الفهرس 
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Abstract
Nowadays, aluminum-silicon alloy A384 is used in automotive industries. This is due to their good casting characteristics, low thermal expansion coefficient, high strength to weight ratio. Unfortunately, this alloy suffers from some challenges, such as, insufficient strength, very poor ductility, high tool wear in the machining process, and low fracture toughness in the as-cast condition which impedes their broader applications.
It has been found that, the improvement of mechanical properties can be achieved by microstructure modification. In this work, friction stir process (FSP) is chosen to this objective. Different friction stir process parameters such as tool rotational speed, travel speed and cooling rate (water, gel and liquid nitrogen) are chosen from literature, trial experimental and capacity of using machine. Then software package, Design-Expert 6.1 are used to estimate the different experimental conditions.
Microstructure investigation is carried out using optical, XRD, SEM and EDS. Also, mechanical properties (UTS, YS, El% and HV) are achieved. The experimental design technique is used to understand the dominant parameter controlling the refining of microstructure during FSP and to create a mathematical model of optimizing the parameters that produce a maximum value of mechanical properties.
Based on the results obtained in this investigation, the following conclusions could be submitted: -
1. The as-cast microstructure consists of α-Al matrix (α-Al grain about 3mm), large needle-like eutectic Si, with average particle size 49.33µm and aspect ratio 29.5, dispersed heterogeneously throughout the α-Al matrix and coarse primary Si particles. Fe-rich intermetallic phases such as α-Fe phase (α-Al15(Fe, Mn)3Si2) with Chinese script morphology and the β-Fe phase (β-Al5FeSi) with platelets appearance are also shown. At the same time, Cu-rich intermetallic phases are observed.
2. X-ray diffraction pattern of the as-cast alloy confirmed the presence of α-Al, Si, Al8FeSi, Al2Cu, Cu3Al2 and Cu9Al4 peaks.
3. Mapping analysis of as-cast alloy indicates heterogeneous distribution of different alloying elements such as Cu, Ni, Mn, Fe, and Si.
4. EDS analysis for α-Al matrix in as-cast condition shows that the atomic percent of Al and Si is 95.49 and 4.51, respectively. Meanwhile, EDS analysis of primary Si is approximately pure Si with little amount of Al (91.06at% Si and 8.94 at% Al). On the other hand, the analysis of eutectic Si containing Al of about 37.13 at% and Si of about 62.87at%.
5. The EDS analysis of script like α-Fe phase, revealed that, Mn/Fe ratio is 1.96. meanwhile, Mn/Fe ratio for β-Fe (platelet shape) is 0.3.
6. The EDS analysis of Al2Cu phase is Al 66.8at% and 29.62at% for Cu. Also, EDS analysis of Cu3Al2 and Cu9Al4 phases are Al 74.96at% and Cu 20.98at%.
7. Mechanical properties of as-cast alloy such as the UTS, YS, El% and HV are 153MPa, 100MPa, 2% and 82.2HV, respectively.
8. The microstructure after friction stir process revealed the modification of long needle-like eutectic Si and α-Fe (α-Al15(Fe, Mn)3Si2) to fine particles and refining of Cu-rich intermetallic phases. So, these phases are fragmented, reprecipitated and redistribution in α-Al. meanwhile, the coarse primary Si and β-Fe (AlFe5Si) are fragmented and eliminated
9. The X-ray analysis for water cooling condition indicates the presence of α-Al, Si, Al8Fe2Si, Cu9Al4, Cu3Al2 and Al2Cu peaks. While, for gel cooling condition the peaks obtained are α-Al, Si, Al2Cu, Cu3Al2, AlCu and Al8Fe2Si. On the other hand, for liquid nitrogen cooling condition α-Al, Si, Cu9Al3, Al2Cu and Al8Fe2Si peaks are observed.
