Chapter 4 Gas. Metal Arc Welding

Transcription

1 Chapter 4 Gas Metal Arc

2 What shielding gases or combinations of shielding gases are used in GMAW? Gas Molecular Weight Remarks Carbon Reactive shielding gas for ferrous metals providing deep Dioxide weld penetration Argon As the amount increases transition current decreases Oxygen Increases puddle fluidity itrogen Increases corrosion resistance in stainless steels Helium High ionization potential increases penetration, when blended with argon, in nonferrous metals 2

3 What kind of properties and characteristics do shielding gases exhibit? Shielding gases are necessary for Gas Metal Arc to protect the weld pool from atmospheric contamination. Depending on the choice of shielding and the flow rate of the shielding gas there is a pronounced effect on: The way the weld puddle wets out The depth of penetration and bead profile (see Figure 4-48) The width of the weld puddle The characteristic of the arc The mode of metal transfer that can be achieved The end mechanical properties of the weld metal The weld metal efficiency The cleaning action on the base metal 3

4 Effects on weld profile with various shielding gases. 4

5 Flow Rate It is important to set the proper flow rate, depending on the shielding gas density (proportional to its molecular weight), to obtain an adequate shielding without wasting gas, creating discontinuities, or changing the mechanical properties of the weld. Insufficient flow rate will cause porosity and a brittle weld metal. 5

6 Surface Tension Selection of the type of shielding gas can reduce surface tension. Surface tension can be reduced by the use of argon gas because it is a cleaning gas. 6

7 What shielding gases are used to weld carbon steel? Shielding Gas composition and flow rates are critical components of a welding procedure. Consider choices for welding carbon steels: Carbon Dioxide Argon/Carbon Dioxide Argon/Oxygen Argon/Carbon Dioxide/Oxygen 7

8 Carbon Dioxide Carbon Dioxide or CO 2 is a common shielding gas for any of the ERXXS-X electrodes. Carbon dioxide is composed of 72% oxygen and 29% carbon. It is the least expensive shielding gases to purchase for welding plain carbon steel with the GMAW process. 8

9 Argon/Carbon Dioxide When two shielding gases are mixed they are may also be called dual blends or binary blends. By optimizing the amount of CO 2 in the argon mixture, the fluidity of the weld puddle can be controlled to give good bead shape in a variety of welding positions. Argon is an inert gas and not reactive like CO 2. 9

10 Argon/Carbon Dioxide Any mixture of the two shielding gases, Argon/CO 2, can be created. The more common mixtures are: 75% Argon/25% CO 2 80% Argon/20% CO 2 85%Argon/15% CO 2 90% Argon/10% CO 2 95% Argon/5% CO 2 10

11 Argon/Carbon Dioxide Short circuited using 85% argon and 15% CO 2 shielding gas Spray transfer mode using 85% argon and 15% CO 2 shielding gas 11

12 Argon/Oxygen There are advantages to using an argon/oxygen The addition of oxygen to argon lowers the spray transition current and thus allows spray arc transfer at lower average currents than Argon/CO 2 mixtures. Oxygen reduces the surface tension of the weld pool and enhances its flow characteristics. 12

13 Argon/Oxygen There are disadvantages to using an argon/oxygen. There can be problems with undercutting and potential lack-of-fusion due to bead rolling at bottom edge of fillet welds. To achieve stable spray transfer and minimum weld porosity, the material surfaces must be clean with no scale or residual oil film 13

14 Argon/Carbon Dioxide/Oxygen Mixing argon/co 2 /O 2 is known as tri-mix shielding gas or ternary blends. A common mixture of this tri-mix gas is: 90% Argon, 8% Carbon Dioxide, and 2% Oxygen 14

15 Shielding Gases for Carbon Steel? Shielding Gas Comparison Chart - Carbon Steels Shielding Gas Short Circuit Spray/Pulse Flow - CFH Characteristics 100% CO 2 Yes o Deep penetrating 75 Ar/25 CO 2 Yes o Ar/20 CO 2 Yes Lower Limit Operates well at higher parameters Marginal at spray transfer parameters A good choice for those wanting all modes of 85 Ar/15 CO 2 Yes Yes transfer with the higher energy of CO 2 Upper limit for FCAW if choosing one shielding gas for everything 90 Ar/10 CO 2 Yes Yes Ar/5 CO 2 Upper Limit Yes Ar/5 O 2 o Yes Ar/2 CO 2 o Yes Ar/8 CO 2 / Yes Yes A good choice for those wanting all modes of transfer Works great with metal cored composite electrodes Mixture more tolerant to mill scale and a more controllable puddle than an Argon-Oxygen mixture Addition of Oxygen to Argon lowers the transition current Good penetration, high efficiency pleasing appearance Ability to use any metal transfer mode to shield carbon steel and low-alloy steel of all thicknesses 15

16 What shielding gases are used to weld stainless steel? Traditionally there have been two types of shielding gases for stainless steel; one tri-mix shielding gas for short circuit and one dual-mixed shielding gas for spray or pulse-spray transfer. 16

17 Helium/Argon/Carbon Dioxide The common mixtures for short circuit welding are: 90% helium, 7.5% argon, and 2.5% CO 2 90% helium, 8% argon, and 2% CO 2 There is little difference between these two mixtures. Both exhibit excellent characteristics. 17

18 Argon/Oxygen Two shielding gases that may be used for spray and pulse spray transfers in stainless steels are: 98% Argon/2% O 2 99% Argon/1% O 2 These mixtures are used for spray transfer on stainless steels. 18

19 Tri-Mixed Shielding Gases There are many different tri-mixed shielding gases for stainless steel. Some mixtures are: Argon/Helium/CO 2 Argon/Nitrogen/CO 2 Argon/CO 2 /Hydrogen 19

20 Argon/Helium/Carbon Dioxide The argon mixture for this shielding gas may range from 60 to 80%; helium from 15% to near 40%; and the balance of CO 2, from 1% to 5%. 20

21 Argon/Nitrogen/Carbon Dioxide This mixture exhibits very good arc stability, low levels of welding fume, improved color match, very good shortcircuiting performance with minimal spatter, and very good performance in pulsed spray with good bead shape and optimized travel speed. 21

22 Argon/Nitrogen/Carbon Dioxide An etched cross section of GMAW stainless steel with the short circuiting mode of transfer. An etched cross section of GMAW stainless steel with the spray mode of transfer. Note the deep root penetration in the etched cross section. 22

23 Argon/CO 2 /Hydrogen This mixture is designed for all-position welding of austenitic stainless steels using axial spray and pulsedspray transfer. 23

24 What shielding gases are used to weld aluminum? There are only two shielding gases used for welding with aluminum electrode wire with the GMAW process: Argon and Helium. A blended shielding gas of argon/helium requires at least 50% argon in order to achieve spray transfer. Common shielding gases are: 100% Argon 75% Argon/25% Helium 50% Argon/50% Helium 24

25 100% Argon Argon provides the best arc starting characteristic compared to Argon/Helium mixtures because of its lower ionization potential. 25

26 Argon/Helium As the helium content increases voltage parameters and travel speeds are increased. 26

27 27 The short circuit mode of transfer on aluminum does not allow adequate penetration due to aluminum s ability to dissipate heat.

28 28 When the spray transfer mode is used on aluminum, root penetration is greatly increased.