Faculty of Science
School of Materials Science and Engineering

Using Al6063 Chips to produce
Porous Material by Powder metallurgy
By

YIFU FANG
A thesis submitted for the Degree of
Master of Engineering

ACKNOWLEDGEMENTS
I would like to express my special appreciation and thanks to my supervisor A/Prof.
Sammy Chan who has been a tremendous mentor for me. I would like to thank you for encouragement of my research and guide me to learn how to think independently like a professional researcher. Your advice on both research as well as on my career have been priceless. And also appreciate the help from Andrew in ADFA who helped me to do the impact test and provided me plenty of background information about how the test worked. I really couldn't analyze the data without it.

I would also like to thank our group member Andrew, Johnson and Alex who have assisted me to do the experiment and offer me some techniques for operating the experimental equipment.

A special thanks to my family. Words cannot express how grateful I am. Thanks to you,
I have this chance to study in this first class university and finally graduate.

I would also like to express my gratitude to lab manager George Yang’s and Rahmat
Kartono’s helps and trainings on my experiments. Thanks for all the supports from the the School of Materials Science and Engineering, UNSW Australia.

ABSTRACT
Recycling aluminium now is fairly popular in the world since more than 15% of the aluminium would be machined down to have the final product. However, melting these
Al rubbish to make the ingots is energy consuming and not environmentally friendly.
Therefore, the way of using powder metallurgy to make use of these scraps could be an economic way of recycling since it doesn't need to be fully melted.

The objective of the project is to use Al6063 scraps to produce porous material by the way of powder metallurgy. Optimum parameters were found for fabrication. Hardness test and impact test was carried out to examine the mechanical properties.

In this work, Al 6063 scraps were recycled to be used as the powders, then being compacted, sintered and aged. It is found that 15 minutes’ ball milling achieves best toughness due to the optimum porosity. Besides, a cabbages like structure is found where a lot of pores distribute along the boundary of the scraps. Precipitates of Mg2Si are also found in the product especially in the scraps boundary which may serve as strengthen structure for the material.

In summary, powder metallurgy is able to fabricate porous material and it has better ductility than pure copper and Al6061

Content
ACKNOWLEDGEMENTS ........................................................................................... ii
1.

Introduction ............................................................................................................. 7

2.

Literature Review ................................................................................................... 8
2.1 Aluminium alloy ..................................................................................................... 8
2.2 Aluminium 6063 ..................................................................................................... 9
2.3 Aluminium Scrap: ................................................................................................. 10
2.4 The role of alumina in Al scrap powder metallurgy ............................................. 11
2.5 The role for SiC in powder metallurgy ................................................................. 13
2.5.1 Increase of hardness ....................................................................................... 13
2.5.2 Increase of toughness ..................................................................................... 13
2.5.3 Decrease of dimensional expansion ............................................................... 16
2.6 Powder Metallurgy................................................................................................ 17
2.6.1 Ball milling .................................................................................................... 17
2.6.1.1 Control of oxide when ball milling ......................................................... 17
2.6.1.2 Ball to powder ratio ................................................................................ 17
2.6.1.3 Milling time ............................................................................................ 18
2.6.1.4 Extent of filling the vial .......................................................................... 18
4

2.7 Powder pre-formation ........................................................................................... 18
2.7.1 Die compaction .............................................................................................. 18
2.7.2 Lubricant for die compression ....................................................................... 19
2.8 Cold isotactic press (CIP) ..................................................................................... 20
2.9 HIP (Hot Isostatic press) ....................................................................................... 21
2.10 Sintering .............................................................................................................. 22
2.10.1 Sintering atmosphere effects for densification ............................................ 23
2.11 Effects of heat treatment on 6063 Al properties.................................................. 25
2.12 Effects of mechanical treatment.......................................................................... 27
3.

