fsw by multi axial trqnsducer

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Measurement 41 (2008) 32–43

www.elsevier.com/locate/measurement

Friction stir weld process evaluation by multi-axial transducer

C. Blignault a, D.G. Hattingh a, G.H. Kruger a,*, T.I. van Niekerk a, M.N. James b

a Faculty of Engineering, P.O. Box 77000, Nelson Mandela Metropolitan University, Port Elizabeth, 6031, South Africab Faculty of Technology, University of Plymouth, Drake Circus, Plymouth PL4 8AA, England, UK

Received 7 July 2006; received in revised form 14 November 2006; accepted 4 December 2006Available online 23 December 2006

Abstract

Friction stir welding (FSW) is a solid state joining technique that is rapidly establishing itself as an approved productiontechnique and it remains the focus of significant international research effort. Scientific understanding of the FSW processis necessary for its successful automation. The interaction between the rotating tool and the alloy is complex, making thedetermination of the contribution of various process parameters (tool geometry, temperature, rake angle, speed and feed)to the desired weld properties difficult. The construction of a rotating multi-axial transducer has been realised. This allowsonline and offline measurement of process responses (force footprint, energy, temperature, etc.) and can be incorporatedinto a feedback control system for the processes technical and economic trade-off regulation. This paper describes thedesign, development and calibration of this multi-axial transducer. The application of the real-time process data to developthe force footprint as an aid for gaining scientific insight into FSW is also discussed.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Friction stir weld (FSW); Transducer; Force footprint

1. Introduction

This paper discusses the development of a rotat-ing multi-axial transducer to measure various loadinputs, torque and temperatures acting on the rotat-ing tool during FS welding. During welding theforces, torque and temperature on the tool fluctuate.Variation in these process variables potentiallyoffers valuable insights into FSW mechanisms andcan be used for process optimization if correlated

0263-2241/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.measurement.2006.12.001

* Corresponding author. Tel.: +27 41 504 3003; fax: +27 41 5049123.

E-mail address: [email protected] (G.H. Kruger).

with weld quality data (microstructure, hardness,etc.). Such data can then be used to interpret aspectsof the microstructure, residual stresses, and perfor-mance of FS welds through sensor fusion. Insightcan also be gained into ways to optimize the FSWprocess, which may have general applicability, andreduce the empiricism generally associated with FSwelding parameter selection.

The FSW process was developed and patented byTWI (The Welding Institute) in 1992 and is cur-rently being applied to an increasing number ofjoining applications worldwide, primarily in alumin-ium alloys. Several facilities have also reportedexperiments on titanium alloys and steels [1–3].FSW is an autogenous solid-state technique thatuses a non-consumable tool to generate frictional

.

mailto:[email protected]

C. Blignault et al. / Measurement 41 (2008) 32–43 33

heat and gross plastic flow of the material aroundthe tool. The joint is made via a keyhole processand has a complex flow pattern and microstructure.The materials to be joined are placed on a rigidbacking and clamped in a manner that preventsthe faying surfaces from separating. The rotatingtool takes the form of a shouldered pin, typicallybeing threaded and/or scalloped, which is slowlyinserted into the joint. The shoulder is plunged asmall distance into the material at a slight forwardrake angle and contributes large amounts of energyand forging pressure to the plastic deformation pro-cess. Kinetic friction between the material androtating tool generates heat energy allowing theplastic flow processes, responsible for welding, totake place around the pin [4]. A short dwell timeensures these processes have stabilized sufficientlybefore the traverse is begun. The tool is then tra-versed along the joint line while a relatively highaxial load (z-force) is maintained.

