1. INTRODUCTION Friction stir welding is a solid state joining process which was invented at The welding Institute (Cambridge, UK) in 1991[1,2].The basic concept is relatively simple which utilize a non consumable rotating tool consisting of a concentric threaded tool pin and tool shoulder, as shown in fig-1. FSW produces a strong metallurgical bond where the severe plastic deformation is caused by the tool pin that plunges or penetrates into the work piece material and travels along the weld line. The shoulder generates the frictional heat which rides on the surface of the work piece. The tool serves three main purposes: (1) Heating of the work piece by friction and severe plastic deformation. (2) Movement of the material to produce the joint. (3) Containment of the hot metal below the tool shoulder. As a result, a solid state joint is produced with no melting. Because of various geometrical features on the tool, material movement around the pin is very complex,ref[3] Fig 1.Schematic representation of FSW[4] The direction where the tool rotation is same as the tool travel direction is the advancing side of the work piece and where the direction is just opposite is the retreating side. The FSW process can be subdivided into 3 phases: (1) the plunge phase or the initial period. (2) The main phase or the steady state period. (3) The termination phase or terminal period. During plunge phase, tool pin plunges axially into the work piece at a specific rate, where the temperature distribution within the welding tool and work piece is established. Once the tool plunges into the work piece, the tool starts moving along the joint line and a steady state period starts where the temperature distribution is uniform. During the terminal period, the tool is withdrawn from the work piece leaving an exit hole at the end of the weld. 1.1 Weld Zone - Thread gill divide the FSW weld zone into 4 distinct regions: the nugget zone (NZ), the thermo mechanical zone (TMAZ), the heat affected zone (HAZ) and the unaffected base metal (BM) [4].The nugget is the fully re-crystallized area, which undergoes severe plastic deformation and frictional
Transcript
1. INTRODUCTION
Friction stir welding is a solid state joining process which was invented at The welding Institute (Cambridge, UK) in 1991[1,2].The basic concept is relatively simple which utilize a non consumable rotating tool consisting of a concentric threaded tool pin and tool shoulder, as shown in fig-1.
FSW produces a strong metallurgical bond where the severe plastic deformation is caused by the tool pin that plunges or penetrates into the work piece material and travels along the weld line. The shoulder generates the frictional heat which rides on the surface of the work piece. The tool serves three main purposes: (1) Heating of the work piece by friction and severe plastic deformation. (2) Movement of the material to produce the joint. (3) Containment of the hot metal below the tool shoulder. As a result, a solid state joint is produced with no melting. Because of various geometrical features on the tool, material movement around the pin is very complex,ref[3]
Fig 1.Schematic representation of FSW[4]
The direction where the tool rotation is same as the tool travel direction is the advancing side of the work piece and where the direction is just opposite is the retreating side. The FSW process can be subdivided into 3 phases: (1) the plunge phase or the initial period. (2) The main phase or the steady state period. (3) The termination phase or terminal period. During plunge phase, tool pin plunges axially into the work piece at a specific rate, where the temperature distribution within the welding tool and work piece is established. Once the tool plunges into the work piece, the tool starts moving along the joint line and a steady state period starts where the temperature distribution is uniform. During the terminal period, the tool is withdrawn from the work piece leaving an exit hole at the end of the weld.
1.1 Weld Zone -
Thread gill divide the FSW weld zone into 4 distinct regions: the nugget zone (NZ), the thermo mechanical zone (TMAZ), the heat affected zone (HAZ) and the unaffected base metal (BM) [4].The nugget is the fully re-crystallized area, which undergoes severe plastic deformation and frictional
heating during the process and is located at the centre of the stir zone. The TMAZ is the zone between the parent metal and the stir zone. This zone mainly consists of elongated parent metal grains that have deformed in an upward flowing pattern around the nugget zone. In TMAZ zone, recrystallization does not occur because of limited deformation.HAZ is located in between parent metal and TMAZ zone, which experiences a thermal cycle but doesn’t undergo any plastic deformation. In this zone the grains are similar to that of base metal, however coarsening of the precipitates occurs. Beyond the HAZ, base metal zone exist, which may have experienced a thermal cycle from the weld but the micro structure and mechanical properties is not affected by the heat input.
Fig-2 Schematic cross-section of a typical FSW weld showing four distinct zones (A) Base Metal (B) Heat-affected (C) Thermo-mechanical affected (D)Stirred (nugget zone).Source:Ref [6]
1.2 Advantages of FSW over other welding process-
In contrast to conventional fusion welding, FSW is a solid state joining process with no melting during the process. Poor solidification cracking, porosity ,kissing bond, Lazy S are the typical weld defects of conventional fusion welding process which can be reduced by FSW process.FSW is considered to be the most significant development in metal joining in a decade and is a “ green” technology due to its energy efficiency, environmental friendliness and versatility. As compared to conventional welding methods, FSW consumes considerably less energy, no consumables such as cover gas or flux and no harmful emissions are created during welding, thereby making the process environmentally friendly. Further because FSW does not involve the use of filler metal with no melting any Al alloy can be joined without concern for compatibility and also dissimilar aluminium alloys and composites can be joined with equal ease.[ref-7-9].
