Dual role of trace elements in magnesium dissolved age hardened A356 alloy on microstructure and peak micro hardness

. The present work re ﬂ ects the effects of trace addition of copper (up to 1 wt.%) and zinc (0.5 wt.%) on the microstructure and hardness property of heat treated A356 (Al-7Si) alloy. Small amount of zinc and copper was introduced into A356 in both atomic form (alloy) and powder form (composite) and its age hardening behaviour were investigated. To enhance the wettability during composite fabrication and solid solution strengthening in alloys, a small quantity of magnesium (1 wt.%) was added. The main objective of this study is to introduce lower melting point zinc reinforcement into A356 matrix alloy by copper coating of zinc particles and then reinforcing it into A356 matrix. All stir cast specimens were subjected to T6 treatment by solutionizing at 520 ° C for 2 h followed by 60 ° C water quench and then aging at 100 and 200 ° C to determine peak hardness value. Microstructure analysis showed that a minimum 1wt.% copper was required to form Al 2 Cu intermetallic phase during solidi ﬁ cation resulting in ﬁ ner grain structure with hardness improvement. Under as-cast conditions, a maximum hardness of 85 VHN was obtained in 1wt.% copper reinforced composite. Under peak aged conditions, aging at 100 ° C showed 116% hardness improvement in 1 wt.% Cu reinforced composite.


Introduction
In recent years aluminium alloys are gaining outstanding performance due to the combination of physical, mechanical and tribological properties over other nonferrous lighter alloys so that this versatile alloy has put a remarkable impression in automotive and aerospace industries [1,2]. External input variables like, alloying elements (change in chemical composition), mechanical deformation (work hardening) and heat treatment (phase modification in solid state) induce positive impact on specific strength, wear resistance, high temperature strength and stiffness with better damping capacity. In general, aluminium alloys are divided into two major categories: wrought alloy and cast alloy compositions which are further differentiated under categories depending on the primary mechanism of property development [3]. Majority of cast alloys used in large volumes have significantly more additions of silicon when compared to wrought alloys. Researchers have proved that silicon addition up to its eutectic composition improves mechanical and wear properties, above which it effects adversely on these properties [4][5][6][7][8]. Therefore, in today's industrial world, Al-Si hypoeutectic alloy (A356) is used as an important alloy in aerospace and automotive applications. Typical applications of this alloy are in aircraft engine and pump parts, airframes and landing wheels, automotive transmission cases, truck chassis parts, water cooled cylinder blocks, aircraft fittings and control parts and structural elements which require high strength material [9][10][11][12][13]. In spite of having excellent properties required for the space applications there is provision to improve its hardness and wear resistance with formability properties. These modifications are possible by the improvement in microstructure as well as the type, number and amount of harder micro-constituents (secondary phases) present in the lattice. Trace addition of ingredients like copper, magnesium, zinc, etc. to the specific base aluminium alloy has two to three fold advantages in the mentioned properties [14][15][16][17]. In addition, impurities like nickel, chromium, and manganese which are present in very small quantities enter into Al solid solution and partially form various strengthening phases like Al 5 Cu 2 Mg 8 Si 6 , Al 5 FeSi, Al 8 Mg 3 FeSi 6 , Al 15 (Mn,Fe) 3 Si 2 and/or other particles under different conditions [18][19][20][21][22]. However, these ingredients (small dosage of alloying elements) go into the base alloy matrix (in liquid or solid state) to improve final properties. This phenomenon also imparts heat treatability of the base alloy by supporting secondary hardening due to precipitation of intermetallics. The ingredients may enter into the base alloy unit cell in the atomic form (alloy) or in the powder form uniformly dispersed in the matrix of the base alloy (composite). This alteration influences microstructure, hardness, wear