The aerospace industry demands a lot from the materials it uses. Demands include improved toughness, lower weight, increased resistance to fatigue and corrosion. The boundaries of material properties are being constantly extended as manufacturers strive to give the next generation of aircraft improved performance while making them more efficient (figure 1). Aluminium is one of the key materials facing these challenges. Aluminium alloy plate is used in a large number of aerospace applications, ranging in complexity and performance requirements from simple components through to primary load bearing structures in aircraft such as the Airbus A340 and Boeing 777 (figure 2). The demands being made on the materials have stimulated significant advances in their development and manufacture at British Aluminium Plate (BAP), a division of The Luxfer Group.
Figure 1. Next generation aircraft beyond the F22 will demand further improvement in the properties form the aluminium alloys from which they are manufactured.
Figure 2. Wing ribs from an A340 Airbus wing box are made from aluminium plate produced by BAP.
The attention of the civil aerospace industry is turned to reducing manufacturing and direct costs. One consequence of this is that complex components machined out of thick plate increasingly are replacing parts previously machined from die forgings or fabricated from sheet and extrusions. This means that aluminium alloy plate producers are facing increasingly challenging performance targets. For example, machined web thicknesses of less than 1mm may ultimately be required if components currently fabricated from sheet are to be replaced by ‘hogouts’ from thick plate. In addition, the plate used in these applications must be free of defects, such as porosity, that, if present, could occupy a major proportion of the cross-sectional area of the thinnest webs.
BAP has made significant improvements to the manufacturing routes for its existing alloys such as 7010, T7651 and 7050 T7451 in response to the increasing demands of airframers. These new routes have resulted in better properties, including short transverse (ST) ductility, fatigue performance and fracture toughness.
Strength is a crucial requirement of any alloy playing a load-bearing role in aerospace applications. However, the potential tensile ductility and toughness of 7XXX series alloys is reduced by the presence of coarse secondary phase particles. Intermetallic phases of concern in alloys such as 7010 and 7050 include Al7Cu2Fe, Mg2Si and Al2CuMg (S-phase). The volume fractions of Al7Cu2Fe and Mg2Si are essentially fixed by alloy chemistry and so can be lowered by altering composition. The volume fraction of S-phase for a given alloy chemistry, on the other hand, must be minimised by optimising the thermal treatments used in plate manufacturing.
Differential Scanning Calorimetry
BAP uses the technique of differential scanning calorimetry (DSC) to quantify the volume fraction of S-phase. DSC is a valuable tool for monitoring the effects of alloy chemistry and thermal processing on the volume fraction of S-phase in the finished plate. What is more, the technique allows the relationship between the volume fraction of S-phase and mechanical properties such as tensile ductility and fracture toughness to be studied.
BAP used this technique to optimise alloy chemistry and processing for thick 7010 T7651 plate, which is used for wing spars in large civil aircraft. DSC showed which thermal processes resulted in the minimum amount of S-phase in the material, and this proprietary information is now used to give an increase in ST ductility of almost 2% for plate thickness ranging from 25mm to 160mm. A similar approach was applied to 7050 T7451 plate in order to increase the plane stress fracture toughness of this material. Again, significant improvements in fracture toughness resulted from reducing the volume fraction of S-phase.
The fatigue performance of thick plate is another area of increasing importance in aerospace applications. Materials specifications are being introduced that require uniaxial fatigue testing for release certification purposes. One such specification for 7050 T7451 plate calls for a minimum average life of 120,000 cycles when tested at a maximum stress of 241MNm-2.
The fatigue life of aluminium alloys is governed primarily by crack initiation, which is accelerated by the presence of microporosity in the alloy structure. Such microporosity can arise during the DC casting of long freezing range 7XXX alloys if gas levels are not minimised and casting parameters are not set to give optimised solidification processes in the solid-liquid zone. BAP has used mathematical models of DC casting to develop processes that ensure minimum levels of microporosity.
Thick Ingot Manufacture
In addition, procedures for casting thick rolling ingots have been developed.
Manufacturing plate using thick ingots gives additional improvements to fatigue performance by increasing the amount of mechanical deformation required to achieve a given plate thickness. This additional working reduces both the size and volume fraction of any microporosity present in the as-cast condition, and so improves the fatigue performance of the finished plate.
Stress Corrosion Cracking
The commercial exploitation of high strength AlZnMgCu alloys is often restricted by their increased susceptibility to stress corrosion cracking (SCC) when in their peak-aged condition. Many aerospace applications demand excellent resistance to SCC owing to the conditions of service. To get round this problem, the materials usually are used in an over-aged condition that provides a significant improvement to SCC resistance and exfoliation corrosion. However, over-ageing results in a loss in strength of 10-15%.
BAP has used alternative artificial ageing treatments, such as retrogression and re-ageing (RRA), to achieve the SCC resistance exhibited by over-aged alloys while maintaining peak aged strengths. These thermal treatments consist of a retrogression or reversion stage in which the near peak-aged material is heated for a short time at 220-280°C, followed by re-ageing at lower temperatures. During retrogression, strength falls rapidly to a minimum, partially recovers and finally decreases. Re-ageing after short retrogression stages restores strength to that of the peak aged material, but extended re-ageing ultimately results in a loss of strength.
BAP has found that for 7150 and other high strength 7XXX series aluminium alloys, strength levels equivalent to or higher than those found in alloys in the T651 condition can be achieved in the laboratory using such RRA processes. However, the thermal processes used to give such properties are not usually commercially practical for the large plates required in upper wing skinning of large civil aircraft, for example.
BAP has therefore developed alternative, commercially viable RRA-type artificial ageing processes. A number of factors must be considered when designing such a process. The heating rate to the retrogression temperature must produce a microstructure suitable for retrogression. The retrogression temperature must have a reasonable tolerance to allow for temperature variations within a furnace load and between different furnace loads. The retrogression time should be long enough to allow the use of conventional ovens. Finally, for economic reasons, the number of separate heat treatments should be kept to a minimum.
With these things in mind, the R&D team ran a broad range of tests to gauge the effects of various heat treatment regimes on conventional 50mm thick 7150 plate. The effect of the various thermal treatments was monitored using electrical conductivity measurements and hardness testing. Electrical conductivity measurement is a straightforward procedure and interpreting the results is simple for AlZnMgCu alloys. The higher the conductivity, the greater the resistance to environment sensitive cracking.
BAP's experiments showed that by carefully controlling retrogression temperature, time and cooling rate after retrogression, it is possible with a two stage heat treatment practice to produce aluminium alloys with properties very similar to those obtained with three stage RRA practices. This was subsequently demonstrated with a full scale manufacturing trial using plate thicknesses from 9.5 to 57.2mm. The heat treatment process used was designed to achieve maximum resistance to exfoliation corrosion in the alloy, while still exceeding tensile property specifications for 7150 T651 plate.
The above has highlighted just a few examples of developments that have been made at BAP in response to increasing performance requirements from customers in the aerospace industries. Improvements to the fatigue, ductility and fracture toughness of thick 7010 and 7050 plate have been achieved through optimisation of chemical composition and casting/processing parameters. Two stage artificial ageing practices for 7150 that provide T651 strength levels with superior stress corrosion and exfoliation corrosion resistance have also been developed. Ultimately, work such as this will ensure that aluminium alloys maintain their wide range of uses in the aerospace industry, and that aircraft of the future will continue to offer improved performance thanks to the advances in the materials from which they are built.