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Unlike the conventional fusion welding process, friction stir welding (FSW) relies on solid-state bonding (SSB) to join metal surfaces. In this study, a straightforward computational methodology is proposed for predicting the material bonding defects during FSW using quantitative evaluation of the in-process thermal-mechanical condition. Several key modeling methods are integrated for predicting the material bonding defects. FSW of AA2024 is taken as an example to demonstrate the performance of the computational analysis. The dynamic sticking (DS) model is shown to be able to predict the geometry of the rotating flow zone near the welding tool. Butting interface tracking (BIT) analysis shows a significant orientation change occurring to the original butting interface, owing to the material flow in FSW, which has a major impact on the bonding pressure at the butting interface. The evolution of the interfacial temperature and the interfacial pressure at the butting interface was obtained to analyze their roles in the formation of material bonding. Four bonding-quality indexes for quantifying the thermal-mechanical condition are tested to show their performance in characterizing the bonding quality during FSW. When the BQI is below a critical value, a bonding defect will be generated. The paper indicates that the simulation-based prediction of a material bonding defect is highly feasible if the developed methodology is extended to quantitatively determine the critical value of the bonding quality index for successful SSB for various alloys. [ABSTRACT FROM AUTHOR]

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]

• Most accurate prediction to date of material transfer during dissimilar FSW of Al/Mg alloys is reported. • For the first time the role of maelstrom and straight-though flow pattern in redistribution of material is revealed. • Maelstrom flow causes downward transfer of Mg alloy from the top of advancing side. • Straight-through flow causes upward transfer of the balancing material. Understanding of the intense in-process material flow during dissimilar friction stir welding (FSW) of Al/Mg alloys has long been limited by the accuracy of the numerical simulation model. In this paper, a computational fluid dynamics (CFD) simulation with state-of-the-art thermo-mechanical-flow fully coupled shear boundary model is employed to predict the material behavior during the dissimilar FSW of AA6061 Al alloy and AZ31B Mg alloy. The prediction on the three-dimensional temperature field and the flow field is obtained with the best accuracy to date. It is shown that the primary motion is rotation around the tool pin in the high-velocity zone (HVZ) near the welding tool, and there is a vertical component of velocity with a magnitude lower than 10 % of the total velocity. It is shown that the material is significantly redistributed during the welding process, with Mg alloy accumulating at the bottom and Al alloy accumulating at the upper part of the joint. The material distribution behavior is attributed to the material transfer in the through-thickness direction. In the HVZ, most material flows in the straight-through pattern and is slightly elevated when it flows through the tool vicinity. A small amount of Mg alloy flows in the maelstrom pattern, encircles the tool pin multiple times, and is transferred to a lower position. In this way, The upward transfer of Al alloy and the downward transfer of Mg alloy lead to the through-thickness redistribution of each alloy. [Display omitted] [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]

Sn-Zn system solder has been considered promising lead-free solder materials to replace Sn-Pb and Sn-Ag-Cu systems in various conditions, due to the performance characteristics of low melting point and high reliabilities. This study investigates the microstructure evolution and mechanical properties of Sn-9Zn-2.5Bi-1.5In (Sn-Zn) solder paste joints on Cu substrates with aging treatment, which are under various conditions of different paste thicknesses and reflow times. It is concluded that the intermetallic compounds (IMC) of Sn-Zn/Cu solder joints are Cu 5 Zn 8 layers at all stages of aging. The microstructure of before-aging joints and evolution during aging is significantly affected by the reflow times, while slightly affected by the paste thicknesses. In all experimental conditions, the IMC layers gradually thicken and reach 4-5 μm when aging for 720 h. While Cu 5 Zn 8 products are generated at the position of the original Zn phases or along the Sn grain boundaries in the solder matrix with the aging process. The unequal diffusion rates of Cu and Zn atoms create two types of joint voids which are accumulated by Zn vacancies in the solder and accumulated by Cu vacancies at the interface between the solder and IMC layer. The interface voids and coarsened crystal structure are identified as the main factor for joint failure during aging. Finally, the shear strength of the joints with the initial 3.5 μm thickness IMC layers can maintain at high values above 45 MPa, exhibiting satisfactory reliability, which indicates that the soldering process determines the initial IMC structure, and affects the joint reliability more than the aging process. [Display omitted] • During aging, the IMC layers of Sn-Zn joints grow slowly and many IMC products are generated in the solder. • Zn vacancy voids are created in the solder and Cu vacancy voids are created at the solder/IMC interface. • The fracture mode of the joints shifts from ductile to brittle with increasing aging time. • The reflow process determines the initial IMC microstructure and affects the joint reliability more than the aging process. [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]

