FINAL YEAR PROJECT REPORT
GOR 001/2011
UNIVERSITY OF NAIROBI
DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING
PROJECT REPORT TITLE:
AN INVESTIGATION OF SOME WELDING CHARACTERISTICS OF
AA6XXX ALLOYS
This project report is submitted in partial fulfillment of the requirement for the award of the
degree of Bachelor of Science in Mechanical Engineering.
Submitted by:
LETIYAN NTANGENOI MARGARET F18/2011/2005
LEMISO KOIYO
F18/1835/2006
Project supervisor:
PROF. G. O. RADING
3rd June 2011
DECLARATION
The content of this document is the original work based on our own research and to the best of
our knowledge it has not been presented elsewhere for academic purposes.
LETIYAN NTANGENOI MARGARET
F18/2011/2005
Signed………………………………….
Date: ……………………………
LEMISO KOIYO
F18/1835/2006
Signed…………………………………...
Date: ………………………………
This project is submitted as part of the Examiners Board requirement for the award of the degree
of Bachelor of Science in Mechanical and Manufacturing Engineering from the University of
Nairobi.
Project supervisor:
PROF G.O. RADING
Signed ………………………………….
Date: ……………………………….
i
DEDICATION
This project is dedicated to our parents for their love and support during our studies.
ii
ACKNOWLEDGEMENTS
We wish to express our sincere gratitude to our project supervisor, Prof. G. O. Rading for his
insight and guidance throughout the undertaking of this project.
We sincerely feel indebted to the following people from the department of Mechanical
Engineering workshop who worked with us tirelessly during this project: The principal
technologist, Eng. Aduol, Mr. Ndulu, Mr. Githome, Mr. Njue and Mr. Niva
Many thanks to Jomo Kenyatta University of Agriculture and Technology (JKUAT) for allowing
us to use their facilities for welding.
Our gratitude also goes to the Kenya Bureau of Standards management and staff for allowing us
to use their facilities and equipment. Mr. Were and Mr. Ndeti were especially of great help.
We are also grateful to African materials science and Engineering Network (AMSEN) for
providing funds for purchase of the microstructure characterization software.
Great thanks to God Almighty for giving us the strength, good health and the opportunity to
carry out this project.
iii
ABSTRACT
The purpose of carrying out this project was to investigate the effect of welding on grain size,
phase volume fraction and hardness of AA6XXX alloys.
To accomplish this, AA6061 in T0, T4 and T6 condition was used. The as received condition
was AA6061-T0. It was then heat treated to T4 and T6 conditions. This was done by solution
heat treatment at 496oC for 10 hours followed by quenching at room temperature.T4 was then
obtained by naturally ageing for a period of 20 days. On the other hand T6 was obtained by
artificial ageing at 176oC for 8 hours.
Varestraint test was carried out to determine the weldability on test specimens measuring 24mm
x 10mm x 1.6mm. The mandrel used had a radius of 80mm. Gas Tungsten Arc welding was used
to produce bead on plate without use of a filler alloy. The weld bead was 19mm long and
immediately the torch was extinguished, the specimen was bent to take the shape of the mandrel.
The cracks so produced were observed using a stereo microscope at a magnification of X10. The
maximum lengths of cracks (MCL) was used as a measure of weldability and it was found that
T4 was the most susceptible to cracking with MCL=183mm while T0 was found to be the least
susceptible with MCL= 13.4mm.
Transverse sections of the specimen was then sectioned and mounted on epoxy resin as a
preparation for metallographic testing. It was then ground on successive grades of silicon carbide
paper (240, 320, 400 and 600) before polishing mechanically on rotating wheels with 7μm, 1μm
and 1/4μm polishing paste successively. Etching was then done using Keller’s reagent before
rinsing in cold water then alcohol and finally hot air drying. The specimens were then observed
under a light microscope at a magnification of X500. The micrographs so produced were then
analyzed using microstructure characterization software (MIC).
From the grain analysis it was found that Grains became coarser as you moved towards the
fusion line from the base metal for T0 and T4 conditions. But for T6 the grains became finer as
you moved towards the fusion line.
On analyzing the volume fractions it was found that at the fusion line there was dissolution of
phase 3 while phase 1 and 2 increased towards the fusion line from the base metal. Phases 2 and
3 were found to be the hardening phases while phase 1 was found to have no effect on the
hardness.
iv
Hardness survey was also carried out at intervals of 0.5mm, starting from the fusion line. The
method used was Vickers Hardness Test. It was found that for T0 and T4 conditions there was an
increase in hardness at the fusion line as compared to the base metal. But for T6 condition,
hardness was seen to decrease towards the fusion line. T6 was found to be harder followed by T4
and finally by T0.
v
TABLE OF CONTENTS
DECLARATION ........................................................................................................................ i
DEDICATION ........................................................................................................................... ii
ACKNOWLEDGEMENTS....................................................................................................... iii
ABSTRACT ............................................................................................................................. iv
LIST OF FIGURES ................................................................................................................ viii
LIST OF TABLES .................................................................................................................... ix
1.0 INTRODUCTION.................................................................................................................1
1.2. Statement of Problem .......................................................................................................2
1.3 Objectives .........................................................................................................................2
2.0 LITERATURE REVIEW ......................................................................................................3
2.1 Background Information ....................................................................................................3
2.1.1 Production of aluminium ............................................................................................3
2.1.2 Properties of aluminium and its alloys ........................................................................4
2.1.3 Aluminium alloys .......................................................................................................5
2.1.4 Effect of alloying elements .........................................................................................5
2.2 Classification of aluminium alloys .....................................................................................5
2.2.1 Heat treatable alloys....................................................................................................5
2.2.2 Non-heat treatable alloys ............................................................................................5
2.2.3 Cast alloys ..................................................................................................................6
2.2.4 Designation of Aluminium Alloys ..............................................................................6
2.3 Applications of aluminium alloys ......................................................................................7
2.4 Welding aluminium ...........................................................................................................8
2.4.1 Welding processes ......................................................................................................8
2.4.2 GMAW (Gas Metal Arc welding) ...............................................................................9
2.4.3 GTAW(Gas Tungsten Arc welding) ............................................................................9
2.4.4 Welding characteristics ............................................................................................. 10
2.5 Weldability of aluminium and its alloys ........................................................................... 11
2.5.1 Methods for determining weldability ........................................................................ 11
2.5.2 The Varestraint test ................................................................................................... 11
2.5.3 Weldability of aluminium alloys ............................................................................... 13
vi
2.6 Heat treatment of aluminium and aluminium alloys ......................................................... 14
2.6.1 Annealing ................................................................................................................. 14
2.6.2 Solution Heat Treatment ........................................................................................... 14
2.6.3 Quenching ................................................................................................................ 15
2.6.4 Age Hardening.......................................................................................................... 15
2.6.5 Precipitation Hardening ............................................................................................ 16
2.6.6 Precipitation sequence in AA6XXX alloys................................................................ 16
2.7 Coherence ....................................................................................................................... 16
2.8 Effect of heat from welding on Mechanical properties of aluminium alloy ....................... 17
2.8.1 Non-heat treatable alloys in class 1XXX, 3XXX, 5XXX .......................................... 18
2.8.2 Heat treatable alloys 2XXX, 6XXX, 7XXX .............................................................. 18
3.0 EXPERIMENTAL PROCEDURE ...................................................................................... 20
3.1 Heat treatment ................................................................................................................. 20
3.2 Varestraint weldability test .............................................................................................. 20
3.3 Metallography ................................................................................................................. 20
3.4 VICKERS HARDNESS TEST ........................................................................................ 21
4.0 RESULTS AND ANALYSIS .............................................................................................. 22
4.1 VARESTRAINT TEST ................................................................................................... 22
4.2 MICROSTRUCTURE ..................................................................................................... 23
4.2.1 MICROGRAPHS OF WELDED AA6061 T0 CONDITION..................................... 23
