Strategies for Welding Aluminium

Strategies for conjoin atomic number 13CHAPTER 1 launching1.1 INTRODUCTION OF THE FSW TECHNIQUEIn todays humourrn world there atomic number 18 nearly(prenominal) opposite join techniques to join everyoys. They go from the conventional oxyacetylene torch weld to optical maser join. The two general categories in which all the types of conjoin give the gate be divided is coalition join and unshakable relegate conjoin.The coalition join answer involves chemical substance bonding of the admixture in the liquid stage and whitethorn necessitate a filler solid much(prenominal) as a consumable electrode or a spool of fit out of the filler genuine, the touch on whitethorn ebulliently need a squashy ambience in order to void oxidation of the molten metal, this could be achieved by a flux material or a dull flub shield in the weld zone, there could be need for adequate surface dressings, examples of partnership conjoin ar metal electroneutral gas join (MIG), tungsten muddy gas weld (TIG) and laser welding. There argon m some(prenominal) disadvantages in the welding techniques where the metal is het up(p) to its thaw temperatures and let it solidify to path the occasion. The thawing and solidification ca consumptions the machinelike properties of the weld to deteriorate such as offset ductile strength, wear down strength and ductility. The disadvantages also include porousness, oxidation, micro requisition, hot pass and other(a) micro morphological defects in the sound out. The process also limits the gang of the metals that mess be conjugate beca wont of the different thermal coefficients of conductivity and elaboration of different metals.The solid state welding is the process where coalescence is produced at temperatures below the melting temperatures of the undercoat metal with out any need for the filler material or any inert ambience beca example the metal does non reach its melting temperature f or the oxidation to expire, examples of solid state welding ar detrition welding, explosion welding, influence welding, hot embrace welding and ultrasonic welding. The collar all- grand(a) parameters time, temperature and pressure individually or in combinations produce the joint in the free radical metal. As the metal in solid state welding does non reach its melting temperatures so there are fewer defects ca utilise payable to the melting and solidification of the metal. In solid state welding the metals world joined retain their original properties as melting does not occur in the joint and the mania affected zone (HAZ) is also precise small compared to fusion welding techniques where much than or less of the deterioration of the strengths and ductility begins. discordant metals can be joined with ease as the thermal elaboration coefficients and the thermal conductivity coefficients are less significant as compared to fusion welding.Friction compel welding (FSW) is an upgraded version of rubbing welding. The conventional clang welding is done by moving the parts to be joined relative to each other along a common embrasure also applying compressive forces across the joint. The frictional heat generated at the interface payable to rubbing softens the metal and the soft metal gets extruded ascribable to the compressive forces and the joint gains in the clear material, the relative motion is stopped and compressive forces are change magnitude to form a sound weld before the weld is tolerateed to cool.Friction stir welding is also a solid state welding processes this rummy upgradation of friction welding was invented in 1991 in The dyers mignonetteing Institute (TWI) 4. The process starts with clamping the plates to be welded to a backing plate so that the plates do not fly a instruction during the welding process. A rotating wear resistant dig is plunged on the interface between the plates to a predetermined information and mo ves fore in the interface between the plates to form the weld. The advantages of FSW technique is that it is environment friendly, vim efficient, there is no necessity for gas shielding for welding Al, mechanized properties as proven by exhaust, tensile tests are excellent, there is no fume, no porosity, no spatter and low shrinkage of the metal due to welding in the solid state of the metal and an excellent way of association miscellaneous and previously unweldable metals.1.2 ALUMINUM ALLOYS AND WELDING OF ALUMINUM ALLOYS aluminium is the most abundant metal available in the earths crust, steel was the most used metal in 19th century that Aluminium has become a strong competitor for steel in plan applications. Aluminium has galore(postnominal) attractive properties compared to steel it is economical and versatile to use that is the reason it is used a lot in the aero infinite, automobile and other industries. The most attractive properties of aluminium and its alloys which make them qualified for a entire variety of applications are their light weight, appearance, frabricability, strength and corrosion resistance. The most important property of aluminum is its ability to change its properties in a very(prenominal) versatile manner it is amazing how much the properties can change from the keen aluminum metal to its most complicate alloys. There are more than then a couple of hundreds alloys of aluminum alloys and umteen are world modified form them internationally. Aluminium alloys call for very low minginess compared to steel it has almost one ternions the density