Influence of deposition strategy and heat treatment on mechanical properties and microstructure of 2319 aluminium WAAM components

Influence of deposition strategy and heat treatment on mechanical properties and microstructure of 2319 aluminium WAAM components Maider Arana a,b, ⇑ , Eneko Ukar b , Iker Rodriguez a , David Aguilar a , Pedro Álvarez a a LORTEK Technological Centre, Basque Research and Technology Alliance (BRTA), Ordizia, Spain b Mechanical Engineering Department, University of the Basque Country UPV/EHU, 48013 Bilbao, Spain highlights WAAM of 2319 aluminium alloy is investigated. Deposition strategy, geometry and heat treatment affects mechanical properties. Without interlayer dwell time, fine equiaxed microstructure is obtained. Low anisotropy is achieved if proper artificial aging is applied. graphical abstract article info Article history: Received 22 March 2022 Revised 11 July 2022 Accepted 20 July 2022 Available online 25 July 2022 Keywords: WAAM Aluminium Mechanical properties Heat treatment Microstructure Anisotropy abstract There is an increasing interest in additive manufacturing of high strength aluminium alloys built with wire and arc additive manufacturing (WAAM) technologies. Al-Cu alloys are susceptible to hot cracking, however, pulse advanced cold metal transfer (CMT-PADV) welding process ensures a low heat input that avoids this defect while reducing the part porosity. This study has demonstrated that porosity below 1 % does not affect mechanical properties of 2319 WAAM walls, whereas the as-built microstructure and applied heat treatment highly influence the properties obtained. Without interlayer dwell time, a fine uniformly distributed equiaxed microstructure is obtained, however, if an interlayer arc stop period is applied, columnar dendritic grains in the interlayer zone is obtained. Heat treatment with low aging temperature and time demonstrated not to be suitable to ensure high mechanical properties and low anisotropy. Instead, if 190 °C and 26-hour aging heat treatment is used, properties up to 324 MPa for yield stress, 452 MPa tensile strength and 8 % elongation can be achieved perpendicularly to the building direction with anisotropy of 1 %, 2 % and 46 %, respectively, for the manufacturing conditions that avoided columnar grains. This fact was critical to ensure highest strength and ductility values and low anisotropy. Ó2022 The Authors. Published by Elsevier Ltd. ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction Wire and Arc Additive Manufacturing (WAAM) is defined as a process by which metallic parts can be manufactured superpositioning welding beads, layer by layer. It is classified into the category of directed energy deposition (DED) processes and as the https://doi.org/10.1016/j.matdes.2022.110974 0264-1275/Ó2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ⇑ Corresponding author at: LORTEK Technological Centre, Basque Research and Technology Alliance (BRTA), Ordizia, Spain. E-mail address: [email protected] (M. Arana). Materials & Design 221 (2022) 110974 Contents lists available at ScienceDirect Materials & Design journal homepage: www.elsevier.com/locate/matdes name describes, and electric arc powered from a welding source is employed to melt and deposit wire shaped feedstock [1–3]. WAAM is well suited for the manufacturing of medium to large size components due to the high deposition rates (usually from 2 to 4 Kg/h), superior material usage (>90 %), outstanding energy efficiencies (between 85 and 90 %) and scalable working environment. Studies have shown that WAAM process can reduce manufacturing cost from 7 % to 69 % compared to conventional subtractive processes such as machining [4]. In fact, WAAM is currently considered as the metal additive manufacturing process with the lowest cost and environmental impact per kg of deposited material [5]. Therefore, this technology is being seriously considered for the repairing and manufacturing of metals components in many sectors such as aerospace, naval, tooling, oil and gas and automotive [2]. In order to attain tight dimensional tolerances and smooth surface, WAAM has to be combined with subtractive technologies, but even with this subtractive post-processing step, the buy-to-fly (BTF) ratio can be greatly reduced in aerospace components like rings, gears, ribs, spars or bulkheads which usually show BTF ratios above 10 for conventional machining processes [2,6]. Many of these components are currently made of high strength aluminium alloys due to their good