Effects and defects of the polypropylene plate for different parameters in friction stir welding process

IJRET: International Journal of Research in Engineering and Technology ISSN: 2319-1163
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Volume: 02 Issue: 02 | Feb-2013, Available @ http://www.ijret.org 143
EFFECTS AND DEFECTS OF THE POLYPROPYLENE PLATE FOR
DIFFERENT PARAMETERS IN FRICTION STIR WELDING PROCESS
K. Panneerselvam1
, K. Lenin2
1
Assistant Professor, 2
Research Scholar, Department of Production Engineering, NITT, Tamil Nadu, India
kps@nitt.edu, leninnaga@yahoo.com
Abstract
Polypropylene is one of the thermoplastic materials used in the lot of engineering applications such as marine, aerospace, automotive,
toys and etc. Friction Stir Welding (FSW) is a solid-state method of used for joining metals. FSW process was successfully extended to
join thermoplastic materials. In this paper an attempt has been made to understand the mechanism of friction stir welding joints of
polypropylene plates. In this study, the role of FSW tool pin profile, rotational speed and welding speed are analyzed with respective
to the quality of the joints. This research outlines the method of welding and optimization of FSW process parameters for
polypropylene material by Taguchi optimization methodology. Experiments were performed at rotational speed of 1500, 1750, 2000
and 2250 rpm, Welding speeds of 30, 40, 50 and 60 mm/min, and tool pin profiles of Triangular, square, Threaded and taper pin
profiles. The experiments are conducted in CNC vertical machining centre with special fixture. Microstructural characteristics were
evaluated by using optical microscopy and hardness are observed in weld joints. The micro structure and Rockwell hardness of the
welded region was created by threaded pin profile with welding speed of 40 and 50 mm/min and rotational speed of 2250 and 1500
rpm act as right friction stir welding parameters to avoid defects in joining of polypropylene materials.
Index Terms: Polypropylene, Microstructure, Hardness, Pin profile, Speed, Feed, and Weld zone
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1. INTRODUCTION
Friction stir welding (FSW) is a novel solid-state welding
process for the joining of metallic alloys and composites; it is
the numerous applications in manufacturing situations [1, 2].
FSW considered to be one of the most significant welding
processes, FSW has the advantage of non-consuming
fabrication of continuous linear welds, the most common form
of weld joint configuration. This joining technique is energy
efficient, environmentally friendly and versatile [3]. In
general, the process is carried out by plunging a rotating FSW
tool into the interface of two rigidly clamped sheets, until the
shoulder touches the surface of the material being welded, and
transverse along the weld line. The frictional heat and
deformation heat are utilized for the bonding under the applied
normal force [4]. This process is illustrated in Fig-1. The
microstructure of copper material with those of the residual
friction stir welded zone was compared and characterized by
using optical microscopy (OM) and transmission electron
microscopy (TEM). Tensile strength and corresponding
microhardness profiles through the weld zone were also
correlated with these microstructures. Hardness variation also
existed in the stir zone from the top to the lower region with
thermal and mechanical conditions [5]. The hardness
distribution in the weld nugget, thermal-mechanical affected
zone, heat affected zone and the base metal were evaluated by
using Vickers hardness tests on the welds. The temperature on
the retreating side were slightly lower than those on the
advancing side. But, there is no significant difference in
hardness between the advancing and retreating sides [6].
Fig-1: Friction stir welding process
The hardness of the stirred zone for 10 to 30mm/sec feed rate
for 1000 to 2000rpm had explained in the magnesium alloys
[7]. The hardness of the material had only evaluated based on
composition of magnesium, silicon and silicon carbide. They
had not explained thoroughly about the changes of hardness in
the nugget, retreating side as well as advancing side for
different compositions [8]. David et al reviewed the current
techniques used for direct bonding of polymers, with a focus
on thermoplastics [9]. Only few reports are there related to
thermoplastics in FSW and Friction stir spot welding (FSSW)
[10-13]. This paper addresses this absence by presenting a

