**1. Introduction**

In recent years, additive manufacturing technologies have become valued processes based on their competitive and comparative advantages in the production of specimens with complex geometries. Regarding the technique developments, one classification could then be done dividing into two big groups in light of the way the material is provided: Powder bed techniques or blown-powder/wire-feed techniques [1–7].

The technology of plasma transferred arc (PTA) is considered one of the most interesting additive techniques to process specimens through layer depositions [8–11] by blown powder. When the wire is employed, the technique could be considered as one of the wire arc additive manufacturing (WAAM) processes, in addition to tungsten inert gas/metal inert gas (TIG/MIG) [12]. The PTA is included in the recently named group 3D Plasma Metal Deposition (3DPMD) that belongs to the category of directed energy deposition processes [13–15]. This manufacturing process permits us to carry out the fabrication of specimens with larger dimensions than specimens produced by powder bed techniques. Furthermore, the production rate might be higher due to the employment of a higher feed rate in comparison to other additive techniques [16,17].

In order to research the possibilities of developing materials with interesting properties, this study was proposed. The flexibility of the employed technique allowed the realization of a detailed investigation of possible changes in the final specimen's properties as consequence of variation in the setup parameters. The selected material was Hastelloy C-22. This alloy presents excellent corrosion behavior as well as mechanical properties, being commonly employed in the industrial sectors [18–23]: Chemical, petrochemical, aerospace, etc.

The main goal of this research was the production of Hastelloy C-22 walls with good mechanical properties by PTA. In pursuing this goal, the aim of this study was threefold: (1) The manufacturing of walls made from Hastelloy with large dimensions by the optimization of processing parameters; (2) the determination of influence degree in the specimens of the manufacturing atmosphere, in air or argon conditions; and (3) the evaluation of thermal treatments on the final behavior of specimens. Therefore, a wall was fabricated in air conditions firstly, and a second wall was built in argon atmosphere subsequently. Thereafter, from the two walls, determined samples were extracted from marked positions to compare their final properties. Moreover, two thermal treatments were defined to evaluate if the specimens would suffer variation in their properties caused by their cooling rate.

## **2. Materials and Methods**

The starting material employed was powder from Hastelloy C-22, supplied by Atomising Systems Limited (Sheffield, UK). This powder was produced by the conventional method of powder manufacturing known as plasma-atomization process. In Figure 1, the spherical morphology of the Hastelloy particles can be appreciated. Furthermore, the chemical composition of the manufactured powder was compared to the standard one; both are listed in Table 1. In the granulometry given by the manufacturer, d50 (average) was 82.74 μm, and d10 and d90 were 57.39 μm and 125.14 μm, respectively.


**Table 1.** Composition of Hastelloy C-22.

**Figure 1.** Circular backscatter detector (CBS)-SEM image of the starting powder of supplied Hastelloy C-22.

After the powder characterization, the production of the specimens was developed. In this research, the additive manufacturing equipment used for this research was based on PTA technology, self-made, and adapted (RHP-Technology GmbH, Seibersdorf, Austria). In the torch, the plasma was generated. Then, the feeding materials were melted with the plasma energy (Figure 2).

**Figure 2.** Scheme of the plasma transferred arc (PTA) torch.

As novelty in this research, an argon atmosphere was employed, in addition to an air conventional atmosphere. When using an argon atmosphere, the specimens were built inside a designed argon box. For ensuring the environmental conditions, an oxygen sensor (Oxy 3. Orbitec GmbH, Seligenstadt, Germany) was placed near the torch [24].

The torch had an internal cooling circuit where water ran, preventing its melting due to the plasma. The distance between the torch and the substrate or specimen during the fabrication was 10 mm. An argon plasma was induced by introducing the gas between the electrode and the copper cylinder (pilot gas, 1.5 L/min), applying a potential of 20 V. The pilot flame was employed to start the plasma arc between the electrode and the substrate connected to ground. Di fferent electric currents could be applied to increase or decrease the intensity of the plasma arc energy.

At the same time, building in air atmosphere, there was another gas that acted as a shield (shielding gas, 15 L/min) preventing the seam from oxidizing during the process. It was applied by coupling an external copper cylinder to the torch. Several gases could be inserted as shielding gases, as argon pure or mixed with a 5 vol% CO2 gas, depending on the material to be processed, and the properties to be reached. In this investigation, pure argon (99.99% purity, Air Liquide, Paris, France) was employed as shielding gas in the manufacturing of all the specimens.

The material was fed as powder to the plasma jet by aligned holes. For a better powder flowing through the ducts, it was injected with a pressurized gas (powder gas, 1.5 L/min), argon.

This additive manufacturing equipment could build large size components thanks to the torch fixed to an XYZ mechanical, which allowed the torch to move through the working table. This working table was made from aluminum profiles, cooled to ease the heat transfer through the system.

Firstly, a flow test was performed to check if the powder size and shape were optimal for the process. Values in g/min after using different engine units (U) of the rotating metering powder feeder were obtained during the flowability test (Figure 3). A clear linear trend was obtained as the value of motor unit's increase. This means that the powder had a good flow through the ducts of the system and there were no clogging problems.

