Effect of sintering insulation temperature on the properties and microstructure of alumina-reinforced aluminum matrix composites

Aluminum matrix composites reinforced with particles have garnered significant attention due to the growing need for advanced structural materials. This research utilized nano-Al2O3 powder (30 nm in size) and pure aluminum powder (40 μm in size) as raw materials to fabricate alumina-reinforced aluminum matrix composites through the hot-pressing powder metallurgy sintering method. The raw powders were amalgamated using mechanical ball milling and ultrasonic dispersion. This paper delves into the relative densities, hardness, and other properties of the alumina-reinforced aluminum matrix composites, emphasizing the impact of the sintering insulation temperature on these attributes and microstructures. The result showed that the optimal sintering insulation temperature was identified as 400°C, where the Vickers hardness (HV0.5) peaked at 31.3 HV and the relative density reached a zenith of 0.986. SEM imagery revealed that the white nano-Al2O3 particles were evenly dispersed across the silver-white aluminum matrix, with no evident clumping. Furthermore, the nexus between the nano-Al2O3 particles and the composite matrix was seamless and robust. Such features enhance the composite’s performance. The research underscores the potential of enhancing these composites and posits that alumina-reinforced aluminum matrix composites can be effectively produced using the hot-press powder metallurgy sintering technique.


Introduction
The relentless advancement of modern science and technology has ushered in heightened expectations for material performance.Aluminum matrix composites, characterized by their low density, high specific strength, low melting point, excellent processing capabilities, thermal conductivity, heat transfer, and corrosion resistance, have found extensive applications in sectors like transportation, aerospace, machinery manufacturing, and electronics [1][2][3].Consequently, there has been a surge in corporate and academic interest in the evolution of aluminum matrix composites.For instance, Alyn (USA) has pioneered the use of these composites in diverse domains such as aerospace, sports, and electronics.Notably, Boeing's next-gen airliner 777X, GE's CFM56 engine, and Audi's RS5 engine have incorporated fiber or particle-reinforced aluminum matrix composites [4].The interplay between different materials, especially the interfacial reactions between the aluminum matrix and its reinforcements, has been a focal point of research.This has led to the emergence of innovative preparation techniques for aluminum matrix composites.Sun et al. delved into the influence of particle size on the microstructures and mechanical attributes of SiC reinforced through the powder metallurgy process [5].Dang et al. explored the density and solidification shrinkage of hypereutectic Al-Si alloys via the stirring casting method [6].Kumar et al.'s research centered on the impact of particle size on SiC particles synthesized by the spray deposition method [7].Meanwhile, Zhao et al. harnessed the potential of laser additive manufacturing (LAM) to probe the microstructural properties and formation mechanisms in SiC-reinforced Al-based composites.To augment the strength of aluminum matrix composites, a myriad of particles have been employed.S.S. et al. enhanced these composites using ceramic particles like SiC, Al 2 O 3 , and B 4 C. Their findings underscored an increase in strength and hardness at the expense of ductility.On another front, intermetallic composites were crafted using metal-reinforced particles such as Ni and Fe.L.K. et al. successfully generated Al 3 Ni and Al 3 Ni 2 intermetallic compounds via FSP and heat treatment.These metal compounds, juxtaposed with Al alloys, exhibited brittleness and a significantly reduced thermal expansion coefficient, suggesting a propensity for interfacial debonding and microporosity.This invariably bolstered the composites' strength but compromised their ductility.
In the present research, we employed nano-Al 2 O 3 in the hot-press powder metallurgy sintering process to fabricate robust aluminum matrix composites.Given the aggregation tendency of nano-Al 2 O 3 particles, achieving a uniform dispersion in the aluminum matrix, especially at high-volume fractions, poses a challenge.This study aims to ascertain the viability of producing alumina-reinforced aluminum matrix composites by scrutinizing the influence of sintering insulation temperature on the composite samples' properties and microstructures.

