Patent Publication Number: US-6209446-B1

Title: Piston for internal combustion engine and process of making same

Description:
This application is a continuation of prior application, U.S. Ser. No. 08/859,536, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a piston for an internal combustion engine and a method of making the piston. 
     BACKGROUND OF THE INVENTION 
     Internal combustion engines are frequently of the piston-type. The piston(s) of these engines are subjected to extreme forces, frictional wear, and high temperature. In addition, the shape and size of the piston greatly affects the performance of the engine. So that the piston and the engine perform optimally, the piston should satisfy several criteria. 
     First, the piston should be light-weight. For among other reasons, reducing the weight of the piston reduces inertial forces generated by the piston as its moves within the engine. Generally, for the piston to be light-weight, it must be thin walled to reduce its mass, and be constructed of a low-density material. 
     The height of the land portion of the piston (i.e. that portion of the piston above the piston ring) should be small. This reduced height increases the compression ratio, which results in increased engine performance. In addition, the shorter land results in a smaller crevice or “squish” volume, causing a reduction in the amount of unburned fuel, improving exhaust emissions. For the piston to have a short land, it is normally necessary for the piston material to maintain its hardness even at temperatures above about 350° C. so as not to thermally fuse the ring(s) thereto and so that the corner of the land does not yield or deform at high temperature (i.e. above about 350° C.). 
     Not only must the land portion of the piston not deform for the reasons described above, but the other portions of the piston, such as the skirt, must also not deform during the piston&#39;s use. This normally requires that the piston be thick-walled and be constructed of a material which retains a high Young&#39;s modulus even at high temperatures (i.e. above about 350° C. on the piston&#39;s top surface). 
     In sum, the piston must have a high fatigue strength, a high proof strength, and a high hardness at high temperatures, and yet be constructed from a material which has a low density and allows the piston to be of a thin-walled construction. Further, the piston material must on the one hand provide high strength and hardness, and yet must be yieldable if the piston is to be forged (as opposed to cast or machined, both of which processes increase the cost of manufacture of the piston). To date, no material and piston configuration has satisfied all of these criteria. 
     As one attempt at satisfying these criteria, it is known to construct a piston having a head portion which is clad with a different material than a material which clads a skirt portion of the piston (such as by having the first cladding material comprise aluminum and the other comprise a compound layer made of aluminum mixed with fibers of material). The claddings comprising different materials are joined together by forging. 
     This arrangement is disadvantageous because insufficient joining strength is provided at the interface between the joined materials. Generally, this is now believed to be, in part, due to the fact that insufficient slip occurs between the two materials during forging. As a result, an oxide film on the surfaces is not destroyed, this film inhibiting strong bonding between the materials. As one means for increasing this bonding strength, fiber reinforcement may be used. This tends to create stress concentrations to occur on the interfaces between the matrix and the reinforcing fibers or material, such that an insufficient fatigue strength at high temperatures is the result. Also, this method of manufacture increases the manufacturing cost, and generally can not be used when its is desired that only a small portion of the piston (such as the area about the piston ring groove(s)) be formed of a different material. 
     In a second arrangement, it is known to make a two-layer composition by powder-forming quenched powder aluminum matrices (powder metal) of a common composition, each layer having a different ratio of included ceramic powder. The two-layer composition is then heat-pressed to form a body. The body is then heat-forged into form a piston, with the head portion containing a higher ratio of ceramic powder and the skirt portion contain a lower ratio of ceramic powder. 
     This arrangement has the disadvantage that insufficient joining strength results at the joining interface between the two compositions, especially in the center. One cause for this is now believed to be that relatively little slip occurs at the interface between the layers during forging. Also, since each layer is constructed from the same matrix material, it is not possible for a lower portion (forming the skirt) to constitute a material which is easily formed, and for a top portion (forming the head) to have high hardness, heat resistance and the like. 
     In a third known arrangement, a head portion of the piston is constructed of forged powdered metal or fiber reinforced metal, the skirt portion is made of an aluminum alloy casting, and the two portions are welded together. When the two parts are welded together, however, a brittle alloy layer is produced in the welded portion, contributing to low joining strength. Also, in the area of the weld, the basic characteristics of the powdered metal, that of high fatigue strength, proof strength and hardness, are lost. When the joining occurs by friction welding, burrs are produced in the welded portion. These burrs can cause stress concentrations and must be removed. However, the removal of the burrs is made difficult, at least on the inside of the piston, because the piston&#39;s irregular shape. Also, when the head portion is constructed of FRM, stress concentrations occur on the interface between the reinforcing materials, such as whiskers and short fibers, and the matrix. As a result, insufficient fatigue strength is provided at high temperatures. 
     An improved piston and method of constructing a piston are desired. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an improved piston of the type utilized in an internal combustion engine and a method of making the piston. 
