TECHNICAL FIELD

The present invention relates generally to consumable designs for contact-start, liquid-cooled plasma arc cutting torches, and, more particularly, to consumable designs that reduce damage to process critical geometric features during contact start ignition of these plasma arc torches.

BACKGROUND

Plasma arc torches are widely used for high temperature processing (e.g., cutting, welding, and marking) of metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, an emissive insert disposed within a bore of the electrode, a nozzle with a central exit orifice, a shield, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The plasma arc torch can produce a plasma arc, which is a constricted, ionized jet of plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g., argon or nitrogen) or reactive (e.g., oxygen or air).

A plasma arc torch can generate a plasma arc using a contact start approach. This involves first operating the torch in a pilot arc mode, which includes establishing physical contact and electrical communication between two electrically conductive torch consumables, such as the electrode and the nozzle, by using a biasing force from, for example, a spring. A current path and a small pilot arc current flow are established between the electrode and the nozzle while they are biased together. A plasma gas is introduced into a plasma chamber/plenum region between the nozzle and the electrode, such that gas pressure builds up in the plasma chamber to break the physical contact between the electrode and the nozzle and separate the two components. The separation causes an electrical arc to be created in the gap between the electrode and the nozzle in the plasma chamber. The electrical arc ionizes the flowing plasma gas in the plasma chamber to produce a plasma arc (i.e., a pilot arc). The plasma gas can be passed through a swirl ring to impart a tangential motion to the gas as it passes through the torch, thereby improving torch performance. Next, in a transferred arc mode, the torch is moved near a grounded workpiece and the plasma arc makes contact with the workpiece. Upon contact, the current transfers from the nozzle to the workpiece and returns from the workpiece back to the power supply. During transferred arc mode, the current flow can be increased to effectively process (e.g., gouge, pierce or cut) the workpiece.

Traditionally, consumable components in a contact start plasma arc cutting torch, such as the nozzle and electrode, have contact/mating features proximate to the nozzle bore through which the plasma arc is ejected from the torch (e.g., to limit the distance the arc needs to travel within the plenum before reaching the workpiece). These contact/mating features—are generally symmetrical about the longitudinal axis of the torch and have very narrow contact points with one another to drive a pilot arc current of high current density therethrough. As the contact between these consumable components (e.g., the nozzle and electrode) breaks during arc initiation, the resistive heating (e.g., Joule heating) caused by the pilot arc current and the separation can result in consumable damage, including localized deformation, and melting and evaporation of the contact/mating features. In addition, materials expelled during this separation process can deposit in the plenum chamber of the plasma arc torch and damage adjacent process critical geometric features of the consumables (e.g., electrode tip and liner/nozzle bore(s)). Further, because such deposition and/or damage frequently occurs close to the liner/nozzle bore(s), which can favor certain region over time and can build up asymmetrically, this can create disturbances in gas flow and to plasma arc generation in that region.

FIGS. 1aand 1bshow exemplary deposition on and damage to a prior art nozzle100and a prior art electrode102, respectively, of a prior art contact start plasma arc cutting system following about 3600 pilot arc initiations (i.e., starts). The surfaces of the electrode102and the nozzle100show significant compromised/degraded conditions. For example, with respect toFIG. 1a, the deep pitting in the plenum104and the liner bore106of the nozzle100is evident and represents a common occurrence with such consumables. Further, with respect toFIG. 1b, the copper around the hafnium insert108of the electrode102is significantly eroded. In addition to consumable surface damage on process critical features, contact starting the nozzle100and the electrode102can also lead to significant deposition as well as disturbances of critical fluid flow and plasma arc generation, thus reducing consumable life as well as impacting cut quality over life.

