The work aims to develop a novel chemical magnetorheological polishing (CMP) technique for high-performance finishing of Ti-6Al-4V artificial joints. The principal innovation is the synergistic coupling of a custom-designed annular Halbach array with an in-situ Fenton-like reaction system, a chemo-mechanical process that overcomes the intrinsic limitation of low material removal rates in conventional MR polishing.
The Halbach array generates a magnetic field with high intensity, high gradient, and exceptional uniformity, which induces the formation of a robust, stable chain-like "magnetic brush" in the MR fluid. This structure enhances abrasive particle control and mechanical polishing stability, while maintaining a constant shear force of approximately 0.8 MPa on the workpiece surface. Concurrently, a Fenton-like reaction is activated in the polishing zone: carbonyl iron powder (CIP) and deliberately added Fe3O4 in the MR fluid act as dual catalysts to decompose hydrogen peroxide (H2O2), generating highly reactive hydroxyl radicals (·OH). These OH radicals preferentially react with Al and V elements on the TC4 surface, reducing metallic bond energy and facilitating controlled etching to form a soft, non-metallic TiO2 layer which is then efficiently stripped by the mechanical shearing of abrasives. This real-time synergy between chemical softening and mechanical removal is the core mechanism enabling high-efficiency material removal.
A comprehensive experimental methodology based on the Response Surface Methodology (RSM) with a Box-Behnken Design (BBD) was employed to model and optimize the process. To eliminate interference from fluid composition, the MR fluid was fixed as 61.76wt.% CIP, 10.20wt.% Al2O3 abrasives, 0.81wt.% glycerol, 0.27wt.% cellulose, 0.11wt.% Fe3O4, and 26.85wt.% deionized water, with continuous supply of 0.4wt.% H2O2. Three critical process parameters (selected based on preliminary experiments to cover practical polishing ranges) were investigated, including working gap (0.6, 0.8, 1.0 mm), spindle speed (300, 400, 500 rad/min), and abrasive concentration (7wt.%, 10wt.%, 13wt.%). Surface roughness (Ra) and material removal depth (DMR) were set as response metrics.
RSM analysis produced highly significant regression models for both Ra (P=0.003 3, R2=92.65%) and DMR (P=0.027 3). The working gap was identified as the most dominant factor: a narrower gap (0.6 mm) amplified the Halbach array-induced magnetic field, directly reducing Ra and increasing DMR. Spindle speed was the second most significant factor, but a key finding was its non-monotonic effect, namely that the speed exceeding 400 rad/min induced centrifugal instability, degrading surface quality. A statistically significant interaction between spindle speed and abrasive concentration was also uncovered (P=0.028 4 for Ra). Additionally, abrasive concentration exhibited a distinct non-linear effect, with an optimal range around 10wt.% and concentrations above this led to excess free abrasives that disrupted polishing stability.
Multi-objective optimization determined the optimal parameter set: working gap=0.6 mm, spindle speed=436 rad/min, and abrasive concentration=9.6wt.%. Validation experiments under these conditions achieved Ra=17 nm (meeting the clinical standard of ≤20 nm for TC4 artificial joints) and DMR=5.091 μm. These values were in excellent agreement with model predictions (Rapredicted =15.5 nm, DMR,predicted= 4.594 μm), with relative errors of 9.7% (Ra) and 10.8% (DMR), both below 11%.
Final validation on a real TC4 prosthetic femoral head demonstrated practical efficacy. After a 1.5-hour polishing cycle, Ra was reduced from an initial 438 nm to 18 nm (over 95% improvement). Profilometric analysis confirmed complete elimination of original milling marks and formation of a uniform, mirror-like finish. This work conclusively demonstrates that the proposed CMP process of leveraging the synergy between Halbach array-enhanced magnetic fields and Fenton-like reactions is a highly effective, scalable solution for achieving superior surface integrity on complex-shaped titanium alloy biomedical implants.
Key words
magnetorheological polishing /
response surface methodology /
titanium alloy /
artificial joint ball head /
Fenton-like Reaction /
Halbach Array
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Funding
National Natural Science Foundation of China (General Program 52475446, Overseas Program 2023); Anhui Provincial Natural Science Foundation Project (General Program 2308085ME170); Anhui Provincial Key Science and Technology Innovation Project (202423i08050035); Wenzhou Major Science and Technology Innovation Project (ZG2022029)
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