Patent Publication Number: US-8980439-B2

Title: Bimetallic forging and method

Description:
TECHNICAL FIELD 
     The present invention relates to forming a bimetallic forging and method. 
     BACKGROUND 
     Components formed of magnesium offer advantages such as high strength to weight ratio when compared to similarly sized components formed of aluminum or ferrous based materials. For example, wheels have been forged from magnesium for specialized applications such as racing vehicle wheels. The use of magnesium wheels for non-specialty vehicles has been limited by the poor corrosion performance of magnesium. Coatings applied to the surface of magnesium components, for example, aluminum diffusion or diffused aluminum coatings, to improve the corrosion performance of the magnesium, have been developed, however spalling and chipping of applied coatings negates the protective effect of the coating. The material, processing time, equipment, handling and transportation and associated costs required for applying coatings in a secondary process such as aluminum diffusion or diffused aluminum coatings to magnesium components represent time and cost disadvantages. 
     SUMMARY 
     A method of forming a bimetallic forging is provided. The method includes providing a first element substantially made of a first metal, a second element substantially made of the first metal, and an insert substantially made of a second metal. A blank is formed comprising the first element, the second element, and the insert. The blank is configured such that the insert may be substantially encapsulated by a shell defined by the first element and the second element. The first element and the second element may be operatively joined to further define the shell. The blank is forged to form a bimetallic forging. The bimetallic forging includes an outer portion defined by the shell, an inner portion defined by the insert, and an interface layer between the inner portion and the outer portion. In a non-limiting example, the first metal is aluminum or an aluminum alloy and the second metal is magnesium or a magnesium alloy. The blank is configurable, in a non-limiting example, to be formed by forging into a wheel for use on a vehicle and configured such that the aluminum outer portion substantially encapsulates the magnesium inner portion, thereby providing a forging with a high strength to weight ratio, and an exterior skin of aluminum for improved corrosion performance. 
     A blank configurable for forming by forging is provided. The blank includes a first element, substantially made of a first metal, a second element substantially made of the first metal, and an insert substantially made of a second metal. The first element and the second element are configured in proximate contact with each other and the insert, such that the first element and the second element define a shell which substantially encapsulates the insert. The first element and the second element may be operatively joined to define the shell. In a non-limiting example, the first metal is substantially comprised of aluminum or an aluminum alloy, and the second metal is substantially comprised of magnesium or a magnesium alloy. The blank may further include a third element in proximate contact with the insert and at least one of the first element and the second element such that the first element, the second element and the third element define the shell which substantially encapsulates the insert. The insert may be configured as a casting. At least one of the first element and the second element may be configured as a casting or an extrusion. The shell and the insert may be configured such that the shell is non-concentric with the insert, or the shell may be of a non-uniform thickness, such that the outer portion of the forging defined thereby may be of non-uniform thickness to provide, for example, supplementary material in some areas of the forging, for example, to improve the strength of the outer portion in those areas, or to provide supplementary stock for secondary finishing operations such as machining or surface finishing treatments. 
     A bimetallic forging formed from a blank is provided. The bimetallic forging comprises an outer portion substantially made of a first metal and defined by a shell portion of the blank, an inner portion substantially made of a second metal and defined by an insert portion of the blank, and an interface layer defining a metallurgical bond between the outer portion and the inner portion. In a non-limiting example, the first metal may be aluminum or an aluminum alloy, and the second metal may be magnesium or a magnesium alloy. The interface layer may be defined by an intermetallic layer comprising at least the first metal and the second metal. The forging may be configured such that the outer portion substantially encapsulates the inner portion. In a non-limiting example, the bimetallic forging may be configured as a wheel for a vehicle. 
     The above features and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective exploded view of a bimetallic blank; 
         FIG. 2  is a schematic perspective illustration of the blank of  FIG. 1 ; 
         FIG. 3  is a schematic perspective exploded view of an alternate configuration of a bimetallic blank; 
         FIG. 4  is a schematic perspective illustration of the blank of  FIG. 3 ; 
         FIG. 5A  is a plan view of a forging formed using the blank of  FIG. 2  or  FIG. 4 ; 
         FIG. 5B  is a schematic cross-sectional view of the forging of  FIG. 5A ; 
         FIG. 6A  is a schematic cross-sectional view of the blank of  FIG. 2  or  FIG. 4 ; 
         FIG. 6B  is a schematic cross-sectional view of an alternate configuration of the blank of  FIG. 2  or  FIG. 4 ; 
         FIG. 7A  is a schematic cross-sectional view of an alternate configuration of the blank of  FIG. 2  or  FIG. 4 ; and 
         FIG. 7B  is a schematic cross-sectional view of an alternate configuration of the blank of  FIG. 2  or  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings wherein like reference numbers represent like components throughout the several figures, the elements shown in  FIGS. 1-7B  may not be to scale or proportion. Accordingly, the particular dimensions and applications provided in the drawings presented herein are not to be considered limiting. 
