Troubleshooting in aluminium welding

There is no question that the use of aluminium is increasing  within the welding fabrication industry. Manufacturers often adopt this material either through innovation, or as a result of pressure applied by their end users. The unique characteristics of  aluminium—light weight, excellent corrosion resistance, high strength, high toughness, extreme temperature capability, versatility of extruding and recycling capabilities—make it one of the current favoured choices of material for many engineers and designers for a variety of welding fabrication applications.

Because of the increased use of aluminium as a manufacturing material, the conversion from steel to aluminium within the welding fabrication industry is becoming increasingly common. The successful conversion from steel to aluminium welding is largely dependent on an understanding of the fundamental differences between these two materials. I have selected some of the most common problems that are encountered when changing to aluminium welding, such as feedability, porosity, cracking and filler alloy selection.

Feedability
This is the ability consistently to feed the spooled welding wire when MIG welding without interruption during the welding process. Feedability is probably the most common problem when changing to the MIG welding of aluminium. Feedability is a far more signifi-cant issue for aluminium than it is for steel. This is primarily due to the difference between the mechanical properties of the material. Steel welding wire is comparatively rigged and can withstand far more mechanical abuse. Aluminium is softer, more susceptible to deformation or shaving during the feeding operation and consequently requires far more attention when selecting and setting up a feed 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 feedability problems of this kind, it is important to understand the entire feed system and its effect on aluminium welding wire. If we start with the spool end of the feed system, we must first consider the brake settings.
Brake setting tension needs to be reduced to a minimum. Only sufficient brake pressure to prevent the spool from free-wheeling when welding stops is required. Inlet and outlet guides, as well as liners, which are typically made of metallic material for steel welding, must be made of a non-metallic material such as teflon or nylon to prevent the abrasion and shaving of the aluminium wire. Drive rolls should have a proper U-type contour with edges that are chamfered not sharp and they should also be smooth, aligned and have the correct drive roll pressure, as excessive drive roll pressure can distort the aluminium wire and increase friction drag through the liner and contact tip.

Contact tip I.D. and quality are of great importance. If the I.D. is too large and there is too much clearance between the wire and the contact tip, arcing can occur. Continuous arcing inside the contact tip can cause a build-up of particles on the inside surface of the tip which increases drag and produces burn-backs.
The deburring and polishing of new contact tips and repolishing or changing contact tips when unsteady feed is noted can improve overall performance.
Aluminium welding wire is used in both push and pull feeder systems; however, limitations are recognized depending on the application and feed distance. Push-pull feeder systems for aluminium were developed to help overcome feed problems and they are typically used in more critical/specialized operations such as robotic and automated applications

Porosity
Porosity is a result of hydrogen gas becoming trapped within solidifying aluminium during welding and leaving voids in the completed weld. Hydrogen is highly soluble in molten aluminium, as seen in Fig 1, and for this reason the potential for excessive amounts of porosity
during the arc welding of aluminium is considerable.
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 substances such 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) which contains hydrogen may be introduced into the welding area through water leakages within the welding equipment cooling system, insufficiently pure shielding gas, condensation on plate or wire from high humidity and changes in temperature (crossing a dew point) and/or hydrated aluminium oxide. Aluminium has a protective oxide layer and this coating is relatively thin and naturally forms on aluminium immediately. Correctly stored aluminium with an uncontaminated thin oxide layer can be easily welded with the inert-gas (MIG and TIG) welding processes which break down and remove the oxide during welding.
Potential problems with porosity arise when the aluminium oxide has been exposed to moisture. The aluminium oxide layer is porous and can absorb moisture, grow in thickness and become a major problem when attempting to produce welds that need to be relatively
free from porosity. When designing welding procedures intended to produce low levels of porosity, it is important to incorporate degreasing and oxide removal. This is typically achieved through a combination of chemical cleaning and/or the use of solvents to remove hydrocarbons, followed by stainless steel wire brushing to remove aluminium oxide. The correct cleaning of the aluminium parts prior to welding, the use of proven procedures, well-maintained equipment, high-quality shielding gas and a welding wire which is free from contamination all become very important variables if low porosity levels are desirable. Porosity is typically detected by the radiographic testing of completed welds. However, there are other methods that can be used without radiography equipment to evaluate porosity levels on test plates. The nick brake test for fillet welds can be extremely useful on test plates when evaluating a new cleaning method and during preliminary procedure development.

