Method for manufacturing piston by forging, forging die and piston

Abstract: A method for manufacturing a piston by using a set of die members (21,22,23) with the steps of forging a metal mass (10) as a raw material into a shaped workpiece (11) having a head portion (2), pin boss portions (3), skirt portions (4) and rib portions (5) extending downward from the underside of the head portion (2) and interconnecting the pin boss and skirt portions (3,4) and applying machining processes to the shaped workpiece (11). Said metal mass (10) is pressed by said die members (21,22,23) such that flashes are formed on the lower surfaces of the pin boss portions (3), skirt portions (4) and rib portions (5) by filling mating surfaces (24) of the die members (22a,22b,21a,21b) for shaping the pin boss portions (3), skirt portions (4) and rib portions (5), which members (22a,22b,21a,21b) are divided along said mating surfaces respectively reaching the lower end surfaces of the pin boss portions (3), skirt portions (4) and rib portions (5).

Description: This invention relates to a method for manufacturing a piston by using a set of die members connected to each other at mating surfaces according to the preamble of independent claim 1, to forging die members for manufacturing a piston, comprising an upper die or punch and a lower die, and to a piston for an internal combustion engine according to the preamble of independent claim 12. When pistons of light-weight aluminium alloy for use in internal combustion engines such as those for vehicles are formed by casting, solidified material texture becomes coarse, which is disadvantageous in strength. Therefore, the forged piston has been conventionally developed through the process of forge-shaping a metal mass as a raw material for the piston into a piston-like workpiece using a set of forging die members including a linearly reciprocating die member (punch) followed by finish-machining of the shaped workpiece, such as boring a piston pin hole, cutting piston ring grooves, etc., to make the workpiece into a product. A conventional, generally known piston for internal combustion engines is constituted for example as shown in FIGs. 1 (A) to 1(C), integrally formed with; a head portion, pin boss portions, skirt portions, and rib portions extending downward from the underside of the head portion and interconnecting the pin boss and skirt portions. When such a piston is manufactured by forging, for example as shown in FIGs. 4(A) and 4(B), an approximately piston-shaped workpiece 11 is forge-shaped by squeezing a thick, disk-shaped aluminium alloy piston material (billet) 10 between a die member (upper die or punch) 21 for shaping the top surface of the head portion and a die member (lower die) 22 for shaping the pin boss, skirt, and rib portions on the underside of the head portion through a cylindrical die member 23. To forge-shape the raw material 10 for such a piston into the workpiece 11, the die member (lower die) 22, for shaping the pin boss, skirt, and rib portions, of the set of die members defines deep furrows to shape the pin boss and the rib portions. Therefore, when the metal material is forced into the furrow portions (in particular the furrows for the thin-walled rib portions) of the die member (lower die) 22, in some cases cracks occur at the bottom corners of the furrow portions. This is one of the factors to reduce the life of the die member. Prior art document GB 415,522 teaches a method for manufacturing a piston by using a set of die members connected to each other at mating surfaces, forging die members for manufacturing a piston, comprising an upper die or punch and a lower die, as well as a piston for an internal combustion engine as indicated above. In particular, said document teaches a method and an apparatus for forging a piston provided with a skirt, piston pin bosses and ribs. The respective die for forging said piston is constituted by a set of die members. Moreover, the specific teaching of said prior art document is directed to forming a flange at a the skirt portion of the piston. In order to form said flange, a core is inserted into the pre-forged piston in order to support the inner surface of the piston when forging the flange. Said core is constituted by a plurality of core elements. Said core elements form mating surfaces on the lower side of the piston pin bosses. In particular, recesses are provided to receive the piston pin bosses. Some parting line is provided associated to said recesses. Accordingly, it is provide a supporting core for forging the flange on the skirt portion. Said core needs to be provided by a set of elements so that the core can be removed after the forging process. Accordingly, it is an objective of the present invention to provide a method for manufacturing a piston by using a set of die members connected to each other at mating surfaces, forging die members for manufacturing a piston, comprising an upper die or punch and a lower die, as well as a piston for an internal combustion engine as indicated above which facilitate to extend the longevity of the dice when a metal mass as a raw material for the piston is forge-shaped using a set of the die members into a piston-shaped workpiece having a head portion, pin boss portions, skirt portions, and rib portions extending downward from the underside of the head portion and interconnecting the pin boss and skirt portions, by preventing cracks from being produced in the furrow portions of