4.2.2 MICROGRAPHS OF WELDED AA6061 T4 CONDITION.................................... 25
4.2. 3 MICROGRAPHS OF WELDED AA6061 T6 CONDITION................................... 27
4.3 GRAIN SIZE ANALYSIS ............................................................................................... 29
4.4 VOLUME FRACTION ANALYSIS ............................................................................... 31
4.5 HARDNESS SURVEY ................................................................................................... 39
5.0 DISCUSSION OF RESULTS.............................................................................................. 44
6.0 CONCLUSION AND RECOMMENDATION .................................................................... 46
7.0 REFERENCES.................................................................................................................... 47
8. APPENDIX........................................................................................................................... 49
vii
LIST OF FIGURES
Fig2.1 Illustration of GMA welding ...........................................................................................9
Fig 2.2 Illustration of GTA welding .......................................................................................... 10
fig2.3 schematic showing the (a) varestraint and (b) transvarestraint test ................................... 12
Fig 2.4 Illustration of the difference between coherent, semi-coherent and incoherent
precipitates ................................................................................................................................ 17
Fig 4.1 comparison of maximum crack lengths for T0, T4 & T6................................................ 23
Fig 4.2 micrographs of the HAZ of welded AA6061-T0............................................................ 25
Fig 4.3 Micrographs of the HAZ of welded AA6061-T4 ........................................................... 26
Fig 4.3 Micrographs of HAZ of welded AA6061-T6 ................................................................. 28
Fig 4.4 Graph showing the grain size trend in AA6061- T0 ....................................................... 29
Fig 4.5 Graph showing the grain size trend in AA6061- T4 ....................................................... 30
Fig 4.6 Graph showing the grain size trend in AA6061- T6 ....................................................... 30
Fig 4.7 Micrograph showing the phase 1 phase 2 & phase 3 ...................................................... 31
Fig 4.8 Graph of % volume fraction against distance from the fusion line for AA6061- T0 ....... 32
Fig 4.9 Graph of % volume fraction against distance from the fusion line for AA6061- T4 ....... 33
Fig 4.10 Graph of % volume fraction against distance from the fusion line for AA6061- T6 ..... 34
Fig 4.11 % Volume fraction variation of phase 1 ....................................................................... 36
Fig 4.12 % Volume fraction variation of phase 2 ....................................................................... 37
Fig 4.13 % Volume fraction variation of phase 3 ....................................................................... 38
Fig 4.14 Graph of VHN at various distances from the fusion line for AA6061-T0 ..................... 40
Fig 4.15 Graph of VHN at various distances from the fusion line for AA6061-T4 ..................... 41
Fig 4.16 Graph of VHN at various distances from the fusion line for AA6061-T6 ..................... 43
viii
LIST OF TABLES
Table2.1 Designation of wrought aluminium alloys .....................................................................6
Table2.2 Designation of cast aluminium alloys ...........................................................................7
Table 2.3 Tensile Strength Properties for heat treatable alloys .................................................. 19
Table2.4 Tensile Strength Properties for non heat treatable alloys ............................................. 19
Table4.1 Maximum crack lengths for the varestraint test .......................................................... 22
Table 4.2 Grain size analysis for T0, T4 and T6 ........................................................................ 29
Table 4.3 volume fraction of phases 1, 2 & 3 in AA6061-T0 .................................................... 32
Table 4.4 volume fraction of phases 1, 2 & 3 in AA6061-T4 .................................................... 33
Table 4.5 volume fraction of phases 1, 2 & 3 in AA6061-T6 .................................................... 34
Table 4.6 Volume fraction variation of phase 1 across T0, T4, & T6 ......................................... 35
Table 4.7 Volume fraction variation of phase 2 across T0, T4, & T6 ......................................... 36
Table 4.8 Volume fraction variation of phase 3 across T0, T4, & T6 ......................................... 38
Table 4.9 Hardness survey on AA6061-T0 ................................................................................ 39
Table 4.10 Hardness survey on AA6061-T4 .............................................................................. 40
Table 4.11 Hardness survey on AA6061-T6 .............................................................................. 41
ix
1.0 INTRODUCTION
Aluminium alloys of the AA6061 group have Magnesium and Silicon as the major alloying
elements. The most common alloys in this group are 6061, 6063 and 6082.
Aluminium alloy 6061 is a precipitation hardening alloy. It has a density of 2700Kg/m³. It is
composed of 0.8% Mg, 0.7% Si, 0.34% Fe, 0.15% Cu and other elements like Cr, Mn and T in
small quantities. It is one of the most common aluminium alloys for general purpose use. This
alloy exhibits good weldability. Addition of Magnesium and Silicon to aluminium produces
Magnesium Silicide (MgSi2) which provides AA6061 with its heat treatability. It extrudes easily
and economically and is found in a wide selection of extruded shapes. AA6061 is available in
pre tempered grades for example 6061-O and also tempered grades such as 6061-T6, 6061-T4.
The different tempers of 6061 have different mechanical properties. AA6061 has good toughness
characteristics as well as excellent corrosion resistance to atmospheric conditions and also sea
water. Annealed 6061 (6061-O temper) has a tensile strength of (125 MPa), and maximum yield
strength of (55 MPa). T4 temper 6061 has an ultimate tensile strength of (207 MPa) and yield
strength of (110 MPa). T6 temper 6061 has an ultimate tensile strength of (290 MPa) and yield
strength of (241 MPa).
[20]
AA6061 has got several uses. It is popular for medium to high strength requirements. It is used
for making wheel spacers for vehicles, manufacture of aluminium cans for the packaging of food
stuffs and other beverages. It is also used in the manufacture of aircraft structures such as wings
and fuselages and also in the construction of small utility boats. It is also common in the
construction of bicycle frames and components.
1
1.2. Statement of Problem
Although welding is a common and faster method of joining metals, it is not commonly used for
making appliances in which AA6061 is involved. This therefore led to the investigation of the
weldability of AA6061 so as to determine whether welding can be used as an alternative for
joining these appliances such as aluminium cans which are normally made by casting.
1.3 Objectives
The objectives of this experiment were to determine the weldability of AA6XXX alloys and to
determine how Gas Tungsten arc welding affects microstructure and hardness of the material.
2
2.0 LITERATURE REVIEW
2.1 Background Information
Aluminium is the most abundant metal on the earth’s crust and the third most abundant element
after silicon and oxygen. On the periodic table it falls into group three with 13 protons and 14
neutrons in its nucleus and therefore a mass number of 27.
Pure aluminium is a silvery- white colored metal with a bluish tinge and with high reflectivity for
both heat and light. It forms a tightly adherent transparent oxide film when exposed to air. This
film is resistant to corrosion in ordinary media. Although this property is useful in resisting
corrosion, it is troublesome when soldering, welding and electroplating.
Aluminium crystals have face centered cubic structure (FCC). This means that it does not suffer
from loss of notch toughness with reduction in temperature. Some of the Aluminium alloys have
shown increase in tensile strength and ductility as the temperature falls. This crystal structure
means that aluminium has good formability and thus products can be produced by drawing,
extrusion and high energy forming.
Due to its reactive nature aluminium is not found in its metallic form but in form of different
compounds. It is a constituent of many silicates and hydrated oxides, the most prolific being
Bauxite. Extracting aluminium from these compounds is difficult because of its high affinity for
oxygen. Therefore normal methods of extracting metals such as reduction are not industrially
employed when it comes to aluminium.
2.1.1 Production of aluminium
Aluminium is commercially extracted from Bauxite using the Hall (Heroult) process which is a
two stage electrolytic process. In the first stage, aluminium oxide, Al2O3 (alumina), is separated
from the ore. In the second stage, electrolytic reduction of alumina at between 950 oC to 1000oC
in cryolite takes place. The aluminium so produced has impurities of about 5% to 10% of silicon
and iron. It is thus further purified either by further electrolysis or by zone melting to produce
pure aluminium (99.9%).
[1]
3
2.1.2 Properties of aluminium and its alloys
Low specific gravity. Lightness of aluminium is its most important property. Pure Aluminium
has a specific gravity of 2.7 as compared to 7.8 for steel.
This makes it the metal of choice where high strength to weight ratios are important. It is
therefore employed in the aerospace industry, in manufacture of automobile parts and for
structures like ladders.
Good Electrical conductivity. Electrical conductivity of aluminium is 60% that of copper. But
since aluminium is lighter than copper, it means that weight for weight, aluminium has overall
better conduction than copper. It is thus used in overhead electrical cables but twisted around a
steel core for strength.
High thermal conductivity and non-toxic. The high thermal conductivity makes it suitable for
heat exchanger components and in machine parts with rapid dissipation of heat. The fact that it is
also non-toxic makes it ideal for manufacturing cooking utensils like saucepans and kettles.
Malleability and ductility .Aluminium is readily rolled into sheets or drawn into wires. It is thus
used to produce aluminium foils used for food packaging.
Good corrosion resistance. The impervious layer of Al2O3 that forms when aluminium is exposed
to air is resistant to corrosion. This resistance to corrosion can be improved by anodizing, a
method of artificially forming an oxide film of controlled thickness. This property makes
aluminium useful in chemical plants which deal with concentrated nitric acid, food processing
industries for packaging, building and marine application and as aluminium paint.