of steel. priggishly treated alloys of aluminum can resist the oxidation process which steel can not resist it can also resist corrosion by water, table salt and other factors.There are many different methods available for fall in aluminum and its alloys. The endurance of the method depends on many factors such as geometry and the material of the parts to be joined, postulate strength of the joint, permanent or dismountable joint, number of parts to be joined, the aesthetic appeal of the joint and the serve conditions such as moisture, temperature, inert atmosphere and corrosion. join is one of the most used methods for aluminum. Most alloys of aluminum are easily weldable. MIG and TIG are the welding processes which are used the most, save there are some problems associated with this welding process like porosity, lack of fusion due to oxide layers, incomplete penetration, cracks, inclusions and undercut, however they can be joined by other methods such as resistance welding, friction welding, stud welding and laser welding. When welding many physical and chemical changes occur such as oxide frame of music, dissolution of heat issue in molten aluminum and lack of color change when heated.The formation of oxides of aluminum is because of its strong affinity to oxygen, aluminum oxidizes very apace after(prenominal) it has been exposed to oxygen . Aluminum oxide forms if the metal is joined using fusion welding processes, and aluminum oxide has a naughty melting point temperature than the metal and its alloys it egotism so it results in incomplete fusion if present when joined by fusion welding processes. Aluminum oxide is a electrical insulator if it is two-ply enough it is capable of pr blushting the emission which starts the welding process, so special methods such as inert gas welding, or use of fluxes is necessary if aluminum has to be welded using the fusion welding processes.Hydrogen has lavishly solvability in liquid aluminum when the weld pool is at high temperature and the metal is still in liquid state the metal absorbs lashings of hydrogen which has very low solubility in the solid state of the metal. The trapped hydrogen can not escape and forms porosity in the weld. All the sources of hydrogen has to be eliminated in order to get sound welds such as lubricants on base metal or the filler material, moistu re on the surface of base metal or condensations interior the welding equipment if it uses water cooling and moisture in the shielding inert gases. These precautions require considerable pretreatment of the usagepiece to be welded and the welding equipment.Hot cracking is also a problem of major concern when welding aluminum, it occurs due to the high thermal expansion of aluminum, large change in the volume of the metal upon melting and solidification and its un proper(postnominal) range of solidification temperatures. The heat treatable alloys cast off greater amounts of alloying elements so the weld crack sensitivity is of concern. The thermal expansion of aluminum is twice that of steel, in fusion welding process the melting and cooling occurs very fast which is the reason for residual stress preoccupancys.Weldability of some aluminum alloys is an issue with the fusion welding processes. The 2000 series, 5000 series, 6000 series and 7000 series of aluminum alloys see diffe rent weldabilities. The 2000 series of aluminum alloys have poor weldability chiefly because of the cooper content which causes hot cracking and poor solidification microstructure and porosity in the fusion zone so the fusion welding processes are not very suitable for these alloys. The 5000 series of aluminum alloys with more than 3% of Mg content is susceptible to cracking due to stress parsimony in corrosive environments, so high Mg alloys of 5000 series of aluminum should not be exposed to corrosive environments at high temperatures to avoid stress corrosion cracking. All the 6000 series of aluminum are quick weldable but are some times susceptible to hot cracking under legitimate conditions. The 7000 series of aluminum are both weldable and non-weldable depending on the chemical composition of the alloy.Alloys with low Zn-Mg and Cu content are promptly weldable and they have the special ability of recovering the strength lost in the HAZ after some weeks of storage after th e weld. Alloys with high Zn-Mg and Cu content have a high tendency to hot crack after welding. All the 7000 series of aluminum have the sensitivity to stress concentration cracking.All these problems associated with the welding of these different alloys of aluminum has lead to the comement of solid state welding processes like Friction Stir Welding technique which is an upgraded version of the friction welding processes. This process has many advantages associated with it, and it can weld many aluminum alloys such as 2000 and 7000 series which are troublesome to weld by fusion welding processes. The advantages of the Friction Stir Welding processes are low distortion even in long welds, no fuse, no porosity, no spatter, low shrinkage, can operate in all positions, very energy efficient and excellent mechanical properties as proven by the fatigue, tension and bend tests.1.3 Conventional Welding Processes of AluminumA brief description of the most common processes, their applicatio ns on aluminum and limitations are given below.1.3.1 Gas Tungsten Arc Welding (GTAW)In gas tungsten firing welding process the heat generated by an arc, which is retained