Wire arc additive manufacturing (WAAM) is an innovative 3D printing technology for producing metal components, including those made from aluminum alloys. This process involves the deposition of material layer-by-layer, utilizing an electric arc to melt and fuse metal wire onto a substrate or previous layers. WAAM has gained popularity in recent years due to its ability to fabricate large-scale, complex, and high-performance parts with relatively lower costs compared to traditional manufacturing methods [1]. In addition, due to the high deposition rate and reduced manufacturing cost, WAAM is gaining popularity worldwide [2]. The various heat sources used for the WAAM process are plasma arc welding (PAW) [3], gas tungsten arc welding (GTAW) [4] and gas metal arc welding (GMAW) [5].

There has been significant interest in the production of aluminum alloys through the WAAM process [6]. Consequently, WAAM of aluminum alloys, including aluminum-copper (Al-Cu) [7], aluminum-magnesium (Al-Mg) [8], aluminum-silicon (Al-Si) [1], and aluminum-magnesium-silicon (Al-Mg-Si) [9] has been the subject of extensive research. Nevertheless, a major obstacle in the commercialization of WAAM for fabricating aluminum alloy components is the presence of substantial porosity [7]. Porosity is a significant problem in the WAAM process because it can cause a decrease in the strength of the deposited structure [10]. Another significant problem related to the WAAM process of aluminum alloys is heat accumulation due to layer-by-layer deposition. Excessive heat accumulation is significant since an increased inter-pass temperature causes delayed melt pool solidification, reducing deposit height and increasing width [11]. This bead dimension effect is an undesirable property in an additive manufacturing process, which requires the geometry of the deposition to be as close to the net shape as possible. Derekar et al. [12] have argued that a higher inter-pass temperature is even desired when producing aluminum alloy components, leading to reduced porosity. Hence, understanding the temperature distribution during the WAAM deposition process is crucial.

In addition, thermal cycling causes dynamic changes in the substrate and previously deposited layers [13]. The WAAM deposition heating and cooling rates affect phase development in the deposited material, which affects mechanical properties [12]. Miao et al. [14] stated that the Si segregation in the samples affects the microhardness of aluminum alloy 4043 (ER4043). The temperature-mechanical properties relationship of the WAAM process is crucial, but there is limited published data on it, notably for ER4043. Therefore, the objectives of this study are to measure the heating and cooling cycle during ER4043 deposition on Aluminum Alloy 6061 using GMAW for the WAAM process and relate the temperature distribution to the microhardness variations of the ER4043 deposited structure.

The experimental investigation was carried out with a robot-supported setup, as shown in Figure 1. The essential components for the WAAM process were an EWM 352 alpha Q GMAW heat source attached to a six-degree-freedom KUKA KR 5 robotic system. They were set to cold arc, an energy-reduced arc mode. The wire feeding system was an EWM M drive 4 Rob 2, and the shielding gas was pure argon. The 1.2 mm diameter ER4043 wire and 150 × 50 × 20 mm (L × W × H) ER6061 plate were used as feeding material and substrate, respectively. The substrate was fixed on a jig attached to a welding table for the additive buildup process. The nominal composition of ER4043 and ER6061 in wt.% is outlined in Table 1.

Figure 1.An EWM 352 alpha Q GMAW heat source attached to KUKA KR 5 with EWM M drive 4 Rob 2 wire feeding system.

Table 1.The nominal composition of ER4043 and ER6061 (wt.%) [14].

The WAAM deposition parameters were based on an optimised set reported by Hussein et al. [5]. The welding current, arc voltage and travel speed were set to 140 A, 18.5 V and 300 mm/min, respectively. The dwell time between layers was 15 seconds, and deposition alternated between the start and stop points. Type-K thermocouples with a data logger were placed at designated locations, as in Figure 2. The thermocouples were placed at 10 mm, 50 mm, and 105 mm from the substrate to capture the temperature distribution throughout the height of the ER4043-deposited wall during the WAAM process. The placement depths were carefully chosen to monitor thermal behavior at different stages of the build: near the substrate (10 mm) to observe initial heat input and early thermal cycles, mid-height (50 mm) to assess heat accumulation and transition in heat flow, and near the top (105 mm) to capture peak temperatures and reduced heat dissipation as the structure grew taller. Holes were drilled to fit in the thermocouples after the location was reached. To ensure accurate temperature readings and minimise air-gap errors, holes were drilled at the designated locations to securely insert the thermocouples. These holes were designed to be within the wall rather than through-holes, which helped reduce atmospheric air circulation around the probes and improved thermal contact. To confirm the accuracy of the recorded temperature profiles during the WAAM process, a validation step was conducted using a calibrated optical pyrometer. The pyrometer was used to perform spot measurements at the surface of the deposited layers at selected stages of the build, particularly during stages when peak temperatures were expected. The deposition was performed to obtain a final thin wall measuring 180 mm in length and 120 mm in height. The average width of the thin wall was obtained at an average of 12 mm.

Figure 2.Thermocouple locations for temperature measurement points of the built structure from the substrate.

Metallography procedures were employed on the samples, which were cut to 25 mm each, cold-mounted in resin, polished, and etched with Keller’s reagent. The microhardness test was performed along the deposited structure using a Shimadzu HMV-G Vickers microhardness testing machine. The indentations were performed with a 500-gram (i.e., 5N) load for 15 seconds. The indentations were spaced 150 µm apart. This is the standard practice for Vickers microhardness testing to ensure accurate and unbiased results.

The microstructural observation and analysis were performed using a Scanning Electron Microscope (SEM) Zeiss Evo 50 and an Energy-Dispersive X-Ray (EDX) spectrometer.

The heating rate, cooling rate, Si-rich intermetallic phase, and porosity are the dominant factors affecting the microhardness of the ER4043-deposited wall. This research shows the temperature-time profile of the new layer reached 263˚C. After 42 layers, the temperature of the beneath layers did not exceed 200˚C. These temperature profiles suggest that the heat flow behaviour has changed substantially as the wall structure has been built higher above the substrate, with a decreased rate of heat loss. However, the results were compromised by the thermocouple's instability within the drilled hole. The peak temperature at 105 mm was 25˚C higher than at 50 mm, indicating that heat accumulation increased as the deposited structure was constructed higher. The heating rate decreases by 2˚C/sec, and the cooling rate decreases by 0.85˚C/sec due to heat accumulation, which reduces heat conduction.

Despite the measured temperature being lower than the actual temperature, the material still melted. It reached the critical temperature for phase transformation. The critical temperature causes partial or complete dissolution of Si precipitation in the HAZ, leading to a decrease in microhardness from 121.2 HV to 52.2 HV. Si precipitations were observed at the interdendritic regions. Areas containing porosity are weaker than the surrounding material, reducing the microhardness to 32.8 HV. The most important finding of this study is that dynamic temperature fluctuations during the WAAM process alter the phase transformation of the ER4043-deposited material, thereby affecting microhardness variations. This research improves process optimisation and control by revealing the significant impact of heat accumulation on WAAM deposition.

The authors would like to thank the Faculty of Industrial and Manufacturing Technology and Engineering at Universiti Teknikal Malaysia Melaka (UTeM) for financial, educational, and technical support throughout this research. This research is funded by a grant from INDUSTRI (URMG)/TPGSB/2024/FTKIP/I00090.