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1.1 Introduction
Friction Stir Welding (FSW) process is relatively a new joining process that is presently attracting considerable interest. FSW is emerging as an appropriate alternative technology with high efficiency due to high-processing speeds. Since the joint can be obtained below the melting temperature, this method is suitable for joining a number of materials those are extremely difficult to be welded by conventional fusion techniques. The process is solid state in nature and relies on the localized forging of the weld zone to produce the joint.
Friction Stir Welding (FSW) is a relatively new technique developed by The Welding Institute (TWI) for the joining of Aluminium alloys.
Friction stir welding is considered to be the most energy efficient, harmless to the environment and produces very effective weld structure such that the mechanical properties are very well retained and improved. For Aluminium conventional welding techniques are least effective so most commonly used technique is friction stir welding. Friction Stir Welding is a solid-state thermo-mechanical joining process (a combination of extruding and forging), invented by The Welding Institute (TWI) in1991, that has become a viable manufacturing technology of metallic sheet and plate materials for applications in various industries, including plate materials for applications in various industries, including aerospace, automobile, defense and shipbuilding.
1.2 Friction Stir Welding
Friction Stir Welding (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, environment friendliness and versatility. As compared to the conventional welding methods, FSW consumes considerably less energy. No cover gas or flux is used, thereby making the process environmentally friendly. The joining, does not involve any use of filler metal and therefore any aluminum alloy can be joined without concern for the compatibility of composition, which is an issue in fusion welding. When desirable, dissimilar aluminum alloys and composites can be joined with equal ease.
In contrast to the traditional friction welding, which is usually performed on small axial symmetric parts that can be rotated and pushed against each other to form a joint, FSW can be applied to various types of joints like butt joints, lap joints, T butt joints and fillet joints. The key benefits of FSW are summarized in Table 1.1.
Before the invention of FSW, there had been some important technological developments of non – fusion welding processes, which have found some limited industrial uses. A significant process of these is friction welding developed at the time just before laser was invented. During friction welding, the pieces to be welded are compressed together and are made to more relative to each other. Thus frictional heat is generated to soften the material in the joining region. The final step is made by applying increased pressure to the softened material to yield a metallurgical joint without melting the joining material. However, the relative movement during the stage of heat generation and material softening can practically only be rotational or linear. Although friction welding operation is simple, the welding geometry is quite restricted and thus its use is also limited.
For solid state welding, the thermo mechanical principle of friction welding had actually laid an important base for the later invention of FSW. The Welding Institute (TWI) in the UK had for years engaged in various R&D and industrial activities of friction welding and surfacing. Wayne Thomas and his colleagues in TWI had long worked on and developed friction extrusion, friction hydropillar processing and third-body friction joining processes.
To date it is with aluminium alloys that FSW is most successfully applied. The reason for the predominant use of FSW on aluminium alloys is a combination of process simplicity in principle and the wide use of aluminium alloys in many major industries. It is especially the case where some aluminium alloys are difficult to fusion weld as, for example, is clearly evident in FSW application made by Boeing for making the Delta 2 rocket tanks. FSW allowed them to dramatically reduce their defect rate to nearly zero. Maximum temperature during FSW can reach just below the solidus of the workpiece alloy. For most aluminium alloys, it is significantly less than 660 ºC. Thus, H13 tool steel or high-speed tool steel, which is quite inexpensive, is a satisfactory tool material. Thus, FSW of aluminium alloys is relatively straightforward, although FS engineering, particularly for components and structures of high geometry complexity, can be quite challenging.
Friction stir welded advanced high strength steel (AHSS) joints are scanty. However, FSW and friction spot stir welding (FSSW) allow the possibility of joining advanced high strength steels and reduce problems associated with resistance spot welding (RSW). In principle, FSW could be applied for welding of all solid metallic materials. During FSW of steels, the local operating temperature generated by both friction and deformation needs to be at 1100 ºC – 1200 ºC so that the workpiece material is sufficiently plasticized for stirring and welding. Such high operating temperatures and the necessary forces acting on the tool during FSW create an extraordinary demand on the mechanical properties of the tool material.
