Compacted graphite iron (CGI) has a number of characteristics that can make it an outstanding alternative to ductile iron or gray iron for engine applications. The material offers 75% increases in both strength and stiffness as compared with gray iron; it is twice as resistant to metal fatigue; and it has thermal and damping characteristics that fall about halfway between those of gray and ductile iron. What’s more, there is an increase in both nodularity and tensile strength as wall section decreases. Several European manufacturers, including Audi, BMW and Ford (with PSA), are using CGI for engines, both diesel and gasoline. According to David C. Woodruff, process development engineer at machine tool manufacturer Makino (Mason, OH; www.makino.com), a difficulty associated with the implementation of CGI is specifically related to machining operations. While CGI has beneficial mechanical and physical properties, there are some issues related to machining it because compared, say, to machining cast iron, reduced cutting speeds can (predicated by the graphite content of the swarf) mean a cycle time of three times as long, which certainly isn’t the sort of thing that one may be looking for as regards output. While roughing can be performed on CGI surfaces comparatively well, semifinishing is more problematic.
Woodruff and his colleagues have been working with engineers at cutting tool manufacturer Sandvik Coromant (Fairlawn, NJ; www.coromant.sandvik.com/us) on tools and process parameters for CGI machining, specially for cylinder boring as this process, wherein the tool is ordinarily engaged continuously, has low tool life. Sandvik has developed a serrated tool for the process, the Long Edge Tool, which is helically moved through the bore. They’re using a Makino a81M horizontal machining center with an integral high torque spindle: it provides 744 lb-ft of torque from zero to 392 rpm. It has a CAT 50-taper toolholder. The spindle runs up to 10,000 rpm. According to Woodruff, they’ve discovered that the surface finish that is left as a result of the boring process they’ve developed, which he says is somewhat “convoluted,” actually reduces the boring requirement.
While the process parameter changes that CGI requires may seem rather off-putting, consider this: Woodruff says that by using CGI for an engine block the mass can be reduced by 22%.—GSV