friction stir welding report ent111

CONTENTS

Introduction to Friction Stir Welding and Processing Literature Review

Process Variables Tool geometry Material flow in FSW Weld micro structure and mechanical property Dissimilar FSW Defects in FSW Multi pass FSW

Objectives Experimental work Results and discussion Conclusions Future work References

INTRODUCTION

Friction stir welding (FSW) is a solid state joining process.

Invented at The Welding Institute (TWI) of Cambridge, UK in 1991.

Utilizes a non consumable rotating tool consisting of a concentric threaded tool pin and tool shoulder.

Transforms the metal from a solid state into a “Plastic like” state and the mechanically stir the materials together under pressure to form a welded joint.

Fig. 1: Schematic representation of FSW [4]

SEQUENCE OF OPERATION

Fig. 2: Contact of the pin produces friction and deformational heating. Contact of shoulder to the work piece increases the work piece heating and expands the zone of softened material and constrained the deformed material. [5]

SCHEMATIC CROSS-SECTION OF A FSW WELD

Fig. 3:A. Unaffected material, B. Heat affected zone (HAZ), C.Thermo-mechanically affected zone (TMAZ), D. Weldnugget (Part of thermo-mechanically affected zone) [6]

APPLICATIONS Aerospace

Ship building

Railway industries

Automobiles

Some of the parts are- Fuel tank for space launch vehicles

Roofing for railway carriages.

Bodies and floors for coaches, buses.

Wings and fuselage panels of aircraft.

Wheel assemblies.

Connectors.

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ADVANTAGES OF FSW OVER OTHER WELDING PROCESS

Retain near-parent material properties across the weld.

Join similar and dissimilar material, difficult by conventional processes.

Weld quality is excellent (no porosity).

No melting of material.

Low residual stresses.

No fumes, no filler material, no shielding gases.

Easily automated on simple milling machine-low setup cost and less training.

FRICTION STIR PROCESSING

LITERATURE REVIEW

Table 1: PROCESS VARIABLES IN FSW

Machine variable Tool variable Other variable

Welding speed

Spindle speed

Plunge force

Tool tilt angle

Tool material

Pin and shoulder diameter

Pin length

Thread pitch

Shoulder and tool feat

Joint design

Material Type and size

Property of work piece

material

Type of fixture material

Author Year Findings

Sato et al. 2002 Significant rise of temperature with rise of rotational speed.

Peel et al. 2006 Both torque and extent of material mixing in the SZ zone displays amuch stronger dependence on the rotational speed than thetraverse speed.

Meran et al. 2006 With const.rpm and varying welding speed finding out theoptimum parameter for defect-free joint

Kwon et al. 2009 Onion ring structure becomes wider as rpm increased. but grainsize decreased with increase in rpm.

Rodrigues et al. 2009 Hot welds obtained with maximum rpm and minimum traversespeed have improved mechanical properties relative to cold weld.

Contd.


Raja manickram et al. 2008 Temperature under the tool was strongly dependent on the toolrotation rate than the welding speed.

Azizieh et al. 2011 With high rpm, higher heat input occur and simultaneously moreshattering effect of rotation cause better nano-particle distribution.

Lakshminarayanan et al. 2011 Quality weld depends on the weld pitch, i.e. tool advance per rev.(welding speed / rpm) and can be increased by increasing thewelding speed at constant rpm or decreasing the rotation speed atconstant welding speed.

Contd.

Contd.

Fig. 4: Schematic drawing of FSW tool [1]

Table 2: A selection of tools designed at TWI [1]

Fig. 5: Tool shoulder geometries, viewed from underneath the shoulder [6]


Scialpi et al. 2007 Used 3 different shoulder geometry (scroll with fillet, cavity with filet, only fillet)and found that best joint has been welded by shoulder with fillet.

Zhang et al. 2011 Tool with three spiral flute w/o pin gives better result than inner concave flute and concentric circle flute.

Forcellese et al. 2012 Used two different tool configuration with different values of shoulder diameter, both with and w/o pin.Large shoulder diameter w/o pin gives strong beneficial effect on both ductility and strength.

