Chapter 16

1. Chapter 16 Gas Tungsten Arc Welding of Plate

2. Objectives • Name the applications for which the gas tungsten arc welding process is more commonly used • Discuss the effects on the weld of varying torch angles • Explain why the filler rod end must be kept inside the protective zone of the shielding gas and how to accomplish this • Tell how tungsten contamination occurs and what should be done when it happens

3. Objectives (cont'd.) • Explain what can cause the actual welding amperage to change • Determine the correct machine settings for the minimum and maximum welding current for the machine used, the types and sizes of tungsten, and the metal types and thicknesses • List factors that affect the gas preflow and postflow times required to protect the tungsten and the weld

4. Objectives (cont'd.) • Determine the minimum and maximum gas flow settings for each nozzle size, tungsten size, and amperage setting • Compare the characteristics of low carbon and mild steels, stainless steel, and aluminum in respect to GTA welding • Describe the metal preparation needed before GTA welding

5. Objectives (cont'd.) • Demonstrate how to properly make GTA welds in butt joints, lap joints, and tee joints in all positions that can pass the specified standard

6. Introduction • Gas tungsten arc welding • Also called GTA welding • Can be used to for nearly all types and thicknesses of metal • Fluxless, slagless, and smokeless • Welders have fine control of the welding process • Used when appearance is important • Setup of equipment affects weld quality • Charts give correct settings • Field conditions affect the variables

7. Torch Angle • Key points • Torch should be held as close to perpendicular as possible • May be angled zero to fifteen degrees from perpendicular for better visibility • As the gas flows out it must form a protective zone around the weld • Too much tilt distorts protective shielding gas zone • Velocity of shielding gas affects protective zone as torch angle changes

8. Filler Rod Manipulation • Filler rod must be kept inside the protective zone • If removed from the gas protection • Oxidizes rapidly: oxide is added to weld pool • Rod tip becomes oxidized: cut it off • Weld is temporarily stopped • Shielding gas must be kept flowing • Rod should enter shielding gas as close to base metal as possible • Angles under 15 degrees prevent air from being pulled in welding zone

9. FIGURE 16-2 The hot filler rod end is well within the protective gas envelope. Larry Jeffus

10. FIGURE 16-5 Filler being remelted as the weld is continued. Larry Jeffus

11. Tungsten Contamination • Most frequent problem • Tungsten becomes contaminated if it touches molten weld pool or filler metal • Surface tension pulls contamination up onto the hot tungsten • Extreme heat causes some of the metal to vaporize and form a large oxide layer

12. FIGURE 16-8 Contaminated tungsten. Larry Jeffus

13. Tungsten Contamination (cont'd.) • Contamination forms a weak weld • Weld and tungsten must be cleaned before any more welding can be done • Tiny tungsten particles will show up if the weld is X-rayed • Contamination can be knocked off quickly by flipping the torch head

14. FIGURE 16-8 Contaminated tungsten. Larry Jeffus

15. Current Setting • Amperage on machine's control is the same at the arc when: • Power to machine is exactly correct • Lead length is very short • All cable connections are perfect • Arc length is exactly right • Remote current control is in full on position

16. Experiments • Designed to help new welders learn basic skills • Help troubleshoot welding problems • Learn more • Subtle changes will become more noticeable • Even experienced welders make changes

17. Figure 16-10 Melting first occurring. Larry Jeffus

18. Gas Flow • Gas preflow and postflow times depend upon: • Wind or draft speed • Nozzle size • Tungsten size • Amperage • Joint design • Welding position • Type of metal welded • Maximum flow rates must never be exceeded

19. Practice Welds • Grouped according to weld position and type of joint • Mild steel • Inexpensive • Requires the least amount of cleaning • Aluminum • Cleanliness is a critical factor • Try each weld with each metal • Determine which metal will be easier to master

20. Low Carbon and Mild Steels • Two basic steel classifications • Most common • During manufacturing small pockets of primary carbon dioxide gas become trapped • Do not affect strength • Porosity: likely when not using a filler metal • Most filler metals have some alloys (i.e., deoxidizers) • Prevent porosity caused by gases trapped in base metal

21. Stainless Steel • Setup and manipulation • Nearly the same as for low carbon and mild steels • Welds show effects of contamination • Precleaning is important • Most common problem • Bead color after the weld • Using a low arc current with faster travel speeds is important • Carbide precipitation

22. Aluminum • Molten aluminum weld pool • High surface tension • Preheat base metal in thick sections • Preheat temperature is around 300 degrees Fahrenheit • Cleaning and keeping the metal clean • Time consuming • Aluminum resists oxidation at room temperature • Rapidly oxidizes at welding temperatures

23. FIGURE 16-15 Aluminum filler being correctly added to the molten weld pool. Larry Jeffus

24. Metal Preparation • Base and filler metals • Must be thoroughly cleaned • Contamination will be deposited into the weld • Oxides, oil, and dirt are the most common • Contaminants can be removed mechanically or chemically

25. Summary • Position yourself to control the electrode filler metal and to see the joint • Experienced welders realize they need to see only the leading edge of the weld pool • Good idea to gradually reduce your need for seeing 100% of the weld pool • Increasing this skill is significant advantage • Welding in the field • May have to be done out of position