For making your own flywheel steel should do fine. You're probably not aiming for an 80,000+RPM flywheel that rides on magnetic bearings. So composites shouldn't be needed. The best material, eventually, for flywheels should be carbon nanotubes. They have huge tensional strength, but lousy compression strength. Tensile loads is what a flywheel has, so they'd work great.
It is important to note that not all batteries associated with these computer models are affected. The company is directing all users to its HP Battery Recall website to go through the online validation and replacement process.
In fact, using 3rd party or refilled cartridge won't affect your warranty, unless HP can prove that the malfunction was due to the refill (click here for details). As in their own words, HP's printer warranty does not cover repairs or services due to use of non-HP cartridge or ink.