       FIG. 1  shows an exploded view of a blank  10 , which may also be referred to as a blank assembly or a forging blank, where the blank  10  is configurable for forming by forging. The blank  10  is comprised of a first element  12  substantially made of a first metal, a second element  16  substantially made of the first metal, and an insert  14  substantially made of a second metal. In a non-limiting example, the first metal may be substantially comprised of aluminum, an aluminum alloy or a material of predominantly aluminum composition and the second metal may be substantially comprised of magnesium, a magnesium alloy or a material of predominantly magnesium composition. 
     The first element  12 , which may also be referred to as a first casing element, defines a surface  13 , which is generally configured as the interior or inner surface  13  of the first element  12 . The surface  13  defines a cavity or opening in first element  12 . The second element  16 , which may also be referred to as a second casing element, defines a surface  17 , which is generally configured as the interior or inner surface  17  of the second element  16 . The surface  17  defines a cavity or opening in element  16 . The first and second elements  12 ,  16  may be, but are not required to be, similarly configured. The first and second elements  12 ,  16  may be configured, by way of non-limiting example, as a casting, forging or extrusion, and may be further configured by secondary processing including but not limited to additional forming, machining, heat treating, or surface treatment operations. 
     The insert  14  defines a surface  15 , which is generally configured as the exterior or outer surface  15  of the insert  14 . The insert  14  may be configured, by way of non-limiting example, as a casting, forging or extrusion, which may be further configured by secondary processing including but not limited to additional forming, machining, heat treating, or surface treatment operations. 
     In the non-limiting example shown in  FIGS. 1 and 2 , the first and second elements  12 ,  16  are configured as generally cylindrical elements, each having one enclosed end and a cavity defined by a respective inner surface  13 ,  17 . The insert  14  is configured generally as a substantially solid cylinder, such that the insert  14  can be inserted into the generally cylindrical cavities defined by inner surfaces  13 ,  17  to form the blank  10 . The insert  14  may be inserted into the casings  12 ,  16  using any suitable method, which may include, by way of non-limiting example, slip-fitting or pressing the insert  14  into the cavities of elements  12 ,  16  defined by surfaces  13 ,  17 . The surfaces  13 ,  15 ,  17  may be configured or modified to facilitate assembly of the blank  10 , for example, by tapering the respective surfaces, or by knurling or relieving one or more of the surfaces. One or more of the interfacing surfaces  13 ,  15 ,  17  may be lubricated with a coating or lubricant, such as a graphite or boron-nitride coating, to facilitate the assembly of the insert  14  and the elements  12 ,  16 . 
     A coating substantially comprised of a third metal may be applied to the outer surface  15  of the insert  14 , such that during the forging process, the third metal may form an intermetallic or a metal matrix composite with one or both of the first metal and the second metal. The third metal comprising the coating may be, for example, one of silver, tin, zinc, copper or alumina. The coating may be applied to the surface  15  of insert  14 , for example, by thermal spraying, cold spraying, plasma spraying or any suitable method. Alternatively, the coating comprising the third metal may be applied to the inner surfaces  13 ,  17  of elements  12 ,  16  instead of or in addition to applying the coating to the surface  15  of the insert  14 . 
       FIG. 2  shows the blank  10  formed by operatively assembling the first element  12 , the second element  16 , and the insert  14 , such that the inner surfaces  13 ,  17  of first and second elements  12 ,  16 , respectively, are in proximate contact with the outer surface  15  of insert  14  to define an interface  19 , and such that the first and second elements  12 ,  16  are in proximate contact to define a joint  18 . The joint  18  may be configured as a seam formed by operatively joining the first element  12  and the second element  16  using any method suitable to provide an operative seam or joint which is formable by forging. For example, the first and second elements  12 ,  16  may be joined by using friction stir welding to form the seam  18 , where the friction welding process provides a seam  18  defined by a fine grained microstructure suitable for forming by forging. 
