Aluminum Alloy in Automotive

Theon going demand for more fuel-efficient vehicles to reduce energy consumption and air pollution is a challenge for the automotive industry. Aluminum has been an increasing interest in automotive applications due to the general need in weight saving for further reduction in fuel consumption in recent years. Especially sheet applications for lightweight structural arts and body-in-white construction are gaining interest and major efforts have been given by all major manufacturers of semi-finished products of aluminum alloys to meet the main requirements which are :

• Enough strength for structural stability and durability, dent resistance, crashworthiness.

• Good formability for stretching, bending and deep drawing operations (including control of anisotropy and spring back).

• Metal-joining techniques, like welding, clinching, glowing,brazing etc.

• Recyclability and low material and fabricating costs

In order to meet the various requirements on mechanical properties a good knowledge of the specific material behavior and an understanding of the underlying metallurgical effects involved are important. The foremost requirement for many sheet applications is to find the most appropriate combination of sufficient strength and good formability. For structural parts and for body-in-white (BIW) application the two main alloy systems used areAl-Mg and Al-Mg-Si which are well accepted due to their good combination of the required properties.

Aluminium Alloy:

We are in a multi-material world where no sole material has power and influence over the automobile. Aluminum is the rising material of selection, offering the rapid, safest, most environmentally friendly and cost-effective way to boost fuel economy and cut total carbon emissions. Decreasing the vehicle weight - without reducing the size of vehicle - will be essential as automakers develop the next-generation vehicle.

Some of the principal properties of this metal are:

• Weight: Aluminum is light. The density of Aluminum is ? = 2,7 g/cm3, which is one third that of steel.

• Strength: Aluminum is strong. Aluminum alloys have tensile strengths vary from 70 to 700 MPa. Unlike steel, aluminum does not become brittle in low temperatures. In fact when cold, the strength of aluminum increases.

• Flexibility: Its strength is combined with flexibility, meaning that it can flex under load and bounce back from the force of impacts.

• Malleability: It is extremely malleable, and can be extruded into any required shape by passing it through a die. It can be extruded either hot or cold and can be further manipulated through operations like bending and forming.

• Conductivity: It is an excellent thermal and electrical conductivity. An aluminum conductor weighs around half the equal amount of copper conductor with the same conductivity.

• Reflectivity: It is a good reflector of both light and heat.

• Corrosion resistance: Aluminum reacts with the oxygen in the air to form a microscopically thin layer of oxide. The layer is only 4 nanometers thick but offers excellent protection against corrosion. [7]

Aluminum alloys are categorized into two types: Cast aluminum alloy and Wrought Aluminum alloy. Table 1 is showing the designation system of Aluminum alloys, which are used by the AluminumAssociation of the United States, for both casts and wrought Aluminum alloys. This designation system uses a four-digit numerical system to distinguish different Aluminum alloys. The nomenclature for wrought alloys has been agreed and accepted by most of the countries and it is also called as theInternational Alloy Designation System (IADS). The alloy group is indicated by the first digits and the last two digits identify the Aluminum alloy or specify the Aluminum purity. The second digit indicates modifications of the original alloy impurity limits. In the cast alloy’s designation system, the first digit is essentially the same as for wrought alloys while the second two digits serve to identify a particular composition.

Cast Aluminum Alloys

As the cast aluminum alloys are cost-effective, environment-friendly lightweight materials, it has an increasing interest in the automotive industries. The properties like better castability, high mechanical properties, ductility and good corrosion resistance, have allowed them to substitute steel and cast iron for the making of critical components[11]. Cast aluminum alloys contain the high percentage of alloying elements; the most important alloying elements are:

Silicon: Silicon is one of the prime alloying elements used for cast aluminum alloys. Generally presents content between 5-12%of weight. Firstly, these alloying elements allow increasing the fluidity of the alloys and as a consequence enhance its castability, reduce the thermal expansion coefficient of alloys. Presents a low density (2.34 g/cm3) determining a reduction of cast components weight and finally, its low solubility in aluminum allows the precipitation of pure, hard Si particles which improve the abrasion resistance of the alloy.

Copper: Copper increases both the mechanical strength and the machinability of alloys; reduces the coefficient of thermal expansion and as most important characteristic has a negative effect on the corrosion resistance of alloys.

Magnesium: Magnesium offers to increase the mechanical properties through the precipitation of Mg2Si hardening precipitates, enhancing the corrosion resistance and the weldability of alloys.

