Comparison of Mechanical Properties of AlSiMg Alloy with Varied Composition

BSR Venu Madhav, Abdul Ahad Mohiuddin, Mohammed Atifuddin Department of Mechanical Engineering
Chaitanya Bharathi Institute of Technology, Hyderabad, India
International Journal of Engineering & Technology Research
Volume 4, Issue 4, July-August, 2016, pp. 12-19
ISSN Online: 2347-4904, Print: 2347-8292, DOA : 12082016
© IASTER 2016, www.iaster.com
ABSTRACT
The AlSiMg 0.3 casting alloy is used in the motor vehicle and aerospace industry, for parts in
mechanical engineering, shipbuilding, fitting etc which require the casting alloy to have very high
strength and hardness along with corrosion resistance. In order to optimize these properties, there is
need to study the effects of individual alloying elements in required proportions at different aging
conditions. With the aim of improving the mechanical properties of LM-25 alloy, studies have been
carried out on the effects of various alloying elements on its mechanical properties. In this process, we
arrived at a particular range of compositions of the alloying elements for LM-25 by conducting several
tests using the software and arrived at the final improved composition which gave superior mechanical
properties.
Keywords: Aluminium, Brinell’s Hardness Test, Elongation, Hardness, LM25, UTM.
1. INTRODUCTION
Aluminium is the world’s most abundant metal and is the third most common element comprising 8%
of the earth’s crust. The versatility of aluminium makes it the most widely used metal after steel.
Aluminium has a density around one third that of steel or copper making it one of the lightest
commercially available metals. The resultant high strength to weight ratio makes it an important
structural material allowing increased payloads or fuel savings for transport industries in particular.
When exposed to air, a layer of aluminium oxide forms almost instantaneously on the surface of
aluminium. This layer has excellent resistance to corrosion. It is fairly resistant to most acids but less
resistant to alkalis. Pure aluminium doesn’t have a high tensile strength. However, the addition of
alloying elements like manganese, silicon, copper and magnesium can increase the strength properties
of aluminium and produce an alloy with properties tailored to particular applications.
2. ALLOYS OF ALUMINIUM
Aluminium alloys are the alloys in which Aluminium is the predominant model. The typical alloying
alloying elements are copper, magnesium, manganese, silicon, tin, and zinc. There are two principal
classifications, namely casting alloys and wrought alloys, both of which are further subdivided into
categories heat treatable and non heat treatable. About 85% of aluminium is used for wrought products, for
example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the
low melting point, although they generally have lower tensile strengths than wrought alloys. The most
important cast aluminium ally system is Al-Si, where high levels of silicon (4-13%) contribute to give good
casting characteristics. Aluminium alloy surfaces will develop a white, protective layer of aluminium oxide
if left unprotected by anodizing and correct paint procedures. In a wet environment, galvanic corrosion can
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occur when the aluminium alloy is placed in electrical contact with other metals with more positive
corrosion potentials than aluminium, and an electrolyte is present that allows ion exchange referred to as
dissimilar-metal corrosion, this process can occur as exfoliation or as intergranular corrosion. Aluminium
alloys can be improperly heat treated. This causes internal element separation, and the metal then corrodes
from the inside out. Aircraft mechanics deal daily with aluminium corrosion.
Aluminium alloys with wide range of properties are used in engineering structures. Alloys systems are
classified by a number system (ANSI) or by the names of the main alloying constituents (DIN & ISO).
Selecting the right alloy for the given application entails considerations for its tensile strength , density
,ductility, formability, workability, weldability, and corrosion resistance, to name a few. Aluminium
alloys are used extensively in aircraft due to their high strength to weight ratio. On the other hand, pure
aluminium metal is too soft for such uses, and it doesn’t have the tensile strength that is required for
the airplanes and helicopters.
