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In advanced solid-state manufacturing processes such as friction stir welding, the metal's temperature ranges from room temperature to the solidus temperature. The material strength in the temperature range is generally required for investigating the mechanical behaviors. In this communication paper, an analytical model is proposed for describing the thermal softening of aluminum alloys for room temperature to solidus temperature, in which the concept of temperature-dependent transition between two thermal softening regimes is implemented. It is demonstrated that the proposed model compares favorably to the well-known Sellars–Tegart model and Johnson–Cook model. The constants of the proposed model for nine typical engineering commercial aluminum alloys are documented. [ABSTRACT FROM AUTHOR]

During the process of friction stir welding, the behavior of interfacial friction holds a pivotal role in shaping both the heat generation and material flow within the workpiece. However, a quantifiable comprehension of how the friction between the tool and the workpiece (T/W) precisely influences the interface contact state and the heat generation, and the material flow remains elusive. This paper strives to address this gap by introducing a mathematical model that couples interface friction and material flow through the utilization of a shear stress boundary condition at the T/W interface, which is able to present sliding and sticking condition, in our computational fluid dynamics simulation. Notably, the simulation underscore a non-uniform distribution characterizing the friction interface contact state. It is found that augmenting the coefficient of friction (CoF) induces a transition from sliding to sticking both locally and averagely at the T/W interface. When the friction interface becomes predominantly characterized by a sticking state, the CoF exerts a nominal influence over the overall heat generation, yet maintains a discernible impact on material flow patterns. This foundation enables the elucidation of the mechanism through which the friction interface contact state impacts material flow behavior. The simulated material flow trajectory also highlight that under a sliding-dominated friction interface, materials simply flow around the tool. The material flow trajectories on the advancing side (AS) and the retreating side (RS) are approximately symmetrical. The materials driven by the shoulder and the pin are converged at the center of the weld behind the tool. Simulation and experimental results demonstrate that under a sticking-dominated friction interface, materials tend to undertake multiple circulations around the tool and migrate downward to the AS. Other materials are observed to migrate upward to the RS. The entrance point for this circular behavior resides in the AS situated beneath the shoulder. [ABSTRACT FROM AUTHOR]

Magnesium alloys are ideal lightweight materials; however, their applications are extremely limited due to their low strength, poor ductility, and weak corrosion resistance. In the present study, a friction stir processing (FSP) treatment was employed to optimize the mechanical properties and corrosion resistance of an as-cast Mg-5Zn alloy. The average grain size of the Mg-5Zn alloy was refined from 133.8 µm to 1.3 µm as a result of FSP. Along different directions, FSP exhibited the enhancement effects on different mechanical properties. Furthermore, according to the potentiodynamic polarization results, the corrosion current density at the free-corrosion potential of the FSPed sample, was 4.1 × 10−6 A/cm2 in 3.5 wt.% NaCl aqueous solution, which was significantly lower than that of the as-cast sample. Electrochemical impedance spectroscopy revealed that the polarization impedance, R p , of the FSPed sample was 1534 Ω/cm2 in 3.5 wt.% NaCl aqueous solution, which was 71.4% greater than that of the as-cast sample. The corrosion morphology of the FSPed sample in 3.5 wt.% NaCl aqueous solution exhibited largely uniform corrosion, rather than severe localized corrosion characteristics, which further reduced the corrosion depth on the basis of reducing the corrosion current density. The results presented herein indicate that FSP is a viable technique for simultaneously improving the mechanical properties and corrosion resistance of the as-cast Mg-5Zn alloy. [ABSTRACT FROM AUTHOR]