resistance and surface finish of the casting obtained as well as heat treatment result. Generally, in composites, reinforcements are coated with copper, magnesium, nickel, borax, etc. Reinforcing particles are coated with metallic or non-metallic compounds, to improve wettability adhesion, mechanical properties and to evade an undesirable chemical reaction between the matrix and reinforcement at elevated temperatures. Coating also helps to incorporate lower melting ingredient to enter into the matrix in solid state during composite preparation. In general, a coating on the reinforcement offers some advantages such as protection of the particles from the reaction with the matrix acting as a diffusion barrier, increases the bonding and wetting between particles and matrix. To improve wettability with the reinforcement and to extract hidden qualities required for age hardening, 1 wt.% of Mg is added to the Al-Si alloy during other alloy additions or composite preparation. Also, it is known that age hardened (T6 heat treated) Al-Si alloy dissolved with small quantity of Mg had major effect on tribological properties by reducing the size of eutectic Si particles and also by enhancing the spheroidization degree of eutectic Si particles contributing to the improvement of ductility property [23][24][25][26]. In Al-Si-Mg alloys, precipitation of Mg 2 Si phase is majorly responsible for hardness improvement during age hardening [27]. In Al-Si-Mg-Cu alloys, several other phases such as Al 5 Cu 2 Mg 8 Si 6 , CuMgAl 2 , Mg 2 Si, and CuAl 2 exist in metastable conditions [28,29]. Addition of Zn promotes the precipitation of Mg 3 Zn 3 Al 2 and Mg 2 Zn type of intermetallics during heat treatment [12].
However, no much work has been carried out on the effect of copper and zinc addition on A356 alloy, especially as reinforcement. Since the melting temperature of zinc is less than aluminium and the solubility of zinc in aluminium is largest among other elements, most of the studies have been carried out by preparing alloys with different zinc content. Here, an effort is made to introduce lower melting point zinc as reinforcement into higher melting point matrix during two-step stir casting. Since lower melting point reinforcement dissolves in the matrix during processing, high melting point heat treatment supportable copper (Cu) coating is provided so that coating does not allow the lower melting point zinc (Zn) to dissolve in the matrix. During stirring of the liquid matrix, the solid coat covered liquid reinforcement maintains separate identity in the matrix at that temperature and even after solidification. The question of weakening the lattice by low melting point reinforcement is overcome due to the controlled formation of high strength and hard copper-nickel intermetallics. Like magnesium, copper coating also improves wettability between matrix and reinforcement [30]. The synergetic effect of heat treatment and the ability to form variety of intermetallics substantially improves hardness and strength of the material, which justifies the use of the composite as bearings and dies in light duty purposes. Hence in the present work, the effect of trace addition of Cu and Zn on A356 (both as alloy and composite) in as-cast and age hardening conditions on microstructure and peak hardness has been investigated.

Material
In the present work, A356 alloy ingots were procured from Laxmi metal exchange, Coimbatore and chemical analysis were done according to ASTM E-1251-2011 standards. Zinc powder and copper powder procured were subjected to particle size analysis. In copper powder 90% of the total volume was between 5 and 30 mm with an average particle size of 9.33 mm and in zinc powder 90% of the total volume was between 20 and 100 mm with an average particle size of 33 mm. To retain zinc as a separate identity in composite (melting point of zinc À480°C), copper coating with a coating thickness of 10-12 mm was done on zinc particles using electrolytic copper coating method. Energy Dispersive X-ray analysis (EDAX) results at different locations confirm the presence of copper coating onto zinc particles as shown in Figure 1. The prepared alloy and composites and designation used in this study are shown in Table 1. Table 2 shows the chemical composition of the prepared as-cast alloy and composites with different wt.% of zinc and copper.

