Alloying for thin section zinc die castings: aluminum additions improve the ability of zinc alloys to be cast in thin sections.

To develop new zinc alloys for thin section diecasting, a commercial zinc alloy was modified by increasing its aluminum content. Improvements in fluidity were achieved, and industrial trials showed the alloy could be used to produce quality thin section parts.

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However, following the additions, the tensile and impact strength of the alloy became a concern. In a recent study, the properties of alloys were therefore explored. Thermodynamic calculations were carried out to help explain the phenomena and to provide insight for future alloy design.

Fluidity and Impact Strength

In the search for an ideal metal for the production of thin-section zinc die castings, ZAMAK Alloy 7 was chosen as a baseline due to its good fluidity. New alloys, both hypoeutectic and hypereutectic, then were prepared to explore the influence of alloy composition, primarily the aluminum content, on fluidity and impact strength (Table 1).

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The fluidity of the alloys first was tested. The molten alloys were held at about 815F (435C), skimmed to remove surface oxides and drawn into a stainless steel tube with a vacuum set at 90 mmHg. The fluidity then was determined by measuring the flow distance of the metal front into the tube by inserting a rod into the unfilled end and measuring its length. Five to six measurements were taken for each alloy composition (Table 2).

Impact tests were carried on unnotched specimens with dimensions of 0.25 x 0.25 x 3 in., which were gravity cast at a temperature of 435C using a steel mold manufactured in-house. Five to six specimens were tested for each composition, and the mean and standard deviation were calculated.

The results of fluidity tests are normally scattered, and although a large standard deviation emerged in this examination, a clear trend emerged showing that fluidity improved with increased aluminum content in the hypoeutectic regime, reached the maximum values at the eutectic composition and gradually decreased as aluminum content increased further in the hypereutectic regime.

Therefore, the aluminum addition to the existing zinc alloys had to be optimized to achieve the greatest fluidity, which makes an alloy most appropriate for thin-section casting.

What Is Fluidity?

Unlike viscosity, which is a pure material property, fluidity is an indicator of a number of material properties involved in melt cooling and solidification.

Two dominant properties affecting fluidity are the heat released from the melt and the volume fraction of solid phases formed during the solidification process. In the fluidity test, the heat released from the melt, in the form of heat capacity and latent heat, balanced the heat loss to the stainless steel tube. The heat loss lowered the melt temperature below the liquidus of the alloy, ushering in the solidification process. The solidification often started with the formation of a dendritic pre-eutectic phase, which hindered the melt flow more effectively than equiaxial or eutectic particles.

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Also contributing to poor melt fluidity was the fact that the pre-eutectic dendrites did not remelt as readily as the pure metals. Pure metals, when exposed to temperatures higher than their melting points, melt themselves. Hypoeutectic and hypereutectic dendrites, when exposed to higher temperatures than the temperature at which they were formed, may melt only partially to re-establish equilibrium with the melt. Their dissolution depends not only on the melt temperature but also on the liquid state diffusion away from the melt/dendrite interface.

The heat released from the melt and the total effective volume of the solid phases were calculated using an in-house thermodynamic database for zinc-based alloys (Fig. 1). It was assumed that there was no diffusion taking place in the solid phases during the solidification. Such a nonequilibrium assumption generated significantly more accurate predictions of the solidification process than an ideal equilibrium assumption.

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In Fig. 1, all curves nearly coincided with the abscissa initially, as there was no solid phase in the melt except for a minimum amount of dross. During this period, the heat released from the melt was determined by the heat capacity of the melt. This period was the longest for 5.03% aluminum alloy, where the composition was almost exactly the eutectic composition.

The period was the shortest for the 5.56% aluminum alloy, mainly because it had the highest liquidus temperature of 390C. The second shortest period was for 4.11% aluminum alloy, where the liquidus temperature was 387C. With the melt temperature lowered to the liquidus temperature, the pre-eutectic phase formed. As a result, the solid volume fraction rapidly increased.

The heat released during the formation of pre-eutectic dendrites was contributed mainly by the latent heat because the temperature range between the liquidus temperature and the eutectic temperature is generally small for diecasting alloys. Following the formation of the pre-eutectic phase was the eutectic transformation, indicated by a decrease in the slope of the solid volume fraction curves (Fig. 1). During the eutectic transformation, the melt temperature remained constant for binary alloys and hardly changed with minor element additions to the binary alloys. The pre-eutectic phase formed in 5.03% and 5.09% aluminum alloys was minimal and verified by micrographic examinations (Fig. 2).

The total heat released up to the point where the solid phase presented significant hindrance to melt flow was a good indicator of the melt fluidity. The accumulated heat released to a half solid and half liquid volume fraction was plotted against aluminum content (Fig. 3) and compared directly to the fluidity measurements (Table 2). The released heat curve resembled the trend shown in the fluidity measurements. Correlation between the released heat and the fluidity measurement was significant (R2 = 0.90).

Making an Impact

The consistently low impact strength of 4.51%, 5.03%, 5.09%, 5.25% and 5.56% aluminum alloys (Table 3) was conspicuous. Because the microstructure of 5.03% aluminum alloy was practically pure eutectic (Fig. 2), its impact strength of 2.4 J could be deemed the impact strength of the zinc-aluminum eutectic. This value was low in comparison to 38.4 J for 4.11% aluminum alloy, which consisted of 19% dendrites. The low impact strength of the zinc-aluminum eutectic revealed the weak interfacial bonding between the hcp-zinc lamella and the fcc-aluminum solid solution lamella.

