Patent Publication Number: US-9849356-B2

Title: Multi-metal golf clubs

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/145,305, filed Dec. 31, 2013, which is a continuation of U.S. patent application Ser. No. 12/902,053, filed on Oct. 11, 2010 (now U.S. Pat. No. 8,616,997), which is a continuation of U.S. patent application Ser. No. 11/960,809, filed on Dec. 20, 2007 (now U.S. Pat. No. 7,811,179), which is a continuation-in-part of U.S. patent application Ser. No. 11/534,724, filed on Sep. 25, 2006 (now U.S. Pat. No. 7,811,180) the contents of each of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to golf clubs, and more specifically to multi-metal golf clubs. 
     BACKGROUND OF THE INVENTION 
     Perimeter weighting in a golf club distributes the mass of the club toward the perimeter, minimizing the effects of off-center hits on the face of the golf club away from the sweet spot and producing more accurate and consistent golf ball trajectories. Perimeter weighting is achieved by creating a cavity in the back of the golf club opposite the face or hitting surface. The material weight saved by creating this cavity is redistributed around the perimeter of the golf club head. In general, larger cavity volumes correspond to increased amounts of mass distributed around the perimeter. Additionally, more of the perimeter weight is moved to the sole of the club to move the center of gravity downward and rearward. 
     Alternative approaches for moving the center of gravity of a golf club head rearward and downward in the club head utilize composite structures. These composite structures utilize two, three, or more materials that have different physical properties including different densities. By positioning materials that provide the desired strength characteristics with less weight near the crown or top line of a golf club head, a larger percentage of the overall weight of the golf club head is shifted towards the sole of the club head. This results in the center of gravity being moved downward and rearward. This approach is advantageously applicable to muscle back iron clubs or fairway woods, as this will help to generate loft and power behind and below the ball. However, composite materials must be bonded together, for example by welding, swaging, or using bonding agents such as epoxy, and may be subject to delamination or corrosion over time. This component delamination or corrosion results in decreased performance in the golf club head and can lead to club head failure. 
     Therefore, there remains a need for a composite golf club head that utilizes components having different densities designed in such a way as to minimize the problems associated with delamination, corrosion, or separation of the components. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to golf club heads constructed from composite materials. The golf club head includes a body portion, for example a cast or forged body portion, made from a first metal to which is attached a face insert made from a second metal. The first and second metals are selected so that the first metal has a higher density than the second metal. An example of suitable metals includes titanium or steel for the first metal and aluminum for the second metal. The face insert is positioned on the front of the body portion adjacent the top line (or crown) and forms at least a portion of the hitting surface of the club head. In order to minimize delamination or separation between the body and the face insert, an interlocking structure is preferably formed in the body portion and arranged to interlock with the face insert when the face insert is fitted onto the body portion. This interlocking structure includes one or more channels running through the top section of the body portion to which the face insert is attached. Upon attachment, the face insert is interlocked with the channels, providing sufficient and stable attachment between the face insert and the body portion. The channel is shaped to further enhance the connection between the two components. These shapes include, but are not limited to, rectangular cross sections and cross sections having overhangs such as dove tail cross-sections. The present invention is also directed at anodizing at least one part of the golf club head, preferably the face insert. In an alternative embodiment, all the components of the club head are anodized. The face insert, the body of the club head or both can be anodized. For example, the face insert can be made from an anodized aluminum, or the body portion can be made from anodized titanium, or both. A polymer such as PTFE, polyurethane or polyurea can be added to the anodized layer to enhance the performance of the clubs. 
     An embodiment of the present invention teaches a golf club head having a body portion and a face insert. The front of the body portion further comprises a cutout designed to receive the face insert. The body portion is preferably comprised of a high-strength metal such as stainless steel, titanium or titanium alloy. The face insert is preferably comprised of a metal having a lower density than that of the body portion. More preferably, the face insert comprises an aluminum metal matrix composite (MMC). The face insert preferably has a plurality of feet to be cold worked into a pocket in the cutout. The feet may have notches or angled surfaces to facilitate their bending into the pocket. 
     The golf club head of the present invention may also include an insert disposed to the top line, said insert comprising a lightweight material. Additionally, the golf club head may include at least one weight member disposed to the back, located behind and below the center of gravity of the club head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of an embodiment of a golf club head in accordance with the present invention; 
         FIG. 2  is a front view of an embodiment of a body portion without the face insert of the present invention; 
         FIG. 3  is a view through line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a cross-section view of the body portion showing another embodiment of the interlocking structure of the present invention; 
         FIG. 5  is a cross-section of the body portion showing another embodiment of the interlocking structure of the present invention; 
         FIG. 6  is a cross-section of the body portion showing another embodiment of the interlocking structure of the present invention; 
         FIG. 7  is another embodiment of  FIG. 2 ; 
         FIG. 8  is a front view of an embodiment of a club head of the present invention; 
         FIG. 9  is a cross-sectional view of an embodiment of a club head of the present invention; 
         FIG. 9A  is a cross-sectional view of another embodiment of a club head of the present invention; 
         FIG. 9B  is a cross-sectional view of another embodiment of a club head of the present invention; 
         FIG. 10  is a cross-sectional view of another embodiment of a club head of the present invention; 
         FIG. 11  is a cross-sectional view of an infused hard-anodic coating applied to a face insert according to the present invention; 
         FIG. 11A  is a cross-sectional view of another infused hard-anodic coating applied to a face insert according to the present invention; 
         FIG. 12  is a front view of an embodiment of a driver-type club head of the present invention; 
         FIG. 13  is a perspective view of another embodiment of a driver-type club head of the present invention; 
         FIG. 14A  is a front plan view of a golf club head of the present invention, shown without a face insert; 
         FIG. 14B  is a front plan view of the golf club head of  FIG. 14A , shown with a face insert; 
         FIGS. 15A and 15B  are a top plan and bottom plan views, respectively, of a face insert of the present invention; 
         FIG. 16  is a cross-sectional view of a portion of the front of a golf club head and a portion of a face insert of the present invention; 
