In general, nickel-based alloys are numbered among materials that are difficult to machine because of their mechanical, thermal and chemical properties [DIN84, KUNZ82]. Due to varying chemical composition and microstructure however, nickel-based alloys show fluctuations in machinability.
On the whole, the high heat resistance and low heat conductivity of nickel — based alloys as well as the abrasive effect of carbides and intermetallic phases in abrasive machining lead to high thermal and mechanical stress on the tools [BRAN81, HABE79, LI97, MASY79, NIEW95]. Due to their high ductility, nickel-based alloys can be assigned to the long-chipping material group.
Depending on the different microstructures of these materials, varying requirements are placed on the grinding process, and machining strategies adapted to the particular machining task are necessary from economical and technological perspectives.
The Use of Conventional Grinding Tools
When machining nickel-based materials, mainly conventional grinding tools made of corundum are utilised. The low heat conductivity of the materials demands a grinding wheel specification suited to the machining case at hand, an appropriate conditioning of the grinding wheel as well as an optimal removal of heat by the cooling lubricant in order to determine thermal damage of the workpiece’s external layer.
Thus, open-pored grinding tools with a granulation of about F60 are used. Preparation for use usually takes place in the CD (continuous dressing) method, in which grinding wheels maintain a state of optimal conditioning by means of continuous in-process dressing. At the same time, it should be taken into consideration that grinding wheel wear is specified by the dressing feed fRd and the workpiece speed vw, and thus by the grinding time. A higher dressing feed frd leads to a high grinding wheel effective peak-to-valley height. In this way, an effective removal of heat from the machining zone is possible. Disadvantageous in this however are workpiece roughness, which rises with the effective peak-to-valley height, and increasing tool wear. Sol-gel corundums are of no advantage in this context.
An increase in the cutting speed generally leads to improved surface quality and smaller cutting forces. Since the dressing feed is moved according to grinding wheel rotation, an increased cutting speed leads to more tool wear. Moreover, thermal workpiece stress rises with higher cutting speeds. The customary range is vc = 20 to 35 m/s.
Conventional grinding wheels are indeed relatively cheap in comparison with superabrasives, however, not only tool costs are to be considered, but also set-up costs necessitated by grinding wheel changing [ADAM98].
In the case of surface grinding with conventional tools, as a rule the creep (feed) grinding process is used, in which the stock allowance is generally machined in one stroke. The advantages of this methodological variant are less surface roughness and less tool wear in comparison with pendulum grinding. When grinding engine blade roots, depths of cut can reach 10 mm or more. Problems develop because chip volumes increase along with the depth of cut and because cooling lubricant addition becomes more difficult, which can lead to thermal stresses, especially in the case of complex geometries.
Increasing the specific material removal rate Q’w generally results in workpiece roughness deterioration, increased thermal stress and more tool wear. In CD — grinding, wear can be compensated with an increased dressing feed. A common range for the specific material removal rate for conventional CD-grinding processes is about Q’w = 20 mm3/mms. In HSCD (high speed continuous dressing) grinding processes, specific material removal rates of Q’w = 100 mm3/mms can be realised.
The Use of Superabrasive Grinding Tools
Besides conventional grinding tools, cBN grinding wheels with vitrified and galvanic bonds have also been tried and tested for machining nickel-based alloys. Depending on the bond-type, usually water-mixed cooling lubricants are used for vitrified bonds and grinding oils for galvanic bonds.
Since nickel-based alloys are considered long-chipping materials, for cBN grinding wheels, in order to remove the chips and add the cooling lubricant, high grinding wheel effective peak-to-valley heights must be adjusted by means of the dressing process. In the case of dressing with a forming roller, dressing speed quotients of qd = 0.5 to 0.8 for low depths of dressing cut of aed = 2 to 4 pm have proven favourable. For crude or simple operations, varying degrees of dressing penetration are set. Similar dressing speed quotients have been proven for profile rollers as well. In this case, radial dressing feed speeds were in the range of frd = 0.5 to 0.7 pm.
The grinding depth of cut has a decisive influence on the chip removal process when machining nickel-based materials, as the contact length goes up with increasing depth of cut). In this way, supplying the contact zone with cooling lubricant and with this the removal of heat is made more difficult. With cBN grinding wheels, pendulum grinding operations with grinding depths of cut of ae < 250 pm tend to be more practical than deep grinding processes. The critical material removal rate (Grenzzeitspanungsvolumina) usually run up from only Q’w = 5 to 10 mm3/mms. By means of speed stroke grinding technology, with table feed rate vw of 200 m/min at small depths of cuts, specific material removal rates could be increased up to Q’w = 100 mm3/mms. Cutting speeds in cBN grinding processes are as a rule located at vc > 100.
Although these grinding conditions have been tried and tested for the machining of many different nickel-based alloys, machinability still depends on the structural state and alloy composition. In comparison to the forging alloy INCONEL 718, in the case of polycrystalline cast alloys like MAR-M247, generally higher grinding forces arise, resulting in increased grinding wheel wear.
The larger amounts of fortifying intermetallic y’-phase and complex M23C6 carbides can be made responsible for this. The highest tool service lives are generally found in monocrystalline nickel-based alloys lacking grain boundaries. For cast alloys in a directional manner, chip removal behaviour is contingent on the direction of solidification. Grinding transversely to the direction of solidification can lead to up 5 times higher tool wear than grinding lengthwise in the direction of solidification.
4.5 Grinding Titanium Materials
Machining titanium materials is generally regarded as difficult. Its machinability is essentially determined by the material type (metallic or intermetallic titanium), as well as by the respective alloy composition and thermomechanical pretreatment.