Composite piston for reciprocating machine

A composite piston and method for forming such a piston for a reciprocating machine such as an internal combustion engine. A blank is formed from a pair of dissimilar alloys, one of which has substantially greater properties such as strength or abrasion resistance. The blank is forged into a piston in such a way that the two materials are bonded together in the forging process. The higher strength and/or abrasion resistance material forms at least a part of the outer surface of the piston in areas where the better properties are required. The other material backs up the higher strength or hardness material in necessary areas so as to provide an integral structure that has lightweight, low costs and nevertheless the desired properties. Various physical constructions and forming operations are disclosed.

BACKGROUND OF THE INVENTION
 This invention relates to a piston for a machine such as an internal
 combustion engine and to a method for manufacturing such a piston.
 The piston in a reciprocating machine is obviously a very critical part of
 the mechanism. This is particularly true with internal combustion engines
 in as much as the piston is the part of the engine that receives the
 explosive force from the combustion and transmits it through the
 connecting rod to a crankshaft for providing an output force. The various
 parts of the piston have specific functions in order to achieve this
 result.
 The head of the piston must be able to withstand the compressive force and
 temperature of combustion. Also the area adjacent the upper surface of the
 head forms a ring groove area where the piston rings are supported. These
 provide a sealing function with the cylinder bore so as to confine the
 combustion products.
 The piston is also provided with pin bosses that receive the piston pin and
 which transmit the force from the piston to the connecting rod through the
 piston pin. Obviously, there are high forced transmissions in this area.
 Furthermore there are considerable frictional forces and loads between the
 piston and piston pin.
 In addition, the piston as a skirt portion that rubs against the cylinder
 bore and which assists in maintaining the piston in an upright condition
 within the cylinder bore. In addition, the side thrusts on the piston are
 taken by the skirt and thus it is also subjected to forces and must have
 high abrasion resistance due to its rubbing action with the cylinder bore.
 Thus, it should be apparent that the different parts of the piston have
 different functions that require optimally different materials. Of course,
 it is possible to form the entire piston from the same material but this
 can give rise to high costs and also high weights. It is important to
 reduce the weight of the piston so as to reduce the inertial loading on
 the engine and provide high power outputs and high engine crankshafts
 speeds. Also, the lighter the weight the lighter the balancing masses in
 the engine can be in order to reduce vibrations.
 Some of these functions can be achieved by changing the dimensions of the
 piston either alone or in combustion with changing the materials. For
 example, the sealing function can be improved if the piston ring area is
 made greater and a greater number or greater size of piston rings are
 employed. However, this causes emission problems in that the area around
 the piston rings may retain combustion products and can cause some
 emission concerns.
 Thus, there has been proposed the formation of pistons with different
 materials, each serving its intended purpose for the particular part of
 the piston in which it is positioned. However, this is quite a difficulty
 in adhering or connecting these different materials to each other to
 provide a unitary structure. Some more methods of connections can be
 employ brazing or welding. However, when applied with these additional
 heats in order to connect the materials together, then deterioration in
 the properties of the associated and affected materials can result thus
 defeating the main purpose of the composite construction.
 It has also been proposed to improve the strength of the piston in certain
 areas by casting in inserts in the areas where stresses is highest. For
 example, it has been proposed to cast in inserts in the area of the piston
 pin bosses so as to increase their strength without adding significantly
 to the overall weight of the piston. However, this also has some of the
 same problems aforenoted in connection with using dissimilar materials.
 Furthermore, the casting process becomes somewhat complicated and thus
 this method does not totally solve the problem.
 Forging is another technique by which composite materials may be used. Some
 methods have been proposed, but they have not been totally successful in
 achieving the desired bonding strength. Therefore we have proposed a
 method and construction that employs a combination of powdered metal
 technology and forging bonding that can produce excellent results. This is
 disclosed in the co-pending application of certain of the applicants
 hereof entitled "Piston For Internal Combustion Engine And Process Of
 Making Same", Ser. No. 08/859,536, Filed May 20, 1997 and assigned to the
 assignee hereof.
