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.
Recertified HP Pavilion 570-P017C Desktop PC with Intel Core i5-7400 Processor, 16GB Memory, 1TB Hard Drive and Windows 10 Home (Monitor Not Included) Product - HP Flyer Red 15.6 15-f272wm Laptop PC with Intel Pentium N3540 Processor, 4GB Memory, 500GB Hard Drive and Windows 10 Home