Mechanical Properties Analysis of WAAM Produced Wall Made from 6063 Alloy Using AC MIG Process

Abstract

Wire and arc additive manufacturing (WAAM) is a promising method of producing medium- and large-sized aluminum alloy structures, though it faces challenges such as porosity, residual stresses and inconsistent mechanical properties. This study investigates the effect of current type (AC and DC MIG welding) and polarity balance (influencing the duration of the positive/negative period of the cycle) on the mechanical and microstructural properties of 6063 aluminum alloy walls produced by WAAM. A TiB₂-refined Al–Mg–Si (6063) filler wire, specifically developed for arc-based processing, was used. Tensile tests, Vickers hardness measurements (HV5), optical microscopy and X-ray diffraction based on cosα method were used to evaluate performance in terms of strength, ductility, hardness, grain structure, porosity and residual stress. The results showed that the balance of AC polarity significantly affects wall geometry, porosity and grain structure. Increasing the negative polarity period resulted in taller and narrower walls, while the widest walls were produced with increased positive polarity. Residual stress measurements revealed a tensile–compressive–tensile distribution, with the DC-MIG samples showing the highest surface stress values. The highest tensile strength (172 MPa) was measured in the lower region of the DC-MIG sample, suggesting that areas near the substrate benefit from faster cooling.

1. Introduction

Wire and arc additive manufacturing (WAAM) is a type of additive manufacturing that works according to the principle of wire arc direct energy deposition (WA-DED). In this process, a metal wire is used as feedstock and melted by an energy source, usually an electric arc, but possibly also a plasma arc, laser or electron beam, and then deposited layer by layer to build up a three-dimensional structure [1]. The additive manufacturing of large-volume components makes WAAM one of the most efficient direct energy deposition (DED) processes when it comes to achieving a low buy-to-fly (BTF) ratio, resulting in significant cost savings [2]. Therefore, WAAM is especially beneficial for producing expensive and high-performance materials such as titanium and nickel alloys, high-strength steels and aluminum alloys, which are normally difficult to machine [3].

This study deals with the limited existing research on WAAM of 6063 alloys, especially using AC MIG with different EN ratios. By evaluating mechanical and microstructural behavior under controlled polarity balance, this research aims to identify process–property correlations and optimize manufacturing quality for potential structural applications.

Aluminum alloys have several desirable properties, including low density, high specific strength, good thermal and electrical conductivity and excellent corrosion resistance and formability. When processed using WAAM, aluminum alloys can exhibit mechanical properties that are comparable to or even better than their wrought counterparts [4,5]. However, despite these advantages, WAAM-produced aluminum components often suffer from process-related defects that limit their wider industrial application. These challenges are primarily related to process stability such as path planning, parameter optimization and melt pool shielding as well as alloy chemistry [6]. The most critical problems with aluminum WAAM are the formation of porosity, residual stress and deformation and solidification cracking [7].

Porosity in WAAM is caused by various factors such as the arc welding process, process parameters, interlayer temperature, wire quality and alloy composition and can be exacerbated in multilayer build-ups by the heat input from the subsequent layers, which promotes pore growth [8]. Pores can reduce mechanical properties such as yield strength, contribute to anisotropy and reduce tensile strength, while the choice of deposition strategy, the type of shielding gas and the gas flow rate have a direct influence on porosity growth [9].

The potential to reduce process-related defects in WAAM can be achieved by using modified GMAW techniques [10]. A significant advance is the development of the Cold Metal Transfer (CMT) process, which minimizes weld spatter by short-circuiting the arc through controlled dipping of the welding wire into the weld pool. This technique has been effectively applied to various aluminum alloys and has evolved into several improved variants, including CMT-P (pulsed), CMT-ADV (alternating polarity) and CMT-PADV (a combination of both), all of which contribute to providing better arc strike behavior and reliability, lower heat input and reduce the porosity and a refined microstructure [11].

An alternative solution that counteracts the cost and complexity of CMT is the implementation of the pulsed variable polarity GMAW (VP-GMAW) process, which provides comparable or better deposition efficiency at a lower process cost. This technique has demonstrated stable arc behavior, high deposition efficiency (up to 82.56%) and isotropic mechanical properties, making it a promising candidate for aluminum WAAM [12].

