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Singh, Chandan Deep, Singh, Rajdeep, Singh, Ripandeep, Singh, Kanwaljit: Effect of
tool pin profile on microstructure and mechanical properties of AL6063 in Friction stir
processing, Hamburg, Anchor Academic Publishing 2017
PDF-eBook-ISBN: 978-3-96067-705-5
Druck/Herstellung: Anchor Academic Publishing, Hamburg, 2017
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i
Table of Contents
Chapter 1 Introduction
1
1.1 FSP
1
1.2 Microstructural zones in FSP
3
1.3 Capabilities of FSP
5
1.4 Tool Profiles
7
1.5 Applications of FSP
12
1.6 Advantages of FSP
13
1.7 Limitations of FSP
13
1.8 Need of the Present Study
14
1.9 Chapter Scheme
14
Chapter 2 Literature Review
16
2.1 Review of related literature
16
2.2 Research Gaps
31
2.3 Methodology
31
Chapter 3 Experimentation
33
3.1 Process Parameters and Procedure
33
3.2 Material
34
3.3 Equipments
35
3.4 FSP Tools
38
3.5 FSP Procedure
41
3.6 Characterization and Testing of Samples
42
Chapter 4 Results and Discussion
49
4.1 Microstructure Results
49
4.2 Results of Micro Hardness
53
4.3 Impact Strength Results
55
4.4 Rockwell Hardness Test Results
56
Chapter 5 Conclusion and Future Scope
59
5.1 Conclusions
59
5.2 Scope for Future Work
59
References
60
ii
List of Tables
Table 3.1
Process Parameters
33
Table 3.2
Specimen Dimensions
33
Table 3.3
Chemical Composition of AL 6063
34
Table 3.4
Specification of CNC Vertical Milling Machine
36
Table 4.1
Microstructure Results
52
Table 4.2
Micro hardness Comparison Results
54
Table 4.3
Impact Strength Results
55
Table 4.4
Rockwell Hardness Results
56
iii
List of Figures
Fig. 1.1
FSP Principle
1
Fig. 1.2
FSP Tool
2
Fig. 1.3
FSP Processed Specimen
2
Fig. 1.4
FSP Zones
4
Fig. 1.5
SEM micrographs showing the SiO2 particle dispersion in the
SiO2/AZ61 composite prepared by FSP
5
Fig. 1.6
Surface Modification after processing
6
Fig. 1.7
FSP Tool Diagram of FSP tool profile design for show all views
8
Fig. 1.8
different tool pin profiles
9
Fig. 1.9
Plates processed with different tool pin profiles (a)-(d)
10
Fig. 1.10
Drawing of the FSP tool threaded pin
11
Fig. 3.1
Base Plate AL 6063
34
Fig. 3.2
CNC Vertical Milling Machine
35
Fig. 3.3
Fixtures Used in experimentations
37
Fig. 3.4
Fixture hold Al plate on bed of CNC vertical milling machine
38
Fig. 3.5
FSP Tool made from HSS
39
Fig. 3.6
Square tool used for FSP
40
Fig. 3.7
Drawing of FSP Tool
40
Fig. 3.8
Pentagonal tool pin profile
41
Fig. 3.9
FSP
41
Fig. 3.10
Optical Microscope
43
Fig. 3.11
AL sample mounted on plastic foil for microstructure examination
43
Fig. 3.12
Buffing Machine
44
Fig. 3.13
micro hardness measurement machine and mechanism
45
Fig. 3.14
Izod Impact Testing Machine
46
Fig. 3.15
AL 6063 sample prepared for impact testing
46
Fig. 3.16
Striking position and specimen fracture after izod test
47
Fig. 3.17
Pyramid shape indenter and indenter marks in processed zone
48
Fig. 3.18
Rockwell Hardness Testing Machine
48
Fig. 4.1
Microstructure of AL base plate
49
Fig. 4.2
Processed zone microstructure for pentagonal tool pin
50
Fig. 4.3
Microscopic image of microstructure of aluminium 6063 sample
processed with square tool pin profile
50
Fig. 4.4
Microscopic image of microstructure of aluminium 6063 sample
processed with threaded tool pin profile
51
Fig. 4.5
Microscopic image of microstructure of aluminium 6063 sample
processed with circular tool pin profile
51
Fig. 4.6
Microstructure comparison with each tool pin profile processed
specimen
53
Fig. 4.7
Micro hardness comparison of processed specimen with each tool
pin profile
54
Fig. 4.8
Impact strength comparison of FSP processed sample with each
tool pin profile
Fig. 4.9
comparison between Rockwell hardness of FSP processed
specimen
57
55
CHAPTER 1
INTRODUCTION
1.1 FSP
Friction stir processing (FSP) is solid state process in which a non-consumable stirring (rotating)
tool plunged into work piece up to half thickness, which causes intense plastic deformation,
material mixing, and thermal exposure, resulting in refinement of micro structural, enhancement
of mechanical properties and homogeneity of the processed (nugget) zone. The FSP technique has
been successfully used for producing the fine-grained structure and surface composite, modifying
the microstructure of materials, synthesizing the composite like metal-metal composites (MMC).