1.3 Principles involved in Friction Stir Welding
Friction stir welding (FSW) produces welds by using a rotating, non-consumable welding tool to locally soften a workpiece, through heat produced by friction and plastic work, thereby allowing the tool to “stir” the joint surfaces. The dependence on friction and plastic work for the heat source precludes significant melting in the workpiece, avoiding many of the difficulties arising from a change in state, such as changes in gas solubility and volumetric changes, which often plague fusion welding processes. Further, the reduced welding temperature makes possible dramatically lower distortion and residual stresses, enabling improved fatigue performance, new construction techniques, and making possible the welding of very thin and very thick materials.
FSW has also been shown to eliminate or dramatically reduce the formation of hazardous fumes and reduces energy consumption during welding, reducing the environmental impact of the joining process. FSW can be used in any orientation without regard to the influence of gravitational effects on the process. These distinctions from conventional arc welding processes make FSW a valuable new manufacturing process with undeniable, economic, and environmental benefits.
FSW is an innovative solid state bonding technique. In early years, it was introduced for light alloys. Recently, high performance tools materials are employed for FSW of high melting temperature materials such as titanium, nickel and steels.
1.4 Comparison of FSW to other welding techniques
Comparison of FSW to other welding processes is typically done within the context of justifying the use of the process over other, more conventional techniques. Successful application of FSW depends upon a clear misunderstanding of the characteristics of the process, so favorable technical and economic justification can be developed.
The unique, favorable characteristics of FSW compared to traditional arc welding methods provide several sources for technical justification for use of the process. The main points for technical justification of FSW compared to arc welding processes are:
• Reduced residual stress, improved fatigue, corrosion, and stress corrosion cracking performance
• Reduced distortion
• Improved weld ability
• High robustness, few process variables
• Improved cosmetic performance
• Elimination of under matched filler metal
• Improved static strength and ductility
• Mechanized process
1.5 Types of Welding Tools
Many of the advanced made in friction stir welding have been enabled by the development of new welding tools. The welding tool design, including both its geometry and the material from which it is made, is critical to the successful use of the process.
Welding tool geometry development led to the first sound welds made in aluminium alloys and this has led to higher weld production speeds, higher workpiece thickness, improved joint properties, new materials and new welding equipment.
Tool geometry is the most influential aspect of process development. The tool geometry plays a critical role in material flow and in turn governs the traverse rate at which FSW can be conducted. Various FSW tool profile are shown schematically in Fig. 1.2. The tool has two primary functions: (a) localized heating, and (b) material flow. In the initial stage of tool plunge, the heating results primarily from the friction between pin and workpiece. Some additional heating are the results from deformation of the material. The tool is plunged till the shoulder touches the workpiece. The friction between the shoulder and workpiece results in the biggest component of heating. From the heating aspect, the relative size of pin and shoulder is important, and the other design features are not critical. The shoulder also provides confinement for the heated volume of material. The second function of the tool is to ‘stir’ and ‘move’ the material. The uniformity of microstructure and properties as well as process loads is governed by the tool design. Generally, a concave shoulder and threaded cylindrical pins are used.
Types of Friction Welding
Friction stir welding has following types:
• Rotary Friction Welding (RFW)
• Linear Friction Welding (LFW)
• Friction Stir Welding. (FSW)
1.7.1 Rotary Friction Welding (RFW)
Rotary friction welding is one, in which one component is rotated against the others, it is the most commonly used of the processes and many carbons steel vehicle axles and sub-axles are assembled in this way. The process is also used to fabricate suspension rods, steering columns, gear box forks, drive shafts, as the well as the engine valves, in which the ability to join dissimilar materials means that the valve stem and head can be made of materials suited to their different duty cycles in service.
1.7.2 Linear Friction Welding (LFW)
Linear friction welding is so named because the relative motion linear across the interface, rather than rotary and is already used to join blades on to discs in the aero engine industry. Low cost linear friction welding machine are now being developed for automotive application such as the fabrication of brake discs, wheel rims and engine parts.