Preparation
Initially, the procured A356 ingots were melted in an electric resistance furnace by heating to 780°C. Simultaneously, 1 wt.% Mg was introduced into the melt mainly to improve the effect of age hardening during the process [31]. The melt is then poured into preheated moulds to get as-cast 'A' bar specimen which has been used as base alloy in the preparation of other alloys and composites. For the preparation of as-cast A1 alloy, bars of as-cast A alloy were melted in graphite crucible using an electric resistance furnace and heated to 780°C. Calculated amount of 0.5 wt.% Zn and 0.5 wt.% Cu was added respectively into the melt and the temperature was raised to 1100°C until copper completely dissolved with the base alloy. The melt was continuously stirred using mechanical stirrer to prevent the formation of lumps or agglomeration within the alloy. It was then poured into preheated moulds maintained at 500°C and allowed to solidify. Similar procedure was used to prepare A2 alloy.
For the fabrication of composites (C1 and C2), alloy A was taken as matrix material with copper and zinc as reinforcements which were fabricated by two-step liquid stir casting technique. In this technique, as-cast A bars were melted in a graphite crucible using an electric resistance furnace and melting was carried out until uniform temperature of 780°C was achieved. The melt was then allowed to cool to 600°C to form a semi-solid state. At this stage, preheated 0.5 wt.% copper coated Zn powder and 0.5 wt.% Cu was introduced into the vortex which was formed during continuous stirring. The molten metal and reinforcements were stirred at 150-200 rpm for 10 min so that reinforcements dispersed uniformly in molten alloy. The melt was then poured into preheated moulds maintained at 500°C and allowed to cool at room temperature. Stir casting setup used in the fabrication of composites is shown in Figure 2.
The prepared bar specimens of both as-cast alloys and composite are shown in Figure 3 which was cut into smaller cube samples of different dimensions (6-14 mm) using wire EDM for microstructure and hardness study.
Test samples were polished suitably to get a flat mirror surface. Each sample was polished on emery paper of different grades for 2 min each starting from 320 to 2500 grit size. Fine polishing was done with velvet cloth saturated with a liquid diamond spray of 1 and 0.25 mm until the mirror finish surface was obtained. Further, the samples were rinsed thoroughly with clean water and dried. Keller's reagent (3 ml HCl + 2 ml HF + 5 ml HNO 3 + 190 ml H 2 O) was used as an etchant [32]. The samples were etched for 25 s (approx.), dried and then used for microstructure analysis and hardness measurement.

Characterization
Microstructure analysis was carried out using Scanning Electron Microscope (SEM) of the prepared samples. It is well supported by the Energy Dispersive X-ray analysis (EDAX) showing chemical analysis as Al, Si, Cu, Zn, etc. at the spot under consideration [33][34][35]. X-ray diffraction (XRD) analysis was performed to identify the intermetallic phases.
Vickers hardness test was performed according to ASTM E384 standard at room temperature, with an average of five concurrent hardness numbers obtained from the indentations with corresponding error bars. Hardness measurements were carried out using Matzusawa micro Vickers hardness tester, model-MMT X 7A with a load of 200 gmf and dwell time of 15 s. Confirmation tests were conducted on as-cast C1 & C2 composite samples to validate the uniform distribution of reinforcements through the chemical analysis method.

Heat treatment
The prepared as-cast A, A1, A2, C1 and C2 specimens were initially homogenized at 520°C for 8 h and then subjected to age hardening treatment process viz., solutionizing at 520°C for 2 h followed by hot water quenching at 60°C and artificially aging at 100 and 200°C separately for different intervals of time to determine the peak hardness value. Before hardness measurements, the surfaces were polished to remove any surface contaminations or oxide layers, which may have formed during the heat treatment process.

Microstructural analysis
The microstructure of the as-cast A specimen obtained through SEM is shown in Figures 4a and 4b. The microstructure of A has spot 1: a-Al, spot 2: eutectic Si and spot 3: Q-Mg 2 Si (Chinese script) phases [36]. Results  show a fine eutectic colony of Al-Si with well dispersed proeutectic aluminium phase a-Al which is present only along the grain boundary. a-Al is the predominant phase present in the microstructure of alloy specimen A forming a dendritic structure and precipitates with several other multiphase eutectic reactions. The silicon phase and small amount of other alloying elements which are soluble in the aluminium phase form a binary eutectic with a-Al [36]. In as-cast A microstructure, the orientation and morphology of dendritic a-Al are observed to be non-uniform whereas the eutectic silicon are present as coarse plates. This can be attributed to the addition of 1 wt.% Mg to base alloy/ matrix which reduces the eutectic temperature with increased undercooling [37].
