Effect of Lead Additions on Microstructure and Casting Properties of AZ91 Magnesium Alloy

In this study, the effect of Pb element addition varied between 0.2 0.4 wt.% on the microstructure and casting properties of AZ91 magnesium alloy were investigated. The microstructural results showed that as increasing Pb additions into the AZ91 alloy, the grains and the Mg17Al12 intermetallic phase becomes thinner. When examining the effect on casting properties; It was observed that the fluidity of AZ91 alloy increased as the increasing of Pb additions. In the hot tear tests two different mold systems were used. Hot tearing were observed in the longest section in the tests using "the mold same diameter, different lengths" and when the "the mold different diameter, same lengths" were used, hot tears were observed in all of the molds with diameters of 6, 8, 10 mm. Hot tearings were observed in 0.2 wt.% and 0.3 wt.% Pb additions, while hot tearing was not observed when this ratio increased to 0.4% for the 12 mm diameter test specimens. There was also not hot tearing were observed in any sample when the sample diameter was 16 mm.


Introduction
Due to its low density, magnesium based alloys are the lightest structural metal in terms of many engineering applications such as portable microelectronics, telecommunications, aeraspace and automobile industry. The weight of Mg is 36% of Al (Al), of iron (Fe) and 78% of that of steel [1]. The main reason why the automotive industry has the biggest interest in the Mg alloys from the automotive sector today is the reduction in vehicle weight with the use of light Mg alloys fuel economy and as a consequence gas emissions can be reduced [2,3]. AZ91 Mg alloy, formed by alloying Mg-Al-Zn elements, is mainly used for casting automobile parts. Compared with other Mg alloys, AZ91 is the most widely used commercial alloy due to its mechanical and casting properties [4]. Casting products are obtained by adding alloying elements in order to improve the structural properties of magnesium. Mg has a hexagonal lattice structure and its particle diameter permits solid solubility with a large number of elements.
When Mg is used as a structural material, it generally has a high strength / weight ratio when alloyed with the elements. The most commonly used alloy element in Mg alloys is Aluminum. In Mg-based alloys Al improves solid precipitation hardening, melt castability and reduces microporosity of Mg cast alloys. Besides, Al improves hardness, stiffness and solidification time of Mg alloys while reducing ductility. Alloy strength of Mg-Al alloys is developed by formation of Mg 17 Al 12 intermetallics at low temperatures (≤ 120°C). However, the tendency of alloys to micropores increases with increasing Al content [5][6][7]. Zn is limited to 2% due to precipitation hardening, which increases resistance to ambient temperatures and at the same time increases castability and precipitation into grain boundaries, resulting in hot tearing. Si reduces the castability and fluidity of Mg alloys [5,6,8]. Si atoms form Mg 2 Si intermetallic in solid state at room temperature [9]. The addition of alloying elements such as Sb, Ca, Bi, Pb and rare earth (RE) to the AZ91 alloy were investigated to improve the casting, microstructural stability or creep properties of the alloy [10][11][12].
In the literature there are few studies on the effect of Pb addition on the casting and microstructure properties of AZ91 alloy. Although Pb vapor is detrimental to health, the addition of a small amount may not be a serious problem as long as proper care is taken [13]. The aim of this work is to investigate the effects of Pb addition on casting properties such as microstructure properties and castability and hot tear on AZ91 alloy.