Further, during pilot arc initiation when the electrode is in physical contact with the nozzle, the narrow, smooth and substantially continuous/even contact formed between the two components can seal and/or inhibit gas flow through the plenum and out the liner/nozzle bore(s) before separation of the electrode and the nozzle occurs, which can cause additional damage to consumable components as a result of this lack of established pre-flow prior to arc initiation.FIGS. 2aand 2bshow an end view and a perspective view, respectively, of another prior art electrode200.FIG. 3shows a cross-sectional view of a portion of a prior art contact start plasma arc cutting torch300that includes the electrode200ofFIGS. 2aand 2b. The torch300also includes a nozzle with a nozzle liner302in physical contact with the electrode200. As shown inFIG. 3, the contact area304between the electrode200and the nozzle liner302extends over almost an entire outer circumference of the electrode200and almost an entire inner circumference of the nozzle liner302, thereby forming a substantially complete seal between the two components. In addition, the contact area304is axially narrow. In some embodiments, the contact area304is sufficiently small that it forms only a continuous circumferential line of contact between the nozzle liner302and the electrode200, as illustrated inFIG. 2b. Specifically,FIG. 2bshows the circumferential contact line304(shown dashed for clarity) around the tip of the electrode200, at which location contact occurs between the electrode200and the nozzle liner302when they are biased together inside of the torch300.

This narrow line of contact304is potentially damaging to plasma arc generation and can exacerbate the deposition and structural degradation issues described above. For example, this line of contact304and the subsequent sealing of (i.e. lack of established) gas flow through the plenum306is detrimental to consumable life and plasma arc operation, as a plasma arc requires a strong gas flow about it in the plenum306to direct the arc out of the liner bore307and the nozzle bore308, and this gas flow cannot be instantaneously developed. As such, once separation between the nozzle liner302and electrode200occurs, the plasma arc formed within the plenum306can only progress out of the bores307,308sub-optimally while the gas pressure and flow rate are still being established. This delay results in further consumable degradation, cut quality limitations, and overall inefficient workpiece processing.

Therefore, there is a need for consumable designs that improve the contact surface and location of contact between certain consumables (e.g., between an electrode and a nozzle) in a plasma arc cutting torch to reduce damage to process critical geometric features and components during contact start ignition of the plasma arc torch.

SUMMARY

The present invention provides consumable designs that control and substantially improve contact surface parameters and locations between two consumables (e.g., between an electrode and a nozzle) in a contact-start, liquid-cooled plasma arc torch such that damage to process critical geometric features during contact start ignition is reduced. Various embodiments of the present invention permit a plasma gas flow and pressure to be established in the plenum chamber of the torch when the consumables are still in physical contact, including permitting the plasma gas flow to be established (e.g., substantially established) in advance of consumable separation (i.e., before the physical contact breaks between the consumables). This is crucial to rapid plasma arc transfer to a workpiece, which minimizes heat energy influx into the consumables and reduces thermal damage, thereby extending consumable life. In addition, various embodiments of the present invention move the contact area between the consumables away from process critical features (e.g., electrode tip and nozzle/liner bore(s)) to minimize damage to these features during contact starting, which also extends consumable life and cut quality over life.

The invention, in one aspect, features a liquid cooled electrode for a contact start plasma arc cutting torch. The liquid cooled electrode includes an elongated body defining a longitudinal axis. The elongated body includes a proximal end shaped to matingly engage a torch body of the contact start plasma arc cutting torch and a distal end located substantially opposite of the proximal end along the longitudinal axis. The distal end includes an emissive insert. The electrode also includes one or more contact surfaces disposed on an external surface of the distal end of the electrode body between the proximal and distal ends. The one or more contact surfaces are shaped to physically contact a nozzle disposed within the contact start plasma arc cutting torch during a portion of a pilot arc initiation process of the torch. The physical contact is configured to support transmission of a pilot arc current between the electrode and the nozzle with a density of at least about 3000 amps per square inch.

In some embodiments, each contact surface has a surface area greater than about 0.002 square inches. In some embodiments, a combined area of the one or more contact surfaces is greater than about 0.015 square inches. In some embodiments, each contact surface is configured to physically contact the nozzle over at least one of an axial distance of at least 0.02 inches. In some embodiments, each contact surface is angled to be substantially parallel to a complementary interior surface of the nozzle.

