Magnesium Alloy AZ91E and its Usage in Bicycle Frame Design

ME 250 Design Project.
Magnesium alloy AZ91E and its usage in Bicycle Frame
Design
Samip Subedi
Ankit Lohani
ME 250, Section 1;
Brigham Young University, Provo, UT
Email:- alexander_emperor@yahoo.com
ankit.lohani7@gmail.com

INTRODUCTION:
A bicycle is a two wheeled vehicle driven by the riders feet pushing on cranks or pedals. A
typical bicycle is comprised of several parts. The principal and essential being: the frame, front
forks, wheels, pedals, saddle, handlebars, chain and brakes. The other important part in a bicycle
is it’s wheels which are responsible for its motion. A bicycle stays upright while moving forward
by being steered so as to keep its center of gravity over the wheels. This steering is usually
provided by the rider, but under certain conditions may be provided by the bicycle itself. The
bicycle is extraordinarily efficient in both biological and mechanical terms. The bicycle is the
most efficient self-powered means of transportation in terms of energy a person must expend to
travel a given distance. From a mechanical viewpoint, up to 99% of the energy delivered by the
rider into the pedals is transmitted to the wheels, although the use of gearing mechanisms may
reduce this by 10-15%. In terms of the ratio of cargo weight a bicycle can carry to total weight, it
is also a most efficient means of cargo transportation and its wheels have a huge role to play for
its efficiency. A bicycle frame is the main component of a bicycle, onto which wheels and other
components are fitted. The modern and most common frame design for an upright bicycle is
based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear
triangle. This is known as the diamond frame. In the diamond frame, the main triangle consists
of the head tube, top tube, down tube and seat tube. The rear triangle consists of the seat tube,
and paired chain stays and seat stays. The head tube contains the headset, the interface with the
fork. The top tube connects the head tube to the seat tube at the top, and the down tube connects
the head tube to the bottom bracket shell. The rear triangle connects to the rear dropouts, where
the rear wheel is attached. It consists of the seat tube and paired chain stays and seat stays. The
chain stays run parallel to the chain, connecting the bottom bracket to the rear dropouts. The
seat stays connect the top of the seat tube (often at or near the same point as the top tube) to the
rear dropouts.
METHODS:

Magnesium, an alkaline earth metal, is the ninth most abundant element in the universe
by mass. The commonness of magnesium is related to the fact that it is easily built up in
supernova stars from a sequential addition of three helium nuclei to carbon (which in turn is
made from a single reaction between three helium nuclei at once). Magnesium constitutes about
2% of the Earth's crust by mass, which makes it the eighth most abundant element in the crust.
Magnesium ion's high solubility in water helps ensure that it is the third most abundant element
dissolved in seawater. AZ91E alloy is a good substitute for the current materials that have
been on use for the manufacturing of the Bicycle frame. Then AZ91E alloy is composed of
Magnesium, Aluminum, Zinc and Manganese. AZ91E alloy is cheap but are also difficult to cast
successfully. Sand castings are prone to outcropping, microporosity, and achieving pressure tight
castings can be a problem unless extensive sealing treatments are carried out. AZ91E is used
primarily in high pressure die cast form particularly by the automotive industry, as car makers
worldwide aim to reduce vehicle weights in order to improve fuel economy. Sand cast Mg-Al-Zn
alloys have reasonable properties in the T6 condition up to 120°C, although strength then
decreases rapidly with increasing temperature and mechanical properties can vary substantially
depending on section thickness. Despite these drawbacks, AZ91E castings are used in some
aerospace applications, including complex aerospace sand castings such as the Westland Sea
King helicopter gearbox. Introduction of high purity variants of these alloys with lower levels of
heavy metal impurities (iron, copper, and nickel) has significantly improved corrosion
performance. The sand casting alloy AZ91C has now been largely replaced by its high purity
variant AZ91E which has a corrosion rate around 100 times better in salt fog tests.
Temperature dependence:- The alloy has a
liquidus and solidus temperature of 595C and 495C
which makes it perfect for usage. Furthermore, the
coefficient of linear thermal expansion, Specific
Heat , latent heat of fusion and thermal conductivity
of the alloy are also perfect for usage.
Stiffness:- The material itself is stiff enough to
handle a load from 0 to 200kg which is good enough
to handle the weights of normal people.

