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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