Manganese: It enhances the tensile properties as well as increases significantly the low cycle fatigue resistance. Addition of manganese also improves the corrosion resistance of the alloy.

Iron: Iron is the most common and unavoidable impurity in Al-Si foundry alloys because it can form different types of inter-metallic compounds; such compounds are brittle and have a deleterious effect on the mechanical strength of components. Several types of Fe-rich phase exists, such as ?-Al5FeSi, a-Al15Fe3Si2 and a’-Al8Fe2Si.

Wrought Aluminium Alloys

The wrought aluminum alloys are widely used in automotive industry to produce different components, because of their mechanical properties, which are higher than those obtained from cast aluminum alloys. Around 85% of Aluminum applications are from wrought aluminum alloys. Initially, they are cast as ingots or billets and subsequently hot and/or cold worked mechanically into the required form. The crystal structure of Aluminum, the face-centered cubic system (fcc) provides a good cold formability. For wrought applications, the addition of alloying elements enhances most of the mechanical properties; even if they have a comparatively less amount of alloying elements, the structure of wrought alloys offers better mechanical properties than cast alloys.

Plastic deformations have increased the degree of grain refinement and homogenize the microstructure. There are four important processes applied to obtain different products:

1) The product produced from rolling: Plates, flat sheets, soiled sheets, and foils.

2) The product produced by extrusion: Extruded rods, solid and hollow shapes, profiles, or tubes.

3) The product produced by forming: rolled or extruded products are formed to achieve complex shapes.

4) The product produced by forging: they have complex shapes with superior mechanical properties.

Due to more and more stringent requirements for vehicle safety and comfort, the size of many vehicle elements increases, leading to increasing the total mass of the vehicle. For vehicles driven by combustion engines, reduction of vehicle mass allows for reducing the consumption of fuel and, consequently, the ownership costs and the amount of carbon dioxide emitted to the atmosphere.

The Aluminium alloys used for the construction of automotive vehicles is one of the methods for reduction of the vehicle mass, as the density of aluminum alloys amounts to 2700 kg/m3 – one-third of the one for steel (7600 kg/m3). To ensure mechanical properties comparable with those for steel, it is necessary to use aluminum elements with cross-sectional areas larger than for steel elements. Therefore, the reduction of average mass is slightly smaller than the reduction resulting from just comparing the specific gravity values for both materials. The reduction of effectiveness of the element made of an aluminum alloy as compared with the steel one amounts to about 50%. The direct reduction of vehicle mass causes the so-called“secondary” mass reduction, being the effect of smaller dimensions and sizes necessary for other structural elements of the vehicle.

The alloys of 5000 and 6000 series, which offers to construct of the virtually entire structure of automotive vehicle body, are of particular interest in the automotive industry. Using the 6060-T6 alloy, space frame-based design of the vehicle was developed which has fulfilled the Federal Motor Vehicle Safety Standards’ requirements for the frontal impact test. [5]

History of aluminum in Automotive:

Today, Aluminum has been a key material for automotive manufacturers. The first sports car presenting an Aluminum body was revealed at the Berlin International Motor Show in 1899. After two years, the first engine with Aluminum components was developed by Karl Benz. Following World War II, Aluminum had become economical enough to be considered for use in mass-produced vehicles. An innovation happened in 1961 when the British Land Rover company produced V-8 engine blocks made with Aluminum cylinders. From there, Aluminum automobile components gained foothold wheels and transmission casings and then moved into cylinder heads and suspension joints. This endlessly reusable metal is now the leading material for use in powertrain and wheel applications and continues to gain market share in hoods, trunks, doors and bumpers - and complete vehicle structures. [6]

Applications Aluminum Alloys In Automotive Industry:

Optimized Aluminium oriented car design has been established in various parts and applications in automotive industry (refer to Figure 3):

• Powertrain- Engine block & cylinder head, transmission housings, fuel system, and radiators: 69 kg

• Chassis& suspension - Cradle, axle, wheels, suspension arms and steering systems: 37kg

• Carbody - Body-In-White (BIW), hoods/ bonnets, doors, front structure, wings, crash elements and bumpers and various interiors: 26 kg

In Figure 5, it has been reported the relative and the absolute mass saving, achieved using aluminum alloys for the manufacturing of automotive components. It also shows the market penetration for each individual component.

