TIG welder: when DC is enough and when you need AC/DC

"I have a TIG welder, so why can't I get a decent bead on aluminium?" I look at the rating plate and the answer is almost always there: it is a direct current machine. It is fine on steel, not on aluminium, and no setting will save it.

DC for steel and stainless, AC for aluminium

Aluminium has an oxide layer on its surface that melts at a much higher temperature than the metal underneath: according to Miller, around 2,000 °C against 650 °C for aluminium. Alternating current is there precisely to break up that oxide while you weld: the electrode-positive half cycle cleans, the electrode-negative half cycle heats and penetrates.

If the machine has balance control, Miller suggests starting at 75% electrode negative, going down towards 65% on old, oxidised aluminium and up towards 80% on clean parts. Below 60% it is better to clean the part by hand: too much positive overheats the tungsten and the arc becomes unstable.

That is why AC/DC machines cost more. In the collection there are two, both Awelco: the TIG 210 AC/DC PFC on 230 V and the TIG 320 AC/DC on 400 V three-phase. If you do not weld aluminium, a good DC machine is more than enough.

The machines we stock, figure by figure

Before the price, look at these figures. They are the ones declared by Awelco:

Machine	Processes and arc start	Current	Power supply	Duty cycle
Mikrotig 170	Stick and lift TIG, DC	30-150 A	230 V single-phase, 7 kVA	20/150
Mikrotig 200	Stick and lift TIG, DC	30-160 A (200 A max)	230 V single-phase, 7 kVA	20/160
TIG 200 i HF-DC Pulse	High-frequency DC TIG with pulse, stick	TIG 10-200 A	230 V single-phase, 6.4 kVA in TIG	TIG 17/200
TIG 210 AC/DC PFC	High-frequency AC/DC TIG, stick	TIG 5-210 A	230 V single-phase, 5.7 kVA in TIG	TIG 35/210
TIG 320 AC/DC	High-frequency AC/DC TIG, stick	TIG 5-320 A	400 V three-phase, 8.5 kVA in TIG	TIG 35/320

• Minimum current. It matters on thin sheet and small tubes: the Mikrotig machines start at 30 A, the TIG 200 i at 10, the AC/DC machines at 5. The maximum is for thickness, the minimum is for not burning through.
• Duty cycle. 35/210 means 35% at 210 A: out of 10 minutes, three and a half minutes of arc at that current, then the machine cools down. EN 60974-1 measures it over 10 minutes at an ambient temperature of 40 °C; at lower current the percentage goes up. On the same data sheet Awelco also gives a second row with higher percentages (for the TIG 210, 210 A at 45%) without saying at what temperature: before ordering, ask for the rating-plate figure.
• Power supply. The TIG 320 needs 400 V three-phase. The others run on 230 V, but 7 kVA is about 30 A at full load: check that your installation can take it.
• Stick. All five also weld with stick electrodes, from 1.6 to 4 mm (the TIG 320 up to 5).

Lift or high frequency

Lift arc starting is done by resting the tungsten on the part and lifting it: simple, no electrical interference, great for stainless fabrication or occasional jobs. That is what the Mikrotig machines use. High-frequency starting strikes the arc without touching the part: the tungsten stays clean longer and precision parts are easier to weld. The TIG 200 i and the two AC/DC machines have it.

High frequency is a burst of very high voltage pulses: EWM, a welding machine manufacturer, gives a peak limited to 15 kV. That is why the machine must be switched off when you change the tungsten or the nozzle. And those pulses radiate: nearby electronics pick them up.

EN 60974-10, the electromagnetic compatibility standard for welding equipment, asks you to look around before installing: computers and machine controls, signal and telephone cables, measuring instruments, people with pacemakers or hearing aids. The remedies in the manuals: short welding cables, kept close together and laid on the floor; a filter on the supply if the interference travels through the mains; anyone with a pacemaker stays away. Next to a control panel with a PLC, lift starting at least removes the interference from high-frequency ignition.

The tungsten: half the defects start here

• Grind it lengthwise, not by spinning it against the wheel: lengthwise marks keep the arc straight. For Diamond Ground Products, a tungsten manufacturer, it is the first rule: crosswise marks make the arc wander.
• Use a dedicated wheel, preferably diamond, not the one you deburr steel on: you contaminate the tip.
• If you dip it in the pool, stop: cut off the dirty part and regrind it from scratch. Carrying on with a dirty tip means a wandering arc and a black bead.
• The angle: the blunter the tip, the deeper the penetration, the narrower the bead and the longer the tip lasts. Between 10° and 25° included angle for low currents, between 25° and 45° for larger diameters and higher currents.
• On AC, pure tungsten does not stay pointed: it forms a rounded ball on its own.