Aluminium has a high affinity for oxygen. This property is put to use in steel manufacture where
aluminium is used as a de- oxidant. It is added to the steel melt where it forms oxides therefore
removing the impurities. This property is also employed in Thermit welding whereby a mixture
of aluminium powder and iron oxide is heated in a mould causing an exothermic reaction and the
heat so produced melts the iron which flows into the crack to be repaired.
Having a higher affinity for oxygen is not always an advantage; it makes it expensive to extract
aluminium from its ore since common extraction methods like reduction cannot be used. Only
electrolysis can be employed for extraction and purification which is costly.
High thermal expansion is another characteristic of aluminium. Its coefficient of thermal
conduction is two times that of steel. Thus adequate allowance has to be made when aluminium
is used in high temperature applications. Otherwise, high thermal stresses will be produced.
4
Hardness. Aluminium is a comparatively soft metal. Its hardness lies between that of tin and
zinc. But cold working and alloying increases hardness to a marked degree.
[6]
2.1.3 Aluminium alloys
Pure aluminium (99.95%) strength is too low to be used structurally. But commercially pure
aluminium (99.2%) containing slight amounts of silicon, iron and manganese is nearly twice as
strong .Working and alloying has the effect of improving the strength of commercially pure
aluminium without affecting its light weight property. The most common alloying elements are
copper, manganese, silicon, magnesium, zinc and lithium.
Occasionally small amounts of other elements such as bismuth, lead, titanium, cadmium, tin,
sodium and vanadium are also added to aluminium.
2.1.4 Effect of alloying elements
Alloying with copper has the effect of increasing strength and hardness while at the same time
making the alloy heat treatable.
Magnesium increases tensile strength, resistance to marine corrosion and ease of welding
Manganese increases strength and resistance to corrosion.
Silicon has the effect of lowering the melting point while improving castability.
Zinc increases strength and hardness.
Aluminium alloys are classified as either wrought (worked by processes such as rolling extrusion
or drawing) or casting alloys. They are further divided as either heat treatable or non heat
treatable within each classification. [6]
2.2 Classification of aluminium alloys
2.2.1 Heat treatable alloys
They are based on; Aluminium-Silicon-Magnesium, Aluminum-Copper and Aluminium-ZincMagnesium alloying systems. They develop high strength by solution treatment then age
hardening at elevated temperatures
2.2.2 Non-heat treatable alloys
These include pure Aluminium, and those based on Aluminium-Manganese, Aluminium-Silicon,
and Aluminium-Magnesium. They can be strengthened only by cold work.
5
2.2.3 Cast alloys
The castings of aluminium may be produced by;
Metal moulds
Sand moulds
Pressure die casting.
The castings are usually rigid and posses corrosion resistance.
Cast alloys are of two main types;
Those, for which heat treatment can be used to enhance properties, like the Aluminium- Copper
alloys.
Those which only depend on alloying for their properties such as Aluminium-Magnesium and
Aluminium-Silicon alloys.
2.2.4 Designation of Aluminium Alloys
The aluminium association developed a designation for aluminium alloys. Wrought alloys are
designated by a four-digit code, the leading digit identifying the major alloying element. [6]
Table2.1 Designation of wrought aluminium alloys
DESIGNATION
MAJOR ALLOYING ELEMENT
1xxx
Commercially pure aluminium
2xxx
Copper
3xxx
Manganese
4xxx
Silicon
5xxx
Magnesium
6xxx
Magnesium and silicon
7xxx
Zinc and magnesium
8xxx
Others( e. g lithium)
6
In addition, condition or temper of an alloy can be indicated by adding a symbol to the alloy
designation preceded by a hyphen. The letters are F for as fabricated, O for annealing, H for
strain hardening and T for heat treatment.
Cast alloys on the other hand have a three digit designation with the leading digit still
identifying the major alloying element.
Table2.2 Designation of cast aluminium alloys
DESIGNATION
MAJOR ALLOYING ELEMENT
2XX
Copper
3XX
Silicon, copper and/or magnesium
4XX
Silicon
5XX
Magnesium
7XX
Zinc
8XX
Tin
2.3 Applications of aluminium alloys
Pure Al (1XXX)
Used in electrical conductors, capacitors, sheet work, tubing and aluminium foil.
Al- Cu (2XXX)
Used in cylinder heads, welded cryogenic tanks, pistons, wheels, aircraft body.
Al-Mn (3XXX)
Used in buildings (sliding gutters), cooking utensils, sheets.
Al- Si (4XXX)
Used in cylinder heads, engine blocks, valve bodies, architectural purposes.
7
Al- Mg (5XXX)
Used in building appliances e.g. fridges, bus bodies, missiles, amour plate, chemical storage
tanks.
Al-Mg-Si (6XXX)
Used for furniture, marine structures, railroad cars, wires, rods, sheets.
Al-Mg- Zn (7XXX)
Used for missiles, aircraft structures and car bodies.
Al-Li (8XXX)
Used in aircraft, spacecrafts.
[1]
2.4 Welding aluminium
Welding is a method of joining metals by coalescences of surfaces in contact. This is achieved
by either melting the two parts a process known as fusion welding or by solid phase joining- the
two parts are brought together under pressure and heat is applied. This forms a metallic bond
across the interface.
Although no solid state phase change occurs when welding aluminium, there are several factors
that influence the weldability of aluminium and its alloys. These factors need to be considered
and dealt with to produce good welds.
2.4.1 Welding processes
Aluminium can be readily welded using a variety of processes such as Arc welding (Gas Metal
Arc welding (GMAW),Gas Tungsten Arc welding (GTAW), pulse arc), Oxy-Gas welding (oxyhydrogen and oxyacetylene), Resistance welding (spot, seam, projection) and specialized high
density processes (Laser and electron beam, friction stir welding). But the choice of welding
method is based on economical or technical reasons.
For most structural and economical quality welds, GMA and GTA are the commonly employed
processes.
8
GTAW is preferred for light gauge work, pipe work and intricate assemblies. Thicker sections
could also be welded using GTAW but it would require higher current with low welding speeds
which is uneconomical.
GMAW on the other hand is preferred for thicker sections and for higher productivity. But
control of penetration is hard when using GMAW therefore, edge welds are not possible.
Some of the advantages that GMAW has over GTAW are such as greater penetration depths and
narrower Heat affected zone (HAZ). In addition, GMAW weld joints have better strength,
penetration, corrosion resistance, durability and finish appearance. GMAW speed is twice that of
GTAW therefore, fast cooling occurs in the weld area resulting in less distortion.
2.4.2 GMAW (Gas Metal Arc welding)
The electrode is Aluminium filler wire fed continuously through the gun from a reel to the weld
pool. The arc is struck between the tip of the wire and the metal being welded. The shielding
gas, Argon or a mixture of Argon and Helium is externally supplied. The inert gas shield
increases penetration while reducing porosity. Standard direct current with reverse polarity is
used in GMAW.
www.afsa.org.za/portal/...../welding
Fig2.1 Illustration of GMA welding
2.4.3 GTAW(Gas Tungsten Arc welding)
Produces welds with good appearance and quality. AC power which is a continuous frequency is
used with water cooled or air cooled GTA torch. The shielding gas (argon or a mixture of argon
9
and helium) is externally supplied. The shielding gas prevents oxidation and also prevents the
tungsten electrode from being consumed. Filler metal is fed into the weld bead from outside.[4]
www.afsa.org.za/portal/..../welding
Fig 2.2 Illustration of GTA welding
2.4.4 Welding characteristics
An oxide film layer forms rapidly when aluminium is exposed to air. If this oxide film is not
removed before welding, it may react with the liquid metal in the weld pool to liberate hydrogen
thus causing porosity. It is removed either chemically or mechanically (brushing or scrubbing).
Aluminium being a good conductor of heat dissipates heat fast from the weld joint. To
compensate for this heat loss, nozzles for welding aluminium should be larger than for mild
steel. Preheating thicker pieces is also advised.
High coefficient of thermal expansion is a characteristic of aluminium alloys. This may cause
distortion and buckling during welding. To counter this proper joint design, edge preparation and
preheating should be employed.
Aluminium suffers reduction of strength in the weld area.
This is because of reduction of mechanical properties across the weld pool when aluminium
alloys are welded. Thus, when stressed local deformation occurs in the weld area first.