     In the method of the present invention, a first block of a first alloy and a second block of a second alloy are press-forged to form the piston. Preferably, the first alloy comprises an aluminum-silicon based alloy, and the second alloy an aluminum-iron based alloy. 
     In the preferred arrangement, the piston is formed to have a head and a skirt, with the blocks arranged so that at least a portion of the head is formed from the second alloy and at least a portion of the skirt is formed from the first alloy during the forging process. 
     In this manner the head comprises a hard, heat resistant material while the skirt comprises a more formable material for ease of forming the thin-walled skirt. 
     In the preferred method, the first and second blocks contact one another along a first, generally planar interface before forging, and then join along a second, non-planar interface after forging. As a result of this transformation, the oxide layers on the first and second blocks are destroyed so that the alloy material forming each block directly bond to one another. 
     Further objects, features, and advantages of the present invention over the prior art will become apparent from the detailed description of the drawings which follows, when considered with the attached figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simple illustration of an internal combustion engine arranged to operate on a two-cycle principle; 
     FIG. 2 is a simple illustration of an internal combustion engine arranged to operate on a four-cycle principle; 
     FIG. 3 illustrates, in cross-section, a piston utilized with an engine such as that illustrated in FIG. 1 or  2 ; 
     FIG. 4 is a flow diagram for a method of constructing a piston in accordance with a first embodiment method of the present invention; 
     FIG.  5 ( a ) illustrates, in cross-section, a stacking step of the method illustrated in FIG. 4; 
     FIG.  5 ( b ) is a cross-sectional view of the piston formed in the method illustrated in FIG. 4 after a forging step thereof; 
     FIG.  5 ( c ) is a cross-sectional view of the piston formed in the method illustrated in FIG. 4 after completion of surface machining; 
     FIG.  6 ( a ) is a top view of a first arrangement for a projection extending from the piston formed in the method illustrated in FIG. 4; 
     FIG.  6 ( b ) is a top view of a second arrangement for a projection extending from the piston formed in the method illustrated in FIG. 4; 
     FIG.  6 ( c ) is a top view of a third arrangement for a projection extending from the piston formed in the method illustrated in FIG. 4; 
     FIG.  6 ( d ) is a top view of a fourth arrangement for a projection extending from the piston formed in the method illustrated in FIG. 4; 
     FIG.  6 ( e ) is a top view of a fifth arrangement for a projection extending from the piston formed in the method illustrated in FIG. 4; 
     FIG.  7 ( a ) is a cross-sectional view of an alternate embodiment piston made in accordance with the method of the present invention; 
     FIG.  7 ( b ) is a cross-sectional view of another alternate embodiment piston ade in accordance with the method of the present invention; 
     FIG.  7 ( c ) is a side view of a piston illustrated in FIG.  7 ( a ); 
     FIG.  7 ( d ) is a side view of the piston illustrated in FIG.  7 ( b ); 
     FIG.  8 ( a ) illustrates a step of making a piston in accordance with a second embodiment method of the present invention; 
     FIG.  8 ( b ) illustrates a second step of the method of the second embodiment of the present invention; 
     FIG.  9 ( a ) illustrates a step of making a piston in accordance with a third embodiment method of the present invention; 
     FIG.  9 ( b ) illustrates a second step of the method of the third embodiment of the present invention; 
     FIG.  10 ( a ) illustrates a step of making a piston in accordance with a fourth embodiment method of the present invention; 
     FIG.  10 ( b ) illustrates a second step of the method of the fourth embodiment of the present invention; 
     FIG.  11 ( a ) illustrates a step of making a piston in accordance with a fifth embodiment method present invention; 
     FIG.  11 ( b ) illustrates a second step of the method of the fifth embodiment of the present invention; 
     FIG.  12 ( a ) illustrates a step of making a piston in accordance with a sixth embodiment of the present invention; 
     FIG.  12 ( b ) is a cross-sectional view of a piston formed in the step illustrated in FIG.  12 ( a ); 
     FIG.  12 ( c ) is a cross-sectional view of a piston formed in the step illustrated in FIG.  12 ( a ) after subsequent machining; 
     FIG.  12 ( d ) is a top view of the piston illustrated in FIG.  12 ( c ); 
     FIG.  12 ( e ) is a cross-sectional view of an alternate arrangement piston from that illustrated in FIG.  12 ( c ); and 
     FIG. 13 is an enlarged cross-sectional microscopic illustration of the material forming the piston manufactured in accordance with the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     The present invention is a piston  20  for use in an internal combustion engine, and a method of making the piston. In general, the piston of the present invention comprises a head portion and a skirt portion. The piston comprises two different alloy materials bonded together to form the unitary body of the piston. 
     In accordance with a method of the present invention, portions of first and second alloys are press-forged together in a manner whereby an original interface between the alloys is elongated or enlarged. Relative movement or slippage between the mating alloys destroys surface oxide layers and promotes bonding between the alloys. 
