Friction Stir Welding Final Project

FRICTION STIR WELDING: AN ENVIRONMENTALLYCLEANER WELDING PROCESS

Abstract: The respiratory effects often seen in full time welders include bronchitis, airway irritation, lungfunction changes, lung fibrosis, and a possible increase in the incidence of lung cancer. Traditionally, control offumes and gases has been by enclosure and local exhaust ventilation, respiratory protective equipment may alsobe necessary in certain circumstances, in particular in confined spaces. In this context, an environmentallycleaner process has been invented at The Welding Institute (TWI), UK, which is popularly known as FrictionStir Welding (FSW) process. This comparatively recent innovation has permitted friction technology to be usedto produce continuous welded seams for plate fabrication. Compared to many of the fusion welding processesthat are routinely used for joining structural alloys, FSW is an emerging solid state joining process in which thematerial that is being welded does not melt and recast. FSW is an environmentally cleaner process, due to theabsence of a need for the various gases that normally accompany fusion welding. FSW process produces nosmoke, fumes, arc glare and it is an eco-friendly welding process. This paper contributes on the variousapplications of FSW and related research findings in the field of materials joining.

Key Words: Friction Stir Welding, Aluminium, Magnesium, Stainless Steel, TensileProperties, Microstructure

1.0 INTRODUCTIONMore than 100,00,000 workers worldwide are currently employed full time as welders,while higher number of workers perform welding intermittently as part of their job. Anumber of epidemiologic studies have reported a higher incidence of respiratory illness inwelders. Respiratory effects observed in full-time welders (Refer Table 1) have includedbronchitis, airway irritation, metal fume fever, chemical pneumonitis, lung infection changes,a possible increase in the incidence of lung cancer, and small opacities on chest radiographsof asymptomatic welders. It is important to reduce welding fume toxicity and exposurewhenever possible.Traditionally, control of fumes and gases has been done by enclosure and local exhaustventilation, respiratory protective equipment may also be necessary in certain circumstances,

in particular in confined spaces. In accordance with good occupational hygiene practice,increasing attention has been paid to the investigation and development of consumables withminimum fume emissions and for the selection of process parameters, which minimizesemissions. Control at source by process modification has particular advantages in developingcountries, where there are limitations on the health and safety infrastructure and on theavailability of appropriate control technology compared to those in developed economies. Forexample, newly commercialized pulsed current power supplies alter the fume formed duringwelding. Pulse welding reduces the quantity of the welding fume and size of the particlesgenerated when compared with conventional spray welding. In turn, the potential of the fumeto affect the respiratory health of workers may be altered. Similarly, an environmentallycleaner process has been invented at The Welding Institute (TWI), UK, which is popularlyknown as Friction Stir Welding (FSW) process.

Table 1 Common Health Disorders due to Welding Emissionsslno

Health Disorders Welding Emissions

1 Acute inflammation of the Lungs, Severe disorder ofnervous sysem, Parkinson disease

Manganese

2 Acute and chronic intoxication, dermatitis and Asthma

Chromium

3 Potentially Carcinogenic and irritating respiratory track

Nickel

4 Irritation of nasal passages, throat and lungs Iron Oxide

5Severe Pneumoconious Aluminium

Oxide6 Pounding of the heart, a dull headache, flashes

before eyes,dizziness, ringing in the ears and nausea

CO

7 Irritation to eyes, nose and throat; Shortness of breath,chest pain and pulmonary edema

NOx

8 Irritation to eyes, nose and throat, Pulmonary edema andbone damage

Fluorides

The earliest reference to the use of frictional heat for solid phase welding and formingappeared over a century ago in a US patent. A period of 50 years then passed before anysignificant advancement in friction technology took place, namely a British patent in 1941that introduced what is now known as friction surfacing. Yet another 50 years went bybefore friction stir welding (FSW) was invented at The Welding Institute (TWI), UK. Thiscomparatively recent innovation has permitted friction technology to be used to producecontinuous welded seams for plate fabrication.Compared to many of the fusion welding processes that are routinely used for joiningstructural alloys, FSW is an emerging solid state joining process in which the material that isbeing welded does not melt and recast. Due to the absence of parent metal melting, the newFSW process is observed to offer several advantages over fusion welding. The benefits thatstand out most are welding of difficult to weld alloys, better retention of baseline materialproperties, fewer weld defects, low residual stresses, and better dimensional stability of thewelded structure. Above all, FSW is an environmentally cleaner process, due to the absenceof a need for the various gases that normally accompany fusion welding. FSW processproduces no smoke, fumes, arc glare and it is an eco-friendly process. Further, no consumablefiller material or profiled edge preparation is normally necessary.

