Rheocasting of Aluminum 201

Aerospace

Engineering.

Aerospace Engineering

Individual Investigative Project

CASTING OF ALUMINUM 201 IN THE SEMI-SOLID

STATE

BY

Latona Nahid Aktare Houssaine

May 2012

Supervisors: Dr P. KAPRANOS

Dissertation submitted to the University of Sheffield in partial fulfilment of the

requirements for the degree of

Bachelor of Engineering

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Abstract

The most prominent technique for the rheocasting of aluminium alloys [Semi-Solid

Rheocasting (SSR)] has been dependent on the effectiveness of processing semi-solid slurries.

Contraction due to solidification and compensation for shrinkage, pouring technique from the

crucible to the mould cavity also dictates its effects on the final microstructures and therefore

the mechanical properties. Six batches of rheocasting were performed at varied temperatures

and stirred for different times and analysis to evaluate the solid fraction, particle density,

particle size, particle shape factor, and particle distribution was performed on the resulting

castings. This project examines the difficulties involved in Semi-solid rheocasting (SSR) of

aluminum alloy and investigates the alternatives these difficulties could be overcome by refined

techniques. The methodology employed in this case is the temperature dropped from the A3

melting point stirred at the required temperature hence poured in metal dies to study the

evolution of the microstructure from dendrite to non-dendritic microstructure in its semi-solid

state during the SSR process. To explore the success of SSR with an Al-Cu based alloy, the

SSR samples were heat treated T6 and undergone tensile testing to achieve appropriate

mechanical properties. Comparison in microstructures of naturally cooled and artificially aged

samples (T6) were part of the study probing in the progression of the microstructures after

homogenization.

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Table of Contents

1. INTRODUCTION .................................................................................................... 1

2. LITERATURE REVIEW .......................................................................................... 4

2.1 History of Semi-Solid Metal Forming............................................................................................... 4

2.2 Applications of SSM ......................................................................................................................... 5

2.2.1 Aluminium-Copper Base Alloys (A201).................................................................................... 5

2.3 Rheocasting Processes ....................................................................................................................... 6

2.4 Structural Evolution during Rheocasting .......................................................................................... 7

2.4.1 Grain Density ............................................................................................................................. 7

2.4.2 Convection ................................................................................................................................. 8

2.4.3 Dendrite Fragmentation .............................................................................................................. 8

2.4.4 Particle Stability ......................................................................................................................... 9

2.4.5 Fluidity of Rheocast Alloy ......................................................................................................... 9

3. EXPERIMENTAL PROCEDURES........................................................................ 11

3.1 Experimental Setup ......................................................................................................................... 11

3.2 Stirring Rod ..................................................................................................................................... 11

3.3 Microstructures................................................................................................................................ 11

3.4 Hydrostatic Weighing ..................................................................................................................... 11

3.5 Heat Treatment ................................................................................................................................ 12

3.6 Tensile Testing ................................................................................................................................ 12

3.7 Metallographic Etchants .................................................................................................................. 12

3. EXPERIMENTAL FIGURES ................................................................................... 13

3.1.1) Experimental Setup ..................................................................................................................... 13

3.2.1) Conventional & SSR Experiment 1 ............................................................................................ 14

3.3.1) SSR Experiment 2 ....................................................................................................................... 15

3.4.1) SSR Experiment 3 ....................................................................................................................... 16

3.5.1) Hydrostatic Machine ................................................................................................................... 17

3.6.1) Tensile Testing Machine ............................................................................................................. 17

3.5.1) Metallographic Etchants ............................................................................................................. 18

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3.5.1.1 Aluminium and Aluminium Alloys ........................................................................................... 18

3.5.1.2 PREPARATION OF ALUMINUM ALLOYS ......................................................................... 18

4. RESULTS AND DISCUSSION ............................................................................. 19

4.1 Microstructures of 1st Experiment ................................................................................................... 19

4.1.1 Microstructures of Conventional Castings ............................................................................... 19

4.1.2 Microstructure of SSR Castings ................................................................................................... 20

4.2 Microstructures of 2nd

Experiment .................................................................................................. 21

4.2.1 Microstructures of SSR Castings ............................................................................................. 21

4.2.2 Evaluation of SSR Casting Particles ........................................................................................ 22

4.3 Microstructures of 3rd

Experiment................................................................................................... 23

4.3.1 Microstructures of SSR Castings ............................................................................................. 23

4.3.2 SSR Recrystallization Behaviour (T6 Heat-Treatment) ........................................................... 24

4.4 Relative Densities Evaluation ......................................................................................................... 24

4.5 Mechanical Properties of SSR Castings .......................................................................................... 25

4. FIGURES OF A201.0 ALLOY .................................................................................. 26

4.1.1) AA201.0 Alloy Conventional Sand Casting ............................................................................... 26

4.1.2) Optical Microstructures at higher resolution .............................................................................. 27

4.1.3) Conventional Sand Castings and SSR (Rheocasting) ................................................................. 28

4.2.1) Semi-Solid Rheocasting (SSR) ................................................................................................... 29

4.2.7) METALLOGRAPHY ANALYSIS ............................................................................................ 32

4.3.1) Semi-Solid Rheocasting (SSR) ................................................................................................... 33

4.3.2) Recrystallization Behaviour of SSR ........................................................................................... 35

4.3.2.1 SSR Microstructures at T6 Conditions .................................................................................. 35

4.4) POROSITY EVALUATION ......................................................................................................... 38

4.5) MECHANICAL PROPERTIES .................................................................................................... 39

4.6) MACROSCOPIC PHOTOGRAPHS ............................................................................................. 42

5. CONCLUSIONS ................................................................................................... 44

Summary of Findings: ........................................................................................................................... 44

Suggested Further Work ........................................................................................................................ 45

Suggested Manufacturing Configurations: ............................................................................................ 46

6. REFERENCES ..................................................................................................... 48

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Acknowledgement

I am sincerely and heartily grateful to my supervisor, Dr. Plato Kapranos, for the support and

guidance he showed me throughout my final year project and dissertation work. His optimum

level of expertise and assistance enabled me to develop the understanding of the subject. Im

truly indebted for his time and devotion to the project notwithstanding the fact of his other

academic and professional commitments.

I would like to thank all the staff in the Department of Engineering Materials who gave me

assistance during project, especially Mr Philip Staton for his help with the rheocasting process.

I do thank all the colleagues in the rheocasting groups including scholars for the helpful

analytical reviews, National Research University Project of Thailand.

I also want to thank Mr. Sam Gascoyne for dedicating his time to facilitate my task in terms of

etching the samples and demonstrating the apt techniques for polishing.

Last but not least, I owe gratitude particularly to my parents and comrades for their continuous

support and encouragement throughout my years of study.

