H. M. MAHAN et al.: THE INFLUENCE OF TiO 2 NANOPARTICLES ON THE MECHANICAL PROPERTIES AND ... 379–384 THE INFLUENCE OF TiO 2 NANOPARTICLES ON THE MECHANICAL PROPERTIES AND MICROSTRUCTURE OF AA2024 ALUMINIUM ALLOY PREU^EV ANJE VPLIV A DODAJANJA NANO DELCEV TiO 2 NA MEHANSKE LASTNOSTI IN MIKROSTRUKTURO ALUMINIJEVE ZLITINE AA2024 Hamid M. Mahan 1,3* , Sergey V. Konovalov 1,2 Kirill Osintsev 1 , Irina Panchenko 2 1 Samara National Research University, Samara, Russia 2 Siberian State Industrial University, Siberian, Russia 3 Middle Technical University, Technical Institute of Baquba, Iraq Prejem rokopisa – received: 2023-05-30; sprejem za objavo – accepted for publication: 2023-06-29 doi:10.17222/mit.2023.898 In this study aluminum alloy 2024 was reinforced with different mass fractions (0 w/%, 2.5 w/%, 5 w/%, and 7.5 w/%) of tita- nium dioxide nanoparticles using the stir-casting method. The main objective was to study an effect of an addition of TiO 2 nanoparticles on the microstructure and the mechanical properties of the 2024 aluminum alloy composite fabricated by stir cast- ing. Scanning electron microscopy, energy-dispersive analysis, as well as X-ray diffraction analysis were implemented to char- acterize the microstructure, elemental and phase composition of the samples. The tensile and Vickers hardness tests were carried out to evaluate the mechanical properties. The results showed that the addition of 7.5 w/% TiO 2 nanoparticles increases the ulti- mate tensile strength by 37 % and elongation by 71 % while decreases the hardness by 14 % comparing with the initial alloy. The highest hardness was demonstrated in the alloy with 5 w/% TiO 2. Keywords: metal-matrix composites, nanoparticles, aluminum matrix, aging V {tudiji smo oja~ali aluminijevo zlitino 2024 z razli~nimi masnimi dele`i (0 w/%, 2.5 w/%, 5 w/% in 7.5 w/%) nanodelcev titanovega dioksida z metodo me{alnega litja. Glavni cilj je bil preu~iti u~inek dodatka nanodelcev TiO 2 na mikrostrukturo in mehanske lastnosti kompozita iz aluminijeve zlitine 2024, izdelanega z me{alnim litjem. Za karakterizacijo mikrostrukture, elementarne in fazne sestave vzorcev so bile uporabljene vrsti~na elektronska mikroskopija, energijsko-disperzijska analiza in analiza rentgenske difrakcije. Za ovrednotenje mehanskih lastnosti so bili izvedeni preskusi natezne trdnosti in Vickersove trdote. Rezultati so pokazali, da dodatek 7.5 w/% nanodelcev TiO 2 pove~a kon~no natezno trdnost za 37 % in raztezek za 71 %, medtem ko zmanj{a trdoto za 14 % v primerjavi z izhodi{~no zlitino. Najve~jo trdoto je izkazala zlitina s 5 w/% TiO 2. Klju~ne besede: kovinski matri~ni kompoziti, nanodelci, aluminijeva matrika, staranje 1 INTRODUCTION One of the most difficult aspects of producing strong, light, and low-cost engineering materials is getting a high strength-to-weight ratio suitable for vehicles. 1 The global need for such products in the automobile and aerospace industries has attracted the attention of re- searchers in the field of composite materials. 2,3 Due to the excellent mechanical properties, aluminum-matrix composites (AMCs) are advanced materials that combine the characteristics of a light and tough matrix material with hard ceramic reinforcement. 4 AMCs satisfy the market need for lightweight, durable, and high-perfor- mance components. 5,6 Traditionally, the most common ceramic reinforce- ments used in AMCs in the last few decades are carbides (SiC, TiC), oxides (Al 2 O 3 , ZrO 2 ,T i O 2 ), and nitrides (TiN, AlN) "e.g.". 7,8 Divagar et al. 9 showed that the addi- tion of 10 w/% SiC and 5 w/% Al 2 O 3 nanoparticles in 7075-T651 aluminum alloy increases the fatigue strength by 13 % compared with the base metal. The presence of ZrO 2 nanoparticles in the aluminum matrix significantly improves the wear resistance and hardness of A356 alu- minum alloy. 10 Jaber et al. demonstrated that the 6063-T6 aluminum alloy reinforced by 7 w/% TiO 2 nanoparticles had 13 % more fatigue strength than the base metal. 