The use of FSP generates significant frictional heating and intense plastic deformation, thereby
resulting in the occurrence of dynamic recrystallization in the stirred zone (SZ). Although there is
still a controversy about the grain-refinement mechanism in the SZ, it is generally believed that
the grain refinement is due to dynamic recrystallization. Therefore, the factors influencing the
nucleation and growth of the dynamic recrystallization will determine the resultant grain
microstructure in the SZ. It has been demonstrated that the FSP parameters, tool geometry,
material chemistry, workpiece temperature, vertical pressure, and active cooling exert a
significant effect on the size of the recrystallized grains in the SZ. [In friction stir processing, a
rotating tool with pin and shoulder is (made from material like HSS, mild steel etc.) inserted up to
1/2 thickness of work piece, and shoulder touches the workpiece surface as shown in following
diagram]
Fig. 1.1: FSP Principle (Mishra R.S et al., 2005)
1
Following diagrams (Fig. 1.2) show the FSP tool and (Fig. 1.3) procesed Al plate sample which
clears the process fundamentals.
Fig. 1.2: FSP tool
Fig. 1.3: FSP processed specimen
The FSP technique is emerging as a very effective solid-state processing (material remains in
plastic state) technique that can provide localized modification and control of microstructures of
2
soft materials in the near-surface layers of processed metallic components. In the relatively short
duration after its invention, increasing applications are being found for FSP in the fabrication,
processing, making composites etc. Friction stir process is quite simple process which can well
controlled by using numerical controlled machines. The use of FSP generates significant frictional
heating and intense plastic deformation, thereby resulting in the occurrence of dynamic
recrystallization in the stirred zone (SZ) or nugget zone (NZ).
Furthermore, the FSP technique has been used for the fabrication of a surface composite on
aluminium substrate and the homogenization of powder metallurgy (PM) aluminium alloys, metal
matrix composites, and cast aluminium alloys. Compared to other metalworking techniques, in
FSP, the major processing parameters are the tool rotating rate, the tool traversing speed and
proper tool pin profile there are different
types
of tool profiles. The intense plastic deformation
around the tool and the friction between the tool and the work piece both contribute to the
temperature increase in the stirred zone (SZ). SZ is processing zone where tool starting stirring
(rotating). Generally, an increase in the ratio of the rotation rate to the traversing speed will
increase the peak temperature in FSP. In the present thesis it is try to find out proper tool pin
profile in friction stir processing. In the present study, the effect of the tool pin profile on
microstructure and mechanical properties is to be examined that how these profile effect
microstructure and mechanical properties of aluminium 6063.
1.2 Microstructural zones in FSP
During processing material divided into following type of zones which arises due to mechanical
action and frictional heat of tool on material microstructure. The microstructure can be broken up
into the following zones (Fratini and Buffa, 2005)
a. Parent material
It is unaffected zone which does not goes under any deformation and remains same before and
after processing This is the zone of parent material away from processed metal which is not
affected by the heat flux in terms of microstructure or mechanical properties. In this zone no
material deformation occurs.
