Wire + Arc Additive Manufacturing of Metals: State of the Art and Challenges

Introduction

Additive manufacturing (AM) a promising technology in reducing the waste, cost and time involved in the manufacturing of the components. A general AM process system has three parts (a) moving system, (b) energy source and (c) feedstock (Cunningham et al., 2018).

Wire + Arc additive manufacturing (WAAM) processes utilizes the electric arc (energy source) and wire feed stock (for melting) to deposit layer by layer and produce/create the components (Williams et al., 2016). WAAM process is also popularly known as shaped metal deposition (SMD) processes. WAAM is quite useful for producing large scale metallic components or parts in a cost-effective way compared to the conventional additive manufacturing techniques such powder bed fusion (PBF), material Jetting, direct energy deposition (DED), material extrusion, binder jetting and sheet lamination, etc., (Derekar, 2018)

The heat sources used in the WAAM processes should have high heat input. Therefore, conventional welding heat sources such as Metal inert gas welding (MIG), Cold metal transfer welding (CMT), Tungsten inert gas welding (TIG) and Plasma arc welding (PAW) are widely used. The conventional MIG welding setup used for the WAAM process is shown in Figure 1 and commonly referred to as Gas metal arc welding (GMAW). The arc is generated between the electrode and the workpiece. The wire is consumable, and no separate feeding is required. The MIG torch itself feeds the wire into the weld pool. Different modes used in MIG welding are globular transfer mode, short circuiting mode, spray mode and pulsed spray mode as shown in Figure 2. Cold metal transfer welding (CMT) is the modified MIG welding process and has better weld pool control, and high deposition rate at comparatively low heat input (J. L. Z. Li et al., 2019). The conventional CMT welding setup with interfacial rolling attachment (for cold work between each deposited layers) used in the WAAM process is depicted in Figure 3.

Figure 1.

Conventional MIG Welding setup for WAAM process.

Figure 2.

Different modes of transfer of molten metal in MIG welding process.

Tungsten inert gas welding (TIG)- a process in which the electric arc is produced in between the non-consumable (tungsten) electrode and workpiece. TIG is generally termed as Gas Tungsten arc welding (GTAW). The conventional TIG welding setup used for the WAAM process is shown in Figure 4. Plasma arc welding (PAW) – similar to the GTAW process but it generates an electric arc which is three times higher than TIG process and the heating zone was quite narrow compared to TIG. PAW is widely used for deposition of smaller components (thin components). When the current in the PAW was less than 30A it is called as micro-plasma arc welding (MPAW) (Rodrigues et al., 2019). The conventional PAW welding setup used for the WAAM process is shown in Figure 5. Unlike in the MIG/CMT process, the wire is externally feed using the feed stock devices for TIG and PAW processes (for consistent deposition). The deposition layers produced in the WAAM processes for example a height of 1-2mm has a roughness of 500µm with single track deposit (Bekker et al., 2016). WAAM process is termed as a near net shaped processes because of the poor surface finish and therefore secondary operations like machining is essential for WAAM components.