Shoulder-affected Zone Research Articles

Dynamics of rotational flow in friction stir welding of aluminium alloys

Al 1050 and Al 6061 plates with the same thickness were configured one after the other along the welding path. During friction stir welding, the rotating tool traversed from one to the other and stopped at locations with various welding distances from the initial interface. The first material was thus taken into the second material by the rotating tool as marker. A hollow inverted truncated cone-shaped rotation flow zone was confirmed, mainly occurring in shoulder-affected zone. The material in the rotation flow zone travelled with the tool and evolved upwards slowly, depositing in the weld upper part finally. EBSD investigation of the inner microstructure revealed that the strain rate at the edge of the rotation flow zone and close to the tool surface is higher than that in the mid-layer.

Origin of unusual fracture in stirred zone for friction stir welded 2198-T8 Al-Li alloy joints

Friction stir welded (FSW) joints of conventional precipitation-hardened aluminum alloys usually fracture in the lowest hardness zone (LHZ) during tension testing. However, all of the FSW joints of a 2198-T8 Al-Li alloy fractured in the stirred zone (SZ) instead of the LHZ with the welding parameters of 800rpm-200mm/min and 1600rpm-200mm/min under the condition that no welding defects existed in the SZ. The experiment results revealed that lazy S was not the dominant factor resulting in the unusual fracture. The SZ consisted of three subzones, i.e., the shoulder-affected zone, the pin-affected zone, and the transition zone between them. While the former two zones were characterized by fine and equiaxed recrystallized grains, incompletely dynamically recrystallized microstructure containing coarse elongated non-recrystallized grains was observed in the transition zone. The transition zone exhibited the lowest average Taylor factor in the SZ, resulting in a region that was crystallographically weak. Furthermore, obvious lithium segregation at grain boundaries was observed in the transition zone via time-of-flight secondary ion mass spectroscopy analysis, but not in the shoulder-affected zone or the pin-affected zone. The combined actions of both the two factors resulted in the appearance of preferential intergranular fracture in the transition zone and eventually caused the failure in the SZ. The lithium segregation at grain boundaries in the transition zone was closely associated with both the segregation in the base material and the partially dynamically recrystallized microstructure resulting from the inhomogeneous plastic deformation in the SZ.

Significant effect of oxide on mechanical properties of friction-stir-welded AA2024 joints

A 30–40 μm thick oxide layer was prefabricated on the top, butt or bottom surfaces of AA2024 sheets by micro-arc oxidation, which were then friction stir welded. Sheets without oxide layer were also welded with blowing oxygen along the weldline during welding for comparison purposes. Results reveal that the tensile properties depend on the amount and distribution of the residual oxide impurities in the joints. The oxide layer on the butt surface leads to continuously distributed oxide impurities in the shoulder-affected zone near the advancing side, and a 159 MPa maximum degradation in tensile strength was noticed. Moreover, the oxygen blowing during welding has a little influence on the joint tensile strength and no oxide was detected in the joint.

Microstructure and mechanical properties in friction stir welded 5A06 aluminum alloy thick plate

Friction stir double-sided butt welding was carried out for the joining of 5A06 aluminum alloys with thickness of 108mm, and sound welds with full penetration were produced. Microstructure characterizations of the welding joint were investigated by optical microscopy, electron back-scattered diffraction and transmission electron microscopy (TEM). The results indicate that void defects are observed in shoulder-affected zone (SAZ) and grain size of equiaxed grains in SAZ and weld nugget zone (WNZ) decreases gradually along the thickness direction, while in SAZ the dislocation density is significantly lower and the sub-grain size is much larger than those in WNZ. Microhardness surveys reveal that on the upper part of the joint hardness profile presents a “V” shape and minimum hardness appears in SAZ whereas that in other regions exhibits a “W” shape and the lowest hardness value occurs in the heat-affected zone adjacent to the advancing side. The investigation of tensile properties on the FSW joint indicates the WNZ exhibited superior tensile properties compared with SAZ, and the analysis of fracture surface using the scanning electron microscopy (SEM) demonstrates that fractures in all tensile specimens are ductile fracture due to the presence of dimples with different sizes.

Effect of Welding Speeds on Mechanical Properties of Level Compensation Friction Stir Welded 6061-T6 Aluminum Alloy

AbstractIn order to eliminate the flash, arc corrugation and concave in weld zone, level compensation friction stir welding (LCFSW) was put forward and successfully applied to weld 6061-T6 aluminum alloy with varied welding speed at a constant tool rotational speed of 1,800 rpm in the present study. The glossy joint with equal thickness of base material can be attained, and the shoulder affected zone (SAZ) was obviously reduced. The results of transverse tensile test indicate that the tensile strength and elongation reach the maximum values of 248 MPa and 7.1% when the welding speed is 600 mm/min. The microhardness of weld nugget (WN) is lower than that of base material. The tensile fracture position locates at the heat affected zone (HAZ) of the advancing side (AS), where the microhardness is the minimum. The fracture surface morphology represents the typical ductile fracture.