3
b. Heat affected zone (HAZ)
In this region the material undergoes a thermal cycle which leads to modified microstructure
and/or mechanical properties. This zone retains the same grain structure as the parent materials.
However, no plastic deformation occurs in this area but it is affected due to heat dissipated in
nugget zone
c. Thermo-mechanically affected zone (TMAZ)
In this zone, the material undergoes plastic deformation by the tool, and the heat flux also exerts
some influence on the material. TMAZ is produced by friction between the tool shoulder and the
top surface of plate, as well as plastic deformation of the material in contact with the tool. In case
of aluminium, no recrystallization is observed in this zone; however extensive deformation is
present in this region
d. Nugget or processed zone (NZ)
The recrystallized area below the tool shoulder is given a separate category, as it has different
grain structure. In this zone, the original grain and sub grain boundaries appear to be replaced
with fine, equi-axed recrystallized grains characterized by a nominal dimension of few micro
meters. In this region intense plastic deformation and frictional heating during FSP result in
recrystallized fine-grained microstructure.
Fig. 1.4: FSP zones
4
1.3 Capabilities of FSP
1.3.1 Fabrication of composite by friction-stir processing
Friction stir process show better result during fabrication of composites, the use of the FSP
technique results in the intense plastic deformation and mixing of material in processed zone;
results in fabrication
of composites, it is possible to incorporate the ceramic particles into the
metallic substrate plate, to form the surface composites. (Mishra et al.,) reported the first result
on the fabrication of a SiCp-Al surface composite via FSP. The SiC powder was added into a
small amount of methanol and mixed, and was then applied to the surface of the plates, to form a
uniform thin SiC particle layer. Due to stirring action of tool silicon powder well mixed and
fabricated in base metal. In general cases aluminium plate used as base plate when silicon
powder is matrix phase.
The aluminium plates with a preplaced SiC particle layer were subjected to FSP. With the
optimized tool and pin profile design and processing parameters, a composite layer of ~100 lm,
with well distributed particles and a good bonding with the aluminium substrate, was generated on
the substrates of 5083Al and A356 aluminium alloys. By adjusting the FSP parameters, 5 to 27
vol. of the SiC particles could be incorporated into the aluminium matrix. Optimum tool design
and process parameter are responsible for properties of composites to be fabricated Slower feed
rates means more stirring action per unit area, more stirring means better material flow rate. So to
faster speed does not show best result as compare to slower ones.
Fig. 1.5: SEM micrographs showing the SiO2 particle dispersion in the SiO2/AZ61 composite
prepared by FSP.
5
Above diagram shows the capabilities of FSP in fabrication of composites. Right hand diagram
shows that matrix particle homogeneously spreaded all over the volume of base metal i.e.
aluminium. Also porosity also reduces and metal composite become very fine. Friction stir
process is suitable for aluminium composites fabrications. (Mishra R.S et al., 2005)
1.3.2 Friction-stir microstructural modification
FSP is unique process for surface modification. Now question is that why? Answer is that because
stirring action of tool flow the metal and fill various casting voids and refines grain boundaries
and grain size due to recrystallization in nugget zone. During FSP, the rotating pin with a threaded
design
produces an intense breaking and mixing effect in the processed zone, thereby creating a
fine, uniform, and dense structure. Therefore, FSP can be developed as a generic tool for
modifying the microstructure of heterogeneous metallic materials such as cast alloys, metal matrix
composites, and nano phase aluminium alloys prepared through the PM technique. Alloys of Al-
Si-Mg (Cu) are widely used to cast high strength components in the aerospace and automobile
industries, because they have good cast ability and can be strengthened by artificial aging. The as-
cast structure of Al-Si-Mg (Cu) alloys is characterized by porosity, coarse Si particles, and coarse
primary aluminium dendrites. These microstructure features limit the mechanical properties of
cast alloys, in particular, toughness and fatigue resistance. Eutectic modifiers and high-
temperature heat treatment are widely used to refine the microstructure of cast Al-Si alloys, to
enhance the mechanical properties of the castings. However, approaches can heal the casting
porosity effectively and redistribute the Si particles uniformly into the aluminium matrix. (Mishra
R.S et al., 2005)
Fig. 1.6: Surface modification after processing (silicon-Al composite)
6
Above diagram shows the capabilities of FSP in modification in surface of metal. FSP tool can
remove casting defects and porosity hence surface of metal become fine and strength will also
increases as shown in above diagrams surface become more porous and defects free. Reason is
that stirring action of tool melts and flow the material of metal hence small casting defects filled
due to flow of material and material microstructure become fine grained and homogeneous.
