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Without doubt, the use of aluminum is increasing within the welding fabrication industry. Manufacturers are often adopting this material, either through innovation, or through pressure applied by their end users. Aluminum’s unique characteristics of being light weight, and having excellent corrosion resistance, high strength, high toughness, extreme temperature capability, versatility of extruding, and recycling capabilities make it one of today’s favored choices of material for many engineers and designers for a variety of welding fabrication applications. Because of the increased use of aluminum as a manufacturing material, the conversion from steel to aluminum within the welding fabrication industry is becoming increasingly common.
The successful conversion from steel to aluminum welding is largely dependent on the understanding of the fundamental differences between these two materials. I have selected some of the most common problems encountered when moving to aluminum welding such as feedability, porosity, cracking and filler alloy selection.
Feedability: This is the ability to consistently feed the spooled welding wire during the welding process. Feedability is probably the most common problem experienced when moving from MIG welding of steel to MIG welding of aluminum. Choosing a welding wire of high quality and consistency, such as the AlcoTec product, can effectively minimize feedability problems. AlcoTec wire has exceptional surface smoothness, extremely tight diameter control, and excellent lubricity, created through the use of a patented feedability process. Feedability is a far more significant issue with aluminum than steel. This is primarily due to the difference between the material’s mechanical properties. Steel welding wire is rigged, can be fed more easily over a further distance, and can withstand far more mechanical abuse when compared to aluminum. Aluminum is softer, more susceptible to being deformed or shaved during the feeding operation, and consequently, requires far more attention when selecting and setting up a feeding system for MIG welding. Feedability problems often express themselves in the form of irregular wire feed or as burn-backs (the fusion of the welding wire to the inside of the contact tip). In order to prevent excessive problems with feedability of this nature, it is important to understand the entire feeding system and its effect on aluminum welding wire. If we start with the spool end of the feeding system, we must first consider the brake settings. Brake tension should be adjusted to a minimum. Only sufficient brake pressure to prevent the spool from free-wheeling when welding has stopped, is required. Electronic braking systems and electronic/mechanical combinations have been developed to provide more sensitivity within the braking system. Inlet and outlet guides, as well as liners, which are typically made from metallic material for steel welding, must be made from a non-metallic material such as nylon, to prevent abrasion and shaving of the aluminum wire. Drive rolls have been developed with U-type contours, edges that are chamfered and not sharp, that are smooth, aligned, and provide correct drive roll pressure. Excessive drive roll pressure can deform the aluminum wire and increase friction drag through the liner and contact tip. Contact tip I.D. and quality are of great importance. We are seeing the availability of contact tips made specifically for aluminum welding that have smooth internal bores and are absence of sharp burrs on the inlet and outlet ends of the tips which can easily shave the softer aluminum alloys.
Aluminum welding wire is used in both push and pull feeder systems; however, limitations are recognized dependent on application and feeding distance. Push-pull feeder systems for aluminum have been developed and improved upon to help overcome feeding problems and may be used on more critical/specialized operations such as robotic and automated applications. More recently, the planetary drive push-pull system (ESAB Mongoose System) has become popular for aluminum welding, providing an extremely positive feeding system capable of delivering aluminum wire over greater distances with minimum burn-back problems.
Porosity: Porosity is a result of hydrogen gas becoming entrapped within solidifying aluminum during welding and leaving voids in the completed weld. Hydrogen is highly soluble in molten aluminum (as seen in Fig 1) and for this reason, the potential for excessive amounts of porosity during arc welding of aluminum is considerably high. Hydrogen can be unintentionally introduced during the welding operation through contaminants within the welding area such as hydrocarbons and/or moisture. Hydrocarbons may be found on plate or welding wire that has been contaminated with such items as lubricants, grease, oil, or paint. It is important to understand the methods available for the effective removal of hydrocarbons and to incorporate the appropriate methods into the welding procedure. Moisture (H2O) contains hydrogen and may be introduced to the welding area through water leaks within the welding equipment cooling system, inadequately pure shielding gas, condensation on plate or wire from high humidity and change in temperature (crossing a dew point), and/or hydrated aluminum oxide.
Aluminum has a protective oxide layer. This coating is relatively thin and naturally forms on aluminum immediately. Correctly stored aluminum, with an uncontaminated thin oxide layer, can be easily welded with the inert-gas (MIG and TIG) welding processes that break down and remove the oxide during welding. Potential problems with porosity arise when the aluminum oxide has been exposed to moisture. The aluminum oxide layer is porous and can absorb moisture, grow in thickness, and become a major problem when attempting to produce welds that are required to be relatively porosity free. When designing welding procedures, intended to produce low levels of porosity, it is important to incorporate degreasing and oxide removal. Typically, this is achieved through a combination of chemical cleaning and/or the use of solvents to remove hydrocarbons followed by stainless steel wire brushing to remove aluminum oxide.
Cracking: A problem that can be easily encountered when aluminum welding is solidification cracking or the hot cracking problem. This form of cracking in aluminum is typically caused by a combination of metallurgical weaknesses of the weld metal as it solidifies with transverse stress applied across the weld. The metallurgical weakness is often a result of the wrong filler alloy / base alloy mixture, referred to as the critical chemistry range and the transverse stress from shrinkage during solidification of the weld. These cracks are called hot cracks because they occur at temperatures close to the solidification temperature. In order to reduce the possibility of hot cracking, we need to understand two issues: the reduction of transverse stresses across the weld and the avoidance of critical chemistry ranges in the weld. The reduction or redistribution of stresses on the weld during solidification may be achieved by the reduction of restraint which may be a result of excessive fixturing, and/or also through the use of filler alloys that have lower melting and solidification points than the base alloy and/or which have smaller freezing temperature ranges. The method of ensuring the avoidance of the critical chemistry range is based on the understanding of the relative crack sensitivity curves as seen in Fig 2. This chart shows the crack sensitivity curves for the most common weld metal chemistries developed during the welding of the base alloy materials.
