WO2014063910A1

WO2014063910A1 - A flat fixed abrasive sawing wire

Info

Publication number: WO2014063910A1
Application number: PCT/EP2013/070710
Authority: WO
Inventors: Roland Groenen; Jan Vaneecke; Tom Baekelandt
Original assignee: Nv Bekaert Sa
Priority date: 2012-10-24
Filing date: 2013-10-04
Publication date: 2014-05-01

Abstract

The invention relates to a fixed abrasive sawing wire wherein abrasive particles are held in a metal matrix fixed to a metal wire. The metal wire has an oblong cross section and particular about the wire is that the abrasive particles are present on the width side of the metal wire. Methods to advantageously produce such a wire are described. The fixed abrasive sawing wire can be used as a single loop in a saw machine or in a reel to reel system. During sawing the work piece is sawn by the width side of the fixed abrasive sawing wire. The inventive fixed abrasive sawing wire and associated saw machine fills the gap between large kerf sawing cords (kerf higher than 4 mm) and small kerf (less than 0.25 mm) multiwire saws.

Description

A flat fixed abrasive sawing wire

Description Technical Field

[0001 ] The invention relates to fixed abrasive sawing wire that can be used to cut hard and brittle materials such as natural or artificial stone, ceramics, crystalline materials such as mono- or polycrystalline silicon, gallium arsenide, or quartz, amorphous materials such as glass or even magnets. The invention further relates to a method to produce such a wire. Also a method and apparatus to use the inventive wire is included.

Background Art

[0002] Sawing is an everyday business wherein material of a work piece is

abraded away by means of saw. A saw is equipped with abrasive elements such as protruding teeth or abrasive particles arranged on a carrier. The abrasive elements abrade the material away and evacuate the debris formed. The carrier can for example be a circular or rectangular blade with the abrasive elements arranged along the rim. Alternatively the carrier can be an elongated element that is closed into a loop such as a chain saw, a band saw or a closed loop sawing wire. When the sawing wire can be supplied in sufficiently long length there is no need to form a closed loop and a wire saw can spool the sawing wire from one reel to another reel while a work piece is sawn in between.

[0003] There is a constant search to find adapted saws for different materials and uses. There is for example stone sawing wherein currently gang saws (reciprocating elongated steel blades with abrasive elements brazed on the edge) are being replaced by sawing cords whereon abrasive elements in the form of sawing beads are laced on a multifilament steel cord (the carrier) and held in place by a polymer injected in between the beads. Such saws result in kerfs of 8 mm or more and thus can only be used in situations where cost of the loss of material is negligible like for example quarry excavation or stone slabbing. But even there it pays off to reduce the kerf width as other benefits like increased speed of sawing can also be attained. However with sawing cords, it is estimated that it will be difficult to reduce the kerf loss below 6.5 to 7 mm given the stretch of the cut that is typically more than 2 meters up to 10 meters.

[0004] At the other end of the spectrum there are sawing wires that have

diameters as small as 120 μιτι that are used for cutting hard and brittle materials such as poly- or mono crystalline silicon ingots (to produce wafers for the electronics or solar cell industry). The carrier is generally a steel wire of high tensile strength on which abrasive particles (such as diamond particles) are attached by means of a metal braze, a resin bond, or by means of indentation in a soft metal layer or incorporation in an electrolytically applied binder layer. Such wires allow for very fine kerfs but at a limited cutting stretch of below 400 mm. Thicker fixed abrasive sawing wires (up to about 0.25 mm diameter) exist for cutting large ingots of polycrystalline silicon into square blocks as there the stretch is longer (typically 1 .0 to 1 .5 meter).

[0005] There is an implicit relation between the stretch of the cut and the kerf: when the kerf is diminished the thickness allowed for the saw, and hence the strength of the carrier, reduces also and consequently the breaking load of the carrier will diminish. On the other hand, when the stretch increases, the force needed to pull the saw through the work piece will increase in proportion to the stretch and the kerf. One way to reduce the kerf while maintaining the strength of the carrier is therefore to have a carrier that is thin in the direction perpendicular to the cutting and axial direction but wide in the cutting direction. With 'movement direction' is meant the direction wherein the carrier is forced to move, with 'cutting direction' is meant the direction perpendicular to the movement direction in which the cut progresses.

[0006] Such saws are popular and are known as 'band saws' or 'blade saws'. In a band or blade saw the abrasive elements are always attached to one or both of the thin rims of a ribbon shaped carrier. A blade saw is of finite length and works in a reciprocal way. The band saw is made in a closed loop and guided over flat wheels that tension the saw while driving it. The axes of the wheels are parallel to the cutting direction. The increased stiffness in the cutting direction helps to push the saw blade through the work piece. The thin blade allows for maximum grip on the wheels. However, guiding wheels sometimes are needed to push and/or guide the band saw through the work piece.