Experimental procedures ..................................................................................... 29
3.1 Sample preparation ............................................................................................... 29
3.1.1 Ingredient ....................................................................................................... 29
3.1.2 Powder metallurgy fabrication flow chart ..................................................... 30
3.1.3 Ball milling .................................................................................................... 30
3.1.4 Preformation .................................................................................................. 31
3.1.5 Compaction .................................................................................................... 31
3.1.6 Sintering ......................................................................................................... 32
3.2

Heat Treatment ................................................................................................ 35
5

3.2.1 Solution Treatment ......................................................................................... 35
3.2.2 Artificial aging ............................................................................................... 36
3.3

Metallographic Preparation ............................................................................. 36

3.3

Mechanical Test .............................................................................................. 37

3.3.1 Density Test.................................................................................................... 37
3.3.2 Hardness test .................................................................................................. 38
3.3.3 Impact test ...................................................................................................... 38
4.

RESULTS AND DISCUSSIONS.......................................................................... 39
4.1 Ball Milling ........................................................................................................... 39
4.2 Cabbages Structure ............................................................................................... 42
4.3 Relative Density .................................................................................................... 45
4.4 Impact Test ............................................................................................................ 47
4.5 Aging ..................................................................................................................... 53
4.6 Hardness................................................................................................................ 54

5.

CONCLUSION ..................................................................................................... 58

6.

FUTURE WORK .................................................................................................. 59

7.

REFERENCES...................................................................................................... 60

6

1. Introduction
Aluminum is the third most abundant elements in earth. The density of aluminum is
2.7kg/m3. Pure aluminum is silver and white and it has good ductility, thermal and electrical conductivity. It is so chemically reactive that Aluminum only exists by the form of compound such as alumina etc. Aluminum is remarkable for its resistance of corrosion due to passivation.

Aluminum alloys is one of the most prevalent materials used in the world. Aluminum alloy is light and of high strength therefore being integrated every corner in life, from windows in our apartment to automobiles etc. China has led the first place in consumption of aluminium in 2010[1]. However, such great consumption doesn't give a high efficiency in making products since the scraps produced accounts for 15% of total Al involved.
These scraps are always gathered and melted to make new ingots. The expense of this is very high compared with recycling which only requires 5% of that used to make new aluminium from raw material [2]. Therefore, in the US, 31% of all aluminium comes from recycled scrap [3] which gives a very good example of recycling and reusing.

Methods for processing aluminium to reuse are various. It can be simply melted to make aluminium ingots which has no loss of properties and qualities [4]. However, the cost of it is high due to involvement of high temperature. The other popular way of recycling is powder metallurgy. Aluminium scrap is used as powder for sintering. The product is not as dense as the previous one due to its porosity. However, this porous structure may offer high fracture toughness since the crack can be blunted when growing.

This work is mainly to use aluminium scraps as raw material to perform powder metallurgy. The process mainly consists of powder making, preforming and sintering. By adjusting experimental parameters, the optimum properties can be achieved.

7

2. Literature Review
2.1 Aluminium alloy
Aluminium alloy mainly categorises to casting alloys and wrought alloys, cast aluminium alloy has low melting point thereby being cost effective when being casted. However, it has lower tensile strength than wrought alloys. Wrought alloys accounts for 85% aluminium alloy and it gives better mechanical properties than cast alloys therefore, it is mostly used as a structural material [5]

The category of wrought aluminium alloy can be classified as 8 series according to alloyed elements:
1000 series are essentially pure aluminium with 99% aluminium
2000 series are alloyed with copper
3000 series are alloyed with manganese
4000 series are alloyed with silicon
5000 series are alloyed with magnesium
6000 series are alloyed with magnesium and silicon.
7000 series are alloyed with zinc
8000 series are alloyed with other elements that are not covered by other series.

One feature of aluminium alloy that superior than most of other alloys is good corrosion resistance due to sturdy oxide layer formed on the surface of aluminium. This protective layer offers the insulation from outside environment thereby reducing the corrosion.