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report on the effects of process parameters such as rotational
speed, welding speed and tool pin profile for polypropylene
plate achieved using FSW. Four different types of FSW pin
profile were rotated at four different rotational speeds and four
different welding speeds during the experiments. The effects
of the process parameters on the weld region and hardness are
measured and the microstructure of the weld zone is
investigated.
2. EXPERIMENTAL PROCEDURE
FSW was performed on samples that were held using a
specially designed clamping fixture that allowed the samples
to be fixed on to a CNC vertical machining centre for welding.
The samples used in this work consisted of two polypropylene
plates with dimensions of 220x100x10mm (length, width,
depth) was prepared to fabricate FSW joints. The initial joint
configuration was obtained by securing the plates in position
using clamps. The direction of welding was normal to the
rolling direction. Single pass welding procedure was used to
fabricate the joints. The non-consumable rotating tools used in
this study had four different FSW tool pin profiles such as
Square pin, taper pin, Triangular pin, and Threaded pin with a
cylindrical shank. The fixed pin type tool made in mild steel
with a nominal pin diameter 6mm and shoulder diameter of
24mm and also pin length of 10 mm was used in the present
investigation. Various nomenclature of tool is shown in Fig-2.
Fig-2: Various nomenclature of FSW tool
Design of experiments (DOE) is a powerful analysis tool for
modeling and analyzing the influence of process variables
over some specific variables, which is an unknown function of
these process variables. The most important stage in the DOE
lies in the selection of the control factors. As many as possible
should be included, so that it would be possible to identify
non-significant variables at the earliest opportunity. There are
three different parameters used in this investigation such as
tool pin profile, spindle speed, and welding speed. Taguchi
defines the quality of a product, in terms of the loss imparted
by the product to the society from the time the product shipped
to the customer. Some of these losses are due to deviation of
the product’s functional characteristic from its desired value,
and these are called losses due to functional variation. The
uncontrollable factors which cause the functional
characteristics of a product to deviate from their target values
are called noise factors, which can be classified as external
factors such as temperature and human factors, manufacturing
imperfections due to variation of product parameter from unit
to unit and product deterioration, etc. the overall aim of
quality engineering is to make products that are robust with
respect to all noise factors. Taguchi creates a standard
orthogonal array to accommodate this requirement. Depending
on the number of factors and their level, an orthogonal array is
selected by the investigator is shown in Table 1. In this
experimental work, L’16 orthogonal array was utilized to
identify the optimized parameters. Details of friction stir
welding conditions are shown in Table 2. Sixteen different
welding conditions were applied to these samples. Welding
speeds were varied from 30 to 60 mm/min, and rotational
speeds were varied from 1500 to 2250 rpm.
Table-1: Different process parameters and their levels
S.No parameter levels
1 FSW tool pin
profile
Square, Triangular,
Threaded, and Taper
2 Welding speed
mm/min
30,40, 50, and 60
3 Rotational speed
rpm
1500, 1750,2000, and
2250
Table-2: Experimental design based on the L’16 orthogonal
array
Run Pin profile Feed rate in
mm/min
Rotating
speed in
rpm
1 Taper 30 1750
2 Taper 40 1500
3 Taper 50 2250
4 Taper 60 2000
5 Triangular 30 2250
9 Square 30 1500
10 Square 40 1750
11 Square 50 2000
12 Square 60 2250
13 Threaded 30 2000
14 Threaded 40 2250
15 Threaded 50 1500
16 Threaded 60 1750
FSW joints were analyzed for microstructural studies.
Microstructural characterization were taken across the cross
section of friction stir welded region (parallel to the rolling
direction). All samples were cut approximately 20mm length.