The unique aluminum–carbon interface without carbide, which is termed as the directly bonded aluminum–carbon interface, is a recently reported remarkable structure for simultaneously enhancing the load/electron transfer properties of aluminum conductors. In this paper, the structure of the directly bonded aluminum–carbon interface in aluminum–carbon composites fabricated by solid-state friction stir processing is reported on an atomic scale for the first time. The atomically resolved structure of two typical interfaces, Al-200 | G-end interface and Al-311 | G-side interface, are investigated by using high-resolution transmission electron microscopy. Interfacial distance analysis shows that both interfaces are bonded via chemical bonds, other than van der Waals forces. The Al-200 | G-end interface is found to be in a semicoherent mode, in which every five Al-(200) planes are bonded directly with three basal planes in graphite. Atomically resolved interfacial matching patterns of the Al-311 | G-side interface are proposed based on the observed atomic plane matching relationship. This work provides new insights into the atomic scale of the aluminum–carbon interface for the development of high-performance aluminum conductors. [ABSTRACT FROM AUTHOR]

Poor ductility is the primary concern of magnesium matrix composites (MMCs) inflicted by non-deformable ceramic particle reinforcements. Metal particles which melt at elevated temperature can be used as reinforcement to improve the deformation characteristics. Ti-6Al-4V particles reinforced AZ31 MMCs were produced through friction stir processing (FSP) which was carried out in a traditional vertical milling machine. The microstructural features as well as the response to external tensile load were explored. A homogenous distribution of Ti-6Al-4V was achieved at every part of the stir zone. There was no chemical decomposition of Ti-6Al-4V. Further, Ti-6Al-4V did not react with Al and Zn present in AZ31 alloy to form new compounds. A continuous strong interface was obtained around Ti-6Al-4V particle with the matrix. Ti-6Al-4V particles underwent breakage during processing due to severe plastic strain. There was a remarkable refinement of grains in the composite caused by dynamic recrystallization in addition to the pinning of smaller size broken particles. Dense dislocations were observed in the matrix because of plastic deformation and the associated strain misfit. Ti-6Al-4V particles improved the tensile behavior and assisted to obtain appreciable deformation before fracture. Brittle mode of failure was avoided. [ABSTRACT FROM AUTHOR]

Carbon nanotubes reinforced aluminum matrix nanocomposites were fabricated by friction stir processing. Raman spectroscopy, scanning electron microscope, transmission electron microscopy and tensile tests were used to characterize microstructures and mechanical properties of the nanocomposites. Effects of energy input during friction stir processing on microstructures and mechanical properties of the nanocomposites were investigated. It was found that the grain of aluminum matrix in the nanocomposites was coarsened slightly with the increase of energy input. Microstructure analysis showed that carbon nanotubes were successfully incorporated into aluminum matrix by friction stir processing and well bonded interfaces between the carbon nanotubes and aluminum matrix were formed. However, the carbon nanotubes dispersion and mechanical properties of the nanocomposites fabricated with different energy inputs were quite different, in which better carbon nanotubes dispersion and higher tensile strength could be obtained by applying higher energy input during the fabrication process. By applying the highest energy input, the tensile strength of the nanocomposite was 53.8% higher than that of the unreinforced aluminum. Meanwhile, the elongation of the nanocomposite was 31.2%, which showed an excellent ductility. The strengthening mechanisms were the synergy of Orowan looping, load transfer and grain refining. The present findings may provide the guidance on the optimization of the processing parameters during friction stir processing for fabricating the high performance aluminum/CNTs nanocomposites. • Effects of energy input during FSP on CNTs/aluminum composites were investigated. • CNTs dispersion was improved with the increase of energy input. • The tensile strength of composites was enhanced with the increase of energy input. • Three possible strengthening mechanisms were discussed. [ABSTRACT FROM AUTHOR]

