Owingto the fact that, it is a mechanical solid state welding technique, it can alsobe applied under water. The welding speed is dependent on the thickness of theplate to be welded.
For thicker plates double sided friction stir welding canbe applied. (Refer fig.3)11Thedesire for better standard of living resulted in continuous development of theexisting manufacturing technologies. Particularly friction stir welding wasdeveloped for welding non-weldable metals and alloys.
The metals which cannotbe welded by conventional technique. Apart from that friction stir welding hadan enormous growth as we can weld similar as well as non-similar metals,polymers, 3D printed objects, etc. Trends in Friction Stir Welding: Verysuitable for robotics and automation applications. High strength alloys like 2XXX and7XXX series alloys are classified as non-weldable by fusion welding technique.So when FSW was invented, it opened new opportunities to weld high strengthaluminum alloys. Rajeev Mishra FSP has replaced riveting technology. FrictionStir Spot welding is used in Automobile technology. Friction Stir Welding hasgrown into an important industrial process that has led to many worldwideapplications, predominantly in the fabrication of aluminum components andpanels.
FSW is being used in all transport industries including shipbuilding,automotive, rail and aerospace industries 10. In shipbuilding industry, theFSW process is used for the production of large aluminum panels, which are madefrom aluminum extrusions. Large tanks for satellite launch vehicles arefabricated by FSW from high-strength aluminum alloys for the aerospaceindustry. Several companies (The Boeing Company, Eclipse Aviation Corporation,Airbus etc.
) manufacture lightweight aluminum airframe structures, fuselage andwing applications using FSW for commercial and military aircrafts. The railwayindustry uses FSW for the production of large prefabricated aluminum panels,which are made from aluminum extrusions. The automotive industry uses FSW inthe production of components like light alloy wheels and fuel tanks.Weldability is one of the most important factors determining the application ofnovel materials.
APPLICATIONS: Corrosion may be a serious issueduring FSW, particularly during unprotected dissimilar alloy combinations. Thisis because metallurgical and microstructural zonal heterogeneities developedduring welding process. The development of pronounced zonal heterogeneities (ordistinctly different metallurgical and microstructural zones) will, mostlikely, lead to the formation of micro-galvanic corrosion cells in the weld.
Pronounced zonal heterogeneities is inevitable in welds involving dissimilarmaterials. The galvanic-coupling effect between the zones can be catastrophicif the materials are both conductive and have widely separated corrosionpotentials. Consequently, it is very important to establish the extent of zonalheterogeneities in the microstructures of welds/dissimilar welds relative tothe welding parameters, the processing history and the compositions of thealloys involved in order to improve in service performance where no additionalcorrosion protection is present (e.g. paint scheme) 2. CORROSIONSUSCEPTIBILITY: Theprocess of friction stir welding is reasonably free from defects which are inexorableto occur in fusion welding processes 1.
Even though defects in friction stirwelded samples are experienced which are the outcome of improper selection ofcombination of different process parameters, the defects found in friction stirwelded samples are mostly subsurface 2. This brings the limitation to visualinspection methods for identification of defects in the welded samples.Available non-destructive methods are applicable in detection of internaldefects but these methods need high investment and precise experience for theanalysis of the collected information. Most of the researchers contributedtowards finding the optimum range of different process parameters to avoiddefects.
Contributions toward detection of defects in friction stir weldingthough non-destructive manner is less. Hence, in the present research work, anattempt has been made to develop procedure for defect identification infriction stir welded samples. Vertical force, traverse force, tool rotationalspeed and main spindle motor current signatures are the four process signalsacquired and analyzed for the same. Signal information extracted though fractaltheory will provide an indicator based on which conclusions can be drawn forsamples to be defective or defect free Defects:6 Oneof the important characteristics of FSP is the different relative speed ofplastic material from advance side and retreating side which results indifferent microstructure. Advance side the speed is greater than the retreatingside, microstructure changes rapidly. Due to lack of necessary transition, thezone between the nugget and TMAZ often has poor mechanical properties, whichcan be tested by mechanical examination. Pin Geometry has significant effect onmechanical properties and joint structure.
Each pin has different transfermanners. It can be seen from the experiment that Void defects were created whenwelded by the pin without screw threads under this weld parameter. Aftertensile test cracks often arise upside voids.5Influenceof Pin Geometry on Bonding, Mechanical Properties and Material Flow: · Relativelylow speed of welding. · Powerfulplates are required for thicker plates.
