Lectropol 5

Firstly, an aluminum (AA 1050) sample was electropolished using a Struers LectroPol-5 (Copenhagen, Denmark). Then, the first step of the anodization process was performed using 0.3 M H₂C₂O₄ at 2 °C and 45 V for 60 min. This step is called long-period anodization. The obtained oxide layer was removed by immersion into a mixed solution of 1.8 wt.% chromic and 6 wt.% phosphoric acid. The second step of anodization consisted of four short anodization cycles in 0.3 M H₂C₂O₄ at 9 °C and at 45 V (for 25 s in the first cycle and 20 s for the next cycles) and a pore widening process in 5 wt.% H₃PO₄ at 30 °C for 12 min. The terms long-period and short-period anodization are used to highlight the difference in duration of these processes. The second step of anodization was also performed at the higher temperature. All reagents were characterized by analytical purity (POCH S.A., Gliwice, Poland). Deionized water was used to prepare the solutions.

The constituent phases and mechanical twins of both the annealed and tensile-deformed specimens were observed using a field-emission scanning electron microscope (FE-SEM; JEOL, JSM-7001F), operated at 20 kV and equipped with an electron backscatter diffractometer (EBSD; Hikari, EDAX-TSL). The step size and the working distance for the EBSD analysis were 0.09 μm and 14 mm, respectively. The surfaces of the EBSD specimens were polished using a suspension including 0.04-μm colloidal silica particles; using an electro-polisher (Struers, LectroPol-5), surfaces were then electrochemically polished at 15 °C for 30 s in a mixed solution of 90% glacial acetic acid (CH₃COOH) and 10% perchloric acid (HClO₄) to remove the mechanically damaged layer. Using point counting analysis of EBSD image quality maps^{17 (link), 43 (link)}, the fraction of mechanical twins was measured as a function of the tensile strain.

In order to be able to correlate the microstructure with the mechanical properties, a more precise knowledge of the chip orientation and the grain structure, which directly affects the strength according to the Hall-Petch relationship [26 (link)], is of importance. Therefore, cast-based and chip-based profiles were cut and cold-embedded perpendicular to the extrusion direction. The profiles were then ground and polished up to a grit size of 0.1 µm using SiO₂ polishing suspension. The microstructure was characterized on cross-sections by means of an electrolytic etching according to Barker [27 (link)]. Fluorophosphoric acid (35%) was used as an electrolyte at a flow rate of 12 L/min. On the profile, poled as the anode, a layer applied by the etching process which enables the detection of the grain orientation. The etching was carried out for 90 s at a DC voltage of 20 V using an electrolytic etching device (Struers LectroPol-5, Willich, Germany). The subsequent microstructural characterization under polarized light was carried out on a light microscope (Zeiss Axio Imager M1m, Jena, Germany). Subsequently, the grain size was determined by the linear intercept method.

A Ø1.2 Inconel 625 metal wire (Kiswel, KW‐M625, Republic of Korea) and 99.95% argon shielding gas were used. The hot mounting resin (Polyfast, Struers, Denmark) was used for cross‐sectional observation of the specimen. The specimen was etched with methyl alcohol: nitric acid with a ratio of 17:3 (CH3OH:HNO3), and electrolytic etching (LectroPol‐5, Struers, Denmark) was performed for 75 s at 6 V.

A mono-crystalline Nickel (Ni) sample oriented in (100) direction was used during this study. Single crystals are preferred to insure the uniformity of the structuring process. The Ni (100) bar was grown by directional solidification and cut into 10 × 10 × 10 mm³ cubes by using a wire saw. Two types of polishing were performed during experiments, the mechanical and electrochemical polishing. The automatic polishing was performed on “Buehler Automet 250” by using a coarse paper of P180, moving successively to P320, P600, P1200 and P2400 followed by a diamond 3 μm and 1 μm and vibratory polishing on “Buehler Vibromet 2” with the colloidal silica 0.05 μm for 17 h, prior to laser irradiations. Electropolishing was performed after automatic polishing on “Struers LectroPol-5” by using stainless steel electrolyte at 25 Volts for 60 s. Both polishing procedures assure mirror-zero scrach samples with an initial arithmetical mean surface roughness (Ra) below 5 nm. The Ra threshold Ra_th for nanostructures formation was found to be 5 nm on the Atomic Force Microscopy (AFM), on a scan of 5 × 5 μm. Crystal orientations were checked by X-ray diffraction prior to laser irradiation to ensure the uniform cutting direction.

The specimen was cut to 15 × 15 × 10 mm, ground with #2000 SiC paper, and polished with a 3 µm diamond paste. Etching was performed using an electrolytic etcher (Lectropol-5, Struers, Denmark). As the etching solution, 10% oxalic acid solution was used. Finally, after washing the specimen with ethyl alcohol using an ultrasonic cleaner, the changes in the microstructure were observed with an optical microscope (AXIOTECH 100 HD, ZEISS, Oberkochen, Germany).