Graphene is widely preferred for reinforcing metal-matrix composites owing to its unique mechanical, physical and thermal properties. Unfortunately, graphene exhibits low load-transfer efficiency due to high specific surface area thereby resulting in poor homogenous distribution in metals. Therefore, researchers have been looking for alternative methods of improving graphene distribution in metals and have identified the cold drawing process as a promising solution.
Among the presently used methods for achieving uniform distribution is powder metallurgy that involves mixing of the matrix powder followed by consolidation of the mixed powders. Generally, mechanical properties of composites are determined by the fabrication process that influences the composite microstructure. The introduction of severe plastic deformation (SPD) process such as friction stir processing helped in eliminating defects during synthesis of graphene/Al composites thereby improving the distribution of nanoscale reinforcements. However, multi-pass drawing process is replacing the SPD process since the latter is limited to small-scale production. Although aluminum alloys using multi-pass drawing process have been widely explored, little have been reported for graphene nano-platelets (GNP)/Al composites.
Harbin Institute of Technology researchers led by Professor Xuexi Zhang fabricated GNP/Al composites reinforced with 0.4 wt.% and 2.0 wt.% GNP by powder metallurgy followed by multi-pass cold drawing at ambient temperature. They investigated the mechanical properties and microstructure evolution of the resulting composite and also analyzed the strengthening effects of GNP on GNP/Al composites before and after the drawing processes. They purposed to improve mechanical properties and graphene distribution in Al-MMC. Their work is published in the journal, Materials and Design.
The authors observed fragments of cracked GNP along the drawing direction due to the accumulation of large shear strain induced by a successive multi-pass cold drawing process. Also, a uniform distribution at shear strain 6.00 was observed in 0.4 wt.% GNP/Al composite and absent in 2.0 wt.% GNP/Al. Furthermore, GNP thickness of 5-10nm was recorded in the as-drawn state down from 30-100nm in the as-extruded state.
The research team successfully implemented the cold drawing process to significantly improve the GNP distribution. This was attributed to the capability of the process to eliminate the pores and GNP aggregates. Consequently, mechanical properties of 0.4 wt.% GNP/Al composites significantly improved due to the strong interfacial bonding and improved load transfer strengthening effect. For instance, ultimate tensile strength was 52% higher than that of aluminum alloy. On the other hand, mechanical properties of 2.0 wt.% GNP/Al deteriorated due to the presence of GNP aggregates. Graphene strengthening efficiency of 80% was recorded in the cold-drawn composites just like in the composites fabricated by wet chemistry methods.
According to Professor Xuexi Zhang, the cold drawing process is an effective technique for uniformly dispersing the GNP in the Al matrix thereby enhancing the mechanical properties of the composites. Unlike the previous techniques, the technique is a promising solution for large-scale applications. Therefore, the study will advance the fabrication of graphene-reinforced aluminum matrix composites.
Li, J., Zhang, X., & Geng, L. (2018). Improving graphene distribution and mechanical properties of GNP/Al composites by cold drawing. Materials & Design, 144, 159-168.Go To Materials & Design