[0007] Band saws or blade saws generally have the abrasive elements on the thin rim of the blade only (the rim spanned by the thickness and length dimension). Apart from teethed saws (that fall outside the scope of this application) the following prior art band saws or blade saws based on abrasive particles have been described:

• JP 2003 231063 describing a fixed abrasive sawing wire wherein diamond abrasives are adhered to a piano wire material having a vertically long cross section;

• US 5438973 showing a cutting blade the core of which is generally tear-drop shaped in cross section and wherein abrasive is provided on the wider of the shortest sides of the cross section;

• JP 2010 167509 discloses a fixed abrasive grain saw wire having a pair of parallel flat parts wherein abrasive grains are fixedly attached only to the curved surface connecting both flat parts;

• GB 1216428 describes a method to provide abrasive particles on the edge of a band saw.

[0008] However, band saws do have drawbacks:

• It is not possible to concurrently make more than two cuts parallel and close to one another;

• The lack of stiffness of the blade in the direction perpendicular to the cutting direction can make the blade 'wander' when cutting anisotropic materials;

• The stretch cannot be too long and/or the cutting force cannot be too high as the blade will tend to tilt leading to a curved cut.

Blade saws additionally have the disadvantages that they have to be used in reciprocating way whereby the continued acceleration and deceleration takes a lot of energy. Also due the reversal of the direction sawing time is lost leading to a lower cutting speed.

One of the goals of the invention is to overcome these drawbacks. Disclosure of Invention

[0009] The main goal of the invention is to provide a fixed abrasive sawing wire that does away the problems associated with blade saws or band saws. More specifally a fixed abrasive sawing wire is provided that better keeps track even when cutting anisotropic materials and allows for multiple parallel and straight cuts. Such a fixed abrasive sawing wire could be used for cropping of silicon ingots, sawing of stone, particularly stone veneer cutting, and even on hand held or portable wire saws. A further goal is to provide a method to produce this kind of wire. The use of the wire itself is also an object of the invention as well as a wire saw to use the wire on.

[0010] For the purpose of this application with 'wire saw' is meant an apparatus to saw work pieces while the wording 'sawing wire' or 'saw wire' is used for the utensil that is driven in a wire saw and that performs the actual sawing.

[001 1 ] The fixed abrasive sawing wire comprises a metal wire and abrasive

particles held in a matrix material fixed to the metal wire. The metal wire has a cross section in the plane perpendicular to the axis of the wire that shows an 'oblong' shape. Within the context of this application an 'oblong' cross section is a cross section having a thickness and a width, the thickness being smaller than the width.

[0012] Any two dimensional cross section can be delimited between two parallel tangent lines a certain distance from one another which is called the Feret distance. Depending on the angle by which the tangent parallel lines are oriented to the shape this distance will vary. When now considering a pair of mutually orthogonal couples of parallel tangent lines, one can select that pair that has the smallest surface area of the rectangle formed by the lines. Put otherwise: the pairs define the smallest rectangle that completely covers the cross section. In what follows the smaller of the two Feret distances will be called the 'thickness', and its associated direction is the 'thickness direction'. The larger Feret distance will be called the 'width' and its associated direction is the 'width direction'. With 'width side' is meant that side visible when viewed in the thickness direction, and with 'thickness side' is meant that side visible when viewed in the width direction.

Obviously there are two width sides and two thickness sides. As the width is associated with the larger Feret distance and the thickness with the smaller distance of the pair of couples of parallel tangent lines, the width will be larger than the thickness.

[0013] Cross sections in the form of an ellipse, a rectangle, an oval, a diamond, a scalene triangle, an isosceles triangle, an egg-shape, a kidney shape are oblong.

[0014] Completely opposite to what is customary in the world of blade saws or band saws the inventors put the abrasive particles on the width side and not on the thickness side of the metal wire. The abrasive particles can be present on both width sides. When present on both sides, both sides can be used for cutting. This can be advantageous when e.g. the sawing wire can be spooled from a pay-off spool to a take-up spool. When re-using the wire on the take-up spool the other side of the wire can be used.

Alternatively when the abrasive particles are only present on one side of the wire, the wire can be used in a reel-to-reel mode but also in a closed loop mode.

[0015] The width 'W of the fixed abrasive sawing wire preferably covers the

range of kerf larger than about 0.25 mm to less than about 4 mm. In this range no wire saws exists build on a single, round carrying wire as the diameter of the carrying wire would not survive the repeated bending during use. As such the sawing wire covers the gap between the relatively thick round sawing wires (of about 0.25 mm) and thin band saws (with kerf losses of more than 4 mm) More preferably a width range of between 1 mm to about 4 mm or even from 1 to 3 mm is envisaged.