Pin thread is one of the most common geometrical features for the friction stir welding (FSW) tools. The main purpose of employing the pin thread is to improve the in-process material flow behaviors during FSW. However, it has not been fully understood how exactly the pin thread influences the material flow because of the lack of in-process observation. In this study, we aim to analyze the effect of pin thread on the in-process material flow during FSW of an Al-Mg-Zn alloy by using numerical simulation based on computational fluid dynamics (CFD). In our numerical simulation, the transient rotation of the threaded pin is implemented explicitly via fully transient control of the zone motion, and the mechanical interaction at the tool-workpiece interface is considered via the recent developed shear-stress-based frictional boundary condition. The numerical simulation has been validated by the experimental measured temperatures at 8 different locations, the distribution of marker materials and the geometry of deformation zone in the weld. Based on the numerical simulation results, three effects of the pin thread on the material flow have been elucidated. First, accelerated flow velocity and enhanced strain rate is induced owing to the use of the pin thread, which is attributed to the fact that the interfacial sticking is preferable inside the thread groove opening. Second, the pin thread has an effect to trap material in the high-velocity zone inside the thread groove opening, which causes a many-circle flow pattern around the threaded pin. Third, the pin thread contributes to a vertical pressure gradient, which is important for the in-process material transfer from the top to the bottom. The approaches and concepts in this study can be applied for further fundamental investigation of FSW and the computer aided design of the welding tools. [ABSTRACT FROM AUTHOR]

Many material manufacturing technologies, such as friction stir welding, rely on rapid solid state bonding to join metal surfaces. In this letter, a differential equation is developed to formulate the growing of the interfacial bonded area owing to the collapse of surface asperities under compressive stress during rapid solid state bonding of metal surfaces. The effect of pressure, temperature and bonding time on the growing of bonded area is discussed. The proposed approach is verified by experimental data. [ABSTRACT FROM AUTHOR]

Material flow during welding is an important factor that affects the quality of friction stir welding (FSW) joints. In this study, a new welding tool/work piece contact model is proposed to simulate material flow near the FSW tool. The material around the tool flows under the driving force of friction non-uniformly distributed on the interface. Friction force is calculated based on modified Coulomb friction law in this model. Wormhole defects under various welding parameters are graphically predicted by the distribution of tracing particles added in numerical models. Five groups of FSW experiments were conducted to obtain temperature and joint morphology results. Simulation results, including temperature and wormhole defects prediction, corresponded with the experimental results. Further analyses imply that under the process parameters selected in this study, an area with very low friction force occurred behind the tool pin. When the material in front of the tool pin moved to this area from the retreating side, the material speed significantly decreased due to the insufficient driving force, and it became difficult for the material to move to the advancing side along the circular path. The study reveals that the non-uniform friction force model provides a potential tool for predicting FSW defects. [ABSTRACT FROM AUTHOR]