 The materials utilized also are important not only to achieve the desired
 properties, but also the proper bond. Basically, pistons for engines are
 generally formed from aluminum or aluminum alloy materials. The aluminum
 has the advantage of light weight and relatively high strength. However,
 the use of alloy materials has been resorted to so as to improve certain
 characteristics.
 For example, silicon (Si) in an alloy with the aluminum to increase
 abrasion resistance and resistance to hardening under temperature. Copper
 (Cu) and Magnesium (Mg) have also been employed for increasing strength.
 At times, however, these alloying elements can present some problems in
 that their inclusion in a casting process can cause difference in particle
 sizes to result which can offset some of the benefits of the alloying.
 It has also proposed, therefore, a method of forming a piston material by a
 form of sintering process which then permits the forging of a piston to
 obtain the desired characteristics. Such an arrangement is disclosed in
 the co-pending application of certain of the inventors hereof entitled
 "Piston For Internal Combustion Engine And Material Therefore", Ser. No.
 09/022,647, filed Feb. 12, 1998, and also assigned to the assignee hereof.
 In accordance with the features hereof these materials are combined with
 lower costs materials to form a composite piston that will provide the
 performance desired along with lightweight and lower costs.
 It is, therefore, a principal object to this invention to provide an
 improved piston construction for an internal combustion engine.
 It is a further object to this invention to provide an improved,
 lightweight, high strength and high abrasion resistant, composite piston
 for a reciprocating machine.
 It is a further object to this invention to provide an improved low cost
 piston having the desired material requirements in the various areas of
 the piston.
 It is a further object to this invention to provide an improved method for
 manufacturing a composite piston of the aforenoted type.
 SUMMARY OF THE INVENTION
 This invention is adapted to be embodied in a composite piston for a
 reciprocating machine comprised of a pair of dissimilar materials bonded
 together by a forging process. A first of the materials has a property
 having characteristics selected from the group of strength and abrasion
 resistance that is substantially greater than the other. The piston is
 comprised of a head portion having an upper surface adapted to experience
 pressure and a peripheral ring groove portion for receiving at least one
 sealing ring below the upper surface. A skirt portion comprised of at
 least a pair of surfaces for sliding engagement with a cylinder bore
 formed below said head portion. A pair of piston pin bosses having piston
 pin receiving openings for connection to a connecting rod small end by a
 piston pin is disposed below the ring groove. The piston pin bosses are
 formed between circumferentially spaced portions of the skirt portion
 surfaces. The one material forms at least a portion of the piston pin
 bosses in the area where engaged by the piston pin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
 Referring now to the first embodiment and that shown in FIGS. 1-9, and
 initially primarily to FIGS. 1-5, a piston 21 constructed in accordance
 with this embodiment is illustrated. The piston 21 has a configuration
 which in external appearance is similar to most conventional pistons. This
 includes a head portion 22 from which a skirt portion 23 depends. The
 interior of the piston 21 in the area of the piston skirt 23 is formed
 with larger thickness material portions that form piston pin bosses 24.
 The area above the piston pin bosses 24 forms a head portion in which
 piston ring grooves 25 are received. A piston pin 26 is received in bored
 openings in the piston pin bosses 24 to provide the connection to the
 associated connecting rod which is not shown.
 In accordance with this embodiment of the invention, the body of the piston
 21 is formed from two materials comprised of a first material indicated at
 21A which is of a higher strength and higher abrasion resistance and which
 is alloyed in a manner to be described and which is formed also in a
 manner to be described.
 Because of the alloying of the material 21A, it has a higher cost and
 higher weight than the remaining base piston material, indicated at 21B.
 The base piston material 21B may be also formed from a known lightweight
 and low cost material. In fact, lower cost materials may be employed then
 with conventional pistons because the material 21A takes the higher
 loading, in a manner which will be described.
 As may be seen primarily from FIGS. 1 through 3, the head area 22 including
 its entire, exposed upper surface and the exposed or exterior ring area 25
 are formed so that the material 21A forms the exposed outer surface of the
 piston 21. In addition the upper or piston head sides of the boss portions
 24 that contact the outer ends of the piston pin 26 are also formed by the
 material 21A. skirt in the area between the piston pin bosses 24 then in
 the area of the piston pin bosses.
