Spark Plasma Sintering As a Solid-State Recycling Technique: The Case of Aluminum Alloy Scrap Consolidation

Abstract

Recently, “meltless” recycling techniques have been presented for the light metals category, targeting both energy and material savings by bypassing the final recycling step of remelting. In this context, the use of spark plasma sintering (SPS) is proposed in this paper as a novel solid-state recycling technique. The objective is two-fold: (I) to prove the technical feasibility of this approach; and (II) to characterize the recycled samples. Aluminum (Al) alloy scrap was selected to demonstrate the SPS effectiveness in producing fully-dense samples. For this purpose, Al alloy scrap in the form of machining chips was cold pre-compacted and sintered bellow the solidus temperature at 490 °C, under elevated pressure of 200 MPa. The dynamic scrap compaction, combined with electric current-based joule heating, achieved partial fracture of the stable surface oxides, desorption of the entrapped gases and activated the metallic surfaces, resulting in efficient solid-state chip welding eliminating residual porosity. The microhardness, the texture, the mechanical properties, the microstructure and the density of the recycled specimens have been investigated. An X-ray computed tomography (CT) analysis confirmed the density measurements, revealing a void-less bulk material with homogeneously distributed intermetallic compounds and oxides. The oxide content of the chips incorporated within the recycled material slightly increases its elastic properties. Finally, a thermal distribution simulation of the process in different segments illustrates the improved energy efficiency of this approach.

Environmental assessment of solid state recycling routes for aluminium alloys: Can solid state processes significantly reduce the environmental impact of aluminium recycling?

“…In terms of functional equivalence, the material properties obtained by the respective solid state recycling processes, as partially reported in [10][11][12][13][14] and summarized in Table 1, allow to conclude that the incorporation of the aluminium oxides present in the production scrap into the recyclates does not negatively affect the functional performance of the obtained materials if dispersion or breakage of these oxides layers during the solid state processing can be assured. It can be observed that the resulting materials in some cases show improved property values.…”

“…More recently spark plasma sintering (SPS) has been demonstrated as a solid state recycling technique for scrap consolidation [14]. Dynamic scrap compaction, combined with pulsed electric current joule heating, cleans and activates the metallic surfaces and achieves efficient fracture of the stable surface oxides and desorption of the entrapped gases, resulting in void-less material.…”

Solid State Recycling of Aluminum Alloy Chips via Pulsed Electric Current Sintering

“…Therefore, relative densities were determined according to two individual absolute densities, to show both upper and lower end of estimations regarding final specimen densities achieved. While Behrens et al [33] used 2.71 g/cm 3 for their calculations on Al EN AW 6082 chips, Paraskevas et al [24] reported of fully dense samples of the same alloy reaching 2.832 g/cm 3 , indicating a much higher amount of oxides. Evaluation of specimens described in this work, based on measurements of the individual samples and calculated relative densities for both above stated cases, is presented in Table 2.…”

“…Some of the bonding mechanisms discussed, like the occurrence of a spark discharge or plasma, are very controversial and others, like diffusion welding, are widely accepted amongst the researchers [11,12,15,22,23]. Although PECS and the other similar processes most commonly are used for powder materials, there already is research available on the successful processing of aluminum chips by Paraskevas et al [24,25] referring to the same effects during consolidation as stated above. By mixing chips from scrap with atomized powders, they were able to produce samples with no porosity exceeding 35 µ m. In comparison to samples produced from powder only, the chip-based specimens could result in an anisotropic material behavior due to a possible alignment of the irregular chips during processing [24].…”

“…By mixing chips from scrap with atomized powders, they were able to produce samples with no porosity exceeding 35 µm. In comparison to samples produced from powder only, the chip-based specimens could result in an anisotropic material behavior due to a possible alignment of the irregular chips during processing [24]. To reduce this anisotropic behavior, it seems to be reasonable to reduce and homogenize the size of the chips by milling, but according to Wan et al [10] the rising ratio between surface oxide and volume needs to be taken into account as a main influence on the consolidation behavior.…”