· Plateshave to be held firmly.· Cannotbe applied for every material. It can only be applied for materials having lowstrength and low melting point( higher melting point materials require specialtools). · Exitholed left when the tool is withdrawn· Criticaltolerances · Lessflexibility as compared to that of arc welding, i.
e. difficulties associatedwith non-linear weldsDIS-ADVANTAGES: · A greenprocess Friction Stir Welding is environmentally friendly, with a process thatfeatures low energy input and requires no consumables, flux, filler material,or shielding gases to run, like conventional welding methods. Friction StirWelding also does not emit smoke, fumes, or gases that need to be exhausted onthe back end.· Joindissimilar alloys Friction Stir Welding may be used to weld dissimilar alloys – evencombinations not compatible with conventional welding methods.· Superior mechanical characteristics Friction Stir Weldingproduces a weld with high weld strength and toughness, plus a fine grainstructure that resists fatigue stress. Due to the low heat and smallheat-affected zone, there is minimal distortion of the joined parts, reducingthe costs associated with preparing the part for subsequent use.
· Limitless panel length and width for large projects: Theflexibility of our Friction Stir Welding process means we can accommodate thewelding of large parts. · Virtuallydefect-free bonding: As a solid-state process, Friction Stir Welding eliminates many of thedefects associated with conventional fusion welding techniques such asshrinkage, solidification cracking, and porosity. The bond between the twopieces is made solely of the original material, giving it similar strength,bending, and fatigue characteristics of the parent material.· Providesopportunities for new solutions to old joining problems: The leading-edge technologyof Friction Stir Welding allows us to continually identify new joiningapplications for extrusions, castings, plate, and sheet for customers rangingfrom railcars to aerospace.
Advantages: Defects:Insufficient weld temperature, resulting from lower rotation speed and hightransverse speed, long tunnel which may be produced on the surface orsubsurface. Lower temperature may also reduce the forging action of the tooland reduce the continuity of the bond from each side of the weld. Light contactbetween the materials has given rise to the name kissing bond for such cases.
This type of defect is extremely difficult to detect by non-destructivetesting. Lack of penetration defect Friction Stir welding has severaladvantages over fusion welding methods as any probable defects from coolingfrom liquid phase are immediately avoided. FSW has found to be to produce lowconcentration of defects and vary tolerant in variation of parameters andmaterials. ADVANTAGESAND DISADVANTAGES OF FRICTION STIR WELDING: During FSW/P, high temperature and workloadexperienced by the tool material results in high amount of material wear andconsiderable damage to material. Failure of the tool as well as tool wear arethe most common issues observed in fabrication of surface composites due tohard reinforcement particles during Additive Friction stir Processing (AFSP).Tungsten-rhenium (W-Re), Iridium Rhenium, cobalt alloys, tungsten carbide andPolycrystalline Boron Nitride (PCBN) are employed successfully for hard alloyssuch as brass, Titanium based alloys and steels due to its high thermomechanical performance.
However, low wearand good weld quality along with some other factors such as strength, hardness,reactivity with work material and ductility that may influence tool materialselection.Tool material Toolpin is responsible for plasticized material flow by stirring action in thejoint area. Pin diameter, surface profile and pin length are important parts ofthe tool pin. Pin length affects the penetration level of plasticized materialin nugget/ stir zone. Tool pin length is generally kept 0.
2 to 0.3 mm less thanthe workpiece thickness so that the shoulder can get proper contact with theworkpiece by giving ppropriate axial plunge load 5. Pin diameter and surfaceprofile features affect the size of stir zone, microstructure and materialflow. Zhao et al. 38 studied experiments with three different tool pinprofiles, threaded cylindrical, taper cylindrical and straight cylindrical andconcluded that, the taper pin is most effective to attain high strengthdissimilar Cu-Al FSW joint.
Selection of the optimum tool pin profile and itsdimension is one of the active research area for dissimilar Cu-Al FSWjoint. The relation between pin andshoulder dimension is defined as shoulder to pin diameter ratio (SPR). SPR ofdissimilar Cu-Al FSW system depends on the type and thickness of alloys beingjoined. However, general range of 2:1 to 5:1 is noted by researchers (referTable 1). This mentioned range of SPR is relatively higher than the similarmaterial FSW system. In a dissimilar system of Cu and Al materials (which ishaving different thermal conductivities and specific heats), the SPR should besuch that the thermal input and the distribution of it can be maintained.