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1. INTRODUCTION

Semi-solid metal processing in recent years has gained more commercial significance in the

world of industrial applications. The needs to produce high quality parts at lower costs are the

centre of attention in the global market. During die filling, from the manufacturing point of

view, round and smaller crystal grains of less than 10 m [1] through a simple methodology of

induced convection (by stirring of the molten metal before introducing it into the die) during

solidification in order to improve the characteristics of metallic materials. Semi-Solid

Rheocasting (SSR) is the process employed whereby the molten metal is vigorously agitated

during solidification before being shaped into the final product. This process rips dendrite arms

due to induced shear forces, creating a highly viscous material that has high fluidity even

though over ~60% of the metal is solidified [2]. Desirable spheroidal microstructures are

formed quickly and efficiently from the molten alloy at the liquidus. SSR creates the possibility

to modify ordinary casting aluminium alloy such as A201 into semi-solid metal slurry with

properties that are conducive to forming into complex shaped dies. This allows SSR to use

conventional sand or die-casting to produce high-integrity, near-net-shape parts with strength

and ductility comparable to squeeze or permanent mould cast components at highly reduced

costs.

The main objective of the semi-solid rheocasting (SSR) technique adopted herewith is to

investigate the evolution of naturally cooled as well as artificially aged (heat treated T6)

microstructures and obtain good mechanical properties. In order to characterize the early stages

of the microstructures, the aluminium alloy A201 will be subjected to two different types of

casting process:

1) Conventional casting (gravity cast) and

2) Semi-Solid Rheocasting (SSR), whereby analysis shall illustrate the breakdown of

dendritic to near non-dendritic microstructures resulting in the desired fine near-

spheroidal grains.

After conventionally sand casting a batch of the 201 alloy, a set of five semi-solid rheocasting

batches at different agitation times were cast to observe the development of the microstructures.

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Subsequently, samples obtained through the SSR castings were heat treated, then machined and

subjected to tensile testing, relating the mechanical properties of the material to the

microstructures and possible defects resulting from the process.

Experimentation on different convection approaches were explored in order to observe the

microstructural behaviour of the Al-Cu alloy. As a result of fast growth of spheroidal of

microstructures at lower temperature when subjected to convection, these factors contribute to a

potentially significant reduction in operating cost when compared to conventional liquid casting

process. The challenge of rheocasting has been the limited knowledge about how to efficiently

process liquid alloy to create non-dendritic metal slurries whilst avoiding possible defects

introduced by the stirring operation.

Over the last 40 years, thixocasting process has been on the verge of establishing itself

as an important route for manufacturing parts quality by the industry [3], making use of

electromagnetically stirred billet as the starting feedstock, produced by continuous casters [4].

Feedstock material for semi-solid processing (generally known as thixoforming) has a very

particular non-dendritic structure that could be easily processed and manufactured into the

desired shaped upon the reheating of the billet into the solid-liquid temperature range before

forming into parts. However, poor estimated supply of the 3 inch diameter billets of the 2.7

million tons of aluminium castings hoard produced by the Magnetohydrodynamic stirring

(MHD) process, proved to be an Achilles heal for the thixoforming process in the North

American, European and Japanese markets; these cost challenges almost extinguishing the

current use of thixoforming in industry [5-6], with only few notable exceptions [7].

Signs are that in order for current exploitation in semi-solid metal forming to succeed advances

must be made in the rheocasting approach.

Rheocasting has immediate advantages over thixocasting:

1) Globule particles can be formed during solidification via applied convection; and

2) High recyclability of scrap on site [8].

Part quality remains the main aspect that is keeping the rheocasting process alive for industries

that pin their hopes on this process. However, in order for rheocasting to become a viable

worldwide industrial success, we need not only to understand and improve the initial stages of

solidification when convection is induced but to have the ability to do this consistently and at a

reduced cost.

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The main aim of this study is to develop an understanding of how particle morphology in

rheocast alloy evolves during the early stages of solidification and compare this to the existing

literature. The results from various sets of experiments are then compared with published work

on particle growth and mechanical properties obtained in the A201 alloy.

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2. LITERATURE REVIEW

2.1 History of Semi-Solid Metal Forming

It all started back in the 1970s when SPENCER and FLEMINGS [9] at the MIT found that by

mechanically agitating a solidifying alloy in the solid-liquid temperature range, the solid phase

would be in the form of dendrite fragments turning into slurry of near-spherical particles in a

liquid matrix. Their investigations into the nature of these semi-solid slurries revealed that they

possessed unique rheological properties; they were thixotropic, i.e. they behaved like solids

when left undisturbed but they flowed like liquids when put under shear and this effect was also

time dependant. The thixotropic behaviour of the slurries allowed their formation into metal

parts of complex shapes, and the higher viscosity associated with them considerably reduced

the turbulent flow during mould filling, producing near net-shape products with better

properties due to the reduction of air entrapment and other inclusions. As the non-dendritic

feedstock microstructure became the key to this new process of metal forming, two semi-solid

processing routes were quickly established in order to exploit the industrial platform:

rheocasting and thixocasting. As the potential of this possible fruitful manufacturing route

for shaping metal alloys was quickly recognised by Prof. M.C. Flemings and his co-workers

they immediately moved to protect their IP and initiated the early development of semi-solid

metal forming route (SSM) [10-11-21]. As already stated earlier, the initial industrial

applications made use of the MHD feedstock production route by electromagnetically stirring

the melt into a continuously cast slurry that solidified into non-dendritic feedstock. This

feedstock could then be re-heated to the semi-solid state when needed and injected into a die

under the appropriate conditions using a forming press. The main drawbacks of the process

were the higher costs in feedstock production and the inability of the part manufacturers to

recycle any scrap material. This lead to the parallel re-development of the original Rheocasting

idea where slurries with dendrite structures disintegrate due to strong stirring agitation resulting

into non-dendritic structures that can be formed into parts either in sequence or by solidifying

and re-heating as required; the main difference here being the recyclability of materials. The

semi-solid metal (SSM) processing has proven to be an interesting route and in some of its

versions has gained commercial significance in the manufacturing world. However, it has never

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fulfilled the early indications of its vast potential; it has remained an interesting but niche

application process. Clearly the most important feature, and probably the factor that made the

economics of the process more complicated, is the production and control of the non-dendritic

microstructures throughout the feedstock. In order to attain the homogenization of

microstructures and the elimination of segregation in a semi-solid alloy feedstock, many

methods have been developed, such as NRC, SLC

, SSR

, rheo-die casting, rapid-

Solidification and several more [12].

There is clearly the potential of developing new methodology in terms of manufacturing cost

effective quality parts such as cylinder engine blocks recently accomplished by Honda in line to

be marketed in Europe [13-21], but the fact remains that semi-solid rheocasting has not yet

established a firm foot hold in producing quality parts needed by todays industries on a large

scale, although it has been successful in the areas of sports equipment & electronic consumer

products [14].

2.2 Applications of SSM

2.2.1 Aluminium-Copper Base Alloys (A201)

Fuel rising market prices and new environmental laws has resulted in a new drive towards

lighter and more efficient vehicles providing impetus for possible SSM applications for

aluminium parts in the automotive industry.

