THE MANUFACTURE OF PISTONS

Abstract: A piston is formed in two parts (10, 11). The main part (10) is formed by gravity die casting from aluminum or aluminum alloy and a second part (11) of the piston is formed by a squeeze casting process to produce a material which is stronger and more resistant than the gravity die cast aluminium alloy. At least one piston ring groove (14a) is formed in the second part (11). The two parts are then electron beam welded together to form the complete piston. The squeeze cast portion may be reinforced with whiskers or fibers to further improve its properties. This method of construction has the benefit that only the minimum amount of the piston is formed by the more expensive and time-­ consuming squeeze casting process so that parts of the piston which do not require the improved properties given by squeeze casting are simply gravity die cast. This is of particular benefit in large diesel pistons.

Description: The invention relates to a method of manufacturing a piston for an internal combustion engine, comprising forming a main body portion with a combustion bowl at an upper end thereof and an annular outwardly-directed rebate extending therearound at said upper end, and forming a second body portion of annular shape with an annular wear-resistant reinforcement in which is provided a piston ring groove, the second body portion being received in said outwardly directed rebate in said main body portion and being connected thereto by a welding technique. A method having these features is shown in DE-B-1251588. It has been customary to manufacture the basic piston shape by a single piston-forming process; by forging, by gravity die casting or, more recently, by squeeze casting methods in which molten metal is gravity fed to an open die, the die then being closed and the metal solidified under a force of many tonnes. Each of these methods has advantages in the formation of certain parts of the piston but disadvantages in the formation of certain other parts of the piston. In the case of forging, this method has the benefit of producing high strength so that highly stressed parts of of piston, such as gudgeon pin bosses, are very strong when forged. It has the disadvantage, however, that it is difficult to produce crown end piston features, such as combustion bowls or piston ring grooves, by forging; these require subsequent machining and, possibly, shaping operations. Gravity die casting has the advantage that pistons can be produced easily and cheaply by this method. The incorporation of inserts can be readily accomplished by using, for example, the Al-Fin bond. Gravity die casting has the disadvantage, however, that it does not have high strength as compared with forging, and is not particularly resistant to the elevated temperatures often found at the crown end of a piston, particularly where that piston is for a diesel engine. Squeeze casting has the advantage that a piston so produced has a more uniform structure than gravity cast pistons, with an absence of voids. This makes the piston mechanically stronger as well as improving generally the properties of material. The production of pistons by squeeze casting is, however, more expensive and time-consuming than their production by conventional gravity die casting. In addition, while the improved properties are desirable or necessary in certain parts of the pistons, such as the crown region, they are not as desirable or necessary in other portions of the pistons, such as the lower skirt region. This is particularly true where the pistons are very large diesel pistons where the crown is subject to high temperatures and where the volume of material involved in the manufacture of the piston makes squeeze casting expensive. Specification GB-A-1260599 discloses a piston in which the crown is formed from a powdered metal by a sintering technique and is connected to a skirt by an electron beam weld. Specification DE-A-2726273 discloses a piston in which a reinforcement is encast into a piston body to reinforce an entrance to a combustion bowl and a piston ring groove. Edges of the piston ring groove are spray-coated with a resistant material. According to the invention, there is provided a method of manufacturing a piston for an internal combustion engine, comprising forming a main body portion with a combustion bowl at an upper end thereof and an annular outwardly-directed rebate extending therearound at said upper end, and forming a second body portion of annular shape with an annular wear-resistant reinforcement in which is provided a piston ring groove, the second body portion being received in said outwardly directed rebate in said main body portion and being connected thereto by a welding technique, characterized in that the main body portion is formed by a forging or gravity die casting process, a third portion is formed by a squeeze casting process in which a mass of temperature-resistant fibres or whiskers are incorporated into an aluminium alloy by a squeeze casting process to produce a generally cylindrical member part-reinforced with said fibres or whiskers, which is then machined to form an annular insert having an inner portion reinforced with said fibres or whiskers and defining a combustion bowl entrance and outer and lower surfaces formed from unreinforced aluminium alloy, the second portion being formed by a squeeze casting process and extending beyond an upper surface of the main body portion to form therewith an inwardly directed rebate, said third body portion then being welded in said inwardly directed rebate to form a temperature-resistant entrance to the combustion bowl, the second and third portions together providing a squeeze cast crown to the piston. The following is a more detailed description of one embodiment of the invention, by way of example, referene being made to the accompanying drawings in which:- Figure 1 is a cross-section of part of a crown end of a piston for an internal combustion engine, which is formed from main, second and third portions connected together by a welding technique, and Figure 2 is a schematic part-section of a squeeze casting die showing the formation of the third portion of the piston of Figure 1. Referring first to Figure 1, the piston, which may, for example, be a large piston for a diesel engine, is formed by a main piston portion 10 and a second piston portion 11, connected to the main portion by a weld 12. The main piston portion is either gravity die cast or forged from aluminium or an aluminium alloy. The first piston portion 10 forms a part combustion bowl 18 but not the combustion bowl entrance 22. The first piston portion 10 is also machined to provide a lowermost piston ring groove 14d and an inner and lower portion 26 of a gallery 13 extending around the crown of the piston for cooling oil. Also provided are two L-shaped rebates 27, 28, one, 27 leading upwardly from the upper edge of the gallery portion 26 and then inwardly to a side wall 16 of the combustion bowl, and the other, 28, leading from the lower edge of the gallery portion to a ring band 15. A second piston portion 11 is formed by a squeeze casting process from a molten aluminium or aluminium alloy. The molten metal or metal alloy is gravity fed into a die which is closed by a movable die member. The molten metal is then solidified under a force of many tonnes to form the second piston portion 11. After solidification, the die is opened and the squeeze cast member of homogeneous metal or metal alloy is removed. A reinforcement 36 of fibres or whiskers is provided in the region of the uppermost piston ring groove 14a so that, when formed, this groove 14a has radially extending surfaces 46 and a base 47 reinforced against wear by the associated piston ring groove. Next, the squeeze cast member is machined to form the second piston portion. In this machining, the portion 11 is shaped to be generally annular with a flat upper surface and a curved outer surface formed with three upper piston ring grooves 14a, 14b, 14c. Each piston ring groove has spaced radially extending surfaces 46 interconnected by a base 47. The inner surface is shaped to provide the remainder 29 of the gallery 13 and is formed with two projecting L-shaped portions 30, 31 corresponding to the two L-shaped rebates 27, 28 on the main portion 10. The third piston portion 35 is formed in the following way (see Fig. 2). First, a wad or mat of fibres or whiskers are placed in a lower die part 41 of a squeeze casting apparatus. These fibres may be ceramic or metallic fibres or whiskers. The die, when closed by an upper die part 42, is of generally cylindrical configuration. The size of mat or wad 40 is such that it does not fill the closed die completely but leaves a space therearound and thereabove. A molten aluminium or aluminium alloy, such as Lo-Ex, is then gravity fed into the die which is closed by the upper die part 42. The molten metal is solidified under a force of many tonnes which forces the molten metal into the mat or wad of fibres to form an intimate bond therebetween. After solidification, the die is opened and the squeeze cast member 43 so formed is removed. This member is generally cylindrical in shape (see Fig. 2) and has a central portion reinforced by the fibres or whiskers. The squeeze casting process ensures that the member is free from voids and other strength reducing defects and also ensures a strong intimate bond between the fibres or whiskers and the metal. The fibres or whiskers improve the heat-resistance of the member and also improve its strength. The member is next machined to produce the third piston portion 35 in the form of an annular insert whose inner portion defines an entrance 22 to the combustion bowl 18 and which has a lower surface 23 and an axially extending outer surface 24 meeting at a circular edge. The lower surface 23 is inclined upwardly towards the centre of the insert. The two portions 10, 11 are first connected together by any suitable welding technique, for example, an electron beam welding technique, with the welds 12 extending only between the vertical surface of the rebate 27 and the corresponding vertical surface of the L-shaped portion 30 of the squeeze casting 11, and between the horizontal surface of the rebate 28 and the corresponding horizontal surface of the other L-shaped portion 31. The remaining horizontal and vertical surfaces are left unconnected, partly because their connection is not strictly necessary and partly because of the difficulty of effecting a connection in these regions. To complete the piston, the third piston portion 35 is connected to the main portion 10 by a weld 12 between the lower surface 23 of the portion 35 and the upper radially inwardly extending surface of the rebate 27. In this way, the completed piston has a reinforced entrance 22 to the combustion bowl 18 which is more heat-resistant than the remainder of the piston, and a reinforced upper piston ring groove 14a which is more wear-resistant than the remaining grooves. This latter eliminates the need for the use of precast piston ring reinforcement inserts and their incorporation by the so-called Al-Fin process. In addition, the gallery 13 is formed without use of a salt core casting process. This simplifies considerably the casting steps. This is of particular benefit in large diesel pistons where the volume of piston material is comparatively high. However, as shown in broken line in Figure 1, the second piston portion 11 may incorporate the whole of the gallery 13, which may be formed in the portion 11 by a salt core process. This has the benefit that the second piston portion 11 is of less complex shape. In addition, it allows unbroken welds 12 between this portion 11 and the main piston portion 10.

