Friction Stir Welded AZ31 Research Articles

Simultaneous enhancement of strength and ductility in friction stir welded AZ31 alloy via multi-pass hot-rolling and subsequent annealing

Friction stir welding (FSW) is a new solid-state joining process for joining light metals, such as magnesium (Mg) alloys. However, the texture-induced softening mechanism (in stir zone, SZ) tends to cause the worsening of mechanical properties in FSW Mg alloys. In this work, multi-pass hot-rolling and subsequent annealing is used to ameliorate the FSW AZ31 alloy plate. The results show that multi-pass hot-rolling leads to the c-axes of most grains in the SZ approximately perpendicular to the normal direction (ND) transforming into these almost parallel to the ND, accompanied with multiple twin types. After subsequent annealing, twins, as nucleation sites, promote the formation of new grains with off-basal texture, resulting in the weak basal texture. Compared with the FSW plate and the initial plate, the rolled-annealed FSW plate exhibits a relatively uniform hardness distribution and a “strength-ductility” synergy, which are mainly attributed to an appropriate texture feature and a homogeneous grain structure. This work provides a new idea for the development of new high-performance FSW Mg alloys.

Effect of ultrasonic vibration and heat pipe assistance on microstructure and mechanical properties of friction stir-welded AZ31 alloy

This work aims to discover the effects of ultrasonic vibration and heat pipe assistance during the friction stir welding (FSW) of AZ31 magnesium alloy. The microstructure and mechanical properties of FSWed AZ31 alloy were investigated. The ultrasonic vibration and heat pipe assistance can promote the dynamic recrystallization of AZ31 magnesium alloy during the FSW process, which reduces the inhomogeneous microstructure and the grains and weakens the texture intensity of the FSWed AZ31 alloy. The microstructure and texture of the joint are regulated by the ultrasonic vibration and heat pipe assistance, to enhance the tensile strength, elongation and microhardness.

Corrosion behavior of friction stir‐welded AZ31 Mg alloy after plastic deformation

AbstractPostweld plastic deformation was reported to be able to largely enhance the mechanical properties of friction stir‐welded (FSW) Mg alloys by changing microstructures in the regions with a soft‐oriented texture. However, few studies have concentrated on the effects of postweld plastic deformations on the corrosion behavior of FSW Mg alloys, which has an impact on their application. In the present study, electrochemical measurements, hydrogen evolution, and mass loss tests were used to study the influences of postweld compression along the transverse direction and subsequent annealing on the corrosion rate of FSW AZ31 Mg alloys. It was found that owing to the grain refinement in the weld zones, an improvement in the corrosion resistance and hardness was observed in the FSW AZ31 sample compared to the base metal (BM) sample. Postweld compression was very harmful to the corrosion resistance of the FSW AZ31 alloys. On the basis of the mass loss results, the corrosion rate of the compressed FSW sample was ~17.62 mm/year, which was ~8.99 times that of the FSW sample (~1.96 mm/year). Subsequent annealing could slightly reduce the corrosion rate of the compressed FSW AZ31 plates, whereas the corrosion rate of the FSW‐C‐T sample (~13.63 mm/year) was much worse than that of the BM sample (~4.73 mm/year).

Effect of the rotation rate on the low-cycle fatigue behavior of friction-stir welded AZ31 magnesium alloy

Effect of the rotation rate on the low-cycle fatigue (LCF) of friction-stir welding joint of AZ31 magnesium (Mg) alloys was studied. The rotation rate had a significant impact on the grain size but only slightly affected the texture in the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ). Grain coarsening and texture softening resulted in the fracture at the boundary between the NZ and TMAZ at the advancing side. Decreasing rotation rate enhanced the fatigue life. The LCF behavior was well expressed by the Coffin-Manson and Basquin relations. The basal slip was the main deformation mechanism associated with coordinated twinning.

Refined Microstructure and Enhanced Hardness in Friction Stir-Welded AZ31 Magnesium Alloy Induced by Heat Pipe with Different Cooling Liquid

The temperature field in welded plates has a significant influence on the microstructure and thereby their properties during friction stir welding (FSW). In this work, a self-designed heat pipe with different cooling liquid was applied in the FSW process for AZ31 magnesium alloy. The temperature fields, microstructures and properties of the welded joints were investigated. The peak temperatures and the durations of high temperature at both the advancing side and the retreating side decrease during the FSW process after applying the heat pipe and adding the ambient temperature water in the condensing tank. The top part of the weld nugget zone of the joint shows a significant decrease as well as its middle part due to the cooling effect of the heat pipe. The microstructure of the weld nugget zone is refined, associated with the increase in the hardness after applying the heat pipe. When the cooling liquid turns into ice water, grains in the weld nugget zone become significantly smaller and have a more homogeneous size. The mean value of hardness increases and the corresponding deviation is declined. Therefore, these results indicate that the application of the heat pipe and the employment of ice water as the cooling liquid can further refine the microstructure and enhance the strength of the material.

