Blowout resistant weld method for laser welds for press-fit parts

A method for reducing “blow-out” of annular welds for attaching press-fit components in a fuel injector assembly is disclosed. The method employs a multi-step welding procedure whereby a first annular weld bead that corresponds to less than 360° of rotation of the assembly about a longitudinal axis leaves a radial void that is thereafter sealed with a second annular weld bead. In an alternative embodiment, a relief region is formed on radially-facing surfaces of the components, the relief region being disposed adjacent to a press-fit region. A sealed gap is thereby formed in the relief region between the welds and the press-fit region. The sealed gap provides a further means for expansion of trapped gases that could otherwise “blow out” the liquid weld bead.

FIELD OF THE INVENTION

The present invention relates generally to the field of welding, and more particularly, to techniques and systems for forming hermetic, blow-out resistant welds between press-fitted components in a fuel injector assembly.

BACKGROUND OF THE INVENTION

A fuel injector includes a pressure vessel, a valve venting the pressure vessel, and a coil-driven magnetic circuit for driving the valve. The pressure vessel must not exhibit external fuel leaks during operation. Most fuel injector designs utilize multiple components that are welded together to create the pressure vessel.

Typically, fuel injector pressure vessel components are welded using a laser. A laser has been used successfully to weld joint configurations such as lap joints where overlapping surfaces of the components are jointed, butt joints where two components are joined end-to-end without overlap, and fillet joints in which material is removed on abutting parts to provide room for a weld bead. Lasers are suitable for welding small precision components together dependably and quickly in a production environment.

In many applications, the laser beam is held stationary as the part to be welded is moved or rotated to form the weld. To weld the hermetic pressure vessel used in fuel injectors, the beam is commonly held stationary while the part is rotated. For a hermetic weld of that type on tubular components such as those of the fuel injector pressure vessel, the “on” time for the laser beam is greater than the time it takes the part to make one revolution. The resulting overlap of the weld ensures that the weld is hermetic.

One common problem associated with such laser welding on tubular components occurs as the overlap of the weld is formed. Certain welding conditions and joint designs tend to result in a “blow out” of the weld bead, usually during final overlap of the weld. That “blow out” is created by rapidly increasing internal pressure on one side of the weld, due to a sudden rise in temperature related to the welding. The “blow out” occurs most commonly as the weld overlap occurs, although under certain conditions it is known to occur elsewhere. If an internal region to either side of the weld joint is undergoing a sufficient pressure increase, the weld “blow out” occurs when the molten weld pool is unable to resist the forces exerted by the pressure differential. The weld “blows out,” leaving a hole or gap in the weld bead. That hole typically leads to an increase in leak-related scrap during the assembly process.

For example, two components may be lap welded together at a continuous “interference fit” or press-fit region. Such welds have been known to exhibit “blow-out” regions at random locations relative as well as in the overlap. Those “blow-outs” are often at multiple radial locations throughout the weld. It has been theorized that in a press fit region, small cavities contain trapped air due to an imperfect surface finish of the components pressed together. When laser welding is attempted over those small cavities, the air inside undergoes a sudden change in temperature and expands. That expansion “blows out” the molten weld pool, leaving behind a void in the weld.

Alternatively, the two parts may be joined without a press fit and with clearance between the facing surfaces. No differential pressure is created, and therefore there are virtually no “blow-outs.” That joint design, however, has two significant drawbacks when used in a fuel injector application. First, any weld slag or oxides created by the welding process can escape from the weld joint into the valve body, creating internal contamination of the fuel injector. Such internal contamination in a precision device such as a fuel injector can have undesirable effects. Secondly, many designs require a press fit between the two components for processing reasons.

There is therefore presently a need to provide a method and system for reliably creating a hermetic weld joining tubular components of a fuel injector. To the inventors' knowledge, no such technique is currently available.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, a method is provided for forming a fuel injector having a fuel inlet, and fuel outlet and a fuel passageway extending from the fuel inlet to the fuel outlet along a longitudinal axis. The method includes the steps of constructing a first fuel injector component comprising a radially outwardly facing annular surface; constructing a second fuel injector component comprising a radially inwardly facing annular surface; assembling the first and second components by press-fitting the first component into the second component with the outwardly facing annular surface abutting the inwardly facing annular surface; rotating the assembled first and second components about a longitudinal axis; first welding the annular surfaces together to form a first annular weld bead along an arc corresponding to less than 360° of rotation about the longitudinal axis so as to leave a radial void in the first annular weld bead; further rotating the assembled first and second components about the longitudinal axis; and second welding the annular surfaces together to form a second annular weld bead along an arc corresponding to the radial void, thereby forming a hermetic weld between the first and second components.