Hence after two decades of development, FSW proves to be an important alternative process in aerospace or aeronautical industries involving aluminium alloys. High joining speed, autogeneous welding, improved metallurgical properties and reduced need for human skill are amongst the most important advantages of FSW in comparison with conventional fusion welding method.[10,11].
FSW can be applied to most geometrical structural shapes and to various types of joints such as butt, lap,T-butt and fillet shapes.[12,13].The most convenient joint configurations for FSW are butt and lap joints. Configurations of other types of joint designs are also applicable to FSW which are illustrated in fig-3.
Additional key benefits of FSW process over fusion welding are summarized below [14]
(1) Low distortion and shrinkage.(2) Excellent mechanical and metallurgical properties.(3) No porosity and spatter.(4) No shielding gas required.(5) No filler wire required.(6) Less surface cleaning is required.(7) Can able to weld 2xxx, 5xxx, 6xxx, 7xxx series which are difficult by fusion welding process.
2. Friction Stir Processing-
Now a days in manufacturing sectors super plastic forming is one of the most critical area of research.[ref-15-18].Therefore FSP can be used as a generic process which enabling technology for unitized structures[ref-19].Friction stir processing is a new developed technique, which is another variant of FSW process. It is a process to modify the micro structure where there is no formation of joint in these applications.
The benefits of FSP include enhancement of material properties of cast and wrought material, Healing of flaws and casting porosity, mechanical mixing of the surface and subsurface layers, grain refinement, homogenization of precipitates in various alloys and composites, enhance super plasticity due to excessive plastic flow of material.
Choi et al (2013) [ref-20] used friction stir processing (FSP) to incorporate SiC particles into the matrix of A356 Al alloy to form composite material. Constant tool rotation speed of 1800 r/min and travel speed of 127 mm/min were used in this study. The base metal (BM) shows the hypoeutectic Al-Si dendrite structure and the microstructure of the stir zone (SZ) is very different from that of the BM.
At this time, FSP is the only solid state processing technique that has unique capabilities to increase the super plastic strain rate as compared to other conventional thermo mechanical processing such as rolling, equal channel angular extrusion.
3. Literature Review
3.1 Process Variables in FSW
There is a complex material movement and plastic deformation involved in friction stir welding or processing which depends on a number of process variables. The main process variables in FSW can be subdivided into 3 categories: Machine variables, tool variables and other variables.
Table-3.1 main process Variables in FSW[21]
Tool variables Machine variables Other variablesTool materialPin and shoulder diameterPin lengthThread pitchShoulder and tool features
Joint designMaterial Type and sizeProperty of work piece materialType of fixture material
Further the process variables are very much important to understand the joint properties which include fatigue strength, toughness, corrosion, hardness and stress corrosion resistance. These process parameters affect the weld joining through heat generation and dissipation.
3.1.1 Welding or Machine Variables.
Among all the parameters tool rotation rate(w,rpm) and traverse speed(v,mm/min) are the most important welding parameters in FSW.The tool rotation results in stirring and mixing of the material around the tool pin and the traverse speed results in movement of material from the front to back and complete the welding process. Higher tool rotation rate generate higher temperature because of high frictional heating results in intense stirring and mixing of material. With increase in temperature there is a frictional coupling occur between the tool surface and work piece. So a monotonic increase in heating with increasing tool rotation rate is not expected as the coefficient of friction at interface will change with increasing tool rotation rate [1].
Sato etal (2002) [22] also observed that there is a significant rise of temperature with rise of rotational speed.
Peel et al (2006) [ref23] investigate the effect of changing the rotational and traverse welding speeds on the tool forces, power input and thermal history throughout the welding cycle. They observed that both the torque and extent of material mixing in the stir zone displays a much stronger dependence on the rotational speed than the traverse speed.
Cemal Meran (2006) [ref-24]had done friction stir welding on brass plates of 3mm thickness with constant rotational speed and different welding speed. They also observed that at constant rotational speed 112mm/min welding speed is the optimum parameter yield defect free weld joint with maximum joint strength.
As per Kwon et al (2009)[ref-25] friction stir welding was performed on 5052 Al plate having thickness of 2 mm with a wide range of rotational speed and constant traverse speed. The results
showed that at all the tool rotational speed defects free welds were obtained. In addition ,the onion ring structure becomes more wider as tool rotation speed is increased and on the other hand grain size decreased with decrease in tool rotation speed.
The mechanical and micro structural behaviour of dissimilar FSW AA6082-AA2024 with different welding parameters was studied by Cavalierea et al (2009) [26].They found that the best tensile and fatigue properties were obtained for the joints with the AA6082 on advancing side and welded with an advancing speed of 115mm/min.
As per rodrigues et al (2009)[27] friction stir welds produced in mm thick plate of AA6016-T4 Aluminium alloy with two different tools were analysed and compared concerning the microstructure and mechanical properties. For each tool, the welding parameters were optimized in order to achieve non-destructive welds. The welds produced were classified as hot and cold welds. The results obtained showed that hot welds obtained with the maximum tool rotational speed and minimum traverse speed, have improved mechanical properties relative to the cold welds and that were in under match condition relative to the base material.