11 An addition of 2 w/% TiN nanoparticles in Al2024 followed by aging increases the tensile strength by 38 %, while the yield strength and elongation de- crease from 376.5 MPa and 12.7 % to 359.1 MPa and 9.4 %, respectively. 12 Aluminum alloy 2024 (AA2024) contains Cu, Mg, Mn, and some other minor alloying elements and has an excellent tensile-to-yield strength ratio at elevated tem- peratures, high ductility, fatigue, and fracture resis - tance. 13 These properties as well as the capability to form second-phase precipitates for improved strength (age-hardening) determine the high demand of AA2024 in the aerospace and automobile industries. It was dem- onstrated that the addition of 10 w/% TiO 2 nanoparticles Materiali in tehnologije / Materials and technology 57 (2023) 4, 379–384 379 UDK 669.715:52-334.2:57.012.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(4)379(2023) *Corresponding author's e-mail: hamid.m.mahan@mtu.edu.iq with an average size of 50 nm in AA2024 matrix by me- chanical milling and hot-pressing increases the Vickers hardness by 54 % and the strength by 13 %. 14 Although the properties of the composite are improved compared with the monolithic alloy, the drawback of this technol- ogy could be a low elongation value, which might nega- tively influence the reliability of the alloy when external stresses are applied. Creep resistance is also enhanced by incorporating of 3 /% TiO 2 nanoparticles (15 nm in size) in an Al matrix since the creep behavior depends predominantly on the diffusional flows of the matrix that are strictly limited by the nanoparticles. 15 After examin- ing the available resources, it was discovered that there is limited research on the impact of incorporating titanium into Al-Cu-Mg alloys through stir casting. The AA2024 aluminum alloy is a part of the Al-Cu-Mg alloy series that relies on s (Al 2 CuMg) and (Al 2 Cu) precipitates as the primary sources of strength. Introducing titanium into this alloy category can facilitate the development of titanium aluminides with high strength. A challenge with Al-Cu-Mg alloys is their vulnerability to thermal insta- bility at high temperatures. Nevertheless, by creating ti- tanium aluminides that possess excellent thermal stabil- ity and ensuring their uniform distribution within the aluminum matrix, the thermal stability of these alloys can be enhanced. Since AA2024 has promising properties in different fields and Al-Cu-Mg alloys can be hardened by adding hard particles during material production, this study aims to investigate the mechanical properties of an AA2024 alloy fabricated through conventional stir casting with the addition of different mass fractions of TiO 2 nano- particles. 2 EXPERIMENTAL PART In this study 2024 aluminum alloy with the composi- tion of 92.8 w/% Al, 1.04 w/% Mg, 0.78 w/% Mn, 5.33 w/% Cu, 0.1 w/% Zn and 0.2 w/% Fe was selected as a matrix material. As a reinforcement material we used nanoparticles of TiO 2 with a purity of 99.8 % and a size of 30 ± 5 nm, as shown in Figure 1. Before adding the TiO 2 nano powder with the differ- ent loading fractions of 2.5 w/%, 5 w/%, and 7.5 w/%, the 2024 aluminum alloy was preheated to 700 °C (more than the matrix melting temperature) in a graphite cruci- ble using an electrical furnace to ensure the complete melting of all its components. Using argon as the carrier gas, it was introduced into the molten material as illus- trated in Figure 2. Subsequently, the resulting molten material was poured into a cylindrical mold measuring 22 mm in diameter and 200 mm in length. The stir cast- ing technique was utilized for 4 minutes at 200 min –1 to ensure proper mixing and dispersion of the reinforce- ment material. The stirring action helps achieve a homo- geneous distribution of the reinforcement, reducing the possibility of agglomeration or clustering. The specified time and rotation speed were determined based on previ- ous studies or experimental optimization to achieve the desired level of dispersion and ensure the quality of the resulting composite material. 