Alloy 201 (Al-4.6Cu-0.7Ag-0.35Mn-0.35Mg-0.25Ti, wt%) [15] is a very competitive material

developed for high mechanical properties, excellent machinability and can survive elevated

temperatures as well as increase damage tolerance through better toughness. Table 1 illustrates

the mechanical properties covered by the American Aerospace Material Specification AMS

4229 [15].

Table 1: Mechanical properties of A201 demonstrated by AMS.

0.2% Proof Stress/

MPa

Tensile Strength/

MPa

Elongation/ %

Separately Cast Specimen 345 414 3

Designated Casting areas 345 414 3

Non-designated casting areas 331 386 1.5

[Courtesy of Casting Technology International][16]

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This particular material has a high response to age hardening due to its main precipitate

phase. The addition of silver in the material considerably changes the precipitation process,

causing orthorhombic form of the tetragonal phase (Al2Cu) to precipitate as very thin,

coherent plates on the matrix planes rather than plane [15-17]. Aluminium alloy

has a known binary eutectic system upon solidification as shown in Figure 2.2.1. This eutectic

reaction occurs during its non-equilibrium solidification phase nonetheless it could be removed

after the homogenisation of the solute content (heated to a temperature below the eutectic

temperature and held for a long time sometimes as much as 24hrs).

Figure 2.2.1(a) & (b) shows alloys with composition (along line bold line) comprising the

eutectic structure which constitutes layers of and phases. Higher eutectic composition in the

alloy indicates significant amount of eutectic it contains. While the alloys on the right from

point O mixture of and the eutectic grains.

2.3 Rheocasting Processes

Several processes for creating non-dendritic structures from liquid alloy have been developed.

Mechanical stirring was the first method employed by SPENCER and FLEMINGS during their

discovery of the non-dendritic structure [19]. More recently a new method was devised known

as SLC as mentioned in section 2.1. In the new rheocasting process, liquid aluminium alloy is

poured into a cooler steel cup at a temperature just above its liquidus. The interaction between

the melt and cold cup results in the formation of many solid particles. Heat extraction is

controlled to cool the volume of metal homogeneously. However, pouring at low temperature

Figure 2.2.1(a): General Phase Diagram [18] Figure 2.2.1(b): Eutectic composition of

alloys. [18]

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such as SLC has some disadvantages and a thermally conductive device must be utilized to

drop the bulk temperature from the entire volume of metal but able to withstand prolonged

contact with liquid aluminium [19-21].

SSR process is more effective compared to SLC

whereby a cool rotating rod is immersed

into the melt during its solidification stage. Hence, this cools the melt rapidly at localized region

when providing vigorous agitation due to continuous stir and drops the bulk temperature of the

melt below the liquidus. Metal slurry could be easily created from liquid metal at varied

temperatures by the SSR process making it the most effective method to produce aluminium

alloy slurries [12-20-21]. Figure 2.3 shows the three basic steps in the rheocasting process.

2.4 Structural Evolution during Rheocasting

2.4.1 Grain Density

In rheocast alloy, the grain density is significantly higher as compared to that in conventional

castings due to a process of dendrite fragmentation and subsequent ripening as required for a

non-dendritic structure to form. The current theory concerning the evolution of a spheroidal

microstructure was reviewed by Flemings, who considers the process to be dependent on the

formation of tiny equiaxed dendrites or dendrite fragments in liquid phase just before

solidification. In order to reduce porosity in the melt, the higher the grain density in the initial

Step 1 Step 2 Step 3

Figure 2.3: Schematic of three basic steps in SSR process.

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melt, i.e. the more and smaller the initial grains are, the higher the chance of reducing porosity

for a given increase in solid fraction. With the bulk of grains remaining small, diffusion lengths

for ripening are also small and hence promote rapid growth of spheroidal particles. Vice-versa

for larger grains, the diffusion length is equally larger and the dendrites will require more time

to breakdown into rosettes and later into a spheroids [20-21].

2.4.2 Convection

Grain refinement in all rheocasting processes is obtained through convection during the process

from the star of solidification. This creates a high density of grains throughout the melt. An

experiment was performed by BOWER and FLEMINGS [22] showing that if convection is

introduced during solidification, a structure with multiple smaller grains will form. The

experiment concludes that if the metal is fed into the mould at high temperature, the formation

of large dendrites would be observed on the surface of the samples and if the metal was fed

with significantly less superheat, multiple sites of fine dendrites are formed. Coarse grains

would be the final outcome if convection is not employed during the process. In the case of a

metal slurry with less superheat, the slurry already starts to nucleate at early stages before the

tip of the stirrer is introduced and engaged into the melt dispersing fine globular grains

throughout the casting [21-22].

2.4.3 Dendrite Fragmentation

A more advanced study was done within the field of dendrite fragmentation by ESAKA et al

[23] who recorded images of particles being generated in the mushy zone of the stirred melt.

The experimentation of ESAKA et al showed that the number of particles generated, and the

rate of generation is simultaneously augmented when stirring rate intensifies [21-23]. Martinez

and Flemings showed that if vigorous convection is applied to a melt during its solidification

state, non-dendritic structures are rapidly formed. Further hypotheses were made by Vogel et

al. and Doherty et al [24]. that bending of dendrite arms arises due to shear forces and leads to

from high-angle grain boundaries at the roots of the dendrite arms, thereby increasing energy at

the root [21-22]. Kattamis et al [25] proposed a coarsening model in which the degeneration of

dendrite arms has a direct link with its arm spacing, where the dendrite arm spacing is

responsible for the rate at which the dendrite arms breakdown [21]. This coarsening mechanism

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proves to be a potential form of multiplication of grains as each detached arm is swept by

convection into the volumetric melt.

2.4.4 Particle Stability

Particle stability plays a vital role when filling the mould cavities in casting. In order to ensure

proper filling of moulds by the metal slurry, we try to maintain small size of near-spheroid

structure of the solid particles during the mould filling operation. In cases where the in-gates

and runners are cooler than the slurry, upon contact, the metal slurry will cool relatively quickly

creating an unstable condition that might result in microstructural effects such as generation of

coarser grains, entrapment of air or oxide and other impurities, hence increasing the probability

of reduced mechanical properties. Therefore, understanding the conditions which maintain

interface stability in rheocast processing is critical [21].

2.4.5 Fluidity of Rheocast Alloy

In the foundry world, fluidity is defined as the length that metal will flow down a channel

before being stopped by solidification. The correlation between the melt flow and

microstructure makes the fluidity test a vital aspect of the rheocast process.

Ragone [26] was the first to perform an arithmetical analysis of the fluidity of pure liquid

metals. During his experiments he observed that the flow of the molten metal entering the sprue

was turbulent due to a difference in pressure. The analysis accounted for frictional losses and

used Bernoullis equation to predict the length the metal would flow before solidifying but

neglected surface tension and viscosity changes [21-26].

