# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 4 Microstructural Characteristics, Mechanical Behavior, and Properties of Hybrid Managing Steels  
The microstructure of an alloy plays a crucial role in determining its functional properties, which can have a significant impact on the material's overall performance. In additive manufacturing, the anisotropy of mechanical properties can be a challenge for applications that experience stresses from multiple directions; thus, making a uniform structure is desirable to achieve isotropic mechanical properties. The contrasting material properties of AM metals compared to conventional metals are mainly due to differences in microstructure characteristics [16]. In the AM process, solidification is influenced by the previously deposited layer and the thermal gradient from the melt pool interior and boundaries, which can impact grain development and, ultimately, the texture of the AM material [16; 82]. Moreover, studies by [PERSON] et al. [83] and [PERSON] et al. [84] using LPBF demonstrated the potential of AM to produce large quantities of retained austenite ranging up to 75%, as evidenced by the mechanical behaviors of the precipitation-hardening steels studied. Such high quantities of retained austenite are not suitable for die and tool applications [47]. Based on the literature data, summarized in Table 3, as-built AM alloys generally possess inferior mechanical properties (i.e., yield strength, ultimate tensile strength, and hardness) compared with their conventional counterparts; the data are associated with the properties of 17-4 PH, including the effect of the processing parameters. The data suggest that, for the PBF process, high laser power values result in inferior mechanical properties, largely because of the retention of a significant amount of austenite, leading to higher ductility at the expense of strength [16]. [PERSON] et al. [85] established that the application of post-heat treatment on 17-4 PH can lead to optimum mechanical properties by transforming the retained austenite to martensite. In their work, they showed that the quantity of retained austenite can decrease from 36% to below 3%. More complex scenarios are presented for dissimilar metals. One such example is the MS1-H13 hybrid produced by Laser Powder Bed Fusion (LPBF) [89], which features a microstructure comprising a large equiaxed grained matrix, oval precipitates on the H13 tool steel side, and ultra-fine martensite grains on the MS1 (LPBF) side. The H13 side of the MS1-H13 material exhibited a constant hardness value of 232 HVN throughout the section, whereas the hardness on the MS1 side was reported to peak at 644 HVN just after the interface (Figure 9), due to the Hall-Petch strengthening mechanism. This increase in hardness is likely attributed to the presence of fine martensite grains, which provide a greater number of grain boundaries and dislocations, which resist the motion of dislocations and enhance the material's strength. In its as-built state, the MS1-H13 material exhibited a lower ultimate tensile strength of 644 MPa, which is 462 MPa less than that of the MS1 side. The failure occurred specifically at the H13 section with a dimple rupture. The lower strength in the H13 counterpart is attributed to the soft annealing heat treatment performed on the conventional H13 steel prior to the material's use in AM [89]. An extension to the previous work [89] involved modifying a similar hybrid material through a heat treatment regime that included Solution Treatment (ST) at 815 \({}^{\circ}\)C and 915 \({}^{\circ}\)C for 15 min each, followed by aging at 490 \({}^{\circ}\)C for 360 min. The resulting material exhibited a uniform hardness averaging 600 HVN across the entire hybrid structure, which was attributed to the coarsening of bath martensite in the MS1 section and solid solution hardening in the H13 steel section [46]. The solution treatment temperatures used minimized the alloy segregation and produced hard martensite after quenching.