[0016] The ratio of the width 'W' to the thickness 't' of the oblong cross section must be chosen in function of the application. For a given width 'W it is advantageous that at least the thickness 't' is less than the width 'W. The smaller 't' becomes, the more flexible the fixed abrasive wire saw will be in the thickness direction. Indeed, the 'stiffness' of a flat wire is equal to ExWxt³/12 wherein Έ' is the modulus of the steel. The ratio of 'W/t' must therefore preferably be at least larger (lower limits) than 1 .5, preferably even larger than 2, or 3, or 5 or even 8. However, the trade-off is that the wire will lose strength when 'W/t' is larger than 10 or 20 or even 50 (upper limits). The ratio is between any of the lower limit numbers and upper limit numbers cited taken in pairs. [0017] The material the wire is made of is a metal. Particularly preferred wires are high strength steel wires, plain carbon steel wire, stainless steel wire such as AISI304 or AISI316, a chrome vanadium steel wire, a chrome

molybdenum steel wire, a chrome molybdenum vanadium steel wire or a chrome molybdenum nickel steel wire. More preferred steel compositions are:

• 51 CrV4, Werkstoff Nr. 1 .8159, SAE6150 with about 0.51 wt% carbon, 0.7 to 1 .1 wt% of manganese, 0.9 to 1 .2 wt% of chromium, 0.1 to 0.25 wt% of vanadium, less than 0.4 wt% of silicon, less than 0.025 wt% phosphorous, less than 0.025 wt% sulphur, the remainder being iron and unavoidable impurities.

• 32CrMoV12-28, Werkstoff Nr. 1 .2365 with about 0.32 wt% carbon, 0.15 to 0.45 wt% of manganese, between 0.10 to 0.40 wt% silicon, 2.7 to 3.2 wt% of chromium, 2.5 to 3.0 wt% of molybdenum, 0.40 to 0.70 wt% vanadium, less than 0.025 wt% of phosphorous, less than 0.005 wt% of sulphur, the remainder being iron and unavoidable impurities.

• 48CrMoNi4.4, with about 0.46 wt% carbon, 0.85 wt% of manganese, about 0.22 wt% silicon, 0.97 wt% of chromium, 0.51 wt% of nickel, less than 0.025 wt% of phosphorous, less than 0.005 wt% of sulphur, the remainder being iron and unavoidable impurities.

[0018] Depending on how the matrix material is applied on the wire, it may be necessary to provide the wire with a coating that enables adhesion of the matrix material.

[0019] The tensile strength of the wire must be as high as possible in order to be able to reduce its thickness 't' while maintaining the necessary strength. For wires with an oblong cross section, tensile strength in excess of 900 N/mm² are easily obtainable, while tensile strengths larger than 1200 N/ mm² and even larger than 1400 N/mm² are available.

[0020] The matrix material that holds the abrasive particles is by preference a metal matrix material. A metal matrix material has the advantage that the wear of the matrix can be adapted to the wear of the abrasive particles. Some ways are possible to adhere the matrix material inclusive abrasive particles to the metal wire each of the methods bringing preferred metals or metal alloys for use in the metal matrix material.

[0021 ] A first method is to use electrolytic deposition of a metal layer from an

electrolytic bath whereby the abrasive particles are concomitantly introduced and bonded into that layer. Typical metals that are used to this end are nickel, nickel phosphorous, nickel tungsten, nickel manganese, nickel cobalt, cobalt or cobalt phosphorous. Typically the metal wire may be coated with a nickel or a copper or an iron layer in order to improve adhesion of the metal matrix to the wire.

[0022] A second method is the method of brazing. In this method a powdery

mixture of abrasive particles and a low temperature melting solder is, possibly held together in organic binder, applied on the wire and

subsequently heated in a furnace where the solder or braze melts and holds the abrasive particles to the wire. Alternatively the powder can be made to melt by means of a laser. Typical solders comprise tin and/or zinc possibly with a further alloying metal such as bismuth, nickel, copper iron, lead, antimony, indium and/or silver. Preferably the wire is coated with copper, or a copper-zinc alloy to improve wetting of the solder to the wire.

[0023] A third and preferred method uses cladding, such as laser cladding,

thermal spray or plasma spraying. Most preferred is the laser cladding technique. In this technique a metal powder and abrasive particles are gas propelled through a nozzle onto a spot on the metal wire. The wire spot is preheated by means of a high-power laser and the fed metal powder melts while the abrasive particles are impelled into it. After cooling a very good bond remains with the metal wire. The bond can even be further improved by pre-coating the wire with a bronze (copper tin alloy). An alternative way to feed the metal matrix is by means of a wire feed instead of a powder feed.

[0024] There are many known alloys of metal matrix materials that are suitable for use with a laser cladding system. Given the constraints imposed by the abrasive particles alloys with melting temperatures between 400°C and 900°C are more preferred. These alloys comprise silver, copper, nickel or cobalt as main alloy element. Additionally, melting temperature reducing elements such as tin, zinc, or indium can be added. In the case of nickel non-metals like phosphorous, silicon or boron can be used to reduce the melting temperature.

[0025] Particularly preferred are brazes (copper and zinc as main elements) and bronzes (copper and tin as main elements). Even more preferred alloys are silver based such Ag-Cu, Ag-Cu-Zn, or Ag-Cu-ln. Important is that the matrix shows sufficient ductility. Ductility is expressed, amongst others, in the elongation the metal matrix can withstand without breaking. As the wire is flexible at least in the direction of its thickness, the abrasive matrix must be able to follow the bending of the wire. Additionally, these alloys give a good balance between abrasive use and matrix wear. If the matrix wears too fast, abrasive particles will be dislodged while not being used to the full leading to premature tool wear. Conversely, if the matrix is too wear resistant, abrasive particles will not protrude sufficient for cutting resulting in too low sawing speeds.

