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Menghani, Jyoti1; Bhushan, Bharat1; Singh,
Balraj1; Suthat, Deepak1; Shah, Dimple1
SVNIT Surat,India
Aluminum, magnesium alloys and plastics have received growing attentions as light-weight materials for automobiles
Al alloys with Si as major alloying addition are one of the most effective way to obtain good Al casting mainly because of high fluidity imparted by presence of relatively large volume of Al-Si eutectic.
A356(al-7Si-0.3Mg) is hypoeutectic heat treatable Al-Si alloy. Presence of Mg in Al-Si alloy makes it heat treatable.
Light weight,
Excellent weldability,
Corrosion resistance,
Good mechanical strength,
Ductility,
Hardness,
Fatigue strength,
Pressure tightness,
High fluidity and Good machinability.
Good plasticity, High durability limit,
Recyclability
Widely used for automotive industry,( for
example for cylinder heads and engine
blocks)
Aerospace parts
Military parts
The microstructure and hence Mechanical
properties of A356 casting alloys consists of
primary aluminium dendrites, eutectic Si,
intermetallic compounds and solidifications
defects, such as gas and shrinkage porosity and
oxides.
The volume fraction of dendrites and eutectic
particles is determined by chemistry, whereas
the size, morphology and distribution of the
dendrites, eutectic particles and intermetallics
depend on the local solidification conditions
On-going research has sought to enhance the
mechanical and wear resistance properties of Al–Si
alloys by controlling their microstructures via
Suitable casting procedures ,
Heat treatment
Addition of minor alloying elements
Grain Refinement
Modification
Two main processes to give better structure and
mechanical properties are :
(i) The addition of alloying elements during melting
and melt treatment of the liquid alloy through
Grain refining and Modification, and/or
(ii) Heat-treatment.
Slow solidification of pure Al-Si-Mg produces a very
coarse eutectic microstructure consisting of large
plates or needles of silicon in aluminium matrix.
The silicon particles appear to be interconnected.
The coarse eutectic results in low ductility because
of brittle nature of large silicon plate.
Most of the mechanical properties of castings are
determined by the eutectic microstructure.
Al-Si Eutectic phase
diagram
Al-Si Eutectic phase
diagram after Na
addition
Conversion of coarse acicular Si needle to fine fibrous structure is called Modification.
Modification of Al-Si-Mg alloys is commonly done as it results in refinement of microstructure, by adding ‘MODIFYING AGENT’ to the melt prior to pouring.
Sodium and Strontium are most widely used as a modifiers.
Main drawback is Sodium is rapidly lost in the molten Aluminium through its high vapor pressure so that modifying effects are transient. Periodic additions are required to maintain modification levels.
Sodium modified alloys are also prone to gas pick up and micro shrinkage.
Strontium additions as Al-Sr master alloy when
added retains the modified effects even on
remelting.
However Sr forms undesirable Fe-Sr
intermetallic. In addition Sr shows fading effect
(Reduction in effectiveness of modifier on
holding modifier in melt for longer time).
Rare earth in form of Misch metal (Ce-55%, Nd-
15%,La-15%,others)increases under cooling
which suppresses growth of Si and fading effect
seemed much less.
There are two principles through which there
is improvement in Mechanical properties:
(1)Elements that satisfies modification
criterion as proposed by Lu and Hellawel.
(2)Elements having atomic size similar to Al.
When Al-Si eutectic solidifies it has been shown that modifying elements become concentrated in the silicon phase rather than Aluminium i.e. a Constitutional effect will be present at the solidification front during solidification.
Silicon is semi metal and solidifies in faceted manner. Twins are easily formed in silicon crystals on <111>plane. Atoms of the modifier are absorbed onto the growth steps of Si solid interface.
A growth twin is created at the interface when the atomic radius of the element relative to silicon size exceeds 1.65.
This multiple twinning may result in the growth directions of primary silicon to change from anisotropic growth to almost isotropic growth The coarse plate like growth was suppressed, coarse primary silicon in hypoeutectic Al–Si-Mg alloy could be modified.
