Investigation of crystal structure formation by electropulse treatment of amorphous Ti50Ni25Cu25 alloy ribbons

The study of the effect of electropulse treatment with a variable duration on the crystallization processes and the structure of a amorphous TiNiCu alloy with 25 at.% Cu in comparison with isothermal annealing and heating at a constant speed was carried out. The alloy was fabricated by rapid-quenching from the liquid state (melt spinning technique) at the cooling rate of the melt of about 106 °C/s in the form of a ribbon with a thickness of 28 μm with a surface crystal layer with a thickness of about 2-3 μm. To remove the crystal layer, the method of double-sided electrochemical polishing was used. The studies were carried out by methods of differential scanning calorimetry, metallography and scanning electron microscopy. It was established that the formation of the crystalline phase in the electropulse treatment of the amorphous ribbon occurs from the surface to the inner part due to the predominant formation and growth of columnar crystals with subsequent nucleation and growth of crystals in the rest of the ribbon.

A Study on Universal Factor Determining the Evolution of Surface Roughness During Electrochemical Polishing

Electrochemical polishing (ECP) is widely used for scratch- and damage-free finishing of metal components. Though the polishing effect of ECP has been confirmed in many researches, the influence of polishing parameters on evolution of surface roughness is still ambiguous owing to the use of different ECP systems. In this paper, the universal factor determining the evolution of surface roughness during ECP is studied by theoretical analysis as well as experiments. Theoretical analysis based on viscous layer mechanism demonstrates that the material removal thickness is the key parameter governing the roughness evolution of the polished surface regardless of other parameters including the voltage, current and electrolyte concentration and so forth. A series of experiments were designed and carried out to verify the proposed hypothesis. Both the experimental results and already published researches proved the validity and universality of the newly developed hypothesis on surface roughness evolution. This work is of great significance for further understanding the finishing mechanism of ECP and process control for its practical applications.

Electrochemical Polishing of 7075 Al Alloy in Phosphoric Acid

7075 aluminum (Al) alloy has been widely used in aircraft structures and other high-end electronic products owing to its excellent mechanical and chemical properties, while its damage-free and highly efficient surface finishing remains a challenge. Herein, we demonstrate a systematic study of the anodic behaviors of 7075 Al alloy during the electrochemical polishing (ECP) process in phosphoric acid under different applied potentials, and the changes of surface morphology, roughness, electric current and resistance are studied intensively. According to the surface morphology and current density, ECP of 7075 Al can be divided into 4 stages including the negative leveling stage, leveling and corrosion stage, levelling and brightening stage, and pitting and corrosion stage. Different factors influencing each stage and the effects of impurity phases in the ECP process are experimentally validated. Under optimized conditions, a mirror surface with a roughness (Ra) of 46.7 nm (decreased from an initial value of 153.2 nm) can be obtained by ECP for 10 min. The presented findings are of great value for the further development of ECP process of multiphase alloys.

Study of Cu Electrochemical Polishing Mechanism With Observation of Water Acceptor Diffusion

The salt-film and water acceptor mechanisms were generally accepted mechanisms for Cu electrochemical polishing (ECP) theory. These mechanisms of Cu ECP are still controversial for a long time. Conventional and new electrochemical analysis methods were used to investigate the mechanisms and behaviors of Cu electrochemical polishing. Two cases of Cu dissolution, with and without polishing, were classified by results of linear scan voltammetry (LSV) and scanning electron microscopy (SEM). The electrochemical impedance spectroscopy (EIS) results showed the main difference in these two cases was in the low-frequency region. However, it was hard to distinguish between the salt-film and water acceptor mechanisms by conventional electrochemical analysis. A scanning electrochemical microscopy (SECM) system, a new electrochemical analysis method that measures the electrolysis currents of the water acceptors along with a set distance from the substrate, was used to investigate the Cu ECP mechanism. Accordingly, the diffusion of the water acceptors was successfully confirmed for the first time. Finally, the mechanisms of the Cu ECP are definitively described by using all analysis results.

Electrochemical Polishing of Extruded and Laser Powder-Bed-Fused Inconel 718

ABSTRACT Electro-chemical polishing (ECP) was utilized to produce sub-micron surface finish on Inconel 718 parts manufactured by Laser Powder-Bed-Fusion (L-PBF) and extrusion methods. The L-PBF parts had very rough surfaces due to semi-welded powder particles, surface defects, and difference layer steps that were generally not found on surfaces of extruded and machined components. This study compared the results of electro-polishing of these differently manufactured parts under the same conditions. Titanium electrode was used with an acid-based electrolyte to polish both the specimens at different combinations of pulsed current density, duty cycle, and polishing time. Digital 3D optical profiler was used to assess the surface finish, while optical and scanning electron microscopy was utilized to observe the microstructure of polished specimens. At optimal condition, the ECP successfully reduced the surface of L-PBF part from 17 µm to 0.25 µm; further polishing did not improve the surface finish due to different removal rates of micro-leveled pores, cracks, nonconductive phases, and carbide particles in 3D-printed Inconel 718. The microstructure of extruded materials was uniform and free of processing defects, therefore can be polished consistently to 0.20 µm. Over-polishing of extruded material could improve its surface finish, but not for the L-PBF material due to defects and the surrounding micro-strain.