In recent decades, additive manufacturing techniques (AM), also referred to as 3D printing or rapid prototyping, have attracted the attention of various industries such as aerospace, automotive, and construction. AM is the process of manufacturing 3-D pieces by adding layerupon-layer of material [1]. The various advantages of AM compared to conventional manufacturing (CM) processes can be discussed in three aspects. First, AM makes it possible to build complex components that are difficult to manufacture by the CM processes  [2]. Second, the AM processes improve the buy-to-fly ratio by reducing the amount of waste material, which reduces the final price of the parts. Third, the AM process can have a significant impact on reducing energy consumption and protecting the environment by reducing both the production time and the weight of parts produced due to new designs or material modifications [3-5]. Wire and Arc Additive Manufacturing (WAAM) refers to a specific group of AM techniques that use an electric arc as the heat source and a metal wire as the feedstock. The WAAM technique uses arc welding processes and more specifically automated arc welding. The three welding methods commonly used in the WAAM technique are Gas Tungsten Arc Welding (GTAW), Plasma Arc Welding (PAW), and Gas Metal Arc Welding (GMAW). Therefore, WAAM technique is divided into three groups, namely GTAW-based WAAM, PAW-based WAAM, and GMAW-based WAAM [6-8]. The main advantage of WAAM over other AM techniques is that its deposition rate is higher, hence WAAM is used to produce large near-net-shape components. Another advantage of WAAM is its lower capital costs compared to other methods [9, 10].

Despite the increasing consumption of aluminum and its alloys in various industries due to its unique properties, such as high strength-to-weight ratio, high ductility, and high durability, most of the research and productions in the WAAM field have focused on the stainless steels, nickel and titanium alloys. The main reason is the gas pores and the coarse dendritic structure formation during the WAAM process, which leads to a severe loss of the mechanical properties of the aluminum alloy components [11, 12].

The research in WAAM field follows two branches to reduce the problems caused by gas pores, dendritic and coarse microstructures, and residual stresses. One area of research is interested in combining other AM techniques with WAAM methods [13-15] or adding equipment such as trailing gas shield [16-18], inter-pass cold rolling [19-21], Peening and ultrasonic impact treatment [16, 22, 23], etc. In other areas, researchers are focusing on improving the quality of WAAM components by improved welding parameters [24-26]. This study focuses on the development of wire and arc additive manufacturing (WAAM) parameters of aluminum alloys. A system consisting of a synergic pulsed GMAW source and a robotic arm was used to perform the WAAM process. The alloy and specimens studied are Al-Si alloy and the thin-walls made by the WAAM process, and each layer was made by depositing ER4043 filler metal.

Additive Manufacturing (AM) or 3D printing is an advanced group of manufacturing processes in which the components are manufactured directly layer by layer. Additive manufacturing enables the production of net-shape or near-net-shape components directly from 3D data and saves time, and tools and production costs [27]. The significant advantage of additive manufacturing versus subtractive manufacturing (traditional manufacturing that involves removing sections of the block of material by machining, milling, etc. for achieving the netshape piece) is the lower buy-to-fly ratio (BTF) [28]. When the topic of discussion is metal additive manufacturing, one of the most commonly used methods is to use a heat source such as an arc, a high-energy electron beam, or a laser beam as a melting source. And a wire of metallic materials is used as feedstock to obtain the designed part by adding a molten wire layer upon layer. The term « Shaped Metal Deposition » (SMD) is commonly used for the mentioned method. Shaped Metal Deposition (SMD) is the name of a fabrication technology presented by Rolls-Royce plc and the University of Sheffield [29]. The main sections of an SMD system consist of: (i) Heat generation unit, which refers to an electric arc, an electron beam or a laser beam unit; (ii) Deposition metal supply unit, which refers to the metal wires and the wire-feed system; (iii) Deposition path movement system, which refers to a welder robot or every system for generating deposition path motion; and (iv) Control movement system, which refers to a control software program for deposition path movement system [30].

Wire and arc additive manufacturing processes 

Wire and Arc Additive Manufacturing (WAAM) is an AM approach that consists of a combination of metal wire feed and arc welding [31]. The type of arc welding process chosen depends on the possibility of its automation. Therefore, Gas Tungsten Arc Welding (GTAW), Plasma Arc Welding (PAW) and Gas Metal Arc Welding (GMAW) are the main options to select as the heat source for the WAAM [32].

The WAAM deposit rate is higher than the other AM methods. For example, Selective Laser Melting (SLM) steel deposition rates reach 0.1 (kg/h), and Laser Metal Deposition (LMD) can reach up to 1 (kg/h) steel deposit, but the steel deposition rate for WAAM technology is 5-6 (kg/h). This advantage leads WAAM to be used to produce large parts [33, 34]. The electric arc has a focused spot radius size in the range of a few millimeters (the focused spot radius for the laser beam is 50 (µm), and the electron beam is 100 (µm)). Hence, the surface roughness of produced pieces by WAAM is high and there is usually a machining step needed after manufacturing by WAAM [35].

GTAW-based WAAM 

Gas Tungsten Arc welding (GTAW) or Tungsten Inert Gas arc welding (TIG) refers to an arc welding technique where an electric arc is formed inside an inert gas atmosphere between a non-consumable Tungsten electrode and the workpiece [68]. When TIG welding is manual, welder uses welding rods as the filler metal.

PAW-based WAAM 

The plasma arc welding (PAW) process is similar to Gas Tungsten Arc Welding (GTAW), with the difference that PAW brings a much higher energy density at the arc, and produces much higher gas velocity and momentum by constraining the arc to flow through a nozzle [71]. The arc energy density in plasma welding can be up to three times that of the GTAW, resulting in less weld distortion and smaller bead welds with higher travel speeds [72]. When using PAW for additive manufacturing, increasing the welding speed can lead to increased production speed. Therefore, it is possible to produce parts with a thinner wall. But the humping phenomenon should not be neglected at high travel speeds [73, 74].

GMAW-based WAAM

Gas Metal Arc Welding (GMAW) is a process in which the electrical arc is the heat source and is formed between a consumable electrode and the workpiece. An inert or active gas stream protects the arc and weld pool during welding [76]. GMAW-based WAAM is very suitable for the production of large parts in a short time due to higher deposition rates [77].