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外 文 翻 譯
畢業(yè)設(shè)計題目： 雙攪拌軸攪拌摩擦焊機設(shè)計
原文1： AN OUTSIDER LOOKS AT FRICTION STIR WELDING
譯文1： 常人眼中的摩擦攪拌焊接技術(shù)
原文2： FIRCTION STIR WELDING AND PROCESSING
譯文2： 摩擦攪拌焊接技術(shù)及其發(fā)展過程
(原文1)
AN OUTSIDER LOOKS AT FRICTION STIR WELDING
BACKGROUND
4.1 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.
4.2 Friction Stir (FS) Technology 5, 6
FSW is a member of the FS technology family. The other members of that family are FS processing for superplasticity, FS casting modification (also referred to as FTMP or friction thermomechanical processing), FS microforming, FS powder processing, FS channeling and FS processing for low temperature formability.
4.3 A Note on Aluminum Alloys
Since the majority of work reviewed in this document pertains to aluminum alloys, it is important to discuss some of the heat treatment aspects of these alloys. A three-step sequence is used to heat treat 2xxx, 6xxx and 7xxx series and other heat treatable aluminum alloys, to higher strength levels. The first step is solution heat treatment and it consists of heating to some prescribed elevated temperature (around 900 F) and soaking there for a prescribed period of time. The second step is to cool the alloy fast enough (e.g., by quenching), so as to retain the elevated temperature microstructure. As will become clear shortly, cold working, forming or straightening of quenched wrought alloys should be performed as soon as possible after quenching. The third step is aging (AKA precipitation heat treatment). Aging involves soaking the alloy for a period of time at some temperature that is lower than that used for solution treatment. For the aluminum alloys of concern here, aging is performed in the room temperature to 375 F temperature range. Aging at room temperature is referred to as natural aging. Aging at temperatures above room temperature is referred to as artificial aging. Aging causes precipitation within the grains, with the attendant increase in strength and hardness, at the expense ductility. Other properties also change as a result of aging.
4.3.1 Natural Aging
After quenching, the alloy is in the unstable -AQ temper. At room temperature, the alloy remains in that temper for a period that ranges from a few minuets to an hour or so, depending on the particular alloy. During that period, the solution treated microstructure remains as it was at the solution treatment temperature; i.e., remains unchanged. At the end of that period, the temper changes to the -W temper, also an unstable temper. This isaccompanied by changes in properties; e.g., the strength and hardness will increase and the ductility will decrease. As more precipitation occurs with time, the properties will progressively evolve; e.g., strength will progressively increase and ductility will progressively decrease with time. After a few days (or about 96 hr), 2xxx and 6xxx alloys reach a stable condition, referred to as the -T4 temper where no further property changes would take place. An additional increment of strength can be obtained in 2xxx alloys if the alloy is cold worked in the -AQ temper or during the early stages of the -W temper, and then naturally aged, for about 96 hr, to a stable condition referred to as the -T3 temper. While it is generally accepted that natural aging for 96 hr is sufficient to develop a stable temper (-T3 or -T4), it is reported, in FSW literature, that natural aging continues for over one month in AA 6013 7 and over 2.5 years in AA 2195. 8 The 7xxx alloys do not reach the stable -T3 and -T4 tempers. Rather, strength and other properties continue to evolve with time for years at room temperature; in fact, it is reported 9 that AA 7050 aluminum alloy age hardens indefinitely at room temperature. In other words, it should be assumed that 7xxx alloys remain in an unstable and evolving -W temper indefinitely, unless the alloy is artificially aged. Therefore, test results obtained in various 7xxx-W alloy investigations cannot be directly compared unless the periods of natural aging indicated (e.g., -W 0.5 hr) are the same. Unfortunately, however, researchers tend not to indicate these periods.
4.3.2 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.
4.4 Abbreviations
Some abbreviations of a general nature are used throughout this document. These are presented alphabetically below, together with what they mean.
EDS: energy dispersive spectrometry. %e: percent tensile elongation. Ftu: ultimate tensile strength. Fty: tensile yield strength. GMAW: gas metal arc welding. GTAW: gas tungsten arc welding. NDI: nondestructive inspection. OM: optical microscope / microscopy. SEM: scanning electron microscope / microscopy. TEM: transmission electron microscope / microscopy.
5.0 INTRODUCTION TO FSW
A brief description of the FSW process for various types of joints is presented in 5.1. Some of the terms and conventions used in FSW are introduced in 5.2. FS welded joint profiles and the various weld zones encountered are detailed in 5.3. The issue of processing variables is tackled in 5.4. An attempt to outline the factors that control weld microstructures is presented in 5.5. Some advanced FSW concepts are discussed in 5.6. The topic of mechanical testing of welded joints is treated in 5.7.