 This is to provide the requisite strength and heat resistance in the piston
 pin groove area 25 and also to carry on this material so that it is in the
 upper area of the piston pin bosses where the piston pins 26 are received.
 This provides wear resistance and strength to take the major part of the
 loading when the piston 21 is being driven downwardly from the expansion
 of the gases.
 The material by which the piston portion 21A is formed is made as set forth
 in the aforenoted co-pending application, Ser. No. 09/022,647. However,
 that method will be described again in detail here.
 First, a powered metal is formed having the desired chemical constituents
 and alloying to be employed in the finished surface portion of the piston.
 This is done by first forming an ingot from which the powered metal is to
 be formed which will be compressed then into a sintered state to form the
 blank from which the piston portion is forged.
 The ingot is formed from an alloy of aluminum and certain alloying
 materials which are added to improve strength, abrasion resistance and
 resistance to deterioration under heat. Basically, this ingot is formed
 from an aluminum alloy that consists of aluminum (Al) as a base material
 and certain alloying materials such as silicon (Si), iron (Fe), and other
 materials as will be noted. As will become apparent as this description
 proceeds, the ingot is subsequently converted into a metal powder state
 which is subsequently heated and extruded to form blanks from which the
 piston portion 21A is forged.
 Certain of these alloying materials may not be included directly in the
 ingot but may be formed as separate powders which are then mixed with the
 ingot powder during the extrusion and heating step that forms the formed
 metal blanks for forging. As will be described below, silicon carbide
 (SiC) is one of such materials that may be separately mixed with the
 powder formed from the ingot.
 EXAMPLE 1
 A first example of the material from which the ingot may be formed includes
 as alloying materials to the base aluminum (Al) the following alloying
 elements:

silicon (Si) 10-22% by weight
 iron (Fe) 1-10% by weight
 copper (Cu) 0.5-5% by weight
 magnesium (Mg) 0.5-5% by weight
 manganese (Mn) 1% or less by weight
 nickel (Ni) 1% or less by weight
 chromium (Cr) 1% or less by weight
 zirconium (Zr) 2% or less by weight
 molybdenum (Mo) 1% or less by weight
 The silicon alloying material improves abrasion resistance and resistance
 to cracking or breaking and is in the form of hard primary crystals or
 eutectic crystals in the metal texture. Iron is added to obtain high
 strength at temperatures of 200.degree. C. or more and by disbursing and
 strengthening the metal texture. Copper and magnesium are added to improve
 the strength at temperatures less than 200.degree. C. It has been found
 that amounts greater than outside the ranges specified may fail to obtain
 the desired abrasion resistance and strength at the varying temperatures.
 EXAMPLE 2
 A specific example of alloying material that falls within the range of
 Example 1 and which is preferred is as follows:

silicon (Si) 17%
 iron (Fe) 5% by weight
 copper (Cu) 1% by weight
 magnesium (Mg) 5% by weight
 manganese (Mn) 0.01% by weight
 nickel (Ni) 0.01% by weight
 chromium (Cr) 0.01% by weight
 zirconium (Zr) 1% by weight
 molybdenum (Mo) 0.01% by weight
 EXAMPLE 3
 Another range of embodiment of alloy that can be employed in connection
 with the invention employs Silicon carbide (SiC) as an alloying material
 and has the following alloying elements:

silicon (Si) 10-22% by weight
 iron (Fe) 1-10% by weight
 copper (Cu) 0.5-5% by weight
 magnesium (Mg) 0.5-5% by weight
 manganese (Mn) 1% or less by weight
 nickel (Ni) 1% or less by weight
 chromium (Cr) 1% or less by weight
 zirconium (Zr) 2% or less by weight
 molybdenum (Mo) 1% or less by weight
 silicon carbide (SiC) 1-10% by weight
 EXAMPLE 4
 A specific preferred embodiment employing silicon carbide as an alloying
 agent and falling within the range of Example 3 includes the following
 components:

silicon (Si) 17% by weight
 iron (Fe) 5% by weight
 copper (Cu) 1% by weight
 magnesium (Mg) 0.5% by weight
 manganese (Mn) 0.01% by weight
 nickel (Ni) 0.01% by weight
 chromium (Cr) 0.01% by weight
 zirconium (Zr) 1% by weight
 molybdenum (Mo) 0.01% by weight
 silicon carbide (SiC) 5% by weight
 In addition to silicon carbide, other materials such as aluminum oxide
 (Al.sub.2 O.sub.3) or aluminum nitride (AlN) may be substituted to improve
 abrasion resistance in the amounts specified in Examples 3 and 4, i.e.
