INTRODUCTION TO HARDMETALS
1.02 Fundamentals and General Applications of Hardmetals
1.02.6 General Applications of Hardmetals .1 Machining
1.02.6 General Applications of Hardmetals
high-productivity, high-speed and high-feed cutting or in dry machining, and when machining of difficult to-machine materials.
Coatings:(a) provide increased surface hardness, for greater wear resistance; (b) increase resistance (abrasive and adhesive wear, flank or crater wear); (c) reduce friction coefficients to ease chip sliding, reduce cutting forces, prevent adhesion to the contact surfaces, reduce heat generated due to chip sliding, etc.; (d) reduce the portion of the thermal energy that flows into the tool; (e) increase corrosion and oxidation resistance; (f) improve crater wear resistance and (g) improve the surface quality offinished parts.
The common coatings for carbides that are applied in single- or multilayers are the following:
TiN: General-purpose coating for improved abrasion resistance. Color, gold; hardness HV (0.05), 2300; friction coefficient, 0.3; thermal stability, 600C.
TiCN: Multipurpose coating intended for steel machining. Higher wear resistance than TiN. Available in mono- and multilayer. Color, gray-violet; hardness HV (0.05), 3000; friction coefficient, 0.4; thermal stability, 750C.
TiAlN and TiAlCN: High-performance coating for increased cutting parameters and higher tool life; also suitable for dry machining. Reduces heating of the tool. Multilayered, nanostructured or alloyed versions offer even better performance. Color, black-violet; hardness HV (0.05), 3000–3500; friction coefficient, 0.45; thermal stability, 800–900C.
WC–C and MoS2: Provides solid lubrication at the tool–chip interface that significantly reduces heat due to friction. Has limited temperature resistance. Recommended for high-adhesive work materials such as aluminum and copper alloys and also for nonmetallic materials. Color, gray-black; hardness HV (0.05), 1000–3000; friction coefficient, 0.1; thermal stability, 300C.
CrN: Intended for copper alloys such as brass, bronze, etc. Color, metallic. Coating fracture toughness is as important as coating hardness in crack retardation. Balance between high compressive stress (poor adhesion) and low residual stress (no crack retardation) is necessary.
Al2O3: High inertness against workpiece materials, high hot hardness, low thermal conductivity, limited toughness, thermally stable >1200C, mainly used for cast iron and steel machining due to the high abrasion resistance. Black color.
Diamond: Mainly used for nonferrous, aluminum, composite, wood and graphite machining. Highest hardness and abrasive wear resistance, but limited toughness which can be compensated by using multilayer coatings.
Black color, thermally stable until about 800C (Figure 23).
A great attempt to correlate the cutting materials and their performance was made byKlocke and Krieg (1999). It was pointed out that there are basically four major groups of coating materials on the market. The most popular group is titanium-based coating materials as TiN, TiC and Ti(C,N). The metallic phase is often supplemented by other metals such as Al and Cr, which are added to improve particular properties such as hardness or oxidation resistance. The second group represents ceramic-type coatings as Al2O3 (alumina oxide). The third group includes superhard coatings, such as CVD diamond. The fourth group includes solid lubricant coating such as amorphous metal carbon. Additionally, to reduce extensive tool wear during cut-in periods, some soft coatings Table 11 Rough guide for selection of carbide grades for metal cutting applications
Cutting conditions ISO code
Finishing steels, high cutting speeds, light cutting feeds, favorable work conditions P01
Finishing and light roughing of steels and castings with no coolant P10
Medium roughing of steels, less favorable conditions. Moderate cutting feeds and speeds P20
General purpose turning of steels and castings, medium roughing P30
Heavy roughing of steels and castings, intermittent cutting, low cutting feeds and speeds P40 Difficult conditions, heavy roughing/intermittent cutting, low cutting feeds and speeds P50
Finishing stainless steels at high cutting speeds M10
Finishing and medium roughing of alloy steels M20
Light to heavy roughing of stainless steels and difficult to cut materials M30
Roughing tough skinned materials at low cutting speeds M40
Finishing plastics and cast iron K01
Finishing brass and bronze at high cutting speeds and feeds K10
Roughing cast irons, intermittent cutting, low speeds and high feeds K20
Roughing andfinishing cast irons and nonferrous materials. Favorable conditions K30 Fundamentals and General Applications of Hardmetals 51
as MoS2or pure graphite are deposited on top of these hard coatings. The basic PVD coatings are listed in Table 12. The effectiveness of various coatings on cutting tools is discussed byBushmann and Gupta (1991).
