CROSS-REFERENCE TO RELATED APPLICATIONSThis application is a continuation-in-part of U.S. patent application Ser. No. 12/091,532 which is a 371 filing of International Patent Application Number PCT/IB2006/003023 filed Oct. 27, 2006 and entitled “Cubic Boron Nitride Compact” and which claims priority benefits of South African Patent Application Number 2005/0766 filed Oct. 28, 2005, the disclosures of which are hereby incorporated by reference.
This invention relates to polycrystalline cubic boron nitride abrasive compacts and the manufacture thereof.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (CBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blende form of boron nitride that has a similar structure to that of diamond. In the CBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds. Methods for preparing CBN are well known in the art. One such method is subjecting hBN to very high pressures and temperatures, in the presence of a specific catalytic additive material, which may include the alkali metals, alkaline earth metals, lead, tin and nitrides of these metals. When the temperature and pressure are decreased, CBN may be recovered.
CBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
CBN may also be used in bonded form as a CBN compact, also known as PCBN. CBN compacts tend to have good abrasive wear, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in contact with a workpiece. Diamond is the only known material that is harder than CBN. However, as diamond tends to react with certain materials such as iron, it cannot be used when working with iron containing metals and therefore use of CBN in these instances is preferable.
CBN compacts comprise sintered polycrystalline masses of CBN particles. When the CBN content exceeds 75 percent by volume of the compact, there is a considerable amount of CBN-to-CBN contact and bonding. When the CBN content is lower, e.g. in the region of 40 to 60 percent by volume of the compact, then the extent of direct CBN-to-CBN contact and bonding is less.
CBN compacts will generally also contain a binder containing one or more of phase(s) containing aluminium, silicon, cobalt, nickel, titanium, chromium, tungsten and iron.
A further secondary hard phase, which may be ceramic in nature, may also be present. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 transition metal, aluminium oxide, and mixtures thereof.
The matrix is defined to constitute all the ingredients in the composition excluding CBN.
CBN compacts may be bonded directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is bonded to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof. A known method for manufacturing the polycrystalline CBN compacts and supported compact structures involves subjecting an unsintered mass of CBN particles, to high temperature and high pressure conditions, i.e. conditions at which the CBN is crystallographically stable, for a suitable time period. A binder phase may be used to enhance the bonding of the particles. Typical conditions of high temperature and pressure (HTHP) which are used are temperatures in the region of 1100° C. or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
The sintered CBN compact, with or without substrate, is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.
High CBN materials (also known as PCBN) are used mainly in machining applications such as grey cast iron, powder metallurgy (PM) steels, high chromium cast irons, white cast irons and high manganese steels. High CBN materials are used normally in roughing and heavy interrupted machining operations. In certain cases they are also used in finish machining, such as finish machining of grey cast iron and powder metallurgy (PM) irons.
Such a wide application area for PCBN places a demand for a material that has a high abrasion resistance, high edge integrity, high strength, high toughness, and high heat resistance. These combinations of properties can only be achieved by a material that has high CBN content, at least 75 volume % and a binding phase that will form a high strength bond with CBN.
Because CBN is the most critical component of the high CBN material which provides hardness, strength, toughness, high thermal conductivity, high abrasion resistance and low friction coefficient in contact with iron bearing materials, the main function of the binder phase is to cement the CBN grains in the structure and complement CBN properties in the composite. Therefore, the weaker link in the high CBN composite design is the binder phase as compared to CBN.
U.S. Pat. No. 6,316,094 and EP 1,043,410 both describe methods of making polycrystalline CBN compacts which contain a low, i.e. less than 70 volume percent, CBN content. These CBN compacts differ materially from compacts of this invention in both overall cBN content and in the function or role of the non-cBN matrix. It is well known in the art that high and low CBN content materials are fundamentally different from one another—evidenced by their use in widely divergent applications.
Low CBN content compact matrix material will include both a secondary hard phase and a binder phase, where the secondary hard phase is the dominant material in the matrix. For these compacts, the matrix phase (particularly the secondary hard phase) plays a significant role in determining, in and of itself, the performance of the compact in application. This matrix phase will be present in sufficient quantity (greater than 30 volume percent) to be continuous in two dimensions. In some examples in the patents cited above, the secondary hard phase, binder phase and CBN are subjected to attrition milling. The purpose of this milling is the reduction in size of the brittle secondary hard phase material and the homogenous dispersion of the binder, secondary hard phase particles and CBN particles.
