zinc castings - brittleness

[rmetzger]

We recently have had a batch of zinc castings begin to fracture during installation on products (they are used as anodes). Initial questions were raised about the material condition of the finished parts and if they could be annealed to increase ductility to head off future occurrences of this issue. I've not found much in regards to a process for annealing to see if an increase in ductility is possible, can anyone provide some insight or a link so that we can anneal a batch of these for some testing? The basic following is the process I've found but I'm unsure if its correct: heating the zinc alloy at a temperature of 250° C.-310° C. for more than 30 minutes, and then cooling to room temperature. We have samples out to a lab to try to determine their temper but the results are not in yet.

the material composition is as follows:
Aluminum 11.4
Cadmium.002
Copper .72
Iron .006
lead .0028
Magnesium .028
Tin .002
Zinc, Balance

thanks

 

[ChuckNorris]

i know that in the case of extruding zinc alloy rods they are annealed to the temperatures that you have previously mentioned in your question, that is 250 degrees to 310 degrees. as you have stated also,i read up on the matter, 30 minutes + is the typical time which you would anneal it for and it should definately cool at room temperature.

"Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to reform more easily, so increases their ductility."

Hope this is of some help!

 

[Metaljon]

I assume you are using sand castings. Sand casting ZA-12 alloy tend to have low ductility of only 1-2% which is a characteristic of the material. The annealing temperature of 250 to 310 deg C is too high considering the liquidus/ solidus range for this material is 377 to 432 deg C. If you are going to anneal, then general rule of thumb is 50% of melting point. This means about 215 deg C for 0.5 hour should be sufficient. This is above the recrystalization temperature for the alloy. If the annealing temperature is too high, the material suffers grain growth and micro-cracking.

 

[rmetzger]

Yes - it is a sand casting applciation. Thanks for the clarification on the annealing temps!