There are several aluminum alloys categories, some of them are used in automotive engineering. These classes are identified by numbers which are called series and vary from 1 to 8 and they are identified by 4 number code. The first class is called one series, it represents a pure aluminum components or material. This is commonly used by sheets, plates, tablets, and blisters and are usually reinforced by work hardening. In fact aluminum alloys are determined not only by the alloy element but also by their reinforcement procedures which are mainly composed by two kinds of processes: a work hardening or and time of treatment. Hence aluminum materials are defined by the one which are alloyed and the one which are pure. The main objective of using alloying elements is to increase the aluminum mechanical properties . However, as more alloying elements are being added, less resistant to corrosion the material are. This is the crucial tradeoff of components based on aluminum. Even though there are several aluminum alloys, neither all them are used on automotive applications. The reason is their characteristics that each alloy presents according to the alloying elements added. Therefore this article proposes a summary of the aluminum alloys used for automotive applications.

Main aluminum grades

Table: The main aluminum alloy grades and their characteristics.

As can be seen the 1xxx series aluminum represents the pure aluminum material. The other classes are all composed by aluminum and its alloying elements. Thus the 2xxx series aluminum alloys called avional, are usually applied on plates and bars and the main reinforcement procedure is the thermal treatment. However work-hardening can also be applied on it. The 3xxx series is an aluminum alloy composed by manganese , it is not so useful for mechanical and automotive design it is usually used by food trays. The procedure of reinforcement is work-hardening. The 4xxx series aluminum alloys are interesting for casting due to the application of Silicon. Finally the five, six and seven series aluminum alloys are the most used for mechanical applications. The inclusion of magnesium (five and six series) increase the corrosion resistance capability. The difference between them is the kind of the reinforcement, work-hardening or thermal treatment ( 5 and 6-7 series respectively ). The most interesting aluminum alloy for automotive application is the 6xxx series, also called anticorodal. In addition the 7xxx series (Ergal) and the 2xxx series (Avional), are also used due to the possibility to be thermal treated.

The heat treatment

The usual procedure of a heat treatment: Solution, Quenching and Aging.

The base heat treatment it is a process which has some steps, each of them has its own purpose. The first one is the solution heat treatment, it is a heating with the objective to improve the solid solution. The temperature reached by this process depends of the objective and the demand of this material. Moreover quenching process is performed which is fast cooling of the material. Now the objective is to retain the solid solution obtained in the previous process. Finally the last and the more important process of the heat treatment is performed, it is call it aging. This is another heating but in a different temperature and time, this last factor is more important in this process , because the temperature is hold constant.

The objective is to improve solubility of the solid solute at high temperature, and motivates a great variation during cooling. These fase diagrams indicates that aluminum alloys can be extracted.
The heat treatment is a sequence of processes with a well defined time and temperature range.
The transformations suffered by the material during the heat treatment.

The process begins with a heat treatment called solution, which is the heating of the material until a define the temperature and after this one is reached, it is held for some moment. This improves the solid solution increasing its solubility. After this cooling a very fast cooling is performance this process is called quenching. The cooling heats the material structure obtained after solution heat treatment and modify it to obtain a composition with a super saturated solution. In addition the quenching process is also performed between a range of temperature which are predetermined. After quenching the material is again heated, but this process is called aging. The aging is also held for a defined time. The objective is, by precipitation hardening, improving the material structure, in other words, refining its grains. the reason why aging change the material structure from a fine grain structure is that supersaturated solid solution are not stable.

The aging process

The material structure transformation during aging process is definitive for the desired Al alloys characteristics.

The aging is the main process of the heat treatment, either hate team of the material under a bread find time and temperature. The objective is to transform the structure from a supersaturated solid solution to a structure with a fine grains. The time and temperature are very important parameters , because they determine if the process is under or overaged. In fact the process has an ideal range of temperature and time The battle result. The ideal result depends of how the precipitates are after the aging process. There are two main precipitates which can be formed coma coherent and incoherent precipitates. The main difference between them is the dislocations (epsilon) inside the secondary phase. The objective of the Asian process is to provide I structure which the precipitates has a coherent structure , because in this case the energy to the formed material must be higher. when a material is under the flexion the dislocations inside the structure, so in a molecular point of view, must win the resistance to these dislocations. In the moment that the dislocation finds a precipitate a coherent precipitate did this location pass inside the precipitate, which requires a great amount of energy today dislocation occur. Generally the amount of mismatch is about 1% coma for a coherent precipitate. When the aging process continues even a coherent precipitate is obtained. A semi coherent precipitate is obtained which also provides a good result in terms of energy required to win the material and precipitates resistance to the dislocations. However in the case of semi coherent precipitates, the dislocations passes through the precipitates but deforms the precipitates thus the energy to win did precipitate is lower. In this situation the aging process is in the middle off underage or overage process. When the aging process is held even with a semi coherent precipitates obtained, the new secondary phase will be constituted by an incoherent precipitates, which means an epsilon bigger than 25%. The problem with incoherent precipitates is that they require less energy to be uh overcome by the dislocations. Actually these dislocations can occur by three means, bowling, climbing and cross slip.

In the aging process its time and temperature range are crucial for material performance. In fact, there are optimum values for these parameters.

