**1. Introduction**

One of the challenges in Additive Manufacturing (AM) of metallic materials is to obtain workpieces free of defects and with excellent physical, mechanical and metallurgical properties [1] to satisfy the strict requirements of engineering applications. Obtaining such mechanical requirements is a hard task, especially in parts fabricated as the result of layer by layer addition of the material. AM of metallic materials involves different techniques (powder bed fusion, binder jetting, sheet lamination, and directed energy deposition) and metals generally must be weldable and castable to be successfully processed in AM [2]. Till now, there have been only a limited number of commercial alloys used in AM [3], so there is a need to increase the number of alloys to be processed by AM techniques in order to

widen the application fields. Most of the current commercial metallic materials for AM are steels [4–7], aluminum [8], and titanium alloys [9,10].

Wire and arc additive manufacturing (WAAM) is a wire-feed AM process and one of the most promising techniques for producing larger components with moderate complexity and relative low costs compared to other AM techniques for metals [11]. WAAM processes have a promising future. Designs are already being made using this technique in different fields where WAAM is performing very well; some of them are lightweight aerospace components (landing gear parts, wing ribs or stiffeners) [12], wind tunnel models [13], bridges [14] and/or complex constructive features (such as the dragon bench in the manufacture of furniture), that could not be made with conventional processes such as casting, or computer numerical controlled (CNC) milling, among others [15].

WAAM processes generally involve high residual stresses due to high deposition rates and heat inputs [16]. The influences of process conditions (for example, energy input, wire-feed rate [17], welding speed and/or deposition pattern [18]) on thermal history, microstructure and resultant mechanical and surface properties of parts need to be analyzed [16] as there is not enough knowledge in the scientific community yet.

As explained in the work by Ge et al. [4], during WAAM processes, the added layers of material suffer a complicated thermal history that includes, among others, melting, fast cooling, solidification, and/or partial remelting, that greatly influence the final properties of the parts produced by these techniques.

A recent study about the microstructure is the one from Wang et al. [19], where mechanical properties of thin-walled parts of the die steel H13 were also analyzed, showing that the tensile properties were anisotropic but could become isotropic after 830 ◦C of heat treatment (annealing) for 4 h. Yan et al. [20] studied the effect of temperature gradient, solidification velocity, and alloy composition on grain morphology in AM of metallic materials. In the overview article of Herzog et al. [21], special attention was paid in analyzing AM specific grain structures, resulting from the complex thermal cycle and high cooling rates. Kok et al. [22] highlighted that anisotropy and heterogeneity in the mechanical properties of metallic AM parts are mainly due to the anisotropy and heterogeneity of the microstructure and material properties. Another work investigating the relationship between the microstructure and mechanical properties of the titanium alloy Ti-6Al-4V fabricated by the WAAM process is reported by Wang et al. [23].

On the other hand, in the work from Szost et al. [24], porosity, microstructure, and micro hardness of Al-6.3%Cu samples fabricated by WAAM were investigated considering cold metal transfer (CMT) variants, pulsed CMT, and advanced CMT. A very interesting paper from Cong et al. [25] explores in depth the influence of the arc mode for different CMT variants (CMT, CMT pulse, CMT advanced, and CMT pulse advanced) on the porosity of an aluminum alloy fabricated by WAAM. In this work, CMT pulse advanced showed excellent performance in controlling porosity, being the most suitable variant for depositing aluminum alloys. Other defects such as lack of fusion, have been also analyzed by the same authors in their work of 2018 [26] for maraging steel.

Mechanical properties obtained by WAAM, including hardness, are also a promising field of study as shown in works from Horgar et al. [27], where AA5183 aluminum alloy wire was deposited on an AA6082-T6 plate as substrate. Wu et al. [28] investigated the influence of the molten pool size on the microstructure and mechanical properties of pieces of Ti-6Al-4V alloy, whereas Lewandowski and Seifi [29] presented a review of mechanical properties for the most common alloys used in AM of metals (Ti-6Al-4V, TiAl, stainless steel, Inconel 625/718, and Al-Si-10Mg).

Micro-geometrical properties such as roughness are also being investigated, as in the case of manufacturing of multi-layer single-pass thin-walled parts [30] and in the work of Li et al. [31].

Arrizubieta et al. [32] presented a novel manufacturing technique combining laser metal deposition, laser beam machining, and laser polishing processes for the manufacturing of a complex Inconel 718 part, resulting in a cleaner technique as conventional machining operations were eliminated. Another interesting work showing a suitable full-dimension sustainability life cycle assessment framework of parts produced by AM is the one presented by Ma et al. [33].

As we can see, due to the great variety of dimensions and parameters involved and their interdependencies [34], AM processes require multidisciplinary research, and many investigation lines are already open to reach a deeper knowledge about these technologies. In the present work, three-dimensional (3D) metallic parts of mild steel wire (AWS ER70S-6) are built with a WAAM process by depositing beads of weld metal layer by layer on a substrate of a S235 JR steel sheet of 3 mm thickness, using as the welding process gas metal arc welding (GMAW) [35] with Cold Metal Transfer technology [36], combined with a positioning system such as a CNC milling machine [37]. The paper will show some interesting results based on hardness measurements, along with complementary values of tensile strength at the working area and microstructure information.
