Aluminum Matrix Composites Reinforced with Alumina Nanoparticles

In this chapter, the state of art of metal matrix nanocomposites (MMnCs) is discussed. In particular, the sintering methods used so far by researchers to prepare different combinations of metal matrix and nanoparticles as well as the possible applications of nanocomposites are described. The strengthening mechanisms involved in the extraordinary mechanical properties of MMnCs are also introduced, reporting the literature formulas used to calculate their contributions to the final strength of the material. In this work, Al-based composites reinforced with nanoparticles were prepared via powder metallurgy methods, first by grinding different mixtures of powders via high-energy ball milling, then by compacting powders through equal channel angular pressing (ECAP) and hot extrusion. Therefore, these processing techniques are thoroughly described in this introductory study.

The experimental work was mainly aimed at producing Al based nanocomposites reinforced with alumina NPs. In order to attain a homogeneously dispersion of nanoparticles, several powder metallurgy routes were adopted. They relied on high-energy ball milling and powder compaction either via ECAP or hot extrusion. Different processing parameters were used in order to break the clusters of NPs and to improve the final properties of the composites. The powders and the consolidated materials were analyzed from the morphological, microstructural and mechanical point of view. Several characterization techniques were used to this purpose and described in this chapter.

Commercially pure micro-sized Al powder, either in as-received condition or after high-energy ball milling processing, was used to verify the capability of ECAP and hot extrusion (HE) to consolidate metal powder. A ball-to-powder weight ratio r = 5:1 was adopted for grinding the metal powder for 5 h using 10 % of ethanol as PCA. Consolidation of powder was performed by ECAP, by hot extrusion and by combining the two process. ECAP was also used for consolidating Al-Al₂O₃ composite powder mixed by ball milling. The samples produced by these PM routes are summarized hereunder: (1) Pure Al powders consolidated by ECAP (3 steps at 200 °C), (2) Pure Al powders consolidated by HE (300 °C), (3) Pure Al powders consolidated by ECAP (3 steps at 200 °C) and subsequently processed by HE (300 °C), (4) Composite powders (2 and 5 wt% Al₂O₃) consolidated by ECAP (3 steps at 200 °C). It is to remark that processing temperatures and number of consolidation steps by ECAP were set based on previous experience and on preliminary tests. Microstructural investigations showed an inadequate dispersion of alumina particles within the matrix and the presence of large alumina clusters. Further samples were then prepared raising the milling time up to 24 h without modifying the ball to powder weight ratio and using 1 % of ethanol as lubricant. By this procedure, the following samples were produced: pure Al powders consolidated by ECAP (3 steps at 300 °C), and composite powders (2 wt% Al₂O₃) consolidated by ECAP (3 steps at 300 °C).

Inhomogeneous dispersion of alumina NPs was achieved by following the PM route described in the previous section. For these processing routes, dry Al₂O₃ nanoparticles and micro-sized Al powder were employed as starting materials. In particular, it was understood that high-energy ball milling was not able to completely break up the large alumina clusters. In the following experiments, an isopropanol colloidal solution of alumina nanoparticles was used instead of the dry alumina nanoparticles that were used for the previous tests. High-energy ball milling was performed to grind two different batches of powder: (1) Aluminum powder without ex situ reinforcement addition; (2) Aluminum powder + 2 wt% Al₂O₃ NPs (after manual stirring, the composite powder was dried by means of an electric stove at 50 °C to remove the isopropyl alcohol). Ball milling with a ball-to-powder ratio of 5:1 was performed for 2 h and ECAP was used to consolidate the pure and composite powders. 12 ECAP passes at 300 °C were then carried out to further break the potential Al₂O₃ clusters.

In order to have a more homogeneous particle dispersion, a much more severe ball milling process was carried out. A higher ball-to-powder ratio (r = 10:1) was adopted and the attritioning process was performed for 16 h. So far, better dispersion was achieve by using colloidal solution of γ-Al₂O₃ nanoparticles in isopropyl alcohol than by using dry-Al₂O₃. Thus, the composites were prepared again by adding 2 wt% of colloidal alumina to the Al powder. The micrographs of the starting materials correspond to those shown in Figs. 3.​1 and 4.​1. Either ECAP at 400 °C or hot extrusion at 400 °C were performed to consolidate the composite powder after milling. A selection of the extruded billets were then cold rolled down to a cross section of 1 mm² to verify the workability of the material. Pure Al powder was also consolidated and cold worked following the same procedure adopted for the nanocomposites so as to investigate the properties of a reference unreinforced sample.

Additional experiments were performed with Al powder and alumina suspension adopting an increased milling time (24 h) to further improve the particles dispersion. The other milling parameters were the same used for the powder sample subjected to 16 h of ball milling. In particular, the same ball-to-powder ratio was used (r = 10:1). The powder were consolidated by a single ECAP pass at 400 °C. Samples reinforced with 0, 2 and 5 % of alumina were produced for comparison with the samples showed in the previous sections. Moreover, Al billets reinforced much higher NP fractions of 10, 20 and 30 wt% were prepared.

In this final set of experiments, Al NPs were employed in the as-received and ball-milled condition to produce in situ reinforced MMnCs. The NPs possess higher surface than the micro-sized counterpart. This means that they may lead to the production of nanocomposites reinforced with a much higher content of in situ reinforcement and, as a limiting and desired condition, highly reinforced composites could be produced even without relying on ex situ addition of oxide NPs. For comparison, Al micro-sized powder was consolidated in the as-received condition and after ball milling as well. Furthermore, a mix of the two above-mentioned powders was also employed to complete the frame of experimental conditions. A ball-to-powder weight ratio r = 10:1 was adopted for grinding the metal powder for 16 h using 1.5 % of stearic acid as PCA. Powder consolidation was performed by BP-ECAP. It was expected that the higher content of non-metallic compound made the consolidation of powder rather difficult. It was also known that in SPD processes, more ductile and bigger particles ease the consolidation process [3] since the driving force is the severe plastic deformation of metal powder particles. On the contrary, nano-sized particles cannot accommodate high shear strains and are inclined to slip on each other instead of being deformed. Since the back-pressure (BP) revealed to be able to more efficiently consolidate powders by ECAP, it was applied for producing the nanocomposite billets. After preliminary attempts at different temperatures, 600 °C was selected as a suitable temperature for producing the following full dense bulk samples: (1) As-received Al micro-powders consolidated by BP-ECAP, (2) As-received Al nano-powders consolidated by BP-ECAP, (3) Ball-milled Al micro-powders consolidated by BP-ECAP, (4) Ball-milled Al nano-powders consolidated by BP-ECAP, (5) Ball-milled Al micro-(50 wt%) and nano-powders (50 wt%) consolidated by BP-ECAP.

Different powder metallurgy routes were designed and investigated to produce Al matrix composites reinforced with Al₂O₃ nanoparticles. They relied on high-energy ball milling and consolidation via hot extrusion and ECAP.