## 1 Introduction  
Vehicle manufacturers continuously strive to develop vehicles with enhanced performance and improved safety features. In recent decades, passenger safety has become an increasingly important focus. For example, despite the fact that there were more registered cars in the US by approximately 30% between 1990 and 2013, the annual traffic accident-related fatality rate decreased by around 32% over the same period [1]. A fundamental factor in mitigating the impact of accidents is the structural design of the vehicle. The ability of a vehicle's structure to absorb dynamic energy and reduce passenger injuries in the event of accidents is referred to as crashworthiness, also known as passive safety [2]. Located between the side rails and the bumper, as illustrated in Figure 1, the crash box plays a critical role in enhancing a vehicle's crashworthiness. In case of accidents, the crash box is designed to absorb dynamic energy, thereby preventing structural damage to other components of the vehicle.  
Crash boxes are traditionally manufactured using methods such as extrusion [3; 4; 5], stamping [6; 7], welding [8; 9], wire erosion [10; 11; 12], CNC water jet cutting [13], and composite processing [14], which restrict the design to relatively simple geometries. This limitation hinders efforts to improve crashworthiness, as it prevents the use of more advanced and efficient designs. Because of this, interest is rising in more sophisticated techniques such as additive manufacturing (AM), sometimes referred to as 3D printing, Solid Free Form (SFF), or Rapid Prototyping (RP). The concept of AM was first introduced by [PERSON] in 1984 [15]. By fabricating materials layer by layer, AM makes it possible to create intricate structures [16]. Unlike traditional manufacturing methods, AM offers increased design freedom, allowing the creation of more creative structures that maximize mechanical properties. This flexibility makes AM particularly suited for producing intricate, high-performance crash box designs [17].  
While several review papers have examined crash box designs and their manufacturing techniques [18; 19; 20; 21; 22], there remains a significant gap in comprehensively assessing the transformative potential of AM in this domain. This paper provides an in-depth exploration of how AM technologies are revolutionizing crash box development. It highlights key design innovations, material advancements, and manufacturing techniques that enhance energy absorption and overall vehicle safety. The categorization of additively manufactured crash boxes is summarized in Figure 2, with detailed discussions in the following sections.  
This paper is organized as follows: Section 2 provides an overview of the working principle of crash boxes. Section 3 discusses various performance metrics for crash boxes, which are essential for evaluating their effectiveness. Section 4 introduces the AM process, outlining the key steps involved in producing components using AM technologies. Section 5 delves into the prominent AM techniques used for crash box manufacturing, dividing them into three categories: liquid-based, powder-based, and solid-based methods. Section 6 explores the different geometric designs used in additively manufactured crash boxes, categorizing them into tubular, origami-inspired, lattice, and bio-inspired structures. Section 7 focuses on the materials used in the AM of crash boxes, discussing the proper  
Figure 1: Crash box.  
ties and performance of polymers, fiber-reinforced polymers, metals, and multi-material structures. In Section 8, this paper reviews the specific AM techniques employed for crash box production. Finally, Section 9 concludes the paper, summarizing the key findings and future directions.