Wire Feed (MIG) So we now have our three wire feed welding processes and their accepted terminology as per the ANSI / AWS A3.

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1 Wire Feed (MIG) Weld Tech News VOL 1. NO. 10 WELD TECH NEWS is a newsletter for welders working primarily in maintenance and repair. Each issue contains useful information on materials (cast irons, steels, aluminum, copper alloys, etc.), welding products and welding techniques. By collecting each issue, the reader will soon have a handy reference manual covering all aspects of welding, brazing and soldering for maintenance repair. WIRE FEED Gas Metal Arc Welding (GMAW) When we speak of "wire feed welding" we can actually mean one of three distinct wire feed welding processes. Wire feed welding started evolving in the late 1940's as an outgrowth of the Tungsten Inert Gas or TIG welding process which had developed greatly during the Second World War. With TIG welding a gas shield is used with a non-consumable electrode. This process is relatively slow and the idea of using a continuously fed solid wire as the electrode and filler metal evolved as being faster. This too would be shielded from contamination from the atmosphere by an inert gas as in TIG welding. Hence the term MIG welding or Metal Inert Gas. This process became successful at welding non-ferrous metals such as aluminum but it took improvements in the welding wires, power sources and shielding gases before the process became popular for welding mild and low alloy steels. The use of C0 2 as a shielding gas for welding carbon steels became common in the 1950's with improvements in the process. This is not an inert gas so the term Metal Inert Gas or MIG welding is not appropriate. A shielding gas mixture of 75% argon and 25% C0 2 is now commonly used to give better arc characteristics. Adding a small percentage of oxygen to argon allowed the development of the spray type transfer. Neither of these gas mixes is inert. Thus the term MIG welding is considered a non-standard term. In 1957 flux core wire welding with the use of C0 2 shielding gas was introduced. In the early 1960's flux core welding wires that did not require the use of shielding gases were introduced. This welding process will be covered in another issue of Weld Tech News. So we now have our three wire feed welding processes and their accepted terminology as per the ANSI / AWS A standard: Gas Metal Arc Welding (GMAW) - an arc welding process that produces coalescence of metals by heating them with an arc between a continuous filler metal electrode and the workpiece. Shielding is obtained entirely from an externally supplied gas. 1

2 Flux Core Arc Welding (FCAW) - an arc welding process that produces coalescence of metals by heating them with an arc between a continuous filler metal electrode and the work. Shielding is provided by a flux contained within the tubular electrode. Additional shielding may or may not be obtained from an externally supplied gas or gas mixture. Flux core welding in effect counts for two different processes because we can have a flux core wire that requires a shielding gas or a flux core wire that is self- shielded. GMAW or solid wire welding is the process we will now focus on. One of the reasons for the popularity of the wire feed process is that the actual welding is usually easier for most people than welding with stick rod. Feeding the stick rod into the arc as it is consumed in a consistent manner to give a good quality weld takes a certain skill and coordination level. With wire feed the welder basically holds the gun a specific distance off the base metal and welds with the machine feeding the wire into the arc as it is consumed. The process also makes welding out of position, vertical or overhead, easier than with stick rod. The problem is that the setup to get to the point where actual proper welding occurs is more complex and has more variables with wire feed than with stick welding. Any welder can take a wire feed gun and run a weld bead that looks okay but if the variables are not properly set up it may not be an effective weld. With stick welding (SMAW), a constant current (CC) power source is used and in most cases the only adjustment the welder can make at the welding machine is amperage. The welder keeps the voltage constant by maintaining a proper and consistent arc length. With GMAW a constant voltage (CV) power source is used and it provides for adjustments to amps and volts. Most GMAW is done using direct current electrode positive (DCEP). Constant current power sources can be used for GMAW but require a special voltage sensing wire feeder. Other critical variables are stickout and shielding gases. All four of these basic variables are interrelated and must be understood to create a proper welding condition. VARIABLES 1. Amperage Amperage refers to the rate of current flow. Amperage and current mean the same thing. With a CV power source amperage for GMAW is controlled by wire feed speed. If wire feed speed is increased, amperage is increased. Amperage settings are basically a function of the metal thickness you are welding, which also determines the appropriate wire size. Adjustments for this are dials labeled either amps, wire speed or WFS and this control is usually on the wire feeder itself. If other factors are constant penetration is affected by the amperage setting. 2

