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What are unweldable aluminum alloys?

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Q - I have heard, on occasion, reference made to some aluminum alloys as unweldable. What does this mean?  Are there such aluminum alloys, and if so, what makes them unweldable?

A - I shall start by saying that the majority of aluminum base alloys can be successfully arc welded when using the correct welding procedures.  However, yes, there are some aluminum base alloys that are sometimes referred to as unweldable.  These groups of alloys, which we will further discuss, are typically well known as being unsuitable for arc welding, and for this reason are joined mechanically by riveting or bolting. Before we start examining the various reasons for the poor weldability of these alloys, we should start by considering the term unweldable.  This is a nonstandard term which is sometimes used to describe aluminum alloys that can be difficult to arc weld without encountering problems during and/or after welding.  These problems are usually associated with cracking, most often hot cracking, and on occasion, stress corrosion cracking (SCC).

When we consider the aluminum alloys that fall into this difficult-to-weld category, we can divide them into different groups.

We will first consider the small selection of aluminum alloys which were designed for machineability, not weldability.  Alloys such as 2011 and 6262 which contain 0.20-0.6 Bi, 0.20-0.6 Pb and 0.40-0.7 Bi, 0.40-.7 Pb respectively.  The addition of these elements (Bismuth and Lead) to these materials greatly assists in chip formation in these free machining alloys.  However, because of the low solidification temperatures of these elements, they can seriously reduce the ability to successfully produce sound welds in these materials.

There are a number of aluminum alloys that are quite susceptible to hot cracking if arc welded.  These alloys are usually heat treatable alloys and are most commonly found in the 2xxx series (Al-Cu) and 7xxx series (Al-Zn) groups of materials.

In order to understand why some of these alloys are unsuitable for arc welding (unweldable), we need to consider the reasons why some aluminum alloys can be more susceptible to hot cracking.

Hot cracking, or solidification cracking, occurs in aluminum welds when high levels of thermal stress and solidification shrinkage are present while the weld is undergoing various degrees of solidification.  The hot cracking sensitivity of any aluminum alloy is influenced by a combination of mechanical, thermal and metallurgical factors. 

A number of high-performance, heat treatable aluminum alloys have been developed by combining various alloying elements in order to improve the materials’ mechanical properties.  In some cases, the combination of the required alloying elements has produced materials with high hot cracking sensitivity.

Coherence Range

Perhaps the most important factor affecting the hot crack sensitivity of aluminum welds is the temperature range of dendrite coherence and the type and amount of liquid available during the freezing process.  Coherence is when the dendrites begin to inter-lock with one another to the point that the melted material begins to form a mushy stage.

The coherence range is the temperature between the formation of coherent interlocking dendrites and the solidus temperature.  This could be referred to as the mushy range during solidification.  The wider the coherence range, the more likely hot cracking will occur because of the accumulating strain of solidification between the interlocking dendrites.

The 2xxx Series Alloys (Al-Cu)

Hot cracking sensitivity in the Al-Cu alloys increases as we add Cu up to approximately 3% Cu and then decreases to a relatively low level at 4.5% Cu and above.  Alloy 2219 with 6.3% Cu shows good resistance to hot cracking because of its relatively narrow coherence range.  Alloy 2024 contains approximately 4.5% Cu which may initially encourage us to suppose that it would have relatively low crack sensitivity.  However, alloy 2024 also contains a small amount of Magnesium (Mg).  The small amount of Mg in this alloy depresses the solidus temperature, but it does not affect the coherence temperature; therefore, the coherence range is extended and the hot cracking tendency is increased. The problem to be considered when welding 2024 is that the heat of the welding operation will allow segregation of the alloying constituents at the grain boundaries, and the presence of Mg, as stated above, will depress the solidus temperature.  Because these alloying constituents have lower melting phases, the stress of solidification may cause cracking at the grain boundaries and/or establish the condition within the material conducive to stress corrosion cracking later.  High heat input during welding, repeated weld passes, and larger weld sizes can all increase the grain boundary segregation problem (segregation is a time-temperature relationship) and subsequent cracking tendency.

The 7xxx Series Alloys (Al-Zn)

The 7xxx series of alloys can also be separated into two groups as far as weldability is concerned.  These are the Al-Zn-Mg and the Al-Zn-Mg-Cu types.

Al-Zn-Mg Alloys such as 7005 will resist hot cracking better and exhibit better joint performance than the Al-Zn-Mg-Cu alloys such as 7075. The Mg content in this group (Al-Zn-Mg) of alloys would generally increase the cracking sensitivity.  However, Zr is added to refine grain size and this effectively reduces the cracking tendency.  This alloy group is easily welded with the high magnesium filler alloys such as 5356 which ensures the weld contains sufficient magnesium to prevent cracking.  Silicon-based filler alloys such as 4043 are not generally recommended for these alloys because the excess Si introduced by the filler alloy can result in the formation of excessive amounts of brittle Mg2Si particles in the weld.

Al-Zn-Mg-Cu Alloys such as 7075 have small amounts of Cu added.  The small amounts of Cu, along with the Mg, extend the coherence range and, therefore, increases the crack sensitivity.  A similar situation can occur with these materials as with the 2024 type alloys. The stress of solidification may cause cracking at the grain boundaries and/or establish the condition within the material conducive to stress corrosion cracking later. 

Be Aware:

It should be stressed that the problem of higher susceptibility to hot cracking from increasing the coherence range is not only confined to the welding of these more susceptible base alloys such as 2024 and 7075.  Crack sensitivity can be substantially increased when welding incompatible dissimilar base alloys (which are normally easily welded to themselves) and/or through the selection of an incompatible filler alloy.  For example, by joining a perfectly weldable 2xxx series base alloy to a perfectly weldable 5xxx series base alloy, or by using a 5xxx series filler alloy to weld a 2xxx series base alloy, or a 2xxx series filler alloy on a 5xxx series base alloy, we can create the same scenario.   If we mix high Cu and high Mg, we can extend the coherence range and, therefore, increase the crack sensitivity.