1.7.3 Friction Stir Welding (FSW)
Friction stir welding also produces a plasticized region of material, but in a different manner. A non-consumable rotating tool is pushed into the material to be welded. Then the central pin, or probe followed by the shoulder is brought in to contact with the two parts to be joined. The rotation of the tool heats up and plasticizes the materials is in contact with. As the tool moves along the joint line, material from the front of the tool is swept around this plasticized annulus to the rear, so eliminating the interface.
1.8 Microstructure Classification of Friction Stir Welds
The first attempt at classifying microstructures was made by P L Threadgill. This work was based solely on information available from aluminium alloys. However, it has become evident from work on other materials that the behavior of aluminium alloys is not typical of most metallic materials, and therefore the scheme cannot be broadened to encompass all materials. A more comprehensive scheme has been developed by TWI, and has been discussed with a number of an appropriate people in industry and academia. This has also been accepted by the Friction Stir Welding Licenses Association. The system divides the weld zone into distinct regions.
1.8.1 Unaffected Material or Parent Metal
This is material remote from the weld, which has been deformed, and which although it may have experienced a thermal cycle form the weld is not affected by the heat in terms of microstructure or mechanical properties.
1.8.2 Heat Affected Zone (HAZ)
In this region, which clearly will lie closer to the weld centre, the material has experienced a thermal cycle which has modified the microstructure and the mechanical properties. However, there is no plastic deformation occurring in this area. In the previous system, this was referred to as the "thermally affected zone". The term heat affected zone is now preferred, as this is a direct parallel with the heat affected zone in other thermal processes, and there is little justification for a separate name.
1.8.3 Thermo Mechanically Affected Zone (TMAZ)
In this region, the material has been plastically deformed by the friction stir welding tool, and the heat form the process will also have exerted some influence on the material. In the case of aluminium, it is people to get significant plastic strain without recrystallization in this region, and there is generally a distinct boundary between the recrystallized zone and the deformed zones of the TMAZ. In the earlier classification, these two subzones were treated as distinct micro structural region. However, subsequent work on other materials has shown that aluminium behaves in a different manner to most other materials. In that it can be extensively deformed at high temperature without recrystallization. In other materials, the distinct recrystallized region (the nugget) is absent, and the whole of the TMAZ appears to be recrystallized. This is certainly true of materials which have no thermally induced phase transformation which will in itself induce recrystallization without strain, for example pure titanium, α-titanium alloys, austenitic stainless steels and copper. In materials such as ferrite steels and α-β titanium alloys (e.g. Ti-6A1-4V), understanding the microstructure is made more difficult by the thermally induced phase transformation, and this can also make the HAZ/TMAZ boundary difficult to identify precisely.
1.8.4 Weld Nugget
The recrystallized area in the TMAZ in aluminium alloys has traditionally been called the nugget. Although this term is descriptive, it is not very scientific. However, is use has become widespread, and as there is no word which is equally simple with greater scientific merit, this term has been adopted. It has been suggested that the area immediately below the cool shoulder (which is clearly part of the TMAZ) should be given a separate category, as the grain structure is often different here. The microstructure here is determined by rubbing by the rear face of the shoulder, and the material may have cooled below its maximum. It is suggested that this area is treated as a separate sub-zone of the TMAZ
Applications
Commercial applications have been reported across many industries, and some selected examples are shown below which illustrate the widening appeal of the process.
Marine
It is believed that the first commercial application of FSW was the joining of 6xxx series alloy extrusions for use in fish freezing plants for fishing vessels. There have been numerous applications of the process for joining 6xxx extrusions for incorporation in bulkheads and decks in various high speed aluminium vessels, and in large steel cruise ships which now often have lightweight aluminium superstructures. In such applications, the FSW panels are very flat due to the low distortion, and are cut up and welded into larger structures, usually by MIG welding. Friction stir welding has been used extensively in the aluminium superstructures of cruise ships such as the 'Seven Seas Navigator' which contain many kilometres of friction stir welds, mostly in 6xxx grade extrusions. The world's largest aluminium vessel, the Japanese fast ferry 'Ogasawara', launched in 2004, makes extensive use of FSW in its superstructure.
