Question. Ed please provide some basic guidelines and general data for establishing automated GTAW weld parameters

Answer. When automated TIG welding, an important first requirement is determine the approximate weld current required for the specific welds. Once the approximate weld current range is determined, then select the correct tungsten electrode size. The following provides a TIG weld start point which can then be fine tuned.

TIG. DCEN. TUNGSTEN SIZE AND
APPROX. WELD CURRENT RANGE

0.040	1 - 70 amps	After 50 amps consider next size.
0.062	10 - 80 amps	After 60 amps consider next size
3/32	20 - 160 amps	After 110 amps consider next size
1/8	25 - 260 amps
5/32	40 - 300 amps
		The ideal tungsten sizeselection is when the optimum weld current selected is found in the 40 to 70% of the tungsten diameter weld current range

 

 

The Cobra TIG 150 is one of the best, small pulsed TIG power souces designed for orbital tube welding that Ed has ever worked with. No bell and whistles, just logical process control features with good product support. Available from MK products California.

 

PULSED TIG DATA:

If using pulsed, set the weld current for the "average pulsed current" For example welding a part 0.040 thick requires approx 40 amps. To attain 40 amps start out with a peak current to back ground ratio of 3 to 1. Back Ground - 20 amps. Peak current - 60 amps, this provides an average weld current of 40 amps.

 

PEAK to BACKGROUND RATIO. Ratio range. 2 to 4 to 1. Try 3 to 1 for start. Background 20 amps Peak 60 amps.

PULSED WIDTH OR TIME. The time at which the peak current is maintained. The
more sensitive the part to weld heat the smaller the width or the less
percentage of time. Start in the range of 30 to 50 %.

PULSED FREQUENCY. Examine the pulsed overlap weld pattern 60 to 80 percent
overlap is good. For thin metals <0.030 its usually beneficial to decrease
the pulsed frequency. For more weld penetration or a smoother more
continuos weld surface, turn up the pulsed frequency.

 

TUNGSTEN POINT. Under 20 amps, grind 20 degrees to a point. Over 20 amps weld applications, grind 20 degrees with a small flat spot (0.005 to 0.010 ) on tip as indicated with the tungsten in the center..

TUNGSTEN TYPE: Use Lanthium or ceriated tungsten.

APPROX ARC LENGTH. Gap between tungsten and weld surface,
Arc length at 15 to 30 amps = 0.025 - 033
Arc length at 30 to 50 amps = 0.030 - 038
Arc length at 50 to 70 amps = 0.040
Arc length at 70 to 150 amps = 0.070 - 0.080WELD SPEED.

WELD SPEED. If you don't know the TIG weld speed, start at 5 or 6 ipm, then change to suit the desired weld size and penetration.

RAMP DOWN FOR CRATER REDUCTION.
About 3 to 4 mm from end of weld, ramp down the weld current to 1 amp for 1 to 2 seconds..

ORBITAL WELD SCHEDULES. Orbital welds typically provide 4 parameter weld
schedule levels to compensate for the increased weld heat that would occur
as the TIG weld goes around the tube. For example a weld schedule may drop the
weld current by 20% between the weld start at level I and the end of the weld at level 4. Example when welding a tube. Start weld current Level One, 12 to 3 o'clock at 60 amps - Level 2. 3 to 6 o'clock at 55 amps. - Level 3. 6 to 9 o'clock at 50 amps. Level 4. 9 to 12 o'clock at 48 amps. Then finish 4 mm back from weld with crater data of 1
amp.

WELD GASES. Be wary of gas salesmanship touting the benefits of argon hydrogen and argon helium mixes. Always consider argon first, its the most economical gas and when provided in liquid form eliminates concern for gas contamination.
Consider argon with 2 to 10% hydrogen if you want more weld wetting or faster faster weld speeds. Remember an increase in weld speed may have little value if the weld cycle time is measured in seconds. The hydrogen addition to argon can increase arc stability on specific low amp application <10 amps. When welding specific alloy steels with hydrogen gas be beware of the potential for hydrogen
embrittlement. Hydrogen can also decrease the potential life of the tungsten, depending on the hydrogen content and tungsten type.

 

Ozone forms in both MIG and TIG arcs. The greater the weld current density and the greater the reflective weld surface, the greater the ozone content. For more info visit weld gases.

Thoriated Tungsten Concerns.Thorium is a radio active alloy used in tungsten welding electrodes to assist in arc starting. Although companies involved in welding have been using thoriated electrodes for many years, the industry is becoming more mindful of their potential health hazards and the mirco amounts of radiation levels found in the enviroment where TIG welders work.
The following are notes, warnings, and recommendations form various organizations on the use of thoriated tungsten welding electrodes

American Welding Society: "Thorium is radioactive and may present hazardous by external and internal exposure. Alternatives tungsten types are available If welding is to be performed in confined spaces for prolonged periods of time or if electrode grinding dust might be ingested, special precautions relative to ventilation and dust diposal should be considered. The user should consult appropriate safety personnel."

Standard Manufacturer's Warning: "Thorium dioxide is a naturally occurring radioactive element. It is an alpha emitter and, as such, its primary hazard lie in inhalation of dust/fumes." "Thorium dioxide has been identified as a carcinogen by the NTP and IARC." (These quotes are from Osram Sylvania MSDS sheets).

The Welding Institute: Thorium is a radioactive element. The HSWE has recommended to factory inspectors that , where thoriated tungsten electrodes are not necessary, users should be encouraged to look for alternatives.