By comparing pulsed AC, pulsed, and CMT modes, it can be concluded that pulsed AC mode (with variable polarity) offers a significant improvement in porosity control, achieving porosity values up to ten times lower than conventional pulsed GMAW. Although the average grain size in pulsed AC mode was slightly higher, it maintained a favorable and uniform microstructure with a balanced distribution of equiaxed and columnar grains, contributing to improved material homogeneity and mechanical performance [13]. Furthermore, image-based symmetry analysis has demonstrated that pulsed AC deposition yields the best bead symmetry, further supporting its suitability for high-quality and consistent WAAM applications [14]. Grain size in WAAM is influenced by the applied power and polarity change frequency, with a higher EN pulse rate promoting a predominant equiaxed structure, reducing columnar growth, improving structural uniformity and increasing deposition weight [15].

Solidification cracks in aluminum alloys during the WAAM process occur within the liquid melt pool and is primarily caused by a wide solidification range, a columnar dendritic grain structure and the inability of the residual liquid to compensate for shrinkage-induced stresses near the solidus temperature [16]. An important prerequisite for the successful use of Al–Mg–Si alloys in WAAM is overcoming their susceptibility to such hot cracking. One of the most effective measures to prevent hot cracking is grain refinement. A promising approach to achieve this is the use of an advanced Al–Mg–Si alloy inoculated with a TiB₂-containing structural refiner [17]. As a result, this modified alloy exhibits no macroscopic cracking in the deposited material. The resulting microstructure is characterized by fine, equiaxed grains smaller than 30 μm. These microstructural features are attributed to preferential heterogeneous nucleation, which reduces the tendency for epitaxial grain growth [18].

Due to its widespread use in structural applications and its favorable response to heat treatment and grain refinement, making it a promising and cost-effective candidate for WAAM technologies, 6063 is selected. To further enhance its processability for additive manufacturing, a TiB₂-refined Al–Mg–Si (6063) filler wire specifically developed for WAAM was used in this study, providing improved grain refinement, reduced cracking susceptibility and better microstructural control.

The high susceptibility of 6000 series aluminum alloys to cracking during solidification poses a challenge for additive manufacturing. However, the use of a TiB-alloyed AlMg0.7Si welding wire (S Al 6063-TiB) enabled the production of aluminum parts with fine, equiaxed grains and low porosity, effectively avoiding solidification cracks. A key advantage of the 6000 series alloys is their ability to undergo heat treatment, which significantly improves mechanical properties. After heat treatment, both the transverse and longitudinal specimens exhibited yield strength, tensile strength and ductility that matched or exceeded those of AW 6063-T6 in wrought form, demonstrating the alloy’s great potential for high performance in WAAM applications [19].

Residual stresses in aluminum components produced by WAAM are primarily caused by uneven heating and cooling during solidification, resulting in forced thermal contraction [20]. These stresses are usually tensile stresses that reduce fatigue strength and potentially lead to deformation if they exceed the yield strength of the material [21,22]. Residual stress in WAAM components is strongly influenced by clamping conditions, the path planning strategy, welding parameters, the interlayer temperature and the component geometry, whereby longitudinal stresses (stresses in the welding direction) generally predominate along the build-up direction. These stresses are commonly measured using techniques such as neutron diffraction, X-ray diffraction (XRD) and hole-drilling [23]. During WAAM production of aluminum alloys, significant residual tensile stresses can occur, especially near the substrate and along the build direction, that approach the yield strength of the material, highlighting the need for stress reduction strategies such as process modifications or post-processing treatments [24].

Mechanical tensile tests, in particular the analysis of stress–strain behavior, provide crucial data for the evaluation of fatigue resistance and fracture performance—key aspects for the structural reliability of WAAM-manufactured parts.

Despite numerous studies on the WAAM process for aluminum alloys, the mechanical performance of 6000 series alloys produced by pulsed AC-MIG welding has been investigated only to a limited extent. This study investigates the mechanical properties of alloy 6063 produced by WAAM using pulsed AC-MIG with different electrode-negative (EN) ratios with the aim of evaluating its suitability for structural applications.

2. Materials and Methods

A Fronius FlexTrack 45 PRO Rail-guided welding carriage and OTC Daihen WB-W400 AC-MIG welding power supply were employed for wire and arc additive manufacturing (WAAM) of four samples. In this study, a 6063 aluminum alloy welding wire, an advanced Al–Mg–Si alloy (tailored to improve processability using a TiB₂-containing structural refiner) with a diameter of 1.0 mm, was selected as the deposition material (chemical composition is provided in

Table 1. Chemical composition of the filler wire and base metal (wt.%).