1.02.6.1.1.5 Tool Wear
Tool wear leads to tool failure. According to many authors, the failure of cutting tool occurs as premature tool failure (i.e. tool breakage) and progressive tool wear.Figure 24shows some types of failures and wear on cutting tools.
Generally, wear of cutting tools depends on tool material and geometry, workpiece materials, cutting parameters (cutting speed, feed rate and depth of cut), cuttingfluids and machine-tool characteristics.
Normally, tool wear is a gradual process. There are two basics zones of wear in cutting tools:flank wear and crater wear.
Flank and crater wear are the most important measured forms of tool wear.
Flank wear is most commonly used for wear monitoring. According to the standardISO 3685:1993for wear measurements, the major cutting edge is considered to be divided into four regions, as shown inFigure 25:
l Region C is the curved part of the cutting edge at the tool corner.
l Region B is the remaining straight part of the cutting edge in zone C.
l Region A is the quarter of the worn cutting edge lengthbfarthest away from the tool corner.
Hardness 28.8±2.3 GPa Hardness 24.7±2.5 GPa Youngs modulus: 444.4±20.1GPa
Grain size: 500–1000 nm
Youngs modulus: 244.3±15.2 GPa Grain size: 50–100 nm
Figure 23 Example of two multilayer CVD coatings for steel machining (Ruppi, Larsson, & Flink, 2008).
Table 12 Basic PVD coating characteristics
Coating Characteristics
Titanium nitride, TiN This gold-colored coating offers excellent wear resistance with a wide range of materials, and allows the use of higher feeds and speeds. Forming operations can expect a decrease in galling and welding of workpiece material with a corresponding improvement in the surfacefinish of the formed part. A conservative estimate of tool life increase is 200–300%, although some applications see as high as 800%.
Titanium carbonitride,
Ti(C, N) Bronze-colored Ti(C, N) offers improved wear resistance with abrasive, adhesive or difficult-to-machine materials such as cast iron, alloys, tool steels, copper and its alloys, inconel and titanium alloys. As with TiN, feeds and speeds can be increased and tool life can improve by as much as 800%. Forming operations with abrasive materials should see improvements beyond those experienced with TiN.
Titanium aluminum nitride,
(Ti, Al)N Purple/black in color, (Ti, Al)N is a high-performance coating which excels at machining of abrasive and difficult-to-machine materials such as cast iron, aluminum alloys, tool steels and nickel alloys. (Ti, Al)N’s improved ductility makes it an excellent choice for interrupted operations, while its superior oxidation resistance provides unparalleled performance in high-temperature machining.
Chromium nitride, CrN Silver in color, CrN offers high thermal stability, which in turn helps in the aluminum die casting and deep- draw applications. It can also reduce edge build-up commonly associated with machining titanium alloys with Ti-based coatings.
Figure 24 Wear characteristics of cutting tools (Sandvik Coromant, 1995).
Figure 25 Types of tool wear according to standardISO 3685:1993.
Fundamentals and General Applications of Hardmetals 53
The width of theflank wear land, VBB, is measured from the position of the original major cutting edge. The crater depth,KT, is measured as the maximum distance between the crater bottom and the original face.
1.02.6.1.1.6 Tool Wear Evolution
Tool wear curves illustrate the relationship between the amount offlank (rake) wear and the cutting time,Tm, or the overall length of the cutting path,L.Figure 26(a)shows the evolution offlank wear VBBmax, as measured after a certain length of cutting path. Normally, there are three distinctive regions that can be observed in such curves. Thefirst region (region I inFigure 26(a)) is the region of primary or initial wear. The relatively high wear rate (an increase of tool wear per unit time or length of the cutting path) in this region is explained by accel- erated wear of the tool layers damaged during manufacturing or resharpening.
