Machinability in the Fixed Abrasive Machining Process

Machinability is understood as a property of a material that allows for chip removal under given conditions. It thus describes the behavior of a material during chip forming. The machinability of a material must always be considered in conjunction with the machining method, the tool, and the machining parameters.
In comparison to machining with geometrically defined cutting edges, grinding manifests clear procedural differences that affect the machinability of the materials. During grinding, machining is achieved by means of a number of individual grain engagements. Together with the strongly negative tool orthogonal rake angle of the grain, there is, in contrast with geometrically defined chip removal, an increased amount of friction and deformation work, from which results a higher conversion of energy in the process. This in turn can lead to heavier thermal stress on the surface layer. Small depths of cut result from the geometrical process characteristics of grinding and the high cutting speeds. Thus, the grain size of the workpiece material as well as the size of inclusions (e.g., carbide) plays a role with respect to machinability. This problem should certainly be considered in process construction as a scaling effect.
A multitude of individual grain contacts are made in the grinding process. Thus, besides the particular abrasive grains, the space between the grain and therefore the entire abrasive coating bond must be taken into account. The machinability of a material is determined by all the elements of the grinding system. All components, i.e., the grinding wheel (specification and preparation), process parameters, and cooling lubricants, have to be adjusted for the respective material and the machining goals (component requirements, productivity, and quality).
The workpiece surface layer created (thermal and mechanical influences) as well as chip formation can be called upon as criteria for the evaluation of machinability. Grinding forces, workpiece roughness, possible clogging of the grinding wheel, and macroscopic and microscopic grinding wheel wear can all occur during chip formation. Chip formation and surface layer properties thus limit potential material removal rates and workpiece roughness.

Grinding Nickel-Based Superalloy

Introduction of Nickel-Based Superalloy

The specific properties of these materials, adapted to the respective field of application, are essentially contingent on chemical composition, possible cold forming, and the heat treatment method. Corresponding to their most important alloying elements, nickel-based alloys can be subdivided into the following main groups (Everhart 1971):

• Nickel-copper alloys
• Nickel-molybdenum alloys and nickel-chrome-molybdenum alloys
• Nickel-iron-chrome alloys
• Nickel-chrome-iron alloys
• Nickel-chrome-cobalt alloys

Nickel-based alloys are highly temperature-resistant, resistant to corrosion, and also very tough. Thus, they are presently the more often exploited material for components that are exposed to nonuniform mechanical strains at working temperatures up to 1,100o C. Because of their high heat resistance, thermal fatigue resistance, and oxidation resistance in the high-temperature range of aviation turbines, their preferred areas of application are in gas turbine construction as well as in the construction of chemical instruments (Konig and Erinski 1987). In turbine construction, compressors and turbines of high power densities expose their components, especially turbine blades, to highly complex stress profiles. They are thermally strained with very high gas inflow temperatures and experience additional mechanical stress from centrifugal forces caused by rotation and have to withstand stress variations due to varying operational conditions. Depending on the request profile, nickel-based alloys of varying microstructures are used in this context.
In general, nickel-based alloys are numbered among materials that are difficult to machine because of their mechanical, thermal, and chemical properties. 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 (Li 1997). Due to their high ductility, nickel-based alloys can be assigned to the long-chipping material group.

Grinding Behavior of Nickel-Based Superalloy

1. Grinding Nickel-Based Materials with Conventional Abrasive Tools
When machining nickel-based materials, mainly conventional grinding tools made of corundum are utilized. 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 = 20–35 m/s.
Conventional grinding wheels are indeed relatively cheap in comparison with superabrasives; however, not only tool costs are to be considered but also setup costs necessitated by grinding wheel changing.
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/mm s (Klocke 2009). In HSCD (high speed continuous dressing) grinding processes, specific material removal rates of Q'w = 100 mm3/mm·s can be realized.

2. Grinding Nickel-Based Materials with Superabrasive 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-tovalley heights must be adjusted by means of the dressing process. In the case of dressing with a forming roller, dressing speed quotients of qd = 0.5–0.8 for low depths of dressing cut of aed = 2–4 μm have proven favorable. 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 = 0.5–0.7 μm.

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 μm tend to be more practical than deep-grinding processes. The critical material removal rate usually runs up from only Q'w = 5–10 mm3/mm·s. 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/mm·s. 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 INCONEL718, 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 γ 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 behavior is contingent on the direction of solidification. Grinding transversely to the direction of solidification can lead to up to 5 times higher tool wear than grinding lengthwise in the direction of solidification.