[0026] Most preferred are alloys that contain an additional active metal such as chromium, titanium, vanadium, tungsten, zirconium, niobium,

molybdenum, tantalum, hafnium or combinations thereof. More preferred are chromium, zirconium, vanadium or titanium of which the latter is the most preferred as it has the lowest melting point. These metals are active in two ways:

• It are carbide formers that work well in combination with carbon

containing abrasives (see below).

• They are also known to improve the wetting of abrasive particles during deposition.

[0027] Most preferred bronzes contain between 5 and 30 wt%Sn, between 0.5 and 15 wt%Ti, the remainder being copper. Good results were obtained with bronzes having between 10 and 20 wt%Sn and between 2 and 10 wt%Ti, the remainder being copper. An example is an alloy containing 14 %Sn, 8 %Ti the remainder being copper, all expressed in weight percentage of the total. Alternatively, a particular example of a silver copper based alloy is Cu 27.25 wt%, Ag 59 wt%, In 12.5 wt% and, 1 .25 wt % of Ti. Such an alloy has a good combination of strength and ductility (tensile strength 455 MPa, Young's modulus 76 GPa, elongation 21 %). [0028] Possible abrasive particles are diamond, cubic boron nitride, silicon carbide, aluminium oxide, silicon nitride, tungsten carbide, titanium carbide or mixtures thereof. Carbon containing particles - diamond, silicon carbide, tungsten carbide, titanium carbide or mixtures thereof - are more preferred as they are easily wettable by all active metals mentioned.

Nitrides (i.e. cubic boron nitride, silicon nitride) are best wetted with titanium. Most preferred is diamond (i.e. almost pure carbon) either man- made or natural, the former being more preferred for its lower cost. Also mixtures of abrasive particles such as e.g. diamond with tungsten powder can be envisaged.

[0029] As an alternative to adding an active metal into the metal matrix material, the active metal can also be provided on the surface of the abrasives for example in the case of diamond. Diamond particles coated with tungsten, chromium or titanium are available.

[0030] When no active metal is present in the metal matrix material or on the abrasive particles, the abrasive particles will not wet easily to the molten metal. This even leads to 'floating' of the abrasive particles on the meniscus of the molten metal pool when they are impinging the metal pool.

[0031 ] The size and type of the abrasive particles are determined by the type of material to be sawn, but also by dimensions of the metal wire. Particles for stone cutting are generally large grit (US mesh sizes 30/40 to 50/60, mesh sizes according to US Standard ASTM E 1 1 , the higher the numbers, the smaller the particles) while for silicon cutting micro grits can be

considered. However, the abrasive particles must have a minimum size to be able to fluidize them in a gas stream for laser cladding. The inventors therefore limit the size of the particles to above 30 μιτι for example with a grit size larger than US mesh 325/400 (mean size larger than 46 μιτι).

[0032] The amount of abrasive particles is also depending on the material to be sawn but for most purposes about 100 to 2000 mg/cm³ of abrasive material taken relative to the total volume of the layer should suffice. More preferred is 100 to 500 mg/cm³. In the case of diamond this is expressed in carat as 0.5 to 10 ct/cm³ (1 carat, 'ct' is 200 mg) or even 0.5 to 2.5 ct/cm³. [0033] A fixed abrasive sawing wire with an abrasive layer obtained by laser cladding shows a metallurgical structure quite different from the layers as obtained by electroplating or brazing that form in a quasi static thermal equilibrium. The latter matrices show a granular or grain structure in a metallographic cross section. In a metal matrix material deposited with laser cladding the components of the metal alloys in the molten metal pool do not get the chance to reach a thermal equilibrium. Due to the presence of a spatial high temperature gradient, the growing solid phase will seek the most effective way to eject superfluous solute components into the liquid again in order to meet the preferred intermetallic phase of the solid. With laser cladding a series of microstructures can be obtained such as cellular, dentritic or columnar growth. See "Laser Cladding" by E.

Toyserkani, A. Khajepour and S. Corbin, Chapter 6, 2005, CRC Press, ISBN 0-8493-2172-7. The inventors have found that the more favourable structure is when the solid grows in the form of a tree as then most surface is available to expel the solute. Then a metallographic cross section will show a dendritic microstructure with stems and branches. The stem and branches of the tree will therefore be formed by an intermetallic phase with a high solidification temperature while the inter-dendritic phase will be a component or intermetallic phase with a lower solidification point. By increasing the spatial temperature gradient, the microstructure can be refined. This temperature gradient is dependent on the relative speeds between the clad track and the substrate: higher relative speeds lead to finer dendritic structures. The fineness of the microstructure is also considered to contribute to the hardness and resistance to wear of laser clad layers: the finer the structure the higher the wear resistance.