         FIG. 17  is a front plan view of a golf club head of the present invention including a top line insert; and 
         FIG. 18  is a back plan view of the golf club head of  FIG. 17  including a plurality of weight members disposed on the back of the club head. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the accompanying  FIGS. 1-7 , exemplary embodiments of the golf club head  10  in accordance with the present invention include face insert  12  and body portion  24 , which is attached to hosel  16 . Hosel  16  is adapted to receive a shaft (not shown). Club head  10  is preferably cast or forged from suitable material such as stainless steel, carbon steel, or titanium. In one embodiment, body portion  24  is a cast body portion. Body portion  24  includes crown or top line  14 , toe  22 , sole  20  and heel  18  that form the perimeter of body portion  24 . Hosel  16  extends generally from heel  18  of body portion  24 . In one embodiment, club head  10  is arranged as muscle-back iron-type club head that has a thicker bottom back portion. Body portion  24  also includes front  32  forming the hitting surface. 
     Improvement in the location of the center of gravity of golf club heads in accordance with the present invention is achieved through the use of a composite construction that utilizes various materials having varying weights or densities. In particular, golf club head  10  utilizes two materials. Body portion  24  is constructed of a first material, for example a first metal, having a first weight or density. Suitable materials for the body portion  24  include, but are not limited to, stainless steel, carbon steel, beryllium copper, titanium and metal matrix composites (MMC). Preferably, body portion  24  is made from a higher density metal such as stainless steel or titanium. Club head  10  also includes face insert  12  attached to front  32  of body portion  24 . Face insert  12  is constructed of a second material, i.e., a second metal having a second density. Suitable materials for face insert  12  include titanium, aluminum and alloys thereof. In one embodiment, the first weight or the first density is greater than the second weight or second density. 
     In order to move the center of gravity of club head  10  downward and to the rear, lightweight face insert  12  is attached to body portion  24  so that face insert  12  is disposed on front  32  of body portion  24  adjacent crown or top line  14 . Therefore, face insert  12  forms a part of the club face or hitting surface of club head  10 . To minimize delamination of face insert  12  from body portion  24 , body portion  24  includes interlocking structure  25  formed on at least a portion of front  32  of body portion  24  adjacent top line  14 . When face insert  12  is attached to or press fit on front  32  of body portion  24 , face insert  12  is secured and anchored in interlocking structure  25 . Optionally, adhesives, welds or other bonding agents can be used to help secure face insert  12  into interlocking structure  25 . The interaction and meshing of face insert  12  with interlocking structure  25  is sufficient to fixedly secure face insert  12  to body portion  24 . 
     In one embodiment, interlocking structure  25  contains at least one channel  26  running through a top of front  32  of body portion  24 . Alternatively, a plurality of parallel channels  26  are formed in front  32  of body portion  24 , further defining a plurality of associated ridges or raised portions  28 . In one embodiment, the plurality of parallel channels  26  are arranged substantially parallel to top line  14  or sole  20  of body portion  24 . In one embodiment, face insert  12  is pressed onto body portion  24 , such that the second metal of face insert  12  substantially fills each channel  26  when face insert  12  is attached to body portion  24 . Although channel  26  can be arranged as any shape including curves and annular shapes, preferably, channel  26  is a generally rectilinear line arranged parallel to sole  20 . 
     By embedding face insert  12  in interlocking member  25  having channel  26 , a stronger more resilient bond is formed between face insert  12  and body portion  24 . Depending on the shape, and in particular the profile in cross section, of the channel, both increased surface area contact and increased mechanical binding is achieved between body portion  24  and face insert  12  when press fit together. In one embodiment as illustrated in  FIG. 3 , each channel has a generally rectangular cross section. In another embodiment, at least one and preferably two undercuts  34  ( FIG. 4 ) are provided in each channel. Undercut  34  is formed by making channel  26  narrow as it approaches its open end. In one embodiment, channel  26  has a dove tail shaped cross section. Alternatively, channel  26  has a generally rounded cross section ( FIG. 5 ), for example circular or oval. Also ridge portion  28  can be rounded or curved outward to facilitate easier engagement between face insert  12  and body portion  24  when the two components are press fit together. Although in these embodiments, each channel  26  opens toward front  32  of body portion  24 , other arrangements are also possible. For example, as illustrated in  FIG. 6 , channel  26  can open towards crown or top line  14  of body portion  24 . Preferably, channel  26  has a dove tail shaped cross section in this embodiment. Face insert  26  will become embedded in this upwardly opening channel when attached to body portion  24 , preferably with adhesives. 
     In another embodiment, interlocking member  25  comprises a plurality of upstanding posts  27  formed by intersecting channels  26 , e.g., one set of horizontal channels  26  and another set of vertical channels  26  as shown in  FIG. 7 . Face insert  12  can be hammered or pressed onto body portion  24 , for example by swaging or cold-forging. This method can also be used with the embodiments shown in  FIGS. 4 and 5 . 
     In one embodiment, in order to form the interlocking structure on the front of the body portion, at least one channel is formed that runs through the portion of the front of the case body. Alternatively, a plurality of parallel channels is formed in the front of the body such that each channel is parallel to at least one of the top lines or the sole of the body portion. The channel can be formed to have a generally rectangular cross section. Alternatively, the channel is formed to have a dove tail shaped cross section. Having formed the interlocking structure in the front of the body, the face insert is pressed onto the front of the cast body to secure a portion of the face insert in the interlocking structure. 
     Exemplary embodiments in accordance with the present invention include a method for making a golf club head by forming an interlocking structure on at least a portion of the front of the body portion of golf club head adjacent a top line thereof. As was described above, the body includes the top line, sole, toe, heel, front and back opposite the front opposite, and the body is made from a first metal. A face insert is attached to the front of the cast body by securing a portion of the face insert in the interlocking structure of the body. The face insert is constructed of a second metal. The first and second metals are selected such that the first metal has a greater density or weight than the second metal. For example, the first metal is selected to be titanium or a titanium alloy, and the second metal is selected to be aluminum or an aluminum alloy. The face insert  12  can occupy between 10% and 40% of the volume of the club head. 
     Low-density, high-strength alloys such as those made from aluminum are particularly suitable for the present invention. The following table illustrates the masses and thickness of corresponding typical face inserts for iron-type golf clubs: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Typical Face  
                 Approx. Mass of  
               