As per Rajamanickram et al (2009)[ 28],temperature under the tool was strongly depend on the tool rotation rate than the welding speed.they also demonstrated that weld speed could be the main input parameter which has the highest statistical influence on the mechanical properties.
Azizieh et al (2011) [ref 29] used friction stir processing to fabricate AZ31/Al2O3 nano-composite for surface application. They observed that with higher rotation speed,inspite of finer particle cluster,grain growth was occurred due to higher heat input and simultaneously more shattering effect of rotation cause better nano-particle distribution.
Lakshminarayanan et al (2011) [ref-30] developed friction stir welding window for AA2219 Al alloy. They conclude that the quality of the welding depends on the weld pitch or tool advance per revolution (ratio of welding speed to rotational speed) and can be increased by increasing the welding speed at constant rotational speed or by decreasing the rotational speed at constant welding speed.
3.1.2 Tool variables.
Tool geometry is the most influential aspect of process development which plays a critical role in the material flow and in turn governs the traverse rate at which it can be conducted. The FSW tool consists of a pin and a shoulder. Contact of the pin with the work piece produces frictional and deformational heating and softens the work material and on the other hand contacting the shoulder to the work piece increases the work piece heating and expands the zone of softened material and constrained the deformed material. Therefore forward motion of the tool produces loads parallel to the direction of travel which is termed as Traverse load: Normal load is the load required for the tool shoulder to remain in contact with the work piece.
Fig-4 Schematic Drawing of FSW tool [souce-ref-1]
Therefore selection of correct tool material is also one of the prime concerns for production of quality welding. Various researchers studied the mechanical and metallurgical aspects of welds using different tool materials.
Tool steels is the most common tool material used in FSW process for aluminium alloy [ref-31-35].The advantages to using tool steel as friction stir tooling material include easy availability, low cost, good machinabilty and established material characteristics.
Colegrove et al [36] and Vaze et al [37] used cobalt-nickel-base alloy MP159 for friction stir welding of aluminium alloy.
Tungsten –base alloys have also been used by many researchers in friction stir welding of copper alloys, nickel aluminium bronze, titanium alloys and steels [ref 38-40].
Table-2 is a summary of the current tool materials used to friction stir the indicated materials and thicknesses which are extracted from the vast literature sources.
Table-3.2 Summary of Current Friction stir welding tool materials [Source-1]
In recent years several new features have been introduced in the design of tools. Several tools designed at TWI are shown in Table-3.3.
Table-3.3 A selection of tool design at TWI [source-1]
The whorl and MX-Triflute have smaller pin volumes than the tools with cylindrical pins. The tapered threads in the whorl design induce a vertical component of velocity that facilitates plastic flow. The flute in the MX-Triflute also increases the interfacial area between tool and the work piece, leading to increased heat generation rates, softening and flow of material. Consequently more intense stirring reduces both the traversing force for the forward tool motion and the welding torque[33].Although cylindrical, whorl and Triflute are suitable for butt welding, they are not useful for lap welding, where excessive thinning of the upper plate can occur together with the trapping of adherent oxide between the overlapping surfaces.Flared-Triflute and A-skew Tools were developed to ensure fragmentation of the interfacial oxide layer and a wider weld than in usual for butt welding. The Flared-Triflute tool is similar to MX-Triflute with an expanded flute, while A-skew TM tool is threaded tapered tool with its axis inclined to that of the machine spindle. Both of these tools increase the swept volume relative to that of the pin, thus explaining the stir region and resulting in a wider weld and successful lap joints. Motion due to rotation and translation of the tool induces asymmetry in the material flow and heating across the tool pin.
Apart from the tool pin design there is a significant impact of tool shoulder profile and tool shoulder feature design on weld quality. Various tool shoulder features design have been used by TWI.These features increase the amount of material deformation produced by the shoulder, resulting in increased work piece mixing and higher quality friction stir welds. Following figure consists of scrolls, ridge pr knurling, grooving and concentric circles and can be machined on any shoulder profile.
Fig-5 Tool shoulder geometries, viewed from underneath the shoulder [source-ref-41]
Scialpi et al(2007) [ref-42]performed FSW using of three different type of shoulder geometry(scroll and fillet,cavity+fillet,only fillet).They investigated the results on micro structural and mechanical properties of friction stir welded 6082 Al alloy. Results showed that for thin sheets, the best joint has been welded by a shoulder with fillet and cavity.
Zhang et al(2011) [43] used the rotational tool without pin using three different tool configuration-inner concave flute, concentric circle flute and three spiral flute. The experimental results showed that tensile strength and grain size attained by the tool with three spiral flute is much better than by the other two which can be used to join thin plate of aluminium alloy.
Forcellese et al (2012)[44] investigated the effect of tool geometry using two different tool configuration with different values of shoulder diameter, both with and without pin. Results indicate that by increasing the shoulder diameter, a strong beneficial effect on both ductility and strength value is obtained using pin-less tool configuration with more homogeneous micro structure.Further Forcellese and simoncini(2012)[45] investigated the plastic flow behaviour and formability of friction stir welded AZ31 thin sheets obtained using pin-less tool configuration and compared the results with the base metal.