[0034] Besides the mechanical strength of the metal matrix material its adherence to the metal wire is of utmost importance. Without adhesion to the metal wire the metal matrix material inclusive the abrasive particles readily will come loose. To this end a metallurgical bond layer is provided between the metal wire and the metal matrix material. Such a metallurgical bond layer can for example be electrolytically applied to the metal wire, but by preference it is applied by laser cladding. Preferably the composition of the metallurgical bond layer is similar to that of the metal matrix material with the exception that the presence of an active metal (such as chromium, titanium, vanadium, tungsten, zirconium, niobium, molybdenum, tantalum, hafnium or combinations thereof) is not desired. It is known that the presence of such active metals deteriorates the bond of the matrix material to the metal wire (EP 1027476). In general this metallurgical bond layer can be smaller than 0.2 mm, or even less than 0.1 mm.

[0035] The abrasive particles in the metal matrix material will - at least locally - add to the thickness of the sawing wire. The maximum thickness of the metal matrix material with abrasive particles will be indicated with 't_mmm' in what follows. Along the wire the total thickness 't_mmm+t' preferably remains smaller than the width 'W of the sawing wire. Preferably the thickness of the metal matrix material with abrasive particles 't_mmm' is about the thickness 't'. A lower thickness such as 't_mmm' being smaller than 0.75, 0.50, or 0.25 times 't' will induce less stress on the metal matrix material during bending of the wire. The thickness 't_mmm' must at least be larger than the mean abrasive particle diameter in order to have at least one layer of abrasive. By preference it is larger than 2 or 5 or 10 times the mean particle size of the abrasive particles. The thicker the matrix material with abrasives is, the more material for abrasion is available, the longer the sawing wire can be used.

[0036] In a further preferred embodiment of the fixed abrasive sawing wire the metal matrix and abrasive material are present over one or both of the complete width sides of the sawing wire.

[0037] Preferably the abrasive particles held in the matrix material are present in discrete sections along the length of the width side of the wire. This to provide space in between the sections to remove debris from the cut. In between the sections the metal matrix material can still be present or alternatively both abrasive particles and metal matrix material are absent in between the sections. The former embodiment has the advantage that the metal matrix is not interrupted leading to a more constant stiffness along the length of the wire. The latter embodiment has the advantage that more space is available between the sections to remove swarf from the cut. [0038] The discrete section can be arranged on the width side of the sawing wire in different forms: the sections can have the shape of a rectangle or a parallelogram when arranged either perpendicular or obliquely to the axis of the wire. The sections can also be alternatively present on the upper and lower side of the sawing wire.

[0039] By preference the sections are at least 3 or 5 or 10 times or more the

thickness ΐ away from one another in order to prevent that - upon bending - the sections would be influenced by one another. The discrete sections should not be separated too much from one another as otherwise the speed of sawing is adversely affected. 5 or 10 or 50 times the width 'W is considered a maximum. The length of the discrete sections is at least the width of the sawing wire. The number of sections per unit length will depend on the type of material sawn. Relatively soft materials - such as marble or travertine - will need lower sections per meter (say below 10 to 35 sections per meter) as the stone will otherwise start to load the abrasive particles. Hard materials - such as granite - will need 40 to 50 sections per meter. The total length of the sections per meter is equal to the sum of the length of the sections per meter and is called 'coverage degree'. By preference the coverage degree - when discrete sections are used - is between 20 to 80 %. More preferred is between 30 and 50%.

[0040] An alternative embodiment is one wherein the abrasive particles in the matrix material are present in a continuous path on one or both width sides of the sawing wire. This continuous path can cover the complete width over the complete length of the wire. Alternatively the continuous path can take the form a zig-zag path meandering from one edge to the other edge of the wire.

[0041 ] An important embodiment is that wherein the fixed abrasive sawing wire is closed on itself i.e. forms a loop. One possibility is to close the loop by welding the ends of the metal wire to one another, prior to application of the abrasive particles in the matrix material. This allows the weld to be made in the best possible conditions. Alternatively, the loop can be closed on the finished fixed abrasive sawing wire. The advantage there is that the loop can be closed on site i.e. on the pulleys whereon the wire will run. An alternative to making a weld is to use a feather edge connection. [0042] In case both sides of the fixed abrasive sawing wire are provided with abrasive particles held in a matrix material, the loop can be made in the form of a Mobius band. This means that one of the ends of the sawing wire is axially twisted 180° prior to welding. The presence of the half-turn will induce a torsion moment to the wire which will make it rotate, at least when the overall cross section of the sawing wire is more or less circular so that it allows rotation. The wire will also turn when the wire is twisted over any integer multiple of 180° and subsequently welded. However, a limit is quickly reached as this is detrimental to the fatigue life of the wire.

[0043] According a second aspect of the invention a production method to

produce a fixed abrasive sawing wire is described. The method comprises the steps of:

• Providing a metal wire of oblong cross section having a thickness and a width, said width being larger than said thickness.