               
                   
                 Face Insert Material 
                 Insert Thickness 
                 Face Insert 
               
               
                   
                   
               
             
            
               
                   
                 High Strength Steel 
                 0.090 in. 
                 50 g 
               
               
                   
                 Titanium 
                 0.120 in. 
                 40 g 
               
               
                   
                 High Strength Aluminum 
                 0.140 in. 
                 30 g 
               
               
                   
                   
               
            
           
         
       
     
     The differences in the thickness of the face inserts for the different materials are necessary due to the varying material strengths; these face inserts have substantially similar strengths. Of the three materials, steel is the strongest, and thus can have the thinnest face, but it has a higher density than both aluminum and titanium. Consequently, even a thinner steel face has a mass greater than either of the titanium or high-strength aluminum faces. Furthermore, the high-strength aluminum face insert&#39;s low density allows more mass to be redistributed for an improved center of gravity location and size of the sweet spot. 
     When a low-density metal such as a high-strength aluminum alloy is used for a face insert, it should be an alloy with suitable material strength and mechanical properties such as yield strength, tensile strength, hardness, elongation, etc., to avoid club failure or performance deterioration. Preferably, a high-strength aluminum alloy such as an alloy containing Scandium and 7-series high strength aluminum alloy (“Sc-7”) or an aluminum alloy containing a percentage of ceramic (“M5C”) is used. Material properties for these alloys, as well as suitable alloys MMC-7 and 13A, are listed in the table below. 
     