Friction stir welding is a continuous, hot shear, autogenous process involving nonconsumablerotating tool of harder material than the substrate material. Fig. 1 explains theworking principle of FSW process. Defect free welds with good mechanical properties havebeen made in a variety of aluminium alloys, even those previously thought to be notweldable. Friction stir welds will not encounter problems like porosity, alloy segregation, hotcracking and welds are produced with good surface finish and thus no post weld cleaning isrequired

Fig. 1 Schematic representation of FSW principle

2.0 DEVELOPMENT OF FSW MACHINESEven a laboratory scale demo FSW machine itself costing more than Rs. 100 lakhs sinceit has to be imported from abroad and also due to the patent regulations governing theprocess. In 2004, we have indigenously designed a prototype Friction Stir Weldingmachine and developed with the financial support of All India Council for TechnicalEducation (AICTE), New Delhi in collaboration with R.V.Machine Tools, Coimbatore(Fig.2a). This was the first of its kind in this country. We have successfully weldedaluminium, magnesium and copper alloys upto 6 mm thickness without any technicaldeficiencies and the weld quality is much better than fusion welding processes. Since, thecapacity of the machine is low and it is difficult to weld mild steel and stainless steelmaterials and further it is restricted to low thickness materials and it is controlled by manualoperation.To overcome these technical deficiencies of the machine, in 2008, we have designed anddeveloped a low cost, large scale, computer numerical controlled, friction stir welding

machine with the financial support of Clean Technology Division, Ministry ofEnvironment & Forests, Government of India. The photograph of the newly builtcomputer numerical controlled eco-friendly welding machine is displayed in Fig. 2b. This isalso the first of its kind in this country. Using the newly developed machine, wroughtaluminium alloys, cast aluminium alloys, magnesium alloys, stainless steels were weldedsuccessfully without any defects and their mechanical and metallurgical properties wereevaluated and they are compared with other welding processes in this paper. Apart from this,the process can be used to perform friction stir spot welding, friction surfacing of materialsand dissimilar materials joining.

FSW m/c

3.0 FSW OF WROUGHT ALUMINIUM ALLOYThe rolled plates of the RDE-40 aluminium alloy (Al-Zn-Mg) were cut into therequired size (300 mm × 150 mm) by power hacksaw cutting and milling. A single ‘V’butt-joint configuration was prepared to fabricate GTAW and GMAW joints. Single-pass

welding was used to fabricate the joints. An AA5356 (Al-5%Mg) grade filler rod and wirewere used for the GTAW and GMAW joints, respectively. High-purity (99.99%) argon gaswas the shielding gas. The square butt-joint configuration was prepared to fabricate theFSW joints. A nonconsumable, rotating tool made of high-carbon steel was used tofabricate the FSW joints. The smooth (unnotched) tensile specimens were prepared toevaluate the yield strength, tensile strength, elongation and reduction in the cross-sectionalarea. The notched specimens were prepared to evaluate the Notch Tensile Strength (NTS)and notch strength ratio of the joints. The transverse tensile properties of the welded jointsare presented in Table 2.

Table 2 Transverse tensile properties of FSW joints of RDE-40 aluminium alloy

Joint Type

Yieldstrength(MPa)

Ultimatetensilestrength(MPa)

Elongation (%)

Reductionin c.s.a(%)

Notchtensilestrength(MPa)

JointEfficiency(%)

GMAW 151 204 8.2 5.36 208 53.26FSW 227 310 13.8 9.41 329 80.93

GMAW

FSW

Fig. 3 Optical micrograph of weld region of various joints and base metalOf the three types of welded joints, the joints fabricated by FSW exhibited higher strengthvalues and the enhancement in strength is approximately 34% compared to the GMAW jointsand 28% compared to the GTAW joints. Hardness is lower in the WM region compared to theHAZ and BM regions, irrespective of the welding technique. Very low hardness is recorded inthe GMAW joints (60 VHN) and the maximum hardness is recorded in the FSW joints (108VHN). The formation of fine, equiaxed grains and uniformly distributed, very finestrengthening precipitates in the weld region (Fig. 3) are the reasons for the superior tensile

properties of the FSW joints compared to the GTAW and GMAW joints. In CEMAJOR,AA1100, AA6061, AA2024, AA2219, AA7039, AA7075 grades of wrought aluminiumalloys were welded using FSW process and their mechanical and metallurgical propertieswere evaluated. The results were published in referred journals.

4.0 FSW OF CAST ALUMINIUM ALLOY

A356 is a kind of Aluminium-Silicon cast alloy used in food, chemical, marine, electricaland automotive industries. Fusion welding of this cast alloy will lead to many problems suchas porosity, micro-fissuring, hot cracking etc. However, friction stir welding (FSW) can beused to weld this cast alloy without above mentioned defects. An attempt was made to studythe effect of FSW process parameters on tensile strength of cast A356 aluminium alloy.Joints were made using different combinations of tool rotation speed, welding speed, andaxial force. The quality of weld zone was analysed using macrostructure and microstructureanalysis. Tensile strength of the joints were evaluated (Fig. 4) and correlated with the weldzone hardness and microstructure (Fig. 5). The joint fabricated using a rotational speed of1000 rpm, a welding speed of 75 mm/min and an axial force of 5 kN showed higher tensilestrength compared to other joints.

