Improvement of Mechanical and Fatigue Properties for Aluminum Alloy 7049 By Using Nano Composites Technique

The aim of present work is to improve mechanical and fatigue properties for Aluminum alloy7049 by using Nano composites technique. The ZrO2 with an average grain diameter of 30-40 nm, was selected as Nano particles, to reinforce Aluminum alloy7049 with different percentage as, 2, 4, 6 and 7 w %. The Stir casting method was used to fabricate the Nano composites materials due to economical route for improvement and processing of metal matrix composites. The experimental results were shown that the adding of zirconium oxide (ZrO2) as reinforced material leads to improve mechanical properties. The best percentage of improvement of mechanical properties of 7049 AA was with 4% wt. of ZrO2 about (7.76% ) for ultimate tensile stress UTS, (9.62% ) for yield stress YS and (9.92%) for hardness HB than other of adding of 2, 6 and 7% of ZrO2. Also the results shown that the fatigue strength of 7049 AA with 4 w % ZrO2 nanoparticles is higher than that of 7049 AA under constant loading 9.86% at 10 cycles as well as the fatigue life factor (IFLF %) at different amplitude stress 400, 350, 300 and 250 MPa was improvement in range 66, 115, 63 and 107% respectively.


Introduction
Only few machine parts are subjected to static loading and the majority of machine parts are subjected to variable loads.Experimentally, when a material is subjected to dynamic loads, it fails at a stress below the yield stress, such type of failure is known as fatigue.Fatigue may occur under constant or variable loads.Constant fatigue loading is defined as fatigue under cyclic loading with constant amplitude and a constant mean stress or load.But in service, the structures or components are subjected to variable amplitude loading, which can be a rather complex load time history [1].
Many Techniques were used to improve mechanical and fatigue properties for metal materials such as Surface cold work, shot penning and laser shock penning……etc.[2, 3 and 4].The aim of present work is Improved mechanical and fatigue properties for Aluminum alloy7049 by Nano composites Technique.The Nano composites material was fabricated by stir casting method using Aluminum alloy7049 as a metal matrix and Nano particles of ZrO2 as a reinforced material with different weight percentage, 2,4,6 and 7w ‫.%ݐ‬ Metal matrix composites were studied which are widely used in an industry for many applications in aerospace, automotive and others so it is request to improve mechanical and fatigue properties of metal matrix composites for use in highperformance applications.
M. Dinesh and R. Ravindran [5] Studied hardness and tensile behavior of 7075 aluminum alloy with different reinforcement by Cr nanoparticles weight as 2,3,4,5 and 6 w ‫%ݐ‬ and zinc nanoparticle of 1 w ‫%ݐ‬ by air casting technique.They found the tensile stress and hardness values of the composite higher than the unreinforced alloy.The stir casting was suitable technique for manufacture of this type of composite material.
C. Kannan and R. Ramanujam [6], Studied Nano composite Al-7075 alloy for two conditions, single and hybrid for 2 w ‫%ݐ‬ and 4 wt.% of SiC produced through stir casting.They observed that the addition of nanoparticles with 30-50 nm grain size, improved the ultimate stress and hardness by 63.7% and 81.1% for single and hybrid respectively.
