Manufacturing of AA7075 Aluminum Alloy Composites Reinforced by Nanosized Particles of SiC, TiN, and ZnO by High-Energy Ball Milling and Hot Extrusion

In this work, composite powders of aluminum alloy 7075 (AA7075) reinforced by 2 weight percent of nanosized particles of silicon carbide (SiC), titanium nitride (TiN), and zinc oxide (ZnO) were produced in a bath of isopropyl alcohol by high energy ball milling during 480 min at 25 ºC, 900 rpm, and Balls-to-Powder Weight Ratio (BPR) of 20:1. The techniques of X-Ray Diffraction (XRD), Laser Diffraction Method (LDM), Scanning Electron Microscopy (SEM), and microanalysis of Energy Dispersive Spectroscopy (EDS) were used to characterize the powders as received and processed. Then, the composites were hot extruded and characterized by XRD, SEM, and microhardness Vickers (HV). The milling process reduces the crystallite and particle size to around 30 nm and 10 µm, respectively. After extrusion, a fine microstructure and good consolidation were found for all bars, except for AA7075 as received. The ranging microhardness values were from 97 HV to 121 HV.


Introduction
A composite combines two or more different materials to obtain a new material with unique characteristics.The type, variation, and interaction of reinforcement in the matrix are vital in determining the final properties 1 .
Nanostructured metallic matrix composites usually combine the ductility of the metal (matrix) with ceramic particles (reinforcements) to enhance the mechanical properties to values found in the material of responsibilities 2 .For example, the aluminum alloy matrix can promote characteristic properties of high strength stiffness to weight ratio, good formability, corrosion resistance, and recycling potential, making it the ideal candidate to replace heavier materials like steel or copper in other applications.In this context, the aluminum alloy 7075 matrix can support high-stress structural due to the formation of MgZn 2 precipitates.At the same time, SiC particle reinforcements can promote a significant strength, TiN increases the self-life of material, and ZnO improves the substrate coating [3][4][5][6][7][8] .
The powder metallurgy route can manufacture nanostructured composites using High-Energy Ball Milling (HEBM) technique.The process has a significant advantage in grain and particle reinforcement because it utilizes high-frequency impacts causing severe plastic deformation, cold welding of the particles, and repeated fracturing 9 .HEBM introduces shear bands that contain a high-density network of dislocations and other crystallite defects that reduces crystallite and particle size and promotes changes in morphology before reaching the equilibrium state [10][11][12][13] .Special attention is paid to studying the influence of the medium on the milling process when developing ball-milling methods.Composites milled in a liquid medium can present finer ground than achieve dry milling due to how the nanoparticles are dispersed in the matrix.Moreover, HEBM in a liquid media can increase the contamination of the process and decrease the surface energy of particles [14][15][16][17][18][19] .
The hot extrusion technique is suitable for consolidating powdered material and excluding the sintering step.Usually, it requires elevated pressures and temperatures to reach desired quality and microstructure.On the other hand, the parameters of extrusion, such as the force applied, the billet temperature, and the extrusion speed, are primary factors influencing the quality of the specimens 20 .It is an efficient route to manufacture aluminum alloy composites reinforced with ceramic phases with good densification 21,22 .
Therefore, this paper focuses on manufacturing AA7075 metal matrix composite reinforced with nanosized particles of SiC, TiN, or ZnO produced by high-energy ball milling and hot extrusion.The aim is to evaluate the effects of milling in a bath of isopropyl alcohol at room temperature and reinforcements on the microstructure before and after extrusion and the microhardness of the extruded bars.
The control of these parameters is fundamental to producing a nanostructured composite with better performance.