     FIG. 1 illustrates an engine  22  of the type which operates on a two-cycle principle and with which the piston  20  of the present invention may be used. This engine  22  has a cylinder  24  in which the piston  20  is movably mounted. A crankshaft  26  is rotatably positioned within a crankcase  28  below the cylinder  24 . The piston  20  is connected to the crankshaft  26  via a connecting rod  30  and crank arm  32 . The crank arm  32  has one end connected to the crankshaft  26  and a second end connected to the connecting rod  30 . The connecting rod  30  extends from the crank arm  32  to a piston pin  34  positioned within an interior of the piston  22 . 
     Air is supplied to the cylinder  26  through an intake passage  36 . The rate of air flow through the passage  36  to the cylinder  24  is governed by a throttle  38 . 
     Fuel is supplied to the engine  22  through a fuel injector  40  or similar charging apparatus. The injector  40  supplies fuel into the air stream flowing through the intake passage  36 . 
     The air and fuel mixture selectively flows through a reed-type valve  42  into the crankcase  28 . The mixture then flows, at the times described below, from the crankcase  28  into the cylinder  24  through primary and secondary scavenging passages  44 , 46 . 
     The air and fuel mixture supplied to the cylinder  24  is ignited by suitable ignition means, such as a spark plug  48 . The exhaust generated within the cylinder  24  is exhausted through an exhaust passage  50 . 
     The operation of this engine  22  will now be described. First, as the piston  20  moves upwardly in the cylinder  24 , low pressure is generated within the crankcase  28  which causes a charge of air and fuel to be drawn into the crankcase  28  through the reed-type valve  42 . At the same time, a charge already in the cylinder  24  is compressed by the upwardly moving piston  20 , the charge prevented from flowing through the exhaust passage  50  and scavenging passages  44 , 46  because the piston covers them. 
     Once near the top of the cylinder  24  (i.e. top dead center) and with the charge compressed, the spark plug  48  is activated, and the resulting spark ignites the charge. The expansion of the charge as it burns within the cylinder  24  forces the piston  20  downwardly (and effectuates a rotation of the crankshaft  26 ). As the piston  20  moves downwardly, the exhaust passage  50  is first uncovered, allowing the exhaust to flow therethrough. 
     At the same time, the piston  20  causes the next charge within the crankcase  28  to be partially compressed. Once the piston  20  has moved downwardly a sufficient distance to uncover the scavenging passages  44 , 46 , the charge flows from the crankcase  28  through the scavenge passages  44 , 46  into the cylinder. 
     After the piston  20  reaches the bottom of the cylinder  24  (i.e. bottom dead center), with the crankshaft  28  still rotating, the piston  20  is driven back upwardly and the process repeats itself. 
     Thus, it may be seen that in an engine  22  operating on a two-cycle principal, a full cycle is completed during each single revolution of the crankshaft  26 . That is, each complete cycle occurs during one piston reciprocation. 
     FIG. 2 illustrates an engine  22   a  of the type which operates on a four-cycle principle and with which the piston  20  of the present invention may be used. This engine  22   a  also has a cylinder  24   a  in which the piston  20  is movably mounted. 
     Again, a crankshaft  26   a  is rotatably positioned within a crankcase  28   a  of the engine  22   a . The piston  20   a  is connected to the crankshaft  26   a  via a connecting rod  30   a  extending from a piston pin  34   a  of the piston to a crank arm  32   a  extending from the crankshaft  26   a.    
     In this engine  22   a , the intake passage  36   a  extends directly to the cylinder  24   a . A throttle  38   a  is utilized to control the flow rate of air through the passage  36   a . A fuel injector  40   a  or similar apparatus as known to those skilled in the art is utilized to deliver fuel into the air. In this engine, at least one intake valve  42   a  controls the passage of the air and fuel charge into the cylinder  24   a , in the manner described in more detail below. 
     A spark plug  48   a  is utilized to ignite the air and fuel charge within the cylinder  24   a , and an exhaust passage  50   a  leads from the cylinder. At least one exhaust valve  44   a  is utilized to control the flow of exhaust from the cylinder  24   a.    
     The operation of this engine  22   a  is as follows. As the piston  20  approaches the top of the cylinder  24   a  (moving upwardly) both valves  42   a , 44   a  are closed and an air and fuel charge within the cylinder  24   a  is compressed. The spark plug  48   a  is activated, with the resultant spark causing ignition of the charge. The expansion force drives the piston  20  downwardly in the cylinder  24   a , thus effectuating a rotation of the crankshaft  26   a.    
     The piston  20  moves to its bottom dead center position, and then rises upwardly again within the cylinder  24   a . As this occurs, the exhaust valve  44   a  opens, and the upward movement of the piston  20  forces the exhaust out of the cylinder  24   a  through the exhaust passage  50   a.    
