Xuelin Lei

As a critical sealing component for rotating machinery, mechanical seals are highly susceptible to friction-induced wear and adhesive failure under the harsh operating conditions encountered in sectors such as petrochemical, aerospace, and marine engineering. In this study, diamond films were deposited on SiC seal rings using HFCVD technique. By systematically manipulating the gas pressure parameters during both nucleation and growth phases, the underlying mechanisms governing the effects of various pressure combinations on the micro-morphology, interfacial adhesion, hardness, and erosion resistance of the diamond films are systematically investigated. The results demonstrate that the deposition pressure exerts a decisive influence on the film’s growth rate and crystalline structure dimensions. Under conditions where the nucleation pressure is equivalent to the growth pressure, increasing the pressure from 600 Pa to 3600 Pa results in a significant reduction in the growth rate and a structural transition from nanocrystalline diamond (NCD) to microcrystalline diamond (MCD) with a (100) preferred orientation. Mechanical characterizations reveal that optimal comprehensive performance (characterized by a peak hardness of 29.636 GPa, a favorable plastic index (H/E = 0.0704), and a high adhesion strength of 36.72 N) is achieved when both the nucleation and growth pressures are maintained at 1600 Pa. Furthermore, erosion tests indicate that erosion resistance and hardness of diamond films are not a simple linear relationship; rather, films with moderate nucleation density and dense grain packing exhibit superior erosion resistance.

The screw rotor constitutes the primary component of a screw compressor, and its rough machining typically employs disk milling cutters. However, due to the intermittent heavy-load cutting nature of the milling process, issues such as large fluctuations in cutting forces, uneven tool wear, and poor machining stability arise, leading to low processing efficiency, short tool life, and high production costs. To address these issues, the objective of this study is to optimize the efficiency and stability of screw rotor rough machining, extend tool life, and reduce processing costs. A novel methodology for the design of disk milling cutters is hereby proposed. This methodology is predicated on the spatial arrangement and coordinated optimization of indexable inserts. First, an approach is put forward that combines two-dimensional overlap rate gradient optimization with three-dimensional spatial arrangement optimization. This approach is intended to address the issue of uneven tool life distribution, the overlap rate is increased at the top of the disc milling cutter and arranged in four densely spaced helical lines, while the overlap rate is reduced at the bottom and arranged in two helical lines, achieving uniform distribution of cutting loads. Second, finite element simulation analysis was used to determine the influence of rake angle and edge inclination angle on cutting force. This analysis was then combined with genetic algorithm optimization to obtain the optimal positional parameters of the insert. Experimental verification has demonstrated that, in comparison with conventional tools, the newly developed disc milling cutter reduces cutting force by 4%, decreases power fluctuation range by 11%, reduces top insert wear by 25%, decreases tool change frequency by 37.5%, and reduces average power per feed by 21%. These findings indicate a substantial enhancement in processing stability and efficiency. The results of this study can be applied to the rough machining production of screw rotors in the mechanical and petrochemical industries, providing theoretical support and engineering references for the design optimization of high-load intermittent cutting tools. This contributes to reducing the manufacturing costs of core components in screw compressors and enhancing their industrial production efficiency.

Cold hole expansion is a crucial technology for enhancing the fatigue life of hole structures. This paper presents the design and optimization of a novel multi-convex hulls expansion tool for deep small holes in Inconel 718 superalloy using the finite element method (FEM) and experimental techniques. The results from the FEM model, which was used to investigate the compressive residual stress (CRS) distribution and contact force during the expansion process, indicate that the curved single convex hull(CSCH) structure is more suitable for deep small hole expansion (DSHE). However, accounting for the strength limitations of the actual expansion tool, a multi-convex hulls expansion tool structure was proposed. The FEM model was employed to compare the effects of various tool structures on the hole wall CRS distribution and the contact force during the expansion process. The obtained optimal parameters for enhancing deep small holes in Inconel 718 superalloy under high temperature and high load conditions are as follows: expansion section hull spacing l₁ = 0.4 mm, taper ratio C = 0.008, number of convex hulls in the sizing section n = 10, and sizing section hull spacing l₂ = 1 mm. Moreover, an optimized convex hull expansion amount allocation scheme was obtained through an experimental study. The results of the FEM simulations and experimental study demonstrate that the optimized multi-convex hulls expansion tool (with an expansion rate of 1.92%) can induce the formation of a high CRS layer and a refined surface without causing significant microcracks in the hole walls.

Inconel 718 nickel-based superalloy has superior thermal and mechanical properties. Nevertheless, it is a hard-to-machine material, and the heat generated by processing is excessive. Reasonable cooling and lubrication (CL) methods have essential effects on its cutting performance and surface integrity. In this study, Dry cutting (DC), flood cutting (FC), cryogenic cutting (CC), and cryogenic minimum quantity lubrication cutting (CMQLC), are used to cutting Inconel 718 under the same parameters. Cutting forces, temperature, tool wear, chip, Ra, microstructure, and residual stress (RS) are measured. It is concluded that the combination of CL is beneficial for processing. The average temperature obtained by CMQLC is 55.47% lower than that of DC. And the workpiece obtained by CMQLC is measured compared with FC, the area of tool chipping is reduced by 25%, and the surface Ra is decreased by 32.05%, the lowest among the four methods. As for RS, although TRS increased by 7.9%, the MCRS and influence depth of the subsurface increased by 3.9% and 10.2% respectively. Furthermore, the machining condition with a high mechanical thermal ratio can obtain a more favorable RS state. Generally speaking, the CMQLC shows the advantages of mild environmental pollution, preferred cutting performance, and superior workpiece surface integrity.

The effect of nanoscale surface texture on the frictional and wear performances of nanocrystalline diamond films under water-lubricating conditions were comparatively investigated using a reciprocating ball-on-flat tribometer. Although the untreated nanocrystalline diamond film shows a stable frictional state with an average friction coefficient of 0.26, the subsequent textured films show a beneficial effect on rapidly reducing the friction coefficient, which decreased to a stable value of 0.1. Furthermore, compared with the nanocrystalline diamond coating, the textured films showed a large decreasing rate of the corresponding ball wear rate from 4.16 × 10⁻³ to 1.15 × 10⁻³ mm³/N/m. This is due to the fact that the hydrodynamic fluid film composed of water and debris can provide a good lubrication environment, so the entire friction process has reached the state of fluid lubrication. Meanwhile, the surface texture can greatly improve the hydrophilicity of the diamond films, and as the texture density increases, the water contact angle decreases from 94.75° of the nanocrystalline diamond film to 78.5° of the textured films. The proper textured diamond film (NCD₉₀) exhibits superior tribological properties among all tested diamond films, such as short run-in period, low coefficient of friction, and wear rate.