8

2.2 Aluminium 6063
Aluminium 6063 is 6000 series aluminium alloy which is characterised by precipitation hardening. This allows the alloy to have better mechanical properties than other series.
The composition of 6063 aluminium is exhibited in following table:

Composition of Al 6063
Zinc

0~0.1

Titanium

0~0.1

Manganese

0~0.1

Copper

0~0.1

Chromium

0~0.1

Iron
Silicon
Magnesium

0~0.35
0.2~0.6
0.45~0.9

Aluminium

99.5

Figure 2.1 Composition of Al 6063
Physical properties of Al 6063:

Major
Properties

Figure

Density

2.69 kg/m3

Melting point

616~654 ℃

Tensile strength(yield) 220MPa

Table 2.1 Major properties of 6063 Al

9

Tensile strength: ultimate

250Mpa

2.3 Aluminium Scrap
Aluminium scrap is the major form of aluminium waste which is usually produced during machining. As table 2.2 indicates, in United States, 2012, aluminium recycled and restored from scraps had achieved to 3.4 million metric tons which accounts for 54% of total aluminium annual consumption [7]. China, also leads to the first place among scraps consumers in the world, recycling more than 5 million metric tons in 2010[8]. The fact shows a big potential in production of Al by using scraps. By recycling aluminium, plenty of metallurgic process is evitable thereby reducing the expense.
Aluminum

Total

Recovered

Aluminum

from

Scrap
Market

scrap Consumption

(mt)

Aluminum
Scrap Exports

Share

(mt)

U.S.

(mt)

(%)
2008

3,320,000

6,408,000

52%

1,982,000

2009

3,000,000

5,697,000

53%

1,658,000

2010

2,700,000

5,053,000

53%

1,913,000

2011

3,110,000

5,386,000

58%

2,144,000

2012

3,410,000

6,310,000

54%

2,037,000

Table 2.2 Data about aluminum scrap in the U.S. aluminum industry [7].

10

2.4 The role of alumina in Al scrap powder metallurgy

Alumina is a most common form of aluminium in the world. it is a product of aluminium reacting with oxygen and usually has 10~15 thickness and extremely high hardness.
These special features lie behind the good corrosion and wear resistance thereby often applying in making milling ball etc.

When making the Al powder, with the collision of stainless steel balls, alumina may be peeled off from the surface of aluminium and further broke up in to smaller pieces by milling balls. This smaller alumina can be served as reinforcement in powder metallurgy by pining the dislocation movement. The strength, as a result, is able to improve to some extent. However, with the increase of alumina, the porosity increases along with the hardness [9].
Data are showed in the table 2.3. This is partly because of the poor cohesion between alumina and aluminium during sintering. And due to the high hardness oxide layer attached on the aluminium surface, it is difficult to deform when being compressed.
Therefore, the porosity increases when amount of alumina goes up.

percentage of

Theoretical

Experimental

density

density

alumina

(g/cm3)

% porosity

Hardness
(VHN)

3

(g/cm )

0%

2.7

2.68

0.74

40.3 ± 0.2

6%

2.76

2.69

2.54

40.5 ± 0.1

11

percentage of

Theoretical

Experimental

density

density

alumina

(g/cm3)

% porosity

Hardness
(VHN)

3

(g/cm )

9%

2.79

2.74

1.79

41.4± 0.4

15%

2.85

2.75

3.51

43.6± 0.2

18%

2.87

2.77

3.48

45.1 ± 0.3

Table 2.3: percentage porosity and hardness data for the AA 6063- Al2O3 [9]

Besides, based on the data being examined, toughness and fracture toughness turn out to be lower as the alumina increases. (see figure 2.2)

Strains for different percentage alumina
0.25

Strain in mm/mm

0.2

0.15

0.1

0.05

0
0%

6%

9%

15%

18%

Volume % alumina

Figure 2.2 Variation of strain to fracture with increase in volume percent alumina particulates [9]

12

2.5 The role for SiC in powder metallurgy
2.5.1 Increase of hardness
SiC is a critical additive often applied in powder metallurgy. It is able to affect the mechanical properties of product such as high hardness, high strength, good wear resistance [10]

. This can be achieved by pinning the dislocation movement. Apart from

pinning effect, for nanometric silicon carbide particles, the reaction is able to occur on the interface of silicon carbide and aluminium matrix, which further improves the mechanical properties of composites [11].
The reaction is: SiC + Al → Si + Al4C3 (reversible)
However, this reaction may be restricted when silicon content is high due to the balance of the reaction. Figure 2.3 shows the hardness changing with the addition of silicon carbide. It is mainly due to the high hardness of silicon carbide.
2.5.2 Increase of toughness
In this case, reinforcement particle serves as obstacles against the crack from growth which brings about the change of direction and shape of the crack. When the crack tends

Figure 2.3 microhardness changes with content of SiC
13

to have secondary cracks, the change of crack direction may absorb a part of energy. This is able to prevent the crack from growing to some extent [12]. The following graphs and figure compares the tissue difference between amount of SiC.

