Strengthening phase and mechanical property analysis of artificially aged Al7075 – Ni coated Al2024 composites

This technical paper demonstrates the possibilities of nickel (Ni) coated Al2024 powder reinforcement in an Al7075 matrix using the liquid stir casting technique. Additionally, the paper focuses on achieving stable properties by implementing artificial aging heat treatment. To apply the Ni coating, the electroless nickel plating technique was utilized, and a minimum coating thickness of 8 μm was determined to effectively prevent the dissolution of Al2024 powder reinforcements within the Al7075 matrix. Stir casting facilitated the uniform dispersion of the coated Al2024 powder up to a weight percentage of 7%. Subsequently, the Al7075 alloy and composites underwent artificial aging through solution heat treatment (SHT) at 450 °C for 4 h, followed by water quenching and aging at temperatures of 120, 150, and 180 °C. Aging at 120 °C was found to yield superior results compared to aging at 150 and 180 °C, thus identified as the optimum aging temperature. When the Ni coating thickness was increased beyond the optimal 8 μm, the resulting enhancements in hardness for both as-cast and peak-aged specimens, as well as the tensile strength, were not significant. The improvements observed were only marginal, ranging between 2 to 3%. Fracture surface analysis revealed that the predominant fracture mode in the Al7075 alloy was ductile, characterized by dimple rupture. In the as-cast Al7075-(7%, 8 μm) Al2024 composite, a mixed fracture mode comprising both brittle and ductile characteristics was observed. In the peak-aged (120 °C) Al7075-(7%, 8 μm)Al2024 composite, the overall fracture mode exhibited a dominant brittle nature. Analytical techniques including XRD, TEM, and EDS confirmed the presence of Mg2Si, MgZn2, CuAl2, and CuAl2Mg phases in the peak-aged (120 °C) Al7075-(7%, 8 μm) Al2024 composite. These phases contributed to the enhancement of the properties of both the Al7075 alloy and its composites. The developed composites can be used in automobile parts and aerospace applications.


Introduction
The quest for materials with superior corrosion resistance, lightweight in nature, and better mechanical properties has always been a goal in the world of advanced engineering domain.In recognition of its excellent properties, aluminium (Al) alloys have become one of the most versatile and broadly employed materials.With the rise in crude oil prices globally, the benefits of employing aluminium alloys in automobiles become more apparent [1].Lowering the weight of vehicles by 10% can boost the economy of fuel by 5% and greatly reduce greenhouse gas emissions [2,3].Due to its excellent machinability and strength-to-weight ratio, the Al7075 and Al2024 alloy have drawn a lot of spotlight.Al7075 is a high-strength aluminium alloy with excellent mechanical characteristics.This alloy belongs to the 7XXX series and is strengthened through precipitation hardening techniques.The inclusion of zinc (Zn) as the principal alloying element, together with a trace of magnesium and copper, improves the Al7075 alloy's strength and corrosion resistance.This alloy is often utilised in vehicle parts, aerospace applications, and structural elements that require great strength while being lightweight [4].The Al2024 alloy is a member of the 2XXX series of aluminium alloys and has outstanding machinability as well as an excellent combination of toughness and strength.The primary alloying element in Al2024 alloy is copper, imparting superior fatigue resistance and strength compared to pure aluminum [5].Metal matrix composites (MMCs) have opened up entirely new possibilities in the search for novel materials.Metal matrix composites (MMCs) are reinforced by high-strength metallic particles, ceramic particles, whiskers, or fibers [6].The composite formed offers greater wear resistance, strength, thermal stability, and stiffness.Matrix and reinforcements are the major components of the MMCs [7,8].In the present work, Al7075 and Al2024 alloys are used as matrix and reinforcement materials.The melting temperature of both the reinforcement and the matrix is nearly identical.In the preparation of composites, a fundamental requirement is that the reinforcement must remain a distinct entity, entering the matrix in a solid state.This ensures the existence of a clear interface between the matrix and the reinforcement.If the melting temperature is less or equal to that of the matrix, during the preparation of composite the reinforcement melts and dissolves in the matrix.This condition is known as alloying.When the melting temperature of the reinforcement is equal to