       
         
           
               
               
            
               
                   
               
               
                   
                 Alloy: 
               
            
           
           
               
               
               
               
               
            
               
                   
                 MMC-7 
                 Sc-7 
                 13A 
                 M5C 
               
               
                   
               
               
                 Al Series: 
                 7xxx 
                 7xxx 
                 6xxx 
                 5xxx 
               
               
                 Chemical 
                 Al—1.5Mg— 
                 Al—1.5Mg— 
                 Al—0.9Mg + 
                 Al—5.0Mg + 
               
               
                 Composition: 
                 4.0Zn + 6SiC 
                 4.0Zn + Sc 
                 Sc 
                 ceramic 
               
               
                   
                   
                   
                   
                 (approx 
               
               
                   
                   
                   
                   
                 0.8%) 
               
               
                 Hardness: 
                 56 HRB 
                 81 HRB 
                 80 HRB 
                 65 HRB 
               
               
                 Tensile 
                 49 ksi 
                 70 Ksi 
                 62 ksi 
                 51 ksi 
               
               
                 Strength: 
                   
                   
                   
                   
               
               
                 Yield 
                 45 ksi 
                 62 ksi 
                 54 ksi 
                 37 ksi 
               
               
                 Strength: 
                   
                   
                   
                   
               
               
                 Elongation: 
                 11% 
                 10% 
                 11% 
                 14% 
               
               
                 Face 
                 3.2 mm 
                 3.2 mm 
                 3.2 mm 
                 3.5 mm 
               
               
                 thickness 
                 (0.1260 in.) 
                 (0.1260 in.) 
                 (0.1260 in.) 
                 (0.1378 in.) 
               
               
                 preferred: 
               
               
                   
               
            
           
         
       
     
     However, aluminum alloys, including high-strength aluminum alloys such as Sc-7 and M5C, can be susceptible to corrosion, and in some cases more than traditional stainless steel or titanium materials. When aluminum alloys are in contact with steel alloys, galvanic corrosion can also adversely affect the aluminum. 
     In accordance with an embodiment of the present invention, the metals of the inventive golf club are oxidized, more preferably anodized, to improve its strength and corrosion resistance. Oxidation of many untreated metals such as aluminum occurs naturally as the metal undergoes prolonged contact with air. Anodization is a process used to modify the surface of a metal, and it produces a much more uniform, more dense, and harder oxidation layer than what is formed by natural oxidation. It can be used to protect the metal from abrasion or corrosion, create a different surface topography, alter the crystal structure, or even color the metal surface. During anodization, a chemical reaction occurs, producing an oxide layer bonded to the surface of the metal. For example, to anodize an aluminum or aluminum alloy object, the object is first pre-treated by an ordinary degreasing. Then the surface is freed of scratches or existing oxides, preferably by an etching process. The object is submerged in a chromic acid or more preferably a sulfuric acid solution. Next, an aluminum oxide layer is made on the object by passing a DC current through the chromic acid or sulfuric acid solution, with the aluminum object serving as the anode. The current releases hydrogen at the cathode and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide. Anodizing at 12 volts DC, a piece of aluminum with an area of about 15.5 square inches can consume roughly 1 ampere of current. In commercial applications the voltage used is usually in the range of about 15 to 21 volts. Conditions such as acid concentration, solution temperature and current are controlled to allow the formation of a consistent oxide layer, which can be many times thicker than would otherwise be formed. This oxide layer increases both the hardness and the corrosion resistance of the aluminum surface. The oxide forms as microscopic hexagonal “pipe” crystals of corundum, each having a central hexagonal pore, which is also the reason that an anodized part can take on color in the dyeing process. Following the formation of a satisfactory oxide coating, the anodized object is often sealed to maximize the degree of abrasion resistance. Sealing can be accomplished by immersing the object in a sealing medium, such as a 5% aqueous solution of sodium or potassium chromate (pH 5.0 to 6.0) for 15 minutes at a temperature from about 90° C. to 100° C., boiling de-ionized water, cobalt or nickel acetate, or other suitable chemical solutions. 
     Different types of anodizing, Type I, II, and III, are explained in MIL-Spec MIL-A-8625F (Anodic Coatings for Aluminum and Aluminum Alloys), which is hereby incorporated by reference. Most preferably, the face insert is hard-anodized with a Type III coating according to MIL-A-8625F. This hard anodic coating is thicker than standard Type I or Type II anodic coatings by up to 0.0035 inches, and penetrates deeper within the coated metal than standard Type I or Type II anodic coatings. The following table from MIL-A-8625F shows the common thickness ranges among the types of anodic coatings. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Coating Type 
                 Thickness Range (Inches) 
               
               
                   
                   
               
             
            
               
                   
                 Type I, IB, IC, IIB 
                 0.00002 to 0.0007 
               
               
                   
                 Type II 
                 0.00007 to 0.0010 
               
               
                   
                 Type III 
                  0.0005 to 0.0045 
               
               
                   
                   
               
            
           
         
       
