**1. Introduction**

Titanium and its alloys have a significant role in the aerospace industry. Demonstrating a density of 4.5 g/cm3, titanium alloys weigh about half as much as steel or Ni-based super alloys, resulting in superior mechanical properties to the latter, while their exceptional corrosion resistance makes them excellent candidates for use in the aviation and aerospace sectors [1]. Currently, this metal and its alloys account for 14% of the total weight of modern aircrafts [2,3]. For non-structural applications in which corrosion resistance, good formability and low weight are the main requirements (e.g., welded pipes and ducts, bolts, seat rails, water supply systems for galleys and sanitary, etc.), Cp-Ti is generally used. However, there are several application areas wherein high strength is important as well. Airframe joints, engine parts (e.g., fan blades, fan case, shaft, compressor) and landing gears require the use of high strength Ti alloys [4,5]. For these purposes, the Ti6Al4V is the most widely used alloy. Titanium-based alloys have the highest tensile strength/density ratio among the metals [6]. Unlike aluminum alloys, Titanium can preserve its strength at elevated (up to 600 ◦C) and cryogenic temperatures as well [7], which is favorable considering the temperature conditions affecting aircraft. Moreover, the ongoing focus on achieving closedloop circularity in the aviation sector for the accomplishment of sustainability objectives [8]

**Citation:** Miko, T.; Petho, D.; Gergely, G.; Markatos, D.; Gacsi, Z. A Novel Process to Produce Ti Parts from Powder Metallurgy with Advanced Properties for Aeronautical Applications. *Aerospace* **2023**, *10*, 332. https://doi.org/10.3390/ aerospace10040332

Academic Editors: Spiros Pantelakis, Andreas Strohmayer and Jordi Pons-Prats

Received: 28 February 2023 Revised: 16 March 2023 Accepted: 21 March 2023 Published: 27 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

can be supported by using recycled titanium alloys. Titanium alloys can be remelted and reprocessed into high purity ingots with lower energy requirements compared to virgin ones. In this context, specialized forging plants that recycle titanium from the aerospace sector are already functioning [9].

Yet, the biggest obstacle to the wider use of titanium is its price. Generally, the cost of the traditional production method is higher compared to other metals, such as steel or aluminum [10]. In addition, cold forming of thin titanium alloys is challenging due to their high yield strength (YS) and their significant strain hardening effect. Furthermore, they present low thermal conductivity, which increases the heat of the tools during machining, while their low Young's modulus causes a significant spring back during traditional processing [11]. Figure 1 shows the price per unit volume of some traditionally produced alloys used in the aerospace industry at different thicknesses [12]. Obviously, the Ti6Al4V alloy has the highest price compared to the unalloyed Ti grade 2, 17-4PH stainless steel and 7075 aluminum alloy. In the case of 7075 Al and Ti grade 2, the price does not increase significantly with decreasing thickness, due to their good formability. However, the specific prices of the 17-4PH stainless steel and the Ti6Al4V increase significantly with the thickness decrease. This is due to the resulting increase in hardness and strength. The latter can make the material more resistant to deformation during the cold forming, which in turn may require more energy to shape it. Additionally, thinner materials may require more precise and specialized equipment and processes to be shaped properly, which can also increase costs. Therefore, cold forming of these alloys is extremely difficult and energy demanding, especially for thinner sheets, something which increases processing costs significantly.

**Figure 1.** Specific price (USD/dm3) of different metal sheets (304 <sup>×</sup> 304 mm) at different thickness.

Furthermore, due to the low yield rate of the traditional Ti metallurgy, 82% of the initial Ti becomes scrap, e.g., in the case of F-22 fighter jet Ti parts [13]. In the above context, reduction of scrap material and consequent manufacturing costs, as well as increases in the mechanical properties of Cp-Ti and titanium alloys, represent the two main challenges of the research community [14]. Within this framework, the powder metallurgical (PM) approach can significantly increase the yield rate and thus reduce manufacturing costs, as it is suitable for the production of ready-to-use parts and components [10]. Among the PM processes, the traditional press and sinter process is the lowest cost method for converting metal powder into a near net shape part [15]. This method involves the cold pressing of the prepared powder into the desired shape in a die, then creating metallic bonding between the cold-welded powder particles via sintering at temperatures below the melting temperature. The productivity of this process is high, but the size and complexity of the cold pressed parts are limited due to technological reasons [16]; therefore, fan blades, cockpit window frames or hydraulic pipes cannot be produced this way. Cold isostatic

pressing (CIP) and hot isostatic pressing (HIP) can be alternative solutions to the press and sinter technique [17]; however, productivity is greatly decreased with these solutions.

However, the use of press and sinter technology has its limitations. The produced parts must be relatively small, as the required press force increases as the surface area increases. In addition, they must have an axisymmetric simple shape to be manufacturable with cold pressing tools. For example, fasteners (nots, screws, clevis pins, washers) can be produced as a finished or semi-finished product this way. These products are traditionally made from rolled sheets and bars by chipping or punching, which results in a lot of scrap material. PM appears as a suitable method for the production of these small parts as it does not generate a considerable amount of scrap. This manufacturing process is particularly economical for expensive materials such as Ti and its alloys. From the observed high costs of small parts produced through conventional methods [18], it becomes clear that the PM technology could emerge as a potential cost-efficient method for the production of small parts such as thin washers, as such parts can be produced without generating any scrap material, thus saving on processing costs and energy.

