Enhanced Mechanical and Fatigue Properties In AA5052 Via TiO2 Nanoparticles Addition Sintering Temperature (ST)

 TiO2 particles reinforced AA5052 composites are successfully fabricated by the stir casting route.  The effect of different sintering temperatures (900, 1000 and 1100°C) on fatigue and mechanical characteristics of the metal matrix AA5052 reinforced with 5% of TiO2 nanoparticles was studied.  Compared to other sintering temperatures, the nanocomposite with a sintering temperature of 1000°C has the highest hardness, ultimate tensile strength, yield strength and the lowest elongation. The goal of the present work is to study the effect of different sintering temperatures (900, 1000 and 1100°C) on fatigue and mechanical characteristics of the metal matrix AA5052 reinforced with 5% of TiO2 nanoparticles. The stir casting process is used for manufacturing of AA5052/TiO2 nanocomposite. The mechanical characteristics of nano composites have been obtained at ambient temperature. The results of mechanical properties showed that the best enhancement in hardness (HB), ultimate tensile strength (UTS) and yield strength (YS) is occurred in nanocomposite with 1000°C sintering temperature (ST). However, the fatigue test results showed that the samples manufactured under 1000°C (ST) have longer fatigue life compared to other materials with different sintering temperatures. The endurance fatigue strength is improved by 7.2% compared to metal matrix. The experimental results showed that the microstructure image of 1000°C (ST) composite has uniformly distributed of TiO2 in AA5052 matrix. A R T I C L E I N F O Handling editor: Muhsin J. Jweeg


