Processing Of Diamond Composites For Cutting Tools By .

Processing of Diamond Composites for Cutting Tools by PowderMetallurgy and Rotary ForgingMarcello Filgueira(1) and Daltro Garcia Pinatti(2)(1) Universidade Estadual do Norte Fluminense. Laboratório de Materiais Avançados. Av. AlbertoLamego, 2000. Campos dos Goytacazes/RJ. 28015-620.(2) Departamento de Engenharia de Materiais. DEMAR/FAENQUIL. Polo Urbo Industrial. GlebaAI6. Lorena/SP. 12600-000.Keywords: In Situ Technology, swaging, sintering, diamond compositeABSTRACT. This paper shows the manufacture route of the bronze 4% weight diamond compositerope for direct application as diamond wires, and linear saws in the slabbing and cutting ofdimension stones. This route consists of powder mix compaction, sintering and rotary forgingtechniques. Tensile tests were performed, reaching an ultimate tensile strength of 230MPa for thediameter of 1.84mm. Scanning electron microscopy showed the diamond crystals distributionalong the composite rope during its manufacture, as well as the diamond adhesion to the bronzematrix. Cutting tests were carried out with the external cutting rope, showing a probableperformance 4 times higher than the diamond sawing discs, however its probable performance wasabout 5 to 8 times less than the conventional diamond wires (pearl system) due to the low abrasionresistance of the bronze matrix, low adhesion between the pair bronze-diamond due to the use ofnot metallized diamond single crystals.INTRODUCTIONLinear and circular saws, grinding wheels, wire saw pearls among others, are diamondcutting tools used in the slabbing, cutting, and polishing of dimension stones, ceramic materials andnonferrous metals in general [1]. There are a wide range of types of materials used in themanufacture of these tools, but the most employed is the system metal bond matrix - diamondcrystals [2]. Diamonds are impregnated in the metal matrix via two ways: electrodeposition orsintering. In the electrodeposited tools there are only a monolayer of diamonds on the tool surface,while the sintered ones show diamond crystals distributed into the bulk and on the surface of thetools, and is cheaper and easier to process. In this sense, sintered diamond cutting tools are morecommon allover the world [2, 3].The metal matrix selection is based on the abrasivity of the material to be cut or polished.For highly abrasive materials such as concrete, SiC, Si3N4, Al2O3, tungsten bond is used. Cobaltbond is employed in the cutting of materials whose abrasivities are similar to the granites. Bronze,cobalt bonds and its alloys are used for marbles. Brass, bronze or copper bonds are employed in thecutting of ceramics, glasses, and nonferrous metals [2].The sintering is normally accomplished by hot pressing. In this case, the powder or greenbody (in the desired form) is submited to sintering at the same time in which it is pressed into amold. A classical example is performed by Contardi [4], explaining a method to process diamondpearls for cutting wires, where the metal matrix bond diamond crystals mix is confined into holesof a graphite mold. This mold is put into a resistive vacuum furnace chamber coupled to a press.The press punches conduct the current for the green bodies sintering, at the same time that pressthem into the mold holes. The productivity is high, about 960 pearls per 8 hours.In this work we present an alternative route to process diamond composites, called "In Situ"Technology, which evolves conventional sintering and swaging (rotary forging) of the metal matrix- diamond mix, instead of the conventional hot pressing route. It was produced wires of bronze 4%weight diamond for use as diamond wires and linear saws in general.

EXPERIMENTALTable 1 gives the typical concentration versus density of diamond in cutting tools. In thepresent work we used a diamond concentration 50 (2.2carat/cm3; 0.44g/cm3; 0.13cm3 ofdiamond/cm3 of tools) since this is the concentration used for cutting dimension stones. Thediamonds used was the De Beers SDA 65 40/50 mesh (425 - 300µm). The atomized bronzepowder used in this work presents 80% of particles in 75/60µm size range, and table 2 shows itschemical analisis, accordingly to the ref.[5]. Figure 1 shows the processing flow chart of thebronze/diamond composite wire, starting with bronze 4 %wt diamond.Table 1- Typical concentration versus density of diamonds in cutting tools.Concentration ofdiamond150125100*7550423025Mass of diamond/cm3 of tool 41.850.371.320.261.100.22Volume of diamond(cm3)/tools n 100 refers to 25 % in volume of diamond per cm3 of tool and has 4.4 carat of diamond / cm3of tools; 1 carat 0.2 g. The fourth column is obtained dividing the third column by the diamond density(ρdiam. 3.48 g/cm3)Mixture of bronze 4 wt% diamond Cold isostatic pressing at 200 MPa, rubber matrix, initial 12.87mm, final 8.00mm Sintering at 8500C/20 min., vacuum 2.10-4 mmHg Capping into eletrolytic copper tube, ext. 13.70mm, int. 9.20mm Rotary forging from 11.45mm down to 2.54mm; lamination down to 1.20mm Annealing at 6500C/20 min., vacuum 2.10-4 mmHg, at each area reduction of 30% Pickling of the copper capping in a HNO3 H2O solutionFigure 1- Processing flow chart of the bronze / diamond composite wire.Table 2- Chemical composition of the bronze powder (%).bronze---- .04N0.01