A small percentage of dendrites (e.g. 6% for 5.56% aluminum alloy and 9% for 4.51% aluminum alloy) failed to improve the impact strength of the alloy significantly, plausibly because a small percentage of dendrites was unable to form a continuous dendritic framework so a fracture could always proceed through the weak zinc-aluminum eutectic. The threshold dendrite percentage for sufficient fortification was about 11% (4.46% aluminum alloy), evidenced by scattered measurements of impact energy ranging from 3.4 J to 47.4 J with a standard deviation larger than the mean value (Table 3). The threshold also was confirmed because the impact strength decreased from 16.0 J for 4.46% aluminum alloy to 2.6 J for 4.51% aluminum alloy.

The impact strength of various alloys was plotted against their aluminum content (Fig. 4). When the dendrite percentage was lower than an 11% threshold, the impact strength of the alloys was consistently low.

Comparing the results of the fluidity and impact strength tests, the study found that a hypereutectic alloy containing more than 11% dendrite to ensure good impact strength should contain at least 6.2% aluminum. The predicted fluidity of this proposed hypereutectic alloy was slightly better than that of 4.46% aluminum alloy. The added benefit of this hypereutectic alloy is that it possesses more stable properties. For the same degree of deviation in alloy composition, the dendrite percentage changed less for hypereutectic alloys than for hypoeutectic alloys.

Adapted with permission of the North American Die Casting Association. Copyright [c] 2008 by NADCA. NADCA members can receive all NADCA transaction papers free through its Technology Resource Library at www.diecasting.org/members.

For More Information

"Diecast Zinc," Engineered Casting Solutions, Casting Source Directory, p. 41-42.

Frank Goodwin, International Lead Zinc Research Organization Inc., Durham, N.C. Artur Filc, Daniel Liu and Nai-Yong Tang, Teck Cominco Metals Ltd., Vancouver, Canada

Frank Goodwin is executive vice president of the International Lead Zinc Organization, Durham, N.C. Artur Filc is development engineer, Daniel Liu is senior research scientist and Nai-Yong Tang is manager of zinc metallurgy for Teck Cominco Metals Ltd., Vancouver, Canada.

Table 1. Chemical compositions of Alloy 7 and experimental
alloys [wt.%]

Alloy       Al    Mg     Cu      Fe     Pb     Cd      Sn      Ni

Alloy 7    4.24  0.014  0.041  <0.001  0.002  0.000  <0.001  <0.01
4.11Al     4.11  0.008  0.031  <0.001  0.001  0.000  <0.001  <0.003
4.46Al     4.46  0.013  0.042  <0.001  0.002  0.000  <0.001  <0.01
4.51Al     4.51  0.008  0.028   0.002  0.002  0.000   0.001   0.003
5.03Al     5.03  0.012  0.042   0.001  0.002  0.000  <0.001  <0.01
5.09Al     5.09  0.012  0.042   0.001  0.003  0.000  <0.001  <0.01
5.25Al     5.25  0.012  0.042   0.001  0.002  0.000  <0.001  <0.01
5.56Al     5.56  0.012  0.042   0.001  0.002  0.000  <0.001  <0.01

Table 2. Fluidity Test Flow Distance at 435C (mm)

Alloy        Trial 1    Trial 2    Trial 3    Trial 4

Alloy 7      327.7      469.9      416.6      353.9
4.11Al       285.8      298.7      304.8      317.5
4.46Al       330.2      340.4      393.7      363.2
4.51Al       420.0      430.0      469.9      381.0
5.03Al       457.2      444.5      465.0      500.4
5.09Al       520.7      482.6      523.2      495.5
5.25Al       444.5      419.1      511.1      469.9
5.56Al       463.3      476.2      473.2      457.2

Alloy        Trial 5    Trial 6    Mean       [sigma]

Alloy 7      358.1      393.7      387        51
4.11Al       330.2        --       307        17
4.46Al       350.5        --       356        25
4.51Al       431.8        --       427        32
5.03Al       508.0      685.8      510        90
5.09Al       500.4      657.9      530        65
5.25Al       520.7      635.0      500        77
5.56Al       463.3        --       467         8

Table 3. Impact Test Results (J)

Alloy       1     2     3     4     5     6   Mean  [sigma]  dendrite

Alloy 7    11.2  20.3  10.2  13.6  7.5   --   12.5    4.9      16%
4.11Al     32.2  22.4  74.6   38    22   41   38.4   19.4      19%
4.46Al     7.8   19.7  10.9  3.4   47.4  6.8   16    16.4      11%
4.51Al     2.7   3.4   2.7    2    2.4   --   2.6     0.5       9%
5.03Al      2    2.7   2.7    2    2.7   --   2.4     0.4       1%
5.09Al     2.7    2     2    3.1   2.7   --   2.5     0.5       2%
5.25Al     3.4   2.4   2.7   2.4   2.7   --   2.7     0.4       3%
5.56Al     3.4   2.7   3.4   2.4   2.7   --   2.9     0.5       6%