Enhancing the microstructural homogeneity and mechanical properties of recycled 7xxx series Al extrudates via grain refiner addition

This study presents a recycling method for 7xxx series Al chips from milling to produce extruded materials with enhanced mechanical properties. Milling chips were compressed and melted with ingots (1:1 weight ratio) to form billets for extrusion. During casting, an Al-5Ti-1B grain refiner (GR) was added, reducing the average grain size of the cast alloy from several millimeters to approximately 90 mu m. Following extrusion, the GR-free extrudate exhibited an average grain size of 138 mu m, while the GR-containing extrudate demonstrated a finer grain size of 36 mu m. This refined microstructure resulted in enhanced grain boundary strengthening. The GRcontaining extrudate achieved a tensile yield strength (TYS) of 273 MPa, an ultimate tensile strength of 439 MPa, and an elongation of 15.8%, surpassing the TYSs of the GR-free extrudate (243 MPa) and commercial AA7075 extrudate (230 MPa). Three-point bending tests revealed that the GR-containing extrudate displayed a limiting bending depth (LBD) of 9.0 mm and failure bending strain of 42.3%, compared to the LBD of 6.5 mm and failure bending strain of 30.7% of the GR-free extrudate. The improved bendability of the former is attributed to its fine, uniform microstructure and slip-favorable textures formed by GR addition, reducing stress concentrations and improving bending formability. Overall, GR addition to recycled Al alloys effectively enhanced their mechanical properties beyond those of commercial AA7075, despite milling chip addition. This method provides a viable approach for Al chip recycling, offering a sustainable solution to reduce waste and improve resource efficiency in the Al industry.