Just spotted this little clip from Boeing about the lightest metal ever, consisting of 99,9 % air.
Wikipedia says that “a metallic microlattice is a synthetic porous metallic material, consisting of an ultra-light metal foam. With a density as low as 0.9 kg/m3 (0.00561 lb/ft3), it is one of the lightest structural materials known to science.” (Source Wikipedia)
Amazing, isn’t it and certainly very hard to grind.
Have a great day and keep grinding with attitude!