© COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON II Direct current electrode negative (DCEN) is produced when the electrode is connected to the negative terminal of the power source.  Since the electrons flow from the electrode to the plate, approximately 70% of the heat of the arc is concentrated at the work, and approximately 30% at the electrode end.  This allows the use of smaller tungsten elec- trodes that produce a relatively narrow concentrated arc.  The weld shape has deep penetra- tion and is quite narrow.  See Figure 8.  Direct current electrode negative is suitable for weld- ing most metals.  Magnesium and aluminum have a refractory oxide coating on the surface that must be physically removed immediately prior to welding if DCSP is to be used. Direct current electrode positive (DCEP) is produced when the electrode is connected to the positive terminal of the welding power source.  In this condition, the electrons flow from the work to the electrode tip, concentrating approximately 70% of the heat of the arc at the electrode and 30% at the work.  This higher heat at the electrode necessitates using larger diameter tungsten to prevent it from melting and contaminating the weld metal.  Since the electrode diameter is larger and the heat is less concentrated at the work, the resultant weld bead is relatively wide and shallow.  See Figure 8. Aluminum and magnesium are two metals that have a heavy oxide coating that acts as an insulator and must be removed before successful welding can take place.  Welding with electrode positive provides a good oxide cleaning action in the arc.  If we were to study the physics of the welding arc, we find that the electric current causes the shielding gas atoms to lose some of their electrons.  Since electrons are negatively charged, these gas atoms now are unbalanced and have an excessive positive charge.  As we learned in Lesson I, unlike charges attract.  These positively charged atoms (or positive ions as they are known in FIGURE 8 Electrode Oxide Heat Polarity Penetration Cleaning Concentration Direct Current Alternating  Current Medium Penetration Medium Width Bead Good Cleans Oxide on Each Half Cycle Alternates  Between Electrode and Work Straight Polarity Electrode Negative Deep Penetration Narrow Bead Direct Current Reverse Polarity Electrode Positive Shallow Penetration Wide Bead Maximum None At Work At Electrode GAS IONS + _ ELECTRON FLOW _ _ + + EFFECTS OF CURRENT TYPE - GAS TUNGSTEN ARC WELDING

Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals
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