Another researcher, Niesse [21-27], extended the work of Ragone and found that adding

small amounts of alloying elements retards the fluidity due to a larger driving force creating less

mobility. Flemings reflecting on these facts concluded that alloying elements generate a

retarding force during the early stages of nucleation resulting in the formation of dendrites in

the liquid state which eventually strive to hinder the rapid flow. Smaller grain size delays

dendrite entanglement and eases the flow of the alloy further until a higher degree of

solidification is reached. Kwon and Lee [28] showed that the fluidity of grain refined Al-Cu

alloy is much greater than non-refined alloy. At temperature above the liquidus, alloy fluidity

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was found to decrease with decreasing temperature. It involves a direct relationship that dictates

the increase in solid content in such a way that fluidity of the non-dendritic slurry continued to

decrease with decreasing temperature occurring below the liquidus temperature [21-22-28].

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3. EXPERIMENTAL PROCEDURES

3.1 Experimental Setup

The main aspect of the experimental setup was to probe in the evolution of the microstructure

as well as the flow behaviour of the aluminium alloy A201. Figure 3.1.1 illustrates the

apparatus utilized during the experiment. In a similar way to the typical rheocast process, the

process has three main components: an open resistance furnace holding the melt, a rotating

copper rod operating at 110rpm (1st Experiment), a rotating stainless steel rod operating at

110rpm (2nd

Experiment), and a preheated mould stacked in sand acting as a source of

insulation. The stirrer rod is mounted in a hand drill acting as the mechanical stirrer. The probe

tip of the thermocouple is immersed in the molten metal to record the evolving temperature.

3.2 Stirring Rod

The copper stirrer was 8 mm in diameter and 200 mm long, with a rounded end. The rod utilized

for the 2nd

experiment was stainless steel with 15 mm diameter and 450mm long. The stirrer

rods were at room temperature (between 220C and 30

0C) before being immersed into the melt.

3.3 Microstructures

In this experiment, during solidification, the dendritic structure was subjected to shear forces

due to convection prompted to rip the dendrites into rosettes and later into a larger number of

spheroidal particles. Figure 3.1.2 below shows the breakoff process from rosette to fine

spheroids. Final spheroidal particles containing entrapped eutectic arise due to the ripening

process of rosette arms whereby liquid is entrapped in between those arms.

3.4 Hydrostatic Weighing

The hydrostatic weighing also known as hydro-densitometry is based on the Archimedes

principle. First the dry weight is determined by hanging the sample freely in air as shown in

Figure 3.5.1. After recording the weight in air, the specimen is then immersed in water without

touching the sides or bottom of the beaker, expels all the bubbles that might arise due to

porosity, the underwater weight is recorded.

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3.5 Heat Treatment

For the semi-solid rheocast (SSR) A201 alloy specimens, conventional T6 heat treatment 2 h at

5130C was employed, followed by 17 h at 527

0C solution treatment, followed by water

quenching and then a 20 h at 1530C ageing period, shown schematically in Figure 3.5 [16].

3.6 Tensile Testing

Specimens, after heat treatment, were machined to specific sizes as shown in Figure 3.6.2, for

tensile testing. The tests method covers the tensile testing of rheocast alloys carried out on the

Hounsfield H100KS material testing machine shown in Figure 3.6.1. The specimens are

gripped by wedge grips at the two extremities and then test parameters were inputted from a

computer. Cross speed was 3 mm/min, and the gauge length was set to 68 mm. Ultimate tensile

strength and elongations to fracture values were obtained. Macroscopic photographs of the

fracture surfaces were taken to observe and relate any factors that may have influenced the poor

mechanical behaviour of these alloys.

3.7 Metallographic Etchants

After preparation of the A201 specimens, obtained from the 1st, 2

nd and 3

rd experiments,

etchants shown in Table 2, were used to reveal the various regions of the resulting

microstructures.

2 h

5130C

5270C

17 h

1530C

20 h

Figure 3.5: Schematic of heat treatment stages.

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3. EXPERIMENTAL FIGURES

3.1.1) Experimental Setup

Copper/

Stainless Steel

Stirring Rod

Ceramic

Crucible

Aluminium

Alloy A201

Thermocouple

Protective

Sheath

With Grounded

tip

Electric hand

drill operating at

110rpm

Figure 3.1.1: Schematic of equipment Setup and built for this study.

Figure 3.1.2: Evolution of structure during solidification with vigorous agitation [20].

Increasing

Shear Rate

Increasing

time

Initial Dendrite Structure

Dendrite Growth

Rosette

Ripened Rosette

Spheroid

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3.2.1) Conventional & SSR Experiment 1

Figure 3.2.1: Aluminium alloy sand casting process at different pouring temperatures.

(a) 9300C Conventional Cast Temperature (b) 8300C SSR Cast Temperature (c) 7300C SSR Cast Temperature

(a) (b) (c)

Figure 3.2.2: Final product from the 1st Experiment from conventional to

rheocast casts respectively.

(a) Conventional Cast Product (b) 8300C SSR Product (b) 7300C SSR Product

Copper Stirrer Rod

(a) (b)

(c)

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3.3.1) SSR Experiment 2

Figure 3.3.1: Semi- solid Rheocasting die cast at two distinct temperatures.

(Pouring temperature 7000C)

(a) SSR Cast at 7000C (b) SSR Cast at 6700C

(a)

(b)

(a) (b)

Figure 3.3.2: SSR final product at different temperatures..


(b) SSR product at 7000C (b) SSR product at 6700C

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3.4.1) SSR Experiment 3

Figure 3.3.2: Samples after heat treatment (T6) and machined to

specification for tensile testing.

Number of samples: 24

Figure 3.3.2: Semi-solid rheocast of 3rd

experiment employing different

stir techniques.

(Pouring temperature at 6700C)

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3.5.1) Hydrostatic Machine

3.6.1) Tensile Testing Machine

Mass

immersed in

H2O

Lid to avoid

disturbance

Mass

hanging

freely Scale

Analogue

Reader

Figure 3.6.1: Schematic illustrating Mechanical test system [31].

Figure 3.5.1: Schematic illustrating the hydrostatic weighing apparatus.

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3.5.1) Metallographic Etchants

3.5.1.1 Aluminium and Aluminium Alloys

CAUTION: Beware of when handling potential hazardous chemicals. Be sure to wear

appropriate clothing and adhere to all warnings on chemical manufacturers. Safety is vital when

etching.

Table 2: Etchants applied to reveal the microstructures of A201.0 alloy.

3.5.1.2 PREPARATION OF ALUMINUM ALLOYS

PREPARATION OF A201.0 SPECIMENS

Grind with 240-grit, 800-grit, 1200-grit 2000-grit and 4000-grti SiC water-cooled paper,

250rpm applying 3lbs per specimen.

Polish with 6-m Diamond (oil based recommended) on ultra-pol (silk) cloth at 250

rpm, using a force of 3lbs/specimen for 5 minutes each (counter rotation).