In high CBN content polycrystalline compacts, the CBN plays the dominant role in determining performance in the application. The role of the matrix is chiefly to facilitate reaction bonding between CBN particles, hence cementing them together. The higher CBN content and required formation of a strong cementing bond necessitates that the matrix mixture in high CBN content compacts contains far higher relative quantities of ductile binder phase material. The compact may still contain some level of secondary hard phase material.
Sintered PCBN materials are usually cut into the desired size and shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques. This cutting process is achieved by EDM (Electric Discharge Machining) or EDG (Electric Discharge Grinding) if the material is electrically conductive or by laser machining if it is electrically non-conductive. Laser cutting is not the generally preferred method due to high degree of surface damage, higher costs and longer cutting times especially when cutting ‘solid’ (unbacked) PCBN inserts. Therefore, it is typically only used if the materials are not EDM-cuttable. In addition EDM-cutting is suitable for machining complicated pin-lock holes in a PCBN insert with tight tolerances, with less surface damage and in a cost-effective manner than laser cutting.
A wire-cut electric discharge machine functions by producing an electrical discharge between the wire and the electrically conductive workpiece. The workpiece is eroded by electric discharges or sparks which on a small scale generate localised shock waves and intense heat. The process generates sufficient heat to melt or sometimes vapourise selectively compounds in the workpiece.
High cBN PCBN materials have particularly poor EDM-cuttability because of the constituent high levels of cBN, which is electrically non-conductive. High cBN PCBN materials typically have EDM cutting speeds roughly ¼ that of the lower cBN content PCBN materials (with a cBN content of 40 to 60 volume %). Improved electrical conductivity can typically be achieved by introducing sufficient quantities of electrically conductive binder materials. However, this tends to degrade overall composite properties by reducing the amount of the effective cBN phase and hence negatively impacting on performance.
According to the present invention, a method of making a powdered composition suitable for the manufacture of a polycrystalline CBN compact includes the step of subjecting a mixture of CBN1 present in an amount of at least 80% by volume of the mixture, and a powdered binder phase to attrition milling.
The powdered mixture, after the attrition milling, and, where necessary, drying, is preferably subjected to a vacuum heat treatment to remove/reduce some of the contaminants prior to subjecting the composition to the elevated temperature and pressure conditions necessary for producing a polycrystalline CBN compact.
The composition typically comprises from about 80 volume % to about 95 volume % CBN. The CBN may be comprised of particles of more than one average particle size.
The binder phase typically includes one or more of phase(s) containing aluminium, silicon, cobalt, molybdenum, tantalum, niobium, nickel, titanium, chromium, tungsten, yttrium, carbon and iron. The binder phase may include powder with uniform solid solution of more than one of aluminium, silicon, cobalt, nickel, titanium, chromium, tungsten, yttrium, molybdenum, niobium, tantalum, carbon and iron.
The binder phase may contain a minor amount of carbide, generally tungsten carbide, which comes from the wear of the milling medium.
The average particle size of the CBN is usually no more than 12 μm and preferably no more than 10 μm.
In one form of the invention, the CBN particles are fine, typically no more than about 2 μm in size. For such fine particles it is preferred that only one particle size (unimodal) is used. The mixture preferably consists of only the binder phase and the CBN particles, with any other components such as tungsten carbide from the milling process, being present in minor amounts which do not affect the performance of the CBN compact which is produced from the mixture. In particular the mixture will be substantially free of any secondary hard phase.
When the CBN comprises particles of more than one average particle size, the CBN is preferably bimodal, i.e. it consists of particles with two average sizes. The range of the average particle size of the finer particles is usually from about 0.1 to about 2 μm and the range of the average particle size of the coarser particles is usually from about 2 to about 12 μm, preferably 2 to 10 μm. The ratio of the content of the coarser CBN particles to the finer particles is typically from 50:50 to 90:10. The coarser particles will preferably be greater than 2 μm in size. For such bimodal CBN particles it is preferable that the mixture also contains a secondary hard phase. The secondary had phase will preferably be present in an amount of no more than 75 percent by weight, more preferably no more than 70 percent by weight, of the combination of binder and secondary hard phase. In this form of the invention it is preferred that the binder phase and secondary hard phase together with the fine CBN particles, be attrition milled, the coarser CBN particles then added to this mixture and mixed using a method which does not involve attrition milling, e.g. high energy mixing such as mechanical stirring or ultrasonic stirring. The binder and secondary hard phases may be mixed and subjected to attrition milling, prior to the addition of the fine CBN particles.