mechanical properties, low density and high corrosion resistance. Therefore, in the last years there has been a growing interest towards the development of WAAM for different aluminium alloys, aiming to reach mechanical performances comparable to wrought and rolled 2000 series aluminium alloys that are widely used in the aerospace industry [7–9].A recent summary of some of the most relevant works in this field can be found elsewhere [10]. Most of the studies, have been carried out with Al-Cu alloys, 2319 specifically, as this is the only one from the 2000 series which is considered readily weldable, because it does not suffer hot cracking [11]. Metal inert gas (MIG) based WAAM has several advantages in comparison with tungsten inert gas (TIG) or plasma arc welding (PAW) such as the use of consumable electrode and coaxial feeding which simplifies the tool path planning and system complexity [12]. However, conventional MIG process usually gives rise to excessive heating which leads to distortion and porosity in aluminium welds. These limitations can be minimized by cold metal transfer (CMT Ò ) technology from Fronius Company [13], i.e., an improvement of short-circuit MIG process with reduced heat input. In fact, many previous works have concluded that CMT is a competent manufacturing process for WAAM of aluminium components. Several research groups have investigated the effect of CMT arc modes or variants on the porosity and mechanical properties of 2xxx (Al-Cu) [8,14–17] and 5xxx (Al-Mg) [18,19] series WAAM aluminium parts. The studied CMT variants include conventional CMT, CMT pulse (CMT-P), CMT advanced (CMT-ADV) and CMT pulse advanced (CMT-PADV), the latter a combination of mechanically continuously pushed and pulled wire feeding movement coupled with alternatively polarity shifts which modify melting rate [20]. According to Fang et al. [21], CMT-PADV based WAAM of 2219 alloy effectively reduces porosity below 1 % reducing pore size below 100 l m, whereas an equiaxed grain structure is obtained. As a result, outstanding mechanical properties are obtained with CMT-PADV arc mode, reaching after T6 thermal treatment a tensile strength up to 283 MPa and almost isotropic behaviour, with only a difference of 5 MPa between the vertical and horizontal tensile testing direction. All previous works concluded that CMT-PADV is the most suitable welding process reducing drastically the porosity of 2319 WAAM samples. Cong et al. were pioneers in drawing this conclusion and it was explained by the lower heat input of CMT-PADV and effective oxide cleaning of the wire [8]. The influence of the quality of aluminium feedstock wire has also been deeply investigated [22,15]. Consistent wire surface quality and properties are required to keep stable process stability and to obtain finest quality in WAAM samples. As-built mechanical properties of 2319 WAAM samples can be greatly enhanced by applying conventional T6 thermal treatment after the complete manufacturing of the part or by applying rolling or peening after deposition of each layer [7,23,24]. Particularly post-deposition heat treatment enhances mechanical properties to values above 300 MPa for yield stress and 450 MPa for tensile strength. These properties overcome the standard specifications requested for equivalent wrought 2219-T6 plates. Interpass rolling is based on applying compressive pressure on the top surface of each layer using a rolling rig with a hydraulic cylinder. Gu et al. [7] studied the effect of different rolling loads (from 15 kN to 45 kN) and T6 heat treatment [25] in the resultant mechanical properties of 2319 WAAM parts. In the as-build condition, as the load increased, the yield stress (YS) and tensile strength (UTS) increased, but elongation was reduced. If combined with the T6 treatment, the elongation was highly improved. Therefore, to obtain the best mechanical properties relation, with YS above 300 MPa, UTS above 450 MPa and elongation greater than 14 %, T6 heat treatment was essential. By this aging thermal treatment, resulting anisotropy was minimum. Guo et al. [26] studied another post welding deformation technique, laser shock peening. It consists on applying a pulse laser with high power density and ultra-short duration to induce large depth and high value compressive residual stress. With this technique, the as-built yield stress was improved, almost doubled up to 178.3 MPa, but the elongation worsened. Al-Cu alloys are heat treatable, but the influence of the heat treatment after WAAM on