5.1 Process Description
Brief process descriptions are given below for butt joints (5.1.1), lap joints (5.1.2) and other joint types (5.1.3). The contents of this section are based on the publications reviewed in this document. 5.1.1 Butt Joints: 4, 10-13
The two workpieces to be welded, with square mating (faying) edges, are fixtured (clamped) on a rigid backplate, Figure 1a. The fixturing prevents the workpieces from spreading apart or lifting during welding. The welding tool, consisting of a shank, shoulder and pin (Figure 1b), is then rotated to a prescribed speed and tilted with respect to the workpiece normal. The tool is slowly plunged into the workpiece material at the butt line, until the shoulder of the tool forcibly contacts the upper surface of the material and the pin is a short distance from the backplate (Figure 1c). A downward force is applied to maintain the contact and a short dwell time is observed to allow for the development of the thermal fields for preheating and softening the material along the joint line. At this point, a lateral force is applied in the direction of welding (travel direction) and the tool is forcibly traversed along the butt line (Figure 1 d), until it reaches the end of the weld; alternately, the workpieces could be moved, while the rotating tool remains stationary. Upon reaching the end of the weld, the tool is withdrawn, while it is still being rotated. As the pin is withdrawn, it leaves a keyhole at the end of the weld. Shoulder contact leaves in its wake an almost semi circular ripple in the weld track, as depicted schematically in Figure 1d.
As the tool is moved in the direction of welding, the leading edge of the pin, aided by certain other tool features, if present, forces the plasticized material, on either side of the butt line, to the back of the pin. In effect, the material
is transferred from the leading edge of the tool to the trailing edge of the pin (i.e., the material is being stirred) and is forged by the intimate contact of the shoulder and the pin profile. Some believe that the stirring motion tends to break up oxides on the faying surfaces, allowing bonding between clean surfaces. It should be noted that, in order to achieve full closure of the root, it is necessary for the pin to pass very close to the backplate, since only limited amount of deformation occurs below the pin, and then only close to the pin surface. An open root (lack of penetration) is a potential failure site. This aside, Figure 1c depicts that the tool axis and the workpiece normal are tilted with respect to each other by a small angle, θ, typically in the 2-4O range; this angle can be achieved by tilting either the tool or the workpieces. It is said that this tilting aids in the compaction of the material behind the tool, but it has the drawback of limiting the ability to execute nonlinear welds and can also limit the welding speed. 12
As a consequence of the FSW method, the start and end of the joint will not be fully welded, particularly at the end of the weld, where the keyhole is left. Furthermore, in FSW steel and other high melting alloys, a small-diameter hole is predrilled in the butt line, to lessen the forces acting on the welding tool during the plunge. It has been recommended, therefore, that the weld start and end regions be machined off. Even with the use of run-on run-off tabs, Ekman et al. 13 report that low joint strengths resulted at the workpiece / tab interfaces (Figure 2), necessitating the removal of material, approximately corresponding to the thickness of the workpiece, from either end.
5.1.2 Lap Joints 14-16
The same operational principles discussed above for butt joints apply to lap welds, except as follows. In a lap joint there is no butt line, where the tool can be plunged between the workpieces and, as such, the pin must penetrate through the top member. Furthermore, it is essential for the stirring motion to break up the scale, oxides and the other contaminants at the interface. This makes lap welds fundamentally different from butt welds. For butt welds, the primary stirring is in plane of the abutting surfaces being welded. By contrast, lap welds need out of plane stirring, across the interface of the two members being welded. This being so, Brooker et al. 14 indicate that the principal difference between a tool for lap welds and one for butt welds is the introduction of a second shoulder, located at the interface between the two details being welded (Figure 3). The lap joint publications reviewed in this document do not specifically indicate that predrilling of a start hole was required.
In lap joints, one must distinguish between the top and bottom members, since the former is in contact with the shoulder. The end of the pin must penetrate completely through the top member, and extend some distance into the bottom member. It is not required, however, that the pin end pass very close to the bottom of the bottom member, since, in contrast to butt joints, there is no root closure to be concerned about. Nevertheless, one must not underestimate the effect of the penetration distance into the lapped (bottom) member on the mechanical properties of the joint. The notches on either side of the joint (Figure 4) are potential sites for crack initiation and, as such, they have a profound effect on mechanical properties. In general, while lap joints are not as strong as butt joints, they have adequate static 14 and fatigue 16 properties to replace fastened joints.
5.1.3 Other Joint Types
FSW has been used to prepare spot joints with and without the end keyhole. Spot welds can be either of the butt or lap type. The specifics are presented in section 8. FSW has been also used to prepare T-joints 16 and corner joints, 17 Figure 5. Based on this figure, a T-joint could be viewed as a special lap joint and, as such, the notches on either side of the weld are potential crack initiation sites. Designing with T-joints is challenging, since care must be taken to avoid compression failure of the web (vertical member). Figure 5 suggests that a corner joint is in essence either a special butt joint (butt configuration) or a special lap joint (rabbet configuration). Apart from the above types of joint, FSW has been used to prepare, among others, fillet welds 18 and hem joints. 19 Not much technical information is published on the T, corner, fillet or hem joints and, as such, they will not be considered any further in this document.