Effects of Welding Speed and Post-weld Hot Rolling on Microstructure and Mechanical Properties of Friction Stir-Welded AZ31 Magnesium Alloy

The as-rolled AZ31 Mg alloy sheets are welded by friction stir welding (FSW) in this study, followed by annealing and hot rolling with different reductions. The effects of welding speed and the rolling reduction on the microstructure, tensile properties, microhardness and fracture surfaces of the specimens are investigated. The results show that the microstructures of the FSW joint in different regions become more and more consistent as the hot rolling reduction increases. The mechanical properties of the FSW joints and the joint coefficient are improved with increasing rolling reduction; however, the hardness value in the stirred zone decreases firstly and then increases. When the rolling reduction is 50%, the tensile strength of FSW joint is close to that of base metal. The sample L3 achieves the best comprehensive properties, with the yield strength, ultimate tensile strength and elongation of 162, 287.9 MPa, and 17.9%, respectively.

Microstructural changes during tension of friction-stir welded AZ31 magnesium alloy

In this work, microstructural changes occurring during transverse tension tests of friction-stir welded AZ31 magnesium alloy were systematically studied in order to clarify the microstructure-properties relationship. To this end, digital image correlation and electron backscatter diffraction (EBSD) were employed. As expected, plastic deformation in the stir zone was found to be preferentially concentrated at its advancing- and retreating sides. In both these locations, the microstructure evolution was shown to be a complex process consisting of two stages. The first stage was characterized by a profuse {101―2} twinning giving rise to a 0001<112¯0> texture component. In the second stage, enhanced slip activity led to extensive formation of low-angle boundaries and to the development of a fiber hkil<112¯0> texture.The twin-induced 0001<112¯0> orientation was shown to promote double {101¯1}–{101¯2} twinning which is known to lead to final failure. It was therefore suggested that the cause of the well-known premature fracture of magnesium welds at the stir zone extremity may be due to the profuse {101¯2} twinning.

Improved properties of friction stir-welded AZ31 magnesium alloy by post-weld heat treatment

Mechanical properties and microstructure of friction stir-welded AZ31 based on variety post-weld heat treatment (PWHT) temperatures were evaluated, and an optimal PWHT condition was identified. At rotational speed of 1200 rev min−1 and welding speed of 300 mm min−1, the average yield tensile, tensile strength and elongation of friction stir-welded joints was 92.5 MPa, 199.1 MPa and 7.3%, respectively. It was found that (300°C – 1 h) heat treatment after welding was more beneficial than other heat treatments in enhancing the mechanical properties and homogenising grain size. The maximum yield and tensile strength was 139.9 and 238.4 MPa, respectively, tensile longitudinal and compressive transverse residual stress could be effectively eliminated, and the fatigue strength increased 34.2% comparing with as-welded joints.

Simulation of microtexture developments in the stir zone of friction stir-welded AZ31 Mg alloys

The evolution of the microtexture in the stir-zone (SZ) portion of AZ31 Mg alloys during friction stir welding (FSW) was investigated via microtexture analysis and polycrystal modeling. The microtexture distribution in as-welded specimens was characterized using electron backscatter diffraction (EBSD). EBSD analysis revealed that the FSW process induced the development of strong fiber textures in the SZ portion, and that the texture development was strongly dependent on the moving path of the material point with respect to the rotating pin. Three dimensional (3D) finite element analysis (FEA) was conducted to extract the thermo-mechanical history of the SZ portion of an AZ31 Mg alloy during the FSW process. In order to simulate the microtexture evolution in the SZ portion during the FSW process, the present study combined a visco-plastic self-consistent (VPSC) polycrystal model in which a dynamic recrystallization (DRX) scheme was implemented and 3D FEA. The thermo-mechanical history was gathered at a particular material point using 3D FEA and the unified microscopic hardening parameters were fitted with a Zener-Hollomon parameter and used as input data for the VPSC polycrystal model. The VPSC polycrystal model successfully predicted the development of strong and heterogeneous fiber textures in the SZ portion of a sample of AZ31 Mg alloy during the FSW process.

Fatigue fracture behaviour and thermographic analysis of friction stir-welded AZ31

The fatigue fracture behaviour and thermographic analysis of friction stir-welded (FSW) AZ31 was studied. It showed that fatigue fracture at the advancing side (AS). Heat-affected zone (HAZ) contained a greater fraction of coarse grains and a small amount of twins. The grains in the thermo-mechanically affected zone were smaller, indicating that grains may originate from incompletely dynamic recrystallisation during the FSW. The nugget zone is composed of equiaxed grains caused by the dynamic recrystallisation. During cyclic deformation, hysteresis loops from the AS of the FSW joints with the stain amplitude were higher than the retreating side of the FSW joints, the temperature of HAZ at AS is higher than other regions.