mechanical properties and anisotropy has not been deeply investigated so far. In fact, different authors and even researchers from the same group, have applied different thermal treatments to these alloys at different aging temperatures and during different soaking times without any detailed justification [7 27]. The ASM Handbook [25] establishes suitable heat treatments for the different aluminium alloys including 2219 wrought or rolled alloy. In addition to the heat treatment, K. F. Ayarkwa et al. [27] studies the effect of deposition strategies on mechanical properties of WAAM samples made of 2319 alloy. They used oscillated and parallel deposition strategies along with single bead deposition. Manufactured samples had different wall thickness ranging from 7 to 11 mm. It was concluded that the best mechanical properties are obtained when using single bead deposition, having YS over 300 MPa, UTS over 450 MPa and elongation over 12 % after dedicated heat treatment. Cong et al. also investigated the effect of wall thickness in thin and block structures of 2319 [28]. In general, one can conclude that deposition strategy, i.e., geometry and torch movement, plays an important role in the manufacture of WAAM parts and resulting microstructure and mechanical properties. In this study, additional rectangular deposition geometry is analysed, with no inter layer dwell time, as well as straight wall depositions with inter layer dwell time to investigate the effect of the heat accumulation in the final properties. The influence of torch movement is also investigated. Additionally, the impact of heat treatment on the mechanical properties is investigated by analysing the influence of aging temperature and time for different deposition strategies. 2. Materials and methods To manufacture the parts to be analysed in this study, a pulse advanced cold metal transfer (CMT-PADV)-based WAAM process M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 2 with ER2319 filler metal of 1.2 mm diameter was employed. The special quality wire so-called ‘‘certified premium Aluminium 2319 wire” was provided by WAAM3D. The chemical composition of the materials used are shown in Table 1. This filler metal was deposited over 10 mm thickness AA6082-T6 substrate using CMT 1369 + P Adv synergic curve. A Fronius CMT Advanced 4000 R welding power source and RA 5000 22G welding torch (Fronius International GmbH, Pettenbach, Austria) assembled in a Fanuc ARC Mate 120iC robot (Fanuc Corporation, Oshino-mura, Yamanashi Prefecture, Japan) were employed. The equipment (Fig. 1) was mounted in the intelligent welding cell of LORTEK. Pure argon (99.999 %) was used as shielding gas with constant flow of 30 L/min. Nozzle to part work distance was fixed at 12 mm (see Fig. 2). The process parameters used to manufacture the parts were kept constant throughout all the study: the intensity was 96 A, travel speed (TS) was 0.6 m/min, and wire feed speed (WFS) was 6 m/ min. CMT-PADV technology has an additional parameter called EP/ EN balance, with a range from 5 to + 5, that changes the balance between positive and negative cycles. This parameter was kept at 0, except for the first layer, where in order to avoid necking defect due to heat sinking through the substrate, it was fixed at + 4 and WFS was increased to 10 m/min. As a result, the heat input increased in the first layer and shrinkage was avoided. Two deposition strategies in terms of torch movement were selected (Fig. 3). Hatching strategy, represents a straight weld bead without any weaving, whereas in circling strategy, the torch oscillates to generate an overlap of round circles with 1 mm amplitude and 2 Hz frequency. For both strategies, travel direction was changed from odd to even layers to provide a stable growth. Two different geometries were manufactured (Fig. 4). First one, called rectangular, is a continuous rectangular spiral, that does not have arc stops, therefore there is no dwell time between layer. The other one is a straight wall geometry with 90 s dwell time between layers. The straight wall geometry was studied for both hatching and circling deposition strategies, whereas the rectangular was only manufactured with hatching strategy. To analyse the microstructure of the parts, samples extracted perpendicularly to the base metal were etched during 20 s with Keller (95 ml H 2 O + 2.5 ml HNO 3 + 1.5 ml HCl + 1 ml HF) reagent and analysed in an Olympus Gx51 microscope. Several micrographs were also taken in the same microscope to determine area