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Lap joining of 6061 aluminum alloy (Al alloy) to 304 stainless steel (SS) by pulse on pulse metal inert gas (MIG) welding-brazing process using 4043 Al-Si filler wire was conducted to study the effect of pulse on pulse frequency on welding process and welding quality. The results show that for high-energy pulse, the effect of pulse on pulse frequency on arc characteristics and behavior of metal transfer is significant. However, for low-energy pulse, this effect is insignificant. The external oscillation frequency of the molten pool induced by variation of arc force and impingement of metal droplets is closer to its natural oscillation frequency when the pulse on pulse frequency is 5.3 Hz; the weld pool is more intensively oscillated. This phenomenon results in the formation of fine and uniform equiaxed grain structures, and thin brazed interface layers, which leads to excellent mechanical properties. The thickness of brazed interface layers varied from about 5 to 9 μm, and the thinner the layer, the better the mechanical properties of Al alloy/SS joints are. When pulse on pulse frequency is less than or equal to 5.3 Hz, the joints fractured at heat-affected zone of 6061 Al alloy with highest strength of 117.4 MPa up to 94.7 % of the tensile strength of Al alloy. However, the joints fractured at the brazed interface layer with much lower strength when the pulse on pulse frequency is larger than 5.3 Hz.

While the MIG arc brazing method provides significant potential for joining aluminum alloy to stainless steel, the welding process is still hard to examine and control quantitatively. In this work, pulse on pulse was applied to improve the stability of conventional pulse gas metal arc brazing (P-GMAB) process, which was determined by measurement of welding current and voltage signals. The results based on time domain and frequency domain analysis demonstrate that the welding process stability of pulse on pulse gas metal arc brazing (PP-GMAB) far outweighs that of P-GMAB. These quantitative results are tested and validated with experimental results. The metal transfer mode of P-GMAB was the blend of one drop per pulse (ODPP) and multiple drops per pulse (MDPP). However, for high energy pulse of PP-GMAB, the droplet transfer mode is primarily ODPP with a few number of two drop per pulse (TDPP); for low energy pulse, the metal transfer maintains ODPP through the whole welding process. The improved weld formation by employing the pulse on pulse mode is due to its stabler welding process.

A method for improving the accuracy of a parallel manipulator with full-circle rotation is systematically investigated in this work via kinematic analysis, error modeling, sensitivity analysis, and tolerance allocation. First, a kinematic analysis of the mechanism is made using the space vector chain method. Using the results as a basis, an error model is formulated considering the main error sources. Position and orientation error-mapping models are established by mathematical transformation of the parallelogram structure characteristics. Second, a sensitivity analysis is performed on the geometric error sources. A global sensitivity evaluation index is proposed to evaluate the contribution of the geometric errors to the accuracy of the end-effector. The analysis results provide a theoretical basis for the allocation of tolerances to the parts of the mechanical design. Finally, based on the results of the sensitivity analysis, the design of the tolerances can be solved as a nonlinearly constrained optimization problem. A genetic algorithm is applied to carry out the allocation of the manufacturing tolerances of the parts. Accordingly, the tolerance ranges for nine kinds of geometrical error sources are obtained. The achievements made in this work can also be applied to other similar parallel mechanisms with full-circle rotation to improve error modeling and design accuracy.

Five-axis hybrid machine tools are widely used due to the advantages of high speed, high precision, and good performance in the aviation and aerospace manufacturing industry. Many scholars have studied the basic theories and key technologies along with the widespread attention of academia and industry thereon. An integrated geometric error modelling method, under the unified coordinate system framework for a general five-axis hybrid machine tool based on a 3-PRS parallel spindle head, is proposed. The kinematic inverse model of a hybrid machine tool is built using a vector chain method. On the basis of the analysis of the various error sources, an integrated geometric error model is developed based on perturbation theory. Then, the influence of each error source of a hybrid machine tool is investigated at the end-effector. Screw theory is applied to research error source separation for the 3-PRS parallel spindle head of a hybrid machine tool, and a sensitivity analysis of the error sources is undertaken using statistical methods. Based on this, the compensable error sources and non-compensable error sources of a hybrid machine tool are separated successfully. The non-compensable error sources must be strictly controlled in the process of design and manufacture, while the compensable error sources can be compensated inexpensively by software after reaching some degree of accuracy in the design and manufacture. This research provides a theoretical reference for this kind of machine tool design and its use in manufacturing.

This paper presented residual stress measurement on two circumferential Variable polarity plasma arc welding (VPPAW) joints and one circular closed Friction stir welding (FSW) joint on the propellant tank of 2219 aluminum alloy using the indentation strain-gauge method. Quite large tensile residual stresses were attached to the center and inner areas of the circular closed FSW joint. There were very large tensile stresses in some points of the two circumferential VPPAW joints, among these points, the maximum value was +253 MPa, which was about 63 % of the yield strength of 410 MPa measured in the base material. In addition, the peak of compressive residual stress was about -160 MPa. Above all, there were two typical peaks of residual stress in the circumferential VPPAW joints, one was located in the middle part while the other one was near the start/end position of the joints. Combining the result of residual stress measurement with the characteristics of the tank structure, it can be concluded that circular closed FSW joint around the flange was a weak spot on the propellant tank. And the most vulnerable point on the circular closed FSW joint has also been found.

