Over the past 30 years, metal additive manufacturing (AM) has advanced rapidly, reaching major milestones that have transformed the manufacturing landscape. This layer-by-layer fabrication technique offers exceptional design freedom and manufacturing flexibility, delivering notable performance and economic benefits across sectors such as aerospace and the automotive industry. This study methodically analyses various printing processes, identifies common defects in metallic materials, and proposes effective strategies for their mitigation. We elucidate the complex relationships among various manufacturing methods, microstructures, and their resulting performances. Considering the rapid advancement of artificial intelligence, we outline its various applications within the field of AM. Within the framework of Industry 5.0, the integration of high-throughput experimentation and materials genome engineering (MGE) is expected to substantially expedite the discovery of novel materials. Moreover, agents developed via the integration of large language models with AM expertise are poised to provide innovative approaches for optimizing process parameters and enhancing decision-making accuracy. With continued advancements in AM agents, cloud computing, renewable energy, and structural design principles, the realisation of smart AM factories based on these technologies is becoming increasingly achievable. This improvement is expected to propel metal AM into a new era characterized by intelligence and customization, fostering substantial progress and transformative shifts in materials science and manufacturing engineering.