Casting processes are among the oldest methods for manufacturing metal goods. In most early casting processes (many of which are still used today), the mold after use is commonly collapsed in order to remove the product after solidification. The need for a permanent mold, which can be used to produce components in large volume quantities which are of high quality, is the obvious alternative. In the middle ages, craftsmen perfected the use of iron in the manufacture of mould [1] . Moreover, the first information revolution occurred when Johannes Gutenberg developed a method to manufacture components in large quantities using a permanent metal mold. Over

the centuries, the permanent metal mold processes continued to evolve. In the late 18th century processes were developed in which metal was injected into metal dies under pressure to manufacture print type production. These developments culminated in the creation of the linotype machine for printing by Ottmar Mergenthaler in 1885, an automated type casting device which became the prominent type of equipment in the publishing industry which led to creation of a die cast machine. Researchers like Adler et al. [2] investigated porosity defects in aluminium cast materials, and used volumetric analysis to identify gas porosity defects. They studied the effect of backscatter in their work on the ultrasonic inspection of aluminium cast materials, similarly Dahle et al. [3] conducted experiments on the effects of pressure on density and porosity in an aluminum cast by applying pressure to the riser in a permanent mold (die) and found a flat distribution this time of density rather than porosity, was observed with the pressurized riser. Dargusch et al. [4] determined the effects of process variables on the quality of high pressure die cast components with the aid of in-cavity pressure sensors. In particular, the effects of set intensification pressure, delay time, and casting velocity were investigated and in turn the effect of variations in these parameters on the integrity of the final part, porosity was found to decrease with increasing intensification pressure and increase with increasing casting velocity, and then Zhu et al. [5] conducted experiments and simulations on the effect of pressure on porosity in cast A356. The alloy was melted under vacuum and pressure was applied in the ceramic mould and found that an increase in pressure reduces the amount of porosity and that the pore size distribution was shifted to smaller pores as pressure increased. Ming et al. [6] conducted experiments on the effect of pressure on the mechanical properties and microstructure of Al-Cu-based alloy prepared by squeeze casting and concluded that hardness, tensile strength and ductility of ZA27 squeezed casting are greatly affected by applied pressure and primary reaction is promoted in squeeze cast ZA27 alloy that solidified at high pressure and a fine microstructure is obtained with the increase of pressure and Wei et al. [7] investigated microstructures and properties of die casting components with various thicknesses made of AZ91D alloy by means of a scanning electron microscope (SEM), transmission electron microscope (TEM), high-reso- lution transmission electron microscope (HRTEM), etc. It was concluded that mechanical properties of the die casting components mainly depend on grain size of Mg phase. Chiang et al. [8] proposed mathematical models for the modelling and analysis of the effects of machining parameters on the performance characteristics in the HPDC process of Al-Si alloys which were developed using the response surface methodology (RSM) to explain the influences of three processing parameters (die temperature, injection pressure and cooling time) on the performance characteristics of the mean particle size (MPS) of primary silicon and material hardness (HBN) value. The experiment plan adopts the centered central composite design (CCD).

The separable influence of individual machining parameters and the interaction between these parameters were also investigated by using analysis of variance (ANOVA). With the experimental values up to a 95% confidence interval, it was fairly well for the experimental results to present the mathematical models of both the mean particle size of primary silicon and its hardness value. Two main significant factors involved in the mean particle size of primary silicon were the die temperature and the cooling time. The injection pressure and die temperature also have statistically significant effects on microstructure and hardness. Li et al. [9] conducted experiments on the effects of specific pressure on microstructure and mechanical properties of squeeze casting of ZA27 alloy and concluded that Hardness, tensile strength and ductility of ZA27 squeezed casting with high height-to-thickness ratio are greatly affected by applied pressure and that Al and Cu elements are homogeneously distributed in matrix of squeeze cast ZA27 alloy. Although many researchers have carried out other works on squeeze casting of Al alloys [8] [10] - [19] , however little work has been reported on die casting of A380 alloy with varying applied pressure.

Microstructures of the samples were investigated by means of a scanning electron microscope available at the physics laboratory at Science and Technology Complex (SHEDA) Abuja and the metallurgical microscope available at metallurgy laboratory available at the Federal University of Technology Minna, Nigeria.

Preparation of the samples for micro examination involved mainly sampling, polishing and etching. Samples measuring 26 mm × 15 mm × 5 mm were cut from the castings with the help of a hacksaw. The samples were filed and ground. Grinding was done in succession on a bench grinder using silicon carbide abrasive papers of 220-, 320-, 400-, and 600-grit, the samples were polished in the usual manner with final polishing being carried out by hand, and they were etched in aqueous solution containing 2.5% HNO₃, 1.5% HCL and 1% HF acid (etched with Keller’s reagent) for 20 to 60 seconds. Etching was done to make visible the grains of the samples under different pressures conditions.