In one embodiment, the step of first welding comprises: applying a laser welding beam as the assembled components rotate through approximately 160 degrees about the longitudinal axis; stopping the laser welding beam for approximately the next 35 degrees of rotation about the longitudinal axis so as to leave a radial void in the first annular weld bead; applying the laser welding beam again for approximately the next 165 degrees of rotation about the longitudinal axis; and the step of second welding comprises applying the laser welding beam in the region of the radial void to complete the hermetic weld between the first and second components.

In another embodiment, the step of step of first welding comprises: applying a laser welding beam as the assembled components rotate through approximately 325 degrees about the longitudinal axis, and stopping the laser welding beam so as to leave the radial void in the first annular weld bead; and the step of second welding comprises applying the laser welding beam in the region of the radial void to complete the hermetic weld between the first and second components.

In an alternative embodiment, the method further comprises shaping at least one of the annular surfaces to form, when the surfaces abut, a non-contact region having a gap between the surfaces, and a press-fit region where the first and second surfaces are in contact, the non-contact and press-fit regions being adjacent.

The substantially sealed portion of the gap may have a volume of at least 0.037 mm3. The surfaces in the non-contact region may be between 0.005 and 0.025 mm apart. A center of the weld bead may be at least 1 mm from the contact region.

In accordance with another aspect of the invention, a fuel injector is provided, which includes a fuel inlet, and fuel outlet and a fuel passageway extending from the fuel inlet to the fuel outlet along a longitudinal axis, comprising: a first fuel injector component comprising a first component inlet, outlet, and passageway, the first component passageway extending from the first component inlet to the first component outlet along the longitudinal axis, the first component further comprising a radially outwardly facing exterior surface; a second fuel injector component comprising a second component inlet, outlet, and passageway, the second component passageway extending from the second component inlet to the second component outlet along the longitudinal axis, the second component further comprising a radially inwardly facing interior surface, the exterior surface of the first component facing the interior surface of the second component; first and second annular weld beads connecting the interior and exterior surfaces, the first annular weld bead being disposed along an arc corresponding to less than 360° of rotation about a longitudinal axis of the first and second fuel injector components, the second annular weld bead overlapping the first annular weld bead in at least two radial locations along the arc.

DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

The inventors have developed a technique and a fuel injector design to address the above-described problems in welding fuel injector pressure vessels. A cross-sectional view of a fuel injector100according to one embodiment of the invention is shown inFIG. 1. While the invention is described in connection with that exemplary fuel injector, one skilled in the art will understand that the inventive method and apparatus are applicable to other fuel injector designs. Embodiments of the invention may further be used in other welding applications where weld “blow out” is a concern.

Referring toFIG. 1, a solenoid actuated fuel injector100dispenses a quantity of fuel that is to be combusted in an internal combustion engine (not shown). The fuel injector100extends along a longitudinal axis A-A between a first injector end200A and a second injector end200B, and includes a valve group subassembly200and a power group subassembly300. The valve group subassembly200performs fluid handling functions, e.g., defining a fuel flow path and prohibiting fuel flow through the injector100. The power group subassembly300performs electrical functions, e.g., converting electrical signals to a driving force for permitting fuel flow through the injector100.