Polish with 1-m Diamond (oil based recommended) on ultra-pol (silk) cloth at 250

rpm, using a force of 3lbs/specimen for 3 minutes each (counter rotation).

Finish with Silico (50% silica and 50% water mix) on micro (cashmere) cloth at 250

rpm, using a force of 5lbs/specimen, 3 minutes (counter rotation).

Etchant Concentration

Mixtures

Conditions Comments

Barkers Etch Distilled Water

Hydrofluoric acid

200 ml

5 ml

Anodize for 40~80s at 20

V dc.

Display dendrites well and

widely used for wrought

alloys

Nitric Acid

Distilled Water

Hydrochloric acid

100 ml

20 ml

3 ml

30-45 seconds

immersion. Add 3ml of

HCl after each dip.

Cast Al-Cu alloys. Over

etching might blemish the

microstructures.

8

7

92

12 12 68

2

Figure 3.6.2: Dimensions of specimen for tensile testing in millimetres (mm).

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4. RESULTS AND DISCUSSION

4.1 Microstructures of 1st Experiment

4.1.1 Microstructures of Conventional Castings

The results of this study consist of three set of experiments. The first set comprises of both

conventional sand and semi-solid rheocasting (SSR) with the remaining three sets of casts being

Semi-Solidly Rheocast at different agitation times. Figure 4.1.1shows the microstructures for

the first set of conventional sand casting of A201 at a pouring temperature of 9300C. As

anticipated, Figure 4.1.1(a) shows the microstructure of a conventional-cast alloy, with large

dendrites and coarsely spaced arms. The dendrites are surrounded by eutectic composition

which appears as darker lines in the micrographs. As a result of conventional casting, shrinkage

macro-porosity is also apparent at the cross section of the sample. The intermetallic Al2Cu

phase believed to be present in the conventional cast sample can also be seen in Figure 4.1.2,

appearing as dark under the Polyvar microscope as shown in Figure 4.1.4.

Grains, after conventional solidification casting, shows that coarse dendrite spaced arms are

predominant. As shown in Figure 4.1.3(a), the microstructure of the conventional cast at a high

temperature has a coarse eutectic structure. This takes place when the solidification follows the

non-equilibrium phase cooling system. Due to the high pouring temperature, the poured

temperature is directly proportional to the bulk free energy and the relationship of bulk free

energy is inversely proportional to the activation energy. Due to low activation energy, the

liquidus starts nucleating faster, speeding up the solidification process due to the high driving

force (rapid cooling rates).

The schematic below shows the relationship of nucleation rate as a dependant factor of the

pouring temperature.

~20~

4.1.2 Microstructure of SSR Castings

Based on the analysis of the conventional castings, in order to reduce the chances of porosity

occurring, low pouring temperatures should be exploited. Figure 4.1.1(b) and (c) shows the

reduction in microstructural defects, i.e. less porosity and finer structures. The current focus of

rheocasting is on forming semi-solid slurries containing high fractions solid. The process

includes mechanical stirring. Initially a copper stirrer was utilized but during extended contact

with the hot molten metal the copper stirrer partially melted at high temperatures. This

technique uses different media to apply the agitation to the melt. Upon contact with the slurry,

due to convection and agitation from the stirrer, dendrite arms caused to be detached by

breaking off. Nonetheless, the nucleation driving force was too high, during 8300C and 730

0C,

in order to create multiple spheroidal structures because of the rapid nucleation rate. Table 3

demonstrates the poured temperature drop after conventional casting, followed by the SSR casts

that were stirred for 40s each, before being poured into the mould cavities.

Type of Cast Pouring

Temperature/0C

Stirring

Time/s

Type of Stirrer Microstructures

Analysis

Conventional

Casting 930 0 Nil Coarse Dendrites

SSR 830 40 Copper Much Finer Dendrites

SSR 730 40 Copper

Dendrite

Fragmentation forming

few globular grains

As ,

Table 3: 1st Set of Experimental data.

Time to form N Nuclei, t=N/I

C-Curve Kinetics

High driving force

Low mobility

Low driving force

High mobility

Tf

T

E

M

P

E

R

A

T

U

R

E

~21~

4.2 Microstructures of 2nd

Experiment

4.2.1 Microstructures of SSR Castings

Figure 4.2.1 illustrates finer microstructures compared to the 1st set of experiments. As a result

of less superheat and more agitation for approximately 3 to 4 minutes, finer grains with a

reduction of pores size were obtained. For melt with less superheat and longer vigorous

agitation to the slurry during nucleation, the convection process forces the dendrite arms found

at early stages of the alloy to create multiple sites of disintegrated solid particles and fine

dendrites. Furthermore, high metal temperatures will accentuate shrinkage problems and

increase the porosity sites.

The micrographs of AA201 alloy show that the microstructure consists of a mixture of globular,

rosette, and fine dendrite particles at the final stages of the rheocasting process as shown in

Figure 4.2.5. These globular and the rosette particles are believed to be the disintegrated

particles resulting from the dendrite fragmentation due to the stirrer mechanism. This

mechanism explains that the bulk liquid still consists of several small pools of superheated

liquid. At early stages, as shown in Figure 4.2.4 (b, c), at 7000C during the first pour, only few

disintegrated particles survived from the dendritic fragmentation. The instance the slurry

attained 6700C after a uniform stir of 3~4 minutes, formation of more disintegrated particles

were observed at lower temperature with longer times of agitation applied as shown in Figure

4.2.4 (e, f). The growth of the fine spheroid and dendrite particles eventually coarsen to yield to

a mixed globular and fine dendritic structure, nevertheless, at times the castings still cracked at

even lower temperatures.

To obtain an effective dispersion of fine globular grains when adopting rheocasting, both time

and temperature plays a critical role in this process. The time during agitation should be

sufficient enough to bring down the temperature and hence augmenting the solid fraction of the

slurry.

~22~

4.2.2 Evaluation of SSR Casting Particles

Figure 4.2.7(a) served to evaluate the important data associated with particles, such as solid

fraction, particle density, and particles shape factor and particles distribution. The data were

obtained from Image J processing software.

4.2.2.1 Total Number of Particles

In this analysis, the number of particles (N) is defined by:

Where AT is the total analysed area of the micrographs and AP is the area of an average solid

particles.

Total Area of micrographs, AT is:

Total Area of particles, AP is:

No. of particles over the analysed surface is 3725.

4.2.2.2 Particle Size

Assuming the shape of the particles are circular and analysis carried out under 2D condition, the

particle size (d) for each primary particle is given by:

Where, AP is the area of an average solid particle.

The diameter of the particles were calculated from an average diameter of particles varying

from 14~23 m in diameter.

Hence, particle size (d) is .

~23~

4.2.2.3 Solid Fraction

The solid fraction (fs) is determined from the image analysis by the following equation:

[32]

The solid fraction (fs) of the AA201.0 alloy is 84.9%.

