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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]

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]

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]

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]

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]

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]

The 1000 MW hydro-generator unit has huge geometrical dimensions and requires high installation accuracy. The control of its installation deviation will directly affect the installation quality and operational performance of the unit and the rotor eccentricity is one of the core problems faced in the installation and operation of the giant hydro-generator unit. This paper presents a method to calculate the asymmetric magnetic field by means of the transient finite element calculation to reveal the effect of the generator rotor radial deviation on the unbalanced magnetic pull of the unit. The effects of rotor dynamic eccentricity on the magnetic field are analyzed in the time and frequency domains, respectively. The results show that both the DC component and the key harmonic component of the unbalanced magnetic pull increase with the increase of the eccentricity value, and this variation relationship is nearly linear. when the eccentricity is no more than 0.15 mm, the unbalanced magnetic pull caused by the rotor eccentricity is calculated to be small, which is acceptable. The conclusions of this investigation can help to straighten out the requirements and standards for the installation deviation control of the large hydro-generator units. [ABSTRACT FROM AUTHOR]