Figure 2.4 Rough tissue of 5% SiC

Figure 2.5 Finer tissue of 10% SiC

The one with more silicon carbide (10%) form a finer tissue compared with the one (5%).
Finer structure has high performance of absorbing energy, which is evidenced by figure
2.4 and figure 2.5. It can be concluded that 10% SiC is able to increase the impact strength by 2.16% [13]. Furthermore, when the content of SiC is increased to 25%, energy (can be evidenced by stress intensity factor) that can be absorbed falls down [14], data can be showed in table 2.

14

Figure 2.6 The relationship between crack growth and impact energy absorbed [13]

Table 2 Stress intensity factor changes with different amount of SiC [14]

15

2.5.3 Decrease of dimensional expansion
By adding silicon carbide of micrometre size, the expansion rate decreases with the amount of silicon carbide addition. As can be seen from the diagram, the one with 30% silicon carbide achieves minimal expansion rate [15]. Therefore, the relative density would be higher when the matrix is added in silicon carbide.

Figure 2.7 Expansion rate changes with temperature [15]

16

2.6 Powder Metallurgy
2.6.1Ball milling
Ball milling is the method of making aluminium powder. By mixing the metal particles and balls (usually made out of stainless steel) in to the bottle of ball milling machine, the machine will shake the container at a certain speed to produce the desired size of metal powder. During the procedure of ball milling, balls in the bottle will impact the particle and make them break up into pieces. The more time ball milled, the smaller sized powder it produces.
2.6.1.1 Control of oxide when ball milling
Aluminium oxide usually serves as a positive role in protection of aluminium due its high hardness and wear resistance. However, in powder metallurgy, this may not be good since it will play as a kind of reinforcement decreasing its toughness especially fracture toughness. The other disadvantage of excessive oxide is able to reduce the internal quality of product since alumina is of high melting point. When the compact sample is sintered, the unmelted alumina prevents aluminium powder from joining each other. This leads to the porosity in the final product which lowers the general mechanical properties of product. 2.6.1.2 Ball to powder ratio
The size of the powder produced has direct relationship with the quality of the products.
This relates to a very important term, ball to powder weight ratio, a term that defines the relative weight of ball and powder. The ratio varies to the type of ball milling machine.
For a small capacity, high energy ball milling such as SPEX mill, the ratio is 10:1. For
BPR, the value ranging from 4:1 to 30:1 has been used. However, for large capacity ball milling, low energy mill, the ratio is up to 50:1 even 100:1 at reasonable time. [16]

17

2.6.1.3 Milling time
The time of ball milling is one of the critical parameter for the quality of products. The more time in milling, the smaller size of particle it will be. Relatively, for the ball milling without shielding gas, oxide formation is also a big problem, which is essential to be considered into controlling milling time. Normally, the time is determined to reach to the state between fracturing and cold welding of particles to facilitate alloying. As a general rule, high energy ball milling requires shorter time while longer time for low energy ball mills. [16]
2.6.1.4 Extent of filling the vial
Powder is made by balls impacting and fracturing the scraps in the vial. If too much scraps are filled in the vial, which means less space for these two things to move freely, it may take long time to break into pieces. However, if there are minimal scraps put into the vial, the production rate is also very low. Therefore, attention needs to be paid on the extent of the vial filling; generally, about 50% or a little more space left to be empty [16].

2.7 Powder pre-formation
Preformation is a step that the powder is compacted to have a relatively uniform structure, after which the sintering is much easier to perform. Compaction in powder metallurgy are mainly using CIP (cold isostactic press) and die compaction. plastic deformation, particle interlock and cold welding will occur during compression in both methods under a certain load that is usually over yield strength of powder.

