210 Modern Welding Copyright Goodheart-Willcox Co., Inc. Each shielding gas and mixture of gases has a dif- ferent effect on the shape of the bead and the penetra- tion. See Figure 8-31. Factors that must be considered when choosing a shielding gas are: • The type of metal transfer desired (short circuiting, globular, spray, or pulsed spray transfer). • The desired bead shape, width, and weld penetration. • The required welding speed. • The undercutting tendencies of the gas. Figure 8-32 and Figure 8-33 list shielding gases for different metals and transfer methods. The shield- ing gases listed for short circuiting transfer are also used for globular transfer. Those gases listed for spray transfer are also used for pulsed spray transfer. Argon and helium are inert gases that do not unite with other chemical elements. Carbon dioxide, oxygen, and nitrogen are reactive gases that mix or react with metals in a weld. Carbon dioxide can be used alone as a shielding gas in both GMAW and FCAW. Other than carbon dioxide, reactive gases are not used alone as shielding gas. The gases and gas mixtures commonly used for GMAW and FCAW–G are discussed in the following paragraphs. Argon Argon causes a squeezing (constricting) of the arc. Spray transfer occurs when welding current is above the tran- sition current. The results are a high-current-density (concentrated) arc, deep penetration, a narrow bead, and almost no spatter. The deep penetration is along the centerline of the weld bead. A cross section, shown in Figure 8-31, shows this deep, fi ngerlike penetration. Argon ionizes more easily than helium, and ionized argon conducts electricity. Therefore, lower arc voltages are required for a given arc length. Argon has a low ther- mal (heat) conductivity, so it conducts heat through the arc more slowly than helium. It is an excellent choice for use on thin metal. Argon is also good for out-of-position welds because of the low voltages required. Argon is the inert gas most commonly used for welding nonferrous metals. It is used for all types of metal transfer. When welding steel and steel alloys with spray transfer, use a high percentage of argon— 90% or greater. An erratic arc can result if pure argon shielding gas is used for welding carbon steel. Pure argon results in undercutting when the spray transfer method is used. Because undercutting is not acceptable, argon is usu- ally mixed with small amounts of oxygen or carbon dioxide. Since argon is heavier than helium, less gas is needed to protect a weld. Helium Helium (He) is an inert gas with a high thermal con- ductivity. It transfers heat through the arc better than argon. Because it transfers so much heat to the weld, helium is used to weld thick nonferrous metal sections. This gas is also used to weld metals that conduct heat well. Metals such as aluminum, magnesium, and cop- per conduct heat away from the weld zone rapidly and therefore require more heat to be applied to the weld. Argon Argon + O2 Helium Argon Carbon Dioxide or + Argon + CO2 Helium Goodheart-Willcox Publisher Figure 8-31. Bead contours and penetration shapes that occur with various gases and DCEP polarity.