The valve group subassembly200includes a tube assembly202extending along the longitudinal axis A-A between the first fuel injector end200A and the second fuel injector end200B. The tube assembly202can include at least an inlet tube204, a non-magnetic shell210, and a valve body206. The inlet tube204has a first inlet tube end202A proximate to the first fuel injector end200A. The inlet tube204can be flared at the inlet end202A into a flange202C to retain an O-ring10. A second inlet tube end202B of the inlet tube204is connected to a first shell end210A of the non-magnetic shell210. A second shell end210B of the non-magnetic shell210can be connected to a generally transverse planar surface of a first valve body end206A of the valve body206. A second valve body end206B of the valve body206is disposed proximate to the second tube assembly end200B. The inlet tube204can be formed by a deep drawing process or by a rolling operation. A separate pole piece208can be connected to the inlet tube204and connected to the first shell end210A of the non-magnetic shell210. The pole piece may comprise a stainless steel material such as SS 430FR (ASTM A838-00). The non-magnetic shell210can comprise non-magnetic stainless steel, e.g., 300-series stainless steels such as SS 305 (EN 10088-2), or other materials that have similar structural and magnetic properties.

As shown inFIG. 1, inlet tube204is attached to pole piece208by means of welds205. Formed into the outer surface of pole piece208are pole piece shoulders208A, which, in conjunction with mating shoulders of a bobbin of the coil subassembly, act as positive mounting stops when the two subassemblies are assembled together. The length of pole piece208is fixed whereas the length of the inlet tube204can vary according to operating requirements of the particular fuel injector design. By forming inlet tube204separately from pole piece208, different length injectors can be manufactured by using different inlet tube lengths during the assembly process. The inlet tube204can be attached to the pole piece208at an inner circumferential surface of the pole piece208. Alternatively, an integral inlet tube and pole piece can be attached to the inner circumferential surface of a non-magnetic shell210.

An armature assembly212is disposed in the tube assembly202. The armature assembly212includes a first armature assembly end having a ferro-magnetic or armature portion214and a second armature assembly end having a sealing portion. The armature assembly212is disposed in the tube assembly202such that a shoulder214A of the armature214confronts a shoulder208B of the pole piece208. The sealing portion can include a closure member216, e.g., a spherical valve element, that is moveable with respect to the seat218and its sealing surface218A. The closure member216is movable between a closed configuration, as shown inFIG. 1, and an open configuration (not shown). In the closed configuration, the closure member216contiguously engages the sealing surface218A to prevent fluid flow through the opening. In the open configuration, the closure member216is spaced from the seat218to permit fluid flow through the opening. The armature assembly212may also include a separate intermediate portion220connecting the ferro-magnetic or armature portion214to the closure member216. The intermediate portion or armature tube220may be attached to the armature214and closure member216by weld beads215,217, respectively.

Surface treatments can be applied to at least one of the end portions208B and214A to improve the armature's response, reduce wear on the impact surfaces and variations in the working air gap between the respective end portions208B and214A. The surface treatments can include coating, plating or case-hardening. Coatings or platings can include, but are not limited to, hard chromium plating, nickel plating or keronite coating. Case hardening on the other hand, can include, but is not limited to, nitriding, carburizing, carbo-nitriding, cyaniding, heat, flame, spark or induction hardening.

Fuel flow through the armature assembly212can be provided by at least one axially extending through-bore214B and at least one apertures220A through a wall of the armature assembly212. The apertures220A, which can be of any shape, are preferably non-circular, e.g., axially elongated, to facilitate the passage of gas bubbles. The apertures220A provide fluid communication between the at least one through-bore214B and the interior of the valve body206. Thus, in the open configuration, fuel can be communicated from the through-bore214B, through the apertures220A and the interior of the valve body206, around the closure member216, and through metering orifice openings of an orifice disk222into the engine (not shown).

As a further alternative, a two-piece armature having an armature portion directly connected to a closure member can be utilized. Although both the three-piece and the two-piece armature assemblies are interchangeable, the three-piece armature assembly is preferable due to its ability to reduce magnetic flux leakage from the magnetic circuit of the fuel injector100according to the present invention. It should be noted that the armature tube220of the three-piece armature assembly can be fabricated by various techniques, for example, a plate can be rolled and its seams welded or a blank can be deep-drawn to form a seamless tube.