Aerospace
The first major application was the use of the process to replace fusion welding in fuel tanks for unmanned Delta II and later Delta IV rockets. The manufacturer (Boeing) has reported virtually zero defect incidence, and significant cost savings over the previous variable polarity plasma arc (VPPA) process. The process has also been adopted for the large fuel tank for the Space Shuttle.
Almost all the major airframe manufacturers are investigating the use of FSW (alongside other welding processes such as laser welding) to replace many of the rivets in current structures. The first aircraft to make extensive use of FSW in its airframe, the Eclipse 500 business jet, has recently completed certification and is now in production. In this aircraft, over 7300 fasteners (approximately 60% of the total) are replaced by 263 friction stir welds.
Rail
High speed aluminium railcars such as the Japanese Shinkansen are normally built from complex double skin extrusions in 6xxx alloys. Since the welds which join these are long (up to 25m) and straight, FSW is an ideal process, and the very low distortion is cited as a major benefit.
Automotive
There are few long straight welds in road vehicles, and so adoption of FSW has primarily been for components such as suspension parts, wheels, seat components, crash boxes, etc. where several leading companies are already using the process in production. The needs of the automotive sector have driven the development of robotic FSW, to cope with the complex shapes and high volume/low cost culture of this market. Significant interest is now being shown in friction stir spot welding, where the linear translation of the tool is either very small or zero. Friction stir spot welding is rapidly gaining acceptance as an efficient method of joining aluminium sheet, and is already in production, for example on the Mazda Rx-8 sports car, where it is used on the aluminium bonnet and rear doors. Friction stir welding is also being developed for lightweight armoured vehicles, where the ability of the process to weld material of around 25-40mm thickness in one pass is being exploited.
LITERATURE REVIEW
1. Rama Narsu M, I Rani M
Among the three base materials considered, AA6061 was found to exhibit better mechanical properties and this alloy is found to be amenable for friction stir welding by different tool profiles. H13 tool material is found to withstand for AA6061 without breakage of tip at the time of welding process. Increase in tool rotational speed causes more heat input and the tensile strength is low for increase in TRS as the TMAZ and HAZ is more. The Square profiled tool facilitates the stirring action from tip to the collar, and due to this the turbulence is avoided, when compared with the use of other tool profiles. The defect-free welds were possible with the square profiled tool for the same reason. The tool rotational speed of 1000 rpm, weld speed of 60mm/min and axial force of 6kN generated good welded joints (with maximum values of mechanical properties that were obtained) when Square profiled tool is used. From the results obtained it can be concluded that the shape of the tool pin and shoulder play a very important role in obtaining better mechanical properties for the weld joints. This is evident from the results obtained for the square pin profile due to flat faces produces a pulsating stirring action in the flowing material. A threaded cylindrical profile is also found to pulsating stirring action. This pulsating action leads to the development of smaller grains with uniformly distributed very fine structure and this, in turn, yielded higher strength and hardness.
2. T. DebRoy, A. De
During FSW of AA7075, fatigue is unlikely to be the mechanism of tool failure except for welding of 8 mm or thicker plates. Although the toughness of the tools varies, uncertainty in the toughness values does not change this finding. Bending stress, which affects the fatigue life of the tools, increases significantly with plate thickness, and somewhat less significantly with the reduction in tool shoulder radius and decrease in tool rotational speed. An increase in tool pin radius results in a higher peak temperature and lower maximum shear and bending stresses and a higher index of tool durability. Butt welding of thicker plates (with longer pins) leads to lower peak temperature, higher shear and bending stresses and considerably lower index of tool durability. Faster welding speed has similar effect as welding thicker plates, however, to a lesser extent for the welding conditions considered in the present work. An increase in axial pressure results in a higher peak temperature, lower shear and bending stresses and a higher index of tool durability. An increase in either the tool shoulder diameter or the tool rotational speed reduces the maximum shear stress and improves tool durability.
