Elastic pre-compensation for welding-induced shrinkage and bending deformations of thick plates by friction stir welding

For ordinary arc welding processes such as MIG, welding deformation, residual stress and inherent strain are important factors to control the reliability of weldments. Friction stir welding (FSW) is a new welding method, especially for light metals. The distortion of FSW is reportedly small but reasons can not be determined in detail. This paper deals with thermal cycles and inherent strains which create both residual stresses and welding deformations, in order to make clear effective factors that lower apparent strains on FSW than on MIG. Inherent strains were measured by using the layer removal technique. Thermal cycles and maximum temperature rise were measured by type K thermocouples. It is concluded from experimental data that the main cause lowering apparent strain in FSW is the compressive load that makes tensile plastic strain in weldments. The second cause is that the heat input is smaller than in ordinary arc welding processes.

Most of the investigations regarding friction stir welding (FSW) of aluminum alloy plates have been limited to about 5 to 6 mm thick plates. In prior work conducted the various aspects concerning the process parameters and the FSW tool geometry were studied utilizing friction stir welding of 12 mm thick commercial grade aluminum alloy. Two different simple-to-manufacture tool geometries were used. The effect of varying welding parameters and dwell time of FSW tool on mechanical properties and weld quality was examined. It was observed that in order to achieve a defect free welding on such thick aluminum alloy plates, tool having trapezoidal pin geometry was suitable. Adequate tensile strength and ductility can be achieved utilizing a combination of high tool rotational speed of about 2000 r/min and low speed of welding around 28 mm/min. At very low and high dwell time the ductility of welded joints are reduced significantly.

Friction stir welding is a novel solid state joining process for making low cost, energy efficient butt welds in aluminum alloy extrusions. The plate edges are clamped against a backing plate and the material is plastically deformed and stirred by a rotating tool moving along the joint line. The resulting weld bead is flush with the surface and exhibits little distortion. The material in the weld and heat affected zone (HAZ) has a fine-grained microstructure and a high tensile strength compared with welds produced by conventional arc welding methods. The present investigation was undertaken to determine the fatigue properties of friction stir welds in 5 mm thick plates in an AA6082 alloy. Extruded plates in the T4 condition were used in the test program. S-N tests in pulsating tension at R = 0.5 were performed on specimens with the weld transverse to the stress direction. Reference tests were made on the base material. Crack growth data were obtained for material in the weld metal, in the HAZ and base material. S-N tests were also made on conventional MIG butt welds from the same batch material to enable a comparison of the two welding methods. The results indicate that the fatigue strength of transverse friction stir welds is approximately 50 percent higher than the fatigue strength of MIG butt welds. The crack growth rates obtained for the weld material were lower than in the base material, probably due to a more fine grained microstructure in the weld region. INTRODUCTION The friction stir welding process has recently been developed as a cost effective alternative to conventional metal inert gas (MIG) and tungsten inert gas (TIG) Transactions on Engineering Sciences vol 8, © 1995 WIT Press, www.witpress.com, ISSN 1743-3533 226 Surface Treatment Effects II welding in aluminum alloys [1]. A major advantage of friction stir welding is that it is a solid state process involving a much lower heat input than that required in conventional arc welding methods. The weld itself and its adjacent narrow heat affected zone both have a very fine-grained microstructure with high mechanical strength. The high tensile strength of the weld material and the favorable geometry would also indicate that friction stir welds could have high levels of fatigue strength. A testing program was implemented to determine the fatigue properties of transverse butt welds of two alloys in the AA6000 series. The data presented in this paper are results from introductory tests on specimens fabricated from extruded plates in AA6082 material in the T4 temper condition. THE FRICTION STIR WELDING PROCESS In friction stir welding the plates to be joined are clamped on a backing plate to prevent movement A cylindrical shouldered tool with a specially profiled pin is rotated at a high speed, see Fig. la. The pin is slowly brought into contact with the joint line, and the material is heated by friction and plasticised in an annular volume around the pin. As the pin is lowered into the plates, soft material is extruded at the surface. Upon further lowering of the pin and movement along the joint line the shoulder face contacts the plate surface and the plasticised material is compressed against the face of the shoulder. The soft material is mashed by the leading face of the pin profile and transported to the trailing face of the pin where it consolidates and cools to form a solid-phase weld. The generation of a friction stir weld has many similarities with extrusion seam welds that form when material is joined in the weld chamber of an extrusion die [2]. The material flow, however, is somewhat different due to the more extensive mechanical mixing of the material from the two plates in the friction stir process. The properties of the weld are closely related to the tool technology. The tool bit shape and material determines the heating, plastic flow and forging pattern. Development of the friction stir welding process has up to now been concentrated mainly on butt and lap joints, however, introductory tests have shown that friction stir welding is suitable for a wide range of joint configurations [4], as shown in Fig. 2. EXPERIMENTAL PROGRAM The specimens were fabricated from AA6082 alloy plate material, in the T4 (as-extruded) condition. The plate thickness was 5 mm. The mechanical properties are listed in Table 1. Transactions on Engineering Sciences vol 8, © 1995 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment Effects II 227 Table 1. Mechanical properties of the AA6082 alloy in T4 temper.

