Lohwasser / Chen Friction Stir Welding

From Basics to Applications
1. Auflage 2009
ISBN: 978-1-84569-771-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 436 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

Friction stir welding (FSW) is a highly important and recently developed joining technology that produces a solid phase bond. It uses a rotating tool to generate frictional heat that causes material of the components to be welded to soften without reaching the melting point and allows the tool to move along the weld line. Plasticized material is transferred from the leading edge to trailing edge of the tool probe, leaving a solid phase bond between the two parts. Friction stir welding: from basics to applications reviews the fundamentals of the process and how it is used in industrial applications.Part one discusses general issues with chapters on topics such as basic process overview, material deformation and joint formation in friction stir welding, inspection and quality control and friction stir welding equipment requirements and machinery descriptions as well as industrial applications of friction stir welding. A chapter giving an outlook on the future of friction stir welding is included in Part one. Part two reviews the variables in friction stir welding including residual stresses in friction stir welding, effects and defects of friction stir welds, modelling thermal properties in friction stir welding and metallurgy and weld performance.With its distinguished editors and international team of contributors, Friction stir welding: from basics to applications is a standard reference for mechanical, welding and materials engineers in the aerospace, automotive, railway, shipbuilding, nuclear and other metal fabrication industries, particularly those that use aluminium alloys. - Provides essential information on topics such as basic process overview, materials deformation and joint formation in friction stir welding - Inspection and quality control and friction stir welding equipment requirements are discussed as well as industrial applications of friction stir welding - Reviews the variables involved in friction stir welding including residual stresses, effects and defects of friction stir welds, modelling thermal properties, metallurgy and weld performance

Autoren/Hrsg.