3. R. Palanivel
The relationships between process parameters for FS welding of AA6351 aluminum alloy have been established. The response surface methodology was adopted to develop the regression models, which were checked for their adequacy using ANOVA test, scatter diagrams and found to be satisfactory. Confirmation experiments showed the developed models are reasonably accurate. The increase in the tool rotational speed, welding speed and axial force leads to the increase in the ultimate tensile strength; and it reaches a maximum value and then decreases. This trend is common for yield strength and percentage of elongation.
4. A. Govind Reddy
Dissimilar aluminum alloys AA 2024 and AA 7075 were friction stir welded under varying TRS and WS, and the TS of the joints were measured. RSM was used to develop a mathematical model (regression) for TS in terms of TRS and WS and the model was used to investigate the effect of TRS and WS on the TS of the joints. The following conclusions are made from the investigations. Friction stir welding can be used to join AA2024 and AA7075 in dissimilar combinations. The tool rotation speed and welding speed are found to affect the tensile strength of the FS welded AA2024-AA7075 joints. Increasing the tool rotation speed increases the tensile strength of the FS welded AA2024-AA7075 joints. Increasing the welding speed resulted in a decrease in the tensile strength of the FS welded AA2024-AA7075 joints. The optimum tool rotation speed and welding speed for joining AA2024-AA7075 as found by using Nelder Mead algorithm are 1087.6 rpm and 14.72 mm/min.
5. Ramona GABOR, Jorge F. dos SANTOS
FSW is considered to be the most significant development in metal joining in the last decades and is a ‘‘green’’ technology due to its energy efficiency, environment friendliness and considerably less energy consume. The joining does not involve any use of filler metal and therefore any aluminium alloy can be joined without concern for the compatibility of composition, which is an issue in fusion welding. For aluminium alloys the process can be considered known, considering the many industrial applications of the process. It is important to evaluate the behaviour of the FSW connections under the HCF action in order to evaluate the possibility of using this type of welding to structures such as bridge decks and also to investigate the performances under LCF actions on such connections for structural components in buildings. New research directions are oriented in the development of new welding tool configurations that allows realizing fillet joints – used on large scale to structural components. The advantages of the process leads to new directions of applications to structural steels and also to high strength steels, which are more and more used in the field of civil engineering.
PROBLEM IDENTIFICATION
FSW is an emergent technology that can be used to overcome significant limitations of other joining processes. Its inherent mechanical properties advantages and operating cost advantages make it ideal for automotive application. Initially, travel speeds where poor as compared to gas metal arc welding, but travel speed improvements have been realized allowing FSW to meet or exceed GMAW travel speeds.
By proper application of the technology, the automotive industry can take advantage of the inherent advantages of the process and make its implementation a success. However, consideration for the process limitations and challenges must be made. The first consideration is to make sure the proper joint design is used. The lap penetration and partial penetration butt weld configuration are most likely to be successful.
The second major consideration is for the high force requirements of FSW. The challenge must and can be overcome. The high force requirement can first be overcome by selecting equipment specialized for the FSW process. Although commercial milling machines can be used and can generate the necessary forces, their closed architecture and inability to deal with the heat generated by the process make them poor sources for production machinery. Another issue with the high force requirements is that the welded product must be able to withstand the high forces at the point of welding resulting in the use of a backup or the optimization of the part geometry. In most cases, automotive products are designed for gas metal arc welding, resistance welding, riveting, etc., but they will be unable to withstand the force generated by FSW. Just as automotive structures are now designed for today’s production joining processes, they can just as well be designed for FSW.