Cancer Assessment: Thorium dioxide has been identified as a carcinogen by the National Toxicology Program and International Agency for Research on Cancer.

TUNGSTEN SAFETY QUESTION. We use Thoriated TIG electrodes in our factory. We have been told by a sales rep that these electrodes are are associated with health hazards. Can we consider switching to Ceriated or Lanthanated TIG electrodes?

What type of tungsten should you replace the thoriated with when using AC and DC TIG welding?

Consider a tungsten with 2 percent cerium or a tungsten with 1 to 2 percent of lanthanum. Ceriated and lanthanated tungsten electrodes are equal to other electrodes in terms of their weld properties and are superior in some areas.

In contrast to pure tungsten the advantages of a ceriated or lanthanated electrode are:

[] Outstanding in the low current range.

[] Excellent ignition and re-ignition performance.

[] More durable a longer service life.

[] Excellent weld current carrying capacity.

[] Maintains a point instead of tendency to balling.

With pure tungsten the AC current will result in a ball at the tungsten tip, this results from the high energy generated from the REP portion of the AC. With an alloyed rare earth tungsten when welding aluminum with AC the tungsten can be pointed with a flat, this provides welding benefits

As mentioned pure tungsten balls up, producing a wider, less intense plasma arc cone that can result in arc wandering. A rare earth tungsten used in combination with square wave technology that enables a greater ratio of EN rather than EP maintains a point and lets you use smaller tungsten. This provides a more focused arc so you can more precisely control heat input and weld bead profile. For a pointed electrode, use a truncated (flat) point as an overheated tip point can melt or fall into the weld.

What benefits are attained from using TIG inverter (balance control power sources) with the rare earth tungstens?

Through the benefits of balance AC control, some power sources allow up to 90 percent EN in the AC cycle with variable output frequency (20 to 250 Hz) you can dramatically reduce the heat at the tungsten tip and direct the majority of electrons to the work piece. This provides;

* Narrower heat affected zones

* Improved control over weld depth-to-width ratios.

* Initiate the weld puddles much faster.

* Faster weld travel speeds.

* Reduction in porosity.

* Less tungsten and gas consumption.

* Eliminate arc wandering.

* What type of tungsten should you use for AC and DC TIG welding?

Aluminum TIG Question: Ed We are trying to AC - TIG weld a plug in an Alum 60 series tube. The tube rotates. The tube is 12 mm od, and it's only 0.050 thick. The plug is the same alloy, however it's solid, 1/8 thick 3/8 in length and fits in the end of the tube. The pulsed weld is made between the plug surface and tube end. We have extensive GTAW issues in controlling the weld fluidity in this single pass weld and frequently melt through the thin tubes.

Aluminum TIG Answer.
This is one of the most difficult, automated TIG applications you may ever come across. The following adds to your weld issues.

[a] The tube is thin aluminum, rapid heat build up..
[b] The tube is small diameter, rapid heat buildup.
[c] The plug thickness is different to the tube creating different weld heat requirements.
[d] The plug length is short creating rapid heat build up in contrast to the tube which is a good heat conductor.
[c] AC with pure tungsten is used. The weld arc width and length may change with variations in the tungsten length and shape.

SOMETIMES WHEN THE PROBLEM IS GENERATED BY WELD HEAT, THE WELD SOLUTION IS MORE WELD PASSES.
The answer to this difficult weld issue may lie in the opposite of what you would expect. Instead of a high current single pass (single tube rotation) weld try two or possibly three small weld passes.

[1] First start out with a low weld current TIG pass. This weld pass will pre-heat the tube and plug and reduce the alum oxides.

[2] For the second pass, slightly ramp up the weld current, just enough to let the tube and plug alum melt and form a weld.

[3] For the third pass if necessary, use one more tube rotation. Use a lower current then the weld pass, this pass is to blend the weld.

[4] Ensure you use at least a 3 - 5 second current tailout with the fiinish weld current less than 5 amps.

 

~~~~~~~~~ AC GTAW Arc Rectification~~~~~~~~

For those of you that use AC current on your TIG aluminum applications and you may wonder about that occasional plasma instability that may occur during the TIG arc. The following is a brief description of AC arc rectification.

During the AC cycle the tungsten is both positive and negative and the electrons will flow in two directions 120 times per-second from the tungsten to work and the work to the tungsten. First the tungsten in the negative mode is a superior conductor than the metals being welding. When the AC cycle is in its negative mode the electrons will flow from tungsten to work. During the negative mode we have more stable electron flow than when the electron flow in the positive cycle in which the electrons flow from negative alum metal surface to the positive tungsten.

Another reason for AC rectification is the condition of the aluminum weld surface. For example when welding multipass TIG welds one weld pass will remove the alum metal surface oxide, the next pass made on top of the weld may present a cleaner weld surface, (a weld surface that presents less surface oxides). When the alum metal surface has less impurities (less oxides) the HF used to reignite the AC arc may have a difficult time as oxides add to arc stability, (that's why we put oxygen or co2 in a MIG gas to weld steel). Remember its the tungsten positive cycle that provides the arc cleaning action. The positive cycle is when the electrons flow from the work (breaking up the minuscule aluminum surface oxides) to the tungsten, this provides the arc cleaning action. Once the alum oxides have decreased from the weld surface its harder for the HF to reignite the arc so we see arc stability issues also affected by the condition of the alum weld surface.

Today we use square wave weld equipment to minimize the effects of AC rectification however the arc rectification will still occur, it's just less noticeable.

 

 

To be continued