High Cu content in A356-T6

[EngineerDave]

What impact would High Cu have in A356. Keep in mind A356 has a low Cu content compared to other Aluminum alloys. The specified is .20 max and we have 0.422.

Thanks in advance for answers. I had some ideas but wanted to confirm them

 

[Vaudou]

In hypoeutectic foundry alloys (Al-Si), which A356 falls in, copper is added to improve tensile strength, machinability and thermal conductivity. However, this increase is at the expense of a reduction in ductility and corrosion resistance.

At 0.42% Cu instead of <0.2%, you might expect lower ductility values, but with slightly higher tensile strength. The processing and casting steps will also have some effects on the mechanical properties of the end-product.

 

[TVP]

I would expect that the excessive Cu will have minimal effect on the mechanical properties of the casting-- the concentration is too low to impart any significant strength to the as-cast of heat-treated component. Maybe a slight bump in yield strength.

The big effect will be on corrosion resistance. Cu forms intermetallic phases that are cathodic to the base aluminum and therefore promote localized pitting corrosion and stress corrosion cracking.

 

[unclesyd]

TVP is correct in stating that ecessive Cu can cause corrosion. We had some very large parts fabricated from A356 when exposed to the elements behaved as 2024 would. No measurable loss of physical properties in our service, but contamination of a product in contact with the part.
Chemical analysis indicated an avgerage of 0.65% Cu in the problem parts. The prsence of Cu particles was confirmed by SEM. We were supposed to be getting virgin melt as per print. Someone from our side allowed the addition of remelt with no constraints to gain a slight cost advantage.