So,by keeping relatively higher SPR, the thermal input can be raised in anappropriate way, and at the same time, the distribution of it can be managed bythe other process parameters such as tool pin offset, position of the workpiecematerial, rotational speed and welding speed. In addition to this, higherDownloaded by University of Colorado – Health Science Library at 04:18 31March 2015 thickness of workpiece materials requires larger SPR because pin ismore responsible for thermal input in case of higher thickness system.Tool Pin 22,29, 69, 70. Conical angle depends on the thickness of workpieces and thediameter of the shoulder. Optimum shoulder design for dissimilar Cu-Al FSW systemmay still consider for strenuous research interest because of limited researcharticles. materialdownward, which, in turn give good surface morphology. But, large amount ofIMCs are formed when scrolled profile is used. These IMCs are responsible toincrease hardness and brittleness in the stir zone and that also causesdefects.
So, scroll surface profile should not be recommended to achieve sounddefect free dissimilar Cu-Al FSW joint. Conical and flat surface shoulderfeatures are favourable profiles for dissimilar Cu Al FSW system. Conical shapearound 2-10? cavity helps to push the material downward through centrifugalforce which gives the proper material flow for joint formation 12-14, scrolled,ridges, grooves, concentrating circles (shown in Fig. 2) can be provided toincrease material deformation and uniform mixing in FSW 5. Selection ofproper shoulder geometry/ feature depends on the workpiece and tool materialsas well as workpiece thickness. For dissimilar Cu-Al FSW system, the toolshoulder geometries and profile affects the material flow, formation of IMCsand mechanical properties of the joint. Galvao et al.
13 reported that,scrolled shoulder is used to force Cu-Al mixed InFSW, the shoulder diameter is maximum responsible for heat generation. It hasbeen found that the shoulder generates around 87% heat by rubbing actionbetween the shoulder surface and the workpiece 9. Tool shoulder diameter andgeometry/surface features affect the quality of weld in FSW as it contributesto maximum heat generation. For achieving good quality FSW joint, the optimum shoulderdiameter is one of the important parameters needed into consideration beforethe welding 5. Tool shoulder diameter affects the peak temperature variation,material deformation, plunge load variation, mechanical properties,microstructural variation and formation of intermetallic compounds (IMCs) indissimilar Cu-Al FSW system. Akinlambi et al.
29 claimed that uniform mixingbetween Cu and Al with a proper material flow pattern can be obtained with 15mm and 18 mm diameters while improper material mixing was observed with 25 mmdiameter for 3.175 mm thick dissimilar (AA5754-C11000) FSW system. Maximumtensile strength (of 208 MPa) and a minimum tensile strength (of 171 MPa) arereported at 18 mm and 25 mm shoulder diameters respectively. Moreover, thelayer of IMCs is found thicker when the larger shoulder diameter (i.e.
25 mm)was applied. Higher hear Downloaded by University of Colorado – Health ScienceLibrary at 04:18 31 March 2015 input is responsible for the thick IMCs layer,material deformation and mechanical properties which consequently deterioratethe material flow of Cu fragments and Al matrix. So, the micro-hardness variesas the shoulder diameter changes. Three shoulder geometries such as concave,convex, flat with special profile features like Tool Shoulder: partsare shoulder diameter, shoulder surface angle, pin geometry, including itsshape and size, and the nature of tool surfaces 11. Tool design and geometryaffects the heat input, force & torque variations and plasticized materialflow in FSW technology 7. Different tool designs and geometries fordissimilar Cu-Al FSW system are discussed as below.
TheFSW tool has two basic parts: (I) Pin and (II) shoulder. Important elements ofthese Tool Geometry: ToolShoulder-the region of tool in contact with work piece surface. To enhance thematerial flow, the tool shoulder can have negative or positive scrolls.Negative Scrolls is a depression in shoulder surface and work piece materialfills this. A positive scroll is protrusion on shoulder surface. Tool pin alsocalled as probe is inserted in workpiece and influences horizontal materialflow from front to back as well as vertical material flow from top to bottom.
Advancingside tool pin surface rotation direction and traverse direction have samevectorial sense. Because of the tools forward movement, the material wants toflow back, but the pins surface rotation opposes the flow on this side of thetool. Retreating side: the tool pin surface rotational direction and tooltraverse direction have opposite vectorial sense. The material flow is easierin this side of the tool pin as the pin surface helps the tool material flow backwards.Friction Stir Welding of aluminum alloys. Aluminum is the third largest elementavailable on the Earth’s crust and second highest consumed element by weight.Low density combined with attractive strength properties of modern Al alloysmakes it ideal for structural applications.
There are different combinations ofelements used in aluminum alloys used for strengthening. Mainly they can beclassified as age hardening, work hardening and casting alloys. PrincipleFriction Stir Welding variables which are under operator control include tooldesign and tool movement parameters. Machine characteristic, workpiece thicknessand control mechanism will also affect the quality of the weld.