2.3 Equipment for microstructural and morphological characterization

Various microstructural characterization techniques were employed to gain a better understanding of microstructural effects of Al-2.0 wt.% Fe alloy LSR-treated under study. These techniques applied were optical microscopy (OM), fieldemission scanning electron microscopy (FESEM) coupled to energy dispersive spectroscopy (EDS) and Vickers microhardness testing, which are described in detail below.

LSR treated samples were analyzed by OM (Olympus BX51) couple to a Q-Color 3 digital camera to capture images. Prior to studying the LSR treated layer, the cross-sections were cut of the samples using a diamond blade and they were sanded and polished. Samples were chemically etched with hydrofluoric acid 0.5% (v/v) at intervals 30 to 45 seconds, after they were polished with metallographic polishing pads, using only water, to ensure LSR treatment would not be impaired.

Laser-treated material and substrate were analyzed by FESEM (MIRA 3 LM) coupled to EDS to examine the microstructural changes caused by laser treatment.

#### 2.4 Vickers microhardness testing

Vickers hardness (HV) tests were performed using a Leica VMHT MOT microhardness tester operating with a load of 0.1 kg at 15 seconds (HV 0.1 15 s). The tester was applied in the cross-sectional area of treated specimen, to different penetration depths until it reached the base material. Penetration depths of the tester from the surface in the treated material region were approximately 50, 100 and 200 μm, however, 300, 500 and 700 μm were in the base material region, as shown schematic in Figure 1. At each of these depths, 15 micro-indentations were made in lines parallel to surface. Average hardness and standard deviation at each of selected depths were calculated based on data obtained.

For preparation of HV tests, a cross-sectional sample was sanded with 600 and 1200 grit sandpaper and polished with colloidal silica to reduce its roughness, thereby preventing roughness that could interfering in results of HV measurements. Besides, microhardness was measured on the laser-treated sample surface, which was cleaned only with water to prevent that it could be modified. Furthermore, the material's hardness was tested on the weld fillets region and between them.

#### 2.5 Electrochemical impedance spectroscopy (EIS) test

The electrochemical impedance spectroscopy (EIS) test was performed in aerated solution of 0.1 M H2SO4 at a temperature of 25 0.5°C, using Autolab PGSTAT 30 potentiostat system connected to a microcomputer. Working electrodes of

#### Figure 1.

Schematic diagram of weld fillets on the sample surface and in the cross-sectional area showing the penetration depth of Vickers indenter in LSR-treated sample.

surface-treated and untreated samples were prepared with epoxy resin to expose a top surface.