Sn-Zn solders have been considered as promising lead-free solders. This study investigated the shear strength and fracture mechanism of Sn-9Zn-2.5Bi-1.5In and Sn-3.0Ag-0.5Cu pastes with Cu-substrate joints. It is found that the intermetallic compounds (IMC) in Sn-Ag-Cu joints are Cu 6 Sn 5 and Cu 3 Sn, while the IMC of Sn-Zn joints are particular Cu 5 Zn 8 , different from other Sn-based solders, which grow more slowly. Both solder joints fracture by a mixed mechanism. The cracks tend to form and propagate at the interface of the solder and IMC in the cases of too short or too long reflow times, resulting in more interfacial fracture and lower shear strength. An optimum soldering zone of reflow time will bring about more reliable joints, which fracture mainly in the solder. Sn-Zn joints have more solder fracture and less interface fracture, therefore have higher shear strength and reliability, compared with Sn-Ag-Cu joints. For Sn-Zn joints, the optimum reflow times are 20 s–120 s with the shear strength of 62 MPa–70 MPa, while for Sn-Ag-Cu joints, the optimum reflow times are 50 s–180 s, with the strength of 50 MPa–62 MPa. • The shear strength of Sn-Zn joints is higher than that of Sn-Ag-Cu joints under the same reflow time. • The shear strength of both joints increased first and then decreased as the reflow time increased. • More fracture occurred at the solder/IMC interface of both solders when the reflow time is too long or too short. • The joints both fractured mainly in the solder under optimal reflow times, corresponding the higher shear strength. [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]

Additive friction stir deposition (AFSD) is a novel solid-state additive manufacturing method developed on the principle of stirring friction. Benefits from its solid-phase properties, compared with traditional additive manufacturing based on melting–solidification cycles, AFSD solves the problems of porosity, cracks, and residual stress caused by the melting–solidification process, and has a significant improvement in efficiency. In AFSD, the interaction between feedstocks and high-speed rotating print heads suffers severe plastic deformation at high temperatures below the melting point, ending up in fine, equiaxed recrystallized grains. The above characteristics make components by AFSD show similar mechanical behaviors to the forged ones. This article reviews the development of AFSD technology, elaborates on the basic principles, compares the macroscopic formability and material flow behavior of AFSD processes using different types of feedstocks, summarizes the microstructure and mechanical properties obtained from the AFSD of alloys with different compositions, and finally provides an outlook on the development trends, opportunities, and challenges to the researchers and industrial fields concerning AFSD. [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]

As the central component in friction stir welding, the design and manufacture of welding tools for aluminum alloys have garnered substantial attention. However, the understanding of tool reliability during the welding process, especially in terms of fatigue performance, remains unclear. This paper focuses on the welding of AA2219-T4 as a case study to elucidate the predominant failure mode of the tool during the friction stir welding (FSW) of aluminum alloys. Experimental methods, including FSW welding and fracture morphology analysis of the failed tool, coupled with numerical simulation, confirm that high-cycle mechanical fatigue fracture is the primary mode of the tool failure. Failures predominantly occur at the tool pin's root and the shoulder end face with scroll concave grooves. The experimental and simulation results exhibit a noteworthy agreement, validating the reliability of the simulation model. The FSW Arbitrary Lagrangian–Eulerian (ALE) model developed in this study analyzes stress distribution and variation under the thermo-mechanical coupling effect of the tool. It reveals that stress concentration resulting from structural changes in the tool is the primary driver of fatigue crack initiation. This is attributed to exposure to alternating cyclic stresses such as bending, tension, and torsion at the tool pin's root, manifesting as multiaxial composite mechanical fatigue. Among these stresses, bending alternating cyclic stress exerts the most significant influence. The paper employs the Tool Life module in DEFORM software to predict the fatigue life of the tool. Results indicate that reducing welding speed or increasing rotation speed can enhance the tool's fatigue life to some extent. The methodology proposed in this paper serves as a valuable reference for optimizing FSW structures or processes to enhance the fatigue performance of welding tools. [ABSTRACT FROM AUTHOR]

In this paper, 5-mm thick 6082-T6 aluminum alloy plates were friction stir welded in a position control mode at rotation rate of 1800 rpm and welding speeds ranging from 200 to 600 mm/min. The metallurgical morphology of root defects was elucidated. As indicated by observation on electropolished samples that contains the root defects, it was found that there are three types of bonding states near the faying surface at the joint root, which are fully bonded state without cracks, incomplete bonding state in the form of non-continuous cracks (the kissing bond), and unbonded state in the form of continuous cracks (the lack-of-penetration defect). The vertical position of welding tool during the welding pocess is recorded, and it is found that, as the welding speed increased, the rebound amount of the welding tool increased during actual welding process because of increasing deformation resistance. It consequently leads to the decrease of insertion depth of the welding tool, which significantly increases formation tendency of the root defect. [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]