     
     Commercial examples of Type III-compliant anodizing processes include the Sanford Hardcoat® process by Duralectra of Natick, Mass. and hardcoat anodizing done by Alpha Metal Finishing Co. of Dexter, Mich., both of which are hereby incorporated by reference. The Type III hard-anodizing process is similar to Type I and II processes, but Type III uses a sulfuric acid bath at a lower temperature, approaching 0° C., as well higher currents. In accordance with MIL-A-8625F, Type III coatings are generally not applied to aluminum alloys having a nominal copper content in excess of 5% or nominal silicon content in excess of 8%. Alloys which have a porosity of greater than about 5% less preferred for Type III coatings. In addition, Because Type III coatings have increased abrasion resistance, sealing or infusing the coating with a polymer in the same manner as Type I and II, as discussed in more detail below, is not required, and the coating can remain somewhat porous. Furthermore, having a porous unsealed structure allows the hard-anodic coating to be infused with a colored dye to change the appearance of the object, or a polymer such as polytetrafluoroethylene (PTFE) or a polyepoxide (epoxy) or polyurethane-based resin to adjust the frictional characteristics of the object. 
     A method for infusing a hard-anodic coating with a polymer is disclosed in U.S. Pat. No. 5,439,712 to Hattori et al. entitled “Method for Making a Composite Aluminum Article,” the entirety of which is hereby incorporated by reference. Once the hard-anodization process is complete, the anodized object is immersed in an infusion solution. This infusion solution contains positively-charged polymer particles dispersed into the solution using a nonionic active agent. The solution and the aluminum object are heated to a temperature ranging from 40° C. to 80° C., and a voltage of 2 to 10 volts is applied. The aluminum object acts as an anode, and the positively-charged polymer particles become absorbed into the hard anodic coating to form a uniform monomolecular layer. As can be appreciated by those skilled in the art, any positively-charged polymer particles can be used, and depending upon the type of alloy or polymer that is used, the temperature and voltage may vary. 
       FIGS. 8 and 9  show an embodiment of the present invention, with face insert  102  attached to body  104  of club head  100 . Face insert  102  is preferably hard-anodized, i.e., Type III, before attachment so that it is coated with hard-anodic coating  110 . After the face insert is hard-anodized, it is preferably attached to the body of the club head via a resin  111  such as epoxy or urethane, with the perimeter of face insert  102  supported on the reverse side by a ledge (not shown) that is part of club head body  104 . However, various other methods of attachment may be envisioned by those skilled in the art, including the attachment methods mentioned in previous embodiments. Other methods of attachment include, but are not limited to, using screws  112  as shown in  FIG. 9 , or cold-forging or swaging a portion  103  of body  104  over face insert  102  shown in  FIG. 9B  to retain face  102 . Insert  102  may have a thin ledge around its periphery sized and dimensioned to receive portion  103 , so that the hitting face is flat. In addition, it may be advantageous to drill larger than normal holes in face insert  102  for screws  112 , as coating  110  will fill in some of the area during the anodizing process, or else use smaller sized screws. 
     Although hard-anodic coatings are often uncolored, gray, or clear, the face insert may be hard-anodized with a colored or dyed coating to create an improved aesthetic effect. The Sanford Hardcoat® process by Duralectra mentioned above has the capability of applying a hard-anodic coat with color to aluminum. Coloring can also be accomplished through a two-step electrolytic method, an integral coloring process which combines anodizing and coloring, organic or inorganic dyeing through polymer infusion as mentioned above, interference coloring, etc. Such a colored coating could be used to effectively outline or shade a hitting area or “sweet spot” on the club head. Sweet spot  114  in  FIG. 8  is an example of such a colored region on the face insert. Coloring only a portion of an object can be done by masking the parts of the object that are not to be anodized with a protective coating mask. Such a coating or masking is often made from vinyl or other polymers and is usually made to be easily applied and removed. A commercially available peelable mask appropriate for hard-anodizing procedures is the PlateOff Mask 4210, available from General Chemical Corp. of Detroit, Mich. 
     The present invention is not limited to examples wherein only the face insert is hard-anodized. Although face insert  102  is preferably constructed from a lighter, less dense material than club head body  104 , it is possible to attach the face insert to club head body  104  prior to the anodization process. As shown in  FIG. 9A , once face insert  102  is attached, then the entire club head  100 , including body  104  and face insert  102 , may be substantially coated by hard-anodic coating  110 . This is especially preferable when face insert  102  is made from aluminum or aluminum alloy, and when club head body  104  is made from titanium or titanium alloy, as these materials may easily be anodized. Whereas aluminum is anodized according to MIL-A-8625F, titanium is anodized according to AMS-2488 or MIS-23545, both of which are hereby incorporated by reference. The Tiodize® Company of Huntington Beach, Calif. processes titanium and titanium alloys according to these specifications under the name of the Tiodize® Processes, all of which are hereby incorporated by reference. The Tiodize® Company produces a brochure titled “Tiodize Process” explaining