10
Aluminium does not show any color change on heating. Care should therefore be taken when
judging the welding temperature. Melting of the dry flux and blistering of the metal surface are
indicators that the welding temperature has been reached.
Aluminium is also non-magnetic. Therefore arc blow is not a problem when welding.
2.5 Weldability of aluminium and its alloys
Weldability is a term used to describe characteristics when a material is subjected to welding.
They include ease of welding, ability of the material to produce a defect free weld, as well as the
required joint properties.
The principle used to test weldability is susceptibility to weld solidification cracking. Cracking in
welds is caused by presence of tensile stress and susceptible microstructure in the weld and heat
affected zone (HAZ).
2.5.1 Methods for determining weldability
Many methods for determining weldability exist. Intrinsic tests promote cracking by naturally
occurring thermal contraction and solidification shrinkage. They include casting pin test and self
restraint tests. On the other hand extrinsic tests promote cracking by external loading of the
solidifying specimen. The varestraint test is an example of an extrinsic test of weldability.
2.5.2 The Varestraint test
This is a method for quantifying solidification cracking susceptibility based on the following;
Maximum crack length (MCL)
Total cracking length (TCL)
Brittleness temperature range BTR) [16]
This method involves welding a bead on a test plate fastened on one end. A sudden load is then
applied at the free end of the plate just after the arc is extinguished. The load makes the test plate
to bend according to a curved mandrel under it in such a way that the solidifying weld pool copes
with the restriction to its shrinkage process.
Cracking propagates in both the travel direction as well as away from it. Forward crack growth
proceeds incrementally at an average velocity similar to the travel speed of the welding gun.
11
Therefore, the growth of the leading edge of the crack occurs at an approximately constant
temperature.
The strain experienced by the test plate is related to the mandrel radius using the relation;
𝑡
ε= 2𝑅
( 2.1)
ε is the augmented strain in the outer fiber
t is plate thickness and
R is the radius of curvature of the mandrel.
a.
b.
fig2.3 schematic showing the (a) varestraint and (b) transvarestraint test
Nicolas Coniglio, Aluminum Alloy Weldability: Identification of Weld Solidification Cracking
Mechanisms through Novel Experimental Technique and Model Development
12
2.5.3 Weldability of aluminium alloys
Commercially pure aluminium (99.0-99.6%) is readily weldable using appropriate filler metals
of matching composition. AlSi, AlMg are some of the filler metals used. All welding processes
such as GMAW, GTAW, Gas welding, Resistance and friction can be used to weld this class of
aluminium metal.
2XXX
This group forms Al-Cu intermetallic bond in the weld metal which renders them brittle. Alloys
with Cu<1% are weldable. Common weldable alloys in this group are, AA2011, AA2014 and
AA2024.
3XXX
This class of alloys is weldable using matching filler metals. Common weldable alloys in this
group include, AA3003, AA3004, and AA3103.
4XXX
Weldable by all processes using AlSi filler metals. Common weldable alloys in this group
include, AA4710, AA4410, and AA4210.
5XXX
Alloys with Mg< 3% are susceptible to cracking but alloys with Mg> 4.5% are readily weldable.
GMAW and GTAW are the most frequently used processes. Common weldable alloys in this
group include, AA5083, AA5454, and AA5251.
6XXX
Alloys with Si< 1% and Mg<1% tend to crack in the HAZ. If high heat inputs are used, liquation
cracking occurs. GMAW and GTAW are the processes most applied. Filler metals of 5% Mg and
5% Si are used. Common weldable alloys in this group include AA6061, AA6063, and AA6082.
13
7XXX
This class has both weldable and unweldable grades. Alloys with Cu< 1% are weldable.
Common weldable alloys in this group are AA7017, AA7020, AA7075
8XXX( Li and other elements)
Not commonly welded.
2.6 Heat treatment of aluminium and aluminium alloys
The term heat treatment in aluminium alloys, both wrought and cast, is restricted to the specific
operations employed to increase strength and hardness by precipitation hardening. Hence, the
term heat treatable is used to refer to alloys whose strength can be improved by heating and
cooling.
Non-heat treatable alloys depend primarily on cold work to increase strength.
2.6.1 Annealing
Annealing is applied to heat treatable and non heat treatable alloys to promote softening. The
non-heat treatable alloys use complete and partial annealing heat treatments. Annealing is done
in the range of 300-410°C depending on the alloy. The time used for heating at temperature
varies from 0.5 to 3 hours, depending on the size of the load and the type of alloy. The time
should not be longer than that required to stabilise the load at a particular temperature. If the
particular material has been solution heat-treated a maximum cooling rate of 20°C per hour must
be maintained until the temperature reaches 290°C.The rate of cooling is not important below
this temperature.
2.6.2 Solution Heat Treatment
This is done to the heat treatable alloys and is concerned with a heat treatment process whereby
the constituents to be alloyed are taken into solution and retained by rapid quenching.
Ageing or natural ageing at room temperature allows for a controlled precipitation of the
constituents therefore obtaining increased hardness and strength.
The time and temperature for solution treatment is dependent on the type of alloy and the furnace
load. Enough time must be allowed to take the alloys into solution for optimum properties to be
obtained. The temperature for solution treatment is important for the success of the process.
14
Solution heat treatment should be carried out as close as possible to the liquidus temperature in
order to obtain maximum solution of the constituents.
Correct furnace temperature and special temperature variation must be within a range of ±5°C
for most alloys. Exceeding initial eutectic melting temperatures must be avoided.
2.6.3 Quenching
Quenching is done so as to ensure that the dissolved constituents remain in solution down to
room temperature. The result of quenching can be affected by excessive delay in transferring the
work to the quench. The time should be 5 to 15 seconds for items of thickness varying from 0.4
mm to 12.7 mm. Rapid precipitation of constituents begins at around 450°C for most alloys and
the work must not be allowed to fall below this temperature before quenching.
Work load and the ability of the quenching liquid to extract the heat at sufficient rate to achieve
the desired results is also another consideration. Water at room temperature is usually used as the
quenching medium. Slow quenching is desirable in some cases as this improves the resistance to
stress corrosion cracking of certain copper-free Al-Zn-Mg alloys.
Parts of complex shapes such as forgings, castings, impact extrusions and components produced
from sheet metal may be quenched at slower quenching rates to improve distortion
characteristics.
A balance of properties must be achieved in some cases where slower quenching is done. Slower
quenching uses fluids such as water heated to 65-80°C, solutions of polyalkalene glycol, boiling
water, aqueous or forced air blast.
2.6.4 Age Hardening
Hardening is achieved at room temperature after solution heat treatment and quenching (natural
ageing). Sufficient precipitation occurs in some alloys in a few days at room temperature to
produce stable products with required properties that are adequate for many applications.
These alloys sometimes are precipitation heat treated to provide increased strength and hardness
in wrought and cast alloys.
Cold working of materials that have been freshly quenched greatly increases its response to later
precipitation treatment.
The time for natural ageing is around 5 days for the 2xxx series alloys to around 30 days for
other alloys. The 6xxx and 7xxx series alloys are usually less stable at room temperature and
continue to show changes in mechanical properties for many years.
Natural ageing may be delayed for several days in some alloys by refrigeration at -18°C or
15
lower.
Forming, straightening and coining are usually completed first before ageing changes material
properties.
2.6.5 Precipitation Hardening
Precipitation hardening is also known as artificial ageing and achieved by re-heating the alloy to
a lower temperature (115⁰C to 200 ⁰C) and holding it at this temperature for a prescribed period
of time usually (5 to 48 hours). This is aimed at producing a metallurgical structure within the
material that provides superior mechanical properties. Larger particles or precipitates result from
longer times and higher temperatures. The objective is to select the cycle that produces the
optimum precipitate size and distribution pattern. The cycle required to maximize one property
such as tensile strength is usually different from that required to maximize others such as yield
strength and corrosion resistance. During heat treatment, if the material is held at a high
temperature for too long, the material will become over aged, resulting in a decrease in tensile
strength. Precipitation hardening process is both time and temperature controlled.
2.6.6 Precipitation sequence in AA6XXX alloys
The precipitation sequence of AA6XXX AlMgSi-alloys without major additions of copper is
given as;
SSSS (supersaturated solid solution) → GPZ → needle-like β” → rodlike β’→ β (Mg2Si).
The metastable phases β’ and β” transform after sufficient long heat treatment into the stable β −
Mg2Si phase.