2.7.1 Die compaction
Preformation by die compaction usually has uneven density distribution due to friction between die wall and powder. Tiny the powder is, more friction there will be. This non uniform structure may not be good for quality of sintering, leading to the inhomogeneous
18

shrinkage and distortion

[17]

. However, die compaction can be effective for the density

from vertical direction. Normally the relative density will go less than 90% with 20min ball milling powder [18].
2.7.2 Lubricant for die compression lubricant in die plays a critical role for the quality of preformed samples. Less or no lubrication may increase the friction between die wall and sticks. This lead to the real pressure applied on powder less than it should be. Therefore, the quality for preformation may not be acceptable and the samples even collapse during CIP. Apart from the effect for quality of samples, this high friction also escalates the wear of the die due to the formation of interatomic bonds. Lubricant is possible to prevent this by forming a lubricant film [19].
Selection of lubricant is very important since preformed samples will be sintered under
600-degree C where some organic lubricants are able to react with the eutectic liquid.
This brings about pressing to swell and pose the threat of cracking

[19]

. Therefore, the

material of it should be stable enough and only gasification allowed during high temperature. According to S. Martsunova et al., acrawax is the best lubricant applied in
USA. [20]

19

2.8 Cold isotactic press (CIP)
More uniform structure can be achieved by applying this method. This method is able to impose a uniform compressive force around the sample which gives a compliment of lacking compaction on radial direction by using die compaction. The applied load of this method is the same as die compression. Figure 2.8 shows the relationship of applied pressure between porosity, number of contacts, and contact area. Figure 2.9 gives the situation that the compaction pressure changes with theoretical density [21].

Figure 2.8 The compaction behaviour of spherical particles of bronze [22]

20

Figure 2.9 Theoretical density and compaction pressure [22]
As can be seen from the figure, powder No.1 and powder No.2 stand for different size.
Particle No.1 has 60% packing density while particle No.2 has 50%. However, this difference in size doesn't give much difference in theoretical density when pressure is over 400MPa. [22]

2.9 HIP (Hot Isostatic press)
HIP can be seen as the combination of CIP and sintering which applies uniform compression and heat on samples. With the heat going high enough close to melting point, the matrix becomes softer and softer thereby offering the condition for easy plastic deformation. The relative density therefore increases to a large extent. The temperature applied on HIP for Al alloys usually ranges from 350 to 500-degree C under 100MPa [23]

21

Figure 2.10 The effect of HIP on properties [24]
As the figure 2.10 indicates, HIP increases the strength of sample for the one that is has
T6 heat treatment before HIP. Besides, the ductility also increases dramatically since the percent of strain rate curve is much longer than the one has not been applied HIP [24].

2.10 Sintering
Sintering is also a critical step of powder metallurgy. This step is mainly to increase the density of the sample to a large extent about 5%~7%

[25]

. The mechanism of sintering

consists of 6 types:

• Surface diffusion: Diffusion along the surface of particles
• Vapor transport: Evaporation of atoms that condense on the different surface
• Lattice diffusion from surface: atoms from surface diffuse through lattice
• Lattice diffusion from grain boundary: atom on grain boundary diffuses through lattice • Grain boundary diffusion: atoms diffuse along grain boundary
• Plastic deformation: dislocation motion brings about flow of material.
22

There are two types of sintering: solid-state sintering and liquid- state sintering. The latter is able to achieve higher densification due to capillary force produced by liquid force [26].
Liquid phase sintering is operated at the temperature above the solidus where aluminium is melted. It is able to fill up the vacancies thereby having high density. In this case, the solid particles are bonded together by capillary force of liquid phase.
2.10.1 Sintering atmosphere effects for densification
Aluminium is readily to be oxidized which affects the cohesion of powder during compression and sintering. Therefore, shield gas needs to be introduced into sintering in order to protect material from oxidation.