5.2 Conventions & Terminology
Following the convention used by Colligan, 11 we define the advancing and retreating sides of a FS weld as follows. The side of the welding tool where surface motion (due to spinning) is in the same direction as the travel direction is referred to as the advancing side. The opposite side, where surface motion opposes the travel direction, is referred to as the retreating side. Some authors refer to the advancing and retreating sides as the shear and flow sides, respectively; this terminology, however, will not be used here. Figure 6 depicts the advancing and retreating sides in a butt weld, together with some other commonly used FSW terminology. As indicated in section 5.1.1, the tool and workpiece are tilted, by an angle θ, with respect to each other. Colligan 11 and Hirano et al. 18 indicate that the tilt is away from the travel direction, as shown in Figure 7. This tilt gives rise to a shoulder plunge, P, defined by Cederqvist et al., 15 as shown in Figure 7; P = 0.5 D sin θ, where D is the shoulder diameter. It is to be noted that the shoulder plunge defined above is for the case where the middle of tool contacts the workpiece; other researchers may use different approaches. The terms and definitions
discussed above and depicted in Figures 6 and 7 apply to all types of FS welded joints. However, the terms advancing and retreating sides, leading and trailing edges, and travel direction are not applicable to spot welds, since no travel is involved. The term joint profile is used throughout this document, for all types of joints. Joint profile is the shape of the outermost boundary of the weld that borders the base metal and it includes the face and root of the weld. Joint profile can be discerned by preparing a weld cross section, perpendicular to the length of the weld, and viewing it as shown in Figure 8 for butt and lap joints.
The terms face, root and toe of the weld, Figure 9, are used with butt joints and occasionally with other types of joints. The terms overmatching and undermatching, respectively, indicate a weld that is stronger than the base metal and a base metal that is stronger than the weld. The term penetration ligament is occasionally used in conjunction with FS welded butt joints. The penetration ligament, as defined by Ding and Oelgoetz, 20 is the distance from the tip (end) of the pin to the backside of the workpiece. Another term that appears in FS welded butt joints is the kissing bond. According to Oosterkamp et al., 21 a kissing bond is a descriptive term for two surfaces lying extremely close together, but not close enough for the majority of the original surface asperities to have deformed sufficiently to affect the formation of atomic bonds. Kissing bonds are extremely difficult to detect by most of the NDI methods that are commonly used for weld inspection. Depending on their location and extent, kissing bonds can have a detrimental effect on fatigue life, impact properties and through thickness load carrying capacity. A third term frequently used with butt joints is joint efficiency. Joint efficiency is defined as the ratio (Ftu)joint / (Ftu)base metal, expressed as a percentage. The ultimate strength of the base metal must be obtained in the same direction in which the joint is tested, using specimens from the same heat; base metal minimum (design)
strength should not be used here. Therefore, if the joint is tested in the longitudinal direction of the product, then the ultimate base metal strength in the longitudinal direction must be used. Similarly, the ultimate base metal transverse strength must be used if the joint is tested in the transverse direction of the product. Note that, so far, we have been referring to the longitudinal and transverse directions of the base metal product. There is also the issue of weld orientation with respect to test direction; i.e., the longitudinal-weld and transverse-weld testing
configurations, to be discussed in section 5.7 and the Appendix. Figure 10 depicts the various weld orientation-working direction combinations in butt welded sheet and plate products. For dissimilar metal butt welding, joint efficiency is computed on the basis of the strength of the weakest member of the dissimilar couple.
Author： Terry Khaled, Ph.D.
Country：U.S.A
Provenance: terry.khaled@faa.gov
（譯文1） 常人眼中的攪拌摩擦焊
背景：
4.1固體焊接，概述：
FSW是本文的主要說明對象，是除了摩擦焊（FRW）外，最新的固態(tài)焊接技術(shù)。固態(tài)焊接，顧名思義，是在固體狀態(tài)下形成的焊縫，并且沒有沒有融合。固態(tài)焊接包括以下類型，如冷焊接，爆炸焊接，超聲波焊接，滾焊，鍛焊，共擠焊接和FRW。在其最簡單的形式中的常規(guī)的FRW涉及兩個軸向?qū)R的部分，一個旋轉(zhuǎn)和一個固定。固定的元件提前與其他元件接觸，在該點施加一軸向力，并保持以產(chǎn)生所需的摩擦熱使與相鄰接面形成固態(tài)焊縫。這個焊縫是通過高溫摩擦產(chǎn)生的。FRW有兩個技術(shù)。第一個是連續(xù)驅(qū)動FRW，為了滿足設(shè)計好的持續(xù)性，恒定能量由一個源提供。第二個是慣性驅(qū)動FRW的，其中的旋轉(zhuǎn)飛輪提供所需的能量。徑向摩擦焊接是傳統(tǒng)技術(shù)的一個延伸，用于空心型材料，如管和管道。這里，固體環(huán)是通過旋轉(zhuǎn)來焊接固定管/管道周圍