1 4 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 When compared to other metal AM methods, the AFSD process does not suffer from significant tool wear issues for aluminum and magnesium alloys since only the AFSD feed rod is consumed during the deposition. In addition, the AFSD process provides a path for reducing costs by alleviating the need for hot isostatic press (HIP) and heat affected zone (HAZ) rework, post-production processes typically required for other advanced manufacturing techniques. Other significant advantages of the solid-state AFSD process are the wide variety of materials that can be deposited, the ability to join or repair dissimilar materials, and the ability to produce functionally graded components. During the AFSD process, maximum temperatures are maintained well below the liquidous state (Td < Tm) of the respective deposited material, preventing solid-liquid-solid phase transformations that may result in material defects such as hot-cracking during fusion-based depositions. Additionally, the lower temperatures during the solid-state deposition allow for a lower amount of distortion for the substrate, to produce primary aluminum from bauxite is approximately eight times greater than energy required for recycling aluminum. Because of the drastic reduction in energy consumption and greenhouse gas emissions, recycling aluminum scrap/waste has found high prominence. At present, conventional recycling of aluminum scrap/cast waste involves segregation, cleaning, shredding, melting, and subsequent casting as ingots, or in some cases, structural members. Production of aluminum granules and powders from scrap/ waste using powder metallurgy is another recycling method, although less common[10]. This secondary recycling process also requires several stages of cleaning, sorting, and preparation of the aluminum scrap/waste. For example, in recycling aluminum machine chips, a relatively clean and high-quality feedstock is required[11], while in cast aluminum alloy recycling production, the scraps/waste are received fromendof-life products and can tolerate high degree of debris and additives[12]. Once aluminum has been recycled, it generally does not retain the same chemical composition or mechanical properties as the feedstock and hence additional secondary manufacturing processes are required to produce required alloys. ADDITIVE FRICTION STIR DEPOSITION To address the shortcomings and achieve the goal of complete recyclability of aluminum scrap/waste, a transformative hybrid solid-state additive manufacturing process, additive friction stir deposition (AFSD), has emerged (Fig. 2). The novelty of AFSD is that it uses a low-power technology (~7 kW) and combines the advantages of AM and microstructure refinement into a single process, allowing fabrication of near-net shape small and large components with similar chemical composition as the feedstock material. AFSD is a thermomechanical non-melting process that can be performed in any ambient environment and has no emission or generation of waste during or after the deposition since the waste chips from machining of near-net shape depositions can be recycled back through the AFSD process. Preliminary analysis reveals that recycling using the AFSD process consumes 77% less energy as compared to existing recycling technologies. Recent research has found several unique advantages of using AFSD to recycle aluminum secondary feedstocks in addition to the energy savings and reduction in greenhouse gas emissions. When recycling secondary feedstocks such as machine chips and damaged strips of metal, the AFSD process has been demonstrated to breakup inclusions and constituent particles because of the high shear stresses and frictional heat generated during processing. Once broken into smaller fractions, these inclusions are widely dispersed throughout the metal matrix which aids in the establishment of a robust metallurgical bond between deposited layers. This means that the waste products do not require the same level of sorting or cleaning that are required for traditional recycling methods but result in superior parts[14–22]. Fig. 2 —Direct additive recycling paradigmdemonstrated to repair an aluminum alloy (AA) part from secondary feedstock (metal strips and compactedmachine chips) at a forward operating base (FOB) using the AFSD solid-state additive manufacturing process. Top le image fromRef 13.
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