May/June_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 8 2 8 aggressive sectioning devices that gen- erate considerable damage at the cut interface. Likewise, metal shears also produce substantial damage. This dam- age must be removed to reveal the true microstructure. When used properly, laboratory sectioning devices produce less dam- age than machine shop devices. In general, two main types of laboratory cutting devices are used by metallog- raphers. The first is the abrasive cutter, which uses consumable wheels. Wheel diameters range from about 9 to 14 in. (229 to 356 mm); laboratory style cut- ters with larger diameter wheels (up to 18 in./457 mm diameter) exist, but they are generally used outside the laborato- ry due to their large physical size. The second device is the low- speed saw, which has evolved over the last 30 years into the precision saw. Ear- ly versions of these low-speed saws had a maximum speed of 300 rpm and used gravity feeding. Current state-of-the-art saws feature a maximum speed of 5000 rpm and use linear feeding along with other options such as automated blade dressing and automated serial cutting. These saws use both consumable and nonconsumable blades. Blade composition. Metal-bonded diamond blades are available with high or low diamond concentrations in a va- riety of particle sizes. High-concentra- tion diamond blades are recommended for cuttingmetals and polymers (ductile materials), which are cut by a ploughing mechanism. Diamonds plough through the sample and hardened strips of ma- terial become brittle and break off. Besides the high or low diamond concentration, blades are made us- ing a variety of mean diamond parti- cle sizes, using an arbitrary scale from 5 (finest) to 30 (coarsest). A blade with a rating of 10 will have larger abrasive particles than one with a rating of 5, yet they are not necessarily twice as large. A general rule for cutting is the small- er the abrasive, the lower the resulting deformation. Control of heat and cutting load. Heat, which is generated by friction from the cutting process, damages the sample surface. Controlling the amount of heat is an effective way to minimize damage. Super thin diamond cutting blades combined with proper lubri- cation can reduce heat, as well as ap- plying just the right amount of cutting head load during the cutting process. By keeping the load low and cutting ca- pability high, along with proper blade selection, the sample can be prepared for analysis under the microscope with virtually no damage to the surface. An experienced lab technician can determine if the cutting load is be- ing properly applied, but consistency is difficult to maintain over the course of a long day of testing. Newly devel- oped software can monitor motor cur- rent and translate that data into cutting head load. The software allows the load to get to a certain point and then, in or- der to prevent the load from increas- ing, instructs the saw to back off on the cutting rate. Because the software is reading the motor current, factors such as material composition and sample thickness do not need to be taken into consideration by the operator. Other cutting system advance- ments developed to enhance precision and efficiency include: • Simplified controls that reduce training time, ensure process re- peatability, and facilitate enhanced control over cutting wheel position • Offline fixturing systems that enable a part to be prepped for mounting while the saw is cutting another part, resulting in reduced downtime • An innovative dressing system that cleans debris from the cutting wheel to allow a fresh layer of diamond coating, saving time and maintain- ing optimum quality. Currently, many machines allow the operator to dress the diamond blade before or after a cut, but this can degrade both cutting speed and quality during a single cut. Automatic dressing, such as the system de- veloped for the IsoMet high-speed precision saw, periodically occurs during the cut and significantly improves speed, quality, and con- sistency. ~AM&P For more information: Michael Kee- ble, U.S. labs and technology manager, Buehler, a division of Illinois Tools Works, 41 Waukegan Rd., Lake Bluff, IL 60044, 847.295.6500, techsupport@ buehler.com, www.buehler.com . (a) (b) (c) Sectioning damage on an hcp zinc sample. a) Band saw recrystallization depth is 50 microns; note that the jagged edge and heavy mechanical twinning will prolong grinding time; b) abrasive wheel recrystallization depth is 30 microns; sample exhibits some mechanical twinning; and c) precision blade recrystallization depth is 20 microns; sample shows minimal twinning.