their processes, which is also hereby incorporated by reference. Titanium is generally anodized in a similar manner as aluminum, by immersing a titanium object in a solution and running an electric current through the solution. However, titanium is typically immersed in an alkaline solution at room temperature, unlike aluminum and its alloys. Although the processes for anodization of aluminum and titanium are not the same, masking may be done during the counterpart anodizing process to avoid interference between the coatings or metals. This embodiment also provides club designers with a wider range of options for attachment methods than if face insert  102  is hard-anodized prior to attachment to club head body  104  to minimize any possible damage to the hard-anodic coating  110  during the attachment process when body  104  and insert  102  have been connected prior to anodization. 
     In yet another embodiment, as shown in  FIG. 11 , a hard-anodic coating may be infused or impregnated with a polymer  117 , preferably a fluorinated polymer such as polytetrafluoroethylene (PTFE), commonly known and available as Teflon® from DuPont, to form low-friction coating  130 . Such a process is commercially available as the Sanford Hardlube® process by Duralectra, which is hereby incorporated by reference. The anodized object is immersed in a solution that contains positive PTFE ions and an electrical current is applied. The positive ions become attracted to the object, which acts as an anode, and become infused into the pores of low-friction coating  130 . Impregnating the hard-anodic coating with PTFE is especially advantageous when low-friction coating  130  is applied to the faces of golf clubs such as drivers or fairway woods, shown in  FIGS. 12-13 , where reduced spin is desired, because PTFE has one of the lowest known coefficients of friction. 
     An optional sole plate  108  may be hard-anodized with regular hard-anodic coating  110  or with a low-friction coating  130  impregnated by a polymer such as PTFE, the latter of which provides a further benefit in fairway woods in that the club will have more protection and encounter less friction when sole plate  108  makes contact with the ground, increasing swing speed and club longevity. The hard-anodic sole plate  108  is also advantageous as applicable to drivers, especially when hitting off a standard plastic driving range mat, due to the reduced friction and extra protection provided by the PTFE-infused coating. This is further applicable to iron-type club heads (as shown in  FIG. 9 ) or putter clubs. As shown in  FIG. 10 , in an alternative to a separate sole plate  108 , a unitary face/sole piece  120  may be provided by the current invention, with said unitary piece  120  preferably being hard-anodized with a low-friction coating  130  infused with PTFE. Unitary piece  120  may act to provide much of the same benefits of the separate inventive face insert and sole plate as seen in previous embodiments, but adds further protection and reduced friction to the lower portion of the club head  100 . 
     As shown in  FIG. 11A , in another embodiment, when increased spin is desired, i.e., in iron-type clubs, the hard anodic coating over the face insert  102  may be sealed with a higher-friction polymer material  137  such as an epoxy-based resin, polyurethane, or polyurea to become hard-anodize increased-friction coating  140 . This is advantageous for highly skilled golfers who desire increased control of the ball when hitting approach shots into greens, because it will increase the friction between the ball and face insert  102 , allowing more control and “workability” for whatever type of shot is desired. The process for infusing the coating with high-friction polymers is similar to the process used for PTFE above. The anodized object is immersed in a solution that contains positive polymer ions and an electrical current is applied. The positive ions become attracted to the object, which acts as an anode, and become infused into the pores of increased-friction coating  140 , sealing the structure. In one example, selected iron-type clubs from a set, such as the short irons and wedges, are constructed with increased-friction coating  140  to increase ball spin and control to the short game. 
     Another embodiment of the present invention is shown in  FIGS. 14A-16 . Golf club head  200  comprises hosel  216 , body portion  224  and face insert  212 . Body portion  224  includes a crown, a skirt, a sole and front  232  having cutout  230 , sized and dimensioned to receive face insert  212 . Cutout  230  can further comprise stepped edge  234  and pocket  226 . Stepped edge  234  comprises a lower ledge  235  positioned between 3.0 and 5.0 millimeters below the surface of front  232 , as shown in  FIG. 14A . More preferably, lower ledge  235  is positioned between 3.5 and 4.0 millimeters below the surface of front  232 . Pocket  226  is preferably machined into front  232  around the circumference of stepped edge  234  and underneath front  232 , so that their openings are not visible from a front plan view of the golf club head. Face insert  212  has upper ledge  213  adapted to be received on top of lower ledge  235  on stepped edge  234 , as best shown in  FIG. 16 . 