the die members for shaping the pin boss and rib portions. According to the present invention, this objective is solved by a method for manufacturing a piston by using a set of die members connected to each other at mating surfaces having the features of independent claim 1. When the die members for forge-shaping the metal mass into the approximately piston-shaped workpiece are divided as described above, even if the metal mass as a raw material is forced into the furrow portions of the die members for shaping the pin boss and rib portions, since the die mating surfaces at the bottom surfaces of the furrow portions part from each other by a very small amount, cracks are prevented from being produced in the furrow portions. According to another aspect of the present invention, this objective is solved by forging die members manufacturing a piston, comprising an upper die or punch and a lower die having the features of independent claim 1. Moreover, this objective is solved by a piston for an internal combustion engine having the features of independent claim 12. Other preferred embodiments of the present invention are laid down in further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: FIG. 1 shows an example of forge-shaped piston for internal combustion engines, manufactured with the method and dice of the invention; in side view (A), in view from under (B) and in cross-section along the line C-C in FIG. 1 (B); FIG. 2 shows a forge-shaped workpiece for the piston shown in FIG. 1; in side view (A), in view from under (B) and in cross-section along the line C-C in FIG. 2 (B); FIG. 3 is an explanatory cross-sectional view corresponding to the cross-section of FIG. 2 (C), showing the forging process in two steps (A) and (B); FIG. 4 is a view corresponding to the cross-section of FIG. 2 (C), showing a process of forge-shaping a metal mass as a piston material into a nearly piston-like workpiece using a conventional method and dice in two steps (A) and (B); and FIG. 5 is an explanatory cross-sectional view corresponding to the cross-section of FIG. 2 (C), showing a set of forging dice in a conventional embodiment. Embodiments of the method for manufacturing the piston by forging and forging die members according to the invention will be hereinafter described in reference to the appended drawings. FIG. 1 shows an example of forge-shaped piston for use in internal combustion engines. FIG. 1 (A) is an external side view. FIG. 1 (B) is a view as seen from under. FIG. 1 (C) shows a cross-section as seen along the line C-C in FIG. 1 (B). A piston main part 1 has a head portion 2 with its top surface to be exposed to the combustion chamber, pin boss portions 3 for supporting a piston pin, and skirt portions 4 with its side surface for coming into sliding contact with the inside surface of the cylinder at least on both sides in the swinging direction of a connecting rod connected to the piston pin . The pin boss portions 3 and the skirt portions 4 are interconnected through rib portions 5 extending from the underside of the head portion 2 downward. Ring grooves 6 are formed in the outside circumferential surface of the head portion 2. Each of opposing pin boss portions 3 is formed with a pin hole 7 into which the piston pin to be inserted. To manufacture the piston main part 1 described above, a metal mass (billet) or the raw material for the piston is forged with dice into the shape of the approximately piston-shaped workpiece having the head portion, pin boss portions, rib portions, and skirt portions. The approximately piston-shaped workpiece is made into the final product by finish machining process of shaping pin holes and ring grooves and, if necessary, plating process. FIGs. 2 (A), 2 (B), and 2 (C) show the half-finished, forge-shaped workpiece 11 for manufacturing the piston main part 1 shown in FIG. 1. The forge-shaped workpiece 11 has integral, respectively paired pin boss portions 3 and skirt portions 4 interconnected through rib portions 5 extending downward from the underside of the head portion 2. FIGs. 3 (A) and 3 (B) show the process of forge-shaping the metal mass as the raw material for the piston into the piston-shaped workpiece 11 in cross-sections as seen along the line C-C in FIG. 2 (B) using the method and dice of the invention. The thick, disk-shaped raw material (billet) 10 of an aluminium alloy is shaped into the approximately piston-shaped workpiece 11 by forging as squeezed between a die member (upper die or punch) 21 for shaping the top surface of the head portion and a die member (lower die) 22 for shaping the pin boss, skirt, and rib portions on the underside of the head portion through a cylindrical die member 23. In that case, according to this embodiment, the die member (lower die) 22 for shaping the pin boss, skirt, and rib portions on the underside of the head portion is divided by parting surfaces 24, reaching the lower end surfaces of the pin boss and skirt portions, into an inside die member 22a and outside die members 22b. As a result, flashes are left after forging on the lower end surfaces of the pin boss portions 3, skirt portions 4, and rib portions 5 of the forge-shaped workpiece 11 corresponding to the positions of the dividing surfaces 24 as shown in FIG. 2 (B). In case of forging with the dice described above, the raw material 10 for the piston placed between the die members is heated with a heater or heaters provided for at least one of the die members up to a temperature