Experimental Studies
The Mg, Al, Zn ingots with a minimum purity of 99.9% were purchased from Karasu Metal Co., Turkey. Pure Mg and Al were melted at 750°C under a protective Ar gas in a graphite crucible. The analyzes of the alloys used in the tests are given in Table 1. Zn and Pb additions were alloyed 1 min before casting to avoid losses of Zn and Pb due to vapourisation. The molten alloy was then cast into a cast iron mould (preheated to 250°C) having 30 mm diameter and 170 mm length under protective SF 6 gas. Casting operations were carried out at 250°C which ensures optimum casting conditions by supplying SF 6 shielding gas [14]. A mold heating furnace which is capable of reaching a temperature of 300°C was used to remove the test molds to the desired temperature. For metallographic inspection, the surfaces of the samples were ground using pure water with 400 and 600 mesh SiC paper and polished with 1µm alumina paste. 5 ml of acidic acid, 10 ml of picric acid, 10 ml of distilled water and 100 ml of ethyl alcohol were used as the etchant. Microstructure studies were carried out on a Nikon Epiphot 200 optical microscope.
Flowing spirals were used for flow tests, and die cast molds with different diameters for hot tearing tests were used. Spirality of fluency used M. Di Sabatino et al. [15] and hot tear mold Cao and Kou [16] are of the same shape and dimensions as those used in their work. In addition, molds were prepared from cast iron for microstructure studies. In Figure 1, mold images and tensile test specimens used in the experiments are given.  Figures 2 (a-d) show microstructure images of AZ91 alloy and additional Pb content. When AZ91 microstructure (Figure 2a) is examined, it is seen that the structure is formed mainly by eutectic and intermetallic phases extending along the grain boundaries in the main matrix of α-Mg. These phases are the Mg 17 Al 12 (β) intermetallics with Mg-Al (α + β) eutectic. The morphological structure of the Mg 17 Al 12 phase is similar to the literature [17,18] and is generally seen as a similar structure to the Chinese script. The addition of Pb to the alloys caused a decrease in the amount of α + β eutectic. Pb in the AZ91 alloy has a high solubility of ~ 45% [19], possibly resulting in a solid solution in the structure rather than Al displacement. It is expected that Mg 2 Pb phase appears in small quantities addition of Pb, as well as intermetallic phase Mg 17 Al 12 in Pb additives. However, there is no information on the formation of Mg 2 Pb phase with Pb addition in AZ91 alloy in literature. Srinivasan et al. [20] investigated the effect of 2% Pb addition on AZ91 on aging and did not mention Mg 2 Pb here.  Figure 3 shows the fluidity values of the alloys as a function of Pb contents. Fluidity increased as increasing Pb contents. The fluidity length of the AZ91 alloy was 17.5 cm while it increased by 20 cm with the addition of 0.4 wt.% Pb. It is known that enthalpy, oxide formation, surface tension, and solidification time are the parameters affecting fluidity. For this reason, it is expected that Pb decreases the surface tension of Mg as a surface active element and accordingly increases the fluidity [21]. The Pb evaporation grade is a low element and may cause the MgO oxide formation on the surface to weaken or break during casting due to evaporation [22,23]. Therefore, weak or broken MgO can lead to increased flow.

Hot Tearing Tests
The hot tear test results were given as two different methods with the same lengths, different diameters and different lengths, same diameters. In the literature, which method is more effective has not been reported, so two different methods will be given comparatively to try to understand which is more effective. Figure 4 shows the results of the hot tear test of the AZ91 alloy and additions of Pb element, after casting in the molds of different diameters, same lengths. Hot tears disappeared in all of the samples of 0.4 mm Pb and 16 mm diameter with a diameter of 12 mm while all the samples with a diameter of 10 mm had a hot tear with Pb additions.  Figure 5 shows the results of the hot tear test after casting in the molds of different lengths same diameters with the additions of the Pb element to the AZ91 alloy. As can be seen in Figure 5, the longest arm in the mold is located at the bottom and hot tears for each alloy are visible on this arm adjacent to the runner. Hot tearing is not visible in other shorter arms. This indicates that the sensitivity of the long arm to hot tear is more effective than that of the other arms, which is explained by the fact that the shortening is longer in the long arm. As the sample diameter increases, hot tearing did not occur. This may be due to the mold design. Casting in the same diameter, different length method is done with large diameter vertical runner and arms are in horizontal position. It may be less common for hot tearing as the upper arms may be subject to preheating and unit shortening may be less during runner filling. On the other hand, casting is done vertically in different diameters and the effect of preheating can be negligible compared to other method.

Conclusion
1. Microstructure studies showed that the addition of Pb to the alloys caused a decrease in the amount of α + β eutectic. 2. Fluidity increased as increasing Pb contents. The fluidity length of the AZ91 alloy was 17.5 cm while it increased by 20 cm with the addition of 0.4 wt.% Pb. 3. Hot tearings were observed in "the mold with different diameter, same lengths" in 0.2 wt.% and 0.3 wt.% Pb additions, while hot tearing was not observed when this ratio increased to 0.4% for the 12 mm diameter test specimens. There was also not hot tearing were observed in any sample when the sample diameter was 16 mm.
4. Hot tearings did not occur in "the molds with different lengths same diameters" as the sample diameter increases. This may be due to the mold design.