In this study, the cross-pass friction stir processing method was applied to a duplex Mg-Li-Al-Zn alloy, and the mechanical and corrosion behaviors were systematically investigated. Through this method, the grain size of the Mg-Li-Al-Zn alloy was greatly refined with uniform distribution of equiaxed Mg-rich α (hexagonal close-packed, hcp) and Li-rich β (body-centered cubic, bcc) phases. After friction stir processing, the tensile strength increased greatly, with an exceptional simultaneous increase in elongation. Moreover, the corrosion current density of the 2 cross friction stir processed alloys after immersion in 0.1 M NaCl solution for 48 h significantly decreased to 2.8 μA/cm2. Through microstructural analyses, it was inferred that the synergistic optimization in strength-ductility-corrosion was closely related to the unique distribution of the α/β phases and ultrafine MgLi 2 Al precipitates introduced by cross-pass friction stir processing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Severe plastic deformation makes graphite exfoliate into graphene in situ. • Clean and tight semi-coherent interface is observed between graphene and aluminum. • Microhardness and electrical conductivity of aluminum were simultaneously enhanced. In this paper, a first successful attempt is demonstrated in using friction stir processing (FSP) for preparing bulk graphene/aluminum composites by in situ exfoliation of graphite. New insight is shed on the resulted enhancing effect and nanostructure of in situ exfoliated graphene. Compared to the FSPed base metal, the prepared composites show 15% and 13% increase in hardness and electrical conductivity respectively. It is found a large number of graphene-aluminum diphasic nano clusters embeded in both grain boundary and grain interior of the aluminum matrix, in which graphene and aluminum is bonded in a semi-coherent manner. HRTEM analysis of a partially exfoliated graphite particle indicated that few-layer graphene can be peeled off from the graphite particle owing to the severe plastic deformation during FSP. The findings in this paper opens new opportunities for fabrication of novel graphene/metal composites via in situ exfoliation of graphite during FSP. [ABSTRACT FROM AUTHOR]

• Friction stir processing (FSP) was used to fabricate graphene/Al nanocomposite. • Uniform dispersion of graphene, clean and intimate interfaces were obtained. • Tensile properties and electrical conductivity of Al were simultaneously enhanced. In this study, we demonstrated simultaneously enhanced mechanical properties and electrical conductivity of pure aluminum (Al) by incorporating graphene as the reinforcement. The graphene/Al nanocomposite was fabricated by friction stir processing (FSP) combined with hot extrusion. It was found that graphene was homogeneously dispersed into Al matrix and directly bonded graphene/Al interfaces were formed, which were the structure basis for the simultaneously improved electrical and mechanical properties. This study provides new strategy for fabricating high-strength and highly conductive graphene/Al nanocomposites. [ABSTRACT FROM AUTHOR]

Next-generation armored vehicles possess external structures embellished with O-tempered 5A06 alloy, exhibiting with strength and lightweight attributes. However, a universally applicable method that can concurrently enhance the mechanical properties and corrosion resistance of O-tempered 5A06 alloy has yet to be developed. This study comparatively analyzed between conventional friction stir processing (FSP) and liquid CO 2 -assisted friction stir processing (LCFSP) on the 5A06 aluminum alloy. The findings revealed that LCFSP stands out as a practical approach, leading to notable refinement and homogeneity in microstructure. After LCFSP at a tool rotation speed of 400 rpm, the coarse grains were replaced by refined equiaxial grains with an average diameter of 6.0±3.3 μm. The increase in the strength of the liquid CO 2 -assisted friction stir-processed samples, compared with conventional FSP, primarily stemmed from grain boundary strengthening, dislocation strengthening, and precipitation strengthening. Furthermore, the fine-grained microstructure played a pivotal role in fostering the formation of an amorphous passive layer on the Al alloy, which contributed to the increase in corrosion resistance. Therefore, LCFSP emerges as an effective method for enhancing the performance of 5A06 alloy in armored vehicles. • LCFSP improved the yield strength of 5A06 Al alloy. • LCFSP resulted in a noteworthy grain refinement due to dynamic recrystallization. • Liquid CO 2 cooling prevented the coarsening of precipitated phases. • LCFSP-400 sample showed optimum corrosion resistance due to finer grains. [ABSTRACT FROM AUTHOR]

