While known as a precision process, many fabricators using the Gas Tungsten Arc Welding (GTAW, or TIG welding) process fight several common problems that hinder quality, slow production, frustrate the operator and otherwise prevent the process from achieving its full potential. These include a limited ability to tailor the weld bead profile, poor control of the arc direction and arc wandering, poor arc starting, unstable or inconsistent arcs in the AC mode, high frequency interference with electronics and tungsten contamination,
Fortunately, new TIG welding technology – made possible by advances with inverter-based power sources and microprocessor controls – can eliminate common productivity gremlins. Further, Miller Electric's new AC/DC inverter-based TIG welder, the Dynasty™ 300 DX, provides advanced arc shaping capabilities. As a result, many fabricators adopting this new technology have experienced phenomenal production increases, taken on new types of projects and reduced costs. Most importantly, the operators enjoy welding more.
Advanced Squarewave Performance
In the mid-1970s, Miller Electric set the standard for AC/DC TIG technology when it introduced the Syncrowave® welding machine with its Squarewave AC output technology. Squarewave technology minimized the problems inherent with AC welding: arc rectification, arc stumbling, wandering and outages.
Non-Squarewave machines usually exhibited these symptoms during the Electrode Negative (EN) to Electrode Positive (EP) transition of the AC sine wave. Sometimes, these older units 1) did not have enough "push" to drive the arc through the zero crossing and then re-establish the arc in the opposite polarity.
If five or six EN to EP cycles failed in a row, it created another problem: the welding output began to resemble DC. If this occurs, a TIG machine reaches for open circuit voltage in an attempt to get enough voltage to re-establish AC welding output. Unfortunately, the excess voltage can cause current overshoots. This may produce tungsten spitting, which degrades weld quality.
Squarewave technology shortened the switching time between EN and EP, so it created a more desirable arc. So desirable, in fact, that all higher end AC TIG machines now feature Squarewave technology (Miller's patent expired in 1994). However, not all Squarewave technology is created equal.
The advanced Squarewave technology employed in an inverter-based TIG machine takes EN to EP switching time a quantum leap forward.
Inverters use advanced power switching semiconductors (you might hear them called IGBTs) and microprocessor controls that operate thousands of times faster than "conventional" power switching devices and controls. As a result, inverters push the arc through the zero crossing very quickly. This very quick transition improves weld quality and consistency.
Not all inverters are created equal, either. Miller's Dynasty 300 DX has a unique design advantage because it uses a full bridge rectifier. This costs more than a half-bridge rectifier other AC/DC TIG inverters use, but it permits using a smaller stabilizer. A stabilizer is the "electricity sponge" that maintains the arc when it transitions through zero. The smaller the sponge, the faster the transition. This gives the Dynasty 300 DX a truer Squarewave output than any inverter in its class, so it has the smoothest, most consistent AC TIG arc in the industry.
Extended Balance Control
Using AC to TIG weld aluminum evolved from the need to remove the oxide layer that forms on its surface. The EP portion of the AC cycle, in which electricity flows from the work to the tungsten, "blasts" off surface oxides. The EN portion of the cycle does the actual welding, directing heat from the tungsten into the metal.
When Miller engineers invented the Squarewave AC output, they also discovered that an unbalanced AC wave form works best for many applications. Lighter-duty machines feature a fixed balance control set for more penetration than cleaning.
Advanced Squarewave machines feature adjustable balance control, another Miller invention. This feature permits tailoring the EN/EP ratio to match an application.
Conventional TIG welders, such as Miller's Syncrowave 250 and Syncrowave 350 LX, lets you adjust EN values from 45 to 68 percent (55 to 32 percent EP). Inverters, because of their power switching capabilities, provide extended balance control. For example, the Dynasty 300 DX allows the operator to fine tune the duration of the EN portion of the cycle from 50 to 90 percent (50 to 10 percent EP; adjusting the EP portion of the cycle beyond 50 percent provides no further benefits).
Increasing time in the EP portion of the cycle removes more oxide and create a shallower, wider bead. On aluminum, increasing the cleaning action improves quality by minimizing the chance of foreign particles becoming included in the weld.