     After the piston  20  moves to top dead center, it begins moving downwardly again. This exhaust valve  44   a  closes, and the intake valve  42   a  opens, and the downward movement of the piston  20  draws an air and fuel mixture into the cylinder  24   a . The piston moves to bottom dead center again, and then moves back upwardly in the cylinder  24   a . With the valves  42   a , 44   a  both closed, the piston  24   a  compresses the newly drawn charge for ignition, and the cycle repeats itself. 
     Thus, it may be seen that an engine  22   a  operating on a four-cycle principle has its crankshaft rotate two complete revolutions per cycle. In other words, the piston must reciprocate two times for each combustion cycle. 
     FIG. 3 illustrates a piston  20  of the type utilized in the engines  22 , 22   a  described above. The right and left hand portions of this Figure illustrate cross-sectional views of the piston  20  at planes passing through the piston at right angles. The piston  20  has a head portion  54  and a skirt portion  56  depending therebelow. A boss  58  is formed within the piston  20 , the boss  58  defining a pin connection for the connecting rod. 
     The boss  58  is positioned below first and second ring grooves  60 , 62  formed in the exterior of the piston  20 . A compression ring (not shown) is preferably positioned in the top groove  60 , and a compression and/or oil sealing ring (not shown) is preferably positioned in the second groove  62 . 
     As illustrated, the portion of the piston  20  near the boss  58  is fairly thick, so as to provide support for the connecting rod connection. The skirt portion  56  of the piston  20 , however, is a generally circular wall having a thickness which reduces moving in a direction opposite the head  54 . 
     FIG. 4 is diagramatically illustrates a method of forming a piston  20  in accordance with the present invention. In a first step (A), an alloy (the term “alloy” herein generally refers to a material which comprises other than a single metal) ingot  64  for the skirt portion of the piston  20  is prepared. Preferably, this ingot comprises an alloy of aluminum (Al), silicon (Si), copper (Cu) and magnesium (Mg). In general, the silicon is added to increase wear and seizure resistance. The silicon causes precipitation of hard initial or eutectic crystals in the metallic composition which allow the alloy to have these features. 
     The copper and magnesium are added to increase the alloy&#39;s strength at high temperatures. In a first embodiment, the alloy additives by weight percent are preferably 5-25% Si, 0.5-5% Cu and 0.5-1.5% Mg. Generally, it has been found that outside of these ranges the intended resistance to wear and seizure, and the high strength at high temperatures are not achieved. 
     In addition, the following specific alloys have been found advantageous for use as the alloy forming at least a portion of the skirt of the piston  20 . These alloy embodiments are preferably manufactured by continuous casting or extrusion forming and then cut into the desired block, and may also be formed from powder metals, as described in more detail below. 
     (1) Al containing the following alloying elements by percentage weight: between 5-25 Si; 1 or less than 1 Fe (iron); between 0.5-5 Cu (copper); between 0.5-5 Mg (magnesium); 1 or less than 1 Mn (manganese); 1 or less than 1 Ni (nickel); and 1 or less than 1 Cr (chromium); 
     (2) Al containing the following alloying elements by percentage weight: between 5-25 Si; 1 or less than 1 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr (zirconium); 1 or less than 1 Mo (molybdenum); and 5 or less than 5 SiC (silicon carbide) or BN (boron nitride) or AIN (aluminum nitride) or Al 2 O 3  (aluminum oxide), where the SiC, BN, AIN and Al 2 O 3  can be combined instead of using only one of them, as long as the total weight of the combination is within the desired range; and 
     (3) Al containing the following alloying elements by percentage weight: between 5-25 Si; 1 or less than 1 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; between 1 and 10 C (carbon) or MoS 2  (molybdenum disulfide); and 5 or less than 5 SiC or Al 2 O 3 , where the C and MoS 2  may be combined instead of using just one of them, as long as the combined weight is within the desired range. 
     Each alloying element may be separately prepared as a powder or ingot and then melted into the base metal. 
     This ingot  64  of alloy is melted and a block for the skirt portion of the piston  20  is prepared by continuously casting or extruding the alloy into a cylindrically shaped extrusion  66 . The alloy formed in this manner has a lower resistance to deformation at high temperatures than the alloy described below for use as the head of the piston  20 . In particular, the yield strength of this alloy at approximately 400° C. is about 50% of the alloy described for use as the head of the piston  20 . Thus, this alloy is relatively formable, making it easier to form the alloy into the skirt of the piston. 
     The cylindrically-shaped extrusion  66  is then cut into individual blocks  68  in step (C). Each block  68  is then prepared for mating with another block, described below, for use in forming the piston  20 . 