The microstructure of an as-cast Mg-5Zn-0.3Ca alloy was significantly modified by friction stir processing, which improved the alloy's corrosion resistance. In electrochemical results, the corrosion current density of the FSPed sample at the open circuit potential in 3.5 wt% NaCl aqueous solution decreased to 4.3 μA/cm2 (about 1/7 that of the as-cast sample). The improved corrosion resistance was attributed to the weakening of galvanic corrosion caused by precipitate fragmentation. In addition, a large number of fine cathodic precipitates tended to accumulate on the corroded surface, thereby forming a denser deposition layer and enhancing the shielding effect against corrosion propagation. • As a grain refinement method, FSP improves the corrosion current density by two orders of magnitudes to 10−6 A/cm2, which is not common in grain refinement treatment. • In this paper, FSP is used to modify magnesium alloy to make the second phase finer in size and more uniform in distribution, and the effect of precipitate refinement on the improvement of corrosion resistance is systematically investigated. The effect of precipitate refinement on corrosion resistance enhancement is described from two aspects: the weakening of galvanic corrosion on the corrosion surface and the stronger corrosion shielding on the corrosion depth. • The mechanical properties and corrosion resistance are simultaneously improved, which provide a reference for the further investigation of improving comprehensive properties. [ABSTRACT FROM AUTHOR]

• A novel simulation method of FSW based on two-phase flow is proposed. • High-velocity rotating flow zone is the channel for vertical migration of material. • The accuracy of the simulation results is verified by experimental measurement. In this letter, combined experimental and computational analysis has allowed new understanding of the vertical material flow behaviors during friction stir welding. It is found that a high-velocity rotating flow zone exists near the welding tool, where horizontal and vertical flow are both significant. Material entering the high-velocity rotating flow zone migrates to the lower location of the workpiece after multiple rotations. Material outside the high-velocity rotating flow zone bypasses the tool and migrates towards the upper locations. The findings of this research offer insights into the regulation of microstructures and the formation of defects in FSW. [ABSTRACT FROM AUTHOR]

In this work, friction stir additive manufacturing was successfully performed to fabricate the dual-phase Mg–Li–Al–Zn magnesium alloy. A fine-grained microstructure decorated by uniformly dispersed precipitates was observed in the stir zone. Through the combined effects of grain refinement and solid solution strengthening, the friction stir additive manufactured Mg–Li–Al–Zn alloy exhibited the synergistic enhancement of strength, ductility, and corrosion resistance. [ABSTRACT FROM AUTHOR]

Friction stir processing (FSP) was applied to develop pure titanium (Ti) particulate reinforced AZ31B magnesium matrix composites (MMCs). Machined groove strategy was used to pack the Ti particulates initially and FSP was carried out using a conventional vertical milling machine. Traverse speed, number of passes and volume fraction of Ti particulates were varied. The microstructural evolution was observed using optical and scanning electron microscopy. Different microstructural zones were identified in the stir zone based on the distribution of Ti particulates. In general, the tendency to form clusters decreased at lower traverse speeds and higher number of passes. The increase in traverse speed and number of passes refined the grains of the magnesium matrix. The variation in the tensile behavior due to the chosen process parameter was explored. The composites fabricated at a traverse speed of 30 mm/min and 5 passes demonstrated highest tensile strength and elongation. The details of fracture surface and fracture mode were further reported. • Development of AZ31/Ti magnesium composites using friction stir processing. • Sintered zone-a unique microstructure was observed in the stir zone. • The increase in number of passes improved the distribution of particles. • Lower traverse speed and higher number of passes were required for higher strength. • Ti particulates retained ductility to that of ceramic reinforcement at optimum conditions. [ABSTRACT FROM AUTHOR]

