Research on the Material Removal Mechanism and Experiments of Electrolytic Plasma Polishing for High-Strength Steel

Technical Summary: Research on the Material Removal Mechanism and Experiments of Electrolytic Plasma Polishing for High-Strength Steel

Problem Statement
The internal rifling of artillery barrels, typically fabricated from high-strength, high-hardness alloy steel, requires exceptional surface quality to ensure projectile accuracy, range, and service life. Current manufacturing relies heavily on mechanical polishing (honing, manual brushing), which suffers from low efficiency, high labor intensity, and poor uniformity, particularly in complex geometries like rifling grooves. Furthermore, mechanical methods generate hazardous metal dust, and chemical polishing alternatives often involve toxic acidic media and struggle to achieve the necessary precision (typically limited to Ra 0.5–10 µm). While Electrolyte Plasma Polishing (EPP) has shown promise in aerospace and medical applications, its application to high-strength steel gun barrels remains underexplored. Specific gaps exist in understanding the material removal mechanisms for this specific material, controlling plasma distribution in deep, high-aspect-ratio holes, and establishing correlations between process parameters and surface quality.

Methodology
This study employs a systematic approach integrating multi-physics field modeling, simulation, and experimental validation to investigate EPP for high-strength steel.

• Theoretical Framework: The research elucidates the EPP process, which involves applying high voltage (250–400 V) in a dilute neutral salt electrolyte to generate a vapor sheath and plasma discharge. The material removal mechanism is analyzed through three distinct phases:

  1. Electrochemical Corrosion: Bubble nucleation and growth leading to a gaseous insulating layer.
  2. Polishing Transition: Bubble collapse and impact, generating shock waves and micro-jets.
  3. Gas Layer Ionization: Formation of discharge channels preferentially at surface peaks, leading to localized melting and material removal.

• Simulation: Finite element analysis (FEA) was conducted using COMSOL Multiphysics to model:

  • Electric and Flow Fields: Solving Laplace equations for potential distribution and Navier-Stokes equations for fluid flow to analyze current density and velocity uniformity.
  • Gas-Liquid Two-Phase Flow: Utilizing an Eulerian-Eulerian framework with a standard k-ε turbulence model to simulate bubble evolution and gas layer formation.
  • "Peak Removal" Process: A moving boundary model was developed to simulate the dynamic evolution of surface asperities, linking current density to material removal rates.

• Experimental Validation: Polishing experiments were conducted on high-strength steel specimens (15mm × 15mm × 5mm) using a custom-built platform with a 4 wt.% ammonium sulfate electrolyte at 85°C and 380 V. Pre-polishing surfaces were ground to Ra ~2.3 µm. Post-polishing characterization included:

  • Macroscopic and microscopic morphology analysis (SEM).
  • Surface roughness (Ra) and 3D topography measurements.
  • Elemental composition analysis via Energy Dispersive X-ray Spectroscopy (EDS).
  • Microhardness testing (Vickers).

Key Contributions and Results

• Mechanism Elucidation: The study confirms that the "peak removal" phenomenon is initiated by electrochemical dissolution, followed by bubble evolution and collapse. The primary material removal mechanism is identified as a synergistic action of fluid erosion, the formation of local discharge channels, and discharge activity concentrated at the tips of surface asperities.
• Simulation Insights:
  • Electric field simulations revealed that while the field is generally uniform, significant concentration occurs at edges and corners (exceeding 140 kV/m), which could lead to non-uniform material removal.
  • Flow field analysis highlighted the formation of vortices in the wake of the workpiece, which may degrade polishing uniformity in those specific regions.
  • Gas layer simulations demonstrated the progressive accumulation of gas volume fraction, stabilizing into a continuous insulating layer over time.
  • "Peak removal" simulations successfully modeled the leveling of surface asperities, showing significant displacement reduction over time.
• Experimental Outcomes:
  • Surface Roughness: The process reduced surface roughness (Ra) from 2.370 µm to 0.879 µm, a 62.9% improvement.
  • Morphology: Macroscopic and microscopic analyses showed the elimination of machining traces, pits, and corrosion. The surface acquired a uniform metallic luster. 3D topography indicated a reduction in maximum peak-to-valley height (Sz) from 57.8 µm to 47.2 µm.
  • Elemental Composition: EDS analysis revealed a significant decrease in surface oxygen content and a slight increase in chromium (Cr) content. The authors attribute the Cr increase to the preferential etching of a chromium-depleted surface layer, exposing the underlying chromium-rich material.
  • Microhardness: A minor reduction in surface microhardness was observed, attributed to the removal of hard surface oxides, defects, and carbon-enriched layers.

Significance
The paper claims that this research deepens the mechanistic understanding of the Electrolyte Plasma Polishing process specifically for high-strength steel. By integrating multi-physics simulation with experimental validation, the study provides a theoretical foundation and experimental data necessary for the optimization and precise control of EPP. The findings support the feasibility of using EPP for finishing deep-hole components with complex cross-sections, such as artillery barrel rifling, offering a pathway to improve manufacturing quality and weapon system reliability through an environmentally friendly and efficient alternative to traditional mechanical and chemical polishing.