The second region (region II inFigure 26(a)) is the region of steady-state wear. This is the normal operating region for the cutting tool. The third region (region III inFigure 26(a)) is known as the tertiary or accelerated wear region. Accelerated tool wear in this region is usually accompanied by high cutting forces, temperatures and severe tool vibrations. Normally, the tool should not be used in this region (Figure 26).
In practice, the cutting speed is of prime concern in the consideration of tool wear. As such, tool wear curves are constructed for different cutting speeds keeping other machining parameters constant. InFigure 26(b), three characteristic tool wear curves (mean values) are shown for three different cutting speeds,v1,v2, andv3. Becausev3is greater than the other two, it corresponds to the fastest wear rate. When the amount of wear reaches the permissible tool wear VBBc, the tool is said to be worn out.
Typically VBBcis selected from the range 0.15–1.00 mm depending upon the type of machining operation, the condition of the machine tool and the quality requirements of the operation. It is often selected on the grounds of process efficiency and often called thecriterion of tool life. InFigure 26(b),T1is the tool life when the cutting speedv1is used,T2, whenv2, andT3, whenv3is the case. When the integrity of the machined surface permits, the curve of maximum wear instead of the line of equal wear should be used (Figure 26(b)). As such, the spread in tool life between lower and higher cutting speeds becomes less significant. As a result, a higher productivity rate can be achieved, which is particularly important when high-speed computerized numerical control (CNC) machines are used.
The criteria recommended byISO 3685:1993to define the effective tool life for cemented carbides tools:
1. VBB¼0.3 mm, or
2. VBBmax¼0.6 mm, if theflank is irregularly worn, or;
3. KT¼0.06þ0.3f, wherefis the feed.
1.02.6.1.1.7 Tool Wear Mechanisms
The general mechanisms that generate tool wear are abrasion, diffusion, fatigue and adhesion. The wear is accelerated at higher speeds and the higher temperatures associated with them. The fundamentals of tool wear have been summarized byShaw (1984)andTrent and Wright (2000).
Figure 26 Wear curves (a) normal wear curve, (b) evolution offlank wear land VBBas a function of cutting time for different cutting speeds.
1.02.6.1.1.8 Tool Life
Tool life is important in machining since considerable time is lost whenever a tool is replaced and reset. Tool life is the time a tool will cut satisfactorily and is expressed as the minutes between changes of the cutting tool.
The process of wear and failures of cutting tools increases the surface roughness, and the dimensional accuracy of the workpiece deteriorates.
1.02.6.1.1.9 Expanded Taylor’s Tool Life Formula
According to the original Taylor tool life formula, the cutting speed is the only parameter that affects tool life.
This is because this formula was obtained using high-carbon and high-speed steels as tool materials. With the further development of carbides and other tool materials, it was found that the cutting feed and the depth of cut are also significant. As a result, the Taylor’s tool life formula was modified to accommodate these changes as
VeTnfadb ¼ C (1)
whered is the depth of cut (mm) and f is the feed (mm/rev). The exponentsaand b are to be determined experimentally for each combination of the cutting conditions. The order of importance of the parameters is cutting speed, feed, and then depth of cut. Using these parameters, Eqn (1)for the expanded Taylor tool life formula model can be rewritten as
T ¼ C1nV1nfandbn or T ¼ C5:83V5:88f4:53d2:18 (2) Although cutting speed is the most important cutting parameter in the tool life equation, the cutting feed and the depth of cut can also be the significant factors. Finally, the tool life depends on the tool (material and geometry); the cutting parameters (cutting speed, feed, and depth of cut); the type and conditions of the cuttingfluid used; the work material (chemical composition, hardness, strength, toughness, homogeneity and inclusions); the machining operation (turning, drilling, and milling), the machine tool (for example, stiffness, runout and maintenance) and other machining parameters. As a result, it is nearly impossible to develop a universal tool life criterion.
