I have not seen M2Al referred to as M57 before, but it could be the same alloy. If so, M57 (or M2Al) is composed of just about the same amounts of Mn, W, Mo, Cr and V as regular old M2 HSS with a little more carbon and 0.8-1.20% aluminum. This alloy has similar properties to M42: good wear resistance, hot hardness, and high toughness. It has slightly less wear resistance compared to M42, but is also slightly tougher than M42 (I have had a tendency to chip M42 endmills if I am not careful). The main advantage, however, is price. As an example Shars sells 1/2 X 1/2 reg 4fl endmills for $5.75 in M42 vs $4.50 in M2Al ($2.30 in M2). I have purchased M2Al endmills sometimes without me even realizing it (from JTS). They seem to work just fine although I can't say I have noticed a big difference. Maybe this has more to do with the type of machining I do, which is nothing out of the ordinary.
