The Zinc-Aluminum Die Casting Alloys

New high performance zinc-aluminum ZA casting alloys (ZA-8, ZA-12 & ZA-27) provide superior mechanical properties which designers can apply using die casting technology. This article is covering the general properties of the zinc-aluminum (ZA) alloy family, their die casting characteristics and several examples where ZA die-castings have lowered manufacturing costs and/or improved product performance.

Initially developed and marketed for gravity casting, the new zinc-aluminum alloys (ZA-8, ZA-12 & ZA-27) are gaining industrial acceptance for die-casting. The new alloys are radically different from the traditional zinc die casting alloys in terms of their chemical composition, die casting procedures and achievable mechanical properties. In general, the ZA alloys are stronger, harder and offer more creep resistance than standard zinc alloys and can be applied where bearing properties are important. During the past few years, ZA-27 had received considerable attention because of its high tensile strength. Recently, ZA-12 and ZA-8, have also gained popularity. ZA-8 in particular is expected to achieve rapid acceptance because it is the only member of the ZA family which can be hot chamber die cast using present zinc die casting equipment and technology. Both ZA-12 and ZA-27 must be cold chamber die cast like aluminum and require modifications to regular aluminum alloy casting practices.

The ZA materials should not be construed as replacements for standard zinc or aluminum die casting alloys where these materials are performing or expected to perform satisfactorily. There is no economic incentive to do so. ZA materials are slightly higher in cost than conventional zinc alloys and therefore higher in cost than aluminums. Instead, ZA materials are extending die casting into higher performance applications where the preferred economics of die casting compete with other manufacturing processes and the special properties of the ZA alloys can be employed to advantage over other materials.

The ZA alloys are quite different from the standard zinc (Zamac) die casting alloys. While the Zamac alloys are based upon nominally 4% aluminum, the ZA alloys all have substantially higher aluminum content. The numbers 8, 12 and 27 represent the approximate weight percent of aluminum in each of the ZA alloys (ZA-8 (Al-8%), 12 (Al-12%), and 27 (Al-27%)). Minor alloying additions include magnesium and copper while impurity levels are kept very low, similar to Zamac alloys.

The ZA alloys, offer higher ultimate tensile strength ranging from 374 MPa for ZA-8 to 426 MPa for ZA-27. Brinnel hardness is also higher. This property becomes important when considering ZA alloys for wear resistance. One key feature is that ZA alloys offer excellent bearing properties.

In the gravity cast state, ZA-l2 and ZA-27 are being used as direct substitutes for bronze industrial bearings. Not only are the ZA alloys less expensive, but in many instances they offer superior bearing performance. The creep properties of the ZA alloys are improved over standard Zamac alloys.

Die Casting the ZA Alloys

Due to the higher aluminum content of ZA alloys versus the standard Zamac alloys, certain melting and handling precautions must be taken. For ZA-12 and ZA-27, it is imperative that refractory lined melting equipment be used to preclude iron attack and subsequent melt contamination by iron. Therefore, ZA-12 & 27 cannot be melted in cast iron kettles. Similarly, ferrous ladles and skimmers must be coated to prevent iron attack. Iron contamination can be a problem since it diminishes both castability and machineability of zinc alloys.

ZA-27 requires the most attention to process detail in order to obtain optimum properties and reduce shrinkage porosity in die castings. ZA-27 as well as ZA-12 must be die cast in a cold chamber machine due to the iron attack problem discussed. To obtain optimum quality, intensification is required for ZA-27 to overcome sluggish feeding behavior. Intensification is the application of higher pressures after the die cavity has been filled.

The die temperature used with ZA-27 is not particularly critical (in the range of 150-200°C) while the melt temperature is typically held at 540-590°C. Die shrinkage value for ZA-27 is 0.8 to 0.85% which is higher than for Zamac’s.