• The wire is guided over a wheel with the inner width side of the wire contacting the wheel. With the 'inner width side' of the wire is meant that side radially inward towards the wheel's axis. The wire is held in close, non-sliding contact with the wheel by applying sufficient tension while being rolled over the wheel.

• Abrasive particles in a metal matrix material are applied onto the outer width side of the wire. The Outer width side' of the wire is that side that is oriented radially outward relative to the axis of the wheel.

• A supply of metal matrix material is molten by a laser beam on said wire.

• A supply of abrasive particles is thrown into the pool of molten metal matrix material.

Specific about the method is that the wheel has a diameter smaller than about 1000 times the thickness 't' of said metal wire but larger than 100 times the thickness 't' of the metal wire.

[0044] Regarding the wire cross sectional shape and preferred dimensions and dimension ratios, reference is made to the sections [001 1], [0012], [0014] and [0015] of this application. [0045] The wire is pulled over the wheel with sufficient force that it follows the curvature imposed by the wheel. The wire is moved over the wheel while a supply of metal matrix material is molten by a laser beam on said wire. A supply of abrasive particles is thrown in the pool of molten metal matrix material. This can go quite fast. The inventors estimate that production speeds of above 10 cm/min or even above 1 m/min are possible.

[0046] The supply of abrasive particles can be intermittent so that sections

containing abrasive particles and metal matrix material alternate with sections of metal matrix material only. Alternatively, the abrasive material and metal matrix material can be supplied synchronously or 'in sync'. In that case sections with abrasive particles and metal matrix material alternate with sections that are free of both.

[0047] In the method use is made of a laser cladding system equipped with:

• A high intensity laser capable of delivering beams of at least 500 W continuous or pulsed power, by preference emitting in the infra-red region of the spectrum. Particularly suited are Nd:YAG (neodymium doped yttrium-aluminium garnet) lasers pumped by flash lamps or solid state lasers or CO₂ gas lasers. The laser-light is guided through suitable wave guides and optics to have a focal point that is adjustably close to the metal wire;

• a supply of metal matrix material in the form of a powder or a wire or a ribbon that is fed into the focal point of the laser. The powder can e.g. be supplied in a carrier gas, preferably a non-oxidising gas or inert gas such as argon;

• a supply of abrasive particles. These particles are by preference carried by a stream of carrier gas. Preferably the type of gas for carrying the abrasive particles and for carrying the metal matrix material is the same. Alternatively and also preferred is that the supply of abrasive particles can simply be done by strewing.

[0048] By preference, each one of the supplies of metal matrix powder, of the abrasive particles or of the heat input by means of the laser can be modulated independently. The supply of metal matrix powder and abrasive particles can be combined although this is not a necessity. [0049] Preferably the stream of carrier gas is directed by means of a gun such as described in US 6,31 6,744 wherein the gas flow is coaxial to the laser beam and converges towards the focal point through a conical annular nozzle. Such a laser gun is also cooled by water in order to prevent heating of the nozzle. The metal matrix material can be fed through the laser gun, while the abrasive is delivered through a separate gas flow or simply by strewing. Alternatively, the metal matrix material can be delivered through a nozzle separate from the laser gun while the abrasive particles are fed through the laser gun. Or both abrasive particles and metal matrix material can be supplied through the laser gun. Preferred is that abrasive particle supply and metal matrix material supply are separate as they do have quite distinct powder - and hence flow - properties.

[0050] Bending the wire while coating it has the following advantageous effects:

• The molten metal pool will shrink when solidifying called

'solidification shrinkage'. After solidification, internal tensile stresses will grow due to thermal contraction of the solidified metal matrix material when cooling to room temperature, putting a strain on the metal matrix material and the underlying metal wire. By now bending the wire during coating, these tensile stresses can be alleviated upon straightening the wire. The degree of shrinking and contraction will depend on the type of metal matrix material and its coefficient of thermal expansion - that can be estimated in the range between 5x 1 0^"6 K^"1 to 20x 10^"6 K^"1 - and the melting temperature of the alloy - typically 400 to 1 000°C - resulting in a linear thermal contraction of between 0.2% to 2 % .

• In case the compression due to the straightening of the wire is larger than the solidification shrinkage and thermal contraction together, a compressive stress will remain in the metal matrix material. Such residual compressive stress may help to keep the integrity of the metal matrix material.

[0051 ] The degree of compression induced by straightening the wire at the outer surface of the wire - i.e. at the interface between metal matrix material and metal wire - is from 0.2 % (for a wheel of 10OOxt) to 5 % (for a wheel of 40xt).

[0052] In order to prevent the deterioration of the strength properties of the metal wire, the wire must be cooled. This can be done by cooling the wheel on which the wire is running. In case the cross section of the wire is not rectangular, a groove is made in the wheel that matches the shape of the inner width side of the metal wire. Possibly a thermoconductive fluid can be introduced between the wire and the wheel to improve the thermal contact even further.