The fracture toughness in a friction stir welded joint of thick plates of structural aluminium alloy type A5083-O is investigated. A joint between two 25 mm thick plates is fabricated by one sided, one pass friction stir welding. The Charpy impact energy and critical crack tip opening displacement (CTOD) in the friction stir weld are much higher than those in the base metal or heat affected zone, whereas mechanical properties such as stress–strain curve and Vickers hardness are not conspicuously different. The effects of the microstructure on crack initiation and propagation are studied in order to clarify the difference in fracture toughness between the stir zone and base metal. The analyses of the fracture resistance curves and the diameters of dimples in the fracture surface after both tensile and bending tests show that the fine grained microstructure in the stir zone helps to increase ductile crack initiation and propagation resistance. It is found that the high fracture toughness value in the stir zone is affected by the fine grained microstructure in friction stir welds.

Beginning this fiscal year, the FCRD research project initiated an investigation on joining thin sections of the advanced ODS 14YWT ferritic alloy. Friction stir welding (FSW) was investigated as a method to join thin plate and tubing of 14YWT since it is a solid state joining method that has been shown in past studies to be a promising method for joining plates of ODS alloys, such as 14YWT. However, this study will attempt to be the first to demonstrate if FSW can successfully join thin plates and thin wall tubing of 14YWT. In the first FSW attempt, a 1.06 cm thick plate of 14YWT (SM13 heat) was successfully rolled at 1000oC to the target thickness of 0.1 cm with no edge cracking. This achievement is a highlight since previous attempts to roll 14YWT plates have resulted in extensive cracking. For the FSW run, a pin tool being developed by the ORNL FSW Process Development effort was used. The first FSW run successfully produced a bead-on-plate weld in the 0.1 cm thick plate. The quality of the weld zone appears very good with no evidence of large defects such as cavities. The microstructural characterization study of the bead-on-plate weld zone hasmore » been initiated to compare the results of the microstructure analysis with those obtained in the reference microstructural analysis of the 14YWT (SM13 heat) that showed ultra-fine grain size of 0.43 μm and a high number density of ~2-5 nm sizes oxygen-enriched nanoclusters.« less

