## 4 PBF-LB/M Process
### Design
#### 4.1.1 Novel Lightweight Compositions
The objective is to attain the necessary mechanical and thermal properties for demanding applications, achieve weight reduction goals, and minimize issues like cracking and solidification defects. In the literature, proposed solutions to address hot cracking include changing the chemical composition to narrow the solidification range and refine the grain structure or modifying alloy solidification conditions reducing the heat flow and the cooling rate by optimizing the process parameters [79]. Finding the optimal process parameter combinations requires extensive studies involving non-cost-effective and time-consuming experiments through a trial-and-error approach. In addition, while optimizing process variables has shown effectiveness in reducing component internal defects, resulting in an improvement of the quality and uniformity of printed parts, this approach has generally failed to completely eliminate hot cracking phenomena in the toughest alloys [80]. This implies that hot cracks can still occur even under the best process conditions. To enhance printability and avoid detrimental hot cracks, it is essential to either develop new alloys or modify existing compositions [81]. Fine equiaxed microstructures proved to exhibit greater capacity to accommodate strain in the semi-solid state [77]. Furthermore, fine grains facilitate efficient liquid feeding and the healing of initial cracks, enhancing their resistance to hot cracking. Additionally, due to their increased grain boundary area within the material bulk, finer grains suppress the accumulation of low-melting elements or phases at grain boundaries. Considering the high hot crack sensitivity of different aluminum alloys, much research has concentrated on studying compositional variations that can result in grain refinement preventing hot cracks [82]. For instance, [PERSON] et al. [77] demonstrated that adding 1 vol% ZrH\({}_{2}\) nanoparticles as nucleation agents in Al-Zn and Al-Si-Mg alloys significantly refined the microstructure by enhancing heterogeneous nucleation and promoting equiaxed grain growth. This method successfully eliminated hot cracking in both Al7075 and Al6061 alloys processed by PBF-LB/M. On the same research line, different studies investigated the impact of incorporating various elements or compounds (including Ti, Zr, Sc, Er, Si, and different compounds such as oxides like TiO\({}_{2}\), carbides like TiC and borides like CaB\({}_{6}\) and TiB\({}_{2}\), as well as carbon allotropes like graphene and carbon nanotubes) on refining the grain structure and so on reducing hot cracking in high-strength Al-based alloys [83]. For example, a study conducted by [PERSON] et al. revealed that adding a Ti/TiN hybrid grain refiner into the PBFed 7050 alloy leads to the formation of ultrafine grains [84]. The approach involves either promoting heterogeneous nucleation or introducing solutes with a high growth restriction factor to rapidly provide sufficient constitutional supercooling for nucleation at the front of the solid-liquid interface. Adjustment to alloy compositions, such as adding grain refiners to promote the heterogeneous nucleation of Al grains, is not merely an academic approach but is increasingly used in industrial fields as well [82]. One of the leading U.S. manufacturers of metal powders, Elementum 3D, features in its portfolio of Al-based compositions enhanced versions of traditional alloys and advanced dispersion-strengthened aluminum powders for PBF-LB/M production formed by a process called reactive additive manufacturing (RAM). The RAM process enables the formation of advanced metal matrix composites that combine a continuous metal matrix with high-strength reinforcing ceramic or intermetallic phases. This approach allows printing materials with unique and advantageous combinations of ductility, strength, toughness, stiffness, fatigue resistance, and high-temperature performance.