Specific alloy metallurgy
3.4.1 Non-heat treatable alloys
126.96.36.199 Pure aluminium (1XXX series)
The principal impurities in ‘pure’ aluminium are silicon and iron, residual elements remaining from the smelting process. Copper, manganese and zinc may also be present in small amounts. The maximum impurity levels vary with the specified purity, e. g. 1098 (Al99.98) contains a maximum impurity content of 0.02%, comprising 0.010% Si max.,0.006% Fe max.,0.0035% Cu max. and 0.015% Zn max. The 1050 (Al99.5) alloy contains a maximum of 0.05% of impurities. In the high-purity grades of these alloys the impurities are in such low concentrations that they are completely dissolved. From the welding viewpoint the alloys can be regarded as having no freezing range and a single phase microstructure which is unaffected by the heat of welding. The less pure alloys such as 1200 (Al99.0) can dissolve only small amounts of the impurity elements and, as the metal freezes, most of the iron comes out of solution to form the intermetallic compound FeAl3. When silicon is present in more than trace quantities, a ternary or three-element compound, Al-Fe-Si phase, is formed. With higher silicon contents free primary silicon is formed. All of these phases contribute to an increase in strength, attributed to slight solution hardening and by a dispersion of the phases.
The effects of welding on the structure of a fusion welded butt joint in an annealed low-purity aluminium such as 1200 is to produce three distinct zones. The unaffected parent material will have a fine-grained structure of wrought metal with finely dispersed particles of Fe-Al-Si. The heat affected zones show no significant change in microstructure except close to the fusion boundary where partial melting of the low melting point constituents along the grain boundaries occurs, leaving minute intergranular shrinkage cavities that result in a slight loss of strength. There will also be a loss of strength in the cold work alloys where the structure has been annealed and softened. The weld metal has an as-cast structure. When the filler metal has the same nominal composition as the parent metal the low melting point constituents such as Fe-Al-Si are the last to solidify and will be located at the grain boundaries.
188.8.131.52 Aluminium-manganese alloys (3XXX series)
When iron is present as an impurity the solubility of manganese in aluminium is very low. The rate of cooling from casting or welding is sufficiently rapid for some manganese to be left in supersaturated solution. Further processing to provide a wrought product causes the manganese to precipitate as FeMnAl6, this precipitate giving an increase in strength due to dispersion hardening. Any uncombined iron and silicon impurities may be present as an insoluble Al-Fe-Mn-Si phase.
The weld zones are similar to those seen in pure aluminium, the only major difference being the composition of the precipitates. The heat of welding has the same effect on the structure as on pure aluminium, with the precipitates arranged along the grain boundaries and a loss of strength in the annealed regions of cold worked alloys.
The 3103 (AlMn1)alloy is more hot short (see Section 2.5) than pure aluminium, despite having a similar freezing range. In practice, however, hot cracking is rarely encountered. Those alloys containing copper (alloy 3003) or magnesium (alloys 3004, 3005 and 3105) are more sensitive to hot cracking. Weld cracking may be sometimes encountered when autogenous welding but this is easily prevented by the use of an appropriate filler metal composition.
184.108.40.206 Aluminium-silicon alloys (4XXX series)
The aluminium silicon alloys form a binary eutectic at 11.7% silicon with a melting point of 577 °C, the two phases being solid solutions of silicon in aluminium, 0.8% maximum at room temperature, and aluminium in silicon. There are no intermetallic compounds. Sodium may be added in small amounts to refine the eutectic and increase the strength by improved dispersion hardening. Iron, even in small amounts, can seriously degrade toughness although manganese may be added to reduce this effect.
The 4XXX series has very high fluidity and is extensively used for casting purposes, often being alloyed with copper and magnesium to provide some degree of precipitation hardening and with nickel to improve high temperature properties. Because of its high fluidity and low sensitivity to hot shortness it is commonly used as a weld filler metal.
220.127.116.11 Aluminium-magnesium alloys (5XXX series)
Up to about 5% magnesium can be dissolved in aluminium to provide a substantial amount of solid solution strengthening: the higher the magnesium content, the higher the strength. The amount of magnesium that can be dissolved under equilibrium conditions at ambient temperature is only some 1.4%, meaning that there is always a tendency for the magnesium to come out of solution when the higher magnesium content alloys are heated and slowly cooled. This reaction is very sluggish and welding processes do not cause any appreciable change in the microstructure except in the cold worked alloys where mechanical strength will be reduced.
The standard aluminium-magnesium alloys have iron and silicon as impurities and deliberate additions of around 0.4-0.7% of manganese to increase strength further, mainly by dispersion hardening. Chromium may be added in place of or in addition to manganese to achieve the same strength increase, 0.2% chromium being equivalent to 0.4% manganese. The iron forms FeMnAl6; the silicon combines with magnesium to form magnesium silicide, Mg2Si, most of which is insoluble.