1.02.6.1.1.10 Recent Trends in Tool Life Evaluation
Although Taylor’s tool life formula is still in wide use today and lies at the very core of many studies on metal cutting, including at the level of national and international standards, one should remember that it was intro- duced in 1907 as a generalization of many years of experimental studies conducted in the nineteenth century using work and tool materials and experimental technique available at that time. Since then, each of these three components has undergone dramatic charges. Unfortunately, the validity of the formula has never been verified for these new conditions. The validity of the equations for cemented carbide is discussed in Chapter 1.14 (Mari &
Gonseth, 1993). Moreover, one should clearly realize that tool life is not an absolute concept but depends on what is selected as the tool life criteria. Infinishing operations, surface integrity and dimensional accuracy are of primary concern, while in roughing operations, the excessive cutting force and chatter are limiting factors. In both ap- plications, material removal rate and chip breaking could be critical factors. These criteria, while important from the operational point of view, have little to do with the physical conditions of the cutting tool.
To analyze the performance of cutting tools on CNC machines, production cells and manufacturing lines, the dimension tool life is understood to be the time period within which the cutting tool assures the required dimensional accuracy and required surface integrity of the machined parts.
Although there are a number of representations of the dimension tool life, three of them are the most adequate (Astakhov, 2006). The dimension wear rate is the rate of shortening of the cutting tip in the direction perpendicular to the machined surface taken within the normal wear period (region II inFigure 26(a)), i.e.
vh ¼ dvr
dT ¼ hrhri
TTi ¼ vhlr 1000 ¼ vfhs
100ðmm=minÞ (3) wherehrandhriare the current and initial radial wear, respectively,TandTiare the total and initial operating time, respectively, and hs is the surface wear rate. It follows from Eqn (3) that the dimension wear rate is inversely proportional to the tool life but does not depend on the selected wear criterion (a particular width of theflank wear land, for example).
Fundamentals and General Applications of Hardmetals 55
The surface wear rateis the radial wear per 1000 cm2of the machined area (S)
hs ¼ dhr
dS ¼ ðhrhriÞ100
ðlliÞf mm=103cm2
(4) wherehriandliare the initial radial wear and initial length of the tool path, respectively, andlis the total length of the tool path. It follows from Eqn (4) that the surface wear rate is reverse proportional to the overall machined area and, in contrast, does not depend on the selected wear criterion.
The specific dimension tool lifeis the area of the workpiece machined by the tool per micron of radial wear.
TUD ¼ dS dhr ¼ 1
hs ¼ ðlliÞf
ðhrhriÞ100 103cm2=mm
(5) The surface wear rate and the specific dimension tool life are versatile tool wear characteristics because they allow the comparison of different tool materials for different combinations of the cutting speeds and feeds using different criteria selected for the assessment of tool life.
1.02.6.1.2 Machining Applications
At the advent of the hardmetal age, hardmetal bits were brazed onto steel bodies and ground to the desired shape. Today most inserts are clamped mechanically onto the tool body and have multiple cutting edges shapes and geometries and are termed“throwaway”inserts. The cutting tool bodies are made from hardened steels. In the case of drills, reamers and sometimes gear hobs, the tools can be made of solid carbide, though the tendency here is also to use replaceable carbide heads or inserts to save on hardmetal costs. All insert types and tools are standardized, but each supplier of tools does have some special types to augment their standard tools.
The number of alternate insert products in metal cutting applications cannot be exactly quantified due to the large number of permutations and combinations that are theoretically and practically possible. Some of the parameters that play a critical role are the following:
l Substrate properties (WC grain size and binder content) l Coating types, coating composition and coating thickness l Insert geometry
l Geometry of the cutting edge l Chip breaker geometry l Tool holder geometry.
Added to this, the workpiece material and geometrical attributes, jigs andfixturing of the workpiece, machining parameters such as feed, speed, depth of cut, coolant type and delivery, machine tool stability (vibrations, elastic deformation under loads), tool holder characteristics (e.g. stiffness, runout, repeatability), and production strategy all have just as much influence on the productivity as the choice of the optimal insert type.
