Effect of ball milling on microstructural evolution during partial remelting of6061 aluminum alloy prepared by cold-pressing of alloy powders

The effect of ball milling on the microstructural evolution was investigated during partial remelting of 6061 aluminum alloy prepared by cold-pressing of atomized alloy powders.The results indicate that the microstructural evolution of 6061 aluminum alloy can be divided into three stages,the dissolution of eutectic phases and the coarsening and growth behavior of the resulting grains,structural separation and spheroidization of primary particles,and the final coarsening behavior of the particles.Compared with the alloy without ball milling,ball milling accelerates the first stage of microstructural evolution due to the energy stored in the powders,but the latter two stages are slowed down because of the formation of large-sized powders.Moreover,the finer the as-cold-pressed microstructure is,the smaller and more spherical the primary particles in the final semisolid microstructure are.Furthermore,properly elevating the heating temperature is beneficial for obtaining small and spheroidal particles.

Regulation of bimodal structure and mechanical properties of powder-thixoformed GO/ZK60 magnesium matrix composite through adjusting GO content

Although remarkable strength enhancements can be achieved in graphene oxide(GO)/graphene nanoplatelets(GNPs)reinforced Mg matrix composites by using the available techniques,their ductility is always quite poor due to the difficultly avoided strength-ductility trade-off.To conquer this dilemma,GO/ZK60 composites with bimodal-grain structure were fabricated using powder thixoforming in this work.The results indicate that the grain size and volume fraction of coarse grains(CGs)first decrease as the GO content increases to 0.2 wt.%and then increase again as the content increases to 0.3 wt.%,while the grain size in the fine grains(FGs)almost does not change.Consequently,the strength of the composites is improved with increasing GO content and reaches the peak values at the content of 0.2 wt.%.The composite with 0.1 wt.%GO content exhibits significantly increased tensile yield strength up to 177±2 MPa while maintaining a high elongation of 23.1%±2.5%,being equivalent to that of the ZK60 matrix alloy.The increased FGs volume fraction,together with the promoted dislocation accumulation and storage via GO and grain refinement of large-sized CGs lead to the improvement of strain hardening ability,thus rendering the composite an excellent ductility.Furthermore,the deformation of the GO/ZK60 composites occurs progressively from the FGs to the CGs,which is opposite to the status of the milled ZK60 matrix alloy.In view of the microstructure characteristics of the composites,a new complex calculation model was proposed and it could well predict the strength of the bimodal GO/ZK60 composites.This study provides a new insight into the microstructure design and fabrication technology of GO/GNPs reinforced metal-based composites with high strength and ductility.