Lohwasser, Daniela

Chen, Zhan

Leseproben

2 The friction stir welding process: an overview
K.J. Colligan Concurrent Technologies Corporation, USA Abstract
This chapter introduces the basic concepts relevant to the use and study of friction stir welding (FSW), including an overview of the process, a comparison to arc welding processes, a discussion of welding tool design and materials, the effect of process parameters, workpiece materials and joint geometries. References are given to point to early contributions in the various areas of study and to the latest progress in the field. Key words: friction stir welding joining solid state joining 2.1 Overview of friction stir welding (FSW) process principles
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. Owing to the typically high forces in the process, FSW is usually practiced as a fully mechanized process, increasing the cost of the equipment compared to arc welding techniques, while reducing the degree of operator skill required. 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. Further, 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 technical, economic, and environmental benefits. Central to the FSW process is the design of the welding tool, as shown schematically in Fig. 2.1. Many variations and new features have been added to this basic tool, as will be discussed further below and in Chapter 4. Conventional FSW, as the process was originally conceived, is done with a welding tool consisting of a shoulder, which rides on the surface of the workpiece, and a smaller diameter pin, which nearly penetrates the workpiece. The shoulder essentially performs the role of the “lid on the pot”, which prevents the escape of softened workpiece material as the tool is rotated and forced along the joint. The pin commonly employs thread-shaped features which act to push the surrounding workpiece material downward, assisting in the retention of material within the weld zone. The downward force applied to the tool to maintain the correct plunge depth also results in forcible contact between the shoulder and the workpiece surface, and relative motion from the tool rotation results in significant heat generation from friction at the shoulder interface. 2.1 Conventional FSW tool and key variables. In conventional FSW the pin accomplishes the breakup of the original faying surfaces of the joint. For this reason, the pin must penetrate to within 0.5 mm of the back of the workpiece to ensure complete penetration of the weld through the workpiece. Features cut into the pin surface, originally demonstrated as downward-pushing screw threads, prevent the formation of pores or voids in the weld. The pin generates heat by both friction and plastic work, and both seizure and sliding contact have been observed and predicted by modeling results. The design of the pin and shoulder has been an area of intense research since the conception of FSW, which has resulted in improvement in throughput, joint strength, and weld quality, and in the range of materials, joint geometries, welding parameters, and workpiece thickness that can be welded. A transverse section from a typical, conventional FSW joint is shown in Fig. 2.2. The weld is bounded on either side by unaltered, base metal (BM). Although BM near the weld zone does experience elevated temperature during welding, this material exhibits essentially the same properties as the workpiece in the as-received condition. Closer to the weld is the heat-affected zone (HAZ), which is heated sufficiently during welding to alter its properties without plastic deformation of the original grain structure. The alteration of properties in the HAZ may include changes in the strength, ductility, corrosion susceptibility, and toughness of the workpiece, but typically will not include changes in grain size or chemical makeup. Heating in the HAZ is generally high enough in aluminum alloys to result in recovery of cold work and coarsening of precipitates, which is the root cause of changes in properties in this region. 2.2 Typical conventional FSW transverse section in 25.4-mm thick 2195 aluminum-lithium plate. The thermomechanically affected zone (TMAZ) encompasses all of the plastically deformed material within the joint region. In this region, the workpiece is sufficiently heated and softened and the process forces are sufficiently high, to result in plastic deformation of the original grain structure. The TMAZ can be further divided into the unrecrystallized TMAZ and the nugget, or recrystallized TMAZ. In aluminum alloys, the unrecrystallized TMAZ may be an important feature in the weld, since it can be of significant size and can represent a region of low microhardness and increased corrosion susceptibility. Further, in aluminum alloys the nugget material is generally composed of fine grain size material and is considered to have experienced severe plastic deformation due to interaction with the welding tool pin and in some cases may actually mimic the shape of the pin profile. However, in materials that experience thermally induced phase transformation, the TMAZ may consist entirely of recrystallized material, while in other materials the TMAZ may be completely unrecrystallized, without regard to the size or shape of the pin. Conventional FSW is typically carried out by first rigidly fixing the plates to be joined in a welding fixture, as shown in Fig. 2.3. Fixture design is a very important consideration in FSW, which is discussed in detail in Chapter 4. The plates are typically fixed with no gap at the joint line. The process requires that the workpieces be prevented from spreading or lifting during welding, so welding fixtures are typically equipped with features which restrain the workpiece. It is common that FSW fixtures are equipped with a removable anvil insert which can be replaced in the event of inadvertent damage to the anvil from contact with the welding tool pin. Since the anvil insert is very closely coupled to the workpiece at the point of welding in terms of heat transfer, it is important to consider the mass and diffusivity of the anvil insert when designing FSW fixtures. 2.3 Conventional FSW fixture requirements. The FSW process can be thought to consist of three phases: the plunge phase, where the weld is initiated; the main phase, where the weld is made; and the termination phase, where the welding tool is withdrawn from the workpiece. The properties of the weld produced are, of course, dependent on the process parameters selected for each phase of the weld, so great care must be taken in establishing these settings. The plunge phase consists of inserting, or “plunging”, the rotating welding tool into the joint. This is typically accomplished by commanding the welding system to drive the tool pin axially into the workpiece at a specific rate or with a specific force. Frictional heating and pressure at the end of the pin induce workpiece material to displace, forming a ring of expelled, plastically deformed material around the pin as the pin enters the workpieces. As the tool is plunged into the joint, heat generated conducts into the surrounding material and the anvil. The plunge phase may be facilitated by drilling a hole at the plunge location, reducing the heat and forces produced. Alternatively, the welding tool may be plunged into the side of the workpiece, although this approach is less commonly applied. Once the welding tool is plunged into the workpiece, the tool is typically driven laterally along the joint without delay, although in some materials, it may be necessary to dwell at the plunge location for some time in order to allow for the welding tool and workpiece to reach a higher temperature. Once the welding tool begins to travel along the joint, friction and plastic work produce heat to maintain sufficient softening in the workpiece to permit material flow around the pin. Features cut into the pin surface, such as screw threads, flats, and spiral grooves, facilitate this material flow by increasing drag between the pin and the surrounding material in such a way as to prevent the formation of internal voids or fractures. Heat from the welding process conducts within the workpiece, serving to precondition the material in front of the tool, producing softening from recovery of work hardening and overaging in materials such as aluminum. This metallurgical alteration may be slight, such as in when welds are made at very high travel speed, or it may dramatically...