linear path a-a is aligned with the first recess  22 , such that moving the first punch  18  along the first linear axis a-a inserts a portion of the first punch  18  into the first recess  22 . 
     The first transfer assembly  14  is adapted to pick up and position titanium blanks and comprises an engagement element  24  and a lubricant application element  26 . The first transfer assembly  14  can be positioned on a robotic arm, movable assembly or other conventional system for moving the titanium blanks. The engagement element  24  comprises a suction element, a magnetic element or other convention means of releasably engaging the titanium blank to position the titanium blank. The lubricant application element  26  can comprise a spray assembly or other conventional system for depositing a quantity of wet lubricant on a titanium blank secured by the engagement element  24 . In certain aspects, the lubricant application element  26  can be operated to apply the wet lubricant while the engagement element  24  is secured to the titanium blank or after the titanium blank is positioned for drawing. In certain aspects, the wet lubricant comprises a graphite component and a mineral spirits component. In various aspects, the graphite component can comprise fifteen to thirty-five weight percent of the lubricant. In other aspects, the graphite component can comprise about twenty-five weight percent of the lubricant. In other aspects, the ratio of graphite component to mineral spirits component is 1:3 by weight. 
     As shown in  FIGS. 4-6 , a titanium blank  30  for use with the drawing system  10  generally comprises a planar surface with at least two faces  32 . The titanium blank  30  can comprise elemental titanium and titanium alloys. More particularly, the titanium blank  30  comprises grade  1  titanium. In certain aspects, the titanium blank  30  can comprise grade  23  titanium. In certain aspects, the titanium blank  30  comprises a formable center portion  34  surrounded by an edge portion  36 . At least one of the faces  32  is roughened to a roughness of 10 Ra to 50 Ra. At least one of the faces  32  is roughened to a roughness of 20 Ra to 40 Ra. In certain aspect, the titanium blanks  30  can be punched from a titanium sheet. 
     In operation, the engagement element  24  of the first transfer assembly  14  engages a titanium blank  30  and maneuvers the titanium blank  30  onto the first die  16  such that the center portion  34  of the titanium blank  30  is aligned with the first recess  22 . During positioning or after the titanium blank  30  is positioned on the first die  16 , lubricant application element  26  is operated to apply a first quantity of wet lubricant onto the roughened face  32  of the titanium blank  30 . In certain aspects, between about 3.0 and about 6.5 ml per sq in of wet lubricant is applied by the lubricant application element  26 . In other aspects, about 4.3 ml per sq in of wet lubricant is applied by the lubricant application element  26 . The titanium blank  30  is positioned on the first die  16  such that the lubricated roughened face  32  is positioned opposite the first recess  22 . In certain aspects, the locking press  20  is closed to engage the edge portion  36  of the titanium blank  30  to the first die  16 . The first punch  18  is moved along the first linear axis a-a and engaged to the lubricated roughened face  32 . The first punch  18  applies a deforming force along the linear axis a-a against the lubricated roughened face  32  to deform the center portion  34  of the titanium blank  30  against the surfaces of the first die  16  defining the first recess  22 . As depicted in  FIGS. 5-6 , in certain aspects, center portion  34  is deformed to define a horizontal base portion  38  and vertical wall portions  39  corresponding to the dimensions of the first recess  22 . In certain aspects, the edge portion  36  retained within the locking press  20  remains unformed. The first punch  18  can then be reversed along the linear axis a-a and the formed titanium piece can then be removed. 
     As depicted in  FIGS. 1-3 , in certain embodiments, the drawing system  10  further comprises a second drawing station  40  and a second transfer assembly  42 . The second transfer assembly  14  can be positioned on a robotic arm, movable assembly or other conventional system for moving the titanium pieces formed at the first drawing station  12 . The second drawing assembly  40  comprises a second die  44  and a second punch  46  movable along a second linear axis b-b. In certain aspects, the second drawing station  40  further comprises a locking press  48  for engaging the edge portions  36  of the titanium blank  30  to secure the titanium blank  30  to the second die  44 . The second die  44  also defines a second recess  50  corresponding to the intermediate or final shape of the formed titanium piece  30 . The second transfer assembly  48  is adapted to pick up and position titanium pieces formed in the first drawing station  12  and comprise an engagement element  52  and a lubricant application element  54 . 