The non-uniform thermal field for thick plates jointed by friction stir welding (FSW) easily causes the complex stress state and affects the mechanical behaviour of welded components. The transient non-uniform temperature field for thick plates by FSW is analytically studied based on the heat conduction problem solved by three-dimensional Green’s function. The asymmetric volumetric heat source model is developed by considering the flux distributions in the thickness direction, which is used to obtain the semi-analytical temperature field for welded plates with nonhomogeneous boundary conditions. Then, the analytical model for welding stress is proposed for thick plates by FSW according to the non-uniform temperature field in the weld. The elastic-plastic boundary and the variations of welding stress in the thickness direction are discussed based on the temperature gradient in the welding process. The non-uniform stress distribution in the weld may be analytically predicted in the welding process. The residual stress in the weld is numerically calculated in the cooling process and compared with experiment data to verify the models for the non-uniform temperature field and stress distribution of thick plates. The results show that the three-dimensional temperature field may be used effectively to provide insights into the mechanical response by FSW.

Ambient, elevated temperature tensile properties and origin of strengthening in friction stir welded 6 mm thick reduced activation ferritic-martensitic steel plates in as-welded and post-weld normalised conditions

Friction Stir Welding (FSW) is a solid-state welding technique used for joining metals and alloys to avoid problems associated with fusion welding. Acoustic Emission (AE) has been successfully used to monitor processes like metal cutting, grinding, electron beam welding and FSW. In this work, an attempt has been made to study the application of AE to monitor FSWs to produce defect free welds. During welding of aluminum alloy AA 2024-T3 5mm thick plates, AE signals were acquired. Patterns of AE signals produced during welding are helpful in identifying the defects produced. The lower and higher values of AE parameters help to decide the quality of welded joints. In order to have a better understanding of the behavior of AE parameters when defects were supposed to have occurred during welding, a time domain analysis of AE signals was carried out. The time domain analysis has resulted in justifying the behavior of the AE signals at the instant of occurrence of defects. The range of values of AE parameters, derived from AE signals found to be helpful in monitoring FSW, was accomplished by identifying the time of occurrence of the defect during welding followed by suitable corrective action to produce defect free welds.

Purpose – The weld region obtained during friction stir welding (FSW) of metallic materials (including aluminum alloys) contains typically well-defined zones, each characterized by fairly unique microstructure and properties. The purpose of this paper is to carry out combined experimental and numerical investigations of the mechanical properties of materials residing in different weld zones of FSW joints of thick AA2139-T8 plates. Design/methodology/approach – Within the experimental investigation, the following has been conducted: first, optical-microscopy characterization of the transverse sections of the FSW joints, in order to help identify and delineate weld zones; second, micro hardness field generation over the same transverse section in order to reconfirm the location and the extent of various weld zones; third, extraction of miniature tensile specimens from different weld zones and their experimental testing; and finally, extraction of a larger size tensile specimen spanning transversely the FSW weld and its testing. Within the computational investigation, an effort was made to: first, validate the mechanical properties obtained using the miniature tensile specimens; and second, demonstrate the need for the use of the miniature tensile specimens. Findings – It is argued that the availability of weld-zone material mechanical properties is critical since: first, these properties are often inferior relative to their base-metal counterparts; second, the width of the weld in thick metallic-armor is often comparable to the armor thickness, and therefore may represent a significant portion of the armor exposed-surface area; and finally, modeling of the weld-material structural response under loading requires the availability of high-fidelity/validated material constitutive models, and the development of such models requires knowledge of the weld-material mechanical properties. Originality/value – The importance of determining the mechanical properties of the material in different parts of the weld zone with sufficient accuracy is demonstrated.

As compared to normal Friction Stir Welding (FSW) joints, the Underwater Friction Stir Welding (UFSW) has been reported to be obtainable in consideration of enhancement in mechanical properties. A 5052-Aluminum Alloy welded joints using UFSW method with plate thickness of 6 mm were investigated, in turn to interpret the fundamental justification for enhancement in mechanical properties of material through UFSW. Differences in microstructural features and mechanical properties of the joints were examined and discussed in detail. The results indicate that underwater FSW has reported lower hardness value in the HAZ and higher hardness value in the intermediate of stir zone (SZ). The average hardness value of underwater FSW increases about 53% greater than its base material (BM), while 21% greater than the normal FSW. The maximum micro-hardness value was three times greater than its base material (BM), and the mechanical properties of underwater FSW joint is increased compared to the normal FSW joint. Besides, the evaluated void-area fraction division in the SZ of underwater FSW joint was reduced and about one-third of the base material (BM). The approximately estimated average size of the voids in SZ of underwater FSW also was reduced to as low as 0.00073 mm2, when compared to normal FSW and BM with approximately estimated average voids size of 0.0024 mm2 and 0.0039 mm2, simultaneously.

