## 3 Literature Review
### Additive Manufacturing and Green Manufacturing
The outcomes of these studies indicate the suitability of AM technology for the production of biodegradable materials and its feasibility for replacing conventional materials without sacrificing the functional requirements. Another merit AM technology offers is its flexibility and freedom of design. The design freedom provided by AM allows for products with improved functional performance [48]. For example, [PERSON] and [PERSON] [49] reported the re-design of a truck engine to lightweight parts that consume less material. The study involves the experimental redesign of a truck engine for AM along with the test prints, as well as the environmental impact assessment to investigate the environmental impacts of AM technology. The outcome of this study indicates that high-density materials, such as nickel-alloys and stainless steel, will contribute to high environmental impacts and should be avoided, while low- to medium-density materials, such as low-alloy steel, will produce a lower environmental impact for the AM technology. In the aerospace and automotive industry, lightweight parts produced with AM technology help to reduce weight, thereby increasing fuel efficiency [49]. AM can eliminate assemblies by producing consolidated parts, thus eliminating the intermediary supply chains, resources and energy used to produce fasteners when conventional processes are used [48]. Eliminating these intermediaries and assemblies will make the manufacturing process more cost- and time-effective than the conventional manufacturing system. Topology optimization has produced high-performance designs using fewer materials [50]. The freedom of design in AM allows the manufacture of products with specialized features, such as integrated sensors or conformal cooling structures [51]. This helps improve the products' functional performance and makes them more energy efficient in their lifecycle. Repair and refurbishment are important strategies for remanufacturing as they allow end-of-life products to be revived. AM can be used to refurbish and repair worn-out equipment [52]. Direct energy deposition (DED) is a group of AM processes that offer the capability to repair worn-out metallic parts. [PERSON] et al. [53] reviewed the use of AM to repair damaged parts and how this contributes to a circular economy. By implication, this will reduce the amount of waste generated and, ultimately, the amount of GHG emitted through indiscriminate waste disposal and landfilling. [PERSON] et al. [54] studied nickel-based aerospace component remanufacturing using DED. Their study concluded that DED is a cost-effective solution for repairing high-value components. [PERSON] et al. [55] presented an overview of the usage of the DED process in the repair of metallic components. The outcome of these studies further lends credence to the fact that AM technology can be used to achieve a circular economy. The global increase in plastic waste is a growing concern as it threatens aquatic life. AM offers a solution to this challenge by recycling polymer-based products, thus contributing to sustainable manufacturing [56]. For instance, fused deposition modelling (FDM) is a popular polymer-based AM process that allows the processing of recycled polymers [42]. [PERSON] et al. [57] conducted a study involving the recycling of materials from fiberglass wind turbine blades. The outcome of the study indicated that FDM is suitable for material recycling. The application of AM technology for component development in the automotive, rail and aerospace sectors have been reported [58; 59; 60; 61; 62]. This can lead to significant savings in the amount of energy consumed due to the development of lightweight components, as well as a reduction in the amount of emissions generated and increase in the environmental friendliness in the transport sector.