# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 1 Introduction  
Additive Manufacturing (AM), an advanced technology, has gained prominence due to its focus on enhancing production efficiency by reducing energy consumption and minimizing carbon footprints [1; 2]. This process utilizes layer-by-layer material deposition in conjunction with a Computer-Aided Design (CAD) model to create 3D objects. Additive manufacturing has been applied in the fabrication of polymeric, ceramic, composite, and metallic materials [3]. Multiple techniques have been developed for AM [4; 5], as shown in Figure 1.  
Vat photo-polymerization, material extrusion, and material jetting are mainly used for producing polymer-based products with UV-curable polymers, thermoplastic polymers, and jetted polymeric material, which is then UV-curable. Techniques labelled M (in red) in Figure 1 are used for Metal Additive Manufacturing (M-AM) or metal printing. In contrast to traditional manufacturing methods, M-AM has enabled the rapid development of intricate and functionally diverse geometries in metallic components used in various service parts [6; 7]. In M-AM processes, various material building approaches are employed. For instance, in the sheet lamination process, metal sheets are accurately cut and then bonded together using metallurgical techniques such as laser welding, diffusion bonding, and resistance welding to create a three-dimensional product. In binder jetting, a binder deposited onto the metal powder consolidates, sinters, or infiltrates another metal into theprinted product. Techniques such as Direct Energy Deposition (DED) and Laser Powder Bed Fusion (LPBF) utilize a focused energy source, such as high-energy electrons or laser, to precisely sinter or melt a layer of powders. Among these techniques, LPBF is commonly preferred in the AM community in the production of nickel, titanium, copper-based alloys, and managing steel components because of its smoother surface finish compared to DED. Moreover, the relative density of parts fabricated by means of LPBF is comparable to that of conventionally produced parts [8]. Metal additive manufacturing processes offer design freedom, which is limited in conventional processes [3]. Complex designs such as topologically optimized cooling channels and consolidated multiple components enable reduced lead and cycle times. Consolidated multiple components refers to the merging of multiple components into one build system and enables the removal of joining surfaces and structures. Consolidating multiple components in a single build has the potential to reduce fabrication and joining costs. In addition, the design freedom offered by AM allows for an optimized material distribution and lightweighting whilst maintaining the minimum mechanical requirements and performance requirements [10]. Metal AM also allows the efficient repair of worn-out parts and saving inventory storage space. Parts can be prepared as per the need or in situ, allowing for an extended component life and efficient servicing of heavy and expensive components such as turbine blades [11]. Despite the novelty of M-AM processes, they have limitations when compared to conventional techniques, specifically in terms of mass production, building rate, build size, and surface quality, as highlighted by [PERSON] et al. [12] and [PERSON] et al. [13]. Service parts produced by the M-AM process often have rough and stair-cased surface finishes, in addition to internal residual stresses and part distortion. [PERSON] et al. [14] reported that non-uniform thermal deformations cause localized stresses in M-AM products, resulting in non-uniform elastic distortions during cooling to room temperature. These mechanical constraints limit a part's ability to meet structural design fittings, leading to premature failure during fatigue loading due to accumulating and propagating dislocations [15; 16]. The presence of a lack of fusion defects was found by [PERSON] et al. [17] to negatively impact the fatigue life of service components. [PERSON] et al. [18] and [PERSON] et al.