The case of lead alloys replaced by tin alloys, always 6xxx, while keeping excellent fast cutting and tool machinability properties
by Giuseppe Giordano
Research aimed at replacing lead in fast cutting alloys led to the development of such alloy systems as Ultra Alloy 6020 by Alcoa, adopted as a replacement of alloy 6262. The latter is a 6xxx alloy with a lead content of about 0.5% sufficient to confer to rods and pipes made out of this material an excellent tool machinability. In the Ultra Alloy the role of the low melting element which allows the breakage of chips during tool machining is played by tin. It is very interesting to note that Ultra alloys in their T651,T8 and T9 states, besides overcoming the environmental danger due to the presence of lead, show higher performances in machinability reaching values of the machinability parameter used in the automotive sector which are higher than those for the 6262 lead alloy (the Ultra Alloys coefficient is 90 on a scale of 100). In parallel with Alcoa and its 6020 Ultra Alloy, Eural developed in Italy an alloy called 6262A whose composition falls within the limits of the 6262 alloy as shown in Table 1, apart from the fundamental replacement of lead with tin.
Table 2 shows the values of the mechanical properties of the different alloys in the physical states (T651;T8;T9) in which they are sold.
Aluminium-lithium alloys for aeronautics and high resistance applications
When the development of air transportation is considered, the industrial clout of the sector is not always correctly perceived. Boeing’s forecasts show there is a need for new civilian aircraft in all regions of the world. The total figure is not far from 40,000 units and the economic value equal to about 6,000 billion dollars. It should be noted that this value is referred to civil aviation only and does not include helicopters and drones. The sector therefore has a strategic value for the aluminium industry, which in the civilian sector also plans to recover some of the clout lost to composite organic materials, both with economic offers aimed at cost reduction and with technologically innovative proposals.
In this scenario a fundamental role is entrusted to aluminium-lithium alloys or more precisely to a new generation of lithium alloys whose composition and production process have been revised. The use of lithium alloys is an interesting opportunity for many applications, especially for aeronautical structures (Figure 1), but the evolution in the alloy family also concerns applications in different sectors, from overland transportation to sports equipment.
Aluminium-lithium alloys allow to obtain a marked increase in the performance of different details. Semis used may be rolled and/or extruded semi products as well as many details which may be obtained from foundry casts. Lithium alloys most widely used come from the Al-Cu-Li family with a concentration of copper by weight greater than the lithium one. Their use represents a considerable innovation in the metallurgy of light alloys, but it should be noted that they are not a novelty in the range of aluminium alloys. The first studies on precipitation hardening of these alloys were carried out as early as the Twenties in the past century.
Interest for lithium alloys may be summarised as follows:
1. the low density of lithium is such that for every 1% of lithium added to the alloy, the volume density with respect to its weight decreses by about 3%;
2. adding 1% of lithium by weight decreases the elasticity module by about 6%;
3. lithium alloys are characterized by an effective precipitation hardening process which determines high strength structures. Besides, the new versions of these alloy containing minor additions of zirconium and/or scandium present a clearly improved resistance to crack propagation and a greater plasticity.
The main critical aspects which should be kept under control deriving from alloying with lithium and which have always been noted in literature are:
1. high Li contents determine a general increase in fragility;
2. a danger condition exists on account of the violent oxidation of lithium in the presence of humidity;
3. lithium’s peculiarities require an accurate control of the composition of scraps.
Actually, the reduction, and regarding some aspects, the complete elimination of the potential negative factors lined to the presence of lithium, are the basis of the new generation of lithium alloys which are today produced using safe foundry techniques, while the recyclability of chips and scraps containing lithium has reached efficiency and cost-effectiveness levels which may be compared to those of other aluminium alloy systems.
Table 3 shows the values of copper, magnesium and lithium contents of some of the alloys most widely used today. It may be noted that the lithium content is never more than 2%. A more detailed analysis has been dedicated to the Constellium Airware® 2065 T8511 Alloy(*), specifically used for extruded components, whose normal composition is reported in Table 4.
The presence of a small amount of silver in the composition of the above lithium alloy has reasons similar to those behind the same presence in high strength Al-Cu-Mg alloys, among them the very well-known alloy for foundry castings, K-01.
The presence of silver helps the precipitation, following solubilization hardening, of a copper and magnesium precipitate. The resulting micro-structure has a resistance to stress corrosion cracking which is remarkably superior to that which may be seen in the same alloy without silver.
Going back to lithium alloys, an effective representation of the difference between latest-generation lithium alloys and the traditional alloys with greater mechanical properties, that is, 7xxx or 2xxx lithium-free alloys, is shown by Constellium, whose aloys for the aeronautical industry al fall within the group brand, Airware®. Figure 2 shows the main performance differences between Airware® 2196 vs 2024 T3511 and Airware® 2065 vs 7150 T77511.
The industrial crisis which followed the financial crisis led to a selection of operators in all industries. Aluminium extrusion was definitely not left out by this process which changed development scenarios and rules for everyone. Many extruders, for instance, had to push forward the boundary of their activity with respect to the finished product by including in their system assembly and finishing activities. This progress towards the finished product does not only concern large groups but also small extruders. Suppliers of the railway or automotive sectors do not stop at extruded semis but provide structures derived from the assembly of different extruded products and even of semis not deriving from the extrusion process. If companies intend becoming suppliers of high technology industries requiring large quantities, such as aeronautics and automotive, they cannot help engaging fully in the quest for maximum efficiency of performances, both in terms of technology and of the continuous cost reduction and control processes. These processes may be helped significantly by starting off with a knowledgeable choice of the raw material and of the relative heat and mechanical treatments, the search for suitable final performances of course begins upstream of the extrusion process and an important support may be obtained by the choice of the ideal alloy composition for the final use. By carefully following the steps and choices of leading groups, even an average-sized extruder may find and accurately define the composition of the extrusion alloys best suited to obtain for each item or component tailor-made performances for the different types of use. In the presentation some examples were provided, showing the optimization of the properties which may be obtained even with small changes and variations in the combination of alloying agents and added elements, and action taken on thermal-mechanical processing cycles of semis, drawing attention to the following cases:
1. development of an alloy as a normally “purer” variation (7068 as a variation of 7075);
2. Possible variations of the 6060 alloy in terms of differences in the extrusion property;
3. Development of 6xxx alloys which by means of thermal-mechanical treatment and the control of the precipitation hardening process provide high shock resistance and energy absorption properties;
4. Complete replacement of one element (lead) with another (tin) which knocks down environmental issues without hindering, but actually improving, specific performances (such as, fast machine tool cutting);
5. Application development of a new generation of high strength alloys (Al-Li alloys) developed for high technology industries such as aeronautics, but which, having overcome production cycle issues, may be increasingly used even for the automotive sector, for instance in forged details on trains, where there are already examples in sports applications.