Discovering zinc alloys for stronger die castings.

Inside This Story:

* Due to the demand for high volumes of zinc die castings, a search is in progress for an alloy that will sustain high temperatures while maintaining strength.

* Investigations examine the properties of zinc alloy prototypes and how well they perform in creep and other mehcanical tests.

* This article outline some of the innvestigations that have been performed and how the results might aid diescasters.

As investigations are in progress to develop alloys with better mechanical properties, the zinc families are no exception. Currently, the two zinc alloys most prominent in applications are the Zamak family, which dates back to the 1930s, and the ZA family, which was created in the 1970s. These alloys offer high strength properties.

However, these alloys also present a significant drawback in that their high temperature performance is not sufficient for many commercial applications. Automotive bracket parts or even toaster ovens would be ideal for zinc applications if it weren't for the alloys' tendencies to deform once confronted with a certain temperature. Therefore, a search is on to develop a zinc alloy that can sustain higher temperatures while maintaining its other mechanical properties.

The Council of Scientific Industrial Research (CSIR) and the International Lead Zinc Research Organization Inc. (ILZRO), investigated more than 40 potential alloys to ultimately create an alloy that is hot chamber die castable and has good creep-resistant properties.

This article discusses results from testing to determine what zinc diecast alloys had the best creep resistant properties and may make the best foundation for future results.

Gauging the Process

CSIR and ILZRO created a multiple step process to discover the best new zinc alloy from their tests. Their objective was to develop a model of creep strengths and other properties of particular alloys and examine the best results to design an alloy with optimal properties.

The first priority of the tests was to establish several properties that would determine the most creep-resistant zinc alloy:

* temperature capability of 284F (140C):

* creep stress of 31 MPa (4,500 psi);

* exposure time of 1,000 hrs;

* maximum creep elongation under these conditions of 1%.

Alloying elements were then decided upon for compositions of the test alloys. These elements, such as aluminum and magnesium, were determined based on past data in existing alloys as well as those that can better form stable precipitates in the diecasting. Thirty-six alloys were created for this process (Phase 1) and were tested for multiple mechanical properties. After these were tested, an additional set of alloys, A-F (Phase 2) were tested for creep properties. Two commercial alloys, Zamak 5 and ZA 8 also were tested concurrently, which made for a total of 44 tested alloys.

Although determining creep-resistance was the essential goal of the tests, the alloys had to exhibit other mechanical properties as well:

* flow distance (mm)--The die design test was to make for a difficult fill and help filter out what alloys can be better injected than others. The distance measured was how far into the die the alloy traveled;

* dimensional change (mm)--gauge length measurements (between the extensometer locators on the creep samples) were made before and after the samples had been aged for one week at 203F (95C) prior to mechanical testing;

* hardness (HB)--measurements were conducted using a Brinell indenter (2.5-mm diameter ball) at a load of 31.25 kg;

* die attack--testing was done to simulate hot chamber diecasting to determine if the composition of elements would gradually harm a ferrous die, such as in chemical reactions of opposite metals. An apparatus capable of rotating pins in liquid zinc was built for the tests;

* alloy stability--these tests were to determine how well a molten alloy can maintain its original composition over time. Because some elements, such as lithium and cerium, are more reactive with air than others, an alloy can lose some of its original chemical properties if left in a liquid state for prolonged periods.

Creeping into the Research

After all the tests were finalized, it was determined that Alloys 4 and 16 from Phase 1 portrayed the best creep properties (Fig. 1). The creep goal of 1% strain in 1,000 hrs. at 140C and 31MPa appeared to have been too ambitious a target as Alloys 4 and 16 fell far short of this. However, in comparison with existing hot chamber diecasting alloys (Zamak 5, ZA 8), the performances off Alloys 4 and 16 were significantly superior and could be used for commercial trials, though none have yet been cast. However, the U.S. Dept. of Energy has asked CSIR and ILZRO to work with diecasters by using these alloys in trials and further investigations.

[FIGURE 1 OMITTED]

Both Alloys 4 and 16 were composed mainly of zinc, aluminum and copper.

The two alloys exhibited only average mechanical properties in the impact, flow distance, tensile, dimensional change and hardness tests (Figs. 2-6), but they are acceptable for zinc alloys because they are within a practical range for use. Diecasters can run trials with these alloys and make successful parts with minimal modifications to their equipment due to the normal zinc properties portrayed.

[FIGURES 2-6 OMITTED]

Testing the Best

Because Alloys 4 and 16 were superior with the creep testing, further tests were taken to see how durable they would be in applications.

New creep tests were conducted on Phase 1 samples in the following conditions:

* solution treated and quenched;

* solution treated, quenched and aged at 284F (140C) (corresponds with test temperature);

* solution treated, quenched and aged at 464F (240C).

The results, however, were disappointing and showed a significant decline in performance compared with the properties obtained in Phase 1 (Figs. 7 and 8). A possible cause for this behavior is embrittlement due to internal oxidation, arising from the heat treatment.

[FIGURES 7-8 OMITTED]

This heat treatment was performed to see if the alloys' properties could improve. The thermal behavior of both alloys was very similar. Only Alloy 4 showed a typical age-hardening response and the peak hardness marches that of the Phase 1 specimens, which were aged for one week at approximately the corresponding temperature, 203F (95C). Alloy 16, however, showed a maximum hardness after solution treating and quenching and softened during aging. This was a more expected property, and it also was found in the other Phase 1 samples.

Zinc About It

Like any investigation, the results of the zinc alloys can be further scrutinized. Because zinc melts at 900F (482C), it makes it more vulnerable to weaken in particular applications. Throughout these tests, investigations sought to improve zinc alloys' ability to withstand loads at 300F (148C) and upward. That is a significantly high temperature when expecting zinc to remain hard and durable and not deform under load, and higher temperatures are more likely to cause creep strains. However, the low melting point also makes zinc alloys economical to cast. There's less work to melt the alloy with minimal pollution to the metalcasting facility and very little energy consumption. The properties presented by Alloys 4 and 16 could help determine future usages of zinc alloys and bring to consideration a better creep-resistant, economical alloy for commercial production.

This article was adapted from a paper presented at the 2003 North American Die Casting Assn. Congress.

For More Information

"Development of a Creep Resistant Hot Chamber Die Casting Zinc Alloy," J.M. Benson, D. Hope and F.E. Goodwin, Die Casting Congress, 2001, Cincinnati, Ohio 

"State-of-the-Art Assessment of Creep Resistant Zinc Alloys--Phase 1: Confirmation of Property Goals," ILZRO Project ZCA-1-2, January, 1997.

Visit www.moderncasting.com for the entire report, "Assessment of Experimental Hot Chamber Die Casting Zinc Alloys."

Duncan Hope is a researcher in the Division of Manufacturing and Materials Technology for the Counsel for Scientific Industrial Research (CSIR), Pretoria, South Africa, and has 12 years experience in mechanical testing.

Jeff Benson works in the Div. of Manufacturing and Materials Technology for CSIR and holds an MSc in Physical Metallurgy.

Cornelius M. Stander in a rebred consultant for CSIR and has authored/co-authored 30 scientific papers.

Frank Goodwin in a researcher for the International Lead Zinc Research Organization (ILZRO), Research Triangle Park, North Carolina and holds an S.M. and an Sc.D. in Materials Engineering from the Massachusetts Institute of Technology.