Flaws arise in most materials joining processes. For example, when arc welding aluminium alloys, weld metal porosity and, depending on the particular alloy, weld metal solidification cracking and HAZ liquation cracking are among the most common flaw types. The occurrence of such problems has contributed to the widely held view that some aluminium alloys, in particular some of the high strength 2xxx and 7xxx series alloys, are difficult, or indeed impossible, to fusion weld successfully. Being a solid state joining process, FSW obviates the problems of porosity and hot cracking. In this respect it is worthwhile to make a distinction between flaws and defects, although the two terms are often used interchangeably within the literature. The usual distinction is that a flaw or imperfection is a feature that one would prefer not to be in the weld, but it may or may not compromise the integrity of the weld. If, after evaluation, the flaw is deemed unacceptable, then it becomes a defect. If it does not compromise the integrity, then it is a tolerable flaw. Flaws or discontinuities should be characterised as defects only when specific acceptance criteria, related to the engineering application, are exceeded, and the presence of the flaw compromises the integrity of the structure. In fact the most common flaw types are caused by use of under optimised parameters or a lack of process control. Since understanding of the causes of these flaws/defects is good, it is usually possible to rectify these problems by changes to parameters, tool designs or operating practice.
Prediction of void formation is a particularly difficult modeling problem, due in part to the limitations of the numerical methods used for flow modeling. Computational fluid dynamics solvers treat the deforming metal as a hot, viscous fluid, neglecting elasticity. Void formation necessarily implies a free surface, and is also strongly influenced by the hydrostatic pressure and the mechanism of cavitations. Computational fluid dynamics models indicate that a region of hydrostatic tension forms on the tool wake on the advancing side, but the neglect of elastic stresses means that the predicted pressures are unreliable quantitatively, and there is in any case no criterion available for the formation of a stable void in hot deforming metal.
Project will show the effect of variations in parameters on friction stir welding and its mechanical properties of the welded dissimilar aluminium alloys. It mainly focuses on the effect of variation in friction stir welding parameters and mechanical properties of the welded alloy, also the microstructure and micro hardness of the aluminum alloy.
EXPERIMENTATION
4.1 Friction stir welding process
In this process, a cylindrical-shouldered tool with a profiled threaded/unthreaded pin is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces of plate materials, which are butted together. The parts to be clamped rigidly onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. The length of the pin is slightly less than the weld depth required and the tool shoulder should be in intimate contact with the work piece surface. The pin is then moved against the work piece or vice versa. Frictional heat is generated between the wear resistant welded tool shoulder pin and the material of the work pieces. This hear along with the heat generated by the mechanical mixing process and the adiabatic heat with-in the material, cause the stirred material to soften without reaching the melting point. As the pin is moved in the direction of welding the leading face of the pin, assisted by a special pin profile, forces plasticized material to the back of the pin whilst applying a substantial forging force to consolidate the weld metal. The welding of the material is facilitated by severe plastic deformation in the solid state involving dynamic recrystallization of the base material. The welded joints will be sliced using power hacksaw and then machined to the required dimensions, ASTM E8M-04 guidelines should be followed for preparing the test specimens.
In FSW, a cylindrical shouldered tool with a profiled pin is rotated and plunged into the joint area between two pieces of sheet or plate material. The parts have to be securely clamped to prevent the joint faces from being forced apart. Frictional heat between the wear resistant welding tool and the work pieces causes the latter to soften without reaching melting point, allowing the tool to traverse along the weld line.
The plasticized material, transferred to the trailing edge of the tool pin, is forged through intimate contact with the tool shoulder and pin profile. On cooling, a solid phase bond is created between the work pieces. In terms of high temperature materials, FSW has been proven successful on numerous of alloys and materials, including high-strength steels, stainless steel and titanium. As what is weldable refers to the material by which the welding tool is made and how the process is applied there are really no limits to what can be achieved. An improvement on the existing methods and materials as well as new technological development, an expansion is expected.