between the workpiece and a non-consumable tungsten, electrode is used to fuse the joint area. The arc is uphold in an inert gas, which serves to protect the weld pool and the electrode from atmospheric taint as shown in Figure 2.3.The process has the by-line featuresIt is conducted in a chemically inert atmosphereThe arc energy density is comparatively highThe process is very controllableJoint quality is ordinarily highDeposition rates and joint completion rates are low.The process may be applied to the joining of a wide range of engineering materials including stainless steel, aluminum alloys and reactive metals such as titanium. These features of the process lead to its widespread application in aerospace, nuclear re affect and might generation industries as well as in the fabrication of chemical process plant, food processing and brewing equipment.1.3.2 Shielded metal arc welding (SMAW)Shielded metal arc welding has for many historic period been one of the most common techniques applied to the fabrication of steels. The process uses an arc as the heat source but shielding is provided by gases generated by the decomposition of the electrode stopping point material and by the slag produced by the melting of mineral constituents of the coating. In addition to heating and melting the parent material the arc also melts the core of the electrode and thereby provides filler material for the joint. The electrode coating may also be used as source of alloying elements and additive filler material. The flux and electrode chemistry may be formulated to stay wear- and corrosion-resistant layers for surface protection as shown in Figure 2.4.Significant features of the process areEquipment requirement are simpleA large range of consumables are availableThe process is extremely portableThe o perational major power is lowIt is labor intensive.For these reasons the process has been traditionally used in structural steel fabrication, shipbuilding and heavy engineering as well as for small batch production and maintenance.1.3.3 Plasma weldingPlasma welding uses the heat generated by a constricted arc to fuse the joint area the arc is formed between the tip of a non-consumable electrode and either the work piece or the constricting nozzle as shown in Figure 2.5. A wide range of shielding and cutting gases is used depending on the sensory system of operation and the application.In the normal transferred arc mode the arc is maintained between the electrode and the work piece the electrode is usually the cathode and the work piece is connected to the positive side of the power supply. In this mode a high energy density is achieved and the process may be used effectively for welding and cutting.The features of the process depend on the operating mode and the current, but in su mmary the plasma process has the sideline characteristicsGood low-current arc stabilityImproved directionality compared with GTAWImproved melting efficiency compared with GTAWPossibility of keyhole weldingThe keyhole technique is the high heat concentration can penetrate completely through the joint.These features of the process make it suitable for a range of applications including the joining of very thin materials, the encapsulation of electronic components and sensors, and high- amphetamine longitudinal welds on strip and pipe.1.3.4 Laser weldingThe laser may be used as an alternative heat source for fusion welding. The focused power density of the laser can reach 1010 or 1012 Wm-2 and welding is often carried out using the keyhole technique.Significant features of laser welding areVery confined heat source at low powerDeep penetration at high power decreased distortion and thermal damageOut-of-vacuum techniqueHigh equipment greetThese features have led to the application of leaders for micro joining of electronic components, but the process is also being applied to the fabrication of automotive components and clearcutness machine tool parts in heavy section steel.1.4 Weld Defects using Conventional ProcessesBecause of a history of thermal cycling and nonessential micro structural changes, a welded joint may develop certain(a) discontinuities. Welding discontinuities can also be caused by inadequate or careless application of established welding technologies or substandard slattern cooking. The major discontinuities that affect weld quality are described below.1.4.1 porosityTrapped gases released during melting of the weld area and trapped during solidification, chemical reactions during welding, or contaminants, cause porosity in welds. Most welded joints contain some porosity, which is generally spherical in fake or in the form of extensive pockets. The distribution of porosity in the weld zone may be random, or it may be concentrated in a ce rtain region. Porosity in welds can be reduced by the following(a) methodsProper selection of electrodes and filler metals.Improving welding techniques, such as preheating the weld area or increasing the rate of heat input.Proper cleaning and preventing contaminants from entering the weld zone.Slowing the welding speed to allow time for gas to escape.81.4.2 impurity inclusionsSlag inclusions are compounds such as oxides, fluxes, and electrode-coating materials that are trapped in the weld zone. If shielding gases are not effective during welding, contamination from the environment may also contribute to such inclusions. Welding conditions are important, and with proper techniques the molten slag volition ramble