Heat and mass transfer in the process of friction stir welding (FSW) determine the weld formation quality. Meanwhile, the formation of voids in FSW limits welding speed improvement and welding efficiency. Although superimposing ultrasonic vibration can be adopted as an effective means to restrain the formation of voids, the potential suppression mechanism was still unrevealed. Herein, a multi-physical model using the shear stress boundary conditions was put forward to quantitatively study the influence of ultrasonic vibration on the heat and mass transfer behaviors and the resulting weld formation which was also validated experimentally. Our results show that ultrasonic vibration in FSW slightly enhances the heat flux at the tool-workpiece interfacial contact surface as well as the plastic deformation heat generation near the tool. Therefore, the high-temperature area (higher than 690 K) near the tool pin side increases from 2.11 to 2.29 mm. The slightly higher heat rate and temperature enhance the fluidity of plastic material, resulting in an obvious increase in the flow velocity. As a consequence, the plastic material moves farther to fill the cavity at the rear advancing side, which is conducive to eliminating the void defects. The maximum strain rate on z = 2 mm horizontal plane at the AS is 206.7 s−1 in UVeFSW, while it is 13.5 s−1 in CFSW. The strain rate of the contact interface on AS increases by nearly 5 times, which implies an enhanced plastic material flow and is the main reason for suppressing void defects by superimposing ultrasonic vibration in FSW. [ABSTRACT FROM AUTHOR]

Part distortion is a technical bottleneck in the field of metal additive manufacturing, which generally depends on the thermo-mechanical behavior the material experienced during the deposition process. However, the transient strain distribution and evolution behavior of the additive manufactured part still remain unclear due to the lack of in-process observation method. This study successfully obtained the continuous full-field strain of a Ti-6Al-4 V thin-wall during the laser engineered net shaping (LENS) additive manufacturing process using Digital image correlation (DIC) method. The evolution characteristic of vertical strain and longitudinal strain of the material was primarily studied during the deposition process. The results shows that the longitudinal strain was found increases rapidly to tensile strain as the laser beam approaches, whereas the vertical strain decreases rapidly to a compressive strain and gradually transform to tensile strain. Both vertical strain and longitudinal strain were found accumulated and rose periodically when depositing multi-layers, which increases the distortion tendency of the deposited part. In situ measurement of the strain field in additive manufacturing process can be an effective verification for theoretic and computational studies, which also provides the possibility of controlling stress and distortion in real time. [ABSTRACT FROM AUTHOR]

Artificial neural networks (ANNs) have been an important approach for predicting the value of flow stress, which is dependent on temperature, strain, and strain rate. However, there is still a lack of sufficient knowledge regarding what structure of ANN should be used for predicting metal flow stress. In this paper, we train an ANN for predicting flow stress of In718 alloys at high temperatures using our experimental data, and the structure of the ANN is optimized by comparing the performance of four ANNs in predicting the flow stress of In718 alloy. It is found that, as the size of the ANN increases, the ability of the ANN to retrieve the flow stress results from a training dataset is significantly enhanced; however, the ability to predict the flow stress results absent from the training does not monotonically increase with the size of the ANN. It is concluded that the ANN with one hidden layer and four nodes possesses optimized performance for predicting the flow stress of In718 alloys in this study. The reason why there exists an optimized ANN size is discussed. When the ANN size is less than the optimized size, the prediction, especially the strain dependency, falls into underfitting and fails to predict the curve. When the ANN size is less than the optimized size, the predicted flow stress curves with the temperature, strain, and strain rate will contain non-physical fluctuations, thus reducing their prediction accuracy of extrapolation. For metals similar to the In718 alloy, ANNs with very few nodes in the hidden layer are preferred rather than the large ANNs with tens or hundreds of nodes in the hidden layers. [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]