Galvao et al(2012) [ref-46] developed a study which aimed was to investigate the influence of shoulder geometry(one is scrolled and another one a conical shoulder tool) on the formation and distribution of brittle structures in friction stir welding of aluminium and copper joint.the author noticed that the nugget of the welds produced using same process parameters but different tool geometry had completely different morphology and intermetallic content.
As per Galvao et al (2013)[ref-47] work has been done to see the influence of the shoulder geometry on friction stir welding of 1mm thick copper-DHP plates. The welds were produced using three different shoulder geometries: flat, conical and scrolled. By varying the tool rotation and traverse speed it was observed that many defects were produced for all weld condition in case of flat shoulder. On the other hand scrolled shoulder tool is more effective than the conical one for the production of defect-free welds. However both geometries required a minimum rotational speed to avoid internal defects.
To overcome these problems TWI recently focused on FSW tool designs that increase the tool travel speed, increases the volume of material swept by pin-to-pin volume ratio and increase the weld symmetry. The skew stir tool increase the volume of material swept by pin-to-pin volume ratio by offsetting the axis of the pin from the axis of the spindle [48,49].
Similarly com-stir tools combine rotary motion (tool shoulder) with orbital motion(tool pin) to maximize the swept volume[50].
The re-stir tool (TWI) avoids the inherent asymmetry produced during friction stirring by alternating the tool rotation, either by angular reciprocation or rotary reversal [51].on the other hand in dual rotation tools, the pin and shoulder rotate separately at different directions[52].
3.1.3 Joint design.
The most convenient joint configurations for FSW are butt and lap joints. Apart from butt and lap joint configurations, other types of joint designs, such as fillet joints are also possible a needed for some engineering applications.
3.2 Temperature Distribution and Heat Transfer in FSW process
In FSW, heat is generated by friction between the tool and the work piece via plastic deformation of the metal. The heat generation mechanism is influenced by the weld parameters, thermal conductivity of the work piece, pin tool, backing anvil and weld tool geometry. The temperature within and around the stirred zone influence the microstructure of the welds, such as grain size, grain boundary character, coarsening and dissolution of precipitates and resultant mechanical properties of the welds. Therefore the study of temperature distribution and the resulting heat input within the work piece material is very important during FSW process.
Hwang and co workers (2008) [53] experimentally explore the thermal histories and temperature distribution within butt joint welds of 6061-T6 aluminium alloy. Four thermocouples of K-Type with data acquisition system connected to a personal computer were used to record the temperature histories during welding. The different types of thermocouple layout i.e same side and equal distance, opposite side and equal distance and same side and unequal distance are devised at different locations on the work piece to measure the temperature distribution during welding process. They concluded that the temperature inside the pin can be regarded as a uniform distribution and that heat transfer starts from the rim of the pin to the edge of the work piece.
3.3 Material flow in FSW
The FSW process can be modelled as a metal working process in terms of five conventional metal working zones (1) Preheat (2) Initial deformation (3) Extrusion (4) Forging (5) Post heat/cool Down. Typical zones obtained during the process are shown in fig-6. In the preheat zone ahead of the pin, temperature rises due to the frictional heating of the rotating tool and adiabatic heating because of the deformation of material. The thermal properties of material and the traverse speed of the tool govern the extent and heating rate of this zone. As the tool move forward, an initial deformation zone form, when material is hated to above a critical temperature and the magnitude of stress exceeds the critical flow stress of the material, resulting in material flow. The material in this zone is forced both upwards into the shoulder zone and downward into the extrusion zone. A small amount of material is captured in the swirl zone beneath the pin tip where vortex flow pattern exists. In the extrusion zone with a finite width, material flows around the pin from the front to the rear. A critical isotherm on each side of the tool defines the width of the extrusion zone where the magnitude of stress and temperature are insufficient to allow metal flow. Following the extrusion zone is the forging zone where the material from the front of the tool is forced into the cavity left by the forward moving pin under hydrostatic pressure conditions. The shoulder of the tool helps to constrain material in this cavity and also applies a downward forging force. Material from shoulder zone is dragged across the joint from the retreating
side towards the advancing side.
Fig-6 (a) Metal flow pattern and (b) metallurgical processing zones developed during friction stir welding [source-ref-54]
Guerra et al (2003)[ref-55] studied the flow of metal during FSW using a faying surface tracer and a nib frozen in place during welding. They observed that material is moved around the nib in FSW by two processes. In first process material on the advancing front side of a weld enters into a rotational zone that rotates and advances with the nib. On the other hand material on the retreating front side of the nib is entrained and fills in material on the RS of the nib wake. They further conclude that material transported by these two processes has very different thermo mechanical histories and properties.
Further Hamilton and his co-workers (2008) [ref-56] proposed a model of material flow during friction stir welding. They found that weld nugget forms as surface material which is extruded from the retreating side into the region of plasticized material around the FSW pin and under the tool shoulder. They further observed that nugget zone is the combination of interleaved layers of particle-rich and particle poor material.
3.4 Weld Microstructure and Weld Mechanical properties.
The microstructure and consequent property distributions produced during friction stir welding of aluminium alloys are dependent on several factors. The contributing factors include alloy composition, alloy-temper, welding parameters other geometric factors. The alloy composition determines the available strengthening mechanisms and how the material will be affected by the temperature and strain history associated with FSW.Similarly the welding parameters(e.g., tool rotation rate and welding speed) dictates for a given tool geometry and the thermal boundary conditions,the temperature and strain history of the material being welded. Plate gage and other geometric factors (such as shoulder size, heat sinks associated with clamping etc.) may affect the temperature distribution within the weld zone and through the thickness of the welded plates.