 1-10% or specifically 5%.
 It has been found that the crystalline size of certain of the alloying
 materials is important in obtaining the desired abrasion resistance,
 resistance to cracking and high fatigue strength. For example, the initial
 crystalline silicon particle diameter should be not greater than 10 .mu.m.
 Also, the average particle diameter of the iron should be not greater than
 10 .mu.m. Where as has been previously noted, these alloying materials may
 be either incorporated in the ingot from which the powder is formed or may
 be formed from separate particles that arc molded into the pellet through
 mixing with the particles formed from the primary aluminum alloy. Either
 method can be employed so long as the resulting crystalline particle size
 is within the range set forth.
 In the examples given as Example 3 and Example 4 it is particularly
 advantageous to add the silicon carbide (SiC) as a separate powder mixed
 with the powder from the ingot before solidifying. If this is done the
 particle size of the silicon carbide (SiC) powder before mixing should
 preferably be 5 .mu.m.
 The particles are formed by melting the ingot from the alloy and the base
 materials at a temperature of 700.degree. C. or more. This molten material
 is then sprayed like a fog and rapidly cooled to solidify at a cooling
 rate of at least 100.degree. C. per second thereby obtaining a rapidly
 solidified powder metal of the aluminum alloy. It has been found that good
 results can be obtained when the specific particle size of the
 wear-resistant material such as the silicon carbide has a diameter of 5
 .mu.m. As has been noted, this is particularly useful when the particles
 are formed separately and combined in the next step which will be
 described.
 After the power has been formed in the manner aforedescribed, then it is
 sintered into a blank from which the final forged piston 21 will be
 formed. The part 21A is formed as a cylindrical blank as shown in FIG. 4
 by a process utilizing an apparatus as illustrated in FIG. 5.
 This cylindrical blank, also indicated by the reference numeral 21A, is
 placed into engagement with another blank, also indicated by the reference
 numeral 21B, which also be formed by a powered sintering process or by
 casting or any other process so as to result in a forging blank having a
 configuration as shown in FIG. 4 and which is identified in this figure by
 the reference numeral 28.
 A specific example of the material 21B may be an aluminum alloy
 conventionally used for casting as a melt production-type (continuous
 casting material) such as an aluminum alloy of a melt production-type
 containing aluminum (Al) as a base material. This may be alloyed with
 10-22% by weight of silicon (Si), 1% by weight or less of iron (Fe),
 0.5-5% by weight of copper (Cu), 0.5-2% by weight of magnesium (Mg), 1% by
 weight or less of manganese (Mn), 1% by weight or less of nickel (Ni) and
 1% by weight or less of chromium (Cr).
 One specific example of such a material is an aluminum alloy of the melt
 production-type containing 19% by weight of silicon (Si), 0.2% by weight
 of iron (Fe), 4% by weight of copper (Cu), 1% by weight of magnesium (Mg),
 0.1% by weight of manganese (Mn), 0.1% by weight of nickel (Ni) and 0.1%
 by weight of chromium (Cr).
 The sintered blank 21A is then formed by an extruding process utilizing an
 apparatus as shown in FIG. 5. Basically, the powder is heated and extruded
 under pressure at a temperature of less than 700.degree. C. and preferably
 in the range of 400 to 500.degree. C. into a hollow cylinder. The
 apparatus by which this is done is illustrated, as has been noted in FIG.
 5, and will now be described by particular reference to that figure.
 The apparatus is indicated generally by the reference numeral 29 and
 includes an extruding cylinder having a bore 31 in which the powder,
 indicated at 32 is charged. A smaller diameter extruding passage 33 is
 formed at one end of the bore 31.
 An annular piston 34 is mounted within the bore 31 and has an extending
 portion 35 that is engaged by a ram for extruding the powder through the
 portion 33 in which it solidifies and results in the formation of a hollow
 cylindrical extrusion 36. This extrusion is then cut off at the desired
 lengths to provide the blank portions 21A.