For each aluminum alloy there is a correct and optimal aging time and temperature, as mentioned before the heat treatment is composed by a solution, quenching an aging. The aging process begins with a solid solution straightening (1), after that a GPZ is obtained with a coherent structure (2), then a precipitation hardening increases the yield stress to the peak level (3) and if the aging process continues, the precipitation hardening makes the material structure weaker (4). In this case it is said that the material is over aged.

The main heat treatments for Al alloy applied in automotive field

The heat treatments are usually called temper. To differentiate the many heat treatments available it is used numbers so T1, T2, and T8. Each of these temper has its own characteristics relative to time and temperature. In addition there are specific tempers which are developed to improve specific characteristics of the material. In this cases the T is followed by more than one number, for instance T76. They are used when the material is applied to a component or system which has a this specific function that requires a new characteristic or an improved characteristic. For automotive applications T3 and T6 are the most used tempers, the first one is usually applied to two series aluminum alloys and the second one is usually applied to six series aluminum alloys. The T6 temper means a solution heat treated and artificially aged. The T3 temper means a solution heat treatment, code worked and naturally aged to a substantially stable condition.

Precipitation Free Zones (PFZ)

A schematic of PFZ which usually occurs when Al is alloyed with another elements.

One of the most important details on alloys is the precipitation free zones or BFZ. Considering an aluminum material, appear aluminum material, there is no grain boundaries, because in this case there are not alloying elements. Kings there is no precipitates. In this case, which is 1 Series aluminum, the corrosion resistance is very high while the mechanical properties is not so interesting. This is the reason that alloying elements are included to the chemical compound of the material. However do it too they are lying elements there is a formation of precipitates, and these results in the so-called PFZ. This characteristics results the increase of cohesion , the secondary phase normally generates a grain boundary with different potential, thus the inter granular cohesion easily occur over the green boundary. Hence an important conclusion about precipitation free zones is that after heat treatment, the aluminum alloy will have a very high mechanical properties, but wheel experiment are lower cohesion resistance. the precipitation free zones strongly affects the corrosion resistance but outside the vicinity of the precipitation free zones there are regions of a very fine grain structure. These provides good mechanical properties. Hence it is a trade off: include alloying elements to improve mechanical properties while the corrosion resistance is progressively reduced. Inside the precipitation free zone there are regions of pure aluminum and this is the reason of the different potential because inside the precipitation free zones there is pure aluminum and outside there are the alloy structure with fine grains. Therefore the difference of potential is what motivates the the lower cohesion resistance in aluminum alloy. The solution to reduce PFZ is reducing these zones which are called vacation. This is a defect on the lattice where nucleation of the secondary phase occurs. The reduction of vacation depends of the aging temperature, but also the starting concentration of the location , which depends of the cold working.

The effects of temperature and cold working on the area of PFZ.

Therefore to improve in aluminum alloy in terms of cohesion resistance there are mainly two ways to obtain the nucleation. One way is working with different temperature variations. In this case it is possible to reduce the concentration of the vacation and then reduce the probability to have a cohesion do it too the different potential. In other words the area with pure aluminum is reduced. an alternative procedure to reduce cohesion yes starting the process with a very high vacant concentration curve , which is usually obtained with gold working. That can be seen in the figure the both concentrations or reduce it, thus the area of pure aluminum is also reduced. Therefore working with optimal temperatures and using cold working it is possible to obtain a reasonable corrosion resistance in aluminum alloy with very high mechanical properties.

Properties

Table: Summary of Al alloys properties, composition and strength.

As can be seen there are several characteristics and properties of the aluminum alloys and as already seen these are a tradeoff. But the most important detail about aluminum alloy properties is observed when the alloying element Cooper Cu is added. It motivates the decreasing of the ductility, weldability and anodizing capability. Cooper is included to improve mechanical property but at the cost of some manufacturability. In fact any alloy element aided to aluminum alloy is responsible for some decreasing on corrosion resistance and some improvement on the mechanical properties coma manganese , magnesium , zinc and silicon are the main ones. But the most pronounced effect is observed when copper is added . For instance it is possible to see how Chu and seven series aluminum alloys have a great variation on some properties after inclusion of copper. The corrosion resistance is reduced as more alloying elements are being added. In addition this same effect is observed with ductility . Actually the best aluminum serious option relative to these properties is the one series. This one is also able to be analyzed for some improvement of the mechanical properties. As can be seen on table there are two types of seven series alloys , the one composed by aluminum, zinc in magnesium and the other one which copper is added. This last one is the ergal aluminum alloy, very common in automotive industry.

The range of improvement between enlongation and ultimate tensile strength for the three main automotive Al alloys, the 2xxx, 6xxx and 7xxx series.

As can be seen on the graph the automotive aluminum alloys have different range of improvement. The six series is the only which presents the best ductility that is the reason why it is mainly used in automotive parts manufacturing. The other two categories, two and seven series, also called avional and ergal, are most used in high performance vehicles manufacturing. The two series aluminum alloys exhibit an intermediate range of elongation and ultimate tensile strength. relative to the seven series aluminum alloys former a higher range enlongation and ultimate tensile strength is obtained , but this material presents a higher strength relative to the other two at cost of a lower ductility.

References

  • Ashby, F. Michael. Materials Selection in Mechanical Design. 3rd Edition, Elsevier, London, 2005;
  • Davies, Geoffrey. Materials for Automotive Bodies. 2nd Edition, Elsevier, 2012. ISBN: 978-0-08-096979-4, DOI: 10.1016/C2010-0-66319-X.