Ranga Komanduri

New observations on the mechanism of chip formation when machining titanium alloys

Abstract Titanium and other aerospace structural superalloys are extremely difficult to machine except at low cutting speeds because of rapid tool wear. To increase productivity it is necessary to understand the mechanics of chip formation when machining these alloys. In this paper we report some new findings towards that goal.

Some clarifications on the mechanics of chip formation when machining titanium alloys

Abstract With the increasing need to use titanium alloys for aerospace structural applications and because of the difficulties experienced in machining them (except at low speed), an investigation on the fundamental mechanism of chip formation when machining these alloys was undertaken. An attempt is made in this paper to clarify various aspects related to the mechanism of titanium chip formation based on a critical review of the literature and machining studies on a Ti-(6Al-4V) work material at various speeds with the aid of high speed photography and in situ machining experiments inside a scanning electron microscope.

Evaluation of carbide grades and a new cutting geometry for machining titanium alloys

Abstract A new cutting geometry consisting of a high clearance angle (from 10° to 15°) together with a high negative rake angle (from −10° to −15°) is proposed for increasing cemented tungsten carbide tool life during the machining of titanium alloys. This geometry would allow the use of a conventional insert (with an included angle of 90°) of any suitable shape (e.g. round, square or triangular) on a modified tool holder. The new geometry is found to yield longer tool life than does a high clearance angle (+15°) alone or a conventional tool with a low negative rake angle (−5°) and a low clearance angle (+5°). Further, the lower cobalt grade (Carboloy grade 999) and finer carbide grain size tools (Carboloy grade 895) are found to yield longer tool life than the higher cobalt grade medium carbide grain size tools (Carboloy grade 883), which are currently the most commonly used grade. A new ceramic tool material, an Si-Al-O-N compound, is found not to be suitable for machining titanium alloys because of rapid wear.

Highlights of the DARPA Advanced Machining Research Program

Results of a four-year Advanced Machining Research Program (AMRP) to provide a science base for faster metal removal through high-speed machining (HSM), high-throughput machining (HTM) and laser-assisted machining (LAM) are presented. Emphasis was placed on turning and milling of aluminum-, nickel-base-, titanium-, and ferrous alloys. Experimental cutting speeds ranged from 0.0013 smm (0.004 sfpm) to 24,500 smm (80,000 sfpm). Chip formation in HSM is found to be associated with the formation of either a continuous, ribbon-like chip or a segmental (or shear-localized) chip. The former is favored by good thermal properties, low hardness, and fcc/bcc crystal structures, e.g., aluminum alloys and soft carbon steels, while the latter is favored by poor thermal properties, hcp structure, and high hardness, e.g., titanium alloys, nickel base superalloys, and hardened alloy steels. Mathematical models were developed to describe the primary features of chip formation in HSM. At ultra-high speed machining (UHSM) speeds, chip type does not change with speed nor does tool wear. However, at even moderately high speeds, tool wear is still the limiting factor when machining titanium alloys, superalloys, and special steels. Tool life and productivity can be increased significantly for special applications using two novel cutting tool concepts – ledge and rotary. With ledge inserts, titanium alloys can be machined (turning and face milling) five times faster than conventional, with long tool life (~ 30 min) and cost savings up to 78 percent. A stiffened rotary tool has yielded a tool life improvement of twenty times in turning Inconel 718 and about six times when machining titanium 6A1-4V. Significantly increased metal removal rates (up to 50 in.3 /min on Inconel 718 and Ti 6A1-4V) have been achieved on a rigid, high-power precision lathe. Continuous wave CO2 LAM, though conceptually feasible, limits the opportunities to manufacture DOD components due to poor adsorption (~ 10 percent) together with high capital equipment and operating costs. Pulse LAM shows greater promise, especially if new laser source concepts such as face pump lasers are considered. Economic modeling has enabled assessment of HSM and LAM developments. Aluminum HSM has been demonstrated in a production environment and substantial payoffs are indicated in airframe applications.

A New Technique of Dressing and Conditioning Resin Bonded Superabrasive Grinding Wheels

Summary A new technique of dressing and conditioning a resin bonded superabrasive (diamond or cubic boron nitride) grinding wheel is presented. It comprises contacting the surface of the wheel with a wetting liquid (in this case a suitable lubricant), forming a film on the surface of the resin, and using the resulting wetted wheel surface to grind a hot pressed silicon carbide or silicon nitride ceramic alongside the workpiece of interest. The resulting thin ceramic chips (whose surfaces are extremely active as they are freshly generated) adhere to the wetted resin face forming a thin slurry layer. Such a layer if rejuvinated protects the resin bonding material from thermal and/or mechanical degradation or damage and the superabrasive from premature pullout, thus increasing the grinding wheel life.