Silicon in an aluminum filler alloy / base alloy mixture (Al-Si) of between 0.5 to 2.0 % produces a weld metal composition that is crack sensitive. A weld with this chemistry will usually crack during solidification. Care must be exercised if welding a 1xxx series (pure Aluminum) base alloy with a 4xxx series (Aluminum – Silicon) filler alloy, to prevent a weld metal chemistry mixture within this crack sensitive range.
Filler Alloy Selection: When welding steel, the selection of a filler alloy is often based on the tensile strength of the base alloy alone. The selection of a filler alloy for aluminum is typically not that simple and is usually not based solely on the tensile strength of the completed weld. With aluminum there are a number of other variables which need to be considered during the filler alloy selection process. The understanding of these other variables and their effect on the completed weldment are of extreme importance.
When choosing the optimum filler alloy, both base alloy type and desired performance of the weldment must be of prime consideration. What is the weld subjected to, and what is it expected to do? The most reliable method of choosing an aluminum filler alloy for evaluation is by using the AlcoTec filler alloy selection chart. The filler alloy selection chart is based on the application variables of the completed weld and rates each variable independently. Some understanding of how the recommendations for filler alloy evaluation within the chart were developed and the possible results of selecting the incorrect filler alloy may prove useful.
The variables which need to be considered during filler alloy selection are:
Ease of Welding (relative freedom from weld cracking) - This is based on the filler alloy / base alloy combination, its relative crack sensitivity and the critical chemistry ranges as discussed in the last section. This rating is based on the probability of producing a crack sensitive filler alloy / base alloy combination.
Strength of the Weld – This rating is based on the ability of the filler alloy to meet or exceed the strength of the as-welded joint. Most often with aluminum the heat affected zone (HAZ) of a groove weld dictates the strength of the joint, and often many filler alloys can satisfy this strength requirement. Unlike groove welds, the joint strength of fillet welds is based on shear strength that can be affected considerably by filler alloy selection. Fillet weld strength is largely dependent on the composition of the filler alloy used to weld the joint. Typically, the 4xxx series filler alloys have lower ductility and provide less shear strength in fillet joints. The 5xxx series fillers typically have more ductility and can provide close to twice the shear strength of a 4xxx series filler alloy in some circumstances.
Weld Ductility – Ductility is a property that describes the ability of a material to plastically flow before fracturing. Fracture characteristics are described in terms of ability to undergo elastic stretching and plastic deformation in the presence of stress risers (weld discontinuities). Increased ductility ratings for a filler alloy indicate greater ability to deform plastically and to redistribute load thereby decreasing the crack propagation sensitivity. Ductility may be a consideration if forming is to be performed after welding, or if the weld is going to be subjected to impact loading.
Service Temperature - When considering service at temperatures above 150 Deg F, we must consider the use of filler alloys that can operate at these temperatures without any undesirable effects to the welded joint. Aluminum / magnesium alloys of over 3 % Mg, that are exposed to elevated temperature, can produce segregation of magnesium at the grain boundaries of the material. This is an undesirable condition which can result in premature failure of a welded component. Consequently, both base alloys and filler alloys with less than 3 % Mg have been developed for high temperature applications.
Corrosion Resistance - Most unprotected aluminum base alloy filler alloy combinations are quite satisfactory for general exposure to the atmosphere. In cases where a dissimilar aluminum alloy combination of base and filler is used, and electrolyte is present, it is possible to set up a galvanic action between the dissimilar compositions. Corrosion resistance can be a complex subject when looking at service in specialized highly corrosive environments and may necessitate consultation with engineers within this specialized field.
Color Match After Anodizing - The color of an aluminum alloy when anodized depends on its composition. Silicon in aluminum causes a darkening of the alloy when chemically treated during the anodizing process. If 5% silicon alloy 4043 filler is used to weld 6061, and the welded assembly is anodized, the weld becomes black and is very apparent. A similar weld in 6061 with 5356 filler does not darken during anodizing, so a good color match is obtained.
Post Weld Heat Treatment – Typically, the common heat treatable base alloys, for example 6061-T6, lose a substantial proportion of their mechanical strength after welding. In order to return the base material to its original strength, it may be an option to perform post weld heat treatment. If post weld heat treatment is the option, it may be necessary to evaluate the filler alloy used with regard to its ability to respond to the heat treatment. Filler alloy 4643, for example, was developed for welding the 6xxx series base alloys and developing high mechanical properties in the postweld heat-treated condition. Other filler alloys have been developed that are designed to respond to thermal postweld treatment, particularly for use with the heat-treatable casting alloys. The important thing to remember here is that the common filler alloys may not respond or even respond adversely to postweld thermal treatments.
Conclusion: I have attempted to provide information within this article that I hope will assist with the understanding of the differences and concerns when welding aluminum compared to other materials. The successful welding of aluminum is not so much difficult as it is different. An understanding of the differences, is in my opinion, the first step in producing successful welding procedures for this somewhat unique material which is continuing to advance in use within the welding fabrication industry.