Short CommunicationEffect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding
Introduction
Friction stir welding (FSW) which was developed by TWI in 1991 is a relatively mature solid joining technology for aluminum alloys and has been successfully applied in many industrial sectors with various joint configurations [1], [2], [3], [4], [5], [6]. Stationary shoulder friction stir welding (SSFSW) is an alternative approach of FSW and was primarily invented by TWI to weld low thermal conductivity Ti–6Al–4V [7]. SSFSW consists of a rotating pin and a non-rotating shoulder, which frictionally slides over the surface of the material during welding [8].
In conventional FSW, the rotational shoulder is believed to generate the majority of frictional heat during the welding process [9]. While in SSFSW, the shoulder is static and no longer acts as a stir component generating frictional heat. The rotational pin produces almost linear heat input throughout the welding [10], and the asymmetrical microstructure and properties of the weld caused by conventional FSW tool are reduced [11]. Besides, stationary shoulder creates a smooth weld surface appearance with no cross section reduction [10]. SSFSW can be easily applied for welding butt joints with different thickness and corner joints. So, it attracted more and more attentions in industrial fields.
SSFSW is one of the advanced solid-state joining technologies, and currently there are only a few open literatures published on SSFSW because of technical confidentiality. Davies et al. studied the crystallographic texture and microstructure of a SSFSW Ti–6Al–4V weld in detail [12]. Ahmed et al. investigated the through-thickness crystallographic texture of a SSFSW aluminum weld joint. They concluded that the use of SSFSW reduced the shoulder-affected region and that the stationary shoulder only affected a very thin surface layer at the top of the weld [11]. Widener et al. successfully performed AA6061-T6 welding experiments under very high rotation rate using SSFSW, and it was found that SSFSW could apparently improve the surface forming of the weldment and reducing weld defects [13]. Martin et al. reported the application of SSFSW on aluminum corner joint and found that SSFSW corner joint had more advantages than conventional FSW corner joint [8]. YU performed paralleled double-pass lap joints for aluminum 7075-T6. The effect of welding control parameters and tool geometry on welding process were investigated. The microstructure, distortion and mechanical properties were studied. They found that SSFSW joint are of fewer defects and has a higher UTS compared to conventional FSW joint [14].
In order to understand mechanism of SSFSW process, Merlin performed both numerical and experimental investigation of the thermal cycle during the butt joint welding of aluminum 2024 using SSFSW [15]. Liu and his co-workers studied the microstructure and the effect of welding speed and tool rotation speed on the mechanical properties of 2219-T6 aluminum welds made by external non-rotation shoulder assisted FSW. They found that the microstructure and hardness profile of the joints are asymmetrical, and the defect-free joint could be obtained when rotation speeds are in the range of 600–900 rpm and the maximum tensile strength of joint could reach 69% of base metal strength for 800 rpm. However, the tool still has a small rotational shoulder and the weld structure is similar to that made by using conventional FSW [16], [17]. Up to now the published literatures on SSFSW seldom reported in detail the influence of welding parameters on the microstructure and mechanical properties of the SSFSW welded joint.
In order to develop and promote the application of the advanced SSFSW technology, a SSFSW tool package was developed independently and installed on the special FSW machine. The welding experiments of SSFSW butt joints for AA6061-T6 sheets were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds, and the influences of welding parameters on weld forming, microstructure, hardness, and mechanical properties during SSFSW process for AA6061-T6 base metal were discussed and analyzed thoroughly. These research results provide the important bases to understand the SSFSW features.
Section snippets
Experimental procedures
The base material used in this study was 5 mm 6061-T6 aluminum alloy sheet. The nominal chemical compositions and the tensile properties are listed in Table 1. The welded sheet was cut to the dimension of 320 × 105 mm. Welding was carried out parallel to the rolling direction of the sheets using the special FSW machine equipped with SSFSW tool package. The welding parameters used in this study are summarized in Table 2. The upset forging pressure of tool package are approximately kept constant
Weld appearance and transverse section macrograph
The appearance of the weld made using 750 rpm and 100 mm/min is shown in Fig. 3. The surface is fine and smooth with an existing hole at the end and only a piece of burr at the beginning of the weld seam. The width of the welding mark is equivalent to the diameter of the stationary shoulder. The stationary shoulder prevented the stirred material at the welding zone escaping to form burrs. Thus, stationary shoulder helps to get a smooth and fine surface which does not need to be further machined.
Conclusions
The welding experiments of SSFSW butt joints for AA6061-T6 sheets were performed using various welding parameters and the influences of welding parameters on the properties of SSFSW welds were investigated. The main conclusions are listed as follows:
- (1)
SSFSW butt welded joints for 5.0 mm thickness of AA6061-T6 sheets were successfully performed and defect-free SSFSW welds with fine and smooth appearance were obtained for all the selected welding parameters of 750–1500 rpm tool rotation rates and
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2022, Journal of Materials Research and TechnologyCitation Excerpt :SSFSW was found to have significantly lower heat input and torque in the 2-mm-thick dissimilar 2024-T3 and 7050-T651 sheets, with a significantly higher SSFSWed joint coefficient (94%) than CFSWed joints (86%) [25]. However, recent research has concentrated more on SSFSW traditional Al alloy sheets such as 5 mm 7075-T651 and 6061-T6 [24,26]. Only one relevant research, which primarily investigated the microstructure heterogeneity along plate thickness and the precipitate evolution along the transverse direction, existed for SSFSW 1.5-mm thick 2A97 Al–Li alloy sheets.