Greater amounts of EN create a deeper, narrower weld bead, better joint penetration and a smaller etched zone. This helps when welding on thick material or when appearance (i.e., a minimal etched zone) is important. Setting an inverter's EN duration to the maximum level creates the potential to deliver 22 percent more heat to the work compared to a conventional TIG machine. Adding more heat in the same amount of time permits faster travel speeds. This means that fabricators using extended balance control technology can often produce more parts per hour.
No hard rules exist for setting balance control. The typical error involves over-balancing the cycle. Too much EP creates a large ball on the end of the tungsten. Consequently, the arc loses stability and then you can't control arc direction or the weld puddle; arc starts also degrade. Too little EP results in a scummy weld puddle. Add more cleaning action if the puddle looks like it has black pepper flakes floating on its surface.
Welding Frequency Control
Advanced inverter-based welding machines give operators another option that dramatically enhances shaping of the bead profile: frequency control. Conventional AC TIG welders have a fixed output of 50 or 60 Hz, but some inverters let the operator adjust the welding output frequency. For example, the output frequency on Dynasty 300 DX adjusts from 20 to 250 Hz (other inverters can adjust frequency, but not quite as broadly).
Decreasing frequency produces a broader arc cone, which widens the weld bead profile and better removes impurities from the surface of the metal. It also transfers the maximum amount of energy to the work piece, which speeds up applications requiring heavy metal deposition (such as building up a worn part or making a fill pass).
Increasing frequency produces a tight, focused arc cone. This creates deeper penetration and it narrows the weld bead, which helps when welding in corners, on root passes and fillet welds.
It lets operators direct the arc precisely at the joint and not have the arc dance from plate to plate.
Best of all, increasing the frequency, combined with increasing EN duration and using a pointed tungsten (see side bar story on pointed tungstens), can dramatically increase travel speed and reduce production time. When applied to the right application, improvements from 10 up to 40 percent are frequently reported.
A good starting point for general welding would be 80 to 120 Hz. These frequencies will be comfortable to work with, increase control of the arc direction and boost travel speed. For a fillet weld application with full penetration in the weld without putting too much amperage in the metal, increase the frequency to 225 to 250 Hz. For build up work, start at 60 Hz and adjust lower from there.
Banishing the Arc Starting Gremlins
Conventional TIG welders may experience arc starting problems for a variety of reasons, but they are all linked to getting the weld current flowing between the tungsten and the work piece. An explanation of what the welding current is trying to do will help.
Whether welding in the DC EN mode (the normal mode for work on ferrous metal) or the AC mode, the current must flow from the tungsten to the work piece. Starts cause problems because the current first must overcome the resistance of the tungsten.
That is, the current must heat the tungsten so it becomes a better emitter of electrons; at that point, the arc can jump from the tungsten to the work piece.
One traditional option for "solving" DC arc starting problems, and the standard method for improving AC arc starts, involves superimposing a high frequency (HF) current over the welding current. Basically, the HF current forms a path for the welding current to follow and so the arc can be established. Unfortunately, HF interferes with CNC machines, computers and other electronic equipment because its frequency is similar to a radio's and can be "broadcasted" (one user of continuous HF reported that it affected the accounting computer...and was changing invoice figures!).
The circuitry of the Dynasty 300 DX provides a unique solution that eliminates this concern while delivering much more consistent arc starts. This Miller technology works by starting the weld current in the DC EP mode for a brief time no matter which welding process is chosen. With heat flowing from the work into the tungsten, the electrode quickly becomes a better emitter. That's why the Dynasty 300 DX has consistent starts time after time.
Through the machine's front panel, operators can tailor the duration of DC EP starting current from 1 to 200 ms and the "force" of the current from 1 to 200 amps. Adjusting these parameters is usually not necessary, as the factory default provides very good starts for most applications. Operators should not have any concerns about this starting mode. All they will notice is consistently positive starts.
Advanced TIG welders offer operators several starting methods, but two are superior. The first superior starting method involves minimizing high frequency for applications that benefit from HF arc starts. In these instances, inverters offer an "HF start only" feature that provides a brief burst of HF at the start of the weld. Once the machine senses the arc has been established, it shuts off the HF to minimize any potential interference with electronic equipment.