percentage porosity by ImageJ program. The applied methodology enabled to quantify pores greater than 10 l m in diameter. Four heat treatments after WAAM process were analysed in this study. Two of them have longer aging time and higher temperature, whilst the other two have lower ones (Fig. 5). Hardness tests were carried out throughout the treatment at different aging times (Fig. 5). To do so, an EMCO-TEST Durascan 20 micro-durometer (EMCO-TEST Prüfmaschinen GmbH, Kuchl, Austria) was used. Five measurements were taken from three zones of each sample (bottom zone 5 mm away from the base metal, middle zone and top zone). The dimensions of the straight walls were different to obtain vertical (90 mm 130 mm) and horizontal (130 mm 70 mm) tensile test specimens, whilst for the rectangular geometry, tensile test parts for both loading directions were obtained from the same part (200 mm 130 mm). Three specimens were extracted from each straight wall and six from the rectangular parts (3 vertical tensile test samples and 3 horizontal ones). They were extracted according to the ASTM E8M standard from the core of the walls, avoiding the arc start and end regions (Fig. 6). Table 1 Chemical composition of base and filler metals. Alloy Chemical composition (wt%) Al Cr Cu Fe Mg Mn Si Ti V Zn Zr Be ER2319 Bal. – 6.39 0.115 0.0073 0.274 0.0366 0.136 0.112 0.0095 0.163 0.0001 Al6082-T6 Bal. 0.25 0.1 0.5 0.6 – 1.2 0.4–1 0.7 – 1.3 0.1–– –– Fig. 1. Manufacturing equipment. Fig. 2. WAAM process main components. M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 3 A ZwickRoell Z100 tensile test machine (ZwickRoell GmbH & Co. KG, UlM, Germany) was used to carry out mechanical testing. By testing three samples along two loading directions (vertical and horizontal) anisotropy was studied. 3. Results With the applied welding parameters, walls between 7 and 11 mm thickness were obtained, depending on the geometry and torch movement. The thickest wall was obtained for the rectangular geometry without interpass dwell time. In addition, the torch movement also influenced the straight wall WAAM part’s thickness. As circling uses an oscillation, the resultant thickness was a couple mm wider than the one obtained with the hatching strategy. Thus walls of around 7 mm were obtained in straight wall geometry for hatching deposition strategy, whereas final average thickness was increased to 9 mm for the same geometry but with circling. By modifying welding parameters in the first layer as explained previously, i.e., +4 EP/EN balance, WFS 10 m/min and intensity 130 A, the shrinkage of the welding bead was effective avoided as shown in Fig. 7. This strategy of adapting process parameters in the first layer to compensate enhanced heat sinking effect due to Fig. 3. Torch movement based deposition strategies in the study. Fig. 4. Deposition geometries of the study. Fig. 5. Soak times of aging treatment at which specimens were extracted for microhardness. M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 4 the proximity of the initially cold substrate has also been employed previously [19] and it is something critical in WAAM of aluminium alloys with high thermal conductivity. Other authors recommend to use MIG-P transference mode in the first layer for 2319 alloy to avoid cracking in the interface and avoid reduction of the load-bearing area [17]. 3.1. Microstructure for different deposition strategies Each deposition geometry (rectangular or straight wall) has a different effect on heat input and therefore in the resultant microstructure (Fig. 8). Straight walls had dendritic structure and columnar grains elongated along the vertical direction whereas equiaxed grains were obtained for the rectangular geometry. As the rectangular one is a continuous spiral without welding process interruptions, the arc never extinguishes, and as a result the part cannot cool down as much as in the straight wall geometry, with 90 s dwell time between consecutive layers. Along this time the arc is switched off and the cumulated heat is evacuated by conduction and convection. Therefore, the lack of dwell time in rectangular geometry can be associated with a decrease of the vertical component of the temperature gradient due to thermal conductivity through the previously formed layers to the substrate, which results in equiaxed grain microstructure. In addition, the deposition strategy (hatching or circling) also changes the heat input of the process. As during circling the torch goes backwards in each oscillation, this