Friction the last two decades. Friction stir

FrictionStir Welding of Dissimilar metals    ABSTRACT:             Friction stir Welding is an upgradedversion of conventional welding process.

It is considered one of the mostimportant one during the last two decades. Frictionstir welding (FSW) is a revolutionary green manufacturing technique due to itsenergy efficiency and environmental friendliness. FrictionStir Welding is more of a forging process than an actual welding process, andsimply uses a spinning pin tool and axial forge force to create a bond betweentwo pieces. FSWparameters such as tool design, tool pin offset, rotational speed, weldingspeed, tool tilt angle and position of workpiece material in fixture fordissimilar Cu-Al system are summarized in the present review paper.Additionally, welding defects, microstructure and intermetallic compoundgeneration.Keywords: Friction Stir welding (FSW), TMAZ (Thermo-MechanicallyAffected Zone), Heat Affected zone (HAZ),  INTRODUCTION: FrictionStir Welding is one of the new off-center welding techniques which has made extraordinaryprogress in Welding technology. Friction Stir Welding was invented at TheWelding Institute (TWI) in United Kingdom in 1991 as a solid state weldingtechnique and was primarily applied to Al alloys. Prevalence of non-heattreatable and powder metallurgy Al alloys to which fusion welding cannot beapplied.

The spot light has been turned to Friction Welding as the joiningtechnology that do not have welds normally associated with fusion weldingprocess 3. Al alloys to which fusion welding cannot be applied. FSW offers alot of advantage over conventional fusion welding process because of its lowheat input and absenteeism of solidification process. Among the manymanufacturing technologies joining has been identified as key enablingtechnology to innovation and sustainable manufacturing.

AsFSW is a solid state welding process it has good dimensional stability, no lossof alloying element, excellent mechanical properties in the mechanical area,fine recrystallized microstructure, no solidification cracks.Joiningof dissimilar materials by any welding process is always difficult because ofthe enormous differences in mechanical and metallurgical properties. The jointsof dissimilar materials are increasingly employed in different sectors ofindustries due to its technical and economic advantages.FSWhas environmental benefits as no shielding gas is required, no filler materialneeded, no detrimental emissions, abolishes grinding waste, etc.                Fig. 1: Principle of Frictionstir welding/processing. Literature reviewFRICTIONSTIR WELDING PROCESS:            It consists of a rotation tool whichis pressed against the surface of two abutting or overlapping plates, betweentwo severely clamped substrates on a backing plate support.

  Friction stir welding can be divided intothree stages: Plunge, Dwell and Traverse. A cylindrical tool rotating at highspeed is slowly plunged into the plate material, until the shoulder of the tooltouches the upper surface of the material. After piercing the tool at therequired depth is held in that position for 5-20 seconds. A downward force isapplied to maintain the contact. Frictional heat, generated between the tooland the material, causes the plasticized material to get heated and softened,without reaching the melting point.  Thetool is then traversed along the joint line, until it reaches the end of theweld. As the tool is moved in the direction of welding, the leading edge of thetool forces the plasticized material, on either side of the butt line, to theback of the tool. In effect, the transferred material is forged by the intimatecontact of the shoulder and the pin profile.

It should be noted that, in orderto achieve complete through-thickness welding, the length of the pin should beslightly less than the plate thickness, since only limited amount ofdeformation occurs below the pin. The tool is generally tilted by 2-4°, tofacilitate better consolidation of the material in the weld. Upon reaching theend of the weld, the tool is withdrawn, while it is still being rotated.

As thepin is withdrawn, it leaves a keyhole at the end of the weld.  This is the main disadvantage of FSW and fewvariants are being used to overcome this aspect.            MICRO-STRUCTURE:            Joint material can be separated intofour visually distinct micro-structural zones according to how they areaffected:Thesezones can be famed as follows:1.SZ: Stirred Zone or weld nugget, which is the central area where the pintraverse between both sheets.

This region is characterised by a fine graineddynamically recrystallized structure.2.TMAZ: Thermally Mechanically Affected Zone, on each side of SZ (these zonesseem to be more narrow in the unconventional – bobbin tool – conditions).3.HAZ: Heat-Affected Zone, situated between the minimum of hardness and the basematerial. Fig.

2: Schematic of thecross-section of the friction stir weld showing the various characteristicregions 

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