Effect of subsequent tension and annealing on microstructure evolution and strength enhancement of friction stir welded Mg alloys

Friction stir welded (FSW) Mg alloys normally exhibit a lower yield strength compared with the base materials, which is a drawback for their application as structure materials. In this study, subsequent tension along the transverse direction (TD) and then annealing were applied on FSW AZ31 alloys to modify the microstructure and texture in weld zone and to improve the joint strength. The results showed that by subsequent tension ~4.5% strain along TD and then annealing, the yield strength (YS) could be greatly enhanced for the FSW AZ31 alloys. Specifically the YS was improved from ~86 to ~177MPa by subsequent 4.5% tension and then reduced to ~156MPa after annealing. For the initial joint sample, fracture occurred in the stir zone (SZ) side during the transverse tensile test. But after subsequent tension, all the samples fractured in the base materials (BMs) whether subjected to annealing or not. The microstructure and texture evolutions after subsequent tension and annealing were examined by electron backscatter diffraction (EBSD) to understand the underlining mechanisms for the improved mechanical properties. It revealed that a lot of {10−12} extension twins were generated in the SZ side by 4.5% tension along TD, which significantly refined the grains in the SZ side and rotated the grains from soft to hard orientations for the transverse tensile test. Therefore, the formation of {10−12} twin lamellae had significant hardening effects on further transverse tensile tests. Schmid factor (SF) analysis revealed that the texture change by subsequent tension had much more impact on the activation of {10−12} extension twinning than basal slip for the transverse tensile test. This might affect the competition of extension twinning and basal slip and reduce the deformation incompatibility in the SZ side, which was beneficial for the improvement of mechanical properties. Nevertheless the present study showed that it was efficient to improve the YS of FSW Mg alloys by texture modification via subsequent tension and annealing.

Changes in texture and microstructure of friction stir welded Mg alloy during post-rolling and their effects on mechanical properties

A strong and complicated microtexture usually forms in stir zone (SZ) of friction stir welded (FSW) Mg alloys, which significantly influences the tensile properties of the joint due to the strong anisotropic mechanical properties of Mg alloys. This study aims to study the evolution of microstructure and texture in SZ during post-rolling of FSW AZ31 alloy and its effect on the tensile properties and fracture of the FSW joint. A detailed characterization of microstructure and texture in SZ and especially in SZ/base material (BM) interface was conducted on the FSW-undeformed and FSW-rolled specimens. Schmid factor (SF) changes for extension twinning and basal slip for transverse tensile tests were analyzed and discussed. It confirmed that the c-axis in SZ-center was parallel to welding direction (WD), but tilted towards transverse direction (TD) with the area moving to SZ-side. After subsequent rolling (by ∼2.5% strain), a large number of extension twins were found in SZ-side, but it was less in SZ-center. The production of extension twins rotated the grains in SZ-side and affected the transverse tensile properties significantly. The electron backscatter diffraction (EBSD) analyses showed that the SF of the twins was decreased compared with the matrix for both extension twinning and basal slip for the transverse tensile tests. So, the extension twins generated by post-rolling could cause texture strengthening for the transverse tensile tests. Moreover, the introduction of twin boundaries subdivided the grains, which also increased the tensile properties of the FSW-rolled specimens. The effect of post-rolling on the tensile properties and fracture was studied. It showed that post-rolling increased the transverse yield strength (YS) of the FSW AZ31 alloy, and the YS was increased further with the increase of rolling strain. The YS was raised from ∼87MPa of the FSW-undeformed specimen to ∼115MPa for the FSW-R-2.5 and ∼171MPa for the FSW-R-7 specimens. Because most extension twins were generated in SZ-side, the strength increase was most significant in SZ-side, which reduced the plastic inhomogeneity between SZ-side and SZ-center of the post-rolled specimens. Fracture occurred in SZ-side of the FSW-undeformed specimens because most grains in there were favorably oriented for extension twinning and basal slip. But it occurred in BM for all the FSW-rolled specimens. This was mainly attributed to the texture strengthening and grain subdivision strengthening by the extension twins generated in SZ-side during post-rolling of the FSW specimens. So, it suggested that subsequent rolling was effective to increase the joint strength of FSW AZ31 alloy.

A study on the SCC susceptibility of friction stir welded AZ31 Mg sheet

In this study, the stress corrosion cracking (SCC) behaviour of a friction stir welded (FSW) AZ31 Mg alloy was examined using the slow strain rate tensile (SSRT) test method. Electrochemical impedance spectroscopy (EIS) and salt spray tests were conducted to understand the general and localized corrosion behaviour of the base material and the FSW joint. It was found that the FSW AZ31 Mg samples exhibited higher SCC susceptibility than the base material under SSRT testing in the corrosive environment. The failure of these FSW AZ31 Mg samples occurred in the stir zone (SZ), whereas failure occurred in the thermo mechanically affected zone (TMAZ) for samples tested in air. The EIS and salt spray test results demonstrate that the SZ exhibits higher uniform and pitting corrosion resistance than the base material. The present study suggests that the hydrogen assisted cracking mechanism predominates in the failure of FSW AZ31 Mg samples during SSRT test in the corrosive environment.