Forcellese et al. 2012 Investigated the plastic flow behavior and formability of FSW AZ31 thin sheet using pin-less tool configuration.

Galvao et al. 2012 Used scrolled and conical shoulder tool. Found that different geometry had completely different morphology and intermetallic content using same process parameter.

Galvao et al. 2013 Further researched to see the influence of 3 different geometry (flat, conical, scrolled) on 1 mm thick copper plate..

RELATED TO SHOULDER GEOMETRY AND PIN GEOMETRY

MATERIAL FLOW IN FSWFSW process can be defined as a metal working process of five

conventional metal working zones.

Preheat

Initial deformation

Extrusion

Forging

Post heat / cool down

Contd.

Fig. 6: (a) Metal flow pattern and (b) Metallurgical processing zones developed during friction

stir welding [54]

WELD MICROSTRUCTURE AND MECHANICAL PROPERTIES

The microstructure and consequent property distribution produced during FSW depends on following factors :

Alloy composition

Alloy temper

Welding parameters

Other geometric factors (Shoulder size, Plate gauge, etc)


Guerra et al. 2003 Studied the flow of metal using faying surface tracer and a nib frozen in place during welding. Material is moved around the nib by two processes both having different thermo mechanical histories and properties.

Hamilton et al. 2008 Proposed a model of material flow during FSW. They observed that NZ is the combination of interleaved layers of particle rich and particle poor material.

Sato et al. 2002 Grain size in the nugget region is determined predominantly by the peak temperature in the weld. Higher the peak temperature larger is the grain size.

DISSIMILAR FSW


Cavaleire et al. 2006 Studied the micro and mechanical properties of dissimilar FSW between 2024 and 7075 Al alloy. Results showed the fatigue behavior is reduced by FSW which proves to be an alternative technology for large part of industrial application.

Somasekharan et al. 2004 Uneven distribution of micro hardness is observed in FSWjoints between Mg alloy and 6061-T6 alloy ,which is due to complex intercalation structure.

Sato et al. 2004 Found that constitutional liquation is the main reason for the formation of large volume of intermetallic compound with higher hardness in nugget zone.


Yan et al. 2005 Studied the complicated lamellar band formed due FSW between 1060 Al alloy with AZ31 Mg.

Yong et al. 2010 Studied the dissimilar joining of aluminum to magnesium with uneven distribution of hardness with higher value than that of base metal. Further tensile fracture locates at the AS side where hardness gradient was sharpest.

Dehgani et al. 2013 Studied the effect of process parameters to control the amount of intermetallic compound for the production of sound weld.

Tan et al. 2013 Found excellent bonding between 5A02 Al with pure copper by lowering one of the process parameter i.e. traverse speed from 40 mm/min to 20 mm/min.


Li et al. 2012 Studied the dissimilar welding through pin-offset technique of pure copper to 1350 Al sheet of 3 mm thickness. Complicated microstructure without intermetallic compound were found in the nugget zone of welded structure with higher hardness in the copper side.

Masyuki et al.

2012 Investigated the effect of pure magnesium and alloying element of ZK60(Mg-Zn-Zr) on the microstructure of dissimilar joint interface with titanium.

Bahrami et al.

2014 Studied the effect of nano-sized particle as well as process parameter on the friction stir welded aluminum metal matrix composite.both mechanical and micro structural properties are enhanced by SiC particle.

Bazmouz et al.

2011 Fabricated Cu-base Sic reinforced composite. They reported that grain size is finer with the addition of Sic particles.

Hsu et al. 2005 Fabricted the in-situ composite by friction stir processing which helps to distribute the Al2Cu particles homogeneously in the composite.

DEFECTS IN FSW WELDS

Formation of defects are mainly due to improper material flow or due to geometric factors.

Lack of penetration

Lack of fusion

Surface grooves

Excessive flash

Tunnels

Voids

Kissing bonds

DEFECTS FROM TOO COLD WELD

Too cold welding condition results in work hardening of the material.

Causes dry slip between the tool and work piece.

Lack of surface fills/ voids, channel defects are the main defects due to insufficient heat generation.