Fig. 1. Mechanical assembly of the measuring device w

A multi-axial transducer has been developed atthe Nelson Mandela Metropolitan University(NMMU) that measures, records and displays theseparameters online during the welding process. It isbased on a modified tool head and consists of sens-ing elements, tool holder, telemetry system, indus-trial computer based data recorder and GUI. Thecomplete mechanical assembly of the sensing ele-ment and telemetry system is illustrated in Fig. 1.The paper by Hattingh et al. introduced the conceptof a ‘force footprint’ for describing the resultanthorizontal force variation during a single revolutionof the instrumented FSW tool [5]. Essentially, thefrictional forces opposing the linear and angularmotion of the tool along the weld seam can berecorded and displayed in the form of a polar plotof resultant force versus instantaneous tool anglevectors. This presentation is useful as it is supposedthat the area of the polar plot is some measure ofthe energy input into the weld, while the angle sub-

ith the degrees of freedom shown in bottom left.

34 C. Blignault et al. / Measurement 41 (2008) 32–43

tended by the direction of travel and the force max-ima and minima is likely to reflect details of the weldflow mechanisms. The force transducer is alsodesigned to provide output data that will facilitatethe construction and on-line video rendering offorce footprint (FF) polar plots, as shown inFig. 9. The FF polar plot allows straightforwardvisual description during a single revolution of thetool, of the maximum force, its angle, and minimumforce. These variations, in principle, offer insightsinto formation of the macrostructure of the weld,e.g. the onion-skin structure observed in certainalloys [1]. FF information can also be used to assessthe effect of tool geometry on weld forces, energyinput and their relationship to FSW process param-eters, such as tool feed and speed. The FF can there-fore be used as a powerful graphical tool to advancethe understanding of process–structure–propertyrelationships in FSW.

2. Overview of the rotating transducer

Fig. 1 illustrates the 3D model of the multi-axialtransducer that measures these complex interactionsduring the weld process. Component 1 in the figureacts both as the tool drive from the machine spindleand as a carrier for the transducer assembly. Thebottom shoulder of this component attaches to therotor coil (Fig. 1, label 2) and telemetry electronics.Power is supplied to the telemetry electronicsthrough induction of a 50 Hz 30 V ac signal fromthe stator to rotor coil. Components 2, 3 and 5 formthe telemetry system providing a high-speed unidi-rectional data link between the rotating and station-ary components. This is achieved by capacitivecoupling of FDM (Frequency Division Multi-plexed) modulated signals. Component 4 is the elas-tic element (load cell), which senses forcemagnitudes, in each axis, by the mechanical dimen-sional variation of the strain gauge foil. The toolholder (6) provides a mount for the tool and trans-mits forces via displacement to the elastic element ofthe load cell. It also ensures that the elastic elementand associated electronic circuitry are not exposedto the full thermal loading during the process. Thetransducer is capable of measuring three orthogonalforces, axial moment, and tool and shaft tempera-tures of the welding tool. The system functions withfour degrees-of-freedom (DOF) namely Fx, Fy, Fz

