Selecting filler metals for fusion welding aluminium

TWI Bulletin, November/December 1990

Mike Gittos is a Principal Metallurgist in the Materials Department. He graduated from Manchester University, joining the then Metallurgy Department at TWI in 1974. Although most of his work has been concerned with thewelding of non-ferrous metals, his published work also relates to ferritic and stainless steels as well as plastics materials.

Mike Scott graduated from Cambridge with a degree in natural sciences, specialising in metallurgy. In his first post, with the Royal Naval Scientific Service, he worked on electroplating applied to manufacturing radar valves and ancillary devices, and on the manufacture of ceramic ferrite.

From there he moved to the Central Electricity Research Laboratory where he worked on the metallurgy of magnesium alloys used for sheathing uranium fuel elements for nuclear power stations. In 1960, he joined the then BritishWelding Research Association, where he spent many years as leader of the Non-Ferrous Group in the Materials Department. On retirement he was a consultant within that Department. During his time at Abington, he worked on various aspectsof joining most of the common non-ferrous metals and their alloys and was actively involved in microjoining.

Mike Gittos and Mike Scott explain how selecting the correct filler helps to achieve sound welds when arc welding aluminium alloys.

Wrought aluminium alloys are divided into eight classes in the international Aluminum Association four-digit system, see Table 1. Of the eight alloy classes, 1XXX, 3XXX and 5XXX alloys are readily arc weldable, 6XXX and 7XXX are weldable with reservations, while 2XXX alloys are not normally welded. 4XXX alloys are not used in forms likely to be welded; in the welding context they occur mainly as filler metals. The only 8XXX alloys of any importance in welding are those containing lithium, eg 8090; these are, like the 7XXX alloys, weldable with reservations.

Table 1: Wrought alloy groups

Casting alloys are not covered by any international designation system. Most contain silicon, often with added copper or magnesium; in some alloys, copper or magnesium are the primary additions. In general terms, the more copper an alloy contains, the less weldable it will be. However, there is an added factor when assessing the soundness of welds in castings: the method of casting. Pressure die castings normally have a high gas content which can cause serious weld porosity. In a somewhat similar vein, wrought alloys produced by compacting powder have a high oxide content which tends to cause unsoundness.

One of the major advantages of aluminium alloys is their resistance to corrosion, which is conferred by their tenacious oxide film. However, this film is something of a disadvantage in welding since it can cause lack of fusion andmust therefore be disrupted. This can be done by using a flux, as in gas welding or manual metal arc welding, but suitable fluxes are corrosive, and their residues must be removed afterwards. The most common welding processes for aluminium alloys are, therefore, the inert gas-shielded processes, TIG and M1G. The oxide is disrupted by the action of the arc and prevented from re-forming by the inert gas cover.

Weld soundness

Filler-metal selection controls weld soundness through its influence on weld cracking phenomena, but a ubiquitous feature of aluminium welds is porosity. Hydrogen is much less soluble in solid than liquid aluminium and, since it is frequently present in the molten pool (originating principally from contamination of the surfaces of the wires and workpieces), pores form as it is rejected during solidification.

Porosity has a relatively minor effect upon mechanical properties - indeed, in many cases, the limitation upon the amount allowed can justifiably be set at the level where it would mask other, more harmful, flaws on a radiograph.However, it should also be borne in mind that porosity is often an indication of poor workshop practice. Low levels of porosity can be achieved by careful attention to pre-weld cleaning and gas shielding. Provision of inert gas backingto the root of aluminium welds is not usually necessary but it has been found desirable for the lithium-bearing alloys.

By far the most serious defect likely to occur in aluminium alloy welds is cracking, of which there are two forms. The more common is solidification cracking in the weld metal. As the name suggests, this occurs while the weld metal is solidifying. Characteristically, it is located along the weld centreline, although other positions are known. It is probably seen most often in craters at the ends of runs. The composition of the weld metal is the most important factor affecting the probability of such cracking, although other factors, eg weld shape, also have an effect.

The Figure shows the typical relationship between crack sensitivity and alloy content.

In general terms, the addition of small quantities of alloying elements to aluminium - eg magnesium, silicon and copper -produces alloys which are sensitive to cracking. The normal solution is to modify the weld metal composition by using filler metals of higher alloy content.

In the heat-treatable alloys, liquation cracking can occur in the heat-affected zone. The severity of such cracking is variable but it is caused by melting at grain boundaries: if these boundaries are still molten when the weld metal solidifies, the resultant strain can open them up. In some cases, the risk of such cracking can be reduced by the appropriate choice of filler metal, eg by using a 4XXX filler metal for welding 6XXX alloys.

Filler metals

Filler metals for the inert gas shielded process are specified in BS 2901: Part 4: 1990 and the range of alloys is given in Table 2. There is no British Standard for manual metal arc electrodes for welding aluminium and its alloys and, although electrodes are commercially available (principally aluminium-silicon types), they are not widely used.