     The blank  10  may be configured such that the insert  14  may be substantially encapsulated by a shell  20  defined by the first element  12  and the second element  16 . The shell  20  may include an outer surface  29 . “Substantially encapsulating” the insert  14  with the shell  20  may include configuring the shell  20  to encapsulate all but an insignificant area of the outer surface  15  of the insert  14 , such that when blank  10  is forged, the bimetallic forging  30  (see  FIGS. 5A and 5B ) which is formed from the blank  10  includes an outer or skin portion  32  defined by the shell  20 , and an inner or core portion  34  defined by the insert  14 . The skin portion  32  of the forging  30  thus formed may substantially encapsulate the core portion  34 , such that, in the non-limiting example provided herein, the aluminum comprising the skin portion  32  defines or provides a corrosion protection layer which substantially covers the magnesium comprising the core portion  34 , thus limiting the exposure of the magnesium-based material comprising the core portion  34  to corrosive factors and environments, thereby improving the corrosion performance of the forging  30 . 
       FIGS. 3 and 4  show an alternate configuration of the blank  10 .  FIG. 3  shows an exploded view of the blank  10  comprised of a first element  22  substantially made of a first metal, a second element  26  substantially made of the first metal, a third element  24  substantially made of the first metal, and the insert  14  substantially made of a second metal. In a non-limiting example, the first metal may be substantially comprised of aluminum, an aluminum alloy or a material of predominantly aluminum composition, and the second metal may be substantially comprised of magnesium, a magnesium alloy or a material of predominantly magnesium composition. 
     The first element  22 , which may also be referred to as a first casing element, defines a surface  23 , which is generally configured as the interior or inner surface  23  of the first element  22 . The second element  26 , which may also be referred to as a second casing element, defines a surface  27 , which is generally configured as the interior or inner surface  27  of the second element  26 . The third element  24 , which may also be referred to as a third casing element, defines a surface  25 , which is generally configured as the interior or inner surface  25  of the third element  24 . The surface  25  defines a cavity or opening in element  24 . The first and second elements  22 ,  26  may be, but are not required to be, similarly configured. The first, second and third elements  22 ,  24 ,  26  may be configured, by way of non-limiting example, as a casting, forging, stamping or extrusion, which may be subject to secondary processing including additional forming, machining, heat treating, or surface treatment operations. 
     The insert  14  is defined as described for  FIGS. 1 and 2 , and defines an outer surface  15 . In the non-limiting example shown in  FIGS. 3 and 4 , the first and second elements  22 ,  26  are configured as generally cylindrical plates, and the third element  24  is configured generally as a hollow cylinder with the hollow portion of the cylinder defined by the surface  25 . The insert  14  is configured generally as a solid cylinder, such that the insert  14  can be inserted or fitted into the generally cylindrical cavity defined by the inner surface  25  of the third element  24 . A hollow space or cavity is defined by each end of insert  14  and the adjacent surface  25  after insert  14  has been fitted into the third element  24 , such that each of the first and second elements  22 ,  26  can be fitted into the hollow space at a respective end of the third element  24  and proximate to a respective end of the insert  14  to form the blank  10  shown in  FIG. 4 . The insert  14  and elements  22 ,  26  may be fitted into the casing  24  using any suitable method, which may include, by way of non-limiting example, slip-fitting or pressing the insert  14  into the inner space of element  24  defined by surface  25 . The interfacing surfaces of elements  22 ,  24 ,  26  and insert  14  may be configured or modified to facilitate assembly of the blank  10 , for example, by tapering the respective surfaces, or by knurling or relieving one or more of the surfaces. One or more of the interfacing surfaces of elements  22 ,  24 ,  26  and insert  14  may be lubricated with a coating or lubricant, such as a graphite or boron-nitride coating, to facilitate the assembly of insert  14  and elements  22 ,  24  and  26 . 
     In a non-limiting example, a coating substantially comprised of a third metal may be applied to the outer surface  15  of the insert  14 , such that during the forging process, the third metal may form an intermetallic or a metal matrix composite with one or both of the first metal and the second metal. The third metal comprising the coating may be, for example, one of silver, tin, zinc, copper or alumina. The coating comprised of the third metal may be applied to the surface  15  of the insert  14 , for example, by thermal spraying, cold spraying, plasma spraying or any suitable method. Alternatively, the coating comprising the third metal may be applied to the inner surfaces  23 ,  25 ,  27  of elements  22 ,  24 ,  26  instead of or in addition to applying the coating to the surface  15  of the insert  14 . 
