15-02-2013, 04:37 PM
AN OUTSIDER LOOKS AT FRICTION STIR WELDING
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ABSTRACT
Friction stir welding (FSW) is a fairly recent technique that utilizes a nonconsumable rotating welding tool to generate frictional heat and plastic deformation at the welding location, thereby affecting the formation of a joint while the material is in the solid state. The principal advantages of FSW, being a solid-state process, are low distortion, absence of melt-related defects and high joint strength, even in those alloys that are considered nonweldable by conventional techniques. Furthermore, friction stir (FS) welded joints are characterized by the absence of filler-induced problems / defects, since the technique requires no filler, and by the low hydrogen contents in the joints, an important consideration in welding steels and other alloys susceptible to hydrogen damage. FSW can be used to produce butt, corner, lap, T, spot, fillet and hem joints, as well as to weld hollow objects, such as tanks and tubes / pipes, stock with different thicknesses, tapered sections and parts with 3-dimensional contours. The technique can produce joints utilizing equipment based on traditional machine tool technologies, and it has been used to weld a variety of similar and dissimilar alloys as well as for welding metal matrix composites and the for the repair of existing joints.
INTRODUCTION
Friction stir welding (FSW) is a fairly recent welding technique, invented by The Welding Institute (TWI), Cambridge, UK. 1 This technique utilizes a nonconsumable rotating welding tool to generate frictional heat and deformation at the welding location, thereby affecting the formation of a joint, while the material is in the solid state. The principal advantages of FSW, being a solid-state process, are low distortion, absence of melt-related defects and high joint strength, even in those alloys that are considered nonweldable by conventional techniques (e.g., 2xxx and 7xxx series aluminum alloys). Furthermore, friction stir (FS) welded joints are characterized by the absence of filler-induced problems / defects, since the technique requires no filler. Finally, the hydrogen contents of FSW joints tend to be low, which is important in welding steels and other alloys susceptible to hydrogen damage.
FSW has been successfully used to weld similar and dissimilar cast and wrought aluminum alloys, steels, as well as titanium, copper and magnesium alloys, dissimilar metal group alloys and metal matrix composites. The technique can be used to produce butt, corner, lap, T, spot, fillet and hem joints as well as to weld hollow objects, such as tanks and tube / pipe, and parts with 3-dimensional contours. Apart from producing joints, FSW is also suitable for repair of existing joint. The primary industrial and research interest, however, has been focused on butt welding aluminum alloy sheet and plate up to 3.00 in. thick. FSW was also used to produce butt joints between metals of different thicknesses and between tapered sections. FSW can be performed in all positions (horizontal, vertical, overhead and orbital), and it can produce or repair joints utilizing equipment based on traditional machine tool technologies.
PURPOSE & LIMITATIONS
The purpose of this document is to review some of the FSW work performed to date. This review should not be considered an all-inclusive or an in-depth one, since it is limited to those publications available to the author. The document is offered on a best-effort basis, due to the proprietary nature of the FSW process and the fact that the author had no involvement in the research or development aspects of this fairly new technology. Accordingly, the reader is urged to seek technical advice from appropriate sources. Furthermore, the document should not be construed as reflecting a Federal Aviation Administration (FAA) position on FSW. FAA applicants are hereby advised that setting requirements and granting approvals are the responsibility of the cognizant FAA certification organizations. Every effort is made here to avoid presenting FS welded joint property data. However, there are cases where presenting such data becomes a logical choice. In such cases, the property values quoted are intended only for information and must not be considered as design allowables. Furthermore, the graphs and depictions offered represent apparent trends or shapes, intended for visualization purposes only. They are approximate, not to scale, and not intended as accurate duplicates of the data reported by the authors cited. All units in the SI system, presented in the publications reviewed, are converted to their approximate equivalent units in the Imperial (in.-lb) system for presentation herein.