 These portions are then placed into engagement with the blank 21B so as to
 provide the configuration as shown in FIG. 4 which forms the final forging
 blank.
 After the blank 28 is formed, it is then placed in the forging apparatus
 shown in FIGS. 6 and 7 and which forging apparatus is indicated generally
 by the reference numeral 37. This forging apparatus 37 includes a female
 die 38 having a cylindrical opening 39 closed by an end wall having a
 recess 41.
 It should be noted that the sections of FIGS. 6 and 7 are taken along the
 same plane as FIG. 3 so that the piston shape can be compared although the
 sides are reversed in this figure.
 The blank 28 may be coated with a release material and also may be heated
 to bring it up to a temperature less than 700.degree. C. and preferably in
 the range of 400 to 500.degree. C. A ram 42 is then pressed into the
 forging die 38 to the position shown in FIG. 7 wherein the final formation
 of the piston is formed. Preferably, the die 38 and forging press 42 are
 also brought up to a temperature less than 700.degree. C. and then in the
 range of 400 to 500.degree. C. If this is done, the blank 28 need not be
 heated but can be left in the dies for a time period until it reaches this
 temperature.
 After the forging has been completed, then finished machining, heat
 treating and other machining steps can be formed. This can include the
 cutting of the ring grooves, final honing of the piston pin holes and any
 other finish machining and surface treatment as may be desired.
 During the forging process, any surface oxides of the material of either of
 the blank materials 21A or 21B will be destroyed by the friction of the
 forging process thus improving the bond between the materials. This
 further increases the strength of the resulting piston.
 The surface properties of the resulting piston and particularly the
 specific areas of the piston comprised of the materials 21A and 21B will
 now be described by references to FIGS. 8 and 9. In these figures, the
 characteristics of Examples 2 and 4 above are compared as materials A2 and
 A4 with an example of a conventional piston material identified at B.
 Basically, the difference between Materials 2 and 4 is that Material 2 has
 no silicone carbide while Material 4 is alloyed with silicone carbide.
 Except for this difference, the constituents of the two alloys are the
 same.
 FIG. 8 shows the results of a conventional fretting type abrasion test.
 This is done by repeatedly scuffing the material. This is done at a
 temperature of 250.degree. C. The greater the area of abrasion marks, the
 less the abrasion resistance. It will be seen that the two alloy materials
 in accordance with the invention, i.e. Materials A2 and A4, have much
 greater abrasion resistance then the conventional piston material B. As a
 result, the areas that are subject to abrasion are formed with this
 surface and the remaining area of the piston can be made from the lighter
 weight, less expensive material.
 FIG. 9 shows the fatigues strengths of the same respective materials at
 various temperatures. It will be seen that the fatigue strength at various
 temperatures is much greater for the materials in accordance with the
 invention then the conventional material which is used for the base of the
 piston. Hence, by utilizing this method it is possible to improve the
 piston performance while not increasing significantly its weight or cost.
 FIGS. 10-16 show another embodiment which differs from the previous
 embodiment in two regards. The first is the relative area of the piston 21
 which is formed from the two materials 21A and 21B and the second, is how
 the blank is formed from which the piston is formed.
 Referring first to FIGS. 10-12, it should be noted that the general shape
 of the piston is substantially the same as the previously described
 embodiment and hence the same reference characters have been applied.
 However, with this embodiment further lightening and the weight without
 sacrifice of strength is made possible by leaving the area at the center
 of the upper surface of the head portion 22 uncovered by the material 21A.
 This open area is indicated at 51.
 This elimination of a portion of the material 21A reduces the cost and
 weight without a significant loss of strength of the piston 21. In fact
 the exposure of a portion of the material 21B on the piston head 22 may
 result in lower piston temperatures because of the possible higher heat
 transfer.
 This embodiment is also formed using a blank which differs from the
 previous blanks and which is shown in FIG. 13 and identified by the
 reference numeral 52. Specifically the blank portion 21A has an open
 hollow cylindrical shape so as to expose the material 21B in the noted
 head area 51.
