**1. Introduction**

Titanium (Ti) and aluminum (Al) alloys are characterized by low density (equal to 4.43 g/cm<sup>3</sup> for Ti and 2.80 g/cm<sup>3</sup> for Al) and good mechanical properties (the average ultimate tensile strength is equal to 300 MPa for Al Alloys and 1205 MPa for Ti alloys) [1]. Al– Ti dissimilar welds are of significant interest in aeronautical and automotive applications, where weight reduction, combined with high mechanical strength and corrosion resistance, is required. Moreover, cost reduction can also be obtained by using Aluminum alloy coupled with Titanium [2]. For example, the passenger seat track after AIRBUS conceptual design was manufactured with Ti6Al4V crown and aluminum alloy web by laser beam welding [3]. The benefits and the recent trend of laser welding of Ti/Al alloys in the academic sector and industry were explored by Quazy et al. [4]. Recent trends also enclose dissimilar Al–Ti laser weldings based on high-strength aluminum–lithium alloys for aviation applications [5,6].

However, joining of Al/Ti alloys remains a difficult technological challenge because of the different physical properties and the formation of intermetallic phases (IMC) that lead to crack formation during cooling and solidification [4,7,8]. The successful joining of aluminum to titanium requires the reduction of the thickness of IMC layer at the interface as well as of the number of IMC particles dispersed on the aluminum fused zone (FZ).

**Citation:** Leo, P.; D'Ostuni, S.; Nobile, R.; Mele, C.; Tarantino, A.; Casalino, G. Analysis of the Process Parameters, Post-Weld Heat Treatment and Peening Effects on Microstructure and Mechanical Performance of Ti–Al Dissimilar Laser Weldings. *Metals* **2021**, *11*, 1257. https://doi.org/10.3390/ met11081257

Academic Editor: Sergey N. Grigoriev

Received: 7 July 2021 Accepted: 2 August 2021 Published: 9 August 2021

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In fact, although intermetallic compounds based on Ti–Al system are attractive, being characterized by good strength at high temperature, the development of IMC leads to restrictions on the available deformation modes. So, the occurrence of those particles is associated with increased strength but reduced ductility and fracture toughness [5].

Both the convective mixing and diffusion phenomena that increases the occurrence of IMC can be limited by diffusion bonding [6] friction welding [7], laser welding [4] of these alloys. In laser welding, the mismatch in thermophysical properties of chosen dissimilar alloy can be adjusted by shifting the laser beam to one of the substrates. For example, according to [4], a shift of the laser beam to the aluminum side promoted the formation of a thin IMC layer and yielded a high tensile strength, whereas a shift to the titanium side promoted the formation of a thick TiAl3 phase that contained cracks and voids. On the contrary, according to [8], the thickness of IMC can be significantly reduced by shift the laser beam on Ti side.

In addition, defocusing the laser beam can modify the IMC thickness of the layer and IMC particles amount due to the change in the beam energy density and temperatures values [9]. The defocus distance is the position of the focal plane relative to the in focus beam plane. A negative defocusing distance occurs when the focal plane is placed under the "in focus" plane, whereas in positive defocusing, the focal plane is above it. The negative defocusing leads to a deeper melt pool due to its convergen<sup>t</sup> nature, by comparing with in focus and positive defocusing. Particularly, when keeping constant laser power and scanning speed, the defocusing distance (positive and negative sense) can affect the microstructure and defects presence [9–11].

Post welding heat treatments (PWHT) are usually applied to the welds. The welds residual stresses may lead to brittle fracture and reduced fatigue life [9,10]. In addition, the increased dislocation density in fusion welds can reduce ductility of the joints [12]. As the PWHT temperature increases, residual stresses and dislocation density are reduced faster. In the case of dissimilar Al–Ti welds the different properties of the alloys, limit the feasibility of an effective stress relief heat treatment. For example, a significative control of the Ti-6Al-4V welds could require heat treatments at temperature higher than the liquidus temperature of Al Alloys [12]. Moreover, the highest temperature suitable must also avoid grain size and IMC coarsening resulting in decreasing of strength and ductility [13,14]. Many studies were devoted to defining the role of PWHT on microstructure and IMC growth for dissimilar welds. For example, for a laser welded AA5754/Ti-6Al-4 V joint, the microstructure of the IMC did not change after heat treatment at 350 ◦C for 336 h [13]. At 450 ◦C for 168 h, nucleation and growth of TiAl3 were observed with consequent decreasing of joint tensile strength [13]. In another study regarding forty-layered Ti–Al composites fabricated in a single-shot explosive welding process, the thickness of the TiAl3 layer was only 0.7 ± 0.1 μm after heat treatment at 500 ◦C for 100 h [15]. Solid-state diffusion between Ti and Al was investigated between 520–650 ◦C in multi-laminated Ti/Al [16]. The authors found that the thickness of the TiAl3 layer reached 10.50 ± 0.78 μm and 24.11 ± 5.85 μm after annealing at 520 ◦C-4 h and 650 ◦C-64 h [16].

As an alternative to the PWHT, mechanical treatments can be applied to reduce residual stress of welds. The ultrasonic peening treatment (UPT) aims to redistribute residual stresses by eliminating tensile stresses and introducing compressive stresses [17,18]. The principle is the same as hammer peening [19,20] and consists of subjecting the material surface to a plastic compression deformation through a hammer gun that vibrates. In the case of UPT, the hammer gun vibrates at high frequency due to a piezoelectric actuator [17,18]. It has been shown [21] that UPT induces a nanocrystalline surface layers with increased microhardness. The nanocrystalline surface layer also enhanced the corrosion resistance of the Zr-2.5%Nb alloy in saline solution [21]. Moreover, this technique allows micro-cracks and pores closure [18] that are well-known to improve the fatigue strength. The efficiency of the UPT treatment obviously depend on the geometry of the impactor, on the percussion intensity and on the duration of the treatment. These parameters need to be accurately regulated to avoid that the treatment is too mild and thus ineffective, but at the same time

not too much severe, to not irremediably damage the material [22]. Most papers study the effects of UPT on fatigue behaviors showing that under correct conditions UPT produces high improvement in fatigue life [23–25] or wear and corrosion resistance [21,22]. However, few investigations aimed to evaluate the effects of UPT on tensile properties of laser welded dissimilar joints.

In this paper, Al-Mg -Si alloy was laser welded to Ti-6Al-4V alloy. The effect of both focused and negative defocused beam coupled with two different laser offset values on fusion zone size, heat-affected zone size, intermetallic compounds occurrence and amount, hardness and tensile performance were investigated. Moreover, the occurrence and distribution along the thickness of the intermetallic particles has also been characterized. Stress relaxation heat treatment effects at 530 ◦C for 2 h followed by air cooling on the joints mechanical properties has been analyzed in term of hardness measurements and tensile test. The stress relaxation heat treatment effects were compared with those induced by the mechanical treatment of the joints. Potentiodynamic polarization test in NaCl solution was used to study the corrosion resistance both of aluminum alloy and welds before and after mechanical treatment. The novelty of this study is that it provides a hitherto unexplored chance to overpass the main limit of the dissimilar joints that is the low ductility by the mean of a mechanical treatment.