and lower than that of the matrix, a protective coating of a higher-temperature metal is applied to the reinforcement.This coating acts as a solid barrier along the interface during processing, preventing the dissolution of the reinforcement into the matrix [9].The surface of Al2024 (reinforcement) powder was coated with nickel (Ni) which has a very high melting temperature (1440 °C) compared to Al7075 (matrix).Coating of Ni over reinforcement was done by electroless nickel plating (ENP) technique.The electroless nickel plating procedure is gaining popularity for improving the surface properties of Al alloys [10].The electroless nickel plating technology provides the coated aluminium surfaces with good corrosion protection, wear resistance, and solderability [11].In current work, stir casting technique is used to fabricate an Al7075-Ni coated Al2024 powder metal matrix composite.Stir casting is a popular method for producing metal matrix composites (MMCs) that involves dispersing reinforcing particulates inside a molten metal matrix.This approach enables the selection of several different reinforcing materials as well as the fabrication of composites with specific properties [12].Al7075 is a precipitate hardenable alloy whose mechanical properties may be improved by artificial aging [13].The age or precipitation hardening (T6) process enhances the mechanical characteristics of Al7075 alloy since it contains magnesium and zinc which has very high solid solubility in aluminium [14,15].Precipitate dispersion and its particle size are influenced by correct solution heat treatment under ideal circumstances [16].During age hardening process of Al7075 alloy, the changes in phases take place in the following order (a) Solid solution (b) Super saturated solid solution (SSSS) (c) GP zones (d) Semi-coherent η′ phase (e) Incoherent η phase [17,18].The rate of precipitation of the stable η phase varies with temperature.Lower aging temperatures result in a higher number of intermediate stages in the process of obtaining the peak hardness value.The higher the aging temperature, the fewer intermediate phases occur, resulting in a lower peak hardness value [19,20].According to researchers [21][22][23], some of the phases that can be formed during age hardening of Al7075 alloys are MgZn 2 , Al 2 Mg 3 Zn 3 , Al 2 CuMg, Al 2 Cu, Al 7 Cu 2 Fe, Al 13 Fe 4 , and Mg 2 Si.The most frequent and widely accepted way of increasing the mechanical properties of Al2024 alloys is age or precipitation hardening (T6).Age hardening in Al2024 alloys occurs at low temperatures and causes the precipitation of intermetallic compounds comprised of the principal alloying elements i.e., magnesium and copper [24].The sequence of artificial aging process in Al2024 alloys are (a) Super saturated solid solution (SSSS) (b) Guinier-Preston-Bagaryatsky (GPB) zones (c) S′ or GPB2 zone phase (d) S′ phase (e) S (Al 2 CuMg) phases [25,26].The crystal structures transformed from tetragonal to orthogonal throughout this process, which was accompanied by a change in lattice parameters suggests that the metastable (semi-coherent) phase changes into the stable (incoherent) phase as the precipitation process proceeds [27].According to Starink [28], the GPB zone is a brief range arrangement of Cu and Mg solute atoms.The S′ phase is commonly thought to be semi-coherent with the Al matrix, with its structure being identical to the S phase but somewhat different in terms of lattice characteristics.The S phase is incoherent with the matrix and is an equilibrium phase [29].The intermetallic phases that form during age hardening of Al2024 alloys are CuAl 2 , and CuAl 2 Mg [30,31].In the work of Clark [32], Al7075 alloys were solution heat treated for 1 h at 420, 450, 480, 510, and 530 °C before being water quenched.When compared to other temperatures, the alloy solution heat treated at 450 °C provided the best results.In another study, the Al7075 alloy was exposed to solution heat treatment (SHT) at 450 °C for 4 h and 480 °C for 1 h, water quenched, and then aged at various temperatures (120, 145, 165, and 185 °C).When compared to SHT at 480 °C, the sample solution heat treated at 450 °C produced better outcomes [33].From the literatures [34][35][36], it was stated that 120 °C is the best aging temperature for the Al7075 alloys since it provides better mechanical properties.Therefore, in the present work 450 °C was taken as the ideal SHT temperature.In order to check whether 120 °C is the ideal aging temperature, in the current work the Al7075 alloy and Al7075-Ni coated Al2024 composites were subjected to various aging temperatures (120, 150, and 180 °C) and the results were examined.
