Patent Description:
Pistons used in internal combustion engines typically include an upper piston part joined to a lower piston part. Various methods are known for joining the piston parts together. One common joining technique is friction welding, which includes continuously rotating at least one of the piston parts about its center axis at a high speed and under pressure against the other piston part. However, friction welding is known to create a significant amount of flash or scrap material in the cooling chamber of the piston, as well as residual stress and/or cracking along the weld. Resistance welding and laser welding have also been used to join piston parts together. However, these joining methods are known to cause residual stress, inadequate strength, and/or cracking along the weld. Another joining technique includes induction welding the piston parts together. An example of this technique is disclosed in <CIT> and <CIT>. However, there is still a need for strong welded pistons produced with less flash and scrap material, as well as less residual stress and cracking along the weld. <CIT> further discloses a method for producing a piston for an internal combustion engine including the steps of producing an upper piston part having at least one joining surface, producing a lower piston part having at least one joining surface, establishing a direct contact between the at least one joining surface of the upper piston part and the at least one joining surface of the lower piston part, heating the upper piston part and the lower piston part in the region of the joining surfaces brought in direct contact by induction or by a direct current flow through the joining surfaces, and connecting the upper piston part and the lower piston part to form a piston by means of a pressing process and optionally finishing the piston.

Methods for manufacturing a piston according to the invention are defined in independent claims <NUM> and <NUM> and a piston according to the invention is defined in subordinate claim <NUM>. One aspect of the invention provides a method of manufacturing a piston by hybrid induction welding to produce a strong weld with little to no flash or scrap material along the weld, as well as a homogenous metallurgical bond across the weld. The method comprises heating an upper joining surface of an upper piston part and a lower joining surface of a lower piston part by induction, bringing the heated joining surfaces toward one another, and allowing the heated joining surfaces to contact one another. The method next includes rotating a least one of the piston parts while the heated joining surfaces contact one another. Typically, the rotating step includes rotating one of the piston parts not more than <NUM> degrees in a first direction while the heated joining surfaces contact one another, and rotating the one piston part not more than <NUM> degrees in a second direction opposite the first direction while the heated joining surfaces contact one another. The method also includes applying pressure to at least one of the piston parts during the rotating steps to form a weld between the upper piston part and the lower piston part. The step of applying the pressure preferably includes increasing the pressure to a maximum pressure level, and applying the maximum pressure level while rotating the one piston part in the second direction. Instead of, or in addition to, controlling the pressure, the method can include bringing the heated joining surfaces toward one another to a part growth compensated position before the volume of at least one piston part reaches a final volume, wherein the part growth compensated position provides a space between the joining surfaces which compensates for the increase in volume.

Another aspect of the invention provides the piston manufactured by said method. The piston comprises the upper piston part including the upper joining surface, and the lower piston part including the lower joining surface welded to the upper joining surface. A portion of the upper piston part and a portion of the lower piston part located along the weld together present an outer surface, and the outer surface of the portions located along the weld are free of an indentation prior to machining.

One aspect of the invention provides a method of manufacturing a piston <NUM> for an internal combustion engine, such as a small diameter steel piston <NUM> with a narrow cooling chamber <NUM>, as shown in <FIG>, for use in a light vehicle diesel (LVD) system. The method is referred to as a hybrid induction welding method and includes a unique combination of position and force control. The method produces a strong weld <NUM> between an upper piston part <NUM> and a lower piston part <NUM>, as well as a homogenous metallurgical bond across the weld <NUM>.

The method begins by providing the upper piston part <NUM> and the lower piston part <NUM> which are used to form the piston <NUM>. The piston parts <NUM>, <NUM> are typically formed of steel, but can be formed of another type of metal or metal alloy. An upper joining surface <NUM> of the upper piston part <NUM> is axially aligned with and spaced from a lower joining surface <NUM> of the lower piston part <NUM>, as shown in <FIG>. The method then includes heating the upper joining surface <NUM> and the lower joining surface <NUM> by induction in an inert, non-oxidizing atmosphere. An induction coil <NUM> or any other type of induction heater can be used to heat the joining surfaces <NUM>, <NUM>, In the exemplary embodiment, the heating step includes heating the piston parts <NUM>, <NUM> to their forging temperature, which is typically a temperature of <NUM>° C to <NUM>,<NUM>° C and at a frequency ranging from <NUM> to <NUM>. The heating step typically lasts not longer than <NUM> seconds, and then the induction coil <NUM> is removed from between the joining surfaces <NUM>, <NUM>. In addition to heating the upper and lower joining surfaces <NUM>, <NUM>, the heating step includes heating a portion of the upper and lower piston parts <NUM>, <NUM> located a distance from the joining surfaces <NUM>, <NUM> to form a heat affected zone in each piston part <NUM>, <NUM>, The heat affected zone typically has a total length of <NUM> to <NUM> micrometers, and a length of <NUM> to <NUM> micrometers in each piston part <NUM>, <NUM>, The volume of at least one of the piston parts <NUM>, <NUM>, and typically both piston parts <NUM>, <NUM>, increases from a starting volume to a final volume due to the induction heating.