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The samples were then manually ground and polishing in the
polishing machine. The polished samples were etched in O-
Xylene. Microstructural image were taken by the optical
microscopy at a magnification of 100x, 200x and 400x.
Rockwell hardness testing machine was used for hardness
testing of FSW joints. The hardness values were measured at
different distances (every 5mm upto 15mm) from the centre of
nugget with the load of 100 kgf, 30 second duration time and
¼” ball indenter.
3. RESULT AND DISCUSSION
3.1 Friction Stir Welded region and their Micro
structural analysis
FSW were successfully produced for different designed
process parameters such as spindle speed of 1500-2250 rpm,
welding speed of 30-60 mm/min, and four different FSW tool
pin profiles (Taper, Triangular, Square and Threaded pin
profile). The polypropylene plates were welded for sixteen
different set of process parameters condition based on the
L’16 orthogonal array in FSW process. In this experimental
work, the friction stir welded joints for different designed set
of parameters were analyzed and those welded region
discussed in Fig-3a-18a. The nugget areas of the welded
region were clearly visible and observed in those figures. Top
of the joint interface and cross section of the welded region
both were explained and for understanding of the welded
region, focused particular region in those figures.
In fusion welding of metal alloys, the defects like porosity, hot
cracks etc., decline the weld quality and joint properties.
Usually, friction stir welded joints are free from these defects
since there is no melting takes place during welding and the
metals are joined in the solid state itself due to the heat
generated by the friction and flow of metal by stirring action.
However, FSW joints are prone to other defects like pin holes,
tunnel defects, piping defects, kissing bond, cracks, etc… due
to improper flow of metal and insufficient consolidation of
metal in the FSW region[14]. But, when the welded region’s
cross section was evaluated in polypropylene materials, some
cracks, pin holes, porosity, cavity and etc… appeared in the
welded region.
The magnified cross sections as micro level of welded region
were evaluated to analyze the defects and optimize the process
parameters in the FSW process. So, the welded region’s cross
section was taken the micro structural images by using the
optical microscopy for 100X, 200X, and 400X magnification
that had discussed in the Fig- 3b-18b for the designed set of
process parameters in FSW. Microstuctural images of nugget
area for different set of process parameters were evaluated for
identifying defects like cracks, blow holes, pin holes, cavity,
porosity and etc.
.
Fig-.3: a&b. Appearance & Microstructure of FSW region for
Taper pin at rotational speed of 1750 rpm and welding speed
of 30 mm/min
FSW tool with Taper pin profile at rotational speed of 1750
rpm and welding speed of 30 mm/min produced concave
shape of the welded region and small crystal form of material
appeared in the retreating side. The molded material
consolidated in the nugget area very well even the poor
contact with retreating side as shown the Fig-3a. The porosity,
cavity and inclusion were appeared in the contact of retreating
side’s parent material as shown in the Fig-3b.

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Fig-4: a&b. Appearance & Microstructure of FSW region for
Taper pin at rotational speed of 1500rpm and welding speed of
40 mm/min
FSW tool with Taper pin profile at rotational speed of 1500
rpm and welding speed of 40 mm/min produced long concave
groove appeared in the joint interface and some amount of
material threw outside. The material consolidation was not
even in the entire length of joint interface. Even though the
material appeared in the joint interface, it is not properly
bonded in the advancing side, the unconsolidated material
stored in the retreating side with poor contact of parent
material. Small crystal form of material deposition was also
appeared in this welding condition as shown the Fig- 4a. Due
to the unmolded material in the retreating side, cavity and
porosity appeared in macro as well as micro level images as
shown the Fig- 4b.
of 50 mm/min
FSW tool with taper pin profile at rotational speed of 2250
rpm and welding speed of 50 mm/min produced long cavity
appearance in the joint interface. It developed concave shape
of the welded region and small amount of the material
accumulation. The material was normally consolidated in the
entire length of the welded region with visible contact on
AS&RS as shown the Fig- 5a. So, when analyzed the micro
images in Fig-5b, more number of blow holes appeared in the
welded region.
of 60 mm/min
FSW tool with taper pin profile at rotational speed of 2000
rpm and welding speed of 60 mm/min produced poor contact
with the retreating side for half of the entire length and then
somewhat better welded region formed due to the temperature
changes in the remaining length as shown in the Fig-6a. Only
few blow holes were appeared in the middle of the nugget.
When the nugget area was analyzed by the microscopy, the
minimum number of defects formed in this region such as
cavity, inclusion and contact of parent material as explained in
the Fig- 6b.

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Fig-7: a&b. Appearance &Microstructure of FSW region for
Triangular pin at rotational speed of 2250 rpm and welding
speed of 30 mm/min
FSW tool with triangular pin profile at rotational speed of
2250 rpm and welding speed of 30 mm/min produced convex
shape of the welded region in the entire length of the joint
interface. Due to the over heat in the advancing side, a small
gap formed in the joint interface and developed rough surface
in the retreating side when the tool shoulder rubbed the
workpiece surface. The poor contact of nugget with retreating
side’s parent material and formed porosity in the cross section
as shown in the Fig- 7a. More number of porosity was visible
in microstructure as shown in the Fig- 7b.
speed of 40 mm/min
The poor contact with the parent material and few porosity
formation were continued in this condition also (FSW tool
with triangular pin profile at rotational speed of 2000 rpm and
welding speed of 40 mm/min). One or two porous formed in
nugget as shown the Fig- 8a. But, more blow holes appeared
in the microstructure of welded region with one inclusion as
shown in the Fig- 8b.
speed of 50 mm/min

