# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 3 Hybrid Additive Manufacturing as a Joining Technique and Its Challenges
### Alloy Compatibility
#### 3.1.3 Intermediate Layer Joining  
Researchers have proposed an intermediate layer technique commonly used in welding to eliminate the precipitation of undesirable phases. Investigations on electron and laser beam welding [60], diffusion bonding [61], and friction welding [62] have been reported to have used inter-layer metals, which form continuous solid solutions with the base materials. The filler material must have matched thermal-physical properties and sufficient thickness to avoid intermixing of the materials involved [23]. Early work by [PERSON] et al. [63] investigated the use of LPBF by directly depositing Ti-6 Al-4V on SS410. The resulting delamination prompted the use of NiCr as an intermediate. This way, the precipitation of detrimental intermetallic phases (Cr\({}_{2}\)Ti, FeTi, and Fe\({}_{2}\)Ti) was avoided. A sound metallurgical bond was formed without delamination and cracks [63]. The choice of an intermediate is critical to prevent the formation of a detrimental phase between the intermediate section and the FGMs, as reported by [PERSON] et al. [64]. The use of V as an intermediate in a Ti-6 Al-4V/SS340L FGM resulted in the formation of FeTi and FeV phases, which actually promoted failure at the interface [64]. [PERSON] et al. [65] modified the Ti-6 Al-4V/SS316L FGM system and used a V/Cr multi-material intermediate instead. This achieved good solubility between the Fe and Ti-containing base materials [65]. Later work by [PERSON] et al. [66] also reported successful bonding of a Ti-6 Al-4V/Inconel 718 FGM with a VC compositional bond layer, which minimized the formation of the Ti\({}_{2}\)Ni and TiNi\({}_{3}\) brittle phases [66]. Not every intermetallic phase is detrimental. Some are preferable, such as the TiAl intermetallic phase with favorable lightweight characteristics [66]. Clearly, it is necessary to understand the design of intermediate FGM materials and how they enhance the properties of a joint. However, this inevitably results in more costs in production. Optimization studies and models for intermediate joining techniques must be developed to predict the resultant phases and determine suitable intermediate materials for joining incompatible metals [52]. Table 2 identifies the discussed findings in the literature on the use of LPBF to combine dissimilar metals using the direct sharp interface, gradient, and interlayer methods. The observations of the interface performances are highlighted. Based on these observations, it can be concluded that, despite the joining technique employed, the performance of each hybrid is unique based on the material used. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Joining Method** & **Materials** & **Observations** & **References** \\ \hline \multirow{8}{*}{Direct sharp interface} & \multirow{8}{*}{316L/C1840} & Sound metallurgical bonding at the interface; \multirow{8}{*}{316L/C1840} & \multirow{8}{*}{[42]} \\  & & however, cracks were evident because Cu \\  & & diffused into the austenitic grain boundaries. & \\  & & As a result, low ultimate strength & \\  & & was achieved. & \\  & & Crack-free interface with a strong & \\  & & metallurgical bond. & \\  & & Al\({}_{2}\)Cu intermetallic phases were observed & \\  & & together with cracks due to non-uniform & \\  & & thermal expansion. & \\  & & A 20\% of mm powder overlap was achieved, & \\  & & but cracks and pores were unavoidable. & \\  & & Cross-finger interface produced improved the & \\  & & horizontal bond strength. & \\  & & Some Al12 Si powders were unmelted, and & \\  & & Al\({}_{2}\)Cu intermetallic phase was precipitated. & \\  & & A good metallurgical bond was achieved & \\  & & regardless of HT. The interfacial bond strength & \\  & & was higher than the Ti-5 Al-2.5 Sn section. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the behavior of dissimilar joints. The degree of compatibility determines the best-suited type of joining.