In the current research work, a top-down approach was employed for the refinement of a micron scale AA2124 alloy powder 40
Composite materials are known for their multiple functionalities due to the ability of tailoring the composite properties through controlling the type, composition, and morphology of the matrix and reinforcement. Metal Matrix Composite (MMC) has been developed to meet specific properties that cannot be met by a monolithic material regardless of its composition or processing technique (thermomechanical treatment). MMC can be tailored in different methods depending on the type and shape of the reinforcing phase (dispersion, particulate, whiskers, or fiber) and the consolidation technique produced to produce the bulk product [
Metal matrix composites reinforced with ceramic reinforcements have been extensively studied over the last two decades for their significant use in aerospace aircraft, automobile, and military applications [
Carbides, oxides, and nitrides powder have been used extensively for the reinforcement of Alalloys. In specific, SiC and A
Nanostructured materials exhibit relatively high interfacial energy compared to microstructural materials. The amount of energy stored by the nanostructured materials depends very strongly on the sensitization and processing techniques used for producing Bulk nanostructures material (BNSM). Several techniques have been employed for the synthesis of nanopowders, such as powder atomization and mechanical milling [
Nanoscale materials have been the subject of major interest in recent years due to the anticipated ultrahigh strength and toughness combination that can be achieved in contrast to materials with conventional meso and microscale structures [
Consolidation of bulk products starting from nanopowders represents an additional challenge for materials designed with superior properties. BNSM produced from the consolidation of mechanically milled powders could produce products with promising superior properties due to the relatively produced fine high energetic hard powders. On the other hand, the high hardness induced in the milled powder particles retards the particles’ deformation during the compaction stages that is necessary for filling the empty spaces formed in between the particles to enhance consolidation. Accordingly, this research aims for the production of sound BNS consolidates of nanopowders with improved mechanical properties over those of the micron powder of the same material. This involves the fabrication of a nanoscale AA2124 powder by mechanical milling of a microscale as-received powder using high-energy ball milling. Influence of the hot compaction parameters on the consolidation behavior of the milled powders compared to the as-received one is investigated. Effect of ceramic nanopowders reinforcement additions on the consolidation behavior of both matrices is also investigated.
In the current research, a top-down approach was employed for the refinement of a gas atomized micronpowders of AA2124 alloy with particle size ranging between 5–85
Chemical composition (wt%) of the AA2124 metallic matrix and TiC powders.
Matrix Powder | Cu | Mg | Mn | Si | Fe | Al | |
AA2124 | 3.8–4.0 | 1.4–1.6 | 0.5–0.7 | 0.1max | 0.1max | Balance | |
Reinforcement Powder | Total C | Free C | Fe | Wax | Ti | ||
TiC | 19.71 | 0.16 | 0.20 | 0.30 | 0.87 | 0.15 | Balance |
X-ray diffraction was used to determine the crystallite size of the milled powders compared to the as-received one. The X-Ray diffractometer used is a Diano Co, energized at 45 kV and 10 mA with a dead time of less than
The as-received and milled powders were consolidated into compacts using cold and hot unidirectional pressing at 624 MPa with a pressing holing time of 30 minutes. Tool steel die with an inner diameter of 20 mm was used to consolidate the samples at 480°C (0.75Tm). Height-to-diameter aspect ratios (h/d) of 0.25, 0.5, and 1.0 for the compact samples were investigated for the micron (as-received) and nanoscale (24 hours MT) matrices with and without TiC additions. A Licka optical microscope was used to investigate the microstructural evolution of the hot compacts (HCs) as a function of the various compaction parameters. Vickers microhardness was used to determine hardness of the consolidates using a Mitutoyo microhardness tester. The VHN measurements was conducted at 200 gm for 15 seconds of dwell time. A minimum of 5 indentations were measured per condition. Compressive strength of the produced consolidates was evaluated using round 20 mm diameter and 5 mm height discs. A 50 tons MTS Universal testing machine was employed for conducting the compression tests at a cross-head speed of 0.5 mm/min. A minimum of 3-samples per condition was tested for compression, the average of which was reported.