Examples of suitable secondary hard phase materials are ceramic hard phases such as carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 transition metal, aluminium oxide and mixtures thereof.
According to another embodiment of the invention, a polycrystalline CBN compact is made by subjecting a powdered composition produced as described above to conditions of elevated temperature and pressure suitable to produce such a compact. The powdered composition may be placed on a surface of a substrate, prior to the application of the elevated temperature and pressure conditions. The substrate will generally be a cemented metal carbide substrate.
According to another aspect of the invention, a polycrystalline cubic boron nitride compact comprises greater than 75 volume % and not greater than 90 volume % cubic boron nitride particles, the cubic boron nitride particles comprising particles of at least two average particle sizes, and a binder phase constituting the balance of the compact and comprising at least one titanium compound selected from titanium boride, titanium nitride, titanium carbide and titanium carbonitride and at least one aluminium compound selected from aluminium oxide, aluminium boride, aluminium nitride, aluminium carbide and aluminium carbonitride.
The cubic boron nitride content of the compact preferably comprises 70 to 85 volume %, and more preferably 70 to 80 volume %, of the compact.
The titanium compound is preferably present in the binder phase in an amount, by mass, greater than that of the aluminium compound. Further, the titanium compound preferably constitutes at least 80 mass % of the binder phase.
There may be more than one titanium compound and more than one aluminium compound in the binder phase.
The titanium is preferably present in the binder phase as titanium carbonitride, titanium nitride or a mixture thereof.
The aluminium is preferably present in the binder phase as aluminium oxide, aluminium nitride or a mixture thereof.
The binder phase may contain a minor amount of carbide of another metal, generally tungsten carbide, which comes from the wear of the milling medium.
The cubic boron nitride particles (cBN) in the compact of the invention comprise particles of at least two average particle sizes. Preferably, the cBN particle size distribution comprises two discrete modes, i.e. it has a differentiable fraction of coarse particles (coarse fraction) and a differentiable fraction of fine particles (fine fraction). The average particle size of the cBN particles in the coarse fraction is preferably at least twice that of the average particle size of the cBN particles in the fine fraction.
The average particle size of the CBN particles in the coarse fraction is preferably less than 20 μm, more preferably in the range 5 to 12 μm. The average particle size of the CBN particles in the fine fraction is preferably at least 0.2 μm, more preferably in the range 1 to 5 μm.
The fine fraction preferably comprises 25 to 75 volume %, more preferably 30 to 70 volume % and still more preferably 35 to 60 volume %, of the CBN particles in the compact.
The cubic boron nitride compact of the invention may be bonded to a surface of a substrate, typically a cemented metal carbide substrate.
According to a second embodiment of this aspect of the invention, there is provided a high cBN content PCBN compact (with a cBN content exceeding 75 volume %), particularly a high content CBN PCBN compact as described above, with enhanced Electric Discharge (ED) Machining or Grinding cuttability such that it may be cut using EDM or EDG techniques at speeds of at least 50% better than those typically obtained for conventional PCBN materials with similar cBN contents whilst still achieving an acceptable surface finish.
The present invention concerns the manufacturing of high CBN content abrasive compacts. The composition or starting material used in producing the polycrystalline CBN compact comprises CBN and a binder phase, in powder or particulate form. The binder phase should at least partially melt and react with CBN and form bonding by reaction sintering during high pressure and high temperature sintering. The CBN content of the powdered composition is at least 80 volume percent. The CBN content of the polycrystalline CBN compact produced from the powdered composition will be lower than that of the composition. Thus, the CBN content of the polycrystalline CBN compact produced from the powdered composition of the invention will be at least 75 volume percent.
The present invention also concerns high cBN content abrasive compacts which contain greater than 75 volume % cubic boron nitride particles. Typically in a polycrystalline cBN compact, where the cBN exceeds about 75 volume % of the compact, there is a considerable amount of cBN-to-cBN contact and bonding.
Typically in a polycrystalline CBN compact, where the CBN exceeds about 75 percent by volume of the compact, there is a considerable amount of CBN-to-CBN contact and bonding. The CBN compact that has a CBN volume percent of greater than about 75 is typically characterised by isolated small binder phase between CBN grains. The binder phase in sintered compact is typically ceramic in nature and formed by reaction sintering between CBN and various metals that can form stable nitrides and borides. At least some of the binder phase material should be liquid or partially liquid during sintering and should wet CBN grains in order to achieve good bonding between CBN grains
The size distributions of the binder phase ingredients are preferably carefully chosen in order to achieve as much binder phase homogeneity as possible so that there is an even distribution of binder phase between CBN grains. This provides the final material with isotropy of properties and increased toughness. Even dispersion of the binder phase tends to provide strong bonding which also tends to reduce ease of removal of CBN grains during machining by abrasive workpiece materials.
