# The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review
## 2 Materials, Processes and Methods
### Materials
#### 2.4.11 Multi-Material  
Different working groups are conducting research on the combination of different materials in one work piece. Two different approaches can be distinguished here. The first one is an in situ alloying approach by different techniques, and the second one is a metallic composite approach with different objectives (e.g., improving material properties for strength, fatigue, corrosion resistance and wear). In situ alloying can be used for different purposes. Among them is attaining functionally graded structures (e.g., as demonstrated by [PERSON] et al. for tantalum, molybdenum and tungsten [365] or [PERSON] et al. for Fe- FeAl material [366] and others for different materials [358; 367; 368]). One promising approach is the usage of a secondary cold or hot filler material to achieve the desired material properties [296]. [PERSON] et al. used this technique to obtain a CuAl\({}_{6.6}\)Si\({}_{3.2}\) alloy [369]. In situ alloying can be performed by various techniques. There are multiple wire processes and modified wires [60; 295] as well as the addition of powders [268; 370] or the modification of filler by surface coatings [182; 371; 372; 373; 374], and furthermore, in situ alloying can be used to generate functionally graded structures. The second approach can be divided into two subsections. First, there is the area of coatings and functionally graded structures, and secondly, there is the area of internal composite-like metallic structures. The basis for these two approaches is the understanding of the metallurgical mechanism to join these different materials. For stainless and mild steel, some occurring challenges have been given by [PERSON], but the occurring phenomena have been known and studied for mixed material joints for most materials in the past and can be transferred from a metallurgical point of view to additive manufacturing [231]. Functionally graded structures are used to generate locally adapted material properties. This is mostly used in the area of wear or corrosion protection. It has been shown that in the area of corrosion-resistant materials, the building strategy can lead to a significant reduction in necessary and cost-intensive cladding material [375; 52]. This can be accomplished by a building strategy where the corrosion-resistant material is printed first so the dilution is reduced significantly (see Figure 29). Here, four different building strategies of alloy 625 on a low-alloy steel with different dilutions were carried out and tested in \"green death\" to get a correlation between the corrosion resistance and the dilution. It was clearly shown that the building strategies through the dilution had a significant impact on the corrosion resistance [375; 52]. Another approach was carried out by [PERSON] et al. They used a double-wire WAM process to achieve a graded material from titanium to 316L steel with the anticipated brittle intermetallic phases of titanium and iron [376]. Overall, it can be stated that WAAM is suitable for the generation of functionally graded structures and that WAAM can lead to the reduction of cost-intensive materials for corrosion resistance and wear protection due to a dilution-related building strategy. The metal matrix composite-like approach is characterized by generating load-dependent and directed materials for fortification or suiting the integral mechanical properties to the occurring loads. It was shown that layered welding-based manufacturing can realize metal matrix compounds like structures with different fiber types such as carbon fibers (see Figures 30 and 31) [232, 377]. Figure 31: Welded specimen according to [52,232]. Figure 29: Loss over dilution for different building strategies [52]. Here, as an example, a soft matrix of FeNi36 was reinforced with a high-strength low-alloy steel in accordance with DIN EN ISO 16834-A G 69 6 M21 Mn4 Ni1, 5 CrMo. Welding-related problems like hot cracks were not observed.