1. Introduction
High-strength aluminum alloys play an important role in the fabrication of structural components for aircraft, such as skins, wing beams, and wing ribs, owing to their advantages of low density, high strength, and good machinability [
1,
2]. The performance requirements of the aircraft materials are shown in
Figure 1 [
3]. The development of modern aviation industry towards large-scale, integrated, and lightweight aircraft structural parts demands a departure from traditional processing methods, such as casting and forging, due to their drawbacks of prolonged cycles and high costs [
4,
5,
6,
7,
8]. Recognizing these limitations, additive manufacturing has emerged as a transformative approach overcoming traditional technology constraints [
9,
10,
11]. Combining additive manufacturing with topology optimization enables the integration of materials, structures, processes and performance, with significant advantages for improving aerospace lightweighting, particularly in the lightweighting of redundant actuation systems for large aircraft [
12].
The primary techniques for aluminum alloy additive manufacturing include laser additive manufacturing (LAM) and wire arc additive manufacturing (WAAM) [
13,
14,
15]. LAM offers high forming accuracy but suffers from low production efficiency and excessive laser energy loss [
16,
17]. In contrast, WAAM exhibits high productivity but faces challenges such as poor forming accuracy and numerous defects [
18,
19,
20]. Combining the advantages of both techniques, laser–arc hybrid additive manufacturing (LAHAM) integrates a laser and an electric arc. The plasma generated by the laser reduces the resistance of the arc channel, thereby increasing the utilization rate of the arc energy. Simultaneously, the plasma dilutes the arc acting on the surface of the molten pool, enhancing the absorption of the laser. Consequently, the LAHAM offers high forming efficiency and good forming quality [
21,
22,
23,
24]. CMT, a spatter-free process developed by Fronius based on metal inert gas (MIG) welding, was later integrated with pulsed MIG to create CMT-P, a hybrid transition alternating between short-circuit and pulsed transition [
25]. The CMT-P process, applicable to various materials, including aluminum alloys, nickel-based alloys, and others, expands the adjustable range of heat input, allowing for precise control over the droplet transition behavior [
26]. Therefore, LCAHAM has emerged as a favorable method for manufacturing aluminum alloy structural parts, holding significant potential for further exploration in the production of high-strength aluminum alloys.
Despite these advancements, the control of the forming quality remains a major challenge hindering the industrialization and large-scale production of metal additive manufacturing. In the actual production process of high-strength aluminum alloy additive manufacturing, issues like low dimensional accuracy and high defects, such as spatter and porosity, persist due to the immaturity of the additive process [
27,
28]. Improving the forming accuracy can effectively reduce machining allowance and improve efficiency. Reducing spatter is crucial to prevent defects that can significantly impact the additive part’s performance [
29]. Additionally, porosity defects lead to stress concentration and induce corrosion, causing a decline in thin-walled parts’ performance [
30]. Therefore, effectively controlling spatter, minimizing porosity defects, and enhancing formation precision are significant concerns for researchers in the additive manufacturing process.
In the LAHAM process of high-strength aluminum alloys, spatter behavior is more complex. Scholars have carried out extensive studies on the spatter characteristics formation mechanism, and the influence of process parameters. Wu et al. [
31] and Zhang et al. [
32] comprehensively investigated the spatter formation mechanism during the laser welding of aluminum alloys, identifying metal evaporation recoil pressure and shear stress from metallic vapor flow as primary factors. Ahmad et al. [
33] studied spatter particle variation during selective laser melting by tracking the trajectories of these particles using Discrete Phase Mode. It was found that the inert gas flow affects the spatter particle trajectories. Zhang et al. [
34] noted differences in the spatter quantity among CMT, pulsed MIG, and both of these hybrid welding processes. In addition, Zhang et al. [
35] also indicated that the spatter during the laser–arc hybrid welding process was significantly less than that during the pure arc welding process. Han et al. [
36] analyzed the spatter in a laser–pulsed MIG hybrid welding and laser–CMT hybrid welding process, finding less or no spattering during laser–CMT hybrid welding.
Porosity defects in the LAHAM process are generated due to the unstable keyhole tip and rapid solidification rate. Scholars have studied porosity in aluminum alloy additive manufacturing parts, demonstrating effective reduction through optimizing additive manufacturing process parameters. Derekar et al. [
37] explored two varieties of WAAM, pulsed MIG and CMT, and found that the pulsed MIG samples showed more pores and a higher volume fraction of porosity than samples manufactured using the CMT. Cong et al. [
38] studied the influence of various CMT modes on porosity in Al-6.3%Cu alloy, discovering that pulsed current in CMT-P increased porosity compared to conventional CMT. Wang et al. [
39] found that pulse frequency and arc current had a significant impact on porosity. Liu et al. [
40] used LAHAM for a thin-wall aluminum alloy, observing a decrease in porosity defects with increased laser power. Meanwhile, Zhang et al. [
41] employed response surface methodology (RSM) to optimize CMT-P arc additive manufacturing aluminum alloy, revealing wire feeding speed’s significant impact on porosity and surface roughness.
To enhance the additive manufacturing quality and mitigate issues such as spatter and porosity defects, an LCAHAM technology was introduced. It is noteworthy that process parameters play a pivotal role in enhancing the formation quality of high-strength aluminum alloys in additive manufacturing. Although there are limited studies on optimizing the LAHAM process for aluminum alloys, research on LCAHAM remains scarce. Additionally, the interaction effects between LCAHAM process parameters and the forming quality of aluminum alloys are not yet clearly understood. Therefore, this study seeks to establish a correlation regulation between the LCAHAM process parameters (including wire feeding speed, scanning speed, and laser power) and the spattering degree, forming accuracy, and porosity using the RSM.
4. Conclusions
(1) A thin-walled structure of high-strength AA2024, characterized by a crack-free, superior forming accuracy, minimal spattering, and a low porosity rate, was successfully fabricated using the innovative LCAHAM technology in combination with RSM, probability statistical theory, and a multi-objective optimization algorithm.
(2) The forming accuracy of LCAHAM AA2024 was significantly more affected by the laser power than by the wire feeding speed and scanning speed. Notably, there was an obvious correlation between the interaction of the laser power and wire feeding speed and the resulting formation accuracy of LCAHAM AA2024. Conversely, the interactions between the laser power and scanning speed, as well as between the wire feeding speed and scanning speed, did not significantly affect the forming accuracy.
(3) The laser power, wire feeding speed, and scanning speed all had noticeable effects on the spattering degree during the AA2024 LCAHAM process. However, the influence of the laser power significantly surpassed that of the other two factors. As the laser power and wire feeding speed increased, the spattering intensified; conversely, an increase in the scanning speed initially led to a decrease and subsequently an increase in the spattering extent. Notably, these three factors exhibited minimal mutual interaction on spattering.
(4) The most significant factor influencing the porosity of LCAHAM AA2024 was the scanning speed when compared with the wire feeding speed and laser power. The porosity exhibited an initial decrease followed by an increase as the scanning speed ranged from 12 mm/s to 20 mm/s. It was crucial to emphasize that the combined effects of the wire feed speed and laser power played an obvious role in reducing the porosity.
(5) Considering the forming accuracy, spattering degree, and porosity comprehensively, the recommended optimal process parameters were as follows: a wire feed speed ranging from 4.2 to 4.3 m/min, a scanning speed between 15 and 17 mm/s, and a laser power set at approximately 2000 W. Under these conditions, the forming accuracy was 84–85%, the spatter level fell within 1.0–1.2%, and the porosity was 0.7–0.9%.