Pistons for combustion engines are usually manufactured by casting or forging processes. The casting technique has the advantage that it allows for the manufacture of complex-shaped pistons of low weight. However, a substantial amount of machining effort must be taken into account for this. This applies in particular if a steel material is to be used in order to manufacture pistons which are suitable for particularly high loading values.
Depending on their size and purpose of application, forged pistons made of steel can be designed as both single-piece and comprising two or more parts. With multi-part pistons assembled from two or more parts, the individual parts are usually connected to one another by suitable joining methods of non-positive and positive fit in such a way that in practical use they resist the forces taking effect on them. Suitable for this purpose are, for example, the welding or conventional screwing or bolting of the individual parts of the piston.
One example of a multi-part piston for a combustion engine is known from DE 102 44 513 A1. This piston comprises a piston upper part forged from steel, into which a combustion recess, a ring wall, and a cooling channel designed in the manner of a hollow throat element are formed. On the other hand, the piston also comprises a piston lower part, which forms the piston shaft carrying the piston upper part and in which are formed hubs for accommodating a piston bolt connecting the piston with a connecting rod. To manufacture this piston, the piston upper part and the piston lower part are pre-formed in separate work processes by forging, and are then finished by machining with the removal of material. The finish-machining of the piston upper part in this situation also comprises the machining with removal of material of the wall sections delimiting the cooling channel, by means of which a metallurgical joining with the piston lower part by welding or soldering is established.
A multi-part arrangement of this type does indeed allow for the provision of a complex form for the piston. Apart from the problems regarding load-bearing capacity which arise from the multi-part design, the associated manufacturing effort and expenditure are, however, considerable.
With the manufacture by forging techniques of a single-piece piston, consisting of one unit, for combustion engines, usually a piston blank is produced in the first instance, which is then finished by machining with removal of material. All the indentations to be formed into the circumferential surface of the piston, such as to save weight, to improve forming behaviour, or hollow throat elements serving as cooling channels, are in this situation usually not produced until the material-removing process.
An example of such manufacture of a single-piece piston is provided in DE 100 13 395 A1. According to the method described therein, firstly a piston blank is forged, in that, starting from one face side, a cut-out is formed into a bolt-type forging blank, referred to as a billet, of which the outer wall forms the piston shaft of the piston blank. A cut-out is then punched into the piston shaft wall. To do this, a milling cutter or turning tool is applied from a lateral direction from the outside onto the circumferential surface of the piston blank.
Despite the advantages achieved by single-piece forming in relation to a multi-part design, single-piece pistons manufactured in a known manner present the disadvantage that they are, generally, heavier than comparable pistons manufactured by casting techniques. In addition to this, the effort and expenditure involved with material-removing finish-machining are high, since in the area of the top land formed at the circumferential edge of the piston base and of the piston shaft large quantities of material must be removed in order to create the required finished shape. This high manufacturing effort leads to high manufacturing costs.