     In operation, the engagement element  52  of the second transfer assembly  42  engages the titanium piece  30  drawn at the first drawing station  12  and maneuvers the titanium piece  30  onto the second die  44  such that the formed portion of the titanium piece  30  is positioned within the second recess  50 . In certain aspects, the second recess  50  comprises a width corresponding to the width of the first recess  22 , wherein the depth d 2  of the second recess  50  is greater than the depth d 1  of the first recess  22 . During positioning or after the titanium piece  30  is positioned on the second die  44 , the lubricant application element  54  is operated to apply a second quantity of wet lubricant onto the roughened face  32  of the titanium piece  30  to supplement or replenish the wet lubricant on the roughened face  32 . In certain aspects, between about 1.1 and about 2.7 ml per sq. in. of wet lubricant is applied by the second lubricant application element  54 . In other aspects, about 1.8 ml per sq. in. of wet lubricant is applied by the second lubricant application element  54 . In yet other aspects, the second amount of wet lubricant is about forty percent of the wet lubricant applied by the first lubricant application element  26 . After the locking press  48  is closed to engage the edge portion  36  of the titanium piece  30 , the second punch  46  is moved along the second linear axis b-b and engages the lubricated roughened face  32 . The second punch  46  applies a deforming force along the linear axis b-b against the lubricated roughened face  32  to elongate the wall portion  39  of the titanium piece  30 . As depicted in  FIGS. 5-6 , in certain aspects, formed titanium piece  30  can comprise a horizontal base portion and vertical wall portions corresponding to the dimensions of the first recess  22 . The second punch  46  can then be reversed along the linear axis b-b and the formed titanium piece  30  can then be removed. 
     In certain embodiments, after forming, the formed titanium piece  30  can be washed with a pressurized water system  56  adapted to apply a pressurized water stream against the surfaces of the titanium piece  30  to strip any remaining wet lubricant from titanium piece  30 . In certain aspects, the water stream can be applied at a pressure between about 900 and 1100 psi. In other aspects, the water stream can be applied to a pressure of about 1000 psi. The cleaning process can comprise an automated system, wherein at least one robotic arm is used to pick up the titanium piece  30  to position it relative to the pressurized water system  56 , and to receive the pressurized water stream and position the titanium strip  30  for cleaning. Alternatively, the pressurized water system  56  can comprise an articulated nozzle  58  mounted on a robotic arm that can be oriented to apply the pressurized water stream to the surfaces of the titanium pieces  30 . 
     In certain embodiments, the system  10  can further comprise a punching die  60  and third punch  62  movable along a third linear axis c-c. The punching die  60  defines a recess  64  for receiving the formed portion  34  of the titanium piece  30 , wherein the edge portion  36  rests on the edges of the punching die  60 . In operation the third punch  62  is movable along the third linear axis c-c to engage the formed titanium piece  30  and cleave the edge portion  36  from the titanium piece  30 . 
     In certain embodiments, the first punch  18  and second punch  46  can be a hardened metal material allowing punches  18 ,  46  to engage the titanium blank  30  and apply a deforming force to form the titanium blank  30 . The punches  18 ,  46  can comprise zinc nitrate; carbide with physical vapor deposition of a diamond like coating; or carbide with a chemical vapor deposition coating of a titancote H, titanium nitrate, titanium cyanide, titanium carbide or combinations thereof. 
     As depicted in  FIG. 7 , a method  70  for forming titanium pieces from generally planar titanium blanks, according to an embodiment of the present invention, comprises a roughening step  72  in which at least one face  32  of the titanium blank  16  is roughened to a roughness of between about 10 Ra to about 50 Ra. In certain aspects, the roughening step  72  can comprise roughening the at least one face to a roughness of between about 20 Ra to about 40 Ra. The method  60  can also comprise a first lubricating step  74  in which a wet lubricant is applied to the roughened faces  32 . In certain aspects, the wet lubricant comprises a graphite component and a mineral spirits component. In various aspects, the graphite component can comprise fifteen to thirty-five weight percent of the lubricant. In other aspects, the graphite component can comprise about twenty-five weight percent of the lubricant. In yet other aspects, the ratio of graphite component to mineral spirits component is 1:3 by weight. 
     The method  60  further comprises a first drawing step  76  in which the lubricated titanium blank  30  is aligned with the first recess  22  and the first punch  18  is moved along the first linear axis a-a to form the center portion  34  of the titanium blank  30  to conform to the dimensions of the first recess  22 . In certain embodiments, the method  30  can further comprise a second lubricating step  78  and a second rolling step  80  to elongate the vertical walls  39  of the deformed portion. 
     In certain embodiments, the method  70  further comprises a cleaning step  82  in which a pressurized water stream is applied to the opposing surfaces  18   a ,  18   b  to remove any remaining wet lubricant from the opposing surfaces  18   a ,  18   b . In certain aspects, the pressurized water stream can be applied with a water pressure between about 900 psi and 1100 psi. In other aspects, the pressurized water stream can be applied with a water pressure of about 1000 psi. 
     In certain embodiments, the method  70  further comprises punching step  84  in which the titanium piece  30  is positioned on the punching die  60  and the third punch  62  is operated to cleave the un-deformed edge portion  36  or excess titanium from the formed portion  34  of the titanium piece  30 . In certain embodiments, the punching step  84  can also comprise additional forming steps such as drilling of additional holes or forming of additional portions of the titanium piece  30 . 
     In certain embodiments, the method  70  further comprises a passivation step  86  in which the formed titanium piece  30  is treated with an anti-corrosion coating. In certain aspects, the passivation step  86  can comprise cleaning the base portion  38  and the wall portion  39  with nitric acid before exposing the titanium piece  30  to oxygen to form a titanium oxide layer on the base portion  38  and the wall portion  39 . 
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and described in detail. It is understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.