1. Introduction
Currently, the additive manufacturing (AM) process is an extensively preferred technique for the development of objectively difficult structures without the use of a mold as it widely expands the manufacturing capability and resilience [
1,
2]. The fabrication and formation of complex three-dimensional (3D) parts are converted into the step-by-step inclusion of lean component layers governed by an automated model using AM techniques [
3]. Heat sources used in the AM of solid structures consist of electric arcs, laser beams, and electron beams. The energy charge is compact and the structural behavior is comparably accurate during the heat source of laser and electron beams [
4]. These two heat source processes of laser and electron beams utilize direct energy deposition and powder bed fusion techniques [
5,
6]. Therefore, metal power as feedstock is essential for these techniques, which in turn limits their production efficiency [
7]. Due to this reason, the production cost of the process increases by limiting the use of laser and electron beams in the fabrication of extensive metallic structures on a larger scale [
8]. Electric arc as a heat source is a promising technique for the fabrication of large-scale intricate metallic structures owing to their high rate of deposition, reduced cost, and minimal wastage rate [
9,
10]. A metal wire is employed as feedstock material in the electric arc method, and its cost relative to metal power for equal weight is very low [
11]. Therefore, the WAAM technique using an electric arc is more suitable than laser and electron beam techniques. Gas metal arc welding (GMAW)-based WAAM is widely preferred due to its capability of fabricating thin multi-layer structures with a lower capital cost, ease of material deposition, and high deposition rate [
12,
13]. However, several challenges arise during the WAAM of metallic structures such as post-processing techniques, reduced surface quality, surface morphology, changes in microstructure, and mechanical properties. This requires additional machining to be performed. Due to the lower wastage of material during WAAM, the entire process still remains economical in comparison with subtractive machining processes [
14]. Appropriate selection of WAAM variables imparts bead geometries with characteristics of multi-layer structures [
15,
16].
The features of the weld geometry and dimensional precision were both affected by the process parameters of WAAM. Construction of a single-layered geometry determines the dimensional precision which is evaluated by its homogeneity and stability [
17]. As a result, suitable design variables to achieve a specified component accuracy are essential and critical for WAAM. Incorrect selection of design variables will result in eminence issues such as partial fusing, hump, and porosity [
18]. Parts with serious flaws will have their mechanical characteristics drastically reduced. Furthermore, past studies often employed design variables that were selected from a specified range [
19]. As a result, adjustment of processing parameters that impact weld bead geometry and quality in the construction of multi-walled components needs to be considered. Thus, it is essential to optimize the design variables of the WAAM process. Optimized parameters give a good quality of properties to the final components. The heat transfer search (HTS) technique was successfully implemented for various manufacturing systems [
20,
21,
22].
Kumar et al. [
23] carried out a parametric study of the GMAW-based WAAM process to manufacture steel structures by employing a copper-coated steel wire. Single-layer deposition was performed by considering the design variables of travel speed (TS), voltage (V), gas flow rate, and current. Bead height (BH) and bead width (BW) were selected as response variables. TS was observed as the highest contributor followed by voltage affecting the BW response with the involvement of 52.29% and 17.08%, respectively. TS followed by voltage was again observed to be the highest contributor affecting the BH response with the involvement of 43% and 15.81%, respectively. For selected responses, the desirability function was utilized as an optimization process. A multi-layer structure was successfully fabricated at optimal combinations of WAAM variables. Dinovitzer et al. [
4] explored the impact of the design variables of WAAM on manufacturing components on an SS 304 substrate by using a metallic wire of Hastelloy X. Taguchi’s technique along with ANOVA was implemented to evaluate the impact of the design variables. Current and TS were observed as the highest contributors affecting the responses. Another study reported by Xiong et al. [
