T 77 condition is solutionazied and overagred to achieve mechanical properties equal to T6 condition and corrosion resistance similar to T76. To achieve this condition the following are important
1. Cu, Mg,Cr, Zn, Ni alloying elements in the solution
2. type of casting process ( S or P)
I hope if you want to achieve the mechanical properties and corrosion behavior of T77 tempered some grade of aluminum with A356, its not possible. A356 with lessthan 1% Cu category alloy, we cannot expect the real purpose of T77.
A242 has the following T77 cycle:
Sand casting process
Sol: 515C for 5 hrs
Aging: 330-355C 2hrs min.
Overaging time can be increase based on corrosion resistance and strength compensation.