The insufficient heat generation causes improper material mixing and thus responsible for non-bonding.


Kim et al. 2006 Evaluate that at lower rotational speed and high welding speed insufficient heat input is generated resulting in cavity/ groove like defects

MULTI-PASS FSW

FSW is capable of producing welds with less defects but stillcomplete elimination of process upset is not possible.

Much researchers has been devoted to understand the effect ofprocess parameters on defect formation in order to optimize theprocess parameters for FSW. Still optimization of processparameters is mostly done by trial and error.

In the past few decades, there has been research going on in thefield of MP FSW/ FSP where it is more desirable to repair thedefective portion of the weld than to throw as a scrap.

One of the technique is to repair the defects is simply

RE-WELDING using nominal process parameter.

MULTI-PASS FSP/FSW


Brown et al. 2009 Significant reduction in feed force when welding is done over the previous weld. Grain size,hardness,temperature remains unaffected with passes. Gradual reduction of residual stress with increasing pass number.

Nataka et al. 2006 Reported an improvement in mechanical properties of Al die castingalloy of MP FSP compared to as-cast BM.

Ma et al. 2006 No effect of overlapping passes on size, aspect ratio or distribution of Sic particle while performed five pass with 50% overlap FSP on cast A356.

Leal et al. 2008 Used two different alloy. Quality and strength is not just a function of parameters but also depend on type of material and condition of treatment.

Surekha et al. 2008 Investigated that MP FSP showed better corrosion resistance compared to base metal irrespective of process parameters.

As FSP is one of the technique for grain refinement,removing flaws,defects,many researchers used MP FSP toimprove the properties of as-cast material.


Johannes et al. 2007 Create large area of super plastic materials with properties using MP FSP. Grain boundary sliding is the most important mechanism to achieve super plastic deformation.

Ma et al. 2009 Two pass FSP resulted in an enhancement in super plastic elongationwith a optimum rate in the nugget zone of the second pass and a shift to higher temperature in both central of second pass as well as transitional zone between passes.

Jana et al. 2010 All single pass runs showed some extent of abnormal grain growth which was removed with multi-pass.Higher rotational speed was found to be beneficial for controlling AGG.


Barmouz et al. 2011 Found that MP FSP reduces the Sic particle size, improve dispersion and separation of Sic particle by severe stirring action in the NZ.

Ni et al. 2011 MP overlapping FSP transforms the coarse cast NiAl bronze alloy (NAB) base metal to get defect free fine micro structure.

Izadi et al. 2012 Study the effect of MP FSP on distribution and stability of carbon nano-tube and to fabricate a MMC based on Al 5059 and MWCNTs.

ADVANTAGES OF CONTRA-ROTATING FSW TOOLS

New variant technique of FSW/FSP.

Requires less clamping and helps to work with high welding speed.

Resultant force counters each other so require low securing force.

Improves the weld integrity by disrupting and fragmenting the residualoxide layer remaining within the first weld region by the follower tool.

Weld over the first run produces further break-up and disposal ofoxides with no loss of mechanical properties.

Second tool does not have to robust as the leading tool.

Motion produced is similar to Re-stir ,but twin stir produces fastertravel speeds and in addition, efficiency of FSW can be furtherimproved with the use of two tools.

OBJECTIVE To determine the effect of two contra-rotating FSW tool

(Tandem Twin-stir) on the friction stir processing/weldingregion on different types of aluminium alloys.

To study the effect of contra-rotating tool on mechanicalproperties and microstructure.

To see the effect of twin tool and single tool with single as well astwo pass and compare the results.

To optimize the process parameters.

EXPERIMENTAL SETUP Fixture design

Fig. 7: Pictorial view of fixture (a) Fixture installed over milling machine bed (b) Welding plates clamped over fixture

EXPERIMENTAL SETUP

Fig. 8: Pictorial view of twin tool attachment(a) Old one (b) New one

(c) (d)

Fig.8(e) Inner assembly and (f) Inner isometric view

Additional changes made against old attachment

In the first attachment the main tool was tightened with the colletbed or hanger of the vertical milling machine. The single tool colletassembly is triangular push type, so when it is tightened from theupper side of the bolt it directly pulled the first tool, for that reasonthere was always a height difference between the two tool eventhough they are identical to each other. This problem is resolved inthe newly fabricated attachment.

