ASTM B91 does not specifically address the heat-treat cycle to be used (or recommended to be used) for wrought ZK60A to obtain a T5 temper. Historically, the cycle used was 300ºF (150ºC) x 24 hours. But if we are not seeing any tensile failures (to ASTM B91), why can’t we change to 300ºF (150ºC) x 1 hour? Is there any value at all in the other 23 hours at temperature for closed-die forgings in ZK60A?
You raise an important question. Obviously, the difference in cycle time (and cost) to which you refer is huge, so minimizing cycle time is a noteworthy goal. It is natural to ask the question, if you can achieve the mechanical properties you need with a shorter cycle time, what is the benefit of a longer cycle? You might also be wondering if the answer is dependent on the end-use application of the product. For example, if it is not to be used in the aerospace industry or in a military application, why should it conform to AMS 4362F (Forgings, Magnesium Alloy 5.5Zn–0.45Zr (ZK60A-T5) Precipitation Heat Treated)?
The answer lies in with how the artificial aging (precipitation-hardening) process works. That is, how the material is actually strengthened. In simplest terms, precipitation of finely dispersed secondary phases is both a time- and temperature-dependent mechanism. This is why 24 hours is specified as the minimum soak time for this material. The proper degree of precipitation optimizes the microstructure and yields a superior product. The Model of Material Science (Fig. 1) tells us, among many other things, that we must optimize the part microstructure in addition to simply achieving mechanical or physical properties.
Read next week’s blog for a more in-depth explanation of this answer.