Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Highlights

• Stationary shoulder friction stir welding (SSFSW) was performed on aluminum.

• Defect-free butt joints with fine and smooth surface were obtained.

• Influence of welding parameters on microstructure of SSFSW joints was studied.

• Influence of welding parameters on mechanical properties of SSFSW joints was studied.

Abstract

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.

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.