The β” phase is coherent and is distributed homogeneously in the Al matrix. It is needle shaped
while the meta-stable β’ phase is rod-shaped with ellipsoidal cross section. This phase is semicoherent with the Al matrix. The equilibrium phase β (Mg2Si) is plate-like in shape and is
incoherent with the Al matrix.
2.7 Coherence
During precipitation, coherent precipitates are said to form if there is continuity of the crystal
lattice from the matrix and through the cluster. The atomic spacing of coherent precipitates is
16
also closely related to that of the matrix. The coherency results in low boundary energy and
therefore the lattice is distorted and strained around the clusters. This makes it difficult for slip to
occur thus resulting in high strength and hardness. In the precipitation sequence this is the β”
precipitate.
Continued precipitation results in overaging which is characterized by relieved coherency strains
and decrease in hardness. The precipitates so formed have no relationship with the crystal
structure of the matrix and are referred to as incoherent precipitate. In the precipitation sequence
this is the stable β precipitate.
Semi coherent precipitates are mostly coherent but are incoherent at the ends. They form during
transition from coherent to incoherent precipitates. These manifests as β’ precipitate in the
precipitation sequence.
Fig 2.4 Illustration of the difference between coherent, semi-coherent and incoherent
precipitates
2.8 Effect of heat from welding on Mechanical properties of aluminium alloy
When welding is done on precipitation hardened aluminium alloys, a reduction in strength is
seen as well as an increase in ductility within an area adjacent to the weld. It is observed that
from the base metal toward fusion boundary coarsening of aluminum grains takes place.
Aluminum grains in the weld metal are finer than that of base metal, HAZ and fusion boundary.
17
Second phase particles and other intermetallic compounds dissolve in aluminum matrix in a
region close to the fusion boundary. This dissolution is also called reversion and is dependent on
the heat input. Less heat input results in partial dissolution of second phase particles, this leaves
behind some amount of these phases in form of network along the grain boundary and a few
partially dissolved nearly spherical particles in the aluminum matrix. Increase in heat input cause
almost complete dissolution of the phases along the grain boundary and a large number of
particles that are round-shaped.
These particles may be the result of re-precipitation of dissolved phases during the cooling after
welding. A region very close to the fusion boundary is subjected to full reversion and GP zones
can be formed during the post-weld natural aging.
There is not a practical way to reintroduce the stiffness into the base material of a fabricated part
after it has been lowered by heating. In theory, this can be done by strain-hardening or heat
treatment. However, due to the procedures required to perform these operations, it is not usually
an appropriate method.
2.8.1 Non-heat treatable alloys in class 1XXX, 3XXX, 5XXX
These alloys are cold worked to increase mechanical strength. The heat produced during welding
will cause the material to return to its original condition before cold work. Control of the HAZ
(Heat affected zone) is therefore advised to keep the material as cool as possible. If excessive
heat is used or very wide weld beads, and the material is not allowed to cool down, HAZ will
become wide and weaken the joint.
2.8.2 Heat treatable alloys 2XXX, 6XXX, 7XXX
These alloys acquire their strength through heat treatment. The heat produced during welding
will give thematerial additional heat treatment resulting in over aging. This is detrimental to the
weld joint and is characterized by a coarse structure.
To counter this effect of heat, welding should be done with low heat and the material should not
be allowed to remain at high temperature for too long.
18
The two tables below illustrate the decrease in tensile strength for heat treatable and non-heat
treatable alloys
Table 2.3 Tensile Strength Properties for heat treatable alloys
Base Alloy and
Tensile Strength - MPa
Temper
Tensile Strength – MPa
(As welded condition)
6061-T6
310.23
186.138
6061-T4
241.29
186.138
2219-T81
455.004
241.29
2014-T6
482.58
234.396
www.esabna.com/us/en/education/knowledge/qa/Heat-Affected-Zone-of-Arc-WeldedAluminum-Alloys
Table2.4 Tensile Strength Properties for non heat treatable alloys
Base Alloy and
Tensile Strength - Mpa
Temper
Tensile Strength – Mpa
(As welded condition)
5052-H32
227.502
186.138
5052-H39
289.548
186.138
5086-H34
324.018
261.972
5086-H38
365.382
261.972
www.esabna.com/us/en/education/knowledge/qa/Heat-Affected-Zone-of-Arc-Welded-AluminumAlloys
19
3.0 EXPERIMENTAL PROCEDURE
3.1 Heat treatment
The as received condition of the alloy was AA6061- T0 condition. It was then heat treated to
AA6061-T4 and AA60061-T6 heat condition.
To get the T6 condition the alloy was first solution heat treated at 496 oC for a period of 10 hours
before being quenched in water at room temperature. It was then artificially aged at a
temperature of 176oC for a period of 8 hours ending up with the AA6061 T6 condition.
For the T4 heat condition the alloy was first solution heat treated at a temperature of 496 oC then
quenched in water at room temperature. It was then left to age harden naturally for a period of 20
days to end up with the T4 condition.
3.2 Varestraint weldability test
Metal strips measuring 240 mm X 10 mm X 1.6 mm were cut from the T0, T4 and T6 conditions
of the aluminium alloy. Small holes of diameter 6.35 mm were drilled on one side 80 mm apart
as shown in appendix B.
The other end was clamped onto the mandrel (radius = 80mm) and onto the work bench. Weights
(4.5kg) were tied onto the metal strip by use of a copper wire through the drilled holes but were
not allowed to fall.
A bead on plate was then produced on the specimen using GTAW torch from a Clarke Tig 200
machine. The travel speed of 3mm/s, a current of 75A and a voltage of 20V were used. When the
torch had moved a distance of 150 mm, the torch was extinguished and the weights released to
bend the specimen to take the shape of the mandrel. This was repeated for the three different
specimens. The lengths of the cracks formed were then examined using a stereo microscope at a
magnification X 10 and used as a measure of relative weldability.
3.3 Metallography
From the Varestraint test specimens, transverse sections from the three test specimen were
sectioned and mounted using epoxy resin for convenience in handling. They were then grinded
using successive grades of Silicon carbide paper (240, 320, 400 and 600) under running water.
20
This was then followed by mechanical polishing on rotating wheels using diamond paste of 7µm,
1µm and 1/4µm successively.
Etching was done using Keller’s reagent (a solution of 3 ml HCl, 5 ml HNO3, 2 ml HF and 190
ml H2O) for 30 seconds after which it was washed in running water, rinsed in alcohol, and dried
in hot air.
The metallographic specimens were then observed under a light microscope(Optika TM Vision
Pro V2.7) at intervals of 0.5 mm, 1.5 mm, 2.5 mm, 4 mm, 5 mm, 6 mm, 8 mm, 11 mm, and 15
mm from the edge of the HAZ. The magnification used was X500. The micrographs taken were
then analyzed using the microstructure characterization software (Image Analysis software for
Metallurgists, revision 2.0 Mic, September 2009 Edition, TCR Advanced Engineering P L.T.D)
3.4 VICKERS HARDNESS TEST
Vickers hardness test was carried out using the WOLPERT-Dual Hardness Tester machine. A
diamond pyramid indenter subjected to a load of 1kg was applied at intervals of 0.5mm from the
fusion line up to a distance of 15mm. The two diagonals were then measured and averaged. The
Vickers hardness values were then read from the Vickers hardness table attached in the
appendix. Graphs of Vickers hardness against the distance from the HAZ were then plotted and
the differences in values for the three heat conditions were then analyzed.
21
4.0 RESULTS AND ANALYSIS
4.1 VARESTRAINT TEST
The criteria used to evaluate the Varestraint test is the maximum crack length (MCL). The
longest crack length in all the three specimens is used as a measure of weldability. The value of
augmented strain applied was found to be 1% calculated from the formula below;
ε=
𝑡
2𝑅
where t = thickness of specimen(1.6mm)
R = radius of curvature of the die block (80mm)
1.6
ε = 2(80)* 100 = 1%
The maximum crack lengths for the 3 heat conditions (T0, T4, and T6) were found to differ from
each other as shown below.
Table4.1 Maximum crack lengths for the varestraint test
Heat condition
Maximum crack length(mm)
T0
13.4
T4
183.0
T6
40.0
These results were then plotted on a bar graph as shown below.
22
MAXIMUM CRACK LENGTH
(mm)
COMPARISON OF THE CRACK LENGTHS FOR THE DIFFERENT
HEAT CONDITIONS
200
150
100
50
0
T4
T6
HEAT CONDITIONS
T0
Fig 4.1 comparison of maximum crack lengths for T0, T4 & T6
From the bar graph it is evident that T4 is the most susceptible to hot cracking followed by T6
and finally by T0.
