This invention relates to heavy duty aluminum pistons, for use in internal combustion engines such as diesel engines, which pistons have reinforced piston ring grooves and particularly to such an aluminum piston and method of making it with a piston ring groove formed in a stainless steel insert therein.
The advantages of using aluminum pistons in internal combustion engines with high compression ratios is well known. However, aluminum pistons have the disadvantage that the walls of piston ring grooves formed therein will deform in use under heavy repetitive impact of the piston rings, which are preferably made of a metal harder than aluminum, thereby shortening the life of the piston and requiring frequent engine overhauling including early replacement of the pistons.
It has been proposed in the prior art to reinforce the piston ring grooves of aluminum pistons with coatings or inserts of a material harder than aluminum such as cast iron or steel. However, since such harder materials melt at temperatures higher than the melting temperature of aluminum, it has been impossible to provide a direct bond between such harder materials and the aluminum piston. If the coatings or inserts are not firmly fixed to the aluminum of the piston, they will tend to move under the influence of the piston rings in use and will eventually produce the same wearing of the aluminum piston as that produced by the piston rings themselves.
Thus, in the prior art, various attempts have been made to mechanically fix steel reinforcing inserts in aluminum pistons. For example, U.S. Pat. No. 3,118,712 to Daub teaches the mechanical embedding of steel inserts in aluminum pistons. Such mechanical bonds are, of course, inherently subject to wear in use.
According to another approach, cast iron rings have been dipped in molten aluminum and placed in the molds for subsequent casting of aluminum pistons thereabout as taught, for example, in U.S. Pat. No. 3,183,796 to Christen et al. However, the resulting bond between the insert and the piston is still essentially a mechanical bond since no alloying of the aluminum with the iron insert can occur at the comparatively low melting temperature of aluminum.
More recently, it has been proposed to flame spray a reinforcing metal such as steel into a preformed groove in the aluminum piston. However, the resulting bond between the piston and the sprayed metal is weak and essentially mechanical. According to the teaching of U.S. Pat. No. 3,715,790 to Reinberger, the bond is improved by first preheating the piston and then spraying a bonding layer of an alloy such as nickel aluminide in the preformed groove prior to filling the groove with a stainless steel spray coating. Even in this case, no alloying of the aluminum with the stainless steel occurs although it may be possible to obtain some fusion of the bonding layer to the aluminum and the stainless steel, respectively, which fusion is sharply limited by the necessity for keeping the temperature of the piston below 800.degree. F. (425.degree. C) which is the creep point of aluminum.
It is the principal object of this invention to provide an aluminum piston and method of making it with a piston ring groove in a stainless steel insert formed in situ in an oversize groove in the piston and fusion bonded thereto by alloying of the steel and aluminum.