S.E.Shin and D. H. Bae [8] Studied fatigue behavior of aluminum alloy 2024 matrix composites reinforced with multi-walled carbon nanotubes under the tension-compression fatigue test.Tensile and fatigue strength of the Al2024 composites increases with increasing multiwalled carbon nanotubes content.Al 2024 with 4vol.%multi-walled carbon nanotubes composite shows the remarkably enhanced fatigue strength of 600 MPa at the 2.5×10 6 cycles and the ratio of tensile strength to fatigue strength of 0.78.When the composite is cyclically loaded, the developed incompatibility between the matrix and the fiber induces the fiber pull-out and it acts as a bridge when the cracks propagate.Thus, as the content of multiwalled carbon nanotubes increases in the composite, the prevailing bridging behavior of multi-walled carbon nanotubes enhances the number of fatigue cycle.
[9] Studied the behavior of 2024 aluminum alloy with A12O3 Nano composite which was fabricated by stir casting at 450 r.p.m and casting temperature of 850°C.The experimental tests showed an improvement in the fatigue strength and life compared to the metal matrix 2024 aluminum alloy.The S-N curves were obtained and the equations of the Nano composite which described the constant S-N curves were established based on Basquin power law equation for 0.2, 0.4, 0.6, 0.8 and 1.0w ‫%ݐ‬ A12O3.The experimental results analysis revealed that the fatigue strength is improved by 5.3% and the fatigue life is increased by 16.02% for 2024 aluminum alloy with 0.4w ‫%ݐ‬ A12O3 Nano composite compared to that for metal matrix.
A.P. Utkarsh Prajapati et al [10] They studied the effect of Nano Al2O3 on the mechanical properties of the aluminum based alloy, they concluded that the mechanical properties of aluminum based alloy can be increase by adding Al2O3 Nano particles into it.Also the stirring speed significantly affects the mechanical properties of the Nano-scattered castings.The speed more than 1500 rpm causes decrease in the elasticity.The distribution of Nano alumina particles in the aluminum from 1.0 to 3.0, the immunization particles are consistently scattered in Al, with alumina content under 4.0 wt.%.
AL Alkawi H. J. M. et al [11] Investigated 7075 Al-alloy metal matrix composites with different w ‫ݐ‬ %, 0.2, 0.4, 0.6, 0.8, and 1.0 of A1203 reinforced particles of 10 nm in grain size, were fabricated using stir casting technique.Discuss the influence of adding A1203 particles content on the mechanical properties of the metal material composites.They concluded the nanomaterial reinforcement lead to improve the BHN hardness, ultimate strength ߪ ௨ and yield stress ߪ ௬ .The maximum improvement in BHN hardness, and was observed with 0.2 w ‫.%ݐ‬ A1203.While the minimum value of ductility was obtained with 0.2 w ‫%ݐ‬ A1203.R. Senthilkumar et al [12] Studied the mechanical and fatigue properties for two different composite material, 90w ‫%ݐ‬ of 2014 aluminum alloy with 10w ‫%ݐ‬ micron size of Al2O3 and 90w ‫%ݐ‬ of 2014 aluminum alloy with 8w ‫%ݐ‬ micron size of Al2O3 and 2w ‫%ݐ‬ Nano size of Al2O3.The results show the mechanical and fatigue properties for composite 90w ‫%ݐ‬ 2014 aluminum alloy with 8w ‫%ݐ‬ micron size of Al2O3 and 2w ‫%ݐ‬ nano size of Al2O3 was higher than that of composite 90w ‫%ݐ‬ 2014 aluminum alloy with 10w ‫%ݐ‬ micron size of Al2O3 due to Nano size of Al2O3 reinforcement which more effectively.
Al-Alkawi et al [13] Studied 7075 Al-alloy with nanomaterial reinforced AI203 of 10 nm in grain size which is used different w ‫%ݐ‬ as, 0.2 , 0.4 , 0.6 , 0.8 and 1.0.It was observed that the best enhancement in fatigue strength and life occurred with 0.2w ‫%ݐ‬ of AI2O3.The fatigue strength increased by 2.86% compared to the base metal alloy.The improvement may be coming from uniform distribution of AI2O3 and minimal porosity.
Hajizamani and Baharvandi [14] Used hybrid Nano composites with A356 aluminum alloy base metal taking four weight percentages 0.5, 1, 1.5 and 2w ‫%ݐ‬ of Al2O3 and 10% volume of ZrO2.The Stir casting technique was used for manufacturing the Nano composites.They observed experimentally that the mechanical and hardness increased for all the above Nano composites in an pproximately linear manner.