In the powdered composition or cubic nitride compact produced by the invention, the CBN may contain multimodal particles i.e. at least two types of CBN particles that differ from each other in their average particle size. “Average particle size” means the major amount of the particles will be close to the specified size although there will be a limited number of particles further from the specified size. The peak in distribution of the particles will have a specified size. Thus, for example if the average particle size is 2 μm, there will by definition be some particles which are larger than 2 μm, but the major amount of the particles will be at approximately 2 μm in size and the peak in the distribution of the particles will be near 2 μm.
The polycrystalline cubic boron nitride compact of the invention is made by subjecting a composition containing cubic boron nitride particles and binder phase components, in particulate form to elevated temperature and pressure conditions. The use of multimodal, preferably bimodal, CBN in the composition, for larger CBN particle sizes, ensures that the matrix is finely divided to reduce the likelihood of flaws of critical size being present in the pre-sintered composition. This is beneficial for both toughness and strength in the compact produced from the composition.
The pre-sintered compositions may be subjected to milling. Milling in general, as a means of comminution and dispersion, is well known in the art. Commonly used milling techniques used in grinding of ceramic powders include conventional ball mills and tumbling ball mills, planetary ball mills and attrition ball mills and agitated or stirred ball mills.
In conventional ball milling the energy input is determined by the size and density of the milling media, the diameter of the milling pot and the speed of rotation. As the method requires that the balls tumble, rotational speeds, and therefore energy are limited. Conventional ball milling is well suited to milling of powders of low to medium particle strength. Typically, conventional ball milling is used where powders are to be milled to final size of around 1 μm or more. In planetary ball milling, the planetary motion of the milling pots allows accelerations of up to 20 g, which, where dense media are used, allows for substantially more energy in milling compared to conventional ball milling. This technique is well suited to comminution in particles of moderate strength, with final particle sizes of around 1 μm.
Attrition mills consist of an enclosed grinding chamber with an agitator that rotates at high speeds in either a vertical or horizontal configuration. Milling media used are typically in the size range 0.2 to 15 mm and, where comminution is the objective, milling media typically are cemented carbides, with high density. The high rotational speeds of the agitator, coupled with high density, small diameter media, provide for extremely high energy. Furthermore, the high energy in attrition milling results in high shear in the slurry, which provides for very successful co-dispersion, or blending of powders. Attrition milling achieves finer particles and better homogeneity than the other methods mentioned.
When the CBN consists of fine particles, typically 2 μm or less, then the CBN and binder phase are milled and mixed together by attrition milling with a controlled amount of wear of milling media. The binder phase may be subjected to attrition milling prior to the addition of the CBN particles.
When the CBN consists of particles of different sizes, where the coarse fraction is typically in the region of greater than 2 μm and 12 μm, the process usually consists of more than one step. The first step being the milling of the powdered binder phase and secondary hard phase, when present, with the fine fraction of CBN, in order to produce a fine mixture and the second step entails adding of coarser fraction of CBN. The mixture to which the coarse CBN particles have been added is then mixed using high energy mixing such as mechanical or ultrasonic mixing. There is no further attrition milling thus minimizing excessive introduction of carbide from the milling media. The binder phase with the secondary hard phase, when present, may be subjected to attrition milling prior to the adding of the fine CBN particles.
In the method of the invention, the binder phase particles are subjected to attrition milling in order to mechanically activate surfaces and optionally decrease particle size of binder phase materials. If the binder phase consists of more than one metallic phase, attrition milling can also provide limited amount of alloying formation, which further homogenize the chemistry of binder phase. The attrition milling of binder phase designed in such a way that wear of milling media, typically tungsten carbide is minimized.
The titanium compound and/or aluminium compound in the binder phase may be produced in the pre-sintered composition, for example by reaction of titanium, aluminium or a compound thereof with the cBN particles under suitable conditions such as a temperature of 800 to 1300° C. under a vacuum. The titanium and/or aluminium compound can be a sub-stoichiometric compound.