BACKGROUND
Solid State Welding, Overview 2-4
FSW, the subject matter of this document, is the newest addition to friction welding (FRW), a solid state welding process. Solid state welding, as the term implies, is the formation of joints in the solid state, without fusion. Solid state welding includes processes such as cold welding, explosion welding, ultrasonic welding, roll welding, forge welding, coextrusion welding and FRW. Conventional FRW in its simplest form involves two axially aligned parts, one rotating and the other stationary. The stationary part is advanced to make contact with the other, at which point an axial force is applied and maintained to generate the frictional heat required to affect welding at the abutting surfaces and form a solid-state joint. The joint is achieved by upset forging at the elevated temperatures generated by friction. There are two FRW techniques. The first is direct / continuos drive FRW, where constant energy is provided by a source for the desired duration. The second is inertia drive FRW, where a rotating flywheel provides the required energy. A variant of the conventional techniques, radial friction welding, is used for hollow sections, such as tube and pipe. Here, a solid ring is rotated and compressed around the abutting beveled ends of the stationary pipes / tubes to be welded. A support mandrel is located at the bore, at the welding position, to prevent the collapse of the pipe / tube ends. Another variant is friction surfacing, where metal layers are deposited on a substrate. Here, a rotary consumable is brought into contact with a moving substrate to affect metal transfer from the consumable to the substrate.
Artificial Aging
Aging at temperatures above room temperature is artificial aging. The properties constantly evolve with aging time at the aging temperature. For example, strength and hardness increase with time to some peak values, beyond which both strength and hardness decrease, with further increases in aging time; strength and hardness peaks may or may not occur at the same aging time. The decrease in strength and hardness is referred to as overaging. For a given alloy, the peak strength (hardness) values that can be achieved by artificial aging are higher than that achieved by natural aging. As the artificial aging temperature is increased, peak strength / hardness shifts to shorter times, and the loss of strength, due to overaging, occurs more rapidly. Peak strength may increase or decrease as the aging temperature increases, depending on the alloy and temperature range. Due to peak shift to shorter times and the more rapid overaging, precise time and temperature control is essential at the higher aging temperatures, to avoid undesirable overaging or underaging. []a In general, the -T4 or -W tempers maybe aged to the -T6 temper (2xxx and 6xxx alloys). The -T3 temper (2xxx alloys) maybe aged to -T8 temper. In 7xxx alloys, the -W temper may be directly aged to the -T6 or -T7 temper. Alternately, the -T6 temper may be artificially overaged to the -T7 temper. The -T7 type tempers are for enhanced corrosion performance, with some sacrifice in strength.
Processing Variables
Joint profiles, microstructures and properties are governed by the thickness and material of the stock being welded and by choice of processing variables. Processing variables include the weld parameters (speeds, tilt, etc.), tool design (configuration and materials) and, in butt joints, even by the material and thickness of the back plate. Weld parameters and tool design, respectively, are discussed in 5.4.1 and 5.4.2. For more specific information, the reader is urged to consult TWI, their licensees or the various users listed in the publications cited in this document.
Welding Parameters
In reviewing the publications cited in this document, a multitude of welding parameters could be identified. These include rotational speed (rpm), travel speed, normal force, lateral force, tool attitude (tilt angle), shoulder plunge, penetration ligament (butt joints), penetration into the bottom member (lap joints); some of these parameters were defined in 5.2. Welding parameters are generally considered proprietary and, as such, are often fully or partially restricted from publication. Of those publications that disclose some weld parameters, only a handful mentions penetration ligament, shoulder plunge or tilt. The most widely disclosed and investigated parameters are the rotational and travel speeds. In general, slower travel speeds and lower rotational speeds are used for harder alloys or thicker sections. Increasing the rotational speed or decreasing travel speeds tends to increase heat input and welding temperatures. However, extremely high or low travel and rotational speeds can adversely affect properties. Nevertheless, the quest for increased travel speeds is relentless, due to economic pressures. The travel speeds quoted in this document do not exceed 51.18 in. / min. It is said, however, that machines capable of up to 100 in. / min are available on the market. In practice, weld parameters have to be adapted / optimized for the particular alloy type and condition / heat treatment, thickness of the stock being welded and the type of joint being produced. In this document, every effort will be made to quote whatever weld parameters published by the various authors cited.