Material and Properties
The material that used in this work was a 7049AA supplied from round bar of 12

Experimental Technique of Fabricating the Nano Composites
Stir casting method was chosen as Technique of Fabricating the Nano composites due to economical route for improvement and processing of metal matrix composites materials.The 7049 AA was selected as the matrix and ZrO2 as nano particles, as a reinforced material with different weight percentage, 2,4,6 and 7w ‫%ݐ‬ with an average grain diameter of 30-40 nm.In stir casting process, the 7049 AA was first heated above its melting temperature (750℃), then keep it in the semisolid condition, at the same time the ZrO2 particles was heated at the same temperature and dropped into semisolid and mixed with 450 r.p.m stirring speed for 4 minutes.The melt was then superheated above liquid temperature and poured into the mold for fabricating the specimens.The test rig which used to prepare the Nano composite material can be seen in Figure -1.The size of the composites product is 150 mm length and 12 mm in diameter.The composite product was fabricated in the rule of mixtures which can be seen in Table-3.

Fatigue Specimens preparation and machine Test
Fatigue specimens were prepared according to DIN 50113, 12 specimens for 7049 AA and 24 specimens for 7049 AA with 4% wt. of ZrO2.The specimen dimensions' configuration of fatigue test was shown as in Figure -2.The silicon carbide papers were using for smooth surface of the specimen.

Fig. 2. The specimen dimensions in mm according to (DIN 50113) standard values
A rotating bending machine fatigue-testing Schenck product type as shown in figure-3 was used to implement all fatigue tests, with constant and variable amplitude stress.The fatigue specimen which is shown above has a round cross-section and is subjected to an applied load from the right side of the perpendicular to the axis of the specimen, developing a bending moment.Therefore, the surface of the specimen is under bending stress, tension and compression when it rotates.

Results and Discussions 5.1 Mechanical Properties of Composites
The experimental of mechanical properties of 7049 AA specimens with using zirconium oxide (ZrO2) as a reinforced material with different w ‫%ݐ‬ was done at State Company for inspection and engineering rehabilitation in Iraq SIER, which is reported in Table-4.The results show the adding of zirconium oxide ZrO2 as reinforced material lead to improve mechanical properties and hardness.The results show the best improvements for mechanical properties and hardness was for composite with 4w ‫%ݐ‬ of ZrO2 compared with that the others w ‫%ݐ‬ of ZrO2 as shown in Table -4

Fatigue Test Results Under Constant Loading
All fatigue specimens were tested under constant loading using the rotating bending machine fatigue.The fatigue Test under constant amplitude stress was for 7049AA without and with 4w ‫%ݐ‬ ZrO2 only to find the improvement for fatigue properties.The S-N curves results illustrated in Table-5 at four different levels stress or amplitude stress.The average of number of cycles to failures of three specimens at each level stress is recorded.It can be observed that the effect of the addition of 4w ‫%ݐ‬ ZrO2 nanoparticles on fatigue behavior by seen the improvement in fatigue life through estimating the improvement fatigue life factor (IFLF %) at different level stress by using equation-1 [16].
…(1) where (ܰ ) and (ܰ ) is number of cycle to failure for base metal or with 0w ‫%ݐ‬ZrO2 and for the composite with 4w ‫%ݐ‬ ZrO2specimens respectively.The (FLIF %) at different amplitude stress was shown in Table -5 Table 5, Improvement fatigue life factor (IFLF) at different amplitude stress.
The empirical fatigue power law regression given by: σ ൌ aܰ ୠ … (2) where, (σ ) is the applied stress, Nf is the number of cycles to failure and (a),(b) are the fitting parameters.The regression constants represent the fatigue trends from the model and the fatigue strength at 10 8 cycles can be observed as shown in Table-6.The results show the Improvement of Fatigue strength was 9.86%.The S-N curve under constant amplitude stress test for specimens 7049AA without ZrO2 or with 0w ‫%ݐ‬ ZrO2 and for the composite 7049AA with 4w ‫%ݐ‬ ZrO2, and the empirical power law equation it can be shown in Figure -6.The results show the introducing 4% of ZrO2 to the7049AA lead to the higher mechanical properties, improve the fatigue life and fatigue strength.The higher mechanical properties of the 4wt% ZrO2 composite could be attributed to the fact that ZrO2 particles acts as obstacles to the motion of dislocation, the increasing in mechanical properties can be attributed to reduce grain size, leading to rise up the fatigue strength and fatigue life of composites and this results agreement with finding by Abdizadeh H., Baghchesara M.A. [17].