3 2. Voltage Constant voltage welding machines used for GMAW set voltage. They are also sometimes referred to as constant potential welders. Arc voltage is one of the main variables that determines heat input into the base metal. The welding machine keeps the voltage level constant as the wire is consumed in the arc. As mentioned earlier in stick welding, the welder in effect keeps the voltage constant by maintaining a consistent arc length off the end of the rod. Increasing or decreasing the voltage at the power source will increase or decrease the arc length off the end of the wire. The voltage control is usually on the power source. For effective GMAW the amperage and voltage must be "in balance." To fine tune or set up for a particular welding application the best approach is to change only one of these variables at a time to achieve the optimum results. The particulars of an application can affect the amp and voltage settings. Fine tuning can be affected by metal thickness, type of joint to be welded, type of shielding gas, wire size, position of the weld, base metal temperature and type of base metal. 3. Stickout This is a "nonstandard" term for electrode extension, which is the length of unmelted electrode extending beyond the end of the contact tube. The wire extending beyond the end of the contact tube conducts the current to the arc. This current also causes the wire to be preheated and enter the weld puddle at a high temperature which increases the melt-off rate. Increasing the stickout increases the deposition rate if the wire feed speed is increased sufficiently to maintain the current. If the stickout is lengthened but the wire feed speed is not increased the current will be reduced. Among other things this can lead to reduced penetration. Changing the stickout after optimum parameters have been set will require a change in those optimum amperage and voltage parameters. 4. Shielding gases The first question to be asked when determining which shielding gas to use is what is the base metal we are welding. There are four gases commonly used for the GMAW welding process. Two of the gases, argon and helium, are inert which means they are not chemically reactive to anything and work to shield the arc and base metal from contamination from the atmosphere. Argon is especially well suited to this because it is heavier than air and blankets the weld zone. The two other shielding gases are carbon dioxide (weld grade) and oxygen. These are not inert gases but reactive gases and when combined with the inert gases those various mixed gases are not inert gases either. C0 2 when used on carbon steels can actually introduce carbon into the weld which on many carbon steels is not detrimental. A common mix of 75% argon and 25% C0 2 is used because it provides good arc stability, good bead appearance and minimal spatter. 3

4 Because of the C0 2 this mix is not good for most stainless steels. Stainless is often welded with argon with a small addition of O 2 (98-2) which serves to help stabilize the arc. Non-ferrous metals such as aluminum are welded with the inert gases. Helium is sometimes combined with argon because it has better thermal conductivity. The type of shielding gas used greatly affects the arc characteristics and the type of metal transfer we can use to weld. METAL TRANSFERS There are four types of metal transfer used for GMAW welding. They are short circuit, globular, spray and pulsed arc. Each type of transfer has specific advantages and applications. The type of transfer is a function of the welding amperage, voltage, wire diameter and shielding gas. Spray transfer requires that at least 80% of the gas mix be argon. Since pulsed arc requires complex equipment and is not commonly used for repair welding it won't be covered here. Short-circuit transfer produces the lowest heat input because it uses the lowest amps and volt settings of GMAW. The weld puddle is small and fast freezing. Shielding gas is usually 100% C0 2 or 75% argon and 25% C0 2. Tri-mix gases of helium, argon and C0 2 ( ) are used on stainless. This transfer works well on materials 3/16" or thinner, or out of position welding and for poor fit up applications. Stickout should be kept to a minimum and variation avoided. The metal transfer occurs when the wire is in contact with the weld puddle and a short circuit occurs. This happens from 50 to 250 times per second. Globular transfer occurs at amp and volt settings above those used for short-circuit transfer but less than those used for spray. All types of gas are used. 100% C0 2 or 75% argon and 25% C0 2 are commonly used for steel. It produces deeper penetration than short-circuit and is good for metal thickness greater than 1/4". Stickout of 3/8" to 1/2" are usually used. The transfer produces more spatter and is more difficult for out of position welding. The transfer occurs when a droplet larger than the electrode forms at the end of the wire and transfers across the arc due to the force of gravity. Spray transfer occurs at higher amp and volt settings and commonly uses 98% argon-2% oxygen for both carbon and stainless steels. It produces high deposition rates and is used on thicker materials. Spatter is minimal with this mode. It is difficult to use for out of position applications. Stickout is usually 1" to 1-1/4" in length and a push technique is used. Transfer occurs when numerous small droplets are formed at the tip of the wire and rapidly pinch off across the arc. 4

5 WIRES FOR GASS MEATAL ARC WELDING Brutus-MIG (130,000 p.s.i.) can be used to join and build up all types of steels including dissimilar steels such as carbon to stainless. It is also ideal for welding unknown types of steels. It provides the highest strength, ductility, corrosion resistance and is machinable. Requires argon-oxygen or Tri-mix gas for shielding. 5

6 Polaris-MIG (100,000 p.s.i.) is ideal for welding low alloy, high strength, heat-treatable low alloy and other problem type steels. It provides superior high strength, impact resistance and excellent out-of-position running characteristics. Excellent for sulfur bearing and free machining steels. Tartan-MIG (80,000 p.s.i.) is a deoxidized copper-flashed wire that is alloyed with silicon, manganese, zirconium, aluminum and titanium. This makes it ideal for dealing with rusty, dirty metals while still providing high strength, high elongation welds. Its fast-freezing weld puddle makes it ideal for out-ofposition welding. For welding mild and medium alloy steels. Neptune-MIG (40,000 p.s.i.) provides high strength welds for the widest range of aluminum alloys. It exhibits good ductility and may be anodized. The alloying avoids porosity and requires minimal skill. Suitable for aluminum sheets, extrusions, pipe and castings. Venus-MIG (60,000 p.s.i.) is for welding bronze, brass, copper, cast iron, steel and dissimilar combinations of these metals. It provides a machinable, low friction long wearing deposit for overlays. Excellent for welding galvanized steel with a corrosion resistant deposit. Requires argon or an argonhelium mix for shielding. Gemini-316L-MIG (88,000 p.s.i.) is for welding type 316L stainless and most other stainless types. It is a high alloy molybdenum bearing stainless that is a superior choice for corrosion resistant deposits. It is especially resistant to cracking. It can be used to weld 304 and 308 type stainless. For more specific information on these or other products, refer to the Rockmount Research & Alloys Welding Manual. 6