# 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
For instance, for the direct joining technique of 316/C18400, cracking occurred due to the differences in the thermal conductivity between Fe and Cu. Copper has four-times greater thermal conductivity than iron, and this difference yields different cooling rates, resulting in higher thermal stresses within the hybrid [78]. The same was observed for the 1.2709/CuCr1 Zr, 18 Ni300/CuSn combinations. Surprisingly, for 316L/Cu10 Sn, no cracking was reported [43] post-printing, and this was attributed to recrystallization refinement, better melt pool dynamics, and heterogeneity in the alloy phases. Considering Al12 Si/Al3.5 Cu1.5 Mg1 Si, poor powder melting was reported, possibly because of a low volumetric energy density together with the presence of copper, which has a high reflectivity. The presence of Al\({}_{2}\)Cu precipitates interrupted the continuous cellular microstructure, ultimately producing low hardness values (<125 HV0.05) in these regions compared with the Al\({}_{2}\)Cu-precipitate-free zones (>130 HV0.05) [70]. The use of the gradient method becomes relevant in such cases to avoid the precipitation of undesirable phases by gradually adjusting the composition of one more element (316L/MS1). However, as observed from Fe/Al12 Si, macro-cracks were observed due to the formation of immiscible regions. For specific compositions, miscibility gaps are formed as a result of spinodal decomposition, promoting segregation and mechanical mixtures [80]. Moreover, the gradient method is not suited for most LPBF processes because of limited powder delivery techniques. The use of filler material is \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Joining Method** & **Materials** & **Observations** & **References** \\ \hline \multirow{8}{*}{Direct sharp interface} & \multirow{3}{*}{MS1/H13} & Brittle intermetallic phase Al\({}_{2}\)Cu was & \multirow{3}{*}{[42]} \\  & & produced with crack formation due to & \\  & & non-uniform CTE. & \\  & & Fair metallurgical bonding at the interface with & \\  & 316L/H13 & improved material intermixing inside the & \\  & & melt pool. & \\  & & HT potentially removed microstructural & \\  & & inhomogeneity, but did not achieve & \\  & & microstructural evolution at the interface. & \\  & & Failure was away from the interface for both & \\  & & as-printed and heat-treated conditions. & \\  & & Fracture at the built AM layer was caused by & \\  & & void expansion. & \\  & & MS1/wrought H13 & Fracture at the interface due to chemical and \\  & & microstructural inhomogeneity. & \\ \hline \multirow{8}{*}{Gradient} & \multirow{3}{*}{[73]} & \multirow{3}{*}{[74]} \\  & & & \\  & & & no visible crack formation. & \\  & & & Composition of W above 20\% promoted the & \\  & & formation of brittle Ti\({}_{2}\)Ni intermetallic phase. & \\  & & Fair metallurgical bonding between the two & \\  & & materials with a presence of unmelted & \\  & & & \\  & & 316L/Cu10 Sn & 316L powder. & \\  & & Materials were immiscible in most sections, & \\  & & and macro-cracks were observed. & \\  & & Good metallurgical bonding due to improved & \\  & & & scan strategy and reduced hatch distance, & \\  & & which improved the melting of In718 powders. & \\  & & Significant portions of unmelted Invar36 due & \\  & & Invar36/Cu10 Sn & to low laser absorptivity and high thermal & \\  & & conductivity of Cu alloy. & \\ \hline \multirow{2}{*}{Intermediate section} & \multirow{2}{*}{Ti6 Al4V/316L} & Copper alloy interlayer insert avoided the & \\  & & intermetallic phase formation & \\  & & between Ti6 Al4V/316. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._an alternative to avoid detrimental phases; however, the design of the filler material is a costly process. The importance of chemical similarity can be observed in gradient joining in M789/N709. These two steels are both precipitation-hardening steels with comparable thermal conductivities and compositions. The interface formed was robust, and failure was located away from its interface in both the as-printed and heat-treated state [55; 72].