PurposeThe purpose of this paper is to propose a method for selecting the position and attitude trajectory of error measurement to improve the kinematic calibration efficiency of a one translational and two rotational (1T2R) parallel power head and to improve the error compensation effect by improving the properties of the error identification matrix.Design/methodology/approachFirst, a general mapping model between the endpoint synthesis error is established and each geometric error source. Second, a model for optimizing the position and attitude trajectory of error measurement based on sensitivity analysis results is proposed, providing a basis for optimizing the error measurement trajectory of the mechanism in the working space. Finally, distance error measurement information and principal component analysis (PCA) ideas are used to construct an error identification matrix. The robustness and compensation effect of the identification algorithm were verified by simulation and through experiments.FindingsThrough sensitivity analysis, it is found that the distribution of the sensitivity coefficient of each error source in the plane of the workspace can approximately represent its distribution in the workspace, and when the end of the mechanism moves in a circle with a large nutation angle, the comprehensive influence coefficient of each sensitivity is the largest. Residual analysis shows that the robustness of the identification algorithm with the idea of PCA is improved. Through experiments, it is found that the compensation effect is improved.Originality/valueA model for optimizing the position and attitude trajectory of error measurement is proposed, which can effectively improve the error measurement efficiency of the 1T2R parallel mechanism. In addition, the PCA idea is introduced. A least-squares PCA error identification algorithm that improves the robustness of the identification algorithm by improving the property of the identification matrix is proposed, and the compensation effect is improved. This method has been verified by experiments on 1T2R parallel mechanism and can be extended to other similar parallel mechanisms.

This paper presents a method for optimizing process parameters of simultaneous double-sided friction stir welding (SDS-FSW). Building upon the thermal pseudo-mechanical mechanism and computational solid mechanics, a heat transfer model is formulated first to investigate the coupling effect of the heat sources and verified by existing experimental data of another study. The nonlinear surrogate models are then constructed to relate three process parameters with maximum temperature at two selected locations and heat-affected zone (HAZ) length, using the data generated by the heat transfer model. Consequently, the spindle speed, feed rate, as well as distance between two welding tools are optimized by minimizing the input energy subject to the constraints in terms of the maximum temperature and HAZ length. Compared to the initial parameters, the optimal case allowed the welding parameters including the HAZ length, input energy, and welding time to reduce by 5.71%, 37.46%, and 20.32%, respectively, thereby having the potential applicability to achieve relatively high welding quality and efficiency.

This paper focuses on identifying the convection heat transfer coefficients of the friction stir welding (FSW) temperature field from the temperature measuring data. Firstly, the numerical model of the FSW temperature field is constructed by utilizing the Lagrangian formulation. Then a novel inversion method combined with the surrogate model is proposed. The surrogate model based on proper orthogonal decomposition and radial basis function (RBF) is applied to approximate the input-output relationship of the numerical model. To improve the calculation efficiency and accuracy of the inversion method, an iterative updating method of the surrogate model combined with parallel computing technology is adopted. Subsequently, the effect of the type of RBF and the measurement position of the thermocouple on the calculation accuracy of the inversion analysis method are analyzed. The conclusions indicate that all the RBFs used in this paper are suitable for constructing the surrogate model of the temperature field. With the thermocouple measuring point close to the weld center, the maximum RMSE calculated by the numerical model decreases from 10.8 °C to 7.0 °C, but the maximum relative error of the peak temperature changes slightly, about 7.5%. The results illustrate that the identified convection heat transfer coefficients allow predictions on the temperature field of FSW which are consistent with the experimental data.

Aiming at the assembly accuracy of a large aircraft transport jig, the effect of component error and the error of work-piece surface on the work-piece position and orientation in the 3-2-1 fixturing scheme is studied with the object pose space description method. The error mapping model between the connecting part of the front frame rack and its support base is modeled using the homogeneous transformation matrix（HTM） method. The probabilistic error is simulated using the Monte Carlo method. The measurement experiment was conducted by the laser tracker to verify the effectiveness of the approach, and the approach has been successfully applied to the production of transport jig.

Purpose The purpose of this paper is to develop a means of the kinematic calibration of a parallel manipulator with full-circle rotation. Design/methodology/approach An error-mapping model based on the space vector chain is formulated and parameter identification is proposed based on double ball-bar (DBB) measurements. The measurement trajectory is determined by the motion characteristics of this mechanism and whether the error sources can be identified. Error compensation is proposed by modifying the inputs, and a two-step kinematic calibration method is implemented. Findings The simulation and experiment results show that this kinematic calibration method is effective. The DBB length errors and the position errors in the end-effector of the parallel manipulator with full-circle rotation are greatly reduced after error compensation. Originality/value By establishing the mapping relationship between measured error data and geometric error sources, the error parameters of this mechanism are identified; thus, the pose errors are unnecessary to be measured directly. The effectiveness of the kinematic calibration method is verified by computer simulation and experiment. This proposed calibration method can help the novel parallel manipulator with full-circle rotation and other similar parallel mechanisms to improve their accuracy.