Chapter 8 Gas Metal and Flux Cored Arc Welding 201 Copyright Goodheart-Willcox Co., Inc. The droplets in spray transfer are squeezed off cleanly and transferred across the arc gap in a straight path. Spray transfer occurs only when at least 80% argon is used as the shielding gas. Most shielding gas mixtures used for spray transfer have at least 90% argon in the mixture. Common shield- ing gas mixtures for carbon and low-alloy steels are 98% Ar plus 2% O2, 95% Ar plus 5% O2, 95% Ar plus 5% CO 2 , and 90% Ar plus 10% CO 2 . The spray transfer method produces deep penetra- tion. The arc can be directed easily by the welder. This is because the arc and metal spray pattern are stable and concentrated. The weld pool is fl uid. Spray transfer is best done in the fl at or horizontal welding position and on metal over 1/8″ (3 mm) thick. See Figure 8-2 for the metal deposition rate possible with spray transfer. Spray transfer with metal cored (not fl ux cored) wire is different than spray transfer with solid wire. See Figure 8-8. In solid electrode wire, the droplets form at the center of the wire. With a metal cored wire, the droplets form around the outside of the electrode wire. The spray arc is a wider, cone-shaped arc. This produces an excellent weld bead. The penetration from spray transfer with metal cored wire is not as great as it is from spray transfer with a solid wire. The wider weld pool with slightly less penetration is excel- lent for bridging gaps in a weld joint. 8.2.4 Pulsed Spray Transfer Pulsed spray transfer gas metal arc welding is similar to the spray transfer method. See Figure 8-9. Electri- cal control circuits within the power source (welding machine) cause the current to pulse from a low cur- rent level to a high current level. There is an arc at the low current level, but no metal transfer takes place. At the high current level, spray transfer droplets leave the electrode and enter the weld pool. Background current is the low-level current that is present to maintain the arc. At regular intervals, the current is increased to the peak current. The peak cur- rent is greater than the transition current level. Since the background current is on for only a short time, no globular transfer occurs. During the peak cur- rent period, spray transfer occurs. In this method, the metal leaves the electrode in a spray of small droplets. Since this type of spray transfer does not occur continually, it is called pulsed spray transfer. In pulsed spray transfer, the rate of metal transfer increases and the droplet size decreases as the pulse frequency increases. See Figure 8-2 for the metal deposition rates possible with the pulsed spray transfer method. Basic welding machines with pulse capabilities allow the welder to select pulse frequencies of 60 or 120 pulses per second. Some machines allow the user to adjust the pulse frequencies. Pulse frequencies can be set from 30 to 400 pulses per second, depending on the manufacturer. Goodheart-Willcox Publisher Figure 8-8. Spray transfer with a metal cored electrode produces a wider weld pool. The droplets form on the outside of the cored electrode. Droplets form at a constant rate Shielding gas envelope Droplet forms Goodheart-Willcox Publisher Figure 8-9. Pulsed spray metal transfer method. Spray transfer only occurs during peak current.