The broadening of the range of aluminium extrusion alloys, with newly formulated compositions, is an extraordinary opportunity to satisfy the growing performances expected from extrusions in many industrial applications
by Giuseppe Giordano
In 2017 the different compositions of aluminium alloys registered at the Aluminum added up to 540, about 7 times as many as the 75 compositions present in 1954. These figures care relative to alloys with a “four digits” numerical denomination, larger numbers would be reached by including the fanciful denominations often used by the lager industrial groups. This situation certifies that the aluminium industry has always tried to develop new alloys capable of satisfying better the requirements imposed by final uses, and this of course also concerns extrusion.
Research and development of new alloys stems from the different demands of the final application and in an attempt to improve the material’s different performances.
Several objectives therefore become motivations for the research of new extrusion alloys, such as:
-obtaining semis with improved tensile properties
- achieving a greater stress resistance of derived products
- reduce wall thickness in hollow extrusions
- increasing extrusion speed
- optimizing hardening process
- improving the quality of the surface of extrusions.
The road map for the development of new alloys normally evaluates the choice of a new alloying element or new combinations of alloying elements without changing the alloy family, by studying the effect of minor anti-crystallizing elements.
Alloys with a new alloying element: aluminium-scandium alloys
Research on scandium as an alloying element in aluminium alloys began in the Sixties and Seventies. The positive effects derived from the addition of this element were soon evident, from the primary foundry operations where scandium behaves as an effective grain refiner. As may be seen in the binary state diagram shown in Figure 1, aluminium and scandium form a compound, Al3Sc. It is relatively simple to obtain saturated solid solutions at high temperature which may undergo heat treatment and turn into supersaturated solid solutions, giving rise to a hardening precipitation process due to natural and artificial aging. The presence of scandium in the alloy also enables interaction with zirconium. If small percentages of the latter element are also added, for instance in 7xxx alloys enriched with small fractions of scandium, materials with a very high tenacity may be obtained (Table 1).
Further advantages deriving from the addition of scandium to aluminium alloys (mainly in the 5xxx and 7xxx ) are:
- a strong increase in the recrystallization temperature;
- an increase of tensile properties even with small percentages of alloying element (Sc = 0.2-0.3%). The increase in the yield strength in different alloys brought about by the addition of small quantities of Scandium, as shown in Figura 2, is noteworthy;
- an excellent behaviour of welds with scandium alloys as fillers, with a marked increase in the working life of the welded junctions.
Alloys of this sort have been used in the Eighties in special aeronautical applications, the best known among them being the strategic details of the renowned Soviet MIG aircraft. After a period in the Nineties characterized by a considerable success, scandium alloy semis were abandoned and replaced by materials which made the finished products less expensive. The main reason behind the lack of popularity of these alloys has always been the cost of the master alloy, in turn caused by the difficulty in producing sufficiently pure scandium compounds. This is due to the chemical-physical properties of so-called “rare earths” (scandium, yttrium and lanthanides), which are actually not rare at all but abundant especially in the northern part of the world. Scandium for instance, is twice as common as lead and ten times more present in the earth’s crust than tin. But producing pure compounds of a single earth clashes with the with huge difficulties in separating them, with a very strong decrease in the efficiency of the reaction aimed at obtaining sufficient purity.
However, the reason behind the increase in the cost of pure compounds is well-known: some rare earths have become over the past few years fundamental additives for strategic technologies in the electronics and battery segments. Without rare earth compounds, widespread modern instruments such as smartphones, hybrid and electric cars, wind farms and many advanced defence systems would not be able to function. The ongoing completion of Rusal’s plant in the Ural region to produce scandium oxide with a purity above 99%, extracting it from red sludge derived from alumina processing, makes it likely that in the medium term there will be a dramatic reduction in production costs. The positive result of the experiment carried out by Rusal is the reduction in the costs of many operations of this process. The Russian pilot plant can produce about 100 kg per year of 99% pure scandium oxide. It should be noted that the global demand of pure scandium oxide for master alloys is estimated at around 15 tons a year, for the production of roughly 3,500 tons of alloys with an alloying element content of 0.2%.