4.2.2.4 Particle Shape Factor

The shape factor (F) of a particle is defined by:

Where P is the perimeter is by: assuming it a perfectly shaped circle. For F=1signifies that

the particle is a perfect circle and if F

~24~

conditions, this experiment also entails a heat treatment process at T6 with the purpose to

provide information about the contrasting microstructure and properties.

4.3.2 SSR Recrystallization Behaviour (T6 Heat-Treatment)

Figure 4.3.2.1 shows the considerable reduction of the shrinkage porosity during 6700C with

the formation of a more desirable spheroidal microstructure which was obtained during

isothermal heating. At earlier stages, due to lack of convection at 6580C, the size of the pores

was reduced to some extent, nonetheless not sufficiently to avoid poor mechanical properties.

During the T6 treatment, extensive recrystallization occurred and more or less a fine equiaxed

spheroidal microstructure was obtained at 6700C compared to the early stages where the grains

were more dendritic. Figure 4.3.2.2 shows the evolution in the microstructures converging to

more disintegrated particles Figure 4.3.2.2(c) but with two long dendrites also seen on the

surface. During the period of recrystallization, the formation of globules were fine, instead of

coarsened, with micro holes shown in the micrographs at 6700C. Consequently the

microstructures at 6700C should result to a better strength of the material rather than the second

condition 6580C consisting of larger holes.

4.4 Relative Densities Evaluation

The evaluation fromTable 3 and Figure 4.4.1 clearly shows that the relative density/ reduction

in porosity and other defects will tend to yield to an increase in yield strength of the material.

As anticipated at early stages, the microstructures and any reduction in defects shall dictate the

mechanical properties. Referring to section 4.1.1, the outcome from conventional casting

resulted into massive shrinkage porosity and thus holds a much lower density in contrast to

other cast temperatures. The two ideal cases were from 2nd

and 3rd

(with or without Heat

Treatment) experiments whereby due to the application of convection to the metal slurry during

solidification, the vigorous agitation not only created fine grain particles but also reduced to

abundant number of porosity into the material. The data evidently display the ideal temperature

7000C for processing the slurry in its semi-solid state resulting in a fine dispersion of stabilised

particles with reduced defects. The relative density of the casts was determined from the

following formula:

~25~

4.5 Mechanical Properties of SSR Castings

Section 4.5 (Figures 4.5.1, 4.5.2 & 4.5.3) shows the average tensile values of the A201.0 alloy

in normal rheocast and T6 temper conditions. Figure 4.5.6 features the maximum elongation

from specimens that performed to their optimum level. The data clearly shows that the tensile

properties decrease with increasing amount of shrinkage porosity, non-metallic inclusions,

formation of oxides and turbulent flow at feeding gates. It is difficult to produce good

mechanical properties as so many factors affect the metallurgical integrity of the castings.

Figure 3.3.2 exhibited a crack, caused by residual stresses induced during contraction.

This study proved that the more refined spheroidal grain structures clearly govern the

mechanical properties whilst heat treatment T6 modifies the structures, e.g. 3rd

experiment,

consequently resulting in better properties when compared to the 2nd

Experiment. The

maximum UTS were achieved at 278MPa at 6700C-T6 with an elongation of 10% which is an

improvement due to less porosity and other inclusions. Micro-cracking occurred during tensile

testing of Al-Cu alloy samples at lower temperatures accounting both experiments (7000C,

6700C, 658

0C-T6) showed a rather poor elongation to fracture marking a reduction in ductility.

Conversely, 6700C-T6 specimens demonstrates a varied elongation to fracture amid 1.7%-10%

which gives hope for improving the mechanical properties of the alloy through refinements in

the manufacturing methodology adapted in the future. It can be seen that a variety of unwanted

inclusions, created at the point of manufacture, are damaging the mechanical properties. Figure

4.6.1 and Figure 4.6.2, illustrates how surface turbulence leads to the creation of oxide

inclusions and shrinkage porosity defects. With the lack of a proper gating system it will

eventually have an impact on the metallurgical integrity.

In this case, the macroscopic images revealed defects such as sand and dross inclusions

in the alloy due to the vigorous manual agitation in the crucible. With inappropriate gating,

flaws such as entrapped gases, localised shrinkage, micro-porosity and cracks were generated.

As a result of those factors, the ultimate tensile strengths and elongations are typically lower

than expected and ductility was reduced by the presence of such inclusions. Demands for

improved metallurgical integrity have emerged and recognised improvements shall be sought

with manufacturing refinements.

~26~

4. FIGURES OF A201.0 ALLOY

4.1.1) AA201.0 Alloy Conventional Sand Casting

(a) (b)

(c)

Figure 4.1.1: Optical Microstructure of Conventional Sand Casting of AA201.0

(a) Conventional Sand Casting at 9300C

(b) Semi-Solid Rheocasting(SSR) at 8300C

(c) Semi- Solid Rheocasting(SSR) at 7300C

200m 200m

200m

~27~

4.1.2) Optical Microstructures at higher resolution

200m 200m

200m 200m

200m 500m

Figure 4.1.2: Microstructure breakdown from coarse dendrite to much finer dendrite

via conventional sand casting of AA201 alloy. (Pouring temperature 9300)

a) 9300C centre, riser sample b) 9300C perimeter, riser sample

c) 8300C centre, riser sample d) 830

0C perimeter, riser sample

e) 7300C centre, riser sample f) 730

0C perimeter, riser sample

(a) (b)

(c) (d)

(e) (f)

~28~

4.1.3) Conventional Sand Castings and SSR (Rheocasting)

Three moulds were used at pouring temperatures of 9300C, 830

0C, and 730

0C respectively. Due

to the degree of high pouring temperature and the manual methodology employed for pouring

the molten metal, turbulent flow resulted as the slurry was poured into the cavity. The result of

reducing temperatures during casting and stirring while doing so, has been an observed drastic

reduction in terms of porosity, vacancy and other defects as shown below.

200m 200m 200m

(a) 9300C Conventional Sand Casting

(b) 8300C SSR Casting (c) 730

0C SSR Casting

Figure 4.1.3: Reduction of crystal defects as grain coarsening temperature

decreases exponentially with the cooling rates.

Figure 4.1.4: Formation of coarse dendritic structure near holes of the

conventional cast sample (9300C).

When subjected to solidification, since the

temperature drops from 9300C and follows a

non-equilibrium phase it results in eutectic

formation. Since composition of melt is

aluminium rich, it crystallizes first as the

temperature continue to drop. The

combination of the liquidus and solidus

forms an intermetallic compound at below

the A1 temperature, hence is

formed.

The solidification near the holes consists of

coarse dendritic microstructures

(conventional casting). Dendritic

microstructures could be broken into a huge

number of fine disintegrated particles when

adapting the convection process. AA201.0 Coarse Dentritic mircostructure Conventional Cast

~29~

4.2.1) Semi-Solid Rheocasting (SSR)

This section consists of the semi-solid rheocasting results whereby metal-moulds and a stainless

stirrer were utilized. The metal-moulds were preheated and stacked in a sand pit, acting as an

insulator preventing heat losses when the molten metal is poured. A stainless steel stirrer was

utilized in order to avoid any possibility of melting and resulting change in the casting

composition of the alloy. Pouring was carried out after stirring the melt for 3 minutes, at 7000C

and 6700C respectively.