The magnesium alloys may all have their microstructure changed by the heat of welding. The microstructure of a butt weld in 5083 (AlMg4.5Mn0.7) in the annealed condition, welded with a 5356 filler shows the following features. The parent metal will have a fine-grained structure composed of a matrix of a solid solution of magnesium in aluminium, dispersion strengthened with a fine precipitate of the compound Mg2Al3 together with coarser particles of Al-Fe-Si-Mn. In the HAZ where the temperature has been raised to around 250 °C further Mg2Al3 will be formed which may begin to coalesce and coarsen. Where temperatures begin to approach 400 °C some of the Mg2Al3 will be redissolved and closer to the weld, where temperatures are above 560 °C, partial melting occurs, causing some shrinkage cavitation. The weld metal is an as-cast structure of a supersaturated solution of magnesium in aluminium with particles of the insoluble inter - metallics such as Mg2Si. The cooling rates of the weld metal are generally fast enough to prevent the precipitation of Mg2Al3.
The strength of aluminium-magnesium weld metal is generally close to that of the annealed wrought parent metal of the same composition and it is not difficult to achieve joint strengths at least equal to the annealed condition. Butt joints in parent metal with more than 4% magnesium sometimes show joint strengths less than that of the annealed parent alloy. In MIG welding this may be due to the loss of magnesium in the arc and it may be advisable to use a more highly alloyed filler such as 5556 (AlMg5.2Cr).
5083 is normally welded with a filler metal of similar composition because the higher magnesium contents increase the risk of stress corrosion cracking. A continuous network of Mg2Al3 along the grain boundaries may make the alloy sensitive to stress corrosion in the form of intergranular corrosion. The alloy can be sensitised by prolonged exposure to temperatures above 80 °C. In service at or above this temperature in mildly corrosive environments the magnesium content should be limited to a maximum of 3%. Alloys for service in these conditions are generally of the 5251 or 5454 type, welded with a 5554 (AlMg3) filler metal. In multi-pass double-sided welds a 5% Mg filler may be used for the root passes to reduce the risk of hot cracking, followed by 5554 filler for the filling and capping passes.
The 5XXX alloys containing between 1% and 2.5% magnesium may be susceptible to hot cracking if welded autogenously or with filler metal of a matching composition. The solution is to use more highly alloyed filler metal containing more than 3.5% magnesium.
3.4.2 Heat-treatable alloys
18.104.22.168 Aluminium-copper alloys (2XXX series)
The aluminium-copper alloys are composed of a solid solution of copper in aluminium which gives an increase in strength, but the bulk of the strength increase is caused by the formation of a precipitate of copper alu - minide CuAl2. To gain the full benefits of this precipitate it should be present as a finely and evenly distributed submicroscopic precipitate within the grains, achieved by solution treatment followed by a carefully controlled ageing heat treatment. In the annealed condition a coarse precipitate forms along the grain boundaries and in the overaged condition the submicro - scopic precipitates coarsen. In both cases the strength of the alloy is less than that of the correctly aged condition.
The early aluminium-copper alloys contained some 2-4% of copper. This composition resulted in the alloys being extremely sensitive to hot shortness, so much so that for many years the 2XXX were said to be unweld - able. Increasing the amount of copper, however, to 6% or more, markedly improved weldability owing to the large amounts of eutectic available to back-fill hot cracks as they formed. The limit of solid solubility of copper in aluminium is 5.8% at 548 °C; at ambient this copper is present as a saturated solid solution with particles of the hardening phase copper alu - minide, CuAl2, within the grains as a fine or coarse precipitate or at the grain boundaries.
The effect of welding on the age-hardened structure is to re-dissolve the precipitates, giving up to a 50% loss in ultimate tensile strength in a T6 condition alloy. The weldable alloy 2219 (AlCu6) can recover some of this strength loss by artificial ageing but this is usually accompanied by a reduction in ductility. The best results in this alloy are obtained by a full solution treatment and ageing after welding, not often possible in a fully fabricated structure. The less weldable alloy 2014 (AlZnMgCu) may also be heat treated to recover some tensile strength but the improvement is not as great as in 2219 (AlCu6) and may exhibit an even greater reduction in ductility.
Filler metals of similar composition such as 2319 (AlCu6) are available and weld metal strengths can therefore be matched with the properties in the HAZ.
22.214.171.124 Aluminium-magnesium-silicon alloys (6XXX series)
The hardening constituent in 6XXX series alloys is magnesium silicide Mg2Si. These alloys contain small amounts of silicon and magnesium, typically less than 1% each, and may be further alloyed with equally small amounts of manganese, copper, zinc and chromium. The alloys are sensitive to weld metal cracking, particularly when the weld metal is rich in parent metal such as in the root pass of the weld. Fortunately the cracking can be readily prevented by the use of filler metals containing higher proportions of silicon such as 4043 or, with a slightly increased risk of hot cracking, the higher magnesium alloys such as 5356.
With these heat-treatable alloys the changes in the structure and mechanical properties, briefly discussed in Chapter 2, are complex and strongly dependent on the welding conditions employed. Welding without filler metal or with filler metal of parent metal composition is rarely practised because of the risk of weld metal hot cracking. A weld metal with a composition close to that of the parent metal may age-harden naturally or may be artificially aged to achieve a strength approaching, but never matching, that of the aged parent metal.