The seat218is secured at the second end of the tube assembly202. An orifice disk222can be used in connection with the seat218to provide at least one precisely sized and oriented orifice in order to obtain a particular fuel spray pattern and targeting. The precisely sized and oriented orifice can be disposed on the center axis of the orifice disk222or, preferably disposed off-axis, and oriented in any desirable angular configuration relative to one or more reference points on the fuel injector100. It should be noted here that both the valve seat218and orifice disk222are fixedly attached to the valve body206by known conventional attachment techniques, including, for example, laser welding, crimping, and friction welding or conventional welding. The orifice disk222is preferably tack welded to the seat218in a fixed spatial orientation to provide the particular fuel spray pattern and targeting of the fuel spray.

In the case of a spherical valve element providing the closure member216, the spherical valve element can be connected to the armature assembly212at a diameter that is less than the diameter of the spherical valve element. Such a connection would be on side of the spherical valve element that is opposite contiguous contact with the seat218. A lower armature assembly guide224can be disposed in the tube assembly202, proximate the seat218, and would slidingly engage the diameter of the spherical valve element. The lower armature assembly guide224can facilitate alignment of the armature assembly212along the longitudinal axis A-A.

A resilient member226is disposed in the tube assembly202and biases the armature assembly212toward the seat218. A filter assembly228comprising a filter230and a preload adjuster232is also disposed in the tube assembly202. The filter assembly228includes a first filter assembly end228A and a second filter assembly end228B. The filter230is disposed at one end of the filter assembly228and also located proximate to the first end200A of the tube assembly202and apart from the resilient member226while the preload adjuster232is disposed generally proximate to the second end of the tube assembly202. The preload adjuster232engages the resilient member226and adjusts the biasing force of the member226with respect to the tube assembly202. In particular, the preload adjuster232provides a reaction member against which the resilient member226reacts in order to close the injector valve100when the power group subassembly300is de-energized. The position of the preload adjuster232can be retained with respect to the inlet tube204by an interference press-fit between an outer surface of the preload adjuster232and an inner surface of the tube assembly202. Thus, the position of the preload adjuster232with respect to the inlet tube204can be used to set a predetermined dynamic characteristic of the armature assembly212.

The valve group subassembly200can be assembled as follows. The non-magnetic shell210is connected to the inlet tube204via the pole piece208, and to the valve body206. The non-magnetic shell210and pole piece208are joined by the weld bead281. Assembly of the non-magnetic shell may be performed using laser welding techniques as described in more detail below with reference toFIGS. 2-4.

The filter assembly228is inserted along the axis A-A from the first end200A of the tube assembly202. Next, the resilient member226and the armature assembly212(which was previously assembled) are inserted along the axis A-A from the injector outlet end200B of the valve body206. The adjusting tube232, the filter assembly228can be inserted into the inlet tube204to a predetermined distance so as to permit the adjusting tube232to preload the resilient member226. Positioning of the filter assembly228, and hence the adjusting tube232with respect to the inlet tube204can be used to adjust the dynamic properties of the resilient member226, e.g., so as to ensure that the armature assembly212does not float or bounce during injection pulses. The seat218and orifice disk222are then inserted along the axis A-A from the second valve body end206B of the valve body206. The seat218and orifice disk222can be fixedly attached to one another or to the valve body206by known attachment techniques such as laser welding, crimping, friction welding, conventional welding, etc. Other preferred variations of the valve group subassembly200are described and illustrated in U.S. Patent Publication No. 20020047054 published on Apr. 25, 2002, now U.S. Pat. No. 6,676,044, which is hereby incorporated by reference in its entirety.

The power group subassembly300comprises an electromagnetic coil302, at least one terminal304, a coil housing306, and an overmold308. The electromagnetic coil302comprises a wire that that can be wound on a bobbin314and electrically connected to electrical contacts316on the bobbin314. When energized, the coil302generates magnetic flux that moves the armature assembly212toward the open configuration, thereby allowing the fuel to flow through the opening. De-energizing the electromagnetic coil302allows the resilient member226to return the armature assembly212to the closed configuration, thereby shutting off the fuel flow. The housing, which provides a return path for the magnetic flux, generally includes a ferro-magnetic cylinder surrounding the electromagnetic coil302and a flux washer318extending from the cylinder toward the axis A-A. The flux washer318can be integrally formed with or separately attached to the cylinder. The coil housing306can include holes, slots, or other features to break-up eddy currents that can occur when the coil302is energized.