A schematic illustrating primary Si growth in Al-Si alloy
(a) Anisotropic growth in unmodified alloy
(b) Isotropic growth in modified hypoeutectic Al-Si due to impurity induced twinning.
Element Atomic Radius Ratio r/rsi Modifying effect
Sr 2.16 1.84 +
Na 1.97 1.68 ++
Ca 1.87 1.59 +
La 1.87 1.59 +
Ce 1.76 1.56 +
Si 1.755 1.00 +
Stirring treatment of molten metal has
significant effects on the solidification
microstructure of A356 alloy, which includes
grain structure, distribution of inclusions,
refinement of secondary phases, etc.
The most important aim of tribology science is the precise determination of the nature of the dependence between the surface interaction behavior and the fundamental properties, which themselves are related to microstructural features.
Wear of a material is controlled by
1.Material characteristics including metallurgical and mechanical properties of alloy
2.Operating parameters such as
applied pressure,
sliding speed,
environment and
the type of sliding interaction.
Coarse and needle-shaped eutectic and large primary silicon particles increase adhesive wear
The grain morphology of various phases in Al–Si
alloys influences the wear and mechanical
behavior.
In general, fine, spherical and uniformly
distributed micro-constituents can improve the
wear and mechanical properties.
Thus wear behavior of polyphase alloys is
complex and not simple function of composition
Considering A356 (Al–7Si–0.3 Mg),an important alloy
for automobile industry, particularly for wheel
production effect of Rare earth (R.E) and Stirring on
A356 in terms of refinement of microstructure,
Wear and hardness was undertaken.
In present study, it was considered that R.E
addition contributes not only as modifier but acts as
alloying element
In this work systematic studies on effect of
individual or combined addition of Rare earth and
stirring on eutectic Si and primary Al solidification
of A356 was performed.
The relationship between mechanical properties
and microstructure are also discussed in order to
improve mechanical properties and microstructure
of alloy and help enterprises enhance the quality of
product.
Fresh ingots of A356 of about 600 gms weight were melted in crucible type electric resistance furnace. Melting was carried out under cover flux.
Lime coated clean iron tools were used. Rare earth (0.5wt%, 1.5wt%) is added in form of pure metal wrapped in Al foil was added to the melt.
Stirrer was attached to vertical drilling machine and stirring was carried out for about five minutes at varying stirring speed of 400 and 600 rpm.
Increasing stirring time beyond 5 minutes resulted in solidification of molten alloy.
Metallography Metallographic samples were prepared
as per normal procedure and etched in 0.5% HF solution. Optical Microscopy was used to see Microstructure. Wear surface was observed at various magnification in SEM (Hitachi 3400).
XRD Observation: The alloyed and unalloyed LM25 have
been characterized by X ray diffraction(XRD) using Cu kα radiation in as cast and heat treated conditions.
Wear tests were carried out,using a Pin-On-Disc-
typewear-testingmachine(TR-20,DUCOM)as per
ASTMG99–90
The normal load/pressure was applied on pin by
dead weight through a pulley string arrangement.
The system had maximum loading capacity of
200N. Disc was rotated by DC motor, having a speed
range of 0–2000 rpm to yield sliding speed of 0–10m/s.
Alloy were machined to produce wear pin of size Wear test specimens are rounded bars with flat surface having dimensions of 20mm length 10mm diameter.
The flat portion of 10mm diameter of the test specimen was in contact with a rotating disc. One end of wear pin was polished to 0.5μ.
Wear pin was held against counter surface of EN24 hardness Rc 63.
Tribological characterization was done by using Pin on
disc wear testing machine with the process parameters
selected were 45N load 240rpm and for distance of 6m
The wear test was carried out at temp of 40C
SEM of wear surfaces was carried out to study the mode of wear. Wear behavior of samples(A356,A356 with0.5 wt%R.E and 1.5wt%R.E)with and without stirring was studied using pin-on flat type wear testing unit (DUCOM pin on disc wear tester).
The microstructure of unalloyed A356 with
stirring and without stirring indicates that
even though rare earth was not added the
morphology of eutectic Si is altered due to
the stirring effect.
