How to harden tin? - Page 2
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  1. #21
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    Can you talk about flux for these alloys or is that not needed?

  2. #22
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    Quote Originally Posted by Rob F. View Post
    Can you talk about flux for these alloys or is that not needed?
    The molten zinc alloys will dissolve iron. The ASTM specifications limit the maximum amount of iron (0.1%), lead (.004%), cadmium(.003%), and tin((.002)% that can be present in the melt.
    If the limits are exceeded there are problems with cracking, corrosion, and casting distortion. The iron-zinc compound stays in the melt and forms hard spots in the casting.

    Other impurities such as nickel, chromium, silicon, and manganese are also limited to small amounts. These metals alloy with the aluminum in the melt and form floating solids that can be skimmed off.

    The magnesium content needs to be controlled within a narrow range of .015 to .03 percent to control intergranular corrosion. Too much causes cracking.

    Flux is used to control some of these impurities, If used when not needed it can reduce the magnesium content below that required for a good casting.

    A foundry will have testing equipment to analyze the melt content. For home casters it would be safer to avoid using recycled zinc alloy and not use a flux. A clay or ceramic crucible will reduce iron contamination. This is important if scrap from the casting process is reused. If a cast iron mold is used, the amount of iron dissolved can be reduced by choosing a zinc alloy with a lower melting temperature. There are also ceramic paints available that can be baked on to a cast iron melting pot to reduce contamination.

    I have looked for a reference on how to artificially age the zinc casting to insure that the dimensions remain stable before machining
    This is one source: Eng_Prop_A_Mechanical Properties - Zinc Die Casting

    It looks like baking the casting for 10 hours at 220 to 300 deg F will do the job.


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