Page 185 - CITS - Welder - Trade Theory

WELDER - CITS

electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the
base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and
appearance.
Alternating current, commonly used when welding aluminum and magnesium manually or semi-automatically,
combines the two direct currents by making the electrode and base material alternate between positive and
negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode
from overheating while maintaining the heat in the base material. Surface oxides are still removed during the
electrode-positive portion of the cycle and the base metal is heated more deeply during the electrode-negative
portion of the cycle. Some power supplies enable operators to use an unbalanced alternating current wave by
modifying the exact percentage of time that the current spends in each state of polarity, giving them more control
over the amount of heat and cleaning action supplied by the power source. In addition, operators must be wary
of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse
polarity (positive electrode). To remedy the problem, a square wave power supply can be used, as can high-
frequency voltage to encourage ignition.

Effect of polarity in DC TIG Welding and application of

straight and reversed polarity and Square wave concept

and Wave balancing, and Pulsed TIG welding and

application

Objectives: At the end of this lesson you shall be able to
• describe the effect of polarity in DC TIG welding
• explain the direct current straight polarity
• explain the direct current reverse polarity.
Power sources

TIG welding power sources have come a long way from the basic transformer types of power sources which
were used with add-on units to enable the power source to be used as a TIG unit, eg high frequency unit and/or
DC rectifying units.The basics of TIG welding has almost remained the same, but the advent of technology TIG
welding power sources have made the TIG processes more controllable and more portable. The one thing that
all TIGs have in common is that they are CC (Constant Current) type power sources. This means only output
adjustment will control the power source amps. The voltage will be up or down depending on the resistance of
the welding arc.
Characteristics of power source : The output slope or volt-ampere curve A, a change from 20 volts to 25 volts
will result in a decrease in amperage from 135 amps to 126 amps. With a change of 25 percent in voltage, only a
6.7 percent change occurs in the welding current in curve A. Thus if the welder varies the length of the arc, causing
a change in voltage, there will be very little change in the current and the weld quality will be maintained. The
current in this machine, even though it varies slightly is considered constant. This is called drooping characteristic
power source. Also called Constant Current (CC) power source. This type of power source is used in GTAW
process.

Effect of polarity in DC TIG Welding and application of straight and reversed polarity
Types of welding current used for GTAW
When TIG welding, there are three choices of welding current. They are: Direct Current Straight Polarity, Direct
Current Reverse Polarity, and Alternating Current with High Frequency stabilisation. Each of these has its
applications, advantages, and disadvantages. A look at each type and its uses will help the operator select the
best current type for the job. The type of current used will have a great effect on the penetration pattern as well as
the bead configuration. The diagrams below, show arc characteristics of each current polarity type.
a DCSP - Direct Current Straight Polarity: (The tungsten electrode is connected to the negative terminal).
This type of connection is the most widely used in the DC type welding current connections. With the tungsten
being connected to the negative terminal it will only receive 30% of the welding energy (heat). This means the
tungsten will run a lot cooler than DCRP. The resulting weld will have good penetration and a narrow profile.

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CITS : C G & M - Welder - Lesson 49 - 60