Manufacturing and characterization of powders
AA7075 powders with 99.7 % purity, supplied by Aluminum Powder Corporate, have been used as matrices.The initial particle size obtained by LDM was 31.7 µm (D50), and the crystallite size measured by XRD was 49 nm.According to manufacturer, the chemical composition in weight percentual is in Table 1.
Three nanometric powders have been used as reinforcements.The nanoparticle sizes ware informed by the manufacturers: SiC, with a D50 of 50 nm (supplied by Iolitec GMBH), TiN, with a D50 of 20 nm (supplied by Iolitec GMBH); and ZnO, with a D50 of 100 nm (supplied by Sigma -Aldrich Corporate).
For each batch, 50 g of nanocrystalline powders AA7075 alloy was put in stainless steel attritor mill developed by the UFPE, equipped with a K-type thermocouple and a temperature controller.In all cases, milling was carried out for 480 minutes at 900 rpm, with a 20:1 ball-to-powder weight ratio (BPR) using 6.4 mm stainless steel balls 100C6 (1 %C, 1.5 %Cr), 1 wt% of zinc stearate (C36H70O4Zn) as process control agent (PCA), a bath of 100 ml isopropyl alcohol (C3H7OH -99.82 %) as liquid media milling, and the process temperature (25 ºC) maintained via a water refrigerated jacket, around the attritor mill 23 .
The reinforcements have the same concentration of 2 wt% for comparative purposes.This final percentage follows a previous work developed using TiC demonstrating that crystallite size diminished as reinforcement concentration increased from 0.5 to 2 wt% 24 .
After milling, all the samples were dried at 100 ºC to evaporate the alcohol completely 25 .Then, the crystallite size, particle size, and morphology were analyzed.
XRD (Shimadzu XRD -7000) investigated the crystallite size in the 2-Theta range from 5º -120º with a scan rate of 0.02 º/s at 40 kV and 30 mA.The linear regression of the Williamson -Hall plot equation 26,27 (Equation 1) was used to calculate the crystallite size and the contribution of micro deformation to the Full Width at Half Maximum (FWHM) of the four principal peaks of aluminum for a confidence factor of more than 92 percent.The experiment did not consider the instrument's effect on crystallite size comparatively.
Where "FWHM" is the full width at half maximum in radians; "k" is a constant (0.94); "λ" is the wavelength of the x-rays (15.4nm); "L" is the average crystallite size; "θ" is the Bragg angle, and "ε" is the micro deformation measured.
Laser Diffraction Method, LDM (Malvern Mastersizer 2000) determined the particle size.The sample was suspended in alcohol or water and agitated by ultrasound 28,29 .
SEM -Hitachi TM 3000 operating at 20 kV, equipped with an EDX probe, analyzed the morphology and composition of the particles.

Hot extrusion consolidation and extruded bars characterization
All the composite powders were cold compressed as billets at the force of 35 kN during 120 s and compression speed of 1 kN/s.The billets were placed in a mold equipped with resistors to 450 ºC.The billets were extruded in a relation of 25:1 at a speed of 1 mm/s to obtain bars of 5 mm diameter [30][31][32][33] (Figures 1-3).Furthermore, boron nitride (BN) was used as a solid lubricant due to its chemical inertness, high-temperature stability (1000 ºC), good thermal conductivity, and electrical insulation to avoid sticking during extrusion 34 .
XRD, SEM, and Vickers microhardness characterized the extruded bars.Shimadzu HMV-2 durometer (HV 0.05) performed the microhardness according to ASTM E-384 35 .XDR and SEM equipment were the same as those used for powder characterization.

Characterization of AA 7075 non-reinforced and reinforced composite powders
All the results are presented to compare the AA7075 powders received, milled, and alloyed milled with the different types of nano-reinforcements (composite powders).Table 2 shows crystallite and particle sizes, Figure 4 shows the diffractograms, and Figure 5 presents the particle size distribution.
After gridding, the diffraction peaks became broader and minor.The crystallite size decreased from 49nm to around 30nm, even without any reinforcement, because of the cold-welding phenomenon and repeated fractures 36 .Besides that, the medium particle size decreased from D50 of 31.7 µm to around 10 µm, and micro deformation increased from 0.02 percent to around 0.12 percent.The process observes similar values among AA7075 milled, AA7075 plus 2wt% of TiN or ZnO.The crystallite and particle size values for the sample AA7075 plus 2 wt% of SiC were superior because of less micro deformation found.
The SEM (Figures 6-9) shows an almost spherical morphology for the as-received powders from the gas-atomized manufacturing process.After milling, small rounded and flat particles show a quasi-equilibrium state found for all cases.
The milling efficiency depends on the characteristics like mechanical parameters applied, the medium used, type, and percentage of reinforcements and alloy elements 37 .Wet milling provides relatively small stress and temperature, and distributes the reinforcement to the whole system, while the energetic conditions (high milling time, rotation speed, and BPR) produce the balanced morphology of the particle 38,39 .
The microanalysis of EDS (Table 3 and Figures 10-14) indicates that alcohol liquid promotes the rise of oxygen percentual.No contamination from the steel balls and the stainless steel attritor mill was detected.