10. Mapping analysis of samples after FSP at optimum conditions shows that, fine structure with homogenous distribution of alloying elements such as Cu, Ni, Mn, Fe, and Si can be obtained. With increasing cooling rate, higher refining and more homogenous distribution of these elements can be achieved.
11. EDS analysis of α-Al after FSP revealed the existence of some alloying elements (Cu, Mg and Si) which are dissolved in α-Al matrix with about 2.54at%, 0.58at% and 8.78at%, respectively, resulting in solid solution strengthening.
12. EDS analysis after FSP of eutectic Si particles shows that, the Si% is about 70at% and Al% is 30at%. Primary Si is not observed; thus, this phase may be dissolved due to hot plastic deformation and cooling process.
13. Mn/Fe ratio of α-Fe phase ranged from 1.3 to 1.7. the β-phase is not recognized. Thus, β-Fe (AlFe5Si) phase is fragmented, dissolved and changes to α-Fe after FSP
14. Silicon particle size decreases to 3.9μm, 2.99μm and 2.13μm for water, gel and liquid nitrogen cooling condition, respectively, with reduction ratios 91.1%, 93.2% and 95.2% compared with as-cast.
15. Also, aspect ratio reduces to 2.75, 2.1 and 1.44 for water, gel and liquid nitrogen cooling condition, respectively, with reduction ratios 90.7%, 92.9% and 95.1% compared with as-cast.
16. FSP improves the mechanical properties of A384 Al alloy. The mechanical properties (HV, UTS, YS and El%) of A384 Al alloy increasing with increasing tool rotational speed, travel speed and in process cooling.
17. The conclusions of mechanical properties at optimum conditions for the three cooling mediums, can be summarized as following; for water cooling conditions, HV, UTS, YS and El% increases by 30, 98.04, 68 and 109% respectively. While, for gel cooling condition, HV, UTS, YS and El% enhances by 33.7, 106.5, 71 and 225%, respectively. On the other hand, for liquid nitrogen cooling condition, HV, UTS, YS and El% increases by 43.4, 135.3, 80 and 300%, respectively.
18. Based on the regression models which revealed the effects of operating parameters (tool rotation speed, travel speed and cooling type) on mechanical properties (yield strength, ultimate tensile strength, elongation percentage and hardness) it can be concluded that, the rotational speed is the first dominant parameter affecting mechanical properties. While, the second dominant parameter is cooling rate. The lowest effect for travel speed.
19. The optimum mechanical properties obtained from the response surface model are predicted by using a rotational speed of 1200 rpm, travel speed of 80 mm/min and liquid nitrogen (cooling rate 200 °C/min). At optimum condition ultimate tensile strength, yield strength, elongation%, and hardness are 360 MPa, 180 MPa, 8% 117.9HV, respectively.
20. regression equations for different parameters are
UTS= 280.8818+31.3*R+13*T+18.3*C-4.45455*R2+2.045455*T2+ 12.54545*C2+1.375*R*T +7.875*R*C+ 0.375*T*C
YS= 160.9+12.5*R+5.5*T+6.5*C-5*R2-1*T2+5*C2-1.625*R*T – 0.375*R*C+0.125*T*C
El%= 5.337273+0.91*R+0.58*T+0.79*C-0.31818*R2+0.031818*T2+ 0.381818*C2+0.1375*R*T +0.3625*R*C- 0.0375*T*C
HV= 104.0973+5.98*R+1.84*T+3.86*C-0.66818*R2+0.631818*T2 +2.831818*C2-0.6125*R*T +1.2625*R*C- 0.3625*T*C
SS= 5.55-1.42* R -0.66 *T-0.94*C+0.10*R2-0.30*T2-0.53*C2 -0.16*R*T-0.50* R*C+0.26*T*C
AR= 2.92 -0.46*R -0.16* T -0.40*C+0.068*R2 - 0.11* T2- 0.24*C2-0.037*R*T -0.31*R*C+2.500E-003* T*C