Next-generation aluminium-lithium alloys
The civilian aeronautical construction industry has ahead of it about two decades of growth with estimated revenues of around 6.000 billion dollars, generated by the building of about 40,000 new passenger and cargo planes. In 2016 over 300,000 tons of aluminium alloys have ben used to build about 1,400 planes. Without considering possible increases in the amount of aluminium per plane, the demand of alloys foreseen for the next two decades for civilian aeronautics adds up to about 9,000,000 tons. Most of these materials are extrusions, rolled and forged products of high-strength alloys; among these, an important role is played by next generation Al-Li alloy semis. Traditional Al-Li alloys require special production techniques to prevent explosive reactions deriving from contact between the liquid metal and air or water. Besides, Al-Li alloys developed in the Seventies and Eighties have problems in terms of low resistance to stress corrosion cracking. New alloys have a lower percentage of lithium with respect to traditional ones and fall within the Al-Cu family (2xxx). Currently these alloys are used in the aeronautical industry, but even for the production of high-strength details for the overland transportation industry, such as, upper bracket brake callipers, and for numerous sports gear items. The main advantages of lithium alloys are linked to the alloying element’s low density, which determines a decrease in the density of the alloy, and to the increase of the modulus of elasticity. Besides, the new compositions have been designed to improve the resistance to stress corrosion cracking and to increase endurance strength. As an example, the composition and properties of some next generation alloys are shown. Table 2 shows the composition of two of the most widespread alloys and Table 3 shows the values of some mechanical properties of the 2099 alloy in T83 temper (Arconic C460). This state is obtained by bringing the metal to a high temperature above the solubilization curve; a % plastic deformation is then applied to end with a natural aging treatment.
The composition of the alloy is not only chosen to maximize its tensile properties but also to enable a controlled hardening precipitation and to prevent unwanted occurrences linked to the precipitation of unbalanced phases capable of affecting stress corrosion cracking, the rate of fatigue crack growth and the values of Rp0,2. This situation is clearly shown by Figures 3A and 3B, where the percentage changes in the values of some properties of alloyAA2050 T84 compared to alloy AA7050 T74 (figure 3A) and alloy AA2219 T87 (figure 3B) are shown. The compositions of the three alloys are shown in Table 4.
“Eco-friendly” free cutting alloys
Research aimed at replacing lead in free cutting alloys led to the development of alloy systems such as Ultra Alloy 6020 by Alcoa adopted as a replacement of alloy 6262. The latter is a 6xxx alloy with a lead content of around 0.5%, sufficient to provide bars and pipes made out of this material with an excellent machinability. In the Ultra Alloy the role of the low-melting element which allows chips to break during machining is performed by tin. It is very interesting to note that Ultra Alloys in the T651 T8 and T9 temper, besides overcoming the environmental danger linked to the presence of lead, also show better machinability performances, reaching values of the machinability parameter used in the automotive industry higher than those of the 6262 lead alloy ((Ultra Alloy coefficient: 90 out of 100). AS well as Alcoa, even Italian company Eural developed the AA6026 LF (Table 5), an innovative material which we described in detail by interviewing Giorgio Di Betta, sales director of Eural, in A&L’s issue 3/2016.
The main properties of the Eural alloy, besides the relevant aspect of the absence of Pb, may thus be summarized:
fit for high speed machining with automatic machines;
good corrosion resistance;
medium-high mechanical properties;
well suited to decorative anodic oxidation;
well suited to hard oxidation for industrial use.
A very interesting property, as can be seen in Table 5, is the absence of tin. It should also be noted that Eural recently developed the high performance 2033 free cutting alloy, which was also described in an interview with Giorgio Di Betta published on issue 1/2019 of A&L. The alloy developed by Eural and inserted in the list of denominations registered in August 2018, has a composition which is compliant with the European RoHSII, ELV and Reach directives. In Table 6 some values of mechanical and physical properties of this new alloy are shown; togehter with 6026LF it is proving very successful on world markets.