Spark plasma sintering (SPS) and friction stir processing (FSP) were used to turn waste WE43 Mg alloy chips into consolidated billets with comparative mechanical properties against the commercial extruded products. Firstly, the chips collected after the machining process were loaded into a mould, without any pre-treatment, and then subjected to the SPS process. Within 1 hour, a consolidated billet (diameter = 80mm, height = 10mm) was obtained (Figure 1). In the SPSed billet, metallurgical bonding between chips was achieved, yet large network-like second-phase particles and oxide layers (Figure 2a) were identified along the chip boundary by scanning electron microscopy (SEM). Pores up to a millimetre scale were also detected by X-ray computed tomography (XCT). These features are adverse to mechanical properties and need to be removed.

Further FSP was applied to the SPSed billet to refine microstructures and remove pores. Due to the heat and plastic deformation generated by FSP, those second-phase networks were no longer observed in the FSPed sample. Most of the large second phases were broken, redissolved and then precipitated, forming fine particles below 100 nm (Figure 2b), though a few particles of micrometre scale remained. The average grain size was reduced to around 1-3 μm via dynamic recrystallisation. Besides, no pore was detected in the FSPed zone. The fine precipitation, grain refinement, and pore closure contributed to an increase of hardness in FSPed material (HV0.5=89), which is higher than that of SPSed material (HV0.5=72) and almost equivalent to the commercial extruded WE43 rod (T5, HV0.5=93).