Introduction
Aluminium matrix composites (AMCs) are well known to be the substances usually reinforced with the matrix by hard particles such as carbides (Al 4 C 3 , SiC, TiC), oxides (Al 2 O 3 ) or nitrides (TiN, AlN) [1]. Only a few parts of the machine are exposed to static loading, and most parts of the machine are exposed to variable loading. Experimentally, this form of failure is referred to as fatigue when a material is subjected to dynamic loads, it fails at a stress below the yield stress. Fatigue can happen under constant or variable loads. Constant fatigue loading is defined as fatigue under cyclic loading with a consistent amplitude and a constant mean stress or load [2]. Abdizadeh et al. [3] examined the mechanical and microstructure characteristics of AA356 reinforced with different percentages of nano mgo particles. The AA356/mgo were produced by using stir casting at different temperatures of 800, 850, and 950℃ and for powder metallurgy at 575, 600, and 625℃. In contrast to the casting samples, they found that powder metallurgy samples show higher porosity sections. Higher porosity portions in sintered composites and more agglomeration and aggregation of mgo nanoparticles in casting samples were seen in scanning electron microscopy images. More homogeneous data and higher values of mechanical properties were expressed in the casting process compared to the powder metallurgy method. T. Rajmohan et al. [4], tested mechanical and microstructural properties of aluminum reinforced with various weight percentages of (CuO) and constant amount of (SiC) using the sintering 1385 process. The experimental results showed that the distribution of SiC and CuO in the Al matrix were relatively homogeneous and the mechanical properties were enhanced by increasing the content of CuO reinforcement nanoparticles. In composite, including (2 CuO+10 SiC wt%), the best improvement was noted. Alalkawi et al. [5] illustrated the effect of adding 10% of alumina Al 2 O 3 with a particle size of 50 nm to the AA6061 metal matrix. The AA6061/Al 2 O 3 were manufactured using the stir casting technique. The experimental results showed that the addition of 10wt% Al 2 O 3 improved the fatigue life and cumulative fatigue power. The outcomes revealed that the fatigue strength improvement was 12.8% at 10 7 cycles due to 10wt% nano reinforcement. Qusay. K. Mohammed, [6], fabricated AA7075/Al 2 O 3 using the stir casting technique with 6 minutes stirring time and 450 rpm stirring speed. The Al 2 O 3 with a particle size of 10 nanometer and different weight percentages of (0.3, 0.5, and 0.7wt%) were used to reinforced AA7075. The outcomes showed that the best improvement in fatigue life and strength occurred at 0.3% of Al 2 O 3 . M. Vaghari et al. [7] used wet attrition milling followed by hot forward extrusion process for fabricating Al reinforced with alumina Al 2 O 3 with different weight percentages of (4, 6 and 8%). The findings showed that the existence of up to 6% of Al 2 O 3 nanoparticles improved the fatigue strength of Al/Al 2 O 3 nanocomposites, while the greater amount of reinforcement has a negative impact on the strength of the fatigue. Al-Alkawi et al. [8] studied the effect of 6% of alumina Al 2 O 3 nanoparticles with AA7100 metal matrix on constant fatigue strength and life at 200℃ under rotating bending loading. The outcomes showed that the fatigue strength improvement factor was obtained to be from 0.84 to 4.84 for 10 3 and 10 5 cycles, respectively. However, increasing the life from 10 3 and 10 5 cycles leads to an increase in FSIF of 4. On the other hand, the FSIF at 200℃ was obtained to be 0.11. Abdulridah et al. [9] investigated the effect of cryogenic temperature on mechanical properties and fatigue behavior of AA2024/Al 2 O 3 nanocomposites. The stir casting technique was used to fabricate the AA2024/Al 2 O 3 with different weight percentages of Al 2 O 3 . It was found that the composite that contains 0.9% Al 2 O 3 showed the best properties and maximum enhancement, which shows an improvement in ultimate strength, yield stress and Brinell hardness of 9.9, 14.63 and 14.28%, respectively compared to the base metal matrix. The fatigue endurance limit of the 0.9wt% Al 2 O 3 composite was taken at 10 7 cycles in two cases without (CT) and with (CT). At 10 7 cycles, the exhaustion limit rises from 83 MPa without (CT) to 89.6 MPa with (CT) resulted in an increase in the fatigue limit of 7.95%. Mamoon A. A. Al-Jaafari [10] studied the fatigue behavior of AA6061 metal matrix reinforced with SiC of (10nm) particle size and with different weight percentages of SiC. The AA6061/SiC nanocomposites were fabricated using stir casting method with rotating bending type of load at room temperature and with a stress ratio of (R=-1). At, 2wt%, the highest fatigue strength and life were made. Due to relatively uniform distribution strengthening of nano particles and limited porosity, percent nano particles were obtained. The percentage of rise in endurance fatigue limits for 10 7 and 5 * 10 8 cycles were 11.48% and 11.05%, respectively. Iman S. El-Mahallawi et al. [11] tested the mechanical properties of A356 reinforced with alumina (Al2O3), titanium dioxide (TiO2) and zirconia (ZrO2) nano-particles (40 nm). They were stirred in the A356 matrix at variable stirring speeds varying from (270, 800, 1500, and 2150 rpm) in both the semisolid (600 °C) and liquid (700 °C) states with a constant stirring time of one minute with different fraction ratios ranging from (0 -5 %) by weight. The experimental results showed that the mechanical properties (strength, elongation, and hardness) of Al2O3, TiO2 and ZrO2 nanoreinforced castings made in the semi-solid state (600 °C) with 2 wt.% Al2O3 and 3 wt.% TiO2 or ZrO2 at 1500 rpm stirring speed were improved. Ali Y. Khenyab et al. [12] manufactured AA7075-T61 metal matrix with different weight percentages of Al 2 O 3 (10%,15%, and 20%) and TiO 2 with 5% were used as reinforcement materials using the stir casting technique. The results revealed improving in fatigue life and mechanical properties. It has been noticed that the well distribution of nano particles on the matrix in microstructure examination, leads to reduce the grain size which leads to improving in fatigue life higher than the base metal and other composites by (3.5%, 7.7% and 9.7%). The objective of the this work is to enhance the mechanical and fatigue properties of AA5052 reinforced with TiO2 nanoparticles using three sintering temperature of (900, 1000 and 1100 ℃). Some of the previous studies used different weight percentages of nanoparticles and variable stirring speed and they also used various temperatures using two methods, namely stir casting and powder metallurgy and they found improvement in mechanical and microstructure properties, but the current study used constant weight percentage of TiO2 nanoparticles with various sintering temperatures of (900, 1000 and 1100 ℃) using the stir casting method. The results of this study revealed an improvement in mechanical and fatigue properties at a sintering temperature of (1000 ℃).

Experimental work
The chemical composition of AA5052, sample preparation and standard geometry of the samples are presented in this section.

Material
Aluminum metal matrix AA5052 is used in the present work. AA 5052 is one of the Al-Mg alloys that, due to its strong properties, is mainly used in the automotive, aerospace and marine industries [13][14][15][16]. AA5052 is enhanced with TiO2 with a particle size of (30nm). The chemical composition of AA5052 is as shown in Table I.

