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Microstructure of Aluminium Alloys Casting Intended for Cyclical Thermal Stress

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The article presents the microstructure and mechanical properties of aluminium cylinder head engine. The cylinder heads are manufactured by gravity casting. Gravity casting (GC) is very good process for making complex mechanical parts of light metal like aluminium alloys. However recently another light metal has come to the forefront in the quest for fighter vehicles and improved fuel economy. The most commonly used aluminium alloy for gravity casting automotive components is the Al-Si-Mg type. AlSi10Mg is a good purity of aluminium alloy with good corrosion resistance, very good mechanical properties and good castability. AlSi10Mg alloy is slightly hypoeutectic with a low content of accompanying elements and impurities. Exhibits excellent casting and technological properties (good machinability and corrosion resistance). During solidification of this alloy is formed a shrinkage, which is necessary to eliminate by risering. This alloy has a wide foundry application, it is used for gravity casting into metal moulds for production of cylinder heads. This alloy is used for precipitation hardening intermediate Mg2Si phase. It was cast a cylinder head of AlSi10MgMn alloy and it was monitored the structure of the casting. © 2016. Published by Manufacturing Technology. All rights reserved.
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Microstructure of aluminium alloys casting intended for cyclical thermal stress
Iva Nova1 , Jiri Machuta1,
1Faculty of Mechanical Engineering, Technical university of Liberec, 461 17 Liberec, Czech Republic.
E-mail: iva.nova@tul.cz, jiri.machuta@tul.cz
The article presents the microstructure and mechanical properties of aluminium cylinder head engine.
The cylinder heads are manufactured by gravity casting. Gravity casting (GC) is very good process for
making complex mechanical parts of light metal like aluminium alloys. However recently another light
metal has come to the forefront in the quest for fighter vehicles and improved fuel economy. The most
commonly used aluminium alloy for gravity casting automotive components is the Al-Si-Mg type.
AlSi10Mg is a good purity of aluminium alloy with good corrosion resistance, very good mechanical
properties and good castability. AlSi10Mg alloy is slightly hypoeutectic with a low content of
accompanying elements and impurities. Exhibits excellent casting and technological properties (good
machinability and corrosion resistance). During solidification of this alloy is formed a shrinkage, which is
necessary to eliminate by risering. This alloy has a wide foundry application, it is used for gravity casting
into metal moulds for production of cylinder heads. This alloy is used for precipitation hardening
intermediate Mg2Si phase. It was cast a cylinder head of AlSi10MgMn alloy and it was monitored the
structure of the casting.
Keywords: aluminium alloy, crystallization, microstructure, cylinder head engine.
Introduction
Castings of cylindrical heads of internal combustion engines are very cyclical thermal stressed components.
Currently some car factories such as VW, BMW, Mercedes Benz-Daimler AC, etc. began to use for
manufacturing of heads of smaller volumes of aluminium alloy internal combustion engines. Aluminium alloys
are light and there is also an assumption of lower engine weight, respectively of the car and thereby saving fuel
mixtures. Another decisive factor for choosing light metal is an excellent thermal conductivity. Head engines are
highly thermally stressed and therefore the heat must be transferred as quickly as possible to the cooling liquid.
Therefor is aluminium, respectively its alloys very suitable material. Simultaneously, it is possible to cast the
aluminium. The whole massif of the head is provided with differently shaped channels in which flows the
cooling liquid. It is also necessary to secured the head against penetration of engine oil. Everything is still
complicated by thermal stress and pressure load of the head while the engine works. In modern turbo-diesel
engines are pressures up to 20 MPa. From foundry perspective it is necessary to choose the appropriate
technology for the production of engine head, which would ensure the consistency and compactness of the head
casting without internal defects and porosity. For this purpose it is used gravity casting into metal moulds with
sand cores. We are engaged with the issue of the production of castings for the automotive industry, at our
department the Department of Engineering Technology-Technical University of Liberec, many years. Currently,
we are focusing on aluminium alloys, their metallurgy and crystallization conditions with minimal internal
defects.
1 Specifics of gravity casting of aluminium alloys into metal moulds
[2,4,5,6,7]
Gravity casting uses the weight of aluminium to create a hydrostatic pressure of the melt and security of the
casting density with minimal porosity. The metal mould has a higher cooling effect, which leads to finer
structures and higher casting mechanical properties of the casting.