     In a step (D), an ingot  70  is prepared for forming the head portions of the pistons  20 . This alloy preferably comprises Al, Fe and Si. The iron is added for increasing the fatigue strength at temperatures above 200° C. The silicon is added for increasing resistance to wear and seizure as described above, and for lowering the melting point of the alloy. The alloying amount of silicon is kept low so that excessive ductility and low strength of the alloy is prevented, and so that the heat resistance is also not lowered excessively. In that regard, the silicon preferably comprises more than 5% by weight of the alloy, and the weight amount of iron preferably comprises more than 5%. 
     Specific alloys contents which have been found suitable are as follows. These first three alloy embodiments are preferably manufactured by continuous casting or extrusion forming and then cut into the desired block. 
     (1) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-3 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Cr; 1 or less than one Zr; 1 or less than 1 Mo; and approximately 0 SiC; 
     (2) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-3 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less Man 1 Zr; 1 or less than 1 Mo; and between 1-10 SiC (the mean diameter of the SiC being between about 1 and 20 microns); 
     (3) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-3 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and between 1-10 SiC, BN, AIN or Al 2 O 3 , where the SiC, Al2O3, BN and AIN can be compounded instead of containing one of them if the compound weight totals within the 1-10% range. 
     The following alloys are preferably made from powder metals: 
     (4) Al containing the following alloying elements by percentage weight: 5 or less than 5 Si; S or greater than 5 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and approximately 0 SiC; 
     (5) Al containing the following alloying elements by percentage weight: 5 or less than 5 Si; 5 or greater than 5 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and between 1-10 SiC (having a mean diameter of about 1-20 microns); 
     (6) Al containing the following alloying elements by percentage weight: 5 or less than 5 Si; 5 or more than 5 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and between 1-10 SiC, BN, AIN or Al 2 O 3 , where the SiC, Al 2 O 3 , BN and AIN can be combined instead of containing only one of them if the compound weight totals within the 1-10% range. 
     (7) Al containing the following alloying elements by percentage weight: 5 or less than 5 Si; 5 or more than 5 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; between 1-10 C or MoS 2 ; and between 1-10 SiC or Al 2 O 3 , where the C and MoS 2  can be combined instead of containing one of them if the combined weight totals within the 1-10% range; 
     (8) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-10 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and approximately 0 SiC; 
     (9) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-10 Fe; between 5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and between 1-10 SiC (with a mean diameter of between about 1-20 microns); 
     (10) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-10 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; and between 1-10 SiC, BN, AIN or Al 2 O 3 , where the SiC, Al 2 O 3 , BN and AIN can be combined instead of containing only one of them if the combined weight totals within the 1-10% range; and 
     (11) Al containing the following alloying elements by percentage weight: between 5-25 Si; between 1-10 Fe; between 0.5-5 Cu; between 0.5-5 Mg; 1 or less than 1 Mn; 1 or less than 1 Ni; 1 or less than 1 Cr; 1 or less than 1 Zr; 1 or less than 1 Mo; between 1-10 C or MoS 2 ; and between 1-10 SiC or Al 2 O 3 , where the C and MoS 2  can be combined instead of containing one of them if the combined weight totals within the 1-10% range. 
     It is noted that carbon and molybdenum disulfide both serve to produce an alloy which is smooth, promoting slidability of the resultant piston. 
     Once again, the alloying elements may be formed into separate ingots or be provided as powder and be melted or mixed together with the base metal to form the alloy. 
     Once the ingot  70  is formed, it is melted and quench-solidified at a high cooling rate of about 100° C. per second. An alloy powder  72  is produced (see step (E)). In step (F), the powder is heated and extruded into a solidified cylindrically-shaped extrusion  76 . The extrusion  76  formed in this manner advantageously provides a metallic composition provides a structure which is generally free from stress concentrations, providing a high fatigue strength. Unlike the cooling in an ordinary casting in which a coarse iron composition is produced in the alloy, resulting in low strength, this formation method prevents the coarse iron composition from forming. 
     Next, in step (G), the extrusion  76  is cut into blocks  78  for forming the head of a piston  20 . 
     In step (H), the blocks  68 , 78  for the head and skirt portions of the piston  20  are stacked, with a parting agent applied. In step (I), the stacked blocks  68 , 78  are heated so as to increase their formability. In step (J), the stacked blocks  68 , 78  are positioned between a pair of dies, and pressed between the dies with high force to form the integral piston shape. During this process, the two blocks  68 , 78  are joined in a manner described below into a single piston member  20 . This piston  20  member as formed includes the skirt and head portions  54 , 56 . 
     In step (K), the piston  20  product is heat-treated to increase its strength. Finally, in step (L), the piston  20  is finished by forming the one or more ring grooves  60  therein, and burrs and other surface irregularities are removed by machining. After this, a surface treatment such as plating may be applied as required on the outside surface of the skirt portion  56  of the piston  20  for increasing the slidability and wear resistance of this portion of the piston  20 . 
