Zinc Diecasting Alloys
Selecting the Best Zinc Alloy for the Job
Alloys ZP5 and ZP3 are by far the most commonly used. Most diecasting companies are set up to use one of these alloys as standard. They frequently have dedicated bulk melting and scrap recirculating systems for their chosen alloy. Using another alloy is possible, but imposes extra metal melting and scrap handling costs. To achieve minimum price for any given casting design therefore either ZP3 or ZP5 should be specified.
ZP5 is stronger than ZP3, but if a normal safety factor has been applied when designing the casting it is highly probable that a ZP3 casting would be perfectly satisfactory in service. ZP3 has superior dimensional stability to ZP5, but it is only where dimensional tolerances are extremely critical that this would need to be taken into account. Therefore ZP3 and ZP5 can be considered to be interchangeable for most applications. The creep resistance of ZP5 is however significantly superior to that of ZP3 so the former is more suitable for use at elevated temperatures under continual stress.
The other two alloys ZP2 and ZP8 can be considered to be special purpose or heavy duty alloys. Their strength and hardness are significantly superior to those of ZP3 and ZP5. Which of these alloys to chose when their extra strength, hardness and wear resistance are required depends to some degree on what other critical attributes are required of the casting. The dimensional stability of ZP8 is excellent, but that of ZP2 is not as good as the other three alloys, although still good enough for the majority of castings. In fact the ageing characteristics of ZP2 are its chief drawback because the impact strength and ductility fall with the passing of time at elevated temperatures (~100°C). For many applications however these are not important factors.
ZP8 is capable of producing good sound castings when sections are reasonably even. However when isolated heavier sections are included in the design these are likely to display more and larger porosity. ZP8 is also slightly more expensive to cast because it is harder on the components of the diecasting machine shot system, which therefore need more frequent replacement.
The creep resistances of ZP2 and ZP8 are approximately equal and significantly superior to those of ZP3 and ZP5. However the creep data for ZP8 has been more adequately characterized than for ZP2, and more confidence can be placed in the results of creep predictions for ZP8. Further creep characterization work is currently ongoing for alloy ZP2.
All four alloys can undergo post casting forming operations and all are especially ductile immediately after the casting operation. When there is a delay between casting and the forming operation ZP3 will be found to be the most readily formed alloy, although the other alloys are not far behind.
All four alloys machine well although ZP3, being softer, has more tendency to pick up on cutting tools, which can lead to burrs and poor finish. This problem can be avoided by adequate flood cooling.
The wear resistance of zinc diecasting alloys is generally in line with their hardnesss. ZP2 and ZP8 are the best followed by ZP5 and then ZP3.
A link to a qualitative alloy selector application is included below. Prioritising the listed attributes against the requirements of the intended application will produce a suggested alloy of choice. Any number of attributes from one to all of them can be prioritised. Equal priorities can be ascribed if desired. The highest priority is 1. Leaving the priority box blank for any attribute means that property will not be taken into account in the selection process. If one (or more) attribute is believed to be far more important than the next priorities do not have to be applied sequentially. For example priority 1 could be ascribed to toughness and priority 4 to strength. After each priority is entered a suggested alloy is shown corresponding to all priorities so far entered. Be sure to hit the return key after typing in the last priority number or that priority will not be included in the selection criteria.
Attributes not included in the selector list are not significantly different for any of the alloys so are not relevant selection criteria.
The selector application is not an infallible guide to the best alloy to use. If in doubt seek further guidance.