Alloy 6061, UNS A96061, ASTM B209, AMS 4027 O - Soft T4 - Solution heat treated and naturaly aged to a substantially stable condition T42 - Solution heat treated and naturaly aged to a substantially stable condition T451 - Solution heat treated then stress relieved by stretching. Equivalent to T4. T6 - Solution heat treated and artificially aged T62 - ..