The present work aims to produce a composite by using Al7075 alloy and Ni coated Al2024 powder as matrix and reinforcement materials.This study presents a novel approach as it explores the manufacturing of Al-based MMCs with matching melting points for both the matrix and reinforcements.Limited prior research has been conducted in this specific area.When subjected to an age hardening process, these MMCs exhibit the formation of intermetallics in both the matrix and reinforcements.This phenomenon results in a combined improvement in the mechanical properties of the composites.Furthermore, the research aims to investigate the long-term stability of the peak aged properties in the investigated composites.This analysis is crucial as it directly influences the longevity and durability of the material properties of the components.

Experimental
In the present work Al7075 alloy and nickel coated Al2024 powder were taken as matrix and reinforcement material.Al7075 alloy billets and Al2024 powder of particle size 50 μm were purchased from Hi-tech steels, Bangalore, India.The chemical composition of Al7075 alloy is shown in table 1. Al2024 alloy powder was coated with Ni of varying coating thickness (4, 6, 8, and 10 μm) by electroless nickel plating (ENP) technique.The coating of reinforcements was done in Balambiga metal finishers, Bangalore, India.The coating thickness was verified using a Fischerscope x-ray XDL 240 instrument.ENP is the chemical reduction technique of applying a Ni-phosphorus alloy coating onto a substrate without requiring the assistance of an external power source [37].The steps followed in this technique include (i) sensitization: Substrate is dipped in an aqueous solution of stinus chloride (ii) Activation: Substrate is immersed in palladium chloride (iii) Metallisation: Substrate is placed in Ni sulphate.The duration of dipping the substrate in these solutions plays important role in providing the required thickness.As we increase the duration of dipping, the coating thickness increases and vice versa.Al7075 -Ni coated Al2024 powder MMCs with the varying amount (3, 5, 7, and 9 wt%) of reinforcements and coating thickness (4, 6, 8 and 10 μm) was fabricated by 2 -step stir casting method.In this method, Al7075 alloy billets (rods) were melted at 750 °C in a crucible using an electrical furnace.Alkaline powder and hexachloroethane tablets were added to liquid melt to remove slag and degasify.Further, the temperature of the furnace was reduced to 600 °C in order to bring the melt to semi solid state.Mechanical stirring was carried out at 400 RPM to form a vortex.Preheated (at 300 °C) Ni coated Al2024 powder (reinforcement) was introduced to vortex and   The Vickers hardness test is performed using an ASTM E384 diamond cone indenter with a 120°conical point to check reinforcement effect on MMCs hardness in as cast conditions, as well as to examine the effect of aging on hardness specimens.For TEM analysis, thin foils of Al7075-(7%, 8 μm)Al2024 composite peak aged at 120 °C with a thickness of ∼0.1 mm were produced by mechanical thinning and slow speed diamond sawing.Then, discs of 3 mm diameter from thin foils were obtained by blanking and further it was ground to reduce thickness.Electro-polishing was conducted to prepare a TEM specimen with the help of a twin jet system and a mixture of perchloric and acetic acids maintained at a temperature of ∼6 °C and voltage of ∼25 V.The process was stopped automatically when the perforation started.The 3 mm perforated disc specimens (TEM specimen) were then studied using a transmission electron microscope (Model: FEI Tecnai) and energy dispersive spectroscopy (EDS).PC-2000/605/06 Electronic tensometer was used to conduct the tensile test.In break test mode, the load cell value was kept constant at 20 KN, while the speed was kept constant at 1 mm/min.Tensile tests were conducted on as cast Al7075 alloy, as cast and peak-aged (120 °C) samples of Al7075-(7%, 8 μm)Al2024 composites.Tensile samples were prepared in accordance with ASTM E8M (figure 1).Tensile samples were subjected to fracture surface analysis.withstand the pressure of molten Al2024 present within it.The coating might have burst during stirring and resulted in mixing of