The microstructure study of as-cast alloy A1 shown in Figures 4c and 4d consists of spot 1: a-Al, spot 2: interdendritic network of eutectic Si plates along with other intermetallic compound particles present between the dendrite arms of a-Al. Iron-rich intermetallic phases are one of the most common phases found in Al-Si. Previous researchers have identified two important phases as spot 4: p-Al 8 Mg 3 FeSi 6 (chinese script) and spot 5: b-Al 5 FeSi (needle shaped) [37,38]. With 1 wt.% Mg present, the peritectic reactions transform b phase into p phase which is visible in the microstructure. In addition, the magnesium atoms present reacts with silicon to give spot 3: Q-Mg 2 Si phase and with copper to give spot 6: S-Al 5 Cu 2 Mg 8 Si 6 phase. The grains are found to be large in size. Partially, some amount of pro-eutectic phase was observed to be within the grain also. Since the melting temperature of zinc is very low compared to silicon, coarse grains of eutectic colony is observed. Hence, there are no other alloying elements remarkably present in the system to control the grain size. Pro-eutectic phase a-Al is distinctly observed in the image.   [41][42][43].
In A1 alloy, containing 0.5 wt.% Cu, all Cu atoms reacted with some Mg atoms to produce Al 5 Cu 2 Mg 8 Si 6 (S) phase during the solidification process [34]. The remaining Mg atoms are formed as Mg 2 Si (Q) and Al 8 Mg 3 FeSi 6 (P) phase along with b-Fe phase. As the wt.% of Cu increases in the alloy (A2), the Mg quantity present in the alloy dominates to form S phase with Cu and Q phase with Si, thus reducing Mg content to generate P phase [3], [41].
The a-Al pro-eutectic phase quantity seems to be more compared to its counterparts (A&A1). Since copper is having a higher melting point, copper added to the alloy supports the precipitation of more amount of proeutectic a-Al phase. The pro-eutectic phase is seen both along the grain boundary and within the grain. The dispersion of eutectic and pro-eutectic phase is excellent compared to other two types of alloy (A&A1). However, no voids or blow holes are observed. Even solute clusters (undissolved) are also not observed in cast alloys. Since the grains are uniform and almost equiaxial, directional solidification mechanism is prevailing to favour dendritic structure [12]. XRD analysis was carried out on as-cast A2 sample to identify the different phases and the results are illustrated in Figure 6. Results show that the diffraction peaks associated with the a-Al and Si phase are clearly seen. As observed in Figure 6, Al 2 Cu (u) peaks are identified in the samples containing 1 wt.% Cu (A2) along with Mg 2 Si peaks supporting the previous studies [44,45].
As-cast composite C1 shows moderate grain size structures whereas C2 shows very fine grain structure which is evident in Figures 4g and 4j. This fine grain is due to high temperature mismatch between the copper and A356 alloy. When the composites are compared with the corresponding alloy category (Figs. 4g and 4j), alloy category (Figs. 4c and 4f) shows coarser grain size. Figure 5 shows the distribution of reinforcements in both C1 and C2 composites. The reinforcement distribution is observed to be uniform throughout the specimen. The corresponding EDAX results of C1 and C2 as in Figure 7a show the presence copper coated zinc reinforcement in the arbitrarily selected spot. Figure 7b shows copper reinforcement in the randomly selected spot. The difference in particle size between copper (10 mm) and copper coated zinc (33 mm) helped in distinguishing the reinforcement particle selection during the EDAX test.