After induction heating the piston parts, <NUM>, <NUM>, the method next includes bringing the heated joining surfaces <NUM>, <NUM> toward one another and allowing the heated joining surfaces <NUM>, <NUM> to contact one another. The joining surfaces <NUM>, <NUM> are maintained in a fixed position about a center axis A1 while bringing the heated joining surfaces <NUM>, <NUM> toward one another, i.e. the piston parts <NUM>, <NUM> do not rotate during this step. The upper and lower piston parts <NUM>, <NUM> are moved axially to a predetermined position, referred to as a part growth compensated position, which accounts for the increase in volume of the piston parts <NUM>, <NUM> due to the induction heating. In the exemplary embodiment, the method includes bringing the heated joining surfaces <NUM>, <NUM> to the part growth compensated position before the volume of the piston parts <NUM>, <NUM> reaches the final volume. Thus, when the piston parts <NUM>, <NUM> first arrive at the part growth compensated position, there is a space provided between the joining surfaces <NUM>, <NUM> of the piston parts <NUM>, <NUM>, as shown in <FIG>, which compensates for the increase in volume that occurs before and after the piston parts <NUM>, <NUM> arrive at the part growth compensated position due to the induction heating. In the exemplary embodiment, after the piston parts <NUM>, <NUM> increase in volume, but while the piston parts <NUM>, <NUM> remain disposed at the part growth compensated position, the joining surfaces <NUM>, <NUM> of the piston parts <NUM>, <NUM> are no longer planar, and only a portion of the upper joining surface <NUM> of the upper piston part <NUM> contacts the lower joining surface <NUM> of the lower piston part <NUM>, as shown in <FIG>.

The part growth compensated position depends on the materials and geometry of the piston parts <NUM>, <NUM>, the heating time and temperatures, and possibly other factors. Various different methods can be used to determine the part growth compensated position. In the exemplary embodiment, the part growth compensated position is obtained by the following steps: (a) providing a test upper piston part formed of substantially the same material and having substantially the same geometry as the upper piston part <NUM> including the upper joining surface <NUM>; (b) providing a test lower piston part formed of substantially the same material and having substantially the same geometry as the lower piston part <NUM> including the lower joining surface <NUM>; (c) heating the test upper joining surface of the test upper piston part and the test lower joining surface of the test lower piston part by induction, wherein the volume of at least one of the test upper piston part and the test lower piston part increases from a starting volume to a final volume due to the induction heating; (d) bringing the test heated joining surfaces toward one another to an estimated part growth compensated position at a constant velocity before the at least one test piston part reaches the final volume, the step of bringing the heated joining surfaces to the estimated part growth compensated position including allowing the heated joining surfaces to contact one another; (e) monitoring an actual pressure level on the test parts for a spike in the actual pressure level; (f) adjusting the estimated part growth compensated position based on the magnitude of an identified pressure spike and position of the test parts when the pressure spike occurs; and (g) repeating steps (a)-(f) until a pressure spike of less than a predetermine value is identified during step (e). When the actual pressure level spikes, at least one of the joining surfaces of the test piston parts is upset into the other test piston part. Typically, the joining surfaces of each of the test piston parts are upset into the other test piston part. The step of adjusting the part growth compensated position then includes increasing the space between the joining surfaces when the test parts are at the estimated part growth compensated position by a distance proportional to the length of the upset formed when the actual pressure spikes.