Typical conditions of elevated temperature and pressure necessary to produce polycrystalline CBN compacts are well known in the art. These conditions are pressures in the range of about 2 to about 6 GPa and temperatures in the range of about 1100° C. to about 2000° C. Conditions found particularly favourable for the present invention fall within about 4 to 6 GPa and 1200 to 1600° C.
Compacts produced from the method of the invention have particular application in machining of grey cast iron, powder metallurgy (PM) steels, high chromium cast irons, white cast irons and high manganese steels. High CBN materials are used normally roughing and heavy interrupted machining operations. In certain cases they are also used in finish machining, such as finish machining of grey cast iron and powder metallurgy (PM) irons.
Further, the polycrystalline cubic boron nitride compacts of the invention have been found to be readily cuttable by ED (electric discharge) machining and grinding). Typically, cutting speeds can be observed which exceed those observed for conventional PCBN compacts of similar cBN content by at least 50%, or more typically 70%; whilst still achieving an acceptable surface finish. This cuttability enables the compacts to be cut easily and effectively into a variety of shapes and sizes for producing tool inserts. Thus, the invention provides, according to another aspect, a method of severing a polycrystalline cubic boron nitride compact as described above by effecting a cut, preferably a fast cut, in the compact using EDM or EDG cutting. Generally, the severing will be such as to produce one or more of, and preferably a plurality of, tool inserts.
The invention will be illustrated by the following non-limiting examples:
Cobalt, aluminium, tungsten powders, with the average particle size 1, 5 and 1 μm, respectively, were attrition milled with CBN. Cobalt, 33 wt %, aluminium, 11 wt %, and tungsten, 56 wt %, form the binder mixture. Cubic boron nitride (CBN) powder of about 1.2 μm in average particle size was added in to the binder mixture in an amount to achieve 92 volume percent CBN. The powder mixture was attrition milled with hexane for 2 hours using cemented carbide milling media. After attrition milling, the slurry was dried under vacuum and formed into a green compact supported by a cemented carbide substrate. After vacuum outgassing, the material was sintered at about 5.5 GPa and at about 1480° C. to produce a polycrystalline CBN compact. This CBN compact (hereinafter referred to as Material A) was analysed and then subjected to a machining test.
Aluminium and tungsten powders, with the average particle size about 5 and 1 μm, respectively, were attrition milled with CBN. Aluminium, 30 wt %, and tungsten, 70 wt %, form the binder mixture. Cubic boron nitride (CBN) powder of about 2 μm in average particle size was added in to the binder mixture in an amount to achieve 94.5 volume percent CBN. The powder mixture was attrition milled with hexane for 2 hours using cemented carbide milling media. After attrition milling, the slurry was dried under vacuum and formed into a green compact supported by a cemented carbide substrate. After vacuum outgassing, the material was sintered at about 5.5 GPa and at about 1480° C. to produce a polycrystalline CBN compact. This CBN compact (hereinafter referred to as Material B) was analysed and then subjected to a machining test.
Aluminium and cobalt powders, with the average particle size about 5 and 1 μm, respectively, were attrition milled with CBN. Aluminium, 30 wt %, and cobalt, 70 wt %, form the binder mixture. Cubic boron nitride (CBN) powder of about 2 μm in average particle size was added in to the binder mixture in an amount to achieve 93 volume percent CBN. The powder mixture was attrition milled with hexane for 2 hours using cemented carbide milling media. After attrition milling, the slurry was dried under vacuum and formed into a green compact supported by a cemented carbide substrate. After vacuum outgassing, the material was sintered at about 5.5 GPa and at about 1480° C. to produce a polycrystalline CBN compact. This CBN compact (hereinafter referred to as Material C) was analysed and then subjected to a machining test.
Cobalt, aluminium, tungsten powders, with the average particle size 1, 5 and 1 μm, respectively, were ball milled with CBN. Cobalt, 33 wt %, aluminium, 11 wt %, and tungsten, 56 wt %, form the binder mixture. Cubic boron nitride (CBN) powder of about 1.2 μm in average particle size was added in to the binder mixture in an amount to achieve 92 volume percent CBN. The powder mixture was ball milled with hexane for 10 hours using cemented carbide milling media. After ball milling, the slurry was dried under vacuum and formed into a green compact supported by a cemented carbide substrate. After vacuum outgassing, the material was sintered at about 5.5 GPa and at about 1480° C. to produce a polycrystalline CBN compact. This CBN compact (hereinafter referred to as Material D) was analysed and then subjected to a machining test.
