TIG WELDING OF ALUMINIUM AND ITS ALLOYS
AC welding with Arc Voltage Control
Alternating current (AC) welding of aluminium and its alloys with Arc Voltage Control (AVC) using the
Polysoude PC 350 AC/DC controller and Tetrix 351 source (Fig.1) is now mastered. This is the direct
result of collaborative work between Polysoude (concerning AVC regulation) and EWM (concerning
source regulation).
The main obstacle with aluminium AC welding is the AVC regulation which does not support any
phase jumps.
The availability of the AVC function in AC gives Polysoude great competitive advantage. Indeed, to
the best of our knowledge, very few or rather no other manufacturer provides a performance
guarantee, of course in “normal” operating conditions.
The word “aluminium” is only a generic term used to describe, apart from pure aluminium, the entire
range of alloys which the majority of applications are composed of.
Orbital welding of tubing systems and pipelines in aluminium alloy prove to be particularly
interesting in the case of cryogenic and nuclear applications which require a high level of quality. The
advantages of an automatic weld process lie in the traceability and the replication of parameters
linked to the electronic control of power sources. This replication allows for the creation of an
elaborated welding procedure resulting in constant high quality welds.
Prerequisites for AC welding of aluminium and its alloys
The Welding Procedures (WP) must be adapted to the following conditions:
-
thermal conduction
-
oxide layers on the surface of the workpieces
-
proneness to the development of porosity.
Thermal conduction
Some recommendations to reduce the effects:
-
welding of sections with equal thickness
-
J-groove weld preparation with collars of low thickness
V-groove weld preparation with an angle of at least 40° (sufficient access and good sidewall
fusion).
Oxide layers
Reasons why oxide layers reduce the weld quality:
-
oxide layers contain crystal water and absorb humidity (thus provoking porosity)
refractory compounds (do not melt): fragments of the oxide layer become inclusions and
cause cracks or prevent penetration.
Measures to be taken before welding:
-
elimination of oxide layers (by machining, pickling, scraping)
-
chamfering the opposite (in case of tubes the interior) edge of the collar.
Porosity
Reasons why porosity must always be taken into account when welding aluminium and its alloys:
-
quite low solubility of H2 in solid Al
-
very fast solidification of liquid Al.
Important:
In 90 % of all cases appearance of H2 is caused by contaminated surfaces or humidity absorbed by
the oxide layer. In some cases H2 comes from the base material or the filler wire.
Recommendations:
-
using exclusively clean and dry materials
-
thorough protection of filler wire against any contamination
-
milling of zones to be welded
-
good shielding gas cover
-
preheating of workpieces (at a temperature of 100 °C).
Fundamentals of AC welding
AC current is used essentially for the welding of aluminium and its alloys. In AC mode, the tungsten
electrode is switched between positive polarity (acting as a cathode) and negative polarity (acting as
an anode). During positive polarity with the electrode as a cathode the oxide layer on the surface of
the workpiece is destroyed (cleaning sequence). During negative polarity with the electrode as anode
the heat necessary to melt the material is transferred to the workpiece, the temperature of the
electrode can decrease during this time (penetration sequence). (Fig. 2)
The percentage of the length of cleaning time in relation to the length of the period is called balance.
In the example in figure 2, the balance is 35 %. With increasing cleaning time a better weld quality
can be achieved (less porosity), but the operating life of the tungsten electrode will be considerably
shortened. To estimate the requested diameter of the tungsten electrode, the following formula can
be used:
(Diameter of the tungsten electrode [mm] x 100 A) – 80 A = max. welding current
The reciprocal of the length of period (T) indicates the number of periods per second and is called the
frequency (f) of the welding current. Higher frequencies (60 – 120 Hz) are used for small wall
thicknesses (≤ 1.5 mm), whereas lower frequencies (50 Hz) are chosen for thicker workpieces to be
welded (≤ 3 mm). AC welding current can be smooth or pulsed, current parameters are to be chosen
according to the thickness of the workpiece to be welded.
Principles of AVC (Arc Voltage Control)
The measurement of the arc voltage between the tungsten electrode and the mass allows keeping a
constant programmed value by regulating the distance between the electrode and the work piece,
which remains constant at the same time. Although welding parameters must not be changed, the
choice of a different welding current e.g. requires the programming of a different arc voltage also if
the distance between the electrode and the workpiece should remain the same. (Fig. 3)
Differences in the electrical behaviour of the tungsten electrode (wear) provoke the same result. The
AVC device maintains the arc voltage constant, so the distance between the electrode and the
workpiece will change. (Fig. 4)
Recommendations for AC welding with AVC
Generally recommended parameter values for AC welding with AVC:
•
Frequency of the welding current between 50 and 120 Hz
•
Max. welding current 250 A, average welding current max. 200 A
•
Min. welding current 20 A
•
Distance between the electrode and the workpiece about 3 mm (the value of arc voltage to
be programmed depends on applied welding currents)
•
Min. arc voltage 10 V
•
Cleaning balance between 30 and 50 %
•
Before welding the oxide layer must be eliminated
•
Good shielding gas protection must be ensured.
What to avoid during AC welding with AVC
•
Use of tungsten electrodes without sufficient diameter when
welding with AC current
•
Use of a tungsten electrode with ball unadapted to the applied welding current
•
Choice of a cleaning balance value which is too high