Cracking
One problem that is frequently encountered when welding aluminium is solidification cracking or hot cracking. This form of cracking in aluminium is typical ly caused by a combination of metallurgical weakness in the weld metal as it solidifies and 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 the 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 can be achieved by the reduction of restraint which may be a result of excessive fixturing and/or also through the use of filler alloys which have lower melting and solidification points than the base alloy and/or smaller freezing temperature ranges.  The method for ensuring the avoidance of the critical chemistry range is based on an 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 aluminium filler alloy/base alloy mixture (Al-Si) of between 0.5 and 2.0% produces a weld metal composition which is crack sensitive. A weld with
this chemistry usually cracks during solidification. Care must be exercised if welding a 1xxx series (pure aluminium) base alloy with a 4xxx series (aluminium-silicon) filler alloy, in order to prevent a weld metal chemistry mixture within this crack-sensitive range.
As can be seen from the chart, copper in aluminium alloys (Al-Cu) exhibits a wide range of crack sensitivity.
Magnesium in aluminium from 0.5 to 3.0% produces a weld metal composition which is crack sensitive and should be avoided. Another issue relating to the aluminium-magnesium base alloys which is not directly related to the crack sensitivity chart but is a very important factor must be addressed. As a rule, the Al-Mg base alloys with less than a 2.8% Mg content can be welded with either the Al-Si (4xxx series) or the Al-Mg (5xxx series) filler alloys, depending on weld performance requirements. The Al-Mg base alloys with more than about 2.8% Mg cannot normally be successfully welded with the Al-Si (4xxx series) filler alloys. This is due to a eutectic problem associated with excessive amounts of magnesium silicide Mg2Si developing in the weld structure, thereby reducing ductility and increasing crack sensitivity.
Perhaps the most common problem associated with hot cracking and the critical chemistry issue is associated with the aluminium, magnesium, silicon alloys (AlMg2Si) or 6xxx series base alloys, as they are known. As purchased, the 6xxx series base alloys, 6061, for example, contain around 1.0% magnesium silicide Mg2Si and, as the chart shows, this is the worst condition, producing maximum crack sensitivity. These base alloys typically crack if they are not welded with sufficient filler alloy additions in order to change their chemistry and reduce their hot-cracking sensitivity. The 6xxx series alloys can be welded with 4xxx series (Al-Si) or 5xxx series (Al-Mg) filler alloys, depending on weld performance requirements. The main consideration is adequately to dilute the percentage of Mg2Si in thebase material with sufficient filler alloy to reduce weld metal crack sensitivity. Care must also be taken when welding the 6xxx series base alloy with the 5xxx (Al-Mg) filler alloys to ensure sufficient additions of filler alloy to prevent the Al-Mg crack sensitivity chemistry range. These types of chemistry cracking problems are usually addressed through weld joint design to ensure maximum filler alloy dilution through increased bevel angles and joint spacing.
Another type of cracking in aluminium is crater cracking or termination cracking. This type of cracking is experienced at the end of the weld and is best reduced by using weld stopping techniques. One method is to remove the crater from the functional area of the weld by using run-off plates which are mechanically removed after welding. Other generally more practical methods are to reduce the size of the weld pool just before the arc is extinguished, so that there is no longer enough shrinkage stress to form a crack.
Some modern welding machines have been developed for aluminium welding and have a built-in crater fill function which is designed to terminate the weld in a gradual manner, thereby preventing a crater from forming at weld termination and thereby eliminating the crater cracking problem

Filler alloy selection

When welding steel, the selection of a filler alloy is of-ten based on the tensile strength of the base alloy alone. The selection of a filler alloy for aluminium is not normally that simple and is usually not simply based on the tensile strength of the completed weld. With aluminium, there are a number of other variables that need to be considered during the filler alloy selection process. An understanding of these other variables and their effect on the completed weldment is of vital importance.
When choosing the optimum filler alloy, both the
base alloy type and the desired performance of the weldment must be areas of prime consideration. What is the weld subjected to and what is it expected to do? The most reliable method of choosing an aluminium filler alloy for evaluation is to use 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 as follows.
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 cracksensitive 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. In most cases involving aluminium, the heat affected zone (HAZ) of a groove weld dictates the strength of the joint and many filler alloys can often satisfy this strength requirement. Unlike groove welds, the joint strength of fillet welds is based on shear strength which can be significantly affected by filler alloy selection. Fillet weld strength is largely dependent on the composition of the filler alloy used to weld the joint. The 4xxx series filler alloys generally have lower ductility and provide less shear strength in fillet-welded 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 flow plastically before fracturing. Fracture characteristics are described in terms of the ability to undergo elastic stretching and plastic deformation in the presence of stress raisers (weld discontinuities). Increased ductility ratings for a filler alloy indicate a greater ability to deform plastically and to redistribute loads, thereby reducing 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° F, we must consider the use of filler alloys which can operate at these temperatures without any undesirable effects on the welded joint. Aluminium/magnesium alloys with more than 3% Mg which are exposed to elevated temperatures can produce a segregation of magnesium at the grain boundaries of the material. This is an undesirable condition which can result in the premature failure of a welded component. Consequently, alloys with less than 3% Mg have been developed for high-temperature applications.
Corrosion resistance - most unprotected aluminium base alloy/filler alloy combinations are quite satisfactory for general exposure to the atmosphere. In cases in which a dissimilar aluminium 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 it comes to service in specialized highly-corrosive environments and may necessitate consultation with engineers from within this specialist
field.
Colour match after anodizing - the colour of an aluminium alloy when anodized depends on its composition. Silicon in aluminium 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 discolour during anodizing, so a good colour match is obtained.

Post-weld heat treatment - typically, the common heat-treatable base alloys, such as 6061-T6, typically 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 that is 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 post-weld, heat-treated condition. Other filler alloys which are designed to respond to thermal post-weld treatment, particularly for use with the heat-treatable casting alloy, have been developed. The important thing to remember here is that the common filler alloys may not respond or may even respond adversely to post-weld thermal treatments.

Conclusion

  • I have attempted to provide information in this article which I hope will assist with an understanding of the
  • differences and concerns when welding aluminium compared with other materials. The ability successfully
  • to weld aluminium is not so much difficult as it is different. In my view, an understanding of the differences is
  • the first step towards producing successful welding procedures for this somewhat unique material, the use of
  • which is continuing to advance within the welding fabrication industry.
by Tony Anderson, Technical Services Manager—AlcoTec Wire Corporation, USA
source: api-iws.org

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