# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.2 Aluminum  
Aluminum is one of the most widely used lightweight construction materials. Therefore, the desire to use aluminum in additive manufacturing to further increase the lightweight potential of additively manufactured structures is even more pronounced. Aluminum also has access to a wide range of additive materials with known properties. In addition, the defects that occur during the welding processing of aluminum have been thoroughly studied. Different working groups are focusing their studies on the usage of aluminum alloys for wire arc additive manufacturing [46; 55; 58; 72; 79; 97; 140; 170; 249; 250; 251; 252; 253; 254; 255; 256; 257; 258; 259; 260; 261; 262; 263; 264; 265; 266; 267; 268; 269; 262; 267; 269; 270; 263; 265; 266; 267; 268; 269; 271; 272; 273; 274; 275; 276; 277; 278; 279; 280; 281; 282]. A wide range of different aluminum alloys is being investigated. It has been shown that the used substrate for additive manufacturing can be preformed [251]. For aluminum alloys, knowledge of the correlations between the welding process variables and the achievable build-up heights and widths is also of decisive importance. [PERSON] et al. showed this for different welding speeds. In addition, they showed that the layer width also depends on the number of layers [253]. In the case of aluminum, due to the relatively high coefficient of thermal expansion, the consideration of residual stresses and distortion is of decisive interest for geometrically accurate production. It turns out that residual stresses can occur up to the yield point. These residual stresses are distributed anisotropically over the component and with dependence on the direction of assembly. They can be significantly reduced by mechanical post-treatment to reduce residual stresses, such as rolling or higher-frequency hammering processes. In particular, rolling also leads to a significant reduction in distortion [256]. The surface quality of the manufactured components can be significantly improved for aluminum alloys by adjusting the process edge sizes, particularly the pulse repetition frequency and the polarity [259]. The formation of pores is one critical point in the welding of aluminum and occurs due to the solvability gap of hydrogen in molten and solidified aluminum [283]. Aside from the hydrogen explanation for pore formation, other hypotheses show that a wider range of gases are responsible for pore formation, such as the hydrocarbon hypothesis [284]. Hydrogen-induced pore formation is no new effect for welding aluminum and does not occur only for WAAM [285]. Nevertheless, control of the porosity is crucial for usable WAAM-made aluminum parts [283]. There are different approaches for controlling the porosity in welding and WAAM as well. One new and promising approach is the usage of a hyperbaric process environment. Here, pore formation can be significantly reduced to nearly zero percent porosity when a pressure of more than 10 bar is applied in a manufacturing chamber [285]. Aside from an increased ambient pressure process, side modifications have been studied to reduce the porosity. It has been shown that an increased interlayer temperature leads to decreased porosity [286]. This supports the general assumption that improved outgassing due to delayed solidification of the melt can lead to reduced porosity of the weld but contradicts the general assumption that slower cooling and a hotter melt absorbs more hydrogen, which leads to greater porosity [287]. The influence of the interlayer temperature and the mode of arc welding (e.g., pulsed or controlled short arc) has been studied by [PERSON] et al. They showed that the effect of the interlayer temperature depends on the welding mode [287]. Therefore, a general assumption on the influence of the interlayer temperature cannot be drawn. Furthermore, they showed that the welding mode itself has an influence on pore formation as well. They stated that an impulse welding mode leads to more pores than controlled short arc processes.