Experimental procedure
The study aimed to prepare aluminum matrix composites with a 2wt.% content of nano-Al 2 O 3 particles.This was achieved by blending nano-Al 2 O 3 and pure aluminum powder using the hot-pressing powder metallurgy sintering technique.The used nano-Al 2 O 3 powder had a granular size of 30 nm, a surface area ranging from 100-300 m 2 /g, and a purity exceeding 99%.The aluminum matrix powder had a particle size of 40 μm and a purity of 99.9%.

Mixing Process:
Initially, 2.4 g of Al 2 O 3 and 117.6 g of aluminum powders were combined in a beaker containing 300 mL of anhydrous ethanol.This mixture underwent ultrasonic treatment for 45 minutes.Subsequently, the mixture was transferred to a planetary ball mill (GQM-5-2) for mechanical ball milling, which lasted 16 hours.In post-milling, the slurry was sieved through a metal mesh and then electrically dried at 60°C.

Pressing and Sintering:
Using an electronic balance, 15 g of the dried composite powder was measured and loaded into a highstrength graphite mold.A pressure of 20 MPa was applied for 15 minutes to form a pre-pressed shape.The sample was then sintered in a vacuum carbon tube sintering furnace (CXZT-50-18Y) with a sintering insulation duration of 420 minutes.Given the constraints of the powder metallurgy method and the vacuum hot-pressing sintering furnace, the maximum temperature was capped at 400°C to prevent the powder from becoming glassy and potentially damaging the equipment.The sintering insulation temperatures tested were 200°C, 225°C, 250°C, 275°C, 300°C, 325°C, 350°C, 375°C, and 400°C.After sintering, the samples were allowed to cool to room temperature before being removed from the mold.

Characterization:
The microstructures of the alumina-reinforced aluminum matrix composites were examined using a Leica microscope (Germany, DMI5000ME).The interplay between the aluminum matrix and the alumina in the composites was further scrutinized using a field emission scanning electron microscope (HITACHI, SU8010).The X-ray diffractometer (Xerept powder) was employed to determine the material composition.The Vickers hardness meter (HVS-1000S) was used to gauge the composite's hardness (with a test load of 1.961 N, dwell time of 10 seconds, and the average taken over 7 measurements).Lastly, the actual density of the alumina-reinforced aluminum matrix composites was assessed using a density test machine (X900).

Effect of sintering insulation temperature on Vickers hardness of the aluminum matrix composites
The Vickers hardness measurements of the alumina-reinforced aluminum matrix composites postfabrication (as depicted in Figure 1) underscored the pivotal role of sintering insulation temperature in determining the composite's strength.At a sintering insulation temperature of 275°C, the Vickers hardness registered at 25.1 HV.As the sintering insulation temperature escalated, there was a marked increase in the composite's Vickers hardness.However, beyond 350°C, this hardness plateaued, reaching its zenith at 31.3 HV at a temperature of 400°C.The initial surge in the Vickers hardness can be attributed to the enhanced diffusion of Al 2 O 3 within the aluminum matrix.The material's augmented deformability facilitated pore condensation, bolstering the composite's compactness and cohesion.However, as the sintering insulation temperature exceeded 350°C, the diffusion of Al 2 O 3 within the aluminum matrix reached a saturation point, leading to a marginal decline in the Vickers hardness.Notably, within the temperature bracket of 275°C to 400°C, the composite's hardness exhibited a near-linear upward trajectory.This trend underscores the activation of powder particle surfaces at specific temperatures, leading to inter-particle bonding and the formation of a cohesive entity during sintering.As the temperature rises, this cohesive entity becomes denser, enhancing material properties and consequently, hardness.The powder metallurgy method, coupled with the vacuum hot-pressing sintering furnace, further corroborates this observation.The sintering process activates the powder particle surfaces at certain temperatures, facilitating bonding and the formation of a more cohesive and denser structure, thereby improving material properties and hardness.In essence, temperature emerges as a critical determinant of composite hardness.However, given the constraints of the current experimental setup, where a sintering insulation temperature of 400°C and a mold pressure of 50 MPa rendered the powder glassy, risking damage to the equipment, the temperature threshold was capped at 400°C.The peak hardness was observed at this temperature, suggesting the potential for further experimentation with equipment capable of withstanding higher temperatures.

