**2. Materials and Experimental Methods**

The chemical compositions of as-received Al and Ti alloys are shown in Tables 1 and 2. Butt joints Al-Mg-Si/Ti-6Al-4V were processed on 2 mm plates. The laser beam was focused on the Titanium side (Figure 1), respectively, 0.1 mm and 0.3 mm away from the aluminum edge, which is the laser offset. For each fixed laser offset, two different laser beam focus positions were used as in the schemes of Figure 1: in focus beam named T as Top (focus at the top of the thickness joint) and negative laser defocusing ( −2 mm) named B as Bottom (defocused beam at the bottom of the thickness joint), respectively.

**Table 1.** AA6061 alloy chemical composition.


**Table 2.** Ti6Al4V alloy chemical composition.


In Figure 1, the macrostructure of the welds cross section is also shown. So, four samples were processed. The welding speed was 2500 mm/min and the laser power 1500 W. An Ytterbium Fiber Laser System (IPG YLS-4000), with a maximum output power equal to 4 kW was used for the welding. The laser beam was delivered with a diameter equal 200 μm and has a beam parameter product (BPP) equal to 6.3 mm/rad. The laser beam has a wavelength of 1070.6 nm. Argon and helium were employed as shielding gas with volumetric flow rate equal to 10 L/min. Particularly, since helium has a specific weight less than atmospheric air, it was employed for the bottom surface, while the heavier Argon was used on top surface.

**Figure 1.** Cross section of the joints 1T (**a**), 3T (**b**), 1B (**c**) and 3B (**d**) coupled with the schemes of the process.

The welds were cut perpendicular to the welding direction to characterize the transverse section. They were analyzed by optical microscopy (OM; Nikon Epipkot 200) and scanning electron microscopy (SEM; Zeiss EVO 40) equipped with energy dispersive spectrometer (EDS Bruker). The cross sections of the samples were prepared using the standard metallographic grinding and polishing techniques and attached using Keller reagen<sup>t</sup> (95 mL H2O, 2.5 mL HNO3, 1.5 mL HCl, HF 1 mL). The grain structure was revealed by electro-polishing (20% perchloric acid and 80% ethanol at 0 ◦C, electro-polishing parameters: 15 V and 60 s), anodic oxidation (Barker etching, anodizing parameters: 20 V and 80 s) and subsequent investigation under polarized light in OM. The area size of FZ, HAZ and intermetallic layer were evaluated using NIS-Element software for imaging analysis. In addition, the width of FZ at different position along the thickness of the joints were evaluated using NIS-Elements. NIS Element is a NIKON software supplied with Epiphot 200 OM. The software is tailored to facilitate image capture, object measurement and counting. The role of the energy density on hardness evolution was investigated by Vickers indentation in the cross section of the joints along a line perpendicular to joint interface. For Vickers microhardness profiles, a Vickers Affri Wiky 200JS2 microhardness tester was used, with load 0.3 kg and time of indentation 15 s (0.3/15 s). The distance between indentations was equal to 300 μm. For the joints T and B, the hardness measurements were done 200 and 1800 μm from upper surface. All the welded joints were subjected to tensile tests. Rectangular samples (length: 20 mm; width: 5 mm; thickness: 2 mm), with sample length perpendicular to the weld line, were cut for tensile test. Tensile tests were carried out on a servo-hydraulic testing machine having MTS810 having a load capacity of 100 kN. The samples were tested using strain rate equal to 0.2 mm/min. The joints 3T and 3B, characterized by having best mechanical performances, were further treated by stress relief heat treatment (SRHT) at 530 ◦C-2 h to relax welding thermal stress. After that, joints 3B and 3T were characterized in terms of hardness at a half of thickness and tensile tests. On the welded joints 3T and 3B, UPT was applied only on the Aluminum longitudinal surfaces for1min/sidebyanUltrasonicPeeningSintesUP600.Thediameterofthepeeningneedle

was 3 mm and the vibration frequency was 20 kHz. The treated zone (about 6 mm from the Al/Ti interface) included both Al FZ, HAZ and BM.

Electrochemical measurements were performed with an AMEL 5000 potentiostat/galvanostat in an aqueous 3.5% NaCl solution. All experiments were conducted in naturally aerated, near neutral solutions at ambient temperature. A conventional three-electrode cell was employed, with a platinized titanium expanded mesh counter electrode, a Ag/AgCl reference electrode and a specimen of AA6061 aluminum alloy, before and after UTP, as a working electrode. All the potentials were referred to Ag/AgCl. The samples were insulated using teflon to mask their cut edges and back sides, in order to leave only a surface area of 0.5 cm<sup>2</sup> exposed to the electrolyte. The steady state potential was determined after 60 min of immersion in the solution at open circuit potential (OCP). Following the determination of the steady state OCP, potentiodynamic polarisation measurements were performed from −0.15 V vs. OCP to +1.5 V vs. Ag/AgCl at a scan rate of 1 mV s<sup>−</sup>1. Each corrosion test was repeated on two samples.
