# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 6 Conclusions  
In this review, we presented the use of hybrid additive manufacturing and its progress over the past decade. The term hybrid additive manufacturing refers to processes that are used in conjunction with an additive manufacturing process in situ or post-printing to improve the quality of the final product or the production process. This review identified several processes as given in the literature with a focus on the use of AM as a joining technique for metals. Direct joining, gradient joining, and inter-layer joining were identified as the main joining techniques that produce interfaces with unique chemistry and mechanical properties. Poor bonding at the interface was a major concern, and the robustness of the interface was found to be governed by alloy compatibility, interface reaction products, and thermal conductivity. Solubility between metals is crucial to avoid segregation due to a lack of solubility. Furthermore, mismatched thermal coefficients and chemical compositions result in accumulated stresses and detrimental phases at the interface, respectively. These parameters contribute to the lack of fusion in joints and premature failure. Hybrid AM was identified as an alternative low-cost production option that can be achieved in the AM of maraging steels by printing on a low-cost substrate material. This reduces the quantity of printable material. In addition, dissimilar metal AM offers customized functionality to meet specific design requirements. However, careful design of the post-processing heat treatment is necessary to ensure good metallurgical bonding at the interface, as well as for the individual materials. Based on the literature surveyed, there is lack of a systematic way of optimizing the heat treatments of the dissimilar metals and their interfaces. The heat treatments are specific for individual materials and often compromise the strength of the interface. The growth of hybrid additive manufacturing for steels depends on the development of competent and specific heat treatment protocols, which are to meet the growing demand for multimaterials in industry. It is evident that multimaterials are crucial for productivity and efficiency, and hybrid AM is a promising field. There is a need for more understanding of the potential afforded by hybrid additive manufacturing.  
Conceptualization, K.N. and C.A.J.; methodology, K.N. and C.A.J.; validation, C.A.J., R.P., Y.T. and J.P.; formal analysis, K.N., C.A.J., R.P. and Y.T.; investigation, K.N.; resources, C.A.J.; data curation, K.N.; writing--original draft preparation, K.N.; writing--review and editing, K.N., C.A.J., R.P., Y.T. and J.P.; visualization, K.N., C.A.J., R.P., Y.T. and J.P.; supervision, C.A.J.; project administration, C.A.J.; funding acquisition, C.A.J. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) (ALLRP 560425-20, ALLRP 571059-21, RGPIN 04006), Canada Foundation for Innovation (CFI) (38944), Atlantic Canada Opportunities (220835), and the New Brunswick Innovation Foundation (NBIF) (EP2021-005, RAI2021-016).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data are available upon request due to restrictions, e.g., privacy or ethical.  
**Acknowledgments:** The authors acknowledge with gratitude the funding received from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), Atlantic Canada Opportunities and the New Brunswick Innovation Foundation (NBIF).  
**Conflicts of Interest:** The authors declare no conflict of interest.