I would be very careful picking the material for this part. The parts around the seat post tend to be very highly stressed. Often these parts are a high strength alloy steel. These parts are typically machined in an annealed condition when it is softer and easier to machine. Then the part is then heat treated to give it a good mix of hardness, strength and ductility. A steel part that is heat treated to full hard can be up to 4 times as strong as the same part in an annealed (soft) condition.