16 Then the molten material was poured into molds and removed after solidification. The samples were then solution annealed by heating to 500 °C for 4 h in an air-circulated furnace, water quenched at room temperature and precipitation an- nealed (aged) at 17 °C for 3 h. Tensile tests were conducted at room temperature us- ing an INSTRON 1125 universal testing machine in ac- cordance with the ASTM standard E8/E8M. The tensile H. M. MAHAN et al.: THE INFLUENCE OF TiO 2 NANOPARTICLES ON THE MECHANICAL PROPERTIES AND ... 380 Materiali in tehnologije / Materials and technology 57 (2023) 4, 379–384 Figure 1: SEM micrographs of TiO 2 nanoparticles specimen used was a flat shape, with a load of 5 kN ap- plied at a deformation rate of 2 mm/min. The yield stress of the composites was determined using the offset method at 0.2 % strain from the stress vs. strain curve. For microstructural characterization the samples were prepared by the standard metallographic procedure and etched for 15 s using Kroll’s reagent (H 2 O: HNO 3 :H F= 92:6:2). Scanning electron microscopy (SEM) (by TESCAN VEGA) and energy disperse spectroscopy (EDS) (by INCA Energy) analyses were carried out to reveal the microstructure and elemental composition dis- tribution of materials. XRD analysis was performed to examine the struc- ture of the phases and precipitates revealed by SEM. These results were obtained using a DRON-7 instrument (Russia) with Cu-K radiation operated at 40 kV , 30 mA and Bragg angles from 20° to 100° with a step size of 0.02° and a scan step time of 5 s. The Vickers hardness was determined on the polished surface using a HV-1000 tester at a load of 0.025 N for a dwell time of 10 s. At least 10 indentations were made on each sample to obtain the average values. 3 RESULTS AND DISCUSSION Figure 3 shows the results of tensile and hardness tests of the obtained samples with the various mass frac- tions of the reinforcing TiO 2 particles. In general, the samples behaved in a brittle manner, as can be deduced from the low values of the elongation at break (not more than 6 %). The initial sample shows the ultimate tensile strength (UTS) of 208 MPa and elongation before frac- ture of 3.5 %. An addition of 2.5 w/% of TiO 2 particles increased the UTS by 43 % to 299 MPa and elongation by 18 % to 4 %. A further increasing of the content of TiO 2 up to 5 w/% slightly decreased the UTS from 299 MPa to 273 MPa and an almost unchanged elonga- tion. In terms of balance between the strength and plas- ticity the alloy obtained by addition of 7.5 w/% of TiO 2 showed the optimal properties, compared with the others, due to the relatively high UTS of 286 MPa and elonga- tion of 6 %. The yield stress of the fabricated composites reaches its maximum value of 240 MPa at the 2.5 w/% TiO 2, which is higher than in the initial material by 20 %. Figure 3b illustrates the plotted values of Vickers hardness as a function of TiO 2 mass content. The addi- tion of 2.5 w/% and 5 w/% of TiO 2 increases the hard- ness up to 13 % and 40 %, consequently, while at 7.5 w/% TiO 2 the value decreases by 14 %. Such an in- crease can be attributed to the presence of TiO 2 particles in the aluminum matrix that hampers the movement of dislocations. Comparing with the results of previous research the composites fabricated in this study have less UTS, YS and Vickers hardness, but better elongation before frac- ture. This might be attributed to the different routes of fabrication of the materials. The route consisted of me- H. M. MAHAN et al.: THE INFLUENCE OF TiO 2 NANOPARTICLES ON THE MECHANICAL PROPERTIES AND ... Materiali in tehnologije / Materials and technology 57 (2023) 4, 379–384 381 Figure 2: Stir-casting furnace for melting Figure 3: a) Stress-strain curves and b) Vickers hardness of 2024 aluminum alloy with the different mass fractions of TiO 2 chanical milling followed by hot-pressing. The external stress during the fabrication led to the higher density of composite (maximum 2.87 g/cm 3 at 10 w/% of TiO 2 nanoparticles) and, consequently, the higher hardness values. Stir casting followed by solution aging used in this study probably provided less dense composites but also more mobile dislocations that contribute to plastic deformation. 