Hardness measurement
Micro Vickers hardness test was conducted to study the hardness variation on the cast specimens with minor addition of zinc and copper as alloying elements and as reinforcement in the composite. The results of all the hardness tests conducted on the as-cast alloy and composite specimens A, A1, A2, C1 and C2 are recorded in the form of tables and graphs. Table 3 shows the measured hardness values of as-cast alloy and composite samples. Figure 8 shows the hardness variation of as-cast A, A1, A2, C1 and C2 samples.
The improved hardness values of A1, A2, C1 and C2 samples can be attributed to the variation in size and morphology of eutectic silicon particles present throughout the structure along with the formation of different intermetallic phases. Under as-cast conditions, hardness values of A, A1 and A2 are found to be 54, 62 and 75 VHN    respectively. Composite samples C1 and C2 showed hardness values of 69 and 85 which is higher than corresponding alloys A1 and A2 respectively. Figure 8 shows that the addition of 0.5 wt.% Zn + 0.5 wt.% Cu as alloying elements, the hardness value of alloy A increases by 16%, whereas at 1 wt.% Cu addition, hardness value increased by 39%. This is attributed to the high melting point of copper, due to which there is provision to form Al 2 Cu intermetallics. Even in the case of A1 alloy, improvement in hardness is seen but is less than that of A2. At the same time, it is observed that there is no prominent role of zinc addition in hardness improvement. Even though there is provision to form MgZn 2 and Al 2 Mg 3 Zn 3 type of intermetallics, the small quantity of zinc added may not be sufficient to form these intermetallics [12]. In the case of composites, addition of 0.5 wt.% Zn + 0.5 wt.% Cu as reinforcement to A356 showed 28% increase in hardness value contrasted with 58% hardness improvement at 1 wt.% Cu addition. Comparing the results of both alloy and composites A, A1, A2, C1 and C2, it is evident that addition of 0.5 wt.% of Zn does not serve as strengthening phase in solid state. The study also shows that wt.% of Mg and Zn available is not sufficient to form required quantity of MgZn 2 intermetallic phase. It is mainly due to the large quantity of Zn solubility (82.8 wt.%) in aluminium which does not strengthen the matrix. Hardness improvement in alloy and composite specimens mainly reflects the effect of copper addition to A356, which is mainly due to the formation of hard Mg 2 Si and Al 2 Cu intermetallic phases. Addition of copper in very small quantity (0.5 wt.%) as observed in A1&C1 may not be sufficient to form Al 2 Cu intermetallics and hence does not show marginal improvement in hardness property. Increase in hardness value with 1 wt.% Cu addition to A alloy may be attributed mainly due to partial refinement of pro-eutectic a-Al dendrites. Solid solution strengthening by precipitation hardening with the formation of Al 2 Cu phases is also major cause for the hardness improvement within the alloy [39]. In C2 and A2 castings, which contain 1 wt.% extra addition of copper over base alloy, copper composites show excellent hardness in as-cast (85 and 75 VHN respectively) and peak aged (119 and 107 VHN respectively) conditions. This explains that copper plays an important role in hardening the base matrix or alloy. Also, Al-Cu binary alloy system shows that aluminium with copper forms a solid solution (alpha with FCC structure) and intermetallic harder phase Al 2 Cu, where Al 2 Cu is the solid solution strengthening phase. Out of all the samples, 1 wt.% copper reinforced composite (C2) showed significant improvement in hardness properties. Addition of copper reinforcement particles into the matrix increases the surface area of the reinforcement and reduces matrix grain size, offering more resistance to plastic deformation thereby increasing hardness value.
To verify the presence of the 1 wt.% copper reinforcement in the fabricated composite, C2 composite was subjected to chemical gravimetric analysis. In this test, samples of alloy A and C2 composite were dissolved in concentrated NaOH (sodium hydroxide). Samples were dried out (at 100°C) before testing, and starting weights were noted. Samples were maintained in individual beakers, and 125 ml of 20% concentrated NaOH was added to each beaker. Beakers were heated on a hotplate to a predetermined temperature of 50°C. Aluminum residue that has been completely evaporated is put onto the filter paper. Now, the filter paper is heated using a Bunsen burner at a high temperature and burned in a crucible.
