## 2 Materials and Methods
These first 9 samples were thus subjected to tensile tests, and the results, which are presented in the following Section 3, in terms of the load-displacement and stress-strain curves, confirmed that this alloy, processed via SLM with the parameters recommended by EOS GmbH, can reasonably be regarded as isotropic. Consequently, the second batch of 30 samples was fabricated only along the vertical direction, using the same process parameters as the first batch; all the specimens were then heat-treated with 3 temperatures, at different durations (for each heat treatment and heating time, three repetitions were tested). In summary, overall, 39 specimens were studied: 3 were not postprocessed, 6 underwent BF and 30 were subjected to heat treatments. The barrel finishing was selected as the surface-finishing operation for its flexibility and easiness to use, while for the heat treatments, different heating cycles were selected, starting from the treatment suggested by the powder supplier [31] and exploring possible alternative toward lower temperatures, to save energy and money in actual applications. The barrel finishing was carried out, using a Rotar EMI 47, an industrial inclined octagonal barrel 400 mm in diameter moved by an asynchronous motor with electronic speed control. The employed media were cylinders made of a synthetic resin matrix with abrasive consisting of powder of alumina, silica and silicon carbide. They measure 15 mm in diameter and 25 mm in height and are angled cut (Figure 3a). The rotational speed and the filling percentage were chosen according to the analysis of the Bed Behavior (BB) diagram obtained following the methodology described in [23]. The BB diagram for this experimentation is reported in Figure 3b. The finishing process was performed at two different rotational speeds, in the rolling and cascading domains, respectively. Namely, the chosen rotational speeds were 30 and 42 rpm, while the filling percentage of the barrel was set at 50%, according to the assessment that, at this value, the active layer has the greatest surface [36]. The conditioned surfaces were analyzed through an Olympus SZH microscope and measured by a Mitutoyo Surftest SJ-412. The roughness profiles were achieved over a 12.5 measuring length and evaluated over 10 mm, according to [37] indications. A spline profile filter with a 2.5 mm cutoff and an 8 \(\upmu\)m short wavelength cutoff was applied to data, in accordance with [38]. Three-dimensional maps 5x3 mm\({}^{2}\) were acquired for overall analysis of BF conditioning. The heat treatment recommended by the M290 EOS material datasheet is 4 h, at 800 \({}^{\circ}\)C [31]. The authors started from that suggested cycle and selected three alternative lower temperatures for the heat treatments: 482, 704 and 788 \({}^{\circ}\)C. Higher temperatures are associated to more marked effects on Ti6 Al4V; nevertheless, the study of the effect of low-temperature treatments is not unusual and can also be found in the literature [32; 39; 40]. Thus, the temperature-range choice was basically made to  
Figure 3: (**a**) Used media; (**b**) Bed Behavior (BB) diagram for the experimented charge; (**c**) zoom of the BB diagram and indication of the chosen process parameters. fulfill the industrial needs of time reduction and energy saving of the postprocessing phase, and it was limited by the operating characteristics of the facilities usually available in the industry. Moreover, the same reasoning was followed to select the durations: for each temperature, three heating durations were applied, trying to mediate between effectiveness and time. All the thermal treatments were carried out, using a Nabertherm LH120/14 electric furnace, at atmospheric pressure. Table 3 reports all the produced samples, along with their orientation, actual dimensions and the type of applied postprocessing. All tensile tests were performed by using a servo-hydraulic MTS universal testing machine, capable of a tensile/compressive load up to 250 kN, with 150 mm of maximum stroke.