According to X-ray diffraction analysis, the sintered materials, Materials A, B, C, and D contained phases of CBN, WC, CoWB, Co21W2B6and small amounts of AlN and Al2O3. These materials were tested in continuous finish turning of K190™ sintered PM tool steel. The workpiece material contains fine Cr-carbides and very abrasive on PCBN cutting tools. The tests were undertaken in dry cutting conditions with the following cutting parameters:
| |
| Cutting speed, vc (m/min): | 150 |
| Depth of cut (mm): | 0.2 |
| Feed, f (mm): | 0.1 |
| Insert geometry: | SNMN 090308 T0202 |
| | (edge radius, r0 = 10 − 15 j-im) |
| |
All cuffing tools from Materials A, B, C, D were tested to failure as a result of excessive flank wear. Flank wears were measured (as Vb-max) at least three different cutting distances and it was found that in general the relationship between flank wear and cutting distance is linear. Least-squares lines were drawn to each set of data points for each PCBN materials. The flank wear rates in μm per meter sliding distance for each example materials were calculated and results are summarized in Table 1.
| TABLE 1 |
|
| Flank wear rates of PCBN cutting tools |
| Materials | Flank wear rate (μm/m sliding distance) |
|
| Material A: Attrition milling | 0.230 |
| Material B: Attrition milling | 0.214 |
| Material C: Attrition milling | 0.230 |
| Material D: Ball milling | 0.238 |
|
The three polycrystalline CBN compacts produced from a composition which had been attrition milled all had lower flank wear rates, indicating better performance due to longer cutting distance for a given total flank wear than the polycrystalline CBN compact produced from the ball milled material, Material D.
Ti(C0.5N0.5)0.8powder was mixed with Al and Ti powders using a tubular mixer, the weight percentage of Ti(C0.5N0.5)0.8, Al and Ti powders were 59%, 15% and 26%, respectively. The powder mixture was attrition milled for four hours with hexane. Cubic boron nitride (CBN) powder of 1.2 μm in average particle size was added in an amount to achieve 24 volume percent in the overall mixture and the mixture was further attrition milled for one hour. Cubic boron nitride (CBN) powder of about 8 μm in average particle size was added in a ratio to achieve 56 volume percent in the overall mixture. The overall CBN content of this mixture was therefore 80 volume percent. The mixture, in the form of a powder slurry, was dried and vacuum out gassed at about 450° C. The dried powder mixture was high energy shear mixed for 30 minutes and freeze dried. The granulated powder was then formed into a green compact and after further vacuum outgassing, the material was sintered at about 5.5 GPa and at about 1350° C. to produce a polycrystalline CBN compact. This CBN compact (hereinafter referred to as Material E) was then analysed.
Ti(C0.5N0.5)0.8powder was mixed with Al and Ti powders using tubular mixer, the weight percentage of Ti(C0.5N0.5)0.8, Al and Ti powders were 59%, 15% and 26%, respectively. The powder mixture was attrition milled for four hours with hexane. Cubic boron nitride (CBN) powder of 1.2 μm in average particle size was added in an amount to achieve 24 volume percent in the overall mixture and the mixture was further attrition milled for one hour. Cubic boron nitride (CBN) powder of about 4.5 μm in average particle size was added in a ratio to achieve 56 volume percent in the overall mixture. The overall CBN content of the mixture was therefore 80 volume percent. The mixture, in the form of a powder slurry, was dried and vacuum out gassed at about 450° C. and dried powder mixture was high energy shear mixed for 30 minutes and freeze dried. The granulated powder was formed into a green compact and after further vacuum outgassing, the material was sintered at about 5.5 GPa and at about 1350° C. to produce a polycrystalline CBN compact. This CBN compact (hereinafter referred to as Material F) was then analysed.
According to X-ray diffraction analysis, the sintered materials, Materials E and F contained phases of CBN, TiCN, WC and Al2O3. In both materials, the cBN content of the sintered materials was 80 to 85 volume % of the material.
Ti(C0.5N0.5)0.8powder was mixed with Al and Ti powders using tubular mixer, the weight percentage of Ti(C0.5N0.5)0.8, Al and Ti powders were 59%, 15% and 26%, respectively. The powder mixture was heat treated about 30 minutes under vacuum at around 1025° C.