       FIG. 4  shows the blank  10  formed by operatively assembling the first, second and third elements  22 ,  26 ,  24  and the insert  14 , such that the inner surfaces  23 ,  25 ,  27  of first, second, and third elements  22 ,  26 ,  24 , respectively, are in proximate contact with the outer surface  15  of insert  14  to define an interface  19 . Further, the blank  10  is formed such that the first and second elements  22 ,  26  are each in proximate contact with the third element  24 , and such that a joint  28  is defined between the elements  22  and  24  and another joint  28  is defined between the elements  26  and  24 . Each joint  28  may be configured as a seam formed by operatively joining one of the elements  22 ,  26  and the third element  24  using any method suitable to provide an operative seam or joint which is formable by forging. For example, the first and third elements  22 ,  24  may be joined by using friction stir welding to form the seam  28 , where the friction stir process provides a seam  28  defined by a fine grained microstructure suitable for forming by forging. The blank  10  may be configured such that the insert  14  may be substantially encapsulated by a shell  20  defined by the first, second and third elements  22 ,  26 , and  24 , as described for blank  10  related to  FIGS. 1 and 2 . 
     Other configurations of a plurality of casing elements are possible, which may define a shell  20  of a first metal which when assembled with an insert  14  of a second metal defines a blank  10 , where the shell  20  may be configured to substantially encapsulate the insert  14 . Other configurations of casing elements and inserts are possible, some of which are shown in  FIGS. 6A-7B  in a cross-sectional view taken through insert  10  through a section A-A shown in  FIGS. 2 and 4 , and described in further detail herein. 
     The blank  10  may be forged to form a bimetallic forging  30  shown in  FIG. 5A , which in a non-limiting example is configured as a wheel  30  adaptable for use on a vehicle.  FIG. 5B  shows a cross-sectional schematic view of the forged wheel  30  taken through a section B-B shown in  FIG. 5A . The blank  10  may be preheated in preparation for forging, and may be forged into the forging  30  using any suitable forging method, including hammer forging and drop forging. The blank  10  is forged to form the forging  30  including an outer or skin portion  32  defined by the shell  20  of blank  10  and as such substantially comprised of the first metal, an inner or core portion  34  defined by the insert  14  and as such substantially comprised of the second metal, and an interface layer  36  therebetween. The interface layer  36  provides a metallurgical bond between the inner portion  34  and the outer portion  32 , and may be further defined by an intermetallic layer comprising the first metal and the second metal. 
     In an alternative configuration, as discussed previously, a coating substantially comprised of a third metal may be applied to the outer surface  15  of the insert  14 , and/or to the inner surfaces of the casing elements, such that during the forging process, the third metal of the coating may form an intermetallic and/or a metal matrix composite with one or both of the first metal and the second metal which defines the interface layer  36 . The formation of the intermetallic by diffusion bonding and/or the formation of the metal matrix composite may be activated when the blank is preheated in preparation for forging, or during the forging operation. In a non-limiting example, the third metal comprising the coating may be one of silver, tin, zinc, copper or alumina, and may combine with the magnesium-based material of the insert and/or the aluminum-based material of the casing to form either an intermetallic which is less brittle than a magnesium-aluminum intermetallic such as Mg 17 Al 12  which may form in the interface layer  36  during the forging process in the absence of the coating, or to form a metal matrix composite which may improve the mechanical properties of the interface layer  36 . 
     In a non-limiting example provided herein, the first metal is a substantially aluminum-based material and the second metal is a substantially magnesium-based material, and the wheel  30  is formed and configured such that the aluminum outer portion  32  substantially encapsulates the magnesium inner portion  34 , thereby providing a forged wheel  30  with a high strength to weight ratio and an exterior skin  32  of aluminum for improved corrosion performance. The aluminum skin portion  32  provides a corrosion protection layer which substantially covers the magnesium comprising the core portion  34 , thus limiting the exposure of the magnesium-based material comprising the core portion  34  to corrosive factors and environments, thereby improving the corrosion performance of the wheel  30 . Improvement in other performance characteristics of wheel  30 , such as thermal shock resistance, may be provided by the bimetallic configuration of wheel  30 . 
     The insert  14  and the shell  20  may be configured such that the insert  14  is non-concentric or non-symmetrical to the shell  20 . The shell  20  may be of a non-uniform thickness. The outer portion  32  of the forging  30  defined by a non-uniform or non-symmetrical shell  20  may be of non-uniform thickness to define a thicker skin  32  in some areas of the surface of wheel  30 , to provide supplementary material to improve the strength of the outer portion  32  in those areas, or to provide supplementary stock for secondary finishing operations such as machining or surface finishing treatments. For example, a thicker skin  32  may be provided on the outboard or appearance face of the wheel  30 , as mounted on the vehicle, or at the rim of the wheel  30 , to provide surplus material to form or finish the appearance face of the wheel  30  or to form or finish the bead or tire mounting surface of the rim, and/or to provide additional corrosion protection against nicks, scratches, stone-impingement, road dirt or other corrosive environmental elements in these areas. 