50m 50m

Figure 4.2.1: Optical Microstructure of SSR casting at 7000 and 670

0 respectively.

(Corresponding Pour temperature at 7000 and 670

0 after agitation of 3~4 minutes each)

a) 7000C centre, bottom section (Fine Dendritic Grain Boundaries with near

definite globular grains)

b) 6700C centre, bottom section ( Much Finer Dendrites attached with multiple

breakoff particles, spheroidal grains)

(a) (b)

Figure 4.2.2: SSR Cast Sample at 7000C,

stirred for 3 minutes


stirred for 3 minutes

~30~

50m 200m

50m 200m

200m

(a) (b) (c)

(f) (e) (d)

Figure 4.2.4: AA201.0 Microstructure across and along the plane as well as the top

and bottom of the samples.

(Pouring temperature 7000C (a, b, c) and 670

0C (d, e, f))

Grains Size reduces from fine dendrite to more single globular

particles at 7000C

Grains Size reduces from finer dendrite to multiple globular particles

at 6700C

200m

~31~

100m

(a)

(b)

(c)

Figure 4.2.5: Different types of particles formed during nucleation of the AA201.0

alloy (SSR casting process).


(a) Formation of rosette (b) Formation of fine dendrite (c) Formation of globular particles

50m

100m

(a) (b)

Figure 4.2.6: Representative of micrographs of SSR at 6700C stirred for 3 minutes


(a) Illustration of multiple spheroidal sites after nucleation

(b) Analytical/mathematical calculations of grain diameter

~32~

(a)

(b)

4.2.7) METALLOGRAPHY ANALYSIS

Figure 4.2.7.1: Representative of metallography evaluation.

(a) Unmask Contours of grain structure used for grain size evaluation.

(b) Focus on the formation of near net shaped globular structures.

Figure 4.2.7.2: Graphical representation of perfectly shaped particles.

~33~

4.3.1) Semi-Solid Rheocasting (SSR)

This set of experiments dwells on the SSR (semi-solid rheocasting) process with7000C as the

commencing temperature for stirring, with a stainless steel stirring the melt for 45s and then

pouring the resulting slurry into two preheated metal moulds. Preheated moulds were utilized to

avoid impurities, such as sand inclusions and stacked in a sand pit to avoid heat losses. The

final temperature recorded prior to pouring was 6580C, lower than the previous experiment.

From 6700C to 658

0C Stirred once for 45s.

50m 50m

(a) (b)

Figure 4.3.1: Representative of micrographs of SSR at 7000C stirred for 45 seconds.

(Pouring temperature 6700C and 658

0C respectively)

(a) Fine dendrite with few vacancy sites.

(b) Dendrite Fragmentation into spheroidal grains with huge porosity sites.


45s stir. Temp=6700C

Figure 4.3.3: SSR Cast Sample at 6580C.

~34~

(e)

(c)

Bottom

Top

(d)

(a) (b)

Top

Bottom

(a) (b)

(c) (c)

(e)

Figure 4.3.3: SSR Cast Sample at 6700C and 658

0C

Stirred for 45s once.

(a) 6700C top section (b) 670

0C bottom section

(c) 6580C top section (d) 658

0C cross section

(e) 6580C cross section

100m

200m

200m 100m

100m

~35~

(a) (d)

(b)

(c)

(e)

(f)

4.3.2) Recrystallization Behaviour of SSR

4.3.2.1 SSR Microstructures at T6 Conditions

Figure 4.3.2.1: T6 SSR Microstructure at 6700C and 658

0C Stirred for 45s once.

A201.0-T6/ 670

0C A201.0-T6/ 658

0C

100m 100m

200m 200m

100m 200m

~36~

Cross

Section

(a) (b)

(c) (d)

200m 200m

200m 200m

Figure 4.3.2.2: Recrystallization Effects reflecting on the grain boundaries at 6700C and 658

0C

respectively.

(a) 6700C Bottom Section Sample (b) 6700C Perimeter of Cross Section (c) 670

0C Long Dendrite at Bottom Area (d) 658

0C Bottom of Sample

Figure 4.3.2.3: 6700C SSR AT T6 Sample

Figure 4.3.2.4: 6580C SSR AT T6 Sample

Bottom Bottom

~37~

(b)

(a)

At Grain coarsening

temperature Al

consume Cu

Saturated solution of

copper in alloy.Active

kinetic particles.

(a)

(b)

(c)

Figure 4.3.2.5: Micrographs represent the effects of heat treatment at pouring temperature of

6700C.

(a) Coarse Dendritic Microstructures at 6700C (b) Finer Dendrite and Spheroidal Particles after heat treatment (T6).

Simulated Normal Grain

Coarsening

TP= 6700C Coarse Dendrite Microstructures

Porosity

100m

Simulated Abnormal Grain

Coarsening

TP= 6700C/T6 Finer Dendrite and Spheroidal Particles

Drastic Reduction InPorosity

100m

~38~

4.4) POROSITY EVALUATION

Section 4.1 (Figures) onwards illustrates the microstructures evolution as well as differentiate

the series of shrinkage porosity and sizes from distinct experiment at specific temperatures.

Table 3: Illustration of density relative to the porosity of the cast specimens.

Experiment Cast

Temperature/0C

Wair/g Wwater/g Density/gcm-3

1

930 30 18 1.6

830 27 15 1.8

730 23 11 2.1

2 700 7 3.5 2

670 6 2.5 2.4

3 670 4 2 2

658 5 3 1.6

Figure 4.4.1: Contrast in porosity reduction at distinct pouring

temperatures.

Schematic of porosity

reduction [33].

~39~

4.5) MECHANICAL PROPERTIES

Figure 4.5.1 represents a simulated model from Ansys 13.0v, displaying the stress variation

when the specimens were subjected to tensile deformation load. The maximum stress

concentration is located round the round of the shank. Previous studies have revealed that

A201.0 has allowable UTS of 485MPa with 7% elongation [26-27].

Figure 4.5.1: Specimen Behaviour under Deformation Load.

Figure 4.5.2: SSR Cast Specimens under 2nd

Experiment Configuration.

~40~

Figure 4.5.3: Heat Treated SSR Samples at pouring temperature 6700C

following the 3rd

Experiment configuration.

Figure 4.5.4: Heat Treated SSR Samples at pouring temperature 6580C

following the 3rd

Experiment configuration.

~41~

Figure 4.5.5: Bar Chart Representation of Experiment 2 Maximum Tensile Strength at

distinct rheocast temperatures.

Figure 4.5.6: Elongation of samples cast at different

configurations (2nd

& 3rd

Experiments). Pie Chart displays

max UTS and Elongation.