In the current work, particle morphology and structure size were characterized for the ball milled as-received AA2124 powder for periods of 6, 12, 24, 36, 48, and 60 hours. Densification of the compact AA2124 powders was characterized for the 0, 24, and 36 hours milling times.
Figure
AA2124 particle morphology evolution with increasing milling time (a) 0 hour, (b) 6 hours, (c) 12 hours, (d) 24 hours, (e) 36 hours, and (f) 60 hours.
Effect of milling time on (a) X-ray diffraction
To determine the powder grain size (
The effect of particle size on the compaction relative density of the compacts was investigated. Milling times of 0, 24, and 36 were selected for this analysis. Figure
SEM micrographs for the (a) as-received powder, (b) 24 hours MT, and (c) 36 hours MT.
Cold compaction was conducted in a die 20 mm in diameter for powder volumes that produced h/d ratios of 0.25, 0.5, and 1.0 at a constant pressure of 624 MPa for 30 minutes. Effect of particle size on the extent of densification was determined by Archimedes’ principle using three randomly selected samples [
Effect of MT on the relative change in density of GC produced at compaction pressures of 624 MPa for 30 minutes and h/d ratios of 0.25, 0.5, and 1.0.
Figure
SEM images at low and high magnifications for the (a) 40
The consolidation behavior of the as-received gas atomized micronpowder (40 nm particle size with 78 nm crystallite size) and the 24-hour milled nanopowders (300 nm particle size with 20 nm crystallite size) were mixed with 0, 5, 10, and 20 TiC wt% nanostructured powders. Figure
SEM images for (a) as-received microcrystalline micronpowder, (b) Nanocrystalline nanopowders milled 24 hours, (c) TiC powder, and (d) compaction conditions of the produced mixtures.
Characterization of the consolidation behavior of the produced nanocomposites was conducted for the green and hot compacts. After mixing of the powders, a compaction load of 624 MPa for 30 minutes and 0.25 h/d ratio were used for cold compaction. Figure
Effect of TiC addition on the density variation of the micro and nanopowder matrices green and hot compacts.
5 wt% addition of TiC did not result in a noticeable change in the densification degree for both types of hot compact matrices. However, increasing the TiC content resulted in a significant decrease in relative density as shown in the figure. Further addition of TiC to both matrices resulted in deterioration in density of the produced compacts; although, hot compactions resulted in a significant enhancement in density. 9% and 7.4% increase in density was achieved by hot compaction of the nano and microscale reinforced matrices with 20 wt% TiC, respectively.
Figure
High magnification SEM micrographs for the micro (a)–(d) and nanoscale (e)–(h) green matrices reinforced with 0, 5, 10, and 20 wt% TiC, respectively. Arrows point at TiC particles.
In the microscale matrix composite green compacts (Figures
The observed decrease in density with addition of TiC to the nanopowder matrix disagrees with the produced increase in density observed by El-Eskandarany, who investigated the effect of adding SiC to Al matrix [
In order to investigate the mechanical behavior of HC micronpowder versus the nanopowder with and without TiC reinforcement, microhardness testing was conducted. Note, the microhardness testing investigates the particle-to-particle bond strength regardless of the densification degree, since the performed indentations were conducted only in dense consolidated regions on the surface of the specimen. Figure
A graph showing the Effect of TiC addition on the microhardness for the as-received micron and nanopowder consolidated HC.
It is observed that the hardness of the monolithic nanopowder HC is higher than that of the micron one by 343%. Similar observations were made by Chen et al., on nanocrystalline NiAl milled powders [
The compressive strength measured for the AA2124/TiC composite materials obtained by hot compaction for the micro and nanopowder compact matrices with and without TiC reinforcement is displayed in Table
Effect of TiC additions to the micron and nanopowder matrices.
TiC wt% | Compressive strength (MPa) (micronpowder matrices) | Compressive strength (MPa) (nanopowder matrices) |
---|---|---|
0 | 313.66 | 389 |
5 | 357.84 | 493.23 |
10 | 415.55 | 538.57 |
20 | 373.65 | 507.47 |
Compressive strength of HC micro and nanopowder consolidates as a function of TiC additions.
