Patent Application
20140261283
1. Field of the Invention
The present invention is related generally to pistons for internal combustion engines.
2. Related Art
Monobloc aluminum pistons for use with internal combustion engines generally include a crown portion, a pair of skirt portions and a pair of pin bosses all formed as one piece. The pin bosses of such pistons are aligned axially with one another, and when the piston is installed in an engine, a wrist pin extends between and is supported by the aligned pin bosses. The wrist pin couples the piston with a connecting rod which transfers mechanical power from the piston to a crankshaft during operation of the engine. The pin bosses of typical monobloc aluminum pistons, which are usually formed through conventional casting or forging processes, are generally too soft to directly support the wrist pin without deforming during operation of the engine, i.e. the pin bosses may deform under the contact forces between the piston and the wrist pin during operation of the engine. Accordingly, most piston manufacturers install a separate bushing of a harder material into each of the pin bosses to reduce the risk of deformation of the pin bosses during operation of the engine. However, the bushings increase the material cost of the piston, and the process of pressing the bushings into the pin bosses may be time consuming and/or laborious.
One aspect of the present invention provides for a piston for an internal combustion engine including a pair of pin bosses formed of aluminum or an aluminum alloy which are hardened relative to the surrounding portions to directly support a wrist pin without deformation during operation of an engine. Specifically, each of the pin bosses presents a zone with increased hardness relative to the surrounding portions of the piston at an inner periphery of the generally circular opening for directly supporting the wrist pin without an intermediate bushing. This leads to cost savings as compared to conventional aluminum pistons with bushings. Preferably, the hardened zones have a dendritic microstructure with primary silicon crystallites dispersed throughout; are substantially free of pores and cracks; and have a Vickers Hardness Value of between 169 and 177.
Another aspect of the present invention provides for a method of making a piston. The method includes the step of preparing a piston body of aluminum or an aluminum alloy and having a pair of pin bosses spaced from one another in an axial direction. Each of the pin bosses presents a generally circular shaped opening for receiving a wrist pin. The method proceeds with melting at least a portion of the inner periphery of at least one of the pin bosses with a laser beam and cooling the melted region. This process alters the microstructure of the piston body at the inner periphery of the pin boss such that it is harder than the surrounding portions of the piston body, thereby allowing the pin bosses to directly support a wrist pin without an intermediate bushing. This process may be carried out very quickly and cost effectively. Additionally, quenching in air or shield gas is sufficient to establish the desired microstructure, so a separate cooling process is not required.
According to yet another aspect of the present invention, the method additionally includes the step of rotating the piston body and reflecting the laser beam in a vertically downward direction such that the portion of the at least one pin boss being melted is substantially the lower-most portion of the at least one pin boss during the rotation of the piston body. This is advantageous because the melt pool is not able to flow downwardly from gravity.
According to still another aspect of the present invention, an apparatus for hardening an inner diameter of an aluminum or aluminum alloy pin boss in a piston body is provided. The apparatus includes a rotary chuck for supporting and rotating the piston body, a laser for emitting laser beam in a direction towards the pin boss and a reflector oriented to reflect the laser beam against the inner diameter of the pin boss to melt at least a portion of the inner diameter of the pin boss. This apparatus is able to quickly and efficiently reinforce harden portions of the pin bosses of piston bodies so that they may directly engage a wrist pin and resist deformation from forces between the piston body and the wrist pin during operation of an engine. Additionally, with little or no changes to the apparatus, the pin bosses of differently sized or shaped pistons may be hardened.
These and other features and advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
One aspect of the present invention is an apparatus 20 for hardening an inner periphery of an opening of a pin boss 22. More precisely, the apparatus 20 according to this aspect of the present invention is configured to harden the inner peripheries of a pair of pin bosses 22 in a pre-formed aluminum or aluminum alloy piston body (such as the piston body 24 shown in
Referring to
During operation of the apparatus 20, the diode laser head 26 is activated and the laser beam 28 is reflected off of the copper mirror 32 in the direction of a lower-most portion of the inner diameter of the pin boss 22 to melt only that portion of the pin boss 22. This location is advantageous because gravity cannot pull the molten aluminum or aluminum alloy downwardly any further. During the melting of the lower-most portion of the inner diameter, the rotary chuck 30 is rotated at a predetermined and generally constant rotational velocity or speed through at least one entire revolution to re-melt substantially the entire inner periphery of the pin boss 22. Among other things, the speed of the rotary chuck 30, the power of the diode laser stack 26, the focal point of the laser beam 28 and the shape of the laser beam 28 may all be varied to adjust the depth of the remelted zone of the aluminum or aluminum alloy material. Preferably, these parameters are selected to remelt the material at the depth of between 1.1 and 1.7 mm.