Once the piston parts <NUM>, <NUM> are disposed at the part growth compensated position, the method includes two short rotational movements of one of the piston parts <NUM>, <NUM> under a controlled pressure, to form the exceptionally strong weld <NUM> between the upper and lower piston parts <NUM>, <NUM>, and the homogenous metallurgical bond across the weld <NUM>. This step includes rotating the one piston part <NUM> or <NUM>, typically the lower piston part <NUM>, not more than <NUM> degrees about the center axis A1 in a first direction, as shown in <FIG>, and then rotating the same piston part <NUM> or <NUM> not more than <NUM> degrees about the center axis A1 in a second direction opposite the first direction, as shown in <FIG>, while the heated joining surfaces <NUM>, <NUM> contact one another. Typically, the rotating steps include rotating the one piston part <NUM> or <NUM> in an amount of <NUM> to <NUM> degrees in the first direction, and then rotating the one piston part <NUM> or <NUM> in an amount of <NUM> to <NUM> degrees in the second direction. The rotating steps also typically include rotating the one piston part <NUM> to the same degree of rotation in the first direction as the second direction. In the exemplary embodiment, the rotating steps include rotating the one piston part <NUM> in an amount of <NUM> degrees in the first direction, and rotating the one piston part <NUM> in an amount of <NUM> degrees in the second direction.

The method further includes applying the controlled pressure to at least one of the piston parts <NUM>, <NUM> during the rotating steps to form the weld <NUM> between the upper piston part <NUM> and the lower piston part <NUM>. When the pressure is applied, only a minimal upset of one or both of the piston parts <NUM>, <NUM> occurs. For example, one of the joining surfaces <NUM>, <NUM> may be upset a longitudinal distance of <NUM> to <NUM> millimeters relative to the position of the joining surface <NUM>, <NUM> at initial contact with the other joining surface <NUM>, <NUM>.

The method further includes gradually increasing the pressure to a maximum pressure level during the rotating steps. This includes applying the pressure at a level less than the maximum pressure level throughout the step of rotating the one piston part <NUM> or <NUM> in the first direction; and obtaining and applying the maximum pressure level only during the step of rotating the one piston part <NUM> or <NUM> in the second direction. In the exemplary embodiment, the step of applying the pressure includes obtaining and applying the maximum pressure level only after the rotating steps are <NUM>/<NUM> complete and before the rotating steps are <NUM>/<NUM> complete.

The maximum pressure level to be applied to the piston parts <NUM>, <NUM> can be determined based on a variety of different factors. For example, the maximum pressure level can be based on an outer diameter D1 of the upper piston part <NUM> at the upper joining surface <NUM>; an outer diameter D2 of the lower piston part <NUM> at the lower joining surface <NUM>; an area presented by the upper joining surface <NUM>; an area presented by the lower joining surface <NUM>; and a desired upset of at least one of the joining surfaces <NUM>, <NUM> after the maximum pressure level is applied. In the exemplary embodiment, when the outer diameter D1 of the upper piston part <NUM> at the upper joining surface <NUM> and the outer diameter D2 of the lower piston part <NUM> at the lower joining surface <NUM> ranges from <NUM> to <NUM>, the area presented by each joining surface <NUM>, <NUM> ranges from <NUM><NUM> to <NUM>,<NUM><NUM>, and the desired upset of one of the joining surfaces <NUM>, <NUM> after the maximum pressure level is applied is not greater than <NUM>, then the maximum pressure level applied is typically about <NUM> N/mm<NUM>.

The steps of applying the pressure and rotating the piston part <NUM> or <NUM> include welding all areas of the joining surfaces <NUM>, <NUM> which are in contact with one another to form the exceptionally strong and homogeneous metallurgical bond across the welded joining surfaces <NUM>, <NUM>, as shown in <FIG> and <FIG>. In addition, by controlling the pressure, as well as the position and degree of rotation of the one piston part <NUM> or <NUM> relative to the other, minimal or no flash forms along the weld <NUM> between the piston parts <NUM>, <NUM>. In a typical friction welding process, flash and an indentation forms along the weld between the piston parts. However, in the method of the present invention, no indentation forms along the weld <NUM> during the steps of bringing the joining surfaces <NUM>, <NUM> together, rotating the one piston part <NUM> or <NUM>, and applying the pressure. An indentation is typically present along the circumference of a weld of a piston formed by conventional friction welding. The indentation is present on both the exposed outer diameter surface and also on the internal gallery surface. The indentation on the outer diameter surface can be removed by finish machining, but the indentation on the internal gallery surface cannot be machined. This indentation along the internal gallery surface acts as a stress riser, which typically leads to the initiation of cracks. Thus, the hybrid induction welded piston <NUM> of the present invention, which is free of an indentation on both the outer surface <NUM> and the internal surface presenting the cooling chamber <NUM>, provides the advantage of reduced cracking and thus longer service life compared to pistons formed by conventional friction welding.

