Silicon carbide fiber reinforced silicon carbide ceramic matrix composite (SiC_f/SiC composite) is characterized by a high strength-to-density ratio, high hardness, and high temperature resistance. However, due to the brittleness of the matrix material and the anisotropy of the reinforcing phase, it is a huge challenge for machining of the material. The milling method has advantages of a high material removal rate and applicability to complex surface geometry. However, no published literature on milling of SiC_f/SiC composite has been found up to now. In this paper, high-speed milling of SiC_f/SiC composites was carried out under dry conditions and cryogenic cooling using liquid nitrogen, respectively. Polycrystalline diamond (PCD) and chemical vapor deposition (CVD) diamond cutting tools were used for the milling work. The cutting performance of the two kinds of tools in high-speed milling of SiC_f/SiC composites was studied. Tool failure modes and mechanisms were analyzed. The effects of the cooling approach on tool wear and machined surface quality were also investigated. The experimental results showed that under identical cutting parameters and cooling approaches, the PCD tool yielded better cutting performance in terms of a longer tool life and better surface quality than that of the CVD diamond tool. In dry machining, the failure modes of the CVD diamond tool were a large area of spalling on the rake face, edge chipping and severe tool nose fracture, whereas for the PCD tool, only a small area of spalling around the tool nose took place. Compared to the dry machining, the wear magnitudes of both PCD and CVD diamond tools were decreased in cryogenic machining. Additionally, the surface quality also showed significant improvements. This study indicates that the PCD tool is highly suitable for machining of SiC_f/SiC composite, and that the cryogenic method can improve machining efficiency and surface quality.

ZrO₂ ceramics are currently used in a broad range of industrial applications. However, the machining of post-sintered ZrO₂ ceramic is a difficult task, due to its high hardness and brittleness. In this study, micro-milling of ZrO₂ with two kinds of diamond-coated end mills has been conducted on a Kern MMP 2522 micro-milling center (Kern Microtechnik GmbH, Eschenlohe, Germany). To achieve a ductile mode machining of ZrO₂, the feed per tooth and depth of cut was set in the range of a few micrometers. Cutting force and machined surface roughness have been measured by a Kistler MiniDynamometer (Kistler Group, Winterthur, Switzerland) and a Talysurf 120 L profilometer (Taylor Hobson Ltd., Leicester, UK), respectively. Machined surface topography and tool wear have been examined under SEM. Experiment results show that the material can be removed in ductile mode, and mirror quality surface with Ra low as 0.02 μm can be achieved. Curled and smooth chips have been collected and observed. The axial cutting force F_z is always bigger than F_x and F_y, and presents a rising trend with increasing of milling length. Tool wear includes delamination of diamond coating and wear of tungsten carbide substrate. Without the protection of diamond coating, the tungsten carbide substrate was worn out quickly, resulting a change of tool tip geometry.

Machining distortion is a recurring problem in the machining of monolithic aircraft parts. This paper aims to study the machining distortion minimization of monolithic aircraft parts. Firstly, the energy principle of machining distortion was analyzed. Then, a rapid prediction model of the final part distortion for beam parts was proposed based on the equivalent stress, and the initial bending strain energy contained in the final part was used to characterize the bending distortion risk of the final part. Numerical simulation and milling experiments verified the effectiveness of the proposed prediction model. The relative error between the experimental and calculated results does not exceed 26.5%. Finally, the influence of initial residual stress fluctuation, part geometry and the part location on part distortion was analyzed from the energy point of view. The obtained results indicated that the expected final part distortion can be minimized by adjusting these three factors.

Optimal fixture configuration is critical to minimise the machining distortion of the workpiece. This study presented an approach for optimisation of fixturing configuration in flexible machining fixtures. Herein, a flexible fixture system composed of adaptive clamping unit, locating unit, and auxiliary locating unit was investigated. Response surface methodology (RSM) was employed to describe the relationship between fixturing parameters (distance between two zero-point fixtures m ₁, the clamping force of float fixture m ₂, and quantity of float fixture m ₃) and machining distortion. Furthermore, sensitivity analysis was implemented to evaluate the effects of fixturing parameters on machining distortion. Adaptive penalty function (APF) was constructed to search optimal settings of fixturing parameters. Comparative analysis of different optimal fixturing parameters obtained by RSM and APF was conducted. Single-sided component was adopted to validate the effectiveness of the proposed approach. Machining distortion was found to first decrease and then increase with the augment of m ₁ and it reduced with the increment of m ₂ and m ₃. Besides, m ₃ exhibited the greatest influence on machining distortion, 61.78%. The interaction between m ₂ and m ₃ resulted in the lowest influence, 0.019003%. Furthermore, the maximum machining distortion through optimised flexible clamping was only 0.312 mm, which reduced by 57.6% compared to traditional clamping.

Thin-walled long stringer made of aluminum alloy 7050-T7451 is prone to deformation during transportation, so a research of residual stress relaxation was launched in this paper. The transport resonance stress of long stringer was analyzed based on the power spectral density of road transport acceleration. The residual stress relaxation experiment of aluminum alloy 7050-T7451 under different equivalent stress levels was designed and carried out. According to the amount of residual stress relaxation in the experiment, an analytical model was established with the equivalent stress level coefficient. The deflection range of long stringer was evaluated under different damping ratios. The results show that when the equivalent stress exceeds 0.8 σ _0.2 , the residual stress relaxation of the thin-walled samples occurs. The residual stress relaxation increases linearly with the equivalent stress, which is logarithmically related to the loading cycle. The deformation caused by residual stress relaxation of the long stringer is proportional to the square of the length and the bending moment caused by stress rebalance, and inversely proportional to the moment of inertia of the structure. As the damping ratio decreases from 0.03 to 0.01, the total deflection of the long stringer increases from 0 to above 1.55 mm.