In the last forty years, it can be observed from Figure 6 that, the percentage of Aluminum cars has had a sharply and continuously increasing, because of the increasing demand by the automotive industry of using light materials.

Two types of alloys are used for most automotive components made from Aluminium:

I) Non-heat-treatable or work-hardening Al-Mg (Mn) alloys (5000 series alloys) that are a solid solution - hardened, showing a fair combination of strength and formability.

II) The heat-treatable Al Mg Si alloys (6000 series alloys) that obtain their desired strength through the heat treatment processes, e.g. for sheets when the car body undergoes in the paint baking process.

In the case of particular components, such as bumpers and crush-zone, the high-strength Al–Zn–Mg–Cu (AA7xxx) is used. These alloys have been developed and currently are widely used in the aerospace industry also, because of their high mechanical performances.

Extrusion:

One more important area of Aluminium solutions and applications is the well-established technology of Aluminium Extrusions. Here very complex shapes of profiles can be achieved allowing an innovative lightweight design with integrated functions. Generally, medium strength AA6000 and high strength AA7000age hardening alloys are used, because the desired quenching occurs during the extrusion process. Formability and final strength are controlled by heating forage hardening. Extrusions are done for bumper beams and crash elements/boxes. The main drivers in new developments are extrudability, tolerances and strength, particularly for strength relevant applications in the automotive vehicles. New alloys are being developed that show higher strength. Simultaneously, it is easier to extrude and even more, complex shapes can be produced, like the drawing of the thin-walled shapes. Today, extrusions are used extensively when tight tolerances can manually be compensated. [3]

Casting:

The increased volume of Aluminium components in automotive applications are castings, such as engine blocks, cylinder heads, wheels and special chassis components. However, due to the high demand for strength and durability, cast iron is still often being used. Significant progress in Aluminium alloy development(Al-Si-Cu-Mg-Fe-type) and better process control and casting methods improved material properties and functional integration that enables Aluminium to meet the specific high requirements. Aluminum castings are also gaining acceptance in the construction of space frames, axle parts, and structural components. Complex parts are produced by high integrity casting methods that ensure optimal mechanical properties and allow enhanced functional integration. [3]

Advanced Multi-Material ‘‘MM’’ Design Concepts

The multi-material design is the innovative automotive vehicle concept, which nowadays is under development by the automobile industry. The basic idea of this concept is that to use the “best” material for each car's components, that allows producing emission reduced lightweight car, without losing performance and first of all the car's passenger safety. The adopted materials could be aluminum together with high and ultra-high strength steels, magnesium and plastics or composites. This is the prime objective of the “Super Light Car”(SLC) project. [3]

Microstructure Evolution (Sheet production):

A processing layout for production of Aluminium sheet alloys by DC – ingot casting, hot rolling, cold rolling and a final annealing treatment is shown in Figure 6. The material is transformed through multiple stages from the cast structure to a fine grain recrystallized structure by hot and cold rolling and final soft annealing or heat treatment solution in a continuous annealing furnace.

1) Cast structure with relatively large grains and random texture usually forms by the homogenization annealing

2) The recrystallized grain structure formed during hot rolling with a typical cube structure and constituent particle stretched out in the rolling direction.

3) Deformedgrains and fine dispersoids after final cold rolling with typical rolling texture.

4) The recrystallized grain structure with relatively weak cube formed after final solution annealing.

Mechanical Properties of Aluminum Alloys:

The unblended Aluminum doesn't have a high tensile strength. Because of the addition of the alloying elements like manganese, silicon, copper, and magnesium, which will enhance the properties like the strength of Aluminum and produce an alloy with properties tailored to particular applications.

Aluminum alloys have been widely used and become a precious material in automobile industries because of its properties such as its lightweight, strength, recyclability, corrosion, resistance, durability, ductility, formability and conductivity. The strength and durability of aluminum alloys vary widely, not only as a result of the components of the specific alloy but also as a result of heat treatments and manufacturing processes. Its strength can be adapted to the application desired by modifying the composition of its alloys. Mixed with a small amount of other metal, it can provide the strength of steel, with only one-third of the weight (The Aluminum Association, 2011). Aluminum alloys increase its strength without loss of ductility. On the other hand, it naturally generates a protective oxide coating and is highly corrosion-resistant. Different types of surface treatment processes such as anodizing, painting or lacquering can further improve this property. It is particularly useful for applications where protection and conservation are desired. Due to this distinctive combination of properties, the variety of applications of Aluminum continues to increase. Table 3 below shows typical properties for Aluminum that normally been used.