The Dynasty 300 DX takes "HF start only" one step further. Because this TIG machine starts in the DC EP mode, it only needs HF for 10ms to establish the arc. Operators using this method report very positive arc starts even at low amperages, and they appreciate the absence of HF wandering and buzzing on the weldment. They also report longer times between tungsten sharpenings; this is because less HF is available to cause erosion.
Note that conventional machines not only use HF to improve arc starts, but that they usually require continuous HF for AC welding. HF helps ensure the arc doesn't stumble as it transitions through the zero crossing.
Inverter-based machines do not experience as much difficulty with arc starts or arc stumbling because the machine's microprocessor control and IGBTs operate so quickly. In fact, all good inverters eliminate the need for continuous HF when AC welding on aluminum and other non-ferrous metals.
The second superior starting method is lift Lift-Arc™, an alternative to scratch starts. Scratch starts may contaminate the weld with tungsten, but welds made with the Lift-Arc starting method can consistently pass x-ray or ultrasonic tests. Lift-Arc enables the operator to touch the tungsten to the work piece, lift it off the work piece, and then have full welding current begin flowing. With the scratch start method, the electrode is hot the instant it touches metal. The Lift-Arc technology in the Dynasty 300 DX offers a further improvement because it works in the DC and AC welding modes.
Even though today's inverter-based TIG machines have more features available, smart manufacturers try to make the control panel as simple and logical as possible. Rather than knobs and switches, some of the newer TIG machines use digital displays and switch pads similar to those on a microwave oven.
At first glance, the control panel may appear intimidating. But at second look, switch pad control panels are very logical, especially if operators approach them the same way they would approach any weld sequence.
For example, an operator must select a trigger function (remote or panel); the switch pads on the top left of the Dynasty 300 DX's front panel control the trigger functions. Next, the operator must select a starting method (HF start only, Lift-Arc, etc.); the switch pads for the start mode are right below the trigger function controls. If the AC TIG process is selected, operators must also specify AC frequency and balance control parameters; the three touch pads controlling these functions are located in the center of the panel. Just to the right of these touch pads are the ones for selecting the DC TIG, AC Stick and DC Stick processes.
The rest of the buttons simply follow the actions that occur on every weld sequence, whether the operator controls them through the machine's front panel or the foot pedal. If an operator wishes to control the weld sequence through the "sequencer panel" (usually a group of switch pads centered on the control panel), values for following functions need to be set (values given are for the Dynasty 300 DX):
- Gas pre-flow time (0 to 25 seconds)
- Initial current (5 to 300 amps)
- Initial slope ( 0 to 25 seconds)
- Main/peak welding amperage (5 to 300 amps)
- Final slope (0 to 25 seconds)
- Final current (5 to 300 amps)
- Gas post-flow time (0 to 50 seconds)
Through the digital display, operators can easily see (and return to) the exact values they set. With rheostats, settings can be approximations. The digital displays may also provide "help messages" to the operator about the status of the machine.
The digital display, coupled with the microprocessor control, provides some inverters with a memory function (the Dynasty 300 DX lets operators store up to four weld sequences for AC/DC TIG and Stick processes). If a shop routinely fabricates certain products or consistently welds three or four types of joints or metals, the memory function speeds changing between projects. A lock-out feature prevents tampering with these pre-programmed settings.
All advanced TIG inverters incorporate pulsed welding capabilities. Pulsed TIG welding is extremely beneficial when welding thin gauge steel and stainless steel. It allows the operator to tailor the amount of heat to the application, decreasing distortion and heat input. Pulsing can also help teach beginning TIG welders because it provides a rhythm for adding the filler rod (i.e., add the filler rod during peak amperage pulse).
For critical applications, discerning operators want precise heat (amperage) control to best prevent burn-through, warping or discoloration.
Welders with true pulsing controls, such as the Dynasty 300 DX, let the operator carefully tailor the pulsed wave form by setting: background amp range, pulse frequency (pulses per second) and peak time adjustment (duration of peak amperage). This gives the operator much more leeway when fine tuning the arc. A series of switch pads lets the operator precisely set parameter values.