The overmold308maintains the relative orientation and position of the electromagnetic coil302, the at least one terminal304, and the coil housing306. The overmold308includes an electrical harness connector320portion in which a portion of the terminal304is exposed. The terminal304and the electrical harness connector320portion can engage a mating connector, e.g., part of a vehicle wiring harness (not shown), to facilitate connecting the injector100to an electrical power supply (not shown) for energizing the electromagnetic coil302.

According to a preferred embodiment, the magnetic flux generated by the electromagnetic coil302flows in a circuit that includes the pole piece208, the armature assembly212, the valve body206, the coil housing306, and the flux washer318. The magnetic flux moves across a parasitic air gap between the homogeneous material of the magnetic portion or armature214and the valve body206into the armature assembly212and across a working air gap between end portions208B and214A towards the pole piece208, thereby lifting the closure member216away from the seat218.

To set the lift, i.e., ensure the proper injector lift distance, several techniques may be utilized. According to a preferred technique, a lift sleeve234is displaced axially within the valve body206. The position of the lift sleeve234is adjusted by moving the lift sleeve234axially. The lift distance is measured with a test probe (not shown). Once the desired lift is reached, the sleeve is welded to the valve body206, e.g., by laser welding. The valve body206is then attached to the inlet tube204assembly by a weld, preferably a laser weld. The assembled fuel group subassembly200is then tested, e.g., for leakage.

The preparation of the power group sub-assembly300, which may include (a) the coil housing306, (b) the bobbin assembly including the terminals304, (c) the flux washer318, and (d) the overmold308, can be performed separately from the fuel group subassembly.

According to a preferred embodiment, wire is wound onto a pre-formed bobbin314having electrical connector portions316to form a bobbin assembly. The bobbin assembly is inserted into a pre-formed coil housing306. To provide a return path for the magnetic flux between the pole piece208and the coil housing306, flux washer318is mounted on the bobbin assembly. A pre-bent terminal304having axially extending connector portions are coupled to the electrical contact portions316of the coil and brazed, soldered welded, or, preferably, resistance welded. The partially assembled power group assembly is now placed into a mold (not shown). By virtue of its pre-bent shape, the terminals304will be positioned in the proper orientation with the harness connector320when a polymer is poured or injected into the mold. Alternatively, two separate molds (not shown) can be used to form a two-piece overmold as described earlier. Additionally, a portion of the coil housing306can extend axially beyond an end of the overmold308to allow the injector to accommodate different length injector tips. The extended portion may be formed with a flange306A to retain a sealing member such as the O-ring10.

The assembled power group subassembly300can be mounted on a test stand to determine the solenoid's pull force, coil resistance and the drop in voltage as the solenoid is saturated during energization of the coil.

The inserting of the fuel group subassembly200into the power group subassembly300operation can involve setting the relative rotational orientation of fuel group subassembly200with respect to the power group subassembly300. According to the preferred embodiments, the fuel group and the power group subassemblies can be rotated such that the included angle between the reference point(s) on the orifice disk222(including opening(s) thereon) and a reference point on the injector harness connector320are within a predetermined angle. The relative orientation can be set using robotic cameras or computerized imaging devices to look at respective predetermined reference points on the subassemblies, calculate the angular rotation necessary for alignment, orient the subassemblies and then check with another look and so on until the subassemblies are properly oriented. Once the desired orientation is achieved, the subassemblies are inserted together. The inserting operation can be accomplished by one of two methods: “top-down” or “bottom-up.” According to the former, the power group subassembly300is slid downward from the top of the fuel group subassembly200, and according to the latter, the power group subassembly300is slid upward from the bottom of the fuel group subassembly200. In situations where the inlet tube204assembly includes a flared first end, bottom-up method is required. Also in those situations, the O-ring10that is retained by the flared first end can be positioned around the power group subassembly300prior to sliding the fuel group subassembly200into the power group subassembly300. After inserting the fuel group subassembly200into the power group subassembly300, those two subassemblies are affixed together, e.g., by welding, such as laser welding. According to a preferred embodiment, the overmold308includes an opening308A that exposes a portion of the coil housing306. This opening308A provides access for a welding implement to weld the coil housing306with respect to the valve body206. Of course, other methods or affixing the subassemblies with respect to one another can be used. Finally, the O-ring10at either end of the fuel injector can be installed.