causes the reheating of the previously deposited metal increasing the heat input and introducing more heat. On the contrary, there is a constant forward movement of the torch with hatching, leading to a reduction of heat input. In this sense, dimensions of horizontal and vertical wall configurations (Fig. 6b and c), significantly influenced resulting microstructure. During WAAM of the horizontal straight wall, less but longer layers were employed compared to the vertical straight wall. Therefore, the arc was continuously ignited during a longer period of time for the horizontal straight wall and the part accumulated more heat resulting in a distinctive grain morphology and microstructure. This effect can be seen in Fig. 8. Vertical samples having more effective heat dissipation and therefore, less heat accumulation, shown larger elongated columnar grains than the ones in the horizontal direction. Based on the analysis of the microstructure, the heat accumulation of each deposition strategy from the lowest to the highest is as follows: straight wall – hatching – vertical, straight wall – circling – vertical, straight wall – hatching – horizontal, straight wall – circling – horizontal and lastly rectangular – hatching. As it has been explained above, because of the lack of dwell time in the rectangular geometry, the vertical temperature gradient through the previously deposited layers will be reduced, favouring the generation of fine equiaxed grains. Fig. 6. (a) Dimensions of tensile specimens in mm. Schematic representation of the specimens in (b) straight wall - vertical, (c) straight wall - horizontal and (d) rectangular parts. Fig. 7. Shrinkage of the first welding bead avoided in by parameter adjustment in 2319 specimen manufactured by WAAM. M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 5 3.2. Porosity The use of different deposition strategies has an impact on the resultant porosity of the parts. It can be seen in the results included in Table 2 that there is a direct relationship between the heat accumulation with the porosity. The reported porosity percentages were determined by measuring the area percentage of pores in several micrographs. Table 2 Deposition strategy effect in the porosity area percentage results in this study. Sample Porosity (area %) ØMax pore ( l m) Straight wall - Hatching - Vertical 0.83 70.12 Straight wall - Circling - Vertical 0.67 60.38 Straight wall - Hatching - Horizontal 0.47 59.19 Straight wall - Circling - Horizontal 0.40 76.48 Rectangular 0.84 92.30 Fig. 8. Microstructure micrographs of each deposition strategy in as-built condition. (1) Straight geometry with Hatching deposition geometry to obtain vertical tensile specimens, (2) Straight geometry with Circling deposition strategy to obtain vertical tensile specimens, (3) Straight geometry with Hatching deposition strategy to obtain horizontal tensile specimens, (4) Straight geometry with Circling deposition strategy to obtain horizontal tensile specimens and (5) Rectangular geometry. (a) Microstructural changes throughout the layers, (b) between layer zone and (c) within layer zone. M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 6 It is worth mentioning that the deposition strategies with most reduced heat accumulation had higher porosity values, and this gradually reduces as the heat accumulation is increased, except for the rectangular process, which has the highest porosity once again (Table 2). However, all the results are below 1 % and the greatest pore is smaller than 100 l m in diameter (Fig. 9). 3.3. Hardness Different heat treatments have been applied to 2319 WAAM walls built with different deposition strategies and geometries to analyse the evolution of the hardness (Fig. 10). As it can be seen, there is an evident effect of aging temperature in the hardness of the samples. Samples aged at 175 °C had a hardness between 138 and 152 HV (Fig. 10 left), whereas samples treated at 190 °C shown lower hardness values from 130 to 143 HV (Fig. 10 right). Anyway, average values of the samples treated at 175 °C (left hand side in Fig. 10) are clearly higher than those of treated at 190 °C (right hand side in Fig. 10). On the contrary, the deposition strategy and geometry did not clearly affect the hardness. 