The colour on the tip tells you the alloy (EN ISO 6848), and this is where people often go wrong. From EWM's tables:

Colour	Code	What for
Green	WP, pure tungsten	AC on aluminium; not suitable for DC
Grey	WC 20, ceriated	DC and AC, low and medium currents
Gold / blue	WL 15 / WL 20, lanthanated	DC and AC
Red	WT 20, thoriated	Slightly radioactive: EWM advises against using it

The Tungsten Electrode Awelco we stock is the green one, 1.6 x 150 mm in packs of 10, like the one in the Awelco kit for the 17 V torch: it is fine on aluminium with the AC/DC machines. For steel and stainless on DC you need grey or lanthanated electrodes: ask for them when you order.

The right torch for your hours

Air-cooled torches are light and need nothing else, but on long jobs at high current they get hot in your hand. Water-cooled torches handle continuous work but need the cooling unit. Before buying the torch, check that the connector is compatible with your machine: it is a question people often ask us afterwards instead of before.

The Awelco torches in the collection all have a 4 m cable and comply with EN 60974-7:

• TIG TORCH 17 V: air-cooled, tungsten from 0.5 to 2.4 mm, D25 connector (D35 with adapter code 90634). Awelco lists it as an option for the Mikrotig 200.
• TIG TORCH HF 26: 180 A on DC and 150 on AC, tungsten from 0.5 to 3.2 mm. It is the one indicated for the TIG 200 i, where Awelco lists it among the options: include it in the budget. The TIG 210 AC/DC PFC and the TIG 320 AC/DC, on the other hand, come with cable and TIG torch according to the Awelco catalogue.
• TIG TORCH HF 18 W: liquid-cooled, an option for the TIG 210 AC/DC PFC and the TIG 320 AC/DC together with the water cooling unit (code 79609).

The sum: the TIG 320 goes up to 320 A, the HF 26 handles 180 on DC. Above that current the HF 26 is not enough: for the TIG 320 Awelco lists the water-cooled HF 18 W among the options, with its cooling unit.

Gas and cleaning

TIG uses pure argon, classified I1 in EN ISO 14175. If the bead comes out grey or blackened, check the flow, the diffuser and draughts before touching the current. And clean the part: on stainless and aluminium a little oil or oxide shows up in the bead. On heavily oxidised aluminium Miller recommends a file or a carbide burr before touching the balance.

Filler rods are chosen by the alloy you are welding, not by eye. The Filler Rods TIG Awelco we stock are ER308L, 1.6 x 330 mm, 60 pieces: the filler for AISI 304 type stainless. Aluminium and carbon steel need rods of a different alloy.

Pulse

The pulse function alternates a high and a low current: you heat the part less, distort thin sheet less and control the pool better. On thin stainless tubes and cosmetic work I use it gladly; on thick sections it makes little difference.

There are four settings: peak current, background current, frequency (pulses per second) and peak time as a percentage. Miller explains why it pays on stainless: it dissipates heat poorly and holds it where the arc is, which leads to warping and rust; pulsing lowers the heat and narrows the heated zone. In their test on stainless (40 A peak, background at 20% of peak, 175 pulses per second, 75% of the time at peak) the pulsed bead took 30 seconds against 45. Among ours, DC pulse is on the TIG 200 i HF-DC Pulse.

Questions I get at the counter

When is TIG better than MIG or stick?

When you need a precise, clean bead on stainless, tubes or thin sheet. It is slower, but you control the heat better.

Dedicated TIG or multi-process machine?

If you do quality TIG work, the dedicated machine has more settings: high frequency, pulse, AC. If you switch between wire, stick and TIG, the multi-process machine is more practical.

What I need to know

• whether you weld aluminium or only steel and stainless;
• typical thicknesses and whether the work is cosmetic or structural;
• how many hours in a row you keep the torch lit, and at what current;
• what socket you have: 230 V single-phase or 400 V three-phase;
• whether there are electrical panels, PLCs or sensitive instruments nearby;
• whether you also need a torch, tungsten and filler rods, and whether you already have an argon cylinder.

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We stock these

A few items from the range we keep. If you cannot find the size or the model you need, just ask: what you see online is only part of what we can source for you.

Browse the whole category: TIG Welders and Welding Accessories