When casting into metal moulds the mould puts up stronger resistance when casting shrinkage at the time of
solidification and cooling. The mould is not permeable, so the focus is on its venting, preheating to a temperature
to 300 °C and a mould face was treating with insulat coat. The best gravity casting alloys are eutectic. To this
condition is approaching the used alloy AlSi10MgMn. The flow velocity of the melt in the mould is generally
less than 3 ms-1 (0.5 m height difference) without respecting pressure losses. Value of the hydrostatic pressure
decreases towards the inlet hole.
Aluminium alloys shrink in volume during solidification. This shrinkage is eliminated by preferable design of
the cast and especially by suitable risering (feeding).
Volume shrinkage can be calculated on the base of density of the cast alloy in the solid state and in liquid state.
You can perform a calculation according to the equation:
(1)
Where: VSC is shrinkage cavity volume;
VL is liquid volume;
VSM is solid metal volume.
On the base of weight equality it is possible express the dependences on the solidificated metal volume in the
context with liquid volume. (2)
Where: WSM is solid metal weight;
WL is liquid weight.
(3)
Where: SM is solid metal density (melting point) [kgm-3];
L is liquid metal density(melting point) [kgm-3].
(4)
For the alloy AlSi10MgMn can be calculated the shrinkage cavity volume. Density SM = 2546 [kgm-3] and L
= 2380 [kgm-3]. After the substitution of values into the equation (4) can be written:
(5)
Substituting this relationship into equation (1) can be written:
(6)
The equation (6) shows that the volume shrinkage is 7%. Aluminium alloys must be risering (feeding) gravity
casting into metal moulds.
2 Aluminium alloys used in the automotive industry and the influence of additive elements
[8, 9, 10, 11, 12, 13, 14]
Aluminium alloys used in the automotive industry are mostly Al-Si-Mg. The magnesium content is in an amount
from 0.25 to 0.45%, and can be precipitating hardened. At equilibrium rate of the cooling is magnesium excreted
as intermediate phase Mg2Si. Castings of these alloys are used for heavy-duty castings in automotive industry.
These alloys can be precipitation-hardened. After hardening, these alloys achieve high mechanical properties
(Rm, Rp0.2 and HB). Compared with the conditions after casting it slightly loses ductility. AlSiMg alloys are
undereutectic and according to the silicon content are mostly used two alloys AlSi10Mg and AlSi7Mg. Higher
silicon content provides better castability.
The elements contained in these alloys operates as follows [3]: Silicon increases the strength of the solid
solution, and also the corrosion resistance. With higher content it is present as a pure Si, thereby increas
brittleness. Casting alloys contain up to 25% of Si, but very common are alloys of eutectic composition (about
12% Si). Magnesium improves strength. Magnesium with silicon forms intermediate phase Si2Mg, which is the
SMLSC VVV
LSM ww
LLSMSM VV
L
SM
SMLVV
07.1
2380
2546 SMSMLVVV
07.0)107.1(07.1 SMSMSMSMSC VVVVV
basis for hardening of aluminium alloys. Magnesium improves corrosion resistance. In foundry alloys is the
content up to 11%. Manganese increases the strength, ductility and corrosion resistance. Its content usually does
not exceed 2%. Zinc increases the strength, but make toughness and corrosion resistence worser, its content in
foundry alloys is up to 6%. Nickel increases the strength and toughness at both normal and elevated
temperatures. It is used up to the content 2% in alloys intended for work at higher temperatures, e.g. for the
production of pistons for combustion engines. For aluminium alloys with Cu improves the corrosion resistance.
Iron make toughness and corrosion resistance worser. In foundry alloys up to 1%. It is significant in the alloys
for die casting, because it reduces the stickiness of the alloy foundry mould. Lead improves machinability of
aluminium alloys.
In the Tab. 1 there are alloys used for preparation of heads of the combustion engine.
