1. good knowledge of the specific material behavior and

1. Introduction

The
on going demand for more fuel-efficient vehicles to reduce energy consumption
and air pollution is a challenge for the automotive industry. Aluminium has
been an increasing interest for automotive applications due to the general need
in weight saving for further reduction in fuel consumption in recent years.
Especially sheet applications for light weight 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 aluminium alloys
to meet the main requirements which are :

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·        
Enough strength for structural stability and durability, dent
resistance, crash worthiness.

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

·        
Metal-joining techniques, like welding, clinching, glewing,
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 are
Al-Mg and Al-Mg-Si which are well accepted due to their good combination of the
required properties.

 

2. 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 environmental friendly and cost-effective way to boost fuel
economy and cut total carbon emissions. Decreasing the vehicle weight – without
reducing size of vehicle – will be essential as automakers develop 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 in two
types: Cast aluminum alloy and Wrought Aluminum alloy. The Table 1 is showing
the designation system of Aluminum alloys, which are used by the Aluminum
Association of the United States, for both cast and wrought Aluminum alloys.
This designation system uses a four-digit numerical system to distinct the
different Aluminum alloys. The nomenclature for wrought alloys has been agreed
and accepted by most of the countries and it is also called as the
International 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
or 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.

Alloy Type

Four Digit Designation

Wrought alloys :

 

99.00
% (min) Al

1XXX

Copper

2XXX

Manganese

3XXX

Silicon

4XXX

Magnesium

5XXX

Magnesium
and Silicon

6XXX

Zinc

7XXX

Others

8XXX

Alloy
Type

Four
Digit Designation

Casting alloys :

 

99.00
% (min) Al

1XX.X

Copper

2XX.X

Silicon
with added copper and/ or magnesium

3XX.X

Silicon

4XX.X

Magnesium

5XX.X

Zinc

7XX.X

Tin

8XX.X

Others

9XX.X

Table 1: Designation system of aluminum
alloys 11.

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 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 increasing 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
exist, such as ?-Al5FeSi, ?-Al15Fe3Si2 and ?’-Al8Fe2Si.

Wrought Aluminium Alloys

The wrought aluminium 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, 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)     
Product
produced from rolling: Plates, flat sheets, coiled sheets, and foils.

2)     
Product
produced from extrusion: Extruded rods, solid and hollow shapes, profiles, or
tubes.

3)     
Product
produced from forming: rolled or extruded products are formed to achieve
complex shapes.

4)     
Product
produced from forging: they have complex shapes with superior mechanical
properties.

Table 2 shows some typical automotive
alloys and their application (share) 10. It also shows the most
representative wrought alloys used e.g. for radiators (3003), forged wheels
(6082), bumper beams (7020), internal (5182, 5457) and external (6016)
structural parts. The wrought alloys used for the calculation are divided into
extrusions (46%), low alloyed rolled products (25%) and high alloyed rolled
products (29%).

Table 2: Alloy composition of
automotive aluminium 10

 

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 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.
Refer Figure 1

 

Figure 1: The relation between
vehicle mass and fuel consumption (Eliezer et al., 1998)

 

The Aluminium
alloys uses for construction of automotive vehicles is one of methods for reduction
of the vehicle mass, as the density of aluminium 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 aluminium 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 effective
mass of the element made of aluminium 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 vehicle.

 

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

3. History of
Aluminium 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 Carl
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 a foothold in
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

4. Applications Aluminum Alloys In Automotive Industry:

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

·        
Powertrain
– Engine block & cylinder head, transmission housings, fuel system, liquid
lines and radiators: 69 kg

·        
Chassis
& suspension – Cradle, axle, wheels, suspension arms and steering systems: 37
kg

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

 

 

Figure
3: Aluminium products for automotive applications.

 

Figure
4: Aluminum application in cars 11

 

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.

 

 

Figure
5: Relative mass saving, absolute mass saving and market penetration obtainable
with Aluminum alloys 11

 

Figure
6: Trend of Aluminum content in cars in the last 40 years 11

 

In
the last forty years, it can be observed from Figure 6 that, the percentage of Aluminum
in 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 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 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 innovative light weight design with integrated functions. Generally, medium strength AA6000 and high strength AA7000
age hardening alloys are used, because the desired quenching occurs during the
extrusion process. Formability and final strength is controlled by heating for
age 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 on 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. Aluminium 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 its
performance and first of all the car’s passenger safety. The adopted materials
could be aluminium 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

5. Microstructure Evolution (Sheet production):

A processing layout
for production of Aluminium sheet alloys by DC – ingot casting, hot rolling,
cold rolling and final annealing treatment is shown 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.

 

 

Figure
6: Processing layout for Aluminum sheet for automotive application

 

Figure 7 shows the corresponding
microstructures and textures typical for most aluminum alloys.

 

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)     
Deformed
grains 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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure
7: Typical microstructure in different processing stages of Aluminum alloy
sheet 1

 

 

 

 

6. 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 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 varies 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
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
increases its strength without loss of ductility. In 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 show typical properties for Aluminum that
normally been used.

 

Table 3: Properties for aluminum.
(http://www.aalco.co.uk, 2012)

 

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 are 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 make them more susceptible to distortions from thermally induced stress
relief. Controlled stress relief can be done during manufacturing by 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 of
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 70
to 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 MPa?m 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 extend 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.

 

7. 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 or
in a variety of shapes (lineal extrusions, roll formed sheet, casting and
forgings), while steel sheet and plate are available mainly as mill products
(flat rolled sheet and plate, shape rolled or formed sheet). When substituting
Al alloys for steel, the following comparative properties need to be considered:

·        
Density
of Al alloy is one third that of steel.

·        
Elastic
Modulus of Al alloys is one third that of steel.

·        
Hardness
of Al alloys is lower

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

·        
Coefficient
of thermal expansion of Al alloys is about 1.5 times greater than that of steel
for an equivalent change in temperature.

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

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

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

·        
Steel
is 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.

·        
Al
alloys can be used unfinished in many applications and can accept a wide range
of mechanical and chemical finishes, while steel require 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.

8. General
Comparison of Aluminum alloys with fiberglass reinforced plastics in automotive

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