# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 1 Introduction  
Additive manufacturing (AM) techniques adopt layer-by-layer accumulation with the aid of CAD/CAM model to develop three dimensional (3D) products. Based on the ISO/ASTM 52900-15 standard, AM processes include vat photo-polymerization (VP), binder jetting (BJ), material extrusion (ME), material jetting (MJ), sheet lamination (SL), powder bed fusion (PBF), and directed energydeposition (DED) [1]. Compared with traditional manufacturing, AM (abbreviations used in the paper are summarized in Table 1) techniques have the ability to produce complex components with less waste of materials and energy, and shorter processing cycle [2; 3]. To date, AM has successfully processed a variety of metal components [4; 5; 6; 7; 8; 9]. Metal additive manufacturing processes utilize laser beam, electron beam, or arc as the heat source and the feedstock material is in the form of powder, wire, or sheet [1; 10]. AM processes used for metals mainly include powder bed fusion (PBF) [11; 12; 13; 14; 15], directed energy deposition (DED) and sheet lamination (SL) [10]. The commonly used metal AM processes include PBF-L (powder bed fusion utilizing a laser as the heat source) [16; 17; 18; 19; 20; 21; 22; 23; 24; 25], DED-GMA (directed energy deposition with gas metal arc), and DED-L (directed energy deposition using laser beam as the heat source) [26; 27]. According to the material feedstock, commonly used metal AM processes can be classified as powder-based and wire-based systems [28]. Additive manufacturing processes based on metal powders, which mainly use an electron beam or laser as the heat source, are usually confined in a sealed chamber due to the high reactivity of the melt pool and the health and safety issues with the fine powders, which limits the dimension of the parts that can be built. In addition, the cost of the fine powder is much higher than wires and the build rate is rather slow, so that it is only suitable for small to medium sized high value components which require a better resolution [29]. As compared to the powder-based AM process, the wire arc additive manufacturing (WAM) technique, which employs an electric arc as the heat source and the building capacity can be expanded to tens of meters, has the potential to fabricate fully dense and large dimensional parts with relatively low forming costs and high manufacturing efficiencies [30]. In general, the primary cost of metal wire is roughly ten percent of the same weight of metal powder [28]. Besides, WAM machines can be easily adapted from arc welding robots, which are usually much cheaper than L-PBF machines and laser-DED machines. In the WAM process, the wire is heated, then melted and transferred to the melt pool, thereby solidifies at the boundary of the melt pool and forms the designed parts layer by layer [28], as shown in Figure 1[31]. WAM is derived from traditional arc welding technology and belongs to the direct energy deposition (DED) techniques [10]. WAM, also known as shape welding (European name) and structural weld build up (American name), has been in use for a long time [32]. Early in 1926 \"Application of electric arc as the heat source to produce bulk objects by spraying molten metal into the deposited layers\" was patented by [PERSON] [32]. In 1983 [PERSON] employed shape welding to manufacture large-scale products from high strength 20 MnMoNi5 steel of 79 ton weight [32]. WAM (shape welding) was used to manufacture primary nuclear components by the German in the late 20 th century [32]. Up to now, WAM has been applied in aerospace (such as stiffened panels, wing ribs), nuclear energy, marine (such as ship's propeller) and architecture (such as steel bridge) industries [33]. Some commercial WAM machines have been developed, as listed in Table 2. WAM technology has attracted the attention of different research institutions around the world [34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49], as listed in Table 3.  
Figure 1: Diagrammatic sketch of WAM process [31].