MWCNT-Reinforced AA7075 Composites: Effect of Reinforcement Percentage on Mechanical Properties

Abstract

Metal–matrix composites (MMC) of aluminium alloy 7075 (AA7075) containing 1 wt.% and 0.5 wt.% multiwall carbon nanotubes (MWCNTs) were developed by powder metallurgy, using a high energy ball milling (HEBM) process for dispersion of the MWCNTs. The powder of the AA7075-MWCNT obtained was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructural changes produced during the milling process, such as the modification of the crystallite size, as well as the micro-deformation of the matrix crystal lattice, were determined using the Scherrer formula. After consolidation into a strip shape using the hot powder extrusion (HPE) process at 500 °C, no porosity was detected and a fine homogeneous dispersion of the reinforcement into the matrix was obtained. After performing a 0.2 HV test and tensile tests in the extruded profiles of both composites, a better combination of properties was found in samples of AA7075-0.5 wt.% MWCNT, with the increase in measured ductility being especially remarkable.

Effect of Milling Time and Reinforcement Volume Fraction on Microstructure and Mechanical Properties of SiCp-Reinforced AA2017 Composite Powder Produced by High-Energy Ball Milling

“…At this milling duration, the welding of flattened pa leads to an increase in the particle size of the composite powder, resulting in the form of larger and coarser particles. Similar observations were reported in the literature milling of particle-reinforced Al composites after 4 h of milling [24,34]. The dispers SiC particles in the AA2017 matrix progressively improved with increasing millin until a uniform distribution was achieved.…”

“…The total weight of the powder mixture was 800 g, and the capacity of the milling tank was 5 L. The ball milling process was performed in the closed container under a residual air atmosphere. The milling speed was cyclically varied between 400 and 600 rpm (5 min at 400 min −1 and 5 min at 600 min −1 ) for different milling times up to 5 h. The cyclic method reduced cold welding and agglomeration between particles [33,34]. The milling process was interrupted at the time intervals of 0.5, 1, 2, 3, 4, and 5 h to take out a small amount of powder for microstructural and mechanical property analysis.…”

“…This suggests that the alloying elements and impurities were not dissolved into the lattice of the aluminum matrix during the HEBM process. Many authors previously reported that the shift in the XRD peak positions is associated with the dissolving of smaller matrix alloying elements, particularly impurities such as iron, and reinforcing particles within the lattice of the aluminum alloy matrix phase during the milling process [34,41,42]. As shown in the right corner of Figure 7d, the first peak of the Al diffraction plane (111) in AA2017 + 5 vol% SiC p is reduced, and its width is significantly broadened after 5 h of milling.…”

“…Hard-reinforcing particles hinder the motion of dislocations, leading to a rise in dislocation density. On the other side, the addition of small size (<1 µm) SiC particles leads to dislocation multiplication due to the Orowan bowing mechanism [34,39,42]. This means that a stronger interaction between SiC particles and dislocations causes a higher degree of dislocation and an accumulation of micro-strains, which consequently contributes to a decrease in crystallite size (grain refinement) in the composite powder particles.…”