Claims: 1. A method of manufacturing a piston for an internal combustion engine, comprising forming a main body portion (10) with a combustion bowl (18) at an upper end thereof and an annular outwardly-directed (27,28) rebate extending therearound at said upper end, and forming a second body portion (11) of annular shape with an annular wear-resistant (36) reinforcement in which is provided a piston ring groove (14a), the second body portion (11) being received in said outwardly directed rebate in said main body portion (10) and being connected thereto by a welding technique, characterized in that the main body portion (10) is formed by a forging or gravity die casting process, a third portion (35) is formed by a squeeze casting process in which a mass of temperature-resistant fibers or whiskers (40) are incorporated into an aluminum alloy by a squeeze casting process to produce a generally cylindrical member part-reinforced with said fibres or whiskers, which in then machined to form an annular insert having an inner portion (22) reinforced with said fibres or whiskers and defining a combustion bowl entrance and outer and lower surfaces (23, 24) formed from unreinforced aluminum alloy, the second portion (11) being formed by a squeeze casting process and extending beyond an upper surface of the main body portion to form therewith an inwardly directed rebate, said third body portion then being welded in said inwardly directed rebate to form a temperature-resistant entrance to the combustion bowl, the second and third portions together providing a squeeze cast crown to the piston. 2. A method according to claim 1, characterised in that the reinforcement of the second piston portion is provided by fibres or whiskers incorporated during said squeeze casting process. 3. A method according to claim 1, characterised in that the reinforcement of the second piston portion is provided by an annular insert of a ferrous material (46) incorporated during said squeeze casting process. 4. A method according to claim 1, characterised in that the second piston portion (11) defines a part of a gallery (13) which extends around the interior of the piston. 5. A method according to claim 1, characterised in that the welding technique is an electron beam welding technique.

Inventors: AVEZOU, JEAN-CLAUDE
Applicants: AE PLC
Application year: 1985
Main classification: B22D15
IPC codes: F02F3, F16J1, B23P15