Figure 2.11 The atmosphere effect on expansion rate [15]
There is a few composition of gas is possible to be applied. Argon gas, pure nitrogen and nitrogen with 5% hydrogen. As the experiment results indicates (shows in following figure 2.11), the one filled up with pure nitrogen has highest shrinkage which may suggest more liquid phase is formed during this process. This is due to the exothermal aluminum

23

nitriding reaction that discharges lots of heat and makes temperature raise over 620degree C. [15] the other thing that may affect the densification of Al is Magnesium. Mg is able to react with alumina to generate aluminium thereby increasing relative density [27]. The reaction is showed below:

3Mg + 4Al2O3 = 3MgAl2O4 + 2Al

During sintering, capillary forces plays a critical part in pulling particles in contact.
Atoms transfer started once they contact with each other. The driving force of this process is reduction of surface energy. When the necks achieve a stable size, the particles are able to form larger grains with reducing pores. Lastly, diffusion of surface and evaporation occur to close and isolate the pores in diffusion paths.

24

2.11 Effects of heat treatment on 6063 Al properties
Aluminium 6063 is an alloy that contains plenty of other elements like Si, Mg etc., therefore, consideration of heat treatment is able to control the precipitation of these elements since they can affect the property of material. Solution treatment is applied on
6063 aluminium to change its properties. By heating up the sample to 525~530 DegreeC where main principate Mg2Si dissolves back to to matrix. Then it is quenched in the water locking the strengthening elements within the matrix. Silicon and magnesium tend to precipitate out but cannot since the room temperature after quenching. In aging, once the temperature raises to high enough, the precipitation is stimulated and starts precipitating the fine particle of Mg2Si thereby increasing the strength to a large extent
[28]

.

Figure 2.12 Three kinds of Mg2Si based on aging time [29]

There are three kinds of Mg2Si according to different aging time. From β’’ to β’ and to β.
The difference between each other is the size which indicates the coherency of the particle and matrix. β’particle is the one that has highest yield strength which is peak aged about
25

12 hours [29]. it has also been found that small amount of magnesium helps to change plates shaped βAlFeSi phase into globular α-AlFeSi phase (see figure where L indicates the length of former β-AlFeSi whereas the l gives the length between α particles after transformation), which improves the better strength of the material [30], [31]

Figure 2.13 Comparison of the shape of β-AlFeSi and α-AlFeSi [31]

As for the aging time and temperature (see figure 2.14), a study of Al 6061 has proved that at the temperature of 185℃~195℃, aging time between 2 to 4 hours may give the highest hardness. (The sample tested is in diameter of 12.7mm and length of 50.8mm)
[32]

26

Figure 2.14 Hardness of Al 6061 changes with time and aging temperature from 175 ℃ to 420 ℃. [32]

2.12 Effects of mechanical treatment
Aluminium is a ductile and soft metal which means it has low dislocation pile-up rate.
However, some mechanical treatment may improve the property of material such as rolling and forging. The result of different treatments is exhibited in following table 3:

UTS (MPa)

Elongation

Processing
Forged

Rolled

Forged

Rolled

Deformed at Ambient temperature.

126.98

212.4

2.49

6.1

Deformed and Homogenized.

99.3

104.5

13

14

73.7

126.7

7.1

19

Deformed and Homogenized, Solution heat treatment and Normalized.

27

Deformed, Homogenized, Solution heat treatment 131.8

127.1

25

24

107.76

93.8

17.2

9.8

114.8

114.8

10.6

10.6

and Water quenched

Deformed, Homogenized, Solution heat treatment, Water quenched and Aged
As-cast

Table 3. The ultimate tensile stress and elongation rate by different processes on samples [33]
Deformation at room temperature gives the highest ultimate strength but the lowest elongation rate due to the precipitation distributed in it. the one with water quenched but without aging has the highest elongation rate. This is because the solution heat treatment and quenching make the precipitates dissolve back to matrix therefore the dislocation may go easier than before and there is increasing volume of other intermetallic compounds giving rise to improve the elongation of samples [33]. To trade off the relationship between ductility (elongation) and strength, deformation along with thermal treatment such as solution hard treatment and water quenching should be operated for enhancement of ductility. 28