4.2 MICROSTRUCTURE
4.2.1 MICROGRAPHS OF WELDED AA6061 T0 CONDITION
23
24
Fig 4.2 micrographs of the HAZ of welded AA6061-T0
4.2.2 MICROGRAPHS OF WELDED AA6061 T4 CONDITION
25
Fig 4.3 Micrographs of the HAZ of welded AA6061-T4
26
4.2. 3 MICROGRAPHS OF WELDED AA6061 T6 CONDITION
27
Fig 4.3 Micrographs of HAZ of welded AA6061-T6
*Positions indicate distances from the fusion line
From the examination of the microstructures, it was found that grain boundaries were not visible
in all the three conditions. The volume fraction of the three phases was found to vary with
distance from the fusion line and also across the three heat conditions. Phase 1 was seen to be
incoherent with the matrix, Phase 2 was seen to be partially coherent with the matrix and phase 3
was found to be coherent with the matrix. The size of grains was also seen to vary with distance
from the fusion line. This means that hardness values will also vary with both grain size and %
volume fraction of phases.
28
4.3 GRAIN SIZE ANALYSIS
Micrographs were analyzed using the microstructure characterization software and the results
tabled in the table below. Charts were then plotted to show the grain size trend in the three test
specimen, AA6061-T0, AA6061-T4 and AA6061-T6.
Table 4.2 Grain size analysis for T0, T4 and T6
GRAIN SIZES ( in µm)
DISTANCE FROM
THE EDGE OF THE
AA6061-T0
AA6061-T4
AA6061-T6
0.5
12.4
13.4
7.8
1.5
14.1
9.9
7.2
2.5
10.9
8.2
8.1
4
10.9
9.2
8.4
5
14.4
9.0
9.1
6
8.1
10.1
12.9
8
7.6
8.6
11.4
11
10.3
9.9
12.0
15
10.3
7.8
12.2
Grain sizes (µm)
HAZ (mm)
16
14
12
10
8
6
4
2
0
0.5
1.5
2.5
4
5
6
8
11
Distance from the edge of the HAZ (mm)
Fig 4.4 Graph showing the grain size trend in AA6061- T0
29
15
16
Grain sizes (µm)
14
12
10
8
6
4
2
0
0.5
1.5
2.5
4
5
6
8
11
15
Distances from the edge of the HAZ (mm)
Fig 4.5 Graph showing the grain size trend in AA6061- T4
14
Grain sizes (µm)
12
10
8
6
4
2
0
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of the HAZ (mm)
Fig 4.6 Graph showing the grain size trend in AA6061- T6
The above results show that close to the fusion line at about a distance of 1.5 mm from the fusion
line T6 has the smallest grain size of about 7µm followed by T4 at 10µm and T0 had the largest
grain size of about 14µm. Away from the fusion line at a distance of 15µm from the fusion line
T6 was found to be the coarsest with a grain size of 12µm followed by T0 at 10.5 µm and finally
T4 which had the finest grain size at 8µm.
30
For T6 since it was artificially aged, the heat produced during welding causes continued aging
and thus the reason for the smaller grain size next to the fusion line. For T4 and T0 the heat
produced during welding causes reversion and thus coarsening of the grains next to the fusion
line. Away from the fusion line the grain size is that of the base metal. And T4 was found to be
finer than both T6 and T0. T4 had been left to age harden at room temperature for a period of 20
days. This caused precipitation of smaller grains than those produced by artificial ageing of T6.
Consequently closer to the fusion line the expectation is that T6 would have the highest strength
due to it small grain size followed by T4 and finally T0. Away from the fusion line T4 is
expected to have the highest strength followed by T0 and finally T6.
4.4 VOLUME FRACTION ANALYSIS
On examining the microstructure, it was observed that three phases were present though the
grain boundaries were not visible. The three phases were designated as phase 1, phase 2, and
phase 3 as shown on the micrograph below.
Fig 4.7 Micrograph showing the phase 1 phase 2 & phase 3
Using the microstructure characterization software, the volume fractions of the phases were
determined for the three test specimen, AA6061-T0, AA6061-T4 and AA6061-T6. The results
were tabled and analyzed further using charts.
31
Table 4.3 volume fraction of phases 1, 2 & 3 in AA6061-T0
% VOLUME FRACTION OF PHASES IN AA6061-T0
Distance from
the edge of the
PHASE 1
PHASE 2
PHASE 3
MATRIX
HAZ(mm)
0.5
1.1
5.9
6.3
86.7
1.5
0.4
2.3
11
86.3
2.5
2.5
3.9
15.4
78.2
4
1.25
2.55
9.6
86.6
5
2.7
4.9
20
72.4
6
1.8
4.4
24
69.8
8
2
5.2
25.9
66.9
11
2.6
9.5
30.6
57.3
15
2.3
6.2
22
69.5
35
% volume fraction
30
25
20
PHASE 1
15
PHASE 2
10
PHASE 3
5
0
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of the HAZ
Fig 4.8 Graph of % volume fraction against distance from the fusion line for AA6061- T0
This graph shows that phase 3 was the most dominant phase and it had an increasing trend with
increasing distance from the fusion line. At about a distance of 11 um it peaked then decreased
32
Phase 2 decreased before increasing as the distance from the fusion line increased similarly
phase 3 decreased before increasing to a constant.
Table 4.4 volume fraction of phases 1, 2 & 3 in AA6061-T4
% VOLUME FRACTION OF PHASES IN AA6061-T4
Distance from the
edge of HAZ(mm)
PHASE 1
PHASE 2
PHASE 3
MATRIX
0.5
7.9
10.4
23.9
57.8
1.5
2.6
5.3
19.3
72.8
2.5
4.7
12
34.9
48.4
4
4.5
12.3
35.4
47.8
5
2.5
6.1
22.1
69.3
6
3.2
6.3
24.2
66.3
8
3.5
6.5
23.8
66.2
11
3.8
7.2
23.7
65.3
15
2.9
5.6
17.1
74.4
40
% volume fraction
35
30
25
PHASE 1
20
PHASE 2
15
PHASE 3
10
5
0
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of the HAZ (mm)
Fig 4.9 Graph of % volume fraction against distance from the fusion line for AA6061- T4
33
The above chart shows that all the 3 phases have the same trend. They first decreased then
increased to a peak before reducing to a constant. But phase 3 was seen to be dominant followed
by phase 2 and finally phase 1.
Table 4.5 volume fraction of phases 1, 2 & 3 in AA6061-T6
% VOLUME FRACTION OF PHASES IN AA6061-T6
Distance from the
edge of the HAZ
PHASE 1
PHASE 2
PHASE 3
Al-MATRIX
0.5
11.6
16.6
20.3
51.5
1.5
10.4
15.9
19.6
54.1
2.5
6
17.3
33.7
43.0
4
5.1
14.6
26.1
54.2
5
5
14.4
31.8
48.8
6
4.1
15.0
36.4
55.5
8
3.4
8.1
22
66.5
11
3.3
9.2
22.8
64.7
15
4
9.1
21.6
65.3
40
% volume fraction
35
30
25
20
PHASE 1
15
PHASE 2
10
PHASE 3
5
0
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of the HAZ (mm)
Fig 4.10 Graph of % volume fraction against distance from the fusion line for AA6061- T6
34
The above chart showed that phase 3 was dominant and its volume fraction increased with
distance from the fusion line to a peak of about 36% before reducing to about 23%. Phase 2 was
seen to remain fairly constant as the distance from the weld increased but at a distance of about 6
mm it dropped to a constant. Phase 1 decreased steadily as distance from the fusion line
increased.
% VOLUME FRACTION OF PHASE 1, 2 & 3 ACROSS AA6061-T0, AA6061-T4 AND
AA6061-T6
Table 4.6 Volume fraction variation of phase 1 across T0, T4, & T6
% VOLUME FRACTION VARIATION OF
PHASE 1
Distance from the edge
of the HAZ (mm)
T0
T4
T6
0.5
1.1
7.9
11.6
1.5
0.4
2.6
10.4
2.5
2.5
4.7
6
4
1.25
4.5
5.1
5
2.7
2.5
5
6
1.8
3.2
4.1
8
2
3.5
3.4
11
2.6
3.8
3.3
15
2.3
2.9
4
35
% volume fraction of
phae 1
14
12
10
8
6
4
2
0
T0
T4
T6
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of HAZ (mm)
Fig 4.11 % Volume fraction variation of phase 1
The above chart shows that T6 had the highest volume fraction of phase 1 followed by T4 and
finally T0. In T6 and T4 phase 1 had a decreasing trend as the distance from the fusion line
increases. On the other hand, it showed an increasing trend in T0 as the distance from the fusion
line increased. The high temperature produced during welding increased the rate of diffusion of
this precipitate from solution in both T4 and T6 but since T0 did not undergo any heat treatment
this high temperature caused this phase to dissolve.