Fracture Toughness:- The bicycle frame should be able to show resistance to crack growth
in the material which makes it less susceptible to fractures. The material has a fracture
toughness exceeding 80-95 Mpa at 1*10 8 cycles which is good enough for using it bicycle
design.
Yield Strength:- The bicycle frame should have sufficient yield strength so that they will be
able to withstand the plastic deformation that will occur in the material because of the weight
of the human body. The yield strength of the material is about 83-97 MPa with the
temperature ranging from 0 to 240C which is good enough to protect the material from
plastic deformation at normal conditions.
Fatigue limit:- The current material steel has an expected life span of 5 to 7 years. Beside
this, it is also an easy target of plastic deformation. So, we are in need of such a material
which will be able to withstand for a longer period of time and with less chances of getting
damaged and this alloy gives us the perfect blend of our needs and it can withstand stress to
RESULTS:-
more than that time frame.
Cost:- The cost of the existing
material steel is about $100-$1000 for
the whole plastic replacement system.
So, for the new material the price
should be around this range or a bit
higher based on its performance level.
The new material price ranges to about
$800 per cubic foot.
Corrosion testing:- The Corrosion behavior of AZ91 magnesium alloy in simulating acid rain
under wet-dry cyclic condition has been investigated. The results show that corrosion potential
shifts positively and the corrosion current density decreases at low wet-dry cyclic time. Further

increase of the cyclic time results in the negative movement of corrosion potential and the
increase of current density. SEM observation indicates that corrosion occurs only in α phase, β
phase is inert in corrosive medium, and the corrosion of AZ91 magnesium appears in uniform
characteristic. XPS analysis suggests that the corrosion product is mainly composed of oxide and
hydroxide of magnesium and aluminum, and a small amount of sulfate is also contained in the
film.
Damping Behavior:- The amplitude-dependent damping of a commercial magnesium cast alloy
AZ91 has been determined at room temperature by measurement of the logarithmic decrement
of free decaying vibrations of bending beams clamped at one side. In order to generate cracks in
the specimens they were subjected to (1) isochronal heat treatments for 1 h at temperatures above
400 °C with succeeding quenching in cold water and (2) controlled fatigue bending loading in
the same equipment also used for the damping measurements. After both treatments, the
amplitude-dependent damping curves show a maximum for strains 10 −5 to 10 −3 , which can be
correlated with the presence of cracks and can be explained by a simple rheological model based
on crack damping. This maximum is enhanced when the number of loading cycles or the
quenching temperature is increased which can be explained by crack nucleation. Crack growth
with increasing number of loading cycles shifts the maximum to lower strains.

From the measured fatigue-affected Young’s modulus the damping curves and the damping
model the following conclusions could be drawn.
1. Increasing number of cycles to fatigue increases the maximum and shifts it to smaller
Conclusion:-
strains. This is reasonable if one assumes that during cycling both, crack nucleation, and
crack growth take place and that the critical strain of a crack is monotonously decreasing
with crack length l, e.g. like a demand of crack growth theory. Therefore, the magnesium
AZ91 alloy acts as a good substitute for steel which has been in current use.
The greatest advantage of choosing magnesium alloys for engineering designs is its low
density, which can contribute significantly to the aspect of weight savings in the design and
construction of bicycles. The AZ91E magnesium alloy, preformed with complete shape, has
been prepared using spray forming technology under a protective atmosphere. The
microstructure and mechanical properties have been investigated. The average tensile ultimate
and yield strength of the spray-formed and extruded AZ91 magnesium alloy samples were
435 MPa and 360 MPa with a room temperature elongation of 9.2%, indicating an enhanced
combination of toughness and strength. So, this is a good material choice for this operation.
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Powder Metall 40 (1997), pp. 126–130
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I.S. Golovin, H.-R. Sinning, Internal friction and damping behavior of metallic foams and
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