OTHER INVERTER IMPROVEMENTS
Though not directly related to controlling the welding arc, inverters provide several benefits over conventional TIG machines, such as accepting a variety of primary input power levels, automatically linking to accept the primary power and light-weight portability.
Flexible Input Power
Conventional AC/DC TIG machines operate only on single-phase power. To weld at 250 amps (230 V primary) requires 92 amps of primary power (and 66 amps with power factor correction). Some companies wanting to add more TIG machines or other equipment cannot do so without expensive modifications to their electrical system. However, inverters can use either single- or three-phase input power.
An inverter using three-phase power, welding at 300 amps (230 V primary), requires just 37 amps of primary current. If 460 V primary current is available, welding at 300 amps requires only 18 amps of primary current. An inverter is also very power efficient.
In addition to using single- or three-phase power, inverters also accept 50 or 60 Hz, 230 or 460 V input power. This provides flexibility when moving the machine between jobs sites or around a large facility. To adapt to different types of input power types, some inverters require removing the housing and manually linking the power leads. Other inverters, including the Dynasty 300 DX, have an Auto-Link® feature that eliminates this hassle. This feature automatically senses the type of input power (both voltage and phase type) and electronically reconfigures the machine. With Auto-Link, you can plug in the machine and begin working immediately.
Location, Location, Location
Is work light enough to bring it to the welding machine, or do you need to move the machine to the job site? An inverter's light weight (the Dynasty 300 DX weighs 90 lb.) makes it easily portable, eliminating this old dilemma.
Conventional TIG machines transform power with a large iron core wrapped with copper and/or aluminum wire. This makes them heavy. To handle the current used to weld thicker sections of metal, a TIG welder's transformer must weigh 200 to 400 lb. or more.
Inverters operate on the principle that increasing the current frequency permits reducing the core size and number of wire turns. Before the current reaches the transformer, an inverter boosts the line frequency to 20 to 100 thousand cycles per second. Thus, the transformer on an inverter such as the Dynasty 300 DX weighs just 5 lb.
Although inverters have been available since the late 1980s, many people hesitated to purchase them because of reliability issues. Fortunately, improvements made in the last few years give all Miller inverters (and some others) a reliability level that approaches that of Conventional welding machines. Miller innovations to improve reliability include:
- Wind Tunnel Technology™, a unique, patented technology which shields an inverter's electronic components from potentially damaging air-borne particles.
- Fan-On-Demand™ cooling; this feature monitors the temperature of your welder and operates the fan only when needed. In dirty environments, it reduces the amount of airborne contaminants pulled through the machine, keeping internal components cleaner.
- More robust designs for circuit boards and power switching devices.
Historically, preparing to AC weld required selecting a pure tungsten electrode and forming a ball at the end of the electrode. Balling, until now a necessary evil (pure tungsten tends to form a ball), promotes arc wandering, less arc focus and poorer arc starts because electricity likes to come off a point. With a ball, the current can dance around the entire surface. That's why Miller now recommends that, for AC welding, operators should sharpen the tungsten as if they are welding in the DC mode. This is true for all advanced TIG machines, and especially for inverters because it optimizes performance.
Guidelines for preparing a tungsten for AC TIG welding are:
- Select a tungsten with 2% cerium (2% thorium as your second choice).
- Grind the electrode to a point (grind in the long direction, make the point roughly two times as long as the diameter).
- Put a .010 to .030 in. flat (land) on the end to prevent balling and to prevent tungsten from being transferred across the arc.
- For welding thin metals, use a 3/32 in. diameter tungsten.
Compared to a balled tungsten, a pointed electrode provides greater arc control and lets you direct the amperage precisely at the joint, minimizing distortion. With a pointed electrode, a skilled operator can place a 1/8 in. bead on a fillet weld made from 1/8 in. aluminum plates. Without using this method, the ball on the end of the electrode would have forced the operator to make a larger weld bead, then grind the bead down to final size. Thus, when fitting welded parts together, a pointed electrode can save time.