molten Al2024 alloy in the matrix (Al7075) melt.Hence coating thickness of 4 and 6 μm was not considered in the present research work.Figures 4((a)-(b)) shows that there is visibility of reinforcement in composite with coating thickness 8 μm on reinforcement and the corresponding EDS report shows there is presence of nickel.So, it confirms that 8 μm is the minimum coating thickness to be provided to successfully fabricate a composite.The composites fabricated with 9 wt% of reinforcement showed that there is a formation of agglomeration. Figure 5 shows the optical microscope image of the agglomeration formed.Hence it was decided to fabricate a composite with a minimum coating thickness of 8 μm and upto 7 wt% only.In order to check whether there is a significant increase in tensile strength and hardness of composite if there is an increase in coating thickness, the composites were fabricated with 10 μm coating thickness and the test results were compared with composites fabricated with 8 μm coating thickness.
Hardness test was carried on as cast and age hardened Al7075 alloy and Al7075 -Ni coated Al2024 MMCs with varying amount (3, 5, and 7 wt%) of reinforcement & coating thickness (8 and 10 μm).Table 2 shows the hardness test result of as cast Al7075 alloy and Al7075 -Ni coated Al2024 MMCs with varying amount (3, 5, and 7 wt%) of reinforcement & coating thickness (8 and 10 μm) samples.It clearly shows that the hardness values of composites are better compared to Al7075 alloy.As the percentage of reinforcement increases there is a significant increase in hardness values.During solidification, there will be immense internal stresses and mismatch strain caused by the thermal expansion mismatch of matrix and reinforcement alloy, altering the mechanical properties and microstructure of composites, as well as plastic deformation of matrix alloy in order to allow lesser volume expansion of reinforcements, resulting in higher dislocation density.Increased dislocation density results in increased resistance to plastic deformation, which increases composite hardness [38].Hence, hardness values rise as the weight percent of reinforcements increases.Composites with 7 wt% reinforcement showed the highest hardness values in terms of both coating thickness (8 and 10 μm) when  c)) it can be clearly seen that the hardness values of age hardened samples were significantly higher than that of as cast samples.Age hardening causes the creation of GP zones, which then convert into semi coherent η′ and incoherent η phases.During the early phases of aging, smaller sized initial GP zones will be formed with larger dispersion density.As a result of the creation of smaller sized GP zones with high surface energy, the hardness of the samples increases.Accumulation of Mg and Zn atoms occurs at the GP zones-Al7075 matrix interface as aging time increases.As GP zones expand, they convert into a semi coherent η′ phase, increasing the hardness effect of the Al7075 alloy matrix.The η′ phases evolve into incoherent phases as the aging time increases, increasing the hardness of the aged samples [39].Even after aging in all three aging temperatures (120, 150, and 180 °C), peak hardness values of composites are much higher compared to Al7075 alloy and also as the percentage of reinforcements increases there is a significant increase in the peak hardness values of the composites.In the study of Ahmet and Yildirim, Al7075 alloy and its composites were made to undergo age hardening at several aging temperatures.Al7075 alloys and composites aged at 120 °C showed better hardness and tensile results compared to other aging temperatures [40,41].Similarly, in current work peak hardness values of composites were better when aged at 120 °C compared to aging at 150 and 180 °C.The time required to attain peak hardness when aged at 120, 150, and 180 °C was 10, 9, and 8.5 h respectively.Hence, it implies that the composites with 7 wt% aged at 120 °C for 10 h have better hardness values when compared to composites with 3 and 5 wt% and aged at 150 and 180 °C.Even after aging at ideal aging temperature (120 °C), Al7075-(7%, 10 μm)Al2024 composite showed marginal improvement (2%) in hardness value compared to Al7075-(7%, 8 μm)Al2024 composite.

