Production of native graphene nano platelets reinforced Al-Cu alloy based composi̇tes wi̇th powder metallurgy, heat treatment and characterization

Application of aluminum and its alloys remains limited because of its low strength and wear resistance in automotive, aerospace and defense industries in which advanced material properties needed. Usage of light weight and high strength materials gains importance in many engineering applications. In addition, there have been significant researches in literature, indicating the usage of some special Al alloys as armor materials due to their lightness and high strength and intense research efforts have been focused towards Al alloy based ballistic armor materials. Al alloy based composite materials enable producing advanced materials to meet these demands. Nano-structured Al alloy powders and ultrafine grained composite structures can be produced by the mechanical alloying through high energy ball milling in the powder metallurgy stages. Together with the formation of ultrafine grained alloys, mechanical alloying enables enhancing the strength by formation of super saturated solid solutions via increasing the solid solubility limits of alloying elements in the matrix phase and formation of metastable intermetallic phases. In addition, mechanical alloying is the most effective method to provide homogeneous distribution of reinforcement additives in the matrix phase, which is the most common problem encountering in the fabrication of composite materials. Graphene with its nano particle size, fine layered structure and being one of the strongest material per unit weight, it has a great potential to become a very important reinforcement additive for Al-based composites. It was stated in the literature that among the carbon-based reinforcement additives graphene shows better distribution and matrix adherence properties due to its wrinkled surface and it can efficiently transfer its high mechanical strength to the matrix. In this thesis,the synthesized graphene nano-platelets were incorporated into the mechanically alloyed Al-5.5wt.%Cu, based alloy having 3 different compositions and fine-grained composites were fabricated. By pressing the mechanically alloyed powdersand pressureless sintering Al-5,5Cu based alloys and graphene-reinforced composites were obtained. Additionally, age-hardening heat treatment was applied to some composites for improving the mechanical properties. Solution and age-hardening heat treatments were exerted on the sintered composites by holding them at 500◦C for 3 h and 170 ◦C for 20 h, respectively. Characterization of the synthesized powders and sintered samples were performed by physical ( X-ray diffractometer (XRD- determination of lattice strain and crystallite size)), compositional (XRD, scanning electron microscope (SEM)), microstructural (optical microscope, SEM,)), mechanical (tensile and compression), corrosion and tribological tests (wear volume loss). In the XRD analysis for Al-5,5Cu based alloy powders, metallic phases or impurities originating from the mechanical alloying process weren't found.With increasing mechanical alloying time and GNP amount, crystallite size values of the mechanically alloyed powders decreased and the lattice stress values increased. In the SEM images, it was observed that the angular, spherical or elliptical particles became flat. Al2Cu phase formation was observed in XRD analyzes of all composites with different compositions obtained by sintering Al-5,5Cu based alloy powders. According to the data obtained after the hardness test, the hardness values increase with the increase of the mechanical alloying times. The hardness value measured with the increase in the amount of graphene increases up to wt.%1 GNP additive, while it decreases in the samples with wt.%2 and wt.%5 GNP additives. Compression results were obtained similarly to the hardness results. A positive effect of mechanical alloying on tensile strength was observed in the tensile test applied to wt.%1 GNP added samples.According to the wear test results, it is seen that the wear rates decrease with the increasing amount of graphene in the samples, but the wear rate of the samples containing wt%5 graphene differs. It is thought that the graphene phase added here reduces the amount of wear by creating a lubricating effect. It is understood that the corrosion resistance of composites decreases somewhat with the addition of graphene. Accordingly, it is seen that the graphene reinforcement did not make a significant change in the corrosion rate up to wt%5, but the corrosion rate increased significantly in the sample with wt%5graphene content. After the T6 heat treatment applied to the Al-5.5Cu-0.5Mn-0.3Mg-xGNP samples, the hardness and compression strengths increased as the Cu and Mg components in their structures, which increase the strength, became stable precipitates.