and Mz, as indicated in Fig. 1.The forces and moment acting on the tool assem-

bly are measured by means of specially arranged,

precision foil-type strain gauges (HBM DK113/350 – bending, HBM XK 51 3/350 – compression,HBM XK11 3/350 – shear). These are connected in afull-bridge configuration, applied to the outer sur-face of the elastic element, on a common centre line.The full bridge configuration assists in compensatingfor superimposed stresses. Tool temperature ismeasured by means of a 0.5 mm type-K thermocou-ple (12-MK-0.5-Class 2 Simplex–I-176-50-250 mmTEF) calibrated up to 800 �C, shown in Fig. 3. Thesmall diameter ensures a rapid response time.Another type-K thermocouple is fixed on the outersurface of the elastic element, near the strain gauges.Principle design criteria were identified as being thatthe transducer should measure forces up to 60 kN onthe Fz-axis (perpendicular to joint line in xz-plane),8 kN on Fx (parallel to joint line in xy-plane) andFy-axes (perpendicular to joint line in xy-plane)and a torsion force (Mz) of 400 N m. These forcesare identified in Fig. 1). Channels 1 through 4 relateto the Fx, Fy, Fz and Mz axes respectively. The designcriteria were determined by empirical FSW investi-gation, using a less accurate prototype telemetry sys-tem [6,14]. To achieve the full potential of the FFtechnique in interpreting process–structure–prop-erty relationships, a more robust telemetry systemwas required which would provide data of high accu-racy and repeatability. The custom-made telemetrysystem was developed by Datatel and is capable ofsimultaneous transmission of eight sensor channelswith a bandwidth of 10 kHz each [7]. Six channelsare configured for strain measurement and two fortemperature. Fig. 10 illustrates the electrical controland monitoring architecture, including the telemetrysystem, of the FSW machine. The receiver and signalconditioner (Datatel dt2005) low-pass filters the sig-nal, with a cut-off frequency of 10 kHz, and has anoutput voltage span of ±10 V dc. The samplingspeed of the data-logger can be set anywhere up to10 kHz, however measurements are taken at 1 kHzto provide computationally manageable acquisition,post-weld data analysis and storage. This providessufficient samples for a high accuracy and resolutionFF. The data recorder uses a series of 12-bit ana-logue-to-digital converters to digitize the signals.Each binary data tuple is time-stamped by a micro-second counter, position-stamped (spindle angle,machine bed position) and buffered in RAM. Thisprevents data loss due to hardware or software jitterand allows the low-speed on-line visualization GUIstream to be generated. The raw high-speed bufferedstream is then spooled to non-volatile storage (hard

Fig. 2. Principal dimensions of the elastic element and axial positioning of the strain gauge elements.

Fig. 3. Typical tool design and thermocouple configuration used for butt-welding of 6 mm thick aluminium alloy plate.


disk). The data recorder software module exists inthe lower layer of the POSIX based soft-PLC archi-tecture developed for monitoring and control of theFSW process [13]. Both the high and low bandwidthsensor data streams are post-acquisition processedaccording to calibration and compensation equa-tions.

3. Load cell design

The transducer design was based on a straingauged hollow steel shaft acting as the elastic ele-ment in a load cell. This component can be replaced

or disassembled from the tool and data telemetryholder. The elastic element is machined from a chro-mium–nickel–molybdenum alloy steel (BS 970 817M40 – EN24) with a 0.2% proof stress of1125 MPa and a UTS of 1550 MPa. The elasticmodulus of the material is 205 GPa and the shearmodulus is 81.9 GPa. The principal dimensions ofthe elastic element as well as the axial positioningand channel number of the strain gauge elementsare shown in Fig. 2.

The load capability required from each measure-ment axis governed the construction and design ofthe elastic element and also defined the necessary


measurement sensitivity for each channel. In orderto ascertain the required ID (internal diameter)and OD (outer diameter) of the elastic element, itwas necessary to trade-off peak measurement strainand the corresponding output sensitivity for eachmeasurement channel.

A typical load cell transducer has a sensitivity of2000 lm/m = 1 mV/V [9]. The elastic element wastherefore sized to give measurement sensitivity closeto 1 mV/V. Table 1 indicates that the highest peakstrain sensitivity occurs in bending, followed bycompression and then shear. Since all the straingauge bridges are applied on a common cross-sec-tion, the requirement for high bending and com-pressive load capability limits the available shearchannel sensitivity. The spreadsheet calculationsshown in Table 1 indicate that an OD of 45 mmand an ID of 37 mm provides a sensitivity of0.523 mV/V in shear, 3.278 mV/V in bending and0.760 mV/V in compression. The design stresses inthe elastic element were also verified by FEA (FiniteElement Analysis).

Table 1 indicates the maximum design stress as apercentage of the alloy yield stress. The design stressincludes a stress concentration tolerance factor of

Table 1Calculated stress, strain and sensitivity values for the transducer design

Properties of elastic element (BS 970 817 M40-EN24)

E (GPa) 205 Bending gauge factor0.2% proof stress (MPa).