     The outer portion  32  of the forging  30  may be of non-uniform thickness to define a thinner skin  32  in some areas of the surface of wheel  30 , such that the aluminum portion  32  in these areas provides nominal corrosion protection to the magnesium portion  34  of the wheel  30 , recognizing aluminum is denser than magnesium, to minimize the weight contribution of the aluminum portion  32  of the wheel  30  to maximize the strength to weight ratio of the wheel  30 . For example, a thinner skin  32  may be provided on sections of the wheel  30  which may be substantially covered by a vehicle tire and a hub cap or decorative trim cover, such that these sections may be minimally exposed to the road environment. 
     The thickness of shell  20  may be varied across the surface of the blank  10  to affect the relative flow characteristics of the aluminum portion  20  and the magnesium portion  14  in the forging die during the forming of forging  30 . Further, the material flow during the forging process may be locally varied, e.g., varied in localized areas of the blank  10 , by varying the microstructure of the shell  20 . For example, certain regions or areas of the shell  20  may be subjected to friction stir processing where increased material flow during forging is desired, resulting in a fine grain structure in the processed areas that will preferentially flow during forging to affect the distribution of the thickness of the skin portion  32  on the forging  30 . These areas of fine grain structure may be characterized by increase fatigue resistance. 
     Multiple configurations of a blank  10  would be possible to provide, for example, varying thickness and distribution of the skin portion  32  over the surface of the core portion  34  of the forging.  FIG. 6A  shows a cross-sectional view of the blank  20  taken through a section A-A shown in  FIGS. 2 and 4 . As discussed for  FIGS. 1-4 , blank  10  is defined by an insert  14 , a shell  20 , and an interface  19  therebetween. Shell  20  is defined by an outer surface  20 . In the non-limiting example shown in  FIG. 6A , the cross-section of the insert portion  14  is generally round and is generally concentric with the generally annular cross-section of shell  20 , such that the interface  19  and the outer surface  29  are both generally circular and concentric to each other. 
     In another configuration shown in  FIG. 6B , the shell  20  is defined in the cross-sectional view shown, by a generally circular inner surface forming the interface  19 , which is eccentric to the outer surface  29  of the shell  20 , thereby providing respectively thinner and thicker areas of the aluminum shell  20 . When blank  10  is formed into wheel  30 , the thinner and thicker sections of the shell  20  may be deformed during the forging process to provide areas of non-uniform thickness in the skin portion  32  of the resulting forging  30 . 
       FIGS. 7A and 7B  show alternative configurations of the insert  14  and the shell  20  forming the blank  10 , where the interface  19  and the outer surface  29 , in the cross-sectional view shown, are of varying shapes to provide thinner and thicker areas of aluminum which may be oriented or configured to coincide with certain features of the wheel  30  formed therefrom. For example, the thinner portions of aluminum in the blank  10  shown in  FIG. 7A  may coincide with the rim sections between the spokes of the wheel  30  (see  FIG. 5A ) to provide a higher magnesium to aluminum content in these areas, for added strength and reduced weight. 
     The forging, the blank, and the method of forming described herein are illustrated using an example of a vehicle wheel as the forged component. The example of a vehicle wheel shown in  FIGS. 1-7B  is intended to be non-limiting. The forging, blank, and the method of forming described herein may be configured to provide other components where a bimetal structure is advantageous, for example, to provide a high strength to weight ratio, or a surface structure differentiated from the core structure for corrosion protection, resistance to thermal shock, or other functional, appearance, or performance characteristics and features. Vehicle related examples include steering knuckles, connecting rods and engine supports, although it would be understood that the blank and method of forming and forging described herein would be useful for non-vehicular components and applications. Material combinations other than aluminum-based and magnesium-based materials may be possible using the methods described herein. 
     A forging blank and/or forged component produced by a method as described herein may be modified by additional processing and/or secondary treatment to enhance, optimize and/or develop certain characteristics and/or features. Non-limiting examples of additional processing and/or secondary treatments which may be applied or used to meet dimensional, appearance, function and/or performance requirements and specifications include machining, burnishing, polishing, pressing, forging, heat treating, anodizing, localized surface treatment such as peening, laser treatment, friction stir welding, friction mixing, etc., or a combination thereof. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.