GMAW FSW

Macrograph of welded joints

Fig. 9 Optical micrographs

The tensile and impact properties of IF steel, FSW and GTAW joints were evaluated andpresented in Table 4. The joints fabricated by FSW process exhibited higher strength valuesand the enhancement in strength is approximately 24 % compared to joint fabricated byGTAW process. The formation of fine, equiaxed grains (Fig. 9) and relatively higherhardness of weld region and HAZ are the reasons for superior tensile and impact properties ofFSW joints compared to GTAW joints.

9.0 FRICTION STIR SPOT WELDINGFriction stir spot welding (FSSW) is a novel variant of linear friction stir welding processcreates a spot, lap weld without bulk melting. FSSW uses a cylindrical tool with pin tipcentered on one circular face. The tool rotates circumferrentially at room temperature andplunges into the samples to be joined with a normal force. There is a backening plate or ananvil on the bottom side of the sample to sustain the normal force. Fig. 12 explains theprinciple and various stages of FSSW process. There are numerous applications for FSSW,especially in the transportation industry, employing aluminum structures. Any applicationthat is currently riveted, toggle-locked or spot welded can often have FSSW substituted withlittle difficulty.Challenges with electrical resistance spot welding (ERSW) includes the need tochemically clean the aluminum alloys within 8 hours of joining, excessive electrodemushrooming causing poor welds to be made, process variability and shunting problemswhich require greater spacing of the welds and its application is limited to 0.25 – 4 mm.Challenges with the mechanical rivets include high cost for fasteners, potentially higher downtime due to feeding issues and need for other operations for non self-piercing rivets.Processes such as toggle-lock are simple and cheap but have less strength than ERSW. FSSW

is not saddled with the problems that are cited above due to the unique nature of the process.The speed of the process is competitive with ERSW but it is much more consistent becauseFSSW is not as sensitive to changing material conditions and surface conditions

Fig. 13 Photographs of FSSW joints

Fig. 13 displays the photographs of FSSW joint and lap shear tensile specimens.AA2024 aluminum alloy (Al- Cu) is very widely used in aerospace and transportationindustries. Single point joining of this alloy by Electrical Resistance Spot Welding (ERSW) iscumbersome. In CEMAJOR, AA1100, AA2024, AA6061 and AA7075 grades of wroughtaluminium alloys were welded using FSSW process and their mechanical andmetallurgical properties were evaluated. The results were published in referred journals.Though FSSW process is seems to be easy, the joint strength is controlled by manyparameters such as tool rotational speed, dwell time, plunge rate, plunge depth, tool pinprofile. The effects of tool rotational speed on lap shear tensile strength of FSSW AA2024alloy are presented in Table 7.

Table 7 Effect of tool rotational speed (Dwell time = 5 sec)

Rotational Failure patternSpeed(RPM)

Spotprofile

Failure patternUpper

Failure patternLower

Lap shearforce &Hardness ofstir zone

1300 8.62 kN128 Hv

1400 9.31 kN197 Hv

1500 8.52 kN178 Hv

The tool rotational speed and dwell time have significant influence on lap shear strength ofthe joints. The joint fabricated using a tool rotational speed of 1400 rpm and a dwell time of 5seconds showed the highest lap shear strength than other joints. The formation of finer grains

in the Stir Zone, higher hardness at the interface and sufficient amount of material flowbetween two mating surfaces are the main reasons for superior performance of the abovejoint.

10.0 FSW OF DISSIMILAR MATERIALSFriction stir welding has a great potential for joining dissimilar materials such asdifferent grades of aluminium (Al) alloys, Al alloys to steels and aluminium alloys tomagnesium (Mg) alloys. However, fusion welding of Al and Mg alloys always producescoarse grains and large brittle intermetallic compounds in the weld metal region.Conventional welding processes such as GTAW, LBW and EBW can be applied to joindissimilar Al and Mg alloys. However, because of high reflectively of Al and Mg alloys, theenergy efficiency of LBW is low. Low melting elements such as Mg and Zn causesevaporation during EBW. High heat exchange ratio cause wider weld bead and coarsergrains in GTA welding. FSW can avoid many problems associated with fusion weldingprocesses, thereby defect free welds having excellent properties can be produced even insome materials with poor fusion weldability. To explore the potential advantages in joiningdissimilar Al-Mg alloys by FSW, an investigation was carried out to study the microstructureand mechanical characterization of friction stir welded AZ31B Mg alloy and AA6061 Alalloy joints. Fig. 14 displays the optical micrographs of various regions of Al-Mg dissimilarjoint fabricated using a tool rotational speed of 400 rpm and a welding speed of 20 mm/min.