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In FSW tool with triangular pin profile at rotational speed of
1750 rpm and welding speed of 50 mm/min, even though the
convex shape of the welded region was formed in the top of
the joint interface, defects were occurred in the form of
porous, cavity and blow holes. This also created larger size
cavity, porous and blow holes formed in the nugget area as
shown in the Fig- 9. and also the poor material contact of
nugget with retreating side as shown in the Fig- 9a. Some of
the cavity and porous had more size in the images as shown in
the Fig- 9b.
speed of 60 mm/min
In this experiment(FSW tool with triangular pin profile at
rotational speed of 1500 rpm and welding speed of 60
mm/min), number of defects reduced such as gap formation,
material storage in both sides, and defects in nugget area. But,
the poor material contact of nugget with retreating side and
porosity appeared in this condition as shown in the Fig-10b.
FSW tool with square pin profile at rotational speed of 1500
rpm and welding speed of 30 mm/min developed convex
shape of the welded region in the entire length and small
groove formed in the advancing side. Small crystal form of
material stored in the retreating side. The porosity created in
the middle of the nugget area as shown the Fig- 11a. Only the
porosity and contact line defects enlarged in the
microstructural images with inclusion. This was explained in
the Fig-11b.
Square pin at rotational speed of 1500 rpm and welding speed
of 30 mm/min
of 40 mm/min

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The porosity, pin hole in the top surface of the welded region,
and poor contact of parent material were developed as the
defects in this parameter condition (FSW tool with square pin
profile at rotational speed of 1750 rpm and welding speed of
40 mm/min) as shown in the Fig- 12a. The few millimeter
interior region of nugget had porosity this was clearly shown
in the microstructure image as shown in the Fig- 12b.
of 50 mm/min
Flat shape of the welded region occurred in this condition
(FSW tool with square pin profile at rotational speed of 2000
rpm and welding speed of 50 mm/min) with some pin holes. If
the welded nugget divided as two part as top portion and
bottom portion, top portion did not have any defects and good
contact with parent material. The bottom portion had porosity
and poor contact with the parent material as shown in the Fig-
13a. A larger size porosity appeared in the welded region with
some inclusion as shown in the Fig- 13b.
In FSW tool with square pin profile at rotational speed of 2250
rpm and welding speed of 60 mm/min, pin holes did not
develop in the joint interface and also good contact of nugget
with parent material. But, few porosity and cavity appeared in
the weld nugget as shown in the Fig- 14a&b. The material
deposition was even in the AS&RS.
of 60 mm/min
Threaded pin at rotational speed of 2000 rpm and welding
speed of 30 mm/min
Due to the over heat in this condition (FSW tool with threaded
pin profile at rotational speed of 2000 rpm and welding speed
of 30 mm/min condition), tool shoulder picked the colloidal
material from the retreating side and developed long cavity in

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the retreating side and also more amount of material
consolidated in the advancing side. Few porous and blow
holes appeared in the entire welded region (see Fig- 15a&b).
Contacting line of parent material in the retreating side had
half of the plate thickness after the tool passed in the joint
interface.
speed of 40 mm/min
In FSW tool with threaded pin profile at rotational speed of
2250 rpm & welding speed of 40 mm/min, 1500 rpm & 50
mm/min, and 1750 rpm & 60 mm/min, material consolidated
as flat form in the entire length. Good welded region
developed in these conditions. Even the advancing side’s
contact was better, the normal contact line of nugget with
retreating side occurred with one or two porosity in the macro
and micro structural images as shown in the Fig- 16-18. Good
nugget area developed in these welded regions without any
porosity, blow holes and other defects.
For the spindle speed except 1500 rpm, the welded region
could not be analyzed as correct way based on the spindle
speed because each spindle speed gave different welded
qualities for different tool pin and feed rate of the designed set
of process parameters in FSW process. Apart from the contact
of nugget area with retreating side’s parent material, the
effects of welded region analyzed and tabulated in Table 3.
When the welded region were analyzed, the contact line of
nugget with retreating side was the major problem in the all
those conditions. If that defect will reduce, the strength of the
welded region mostly improve in all experimental condition.
Table-3: Effects of welded region based on the spindle speeds
Spindle
speed/ Pin
profile
1500 rpm 1750
rpm
2000
rpm
2250
rpm
Taper Normal Poor Poor Ok
Triangular Normal Ok Poor Good
Square Normal Poor Poor Good
Threaded Normal Good Good Good
3.2 Analysis of Rockwell hardness in the welded
region
Rockwell hardness of the welded polypropylene plate for all
set of process parameters were evaluated in the nugget area
and 15 millimeter with 5 millimeter interval on the both side
parent material from the middle of joint interface. Those
values are tabulated in the Table 4 and presented in Chart-1.
The left side data mentioned the advancing side hardness and
the right side data mentioned the retreating side hardness in
the Chart-1.
speed of 50 mm/min

