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This article investigated the microstructure of Ti6Al4V that was fabricated via selective laser melting; specifically, the mechanism of martensitic transformation and relationship among parent β phase, martensite (α’) and newly generated β phase that formed in the present experiments were elucidated. The primary X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile test were combined to discuss the relationship between α’, β phase and mechanical properties. The average width of each coarse β columnar grain is 80–160 μm, which is in agreement with the width of a laser scanning track. The result revealed a further relationship between β columnar grain and laser scanning track. Additionally, the high dislocation density, stacking faults and the typical (10true1¯1) twinning were identified in the as-built sample. The twinning was filled with many dislocation lines that exhibited apparent slip systems of climbing and cross-slip. Moreover, the α + β phase with fine dislocation lines and residual twinning were observed in the stress relieving sample. Furthermore, both as-built and stress-relieved samples had a better homogeneous density and finer grains in the center area than in the edge area, displaying good mechanical properties by Feature-Scan. The α’ phase resulted in the improvement of tensile strength and hardness and decrease of plasticity, while the newly generated β phase resulted in a decrease of strength and enhancement of plasticity. The poor plasticity was ascribed to the different print mode, remained support structures and large thermal stresses.

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Control of workpiece machining error (WME) is a key concern in the design of a fixture system. In this paper, source errors, which are categorized into workpiece-fixture geometric default and workpiece-fixture compliance, are systematically investigated to reveal their effects upon the WME. The underlying mechanism is that source errors lead to the workpiece position error (WPE), the workpiece elastic deformations (WED), and the inconsistent datum error (IDE), and all of them will contribute together to the WME. Here, the IDE refers to the dimension deviation of the processing datum from the locating datum once two references do not coincide. An overall quantitative formulation is proposed for the computing of WME in terms of WPE, WED, and IDE for the first time. In detail, the WPE raised in the workpiece-locating and clamping process is evaluated based on the geometric defaults and local deformations of workpiece-fixture in the contact region. The WED relative to the workpiece-clamping process is determined by solving a nonlinear mathematical programming problem of minimizing the total complementary energy of the frictional workpiece-fixture system. Some numerical tests are finally demonstrated to validate the proposed approach on the basis of both theoretical and experimental data given in the references.

It is well recognized that the cutter run-out appearing in the milling process will cause an uneven redistribution of the instantaneous uncut chip thickness through the cutter flutes and thereby will generate an irregular distribution of the cutting forces in different tooth periods. This work aims to develop a new approach able to identify the cutter radial run-out and cutting-force coefficients in the flat end milling. It is shown that the total cutting forces can be considered as the sum of a nominal component that is independent of the run-out plus a perturbation component induced by the run-out. Mathematical formulations of both components are developed, accounting for the cutting geometry and the radial run-out parameters. Firstly, to calibrate the cutting-force coefficients, a generic procedure is proposed using the instantaneous value of the nominal component instead of the average value. Secondly, considering the fact that the perturbation component of the cutting force depends non-linearly upon the run-out parameters, the identification of run-out parameters is carried out by solving the linearized equation. In the identification procedure, some key techniques such as the calculation of the immersion boundary at any cutting instant and the reasonable selection of the depth of cut are discussed in detail. Finally, based on simulation and experimental results, the validity of the identification approach is demonstrated.

Considering the great impacts of the application sequence of multiclamps on the workpiece machining accuracy, this paper analyzes and optimizes clamping sequence. A new methodology that takes into account the varying contact forces and friction force during clamping is presented for the first time. A new analysis model is established to capture the effect of clamping sequence on contact force distributions and workpiece machining accuracy. It reveals that the historical accumulation of clamping steps influences heavily the final distribution of contact forces in the workpiece-fixture system. Therefore, the present contact forces in each clamping step are solved incrementally in terms of contact forces of the precedent step by means of the principle of the total complementary energy. Furthermore, based on the fact that the variation of contact forces from one step to another results in different workpiece deformations and position, a novel design model is formulated to select optimally the clamping sequence in order to minimize the workpiece deformation and position errors. Workpieces of low stiffness and high stiffness are investigated separately in order to simplify the modeling of clamping sequence optimization. Some numerical tests are finally demonstrated to validate the proposed model and method. Computational results are discussed and compared with experimental results available in the reference.

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