In addition, another advantage is that no material is removed from the joining surfaces <NUM>, <NUM> of the piston parts <NUM>, <NUM> during the steps of allowing the heated joining surfaces <NUM>, <NUM> to contact one another, rotating the one piston part <NUM> or <NUM>, and applying the pressure. After the rotating steps, the method typically includes maintaining the upper piston part <NUM> and the lower piston part <NUM> in a fixed position about the center axis A1 while still applying the pressure for <NUM> seconds to <NUM> seconds to further promote the strength of the weld <NUM>.

Yet another advantage provided by the hybrid induction welding process of the present invention is accurate radial positioning of the upper piston part <NUM> relative to the lower piston part <NUM>. The radial positions of the piston parts <NUM>, <NUM> at the end of the hybrid induction welding process are equal to, or approximately equal to, the predetermined, desired radial positions of the piston parts <NUM>, <NUM> set at the start of the process. Preferably, the radial position of the upper piston part <NUM> relative to the lower piston part <NUM> at the end of the process is not more than +/- <NUM> degrees different from the radial position of the upper piston part <NUM> relative to the lower piston part <NUM> at the start of the process. In other words, the radial position of the upper piston part <NUM> relative to the lower piston part <NUM> after applying the pressure is not more than +/- <NUM> degrees different from the radial position of the upper piston part <NUM> relative to the lower piston part <NUM> before heating the piston parts <NUM>, <NUM>.

Thus, the hybrid induction welding process of the present invention is especially beneficial when the upper piston part <NUM> has a pre-forged geometry, such as when the upper piston part <NUM> includes a crown feature which is preferably disposed at a predetermined radial position relative to a pin bore axis A2 of the lower piston part <NUM>. For example, the upper piston part <NUM> may have a crown feature that is preferably disposed <NUM> to <NUM> degrees +/- <NUM> degrees from the pin bore axis A2. The desired radial position of the crown feature is set at the start of the process, and at the end of the process, the radial position of the crown feature is within +/- <NUM> degrees of that desired radial position. One of ordinary skill in the art will understand that the radial position of the pin bore axis A2 is determined by a line extending along the pin bore axis A2 and through the center axis A1 of the piston <NUM>; and the radial positon of the crown feature is determined by a line extending through the crown feature and through the center axis A1 of the piston <NUM>. The difference between two radial positions is determined by the angle between those two lines,.

Another aspect of the invention provides the hybrid induction welded piston <NUM>, as shown in <FIG>, and <FIG>, which is produced by the method described above. The piston parts <NUM>, <NUM> can comprise various different geometries. However, in the exemplary embodiment, the upper piston part <NUM> extends annularly around the center axis A1 and longitudinally along the center axis A1 from an upper wall <NUM> to a first portion of the upper joining surface <NUM>, referred to as a first upper joining surface, and a second portion of the upper joining surface <NUM>, referred to as a second upper joining surface. The upper piston part <NUM> is typically formed of a steel material, but can be another type of metal material. The upper wall <NUM> of the upper piston part <NUM> typically presents a bowl rim extending annularly around the center axis A1 and a combustion bowl <NUM> extending inwardly and downwardly from the bowl rim toward the center axis A1. The upper wall <NUM> also presents an apex at the center axis A1 surrounded by the combustion bowl <NUM>.

As best shown in <FIG>, the upper piston part <NUM> includes an upper outer rib <NUM> depending from the bowl rim of the upper wall <NUM> and extending annularly around the center axis A1 and longitudinally along the center axis A1 to the first portion of the upper joining surface <NUM>. The upper outer rib <NUM> presents a first portion of an outer surface <NUM> of the piston <NUM>, which extends annularly around the center axis A1 and faces away from the center axis A1, The upper outer rib <NUM> has a thickness t extending from the outer surface <NUM> to the cooling chamber <NUM>, and the thickness t of the upper outer rib <NUM> can be made smaller compared to ribs of pistons formed using other welding methods. The annular outer surface <NUM> of the upper outer rib <NUM> includes at least one ring groove <NUM> for retaining at least one piston ring (not shown). In the exemplary embodiment, the upper piston part <NUM> also includes an upper inner rib <NUM> spaced radially inwardly from the upper outer rib <NUM>. The upper inner rib <NUM> depends from the upper wall <NUM> beneath the combustion bowl <NUM> and extends annularly around the center axis A1 and longitudinally along the center axis A1 to the second portion of the upper joining surface <NUM>. The first and second upper joining surfaces <NUM> are flat and extend perpendicular to the center axis A1.

