February 2021 Volume 3

FORGING RESEARCH

can “jacketing” the extruded shape. A final step of removing the can from the extruded shape is necessary [11]. The primary downside to this consolidation route is the cost associated with the canning process due to filling, vacuum treatment, and removal of the can material. [5]. Several novel solid-state processing paths for powder consolidation may provide additional and practical processing pathways for bulk material. Processes of interest include additive friction stir deposition (AFSD), and friction extrusion [5,12–14]. These two consolidation methods are unique from extrusion in that the deformation occurs simultaneously with consolidation and is performed open to atmosphere [15,16]. Both of these solid-state processes start with loose powder and do not require the canning as in a traditional extrusion process thereby, eliminating multiple steps in the consolidation process. Additionally, all of the heat required to soften the material is generated by friction and plastic deformation during the process in comparison to conventional extrusion where the majority of heat comes from pre-heating the billet and tooling. AFSD is a solid-state powder consolidation method to additively manufacture, repair, and join components. The process derives from

the hollow rotating tool friction stir cladding technique developed and introduced by Stelt et al. and Liu et al. [17–19]. This process is based on the friction stir principle, where feed material is deposited layer-by-layer through a hollow shoulder tool shown in Figure 1 [20–24]. Multiple forms of feedstock can be used including solid rod, metal strips, powder, or machine chips shown in Figure 1a and b. At the tool/deposition interface heat is generated through friction and severe plastic deformation to bond consolidated material to previous layers and build a three-dimensional part in an open atmosphere as shown in Figure 1c [15,25]. In lieu of an auger system in Figure 1c powders and chips can be pressed into a green body before deposition through the tool head. Although the feed material in this case is porous, the intense deformation and material flow in AFSD render fully dense material in the as-printed state. This route has successfully demonstrated both with powder feed andmachined chips[26]. This solid-state process allows the production of non weldable alloys that are restricted in beam-based additive process [15]. Additionally, with near net shape capability [15], AFSD has the advantage of creating preforms for closed die forging that could reduce material waste or processing time helping to reach forging industry efficiency goals [3].

Figure 1: Schematic of the Additive Friction Stir deposition (AFSD) process showing the hollow shoulder where filler material is added to the build for compacted powder, solid rods, and strips of material depositions (a) and loose chips and powder depositions (b) [21]. Deposition of 8-inch diameter and 2-inch tall circular build made from AA6061 with AFSD (c).

FIA MAGAZINE | FEBRUARY 2021 79