In the overheated zone in the HAZ closest to the fusion line, partial melting of the grain boundaries will have taken place. Temperatures have been high enough and cooling rates sufficiently fast that solution treatment has taken place, enabling some ageing to occur after welding. Adjacent to this is the partially solution-treated zone where some of the precipitates have been taken into solution, enabling some post-weld hardening to occur, but those not dissolved will have been coarsened. Outside this will be the overaged zone where precipitate coarsening has taken place and there has been a large drop in strength.
The strength losses in the 6000 alloys are less in the naturally aged metal than in the artificially aged alloys. The strength of the weld and HAZ in the artificially aged condition generally drop to match that of the naturally aged alloy with a narrow solution-treated zone either side of the weld and an overaged zone beyond this, which is weaker than the T6 condition. With controlled low-heat input welding procedures the strength of the weldment
will not drop to that of an annealed structure but will be close to that of the T4 condition.
126.96.36.199 Aluminium-zinc-magnesium alloys (7XXXseries)
7XXX series alloys may, from a welding point of view, be conveniently divided into two groups. The first group is the high-strength alloys containing more than 1% copper, normally used in the aerospace industry and joined by non-welding methods. The second group is the medium strength alloys which have been developed for welding.
Aluminium and zinc form a eutectic containing solid solutions of 83% zinc in aluminium and 1.14% aluminium in zinc. The addition of magnesium complicates the situation with additional ternary eutectics and complex intermetallics being formed, these intermetallics providing dispersion hardening and precipitates of composition MgZn2. Copper provides further precipitation hardening, forming CuAl2 and an intermetallic of the copper-zinc system.
Welding of the hardened high-strength alloys results in a major loss of strength, the high-strength alloys such as 7022 (AlZn5Mg3Cu) or 7075 (AlZn5.5MgCu1.6) in particular suffering a considerable reduction in strength. Although almost all of this strength loss can be recovered by a full heat treatment, the loss in ductility is so great that brittle failure is a real possibility. The alloys are also very prone to hot cracking and the combination of these adverse features is such that the high-strength alloys are rarely welded. Joining techniques such as riveting or adhesive bonding are often used to avoid these problems.
The lower-strength non-copper-containing alloys such as 7017 (AlZn5Mg2.5Mn0.7), 7020 (AlZn4.5Mg1) and 7039 (AlZn4Mg2.5Mn0.7) are more readily weldable. The response of these alloys is very similar to that of the 6XXX series, with a loss of strength in the heat affected zones, some of which can be recovered by suitable heat treatment. The alloys will age naturally but it may take up to 30 days for ageing to proceed to completion. The strength loss in the 7XXX alloys is less than that in the 6XXX series and this, coupled with the natural ageing characteristic, makes this alloy a popular choice for structural applications where on-site repair and maintenance work may be required.
One problem peculiar to the 7XXX series is that the zinc rapidly forms an oxide during welding, affecting the surface tension of the weld pool and increasing the risk of lack of fusion defects. This requires the use of welding procedures in which the welding current is some 10-15% higher than would be used for a 5XXX alloy. It has also been found to be beneficial to use a shorter arc than normal so that metal transfer is almost in the globular range.
188.8.131.52 Unassigned (or other alloys) (8XXX series)
The 8XXX series is used to identify those alloys that do not fit conveniently into any of the other groups, such as 8001 (Al-Ni-Fe) and 8020 (Al-Sn). However, contained within this 8XXX group are the aluminium-lithium (Al-Li) alloys, a relatively new family that gives substantial weight savings of up to 15% and a higher Young’s modulus compared with some of the other high-strength alloys. Each 1% of lithium added results in an approximate 3% reduction in weight. These advantages mean that significant weight savings can be achieved in the design of aerospace structures and that the very high-strength unweldable alloys, such as those in the 2XXX series, may be replaced by the weldable, lighter Al-Li alloys.
The Al-Li alloys generally contain some 2-3% of lithium and small amounts of copper and magnesium. They are fully heat treatable, with a number of different precipitates, the principal one being Al3Li. Typical of these alloys are 8090 (AlLi2.5Cu1.5Mg0.7Zr) and 8091 (AlLi2.6Cu1.9Mg 0.8Zr). Lithium has a great affinity for oxygen and this reactivity requires great care to be taken during any process that involves heating the alloy. These processes comprise melting, casting, high-temperature heat treatment and welding. Failure to remove the oxidised layer will result in gross porosity - some 0.2 mm should be machined off to be certain of complete removal. It may also be necessary to purge the back face of the weld with an inert gas to prevent oxidation and porosity. As with the 7XXX alloys the Al-Li alloys have a similar response to the heat of welding, losing strength in the HAZ, although a post-weld artificial ageing treatment can restore a large proportion of this strength.
A further family of alloys that may fall into this group once they have been assigned a designation are those containing scandium. These are new alloys, still to a great extent in the development phase. Scandium is a rare earth element that has been found to be highly effective in increasing strength by age hardening and by grain refinement, the latter being particularly useful in weld metal. Scandium is likely to be used in conjunction with other alloying elements such as zirconium, magnesium, zinc or lithium where tensile strengths of over 600N/mm2 have been achieved in laboratory trials.