**Figure 3.** Flowability test of the Hastelloy C-22 powder on the PTA device.

After checking the flowability of the powders, a preliminary bead on a plate welding test was conducted with different parameters (see Table 2), to properly select the more efficient ones and to produce the best quality seam for a subsequent application to the final sample structure. Several parameters' combinations were tested in order to meet a range of processing conditions. By varying the most significant processing parameters (current 120–250 A, speed 100–400 mm/min, material feed rate 11–43.5 g/min), different single seams were welded on a steel AISI 1015 (Figure 4). The dimensions of this substrate were 200 mm × 300 mm × 10 mm. The surface was previously cleaned by brushing.


**Table 2.** Test processing parameters.

**Figure 4.** Processing parameters test.

There were no initial geometrical specifications (seam width and height) predefined in this work. The final height resulted from the layer deposition, and the seam width was the obtained under the fabrication parameters.

By visual inspection, the parameters of the seams with better geometrical and surface quality were selected (6, 7, and 10 from Figure 4). Then, oscillation bead on plate welding tests were performed with these conditions (Figure 5). The oscillation movement helped to improve the quality of the different parts because the torch stayed longer on top of the hot spot after the melting pool. The oscillation parameters were: (1) The amplitude, 7.5 mm and (2) the overlapping, 2.5 mm. The oscillation amplitude resulted in 15 mm for the walls width. The shielding gas protected more during the very beginning of the cooling of the pool, reducing the oxidation due to high temperature. Considering the most promising parameters, seam number 14 from Figure 5, the constructions of both walls were conducted in air and argon. This sample was the candidate selected based on a visual checking of the first manufactured specimens, thus there were neither cracks nor pores.

**Figure 5.** Oscillation bead on plate welding test.

Obviously, the height of the resulting wall was set considering the height of each layer deposition. Setting the parameters listed in Table 3, the walls of dimensions 120 mm × 40 mm × 15 mm were manufactured. To reach this height of 40 mm, 16 layers were necessary, as can be appreciated in Table 3 and Figure 6. With the purpose of obtaining a proper comparison in the study of the effect of the manufacturing atmosphere on the material properties, the similar parameters, strategy, and the number of layers were used to build both walls, in air and argon atmospheres.


**Table 3.** Fabrication parameters for Hastelloy C-22 wall produced in air and in argon atmosphere.

**Figure 6.** Walls in as-built conditions: (**a**) Produced under air atmosphere, (**b**) produced under argon atmosphere.

Next, the extraction of the specimens was conducted following a determinate distribution plan and marks of the location of the cut specimens from each wall. The cutting machine employed was an electrical discharge machine Mitsubishi FX-20 (Mitsubishi, Ratingen, Germany). The geometry of the tensile samples is shown in Figure 7, as well as the sample disposition on the wall. Three sets of samples were extracted.

**Figure 7.** Tensile test sample geometry (in millimeters) and positioning of tensile test samples extracted from the produced sample walls.

The extracted specimens were studied under determined conditions: (1) As built, (2) after thermal treatment 1 (TT1), and (3) after thermal treatment 2 (TT2). Therefore, the heat treatment parameters were previously set up according to recent bibliography [25]. The cooling stage was the main difference between them. The two sets of samples were heat-treated at 1120 ◦C for 20 min in a high vacuum furnace under an argon atmosphere. On the one hand, the set named TT1 suffered a cooling down by rapid air cooling (RAC = TT1). On the other hand, the remaining treated set (named TT2) was cooled down by water quenching (WQ = TT2) after the heat treatment (Figure 8).

**Figure 8.** Thermal treatments applied to the specimens.

The study of the specimens was conducted through a detailed characterization. The microstructural study was performed by scanning electron microscopy (SEM) using a TENEO 6460LV microscope (FEI Teneo, Hillsboro, OR, USA), equipped with an energy dispersive X-ray spectrometry (EDS) system to carry out the phase analysis. X-ray diffraction (XRD) analysis was performed by a Bruker D8 Advance A25 (Bruker, Billerica, MA, USA) with Cu-Kα radiation. In a tester model, Struers-Duramin A300 (Struers, Ballerup, Denmark), the measurement of the mechanical properties was performed to ascertain the Vickers hardness (HV2). Room-temperature tensile tests were performed on a universal testing machine Instron 5505 (Instron, Norwood, MA, USA) with a strain rate of 0.5 mm·min−1. Concluding the characterization, the tribological behavior of the samples was determined by a ball-on-disc tribometer (Microtest MT/30/NI, Madrid, Spain) using aluminum balls (6 mm in diameter) with a sliding speed of 200 rpm and a normal load of 5 N on the ball during 15 min on a circular path of 2 mm in radius. The surface morphology was studied by optical microscopy (OM) with a Leica Zeiss DMV6 (Leica Microsystems, Heerbrugg, Switzerland).