Grinding Titanium Alloy

Introduction of Titanium Alloy

Titanium and titanium alloys have a low density (ρ = 4.5 g/cm3) and high tensile strength (Rm = 900–1,400 N/mm2). They exhibit good heat resistance up to temperatures of 500o C. In addition, they are resistant to many corrosive media. From these properties are derived the main applications of titanium material, these being in air and space travel and the chemical industry. More universal use is prevented by the price, which is several times higher than steels and aluminum alloy. Titanium materials are subdivided into four groups:

• Pure titanium
• α-Alloys
• (α + β)-Alloys
• β-Alloys

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.

Grinding Behavior of Titanium Alloy

In order to machine titanium materials economically, the physical properties of this material group must be carefully considered. Its strength is high, while its facture elongation is, with A5 = 5–15 %, low (for alloys). The elasticity modulus is about 50 % lower than that of steel, while its heat conductivity is about 80 % lower. In addition, high temperatures arise in the contact zone. The heat generated can only be removed to a small degree with the chips. In titanium machining, a much larger amount of heat in comparison with steel machining drains off through the tool and the coolant.
Grinding can lead to the formation of cracks, which influence the ultimate component in its function and service life (Aust and Niemann 1999). Moreover, the mechanical processing of TiAl can lead to hardening zones in areas near the surface, which are characterized by a microhardness of up to 800 HV0.025 and a maximum thickness of 180 μm. These have a negative effect on the tool component’s service life.
Titanium’s reactivity with oxygen, nitrogen, hydrogen, and carbon, together with the high temperature of the contact zone, increases grinding wheel wear. In this case, two different types of wear can be distinguished. The first is grain wear, which is also called fatigue-adhesive wear. If the thermal and mechanical resistance of the bond is insufficient at the high temperatures of the contact zone, a second wear-type comes into play, bond wear. This is characterized by abrasive grain breakaway under excessive strain of the bond or thermal or chemical wear of the binder. In an experiment involving grinding the titanium alloy TiAl6V4, bond wear was responsible for up to 80 % of the total grinding wheel wear. In this case, most of the wear was caused by grain breakaway and bond fracture. These effects, however, were decisive in the failure of the bond, since the grains were dulled by use and caused higher cutting forces. Silicon carbide and diamond grinding wheels have proven advantageous in the grinding of titanium alloys. In grinding experiments with alloy TiAl6V4, two to three times higher grinding forces were measured using corundum grinding wheels as opposed to silicon carbide at a constant specific material removal rate of Q'w = 3 mm3 /mm·s. This leads to more than doubled wear in the case of the corundum grinding wheel. The use of corundum grinding wheels is not to be recommended due to their low heat conductivity (Kumar 1990).
In comparison to CD grinding methods, better surface qualities were realized with metallically or vitrified bonded diamond grinding wheels without continuous dressing (Aust and Niemann 1999). Pendulum grinding operations with small depths of cut and high table velocities tend to be better suited to grinding Ti-6Al-4V than deep-grinding processes due to improved thermal marginal conditions. The reaction of titanium chips with atmospheric oxygen and atomized grinding oil can lead to deflagration or to inflammation of oil spills in the machine.
Compared with grinding the titanium alloy TiAl6V4, machining titanium aluminides tended to have lower machining forces, lower tool wear, less grinding wheel clogging, and improved surface quality. Machining with formal and dimensional accuracy with conventional grinding wheel specifications is more realizable with titanium aluminides than with metallic titanium alloys. In this case, up to ten times higher grinding ratios can be achieved.
In the grinding of titanium aluminides, grinding wheels with high effective peak-to-valley heights, obtainable by high positive dressing feed rate conditions and high depths of dressing cut, has proved effective. Machining titanium materials created chips with highly reactive surfaces, which lodge themselves in the chip spaces of the grinding wheel and cause clogging in the grinding wheel. Therefore, a large chip space is required. Due to this high tendency to clog the grinding wheel, cleaning nozzles should be used, which rinse the chips and impurities from the pore space of the grinding wheel during the process.
When using vitrified bonded diamond grinding wheel to machine DP-titanium aluminides, grinding ratios of over 500 can be realized at a specific material removal rate of Q'w = 5 mm3/mm·s and a grinding depth of cut of ae = 25 μm. In the case of pendulum grinding operations, higher specific material removal rates lead to uneconomically high wear and low tool service lives.
By using speed stroke grinding technology, with which table speed of vw = 200 m/min can be realized, specific material removal rates of up to Q'w = 70 mm3/mm·s at grinding ratios of 200 could be reached in machining titanium aluminum with vitrified bonded diamond grinding wheels. Due to the high table feed speeds of this technology, low performance intensity leads to minimal thermal influence on the surface layer. Crack formation in surface layer of the workpiece can thus be prevented. Since high table velocity and high material removal rates create chips that are very thick, high cutting speeds should be selected. But because the cooling lubricant should be added at the rotational speed of the grinding wheel, a flexible choice of cutting speed is only possible within limits. Values of vc = 125–140 m/s have proven effective at a feed velocity of vkss ≈ 130 m/s. A further increase in the grinding wheel’s circumferential speed leads to problems in cooling lubricant supply and an uneconomical increase in grinding wheel wear and crack formation in the material.