Fig. 14 Micrographs of various region of dissimilar joint of AZ31B Mg - AA6061 AlOf the five tool rotational speeds used (varying from 300 rpm to 600 rpm), the jointfabricated using a rotational speed of 400 rpm yielded a maximum tensile strength of 192MPa and joint efficiency of 89 % compared with the weaker base metal. Complexintercalated microstructures in the weld zone, with swirls and vortices indicative of the flowpattern of the dissimilar metals (Fig. 14). Complex intercalated microstructures in the FSWzone contribute to elevated hardness readings in the weld zone and the EDS analysis showsthe mixing of the materials in the weld zone

11.0 CONCLUSIONSIn, the Friction Stir Welding (FSW) process wasused to weld different materials such as wrought aluminium alloy, cast aluminium alloy,magnesium alloy, IF steel, mild steel and stainless steel and the important conclusions are:1) The wrought aluminium alloy joints fabricated by FSW exhibited higher strengthvalues and the enhancement in strength is approximately 34% compared to the GMAWjoints and 28% compared to the GTAW joints.2) Defect free weld region, higher hardness of weld region and very fine, uniformlydistributed eutectic Si particles in the weld region are found to be the importantfactors attributed for the higher tensile strength of the cast aluminium alloy jointsfabricated by FSW process.3) The magnesium alloy joints fabricated by LBW exhibited higher strength values, andthe enhancement in strength was approximately 14% compared to GTAW joints and 2% compared to FSW joints. FSW process is cheaper compared to LBW process.4) The IF steel joints fabricated by FSW process exhibited higher strength values and theenhancement in strength is approximately 24 % compared to joint fabricated byGTAW process.5) Tensile strength and hardness in mild steel joints indicated the overmatching offriction stir welded joints compared with the base metal. The joint efficiency wasfound to be 108 %. This is due the fine equiaxed structure of ferrite and pearlite of thestir zone.

6) The stainless steel joints fabricated by FSW exhibited higher strength and theenhancement in strength is approximately 40 % compared to CCGTAW joints, and 35% compared to PCGTAW joints.7) The tool rotational speed and dwell time have significant influence on lap shearstrength of the friction stir spot welded joints. The joint fabricated using a toolrotational speed of 1400 rpm and a dwell time of 5 seconds showed the highest lapshear strength than other spot welded joints.8) All the dissimilar welds of Al and Mg exhibited dynamic recrystallisation of the base

materials. Dynamic recrystallisation was enabled by the frictional heat from the toolshoulder and tool nib, heat generated by mechanical string of the materials by the nib,mostly adiabatic heat contributing to DRX through deformation.9) From various investigations, it is found that the friction stir welding (FSW) processdidn’t produce gaseous emission, particulate emission and radiation during welding ofabove materials and hence it could be very much called as Eco-Friendly Weldingprocess. Moreover, the joints fabricated by FSW process exhibited superiormechanical and metallurgical properties compared to other conventional weldingprocesses.

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6. Jayaraman, M., Sivasubramanian, R., Balasubramanian, V., Babu, S. Influences ofprocess parameters on tensile strength of Friction Stir welded cast A319 aluminiumalloy joints 2009, Metals and Materials International 15 (2), pp. 313-3207. Jayaraman, M., Sivasubramanian, R., Balasubramanian, V. Establishing relationshipbetween the base metal properties and friction stir welding process parameters of castaluminium alloys 2010, Materials and Design 31 (9), pp. 4567-45768. Karthikeyan, L., Senthilkumar, V.S., Balasubramanian, V., Arul, S. Analysis of firstmode metal transfer in A413 cast aluminum alloy during friction stir processing2010, Materials Letters 64 (3), pp. 301-3049. Padmanaban, G., Balasubramanian, V. Fatigue performance of pulsed current gastungsten arc, friction stir and laser beam welded AZ31B magnesium alloyjoints 2010,Materials and Design 31 (8), pp. 3724-373210. Padmanaban, G., Balasubramanian, V. An experimental investigation on friction stirwelding of AZ31B magnesium alloy 2010, International Journal of AdvancedManufacturing Technology 49 (1-4), pp. 111-12111. Karthikeyan, R., Balasubramanian, V. Predictions of the optimized friction stir spotwelding process parameters for joining AA2024 aluminum alloy using RSM 2010,International Journal of Advanced Manufacturing Technology , pp.1-1112. S. Malarvizhi and V.Balasubramanian, Microstructure characteristics, tensileproperties of dissimilar friction stir welded AZ31B mg alloy and AA6061 alloy joint.(Accepted for Presentation), National Welding Seminar (NWS 2010) to be held atviskapatnam during 21 – 23, December 2010.