Another aspect of the present invention is a method of making an aluminum or aluminum alloy piston body 24 with hardened pin bosses 22 for directly supporting a wrist pin without a bushing and resistant to deformation from forces between the piston body 24 and the wrist pin during operation of an engine. The exemplary method includes the step of preparing a piston body 24 having a pair of pin bosses 22 of aluminum or an aluminum alloy and extending in an axial direction. Each pin boss 22 also has an inner periphery which encircles and opening for receiving the wrist pin. In the exemplary embodiment shown in
The exemplary method continues with the step of applying a light absorbing coating 38 (best shown in
The exemplary method then proceeds with the step of emitting a laser beam 28 having a generally rectangular profile in a generally horizontal direction (i.e. perpendicular to gravity) and reflecting the laser beam 28 by approximately ninety degrees (90°) with a copper mirror 32 into a vertically downward direction (i.e. parallel with gravity). The mirror 32 is preferably placed within the circular opening of one of the pin bosses 22 and oriented at a forty-five degree (45°) angle relative to the horizontal direction. As such, the reflected laser beam 28 contacts the lower-most portion of the inner diameter of the pin boss 22. In the exemplary method, the laser beam 28 has a wavelength of approximately eight hundred and eight nanometers (808 nm) and has a generally rectangularly shaped profile and is emitted by a diode laser stack 26 with approximately four kilowatts (4 kw) of power and a focus of approximately twelve by one half millimeters (12 mm×0.5 mm). These parameters have been found to melt the inner periphery of the aluminum or aluminum alloy pin bosses 22 to a depth of approximately one to two millimeters (1 mm-2 mm) and most preferably to a depth of approximately 1.1-1.7 mm.
During the melting step the method continues with the step of rotating the piston body 24 (for example, with a rotary chuck 30) about an axis which extends in the horizontal direction through at least one full revolution. Specifically, the axis extends through the centers of the openings in the pin bosses 22. As such, the laser beam 28 remains generally stationary and the piston body 24 rotates so that substantially the entire inner periphery of one of the pin bosses 22 may be re-melted. As the re-melted portions of the pin boss 22 rotate away from the reflected laser beam 28, they cool and solidify in the ambient air with a different microstructure and an increased hardness as compared to the surrounding portions of the piston body 24. Because the laser beam 28 contacts the lower-most portion of the inner periphery of the pin boss 22, the melt pool does not flow out of the treated zone via gravity before solidification takes place, i.e. the melt pool remains in the lower-most portion of the inner periphery. As such, the hardened zone 25 has a generally uniform thickness around the opening. Depending on the length of the pin boss 22 and the width of the laser beam 28, it may be necessary to rotate the pin boss 22 through more than one full revolution while either moving the piston body 24 and/or the mirror 32 axially between rotations so that the entire length of the inner periphery of the pin boss 22 may be re-melted. After the re-melting and cooling processes are completed, it may be desirable to smooth and finish the inner surface of the pin boss 22. The above-described steps may then be repeated for the other pin boss 22 so that both pin bosses 22 are hardened. The hardened zones 25 preferably have a dendritic microstructure with primary silicon crystallites dispersed throughout; are substantially free of pores and cracks; and have a Vickers Hardness Value of between 169 and 177.
The re-melting process may be carried out in atmospheric air or in a shield gas without the need for additional cooling. As such, the re-melting and cooling processes may be carried out very quickly and cost effectively. This provides for cost savings as compared to conventional aluminum pistons which require a separate bushing to be installed into the openings of the pin bosses to support the wrist pin. Rather than re-melting and cooling the entire inner periphery, the laser head 26 could be synchronized with the rotary chuck 30 to only re-melt selective zones of the inner periphery of the pin boss 22.
An exemplary piston body 24 having pin bosses 22 which were hardened according to the above-described method using the above-described apparatus 20 is generally shown in
Referring now to the cross-sectional view of
Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims.