     In accordance with this embodiment, face insert  212  is attached to front  232  at cutout  230  so that the top surface of face insert  212  is flush with the surface of front  232 . Preferably, the thickness of face insert  212  is substantially the same as the thickness of front  232 . To retain face insert  212  to front  232 , upper ledge  213  and feet  228  of face insert  212  rest on lower ledge  235  of stepped edge  234  and feet  228  are inserted into pocket  226 . As shown in  FIG. 16 , feet  228  are positioned substantially downward and pocket  226  is oriented substantially sideways. To ensure proper attachment, feet  228  are at least partially plastically deformed into pocket  226 . Optionally, some residual elasticity in feet  228  after being bent can ensure a tight fit. To assist the bending of feet  228  in the proper direction, feet  228  can be initially oriented outward toward pocket  226  (not shown). Alternatively, to assist in the outward bending of feet  228  notch(es)  215  or other weakened sections can be included on feet  228  to assist the bending, or angled surface  239  can be used. Preferably, feet  228  are securely disposed in pocket  226  by swaging or cold-forging, causing feet  228  to plastically deform to fit pocket  226 . More preferably, feet  228  are inserted into pocket  226  by the process of micro-swaging, wherein approximately 15 tons of force are used to bend said feet into said pocket. This process requires significantly less force than typical swaging processes, which require about 80 tons of force to plastically deform a part. Feet  228  may have a substantially rectangular shape or may have any shape suitable for swaging. Pocket  226  may comprise a plurality of pockets having a substantially similar shape to feet  228 . Main portion  240  of face insert  212  may have a substantially oval shape or any suitable shape to create a hitting surface on front  232 . After insertion and swaging, feet  228  are preferably not visible from any exterior view of club head  200 , as is illustrated in  FIG. 14B . 
     To further secure face insert  212  to front  232 , an adhesive or glue, such as 3M® Scotch-Weld® Epoxy Adhesive DP420, may be used to adhere upper ledge  213  of face insert  212  to lower ledge  235  of front  232 . The addition of glue to the face insert-body portion subassembly not only enhances the attachment of said components, but also improves the sound and feel of the impact between club head and ball. Furthermore, the sound at impact can be controlled (hard vs. soft) by controlling the amount of glue used. It should be noted that during testing, a model club head made according to the present invention without the use of glue or adhesive was subjected to 3000 hits and produced no adverse feel or sound (rattling, looseness, etc.). 
     Golf club head  200  may further comprise top line insert  244 , as shown in  FIG. 17 . Cavity  242  may be machined into or otherwise created in the top line of golf club  200  such that insert  244  may be received into cavity  242 . Top line insert  244  preferably comprises a material having a density less than the density of face insert  212  and may have any shape suitable for positioning at the top line of an iron-type golf club head. For example, top line insert  244  may comprise aluminum, an aluminum alloy or a polymer. More preferably, top line insert  244  comprises a material having a density less than 2.85 g/cm 3 . The placement of the lightweight insert at the top line of golf club head  200  causes the center of gravity of the golf club head to move downward to a more optimal position. 
     In addition to top line insert  244 , golf club head  200  may also include any one of or any combination of high density weight members  248 A-C, disposed to back  246 , as shown in  FIG. 18 . Golf club head  200  is depicted as a muscle-back iron type club in  FIG. 18 , however, in accordance with this and all previous embodiments, golf club head  200  may also be a cavity-back iron type club head. Weight members  248 A-C are preferably positioned behind and/or below the center of gravity of golf club head  200  to increase the moment of inertia of the club head. Golf club head  200  may include cavities located on back  246  toward the toe and the heel, designed to receive weight members  248 A and  248 B, respectively. Golf club head  200  may also include weight member or cup  248 C disposed on back  246  along the perimeter of the sole of the club head. Weight members  248 A-C preferably comprise a material having a density greater than the density of the material comprising body portion  224 . In particular, weight members  248 A-C may comprise tungsten. 
     As in previous embodiments of the present invention, the club head comprises multiple metals to optimize its performance. Body portion  224  comprises a first metal having a first density, while face insert  212  comprises a second metal having a second density. According to this aspect of the present invention, the first metal preferably has a greater density than the second metal to keep the center of gravity downward and aftward. Body portion  224  preferably comprises a high-strength metal or metal alloy, such as stainless steel, titanium or titanium alloy. More preferably, body portion  224  comprises stainless steel 17-4. Face insert  212  preferably comprises a metal or metal alloy exhibiting both high-strength and low density, such as aluminum, aluminum alloys or aluminum metal matrix composites (MMCs), such as those described above. More preferably, face insert  212  comprises an aluminum metal matrix composite or MMC, known as the M9 MMC. 
     The use of M9 in face insert  212  provides for a strong and lightweight hitting surface. M9 is a member of the 7000 series aluminum alloys, and typically includes certain amounts of magnesium, zinc and copper, with a small percentage of scandium precipitated into the metal matrix. More specifically, M9 contains approximately 0.4 percent scandium, the addition of which improves characteristics such as the tensile strength, yield strength and hardness of the alloy. The scandium can be present in the range of about 0.2% to about 0.8%, preferably from about 0.3% to about 0.6%, and more preferably about 0.4%. An amount of zirconium less than but comparable to the amount of scandium is also precipitated into the M9 metal matrix composite. Approximate attributes of M9 are shown in the table below. 
     