In operation, the electromagnetic coil302is energized, thereby generating magnetic flux in the magnetic circuit. The magnetic flux moves armature assembly212(along the axis A-A, according to a preferred embodiment) towards the integral pole piece208, closing the working air gap. That movement of the armature assembly212separates the closure member216from the seat218and allows fuel to flow from the fuel rail (not shown), through the inlet tube204, the through-bore214B, the apertures220A and the valve body206, between the seat218and the closure member216, through the opening, and finally through the orifice disk222into the internal combustion engine (not shown). When the electromagnetic coil302is de-energized, the armature assembly212is moved by the bias of the resilient member226to contiguously engage the closure member216with the seat218, and thereby prevent fuel flow through the injector100.

Referring now toFIG. 2, a pole piece assembly280includes the pole piece208and the non-magnetic shell210. The pole piece208includes an external, outwardly facing annular surface286. The non-magnetic shell210has an internal, inwardly-facing annular surface285. The pole piece208and the non-magnetic shell210are initially assembled by pressing the two components together along the longitudinal axis A-A. The external annular surface286has a diameter slightly greater than the internal annular surface285, resulting in a press fit or interference fit. The relative diameters are controlled closely to control the press force required to assemble the parts, to avoid galling and other damage to the parts. A lubricant such as oil may also be used to ameliorate those problems.

A weld bead281connects the pole piece208and non-magnetic shell to form the assembly. The weld bead281is annular; i.e., it extends in a ring-like manner around the joined components. The weld bead, as that term is used herein, is a mass of material originating from both components that are joined by the weld. The material has been liquefied by energy from a welding energy source such as a laser beam (shown schematically at299). The materials from the joined components are commingled to some extent, although in most cases not completely. The liquefied material is allowed to cool back to the solid phase. In a completed assembly, the weld bead281is distinguishable from the parent parts because of its appearance, crystalline structure and other metallurgical characteristics.

The present invention addresses the problem of “blow-out” of the weld bead281. It is believed that “blow-out” is caused by gases trapped in small voids or pockets between the surfaces285,286. For example, heated air or vaporized lubricant trapped in imperfections on the surfaces may cause a differential pressure across the molten weld bead281.

Referring now toFIG. 3, there is depicted a sectional view along lines3-3inFIG. 2showing an illustrative embodiment of a multi-step welding procedure in accordance with an aspect of the invention to minimize blow-out. Specifically, as the assembly comprising the components208,210is rotated about longitudinal axis A-A, a first annular weld bead281ais formed corresponding to an arc less than 360° of rotation of the assembly. In a first expedient, datum “A” signifies the 0° location and the first annular weld bead281ais applied for approximately the first 160° of rotation, stopped for the next approximately 35° of rotation and reapplied for the next approximately 165° of rotation, thereby leaving a radial void corresponding to an arc of approximately 35°. This enables trapped gases to be relieved through the void. Subsequently, a second annular weld bead281bis applied over the radial void to provide a hermetic seal between components208,210. The second weld bead281boverlaps the first weld bead281ain two radial locations as shown in the drawing to provide a hermetic seal. This is also depicted graphically inFIG. 4a. In a second expedient, datum “B” signifies the 0° location and the first weld bead281ais applied for approximately 325° of rotation and then stopped, and the second weld bead281bis thereafter applied over the radial void in the same manner when the radial void passes approximately the 0° location on the next pass. The second expedient is depicted graphically inFIG. 4b. In either implementation, the goal is to leave a radial void for trapped gas to vent and to then subsequently seal up the void with a second weld that closes the gap. It will be understood by those skilled in the art, that the tapered lines depicted inFIGS. 4aand4bcorrespond to the laser being powered on as power is increased in the welding operation to 800 W and back off again to 0, thereby leaving a tapered weld portion in the region of the overlapping first and second weld beads.

Referring now toFIG. 5, in an alternative embodiment, a small step290is formed in either component208,210. The step290may be formed in a machining operation such as a grinding or turning operation, or may be formed by a die in a press operation.