17 Therefore, the maximum elongation was higher (by 2.5 %). Figure 4 shows the microstructure of the fabricated samples after casting and heat treatment. The secondary phases composed of the alloying elements are distributed alongside the grain boundaries and as separate particles inside the boundaries. The area fractions of the intermetal ides (white regions) obtained using image analysis software ImageJ revealed the following results: initial sample has 4.4 % of intermetal ides, at 2.5 w/% TiO 2 – 7.3 %, at 5 w/% TiO 2 – 3.5 %, and at 7.5 w/% TiO 2 –5% . Solution annealing and aging of 2024 aluminum al- loy result in the formation of CuAl 2 Mg and CuAl 2 pre- cipitates Figure 5 and 6. The addition of TiO 2 particles may increase the number of precipitates after a homoge- nization and aging treatment. 18 However, Figure 4 shows that Ti particles are distributed uniformly in the microstructure of the 2024 alloy without the formation of precipitates even alongside the grain boundaries where the accumulation of particles and precipitates usually oc- curs. This might be related to the high solubility of TiO 2 nanoparticles in the structure of the matrix. 19 According to the X-ray diffraction patterns for the 2024 aluminum composite presented in Figure 6, the in- tensities of the peaks gradually rise as the amount of H. M. MAHAN et al.: THE INFLUENCE OF TiO 2 NANOPARTICLES ON THE MECHANICAL PROPERTIES AND ... 382 Materiali in tehnologije / Materials and technology 57 (2023) 4, 379–384 Figure 4: SEM micrographs of 2024 aluminum alloy with the differ- ent mass fractions of TiO 2 Figure 5: SEM image of AA 2024 alloy with the different loaded TiO 2 fraction. Elemental mapping was utilized to prove the presence of Ti at- oms in the microstructure. TiO 2 increases due to the broadening effect. 20 Al peaks are observed at 38.50°, 44.64°, 65.14°, 78.18° and 82.5°. The major peaks of precipitates of the second phases are related to Al 2 Cu, MgCuAl 2 . The peaks related to TiO 2 nanoparticles have low intensities, even with the increas- ing of the mass fraction. In the literature, there are re- ports of reactions between Al and TiO 2 , which yields Al 3 Ti and Al 2 O 3 as final results. 21 In this study, however, no obvious reacted product is apparent, which is in good agreement with the results of the SEM-EDS analysis. To sum up, stir-casting technology is a suitable tech- nology for producing AA2024-TiO 2 composites. The mechanical properties of these composites are slightly lower than those obtained by mechanical milling, but due to the lower cost in some cases this technology might be more economic. A further investigation should be carried out to evaluate other properties of the AA2024-TiO 2 composites fabricated by this method, such as wear and corrosion resistance, fatigue strength, etc. 22 4 CONCLUSIONS In this study, AA2024 was reinforced by TiO 2 nano- particles with different mass fractions using the stir-cast- ing process. Aging was implemented to improve the me- chanical properties of the alloys. The following conclusions can be drawn: 1. The strength and plasticity of the composite in- creases with the increasing mass of TiO 2 nanoparticles. The highest UTC of 299 MPa was obtained in the alloy with 2.5 w/% TiO 2 . The optimal combination between strength and elongation is obtained at 7.5 w/% TiO 2 , which showed UTS of 286 MPa and elongation of 6 %. 2. The Vickers hardness was increased by 40 % when 5%T iO 2 was added, compared with the initial A2024 al- loy. 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