In ‘Z’ condition1125 Compressive gauge factor

G (GPa) 81.9 Torsion gauge factorPoison’s ratio 0.28 Cross. area (m2)OD (m) 0.045 lyy (m4)

ID (m) 0.037 J (m4)

Applied load Estimated sensor output

Vertical force, (N) Fz 60000 Compressive strain(micro), total indicated

Horizontal force, (N) Fx/y 8000 Shear strain (micro), totalindicated

Applied torque, (N m) T 400 Bending strain (micro),total indicated

Calculated FEA

Verification of structural integrity

Normal compressive stress(MPa)

116.45 148.07

Max shear stress (MPa) 41.17 53.08Bending stress (MPa) 329.39 512.84Combined stress, 1st

principal (MPa)449.61 535.66

1.95 to compensate for dynamic effects. Under typ-ical loading conditions the elastic element is sub-jected to a stress approximately 35% of the yieldstrength of the material.

4. Load cell calibration

To find the mathematical relationship betweenthe digital output from the data acquisition systemand the actual loads and temperatures, a calibrationprocedure had to be developed. Calibration wasdone with the system completely assembled to min-imize reproducibility errors. Each measurementchannel was cycled four times over its workingrange to determine the hysteresis and assess the lin-earity and repeatability of the monitoring system.Data from the other channels were recorded simul-taneously during these tests in order to establish anycrosstalk effects.

Avery torsion and compression testing machineswere used to calibrate the compression and torquechannels (channels 3 and 4, respectively). Fig. 4shows the setup used to calibrate the bending chan-nel. The applied load was amplified on a secondaryload cell by means of an extension rod. The loading

under maximum loading conditions

2.04 Fillet radius (m) 0.0032.09 Bend. K factor 1.95

2.08 Comp. K factor 2.20.000515221 Torsion K factor 1.71.09291E-07 Distance for bending

moment (m)0.2

2.18583E-07

1454.265 Sensitivity (m V/V) 0.760



Max design stress(adjusted with K factor)

% of yield for adjustedstress

325.75 28.96

90.23 8.021000.04 88.891179.3 104.83

Fig. 4. Experimental setup for bending 1 (ch 1) and bending 2 (ch 6).


and unloading procedure for torque, compressionand bending was conducted in 10 increments overa range of 400 Nm, 60 kN and 2500 N respectively.The calibration data obtained for each channel isshown in Fig. 5. Eqs. (1)–(4) show the third order

Fig. 5. Calibration curve used for each

polynomial equation that was fitted to obtain a rela-tionship between digital output and applied loading.This provides improved accuracy over the secondorder equation typically recommended in ASTME74 and E04 [10,11].

channel of the transducer output.

Table 2Data verifying calibration equations (bold values indicate maximum percentage error)

Ch 4 – Torque Ch 5 – Compression

Actual torque applied (N m) Formula results (N m) % Error Applied compressive load (kN) Formula results (kN) % Error

58.75 57.8 1.62 10 10.32 3.1

126.54 126.35 0.15 20 20.12 0.6180.77 180.1 0.37 30 30.15 0.5228.23 227.5 0.32 40 40.28 0.7302.8 304 �0.4 50 50.44 0.87409 409 0 60 60.61 1.01

Ch 1 – Bending 1 Ch 6 – Bending 2

Applied load (N) Formula results (N) % Error Applied load (N) Formula results (N) % Error

345.8 346.88 0.31 353.16 354.8 �0.46851.02 845.03 �0.7 712.45 714.2 �0.251145.32 1137.72 �0.66 1164.94 1169.72 �0.411665.25 1658.39 �0.41 1654.21 1664.5 �0.62

2203.57 2198.35 �0.24 2212.16 2224.05 �0.54


F xðNÞ ¼ 0:511938853xþ 1:46� 10�6x2� 1:62� 10�10x3

ð1ÞF yðNÞ ¼�0:642379856x� 1:21� 10�6x2þ 3:94� 10�10x3

ð2ÞF zðkNÞ ¼ 0:0148034x� 2:61� 10�7x2þ 2:17� 10�11x3

ð3ÞMzðN mÞ ¼ 0:0513037x� 2:64� 10�7x2� 2:28� 10�11x3

ð4Þ

In these formulae ‘x’ represents the binary valuethat corresponds to the physical quantity f(x), inN m, kN and N, respectively. These equations wereincorporated into the data-recording and visualiza-tion sub-systems. The calibration was verified usingknown loads and torques, as shown in Table 2.