Grinding Brittle Material

Introduction of Brittle Material

The designation “brittle,” often used in manufacturing, characterizes a certain material group according to their mechanical properties. High brittleness, i.e., low fracture resistance, and hardness represent a combination of material properties that, on the one hand, influences the range of uses of these materials but also determines their machinability and workability properties.
Several factors influence the mechanical behavior of a material. Firstly, there is the atom arrangement of the solid body. This can have an amorphous or a crystalline structure. In the case of amorphous structures, the atoms are arranged randomly. Glasses and many plastics and rubbers are amorphously structured. One speaks of a crystalline structure if the atoms form a regular three-dimensional lattice. Ceramics can exhibit both structures. The dominant atomic bond type is decisive for the inclination toward ductile or brittle material behavior. Covalent bonds lead to limited electron movement potential. For plastic forming processes, position changes are, however, extremely necessary. For this reason, large amounts of covalent bonds facilitate brittleness and hardness, while metallic bonds (ionic bonds) cause ductile material behavior.

Grinding Behavior of Brittle Material

The machinability of brittle materials significantly variables in contrast to metallic materials due to the characteristics described above. When machining brittle materials, as opposed to machining ductile materials, the assumption is that, with increasing penetration depths, material separation becomes dominated by the characteristic behavior of brittle materials, i.e., microcrack formation and resultant fragment breakaway. Fundamentally, it can be stated that, in case of a local loading of brittle materials (observing the microscopically small chip formation zone), the same action mechanisms are always prevalent – crack initiation and propagation and plastic material deformation. 

1. Grinding of Ceramic Material
With respect to their material characterization, ceramics are subdivided into oxide, non-oxide, and silicate ceramics. In the case of oxide ceramics, aluminum oxide and zirconium oxide or zirconia (ZrO2) represent the industrially most important materials. Oxide ceramics have mostly ionic bonds (>60 %), exhibit favorable sintering properties, and are disadvantageous compared to other ceramics with respect to their heat resistance.
As opposed to metal machining, when processing ceramics, the process forces are higher as a rule, especially in the normal direction. It is imperative that these forces be absorbed by correspondingly rigid machines and spindle systems. Otherwise, excessively soft, resilient systems lead to decreased dimensional and formal accuracy in the functional surface. Furthermore, ceramic machining is more demanding on machine protection. Grinding sludges have a highly abrasive effect due to the hardness of the removed particles. There is a lot of research available pertaining to the use of diamond grinding wheels for grinding ceramics. Since diamond reacts sensitively to strong thermal loads, increased demands are placed on cooling lubricant supply.
Concerning the type of bond, tools bonded with synthetic resin and metal are the most often used in ceramic grinding. Grinding wheels bonded with synthetic resin exhibit more wear during the process, but lower process forces, and therefore result as a rule in better surface quality and formal accuracy. The size of the diamond grains used varies between D7 and D252, whereby grain sizes between D91 and D181 are selected for most machining operations. Grain concentrations in ceramic machining are usually between C75 and C100 (Verlemann 1994).
With respect to the maximum obtainable specific material removal rates, values up to 50 mm3/mm·s are cited for machining high-performance ceramics with external grinding. Higher specific material removal rates are realizable with other grinding kinematics, such as surface grinding, but these cannot be considered representative methods. In ceramic chip removal, increasing machining performance is accompanied by concurrently increasing process forces.

2. Grinding of Silicon
Silicon is primarily used as a wafer material. The mechanical properties of silicon are informed by the anisotropic bonding forces prevalent in the monocrystalline corpus. The wafers are separated from a silicon monocrystal (ingot) and undergo chip removal on planar surfaces. Established praxis when grinding the isolated wafers dictates a two-step process comprising a preprocessing and a post-processing stage. First, there is a rough grinding process, which is required to remove the wafer surface, which is quite faulty after separation, as well as to smooth out the grooves. For this process, a relatively coarse grain is often selected (D46 in a synthetic resin or ceramic bond) in order to realize a high material removal rate (Qw = 100–200 mm3/s). In this case, brittle machining mechanisms are acceptable if the damage depth of the external zone is less than the depths of cut of the subsequent fine-grinding process. The state of the art for fine grinding is D6 grits bonded in synthetic resin. Synthetic resin bonds are preferred to vitrified bonds. In fine grinding, low material removal rates of Qw = 5–15 mm3/s tend to be chosen. Using a ductile machining mechanism in this way, a surface quality of Ra < 10 nm and external zone damages smaller than 3 μm (Klocke et al. 2000) can be realized, lessening post-processing costs.