In case of FSW/FSP the weld micro structure and property distribution also depends on the type of alloy such as in case of Al alloy is it non-heat treatable, heat treatable (precipitation-Hardening) alloys etc.
The weld nugget is typically described as the region of the thermo mechanically affected zone that has experienced sufficient deformation at elevated temperature to undergo re-crystallizarion.The two key variables that determine the properties of the material in the weld nugget are the peak temperature and
the quenching rate from that temperature.
According to Sato et al.[ref-57],the statistically re-crystallized grain size in the nugget region is determined predominantly by the peak temperature in the weld; the higher the peak temperature ,the larger the grain size. Some effect of welding speed may also be involved, but because the grain size (for static grain growth) is exponential with temperature and linear with time, the peak temperature will exert the dominant influence.
However a wide range of nugget grain sizes can be achieved by manipulation of welding process parameters. Grain sizes on the order of 10s of micrometers and less than 1µm have been reported by Su and et al (2000) [ref-58] and Heinz and et al (2002) [ref-59].
3.5 Defects in FSW Welds
Compare to fusion welding process of aluminium and its alloy, the FSW does not suffer from problems such as weld porosity, solidification cracking or heat affected liquation cracking. This is because in FSW there is no bulk melting of the parent material. However obtaining a defect-free joint with good mechanical properties is critical for industrial application. The formation of defects such as lack of penetration, lack of fusion,tunnels,voids,surface grooves, excessive flash, surface galling, nugget collapse and kissing bonds are mainly related due to imbalance in material flow or due to geometric factors i.e process parameters (tool design, tool rotation speed, tool travel speed, shoulder plunge depth or axial force, spindle tilt angle)[ref-60] . The temperature below melting point of the parent material is the main source of plastic deformation of the material at the joint line. Due to which micro structural change like re-crystallization, coarsening and or dissolution of strengthening precipitates, grain orientation and growth occurs. The improper process parameters in FSW giving rise to too hot or too cold welding condition. Too cold weld condition is responsible due to insufficient material flow and giving rise to defects like void formation and nonbonding. On the other hand too hot weld condition, giving rise to excessive material flow leading to material expulsion like flash formation and the collapse of the nugget within the stir zone[ref-61] .
3.5.1 Defects from too hot welds
The defects which are generates under such processing conditions are visually indentified through the surface appearance of the welded joint. The improper parameter settings cause too much thermal softening. The surface of the welded joint appears to contain blisters or surface galling. Furthermore, excessive heat generation can lead to thermal softening in the work piece material beyond the boundary of tool shoulder. Therefore, the tool shoulder rather than actively participating as a mean of material containment, it is giving rise to material expulsion in the form of excessive flash formation. Too much thermal softening can also lead to the thinning of the work piece material. The work piece material below the tool shoulder will reaches a point where it is no longer able to support the axial load placed upon it. Such a condition during processing causes excessive flash of the work piece material. A weld nugget collapse under too hot welding condition is another serious defect in FSW joint. It is not expected all the times that increase of tool rotational speed at constant tool travel speed causes increase in size of the weld nugget [62].
Kim and et al (2006) [ref-62] observed that excessive heat input had generated due to higher rotational speed with a lower welding speed. As a result large mass of flash was ejected to the outside due to the softening of the metal and also the tip of the probe sometimes touches the backing plate.
3.5.2 Defects from too cold weld
Too a cold welding condition result in work hardening of the material.This causes the dry slip between the tool pin and the work piece material. The lack of surface fills or voids and channel defects are the main defects arising due to insufficient heat generation. The insufficient heat generation causes improper material mixing and thus responsible for non-bonding [ref-63].
Kim and their co-workers also evaluate that at lower rotational speed and high welding speed insufficient heat input is generated. As a result cavity or groove-like defects are formed.[ref-63].
Excluding the defects due to excess and insufficient heat input one more defect is analysed by Kim and his co-worker which is termed as defects due to abnormal stirring. For the abnormal stirring defects are formed at higher rotational speeds and higher welding speeds. They found that the abnormal stirring is caused due to the different temperatures between the upper part near the surface and the lower part. Due to discontinuous flow of material shape of the top part on the advancing side in the stir zone is completely different than the shape due to excess or insufficient heat input
FSW is capable of producing weld with a very less defects but still elimination of complete process upset is not possible. Much research has been devoted to understanding the effect of process parameters on defect formation in order to optimize the process parameters for FSW. Still optimization of process parameters is mostly done by trial and error.
In past few decades, there has been research going on in the field of multi-pass welding and processing where it is more desirable to repair the defective portion of the weld than to throw as a scrap. One of the techniques that can be used to repair defects arising from process upsets is simply re-welding using the nominal process parameter [ref-64]. Brown et al (2009) [ref-65] performed five overlapping passes and found that there is no need of any adjustment for multi pass welding. Further there is a significant reduction in feed force when welding over the previous weld. However grain size, hardness and temperature during welding is unaffected with number of passes. Again there is a gradual reduction of residual stress with increasing pass number.