5. Crosstalk compensation between channels

During the application of a uni-axial load to asingle channel, all other channels should ideallyregister zero loads. In practice, however, the chan-nels displayed a certain amount of crosstalk due toinduced voltages or loads. This crosstalk arises

Table 3Crosstalk effects for individual loading of each channel

Applied load

400 N m

Torque

Cross-talk Torque 100%Compression 5%Bending 1 1%Bending 2 2%

from strain gauge positioning artefacts andresponse of the elastic element under dynamic loadconditions. Table 3 indicates that the crosstalk out-put for the bending and compression channelsunder nominal output is within acceptable levels(1–2%), when a pure torque is applied to the trans-ducer shaft. When a pure compressive load isapplied the crosstalk experienced on the bendingchannels is similar in magnitude to the torquechannel. However, there is significant crosstalk onthe torque channel. A 3rd order compensationequation was used to correct this. From this datait was recognized that minimal crosstalk errorsarise on the bending output channels when torqueand compression loads are applied. Despite thestrain gauges being connected in full bridge config-urations it was still found that the transducer didnot completely compensate for superimposed bend-ing stresses. When a pure bending load was appliedthe torque and compression channels registered asignificant crosstalk output. A dynamic calibrationexperiment was performed to clarify the crosstalkrelationship for each channel during spindle rota-tion under only an applied bending load. This

60 kN 3.2 kN 3.2 kN

Compression Bending 1 Bending 2

28% 34% 34%100% 53% 53%1% 100% 100%2% 100% 100%

Fig. 6. Bearing test used to apply only a bending load on thespindle during one revolution.


dynamic test set-up will further be referred to as a‘bearing test’, illustrated in Fig. 6.

The strain gauge bridges were found to fullycompensate for these superimposed bending stressesonly at certain angles during a revolution and thatthe crosstalk voltages in channels 4 (torque) and 5(compression) were non-linear, as illustrated inFig. 7. The mathematical relationship for compen-sating between the applied bending load and theinduced compressive crosstalk needed to be found.Due to the complex non-linear nature of thesecurves this was not an optimum solution. Therefore,a novel algorithm was developed to compensate forthe induced crosstalk voltages. Initially three identi-cal ‘bearing tests’ were performed except for varyingloads. Fig. 8a and b illustrate the data recorded overa complete tool revolution. The induced load read-

Fig. 7. One revolution of torque (Ch 4) and compression (Ch 5) ve

ings vary in amplitude, but have constant zero-crossing phase angles with respect to the bendingchannels. It would seem that the zero-crossingpoints have a strong correlation with the straingauge positions on the circumference of the elasticelement.

An optical incremental encoder was coupled tothe spindle to provide direct feedback on its angularposition. This makes the recording of loads at spec-ified tool angles possible. A dynamically allocatedcircular buffer is used to store angle-force data pairs.After each spindle revolution the peak bendingforce is located and used as a reference. The pre-cal-culated constants for the force and torque zero-crossing points are then added to this reference.Torque and compressive force readings correspond-ing to each of the four zero-crossing points overevery revolution and their averages are calculated.These average values represent the true applied val-ues of compressive force and torque. The averagingcompensates for errors due to slight variations inbending peak detection. Table 4 demonstrates thesuccess of this compensation method in reducinginduced crosstalk load readings.

Table 5 provides a summary of the measurementerrors for the rotating multi-axial transducer. Dur-ing FS welding typical values for torque, compres-sion and bending on the tool were found to be60 N m, 10 kN and 3000 N, respectively, althoughthese values depend both on tool design and theselected welding parameters. In this table the errorsare summarized as a percentage of the FSO of thetransducer. Load readings obtained with the trans-ducer are sufficiently accurate and repeatable to

rsus bending (Ch 1), illustrating their non-linear relationship.