       
         
           
               
             
               
                   
               
               
                 M9 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 MMC 
                 Mg 3% 
               
               
                   
                 composition 
                 Zn 7% 
               
               
                   
                   
                 Cu 2% 
               
               
                   
                   
                 Sc + Zr 0.1-0.5% 
               
               
                   
                   
                 Al balance 
               
               
                   
                 Density 
                 2.85 
               
               
                   
                 (g/cm 3 ) 
                   
               
               
                   
                 Elongation 
                 12 
               
               
                   
                 (% in 2 in.) 
                   
               
               
                   
                 Melting range 
                 640-680 
               
               
                   
                 (C. °) 
               
               
                   
                   
               
            
           
         
       
     
     Compared to other aluminum alloys and MMCs, M9 has better strength and hardness. Moreover, M9 has a low density of about 2.85 g/cm 3 , making it much lighter than stainless steel, titanium and titanium alloys, and other high-strength metals. M9 reaches its peak strength after rolling and heat-treating. The following table illustrates a number of characteristics of M9 as compared to other aluminum alloys and MMCs. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                 M9 
                 MMC-7 
                 Sc-7 
                 13A 
                 M5C 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Al series 
                 7000 
                 7000 
                 7000 
                 6000 
                 5000 
               
               
                   
                 Hardness 
                 85-95 
                 56 
                 81 
                 80 
                 65 
               
               
                   
                 (HRB) 
                   
                   
                   
                   
                   
               
               
                   
                 Tensile 
                 94-98 
                 49 
                 70 
                 62 
                 51 
               
               
                   
                 strength 
                   
                   
                   
                   
                   
               
               
                   
                 (Ksi) 
                   
                   
                   
                   
                   
               
               
                   
                 Yield 
                 85 
                 45 
                 62 
                 54 
                 37 
               
               
                   
                 strength 
                   
                   
                   
                   
                   
               
               
                   
                 (Ksi) 
                   
                   
                   
                   
                   
               
               
                   
                   
               
            
           
         
       
     
     In contrast to more dense metals typically used for body construction, face insert  212  comprising M9 is very light, allowing more weight to be apportioned to the back and side perimeters of body portion  224 , a preferred method of weight distribution to optimize moment of inertia and center of gravity. The strength of the M9 material is similar to that of 431 stainless steel, but with much lower density. The M9 material also has better vibration absorption than forged iron. The table below shows strength and density characteristics of M9 as compared to other high-strength metals. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                 M9 
                 17-4 
                 431 
                 8620 
                 Ti 6-4 
               
               
                   
               
             
            
               
                 Metal 
                 Aluminum 
                 Stainless 
                 Stainless 
                 Stainless 
                 Titanium 
               
               
                   
                 MMC 
                 steel 
                 steel 
                 steel 
                 alloy 
               
               
                 Density (g/cm 3 ) 
                 2.85 
                 7.75 
                 7.68 
                 7.80 
                 4.43 
               
               
                 Hardness 
                 85-95 HRB 
                 28-38 HRC 
                 18-25 HRC 
                 — 
                 35-45 HRC 
               
               
                 Tensile strength 
                 94-98 
                 140 
                 125 
                 85 
                 140 
               
               
                 (Ksi) 
                   
                   
                   
                   
                   
               
               
                 Yield strength 
                 85 
                 120 
                 95 
                 60 
                 134 
               
               
                 (Ksi) 
                   
                   
                   
                   
                   
               
               
                 Strength/Density 
                 237 
                 125 
                 112 
                 75 
                 218 
               
               
                 (MPa/g/cm 3 ) 
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     As discussed above, M9 is rolled and subjected to heat-treating to increase its strength and hardness. After the hardening process, the average grain size of the M9 MMC is decreased from about ten micrometers to between three and five micrometers. To further enhance strength and durability, face insert  212  may be anodized. Preferably, face insert  212  is anodized using the Type I process discussed in previous embodiments, as the chromic acid bath of the Type I process is able to produce an oxidization layer on the surface of parts with complex geometries, such as face insert  212 . Body portion  224  may also be anodized, particularly if body portion  224  is composed of titanium or titanium alloy. 
     While it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives of the present invention, it is appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Additionally, feature(s) and/or element(s) from any embodiment may be used singly or in combination with other embodiment(s) and steps or elements from methods in accordance with the present invention can be executed or performed in any suitable order. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments, which would come within the spirit and scope of the present invention.