Metal additive manufacturing (AM) is a revolutionary technological advancement that has made significant inroads in a wide range of sectors, including aerospace, defense, automotive, health care, and engineering applications. It offers unprecedented design freedom, reduced material waste, and enhanced performance, in addition to significant enhancements to fabrication processes. Microstructural defects and internal stresses formed during fabrication directly affect the fabricated product's surface integrity, quality, and service life. Identification, characterization, and prediction of these defects help significant and direct production of defect-free structures with high density. This article provides detailed insights concerning the common defects, mitigation techniques, and challenges reported in both powder bed fusion-based and wire arc AM methods. Defects such as porosity may develop due to the powder sphericity, roughness of the powder, preheating, process parameters, build environment, postprocessing techniques, and environmental factors. Therefore, a critical study of the techniques, alloys, process parameter optimization, and different postprocessing techniques to tone down the defects is made from their formations.

High-performance lightweight materials give rise to a demand for magnesium-based composites. Magnesium metal matrix composites have found extensive applications in the aerospace and automotive industries due to their remarkable mechanical properties. Limitations in dispersion, strength, and interface strength of magnesium-based composites are seen through the addition of some hybrid reinforcements and different fabrication techniques. In this regard, previous studies have demonstrated the ability of various reinforcements such as graphene, carbon nano tubes, silicon carbide, and titanium carbide to a magnesium matrix for the enhancement of its metallurgical properties. There are still several challenges in the development of magnesium metal matrix composites, such as non-homogeneous distribution, poor creep resistance at elevated temperatures, limited cold work ability, and low corrosion resistance. The challenges that need to be overcome and suggestion for improving the wear resistance and frictional properties of magnesium metal matrix composites were studied and discussed. This review evaluates the importance of different reinforcement percentages as well as the development of magnesium metal matrix composites. The different types of fabrication techniques that are well suited to overcome the challenges of poor dispersing, non-homogeneous distribution, interfacial problems, and poor wettability are discussed. Microstructure analysis, the agglomerating effect, and matrix bonding strength are also discussed. The challenges and future scope of research are discussed for the demonstration of the importance of more scientific studies in magnesium metal matrix composites.