Figure 4.5.7: SSR Heat Treated Samples Mechanical Properties Relative with Varied

Cast Temperatures 7000C and 658

0C respectively.

~42~

4.6) MACROSCOPIC PHOTOGRAPHS

Macroscopic defects found at 6700C are shown below.

Regions of

micro pores

(a)

Figure 4.6.1: Defects from pouring temperature of 6700C-T6.

(a) Micro Pores throughout the casts

(b) Layer of impurities at the edge

(b)

Regions of

oxides or other

impurities

Micro holes are

located

throughout the

specimen

8mm

MACRO SCALE

~43~

Macroscopic defects found at 6580C are shown below.

Shrinkage Porosity

Non- metallic

inclusions and oxides

(a) (b)

(c) (d)

Oxides, non-metallic

inclusions and porosity

leading to poor

mechanical properties.

Regions of

localized Shrinkage

Porosity

Figure 4.6.2: Defects from 6580C-T6 leading to poor mechanical properties.

(a), (b), (c) and (d) shows different defects influencing the metallurgical integrity.

~44~

5. CONCLUSIONS

Summary of Findings:

5.1 This research reveals the important aspects that influence the formation of globular

structures during the rheocast process. Unique experiments were setup to observe the

evolution of those structures in the semi-solid condition. They show how time and

temperature, at which the alloy is processed, are vital for an initial equiaxed grain to

ripen sufficiently into a spheroidal one. The unique aspect of this study is how vigorous

agitation potentially changed the initial microstructures of A201 from dendrite to

partially non-dendritic structures during its solidification stage.

5.2 The experimental setup comprised of a furnace, stirring rod (Copper/Stainless Steel),

and preheated. Convection was then applied to the low superheat melt during rapid

solidification of A201 alloy creating stable spheroid and fine dendrite arms. This

signifies the importance of vigorous agitation for the globule formation.

5.3 Evaluation of micrographs shows spheroid particles roughly varying from 14~20 m in

diameter. The equiaxed dendrites ripened into spheroids with entrapped eutectic during

the period of recrystallization. Metallography revealed a small quantity of near-

spheroidal particles (shape factor near to 1) followed with the fine dendritic arms.

5.4 Different rheocasting approached were implemented during this study. With the

application of convection to induce solidification during rapid cooling of the A201

alloy proving to be essential in increasing the density of the alloy.

5.5 The results of the rheocasting process clearly indicate that the mechanism for reducing

porosity in the alloy depends on temperature, convection, as if the initial particle

density in the metal slurry is large and dendritic, ripening shall occur rapidly into

spheroids/ fine dendrite arms with micro holes. Nonetheless, if the particle density is

~45~

low and convention is retained, ripening will result in the growth of dendritic particles

with large porosity with entrapped eutectic in the final structure.

5.6 Self-improvised etchants based around Barkers Etch allowed metallographic of

resulting microstructures.

5.7 Standard heat treatment T6 procedure showed improvement, both in terms of the

evolution of the grain structures alongside with the mechanical properties. The tensile

properties of different rheocast conditions showed wide variation in properties with

some reasonably good results together with some low and irregular ones.

5.8 Good elongation to fracture was obtained from SSR specimen at 6700C-T6 with an

elongation of 10% with a tensile strength of 278 MPa. Several defects contributed to

the behaviour of the SSR by adversely affecting the metallurgical integrity of the cast

alloys and ways of reducing their incidence have been proposed.

Suggested Further Work

The following is a list of ways to continue this investigation:

Repeating the rheocasting process at different suggested times at 7000C (if considering

the usage of two moulds during each experiment): 45s (between each feed), 1 minute

(between each feed) and 3~5 minutes at 7000C only one feed to the mould. Observation

of the grain structures should indicate the progress before heat treatment of the samples.

A good gating system should be considered next time if casting scrap is to be minimised

and metallurgical integrity improved. A filter should be utilized to physically trap the

non-metallic inclusions but then should be used up to temperatures not less than 6500C

to avoid the possibility of blockage during the pour.

A pouring basin should be considered since the initialflow from the crucible was

turbulent. A hole from the basin shall impede the turbulent flow at a certain height

allowing the slurry to stabilise at that point of time. Those aspects should hinder any sort

~46~

of defects to the microstructure and eventually obtain better properties compared to this

study.

It was not possible to record the cooling rates in between the normal solidification and

induced solidification. Determining the cooling rates would help to understand the

particle generation during rheocasting and the time the particles spheroidize. Figures

5.1, 5.2 and 5.3 demonstrates different manufacture configuration that could mitigate the

adverse of inclusions happening.

Suggested Manufacturing Configurations:

Furnace

Ceramic

Crucible

Preheated

Metal Die

Turbulent flow

of melt

Ceramic Pot

Stabilizer

Laminar flow

of liquid metal

Feed channel

at an angle

Figure 5.1: Schematic of typical gating slanted at an angel with a stabilizing ceramic

pot limiting turbulent flow.

~47~

Furnace

Ceramic Crucible

Preheated

Mould

Turbulent flow

of molten

metal

Ceramic Filter

v-channelsprue

Laminar flow

of melt

Figure 5.2: Schematic of feeding system including a filter separating non-metallic

inclusions leaving pure metal in mould.

Figure 5.3: Schematic of feeding system tilted at an angle. Feeding takes place right

after vigorous stirring is applied.

Ceramic

Crucible

Laminar flow

neglecting

induce gust

Preheated

Mould

Flow behaviour rely

on the operator

feeding the mould

Slanted

channel

~48~

6. REFERENCES

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http://www.foundrymag.com/feature/feature/49867/semisolid_rheocasting_moves_into_

commercial_production

[Accessed 06 03 2012].

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4. P. Kumar, H. Lakshmi and P. Dutta, 2008. Solidification of A356 Alloy in a Linear Electromagnetic Stirrer. Volume 141-143, pp. 563-568.

5. M.C. Flemings et al, 1997. Advanced Casting Technologies in Japan and Europe, Maryland: American Foundrymen's Society.

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&hl=en&gl=uk&pid=bl&srcid=ADGEEShlYQvbb0iUGvPXcSu7fYhA08pqIJJdhRavI

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Switzerland.

8. Basner, Tim, 2000. Rheocasting of Semi-Sloid A357 Aluminum. [Online] Available at:

https://docs.google.com/viewer?a=v&q=cache:7oaJBHeHQwIJ:stealth316.com/misc/del

phi_rheocasting.pdf+&hl=en&gl=uk&pid=bl&srcid=ADGEESgUBD0tqEeMMKkJiP8

A3sDGmXhTSmgAzsjDp8NoxWFLf5Afny3etc_uql_eG9aIGa8zvyIDBMSrWLhzrxye

GDRQZGUHxHG0lqv6BaVEBoaQXq2zk9QIFUxIXx6C8v

[Accessed 11 02 2012].

9. M.C. Flemings: Metall. Trans. A, 1991, 22A, 957-981.

~49~

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