The stirring has direct effect on grain
refinement and indirect effect on
modification, morphology of eutectic
structure.
I(A)0wt%R.E without Stirring I(B)0 wt %R.E Stirring at 400 rpm I©0 wt %R.E Stirring at 600 rpm
II(A)0.5wt%R.E without Stirring II(B)0.5 wt %R.E Stirring at 400
rpm
II©0.5wt %R.E Stirring at 600 rpm
III(A)1.5wt%R.E without Stirring III(B)1.5 wt %R.E Stirring at 400
rpm
III©1.5wt %R.E Stirring at 600 rpm
The as cast specimen clearly shows the large and
steep silicon needles present in the microstructure
of A356 base alloy. It decreases the tensile strength
of the material.
After stirring the base alloy the specimen produced
by casting is seen with some blunt and small
needles which consequently increase its properties
up to a certain level.
The modifiers produce the blunt and somewhat
globular needles which can be seen in the image
above. This type of grain structure is considered to
be great in terms of strength analysis
The stirring has direct effect on grain refinement and
indirect effect on modification, morphology of eutectic
structure.
It can be seen from microstructure that the case where
metal is stirred the volume of interdendritic liquid
between primary islands is reduced. This physical
constriction put constraints on the size of Silicon
needles upon eutectic transformation.
The effect of varying amount of rare earth (0.5 wt%
and 1.5 wt %) on as cast microstructure and
microstructure obtained after stirring is shown .
Rare earth addition seems to modify structure
appreciably in both with and without stirring. However
addition of 1.5 wt% rare earth results in recoarsening of
Si needles and new phases may have formed.
A typical XRD diffractogram for 0.5wt% rare earth inoculated A356 with stirring of 400 rpm is shown in Figure
The search done by XRD software doesn’t give any lanthanum-aluminium based compounds. Following compounds are found on the basis of test carried out: Al4Ce, CeMg2Si2, CeMg, MgAl2O4, AlCe3, MgO.
It is the limitation of software used to not being able to identify the less quantity lanthanum based compound.
If this is compared to unalloyed A356, then new rare earth based compound phases are observed which gives higher strength to alloy and cause discontinuous dendrites.
Fig (a) 0%R.E 400 rpm
Fig (b) 0%R.E 600 rpm
Considering the case of unalloyed A356 and 400 rpm
stirring on examination of the worn surfaces at
higher magnification, (fig a) evidence of extensive
plastic flow and cracking was observed.
Cracks may initiate in the highly work-hardened
layer, starting from the subsurface region.
When cracks grow and get interconnected, a layer
of metal is removed leading to delamination wear.
Extent of deformation is more however cracks are
less in case of unalloyed A356 and 600 rpm (fig (b))
Fig (c)0.5%R.E Without stirring
Fig (d)1.5%R.E Without stirring
Considering case of Rare earth added A356, (fig c & d) observing the worn surface at high magnification evidence of extensive plastic flow was observed.
However in fig (c) the hollow portion is seen. It may be blow hole or any other casting defect. In Fig (d) small porosities are observed. Also extent of work hardening of surface is observed.
Wear debris were not observed because delaminated wear debris particles were severely work-hardened and fractured and they did not stick to the sliding surface and were removed.
The combined effect of stirring at varying speed and varying amount of rare eath (0.5wt% and 1.5wt%) on the wear behavior of A356 gravity castings has been studied. Based on the wear testing and metallographic examination conducted for the specimens, the following conclusions can be drawn:
1. Microstructural analysis shows that rare earth addition is able to refine grain size by reducing the dendrite size and produces more fibrous eutectic silicon phase.
2. Wear behaviour of A356 alloys mainly depend on the shape, type, size and size distribution of α-Al grains and silicon particles in the matrix.
3. The A356 alloys investigated exhibit plastic deformation and work hardening during wear testing. A356+1.5wt.R.E cast alloys work-hardened to a greater extent than the unalloyed A356.
4. Wear occurs by plastic deformation and cracking of the matrix followed by delamination of flakes in case of unalloyed A356 at 600 rpm stirring
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