Contents
Chapter 1 Introduction
1.1 Background and Motivation
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Chapter 1
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joined because of the different thermal coefficients of conductivity and expansion for different metals.
The solid state welding is the process where the joint is produced at temperature below the melting point temperature of the base metal without any filler material or inert ambience.
In this type of welding the metals being joined retain their original properties as melting does not occur in the joint and the heat affected zone (HAZ) is also very small compared to fusion welding techniques where most of the deterioration of the strengths and ductility begins. Dissimilar metals can also be joined with ease as the thermal expansion coefficients and the thermal conductivity coefficients are less important as compared to fusion welding.
Friction stir welding (FSW) is a newly developed solid state joining method in which the joined material is plasticized by the frictional heat generated between the surface of the plates to be welded and the contact surfaces of a special rotating tool. The tool is composed of two main parts, namely shoulder and pin. Shoulder is responsible for the generation of heat and containing the plasticized material in the weld zone, while pin mixes the material of the components to be welded, thus creating a joint. Though tools are designed for different applications may have slightly different shapes of the tool pin and shoulder, all tools maintain this same two elemental design. The schematic diagram of FSW process is shown in Fig. 1.1.
Figure 1.1 Friction stir welding process (a) positioning of plates, (b) pin and shoulder penetration, (c) tool travel and (d) pin removal.
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The four basic steps involved in the process (shown in Fig. 1.1) can be stated as follows:
a) Positioning of the workpiece material so that the tool should travel along the joint line.
b) Tool penetration or plunging into the workpiece in rotating condition.
c) Tool travel along the joint line.
d) Tool removal at the end of the process leaving a hole of size equal to tool pin.
FSW has a number of process parameters that influences the joint quality both in terms of mechanical and metallurgical properties. The different process variables involved in FSW are shown in Fig. 1.2. The involvement of large numbers of process parameters makes it difficult to control the joint properties. However, all of them may not have equal influences on the weld qualities. Therefore, studying the relative importance of process parameters on the individual weld quality parameter and also to the collective quality characteristics are of great important. This may help to identify fewer significant parameters which control the weld properties. The selection of optimal process parameters settings which will ensure the desired weld quality characteristics is another important issue. The minimization of post welding works and material wastage is always important for a production unit. So for FSW process finding an appropriate starting point and elimination of end hole by some low cost methodologies is always going to be useful. Use of soft computational methods to model FSW process can be beneficial as these methods deal with complex and highly interactive processes.
Figure 1.2 Friction stir welding process parameters
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Due to the combined effect of frictional heating and plastic deformation, various zones are formed in FSW process. The different zones are shown in Fig. 1.3. Unaffected material or base material (BM) is remote from the weld, which has not been deformed. In this zone there is no change in microstructure as it is not affected either thermally or mechanically.
HAZ which clearly lays close to the weld center material and experiences transient thermal cycle, which has modified the micro-structure and mechanical properties. In this zone expanded grains are available. Thermo-mechanically affected zone (TMAZ) lies between the HAZ and the nugget zone (NZ). In this region the material is plastically deformed by the stirring action of the friction stir welding tool and also the material is undergone a thermal cycle by the heat from the process. So the grains of the original microstructure are in a deformed state. Usually elongated grains are found in TMAZ. The recrystallized area in the TMAZ in aluminum alloys is called the nugget. It is the one which experiences the most severe plastic deformation and is a consequence of the way in which a tool consolidates material from the front to the back of the weld. The grains of the original microstructure in this region are also more refined than the TMAZ.
Figure 1.3 FSW joint with four distinct zones
The solid state nature of FSW immediately leads to several advantages over fusion welding methods since any problems associated with solidification are avoided. Issues such as porosity, solute redistribution, solidification cracking and liquation cracking are not prone in FSW. In general, FSW has been found to produce a low concentration of defects and is very tolerant to variations in parameters.
A number of potential advantages of FSW over conventional fusion welding processes have been identified as:
• The joint shows good strength, ductility and fracture toughness.
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• Operating temperature below the melting point of the parent metal avoids most of the thermal defects related to conventional fusion welding.
• Very less deformation which makes the process suitable for welding of relatively thin plates.
• Improved safety due to the absence of toxic fumes or the spatter of molten material.
• No consumables - conventional steel tools can weld over 1000 m of aluminum and no filler or gas shield is required.
• Easily automated on a simple milling machine, hence lower setup costs and less training.
• Shows almost similar properties as in parent metals in weld which leads to obtain a structure of different parts as a single one.
• Simplifies dissimilar alloy welding.
• Can operate in horizontal and vertical positions, as there is no molten weld pool.
• Generally good weld appearance and minimal thickness under/over matching, thus reducing the need for expensive machining after welding.
• No grinding, brushing or pickling required in mass production.
• Low environmental impact.
Nevertheless, FSW is associated with few unique limitations. Insufficient weld temperatures, due to low rotational speeds or high traverse speeds, may result in long, tunnel defects running along the weld which may be in surface or subsurface level. Low temperatures may also limit the forging action of the tool and reduce the continuity of the bond between the materials from each side of the weld. Below listed are some of the limitations of FSW process.
• Cannot make joints which required metal deposition.
• Less flexible than manual and arc processes (difficulties with thickness variations and non-linear welds).
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• Often slower traverse rate than some fusion welding techniques although this may be offset if fewer welding passes are required.
The FSW process is most suitable for components, which are flat and long (plates and sheets) but can be adapted for pipes, hollow sections and positional welding. The applications of the process are discussed below in detail.
• Ship Building and Marine Industries: The shipbuilding and marine industries are two of the first industry sectors, which have adopted the process for commercial applications. The process is suitable for the following applications in: panels for decks, sides, bulkheads and floor, helicopter landing platforms, marine and transport structures, refrigeration plant etc.
• Aerospace Industry: At present the aerospace industry is welding prototype parts by FSW. Opportunities exist to weld skins to spars, ribs, and stringers for use in military and civilian aircraft. This offers significant advantages compared to riveting and machining from solid, such as reduced manufacturing costs and weight savings. The process can therefore be considered for: wings, fuselages and empennages, cryogenic fuel tanks for space vehicles, aviation fuel tanks, military and science rockets etc.
• Railway Industry: The commercial production of high speed trains made from aluminum extrusions which can be joined by FSW has been published.
Applications include: high speed trains, railway tankers and goods wagons, container bodies etc.
• Land Transportation: The FSW process is currently being experimentally assessed by several automotive companies and suppliers to this industrial sector for its commercial application. Potential applications are engine and chassis cradles, attachments to hydro formed tubes, truck bodies, mobile cranes and fuel tankers, buses and airfield transportation vehicles, motorcycle and bicycle frames.
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• Other Industry Sectors: FSW can also be considered for electric motor housing, cooking equipment and kitchens, gas tanks and gas cylinders and connection of aluminum or copper coils in rolling mills.