PSI - Issue 69

C.A. Biffi et al. / Procedia Structural Integrity 69 (2025) 47– 52

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1. Introduction The development of high temperature shape memory alloys (HTSMAs), operating at temperatures above 100°C, is a hot research topic, which is recently attracting considerable attention in several challenging fields, such as the automotive, aerospace and robotic ones [1-2]. In particular, novel compositions are currently being researched and proposed: NiTiHf-based compositions appear as the most promising ones [3], as they are characterised by a distinctly lower cost than HTSMAs containing precious elements (Au, Pt, Pd [4]) and exhibit superior properties with respect to NiTiZr or CuZr-based alloys [5-6]. Unfortunately, most HTSMAs, including NiTiHf formulations, display extremely poor workability, related to high cracking susceptibility, which limits the possibility of using cold deformation processes and restricts the feasible applications [7-8]. In this respect, the use of melting-based processing, such as welding, represents an interesting perspective, as it would allow to join semifinished products, thus creating more complex geometries and possibly overcoming some of the workability limitations affecting HTSMAs. Among the most widespread joining techniques, laser beam welding appears to be a particularly interesting one, as it offers high throughput and flexibility; moreover, the use of a high-power, extremely focussed heat source allows to precisely tune thermal cycles, preventing the formation of an extended heat affected zone and controlling the local microstructure [9-10]. Nowadays, one of the most promising novelties in laser technology lies in beam shaping, which indicates the option of adjusting the spatial power distribution of the laser beam, obtaining irradiance profiles whose shape can deviate from the conventional gaussian one [11]. This, in turn, enables more precise control over the temperature distribution and thermal history experienced by both the melted and heat-affected zones—particularly by allowing the adjustment of cooling rates. Notably, static laser beam shaping has been successfully investigated in both laser powder bed fusion [12] and laser welding [13–14], demonstrating promising results for joining materials that are typically difficult to weld. Therefore, the goal of this work is to explore the opportunity of using different laser power distributions, obtained through in-source laser beam shaping, to control the melting and solidification of a high-temperature, shape-memory NiTiHf alloy, thus exploring its feasibility map and its weldability. 2. Materials and experimental procedures Ni50Ti40Hf10 [at %] ingots were prepared by plasma arc melting starting from commercially pure raw materials: a laboratory scale plasma arc furnace (Leybold—Plasma Lab 3000), equipped with a transferred plasma torch of 150kW (PEC Plasma Electric Company) and operating under high-purity He atmosphere, was used to melt feeding materials into a cylindrical (60mm in diameter), water-cooled, copper crucible. 1-mm thick slices were then cut by wire-EDM and appropriately grinded to remove surface oxidation. An industrial LPBF system with an open architecture (3D-NT LLA150R, 3DNT) was used in the present work to perform laser beam welding experiments. The system is equipped with a double-core fiber laser source with in-source beam shaping capabilities (mod. AFX 1000 from nLIGHT). The laser source emits at a wavelength of 1070 nm (±10 nm) and has a maximum power of 600 W when emitting with the inner core of the fiber and 1200W with the outer core. During the presently described tests, laser power was kept constant at 600 W, while at the same time scanning speed was varied, achieving 20 mm/s, 50 mm/s, and 100 mm/s values. Finally beam shaping was used in three conditions, hereafter named BS0 (gaussian shape), BS3 (intermediate condition), and BS6 (quasi-uniform distribution). Figure 1 shows the intensity distributions related to the mentioned beam shaping conditions. From theorical calculations, the beam waist diameter with BS0 corresponded to 70 µm whilst BS6 corresponded to a beam waist diameter of 200 µm [15].