In some embodiments, the contact surfaces are disposed radially around a circumference of the external surface of the electrode. The one or more contact surfaces can be radially discontinuous and form discontinuous circumferential contacts between the electrode and the nozzle during the portion of the pilot arc initiation process. In addition, the electrode can further comprise one or more channels disposed on the exterior surface of the electrode. The one or more channels can be interspersed radially between the one or more contact surfaces to permit a pre-flow of plenum gas between the nozzle and the electrode during the portion of the pilot arc initiation process. In some embodiments, each channel is positioned radially between a pair of the contact surfaces. In some embodiments, each channel is substantially arcuate relative to the one or more contact surfaces. In some embodiments, each channel has a cross-sectional width of greater than about 0.05 inches. In some embodiments, the one or more contact surfaces and the one or more channels are uniformly spaced about the circumference of the external surface of the electrode to facilitate substantially symmetrical plasma arc initiation and transition through a bore of the nozzle. Alternatively, the one or more contact surfaces and the one or more channels are asymmetrically distributed around the circumference of the external surface of the electrode to influence a direction of plasma arc initiation and transition through a bore of the nozzle.

In some embodiments, the one or more contact surfaces are radially contiguous to each other around the distal end to form a continuous circumferential contact between the electrode and the nozzle during the portion of the pilot arc initiation process.

In some embodiments, the distal end of the electrode body comprises a frustoconical section that tapers toward the emissive insert. In some embodiments, the one or more contact surfaces are disposed on a plurality of pads raised above a surface of the frustoconical section, the plurality of pads shaped to seat against an internal surface of the nozzle during the portion of the pilot arc initiation process. In some embodiments, the one or more contact surfaces are located at least 0.07 inches distally aft the emissive insert in the frustoconical section.

In some embodiments, the current transmission density of the pilot arc current between the electrode and the nozzle is about 5000 amps per square inch. In some embodiments, the electrode body is substantially hollow for conducting a liquid coolant therein.

In another aspect, the present invention features a liquid cooled nozzle for a contact start plasma arc cutting torch. The liquid cooled nozzle comprises a body defining a longitudinal axis. The body includes a proximal end shaped to matingly engage a torch body of the contact start plasma arc cutting torch and a distal end located substantially opposite of the proximal end along the longitudinal axis. The distal end defines a plasma exit orifice. The nozzle also includes one or more contact surfaces disposed on an interior surface of the distal end of the nozzle body. The one or more contact surfaces extends over an interior conical surface of the nozzle and shaped to physically contact an electrode of the contact start plasma arc cutting torch during a portion of a pilot arc initiation process of the torch. A combined area of the one or more contact surfaces is at least 0.015 square inches.

In some embodiments, the physical contact between the electrode and the nozzle is configured to support transmission of a pilot arc current between the electrode and the nozzle with a density of at least about 3000 amps per square inch.

In some embodiments, the one or more contact surfaces are disposed radially around a circumference of the internal surface of the nozzle. The one or more contact surfaces can be radially discontinuous and form discontinuous circumferential contacts between the electrode and the nozzle during the portion of the pilot arc initiation process. The nozzle can further include one or more channels disposed on the internal surface of the nozzle. The one or more channels can be interspersed radially among the one or more contact surfaces to permit a pre-flow of plenum gas between the nozzle and the electrode during the portion of the pilot arc initiation process.

In some embodiments, the one or more contact surfaces are radially contiguous to each other around the distal end and form a continuous circumferential contact between the electrode and the nozzle during the portion of the pilot arc initiation process.

In some embodiments, the one or more contact surfaces are disposed on a plurality of pads raised relative to the inner surface of a frustoconical section of the nozzle. The plurality of pads are shaped to seat against a complementary external surface of the electrode during the portion of the pilot arc initiation process.

In some embodiments, each contact surface is angled to be substantially parallel to a complementary exterior surface of the electrode.