Manufacturing of specimens
The AA5052/TiO 2 were fabricated by using the stir casting method. The procedure of the process can be as follows [17]: Initially, the desired sample weight is determined by the rule of volume x density m=V×ρ→m=0.6×12×5×3→m=108gm (1) 1386 Where (m) is mass in (gram), (V) is the volume in cm 3 and (ρ) is density in g/ cm 3 =3, (h) thick in cm=0.6, (L) length in cm=12 and (b) wide in cm=5. Calculate the proportions of added elements (magnesium and chromium). Mg weight=108×0.0317→Mg weight=3.4236g Cr weight=108×0.0043→Cr weight=0.4644g Al weight=3.4236-0.4644→Al weight=2.9592g After that, put the aluminum, which is in the form of pure wires, in the crucible and then start fusion it up to 750℃ for 5 minutes by using a gas furnace. During the aluminum smelting process, a sand mold is prepared according to the dimensions of the sample. Chromium is added to the melt, which is placed in aluminum foils, wrap it well and then immerse it in the molten metal. Leave it about three minutes, then mixing is done by using an electric mixer at a speed of 600 r.p.m for a minute. After mixing is completed, magnesium is added which is in the form of strips and cover it with aluminum foils, then expel the air, then add it to the inside of the molten, keeping it under the slag in the melt as well.
Then, mix it until the magnesium and chromium are distributed well, then add 1 gram of aqueous aluminum chloride (AlCl 3 ) for the purpose of removing slag and expelling gases. After that, the slag is removed and the first casting is poured without adding nano titanium oxide after it has frozen and then, remove it from the mold. For the second, third and fourth castings, the same previous steps are used, but nano titanium oxide is added after coating it with aluminum foil to expel the gases before adding the magnesium. It is mixed for five minutes using an electric mixer, then magnesium is added in the same way as the previous one also mix well and aluminum chloride is added as an aid to remove slag. Then, leave it for10 Minutes until the sample freezes, after that it is removed from the sample. Subsequently, the mixture is poured into the mold and according to the temperature, where the second casting was added at a temperature of 900, the third casting at a temperature of 1000 and the fourth at a temperature of 1100.

Tensile test
In order to determine the mechanical behavior of cast AA5052 and fabricated nanocomposites, tensile tests were performed. A tensile test specimen according to ASTM D 638-97 [18] is shown in Figure 1. The tensile samples were manufactured from the cast AMCs. Specimens have been tested using a tensile machine at room temperature.

Fatigue test
The samples were manufactured according to D 3479/D 3479M-96 ASTM [19]. In order to satisfy the machine test section that is appropriate for flat plate specimens, fatigue specimens were cut to acceptable dimensions. Figure 2 demonstrates the shape and dimensions of the fatigue sample [20]. The AVERY fatigue testing machine of type-7305 was used to conduct all the fatigue testing.
The AVERY fatigue testing machine of type-7305 was used to apply reverse loads. For the bend test, grips are provided where the load is imposed by an oscillating spindle driven by a connecting rod, crank, and double eccentric attachment at one end of the specimen. To give the required range of bending angle, the eccentric attachment is adjustable. From the deflection, the applied stress is measured. The motor is equipped with a revolution counter to record the number of cycles. The cycling rate is 1400 rpm [20]. The stress ratio of R= -1 was adjusted for the device. On each collection of specimens, a series of experiments were carried out by altering the deflection angle each time and recording the number of cycles to failure. Figure 3 shows an example of the specimen after failure of the fatigue test.