Composition	Si	Fe	Cu	Mn	Mg	Cr	Zn	Be	Ti	B	Other	Al
6063	0.2–0.6	<0.35	<0.1	<0.1	0.45–0.9	<0.1	<0.1	<0.0003	0.2–0.3	0.04–0.06	<0.05	balance

). As the substrate material, a 5083 aluminum alloy with dimensions of 300 mm × 70 mm × 8 mm was used, while the total length of the fabricated samples was approximately 250 mm.

Prior to welding, the substrate surface was cleaned with alcohol to remove any surface contamination that could compromise the integrity of the weld metal. The selected deposition parameters, along with the corresponding sample designations, are presented in

Table 2. Welding parameters used for sample processing.

Sample	Current, A	Voltage, V	EN Ratio	Wire Feed Rate, m/min	Heat Input, kJ/mm
11	170	19.5	50	18	2.09
12	170	22.6	0	17.2	2.43
13	170	22.9	−50	15.2	2.46
14	170	21.8	DC	15.4	2.34

. The interpass temperature was maintained at 100 °C. To ensure a constant interpass temperature, the time between individual layers was adjusted, with temperature measurements taken using an infrared thermometer (Fluke 568 Infrared Thermometer, Fluke Corporation, Everett, WA, USA). The contact tip-to-workpiece distance was fixed at 10 mm, and the welding speed was set to 76 cm/min. Argon (Ar ≥ 99.996%, Group I1 according to EN ISO 14175) was used as the shielding gas, with a constant flow rate of 19.5 L/min. All samples were fabricated with 50 overlapping layers using a zig-zag method, with the starting position for each subsequent layer placed at the opposite side of the previously deposited wall. To investigate the effects of polarity balance while maintaining process stability, the current was kept constant during all experiments. Accordingly, the wire feed rate and other parameters were adjusted to ensure arc stability.

Sample 11 represents an aluminum wall fabricated using an alternating current (AC) with an electrode-negative (EN) ratio of 50. This indicates that, within a single cycle, the duration of the negative polarity is maximized, while the positive polarity is minimized. In contrast, Sample 13 was also fabricated using an AC, but with an EN ratio of −50, signifying a cycle in which the negative polarity duration is minimized, and the positive polarity is maximized. Sample 12 was produced using an EN ratio of 0, corresponding to a conventional AC waveform, where the durations of the positive and negative polarities are equal within each cycle.

Adjustments to the EN ratio inherently modify the welding voltage and wire feed speed, thereby contributing to improved process stability and weld quality. Sample 14 was fabricated using a direct current (DC). Based on the sample height data presented in Table 2, it may be concluded that an increased duration of the negative polarity within the pulse cycle correlates with reduced heat input and enhanced material deposition.

Upon completion of the welding process, residual stress measurements were conducted on both the upper and lateral surfaces of the samples using the cosα X-ray diffraction method [25,26]. The measurements were performed using the Pulstec μ-X360 system (Pulstec Industrial Co., Ltd., Hamamatsu, Japan).

Tensile test specimens were extracted from the additively manufactured thin-walled structures, as shown in Figure 1. All specimens had identical dimensions of 175 mm in length, 22 mm in width and 5 mm in thickness, with all geometrical parameters defined and prepared in accordance with the EN ISO 6892-1 standard. All specimens were oriented horizontally. Specimens labeled “A” were obtained from the upper region of the aluminum walls, whereas those labeled “B” were extracted from the lower region. The start and end sections of the welding process were cut off and discarded and excluded from testing.

Tensile testing was conducted using the Aramis optical measurement system, which enables high-precision displacement tracking and surface deformation analysis through the use of cameras, LED illumination and dedicated software (Figure 2). This system effectively replaces conventional devices such as extensometers and strain gauges and can be readily integrated with existing testing infrastructure.

Metallographic specimens were prepared from the central section of the tensile samples for macrostructural analysis. The samples were mechanically polished and chemically etched using Keller’s reagent. Vickers hardness measurements were performed across various layers of the deposited structure (from the bottom to the top), applying a test force of 49.03 N (HV 5) with indentation time of 10 s.

3. Results and Discussion

3.1. Macrostructure Characterization

The results of the basic macrostructural characterization of the as-built samples (11, 12, 13 and 14) are presented in

Table 3. Macroscopic characterization.

Sample	Sample Width, mm	Sample Height, mm
11	9.1	92.3
12	10.6	71.6
13	12.1	57.1
14	12.5	64.5

, while images of the as-produced samples are shown in Figure 3. Analyzing the values obtained from measurements of the height and width of the samples produced with an AC, a linear correlation with the AC balance settings is evident. When the AC balance is set to 50 (the longest negative period), the highest and narrowest walls are produced. Conversely, when the AC balance is set to −50 (the longest positive period), walls with the smallest height and the greatest width are produced.