TEM analysis
TEM with SADP (Selected Area Diffraction Pattern) was used to identify the presence of intermetallic phases.Due to the excellent performance in all the tests conducted, the Al7075-(7%, 8 μm)Al2024 composite peak aged at 120 °C sample was considered for TEM analysis.According to Jin [13] and Giovanni [42], η′ and S′ phases present in the matrix of Al alloy can be seen as satellite spots and streaks in SADP images.Moreover, several small dots indicating the S′ needles can be seen in the bright field images.

XRD analysis
Identification of the metastable intermetallic phases that develop during precipitation hardening treatment is the main objective of XRD analysis.Figure 8 shows the XRD patterns of peak aged (at 120 °C) Al7075-(7%, 8 μm)Al2024 composite sample.The results show that the sample comprised of different phases such as Al 2 Cu (θ), Al 2 CuMg (α), Mg 2 Si (δ), and MgZn 2 (η).These metastable intermetallic phases are primarily responsible for the improvement in mechanical properties of aged composites.Intermetallics act as a barrier to dislocations, enhancing the material's strength [43,44].According to Tello [45] and Rajaram [22], the major phases that form during age hardening of Al2024 alloys are CuAl 2 , CuAl 2 Mg and Al7075 alloys are Mg 2 Si, MgZn 2 .

Tensile test
Figure 9 shows the tensile strength comparison of as cast and peak aged (at 120 °C) Al7075 alloy & Al7075 -7 wt% Ni coated Al2024 powder composites of coating thickness 8 and10 μm.It shows that, the tensile strength of composites was significantly high in composites compared to Al7075 alloy in both as cast as well as peak aged samples.With the increase in the percentage of reinforcement, there is a substantial increase in tensile strength values.During solidification, considerable internal stresses and strain mismatches occur due to the thermal expansion differences between the matrix and reinforcement alloy.These factors alter the mechanical properties and microstructure of the composites.Additionally, the matrix alloy undergoes plastic deformation to accommodate the lesser volume expansion of reinforcements, leading to a higher dislocation density.This increased dislocation density, in turn, raises the resistance to plastic deformation, ultimately resulting in enhanced tensile strength in the composite.As a result, it is clear that the composites with a 10 μm coating on Al2024 powder had slightly higher tensile strength than those with an 8 μm coating in both as cast and peak aged conditions.However, this marginal improvement of 1%-2% is not deemed significant and is not worth consideration for the following reasons: • With an increase in coating thickness, the properties of the coat metal become more pronounced within the composite.Specifically, the coating should serve as a sacrificial protective layer for the reinforcement to prevent any mixing with the matrix.
• The cost associated with the coating process escalates due to the extended duration required for applying a thicker coating.• Additionally, Ni is a costly metal, further contributing to the overall expense.
Hence it is decided that Al7075 -7 wt % Ni coated Al2024 powder composites of coating thickness 8 μm on Al2024 powder aged at 120 °C is the ideal composite for current work.

