# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 1 Introduction
[19] performed studies on Ti-6 Al-4V and IN 718 using the DED--L process, and both affirmed that metallurgical defects act as stress concentrators, which lower fatigue endurance. For the LPBF process, internal pore defects are categorized into Lack Of Fusion (LOF) and gas defects. Lack of fusion defects are often found in the vicinity of spatter particles in between the laser track and powder layers. This is associated with insufficient volumetric  
Figure 1: The classification of additive manufacturing technologies [5; 9]. energy density and poor powder spreading [20; 21]. On the other hand, gas pores include keyhole pores, which are caused by instable melt pools, powder feedstock, and hydrogen pores, which arise from trapped gas and precipitating hydrogen during processing, respectively [21]. Moreover, studies by [PERSON] et al. [22] showed the importance of the surface quality for the mechanical properties. In situ laser polishing of the IN 718 alloy produced by selective laser melting reduced the surface roughness by about 34%. Laser polishing has the capability of reducing the surface cracks of alloys, fuse surface pores, and melt surface residual powder. The scalability and mass production of these M-AM continue to be a challenge, and these limitations for M-AM pose significant drawbacks in its adoption and application. Combining traditional manufacturing techniques (milling, casting, etc.) and M-AM could potentially provide new manufacturing solutions relevant to mass production, improved geometrical collaboration, accuracy, and damage repair using net-shape construction [4; 13]. From this point forward, the fusion of traditional manufacturing and M-AM will be referred to as hybrid manufacturing or Multimaterial Additive Manufacturing (MMAM) throughout the document. Hybrid manufacturing is relatively in its infancy, and its governing mechanisms are still subjects of great interest. The demand for multi-service superior alloys with multiple properties is rising to suit lightweight functionality, corrosive environments, high-temperature applications, and prolonged exposure to radiation [23]. Additive manufacturing shows a great promise as an alternative to traditional joining techniques for different materials. Inevitably, complexities arise in these hybrid solutions because of incompatibilities, which may arise due to complex thermal histories, which involve directional heat extraction, varying coefficients of thermal expansion (CTE), repeated melting, and fast localized solidification for the considered metals [7]. In addition, the mechanical behavior of the interface is crucial to developing sound MMAM to avoid premature failure of such components. Components manufactured by metal joining processes are prone to premature failure due to the presence of joints and interfaces between different materials [24]. This behavior can be attributed to the nature of the compositional gradient at the interface. This review paper explored the types of processes and complexities involved in hybrid manufacturing encompassing joining techniques and combinations with an emphasis on the utilization of AM as a potential joining method for dissimilar metals, in particular, managing steels. Notably, there has been limited research on joining dissimilar materials, especially of this material type. The discussion will also cover other alloys including stainless steel, Cu-based alloys, and Ni-based steels. Subsequent sections will evaluate different hybrid techniques and their respective performances.