These results are directly correlated with the heat distribution during the welding process. With the balance set to 50 and the longest negative period, heat is more directed toward the filler material, increasing the melting rate of the wire and resulting in a higher deposition rate. Consequently, in the same period of time, higher and narrower walls can be produced compared to those fabricated using an AC with a balance of 0 or −50.

Additionally, when comparing the sample produced using a DC to those produced with an AC, it can be observed that the width is greater, while the height falls between the values observed for the AC with balance set to −50 and 0. The width is largest when using a DC due to the heat distribution being concentrated on the wall, which increases the fluidity of the weld pool. As a result of the enhanced fluidity of the weld pool and the stability of the electric arc, the profile of the sample produced with a DC is significantly more uniform. When analyzing the homogeneity of the AC-produced samples, it is evident that the sample produced with the balance set to −50 has the fewest pores, and these pores are the smallest in diameter. This is also a direct consequence of the heat distribution; in this case, the heat is more concentrated on the wall, leading to a slower cooling rate. This allows gas bubbles to migrate through the molten pool more effectively.

These results are consistent with those of Lee et al. [15], who showed that an increased electrode-negative pulse ratio leads to higher and narrower deposits with lower porosity. The improved geometry was primarily attributed to a more stable metal transfer, which enables uniform and efficient material deposition.

3.2. Microstructure Characterization

The microstructural analysis of the samples was conducted using an Olympus GX51 light microscope with a depth of field of 0.2 µm. The microstructural observations were carried out on macrosections made from cross-sections of the walls produced with WAAM. The observations were carried out at several points, particularly in the middle and upper areas of the walls, in order to determine representative microstructural features and the presence of specific defects. Figure 4a illustrates the microstructure of aluminum alloy 6063 in sample 11, which was produced using WAAM. The grains are predominantly regular in shape, polygonal, and uniform in size. Within the deposited layer, a uniform and homogeneous microstructure is observed, characterized by clearly defined aluminum grain boundaries along which intermetallic phases (precipitates) have formed. This suggests favorable and stable thermal conditions during solidification. Among the typical defects associated with WAAM processing of aluminum alloys, only gas pores of varying sizes were observed.

Similar grain morphology and refinement trends were also observed in studies by Klein et al. [18] and Winterkorn et al. [19], who investigated WAAM processing using a structurally refined Al–Mg–Si filler wire and reported fine equiaxed grains with minimal columnar growth, supporting the effectiveness of TiB₂ additions in promoting a uniform microstructure.

Figure 4b shows three successive layers of sample 11, each exhibiting differences in grain size and morphology. These variations are attributed to the partial recrystallization of the previous layer, induced by repeated thermal cycling during deposition. The distinct separation between the layers indicates local thermal gradients and varying solidification conditions. Some porosity is also observed, likely caused by gas entrapment during solidification. This, along with the microstructural inhomogeneity, may influence the overall mechanical performance of the part.

Figure 4c presents the microstructure of sample 12, which shares similarities with the features observed in sample 11. The solidification patterns seen can be attributed to the heat dissipation trajectories resulting from subsequent thermal passes, suggesting directional solidification influenced by localized thermal gradients.

In the Al 6063 alloy, the central aluminum matrix has a higher melting point than the intermetallic precipitates, which normally form at the grain boundaries. During the WAAM process, additional segregation and precipitation of these phases occur at the boundaries of the individual deposition layers due to multi-layer welding (Figure 4d,f).

Figure 4e displays the microstructure of sample 14, produced via DC-MIG welding. A high density of medium-sized, rounded pores is clearly visible. The average pore diameter ranges from 30 to 50 µm, with some pores locally exceeding this range. The microstructure is characterized by a predominance of equiaxed grains, indicating relatively uniform solidification conditions. However, compared to the microstructures obtained during AC-MIG welding, this structure appears coarser, suggesting a different heat input and cooling behavior during DC-MIG deposition.

Figure 5 shows a comparative microstructural analysis of four different WAAM-fabricated samples (samples 11 to 14) at a higher magnification. All samples exhibit predominantly equiaxed grain structures with visible grain boundaries and varying densities of pores or inclusions. The samples produced with AC polarity show a clear trend: sample 11 has clean grain boundaries, fine equiaxed grains and minimal porosity. Sample 12 (balanced) has slightly coarser grains with occasional gas porosity. Sample 13 shows coarse grains, higher porosity and sometimes irregular grain boundaries. In contrast, sample 14, produced with DC MIG, displays the coarsest microstructure, with numerous gas pores the size of the grains, incomplete fusion and uneven solidification.

