There are many different types of coated wires, but the most common type of high-speed coated wire available today is called gamma phase wire. Gamma phase wire represents the newest generation of stratified wire and utilizes a special outer enriched zinc brass coating. The term ‘gamma phase’ refers to the metallurgical characteristics of the coating’s brass alloy, which contains even higher levels of zinc than the coating found on older D-type wires. Gamma phase wire builds upon the proven D-type wire technologies and adds a double layer enriched zinc coating to the wire. Gamma phase wire was designed to further improve machining speeds over D-type wire, and it cuts 20~30% faster than plain brass wire. Gamma phase wire is an excellent choice when wanting to improve cycle time of both good and poor flushing applications.
As stated by edmclub, the coating is very important.
The coating of a wire is responsible for its eroding yield, that is for the cut surface (mm²/min) per current unit (A). The best coating at the moment is a composite of beta brass and gamma brass.
But the cutting speed also depends on the EDM current (A). An high conductivity core will withstant more current than a standard brass core. That is why a core containing 80% of copper such as “D” wire is able to cut faster than a core containing only 63% of copper.
A Pure copper core will be the fastest: it is the old family of the “X” wires