12] explored the impact of GMAW-based WAAM design variables on surface quality. It was observed that lower wire feed speed (WFS) improved the surface quality of the multi-layer structure. A study pertaining to the parametric optimization of the WAAM technique was carried out by Zhao et al. [
19] for the enhancement and better quality of weld bead geometries. Geometrical features of the weld bead structure of the WAAM process are largely dependent on the selection of design variables and their appropriate values. Yuan et al. [
24] established a parabola model to acquire the favorable path geometry and suitable process parameters of the WAAM technique. They concluded that the minimum values of WFS and TS will lead to a higher rate of production. As per the study reported by Kannan and Murugan [
25] and Teixeira et al. [
26], a higher deposition rate with simultaneous dimensional accuracy is largely dependent on the geometric structure of fabricated parts. Cadiou et al. [
27] presented a 3D numerical model of WAAM to acquire the shape of the component as well as its temperature field. In the event of pulsed currents, this model tries to replicate the formation of a 304 SS rod beginning with operational variables. The geometry of the component was predicted by modeling the detachment of deposited metal droplets. Mai et al. [
16] optimized the design variables (current, voltage, and TS) of GMAW-based WAAM for the weld bead geometry of 308L steel. Experimental results show that the required geometry of weld bead was successfully fabricated at optimized parameters. Voltage was observed as the highest contributor followed by TS affecting the BW response with the involvement of 70.18% and 18.54%, respectively. TS followed by the current was observed to be the highest contributor affecting the BH response with the involvement of 48.11% and 38.27%, respectively. The fabricated structure at optimized conditions at a current of 122 A, TS of 368 mm/min, and voltage of 20 V was found to be without the presence of cracks. This shows the suitability of the proper selection of parameters for bead geometries and surface quality. Vora et al. [
28] employed a metaheuristic TLBO algorithm for optimizing the design variables of the GMAW-based WAAM technique to acquire better geometrical weld beads for multi-layer structures. They used 2.25Cr-1.0Mo as a substrate with a metal-cored wire as feedstock. Optimization results yielded successful fabrication of a thin multi-layered structure with the optimal BW of 7 mm and BH of 6.07 mm with the optimal parameter settings as follows: TS of 476 mm/min, voltage of 18 V, and WFS of 5.9 m/min. The multi-layered structure obtained at optimized parameters was found to be free from disbonding, and seamless fusion was detected between the obtained layers of the structure. Thus, the literature demonstrated the necessity of a parametric study for obtaining the desired quality of the multi-layered structure.
The austenitic stainless steel 316L (SS316L) was created over three decades ago for use in fast breeder reactors [
29]. SS 316L is an austenitic stainless-steel grade having a lower carbon content of 0.03% by wt. It is utilized in various industrial applications including marine and offshore applications, biomedical equipment, automobiles, petrochemical facilities, and nuclear reactors owing to its excellent characteristics of superior corrosion resistance, good weldability, high strength and ductility, strong biocompatibility, and comparatively cheap cost [
30,
31,
32]. Studies pertaining to the parametric study of the weld bead geometries of the GMAW-based WAAM process have not been conducted appropriately on SS 316L substrates. The current study focused on the WAAM of 316L stainless steel. As per the studied literature, parametric studies on bead geometries for multi-layer structures employing the GMAW-based WAAM multi-layer structure of SS 316L have not been comprehensively reported. In the current study, we built an experimental platform and then performed WAAM with 316L stainless steel on it with optimized parameters.
In the present study, GMAW-based WAAM was employed to perform bead-on-plate trials on an SS 316L substrate by considering the TS, WFS, and V as design variables, while BH and BW were considered as responses. Multivariable regression equations were generated through results generated from the experimental matrix followed by the BBD approach of RSM. Analysis of variance (ANOVA) was used to investigate the feasibility of the regression equations. The HTS algorithm was employed to obtain the optimal combinations of design variables by considering single- and multi-objective optimization of BH and BW. A multi-layer structure was then fabricated by WAAM at the optimized process parameters. The authors believe that the present study will be beneficial for industrial applications for the fabrication of multi-layer structures.