Effect of sintering insulation temperature on the relative density of the aluminum matrix composites
Figure 2 delineates the correlation between sintering insulation temperature and the relative density of the composites.A discernible trend emerges: as the sintering insulation temperature escalates, so does the relative density of the composite material.This relationship is predominantly linear, with significant increments in value, underscoring the direct proportionality between material density and sintering insulation temperature.The pronounced influence of temperature on the composite's relative density can be attributed to the behavior of aluminum particles during the sintering process.As the temperature rises, these aluminum particles assume the role of a liquid phase.Elevated temperatures enhance the surface activity of the powder particles, fostering more robust inter-particle bonding.Upon subsequent cooling, these bonds solidify, resulting in a tightly knit structure.In essence, the sintering insulation temperature plays a pivotal role in determining the relative density of the aluminareinforced aluminum composites.The aluminum particles, acting as a liquid phase during sintering, facilitate the formation of a dense, cohesive structure, especially at higher temperatures.This phenomenon underscores the importance of optimizing sintering temperatures to achieve desired composite properties.

Effect of sintering insulation temperature on the microstructure of the aluminum matrix composites
Figure 3 presents the microstructure images and SEM photographs of the alumina-reinforced aluminum matrix composites at different sintering insulation temperatures.A striking observation from these images is the absence of discernible pores in the aluminum matrix composites, indicative of the close-knit formation achieved during the sintering process.The composite predominantly showcased two distinct tissues: the white Al 2 O 3 reinforcement tissue and the silver-white aluminum matrix tissue.The Al 2 O 3 tissue was uniformly dispersed throughout the composite, suggesting an even distribution.Furthermore, there was a harmonious integration between the Al 2 O 3 tissue and the aluminum matrix tissue, with the former exhibiting a more homogenous spread.However, despite this overall uniformity, the SEM images revealed the presence of minute gaps at the interface between the aluminum and Al 2 O 3 tissues.These gaps, when subjected to external loads, can act as initiation points for fractures, making them susceptible to propagation.Such microstructural imperfections can significantly compromise the mechanical properties of the composites, underscoring the need for meticulous processing to minimize or eliminate such defects.

Figure 1 .
Figure 1.The Vickers hardness of composites with various sintering insulation temperatures.

Figure 2 .
Figure 2. The related density of composites with various sintering insulation temperatures.

Figure 3 .
Figure 3.The microstructure of composites with various sintering insulation temperatures.4.ConclusionsThis research utilized nano-Al 2 O 3 powder (30 nm in size) and pure aluminum powder (40 μm in size) as raw materials to produce alumina-reinforced aluminum matrix composites through the hot-pressing powder metallurgy sintering technique.The raw powders underwent a combination of dispersion and mechanical ball milling to ensure a homogeneous mix.Subsequent evaluations focused on the relative density and hardness of the samples, emphasizing the influence of the sintering insulation temperature on the properties and microstructure of the alumina-reinforced aluminum matrix composites.The sintering insulation temperature of 400℃ emerged as the most favorable, as evidenced by the Vickers hardness (HV0.5)peaking at 31.3 HV and the relative density attaining a maximum of 0.986 at this temperature.SEM analyses revealed a commendable microstructure.The composites were devoid of pores, and the white nano-Al 2 O 3 particles were evenly dispersed across the silver-white aluminum matrix.There was no noticeable clustering, and the boundary between the nano-Al 2 O 3 particles and the composite matrix was seamless and robustly bonded.The observed microstructural features, particularly the uniform distribution of nano-Al 2 O 3 particles and their strong bonding with the aluminum matrix, are conducive to superior composite performance.The research unequivocally demonstrated that the alumina-reinforced aluminum matrix composites were significantly fortified.The study validated the viability of fabricating alumina-reinforced aluminum matrix composites using the hot-pressing powder metallurgy sintering method, with nano-Al 2 O 3 serving as the reinforcing agent.In summary, this research not only underscores the potential of nano-Al 2 O 3 as an effective reinforcement in aluminum matrix composites but also highlights the importance of optimizing sintering insulation temperatures to achieve desired composite properties.