     As formed, the piston  20  comprises a single member formed of two different alloy materials joined together. 
     FIGS.  5 ( a-c ) illustrate in more detail the process steps (I-L) of FIG.  4 . As illustrated, the stacked blocks  68 , 78  are positioned within a main recess  80  of lower die  82 , the top block  68  comprising the formable aluminum-silicon alloy and the bottom block  78  comprising the durable aluminum-iron alloy. This lower die  82  preferably also includes a sub-recess  84 . A top die or punch  86  having a projected portion  88  is utilized to press the stacked blocks  68 , 78 . 
     Before forging, the stacked blocks  68 , 78  are in contact with one another at a contact face  90 . Before forming, this face or surface  90  is planar. 
     The shape of the top die  86  is chosen, as known to those skilled in the art, to cooperate with the lower die  82  to form the blocks  68 , 78  into a single mass having the desired piston shape with high dimensional accuracy, but without deteriorating the characteristics of the alloys forming the two blocks. 
     In the process step (I), as illustrated in more detail in FIG.  5 ( a ), in a hot forging step the top die  86  is moved downwardly into the recess  80  in the lower die  82 . In this process, alloy comprising the first block  68  rises upwardly along the sides of the top die  86  to form the skirt portion of the piston. In addition, as the top die  86  presses below the level of the original interface  90 , the periphery of the block  78  also rises, further driving the skirt portion upwardly and thinning the block  78  to form a piston head with a thin or short land. 
     After this step, the piston  20  shape is formed, as illustrated in FIG.  5 ( b ). As illustrated, the piston  20  has a projected part  92  corresponding to the alloy material which was pressed into the recess  84  within the lower die  82 . 
     After forging, an interface  94  is formed between the two alloys which is different from the original surface or interface  90 . In particular, the new interface  94  includes a peripheral part  94   a  which is positioned higher than the original interface  90 , a dome portion  94   b  which extends downwardly in convex fashion to a point lower than the original interface  90 , and a apex portion  94   c  dipping towards the projection  92 . It is now apparent that the recess  84  within the lower die  82  contributes to the formation of this interface  94  shape by permitting the alloys forming the piston to flow into the recess as the top die  86  moves downwardly. 
     FIG.  5 ( b ) also illustrates that a grain or fiber direction is introduced into the alloys forming the piston  20  during the forging step. These grains of the alloys are also elongated. 
     Notably, the surface area of the new interface  94  is enlarged as compared to the area of the original interface  90 . During this stretching or elongation of the original interface  90 , relative movement between the two alloys occurs at the interface. This slippage and enlargement contributes to the destruction of oxide films which exist on the outside surfaces of the blocks  68 , 78 . When the oxide films are destroyed, the alloy of the first block  68  comes into direct contact with the alloy of the second block  78 , resulting in a strong joining between the two alloys after forging. 
     FIG.  5 ( c ) corresponds to step (L) in FIG.  4 . As illustrated, various machining steps are performed on the piston  20  after forging. First, the projection  92  is removed from the head portion  54  of the piston  20 . Next, one or more piston ring grooves  60  are formed in the exterior of the piston  20 . In this step the grain or fiber flows illustrated in FIG.  5 ( b ) remain undisturbed. 
     In conjunction with FIGS.  6 ( a-e ), it is noted that more than one projection ( 92 ) may be formed during the forging process, and the projection need not necessarily be formed in the center of the head portion  56  of the piston  20 . 
     FIG.  6 ( a ) is a top view illustrating the embodiment piston described above where the projection  92  is in the center of the head portion  54 . The broken lines in this drawing illustrate the positions of a pair of main scavenging passages  96 , 98 , a secondary scavenging passage  100 , and an exhaust passage  102  when the piston  20  is positioned within a cylinder of a two-cycle engine. 
     FIG.  6 ( b ) illustrates an arrangement in which the projection  92  is formed offset from the center of the head portion  54  of the piston  20 . This arrangement is advantageous since the portion of the piston  20  adjacent the exhaust passage  102  (where the temperature is very high) is strengthened. Of course, a similar circular projection  92  may be formed in other locations about the head portion  54 . 
     FIG.  6 ( c ) illustrates an example of a circular projection  92  formed in the center of the head portion  54  and surrounded by an annular projection  92   a.    
     FIG.  6 ( d ) illustrates an example of a non-circular projection  92 . 
     FIG.  6 ( e ) illustrates an example in which only an annular-shaped projection  92  is formed and extends from the head portion  54  of the piston  20 . 
     In each of these figures, lines  104  are used to illustrate the direction of grain or fiber flow within the alloy. In general, the flow direction is generally radial from the extruded projections  92 . 
