# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 3 Hybrid Additive Manufacturing as a Joining Technique and Its Challenges
### Alloy Compatibility
#### 3.1.1 Direct Joining  
[PERSON] et al. [53] applied the direct joining technique on a 316/C18400 alloy using LPBF. Cracks were observed on the stainless steel section close to the interface because the highly conducting Cu diffused into the austenite grain boundaries, resulting in thermal stress inside the matrix. Fused Ion Beam imaging (FIB) revealed an intermix region, approximately 750 \(\upmu\)m in length, with a mixture of fine-grained structures (Figure 8). Inadequate melting caused substantial porosity in the Cu alloy (Figure 8c). This was most likely due to Cu's high reflectivity and heat conductivity at a 300 W laser power. The copper alloy was melted directly on top of the stainless steel, and a large quantity of copper was presumably present in the melt pool, causing micro-cracks to occur during cooling [53].  
LPBF's slow layer-by-layer building strategy produced an excellent metallurgical bond between 316L and Cu10 Sn. In contrast to prior reports, a crack-free interface was obtained [24]. Appropriate melt pool agitation (Marangoni convection) and an equal distribution of alloying components in the heterogeneous alloy phases were acclaimed by [PERSON] et al. [43] for the strong metallurgical bonding. Furthermore, grain refinement and recrystallization improved the bonding performance at the interface with the following operating parameters: a laser power of 400 W and a scanning speed of 500 mm/s [43]. In another work by [PERSON] et al. [54], a 316L/H13 MMAM produced a sound metallurgical bond at the interface with a build platform temperature of 200 \({}^{\circ}\)C. The build temperature was chosen to minimize thermal stresses at the bonding interface. In addition, the good bonding was attributed to the efficient agitation and distribution of elements inside the melt pool. Epitaxial growth at the interface was observed and attributed to the almost similar chemical composition of the two materials. The nickel content in 316L was reported to decrease in each layer scan gradually, and no Ni was detected in the H13 layer. Moreover, the fractography revealed inclusions and pores at the fracture surface on  
Figure 7: Material joining strategies [52].  
the 316L section [54]. In this work, no post-Heat Treatment (HT) was performed under the premise that localized heat treatment occurred during laser scanning. The benefits of joining materials with similar chemistries was evidenced by a robust interface achieved for a the M789-wrought N709 hybrid alloy using LPBF. The interface was reported to be intact in both printed and heat-treated conditions, and no cracks were observed. The interface revealed a fine microstructure with possible precipitation of the ETA-Ni\({}_{3}\)Ti (\(\eta\)-phase) and \(\beta\)-NiAl phase associated with M789 and N709, respectively [47; 55]. In contrast, void nucleation and coalescence were reported at the interface of a Corrax-420 SS hybrid due to mechanical incompatibilities and decohesion [56]. The conclusion from these reports indicated that, with carefully studied elemental diffusion and processing parameters, direct joining techniques are a possible utility for MMAM.