Nataka et al. (2006) [ref-66] reported an improvement in the mechanical properties of aluminium die casting alloy of multi-pass fsp compared to the as-cast base metals. As compared to base metal, hardness increased by 20HVand the tensile strength of the multi-pass specimens were significantly increased to about 1.7times of as-cast base metal. The main reason for improvement of mechanical properties is due to the elimination of cold flake, uniform dispersion of the finer Si particles and grain refinement of aluminium matrix. On the other hand Ma et al(2006) [ref-67] reported that there is no effect of overlapping passes on size, aspect ratio or distribution of the Si particle while performed five-pass with 50% overlap FSP on cast A365.
Leal and Louriero (2008)[ref-68] investigated the effect of overlapping FSW passes using two Al alloys(AA5083-O and AA 6063-T6).They found that the quality and strength of the welds is not just a function of the welding parameter but also depend on the type of material and its condition of treatment. Lastly they found that weld polishing improved the mechanical efficiency of the welded joint.
As Friction stir stir processing is one of the most promising techniques for grain refinement, removing flaws, defects and all many researchers used multi-pass friction stir processing to improve the properties of as-cast material.Fsp been applied by Johannes and Mishra (2007) [ref-69] to demonstrate the effectiveness of multiple passes to create large area of super plastic materials with properties.
They conclude that GBS is the most important mechanism to achieve super plastic deformation. Similarly Ma et al (2009) [ref-70] noted that two pass FSP resulted in an enhancement in super plastic elongation with a optimum rate in the nugget zone of second pass and a shift to higher temperature in both central of second pass as well as transitional zone between two passes.
Surekha et al (2008) [ref-71] reported that multi-pass FSP showed better corrosion resistance compared to the base metal.
Using cast Al alloy, Jana et al (2010) [ref-72] reported that all single pass runs showed some extent of abnormal grain growth which was removed with multi-passes. They also examined and found that higher rotational speed was found to be beneficial for controlling the AGG .
Barmouz et al. (2011) [ref-73] Fabricated cu/sic composites using MFSP.results found that multipass FSP reduces the Sic particle size,improve the dispersion and separation of Sic paticle by severe stirring action in the nugget zone which reduces the grain size of the copper matrix and created strong interfacial bonding by removing the porosity content.
MPO Fsp has been applied by Ni and et al.(2011) [ref-74] to transform the coarse as-cast Nab alloy base metal to get defect free material with fine microstructure, which was feasible to modify the large sized plates.
Similar type of study had been conducted by Izadi and Gerich (2012)[ref-75] to study the effect of multi-pass FSP on distribution and stability of carbon nano-tube and to fabricate a metal matrix composite(MMC) based on AL 5059 and MWCNTs.
To avoid the use of multi-pass FSW/FSP, the two-tool-FSW concept is being developed at TWI in several variations and is referred to as Twin-Stir [76,77, 78]. One of those techniques is Tandem twin-stir technique. Tandem Twin-Stir uses two FSW tools (with or without counter rotation) positioned one in front of the other.
3.6 Advantages of contra-rotating FSW tools
• It is new variant techniques of FSW/FSP which require less clamping and improve the welding speed. With two contra-rotations the resultant force counters each other so that the parts to be welded require relatively low securing forces.
• The tandem technique improves the weld integrity by disrupting and fragmenting the residual oxide layer remaining within the first weld region by the follower tool.
• As the weld made over the first run, but in reverse direction, creates no loos of mechanical properties but produces further break-up and dispersal of oxides.
• Since the second tool travels over the first weld region; it does not have to be as robust as the leading tool.
• Further the motion produced by the counter rotating tandem Twin-Stir is similar to Re-stir tool, but the Twin stir produces faster travel speeds and in addition efficiency of FSW can be improved with the use of two FSW tools [111].
3.7 Modelling and optimization of FSW using statistical methods, FE model, and soft computing tools.
In order to accelerate, support and guide experimental development work with cost Process modelling is one of the most innovative techniques. Modelling based on scientific understanding of the mechanisms and physical phenomena of FSW has great potential for guiding tool design, predicting likely operating conditions in new materials or joint geometries and then optimizing process conditions for maximum process speed. Further it helps to predict the occurrence of voids and defects, the extent of micro structural and property changes in the deformed and heat-affected regions and the development of residual stress. Friction stir welding presents a multi physics modelling challenge, because it combines closely coupled heat flow, plastic deformation at high temperature and microstructure and property evolution. All three contribute to the processability of a material by FSW and to the subsequent properties of the weld.
Analytical and numerical methods each have a role to play although numerical methods dominate due to the power and ease of use of modern workstations and software.
The conventional experimental design techniques such as regression method, response surface methodology (RSM) focus mainly on the mean of the performance characteristics, where as Taguchi method takes the variance into consideration for the model development. These tools use experimental data for the model development..
