GTA WELDING EQUIPMENT AND SUPPLIES
The equipment and supplies required for gas tungsten-arc welding consist of an electric power unit, shielding gas, a pressure-reducing regulator and flowmeter, an electrode holder (commonly called a torch), nonconsumable tungsten electrodes, filler rods, a supply of cooling water (when required), and personal protective gear.
Electric Power Unit
Most welding power sources can provide the current needed for GTA welding. The common welding machines, whether alternating current (at) or direct current (de), have their advantages in certain welding applications; however, they can be cumbersome and their hose and cable connections can create difficulties. Because of this, specially designed machines with all the necessary controls are available for gas tungsten-arc welding (fig. 8-3).
GTA power units are equipped with solenoid valves that turn the flow of shielding gas and cooling water on and off. They are also equipped with a hand- or footoperated remote-control switch that turns the water and gas on and off. Some of these remote-control devices
Figure 8-4.-Effects of polarity on the weld.
also turn the main welding current on and off at the same time. This not only allows the operator to start and stop without leaving the work but also to adjust the current while welding.
Most of these welding machines can produce both ac and do current. The choice of ac or do depends on the welding characteristics required.
DIRECT CURRENT.- As you learned in chapter 7, a direct-current welding circuit maybe either straight or reverse polarity. When the machine is set on straight polarity, the electrons flow from the electrode to the plate, concentrating most of the heat on the work With reverse polarity, the flow of electrons is from the plate to the electrode, thus causing a greater concentration of heat at the electrode. Because of this intense heat, the electrode tends to melt off, therefore, direct-current reverse polarity (DCRP) requires a larger diameter electrode than direct-current straight polarity (DCSP).
The effects of polarity on the weld are shown in figure 8-4. Notice that DCSP produces a narrow, deep weld. Since the heat is concentrated on the work, the welding process is more rapid and there is less distortion of the base metal. Overall, straight polarity is preferred over reverse polarity because you can achieve better welds.
DCRP forms a wide and shallow weld and is rarely used in the GTAW process. The exception to this is when it is used to weld sections of aluminum or magnesium. DCRP has excellent cleaning power that results from the
Figure 8-5.-The ac welding cycle.
Figure 8-6.-ACHF combines the desired cleaning action of DCRP with the good penetration of DCSP.
action of positive-charged gas ions. When these gas ions strike the metal, they pierce the oxide film and form a path for the welding current to follow. This same cleaning action occurs in the reverse polarity half of an alternating-current welding cycle.
ALTERNATING CURRENT.- AS shown in figure 8-5, ac welding is actually a combination of DCSP and DCRP; however, the electrical characteristics of the oxides on the metal often prevent the current from flowing smoothly in the reverse polarity half of the cycle. This partial or complete stoppage of current flow (rectification) causes the arc to be unstable and sometimes go out. Ac welding machines were developed with a high-frequency current flow unit to prevent this rectification. The high-frequency current pierces the oxide film and forms a path for the welding current to follow. The effects of alternating current high-frequency (ACHF) are shown in figure 8-6. Notice that ACHF offers both the advantages of DCRP and DCSP. ACHF is excellent for welding aluminum.
Table 8-1.-Current Selection Guide for GTA Welding of Common Metals
You can use table 8-1 as a guide for selecting the current for welding some of the more common metals. For more specific information, refer to the operator's manual for the specific machine you are using.