between 400 and 500 °C and hot - forged. In this way, ductility of aluminium alloy is fully utilized to forge shape the workpiece 11 with a high precision. Alternatively, the hot forging may be carried out by heating the raw material 10 for the piston, before being placed in the dice, up to a temperature between 400 and 500 °C and then placed in the dice and forged, or even in that case, forging may be carried out while the dice are being pre-heated up to a temperature between 400 and 500 °C. Arranging the forging process and the heating process for the raw material 10 for the piston in parallel can shorten the period of time for forging. With the forge-manufacturing method and the dice for the piston of the embodiment described above, the die member 22 is divided along parting surfaces 24 reaching the lower end surfaces of the pin boss, skirt, and rib portions. Therefore, when a piston material of a metal mass 10 is forge-shaped using a set of the dice into a piston-shaped workpiece 11 having a head portion 2, pin boss portions 3, skirt portions 4, and rib portions 5 extending downward from the underside of the head portion 2 and interconnecting the pin boss and skirt portions, cracks are prevented from being produced at the furrow bottom portions of the die member 22 for shaping the pin boss and rib portions, as the parting surfaces 24 part from each other by tiny amounts when the metal material is forced into the furrows. As a result, the life of the forging dice is extended. When the forging die is divided as described above, flashes are left after forging on the lower end surfaces of the pin boss portions 3, skirt portions 4, and rib portions 5 of the forge-shaped workpiece 11 corresponding to the positions of the dividing surfaces 24. However, even if the flashes remaining after forging are left without being scraped in the finish machining process, they do not impair the function of the piston itself. Leaving the forge-shaped flashes as they are can prevent the machining process from becoming more troublesome. Incidentally, in case that the rigidity of the cylindrical die member 23 as shown in FIG. 3 cannot be made high enough, the wall thickness of the skirt and pin boss portions after forging increases and the necessary volume of the piston material 10 increases. Therefore, the connection between the cylindrical die member 23 and the outside lower die 22b must be made stronger, which increases the cost for the die. In that case, the cylindrical die member 23 and the outside lower die 22b may be made as a single member: Alternatively as shown in FIG. 5, the forging die set may be arranged upside down so that the skirt, pin boss, and rib portions are located over the head portion. In that case, the lower die member 22 and the cylindrical die member 23 are secured to a stationary bed of a forging machine, while the upper die 21 comprising the inside die member 21a, the outside die member 21b, a base member 21c, and a connecting member 21d and attached to a press punching member is moved downward to perform forging. In that case too, the pin boss and rib portions are formed by using the two die members, the inside die member 21 a and the outside die member 21 b. The piston material (billet)10 used for the forge-shaped piston as described above is cut from a continuous, cylindrical bar-shaped cast material of aluminium alloy containing, for example, 10 - 25 % by weight of silicon (Si), 1 % by weight of iron (Fe), 0.5 - 7 % by weight of copper (Cu), 0. 1 - 2 % by weight of magnesium (mg), 1. 5 or less % by weight of manganese (Mn), 1. 5 or less % by weight of nickel (Ni), and 1 .5 or less % by weight of chromium (Cr) in aluminum base. Alternatively, the piston material 10 may be obtained as follows: Along with drawing out the aluminium alloy of the above-described composition in the form of a cylindrical, continuously cast body out of the bottom area of a smelting furnace, an agitator constituted with electromagnets or an ultrasonic oscillator is placed around part of the outside circumferential area of the material that has just come out the smelting furnace and has not solidified yet to agitate and mix together the materials in the central and outside circumferential areas of the cylindrical, continuously cast body, and the material is solidified while the agitation continues. In this way, the crystallized grains are restricted from growing and the small-sized grains are evenly distributed across the central and outside circumferential areas of the cylindrical material out of which the piston material 10 is cut to an appropriate size. When the piston material 10 is used that is obtained from the cylindrical, continuously cast body solidified while its inside and outside circumferential areas are mixed together by agitation as described above, since the crystallized grain size is small and the crystals are evenly distributed, cracks are less likely to occur during the forging process, and the forging yield increases. Furthermore, when such a piston is used in engines and operated, it exhibits a high fatigue strength in the skirt portion 4. An alternative piston material 10 may also be used that is made of rapidly cooled, solidified aluminium alloy powder of initial crystal grain size of 10 micrometers or less and containing 10 - 22 % by weight of silicon (Si). One composition of such rapidly cooled, solidified aluminium alloy powder contains, for example, 10 - 22 % by weight of silicon (Si), 1 - 10 % by weight of iron (Fe), 0. 