Fig. 8. Zero-crossing angles for crosstalk compensation. (a) Bending influence on torque, (b) bending influence on compression.

Table 4Crosstalk compensated measurements for torque and compres-sive loads during four ‘bearing tests’

Test no. Bending 2 (N) Torque (N m) Compression (kN)

1 3676.02 2.87 �0.442 3866.85 1.95 �0.67

3 4074.19 1.59 �0.554 3760.24 0.46 �0.37


allow detailed investigation into the utility of thepolar force footprint plot for FSW research.

6. Development of a polar plot of force footprint

While the sensor is rotating, an ideally asymmet-ric zero mean sinusoidal waveform is produced, the

actual waveform produced is illustrated in Fig. 8aand b. It is essential that a specific mechanical toolreference position with respect to the strain gaugesbe maintained during experimentation. When theabsolute values of this waveform are calculatedand plotted in polar format, the FF is generatedas illustrated by Fig. 9. FSW is an inherently asym-metric process due to the tool rotation and profilechanges. A retreating and advancing side of theweld run can therefore be identified (the advancingside is defined where tool rotational and feed veloc-ities are additive). The peak lobe forces in the FFand their angles relative to the direction of tool tra-vel could contain information reflecting weld flowprocesses and microstructure. In this work, theangle where the maximum lobe peak occurs istermed the characteristic force footprint angle. Ear-

Table 5Summary of errors showing overall measurement accuracy of the transducer

Error category(confidence limit 95%)

Ch 1Bending 1

Ch 4Torque

Ch 5Compression

Ch 6Bending 2

Ch 7 & 8Temperature

FSO 5100 N 400 N m 60 kN 5100 N 800 �CStandard deviation 0.04% 0.35% 0.56% 0.12% 0.26%Uncertainty 0.09% 0.85% 1.33% 0.29% 0.61%Non-linearity 0.08% 1.41% 3.92% 0.31% –Hysteresis 0.98% 2.44% 2.06% 1.91% –Repeatability 0.25% 1.71% 0.41% 1.39% 0.71%Cross-talk 0.22% 0.72% 1.12% 0.22% –

Fig. 9. Characteristic Force Footprints comparing dynamic calibration test with typical weld data.


lier work at the Nelson Mandela Metropolitan Uni-versity (NMMU) has indicated that the FF angle isinfluenced by tool design and weld process parame-ter selection [5]. Variations in its profile and anglecan be observed over a series of revolutions. Workby Blignault and Kruger has produced force foot-print videos, which graphically show these varia-tions during a weld run [8,12].

An important consideration during developmentof the FF was the correction of the FF angle at var-ious spindle speed settings. Since the transducer is

rotating and its angle is measured by means ofanother signal path (encoder) these signals have tobe synchronized in order to represent the true angleor direction of the applied load. A constant multi-path delay of 15 ms between the two signals (enco-der and transducer) is sufficient to cause a consider-able error. The encoder is interfaced directly to thedata recording system via an RS422 to TTL levelconverter, making position acquisition almostinstantaneous. However, the force readings mustbe acquired on the transducer, transmitted, received

Fig. 10. Architecture of FSW control and monitoring system.


and signal conditioned and then sampled by thedata recorder. This leads to a certain error in themeasured FF angle and its associated force. A spin-dle speed of 500 rpm has an angular error of about45�. It was found that the angular orientation of themaximum bending moment increases linearly withincrease in spindle speed and it can be compensatedusing Eq. (5):

Encoder count ¼ 0:68963� spindle speedðrpmÞð5Þ

This encoder count is used to rotate the FF in theopposite direction to that of the spindle rotation.This equation and compensation procedure was ver-ified through a bearing test performed at variousspindle speeds. During these tests a constant bend-ing load of 2500 N was applied and it was foundthat the FF angle remained constant at this angleregardless of the change in spindle speed. Fig. 9indicates that a slight eccentricity error within thespindle tool and holder causes a 150 N deviationin plot symmetry during a ‘bearing test’. The FFdata recorded during FSW using a typical tool(Fig. 3) is also shown in Fig. 9, and it can be seenthat the lobe angle has shifted through 45� relativeto the feed direction (0�). This rotation arises fromthe interaction between the tool and the alloy andis specific to weld process parameters and the toolgeometry. Further ongoing research is seeking todevelop fundamental understanding of this tool-al-loy interaction that will facilitate development ofanalytical models for the FSW process.