Tensile test results
For the evaluation of ultimate tensile strength, yield tensile strength and other properties, tensile tests were used. The experimental test established stress-strain curves for pure AA5052 and AA5052 reinforced with constant weight percentage of (5wt%) of TiO 2 nanoparticles, as shown in Figure 4. The results of the mechanical properties of pure AA 5052 and its composites with various sintering temperatures calculated from Figure 4 are shown in Table II. Jufu Jiang et al. [21] tested mechanical properties of AA2024/Al2O3 by using ultrasonic assisted semisolid stirring (UASS) method and rheoformed into a cylinder component. The outcomes showed that at the bottom of the rheoformed composite components, the optimum ultimate tensile strength (UTS) of 358 MPa and YS of 245 MPa was obtained after 25-min stirring of the composite semisolid slurry with 5% Al2O3 nanoparticles at 620 °C. Table II shows that the mechanical properties of nanocomposite were affected by the sintering temperature. It can be inferred that nanocomposite AA5052/TiO 2 at 1000(ST) has the best mechanical properties. The above improvements may be coming from the existence in the matrix of hard TiO2 particles and the low degree of porosity and uniform distribution of the nano particles. The introduction of TiO2 nanoparticles into the aluminum matrix is believed to provide some heterogeneous nucleation sites during solicitation resulting in refined grains [22,23] and the sintering temperature, which leads to high dislocation and this will reduce the grain size.

Fatigue S-N curve results
The specimens were tested under constant amplitude fatigue. The results of this test are illustrated in Tables III and IV. The applied stress can be calculated from the following equation [24]: Where is applied stress in (MPa), modulus of elasticity is E in (Gpa)=70 Gpa, h is thickness of specimen=4mm, maximum deflection is in (mm) and 0 is the effective length in (mm). The effective length can be calculated from the equation [24]: Where L is half length = 32mm, deflection is calculated from the calibration curve [24], (84) specimens were tested to get the S-N curve, (21) specimen for condition as received AA5052 and other samples for conditions with 5wt%TiO 2 and different sintering temperatures. The S-N curve was plotted depending on fatigue equations, as shown in Figure 5.
The (S-N) curves equations are calculated according to Basquin equation of the form of σ=A〖Nf〗^bwhere A and b are material constant fatigue parameters and endurance limit, as shown in Table IV.     The results of endurance limit shows that the higher fatigue strength occurred for the nanocomposite under 1000 ℃ (ST) then when the sintering temperature increases to 1100℃ the endurance limit decreases due to less bonding between matrix and nanoparticles and less distribution of TiO2 compared to other nanocomposites. Using Basquin's formula, it is easy to express the fatigue strength in terms of the stresses corresponding to a given lifetime on the mean S-N curve. The results of the fatigue strength are shown in Figure 6. It is clear that the endurance fatigue limit of 1000℃ (ST) nanocomposite is higher than the matrix by 7.2%.

Microstructure
The distribution of the nanoparticles in the matrix plays a significant role in determining the mechanical properties. If there are fewer porosities after casting, the nanocomposite gets excellent characteristics. The pure AA 5052 and AA5052/TiO2-5wt% at 1000℃ sintering temperature (of the third) composite samples were carried out with optical micrographs. Figure 7 shows the optical micrographs of the base metal and 5wt% TiO2 nanocomposite at 1000℃ (ST). Figure 7(b) reveals the presence of TiO2 with homogenous dispersion of TiO2 into the base metal. Also, the microstructure of nanocomposite reveals small discontinuity and a reasonably uniform distribution of TiO2 particles in the aluminum matrix. Due to the nanosize particles and sintering temperature, the higher fatigue intensity increases and the surface morphology is shown in Figure 7. The density of dislocation and the minimization of grain size defects contribute to an improvement in the mechanical properties and strength of fatigue.

Conclusions
The objective of this work is to enhance the mechanical and fatigue properties of AA5052 reinforced with TiO2 nanoparticles using three sintering temperatures of (900, 1000 and 1100 ℃). The main problem in the present study is the manufacturing of nanocomposite in a high sintering temperature of (1100 ℃) because this leads to evaporate the alloys, which leads to generate oxides. The major conclusions extracted from the present work can be summarized as follows: TiO 2 particles reinforced AA5052 composites are successfully fabricated by the stir casting route. It has been also shown that the sintering temperature of 1000℃ with 5wt% TiO 2 nanocomposites has better mechanical, hardness and fatigue properties than other sintering temperatures of (900 and 1100℃) and has a major effect on the above characteristics. Compared to other sintering temperatures, the nanocomposite with a sintering temperature of 1000℃ has the highest hardness, ultimate tensile strength, yield strength and the lowest elongation. Compared to the other manufactured nanocomposites and base metal, the nanocomposite AA5052 supported with 5wt% TiO 2 with 1000℃ (ST) showed high strength and fatigue life. The improvement of mechanical and fatigue properties is attributed to the grain refinement and uniformly distribution of TiO 2 particles into a metal matrix.