An attempt was made to selectively alloying Mg base matrix with 3%, 6% and 9% volume of Al particulates respectively, using friction stir selective alloying (FSSA). The objective of this investigation is to enhancing the mechanical and metallurgical properties of AZ31 Mg using solid state alloying process. Tensile properties revealed an enhanced strength of 249.4 MPa at 6% Al alloying sample and which is 95% strength of base material. The fractography detects a ductile fracture consisting of dimples, cup and cone shape counterparts with the fracture initiation area and propagation direction. Hardness indentation depicts improved micro-hardness properties and 185% of base alloy in case of 6% alloying due to presence of favorable reinforcing intermetallic particles in the stir area. The hardness relatively varies corresponding to % of Al alloying element at the stir zone. The 6% alloying sample gives best mechanical properties compared to the 3% and 9% alloying. Line scan/mapping technique detects the uniform distribution of the alloying elements. The EDX analysis spot formation of different intermetallic phases, namely AlMg, Al 12 Mg 17 and Mg 2 Al 3. The XRD analysis also informs the intermetallic phase formation and well agrees with EDX analysis. Alloying at different Al% to the Mg base matrix helps in microstructural modification, including uniform dispersion of the alloying element, finer grain and minimum assembled particles at the stir zone leads to improved metallurgical and mechanical properties. The stirred zone consists of metal matrix solid solution of martensitic structure and relatively finer grain compared to base alloy. [ABSTRACT FROM AUTHOR]

Conventional ceramic particulate reinforcements cause a major loss in ductility of magnesium matrix composites (MMCs). Metallic particles possessing higher melting point can offer a solution to this issue. Titanium (Ti) particles (0,7,14 and 21 vol%) were reinforced into magnesium alloy AZ31 using friction stir processing (FSP) performed by a conventional sturdy vertical milling machine. The microstructure and the tensile behavior of the fabricated composites were studied in detail. The micrographs revealed a uniform distribution of Ti particles all over the stir zone irrespective of the volume content of Ti. Ti particles did not decompose or react with the matrix and its alloying elements. Ti particles established a proper interface with the matrix AZ31. Ti particles survived the severe plastic strain without breakage. The grains in the matrix were refined extremely because of dynamic recrystallization and the pinning effect of Ti particles. A large number of dislocations are found in the composite. Ti particles improved the tensile strength of the composite and helped to retain appreciable ductility. [ABSTRACT FROM AUTHOR]

In this study, graphite-reinforced 6201 aluminum alloy matrix composites were fabricated by multi-pass friction stir processing. The mechanical properties and electrical conductivity of the composites were tested, the microstructure and composition of the composites were analyzed by scanning electron microscope and Raman spectroscopy. The results showed that the graphite-reinforced 6201Al matrix composite achieved simultaneous improvement of mechanical and electrical properties compared to the base material, with a 19.5 % increase in ultimate tensile strength and a 1.2 % increase in electrical conductivity. Microstructural and compositional analysis revealed that the graphite reinforcement in the composites was significantly fragmented during FSP, and its size decreased while the number of atomic layers reduced with increase in FSP passes. A directly bonded Al-C interface is observed between the graphite reinforcement and the matrix, and no interface products such as Al 4 C 3 were generated. The graphite reinforcement in the matrix simultaneously serves as a hindrance to dislocation motion and the expansion of microvoids during the tensile process, thereby leading to an enhancement in the tensile strength of the composite. The improvement in the electrical conductivity of the composite is attributed to two factors. On one hand, the structure and properties of graphite-reinforced phase is transformed to be closer to graphene after the severe plastic deformation during FSP process. On the other hand, the directly bonded Al-C interface between the matrix and the reinforcement is able to transfer electrons across the interface. • Mechanical and electrical performance of aluminum were simultaneously enhanced. • Severe plastic deformation decreased the number of atomic layers of the graphite. • Directly bonded Al-C interface between matrix and reinforcement is observed. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced AA5052 bulk composites were successfully fabricated by multi-pass friction stir processing (FSP), aiming to improve the wear-resistance of AA5052. The microstructural, mechanical and tribological performances of the composites were documented and investigated. Microstructure observations indicated that, in the composites, carbon fibers were homogeneously dispersed in large volume, where no obvious Al 4 C 3 layer was detected between the matrix and the carbon fibers. The orientation of carbon fibers in the composites were random owing to the severe plastic deformation brought on by FSP. Further mechanical tests showed that the hardness of the composites increased by 46.8% comparing to the base metal, and that the composite fabricated at 1000 rpm and 75 mm/min showed 18.6% higher UTS and 13.0% higher elongation in comparison with the base metal. The wear tests illustrated that wear process of the composites was more stable and the wear volume loss was reduced by more than 70%. The strengthening of mechanical properties was attributed to formation of GNDs, crack deflection and load transfer of carbon fibers. The further analysis on the worn surfaces revealed that abrasive wear occurred in the composites, while adhesive wear occurred in both the based metal and FSPed matrix. The addition of carbon fibers segments in the aluminum could suppress the nucleation and propagation of micro-cracks, which effectively prevented the material peeling during the wear process and thus improved the tribological properties. In addition, the formation of mechanical mixing layer would be another contributing factor to the improvement of wear resistance. [ABSTRACT FROM AUTHOR]