RESULTS AND DISCUSSIONMicrostructure and Tensile BehaviorFig.2 shows the longitudinal microstructure of the initial sintered bronze 4 %wt. diamond at 8.00mm. The distribution of diamond is relatively spaced. The samples were cupped in Bakeliteand ground by 80 mesh emery paper. 1H2O 1HNO3 pickling did not work because corrosion wasnot uniform and pitting takes place in the bronze/diamond interface. Fig.3 shows the cross sectionof the diamond wire at 1.84mm (R 82/1.842 18.90X). As can be seen, the swaging promotedan elevated area reduction, and the diamond crystals remained intact without further breakage. Thisis very important because the cutting capacity of the tool is strongly dependent on the diamondsintegrity.For bronze 4wt.% diamond the rupture load is shown as a function of the external capdiameter along the various stages of the processing (fig.4). The curve of fig.4 does not extrapolateto zero, but to 1.20mm, whose diameter is the limit of the contact diamond to diamond. At thisdiameter, the rupture load is 180N, indicating some adherence between bronze and diamond of theorder of 160MPa (σ 180N/[π. 2/4] 180N/[(π/4).(1.20)2.10-6 m2] 160MPa). In fig.5 the UTSremains constant at 230MPa, up to 1.84mm, after which it decreases. The data of the fig.8 refersto the UTS values for the same material. This indicates that it is the optimum diameter for thiscomposition and grain size of the diamonds (350 µm).Fig.2Fig.3Figure 2 and 3- Longitudinal section of the sintered bronze 4wt% diamond. 8.00mm (fig.2).Cross section of diamond composite. 1.84mm (fig.3).The measurements were made in the annealed condition (6500C/20min.) and data are anaverage over 5 to 7 samples. The average modulus of elasticity was E 11.5GPa. For annealedcommercial bronze, UTS 260MPa and E 16GPa [3]. The presence of diamond up to 1.84mmhas a minor influence in UTS, but it decreases E (more flexible material) since it is now distributedas a composite. All the tensile tests were performed accordingly to the reference [6].Bronze – Diamond AdherenceFigs. 6 and 7 show the interface bronze-diamond in the various stages of the processing (assintered- 8.00mm, and 5.00mm respectively). The Cu layer is removed with 1H2O HNO3in order to expose the diamond cutting faces. There are no gaps (faults) between bronze anddiamond. There is no interdiffusion between them, and the adherence is smaller than theUTS 230MPa of the bronze 4wt.% diamond.

Fig.4Fig.52401600220Rupture Tension, UTS (MPa)Rupture Load (N)1400Bronze 4wt% Diamond12001000800600400200200Bronze 4wt% Diamond18016014012010001,01,52,02,53,0Diamond External Cap Diameter (mm)1,01,52,02,53,0Diamond External Cap Diameter (mm)Figure 4 and 5- Rupture load as a function of diamond external cap diameter (mm) (fig.4). Rupturetension (UTS) of diamond external cap diameter (mm) (fig.5).Fig.6Fig.7Figure 6 and 7- View of a diamond crystal spiked in the bronze matrix. Sample as sintered (fig.6).Bronze-diamond interface. 5.00mm. longitudinal view (fig.7).Preliminary cutting of marbleTable 3 presents the data of marble cutting by a monolithic bronze 4 wt% diamond wire.Figs.8 a, b and c shows micrographs of the wire before, in the middle and in the end of the cuttingtrial, respectively. This was made in a linear saw frame of 0.22 m in lenght, resulting in 1188 X10-4 m3 /0.22m 0.54 m2 /m of cut area per meter of wire. This is 5 to 8 times less than the pearlsdiamond wire. The reason for this poor performance is the low abrasion resistance of the bronzematerial shown in Fig. 8, where abrasion of the loose diamond powder is clearly seen. Anothercomparison can be made with results of De Beers [7] for granite cutting with a circular saw of φ 0.7 mm with a diamond volume of v 1.7 X 10–5 m3 and a perimeter of λ 1.7 mm (v 10–5 m3 /m,,effective diamond DAS 100,40/50 mesh and concentration 30). The measured cutting capacity ofthe circular saw was 0.62m2 resulting in 0.62/1.7 0.35 m2 /m. For marble, the cutting capacity isdoubled (0.73m2/m). The cutting capacity of the “in situ” linear saw frame (0.54 m2/m for diamond

volume of v 1.7 X 10 –6 m3 ) is 4.4 times higher than the De Beers results [(0.54/1.7 X 10–6):(0.73/10–5 )]. It is recognized that the critical point is the adhesion between the diamond and thematrix . Metal coated diamond will improve the cutting area capacity of the “ in situ” wire.A new characteristic of the “in situ” diamond wire is its possibility to be welded afterrupture . Fig.8.d shows the micrographs of a silver welded part of the “in situ” monolithic wire (φ 1.69 mm) Mechanical resistance is maintained (UTS 226 MPa) in the welded area and diamondof this region is not lost. This characteristic is a considerable improvement over the pearl-diamondwire that is completely lost after rupture.Table 3 - Marble cutting data with the “In Situ” Diamond wire.Nφ, mmσ, 8341.440.10331.380.11281.310.14181.230.12N – Number of measurements φ during cuttingφ - average diameter φ ( φi)/N(a)2A , cm ε, 56.25ε - error σ(N) ½σ - variance: σ ( [φi - φ] 2) 1/2N–1(b)