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قديم 11-10-2012, 06:10 PM
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افتراضي tig welding

 

tig welding
Tig welding

Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining magnesium and aluminium. Using an inert gas shield instead of a slag to protect the weldpool, the process was a highly attractive replacement for gas and manual metal arc welding. TIG has played a major role in the acceptance of aluminium for high quality welding and structural applications.
Process characteristics
In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is **t consumed during welding, the welder does **t have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weldpool.


Gas Tungsten Arc welding (GTAW) is frequently referred to as TIG welding. TIG welding is a commonly used high quality welding process. TIG welding has become a popular choice of welding processes when high quality, precision welding is required. In TIG welding an arc is formed between a **nconsumable tungsten electrode and the metal being welded. Gas is fed through the torch to shield the electrode and molten weld pool. If filler wire is used, it is added to the weld pool separately.
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TIG welding Benefits:
Superior quality welds
Welds can be made with or without filler metal
Precise control of welding variables (heat)
Free of spatter
Low distortion

Shielding Gases :
Argon
Argon + Hydrogen
Argon/Helium
Helium is generally added to increase heat input (increase welding speed or weld ***********). Hydrogen will result in cleaner looking welds and also increase heat input, however, Hydrogen may promote porosity or hydrogen *****ing.


GTAW welding Limitations:


Requires greater welder dexterity than MIG or stick welding
Lower deposition rates
More costly for welding thick sections
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Common GTAW welding Concerns

We can help optimize your welding process variables. Evaluate your current welding parameters and techniques. Help eliminate common welding problems and discontinuities such as those listed below:

Weld
Discontinuities
Undercutting
Tungsten inclusions
Porosity
Weld metal *****s
Heat affected zone *****s
TIG welding Problems
Erratic arc
Excessive electrode consumption
Oxidized weld deposit
Arc wandering
Porosity
Difficult arc starting


Power source
TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the workpiece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the a**de (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium.

 


Arc starting
The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts which last for a few microseconds. The HF sparks will cause the electrode - workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source.
**te: As HF generates ab**rmally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding.
HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the beginning of each half-cycle.
Electrodes
Electrodes for DC welding are **rmally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be **ted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or 'ball' profile.
Shielding gas
Shielding gas is selected according to the material being welded. The following guidelines may help:


  • Argon - the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium.
  • Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen *****ing in carbon steels and weld metal porosity in aluminium alloys.
  • Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld ***********. Disadvantages of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc.

Applications
TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled *********** (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.
TIG is also widely applied in mechanised systems either autoge**usly or with filler wire. However, several 'off the shelf' systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require ** manipulative skill, but the operator must be well trained. Because the welder has less control over arc and weldpool behaviour, careful attention must be paid to edge preparation (machined rather than hand-prepared), joint fit-up and control of welding parameters.
For further information contact


System and Method for Improved TIG Arc Starting

Abstract:

A system and method for performing a welding-type process includes a welding-type torch having a first electrode connected to receive a welding-type power to effectuate a welding-type process. The welding-type torch also includes a second electrode connected to receive a starting power to initiate the welding-type process and a **zzle surrounding at least a portion of the first electrode.

Claims:

1. A welding-type torch comprising:a first electrode connected to receive a welding-type power to effectuate a welding-type process;a second electrode connected to receive a starting power to initiate the welding-type process; anda **zzle surrounding at least a portion of the first electrode.

2. The welding-type torch of claim 1 wherein the second electrode is electrically connected to the first electrode.

3. The welding-type torch of claim 2 wherein the second electrode is connected to the first electrode through a filter configured to restrict the welding-type power from the second electrode.

4. The welding-type torch of claim 3 wherein the filter includes a capacitor.

5. The welding-type torch of claim 1 wherein the first electrode and the second electrode are composed of different materials.

6. The welding-type torch of claim 5 wherein the first electrode includes tungsten and the second electrode includes at least one of nickel and chromium.


7. The welding-type torch of claim 1 wherein the welding-type power includes a direct-current (DC) power and the starting power includes an alternating-current (AC) power.

8. The welding-type torch of claim 1 wherein the **zzle is electrically **n-conductive and wherein the second electrode extends through the **zzle and proximate to the first electrode.

9. The welding-type torch of claim 1 wherein a tip of the second electrode is recessed in the **zzle.

10. The welding-type torch of claim 1 wherein the second electrode has a diameter of less than the first electrode.

11. The welding-type torch of claim 1 wherein the first electrode extends coaxially through an opening in the **zzle, the **zzle includes a bridge extending within the opening, and the second electrode extends from the bridge.

12. The welding-type torch of claim 1 wherein the first electrode extends coaxially through an opening in the **zzle and wherein the second electrode extends from a wall of the **zzle surrounding the opening.

13. The welding-type torch of claim 12 wherein the second electrode is at least partially recessed in the wall of the **zzle.

14. The welding-type torch of claim 1 further comprising a collet having a threading configured to engage a coupling of the **zzle and electrically coupling the first electrode to a power source configured to deliver the welding-type power and the starting power.

 



15. The welding-type torch of claim 14 wherein the **zzle includes an electrically **n-conductive material and wherein the second electrode extends through the **zzle to engage the collet to receive at least the starting power.

16. The welding-type torch of claim 15 further comprising a filter arranged between the collet and the second electrode to restrict the welding-type power from being delivered to the second electrode.

17. The welding-type torch of claim 1 further comprising at least one conductive pad formed on the **zzle and configured to engage a reciprocal conductor to receive the starting power, and wherein the second electrode is electrically connected to the at least one conductive pad.

18. The welding-type torch of claim 17 further comprising a filter arranged in a body of the torch and connected between the reciprocal conductor and a source of the starting power to preclude the welding-type power from being delivered to the second electrode.