[0053] When spill-over of material is to be prevented, the wheel can be equipped with double flanges where in between the metal wire is guided. By preference the wheel is made of a metal that allows for a good heat conduction but also that reflects the laser light efficiently. This is to be prevent that the wheel would become easily coated with metal matrix material.

[0054] According a third inventive aspect of the invention a method to saw a hard and brittle work piece is presented using the above described fixed abrasive sawing wire. Particular about the method is that the sawing wire is used with a width side comprising the abrasive particles facing the work piece. This in contrast with a band saw that is sawing with the thickness side facing the work piece.

[0055] In a fourth inventive aspect a wire saw is disclosed for use with the

inventive fixed abrasive sawing wire described above. The wire saw comprises pulleys that guide the sawing wire to the work piece. At least one of the pulleys is driven to transfer motive force to the wire. The wire saw presents the sawing wire to the work piece with the width side comprising abrasive particles facing towards the work piece. The wire saw fills the gap of sawing stretches between 400 mm to 2000 mm.

[0056] In a first preferred embodiment of this wire saw, a loop of sawing wire is used tensioned between at least two pulleys of which at least one is driven. In an alternative embodiment a large length of fixed abrasive sawing wire is unreeled from a pay-off spool guided through the wire saw, saws the work piece and is wound on a take-up spool after use. Brief Description of Figures in the Drawings

[0057] Figures 1 a and 1 b show a conventional band saw.

[0058] Figures 2a to 2d show possibly different cross sections of the fixed

abrasive sawing wire according the invention.

[0059] Figure 3 is an illustration of the dendritic microstructure obtained in a

metallographic cross section of inventive fixed abrasive sawing wire.

[0060] Figure 4a to 4d show different ways in which the abrasive particles can be deposited on the inventive fixed abrasive sawing wire.

[0061 ] Figure 5 shows an exemplary way to produce the fixed abrasive sawing wire according the invention.

[0062] Figure 6 shows an embodiment of the inventive fixed abrasive sawing

sawing wire.

[0063] Figure 7a and 7b show two possible ways how the fixed invention abrasive sawing wire can be welded.

[0064] Figure 8a and 8b shows a saw machine using the invention fixed abrasive sawing wire must be used.

Mode(s) for Carrying Out the Invention

[0065] In a prior art band saw 100, as depicted in Figure 1 a and 1 b, a closed loop band saw 102 runs over two pulleys 104, 104' driven by a motor. The band saw is provided with a rim 106 of abrasive particles on a closed metal band 1 10. Possibly guiding pulleys 103 are provided to guide the band saw 102 when sawing through the work piece 108. The band saw saws in the direction V i.e. in the plane of the band saw.

[0066] Figure 2a to 2d shows different embodiments of the invention in cross

section. Each of the embodiments shows a metal wire having an oblong cross section: 210a rectangular, 210b oval, 210c kidney shape, 21 Od triangular. The cross section has a width 'W and a thickness T. On one of the width sides abrasive particles 214 are present that are held in a metal matrix 214 in each of the embodiments.

[0067] The embodiment of Figure 2d is made by means of electrolytic co- deposition of abrasive particles and a matrix metal. In this example the abrasive particles are diamonds that are nickel coated (prior to co- deposition) and the matrix metal is nickel. The pre-coating helps to deposited the diamond particles. [0068] The embodiment of Figure 2b shows a roughened interface 218 while the abrasive layer is deposited by means of brazing. The brazing is preferably done with a low melting point solder such as a tin silver (4 wt% of silver) or other compounds having a low melting point. The solder in the form of a metal powder and organic binder paste is applied together with the abrasive particles to the width side of the metal wire. By first baking out the binder and subsequently heating the wire above the solder melting point the solder melts thereby fixing the abrasive particles.

[0069] The embodiments of Figure 2a and Figure 2b show a fixed abrasive

sawing wire wherein the abrasive particles are bound in the metal matrix by means of a laser cladding. A binding layer 216 forms that is an alloy of the material of the metal wire and the metal matrix.

[0070] The wire of Figure 2a is made as follows:

A length of wire of 1 km with steel composition 51 CrV4 (Werkstoff Nr.

1 .8159) is provided. The nominal composition is between 0.47 to 0.55 %wt carbon, max. 0.4 %wt silicon, between 0.7 and 1 .1 %wt Mn, maximum 0.025 %wt phosphorous, maximum 0.025 %wt sulphur, between 0.9 and 1 .2 %wt chromium, between 0.1 and 0.25 %wt vanadium and the remainder being iron. The wire has a rectangular cross section of 0.3 mm in thickness and a width of 2 mm and a tensile strength of 1 100 N/mm² and is provided on a spool.

[0071 ] The wire 510 (Figure 5) is continuously unwound and guided over a brass wheel 534 of diameter 'D' 10 cm (i.e. 333 times the thickness). The brass wheel is cooled by means of water. The brass wheel is polished in order to reflect the laser light as much as possible. The wire is held tight on the wheel with a force of about 200 N. The brass wheel 534 is provided with flanges 535 to ensure that not too much material is spilled sideways. A laser gun 502 is provided with a central laser waveguide 530 with a coaxial gas powder feed 504.

