weld metal transfer

8/2/2019 Weld Metal Transfer

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Introduction

In Gas Metal Arc Welding (GMAW), also known as Metal Inert Gas (MIG) welding, an electricarc isestablished between the workpiece and a consumable bare wire electrode. The arccontinuously melts thewire as it is fed to the weld puddle. The weld metal is shielded from the

atmosphere by a flow of an inertgas, or gas mixture. Figure 1-1 shows this process and aportion of the welding torch.The mig welding process operates on D.C. (direct current) usually

with the wire electrode positive. This isknown as reverse polarity. Straight polarity, is seldomused because of poor transfer of molten metal fromthe wire electrode to the workpiece. Weldingcurrents of from 50 amperes up to more than 600 amperes are commonly used at weldingvoltages of 15V to 32V. A stable, self correcting arc is obtained by using theconstant potential(voltage) power system and a constant wire feed speed.Continuing developments have madethe mig process applicable to the welding of all commerciallyimportant metals such as steel,aluminum, stainless steel, copper and several others. Materials above .030in. (.76 mm) thickcan be welded in all positions, including flat, vertical and overhead. It is simple to choose theequipment, wire electrode, shielding gas, and welding conditions capable ofproducing high-quality welds at a low cost. Basic Mig Welding Process

ADVANTAGESThe mig welding process provides many advantages in manual and automatic metal

joining for both low and high production applications. Its combined advantages when

compared to covered (stick) electrode,submerged arc, and tig are:

1) Welding can be done in all positions.

2) No slag removal required.

3) High weld metal deposition rate.

4) Overall times for weld completion about 1/2 that of covered electrode.

5) High welding speeds. Less distortion of the workpiece.

6) High weld quality.

7) Large gaps filled or bridged easily, making certain kinds of repair welding more

efficient.

8) No stub loss as with covered electrode.


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Figure 1- 2 illustrates one complete short arc cycle. As the wire touches the weld pool

(A), current begins torise to a short circuit current. When this high current is reached, the

metal is transferred. The arc is thenreignited. Because the wire is being fed faster than

the arc can melt it, the arc will eventually beextinguished by another short (I). The cycle

begins again. There is no metal transferred during the arcing period; only at the short

circuits.To insure good arc stability, relatively low welding currents must be employedwhen using the short arctechnique. Table 1-1 illustrates the optimum current range for

short circuiting metal transfer with several wiresizes. These ranges can be broadened,

depending upon the shielding gas selected.

Globular Transfer

As the welding current and voltage are increased above the maximum recommended for

short arc welding, metal transfer will begin to take on a different appearance. This

welding technique is commonly known as globular transfer, with metal transferring

through the arc. Usually, the drops of molten metal have a greater diameter than the

wire itself. This mode of metal transfer can be erratic, with spatter and occasional short

circuiting being common.

Spray Arc Welding

By raising the welding current and voltage still further, the metal transfer will become a truespray arc. Theminimum welding current at which this occurs is called the transition current.Table 1-2 shows typical valuesof transition current for various filler metals and shielding gases.

As seen in this table, the transition currentdepends on the metal wire diameter and shieldinggas. However, if the shielding gas for welding carbonsteel contains more than about 15% CO2

there is no transition from globular transfer to spray transfer.Figure 1-3 shows the typical finearc column and pointed wire of the spray arc. The molten drops from thewire are very small,affording good arc stability. Short circuiting is rare. Little spatter is associated with thiswelding

technique.Spray arc welding can produce high deposition rates of weld metal. This weldingtechnique is generally usedfor joining materials 3/32 in. (2.4mm) and greater in thickness.Except when welding aluminum or copper,the spray arc process is generally restricted towelding in the flat position only because of the large weldpuddle. However, mild steel can bewelded out of position with this technique when small weld puddles areused; generally with a.035 in. (.89mm) or .045 in. (1.1mm) diameter wires. Figure 1-3Spray Arc Welding Technique


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weld metal transfer

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