Tab. 1 Commonly and occasionally used aluminium alloys for cylinder heads (composition of the casting
according to EN 1706)
Alloy
Si
Cu
Mg
Mn
Fe
Ni
Zn
Ti
AlSi10MgMn
9.0-11.0
0.05
0.20-0.45
0.45
0.55
0.05
0.10
0.15
AlSi10Mg(Cu)
9.0-11.0
0.35
0.20-0.45
0.55
0.65
0.15
0.35
0.15
AlSi8Cu3
7.5 - 9.5
2.0-3.0
0.05-0.55
0.15-0.65
0.8
0.35
1.2
0.25
AlSi6Cu4
5.0-7.0
3.0-5.0
0.55
0.2-0.65
1.0
0.45
2.0
0.25
AlSi7Mg0.3
6.5-7.5
0.05
0.25-0.45
0.10
0.19
0.03
0.07
0.08-0.25
AlSi7Mg
6.8-7.2
0.15
0.25-0.65
0.35
0.45
0.15
0.15
0.05-0.20
AlSi7MgCu0.5
6.5-7.5
0.4-0.6
0.25-0.45
0.10
0.19
0.03
0.07
0.08-0.25
AlSi9Mg
9-10
0.05
0.25-0.45
0.10
0.19
0.03
0.07
0.15
AlSi10Mg alloy is slightly undereutectic with a low content of accompanying elements and impurities. It
exhibits excellent casting and technological properties (good machinability and corrosion resistance). It does not
form inside shrinkage while solidification. This alloy has a wide foundry application, it is used for gravity
casting into metal moulds. To achieve better crystallization of silicon is suitable melt modification. Fig. 1 shows
an equilibrium diagram of the Al-Si and a part of ternary diagram of the Al-Si-Mg.
1a) 1b)
1c) 1d)
Fig. 1 The binary Al-Si system (1a - on the left); a part of ternary Al-Si-Mg system, temperature 20 °C, pressure
0.1 MPa (1b on the right); 1c, 1d structure of AlSi10MgMn alloys [1], [3]
Ternary system Al-Si-Mg belongs among relatively simpler types of diagrams. In equilibrium with the solid
solution (Al) is a compound Mg2Si. In the system Al-Si-Mg are no ternary phases. The stable phases are given
in Table 2.
Tab. 2 Solid phases in the Al-Si-Mg system
Phase of Al-Si-Mg system
Temperature
[°C]
450
370 - 450
460
1085
Phase
Al3Mg2
Al30Mg23
Al12Mg17
Al5Mg4
Mg2Si
Indication
-
-
Fig. 1 b) shows the isothermal section of the system Al-Si-Mg by normal temperature designating areas of the
individual phases. Balance with Mg2Si are dominant in the concentration triangle. At room temperature, are all
phases in equilibrium with Mg2Si [1].
3 Experimental part
The head of the internal combustion engine was cast into a metal mould made of tool steel 1.2343 by gravity
casting aluminium alloy EN AC 43000, chemical designation EN AC AlSi10Mg (a) (the CSN 42 4331 it is
AlSi10MgMn alloy). Metal mould was during casting preheated to 260 °C. The mould was provided with an
insulating coating DYCOTE 6 cores for the production of a cylinder head engine were manufactured by Cold
Box. The melt was melted in a gas furnace, the alloy was before casting metallurgically treated, vaccinated and
modified. For the vaccination was used ligatures AlTiB and for the modification was used strontium. Before
casting the melt was refined and degassed. For degassing was used nitrogen. The melt was cast from a
temperature of 700 °C. The chemical composition of the alloy AlSi10MgMn was evaluated by using the
analyzer Q4 Tasman and is shown in the Tab. 3. In the Fig. 2 is a cast and analyzed cylinder head.
Tab. 3 Chemical composition of alloy AlSi10MgMn for cylinder heads (composition of the casting according to
EN 1706)
Alloy
Chemical composition
Si
Mg
Mn
Cu
Fe
Ni
Zn
Ti
Al
AlSi10MgMn
10.2
0.28
0.22
0.05
0.15
0.1
0.03
0.11
rest
Fig. 2 Cylinder head of an internal combustion engine made of AlSi10MgMn alloy,melt by gravity casting in a
metal mould of steel 1.2343
4 Metalographic evaluation of the structure of the cast
The samples for metallographic observation were carefully polished, as the final step was used mechanical-
chemical polishing with colloidal silica (OP-S). The structure was examined using scanning electron microscope
(SEM) Zeiss Ultra Plus equipped with energy-dispersive spectrometer (EDS) Oxford X-Max for local chemical
analysis and EBSD detector Oxford Nordlys Nano for crystallographic analysis. SEM images were taken in
compositional contrast, EDS analysis were carried out at accelerating voltage 10 kV; for the EBSD was used
accelerating voltage 20 kV and sample tilt 70°.