Aluminum can be processed in a number of ways when it is in a molten condition because it is ductile and has a low melting point and density. Its ductility allows products of aluminum to be basically formed close to the end of the product’s design. Another key property of Aluminum alloys is their sensitivity to heat. Workshop procedures involving heating are complicated by the fact that aluminum, unlike steel, melts without first glowing red. Forming operations where a blow torch is used therefore require some skills since no visual signs reveal how close the material is to melting. Aluminum alloys, like all structural alloys, also are subject to internal stresses following heating operations such as welding and casting. The difficulty with aluminum alloys in this regard is their low melting point, which makes them more susceptible to distortions from thermally induced stress relief. Controlled stress relief can be done during manufacturing by the heat-treatment process of the parts in an oven, followed by gradual cooling - in effect annealing the stresses.

Aluminum alloys are divided into two types, castings and wrought (mechanically worked products). The main groups aluminum alloys which are the most often used in practice besides technically pure aluminum are Al-Mn, Al-Mg, Al-Mg-Mn, Al-Mg-Si, Al-Zn-Mg, and Al-Zn-Mg-Cu alloys. These are wrought alloys which are shaped into products by rolling, extrusion, forging and drawing. Each of the mentioned groups consists of numerous subgroups, depending on amounts of main and additional alloying elements, and they have tensile strength values varying in a wide range from 70to 600 MPa. The tendency with standard wrought aluminum alloys is to achieve better strength values, which imposes a great challenge for metallurgists. In this group, there are mainly alloys of Al-Cu-Mg and Al-Zn-Mg-Cu type. With the later, the strength values of over 600 MPa at plane-strain fracture toughness of 30 MPavm have been achieved. These alloys are used for the most demanding purposes like vehicles and airplanes, due to their high strength/weight ratio. Further improvements are in progress for alloys for sections, rods, tubes (Al-Mg-Si, Al-Cu-Mg,free-cutting alloys), for deep drawing purposes, for heat exchangers, and for wrapping materials. Research efforts to optimize production processes and properties of alloys will be extended in future since many of existent alloys are completely acceptable for general use.

One important structural constraint of Aluminum alloys is their fatigue strength. Unlike steels, aluminum alloys have no clearly stated fatigue limit, means that fatigue failure occurs in due course, under even very small cyclic loadings. This implies that design engineers must assess these loads and design for a fixed life rather than an infinite life.

General Comparison of Aluminum alloys with steels in automotive

In light weighting automotive vehicles, the most common substitution for steel is made with Aluminum alloys. Aluminum alloys are available to the transportation industry as flat sheet and plate in a variety of shapes (linear extrusions, roll formed sheet, casting and forging), while steel sheet and plate are available mainly as mill products(flat-rolled sheet and plate, shape rolled or formed sheet). When substituting alloys for steel, the following comparative properties need to be considered:

• The density of Al alloy is one third that of steel.

• The elastic modulus of Al alloys is one third that of steel.

• The hardness of Al alloys is lower

• Specific fatigue strength of Al alloys is about one half that of steel

• The coefficient of thermal expansion of Al alloys is about 1.5 times greater than that of steel or an equivalent change in temperature.

• Ductility, as measured by % elongation of Al alloys in the annealed condition, is about two-thirds less than that of annealed low carbon steel.

• Formability of Al alloy sheet is lower than that of annealed low carbon steel.

• Alloys can be used to cryogenic temperatures without loss of ductility, while carbon steels suffer from embrittlement at low temperatures.

• Steelies strain rate sensitive while Al alloys are not and Al alloy structures have been shown to absorb more energy than steel structures upon impact.

• Unlike steel, Al alloys are non-magnetic.

• Unlike steel, Al alloys are non-sparking.

• Thermal and electrical conductivities of Al alloys are about four times that of steel.

• Damping characteristics of Al alloys and steel are similar.

• Atmospheric corrosion resistance of Al alloys is much higher than steel.

• Alloys can be used unfinished in many applications and can accept a wide range of mechanical and chemical finishes, while steel requires paint or electroplated finishes to ward of atmospheric corrosion.

• Galvanic corrosion resistance of Al alloys is lower than Al alloys.

• Recycling value of Al alloys is higher than steel.

The above properties of Al alloys in comparison with steel should be taken into consideration by vehicle designer while joining these particular materials.