They used a high magnification video camera system to record the process and then examined the machined samples with optical and scanning electron microscopes. The tapes clearly demonstrate that cracking occurs along the graphite flakes and that severe deformation occurs in the surrounding matrix structure. The longer the graphite flakes, the longer the fracture distance ahead of and below the cutting tool. The graphite at the interface forms a thin solid film that separates the tool from the work and reduces strain and friction. The researchers also found that the type of matrix or fine microstructure in which the graphite was embedded played a role in machinability.
Gray irons have different machining characteristics than ductile irons and leaded steels.
Ductile irons are more plastic and form longer chips than gray irons. Leaded steels show a much higher ductility than ductile irons, longer deformation distances, and the formation of continuous chips during machining. Voigt notes that the researchers are currently forming a consortium of companies to further examine the role of graphite and structure on cast iron machinability. Their goal is a consistently machinable cast iron that will provide better performance when machined rapidly by automated machining cells.