5 - 5 % by weight of copper (Cu) , 0. 5 - 5 % by weight of magnesium (Mg), 1 or less % by weight of manganese (Mn) , 1 or less % by weight of nickel (Ni) , 1 or less % by weight of chromium (Cr) , 2 or less % by weight of zirconium (Zr), and 1 or less % by weight of molybdenum (Mo), in aluminum base. Of the constituents in such an aluminium alloy of rapidly cooled powder, silicon (Si) is added to improve resistance against wear and seizure by precipitating hard initial crystal and eutectic silicon grain in the metallic composition, iron (Fe) to increase strength at temperatures of 200 degrees C or higher by strengthening metallic composition by dispersion, copper (cu) and magnesium (Mg) to increase strength at temperatures of 200 °C or lower. Intended resistance against wear and seizure, and strength at high temperatures cannot be obtained when the amounts of addition deviate from the ranges given above. The rapidly cooled solidified powder aluminium alloy for the piston material 10 is made by dispersing molten aluminium alloy in the state of mist, made into powder by rapid cooling and solidification, and shaped. Therefore, the average grain size of the aluminium alloy powder is about 100 micrometers and the average grain size of silicon (Si) contained in the powder is 10 micrometers or smaller. Silicon is contained evenly in each aluminium alloy grain. When the piston material 10 is forge-shaped into the nearly piston-shaped workpiece 11, the material is extended and becomes thin especially in the skirt portion 4. However, since silicon (Si) is made into fine grains and dispersed as described above, cracks do not appear in the skirt portion 4 as a result of cracking of the initial crystal grains. Thus, the fatigue strength of the skirt portion 4 of the piston main part 1 is high. The rapidly cooled solidified powder aluminium alloy for the piston material 10 is not limited to that described above but may be an alloy that further contains silicon carbide (SiC) which is harder than silicon (Si) to further increase the wear resistance. An example of the rapidly cooled solidified powder aluminium alloy containing silicon carbide (SiC) and other elements contains in the aluminium base; 10 - 22 % by weight of silicon (Si), 1 -10 % by weight of iron (Fe), 0.5 - 5 % by weight of copper (Cu), 0.5 - 5 % by weight of magnesium (Mg), 1 or less % by weight of manganese (Mn), 1 or less % by weight of nickel (Ni) , 1 or less % by weight of chromium (Cr), 2 or less % by weight of zirconium (Zr), 1 or less % by weight of molybdenum (Mo), and 1 - 10 % by weight of silicon carbide (SiC). The rapidly cooled solidified powder aluminium alloy for the piston material 10 contains silicon (Si) in the state of fine particles of the size of 10 micrometers or smaller, dispersed in the metal composition, and further contains silicon carbide (SiC) 1 an insoluble, non-metallic substance which is harder than silicon (Si), of an average particle size of 10 micrometers or smaller to increase resistance against wear and seizure. The piston main part 1 forge-shaped from the piston material 10 contains the fine particles of silicon carbide (SiC) evenly dispersed in the aluminium base. This provides a high resistance against wear. To manufacture the piston material 10 of the rapidly cooled solidified powder aluminium alloy, first an aluminium alloy ingot is prepared with an aluminum (A1) base containing necessary amounts of ingredients (such as silicon, silicon carbide, etc.). The ingot is melted by heating at a temperature of 700 °C or higher, sprayed in the state of mist, and rapidly cooled at a cooling speed of 100 °C per second or faster to solidify into powder. Another method of forming the powder aluminium alloy is to make aluminium powder by melting and rapidly cooling aluminium alloy not containing necessary ingredients, and adding specified amounts of powder of necessary ingredients of average particle size of 1 - 10 micrometers, to obtain the aluminium alloy powder before solidification. Using the aluminium alloy powder, the piston material 10 is either directly formed by packing a mold of desired shape and size with the powder under pressure and temperature of 400 - 500 (below 700) degrees C, or the aluminium alloy powder is heated and extruded as a round bar, solidified, and the round bar is cut into a thick, disk-shaped material 10 of an appropriate volume corresponding to a piece of piston. Other alternative methods for making the piston material 10 of aluminium alloy powder include rolling aluminium alloy powder between a pair of rolls while heating the aluminium alloy powder at a temperature of 400 - 500 degrees C. The rolled product is punched with a press to produce a thick, disk-shaped piston material, or sheared to produce a rectangular material and shaped. It is also possible to apply preliminary forging to the rectangular product to shape a disk-shaped piston material. With the forging method and forging dice for manufacturing the forged piston according to the invention, when the metal mass as the material for the piston is forge-shaped into the shaped workpiece of the constitution in which the pin boss portions and the skirt portions are interconnected through the rib portions extending downward from the underside of the head portion of the piston, cracks are prevented from appearing in the furrow portions of the die members for shaping the pin boss portions and the rib portions. This extends the life of the forging dice.