     In each of the variations illustrated in FIG. 6, commonality exists in that the goal during press-forging is to cause the interface between the original alloy block  68 , 78  to increase, whereby relative slippage or movement between the each contacting surface of the blocks  68 , 78  occurs. Again, this contributes to the break-up of oxide films on the contacting surfaces  68 , 78  of the blocks, permitting the alloys to bond securely to one another. 
     While the above-described piston  20  has its entire head portion  54  formed from the aluminum-iron alloy, it is possible to form the piston so that only a part of the head portion  54  of the piston comprises this alloy. FIGS.  7 ( a-d ) illustrate such arrangements. 
     FIGS.  7 ( a ) and ( c ) illustrates an arrangement in which only the periphery of the head portion  54  of the piston  20  is formed from the aluminum-iron alloy (i.e. from the alloy forming block  78  described above), while the remainder of the head portion and the entirety of the skirt portion  56  is formed of the aluminum-silicon alloy (i.e. from the alloy forming block  68  described above). In this manner, the periphery of the head portion  54  comprises the alloy which has a high heat resistance, while the remainder of the piston  20  is formed of the alloy which has good forming properties. In fact, the peripheral edge of the head portion  54  will withstand a temperature of about 350° C. without deformation. 
     Also, this arrangement causes the two alloys to join along a curved surface after forging. As described above, when the generally planar interface between the two alloy blocks is increased (as occurs when the surfaces are stretched to form the elongated curved surface) bonding of the alloys results with the oxide layers destroyed. 
     This arrangement also permits the land portion of the piston  20  to be thinner than in he ordinary case. This reduces crevice or squish volume within the cylinder when the piston is in use, and thus reduces the amount of unburned gas, improving engine emission quality. 
     FIGS.  7 ( b ) and ( d ) illustrate an example in which the portion of the aluminum-iron alloy (from block  78 ) is made thicker in the part of the head portion  54  of the piston  20  corresponding to the intake and exhaust passages of the cylinder in which it is to be utilized. FIG.  7 ( d ) illustrates how the thickness of the aluminum-iron layer is varied so as to be wavy, being thin at the piston pin bosses  58 , and thick in the remaining areas. 
     FIGS.  8 ( a ) and ( b ) illustrate a second embodiment method of making a piston  20  in accordance with the present invention. In arrangement, the bottom side of the block  78  of the aluminum-iron alloy is formed with a recess  106 . In the forging process, the top die  86  is moved downwardly until the recess  106  is filled with material (the top die  86  moves to a distance HI above the bottom die  82  which is less than a depth H of the original interface  90 ). This arrangement causes the original interface  90  to transform into the three-part interface  94   a , 94   b , 94   c  having a shape similar to that described above. Once again, the transformation of this interface results in oxide layer destruction and then strong bonding between the alloying materials of the two blocks  68 , 78 . Also, even though this forging process creates a piston having the same advantageous structure as described above, no projection is formed from the head portion which must be removed in a later machining step. Also, the recess  106  may be formed in the block  78  during a sintering step during which a pre-forging alloy layer is provided. 
     FIGS.  9 ( a ) and ( b ) illustrate a third embodiment method of making a piston  20  in accordance with the present invention. In this arrangement, the bottom die  82  has a generally annular recess  108  extending from the main recess  80 . As the top die  82  is lowered, the periphery of the alloy forming the block  68  rises to form the skirt portion  56  of the piston  20 . As the die  86  is lowered further, a ledge  112  of the die restricts further upward movement of the alloy forming the block  68 , and the periphery of the alloy forming the block  78  is forced into the recess  108 , forming a projection  110 . 
     In this case, the original interface  90  between the blocks  68 , 78  rises to an outer interface section  94   a , an even higher section  94   b  inwardly thereof, and then drops into a low central portion  94   c . Once again, this elongation of the interface causes oxide layer destruction and results in strong bonding between the alloys forming the original blocks  68 , 78 . Once forging is complete, the projection  110  is machined off of the piston  20 . 
     FIGS.  10 ( a ) and ( b ) illustrate a fourth embodiment method of making a piston  20  in accordance with the present invention. This method is primarily directed to making a piston  20  arranged as illustrated in FIG.  7 ( a ). In this method, the bottom die comprises a mating right and left die halves  82   a , 82   b . The halves  82   a , 82   b  cooperate to form the recess  80 , and each half  82   a , 82   b  has a separate recess  114  extending generally radially outward from the main recess  80 . 
     In this case the bottom alloy block (i.e. the aluminum-iron alloy)  78  is generally ring-shaped, and the top alloy block  68  has a projecting portion  68   a  which fits within the otherwise hollow center portion of the bottom block  78 . The blocks  68 , 78  contact one another along an interface  90  which has portions extending generally at right angles to one another. 