Higher EN polarity directs more heat into the filler wire rather than the base material, resulting in a higher melt rate but lower net heat input into the wall. This promotes faster cooling and solidification, which in turn favors grain refinement and the formation of equiaxed microstructures. The observed refinement of grains with increasing EN ratio is consistent with previously reported results [15], where higher negative polarity promoted directional solidification and favored the formation of equiaxed grains.

3.3. Hardness

Each sample was produced in 50 layers, with hardness measurements taken approximately every two layers. The hardness distribution of the samples under different processing parameters using AC welding current is shown in Figure 6. Testing was performed on the sample along the path represented by the dotted line, as indicated in the chart. The arrow above the sample represents the direction of testing. Along the mid-width line of the samples, the average hardness values are 48.9 HV5 for sample 11, 51.7 HV5 for sample 12 and 50.3 HV5 for sample 13.

When comparing the extreme average hardness values, it is evident that the maximum hardness is achieved when using an AC with the balance set to 0 (equal duration of the positive and negative periods in one cycle), while the minimum hardness is observed when using an AC with the balance set to 50. The difference in these extreme values is 5.3%, suggesting that variations in the balance of polarity distribution during the cycle do not significantly influence the overall hardness of the WAAM-AC MIG-produced 6063 aluminum alloy walls. Due to the relatively high standard deviations (± 5.03 for sample 12, ±4.75 for sample 13 and ±3.83 for sample 11), the overlap in variability also means that the differences between the samples are not statistically significant. Therefore, although it can be assumed that sample 12 has the highest average hardness, this conclusion should be interpreted with caution. It can also be concluded that the maximum hardness values are achieved when using AC with a balance set to 0.

Along the mid-height line of the samples, the average hardness values are 47.8 HV5 at the middle and 50.6 HV5 at the top of sample 11, 50.7 HV5 at the middle and 58.5 HV5 at the top of sample 12 and 49.9 HV5 at the middle and 54.8 HV5 at the top of sample 13. As observed from all obtained values, a linear increase in hardness is evident from the bottom towards the top of all samples produced using an AC, with no significant influence of the polarity balance.

3.4. Tensile Properties

Tensile specimens were extracted from different samples, all oriented horizontally to the deposited build. Figure 7 presents the stress–strain curves for all samples, while

Table 4. Comparison of tensile test results.

Sample	UTS, MPa	ε, /	YS, MPa	Sample	UTS, MPa	ε, /	YS, MPa
11A	135.72	0.103	99.93	11B	164.84	0.204	70.06
12A	151.42	0.182	70.03	12B	151.43	0.173	69.95
13A	145.83	0.173	69.98	13B	161.53	0.172	69.90
14A	140.42	0.154	69.90	14B	171.99	0.221	70.02

provides a comparison of the ultimate tensile strength (UTS), yield strength (YS) and elongation (ε) at room temperature for the different samples. The narrow ranges of UTS and YS (excluding the value for sample 11A) indicate stable strength properties of the deposited material. In contrast, the comparatively broader range of elongation suggests lower stability in ductility properties relative to strength. Overall, the maximum UTS values were obtained in samples extracted from the bottom part of the WAAM-produced wall (designated as “B” samples).

Due to slippage during gripping, sample 11B exhibited an unusually high initial stress of about 40 MPa, in contrast to the other specimens, which started near zero. This probably affected the accuracy of the yield strength. However, the ultimate tensile strength (UTS) is still considered reliable as the fracture occurred within the gauge length, indicating that the sample experienced proper deformation and failure despite the initial slippage.

In the WAAM production process, cooling and heating rates are not uniform along the build-up direction. Samples taken from the bottom region, near the substrate, experience rapid cooling (compared to samples extracted from the upper region of the wall) as layers are deposited onto the cooled substrate. This cooling effect promotes fine grain formation, leading to increased strength (up to 172 MPa) in the bottom region. Additionally, the subsequent “heat treatment” from the electric arc of the following deposited layers positively affects the mechanical properties.

The effect of heat input and distribution can also be observed when analyzing the UTS value of sample 14B. Although the heat input is almost the lowest, the distribution of heat is primarily directed to the deposited layer, which acts as a heat treatment for the produced layers and ultimately plays a beneficial role in improving mechanical properties.