Table 4.7 Volume fraction variation of phase 2 across T0, T4, & T6
% VOLUME FRACTION VARIATION OF
PHASE 2
Distance from the
edge of the HAZ (mm) T0
T4
T6
0.5
5.9
10.4
16.6
1.5
2.3
5.3
15.9
2.5
3.9
12
17.3
4
2.55
12.3
14.6
5
4.9
6.1
14.4
6
4.4
6.3
15
8
5.2
6.5
8.1
11
9.5
7.2
9.2
15
6.2
5.6
9.1
36
% Volume fraction of
phase 2
20
15
10
T0
5
T4
T6
0
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of the HAZ (mm)
Fig 4.12 % Volume fraction variation of phase 2
From the above chart it was observed that T6 had the highest fraction of phase 2 followed by T4
and finally T0. T4 and T6 show a decreasing trend with distance while T0 shows an increasing
trend the further you move away from the fusion line. The reason for this trend is the same as
that given for phase 1.
37
Table 4.8 Volume fraction variation of phase 3 across T0, T4, & T6
% VOLUME FRACTION VARIATION OF
PHASE 3
Distance from the
edge of the HAZ
T0
T4
T6
0.5
6.3
23.9
20.3
1.5
11
19.3
19.6
2.5
15.4
34.9
33.7
4
9.6
35.4
26.1
5
20
22.1
31.8
6
24
24.2
36.4
8
25.9
23.8
22
11
30.6
23.7
22.8
15
22
17.1
21.6
% volume fraction of
phase 3
(mm)
40
35
30
25
20
15
10
5
0
T0
T4
T6
0.5
1.5
2.5
4
5
6
8
11
15
Distance from the edge of the HAZ (mm)
Fig 4.13 % Volume fraction variation of phase 3
The volume fraction of phase 3 was observed to be almost the same in both T4 and T6. The trend
was such that it decreased slightly before increasing to a peak then decreasing to a constant.
On the other hand in T0, it was observed that the volume fraction increased the further one
moved from the fusion line. The heat produced during welding causes this phase to dissolve.
Leading to the conclusion that this phase is metastable.
38
4.5 HARDNESS SURVEY
Table 4.9 Hardness survey on AA6061-T0
AA6061-T0
Distance
Distance from
Diagonal
from the
Diagonal
the edge of the
(Average)
edge of
(average)
HAZ
mm
HV1
HAZ
mm
HV1
0.5
0.18
57.2
8
0.22
38.3
1
0.17
64.2
8.5
0.22
38.3
1.5
0.165
68.1
9
0.22
38.3
2
0.17
64.2
9.5
0.22
38.3
2.5
0.18
57.2
10
0.22
38.3
3
0.195
48.8
10.5
0.21
42.1
3.5
0.215
40.1
11
0.21
42.1
4
0.22
38.3
11.5
0.21
42.1
4.5
0.22
38.3
12
0.21
42.1
5
0.22
38.3
12.5
0.21
42.1
5.5
0.22
38.3
13
0.215
40.1
6
0.22
38.3
13.5
0.215
40.1
6.5
0.22
38.3
14
0.215
40.1
7
0.22
38.3
14.5
0.215
40.1
7.5
0.22
38.3
15
0.22
38.3
39
Vickers Hardness Number
80
70
60
50
40
30
20
10
0
Distance from the edge of the HAZ
Fig 4.14 Graph of VHN at various distances from the fusion line for AA6061-T0
Table 4.10 Hardness survey on AA6061-T4
40
Distance from the Diagonal
(average) mm
edge of the HAZ
HV1
Distance
Diagonal
from the
(average) mm
HVI
edge of
(mm)
HAZ (mm)
0.18
57.2
8
0.175
60.6
1
0.17
64.2
8.5
0.175
60.6
1.5
0.16
72.4
9
0.175
60.6
2
0.16
72.4
9.5
0.175
60.6
2.5
0.16
72.4
10
0.175
60.6
3
0.165
68.1
10.5
0.18
57.2
3.5
0.165
68.1
11
0.18
57.2
4
0.165
68.1
11.5
0.18
57.2
4.5
0.165
68.1
12
0.18
57.2
5
0.17
64.2
12.5
0.175
60.6
5.5
0.165
68.1
13
0.175
60.6
6
0.165
68.1
13.5
0.175
60.6
6.5
0.165
68.1
14
0.18
57.2
7
0.165
68.1
14.5
0.179
57.2
7.5
0.17
64.2
15
0.18
57.2
Vickers Hardness Number
0.5
80
60
40
20
0
0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5
Distance from the edge of the HAZ
Fig 4.15 Graph of VHN at various distances from the fusion line for AA6061-T4
Table 4.11 Hardness survey on AA6061-T6
41
AA6061- T6
Distance from the
edge of the HAZ
Diagonal
(average)
mm
HV1
(mm)
Distance
Diagonal
from the
(average)
edge of
mm
HV
I
HAZ (mm)
0.5
0.16
72.4
8
0.175
60.6
1
0.16
72.4
8.5
0.175
60.6
1.5
0.165
68.1
9
0.17
64.2
2
0.165
68.1
9.5
0.17
64.2
2.5
0.165
68.1
10
0.17
64.2
3
0.17
64.2
10.5
0.165
68.1
3.5
0.17
64.2
11
0.165
68.1
4
0.17
64.2
11.5
0.165
68.1
4.5
0.17
64.2
12
0.165
68.1
5
0.165
68.1
12.5
0.16
72.4
5.5
0.165
68.1
13
0.165
68.1
6
0.165
68.1
13.5
0.16
72.4
6.5
0.17
64.2
14
0.16
72.4
7
0.16
72.4
14.5
0.16
72.4
7.5
0.17
64.2
15
0.16
72.4
42
74
Vickers Hardness Number
72
70
68
66
64
62
60
58
56
54
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5 10.5 11.5 12.5 13.5 14.5
Distance from the edge of the HAZ
Fig 4.16 Graph of VHN at various distances from the fusion line for AA6061-T6
43
5.0 DISCUSSION OF RESULTS
From the Varestraint test, it was observed that at an augmented strain of 1 %, T0 was the least
susceptible to hot cracking followed by T6 while T4 was found to be the most susceptible. The
T4 condition was obtained by solution heat treatment followed by natural ageing. T6 condition
on the other hand, was also obtained by solution heat treatment but unlike T4, it was artificially
aged. Full properties of the material are reached at the T6 condition.T6 is therefore strong and
ductile and thus less susceptible to hot cracking. T4 on the other hand is also hard but the coarse
structure causes loss of ductility and thus increased susceptibility to hot cracking. T0 did not
undergo any heat treatment and the reason why it is the least susceptible to hot cracking during
welding.
From the Grain size trend charts for AA6061-T0 and AA6061-T4, it was observed that the grain
size decreased with increase in distance from the edge of the HAZ. This trend could be as a
result of heat produced during welding which causes overaging and therefore results in a coarse
grain structure close to the fusion line. The trend exhibited by AA6061-T6 shows increase in
grain size with increase in distance from the fusion line. Since T6 had been artificially aged, the
heat produced during welding caused further aging of T6 and thus the reason for the smaller
grain size next to the fusion line. Holding the distances from the HAZ constant and examining
the grain size trend across the three test specimen, it was seen that close to the fusion line
AA6061- T6 had the finest grain size while AA6061-T0 had the coarsest. But away from the
fusion line T4 was found to have the finest grain size followed by T0 and finally T6. This
observation could be attributed to the heat treatment the specimens were put through to produce
the T6 and T4 conditions. T4 was left to age harden at a room temperature for a period of 20
days resulting in very fine precipitates. T6 on the other hand was artificially age hardened but
clearly it did not reach its optimal grain size and that is why the heat from welding caused the
grains to be finer. T0 did not undergo any heat treatment and thus the reason for the coarse grain
structure.
From the analysis of volume fraction of the phases present, it was observed that AA6061-T6 had
the highest volume fraction of phase 1 followed by AA6061-T4 and the lowest volume fraction
was in AA6061-T0.