19. A welding torch **zzle assembly comprising:a first electrode having a first diameter and formed of a first material capable of receiving a welding-type power and effectuating a welding-type process;a second electrode having a second diameter, which is less than the first diameter, and formed of a second material configured to initiate a corona discharge proximate to the first electrode to initiate the welding-type process between the first electrode and a workpiece.

20. The welding torch **zzle assembly of claim 19 wherein the second electrode is electrically connected to the first electrode.

21. The welding torch **zzle assembly of claim 20 wherein the second electrode is connected to the first electrode through a filter configured to restrict the welding-type power from the second electrode.

22. The welding torch **zzle assembly of claim 21 wherein the filter includes a capacitor.

23. The welding torch **zzle assembly of claim 19 wherein the first material includes tungsten and the second material includes at least one of nickel and chromium.

24. The welding torch **zzle assembly of claim 19 wherein the **zzle is electrically **n-conductive and wherein the second electrode extends through a side wall of the **zzle.

25. The welding torch **zzle assembly of claim 24 wherein a tip of the second electrode is recessed in the side wall of the **zzle.

Description:

BACKGROUND OF THE INVENTION
The present invention relates generally to welding-type systems and, more particularly, to an apparatus for improved high-frequency arc starting of a welding process.

There are a large number of welding processes available for use in industry. For example, some welding processes include gas tungsten arc, oxygen gas welding, and shielded metal arc welding. The gas tungsten arc welding process is generally referred to as tungsten inert gas (TIG) welding. A typical TIG welding apparatus includes a welding component that is commonly referred to as welding torch and is designed to control a tungsten electrode during a welding process. The electrode is heated to extremely high temperatures by electrical power received from the power supply. At appropriate voltage and current, a welding arc is created between the electrode and a workpiece to be welded.

It is well k**wn that TIG welding is often preferably started using a high-frequency (HF) starting system. High-frequency starting is a method of generating an arc without moving parts or the wear associated with shorting and breaking. To perform HF starting, a welding torch is connected to a power supply having an HF starting circuit. The circuit typically includes a high-voltage transformer, capacitors for power conditioning, and a **zzle assembly configured to generate a high-voltage spark at the torch electrode. When sufficient voltage is provided by the power supply to the torch, a spark fires from the electrode and traverses a gap between the electrode and the workpiece.

However, while HF starting systems serve to protect the electrode from the wear associated with shorting and breaking of a contact starting system, it is often less reliable than contact starting systems. For example, it is **t uncommon that HF starting of a TIG welding process may fail on more than 25% of starts.

As such, some operators have attempted to improve the reliability of HF starting by decreasing the distance between the electrode and workpiece during the HF start. However, as the gap between the electrode and workpiece is decreased, the probability of the electrode contacting the workpiece increases. Should the electrode strike the workpiece, the advantage of decreased wear afforded by HF starting is *******. Furthermore, in TIG welding processes, should the tungsten electrode strike the workpiece, some of the tungsten may be transferred to the workpiece and reduce the integrity of the weld.

Accordingly, some operators have designed a way to "move" the workpiece electrically closer to the electrode without reducing the gap between the electrode and the workpiece. That is, some operators have connected a wire from the workpiece to the **zzle where it is clamped thereto. While this piecemeal system increases HF starting performance it has many drawbacks.

Specifically, the system is insecure, unstable, and may be unintentionally separated or dismantled during **rmal welding operations. That is, these systems are prone to inadvertently disassembly because the wire connecting the workpiece to the **zzle is merely clamped in place and dangling from the welding torch. As such, the wire may be inadvertently ******* from the clamp or may interfere with the welding process. Furthermore, the clamp may obscure the operator's view of the workpiece or interfere with the welding process. Additionally, the clamp may be easily moved or dislodged from the **zzle during **rmal welding operations.

Also, the clamp securing the wire to the **zzle, while integral in creating the advantages of these piecemeal systems, is undesirable. That is, the clamp is electrically charged and, therefore, creates a system that may **t comply with applicable workplace standards and regulations.

Furthermore, the welding cable and the wire connecting the workpiece and **zzle carry a potential difference equal to the potential difference between the tungsten electrode and the workpiece desired for effectuating a welding process. This relatively high potential difference can result in leakage of the HF voltage and diminish the peak voltage available to establish an initial welding arc if the welding cable and the wire leading from the workpiece to the **zzle are too close in proximity.

Therefore, it would be desirable to have a system and method for improving the consistency of HF starting that does **t interfere with initiating a welding arc, does **t interfere with the welding process, is **t susceptible to inadvertent movement or disassembly, and is compliant with acceptable workplace standards and regulations.

BRIEF SUMMARY OF THE INVENTION
The present invention overcomes the aforementioned drawbacks by providing a high-frequency arc starting system that includes at least one electrode that is specifically designed to initiate a welding process. The system may be integrated into a **zzle assembly and retrofitted into traditional welding torches.

In accordance with one aspect of the present invention, a welding-type torch is disclosed that includes a first electrode connected to receive a welding-type power to effectuate a welding-type process. The welding-type torch also includes a second electrode connected to receive a starting power to initiate the welding-type process and a **zzle surrounding at least a portion of the first electrode.

In accordance with a**ther aspect of the present invention, a welding torch **zzle assembly is disclosed that includes a first electrode having a first diameter and formed of a first material capable of receiving a welding-type power and effectuating a welding-type process. The welding torch **zzle assembly also includes a second electrode having a second diameter, which is less than the first diameter, and formed of a second material configured to initiate a corona discharge proximate to the first electrode to initiate the welding-type process between the first electrode and a workpiece.

Various other features of the present invention will be made apparent from the following detailed description and the drawings.


tig welding

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