     During forging, material from both blocks  68 , 78  is forced into the recess  114  of the lower die halves  82   a , 82   b , forming a circumferential projection  116 . The forging also transforms the original interface  90  into an elongate curved interface  94 . Once again, the later interface  94  has a greater surface area (and length in a single dimension) than the original interface  90 , with relative movement or slippage between the blocks  68 , 78  during forging causing the bonding as described above. The projection  116  is then removed in a post-forging process. 
     FIGS.  11 ( a ) and ( b ) illustrate a fifth embodiment method of making a piston in accordance with the present invention. This embodiment is similar to the last, except that an annular or donut-shaped recess  118  (instead of recess  114  as in FIGS.  10 ( a ) and ( b )) is provided in the bottom die  82  extending from the main recess  80 . Preferably, blocks  68 , 78  having the same shapes as those described in conjunction with the method of FIGS.  10 ( a ) and ( b ) are utilized in this method. 
     In this method, when the top die  86  is moved downwardly, the alloy material of both blocks  68 , 78  is pressed downwardly into the recess  118 , forming projection  120  from the formed piston  20 . Once again, the movement of this alloy material into the recess  118  has the effect of lengthening the original interface  90  between the blocks  68 , 78  into a longer interface  94 , thereby bonding the two alloys together. Once formed, the projection  120  is removed by machining. 
     FIGS.  12 ( a-e ) illustrate a method of forming a piston  20  in accordance with a sixth embodiment of the present invention. The piston  20  created as a result of this method is best suited to use in an engine operating on a four-cycle principle. 
     As illustrated in FIG.  12 ( a ) (corresponding to step (I) in FIG.  4 ), a block  68  of the aluminum-silicon alloy is placed upon a block  78  of the aluminum-iron alloy of the type described in detail above. The stacked blocks  68 , 78  are positioned within the recess  80  of the heated lower die  82 , and then pressed with a top die  86  to forge-form the piston  20 . 
     Preferably, four projections  122  extend upwardly from the bottom of the recess  80  in the lower die  80  for forming four recesses  124  in the head portion  54  of the produced piston  20  (see FIG.  12 ( d )). These recesses  124  to correspond to two intake and two exhaust valves of a four-cycle engine. 
     In addition, recesses  128  are provided in the top die  86  for producing projections  126  inside the piston  20 . 
     As the top die  86  is pressed downwardly, the alloy material from both blocks  68  rises upwardly into the recesses  128  in the top die  86 . In this manner, the original flat interface  90  between the blocks  68 , 78  is changed into a wavy or non-planar interface. Most importantly, the surface area of the interface is enlarged, for greater bonding area. Because the elongation (when viewing a single dimension) of the blocks  68 , 78  is different in different areas, the two blocks  68 , 78  must move relative to one another during the forging. This destroys the oxide layers on the blocks  68 , 78  at the interface  94 , thus contributing to the bonding of them. Once again, grain or fiber flows are introduced into the alloy materials as a result of the pressing force of the top die  86 . 
     FIG.  12 ( c ) illustrates a step corresponding to step (L) of FIG.  4 . Here, at least one piston ring groove  60  has been formed into the piston  20 . This processing is done in a manner which prevents disruption of the grain or fiber flows. 
     As an alternate arrangement, and as illustrated in FIG.  12 ( e ), the piston  20  may be formed without the projections  126  with use of a top die  86  which does not include the recesses  128 . In this case, a wavy interface (as defined by sections  94   a  and  94   b ) is still created, as a portion of the alloy in the bottom block  78  is pressed upwardly into the top block  68  as a result of the projections  122 . 
     FIG. 13 is an enlarged view of an interface texture drawn in reference to a microscopic photograph of the joining interface of the piston  20  of the present invention. In this case, the interface  94  is wavy, and created during forging which results in a relative slip of the two alloy materials. 
     The piston of the present invention has a head which comprises at least a portion of an alloy which has very high strength, low deformation at high temperatures, and is slidable. At the same time, the skirt portion comprises at least a portion of an alloy which, while also strong so that the skirt may have a thin wall thickness, is also formable so that the piston may be easily formed in press-forging operation. The piston is light-weight, since the alloy is an aluminum alloy. The piston has a short land due to the manufacture of least a portion of the head of the high-strength and low deformation alloy. The two alloys are securely bonded to one another to form an integral piston member. 
     While specific examples of the alloy contents for the alloys which are utilized to form the head and skirt portions are described above, it is contemplated that other alloy variations may be found satisfactory. 
     Also, specific press-forging arrangements have been described for use in causing the relative slippage and grain elongation resulting in oxide layer destruction and strong bonding between the alloys. It is contemplated that a wide variety of other die arrangements and the like may be utilized to achieve this result. 
     Further, while the two alloys have been described as used as blocks which are press forged, the alloys may have a variety of shapes or forms. Also, it is contemplated that more than one separate alloy may be used in the method of the invention, resulting in a piston comprising three of more bonded alloys. 
     Of course, the foregoing description is that of preferred embodiments of the invention, and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.