From the results obtained for the number of grains and grain size in the samples of A380 alloy, an analogy was deduced and porosity levels were predicted in relation to the number of grains and grain size of the cast samples. The relation shows lower porosity levels at finer grain sizes and more number of grains. This result agrees with work carried out by Dargusch et al., [4] .

The regression was carried out using the multiple regression equation

where Y = porosity, X₁ = grain size and X₂ = number of grains. The porosity follows this regression equation model:

The microstructures of the samples of A380 alloy at different pressures consisted of a primary α phase (Al), peritectic β phase (FeSiAl₅) and ternary eutectic (β + η + ε) (Al₂Cu) where α phases solidified as coarse grains (Figure 8). The primary α phases became finer in higher pressure (seen in Figure 9 and Figure 10) and with the increase of pressure, the microstructure became finer when the pressure reached 1400 kg/cm² (see Figure 9).

In the lower pressure samples, scanty grains were seen and were not homogeneously distributed (Figure 8), also the eutectic structure (β + η + ε) was not found in the samples at 1400 kg/cm², while the hypo-eutectic (η + ε) phases appeared between grains (Figure 9). In the solidification process, the primary phase α precipitates first from liquid phase and then the peritectic reaction follows. However, at high pressure, the degree of these two

reactions becomes greater due to the fact that the eutectic point moves to the direction of rich Al, thus the quan- tity of remaining liquid phase is reduced greatly. On the other hand, because the melting points are elevated at high pressure, the degree of super-cooling increases, thus the nucleate rate of primary reaction increases largely during solidifying. This is also the reason of microstructure refining. In addition, the remaining phase is in deep super-cooling state when temperature is dropped to the eutectic point.

Therefore the improvement of mechanical properties was attributed to eliminating of micro-pores caused by higher pressure. On the other hand, it is because of the microstructure refining as the applied pressure is increased as seen below that increased tensile strength and hardness are attributed. From the above results, it can be deduced that the eutectic reaction was restrained while the primary reaction was promoted at higher pressure. This viewpoint agrees with other works [5] -[7] .

In Figure 9, almost 95 percent of grains occupy the micrograph showing fine grain sizes. The much concentrated grains were cohesively arranged and evenly distributed due to good compatibility of the grain structures which is absolutely, evenly distributed in an attractive manner and perfectly embedded with one another. The improvement of mechanical properties is attributed to eliminating of micro-pores in the alloy caused by applied pressure and this is because of the microstructure refining as the applied pressure is increased as mentioned above.

In Figure 11, the micrograph shows spherical dimples characteristics of grain types which are still shown occupying almost 85 percent the micrograph showing also fine grain sizes and also the much concentrated grains were cohesively arranged and evenly distributed also due to good compatibility of the grain structures. The improvement of mechanical properties is attributed to eliminating of micro-pores in the alloy caused by applied pressure.

In Figure 10, microstructure shows the much concentrated grains are cohesively arranged and evenly distributed, also almost 75 percent of grains occupy the micrograph showing not very fine grains.

In Figure 12, the micrograph shows spherical dimples characteristic of grain type and almost 70 percent of grains occupy the micrograph showing big grain sizes. It was also clearly shown that the grains were scanty because of bigger grain sizes that depict porosity susceptibility over time.

In Figure 10, it is seen that the grains are very scanty, also the regions at which grains occupies was lower than 50 percent showing presence of bigger grain sizes. The micrograph also shows deformation that is worsened and the degree of deformation so great. The microstructure clearly shows obvious porosity in the sample that solidified at low pressure.

From the results of this research, the following conclusions can be drawn:

Ÿ Hardness values increased from 76 to 85 HRN as applied pressure was increased from 0 to 1400 kg/cm². This justifies our expected outcome.

Ÿ Number of grains increased from 20.31 to 87.05 g/a² as applied pressure was raised from zero pressure to 1400 kg/cm².

Ÿ Tensile strength, yield strength and elongation increased with increased applied pressures.

Ÿ The impact strength was also increased with increase in applied pressure from 3.93 to 4.39 joules as pressure was raised from zero to 1400 kg/cm².

Ÿ The microstructures obtained in the A380 alloy cast with 1400 kg/cm² pressure show that the eutectic reaction was restrained, and the final solidified structure was (η + ε) phases instead of eutectic phase (β + η + ε). Also the primary solid reaction (α) was promoted and a finer microstructure was obtained.

Ÿ The micrographs of the A380 alloy casts show different morphologies distributed across the samples under the varying applied pressure as fine grains which were seen homogenously distributed across the micrograph of the sample that was cast with 1400 kg/cm² pressure, can effectively block the movement of dislocation, and increase the strength and plasticity.

Ÿ The microstructures also show structure changes due to applied pressure as some appeared granular, lamellar, coarse etc. It was also seen that porosity susceptibility was lower in the sample that solidified at high pressures due to finer grain sizes.