The lower piston part <NUM> also extends annularly around the center axis A1 and longitudinally along the center axis A1 from a base wall <NUM> surrounding the center axis A1. to a first portion of the lower joining surface <NUM>, referred to as a first lower joining surface, and from the base wall <NUM> to a second portion of the lower joining surface <NUM>, referred to as a second lower joining surface. The first portion of the lower joining surface <NUM> is welded to the first portion of the upper joining surface <NUM>, and the second portion of the lower joining surface <NUM> is welded to the second portion of the upper joining surface <NUM>.

The lower piston part <NUM> is also formed of a metal material, which is also typically a steel material. However, the hybrid induction welding process provides for the joining of different alloys, in which case the lower piston part <NUM> is typically formed of a steel material having a hardness less than the hardness of the steel material of the upper piston part <NUM>, For example, a very hard and temperature resistant alloy can be used to form the upper piston part <NUM> where combustion occurs, while a tougher, less costly alloy can be used to form the lower piston part <NUM> where cylindrical loading is present,.

In the exemplary embodiment, the lower piston part <NUM> includes a lower outer rib <NUM> extending upwardly from the base wall <NUM> toward the upper piston part <NUM> and extending annularly around the center axis A1 and longitudinally along the center axis A1 to the first portion of the lower joining surface <NUM>, The lower outer rib <NUM> presents a second portion of the outer surface <NUM> of the piston <NUM>, which extends annularly around the center axis A1 and faces away from the center axis A1. The lower outer rib <NUM> also has a thickness t extending from the outer surface <NUM> to the cooling chamber <NUM>, and the thickness t of the lower outer rib <NUM> can be made smaller compared to ribs of pistons formed using other welding methods. The annular outer surface <NUM> of the lower outer rib <NUM> includes at least one ring groove <NUM> for retaining at least one piston ring (not shown). The lower piston part <NUM> also includes a lower inner rib <NUM> spaced radially inwardly from the lower outer rib <NUM>. The lower inner rib <NUM> extends upwardly from the base wall <NUM> toward the upper piston part <NUM>, annularly around the center axis A1, and longitudinally along the center axis A1 to the second portion of the lower joining surface <NUM>. Like the first and second portions of the upper joining surfaces <NUM>, the first and second portions of the lower joining surfaces <NUM> are flat and perpendicular to the center axis A1.

The first portion of the lower joining surface <NUM> is radially aligned with the first portion of the upper joining surface <NUM>, and the second portion of the lower joining surface <NUM> is radially aligned with the second portion of the upper joining surface <NUM>, Each of the joining surfaces <NUM>, <NUM> are symmetric relative to the center axis A1 and concentric about the center axis A1. In addition, the joining surface <NUM>, <NUM> of at least one of the piston parts <NUM>, <NUM> may be upset a longitudinal distance of <NUM> to <NUM> millimeters.

The welded inner ribs <NUM>, <NUM> and the welded outer ribs <NUM>, <NUM> and the upper wall <NUM> and the base wall <NUM> form the cooling chamber <NUM> therebetween. The joining surfaces <NUM>, <NUM> of the outer ribs <NUM>, <NUM> are welded continuously from the outer surface <NUM> of the piston to the cooling chamber <NUM>, and the homogenous metallurgical bond extends across the welded ribs <NUM>, <NUM>, <NUM>, <NUM>. The cooling chamber <NUM> is closed and extends annularly around the center axis A1. The closed cooling chamber <NUM> presents a width w extending from the inner ribs <NUM>, <NUM> to the outer ribs <NUM>, <NUM> at the weld <NUM> and a volume which is free of flash or scrap metal material removed from the welded piston parts <NUM>, <NUM> during the welding process. This is an advantage over friction welded pistons which typically contain scrap metal material in the cooling chamber due to the welding process. Since the piston <NUM> of the present invention includes no flash or scrap metal material in the cooling chamber <NUM>, the volume of the cooling chamber <NUM> can be smaller than cooling chambers of other types of welded pistons. For example, the width w of the cooling chamber <NUM> is typically from <NUM>% to <NUM>% of the outer diameter D1, D2 of the piston parts <NUM>, <NUM> at the weld <NUM>. The thickness t of the outer ribs <NUM>, <NUM> can also be made smaller compared to pistons formed using other welding methods. In addition, minimal residual stress is formed in the piston, thus eliminating the concern for cracking of the piston parts <NUM>, <NUM> after welding, which oftentimes occurs in friction welded pistons.