3. Experimental procedures
3.1 Sample preparation
3.1.1 Ingredient
Main ingredient of this study is aluminium 6063 scrap offered by Shanghai Jiao Tong
University with average sized of 6mm. The metal scraps are made when machining the
Al window frame. And it was cut into the final size of around 6mm x 1mmx 0.2mm by ball milling showed in figure 3.1b. The raw material is cleaned by ultrasonic vibrating within solution of ethanol to guarantee that most of the dirt and grease is removed to not affect the result of samples. Besides, all the scraps are heated in oven for 2 hours to dry completely and stored in the drying chamber which keeps the ingredient in a best condition. The composition of cleaned and dried scraps is showed in the table below:
Elements

Al

Mg

Si

Fe

Others

Weight %

98.55

0.34

0.87

0.14

Al 6061> Al6063 scraps-made sample. Yield strength of pure copper is only 70 MPa, however, its ductility is much larger than Al6061.
Therefore, the product of strain and stress for copper is larger than Al 6061. As for the reason that Al 6063 scraps-made sample has lowest elastic energy, it is due to the porosity inside. The existence of pores leads to less contact between particles or scraps. When it is compressed, little load will make it have plastic deformation. Therefore, it has the least elastic energy.
The plastic energy follows the same order, pure copper > Al 6061> Al6063. The strain of
6063 scraps-made sample has the largest strain but the lowest ultimate strength. Therefore, it still ranks the last in terms of the energy absorption.
In conclusion, copper has best toughness. However, its ductility is not as good as 6063 scraps-made sample.

52

4.5 Aging

Figure 4.20 SEM of the precipitates and elements analysis
Aging is done for 12 hours at 160-degree C which is theoretically peak aged. The white dots in figure 4.20 are precipitate and the chemical composition can be determined from
EDS technique which is Mg2Si. The shape of the Mg2Si is different from each other. Some are like tiny white spots and some are plates like while some are much bigger than any others. this situation actually is the case for peak aging where all kinds of different shape precipitates are presented. As Rachael. D et. al. has found, there are three forms of Mg2Si forming with different aging time. When being under aged, the precipitates are tiny. When being over aged, the participates tends to be bigger. Specially, when it is peak aged, the precipitates are actually the mixture of under and over aging precipitates which looks exactly like the way in the picture.

Apart from this, one interesting thing from the element analysis is the precipitation between the boundary. both of the precipitates are made out of Mg and Si. However, the colours of them tends to be different as the one in boundary is not as white as the one scattered in flakes. Therefore, it should be two different compound but both of them are made out of elements Mg and Si.

53

4.6 Hardness
4.6.1 hardness test on top surface
Sintering time

Ball milling time
10 minutes

aging at 160℃
2 hours sintering

31.3±0.3

10 minute

37.67±1

15 minutes

39.9±0.8

20 minutes

41.73±1

10 minutes

by 12 hours artificial

28.1±0.2

20 minutes
1 hours sintering followed

27.5±0.2

15 minutes

1 hours sintering

Hardness(HV)

34.6±0.6

Table 9 Hardness in different conditions
Original hardness for Al 6063 chips is around 86 HV. After ball milling for 10 minutes,
15 minute and 20 minutes, the hardness drops a lot to the approximate 30 HV. It should be mentioned that the hardness shows a slight trend of going up while the ball milling time goes up. This is because long time ball milling makes flakes become smaller which promotes the flakes piling up in a denser arrangement.

Figure 4.21 10min with 2-hour sintering

Figure 4.22 10min with 1-hour sintering
54

The sets that are age treated have significant increase in hardness by approximately 10HV.
This is precipitation hardening where plenty of Mg2Si come out which serves as the prevention when the material is deformed.
Specially, for 2 hours sintering of 10 minutes’ ball milled flakes, the hardness is even higher than 20 minutes set because long sintering time promotes the diffusion of atoms from the flakes into vacancy which increases the relative density. Besides, the thicker boundary between flakes may be another reason that contributes to the higher hardness than 1-hour set. (see figure 4.21 and 4.22)
Hardness is also tested after impact test since the length has decreased by around 1mm among all the samples.
10 min ball milling