Table 2: Al-alloy filler metals to BS 2901: Part 4: 1990

As already noted, the choice of filler metal is influenced primarily by the parent metal. Fortunately, many parent metals are sufficiently similar in composition for them to be grouped as in Table 3. Similarly for most purposes, the filler metals can be grouped as in Table 4: where this is not true, it is noted in Table 5, which gives the appropriate filler metal group for both similar and dissimilar welds. It has been kept as simple as possible butgeneralisations as to the type of filler metal required have been made in order to achieve this. Thus the filler metals nominated are those most suitable for general use; they are not necessarily the only applicable types and, where there are special requirements - eg application of post-weld heat treatment or service in aggressive environments - other choices might be more appropriate.

Table 3: Parent metal groups used in Table 5

Table 4: Filler metal groups used in Table 5

*4047A is specifically used to prevent weld metal cracking in joints involving high dilution and restraint. In most other cases, 4043A is preferable.

Similar alloys

Pure grades of aluminium (1XXX series) are usually welded either with a matching grade of pure aluminium or with one of higher purity. This optimises corrosion resistance, which is usually the important criterion when these materials are welded. 3103 and 3105 are best welded with 3103, although 4043A or pure aluminium can be used and are likely to be more readily obtainable: 4043A will give higher strength; pure aluminium better corrosion resistance.

Lean 5XXX alloys are sensitive to solidification cracking and are often welded with the more highly alloyed aluminium-magnesium group of fillers (Table 4). However, corrosion resistance considerations may demand that the magnesium content of the weld is restricted. For example, 5454 (up to 3% magnesium) was developed for resistance to stress-corrosion cracking and, to maintain this, it is normally welded with the matching 5554 filler. For 5005, the magnesium content is sufficiently low (0.8%) for it to be welded with 4043A filler which may give a higher resistance to cracking than the aluminium-magnesium group.

The specifications of the aluminium-magnesium group fillers overlap to some extent but the strength of the weld metals made with 5556A or 5183 will tend to be greater than those made with 5056A and 5356. This is because the former contain a higher and mandatory level of manganese, and 5556A will tend to have a higher magnesium content. In particular, 5556A was specifically developed to match the strength of 5083 and is therefore the traditional choice for this widely-used structural alloy.

Because of the combination of a high susceptibility to weld metal cracking and a severe loss of strength in the weld zone, the high-strength heat-treatable 2XXX alloys are not generally recommended for welding. If the strength penalty can be accepted, 4047A gives the best chance of avoiding cracking.

Heat-treatable alloys in the 6XXX and 7XXX series also suffer reductions in strength in the weld zones compared with the fully heat-treated parent metals. 7XXX alloys have a greater potential to recover strength by ageing after welding than those of the 6XXX series and higher joint strengths may therefore be obtained. The choice of filler metal for 6XXX alloys lies between the aluminium-silicon and aluminium-magnesium groups. Broadly, weld metal solidification and HAZ liquation cracking are resisted better by aluminium-silicon filler metals but aluminium-magnesium weld metals are more ductile and give a better colour match when anodised after welding. The aluminium-zinc-magnesium alloy 7020 should be welded with aluminium-magnesium group alloys, 5556A being the most appropriate from the point of view of weld strength.

Casting alloys

Some casting alloys can be welded to themselves or to wrought alloys as recommended in Table 5 although in some cases, where the alloy is heat treatable, the mechanical properties will be affected. The compositions of most other alloys are inherently crack sensitive and such alloys are not recommended for welding, although it is often possible to repair castings, eg to remove shrinkage defects, with filler of the same composition as the parent metal.

Table 5: Selection of filler wires and rods

Table 5 - notes:

1. The welding of alloys containing approximately 2% or more of Mg with Al-Si filler metal (and vice versa) is not recommended because sufficient Mg ₂Si precipitate is formed at the fusion boundary to embrittle the joint.
2. 2XXX alloys covered by British Standards are not regarded as weldable, but 4047A gives the best chance of success.
3. The corrosion behaviour of weld metal is likely to be better if its alloy content is close to that of the parent metal and not markedly higher. Thus for service in potentially corrosive environments it is preferable to weld 5154A with 5154A filler metal or 5454 with 5554 filler metal. Sometimes this is possible only at the expense of weld soundness so that a compromise will be necessary.
4. The choice of filler is discussed more fully in the text.
5. For welding 1080A to itself, 1080A filler metal should be used.
   

Dissimilar alloys

In deciding whether dissimilar materials can be joined, an important consideration is that the joints between alloys high in silicon and magnesium are likely to be unsuccessful since, even if resistant to solidification cracking, extensive formation of the Mg ₂Si phase at the edge of the weld causes embrittlement.

If Industrial Members have any questions or doubts about the appropriate filler for their particular applications, TWI will be pleased to offer specific advice on this or any other aspects of the fabrication.