Heat resistant super alloys are considered very important in the aerospace industry, mainly for the application of engine components. Approximately 50% of ‘’hot section’’ gas turbine components are manufactured from nickel-based super alloys, as reported in . Nickel-based super alloys are also logical choices in other key industrial sectors such as oil and gas industry components  and nuclear power plant components . The main reason for using nickel-based super alloys is their exceptional material characteristics, such as mechanical strength, resistance to surface degradation and creep resistance at elevated temperatures. Many types of nickel-based super alloys have been developed over the years . One commonly used nickel-based super alloy is Inconel 625. Because it contains niobium and molybdenum in a nickel chromium matrix, this super alloy is able to maintain its mechanical properties and corrosion resistance at temperatures up to 650°C . These excellent properties are retainable even without precipitation heat treatment. Components made from Inconel 625 are widely used in the aerospace industry in the ‘’hot section’’ of gas turbine engines and in nuclear reactors, the chemical industry and other high temperature and corrosion applications .
The most common manufacturing method for final products is machining. In general, nickel-based alloys are considered to be difficult-to-cut materials, mainly due to their low thermal conductivity, high hardness and low elastic modulus . This results in a high thermo-mechanical load of the component and the cutting tool. Subsequently, progressive tool wear is observed in machining of nickel-based alloys . In response, new part manufacturing approaches are being explored with the aim of minimizing material removal through the cutting process, which can save both input material and tool costs. Considering current industrial and economical demands, additive manufacturing (AM) of Inconel 625 complex shaped components is one reasonable option .
Wire arc additive manufacturing (WAAM) is a subtype of AM that combines wire as a feedstock and electric arc as a heat source . WAAM excels over other metal AM technologies in particular owing to its high deposition rates and low equipment costs . On the other hand, it cannot fully match the geometrical complexity and precision of powder bed AM methods (e.g. Selective Laser Melting or Laser Metal Deposition) . Hence, WAAM is suitable especially for manufacturing or repairing larger components which would typically be machined after an additive process .
WAAM techniques can be further categorized by the type of arc welding process. According to current studies,  three basic techniques are being examined and used for research and development of prototypes. These welding techniques are gas tungsten arc welding (GTAW) , gas metal arc welding (GMAW)  and plasma arc welding (PAW) . The advantage of the GMAW process lies in the fact that an electrode is a consumable wire which is standardly coaxial with the welding torch. In contrast, GTAW has the consumable wire added non-coaxially, rendering it more difficult for 5-axis control. While PAW can achieve very high productivity, the heat input is also quite high . It is not beneficial for smaller, more precise parts. Moreover, the PAW process lead and tilt angles of the welding torch could be harmful for the manufacturing result. Thus, the PAW process is not suitable for 5-axis WAAM technology. For the purposes of WAAM, GMAW modification, known as Cold Metal Transfer (CMT), is suitable .
CMT is a modified GMAW process based on short-circuiting transfer, where movement of the wire feedstock at high frequencies (up to 80 Hz) combined with the welding current and voltage control improves the properties of the welding process. The key improvements are elimination of spatter and considerably lower heat input . As an alternative to a standard continuous welding strategy, a CMT welding unit is also capable of a spot welding strategy. The spot welding strategy has lower productivity and higher consumption of shielding gas. However, it excels in terms of very low heat input and very precise material cladding in comparison with a standard continuous welding strategy. Fine details can be manufactured even on thin walled components or in sections with problematic heat dissipation .
In relation to the current state of the art, numerous papers focus on WAAM manufacturing of Inconel alloys. Most papers address Inconel 718 (IN718) alloy, and only  focus on Inconel 625. WAAM components from Inconel 718 require post-process heat treatment , which is no obstacle for manufacturing blank parts. According to , Inconel 625 (IN625) does not require post-process heat treatment. This property makes IN625 more suitable for hybrid WAAM  of a finished component in one machine. Analysis of material properties is a common step in the process of developing a technology strategy. Analysis enables an understanding of the elementary principles of the manufacturing process. To develop a reliable manufacturing method, it is necessary to study the material properties of samples manufactured using various complementary operations and methods.
Summarizing the current state of the art of Wire Arc Additive Manufacturing of Inconel, information from research papers can be sorted into these categories:
(i) Shielding gas influence
In , the influence of 4 different shielding gases for WAAM of IN625 was studied. The gas mixture of 97.5% Ar and 2.5% CO2 performed better than the other gas mixtures in terms of YS, UTS and hardness. However, other studies  mentioned the importance of having a helium proportion of about 20–25%. A proper solution could be to use a gas mixture  designed for nickel based super alloys from a specialized gas company (Messer Group GmbH).
(ii) Welding strategy
Study  focused on comparison of the material properties of a standard CMT welding process and a pulse CMT welding process during the WAAM of IN625. Both techniques demonstrated YS and UTS results that were about 20% better than a casted sample. The CMT pulse manufactured sample was even slightly better (5–15%) than the standard CMT in YS, UTS and hardness. There are similarities between CMT pulse welding and CMT spot welding. Investigation of the material properties of a IN625 WAAM CMT spot welding strategy can further confirm or contradict the results from .
(iii) Inter layer technological operations
Various other technological factors affecting WAAM of IN718 have been examined in published papers. These are interpass cleaning of the weld surface  and interpass rolling of the weld seams with 50 kN and 75 kN . In both cases, the results show that the positive influence of these treatments is not quite clear. Interpass rolling increased hardness but other mechanical properties were worse than without interpass rolling. Effects of interpass cleaning have not been observed.
(iv) Material analysis
Standard analytical tools in contemporary research include optical microstructure and macrostructure analysis, hardness analysis and SEM analysis. Many studies also rely on tensile strength testing and EDS analysis. To keep the present research relevant to the current state of the art, all of these standard types of analyses were performed. It is clear that the properties of Inconel 625 alloy, which should retain its tensile strength at elevated temperatures, make a tensile strength test at 650°C  very relevant for this research.
The main goal of this paper is to expand the state of the art in WAAM of Inconel 625. A GMAW CMT spot welding strategy will be tested and compared with a standard GMAW CMT continuous welding strategy. Complex material analyses will be performed, including: metallography, SEM (Scanning Electron Microscope), x-ray tomography, hardness, wear resistance and tensile strength. The last two material properties will be investigated at room temperature (20°C) and also at an elevated temperature of 650°C.