# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 3 Hybrid Additive Manufacturing as a Joining Technique and Its Challenges  
To achieve a reliable component fabricated by joining processes, a thorough understanding of the interface behavior of the joined materials is necessary. Traditional joining techniques, such as welding or fastening, have design limitations at the joints. It is well known that there can be localized stresses, corrosion, and inferior mechanical properties in the joints, compared to the parent metals. During welding, materials are heated to a suitable  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Hybrid Technique** & **Fe-Based Systems** & **Other Systems** & **References** \\ \hline AM + CNC part/CNC as post-processing & Maraging steels, tool steels, and SS316L. & & [25; 29] \\ AM as a repairing technique & Maraging steels, SS316L, and tool steels. & Al6061, Ti-6 Al-4V; Ni-super-alloys, and brass. & [37; 38] \\ Combined AM processes & SS316L & Ti alloys, resin, polymers (PLA, PET), and ceramics. & [37; 39; 41] \\ Hybrid machine & SS316L & Inconel, PLA, and ABS. & [44; 45] \\ AM process on a conventionally made part & Maraging steels & & [34; 46; 47] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of hybrid solutions.  
Figure 5: Different zones created by WAAM and SLM. Reprinted with permission from [33]. Copyright 2016 Elsevier B.V.  
temperature to combine them. A melt pool is created, which solidifies at the rear of the weld pool to create a fusion zone; then, a Heat-Affected Zone (HAZ) is generated adjacent to the base metal. Both AM and welding require localized heat sources to create melt pools. In addition, both have similar behaviors in reacting to the temperature--steep thermal gradients formed at temperatures above the liquidus temperature of the alloy. The cooling behavior of each material depends on the individual chemical composition and thermal expansion coefficient [16]. Regardless of the techniques used, the joining of dissimilar alloys inherits challenges because the areas in the vicinity of the interface tend to produce inhomogeneous structures [48,49]. On the other hand, similar melting joining techniques also develop heterogenous structures at their interfaces. Constricted Arc Welding (GTCAW) was used to join a maraging 250 steel [50], and the resulting microstructure varied across the similar alloy joint. The weld zone region for the maraging steel joint had fine-grained martensite adjacent to a structured dendritic interface. Further away from the weld zone, the HAZ exhibited a mixture of coarse-grained austenitic-martensitic structures. Similarly, a PH 13-8 Mo stainless steel joint had a coarse weld [50], resulting in an interface similar to the parent material because of a filler material of similar composition. Such inhomogeneous structures result in various material properties, which are usually undesirable for uniformly loaded structural components [23].  
Thermal conductivity differences have been observed in welding to contribute to the lack of fusion in joints, as in AM. Dissipating heat moves faster in the high-conducting material than the low-conducting material, resulting in poor fusion.  
To help future studies on hybrid/MMAM and expand the usefulness of alloy compatibility, interface reaction products and thermal property mismatch need to be well understood [23]. The following sub-sections discuss the effects of these parameters based on previous studies.