Improving the structure-property of aluminum alloy friction stir weld by using a non-shoulder-plunge welding tool

In order to improve the joint performances by controlling the thermal effect of tool shoulder, a non-shoulder-plunge (NSP) welding tool is utilized to conduct friction stir welding (FSW) of an Al-Mg aluminum alloy in this paper. Sound FSW joint is successfully produced by using the NSP welding tool. In contrast to conventional FSW (C-FSW), the thermal effect of tool shoulder is effectively reduced by NSP-FSW, leading to a significant size reduction of shoulder-affected zone in the joint. Besides, the NSP-FSW joint shows finer grain structures in the stir zone and thermo-mechanically affected zone, and exhibits lower extents of grain coarsening, dislocation polygonization, and particle dissolution in the heat-affected zone. Hardness maps show that the softening region width of NSP-FSW joint is only nearly half that of C-FSW joint and the NSP-FSW joint exhibits higher hardness in the heat-affected zone than C-FSW joint. The tensile test results indicate that the joint efficiency is improved from 89 % of C-FSW joint to 97 % by the NSP-FSW process.

Elimination of tunnel defect in ultrasonic vibration enhanced friction stir welding

Through observing the marker material (MM) distribution and the exit holes left by the “tool sudden stop action” technique, the formation mechanism of tunnel defect in friction stir welding (FSW) is elucidated. Based on the MM distribution on the transverse cross-sections, the weld nugget zone is divided into three subzones, i.e., shoulder affected zone (SAZ), pin affected zone (PAZ), and weld bottom zone (WBZ). There is a time-delay feature of material flow which is considered as the inducement of tunnel defect formation. When a tunnel defect occurs in FSW joints, three subzones connect with each other and there exists a converging point. The insufficient material flow from retreating side to advancing side in the PAZ and the shortage of downward material transfer in the SAZ are the primary and secondary reasons for tunnel defect occurrence, respectively. Under the same welding conditions, ultrasonic vibration enhanced friction stir welding (UVeFSW) can eliminate the tunnel defect in the weld nugget zone where the SAZ and WBZ are separated by the PAZ, because the exerted ultrasonic vibration improves the material flow in the PAZ and reduces or even suppresses the time-delay of material flow in the PAZ.

Effects of the Reversely Rotating Assisted Shoulder on Microstructures During the Reverse Dual-rotation Friction Stir Welding

The reverse dual-rotation friction stir welding (RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin. The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin, so the total welding torque is reduced, which is beneficial to reducing the clamping requirement of workpieces. In the present paper, a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW, and microstructures in various zones were investigated by comparison, aiming to highlight effects of the reversely rotating assisted shoulder. Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined, the thermal effect of RDR-FSW was smaller than that of the conventional FSW. Moreover, the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface, and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW. In the heat-affected zone (HAZ), the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates. By contrast, the dissolution degree of precipitates increased significantly in the thermo-mechanically affected zone (TMAZ), and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates. Precipitate evolutions in the shoulder affected zone (SAZ) and the weld nugget zone were similar, i.e. the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates, though the dissolution degree was greater in the SAZ. Compared with the conventional FSW joint, the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller, as well as the dissolution degrees of precipitates in all four specified zones.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF BANDED TEXTURES OF FRICTION STIR WELDED 7075-T6 ALUMINUM ALLOY

Friction stir welding (FSW) is a ideal method for joining light metal materials, especially for AA2000 and AA7000 series aluminum alloys. In the welding field, many aspects of FSW have been studied quite extensively and comprehensively. However, the origin of the banded texture is still a subject of current research. One of the phenomena that strike researchers today is the occurrence of banded texture on the top surface in FSW joints. Furthermore, many waves also emerged in shoulder affected zone (SAZ) after a joint being polished and etched. The correlation between the banded textures and the waves is not clear. In this paper, 1.6 mm thick 7075-T6 aluminum alloy sheets were friction stir welded, microstructures and hardness profile of banded textures on the top surface of the joint were investigated. The results of metallographic observation showed that alternately light and dark waves presented in SAZ, the light waves coincided with valleys of the banded textures, whereas the dark waves coincided with peaks. SEM and TEM analyses revealed that compared with the valleys, the peaks were characterized by more secondary phase particles and small grains, resulting in higher hardness at the peaks.