ZA-12 must also be cold chamber die cast. Although intensification can be beneficial for ZA-12, it is not critical. A number of ZA-12 die castings are being made without intensification and the shrinkage porosity is minimal. Both ZA-12 & ZA-27 castings can be gated using standard aluminum practices. A heavier gate is sometimes preferable to ensure good feeding of heavy sections. ZA-12 die temperature is the same as ZA-27, in the 150-200°C range. Die casting temperature for the alloy is approximately 50°C lower than for ZA-27 (480-540°C) and die shrinkage is somewhat less than ZA-27 (0.7-0.8%). Because of the narrower freezing range of ZA-12, sounder sections are more easily obtained than for ZA-27.

ZA-8 is the easiest of the ZA alloys to die cast. Both cold chamber and hot chamber die casting is possible. ZA-8 is similar to the Zamac alloys in terms of aluminum content and die casting temperatures, allowing the use of standard zinc hot chamber die casting equipment as well as Zamac gating practices. The general die casting practices followed for Zamac alloys works well for ZA-8. This includes the use of reducing area runner systems to minimize air entrapment and relatively low die temperatures. The die shrinkage value of 0.7% is only slightly higher than that of the Zamac alloys (0.6%).

Technical Concerns

As with any alloy system, ZA alloys will not overcome deficiencies in either product design or process is control. One cannot take a poorly designed die casting and produce it in ZA alloys and expect to obtain a better die casting than would be produced in 380 aluminum or in Zamac #3.

Designers of die castings should apply good design rules. The best properties and soundest castings will occur if fairly uniform section, thicknesses are incorporated into castings. Good product design includes the use of radii where sections join and good draft allowances. Draft allowances used with ZA alloys tend to be those used for the Zamac alloys. Some small components that have short core pulls have been run with zero draft. Large castings produced in ZA-27 require draft allowances similar to those used with 380 aluminum.

Dimensional change upon aging should be considered. At elevated temperatures there is a slight growth (0.14%) for ZA-27 die castings. Designers should be aware of this fact if a large component requires critical dimensions. At, room temperature ZA-27 die castings are stable with only slight shrinkage noted after 10,000 hours. At 95°C there is a growth typical of zinc alloys with high copper content.

Casting soundness is often very important. The wide freezing range of ZA-27 yields sluggish feeding characteristics and makes it necessary to use intensification to achieve optimum soundness. These limitations can be readily overcome, however, for critical applications ZA-12 or ZA-8 may be preferred. On the other hand, ZA alloys are not susceptible to hydrogen gas porosity that can be a problem with aluminum alloys. Soundness, however, is more often a function of gating and part design rather than metal characteristics.

Machining of ZA die casting is similar to that of zinc alloys under most circumstances. Although high speed steel tools have been used successfully to machine ZA die castings, it is recommended that carbide or diamond tooling be used for high production runs.

Any problems that have been encountered regarding poor machineability, have been attributed to high impurity levels. These impurities (usually silicon and iron) are normally due to contamination with aluminum alloy scrap or due to dissolution of iron components (ladles and skimmers). The impurities form hard intermetallics, which cause excessive tool wear and subsequent machining difficulties. Specific machining guidelines have been developed for ZA alloys.

The corrosion resistance of ZA alloys tends to fall between the corrosion resistance of the Zamac alloys and aluminum. In comparison to aluminum alloys and special high grade zinc and zinc alloys, the ZA alloys fall in between. It is recommended under aqueous exposure that the pH range be kept close to neutral with slight excursions into the acidic or basic conditions.

Salt spray testing is another method to evaluate corrosion resistance of alloys. If corrosion resistance is an important requirement, then finishing of ZA die castings may be required.ZA alloys can be chromated, electroplated, painted or protected by the special zinc anodizing process.

Standard paint systems have been successfully used. Phosphate pretreatments yield the best adhesion while chromate treatments give the best corrosion resistance. If the ultimate corrosion resistance is required, then the zinc anodizing process offers exceptional performance. The process is not to be confused with aluminum anodizing. It is completely different requiring special solutions and processing. The finish is extremely resistant to aggressive salt environments such as a marine atmosphere and winter road conditions. The coating has withstood several thousand hours neutral salt spray testing (ASTM 8117) without failure.