Reproduced courtesy of the Welding Design and Fabrication Magazine  

These are guidelines for the selection of the right equipment for welding, applying the right technique, preparation of base materials and examining the weldments visually so as to ensure high-grade gas metal and also tungsten inert gas welding on any aluminum alloy.  Welding of aluminum alloys may prove to be quite challenging for even those experienced welding steels. Due to the lesser melting point and its greater thermal conductivity, aluminum alloys are easily prone for burn-through provided welders carry out specific procedures. Besides feeding the aluminum welding wires during the process of (GMAW) gas-metal-arc welding is a test since the wire has a lesser column strength, happens to be softer than steel and is prone to get entangled in the drive’s roll. In order to get over these difficult issues, operators would find that following the thumb rules and selection of equipment guidelines given below very useful.

GMAW or Gas-Metal-Arc Welding

Preparation of the base-metal: While welding aluminum, it is very important that operators remember to get rid of any presence of aluminum oxide, the hydrocarbon pollution from the cutting solvents or oils and to always cleanse the material’s base. It is well known that aluminum oxide melts at 3,700 F on the surface of material whereas the aluminum of the material at the base beneath melts at 1,200 F. So, it is obvious that any oxide left on the exterior of the base material can cause obstruction for the penetration of filler metal right into the piece of work.

Aluminum oxides can be effectively got rid of with the help of a wired brush made of stainless steel bristles or using other etching solutions or solvents. If a stainless steel brush is being used, remember to brush it only in a particular direction and remember to brush gently since brushing roughly helps to further lodge the oxides inside the work-piece. It should also be kept in mind never to use a brush that has been used to clean carbon or stainless steel when cleaning out aluminum.  If chemical etching solutions are used, ensure that they are removed from work before beginning to weld. To reduce the danger of the hydrocarbons from the cutting solvents and other oils from getting into the weld, take them out with the help of a degreaser, after checking that it doesn’t have any hydrocarbons.

The Push Technique: While welding aluminum, it is better to keep the gun at a distance from the weld puddle than pulling it, as it gives better results for the cleaning action, reducing the weld contamination and improving the coverage of the shielding-gas.

Shielding Gas: Because of its penetration and high standard of cleaning action, Argon, happens to be the most popular and common shielding gas being used while welding aluminum. There will be minimum accuulation of magnesium oxide when welding with 5XXXX-series aluminum alloys, which is a mixture of shielding gas combining helium with argon – 75% helium maximum.

Preheating: This can help to prevent weld cracking an aluminum work-piece. Since the preheating temperature must never go over 230 F, monitoring it with an indicator which displays the temperature so that overheating can be avoided. Keeping tack welds both at the starting and ending parts that are to be welded, will go a long way to help in the effort of preheating.  Before welding aluminum to a thin piece, welders need to first heat, a rather thick piece of it and in case of a cold lapping, they should try utilizing the run-off and run-on tabs.

Welding Wires: Choose aluminum filler wires that have a melting temperature same as that of the base material. It the operator can judiciously bring down the range of the melting point of the metal, the easier will it be for the alloy to be welded. Procure wires that is either 3/64 or 1/16 inch diameter, since the bigger the diameter of the wire, the easier will it feed. A wire with a 0.035 inch diameter, in combination of a pulsed- welding process with a lesser speed of the wire-feed may ideally be between 100 and 300 inches/minute would be ideal to weld a thin-gage material.

The Convex-Shaped Welds: It is the crater cracking that causes most of the failures while welding aluminum. This cracking is due to the high-level rate of thermal expansion of aluminum along with the substantial contractions which occurs as the welds cool.  The chance to crack is highest with the concave craters, as the exterior of the craters tears and contracts while it cools. So, welders must buildup the craters so that it forms a mound or convex shape as this will make up for the forces of contraction while the weld becomes cool.

Travel Speed: While welding aluminum, the process is to be “fast and hot”. Contrary to steel, aluminum’s high-level thermal conductivity prescribes greater weld travel speeds, settings for voltage and the use of extra hot amperage. In case the travel speed happens to be very slow, then the welder will risk excessive burnthrough, especially when it comes to aluminum sheets having a thin-gage.

Selection of Power-Source: When deciding on a power source for the GMAW of aluminum, what has to be considered first is the system of pulse or transfer-spray-arc. Constant voltage (cv) and constant current (cc) machines may be utilized for welding spray-arc. The spray-arc absorbs a very small stream of the liquefied metal and then sprinkles it along the arc to the material’s base, from the wire of the electrode. For the thick aluminum which may require a welding current of over 350A, cc will provide the maximum results.

Pulse transfer is normally executed with the power supply from an inverter. The latest power supplies have filler-wire type and diameter and built-in pulsing procedures based on. With pulsed GMAW, a tiny droplet of filler metal gets transferred to the work-piece in the electrode with every pulse of current and creates a droplet transfer that is positive resulting in a low splatter and quicker follow speeds when compared to spray-transfer welding. Making use of pulsed GMAW method on aluminum helps to better control the input of heat, which eases the welding which is out-of-position and permitting the welder to weld in the material with a thin-gage at less wire-feed currents and speeds.

Wire Feeder: The push-pull system, which employs a wire-feed cabinet that is enclosed in order to protect the wire from the environment. It has proved to be the most preferred process when it comes to feeding soft aluminum wire over long distances. The constant-torque variable speed motor present in the cabinet of the wire feed helps to guide and  push the wire from one side to the other end of the gun with a steady speed and force. The high torque motor that is within the gun used for welding draws the wire into and out and help to keep the wire feed speed along with the length of the arc uniform. In some of the shops, welders utilize selfsame wire feeders for delivering both aluminum and steel wire.

In such a case, using Teflon or plastic liners can help to ensure a smooth and consistent aluminum wire feeding. Plastic tubes that are incoming and Chisel outgoing tubes can be used for guide tubes for supporting the wire to be as near as possible to the drive roll so that the wire is prevented from entangling. During the weld, the cable of the gun should be kept as parallel as possible so as to reduce the resistance of the wire feed. Check for the right alignment between the drive rolls along with the guide tubes so as to avoid aluminum shavings.   

Make use of drive rolls that are specifically meant for aluminum and set the drive roll tension for delivering a uniform rate of wire feed. If there is excessive tension, the wires will become deformed and lead to a rough and erratic feeding and too low a tension will result in uneven feeding. So, either of these conditions may lead to an arc that is unstable and to weld porosity.

Welding Guns: Separate gun liners need to be used for aluminum welding. For avoiding wire chaffing, restrain can be put on either ends of the liner so that the gaps between the gas diffuser on the gun and the liner can be eliminated. Changing liners frequently helps to reduce the potential for abrasive aluminum oxide which causes problems in wire feeding. Using a contact tip that is about 0.015 inch bigger that the filler metal’s diameter will help to heat the tip quickly thus expanding to an oval shape, probably restricting the wire feeding. Normally, when the welding current goes above 200 A, a water cooled gun is used so that it minimizes the building up of heat and thus lessens other wire feeding issues.