Melting and Corrosion Behavior of Al-9 Zn Based Alloys with Cu and Mg Additions

In the present paper, the effect of Mg and Cu additions on melting, mechanical and corrosion behavior of Al-9Zn alloy was studied and analyzed. It was found that, addition of Cu and Mg led to form the Al-Cu, Al-Mg and Mg-Zn intermetallic compounds. These compounds tend to increase the hardness values due to the precipitation strengthening hardening mechanism of these compounds. Also corrosion behavior of the Cu and Mg content alloys measured by the weight loss method is better than those of the Al-9Zn alloy. Furthermore, corrosion resistance of the Cu content alloys is superior to that of the Mg content alloys.


Introduction
The study of the material strength is an important subject because it is the first characteristic comes in mind when used in industrial applications specially that subjected to shock loading.Steel is a good example for the most strength materials, but its high density restricts its uses.Recently, aluminum alloys are increasingly employed in many important manufacturing areas, such as the automobile industry, aeronautics and the military [1].Currently, it offers the greatest potential for cost effective weight savings in automotive body structures and closures.With a density of only 33% of that of steel and a greater strength-to-weight ratio, there is the possibility for a weight savings of 40% to 50%.Also, Mg alloys are very attractive materials for producing lightweight components for automobiles because they have densities that are 66% of Al alloys and 22% of steel.With their lower density and moderate strength, Al-Mg alloys are well suited for a number of applications, ranging from steering wheels and instrument panels to engine and transmission components.The mechanical properties of the Al-Mg plastically processed alloys depend on the content of magnesium in the alloy.With an increase of magnesium from 0.5% to 5% the properties increase; this rise is greater when magnesium increases from 3% to 6% [2].There are many studies characterize the strength and mechanical properties of Al-based and Mg-based alloys with different elements [3][4][5][6][7][8].The present paper aims to characterize corrosion and mechanical properties of the quaternary alloy Al-1.5Cu-9.5Zn-3Mgas an example for a high strength material.

Experimental Procedures
Four alloys of chemical composition as illustrated in Table 1 were prepared in the as-cast condition.The required quantities were weighted out and melted in an electrical furnace then thermally agitated to ensure the homogenization.The molten alloys were cast into graphite molds to produce rods of 25 mm length and 10 mm diameter.Then the rods were cut in to small discs of about 1 cm length.X-ray diffraction analysis was performed to identify the phases presented in these alloys using a 1390 Philips X-ray Diffractometer with Cu-radiation.DSC tests were carried out during heating using a heating rate of 10˚C/min to evaluate the melting behavior of these alloys.Corrosion behavior of these alloys was determined chemically by the weight loss method.The test pieces were cut into 1 × 1 cm and mechanically polished with emery paper and weighted.Then the samples were suspended in a test solution of 1 mole of HCl with doubly distilled water.The measurements were carried out after suspension time of 15, 30, 45, 60 and 75 min.The average weight loss at the specific time was calculated.Measurements of hardness were done using Vickers hardness tester with the test samples placed against a glass slide with a constant load of 4.9 N for 10 sec.

Melting Behavior
DSC endothermic peaks of the Al-9Zn, Al-9Zn-1.5Cu,Al-9Zn-3Mg, Al-9Zn-1.5Cu-3Mgalloys during continuous heating are demonstrated in Figure 2. It shows that, the DSC curve is not discernible exothermic/endothermic peaks occurred until the remelting temperature range was reached.In alloy 1 there is a large endothermic peak at 657.7˚C, corresponds to the melting reaction of the binary Al-9Zn alloy.DSC curve of alloy 2 consists of three endothermic peaks, as labeled 1, 2 and 3, respectively.Peak 1 is the smallest one with an onset temperature of about 532.7˚C followed by the second peak.Peak 2 appeared at 640.3˚C, while the third peak, peak 3 started at compound formed at this alloy composition.While, the second peak which is the largest one observed at a temperature of about 634.7˚C is related to the melting reaction of this alloy.

Hardness
In solid mechanics, the material hardness is described as the resistance of deformation and should be independent from the applied load.So, it was measured using a fixed load of 4.9 N for 10 sec and the results are illustrated in Figure 3.It was observed that, the hardness with the corresponding values of the yield strength [9] is increased continually with the Cu or Mg content alloys.The maximum value obtained for alloy 4, can be attributed to the more precipitations of Al-Cu, Al-Mg and Mg-Zn IMCs, which act as hard inclusions in soft matrix.
The continuous increase of Hv can be attributed to the precipitation hardening strengthening mechanisms of Al-Mg and Al-Cu and Mg-Zn compounds.

Corrosion Behavior
Corrosion behavior of metals in an aqueous environment is characterized by the extent to which it dissolves in the solution.This can be quantified using the relationship W 0 = W B − W A , W 0 ; weight lost in test solution, W B ; weight before exposure, W A ; weight after exposure.pends on the alloy composition.The maximum value of the weight loss is obtained for the Al-9Zn alloy.All additions to this alloy decrease the weight lost per unit area than that of the binary alloy and the minimum value is obtained in the Al-9Zn-1.5Cualloy.This behavior can be attributed to the presence of Al-Cu, Al-Mg and Mg-Zn compounds, which might act as a high corrosion resistance compounds.

Conclusions
The effect of 1.5 Cu and 3 Mg as third and quaternary additions to the Al-9Zn alloy on structure, melting reaction, corrosion behavior and mechanical properties were studied and analyzed using X-ray diffraction, DSC tests, weight loss system and Vickers micro hardness tester, respectively.It was found that, adding of Cu and Mg led to precipitation of Al-Cu, Al-Mg and Mg-Zn compounds.These compounds led to increase the Vickers hardness number and corrosion resistance.Furthermore, corrosion resistance of the Cu-content alloys is superior to that of the Mg content alloy, which concluded that the Al-Cu compound is a more corrosion resistance than the Al-Mg or Mg-Zn compounds.

Figure 3 .
Figure 3. Hardness with the corresponding values of the yield strength of Al-Zn-Cu-Mg alloys.

Table 2 .
1.5Cu-3Mg alloys.All diffraction patterns contain peaks due to Al matrix phase.Accordingly, the Al-9Zn alloy shows the presence of only the Al phase, suggesting that all the Zn content has gone in to Al to form supersaturated solid solution.This is inferred from the shift of Al peaks to higher angular positions and consequently the Al solid solution phase had smaller lattice parameters than that of pure Al.The pattern of the Al-9Zn-1.5Cualloyindicatestwo X-ray diffraction peaks at 2θ ≈ 43.25and 43.96˚ refer to the AlCu 4 and Al 4 Cu 9 compounds, respectively.The pattern of the Al-9Zn-3Mg alloy indicates precipitation of Al 2 Mg compound at 2θ ≈ 36.9, 39.13 and 43.26˚, and MgZn 2 compound at 2θ ≈ 45.18˚.This means adding of Mg decrease the solubility of Zn in Al matrix phase by forming the MgZn 2 compound.The quaternary Al-9Zn-1.5Cu-3Mgalloy indicates precipitations of Al 12 Mg 17 , MgZn 2 , Al 2 Mg and Al 4 Cu 9 compounds as illustrated in Increasing the amount of the intermetallic compound MgZn 2 indicates that, the solubility of Zn in Al-matrix was decreased.