The powder mixture was attrition milled for four hours with hexane. Cubic boron nitride (cBN) powder of 1.2 μm in average particle size was added in a ratio to achieve 24 volume in the overall mixture and the mixture was further attrition-milled for one hour. cBN powder of about 8 μm in average particle size was added in a ratio to achieve 56 volume in the overall mixture and the mixture was further attrition-milled for five minutes. The powder slurry was dried and vacuum out gassed at about 450° C. and dried powder mixture was high energy shear mixed for 30 minutes and freeze dried to form granules. The freeze granulated powder was formed into a green compact and after further outgassing under vacuum, the material was sintered at about 5.5 GPa and at about 1350° C. to produce a polycrystalline cBN compact.
This CBN compact is hereinafter referred to as Material A was analyzed and then subjected to a machining and EDM cuttability test.
Material B is a commercially available a high CBN PCBN material, Amborite DBW85™, with average CBN grain size of about 1.3 micron and containing about 80 volume % CBN. The material contains CBN, WC, CoWB, Co21W2B6and Al2O3according to XRD phase analysis. The thickness of PCBN layer is about 0.7 to 0.9 mm and remaining is cemented carbide with 13 weight % cobalt. This CBN compact is hereinafter referred to as Material B.
Material C is SECOMAX CBN350™ commercially available a high CBN PCBN material from Seco Tools. This material is a high CBN PCBN containing about 90 vol % CBN with AlN and AlB2 binder phases according to SEM and XRD phase analysis. This CBN compact is hereinafter referred to as Material C.
Material D is a cemented carbide material with 13 wt % Cobalt and remaining tungsten carbide with sintered grain size ranges from 1 to 3 μm. The coercivity of Material D is between 9 and 10.5 kA/m and magnetic cobalt content according to magnetic saturation measurement method varies between 11.5 and 12.5. Typical hardness (HV30) of this material is between 1170 and 1270 kgf/mm2. This cemented carbide material is hereinafter referred to as Material D.
Material A and Material C (comparative example) were cut and ground to form cutting inserts according to standard ISO insert geometries as RNMN1204S0220 with 200 μm chamfer width and 20 degrees angle and a hedge hone of 15 to 20 μm.
A05™ is a high chromium white cast iron from Weir Warman Ltd. It confirms to ASTM A532 Grade IIIA specification for high chromium white cast iron. It has about 3 weight % carbon and 27 weight % chromium with silicon, manganese, phosphorous, sulphur and molybdenum as alloying elements. The overall hardness of the material is about 650 Brinell hardness.
A05™ was machined using cutting speeds of 70 m/min with a feed rate of 0.15 mm/rev and depth of cut of 1.0 mm. Machining operation was a continuous turning of a cylindrical bar with outside diameter of 140 mm and length of 400 mm. Machining tests were run until catastrophic failure of the cutting edge. Machining time was recorded as an indication of the performance of the cutting tool. Material C failed by catastrophic fracture of the cutting edge after about 38 minutes whereas the machining test with Material A was discontinued due to very long machining time. Material A did not fail after about 100 minutes and maximum flank wear of Material A was about 1.4 mm.
Material A, B and D were subjected to WEDM cuttability test using Fanuc 0 iA WEDM machine. Material A, B and D were prepared by conventional lapping and grinding methods to 3.18 mm overall thickness. The machine settings were described in Table 1. Settings were selected in such a way that all three materials can be cut without any wire breakage and low surface damage. A square piece from each material with dimensions 10×10 mm was EDM cut using the set conditions in Table 1 with manual override setting of 60 out of range of 0 to 200. Cutting speed readings were taken at regular intervals along each cut edge; i.e. at 1, 5 and 9 mm along the edge being cut.