3.4. Tensile test Mechanical properties obtained from tensile test samples machined from WAAM walls treated at different aging conditions are included in Table 3. From these data, one can conclude that the deposition geometry directly affected the mechanical properties. Straight wall samples shown null ductility in vertical direction after short (3–6 h) and low temperature (175 °C) aging treatments. Therefore, one can conclude that this short heat treatment is not able to dissolve the phases precipitated in the grain boundaries (Fig. 11), which in fact are bigger in size than in as-built state. The combination of this phase along grain boundaries with the Fig. 9. Porosity micrographs of the different deposition strategies: (a) Hatching - Vertical, (b) Circling - Vertical, (c) Hatching - Horizontal, (d) Circling - Horizontal and (e) Rectangular. Fig. 10. Microhardness evolution along aging time at 175 °C (left) and 190 °C (right). M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 7 columnar grain structure in vertical direction leads to a premature fracture without reaching plastic deformation. On the contrary, for 26 h at 190 °C, the original phases in the grain boundaries are almost dissolved and the ones remaining are smaller in size and spherical in shape, not limiting elongation results. The strength and elongation of the rectangular geometry – hatching, are significantly higher in comparison with straight walls (up to 4 % higher for yield stress and 5 % for ultimate tensile strength). Moreover, for this particular condition, it is remarkable the fact that there are no differences in yield stress and tensile strength between vertical and horizontal orientations, meaning that strength is very isotropic. Note that this WAAM manufacturing conditions led to fine and equiaxed grains in as-built state (Fig. 8). Samples with these microstructures gave rise to acceptable mechanical properties even when short and low temperature aging treatments were applied. However, a better balance and isotropy was reached for the long and high thermal treatment. 4. Discussion In this study the influence of both deposition strategy (geometry, interpass dwell time and torch movement) and thermal treatment for different 2319 WAAM geometries has been investigated. Table 3 Mechanical properties obtained in this study. Sample PWHT Extraction Yield Stress Tensile Strength Elongation Rp 0.2 (MPa) Rm (MPa) e % Rectangular Hatching 175 °C – 3 h Vertical 272 ± 1 386 ± 6 5.60 ± 0.1 Horizontal 283 ± 1 452 ± 3 15.25 ± 2.7 175 °C – 6 h Vertical 306 ± 4 396 ± 15 3.59 ± 1.1 Horizontal 303 ± 3 434 ± 3 10.16 ± 1.1 190 °C – 18 h Vertical 319 ± 2 414 ± 2 4.15 ± 0.4 Horizontal 322 ± 5 439 ± 4 9.41 ± 0.2 190 °C – 26 h Vertical 321 ± 2 443 ± 10 6.12 ± 2.3 Horizontal 324 ± 3 452 ± 1 8.93 ± 0.8 Straight wall Hatching 175 °C – 3 h Vertical 0 163 ± 13 0.00 Horizontal 287 ± 4 459 ± 14 18.36 ± 0.8 175 °C – 6 h Vertical 0 251 ± 65 0.08 ± 0.1 Horizontal 332 ± 8 470 ± 18 9.10 ± 2.1 190 °C – 26 h Vertical 304 ± 2 405 ± 18 3.09 ± 1.1 Horizontal 321 ± 8 445 ± 5 8.44 ± 0.8 Straight wall Circling 175 °C – 3 h Vertical 0 131 ± 14 0.00 Horizontal 317 ± 10 463 ± 11 14.55 ± 1.3 175 °C – 6 h Vertical 0 150 ± 16 0.00 Horizontal 300 ± 8 436 ± 30 7.83 ± 2.9 190 °C – 26 h Vertical 298 ± 1 415 ± 10 5.45 ± 1.5 Horizontal 322 ± 8 446 ± 5 8.38 ± 0.8 Fig. 11. Microstructure and phase analysis of specimens through heat treatments for straight wall geometry. M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 8 The mechanical properties obtained in this work, are higher than the ones reported in the bibliography for 2319 alloy [7 9 14 26 29 24]. These previous works have associated limited mechanical properties and high anisotropy to the following factors: porosity, microstructure and heat treatment or precipitation state. Regarding porosity, different porosity levels have been reported for this alloy by several authors. Zhang et al. [14] reported porosity up to 0.98 % even for the best welding condition (CMT-PADV), whilst Cong et al. [8] stated that no pores greater than 10 l min diameter were found in a 15 mm representative section of part, 100 mm in length, manufactured with CMT-PADV arc mode, however, no area percentage porosity was given. As for Fang et al. [24], porosity was measured by tomography and a porosity volume of 0.46 mm 3 was obtained. The influence of CMT variant or arc mode in reducing porosity has been deeply investigated [8] for this alloy. Moreover, the effect of shielding gas flow rate [25] and wire surface quality [15] has been studied for Al-Mg alloys. In this study, recommended CMTPADV mode and high flow rates of 30 L/min have been employed and resulting porosity was efficiently reduced to less than 1 %. Moreover, the part with higher porosity (0.84 area %) is the one with greatest mechanical properties and the lowest anisotropy. Many authors have considered that if pores are aligned and concentrated in interlayer regions, this can reduce and limit the plastic strain capability. Therefore, the reduced ductility determined in samples machined along vertical orientation has been usually linked with this aligned porosity issue in aluminium WAAM parts [18]. Aligned pores reduce the effective area and ease stress concentration and crack growing in this region. In this regard, porosity observed in current work was randomly distributed (Fig. 9) and therefore, this ductility limiting factor can be discarded. On the other hand, the microstructural characterisation has demonstrated that resulting microstructure greatly depends on the deposition strategy and geometry. The rectangular geometry deposited by hatching, had equiaxed grains that did not change their size between layers, therefore it was concluded that this sample had a uniform microstructure along its height. The straight wall geometry however, had a dendritic microstructure, with columnar grains in the interlayer zone, irrespectively of the length and the torch movement (hatching or circling). As explained before, the columnar nature of the microstructure was enhanced in shorter straight WAAM wall (vertical). Results have shown that the solution and aging heat treatment did not change the microstructure, in terms of neither grain size nor morphology. In this sense, columnar and vertically elongated grains obtained in straight wall samples kept their shape after full aging treatment. It must be highlighted that segregations resulting from dendritic solidification structures were homogenized after thermal treatment and the grains looked cleaner in comparison with the as-built state (Fig. 12). As Ayarkwa et al. [27] reported, columnar grains results from a unidirectional oriented heat flux during heat transfer from a melt into a cooler solid. Since straight walls shown columnar grains pronounced vertical thermal gradients in the part will explain the preferential growth of grains along this direction. Conversely, the coarse equiaxed grain structure in rectangular geometry would be the result of low temperature gradient and slower cooling rate due to a higher heat accumulation. It is more than clear looking at Table 3 that samples with these equiaxed grains shown more balanced mechanical properties and reduced anisotropy after proper aging thermal treatment. In terms of thermal treatment, the effect of aging temperature and time is evident from the data shown in Table 3. From this study, it is concluded that longer aging times result in an increase in the mechanical properties as compared to work performed on single bead width walls by Gu et al. [7]. These authors [29 7], used a short artificial aging treatment keeping the samples at 175 °C for 3 h after solution treatment at 535 °C during 90 min. As for Ayarkwa et al. [29] used for the same 2319 alloy, an alternative thermal treatment consisting of 12 h aging time at 171 °C with stepped heating. In this case the solution temperature and time were similar. In the current work, comparable short and long heat treatments at 175 °C and 190 °C, being the latest the recommended aging temperature by ASM Handbook [30], were used. The comparison, has enabled to conclude on the effect of the thermal treatment on 2319 WAAM samples manufactured with the same welding parameters. From these results, it has been observed, that long aging times and 190 °C temperature have a beneficial effect on anisotropy reduction, increasing both strength and ductility. Microhardness and tensile test results demonstrated a direct relationship between hardness and elongation capability in the vertical direction against aging time and temperature. Short precipitation times lead to high hardness and low or null elongation. For straight wall geometry, heat treatment further affected the mechanical properties in comparison with rectangular geometry. Short precipitation times during the aging phase led to premature fracture during the tensile test, without reaching plastic deformation or yield stress, while rectangular geometry shown better mechanical properties. In this case, the anisotropy after 175 °C aging temperature was greater than the one obtained with 190 °C(Table 4). On the other hand, for straight wall geometry Fig. 12. Microstructure before and after heat treatment. M. Arana, E. Ukar, I. Rodriguez et al. Materials & Design 221 (2022) 110974 9