to the surface of the molten weld metal and not be entrapped. Slag inclusions may be prevented byCleaning the weld-bead surface before the near layer is deposited by using a hand or power wire brush.Providing adequate shielding gas.Redesigning the joint to permit suffici ent space for proper manipulation of the puddle of molten weld metal.1.4.3. Incomplete fusion and penetrationA better weld can be obtained byRaising the temperature of the base metal.Cleaning the weld area prior to welding.ever-changing the joint design and type of electrode.Providing adequate shielding gas.Incomplete occurs when the depth of the welded joint is insufficient. Penetration can be improved by change magnitude the heat input.Lowering travel speed during welding.Changing the joint design.Ensuring that surfaces to be joined fit properly.81.4.4 Weld profileWeld profile is important not only because of its effects on the strength and appearance of the weld, but also because it can indicate incomplete fusion or the figurehead of slag inclusions in multiple-layer welds. Under filling results when the joint is not change with the proper amount of weld metal Figure 2.7. Undercutting results from melting away the base metal and subsequently generating a groove in the shape of recess or notch. Unless it is not deep or sharp, an undercut can act as a stress raiser and reduce the fatigue strength of the joint and may lead to premature failure. Overlap is a surface discontinuity generally caused by poor welding practice and selection of the wrong materials. A proper weld is shown in Figure 2.7c.51.4.5 CracksCracks may occur in various locations and direction in the weld area. The types of cracks are typically longitudinal, transverse, crater, and toe cracks Figure 2.8. These cracks generally result from a combination of the following factorsTemperature gradients that cause thermal stresses in the weld zone.Variations in the composition of the weld zone that cause different contractions.Embitterment of grain boundaries by segregation of elements, such as sulfur, to the grain boundaries as the solid-liquid boundary moves when the weld metal begins to solidify.Hydrogen embitterment.Inability of the weld metal to contract during cooling is a situation similar t o hot tears that develops in castings and related to excessive restraint of the work piece.(a) crater cracks. (b)Various types of cracks in butt and T joints.8Cracks are classified as hot or cold cracks. Hot cracks occur while the joint is still at elevated temperatures. Cold cracks develop after the weld metal has solidified. Some crack prevention measures areChange the joint design to minimize stresses from shrinkage during cooling.Change welding-process parameters, procedures, and sequence.preheat components being welded.Avoid rapid cooling of the components after welding.81.4.6 Lameller tearsIn describing the anisotropy of plastically deformed metals, we stated that because of the alignment of nonmetallic impurities and inclusions (stringers), the work piece is weaker when tested in its thickness direction. This condition is particularly evident in rolled plates and structural shapes. In welding such components, lamellar tears may develop because of shrinkage of the members in t he members or by changing the joint design to make the weld bead penetrate the wearer member more deeply.81.4.7 Surface damageDuring welding, some of the metal may spatter and be deposited as small droplets on adjacent surfaces. In arc welding possess, the electrode may inadvertently contact the parts being welded at places not in the weld zone (arc strikes). Such surface discontinuities may be objectionable for reasons of appearance or subsequent use of the welded part. If severe, these discontinuities may adversely affect the properties of the welded structure, particularly for notch-sensitive metals. Using proper welding techniques and procedures is important in avoiding surface damage.81.5 Skill and Training requirementsMany of the traditional welding processes required high levels of operator scientific discipline and dexterity, this can involve wooly training programs, particularly when the procedural requirement described above need to be met. The newborner processes can o ffer some reduction in the overall skill requirement but this unfortunately been replaced in some cases by more complex equipment and the time involved in establishing the process parameters has brought about a reduction in operating factor. Developments, which seek to simplify the operation of the equipment, will be described below but effective use of even the most advanced processes and equipment requires appropriate levels of operator and support staff training. The cost of this training will usually be recovered very quickly in improved productivity and quality.1.6 Areas for developmentAdvances in welding processes may be justified inIncreased deposition rate bring down cycle timeImproved process controlReduced reinstate rateReduced weld sizeReduced joint preparation timeImproved operating factorReduction in post-weld trading operationsReduction in potential safety hazardsRemoval of the operator from unsettled areaSimplified equipment setting.Some or all these requirement ha ve been met in many of the process developments which have occurred in the ten long time these will be described in detail in the following chapters but the current trends in the of this applied science are examined below.1.7 New processesThe primary coil