Fig .3 The structures with EDS analysis in two specific places for cylinder head of an internal combustion
engine made of AlSi10MgMn alloy melt by gravity casting in a metal mould of steel 1.234.
Fig. 4 EDS map of sample from cylinder head of an internal combustion engine made of AlSi10MgMn alloy
showing element distribution.
5 Simulation calculation of solidification and cooling
[15, 16]
Simulation calculation of solidification and cooling was made at a the cylinder head engine by using simulation
software MAGMA. [6], [7], [8]. On the fig. 5 are shown used temperature dependence of temperature-physical
quantities of the metal form of steel 1.2343. On the fig. 6 are shown temperature dependence of temperature-
physical quantities of the melt alloy AlSi10MgMn. On fig. 7 and 8 are shown demonstration of simulation
calculations.
Fig. 5 Values of the thermal properties metal mould material 1.2343, depending on the temperature (density,
thermal conductivity, specific capacity and heat transfere coefficient) [18]
Fig. 6 Values of the thermal properties AlSi10MgMn alloy depending on the temperature (density, thermal
conductivity, specific capacity anf heat transfere coefficient) [18]
Fig.7 Filling of the foundry mould with the melt with current distribution of temperatures, which corresponds
temperatures 640 620 °C (on the left. Distribution of temperatures in solidifiing cast of the cylinder head in the
time 34 [s] after casting TL =595 °C, TS = 555 °C (on the right)[17]
Fig. 8 Distribution of temperatures in solidifiing cast in the time 115 [s] after casting(on the left), Distribution of
microporosity in definite places of the cast (on the right) [17]
Conclusions
The alloy selection for cylinder head castings requires the consideration of various criteria, some of them are
similar to those used in case of engine blocks. Castings of cylinder head are farther heat treat with a subsequent
aging. The effects of aging is also possible to take in account during operation of the engine over a long period.
The best combinations of strength and ductility are offered by casting alloys with low iron content such as
AlSi7Mg0.3 (A356). Therefore, in the past, most cylinder heads were made from primary aluminium alloys.
Today it is attempted to use for the manufacture of heads also alloys, which are made of recycled aluminium (i.e.
with a slightly increased content of impurities) such as AlSi10Mg or AlSi7Mg. These alloys provide sufficient
ductility and can be used for low engine power. For high performance of diesel engine heads were developed
special alloys (eg. AlSi7MgCu0.5, AlSi9Cu1Mg and AlSi7MgCuFeNi). They provide higher strength at elevated
temperatures, while maintaining ductility and fatigue performance. Cylinder heads made of these alloys are
generally applied in the T6 condition. Next to the selection of a suitable alloy, it is important to use the correct
casting technology and metallurgical preparation of the melt (including vaccination, modification and
degassing). To obtain compact castings is suitable technology of gravity casting in a metal mould preheated to a
temperature of 280 °C, and treated with an insulating coating.
The project is financially supported by SGS 21 122.
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... For the fabrication of light-vehicle cylinder heads, light aluminium alloys are used by means of a casting process [1], these alloys are used for their low weight, better heat dissipation, low density and excellent thermal conductivity [2], the cylinder head is subjected to high mechanical and thermal stresses during operation, causing fractures such as: cracks, fissures, discontinuities, impurities, roughness and pitting due to corrosion, fatigue and caloric performance [3][4][5][6]; conventional nondestructive tests are used for the detection of these defects, which allow the material to be evaluated without the need to damage or cause alterations in the physical or chemical state, in order to detect imperfection [7]. ...
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Structural analysis of AlSi cast alloys
  • E Tillová
  • M Chalupová
TILLOVÁ, E., CHALUPOVÁ, M. (2009) Structural analysis of AlSi cast alloys. Žilina: EDIS.