3. PHYSICAL PROPERTIES
Coefficient of Thermal Expansion (per *C at 20 -100 *C) 0.000022
Thermal Conductivity (Cal/cm2/cm/*C/s at 25*C 0.36
Electrical Conductivity (% copper standard at 20*C) 39
Specific Gravity 2.68
4. MECHANICAL PROPERTIES [3]
Tensile Stress (N/mm2)* 130-150 150-180 160 230-280
Elongation (0%)* 2 1 2.5 -
Brinell’s hardness Number 55-65 70-75 65-75 90-110
Endurance Limit (5 X 108 70-100 55 60 60
Cycles + N/mm2)
N/mm2)
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0.2% Proof Stress (N/mm2) * 80-100 130-200 90-110 220-260
Tensile Stress (N/mm2)* 160-200 190-250 230 280-320
Elongation (%) 3 2 5 2
Brinell Hardness Number 55-65 75-95 65-75 90-110
Endurance Limit (5 X 108 80-110 75 95 95
Cycles + N/mm2)
N/mm2)
5. SILUMIN
Silumin is the name that is used is some countries for alloys based on Al-Si system. Silumin is a series
of light weight, high-strength aluminium alloys with silicon content within the range of 3-50%. Most
of these alloys are casting ones, but also it is produced by rapid solidification processes and powder
metallurgy.
5.1. Advantages
1- Its high resistance to corrosion, making it useful in humid environments.
2- The addition of silicon to aluminium also makes it less viscous when liquid, which together with its
low cost, makes it a very good casting alloy and a fresher metal.
3- It is also used on 3 phase motors to allow speed regulation, Another use is rifle scope mounts
5.2. Characteristics
1. High castibility, high fluidity, high corrosion, high ductility, low specific gravity, high
machinabilty
2. Used for large casting, which are to be operated under heavy load conditions.
3. Strengthened by solution treatment, e.g. adding 0.01%Na (in the form of NaF & NaCl) to melt just
before casting.
6.1. Magnesium
Magnesium is the major alloying element in the 5xxx series of alloys. Its maximum solid solubility in
aluminum is 17.4%, but the magnesium content in current wrought alloys does not exceed 5.5%. The
addition of magnesium markedly increases the strength of aluminum without unduly decreasing the
ductility. Corrosion resistance and Weldability are good.
6.2. Silicon
Silicon, after Iron is the highest impurity level in electrolytic commercial Aluminum (0.01 to 0.15%).
In wrought alloys, silicon is used with magnesium at levels up to 1.5% to produce Mg2Si in the 6xxx
series of heat-treatable alloys.
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6.3. Iron
Iron is the most common impurity found in aluminum. It has a high solubility in molten aluminum and
is therefore easily dissolved at all molten stages of production. The solubility of iron in the solid state
is very low (~0.04%) and therefore, most of the iron present in aluminum over this amount appears as
an intermetallic second phase in combination with aluminum and often other elements.
6.4. Zinc
Aluminum-zinc alloys containing other elements offer the highest combination of tensile properties in
wrought aluminum alloys.
6.5. Copper
Aluminum-copper alloys containing 2 to 10% Cu, generally with other additions form important families of
alloys both cast and wrought aluminum-copper alloys respond to solution heat treatment and subsequent
aging with an increase in strength and hardness and a decrease in elongation. The strengthening is
maximum between 4 and 6% Cu, depending upon the influence of other constituents present.
6.6. Manganese
It decreases resistivity. Manganese increases strength either in solid solution or as a finely precipitated
intermetallic phase. It has no adverse effect on corrosion resistance. Manganese has a very limited
solid solubility in aluminum in presence of normal impurities but remains in solution when chill cast so
that most manganese added is substantially retained in solution, even in large ingots.
6.7. Nickel
Nickel (up to 2%) increases the strength of high-purity aluminum but reduces ductility. Nickel is added
to aluminum-copper and to aluminum-silicon alloys to improve hardness and strength at elevated
temperatures and to reduce the coefficient of expansion.
6.8. Lead
Lead is added at about the 0.5% level with the same amount as bismuth in some alloys to improve
machinability.
