How work materials affect machinability:
Properties of work materials that may affect machinability are: microstructure, grain size, heat treatment, chemical composition, fabrication, hardness, yield strength, and tensile strength as well as physical properties such as the modulus of elasticity, thermal conductivity, thermal expansion, and work hardening.
Crystal structure, arrangement of atoms such as BCC (body centered cubic), FCC (face centered cubic), and hexagonal, may make separation of atoms easier or harder and affect machinability. Atoms are not arranged in a regular way and a piece of metal is made of regulatory regions so-called "crystal grains" and at grain boundaries atoms are misaligned. Heat treatment may change crystal structure and this way affect machianability.
Chemical composition may affect machinability, for instance If you take steel, the percentage of carbon present in the steel, which determines generally the classification and strength of the steel (like low-carbon steel, medium carbon steel and high carbon steel) has a direct influence on the machinability – the higher the carbon content, the more difficult it is to machine. In alloy steels, the presence of elements like Chromium, Nickel, molybdenum, and Vanadium, etc. can cause decreased machinability.
fabrication methods like casting, hot rolling, cold rolling, forging may change structure, grain size and other properties of metals and affect machinability.
Generally speaking, any change in metal properties, due to nature of change, can have positive or negative effect on machinablity.