1 3 A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 3 As the global scientific and engineering community struggles with minimizing logistical and economic challenges for metal part production and repair, there is a strong demand to incorporate more sustainable additive manufacturing (AM) solutions on demand at the location of use, or point-of-need. Within the United States, multiple government agencies seek innovative ways to reduce the strain on supply chain, natural resources, and active sustainment. AM is sought as a pathway to these goals, which provides: (1) resource management in that less waste is produced per component, (2) economic-design optimization to develop complex, yet lightweight parts to improve efficiency, and (3) reduction in chemical byproducts typical of traditional manufacturing[1]. More recently, proposed efforts have included multiple investigations on AM as a means of recycling[2]. Current hailed objectives have focused on the most popular additively manufactured material, plastic[3]. The advancements in these methods have developed the capability to employ polypropylene as a feedstock material, which is most commonly known for use as water bottles[4]. With the success of improving overall sustainability, and thus active readiness, these same agencies have expanded the desire for stronger materials. The Committee on Armed Services[5] specifically outlines particular interests in structural materials such as metals, ceramics, and composites, which are envisioned as recyclable lowcost, low logistical, and high strength to be utilized in military vehicle applications. As such, multiple means have been proposed to attempt to accomplish these challenges for both environmental and practical purposes. Currently, the Strategic Environmental Research and Development Program (SERDP) has been supporting multidisciplinary research to recycle unwanted waste materials, or secondary feedstocks, through low-power approaches that are applicable to austere locations and traditional manufacturing environments. From this SERDP research, a new direct additive recycling (DAR) is responsible for 1% of annual greenhouse gas emissions, and production of aluminum from ore-bauxite consumes more energy than any other metal[7]. In the lifetime of an aluminummade product, researchers have reported that 1 kg of aluminum in a car reduced CO2 emission by an equivalent 19 kg. This environmental benefit is also compounded by the 5 to 7% fuel savings that can be realized for every 10% weight reduction through substituting aluminum for heavy steel-based alloys[8]. How- ever, the greenhouse gas emissions per kilogram production of primary aluminum range from 5.9 to 41.1 kg CO2 equivalent and the “break-even point” of vehicle lightweighting in terms of its required lifespan ranges from 50,000 to 250,000 km[7]. Estimates indicate that recycling 1 kg of aluminum can potentially save 4 kg of bauxite, 2 kg of chemicals and 7.5 kWh of electricity[9]. As reported in literature[8] and presented in Fig. 1, the average energy consumption paradigm has evolved for using various waste streams (machine chips and damaged scrap) as feedstock for metal AM and repair. The focus of the initial research was aluminum alloys, which saw significant advances; however, preliminary studies show significant promise for the adaptation of these techniques to hard alloys including steel castings, wrought high strength steels, and even titanium alloys. DAR MOTIVATION FOR ALUMINUM ALLOYS Aluminum is the most widely used metal in the aerospace and automobile industries because of its combination of lightweight and other desirable properties[6]. With increasing demand for aluminum around the globe, the scientific communityhas estimated that nearly 80 new smelters with 400 kt production capacity each must be established. Globally aluminum production Fig. 1 — Energy requirements for production of (a) primary aluminum- 113 GJ/t and (b) secondary aluminum- 13.6 GJ/t. Recreated fromRef 8.
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