Soft computing techniques, such as artificial neural network (ANN),genetic algorithm(GA),fuzzy logic(FL) and their combinations provide an alternative solution for predictive learning, modelling and optimization of process parameters for achieving good weld quality. These evolutionary algorithms consider the uncertainty features of the welding processes, which cannot be expressed by mathematical equations. Thus, they are better as compared to conventional mathematical and statistical techniques. These tools can handle large number of data to generate the model and optimize it with a short time span. These tools are also adaptable for incremental learning, enabling the models to be improved incrementally as new data become available.
3.7.1 Using statistical method
Various statistical tools have been applied for the modelling and optimization of FSW process with weld parameters.
Jayaraman et al (2009) [Ref-80] analysed the effect of process parameters using full factorial design technique for optimum tensile strength. Further they developed a mathematical model using nonlinear regression analysis to correlate the process parameter with measured tensile strength.
Central composite Design with four parameters, five levels and 31 runs are used by Sundaram and Murugam (2010) [Ref-81] to conduct the experiment on dissimilar Al alloy where five different pin profiles are used to fabricate the joints. Further response surface methodology is employed to develop the model.
Heidarzadeh et al (2012) [82] used response surface methodology based on central composite rotatable design to develop a mathematical model predicting the tensile strength of friction stir welded AA6061-T4 Al alloy joints at 95% confidence interval.
Bozkurt (2011) [83] used taguchi approach of parameter design to set the optimum welding parameter. The experiments were performed using L9 orthogonal array method.
Koilraj et al (2012) [84] found out the optimum process parameter with reference to the tensile strength of the joint using taguchi L16 orthogonal DOE method.
3.7.2 Using soft computing method
Buffa et al (2006) [85] proposed a continuum based FEM model which is capable of predicting non-symmetric nature of FSW process and the relationships between the tool forces and the variation in process parameters. Predicted results are validated by comparing with experimental data of force and temperature distribution using AA7075 Al alloy of 3mm thick plate. From the simulation results it is found that temperature distribution about the weld line is symmetric which is due to the resultant of heat generation where rotational speed of the tool is the dominant factor than advancing speed. But on the other hand material flow and effective strain distribution is non-symmetric about the weld line which is controlled by both advancing and rotating speeds.
Okuyucu et al (2007) [86] developed an ANN model to make a correlation between the fsw parameters of Al plate and mechanical properties which were obtained experimentally. Input parameter taken in this model are weld speed and rpm and output include tensile strength, yield strength, elongation, hardness of weld metal and hardness of HAZ.Results showed that the calculated values were in good agreement with measured one. So the model can be used as an alternative way for the calculation of the mechanical properties of the welded Al plates by FSW method.Boldsaikhan et al (2011) [87] proposed an innovative algorithm using discrete fourier transform and a multilayer NN.This approach used to detect wormhole defects and important feedback information about weld quality in real time to a control system for friction stir welding.
Laxminarayanan and balasubhramanian (2009) [88] used both RSM method and ANN to predict the tensile strength of friction stir welded AA7039 aluminium alloy. Results obtained through response surface methodology were compared with artificial NN.
Hattel et al (2012) [89] developed a step wise modelling approach to combine an in situ weld simulation with a post welding failure analysis. By using the commercial software ANSYS, a thermo mechanical model is developed to predict the thermally induced stresses and strain during welding and finite element code is used to study the plastic flow localization and failure in a subsequent structure analysis. They observed that there is a remarkable influence of post welding stress-strain condition when the welded plate is subjected to tension, and it is largest when the specimen cut in transverse to the weld line.
Veljic et al ( ) [90] developed a coupled thermo mechanical model to study the temperature field, plunge force and plastic deformation of AA 2024-T351 Al alloy under different rotational speed during the friction stir welding process. Three dimensional FE model has been developed in ABAQUS/EXPLICIT using the arbitrary Lagrangian-Eulerian-formulation,the Johnson-cook material law and the coulomb’s law of friction. In this study, they observed that the maximum temperature in the welding process is lower than the melting point temperature of the base metal and the temperature field is approximately symmetrical along the line of welding. With increase of rotational speed, the plunge force is reduced. They further observed that the plastic strain is more in the advancing side and even with increase of rpm; the low plastic strain region is on retreating side.
4. Objective of the research work
Though the twin stir techniques was proposed by TWI,but no detailed research on micro structure, mechanical properties and process optimization has been carried out till today.
Therefore, the objective of this work is placed on to determine the effect of two contra rotating FSW tool (Tandem Twin-stir) on the friction stir processing/welding region.
In order to demonstrate the characteristics of twin tool, a tool system was designed and used initially for friction stir processing. When experiments was conducted both the tools rotated independently but in opposite direction to each other and pass over the sample one after another.
5. Experimental Setup
5.1 Fixture Design
For conducting actual experiments it requires a fixture which can hold the welding plates firmly and prevents the rotary and translator motions. So a properly designed fixture was manufactured and installed over the milling machine bed as shown in figure- which has higher damping coefficient and shock absorbing capability.
Fig- 7 Pictorial view of fixture (a) Fixture installed over milling macine bed (b) Welding plates clamped over fixture
5.2 Machines/Instruments used during experiments
VF3.5 knee type vertical miiling machine has been used to fabricate the joints is shown in fig-8.This has a facilty of RPM ranges from 50-1800 RPM and traverse speed ranges from 16 to 800 mm/min.So a large number of experiments by varying the welding speed and RPM.
