Recyclability assessment of Al 7075 chips produced by cold comminution and consolidation using spark plasma sintering

Abstract

A solid-state recycling route for processing aluminium alloy 7075 chips is proposed. This process involves pulverizing the chips using an industrial scale comminution line to produce a broad particle size distribution of granulated products and powders. Upon sieving, the −140 mesh fraction of the pulverised material was consolidated by spark plasma sintering into fully dense compacts, i.e. >99%. The properties of the pulverised powders and compacts were compared to commercially available aluminium alloy 7075 powder used in industrial press-sinter applications. The comminution line produced powder particles which possessed a 7•9 ± 0•6-nm thick native oxide layer with adhering exogenous contaminants, and aluminium oxide particles dispersed throughout the powder. The presence of the oxide layer and exogenous contaminants was found to be deleterious to the powder consolidation and mechanical properties. A flexural strength and strain and hardness of 94•7 ± 7•2 MPa, 2•1 ± 0•3% and 69•3 ± 5•2 HV were obtained, respectively.On suggère une voie de recyclage à l'état solide pour le traitement de copeaux d'alliage d'aluminium 7075. Ce procédé implique la pulvérisation des copeaux en utilisant une ligne de broyage à l'échelle industrielle afin de produire une large distribution de taille de particule de produits granulés et de poudres. Après tamisage, la fraction d'ouverture de maille de -140 du matériel pulvérisé était consolidée par frittage par étincelage au plasma en comprimés complètement denses, i.e. > 99%. On a comparé les propriétés des poudres pulvérisées et des comprimés à la poudre d'alliage d'aluminium 7075 disponible commercialement et utilisée dans les applications industrielles de pressé-fritté. La ligne de broyage produisait des particules de poudre qui possédaient une couche d'oxyde natif de 7.9 ± 0.6 nm d'épaisseur avec contaminants exogènes adhérents, et des particules d'oxyde d'aluminium dispersées à travers la poudre. On a trouvé que la présence de la couche d'oxyde et de contaminants exogènes était nuisible à la consolidation de la poudre et aux propriétés mécaniques. On a obtenu une résistance à la flexion, une déformation et une dureté de 94.7 ± 7.2 MPa, 2.1 ± 0.3% et 69.3 ± 5.2 HV, respectivement.

Spark plasma sintering and spark plasma upsetting of an Al-Zn-Mg-Cu alloy

“…In the case of Alumix 431D, which is equivalent to AA7075, the oxide layer is different than the one present on pure Al. It has a composite type of oxide layer coming from magnesium oxide and aluminum oxide [20]. Therefore, the pressure during the initial sintering of Al alloy might not have been enough to disrupt the oxide layer unlike that present on soft, pure Al [17,20].…”

“…It has a composite type of oxide layer coming from magnesium oxide and aluminum oxide [20]. Therefore, the pressure during the initial sintering of Al alloy might not have been enough to disrupt the oxide layer unlike that present on soft, pure Al [17,20]. The application of forming processes to sintered Al products, such as forging, can disrupt the oxide film and enhance the metallurgical bonding [1].…”

“…However, it has been shown by Garbiec and Siwak [18] and Rudinsky et al [19] that Alumix 431 powder sintered by SPS exhibits low ductility during tensile or bending tests due to the lack of bonding between particles. It has been suggested that the formation of the spinel layer arising from the reaction of Mg and the alumina layer would impose the requirement for a higher applied load to break the oxide layer [20]. Alternatively, forging of aluminum P/M products is known to close the residual porosity and disrupts the oxide film around the powder particles because of the high strains and strain rates involved [1].…”