The properties of PM products are significantly influenced by the size, morphology, and hardness of the initial powders. Duriagina et al. has investigated the effect of the size and morphology of the VT20 Ti alloy powders on the mechanical properties of coatings. The coatings deposited from the −160 + 40 μm fraction showed an optimum ratio of strength and plasticity. Nonspherical VT20 titanium alloy powders are characterized by a finer structure than the coatings produced from spherical powders [19].

Although powders with spherical morphology are ideal for the additive manufacturing (AM) processes such as metal injection molding (MIM), they are not favorable for cold pressing due to the weak compressive bonds between the pressed powder particles. Powders with sponge-like particles ensure the highest green (i.e., not yet sintered part) strength [15] by promoting higher plastic deformation during cold compaction and better interlocking behavior compared to spherical powders [20]. The morphology of the Cp-Ti sponge results from the reduction of the Ti ore using the Kroll, Hunter or Armstrong method, which is the first main step of commercial titanium's production route [10]. The size of the coral-like spongy particles can be decreased to specified ranges by using a crushing or milling technique. This size reduction can easily be carried out after hydrogenating the powder. The resulting TiH2 can be easily crushed to different particle sizes, ranging from 45 μm to 300 μm. After this relatively inexpensive hydrogenation-dehydrogenation process (HDH), we obtain a powder with irregularly shaped particles with high purity [21]. Currently, the spherical Ti6Al4V powder is also produced from alloyed Kroll sponge; it is formed into a wrought product which is then reduced to powder by an atomization process (e.g., the plasma rotating electrode process (PREP)). The resulting spherical powder's cost is approximately 15–30 times of the cost of sponge Cp-Ti powder [14]. This feedstock ensures much higher sintered strength, but the cost could be higher than the production cost of the same wrought alloy. In the case of PM parts, the effect of porosity has an important role. In order to increase the relative density close to its theoretical maximum, most studies used high sintering temperatures and long sintering times, but in these cases, coarsening is a significant issue [22]. The applied value of the cold pressing has also an important role in the green density [23]. Decreasing the porosity from 35% to 5%, the strength (YS, UTS) and the elongation to fracture value increases by about 5 to 10 times, respectively [24]. The difference between the green density of the samples made of Ti6Al4V and the Cp-Ti is ~20% [20]. However, if we compare the tensile properties of the Cp-Ti and Ti6Al4V (Table 1), it can be stated that the strength (YS, UTS) of the sintered/wrought Ti6Al4V are double, while the elongation is half of the Cp-Ti's values [25].


**Table 1.** Tensile properties of Cp-Ti and Ti alloys.

From the above table (Table 1), it is clear that the alloying elements play an important role on the strength of the alloys. There are several studies which have investigated the role of the different alloying elements on the strength of the titanium. By adding 42 wt.% Nb, the tensile yield strength increases to 675 MPa [26], and by adding 7 wt.% Fe, the tensile strength increases to 916 MPa [27]. The strength can even be increased more by adding ceramic reinforcement to Ti. Jeong et al. mixed pure titanium powder with TiB2 and B4C powder, then produced bulk samples using the press and sinter approach. The compressive yield strength of the in situ processed composites was higher than that of the Ti6Al4V alloy at ambient temperature. The highest compressive yield strength obtained was 1400 MPa [28]. The oxygen content has also an important role on the strength of the Ti. Chen et al. increased the oxygen content of the Cp-Ti up to 0.8 wt.%. using different PM methods. The highest tensile YS was measured around 900 MPa [20].

Besides the alloying and reinforcing elements, the role of the grain structure and the grain size are crucial as well, and this will be the focus of the present work. Nanograined (NG) and ultrafine-grained (UFG) metals and alloys show significantly higher strength compared to coarse-grained metals [29]. However, with the increased strength, the deformability and the room temperature ductility significantly decrease as well, limiting the applicability. To tackle the latter issue, dual scale grain size can be considered. The big advantage of a dual scale grain size is that the coarse grains retain the toughness of the material, while the fine grains improve its strength [30]. This microstructure can be achieved by PM method. Sun et al. prepared non-milled and cryo-milled Ti6Al4V alloy by plasma-activated sintering. The highest hardness measured was 470 HV, and the highest compressive YS was 1706 MPa [31]. Li et al. made a Ti-Bi bimodal alloy by using highenergy ball-milled Ti-Bi and spark plasma sintering, achieving 1080 MPa tensile YS [32]. Attar et al. made in situ titanium–titanium boride composites using the mixture of fine TiB2 and coarse CP-Ti powder. The solidification was carried out with the selective laser melting (SLM) method, achieving 1400 MPa compressive YS [33].

Based on the cited literature, the economically feasible production of Ti parts with satisfactory mechanical properties can still be further improved. In this study, Cp-Ti was considered the initial material, with the aim being to improve its performance using a dual-scale microstructure which was produced by mixing fine and coarse titanium grade-2 type powders through the PM process. The goal of the study was to produce a material with increased mechanical properties, to be considered as a potential substitute to commercially produced Ti alloys widely used in the aviation sector, such as the Ti6Al4V (grade5) alloy. To achieve the above, the cavities of Cp-Ti sponge powders were filled with fine nanoscale-milled Cp-Ti powder during the mixing process, which resulted in a special feedstock for the applied cold press and sinter process. The density, hardness, yield strength, compressive strength and strain were systematically investigated on the sintered parts. The results showed that the fine nanoparticles of the dual-scale grain have significantly enhanced the strength of the material, while in several cases, the corresponding strength exceeded the values of the Ti6Al4V alloy. The novelty of the work lies in the simplicity and inexpensiveness of the production route.