 The blank 52 is formed by the apparatus shown in FIG. 14. Like the previous
 example, the sintered blank formed by an extruding process utilizing an
 apparatus as shown in FIG. 14. Basically, the powder is heated and
 extruded under pressure at a temperature of less than 700.degree. C. and
 preferably in the range of 400 to 500.degree. C. into a cylinder. The
 apparatus by which this is done is illustrated, as has been noted in FIG.
 14, and will now be described by particular reference to that figure.
 The apparatus is indicated generally by the reference numeral 61 and
 includes an extruding cylinder having a bore 62 in which the powder,
 indicated at 63 is charged. A fixed core rod 64 extends through the end of
 the cylinder 62 and through a smaller diameter extruding passage 65.
 An annular piston 66 is mounted within the bore 62 and has an extending
 portion 67 that is engaged by a ram for extruding the powder through the
 portion 65 in which it solidifies and results in the formation of a hollow
 cylindrical extrusion 68. This extrusion is then cut off at the desired
 lengths to provide the blank portions 21A.
 These portions are then slipped over the smaller diameter portion of the
 blank 21B so as to provide the configuration as shown in FIG. 13 which
 forms the final forging blank. The resulting blank 52 is forged by the
 apparatus shown in FIGS. 15 and 16 in the manner previously described.
 Except for the end wall that forms the piston head 22 this apparatus is the
 same as that shown in FIGS. 6 and 7. Thus like reference numerals have
 been employed to identify like parts. Rather than a central recess at the
 closed end of the die cylinder 39 there is formed an annular groove 70.
 This is at the areas where the two blank materials 21A and 21B abut. This
 helps to insure that there will be full metal to metal bonding in this
 area as seen in FIG. 16.
 FIGS. 17-23 show another embodiment of the invention which is generally
 similar to the embodiments previously described. In this embodiment,
 however, the piston material 21A forms the entire outer surface of the
 resulting piston, indicated again at 21, except for the exposed area of
 the material 21b in the head area 51 as seen in FIGS. 18 and 19. Thus, in
 this embodiment, the entire exterior surface of the piston 21 is formed
 from the harder more abrasion resistant material except for this head void
 area 51. Also and as seen best in FIG. 19, the outer portions of the
 piston pin receiving bores of the bosses 24 arc also covered completely
 around their circumference by the material 21A.
 The blank utilized for this embodiment has a different shape and is also
 formed in a different manner as shown in FIGS. 20 and 21. In this
 embodiment the portion 21B is also formed simultaneously by an extrusion
 process. Hence, the extruding apparatus, indicated generally by the
 reference numeral 71 includes a first cylindrical chamber 72 wherein a
 solid blank of the material for the blank portion 21B is positioned. This
 is heated to a temperature in the range of 400.degree. C. to 500.degree.
 C. A piston 73 acts to extrude this material through a restricted
 extruding opening 74. Thus, there will be a solid core 75 formed around
 which the piston material 21A is extruded.
 Thus, the core 75 that is formed is extruded into a chamber 76. This
 chamber 76 has a tapering portion 77 that leads to an extruding portion
 78.
 One or more side pistons 79 compresses the metal particles of the material
 21A in the chamber 76 and extrudes them through the opening 78 so that a
 composite blank 81 is formed. This blank 81 will thus not only facilitate
 the extrusion, but also will permit some initial bonding of the material.
 The piston is then forged from this blank in the manner previously
 described and using the same type of apparatus as shown in FIGS. 22 and
 23. The piston 21 in the previous embodiments has been forged in an
 inverted position in the previous embodiments. That is the pressing
 element 42 has formed the interior of the piston 21. That arrangement is
 reversed in this embodiment. That is the interior of the piston 21 is
 formed by an extension 91 of an end cap 92 that closes the lower end of
 the cavity 39 of the die 38.
 Thus, from the forgoing description, it should be readily apparent that the
 described constructions and methodology permits the formation of
 lightweight pistons having the appropriate surface properties and metal
 characteristics without significant increase in weight and/or cost. Of
 course, the foregoing description is that of preferred embodiments of the
 invention, and various changes and modifications can be made without
 departing from the spirit and scope of the invention, as defined by the
 appended claims.