Fig-8 VF3.5 Knee type vertical milling machine.
5.3 Twin Tool Setup
The self designed twin tool setup is manufactured for FSP/FSW is shown in fig-9.The twin tool setup was mounted over the vertical milling machine. The twin tool system is composed of two tools which are rotating in opposite direction to each other. The primary tool is mounted on the main shaft and the main shaft is connected directly with spindle of the milling machine. Therefore, the main tool rotates at the same rotational speed and in the same rotation direction as the spindle during the welding process. The secondary tool is connected just like a cantilever beam with the primary tool with the help of gear assembly. The power transmission from the primary to secondary tool is similar to the transmission of power from driver to driven gear, So that the rotation of the secondary tool is just opposite to the primary tool.
Non-threaded cylindrical pin with non-consumable tool was used to fabricate the process. Each of the FSW tools is placed inside collet with a collet cover. With the help of tapered roller contact bearing, the gear is assembled with the main shaft or the spindle. Hence when the m/c power is on, the main spindle rotates, which transmit power to the primary tool through the driver gear and on the other hand, the main gear transmits power to the driven gear so that the second tool also rotates simultaneously with the fist tool but in opposite direction.
Fig-9 twin tool attachment
5.4 Experimental Work
The rolled plates of 2.5 mm thickness, commercial pure aluminium alloy have been cut into the required size (200mm×50mm×2.5mm) by power hacksaw cutting and milling. The joint configuration is obtained by securing the plates in position using mechanical clamps. Both FSP and FSW were performed along the longitudinal direction and perpendicular to the rolling direction of the plate.Non-consumable tools made of stainless steel SS316 have been used to fabricate the joints. The tool dimensions are shown in fig-10.
The chemical composition and mechanical properties of base metal are presented in table-5.1 and 5.2.The chemical composition of work piece material and tool material was analyzed using OES analysis. The chemical composition of tool material is shown in table 5.3.From OES analysis it is confirmed that the tool material is SS316 type. The tensile specimen of base material is also tested to check the mechanical properties of the base material. Vickers micro hardness test is also performed to check the micro hardness of base material.
Table-5.1 chemical composition (weight %) of work piece material
Chemical composition (weight %) of work piece material
Elongation in %age Hardness at 200 gmf load in VHN45-55 HV
Table 5.3: Chemical composition (weight %) of Tool Material SS316
Si P Mn Cr Ni Mo Fe2.13 0.27 8.95 16.29 0.2 0.14 72.01
Friction stir processing has been carried out both by using single tool as well as twin tool attachment. The welding parameters and tool dimensions are presented in table-5.4.
Table-5.4: Welding parameters and tool dimensions
Process parameters ValuesRotational speed (rpm) 900,1120,1400,1800Welding speed (mm/min) 16,20,25D/d ratio of tool 3.2Pin length (mm) 2Tool shoulder, D (mm) 16Pin diameter (mm) 5
Fig-10 FSP/FSW tool dimensions
By using four rotational speed (900, 1120, 1400, 1800 rpm) and three welding speed (16, 20, 25 mm/min) total 12 experiments were performed both by single tool and twin tool attachment. Therefore total 24 experiments have been carried out in this process.
5.5 Measurements
After processing, specimens are prepared for macro and micro structural analysis, tensile test and Vicker’s hardness test from the processed region perpendicular to the welding direction.
Macrostructural analysis has been carried out using a light optical microscope (LEICA DFC-295) as shown in fig-11(a) in corporate with an image analysing software (LEICA QWin-V3) as shown in fig-11(b).The specimen for metallographic examination are sectioned to the required sizes from the joint or region comprising FSP zone, thermo-mechanical zone, heat-affected zone and base metal regions and polished using different grades of emery papers. Final polishing has been done using the diamond paste in variable speed grinder polishing machine as shown in fig-12 and is etched with Keller’s reagent to reveal the macrostructures.
The micro hardness profiles of the FSW joints were measured in the cross sections in order to evaluate the material behaviour as a function of the different welding parameters. Microhardness testing was done on Vickers micro hardness testing apparatus as shown in fig- 13. The Vickers hardness was measured on the polished cross-section with a spacing of 200µm between two adjacent indentations. The hardness test was taken perpendicular to the direction of welding with testing load of 200gmf and dwell time of 15sec.
Fig-13 Vickers microhardness testing apparatus
5.5.3 Tensile properties
The welded joints are sliced using band saw and then machined perpendicular to the welding direction with a gauge length of 16mm and a width of 6mm, as shown in fig-14 below. Three specimens were prepared and tested for each joint and the average is used to estimate the tensile property. The Tensile test was performed at room temperature using universal testing machine (INSTRON-8862) as shown in fig-15.the specimen is loaded at the strain rate of 1.0mm/min as per ASTM specification so that tensile specimen undergoes deformation as shown in fig-15(b).the specimen finally fails after necking and the load versus position has been recorded. The 0.2% offset yield strength; ultimate tensile strength and percentage of elongation have been evaluated.
6.2 TENSILE STRENGTH, UTS AND % ELONGATION OFBASE METAL AND 12 SAMPLES USING SINGLE TOOL AS WELL AS TWIN
TOOL
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