As shown in <FIG>, the inner ribs <NUM>, <NUM> of the hybrid induction welded piston <NUM> surround the center axis A1 and form a combustion bowl <NUM>. The joining surfaces <NUM>, <NUM> of the inner ribs <NUM>, <NUM> are welded continuously from the cooling chamber <NUM> to the combustion bowl <NUM>, and the homogenous metallurgical bond extends across the welded inner ribs <NUM>, <NUM>.

A portion of each of the upper ribs <NUM>, <NUM> and a portion of each of the lower ribs <NUM>, <NUM> located along the weld <NUM> include the heat affected zone. In the exemplary embodiment, when both the upper and lower piston parts <NUM>, <NUM> are formed of steel material, the steel material of the heat affected zone includes a microstructure of tempered martensite. The steel material surrounding the heat affected zone has a microstructure different from the tempered martensite of the heat affected zone. In the exemplary embodiment, the martensitic material of the heat affected zone is harder than the surrounding material.

As shown in <FIG>, the heat affected zone has a length HAZ extending along and parallel to the center axis A1. In the exemplary embodiment, the heat affected zone length HAZ is <NUM> to <NUM> micrometers. The piston <NUM> presents also presents a total cross-sectional area at the weld <NUM>, which includes the area of the joining surfaces <NUM>, <NUM>, the area of the combustion bowl <NUM>, and the area of the cooling chamber <NUM>. The joining surfaces <NUM>, <NUM> of the piston <NUM> at the weld <NUM> together present a cross-sectional area of <NUM>% to <NUM>% of the total cross-sectional area of the piston <NUM>.

As discussed above, immediately upon completion of the welding process, the outer surface <NUM> along the weld <NUM> and along the heat affected zone are free of any type of visible weld parting line or indentation. The outer surface <NUM> of the portions along the weld <NUM> and including the heat affected zone are also free of flash, prior to any machining, which is an advantage over other types of welded pistons. In the exemplary embodiment of <FIG>, the outer surface <NUM> of the piston <NUM> presents a flat surface along the heat affected zone and the weld <NUM>. In this case, the flat outer surface <NUM> extends continuously around the center axis A1 along the weld <NUM> and longitudinally along the heat affected zone.

The desired flat outer surface <NUM> is preferably formed during the hybrid induction welding process. However, the outer surface <NUM> of the piston <NUM> typically presents a convex surface having a spherical radius of at least <NUM>,<NUM> millimeters along the heat affected zone and the weld <NUM>, as shown in <FIG>, at the end of the welding process and prior to any machining. In this case, the outer surface <NUM> located along the heat affected zone and the weld <NUM> is convex and presents a bulge extending radially outwardly and continuously around the center axis A1. The outer diameter D1, D2 of the piston <NUM> at the bulge is <NUM> to <NUM> millimeters greater than the outer diameter D1, D2 of the piston <NUM> adjacent the bulge. Typically, any spherical radius or convex surface formed along the weld <NUM> and present at the end of the welding process is machined to provide the desired flat outer surface <NUM> shown in <FIG>.

In the exemplary embodiment, the lower piston part <NUM> includes a pair of pin bosses <NUM> extending downwardly from the base wall <NUM> away from the upper piston part <NUM>. Each pin boss <NUM> presents a pin bore <NUM>, and the pin bores <NUM> are aligned with one another along a second axis A2 perpendicular to the center axis A1. The lower piston part <NUM> includes a pair of skirt sections <NUM> each depending from the base wall <NUM> and spaced from one another by one of the pin bosses <NUM>.

Claim 1:
A method of manufacturing a piston (<NUM>), comprising the steps of:
heating an upper joining surface (<NUM>) of an upper piston part (<NUM>) and a lower joining surface (<NUM>) of a lower piston part (<NUM>) by induction;
bringing the heated joining surfaces (<NUM>, <NUM>) toward one another and allowing the heated joining surfaces (<NUM>, <NUM>) to contact one another;
rotating one of the piston parts (<NUM>, <NUM>); and
applying pressure to at least one of the piston parts (<NUM>, <NUM>) during the rotating step to form a weld (<NUM>) between the upper piston part (<NUM>) and the lower piston part (<NUM>) characterized in that
the step of rotating one of the piston parts (<NUM>, <NUM>) includes rotating the one of the piston parts (<NUM>, <NUM>) in a first direction while the heated joining surfaces (<NUM>, <NUM>) contact one another;
the step of applying the pressure includes increasing the pressure to a maximum pressure level and applying the maximum pressure level while rotating the one piston part in a second direction opposite the first direction.