Abstract: Fatigue crack propagation (FCP) at different locations of friction stir welded AA6N01 joint was investigated. FCP experiments were performed to study crack propagation rates in the Paris region. The highest FCP rate was in the stir zone (SZ) which composed of much finer grain compared with the other areas of joint. Fracture surfaces at different locations were examined. More brittle fracture characteristic was observed in SZ. To further investigate the mechanism of high FCP rate in SZ, differential scanning calorimetry (DSC) analysis was conducted. The results showed that there was relatively high energy stored in SZ. After releasing the stored energy, the resistance of FCP in SZ improved significantly while the grain size kept the same, the fatigue fractographs had changed from quasi-cleavage to plastic fatigue striation fracture. These proved that the stored energy affected fatigue crack propagation in FSW joint significantly. [Copyright &y& Elsevier]

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]

Wear behavior of friction stir alloyed (FSAed) AZ31-Mg with externally added Al has been investigated. The phase distribution pattern in the case of 6% Al alloying was best with honey-comb like structure with enhanced metallurgical properties. The major wear occurs due to abrasion, adhesion, delamination, plastic deformation and oxidation. The friction coefficient and wear characteristics at 6% Al alloyed sample is comparatively less than 3 and 9% alloying. With an increase in Al volume 3–6% the hardness increases 10%. The wear loss of the alloyed specimen at 6% Al are 16% and 11% less compared to the base metal and processed specimen respectively. In brief, 6% Al alloying sample gives the best tribological, metallurgical and mechanical properties. Main contribution • The number of previous research works is very limited in wear behavior of AZ31 Mg with added different percentage of Al alloying and its effect on mechanical properties. • This research work can provide useful information regarding the development of future upgraded Mg alloy on the basis of external alloying at the selected area depending on the application requirements. • The current technique is innovative and it is first try of using Al particulates at different percentage in AZ31 Mg matrix by friction stir alloying (FSA) and observed enhanced mechanical properties by the formation of solid solution phase constituent. The Al is selected for better reaction feasibility with Mg and formation of favorable binary and ternary compounds. Novelty • This process has novelty regarding the experimental techniques. As in case of fusion welding process, we can also add a filler (alloying) material to the base matrix using solid state processing named as FSA process to improve the metallurgical and mechanical properties, more than without alloying case, which is rare in earlier research. Industrial applications • The likely users of this AZ31Mg+Al alloy are light industry, household and electrical appliances. This alloy extensively used for high specific strength, excellent machinability, wonderful biocompatibility, outstanding electromagnetic shielding and ease of recyclability. Also application includes fluid connector and coupling, actuators, safety valves and other safety devices. This alloy has appreciable weight reduction, performance enhancement and cost reduction. [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]

Tool tilt angle in friction stir welding (FSW) is an important factor that influences the joint quality, but the underlying mechanism has not been fully understood due to the lack of in-process observation. In this study, we present a CFD model, in which a geometrical model and an incomplete contact boundary condition are proposed, to investigate the effects of tool tilt angle on the in-process heat transfer and mass transfer during FSW. Three effects of tool tilt angle on heat transfer and mass transfer have been concluded based on the simulation results. First, a higher temperature is generated by the tilt welding tool on the advancing side (AS), which is attributed to the incomplete contact at the shoulder/workpiece interface. Second, the tilt welding tool generates a higher frictional force at the tool/workpiece interface, which significantly improves the interfacial material flow velocity behind the tool. Third, the tilt welding tool generates a stronger stirring action to the material in the vicinity of the welding tool, which is beneficial for the mixing of materials and the formation of friction stir welds. The approaches and concepts in this study can be used for the optimization of the application of tool tilt angle in FSW. [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]

Two kinds of atom distribution in Mg-9wt.%Al melt, undissolved particles and atom aggregation, were investigated by synchrotron small-angle X-ray scattering (SAXS) and molecular dynamics simulation, respectively. The SAXS results indicated that no undissolved particles exist in Mg-9wt.%Al melt after short-time equilibration. The simulation results indicated that atom Mg and Al have a tendency to form bonds in Mg-9wt.%Al melt and the sizes of aggregated Al-centred clusters are within the characteristic scale of medium-range order; the degree of atom aggregation in Mg-9wt.%Al melt increases slightly with increasing temperature. [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]