EFFECT OF COPPER AND MAGNESIUM

The addition of copper and magnesium to Al-Si alloys aims at improving both the strength and hardness of the alloy. These elements are commonly known as strengthening elements, due to their immediate positive effect on the alloy strength and hardness upon

addition, which can then be further enhanced and/or optimized through the application of adequate heat treatment procedures.^{11, 36, 44}

Copper is considered as an effective hardening element which improves alloy strength and hardness^45-47 at both room and elevated temperatures; these improvements are attributable to the formation of copper intermetallic phases. These copper intermetallic phases may appear in the form of (i) eutectic-like Al-Al₂Cu, (ii) block-like Al₂Cu, and (iii) blocky Q-Al5Mg8Cu2Si6.

Samuel et al.⁴⁸ proposed the precipitation mechanism of Al₂Cu phase as follows. At the start of solidification, the dendritic network of α-Al is formed associated with the segregation of both Cu and Si in the liquid ahead of the solidification front. Upon reaching the eutectic temperature, the silicon particles precipitate eventually, resulting in areas with higher Cu concentrations which subsequently solidify as copper intermetallic phases.

Lemon and Howle³⁷ investigated the effect of Cu content on the ambient temperature tensile properties of Al-9%Si-0.5%Mg alloy after being heat treated according to T6 and T62 procedures. The results of their study are shown in Figure 2.6, where the straight lines are connecting the values obtained from T6 and T62 tempers. The authors found that the optimum compromise between the strength, ductility and the overall quality of the alloy is achieved when the copper content varies between 1.6 and 2 wt%. Based on the data shown in Figure 2.6, Sigworth⁴⁹ observed that a copper content up to 1.8 wt% has a beneficial effect on the quality index of Al-9%Si-0.5%Mg cast alloys; the author credited this enhancement to the significant increase in alloy strength associated with a slight decrease in the ductility.

Figure 2.6 Ambient temperature tensile properties of Al-9%Si-0.5%Mg cast alloy with different Cu contents and subjected to T6 and T62 tempers.³⁷

The addition of magnesium to Al-Si alloys enhances the yield and ultimate strength as well as the impact toughness of alloys; however the presence of magnesium significantly reduces the ductility of this category of alloys.11, 36, 44

The presence of Mg leads to the segregation of Cu in areas away from the silicon-rich regions during solidification. This practice results in the formation of the block-like Al₂Cu as well as the Q-Al₅Mg₈Cu₂Si₆ intermetallic phase. The Q-Al5Mg8Cu2Si6 phase may form out of the block-like Al2Cu phase along its edges, during the last stage of solidification.

The presence of Cu in Al-alloys leads to the formation of Al2Cu during solidification; this phase can exist either in blocky form or as finely dispersed particles within the interdendritic regions. If the cooling rate is high and if Al₅FeSi platelets exist in the microstructure, the fine Al2Cu phase will form, accordingly. The fine Al2Cu phase dissolves easily within two hours of solution treatment. On the other hand, the block-like

Al₂Cu phase is not that easy to dissolve under the same conditions (see Figure 2.7).⁵⁰ Practically the same situation occurs in the case of Mg addition, the phase Mg2Si is the non-equilibrium phase responsible for strengthening age-hardenable Al-Si-Mg alloys. In the absence of Cu, high Fe and Mg contents lead to the formation of π-FeMg₃Si₆Al₈ phase which is difficult to dissolve during the solution treatment process.^{51, 52}

Figure 2.7 Cu-rich phases in as-cast 319 alloy: (a) Eutectic Al2Cu and (b) blocky Al2Cu.⁵⁰

This segregation behavior of Cu may lead to incipient melting during solution treatment which will apparently reduce the alloy strength; yet if it is possible to avoid the segregation of Cu, it is possible to combine the strengthening effect of Cu by forming Al₂Cu precipitates besides the strengthening effect of Mg by the formation of Mg₂Si precipitates which will lead to a very high level of strengthening. ^{53, 54}

Dunn and Dickert⁴³ studied the influence of adding up to 0.55% Mg on the mechanical properties of A380 and 383 cast alloys. The authors found that the ultimate tensile strength, yield strength and hardness values improved in the presence of Mg.

However it was also clear that increasing the Mg content led to reduction in the ductility of the alloys, and acceptable ductility values were attained with max 0.35% Mg content.⁴³ Mg

addition was also found to have a negative effect on Si modification using Sr, as it resulted in changing the Si morphology from a well-modified to a partially-modified one. This reduction in modification level was attributed to the formation of a complex Mg2SrAl4Si3

intermetallic phase, which probably formed prior to the eutectic reaction.⁴²

Magnesium content may also affect the Fe-containing intermetallic phases.

Narayanan et al.⁵⁵ found that increasing the level of Mg in Al-Si alloys will lead to reduction in the eutectic temperature; this will cause difficulty in the formation of the α-Fe phase in alloys with high Mg content even if the melt is superheated to 900°C. The same effect was reported by Awano and Shimizu.⁵⁶ The authors found that it is difficult to force the β-Al5FeSi phase formation temperature to occur below the eutectic temperature in high-Mg alloys employing melt super-heating to a very high temperature or using a high cooling rate or even employing both together. In contrast, Samuel et al.⁵⁷ found that the addition of Mg to 319 type alloys transformed a large proportion of the β-Al₅FeSi needles into the π-Al8Mg3FeSi6 compacted Chinese-script phase.

Cáceres et al.⁵⁸ studied the influence of the content of Cu and other elements, such as Mg, Si, Fe and Mn, as well as that of the cooling rate on the mechanical properties and quality index of T6-tempered Al-Si-Cu-Mg casting alloys. The authors concluded that the overall effect of Cu and Mg is to lower the quality index values of the alloys, as may be seen in the quality chart shown in Figure 2.8. The loss in the quality in this case is directly related to the decreased ductility as a result of the cracking of second phase particles occurring in the strengthened alloys. It was also observed that the degree to which the quality index is affected by the addition of copper depends not only on the Cu content itself

but also on the presence of other elements such as Mg, Si, and Fe, as may be seen from Figure 2.8.

Figure 2.8 Quality chart illustrating the influence of the content of Cu and other elements (Mg, Si, Fe, and Mn) and cooling rate, as indicated by arrows, on the strength and quality index of Al-Si-Cu-Mg alloys. The numbers 1 through 21 located in the chart represent various alloy compositions.⁵⁸