Claims: Method for manufacturing a piston by using a set of die members (21,22,23) connected to each other at mating surfaces (24), with the steps of: forging a metal mass (10) as a raw material into a shaped workpiece (11) having a head portion (2), pin boss portions (3), skirt portions (4) and rib portions (5) extending downward from the underside of the head portion (2) and interconnecting the pin boss and skirt portions (3,4) and applying machining processes to the shaped workpiece (11), wherein the metal mass (10) is pressed by said die members (21,22,23) such that flashes are formed on the lower surfaces of the piston, characterized in that said flashes are formed on the pin boss portions (3), skirt portions (4) and rib portions (5) by filling the mating surfaces (24) of the die members (22a,22b;21a,21b) for shaping the pin boss portions (3), skirt portions (4) and rib portions (5), which members (22a,22b;21a,21b) are divided along said mating surfaces (24) respectively reaching the lower end surfaces of the pin boss portions (3), skirt portions (4) and rib portions (5). Method according to claim 1, characterized in that the die members comprising an upper die or punch (21) for shaping the top surface of the head portion (2) and a lower die (22) for shaping the pin boss portions (3), skirt portions (4) and rib portions (5) and that the metal mass (10) is placed into the lower die (22) and punched by the upper die (21). Method according to claim 1, characterized in that the die members comprising an upper die or punch (21) for shaping the pin boss portions (3), skirt portions (4) and rib portions (5) and a lower die (22) for shaping the top surface of the head portion (2), and that the metal mass (10) is placed into the lower die (22) and punched by the upper die (21). Method according to one of claims 1 to 3, characterized in that said die members (21,22,23) and/or said metal mass (10) are heated up to a temperature between 400° and 500° C before the forging process. Forging die members (21,22,23) for manufacturing a piston, comprising an upper die or punch (21) and a lower die (22), wherein said die members are connected to each other at mating surfaces (24), wherein at least said die members (22,23;21,23) for shaping pin boss portions (3), skirt portions (4) and rib portions (5) of said piston being divided along mating surfaces (24) respectively receiving the lower end surfaces of the pin boss portions (3), skirt portions (4) and rib portions (5). Forging die members according to claim 5, wherein the upper die or punch (21) being adapted to form the top surface of a piston head portion (2), and that the lower die (22) being adapted to form said pin boss, skirt, and rib portions (3,4,5) and being associated to a cylindrical die member (23). Forging die members according to claim 6, wherein said lower die (22) comprising an inside die member (22a) and outside die members (22b). Forging die members according to claim 7, wherein the outside members (22b) of the lower die (22) being removably connected or integral with the cylindrical member (24). Forging die members according to claim 5, wherein the upper die or punch (21) being adapted to form said pin boss, skirt, and rib portions (3,4,5), and that the lower die (22) being adapted to form the top surface of a piston head portion (2) and being associated to a cylindrical die member (23). Forging die members according to claim 9, wherein the upper die (21) comprising an inside member (21 a) and outside members (22b). Forging die members according to claim 9 or 10, wherein the lower die (22) being removably connected or integral with the cylindrical die member (23). Piston for an internal combustion engine, comprising a head portion (2), pin boss portions (3), skirt portions (4), and rib portions (5) extending downward from the underside of the head portion (2) and interconnecting the pin boss (3) and skirt portions (4), having flashes on the lower surfaces of the forged piston, said flashes being on pin boss portions (3), skirt portions (4), and rib portions (5), said flashes being formed by mating surfaces (24) of forging tools.

Inventors: KOIKE TOSHIKATSU, YAMAGATA HIROSHI, KOIKE, TOSHIKATSU, YAMAGATA, HIROSHI, Koike, Toshikatsu, c/o Yamaha Hatsudoki K.K., Yamagata, Hiroshi, c/o Yamaha Hatsudoki K.K.
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1999
Main classification: F02F3
IPC codes: B21K1, B22D18, F16J1, F02F3