incentive for welding process development is the need to improve the correspond cost effectiveness of joining operations in requirement for new processes. Recently, concern over the safety of the welding environment and the potential shortfall of skilled technicians and operator in many countries have become important considerations.Many of the traditional welding techniques described in this Chapter are regarded as costly and hazardous and it is possible to improve both of these aspects significantly by employing some of the advanced process developments described in the following chapters.The use of new joining techniques such as Friction Stir Welding appears to be increasing since it does not involve melting. The application o f these processes has in the bypast been restricted, but with the increased recognition of the benefits of automation and the requirement for high-integrity joints in newer materials it is envisaged that the use of these techniques will grow.This is a new process originally intended for welding of aerospace alloys, especially aluminum extrusions. Whereas in conventional friction welding, heating of interfaces is achieved through friction by rubbing two surfaces, in the FSW process, a third body is rubbed against the two surfaces to be joined in the form of a small rotating non-consumable tool that is plunged into the joint. The contact pressure causes frictional heating. The probe at the tip of the rotating tool forces heating and mixing or stirring of the material in the joint.1.8 Research objectivesThe objectives of our project are toAdopt FSW to a mill about machine throw the FSW tools, select its material and have it manufacturedDesign the required clamping systemApply FSW to plates of an alloy that is not readily weldable by conventional methodsInvestigate FSW parameters (RPM, Feed Rate and Axial force)Analyze conventionally welded and Friction Stir welded sections then compare their properties.The objective of this research is to characterise the mechanical properties of friction stir welded joints and study the micro structure of the base metal and the weld nugget evolved during the friction stir welding of similar and dissimilar alloys of Aluminum.Aluminum 2024 and 7075 are considered for this investigation. The mechanical properties such as ultimate tensile strength, yield strength, formability, ductility and vickers hardness are measured and an effort is do to find out a relation between the process variables and properties of the weld. The best process parameters for the Friction-Stir welding of AA2024 and AA7075 will be defined based on the experimental results.Having understood the significance of FSP, the main objective of this thesis is to go over the effect of process parameters like rotational and translational speeds on the forces generated during FSP of aluminum alloys and relate these forces with the microstructure evolved in order to optimize the process.The specific objectives of the work presented areDesign and conduct FS processing experiments on aluminum alloy for different combinations of rotational and translation speeds.Measuring the generated processing forces during FSP of aluminum alloysExamine the microstructural of the touch on sheets using transmission electron microscope (TEM).Attempt to establish a correlativity between these measured forces and the resulting microstructure.Chapter 2 Review of Literature2.1 General idea of the Friction Stir TechnologyThis section gives an insight into the innovative technology called friction stir technology.The action of rubbing two objects together do friction to provide heat is one dating back many centuries as stated by Thomas et.al 1. The principles of t his method now form the basis of many traditional and novel friction welding, go up and processing techniques. The friction process is an efficient and controllable method of plasticizing a specific area on a material, and thus removing contaminants in preparation for welding, surfacing/cladding or extrusion. The process is environmentally friendly as it does not require consumables (filler wire, flux or gas) and produces no fumes. In friction welding, heat is produced by rubbing components together under load. Once the required temperature and material deformation is reached, the action is terminated and the load is maintained or increased to create a solid phase bond. Friction is ideal for welding dissimilar metals with very different melting temperatures and physical properties. Some of the friction stir technologies are shown in the Fig.2-1.Work carried out at TWI by Thomas et.al 2,3 has demonstrated that several alternative techniques exist or are being developed to meet the r equirement for consistent and reliable joining of mass production aluminum alloy vehicle bodies. Three of these techniques (mechanical fasteners, lasers and friction stir welding) are likely to make an impact in industrial processing over the next 5 years. FSW could be applied in the manufacture of straight-line welds in sheet and extrusions as a low cost alternative to arc welding (e.g. in the fabrication of truck floors or walls). The development of robotized friction stir welding heads could extend the range of applications into three dimensional components.Mishra et.al 4 extended the FSW innovation to process Al 7075 and Al 5083 in order to render them superplastic. They observed that the grains obtained were recrystallized, equiaxed and homogeneous with average grain sizes