• A novel integrated thermal-fluid-structure coupling model of the friction stir welding process was proposed for simultaneous prediction of the weld formation and tool service life. • A new non-uniform distribution model was proposed to describe the interaction at the tool-workpiece contact interface. • The severe reduction of tool-workpiece contact interfacial frictional shear stress is the main reason for the formation of the void defect in friction stir welding. • The difference between the maximum and the minimum tool-workpiece contact pressure could serve as a numerical criterion to predict void defect formation. • The tool is apt to fracture at its root under an improper welding condition since severe stress concentration is located there. Understanding the void defect formation mechanism and simultaneous predicting the tool service life in friction stir welding are critical for optimizing the welding parameters. However, the void defect formation mechanism in friction stir welding is not yet elucidated. In this study, a novel integrated thermal-fluid-structure coupling model of the friction stir welding process was proposed for simultaneous prediction of the weld formation and tool service life. A new non-uniform distribution model of the tool-workpiece contact pressure was proposed to describe the interaction between the tool and the workpiece. The void defect formation mechanism was quantitatively studied using the proposed integrated thermal-fluid-structure coupling model. The results show that the plastic material flows in the horizontal direction and can completely fill the cavity behind the tool for the welding condition of forming a sound weld. While the tool-workpiece contact interfacial frictional shear stress in the rear of the tool is decreased significantly which leads to a severe decrease in the plastic material flow velocity. Therefore, after bypassing the tool from the retreating side, the plastic material at the bottom of the weld stagnates, and void defect forms in the middle and lower part of the weld at the advancing side. The difference between the maximum and the minimum tool-workpiece contact pressure could serve as a numerical criterion to predict void defects. A sound joint is formed when the difference is lower than the critical value of 15 MPa, while a void defect is formed in the weld if it is higher than this critical value. The maximum equivalent stress acting on the tool is located at the pin root with severe stress concentration at a high welding speed. The front of the tool is subjected to tensile stress while its rear is subjected to compressive stress, therefore the tool is apt to fracture at its root under an inappropriate welding condition. The average normal stress of the tool varies periodically with its period consistent with the rotation period of the tool. The service life of the tool is decreased with the increase in welding speed and the decrease in rotation speed. The model is validated by experimental results. [Display omitted] [ABSTRACT FROM AUTHOR]

This study employed the bobbin tool friction stir welding (BT-FSW) technique to weld 2219 aluminum alloy with 6 mm thickness. The microstructure investigation revealed that the weld nugget zone (WNZ) has a small number of coarse θ phases, whereas the heat-affected zone (HAZ) exhibits colossal amount of θ' phases. The impedance analysis demonstrated that the corrosion resistance of the WNZ is better than that of other zones. Also, the salt spray corrosion test showed that the WNZ has the best corrosion resistance, whereas the base metal (BM) has the worst corrosion resistance. Therefore, the poor corrosion resistance of the BM can be attributed to a high content of the precipitates. • 2219 aluminium alloy was joined using the bobbin tool friction stir welding method. • The corrosion prerformance of BT-FSW joints was investigated using the LEIS method and salt spray corrosion test. • Corrosion attack was galvanic corrosion between the precipitates and matrix. • The relation between corrosion susceptibility and the content of precipitates was revealed. [ABSTRACT FROM AUTHOR]