Selection of Process Parameters
Welding process where metallic bonding is produced at temperature lower than the melting point of the base metals. Friction time, friction pressure, forging time, forging pressure and rotation speed are most interesting parameters in the friction sir welding method. Friction pressure (MPa), Friction time (s), forging time (s), forging pressure (MPa), rotation speed (rpm), work piece diameter (mm). The main variables in direct drive friction welding
• Rotation Speed (rpm)
• Axial force (kN)
• Feed rate (mm/min)
The process parameters workable range for the experiment was chosen in order to control the weld seams quality including defects in the root, type of defect more difficult to eliminate in sound welds. Therefore three levels of FS welding parameters
The FSW process involves joint formation below the base material’s melting temperature. The heat generated in the joint area is typically about 80-90% of the melting temperature. With arc welding, calculating heat input is critically important when preparing welding procedure specifications (WPS) for the production process. With FSW, the traditional components – current and voltage – are not present as the heat input is purely mechanical and thereby replaced by force, friction, and rotation. Several studies have been conducted to identify the way heat is generated and transferred to the joint area.
Weld ability of the materials can be varied in terms of shapes, comprising steps of urging and securing the assembled members towards each other, entering the assembled members along the joining line by a probe of material harder than the material of joined members under rotating movement which generates a frictional heat, thereby creating a plasticized region in the adjacent members' material, the method further comprising a homogenization of the resulting weld seam ensured by an enhanced flow of plasticized material both perpendicularly and vertically to the longitudinal extension of the adjacent assembled members by exposing the created plasticized material to a perpendicular pressure along the surface of the members and causing simultaneous material flow along the probe pin in the vertical direction allowing the plasticized material to solidify behind the probe.
The welding of aluminium and its alloys has always represented a great challenge for researchers and technologists. Friction stir welding (FSW) is a new welding process that has produced low cost and high quality joints of aluminium alloys.
To select an appropriate orthogonal array for experiments the total degrees of freedom need to be computed. The degrees of freedom are defined as the number of comparisons between process parameters that need to be made to determine which level is better and specifically how much better it is.
The Friction stir welding parameters are chosen from the Literature survey as L9 Orthogonal array (3³ = 27) is listed in table 4.4.
The three factors used in this experiment are the Tool rotation speed (rpm), axial force (kN) and Feed rate (mm/min).
Microhardness
The Vickers hardness test method, also referred to as a microhardness test method, is mostly used for small parts, thin sections, or case depth work. The Vickers method is based on an optical measurement system. The microhardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value. It is very useful for testing on a wide type of materials as long as test samples are carefully prepared. A square base pyramid shaped diamond is used for testing in the Vickers scale. Typically loads are very light, ranging from a few grams to one or several kilograms, although "Macro" Vickers loads can range up to 30 kg or more. The microhardness methods are used to test on metals, ceramics, and composites - almost any type of material.
Since the test indentation is very small in a Vickers test, it is useful for a variety of applications: testing very thin materials like foils or measuring the surface of a part, small parts or small areas, measuring individual microstructures, or measuring the depth of case hardening by sectioning a part and making a series of indentations to describe a profile of the change in hardness. The Vickers method is more commonly used.
ADVANTAGES AND DISADVANTAGES
6.1 Advantages
The advantages of FSW for welding aluminium can be summarised as follows:
1. As a solid state process it can be applied to all the major aluminium alloys and avoids problems of hot cracking, porosity, element loss, etc. common to aluminium fusion welding processes
2. As a mechanised process, FSW does not rely on specialised welding skills; indeed manual intervention is seldom required.
3. No shielding gas or filler wire is required for aluminium alloys.
4. The process is remarkably tolerant to poor quality edge preparation: gaps of up to 20% of plate thickness can be tolerated, although this leads inevitably to a reduction in local section thickness since no filler is added.
5. The absence of fusion removes much of the thermal contraction associated with solidification and cooling, leading to significant reductions in distortion; however, it is not a zero distortion technique.
6. It is very flexible, being applied to joining in one, two and three dimensions, being applicable to butt, lap and spot weld geometries; welding can be conducted in any position.
7. Excellent mechanical properties, competing strongly with welds made by other processes (see the section on 'Comparison with other joining processes').
8. Workplace friendly: there are no ultraviolet or electromagnetic radiation hazards as the absence of an arc removes these hazards from the process; the process is no noisier than a milling machine of similar power, and generates virtually zero spatter, fume and other pollutants.