Influence of Traverse Speed in Self-Reacting FSW of AA6061-T6

Microstructure and fatigue properties of double-sided friction stir welded Ti-4.5Al-2.5Cr-1.2Fe-0.1C alloy plate for aerospace applications

Friction stir welding (FSW) has now reached a technological impact and diffusion that makes it a common joining practice for several classes of metallic materials. These include light alloys (aluminum, titanium, magnesium), steels, and other metallic alloys. In addition, the combination of FSW with pre- or post-welding heat treatments or plastic deformation, such as cold rolling (CR), can favor minimal necessary plate thicknesses and induce effective alloy strengthening mechanisms that make the FSW joint lines as plate reinforcing zones. Process parameters, such as pin rotation and transverse speed, can be tuned to optimize the mechanical properties of the resulting joint. This work presents a microstructural study of the mechanical response of different sequences of heat treatment, FSW, and CR in a non-age hardened Al-Mg AA5754 alloy. By using polarized optical microscopy and microhardness tests, two FSW conditions were used to fabricate a joint; and were than subjected to different sequences of heat treatment and cold rolling. The results suggest that FSW conditions have a limited effect on the microstructure, while microhardness profiles show a higher variability of the different datasets related to the low welding speed investigated.

Heat-treatable aluminum alloys are difficult to fusion weld because of easy formation of some welding defects such as crack and porosity in the weld [1]. Friction stir welding (FSW) is a solid state welding process in which the crack and porosity often associated with fusion welding processes are eliminated [1, 2]. Therefore, the FSW process is being studied to weld heat-treatable aluminum alloys in order to obtain high-quality joints [3–10]. However, some studies have indicated that FSW gives rise to the softening of heat-treatable aluminum alloys, thus resulting in the degradation of the mechanical properties of the joints. The degradation extent is related not only to the alloy type [9–11], but also to the alloy thickness [12–16]. 2017-T351 aluminum alloy is one of the 2xxx-series heat-treatable aluminum alloys, and a 5-mm thick 2017-T351 plate has been friction stir welded to examine the tensile properties and fraction locations of the joints [9]. This letter aims to further demonstrate the FSW characteristics of a 3mm thick 2017-T351 sheet to comprehend the effect of alloy thickness. The base material used in this study was a 3-mm thick 2017-T351 aluminum alloy sheet with the chemical compositions and mechanical properties listed in Table I. The sheet was cut and machined into rectangular welding samples, 300 mm long by 80 mm wide, and they were longitudinally butt-welded using an FSW machine. The designated welding tool size and welding parameters are listed in Table II, in which the revolutionary pitch (RP) is defined as the travel speed divided by the rotation speed. After welding, the joints were cross-sectioned perpendicular to the welding direction for metallographic analyses and tensile tests. The crosssections of the metallographic specimens were polished with an alumina suspension, etched with Keller’s reagent and observed by optical microscopy. The configuration and size of the transverse tensile specimens were prepared according to Fig. 1, in which RS and AS denote the retreating side and advancing side of the joint, respectively. Prior to the tensile tests, the Vickers hardness profiles across the weld nugget (WN), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and partial base material were measured along the centerlines of the cross-sections of the tensile specimens under a load of 0.98 N for 10 s, and the

Modeling the strongly localized deformation behavior in a magnesium alloy with complicated texture distribution