7. Conclusions

The real-time measuring device described in thispaper enables on-line sensing, transmission, pro-cessing, recording and graphical visualization ofmultiple process response variables during FSW. Itwill therefore be of significant use in overload pro-tection, for commercialisation of the process andin providing an understanding of tool-process inter-actions and dynamics. This type of multi-axialtransducer enables the development of force foot-prints, contains characteristic process information,and can be analysed to possibly yield a measure ofenergy input into the weld and insightful processknowledge. The peak magnitudes and angular posi-tions recorded for the FF lobes seem to closelyrelate with tool-weld parameters.

Multiple force and temperature responses duringwelding provide the possibility for automated intel-ligent control of the weld process and enable bettermodelling of the effects of process parameterchanges, current and optimal process states, interms of mechanical properties and the dynamicperformance of the FS welds, can be found. Thesehypotheses are being actively investigated at theManufacturing Technology Research Centre at theNMMU.

Acknowledgements

The authors would like to thank Datatel for themanufacture and support of the Telemetry system


hardware and the NRF (National Research Foun-dation of South Africa) for their financial supportof this research.

References

[1] W.M. Thomas, Friction Stir Welding, International PatentApplication PCT/GBG2 GB125978.9, December 6, 1991.

[2] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch,P. Temple-Smith, C.J. Dawes, Improvements relating tofriction stir welding, European patent specification 0615 480B1.

[3] D.J. McMullan, A.S. Bahrani, The mechanics of frictionwelding dissimilar metals, in: Second International Sympo-sium of the Japan Welding Society on Advanced WeldingTechnology, Osaka, Japan, August 25–27, 1975.

[4] W.M. Thomas, Friction stir welding and related frictionprocess characteristics. Seventh International Conference,Joints in Aluminum, Inalco, April, 1998.

[5] D.G. Hattingh, T.I. van Niekerk, C. Blignault, G. Kruger,M.N. James, Analysis of the FSW force footprint and itsrelationship with process parameters to optimise weldperformance and tool design, IIW Journal: Welding in theWorld 48 (1–2) (2004) 50–58.

[6] C. Blignault, Masters Thesis: Design, Development andAnalysis of the Friction Stir Welding Process, PE Technikon,January 2003.

[7] Strain and Temperature Measurement on a Chuck, inTelemetry User Manual, Datatel, Germany, May 2004.

[8] G. Kruger, Doctoral Dissertation: Intelligent Architecturefor Manufacturing Process Optimization, NMMU (2005).

[9] Guide to the measurement of Force, Institute of Measure-ment and Control (ISBN 0904457281) (1998).

[10] Standard Practice of Calibration of Force-Measuring Instru-ments for Verifying the Force Indication of TestingMachines, ASTM international standard E74-02, April 2002.

[11] Standard Practices for Force Verification of TestingMachines, ASTM international standard E4-01, May 2001.

[12] C. Blignault, Doctoral Dissertation: A Friction Stir WeldTool-force and Response Surface Model CharacterizingTool Performance and Weld Joint Integrity, NMMU (2005).

[13] G.H. Kruger, T.I. van Niekerk, C. Blignault, D.G. Hattingh,Software architecture for real-time sensor analysis andcontrol of a friction stir welding process, IEEE Africon2004, International Conference Centre, Botswana, pp. 461–466.

[14] G.H. Kruger, Masters Thesis: Intelligent Monitoring andControl System for the Friction Stir Welding Process, PortElizabeth Technikon, January 2003.

fsw by multi axial trqnsducer

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