For 1500 g of A alloy, 15 g (1 wt.%) of copper was added during fabrication of C2 composite. Initial weight of the samples used in this study is 2 g. Theoretical weight of the copper reinforcement in the 2 g of C2 composite by calculation is 0.02 grams. From the test the following weight was noted:  So, based on the calculations, it can be concluded that the quantity of foreign particles present in the alloy is extremely low (viz., 0.24%), indicating that the alloy is pure with almost no impurities and that almost 98% of the reinforcements are present in the matrix of the fabricated C2 composite. Similar tests were conducted on 2 g of C1 sample where the results showed 0.93 wt.% reinforcement in the composite. The process of chemical analysis test conducted is shown in Figure 9.

Effect of age hardening on alloy and its composites
Age hardening treatment (T6) was carried out on all the prepared samples A, A1, A2, C1 and C2 and peak hardness value was determined. Figure 10 shows the hardness variation of T6-treated A, A1 and A2 alloys and corresponding composites. Results clearly show the increase in hardness value with increasing time for all the samples until peak aged condition is reached. Longer time holding beyond peak aging decreased the hardness value and is known to be over aged condition. Hardness during solutionizing, followed by peak aged condition increased with increasing Cu content. This increase in hardness value at as-quenched condition with increasing wt.% of Cu clearly shows that solid solution strengthening of A356 can be enhanced by increasing wt.% of Cu. The hardness results with aging time obtained are consistent with the work reported by previous researchers in which small amount of copper was introduced to Al-Si base alloy [33,40,41]. Moreover, the intentional addition of 1 wt.% Mg not only improved wettability in composites but also improved solid solution strengthening of A356 by the formation of Mg 2 Si intermetallic phases. Figures 10a-10d show the hardness variation graph of all five specimen samples A, A1, A2, C1 and C2 aged at 100 and 200°C.
In the case of alloys A1&A2 aged at 100 and 200°C, the time required to reach peak aged condition is found to decrease with increasing weight percentage of copper although 0.5 wt.% zinc addition did not show much significant effect in hardness improvement at peak aged condition. For samples aged at 100°C, as-cast sample A showed peak hardness value of 64 VHN at 10 h whereas A1 and A2 samples had a peak hardness of 95 and 107 VHN at 9 and 8 h respectively (Fig. 10a). The 48% increase in hardness of A1 compared to A is primarily due to solid solution strengthening of Cu on A356 by the formation of Al 2 Cu intermetallic phase. It indicates that Cu addition mainly increases age hardening ability along with age hardening rate. The formation of intermetallic phases increases with increase in wt.% of Cu which is responsible for 67% increase in hardness for A2 compared to A. Aging at 200°C resulted in lower hardness value which can be explained through aging kinetics [33]. The formation of solute-rich phases from supersaturated solid solution contributed to hardness improvement in aging. Lower aging temperature (100°C) resulted in higher hardness value than higher aging temperature (200°C) however the time required to reach peak hardness when aged at 200°C was faster. Aging at 100°C showed 72-95% hardness improvement in alloys and at 200°C, 37-47% hardness improvement was observed at peak aged condition when compared to the as-cast hardness of alloy A (Figs. 10a and  10c). In the case of composites C1 and C2 aged at 100 and 200°C (Figs. 10b and 10d), the graphical results showed similar to alloys but with much higher peak hardness values than alloys obtained at much lesser time. Compared to alloy with similar composition, composites C1 and C2 showed much higher peak hardness values in lesser time.