Interest in friction stir processing rapidly growing as it is refining materials microstructure and
internal properties through single pass of non-consumable tool without any addition of alloying
elements for aluminium and its alloys. Aluminium is the most popular metal that is widely used.
About 85% of aluminium is used for wrought products, for example rolled plate, foils and
extrusions. Aluminium has light weight, resistance to corrosion and has low melting point but its
severe limitation is the difficulty associated with welding of aluminium/aluminium alloy
structures many times aluminium failed to
sustain heavier loads and also having poor surface
hardness.
The research work carried out by some of the investigators reveals that friction stir processing
technique can be successfully used for micro structural modification, enhancement of surface
properties such as micro hardness, wear resistance etc. of aluminium or its alloys. Materials
processed by friction stir process used in complex aerospace part where high specific strength per
unit area required, best suited for manufacturing composites. In the light of above mentioned facts
mostly circular tool profile is used which show better results on aluminium composites. Now, the
effort shall be made to find out the effect of other tool profiles either than circular tool profile. If
other tool profile show better result than circular then these tool profile can be used instead of
circular tool profile So effort is made to find out effect of four tool profiles on aluminium 6063 so
that desired tool can be selected for desired results.
1.4 Tool profiles
The tool geometry plays a critical role in material flow and in turn governs the traverse rate at
which FSP can be conducted. Tool has two primary functions
1. Localized heating
2.
Material flow
7
Fig. 1.7: FSP tool diagram of FSP tool profile design for show all views
In first function tool provide heat which melts the material and this heat is due to centrifugal force
friction between tool shoulder and work piece. The tool is plunged till the shoulder touches the
workpiece. The friction between the shoulder and workpiece results in the biggest component of
heating. From the heating aspect, the relative size of pin and shoulder is important, and the other
design features are not critical. Now it is clear that tool pin and shoulder are responsible for
localized heating the shoulder also provides confinement for the heated volume of material. The
second function of the tool is to stir` and move` the material. The uniformity of refined fine
grained microstructure and mechanical properties as well as process loads. In present research it is
try to find out effect of tools profiles (Circular tool pin profile Pentagonal profile, Threaded tool
profile, Square tool profile)
threaded pin profile
square pin profile
8
circular pin profile
pentagonal pin profile
Fig. 1.8: different tool pin profiles
Following are specimen processed with each profile
(a) square
(b) pentagonal
(c) circular
9
(d) threaded
Fig. 1.9 Plates processed with different tool pin profiles
All above diagrams shows four plates processed with four tool pin profiles. It is show that every plate
having different effects with each profile i.e. type of chips etc.
1.4.1 Circular tool pin profile
Mostly used tool profile is circular tool pin profile as shown in (Fig. 1.6). It simpler in design and
can be machined easily on center lathe. Tool pin length varied according to the thickness of work
piece, it should be ½ thickness of work piece and shoulder should touch with surface of material
to be processed as shown in first diagram Now using the working material get deformed and melts
during single pass of tool and after pass complete material solidified during process with circular
tool profile because to pin is round less power is required and also less wear tear of pin than that
of pentagonal tool profile.
1.4.2 Square tool pin profile
With increasing experience and some improvement in understanding of material flow, the tool
geometry has evolved significantly. Complex features have been added to alter material flow,
mixing and reduce process load. Another tool pin profile is square tool pin profile which having
pentagonal base of pin tip, remaining dimensions same as circular profile so we can say that
square tool profile is modified form of circular tool profile. Four sharp edges of square tool profile
are responsible for material flow rate. If four edges of tool profile replace with six edges a new
profile will formed known as pentagonal tool pin profile. These edges are more responsible in tool
performance due material flow rate and heat generation during stirring of tool in nugget zone
10
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