Characterization of extruded bars
Figure 15 shows the diagram of the relationship between the applied force and the displacement of the piston during the extrusion of all materials.For the composites and powder AA7075 milled, the forces required to extrude the bars (800 kN) were more significant than the ones found to extrude the powder AA7075 as received (540 kN).However, the crystallite decreased after milled, so the microhardness increased due to the Hall-Petch effect.The reinforcements also give extra resistance to the AA7075 milled powders changing the forces to extrude.The TiN reinforcement is the one that promotes higher force due probably to an increase of the friction between particles by its chemical nature and initial size of 20 nm.
Figure 16 shows a significant difference among the diffractograms of the specimens extruded compared to diffractograms of powders samples AA7075 (Figure 4).The bars extruded maintain the same peaks found at the origin powders, except for an increase of intensity from the crystallite size growth and an amorphous phase indicator at 20 º degrees.The amorphous phase reduces the intensity of the diffraction peak when compared to the crystalline response due to the characteristics applied to the extrusion process (extrusion rate of 25:1, temperature of 450 ºC / 2 minutes, extrusion speed of 1 mm/s).These parameters provide great frictional forces during extrusion and promote less homogeneous structural changes 40 .Specifically, the Al secondary reflections of AA7075 as received, reduced due to significant changes in the structure of the spectrum, suggesting that the portion of alloying elements was dissolved in the matrix to form a solid solution.Therefore, there is a critical temperature at which grain growth decrease due to pre-existing alloy precipitates acting as a barrier in the matrix.With the increase in temperature by extrusion, there is the dissolution of precipitates considered anchors of the grain boundaries 41,42 .
According to Table 4, crystallite size grows about ninety percent of the value found in powders except for the asreceived one.The results indicate that the composite extruded bars and the AA7075 milled extruded bar have less thermal stability than the bar of AA7075 as-received because the crystallite size is inversely proportional to the volumetric modulus and an increase in temperature 43 .
The SEM micrographs (Figure 17) show a fine microstructure and good consolidation for all milled extruded bars, except for AA7075 as-received.In this case, the bar was deformed heterogeneously, and superficial agglomerates formed in the extrusion direction due to the thermal stability found.Table 5 shows that high-energy ball milling increases powders' microhardness from 21 % to 52 % compared to the as-received material.The results suggest the combination of two mechanisms related to the microhardness of extruded composites: the reduction of crystallite size and the inhibition of the movement of dislocations by the dispersion of reinforcements originating from the milling process 44,45 .The highest hardness values are related to the smaller crystallite sizes obtained after extrusion and the kind of reinforcement used.

Conclusions
Powders of AA7075 unreinforced and reinforced by nanosized particles of SiC, ZnO, and TiN in 2 wt% were produced by high-energy ball milling, and bars were extruded with relative success.The results can be resumed as follows: 1.For all samples of milled powders, the crystallite and particle sizes achieved values of the same magnitude independent of reinforcement, about 30 nm and 10 µm, respectively.2. Small rounded and flat particle powders show a quasi-equilibrium state achieved, and no contamination from the steel balls and the stainless steel attritor mill was detected.3.All extruded bars obtained a fine microstructure and good consolidation, except for AA7075 as-received, which was deformed heterogeneously, and superficial agglomerates were formed in the extrusion direction.4. The highest hardness values are related to the smaller crystallite sizes obtained after extrusion and the kind of reinforcement used.

Acknowledgments
This work has been carried out under the projects: "Functional" -Partnership between the Federal University of Pernambuco and European Union's Seventh Framework Programme for Research and "Aluminum Metal Matrix Composite Ceramic Particulate Reinforcement" of the Federal Rural University of Pernambuco, National Council for Scientific and Technological Develop (CNPq) -Brazil and Pernambuco Science and Technology Support Foundation (FACEPE) -Brazil.

Figure 2 .
Figure 2. a) Extrusion mold equipped with resistors; b) Mold placed in a compression machine.

Figure 3 .
Figure 3. Example of the bar extruded.

Figure 15 .
Figure 15.Force VS Piston displacement for all powders.

Table 1 .
Values of chemical composition of AA7075 powders.

Table 4 .
Values of crystallite size for all billets extruded.

Table 5 .
Values of microhardness Vickers for all extruded bars.