Fracture surface analysis 4.1. As cast Al7075 alloy
In as cast Al7075 alloy, the predominant mode of fracture is of ductile nature in which dimple rupture was observed.It is observed in the fractographs (figures 10((a)-(b))) that few micro voids are formed at locations on the fracture surface including the grain boundaries.Cup-like depressions were also observed which indicate the dimple rupture.A river-like pattern formed on the fracture surface is also an indicator of ductile mode of fracture.Fracture surfaces observed are bumpy, and matrix shear has been observed; however, despite these processes occurring simultaneously, dimple rupture is the dominant mode of fracture.The fracture surface exhibits relatively finer and smoother dimples revealing ductile failure.Hence Al7075 alloy exhibits lower UTS and higher ductility values.), mixed mode of fracture (both brittle and ductile) is observed.As the volume percentage of reinforcements added to the Al7075 alloy matrix increases, it changes the mechanism of fracture from ductile to brittle.This leads to a decrease in the percentage of elongation in composite samples [46].The presence of micro voids, dimples, and some river-like patterns indicates the ductile mode of fracture is dominant [47], whereas at some portion minor brittle fracture symptom (mirror facets) is also observed which is because of higher dimple density.The Fractograph of the selected location in the matrix EDAX reveals the presence of Ni coated Al2024 powder.Fractography of fractured surface in a few places displays shear/brittle fracture.An increase in the total number of dimples and Fractures in a localised area may be caused by a tear or shear, leading to the creation of elongated dimples.Furthermore, it was observed that few of the dimples were shallow, which might be attributable to the shearinduced coalescence of micro voids.In a few places, there is also evidence of quasi-cleavage fracture.Quasi cleavage is localised and displays cleavage and plastic deformation characteristics.
4.3.Al7075-(7%, 8 μm)Al2024 composite peak aged at 120 °C Figures 12((a)-(c)) shows SEM fractographs of Al7075-(7%, 8 μm)Al2024 composite peak aged at 120 °C.The figures clearly demonstrate a higher density of dimples, smaller dimple sizes, and a uniform distribution of dimples which help in the creation of a large number of micro voids at precipitated zones.The finer the dimple size, the greater will be the ductility and strength of the particular joint [48].As a result, the artificially aged specimen has greater UTS values than the untreated Al7075 alloy [49].The presence of furrows or a linear mark pattern in the region of crack propagation indicates peak aging (figure 12(d)), resulting in an improvement in UTS [50].The presence of Ni coated Al2024 reinforcement particles causes shrinkage cavities as a result of coarser inter dendritic segregation, which may lead to brittle failure.The relation between the strength of the matrix and the strength of the particle/matrix interfacial bond is the fundamental criteria for defining the mode of fracture in composites.If the particle/matrix interfacial connection is strong, particle fracture generally happens during the permanent deformation stage.Decohesion between the particles and the matrix happens prior to particle fracture if the particle/matrix interfacial interaction is weak.Because of the strong particle/ matrix bonding, the presence of dimples and river patterns indicates that the overall fracture mode is of a mixed nature, with brittle nature outplaying [51].

Conclusion
o Al7075-Ni coated Al2024 composites were successfully manufactured by liquid stir casting method.o A coating thickness of 8 μm on the reinforcement is ideal for visualizing the clear identity of undissolved Al2024 powder in the Al7075 alloy matrix.Coating thickness greater than 8 μm on the reinforcement does not assist in enhancing the properties of age-hardened Al7075-Al2024 composites significantly.Similarly, a coating thickness of less than 8 μm on the reinforcement allows for reinforcement dissolving in the matrix and limits the chances of composite formation.
o Increased Ni coating thickness above ideal (8 μm) doesn't significantly enhance the hardness and tensile strength much (2 to 3%).An increase in coating thickness doesn't influence much on the enrichment of property, there is the possibility of reflection of Ni properties on the composite and it also causes additional financial burden.Hence the thickness of coating is limited to 8 μm.
o Quantity of reinforcements greater than 7 wt% leads to Al2024 powder reinforcement agglomeration in the matrix.Therefore, the maximum amount of reinforcement considered is limited to 7 wt%.
o In as cast conditions, composites showed better hardness and tensile strength compared to Al7075 alloy.Composites with 7 wt% reinforcement showed 20 and 9% improvement in hardness compared to 3 and 5 wt% reinforced composites.Composites with 7 wt% reinforcement showed 74 and 36% improvement in hardness and tensile strength compared to Al7075 alloy.
o Composites aged at 120 °C showed better results when compared with composites aged at 150 and 180 °C.In peak aged (120 °C) condition, composites with 7 wt% reinforcement showed 13 and 5% improvement in hardness compared to 3 and 5 wt% reinforced composites.Composites with 7 wt% reinforcement showed 58 and 31% improvement in hardness and tensile strength compared to Al7075 alloy.