15 min ball milling

20 min ball milling

0.92

0.97

0.95

Decrease Rate

7.0%

7.4%

7.2%

Hardness (HV)

57±3

61.5±2

55±2

Length
Decreased(mm)

Table 9 Hardness test after impact
As can be seen from the table, 15 minutes’ ball milling set achieves highest hardness. The reason for this could be explained by the largest decrease in length which leads to the reduction of porosity in the billets. Therefore, the hardness could be higher than other sets. 4.6.2 Hardness test on cross section after impact test hardness test over the cross section is conducted to see the pattern for hardness distribution. Tested region is selected from the top to the bottom of the cross section. And every section is done for 10 times from the left to right. The schematic graph can show how it is done.
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Figure 4.23 Arrangement for hardness test over cross section
The test region is divided to three, upper region, middle region and bottom region. The hardness data and distribution is showed in following graphs.

10min Ball Milling
80
70
60
50
40
30
20
10
0
1

2

3

4

5

6

7

Distance from the left to right

upper part

middle part

8

9

10

bottom part

Figure 4.24 Hardness distribution over cross section of 10min ball milling billet

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15 min Ball Milling
70
60
50
40
30
20
10
0
1

2

3

4

upper part

5

6

7

middle part

8

9

10

bottom part

Figure 4.25 Hardness distribution over cross section of 15min ball milling billet

20 min Ball Milling
70
60
50
40
30
20
10
0
1

2

3

4

5

6

7

Distance from the left to right

upper part

middle part

8

9

10

bottom part

Figure 4.26 Hardness distribution over cross section of 20min ball milling billet
As we can see from three figures, middle region of the samples tends to have better hardness than the other two regions. This is consistent with the less porosity on middle cross section which gives much denser tissue. Besides, the central hardness is larger than side hardness which may be caused by uneven load applied during impact test. The loading surface have different extent of deformation therefore partial porosity tends to be different. As a result of this, the hardness of less porous region (mid-region) is higher.
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5. CONCLUSION
The sintered Al6063 compacts produced by processes including ball mill, cold press, cold isostatic press, sintering were studied in this work. The following conclusions can be made: 1.15 min ball milling relative density 95.6% ） achieves best strain among all different
（
set ball milling set which indicates the amount of porosity in sample is optimum for energy absorption. 2. Aging is able to increase the hardness to a large extent and it is able to increase the hardness by 10 HV compared with the one has not done which means an effective way of strengthening.
3.Impact test (of the strain rate of 1000/S) decreases the length of the sample by around
1mm and increases the hardness by further 10 HV which implicates less pores inside may increase the hardness.
4. From the hardness profile of cross section, side hardness is averagely lower than centre among three samples. And this means there is higher porosity near the surface.
5. 6063 scraps-made sample is better than pure copper and 6061Al in terms of strain since pores inside the billets may contribute to some amount of the deformation.
6. HIP doesn't improve much relative density on cold pressed sample (density less than
92%) which indicates HIP can be applicable over a certain relative density.

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6. FUTURE WORK
The study of producing porous material from Al6063 scraps has successfully found out the parameters of optimum strain for the material and the property test such as impact test and hardness test has been conducted. However, there are still work to be done in the future: 1. Add SiC to find out if it plays a positive role in promoting fracture toughness.
2. Optimum porosity needs to be found to reach the best energy absorbing capacity.
3. Role of the pores for absorbing the fracture energy should be investigated and the assumption of the mechanism should be proved.
4. Argon atmosphere should be investigated to avoid high oxygen content in ball milling.
5. Precipitates in boundary and in flakes should be investigated to see if they are two different compounds which are consisted of Mg and Si.
7. Control of the uniform thickness for ball milling product to have the stable properties.
8. Produce the billets by CIP without cold pressing in order to avoid fold of ball milling flake therefore avoiding cabbages structure. And see how the performance will change without cabbages structure.
9. 6063 scraps-made sample is better than pure copper and 6061Al in terms of strain but the comparison between 6063 scraps-made sample and 6063 billet should be made to determine the change for the performance between different inner structure.
10. the hardness of cross section needs to be compared with the one hasn't done impact test to see the change of the hardness profile.

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