In all specimen samples, aging at 200°C showed double peak where the 1st peak is of lower hardness as compared to the 2nd peak. After the second peak, 3rd peak is not observed with further increase in aging duration, rather hardness value decrease trend is gradual after peak aging (2nd peak) and is not ascending. The lower hardness in the first peak is due to metastable transition state of uneven size growing embryo into stable intermetallics and such phenomenon is observed in Al-Cu and Al-Zn-Mg alloys under T6 treatment at certain temperature range during aging [13]. During age hardening process, strengthening of alloys mainly due to GP zone and metastable phases formed leading to peak aging condition. Initially, the GP zones formed homogeneously distribute within the matrix in which strengthening effects are significant. However, the metastable phases which are formed during intermediate state of aging resist the movement of dislocation having certain strengthening effect [46,47]. The GP zones are remarkably dissolved well before the formation of metastable phases which are nucleated and grown up on dislocation sites instead of fine and uniformly distributed GP zones. This is responsible for the temporary reduction in the strengthening process at the 1st peak [13]. It is found that u phase (Al 2 Cu) to have as nucleated on the dislocation sites which is responsible for higher hardness value before the peak aged condition [3,33,34,47]. But the metastable phases formed may not have grown up or are too small in size for the effective dislocation mobility which is responsible for lower hardness value before the peak aged condition. Figure 11 shows the peak hardness value obtained for alloy and composites under as-cast and heat treated condition.
Comparing Figures 8 and 11, aging at 100°C showed a maximum hardness of 119 VHN for composite with 1 wt.% Cu addition. Overall aging at 100°C showed 100-116% hardness improvement in composites on the other hand aging at 200°C showed 50-65% increase in hardness value. This enhanced property uptake over the respective composite category is due to the thermal coefficient mismatch between solid copper particles and melt A [46,47]. In base alloy, during aging at higher temperature (200°C) shows marginal improvement in peak hardness is observed. The small quantity of magnesium (1 wt.%) in the alloy may not be sufficient to support to a considerable extent for solid solution strengthening. Also, the solid copper particles give extra nucleation sites for the phase transformation and increase the dislocation density. Increased dislocation density is responsible for the increase in hardness property [33].

Conclusion
The study conducted shows the effect of zinc and copper addition on microstructure, precipitation kinetics and hardness property of custom made A356 alloy. From the experimental results, following conclusions can be made.
-Copper coated zinc particles were successfully reinforced into A356 alloy by two-step stir casting process. The study also proves that a lower melting point reinforcement (zinc) is able to be incorporated into higher melting point matrix to fabricate A356 composite. -Microstructure study shows that in as-cast conditions, addition of zinc to A356 as alloying element has no role in controlling the grain size during solidification whereas 1 wt.% copper when added as alloying element produced more pro eutectic a-Al phase giving finer eutectic colony of Al-Si. -In addition, 0.5 wt.% copper addition as alloying element to A356 was not sufficient to produce Al 2 Cu intermetallic phase. The presence of Cu atoms reacted with Mg atoms to form Al 5 Cu 2 Mg 8 Si 6 phase during the process. Although Al-Fe intermetallic phase was observed in composite samples, the eutectic Al-Si phase remained almost similar to A356 alloy with finer grain size. -In as-cast condition, a maximum hardness value of 75 VHN was obtained in alloy specimens with 1 wt.% copper addition which is attributed to the precipitation of Al 2 Cu intermetallic phase. Compared to alloy samples, composites in as-cast condition showed the highest hardness value of 85 VHN due to the formation of finer grains. -Addition of copper to A356 alloy promotes age hardening under as-quenched and aging conditions by solid solution strengthening and formation of different intermetallic phases. Aging of alloy at 100°C displayed 95% hardness improvement at peak aged condition although time taken was longer. However, 1 wt.% copper reinforced composite showed maximum peak hardness of 119 VHN (116% more than as-cast alloy A) at much lesser time. -Age hardening curves of A356 alloy and composite when aged at 200°C show double aging peaks. The 1st peak is found at early stages of aging (2-4 h) with lower hardness value attributed to the formation of non-uniform metastable phases which interrupts the dislocation sites for the growing GP zones.