When the components208,210are assembled and the surfaces285,286are overlapped, an annular press-fit region292is created, wherein the two surfaces285,286interfere, locking the parts together and creating a substantially air-tight seal. A total length (not shown) of the press-fit region is preferably about 1.3 mm.

The step290results in a non-contact region291of the surfaces285,286. A gap289between the surfaces285,286in the non-contact region has a preferred width288of between 0.005 and 0.025 mm.

The annular weld beads281a,281bformed by the laser beam299join the external surface286with the internal surface285in the non-contact region291. The weld beads281a,281bform a hermetic seal preventing liquids and gases from passing between the components208,210. In a preferred embodiment, a nominal longitudinal distance287from a centerline293of the weld beads281a,281bto the press-fit region292is 1 mm. In other embodiments, the distance287may be from 0.5 mm to 5.0 mm.

The weld beads281a,281band the press-fit region292demarcate a substantially sealed portion295of the gap289. The sealed portion295is sufficiently large so that the vapors inside the sealed portion have sufficient time and volume to minimize the pressure differential across the weld beads281a,281bduring the welding process. For example, the sealed region may be approximately 0.75 mm in length in a longitudinal direction.

The inventors have found that a narrower gap is preferred (without becoming a press fit) for reducing “blow-out” at the weld overlap. For example, the sealed region may have a ratio of length to width of greater than 10. Preferably, the ratio is about 50. It has been theorized that as the gap width increases, vapor inside the sealed region has more exposed area to the molten weld pool. The larger exposed area to the weld pool leads to a more sudden increase in temperature, and consequently, vapor expansion and higher pressure. Conversely, a small gap minimizes the rate at which the pressure increases due to the small exposed area to the molten pool.

In general, the longer the longitudinal length of the sealed portion295, the more resistant the design is to “blow-out” at the weld overlap. It has been theorized that the relatively cool facing walls of the two components208,210cool the expanding gas, slowing the rate at which the internal pressure increases.

The press-fit region292provides an effective seal for preventing weld slag and oxides created during the welding operation from entering the valve body206(FIG. 1) and potentially contaminating the precision components contained there. The press fit further is helpful in several processing steps of the fuel injector, such as handling the assembly280before welding.

FIG. 6ais a flow chart illustrating a method600according to one embodiment of the invention. The method is for forming a fuel injector having a fuel inlet, and fuel outlet and a fuel passageway extending from the fuel inlet to the fuel outlet along a longitudinal axis. The method includes the step of constructing (step610) a first fuel injector component comprising a radially outwardly facing annular surface. The first fuel injector component may, for example, be a pole piece. A second fuel injector component is also constructed (step620), comprising a radially inwardly facing annular surface. The second component may be a non-magnetic shell.

The first and second components are assembled (step630) with the annular surfaces overlapping. Assembly is performed by aligning the parts along their longitudinal axes, and pressing the parts together to a predetermined length. The parts are in contact in the press-fit region.

The assembled first and second components are rotated about a longitudinal axis (step640) and the annular surfaces are welded together (step650) in the non-contact region to form an annular weld bead along an arc corresponding to less than 360° of rotation about the longitudinal axis, so as to leave a radial void in the first annular weld bead. In a preferred embodiment, the welding operation is performed by rotating the assembled components about a longitudinal axis, and applying a stationary laser beam at 800 W to the overlapping surfaces. In an exemplary expedient, the laser is maintained “on” for approximately 325° of the revolution of the assembly (as shown inFIG. 3) with a rotation speed of 200 RPM.

The partially welded assembly is further rotated (step660), and thereafter a second weld bead is applied across the radial void to hermetically seal the assembly.

In an alternative embodiment depicted in the flow diagram ofFIG. 6b, all the steps described above with respect toFIG. 6aare identical, with the addition of step625. Here, at least one of the annular surfaces is shaped (step625) to form, when the surfaces are overlapped, a non-contact region having a gap between the surfaces, and a press-fit region where the first and second surfaces are in contact. The non-contact and press fit regions are adjacent. The shaping step625may be done in combination with forming steps610,620, or may be done as a subsequent operation.

The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. For example, while the method is disclosed herein with respect to tubular components of a fuel injector, the techniques and configurations of the invention may be applied to other tubular components where a hermetic weld is required. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.