In heat treated condition, tin reduces hardness increases elongation, reduces yield strength but did not
affect Ultimate Tensile Strength (UTS).
6.10. Titanium
Titanium is added to aluminum primarily as a grain refiner. The grain refining effect of titanium is
enhanced if boron is present in the melt or if it is added as a master alloy containing boron largely
combined as TiB2. Titanium is common addition to aluminum weld filler wire as it refines the weld
structure and helps to prevent weld cracking.
6.11. Strontium
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7. METHODOLOGY
Procurement of the two Alloys- LM 25 and Varied Composition One
Acid Pickling of Casting
Aging
Machining
Fig 1 : Brinell’s Hardness Machine Fig 2 :Universal Testing Machine
8. TESTING OF MECHANICAL PROPERTIES
Tensile strength
Yield Strength
Compressive strength
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9. RESULTS
The studies conducted hereby were to improve the mechanical properties and microstructures of the
AlSiMg alloy with varied composition.
9.1. Ultimate Strength and Percentage Elongation Values
Table No 1. Variation in Tensile Strength and the Percentage
Elongation at Constant Temperature with Varied Alloy Composition
Temperature Composition Tensile
Strength(N/mm2) % of Elongation
For Specimen 1
Maximum load = 21948.00N
9.2. Brinell’s Hardness Values
Table No. 2. Variation in Hardness after Solutionizing at Constant Temperature
with Varied Alloy Composition
Hardness Value Indentation (mm) Hardness Value
500±10°C Standard 80.17 2.8 79.57
500±10°C Standard 80.17 2.82 78.42
500±10°C Varied 117.36 2.32 116.66
500±10°C Varied 117.36 2.31 117.69
BHN = 2 P / (π D (D - (D 2 -d
2 ) 1/2
)) Where P = Force applied = 500kg; D = Diameter of indenter = 10mm
d = Diameter of indentation (in mm); Test Specs: LM-25 Percentage of Elongation: 2.88; Composition: STANDARD
For Specimen 1(UTS value of bar–1)
Load at Peak: 21948.00N
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For Specimen 2(UTS value of bar-2) Load at Peak: 23644.00N
Tensile Strength @ of peak: 283.36Mpa
Fig 3 Microstructure of AlSi7Mg (Varied) Fig 4 Microstructure of AlSi7Mg (Standard) [2]
LM25 (AlSi7Mg) ALLOY (STANDARD)
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10. CONCLUSIONS
It can be concluded that the varied composition specimen obtained by carefully varying elemental
composition of the LM 25 alloy gave superior mechanical properties like improved ultimate strength,
% of elongation, hardness and a refined grain structure as shown in Fig 3 compared to the standard
composition specimen of LM 25 alloy specified in Fig 4. Hence, it finds a wider area of applications
in industries which require improved properties.
REFERENCES
[1] Hadleigh Casting Aluminum Technology. “LM25 Aluminum Casting Alloy (Al – Si7Mg).
[2] Stanislava Fintová, Radomila Konená, Gianni Nicoletta. “Microstructure, Defects and Fatigue
Behavior of Cast Alsi7mg Alloy”. Acta Metallurgica Slovaca, Vol. 19, 2013, No. 3, p. 223-231.
[3] Venkatachalam, Kumaravel, Arum Kumar, and Dhanasekaran Raja Gopal, “Studies on
Microstructure and Mechanical Properties of Modified LM25Aluminium Alloy” ISSN (online):
2348 – 7550.
[4] R. Harikrishnan et al., “Evaluation of Mechanical Properties of Aluminum Metal Matrix
Composites”, Intl. J. of Research in Mechanical Engineering, Vol.3, Issue 3, May-June, 2015,
pp.14-18.
[5] Akhilesh Jayakumar, Mahesh Rangaraj. “Property Analysis of Aluminium (LM-25) Metal
Matrix Composite”. ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 2,
February 2014.