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speed of 60 mm/min
Table-4: The Rockwell hardness distribution middle,
advancing side as well as retreating side for each 5mm upto
15mm
S.
No
Advancing side
in mm
Middle Retreating side
in mm
15 10 5 5 10 15
1 76 71 70 55 68 71 76
2 76 71 70 55 70 72 76
3 76 71 70 57 70 72 76
4 76 71 69 60 69 71 76
5 73 70 68 65 66 69 72
6 72 70 68 66 67 70 72
7 72 70 69 66 67 69 71
8 72 71 70 67 67 69 71
9 72 70 68 67 70 71 72
10 72 70 68 68 70 71 72
11 72 70 68 68 69 70 71
12 72 70 69 69 69 70 72
13 73 71 70 69 69 70 72
14 73 72 71 70 70 71 72
15 73 72 71 70 70 70 72
16 73 72 71 69 70 71 73
Process heat is not the only factor for a successful FSW
process. Other factors, such as welding speed, rotational
speed of the tool and tool pin profiles are all important factors
for a successful FSW process. These factors influence the
temperature distribution in the workpiece, and eventually, the
control of the temperature becomes the key factor that affects
the final properties of the weld [14]. The temperature changes
were occurred due to the different process parameters that
were focused in this hardness investigation. Normally, the
temperature profiles on the retreating side are slightly lower
than those on the advancing side. Those similar conditions
occurred in hardness profiles of polypropylene material in
FSW process.
Chart-1: Effects of different process parameters on hardness
The hardness of base material has a range of 74-76 RH.
However, the hardness near welded region shows variables
value from 55-70 RH. The hardness on the retreating side was
slightly lower than those on advancing side. But, there is no
much difference in hardness between the both sides as shown
the Chart 1. The left side of the chart mentioned advancing
side and right side mentioned retreating side. The most of the
nugget area’s hardness had only 65 -70 RH except three
nugget area. Taper pin created very poor welded region with
low hardness. Depend upon the heat affection on the both
sides, the hardness changes occurred upto particular length.
Taper pin did not create more hardness in the middle. So, the
temperature rises was also very low in the middle and both
sides. But, for other pin profiles, hardness increased upto 70
RH in the middle and heat affected on the both sides (see the
Table 4 and Chart 1). The threaded tool pin created more
hardness for all conditions without bothering of welding speed
and spindle speeds as well as the threaded pin produced good
welded region without more defects in the joint interface and
nugget area.

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CONCLUSIONS
The study has demonstrated that polypropylene plate material
can be joined using FSW process. The effects of the
processing parameters were investigated by microstructure
and Rockwell hardness. From this investigation, threaded tool
pin profile created good welded region and more hardness
without bothering of welding speed and rotational speed. The
poor contact line of nugget area with advancing side’s parent
material is followed for all designed process parameters with
various effects. Even the square pin produced good nugget
area, some pin holes appeared in the joint interface. That may
be affected the strength of the joints. Triangular pin developed
good outer look as convex profile for all experimental
conditions. But, those had more interior defects such as
porous, poor contact, blow hole, and etc. The taper pin did not
create good welded region for all the conditions. In the
microstructure and hardness testing based investigation, the
welding speed and rotational speed both the parameters were
depended only the tool pin profile. There is no much
difference occurred based on the welding speed and rotational
speed.
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BIOGRAPHIES:
K. Panneerselvam is currently working
as Assistant Professor in Department of
production Engineering, NITT,
Tamilnadu, India. He did his Ph.D.
(Welding) from NITT, Tamilnadu,
India. He has published nine research
papers in national and international
journals and twenty one papers in
conferences. His research interests are
Advance composite material processing, modeling and
optimization of process parameters.
K. Lenin is Research scholar in
Department of production
Engineering, NITT, Tamilnadu, India.
He is doing Ph.D. from NITT,
Tamilnadu, India. His research
interests are Advance composite
material processing.

Effects and defects of the polypropylene plate for different parameters in friction stir welding process

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Effects and defects of the polypropylene plate for different parameters in friction stir welding process