| TABLE 1 |
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| WEDM setting conditions for measuring EDM cuttability |
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| Cutting conditions | NUM | PM | VS | VM | ON | OFF | SV |
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| Machine settings | 1 | 1 | 4 | 14 | 8 | 120 | 38 |
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| Cutting conditions | WP1 | WP2 | T | WF | FR | FC | SPD |
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| Machine settings | 8 | 6 | 1500 | 5 | 10 | 0 | 3.2 |
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| Details of WEDM machine settings |
| NUM Number of cutting passes: This item specifies one cutting pass to be performed |
| PM Pulse mode: This item specifies a machining pulse, selected value represents ordinary roughing and semi-finishing. |
| VS No-load voltage: This item specifies a voltage applied to trigger discharge across the wire electrodes. The range is 1 to 8. The higher the set value, the higher the voltage. |
| VM Cutting voltage: This item specifies the intensity of the cutting pulse. The setting range varies from 3 to 18. The higher the set value, the higher the cutting pulse peak, and the higher the cutting speed, but the more likely becomes the cutting wire to break. |
| ON On time: This item specifies the intensity and width of the cutting pulse. The setting range is 1 to 16. The higher the set value, the wider the cutting pulse, and the stronger the discharge. As a result, the cutting speed is increased, but the cutting wire becomes more likely to break. |
| OFF Off time: This item specifies the discharge pause time, between the end of discharge and the beginning of the next voltage application. The setting range varies from 6 to 300. |
| SV Servo voltage setting range: This item specifies a reference voltage used to keep the cutting voltage nearly constant during cutting operation. The range is from 1 to 255. The lower the set value, the higher the cutting speed. However if the set value is too low, the discharge becomes unstable, possibly leading to a broken cutting wire |
| WP1 Power control setting range: The setting range is between 0 and 10. This item specifies a cutting wire breakage protection function by adjusting the intensity of the cutting pulse for stable cutting operation. The higher the set value, the less likely becomes the cutting wire to break, and the lower becomes the cutting speed. |
| WP2 Off time control: The setting range is from 0 to 10. This item specifies a cutting wire breakage protection function by adjusting the discharge pause time for stable machining operation. The higher the set value, the less likely becomes cutting wire to break, and lower becomes the cutting speed. |
| T Tension setting: The range is between 1 and 2500. The cutting wire vibrates as the discharge progresses and the dielectric fluid flows. When the cutting wire vibrates it can decrease cutting precision. The higher the set value, the stronger the wire tension and more likely becomes the cutting wire to break during rough cutting. |
| WF Wire feed: This item specifies the speed at which the cutting wire is fed. The setting range is between 1 and 15. The cutting wire becomes thinner during cutting operation because of erosion caused by discharge. So, the product straightness becomes lower if the cutting wire speed is low. |
| FR Water flow: The setting range is between 1 and 15. This item specifies the strength of dielectric fluid jets from the upper and lower nozzles. Dielectric fluid is supplied to take away heat generated by discharge, remove sludge, and stabilize discharge |
| FC Dielectric fluid control: This item specifies which nozzle is used to spout dielectric fluid. Selected setting for “0” means that both the upper and lower nozzles spout dielectric fluid. |
| SPD Set Feedrate: This item specifies the federate for cutting. Setting range changes from 0.1 to 50 mm/min. |
The results of this WEDM cutting test are given in Table 2. Material A has substantially higher WEDM cutting speed than Material B and Material D, according to results in Table 2. Surface and edge quality of the WEDM cut surfaces were investigated.
| TABLE 2 |
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| WEDM cutting speeds of Material A, B and D in [mm/min] |
| Distance from | | Material B | Material D |
| Cutting Edge | edge [mm] | Material A | (Compar.) | (Compar.) |
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| Edge 1 | 1 | 9.9 | 3.9 | 3.3 |
| 5 | 9.0 | 3.8 | 3.2 |
| 9 | 9.0 | 3.7 | 3.2 |
| Edge 2 | 1 | 9.0 | 3.9 | 3.3 |
| 5 | 8.9 | 3.8 | 3.1 |
| 9 | 8.9 | 3.8 | 3.1 |
| Edge 3 | 1 | 9.0 | 4.0 | 3.2 |
| 5 | 8.9 | 3.8 | 3.1 |
| 9 | 8.9 | 3.8 | 3.1 |
| Edge 4 | 1 | 9.1 | 4.0 | 3.2 |
| 5 | 8.9 | 3.9 | 3.2 |
| 9 | 8.9 | 3.8 | 3.2 |
| Average | 9.0 | 3.9 | 3.2 |
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Average WEDM cut surface roughness (Ra) was measured on each cut side as well as any chipping damage was monitored and measured. Any edge chipping larger than 50 μm was measured and recorded. The results of this test are summarized in Table 3. As can be seen in Table 3, Material A is not only twice as EDM cuttable as Material B and D, it has a comparable surface quality to Material B and better surface finish than Material D.
| TABLE 3 |
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| Average WEDM cut surface roughness and surface damage |
| Surface | |
| Roughness, | Surface |
| Ra, [μm] | damage |
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| Material A | 2.98 | No |
| Material B | 2.95 | No |
| Material D | 2.53 | 74 μm chip |
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Having thus described in detail various embodiments of the present invention, it is to be understood that the invention defined by the above paragraphs is not to be limited to particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope of the present invention.