In this case, the rotor plate of the attachment is directly tightenedwith the movement unit of the milling machine. So there is nocontact with the collet assembly of the machine and no heightdifference between the two tools.

In the first attachment two normal ball bearings was used near tothe collets of the two tools. But in new attachment four ZZ ballbearings is used, two pushed upward inside to the pressure plateand two to the upper side of the gear assembly. Two bearingssupport the secondary tool which helps in rotating the tool verysmoothly and independently without any vibration.

Distance between the two tools in the old attachment was 38.6 mmwhich is more than the new attachment and is 36.8 mm.

Collet length and hub unit is small as compared to the older onewhich eliminates the slack and vibration while running.

Contd.

WORK MATERIAL

Work piece material – commercially pure aluminium alloy

Work piece size – 200 mm x 80 mm x 2.5 mm

Chemical composition (weight %) of work piece material

Si Fe Cu Mn Mg Zn Ti Ga Na Others, eachRemainder

Aluminium

0.7055 0.831 0.00505 0.013 0.00465 0.0031 0.0048 0.0118 0.00245 Max. 0.05% 98.7

Mechanical properties of base metal

Yield Strength in MPa Ultimate strength in MPa Elongation in % ageHardness at 200 gmf load in

VHN

106.47 119.79 16.39 35-46 HV

TOOL MATERIAL

Tool material – SS316

Shoulder diameter – 16 mm

Pin diameter – 5 mm

Pin length – 2 mm

D/d ratio of tool – 3.2

Chemical composition (weight %) of Tool Material

SS316

Si P Mn Cr Ni Mo Fe

2.13 0.27 8.95 16.29 0.2 0.14 72.01

Fig. 9: FSP/FSW tool dimensions

PROCESS PARAMETERS

No of tools – 2

Rotational speed – 4

Welding speed - 3

Total experiments - 36

Process parameters Values

Rotational speed (rpm) 900, 1120,1400,1800

Welding speed (mm/min) 16,31.5,63

D/d ratio of tool 3.2

Pin length (mm) 2

Tool shoulder, D (mm) 16

Pin diameter (mm) 5

MEASUREMENTS Metallographic Observations (Macrostructure Analysis)

Fig. 10: Optical microstructure (LEICA DFC-295)

Fig. 11: Variable speed grinder polisher

Micro hardness

Fig. 12: Vickers micro hardness testing apparatus

Tensile test specimen

Fig. 13: Dimension of the tensile test specimen

Tensile properties

Fig. 14: (a): Universal Testing Machine (INSTRON) (b): Specimen mounted over UTM

RESULTS AND DISCUSSIONFollowing weld joints properties were studied:

Macrostructure analysis of welded samples

Micro-hardness

Ultimate tensile strength

Yield strength

% elongation

Joint efficiency

Macro and microscopic Study of fractured tensile test pieces using optical microscope and SEM

RESULTS AND DISCUSSION

Sl.

No

Sample

parameter

TT ST-SP ST-DP

1 900-16

2 1120-16

3 1400-16

4 1800-16

Tunnel at middle

Tunnel with pinhole

Pin hole at middle

Tunnel at upper side

Table 6.1: Effect of TT, ST-SP, and ST-DP on macrostructure of the FSW zones

Sl.

No

Sample

parameter

TwinTool Single Pass Multi Pass

5 900-31.5

6 1120-31.5

7 1400-31.5

8 1800-31.5

Elongated tunnel at middle

Small tunnel at middle

Tunnel at bottom

Tunnel defect

Worm hole at center

Contd.

Sl.

No

Sample

parameter

TwinTool Single Pass Multi Pass

5 900-63

6 1120-63

7 1400-63

8 1800-63

Defect free pinhole at bottom Pinhole at bottom

Defect free

Tunnel at middle

Defect free

Contd.

Defect free Defect free

friction stir welding report ent111

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