44
The trend exhibited by phase 1 in both T4 and T6 conditions showed it decreases with distance
from the fusion line while in T0 condition, phase 1 increased as one moved away from the fusion
line. Therefore it seems welding favors the formation of phase 1 in T4 and T6 conditions but
causes the dissolution of this phase in T0 condition.
It was also observed that T6 still had the highest volume fraction of phase 2 followed by T4 and
finally T0. The trend shown is similar to the distribution of phase 1. T4 and T6 shows a
decreasing trend with distance from the fusion line, while T0 shows an increasing trend the
further you move away from the fusion lines. Heat treatment of T4 and T6 causes the Phases 1
and 2 to precipitate out of solution. Therefore, the heat produced by welding causes an increased
rate of diffusion of these phases, and therefore the reason for the increasing trend as you move
towards the fusion line from the base metal.
The volume fraction of phase 3 was observed to be almost the same in both T4 and T6 condition.
It was also observed that it increased marginally with distance from the fusion line. The same
trend was also observed in T0. This phase is metastable and therefore the high temperatures
produced during welding causes dissolution of this phase close to the fusion line.
From the Vickers hardness test the hardness of T0 increased to a peak of 68 HV then decreased
to a constant of about 40 HV as the distance from the fusion line increased. For T4, the hardness
trend was found to be the same as for T0 in that it increased to a peak of 72 HV then decreased to
about 57 HV as the distance from the fusion line increased. The hardness for T6 decreased from
a value of 72 HV to 62 HV before increasing again to 72 HV as the distance from the fusion line
increased. The trend for T0 and T4 was caused by the fact that as the distance from the fusion
line increased, the grains became finer and hence the increase in hardness but beyond an
optimum grain size the strength and therefore the hardness decreased. On examination of the
microstructures it was observed that phase 1 was incoherent with the matrix therefore it does not
contribute much to hardening. On the other hand phase 3 was seen to be coherent with the matrix
and therefore associated with coherency strains which causes hardening of the precipitates. Phase
2 was seen to be partially coherent and therefore a combination of phase 2 and 3 results in
hardening of precipitates. For T6, close to the fusion line combination of fine grain size and a
high volume fraction of phase 2 and 3 was the cause for the high values of hardness. Though the
grain size became coarser with distance from the fusion line the high volume fraction of phase 3
caused the high hardness values.
45
6.0 CONCLUSION AND RECOMMENDATION
From the analysis and discussion of results it was concluded that;

The heat treatment condition least susceptible to hot cracking was found to be T0
followed by T6. T4 was found to be the most susceptible to hot cracking.

Close to the fusion line T6 and T4 had the same hardness of about 72 HV. T0 was found
to be less hard at a value of 68 HV

From the microstructure, phase 1 was found to be incoherent with the matrix, Phase 2
was found to be partially coherent with the matrix and finally phase 3 was found to be
coherent with the matrix.

Phase 2 and phase 3 were found to be the hardening phases.
46
7.0 REFERENCES
1. King F. Aluminium and its alloys, Ellis Horwood, England, 1987.
2.
Khanna O.P. Welding Technology, Dhanpat Rai& sons, Delhi ,1994
3. www.nalco.co.nz/userdocument/welding%20aluminium
4. www.afsa.org.za/portal/...../welding
5. www.bbssystem.com/manuals/aluminiumwelding
6. George O. Rading, Concise notes on materials Science and Engineering, Trafford
publishing, Canada, 2007 pg 86-96.
7. Higgins, R. A. Engineering Metallurgy Part 1, 5 th Edition, Hodder and Stoughton,
London, 1983.
8. Wulff, J. Metallurgy for engineers, John Wiley & sons, 1952.
9. Stoughton, B. Engineering Metallurgy, 4th Edition, McGraw Hill, New York, pg 280-290
10. Albert G. Guy, Physical Metallurgy for Engineers, Addison- Wesley 1963, pg 278-281
11. Mondolfo L. F. Aluminium Alloys: structure and Properties, Butterworth, London, 1976
12. King, F. Aluminium and its alloys, Ellis Horwood, England, 1987.
13. Finn/Trojan, Engineering Materials and their applications, Houghton Miffin co. Boston
(1981)
14. Verhoeven J.D Fundamentals of Physical metallurgy, john Wiley& Sons, New York
(1975)
15. Lampard S. Weld Integrity and performance, ASM international, 1997
16. Matsuda F. Nakata K. Solidification crack susceptibility of aluminium weld metal-
characteristics of ductility curves using transvarestraint test
17. Van Vlack, Material science for Engineers, Addison- Wesley Publishing co. New York
(1969)
18. M. G. Mousavi, C. E. Cross, and Ø. Grong, The High-Temperature Effect of Eutectic-
Forming Impurities on Aluminum 7108 Weldability vol. 88 pg 103-108.
47
19. Jastrzebski Z.B. Nature and properties of Engineering Materials, 2 nd Edition, John Wiley
& sons, New York 1977.
20. www.tppinfo.com/metallurgical_lab/grainsize
21. www.esabna.com
48
8. APPENDIX
Appendix A: From operating instructions for the Wolpert-Dual Hardness tester
Vickers Hv1
F=9.804N ∆1kp
Diagonal(mm)
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.2
0.21
0.22
0.23
0.24
0.25
0.26
0.27
0.28
0.29
0.3
0.31
0.32
0.33
0.34
0.35
0.36
0.37
0.38
0.39
0.4
0.41
0.42
0.43
0.44
0.45
0
185
153
129
110
94.6
82.4
72.4
64.2
57.2
51.4
46.4
42.1
38.3
35.1
30.9
29.7
27.4
25.4
23.6
22
20.6
19.3
18.1
17
16
15.1
14.3
13.5
12.8
12.2
11.6
11
10.5
10
9.58
9.16
1
182
151
127
108
93.3
81.3
71.5
63.4
56.6
50.8
45.9
41.7
38
34.8
30.7
29.4
27.2
25.2
23.5
21.9
20.5
19.2
18
16.9
15.9
15
14.2
13.5
12.8
12.1
11.5
11
10.5
9.98
9.53
9.11
2
178
148
125
106
92
80.3
70.7
62.7
56
50.3
45.4
41.3
37.6
34.5
30.5
29.2
27
25.1
23.3
21.7
20.3
19
17.9
16.8
15.9
15
14.1
13.4
12.7
12.1
11.5
10.9
10.4
9.93
9.49
9.07
3
175
145
123
105
90.7
79.2
69.8
62
55.4
49.8
45
40.9
37.3
34.2
30.2
29
26.8
24.9
23.1
21.6
20.2
18.9
17.8
16.7
15.8
14.9
14.1
13.3
12.6
12
11.4
10.9
10.4
9.89
9.45
9.03
4
17
143
121
103
89.4
78.2
68.9
61.2
54.8
49.3
44.6
40.5
37
33.9
29.9
28.7
26.6
24.7
23
21.4
20.1
18.8
17.7
16.6
15.7
14.8
14
13.3
12.6
11.9
11.4
10.8
10.3
9.84
9.4
49
5
168
140
119
102
88.2
77.2
68.1
60.6
54.2
48.8
44.2
40.1
36.6
33.6
32.2
28.5
26.4
24.5
22.8
21.3
19.9
18.7
17.6
16.5
15.6
14.7
13.9
13.2
12.5
11.9
11.3
10.8
10.3
9.8
9.36
6
165
138
117
100
87
76.2
67.3
59.9
53.6
48.3
43.7
39.7
36.3
33.3
31.9
28.3
26.2
24.3
22.7
21.2
19.8
18.6
17.4
16.4
15.5
14.6
13.8
13.1
12.4
11.8
11.2
10.7
10.2
9.75
9.32
7
162
135
115
9.8
85.8
75.2
66.5
59.2
53
47.8
43.3
39.4
36
33
31.7
28.1
26
24.2
22.5
21
19.7
18.4
17.3
16.3
15.4
14.5
13.8
13
12.4
11.8
11.2
10.7
10.2
9.71
9.28
8
159
133
113
97.4
84.7
74.3
65.7
58.5
52.5
47.3
42.9
39
35.7
32.7
31.4
27.9
25.8
24
22.4
20.9
19.5
18.3
17.2
16.2
15.3
14.5
13.7
13
12.3
11.7
11.1
10.6
10.1
9.66
9.24
APPENDIX B: VARESTRAINT TEST SPECIMEN
50
APPENDIX C: SCHEMATIC OF A VARESTRAINT TEST SPECIMEN ON A
MANDREL
51