Investigation of the hot compacts microstructure at low and high magnification (Figure
Optical micrographs at low and high magnifications showing the microstructure of the hot compact discs produced for the monolithic (a), (b) micronpowder and (c), (d) nanopowder HC AA2124.
Figure
OM micrographs showing low magnification images for the (a)–(d) micronpowder 0, 5, 10, and 20%, and (e)–(h) nanopowder 0, 5, 10, and 20% consolidated HC matrices, respectively.
Figure
OM micrographs showing high magnification images for the (a)–(d) micronpowder 0, 5, 10, and 20%, and (e)–(h) nanopowder 0, 5, 10, and 20% consolidated HC matrices, respectively.
Comparing the reinforced nanopowder HC matrices (Figures
The observed intense matrix deformation around the TiC particles in addition to the strain hardening effect induced by ball milling of the matrix powder explains the exhibited increase in VHN values and compressive strength measured for the Nanopowder consolidated matrices compared to the micronpowder one. AA2124 is a precipitation-hardenable alloy that contains different kind of second phase particles most important of which is the Cu-rich phases which are responsible for the strengthening of the alloy on aging [
Further investigation is required to understand the consolidation behavior and deformation mechanism of the nanopowder matrix compared to the micropowder one with and without reinforcement. One of the major causes that might have contributed to the formed cavities is the problem of entrapped air pockets within the individual particles and agglomerates. The powder compaction stage was done in open air without the use of vacuum or inert gas. A much improved consolidation behavior is expected under controlled atmosphere. Moreover, metal forming after cold or hot compaction will without doubt increase density and hence enhance the mechanical behavior of the consolidated bulk specimens.
Investigation of the effect of aging time at 175°C was also investigated for the micron and nanopowder matrices in the monolithic and reinforced conditions. Table
Effect of aging at 175°C on peak aging time, hardness, and compressive strength.
TiC (wt.%) | Micronpowder matrices | Nanopowder matrices | ||||
Peak aging time (min) | VHN | Peak aging time (min) | VHN | |||
0 | 560 | 190 | 377 | 530 | 305 | 458 |
5 | 430 | 195 | 453 | 360 | 365 | 540 |
10 | 300 | 220 | 508 | 270 | 375 | 636 |
20 | 190 | 242 | 464 | 180 | 425 | 624 |
From Table
Comparing the continuously cast and rolled sheets of AA2124-T851 properties with those produced for the investigated 24 hours milled powder consolidates in the T6 temper condition, it is found that a hardness of 146 HV versus 309 HV is produced, respectively. In addition, the hot compact nanopowders reinforced with only 10 wt% TiC displayed compressive strength that is higher (636 MPa) than MMC of AA2124 micronpowder (75
In the current paper, we have what follows.
Nanocrystalline-nanopowders
The consolidation behavior of the micro and nano consolidated matrices was characterized in the monolithic condition and with addition of TiC ceramic nanostructured particles.
The green compact densities of the nanoscale powder decreased to 92% compared to 97% for the micronpowder as-received powder. This was due to the resistance of the strain hardened agglomerates to the applied pressure used for compaction.
The degree by which the density decreased with the addition of 5 wt% TiC to the matrix was much lower for the nanoscale consolidated matrix compared to the micronpowder consolidated one due to the low RPS ratio between the matrix and the reinforcement, which promoted uniform distribution of the TiC particles within the matrix agglomerates.
Increasing TiC content up to 10 wt% results in the formation of large voids and cavities, which reduces the compressive strength of the produced compacts, although hardness increases with TiC addition regardless of the content.
Hardness and compressive strength increased by peak aging at 175°C for monolithic and composite consolidated nanostructured AA2124, which qualifies them for high performance load bearing light weight applications.
Based on the current research nanopowder of Alalloys produced by 24 hours mechanical milling of gas atomized micronpowders reinforced with 10 wt%. TiC is recommended for products suitable for high wear and erosion resistance applications.
Further investigation is required using TEM for microstructural analysis on the nanoscale level. In addition, investigation of the compressive strength of the produced compacts with and without heat treatment is also planned.
The authors of the work would like to acknowledge the Yousef Jameel Science and Technology Research Center (YJ-STRC) at the American University in Cairo, Egypt for the financial support of the current research.