o Fracture surface analysis confirms that in Al7075 alloy the predominant mode of fracture is of ductile nature in which dimple rupture was observed, in the case of as cast Al7075-(7%, 8 μm)Al2024 composite mixed mode of fracture (both brittle and ductile) was observed and in the case of peak aged (120 °C) Al7075-(7%, 8 μm)Al2024 composite the overall fracture mode is of mixed nature, with brittle nature outplaying.
o XRD, TEM, and EDS analysis confirm that in peak aged (120 °C) Al7075-(7%, 8 μm)Al2024 composite sample there was the presence of Mg 2 Si, MgZn 2 CuAl 2 and CuAl 2 Mg phases which leads to enhancement in the properties of Al7075 alloy and its composites.

Figure 1 .
Figure 1.Specimens used for tensile test prepared as per ASTM-E8M.
10 min for uniform mixing reinforcements in the semi solid matrix melt.The furnace temperature was increased to 750 °C to obtain the mixture of matrix and reinforcements in liquid melt state.Al7075-Ni coated Al2024 powder composite in the liquid state was poured into rectangular preheated (at 520 °C for 2 h) moulds and allowed to cool down to obtain the cast composites.The shortcut name given to Al7075-Ni coated Al2024 powder composites was represented by equation Al7075-(x%, y μm)Al2024, where (x%) is the percentage of Al2024 powder reinforcement added and (y μm) is the thickness of Ni coating on reinforcement.EDM was used for machining all as cast composites with weight percentages (3, 5, 7, and 9%) of varying coating thickness (4, 6, 8, and 10 m) reinforcement.Machined samples have been efficiently polished with emery sheets with grit sizes ranging from 100 to 2000 and then disc polished with diamond paste to achieve a mirror finish.These are etched with Keller's reagent, dried, and captured using an optical microscope (OM) model: Olympus, MLX B-Plus, a scanning electron microscope (SEM) model: JEOL JSM 840 A, and also subjected to hardness test.To test hardness, a Vickers Hardness Tester (Mod: MMT X7A) with a load of 200 gmf and a dwell time of 15 sec was employed.
Figure 2 shows the process of age hardening treatment.As cast Al7075 alloy and Al7075-Ni coated Al2024 MMCs with varying amount (3, 5, and 7 wt%) of reinforcement & coating thickness (8 and 10 μm) were made to undergo SHT at 450 °C for 4 h, subsequently water quenched and later aged at 120, 150 and 180 °C for different time intervals followed by water quenching.Aged samples were subjected to hardness and tensile tests.Peak aged (120 °C) Al7075-(7%, 8 μm) Al2024 composite samples were subjected to TEM and XRD analysis.

Figure 2 .
Figure 2. Process chart of age hardening treatment.

3. Results and discussion 3 . 1 .
Microstructure and hardness analysisAl7075-Ni coated Al2024 MMCs with varying amount (3, 5, 7, and 9 wt%) of reinforcement & coating thickness (4, 6, 8, and 10 μm) were subjected to SEM analysis.Figures3((a)-(b)) showed that there was no presence of reinforcement in composite fabricated with 4 and 6 μm thickness coating on Al2024 alloy powder.It confirms that the coating thickness of 4 and 6 μm on Al2024 alloy powder is not enough to provide protection from mixing of Al2024 alloy with Al7075 alloy matrix at casting temperature.The high temperature coating couldn't

Figure 7 (
a) depicts the dispersion of precipitated phases, whereas figure 7(b) displays a single precipitate at a greater magnification.Both figures show that the precipitates were generated during aging and had a needle, rod, or rectangle shape.EDS analysis from figure 7(c) confirms the existence of Cu and Al, representing them as Al 2 Cu (S) phase.Similarly, figure 7(d) confirms the existence of Mg and Zn, representing them as MgZn 2 (η) phase.Figure 7(b) shows the SADP analysis conducted on precipitated particles.SADP images clearly show the