Fatigue Test Results Under Variable Loading
Fatigue test under variable loading was done by testing specimens under variable loading for two different conditions at low to high sequences loading and high to low sequences loading by using fatigue test machine type rotary, the machine run 10000 cycles for each stress level and so on to the failure of specimen, 6 specimen for each case.Experimental fatigue test under variable loading for base metal with 0wt% ZrO2 and Nano composite 4w ‫%ݐ‬ZrO2 was recorded in Table-7.The results show improvement in fatigue life factor 33.7% and 15.8% of Nano composite with 4wt% ZrO2 for both cases respectively through estimating the improvement fatigue life factor (IFLF %) for two cases by using equation-1.

Conclusions
The results show the improvement of using different w ‫%ݐ‬ of zirconium oxide ZrO2 Nano particles reinforced Aluminum alloy7049 as Nano composites material on mechanical and fatigue properties.The significant findings of this investigation are as follows: 1.The mechanical properties of 7049AA specimens with zirconium oxide (ZrO2) reinforced material lead to improving mechanical properties and hardness in different percentage according to different w ‫%ݐ‬of ZrO2 reinforced 7049AA.The best improvement of mechanical properties and hardness was occurred with 4w ‫%ݐ‬ of ZrO2 in the rate of (7.76ߪ ௨ %) UTS, (9.615ߪ ௬ %) YS and (9.92%) HB. 2. Fatigue properties of 7049AA with 4w ‫%ݐ‬ ZrO2 are higher than that of the nonreinforced 7049AA under constant loading.Also the Fatigue life of Nano composite with 4wt% ZrO2 was higher than that for base metal or with 0wt% ZrO2 under variable loading sequence for both high to low and low to high sequence.3. The higher mechanical properties of 7049AA with 4w ‫%ݐ‬ ZrO2 composite could be attributed to the fact that ZrO2 particles acts as obstacles to the motion of dislocation and particles of ZrO2

Fig. 1 .
Fig. 1.Stir casting process; a: furnace Ignition and metal melting; b: move the nanoparticles with molten metal; c: the melt poured into the mold; d: composites product into mold.
and Figure-4.The tensile stress-strain diagram up to ultimate stress for 7049AA and Nano composite with 4w ‫%ݐ‬ of ZrO2 was shown in Figure-5.

Fig. 6 .
Fig. 6.S-N curves of the Nano composite with 4 ‫ܟ‬ % ZrO2 compared with the base metal 7049AA with 0% ZrO2 under constant amplitude stress .
Microstructures of 7049 AA as received and composites with the content of 4 wt.%ZrO2, are shown in Figures-7, Dark regain represent AL matrix and bright particles represent particles of ZrO2.It can be mentioned that Nano-particle ZrO2 represents appropriate wettability with molten metal and good stability as well.It certainly should be noted that not only the wettability has an important role in the distribution of reinforcement particles in the matrix, but also, other factors such as pouring condition, stirring rate, solidification rate, etc. have profound influences on the even distribution of ZrO2 in the metal matrix, fabricated by stir casting.
(a) SEM micrograph of 7049AA as received at different magnification 50 and 20μm respectively (b)SEM micrograph of composites fabricated of 7049AA with 4wt% ZrO2 at different magnification 50 and 20μm.

Table 1 , Experimental and standard chemical composition of 7049 AA in ‫ܟ‬ ‫%ܜ‬
mm in diameter.The experimental of chemical and mechanical properties of 7049AA was done at State Company for Inspection and Engineering Rehabilitation in Iraq SIER, which is reported in Table-1 and Table-2 respectively.