9. The energy required at the weld for FSW lies between laser welding (which requires less energy) and metal inert gas (MIG) welding (which typically needs more)
10. High welding speeds and joint completion rates: in single pass welds in thinner materials, FSW competes on reasonable terms with fusion processes in terms of welding speed; in thicker materials, FSW can be accomplished in a single pass (e.g. the 50 mm tool), whereas other processes need multiple passes. This leads to higher joint completion rates for FSW, even though the welding speeds may be lower. Thick plates can also be joined by FSW on either side
11. Various mechanical and thermal tensioning strategies can be applied during welding to engineer the state of residual stress in the weld (see the section on 'Residual stress control').
6.2 Disadvantages
There are of course disadvantages to FSW; indeed, some of the advantages listed above can be viewed in a less positive light in certain circumstances. For example, the absence of a filler wire means that the process cannot easily be used for making fillet welds. Similarly, the fully mechanised nature of the process prevents its use for applications where access or complex weld shape is best suited to a manual process. The presence of a hole at the end of the weld from which the probe was withdrawn is often quoted as a disadvantage. In practice, this has seldom been a significant problem, as there are many possible solutions, which have been considered elsewhere. The work piece also needs to be restrained in well-designed support tooling, both to react to the forces applied, and to prevent the probe from pushing the work piece materials apart. Although the process may reduce the strength of aluminium alloys, this can be compensated for if necessary by appropriate design of the joint, for example by locally increasing the thickness, but in most cases no changes are made. Process economics are generally considered favorable, but specific published data are lacking. However, it is known that the process drastically reduces weld preparation costs, skilled welder requirements and repair rates. Efficient power consumption is dependent on matching the size of machine being used to the size of weld being made, although this is not always a practical option.
CONCLUSION
The butt joining of two dissimilar alloys of aluminum from AA6063 and AA7075 series is successfully completed by the technique of friction stir welding. The characteristic welded samples are evaluated on the basis of mechanical properties such as mechanical strength, yield strength, percentage of elongation, microstructure and micro-hardness on the HAZ as well as the welded zone. The testing of the samples yielded these conclusions:
• The overall mechanical testing was conducted based on the ASTM E8 M standard sample shape and the recorded results gave the result with sample WY9 with the optimum Tensile strength of 153.86 (MPa) and % elongation of 19.
• The process parameters considered for the weld are the Tool Rotation Speed, Axial Force and the Feed Rate. The optimum range where the obtained weld was strong when the Tool Rotation Speed -1200rpm, Axial Force -5kN and Feed Rate -60mm/min.
• The Micro-hardness test is carried out on the heat affected zone of both the parent metal as well as the welded region. The hardness value in the 6063 HAZ range from 54-74 HV. The hardness value in the 7075 HAZ range from 122-154 HV.
• The tool used for the welding process is a cylindrical tool with a straight shoulder. The tool is H13 steel which has high toughness and stability. The cylindrical tool is advantageous as is generates pulsating stirring action which leads to the development of finer grain and uniformly distributed structure throughout the weld thus increases better mechanical properties.
• The test results show the Micro-hardness of the welded zone range from 85-137 HV. The highest hardness value recorded is on the sample WY4.
• Microstructure consists of Fe3SiAl12 and Mg2Si particles distributed in a matrix of aluminium solid solution throughout the structure.
FUTURE SCOPE
The research could be taken further by inspecting the weldability of dissimilar metals with the help of friction stir welding. Friction stir welding is a recently developed technology so further development in the field is highly plausible. Further studies can be conducted by considering more number of process parameters and generating different iterations of results can lead to positive outcomes.
The main aim of our project was to combine two dissimilar aluminium alloys of varying mechanical properties to improve its overall mechanical property. The 7-series aluminium is considered to be the strongest of all aluminium alloys and it has low weldability. The 6-series is commonly available and it has higher weldability, so combining the two would provide a perfect combination and it can use widely. The end product can be widely used in tool development and also in the construction of heavy load automobiles.