The mechanical properties achieved in this study—particularly the ultimate tensile strength (UTS) of up to 172 MPa—are in good agreement with the results of Winterkorn et al. [19], who also investigated WAAM-manufactured components using a TiB₂-modified Al–Mg–Si filler wire. Their study yielded comparable UTS and yield strength values, confirming that the use of structurally refined filler wire enables stable and competitive mechanical performance in WAAM-produced aluminum alloys.

Among the tested samples, those with higher porosity also tended to have lower UTS values. This can be explained by the fact that increased porosity leads to stress concentrations that act as crack initiation points under tensile loading. This reduces the effective load-bearing cross-section and thus leads to a measurable reduction in ultimate tensile strength. This correlation reinforces the importance of controlling the process parameters, not only for the geometry but also for the mechanical performance.

3.5. Residual Stresses

After the additive manufacturing of the samples was completed and cooled to ambient temperature, residual stresses were measured using X-ray diffraction based on the cosα method. The measurements were performed using the Pulstec μ-X360 device (Pulstec Industrial Co., Ltd.), as shown in Figure 8. The measurements of longitudinal residual stresses (i.e., in the direction of welding) were conducted on the last layer at the top of the samples, as well as along line A-B (Figure 8). The measurements on the last layer were taken immediately after sample preparation, while the measurements along line A-B were performed after cutting the substrate from the sample due to the geometrical limitations of the measurement equipment. For the last layer, the distance between measurement points was 20 mm, while on the centerline, the distance was 5 mm.

For the last layer, the highest normal residual stresses were observed in sample 14, produced with DC-MIG, amounting to −110 MPa in compression and 80 MPa in tension. The residual stresses in the samples obtained with AC-MIG were slightly lower, with maximum tensile stresses of 55 MPa in sample 12 and compressive stresses of approximately −80 MPa in sample 13. The obtained results correlate with findings from the literature [27], showing slightly higher tensile stresses but significantly lower compressive stresses. This discrepancy is attributed to the fact that in the referenced literature, the stresses were measured after 20 layers, while in our study, they were measured after 50 layers.

The distribution of residual stress along line A-B, as presented in Figure 9, reveals a tensile–compressive–tensile pattern extending from the top layers of the deposit down to the substrate. This pattern results from the complex thermal cycling and mechanical constraints inherent in the WAAM process.

The overall distribution indicates that tensile residual stresses occur in the middle of the deposition layers and near the top, while compressive stresses are intermittently present in certain areas, particularly at the ends of the last deposition layer in samples 11 and 12. Notably, sample 11 exhibits the highest tensile residual stress of approximately 40 MPa, located several layers below the top surface.

Among the analyzed samples, sample 11 not only achieved the greatest height with the same number of layers but also exhibited the highest residual stress values among the samples produced by AC-MIG. Furthermore, the frequency of transitions between tensile and compressive stresses becomes more pronounced as the height of the structure increases. Sample 14, produced with DC-MIG, also exhibited tensile stresses of around 40 MPa.

Overall, the determined residual stress distributions and magnitudes are consistent with the results reported in the literature [28], confirming the expected stress distributions in WAAM-manufactured components of aluminum alloy 6063.

4. Conclusions

In the present work, the mechanical properties and microstructural characteristics of WAAM walls made of aluminum alloy 6063 using the AC-MIG process were investigated. The results show that polarity balance significantly influences wall geometry, porosity and grain structure. A higher electrode-negative (EN) ratio (AC balance 50) resulted in taller and narrower walls with reduced porosity, while a higher positive polarity (AC balance −50) resulted in wider walls. Microstructural analysis revealed predominantly equiaxed and polygonal grains with minimal defects, with gas pores being the only typical WAAM-related defects. Although the DC-MIG samples exhibited slightly coarser grains, they also showed a higher density of gas pores. The highest average hardness was measured in samples produced with balanced polarity (EN = 0), but due to the relatively high standard deviations, the observed differences were not statistically significant. The tensile test showed stable mechanical performance for all samples, with the highest tensile strength (172 MPa) measured in the lower region of the DC-MIG sample, which is probably due to rapid cooling near the substrate. Residual stress analysis showed that the DC-MIG samples had the highest surface stress values, while the AC-MIG samples showed lower stresses with a characteristic tensile–compressive–tensile pattern. Among the AC-MIG samples, sample 11—which reached the highest build-up height—also exhibited the highest tensile residual stress, suggesting a correlation between structure height and residual stress accumulation.