1.02.6.1.3 Machining Trends
The trend in modern machining is toward one of part production lot size, reduced part tolerance, near net shaping capabilities coupled with an increasing number of workpiece materials with enhanced mechanical properties that earn the description“difficult-to-machine”.
Whereas mass-produced dedicated inserts are predestined for large production run lots, there is a sizeable demand in general machining applications, that call for a reduction in the complexity of insert choice and insert logistics by engineering“universal inserts”with optimized insert geometry, coating and substrate that could be used for machining a variety of workpiece materials by the proper choice of the cutting parameters like feed, speed and depth of cut. This dedicated“Multimat”universal insert concept was successfully introduced more than a decade ago, e.g. by Lamina Technologies SA and also reported (Prakash, 2010).
This concept foresees just one submicron straight WC–Co carbide substrate for turning applications and one straight WC–Co submicron substrate for milling and drilling applications, with a proprietary nanostructured PVD coating based on the TiAlN family with high hardness, toughness and oxidation resistance coupled with an optimized edge and chip breaker geometry that can handle a wide range of materials like brass, cast iron,
stainless steels, nickel based alloys and other exotic materials. The number of insert types required to handle a majority of machining applications is rather limited to about perhaps 100 standard ISO insert styles compared to the thousands of types offered by dedicated insert manufacturers.
The growth in demand of such“universal”inserts for general machining applications is reflected by the fact that many major insert manufacturers like Böhlerit, Ceratizit, Kennametal, Safety, Sandvik, Taegutec amongst others have refined this concept and offer similar types of inserts for turning, milling, drilling, parting, etc. of standard and exotic workpiece materials under dry and wet machining conditions for medium to low chip load appli- cations. This concept of universal types of tools has been also extended to drills and routers (Sandvik Coromant).
1.02.6.1.4 Special Tools
As machining conditions such as machining speed, feed rate, depth of cut, etc. are being driven to even higher benchmarks, the actual machining time is just a fraction of the total machining time for component production time, since tool and component set up times play the lions share in each machining operation using standard tools. This can be combated by the use of tailored, multimachining operation capability of specially engineered complex tools designed to combine many machining operations into a few. Such specialized tooling are offered by many tool suppliers, e.g. Kyocera Unimerco (Figure 28), Walter, Komet, Mapal, Rübig, etc. just to mention a few big players in Europe and a very large number of small specialized tool manufacturers. The higher purchasing price of such special tooling is offset by the possibility of increased productivity as well as the possibility of regrinding and recoating (ReNew) of such “specials”leading to a better utilization of material resources compared to the“once use”and throwaway concept.
As a rule of thumb, the tooling cost makes up only about 3–4% of total machining costs (Figure 29), but proper choice of tools, cutting parameters, machine tools, production engineering concepts, etc. could lead to large cost savings of 40% and more.
1.02.6.1.5 High-Pressure Coolant Supply
As early as 1950, the idea of delivering coolant under high pressure to the cutting region in order to increase tool life during machining was born. The primary objective of this machining technique is to reduce the tool–- workpiece and tool–chip interface temperatures during machining. This can be achieved by directing high- pressure coolant at the chip–tool interface. This can also aid chip breakages and control by chip curl and compressive stress. Flood cooling of the cutting zone should reduce the interface temperatures when machining at lower speeds with significant sliding region and where relatively low cutting temperatures are generated. The coolant also has the function of a lubricant to minimize friction and lower component forces (Figure 30). In addition to the primary function of workpiece cooling, lubrication and chip transport, it also affects the pro- ductivity and process reliability. Concepts of high-pressure cooling as well as cyrocooling (using liquid nitrogen or carbon dioxide) have been introduced to improve productivity and process reliability for difficult-to-machine materials. This is reflected in an improvement in chip fracture and longer tool life.
Figure 27 Evolution of theflank wear land VBBas a function of cutting time for different cutting speeds (Sandvik Coromant, 1995).
Fundamentals and General Applications of Hardmetals 57