# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 4 Microstructural Characteristics, Mechanical Behavior, and Properties of Hybrid Managing Steels
[34] employed a combination of solution treatment and aging to modify the properties of a similar MS1-H13 hybrid material, as demonstrated in Figure 10. The resulting microstructure consisted of a coarse-grained H13 substrate and a fine-grained MS1 layer (Figure 11). The solution treatment involved heat treatment up to 1030 \({}^{\circ}\)C, followed by quenching, which caused the H13 alloy to revert to austenite and form a hard martensitic structure. The MS1 layer, on the other hand, experienced a reduction in hardness due to austenite formation. Aging at 650 \({}^{\circ}\)C for 60 min further enhanced precipitate formation and resulted in a maximum hardness of 420 HVN for MS1 and 520 HVN for H13, respectively. In the as-built state, the H13 layer had a lower hardness of 220 HVN than MS1, which averaged 420 HVN. The solution treatment improved the hardness of H13 to about 600 HVN and lowered the hardness of the AM layer, which is in agreement with the findings of [PERSON] et al. [46] and [PERSON] et al. [85]. Interestingly, this finding contradicts previous reports suggesting the formation of austenite, which typically has lower mechanical properties [16]. In the case of the H13 alloy, it contained approximately 0.40% C, which resulted in the formation of a hard and brittle martensitic structure after undergoing solution treatment. During tensile testing of the MS1-H13 hybrid material, the fracture occurred at a stress and fracture strain of approximately 1100 MPa and 11%, respectively. Before the fracture, a considerable amount of strain was concentrated in the dissimilar joint. The inhomogeneous microstructure resulted in strain concentration in the dissimilar joint. In conclusion, the impact of solution treatment on the two alloys varied. It caused H13 to revert to austenite and form a hard martensitic structure upon cooling. After solution treatment, MS1 experienced a reduction in hardness due to austenite formation while enhancing precipitate formation (Figure 12) [34]. Figure 10: Heat treatment profile for MS1–H13 hybrid. AQ: Air Quench [26]. The properties of an MS1-C300 hybrid material were compared in another study, where the C300 substrate was employed and the MS1 was deposited using LPBF. The individual hardness values of the MS1 and C300 after aging were found to be similar to the MS1-C300 hybrid material, with a peak value of 450 HVN achieved after solution treatment at 850 \({}^{\circ}\)C for 90 min, followed by aging at 650 \({}^{\circ}\)C for 10 min. During aging at 490 \({}^{\circ}\)C, it was observed that the hardness value continued to increase steadily beyond 67 h of aging time. The peak hardness was not determined, indicating that no over-aging had occurred. Mechanical testing of the MS1-C300 material after aging at 650 \({}^{\circ}\)C revealed a fracture in the MS1 portion with an ultimate tensile strength of 1200 MPa, which was 900 MPa higher than that of the individual C300 alloy aged at 490 \({}^{\circ}\)C. The optimal temperature for good mechanical properties was found to be 650 \({}^{\circ}\)C, which increased the hybrid material's ductility to 10% due to a loss of strength at higher temperatures. The failure in the MS1 section was attributed to about a 10% lower flow stress and initial porosity, which facilitated void expansion and coalescence. Pore defects acted as stress concentration points, which exacerbated failure [21]. The strain was localized in the MS1 region after the peak strength was reached [34]. The studies mentioned above have shown that the primary objective of hybrid materials is to attain maximum hardness after adequate aging of the printed alloy and tempering  
Figure 11: Microstructure variation between wrought H13 and printed MS1. Reprinted with permission from [26]. Copyright 2019 Elsevier B.V.  
Figure 12: Hardness variation at different conditions. Reprinted with permission from [26]. Copyright 2019 Elsevier B.V.  
of the conventional material to achieve uniform properties.