Manufacturing of aluminium metal matrix composites by high pressure torsion.
Aluminium (Al) and aluminium alloys (AA) are widely utilised in the industry because they are lightweight, inexpensive, ductile, thermally stable, and possess high electrical conductivity. However, poor stiffness, strength, and wear limit their use in aerospace, automobile, and transmission. Metal matrix composites (MMC) have shown the capability to make tailor-made materials to overcome the issues mentioned above. Moreover, nanomaterial-based MMC provides better mechanical properties and wear resistance. However, available methods of producing AA MMC have challenges such as porosity, brittle interfaces, and defects in nano-reinforcement. In the current study, a novel approach has been used to produce nanomaterial-based AA MMC to overcome these challenges. Graphene (Gr), Nanodiamond (ND), and Molybdenum disulfide (MoS2) have been added to CP Al and 7xxx alloys using severe plastic deformation (SPD). The current study provides deep insight into the nature of nanomaterials in metal nanomatrix and the resulting properties for industrial applications, particularly electrical transmission and self-lubrication in the automotive industry. The potential of graphene to improve the strength of aluminium alloy powder has been experimentally studied. Gr AA 7075 MMC is produced by high pressure torsion (HPT) at room temperature. Variations in the concentration of graphene nanosheets and HPT process parameters are applied to optimise the fabrication route. 0.5-1 wt.% Gr AA 7075 MMC were processed through 0.5 to 100 turns of HPT. Also, graphene-free AA 7075 samples were produced using the same process. The samples were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) line broadening analysis, which revealed the dislocation density and the crystallite size. XRD line broadening revealed a crystallite size of 28 nm for the 1 wt.% Gr AA 7075 MMC after 50 turns of HPT, which illustrates the capacity of HPT to produce a genuinely nanostructured material. Additionally, a comparison of the strength modelling of (1-5%) ND AA 7075 MMC and (1-10%) MoS2 AA 7075 MMC has been done using a similar methodology. The porosity evolution in the Al matrix during the early stages of HPT was analysed. The hardness of the material improved up to 300 % by HPT as compared to annealed bulk AA 7075 to reach a maximum hardness of 310 HV for the 0.5 wt.% Gr AA 7075 MMC processed by 20 turns of HPT. In contrast to cast and solution treated AA 7075, no consistent improvement in hardness is recorded in HPT processed AA 7075 due to artificial ageing. However, XRD results show that the microstructure changed after artificial ageing. In comparison, the addition of ND resulted in softening AA 7075. In contrast, MoS2 AA 7075 MMC have shown higher hardness than two predecessors AA 7075 MMC. A volume-averaged dislocation density model was applied for strength modelling of HPT processed samples. Considering future applications such as aerospace, and automobile parts, a focus on tribological applications of Gr AA 7075 MMC, ND AA 7075 MMC, and MoS2 AA 7075 MMC has been extensively studied. For comprehensive understanding, dry sliding ball on disc wear tests and post-wear analysis were done using tribometer, profilometry, and electron microscopy. Compared to currently available Al alloys, the HPT processed AA 7075 MMC demonstrated a 50% improvement in friction coefficient (0.2–0.3) and wear properties. Generally, Gr and MoS2 reduce the coefficient of friction (COF), whereas ND improves wear resistance in the coarse-grained AA MMC prepared by industrial methods. However, ND AA 7075 MMC shows better friction properties than Gr and MoS2-based AA 7075 MMC. Moreover, MoS2 AA 7075 MMC have a relatively poor wear rate. This study involves a descriptive analysis of the possible unusual role of nano-reinforcements in wear mechanisms. AA are used for electric transmission in many Asian countries, such as India, because of its low cost and good electrical conductivity. Earlier attempts to improve electrical conductivity included alloying and reinforcing with gold and silver. Moreover, considering the electrical properties of graphene, graphene nanosheets have been added to commercially pure Al to make 1% Gr AA 1050 MMC. However, the 4-point probe test shows a 40–60 % reduction in electrical conductivity (IACS). The current study involves the assessment of form factors for measuring the electrical conductivity of AA MMC. Overall, the current study shows the potential of HPT to create a new class of defect-free nanomaterials. Furthermore, experiments are conducted to examine the application-related roles of Gr, ND, and MoS2 in the AA MMC.
https://eprints.soton.ac.uk/478335/
https://eprints.soton.ac.uk/478335/1/Kumar_29872707_PhD_thesis_FEPS_MECH_Engg.pdf