[0072] As metal matrix alloy powder 514 an alloy compositon CuSnTi 78/13/9 (wt%), gas atomised with a median diameter of 50 μιτι is used. As abrasive particles, saw grit diamond of mesh 40/50 (315 to 400 μιτι particle size) of the MBS series of Diamond Innovations is used. The concentration of the diamond is 7% in volume related to the metal matrix. The reactive titanium in the metal matrix powder ensures good bonding of the diamond to the metal matrix. Argon is used as a protective gas.

[0073] When the laser is switched on and the powder with abrasive is supplied through the nozzle a pool of molten metal is formed on the wire surface. The pool gradually advances thereby forming a section 506 of an abrasive layer. The speed of the wire was set to 36 cm/min. By regularly switching on the laser and/or the flow of metal powder abrasive discrete sections are produced along the length of the fixed abrasive sawing wire as depicted in Figure 4a. The fixed abrasive sawing wire has covered sections 41 1 a alternating with not covered sections 422. The sections have sizes 5 mm long, 2 mm wide and a matrix material thickness of 1 mm. Possibly a thermal post-treatment must be given to the wire in order to relieve residual stresses.

[0074] Alternative ways to from the fixed abrasive sawing wire are depicted in Figures 4b, 4c and 4d. By introducing a movement of the laser head parallel to the axis of the pulley in combination with the movement of the metal wire oblique sections 41 1 b can be formed instead of rectangular discrete sections. Or a zig-zag path 41 1 c, 41 1 d can be traced as shown in Figure 4c and 4d.

[0075] In another preferred embodiment the coated discrete sections can be

alternated on both width sides of the fixed abrasive sawing wire as shown in Figure 6. Discrete sections 612, 614 alternate on both sides of the metal wire. There the wire is guided over two wheels wherein the second wheel is provided with 'pockets' for receiving the coated sections.

[0076] The metallographic structure 312 of the matrix is depicted in Figure 3.

After polishing a typical dendritic structure 340 appears which is indicative for laser cladding. It is not the only metallographic structure that can be obtained by laser cladding but is most preferred for this kind of

applications as it offers a tougher structure.

[0077] The fixed abrasive sawing wire can be made endless by welding two ends of the wire together as shown in Figure 7. In Figure 7a the ends are welded flush at weld 720 resulting in loop 710. In Figure 7b, a half turn has been added to the ends prior to welding at 740 resulting in Mobius band 730. In principle this wire should have cutting sections at both width sides as a Mobius band is one sided.

[0078] In an alternative way for making the fixed abrasive sawing wire in loop form, the loop can be welded prior to laser cladding the cutting sections on the metal wire. In that case the loop is first formed and then held tight between wheels.

[0079] The fixed abrasive sawing wire can be used as depicted in Figures 8a and 8b. Opposite to the band saw, the wire is used now in the width direction and the wider part 814 of the wire 810 now cuts its way through the work piece 808 in direction V. Contrary to a band saw, it is possible to mount wires side by side and to use this kind of wire for slabbing. In that case the driving pulleys 804 of the machine is equipped with parallel grooves. In an alternative embodiment of the saw machine, the wire is dereeled for a payoff spool onto a take up spool. In case sufficient length can be

accumulated in this way on a single spool, the weak spot that is formed by welding can be eliminated.

[0080] The inventive fixed abrasive sawing wire fills the gap in sawing wires

having a kerf loss between 0.25 mm and about 4 mm. The associated machine provides an intermediate between a large kerf sawing cord slicer and a small kerf multiwire saw.

Claims

1 . A fixed abrasive sawing wire comprising a metal wire and abrasive particles held in a matrix material fixed to said metal wire, said metal wire having an oblong cross section with a thickness and a width, said width being larger than said thickness

characterised in that

said abrasive particles are present on a width side of said fixed abrasive sawing wire.

2. The fixed abrasive sawing wire of claim 1 wherein said abrasive particles are present on one width-side of said metal wire.

3. The fixed abrasive sawing wire according to any one of claim 1 to 2 wherein the ratio of the width to the thickness of said oblong cross section is larger than 1 .5.

4. The fixed abrasive sawing wire according to any one of claims 1 or 3 wherein said matrix material is a metal or metal alloy matrix material comprising one or more metals selected out of the group comprising iron, nickel, cobalt, tin, copper, zinc, chromium, titanium, vanadium, tungsten, zirconium, niobium, molybdenum, tantalum and hafnium.

5. The fixed abrasive sawing wire of claim 4 wherein said metal or metal alloy matrix material shows a dendritic microstructure in a metallographic cross section.

6. The fixed abrasive sawing wire according any one of claims 1 to 5 wherein said abrasive particles are one or more selected out of the group comprising articificial diamond, natural diamond, cubic boron nitride, tungsten carbide, silicon carbide aluminium oxide or silicon nitride.

7. The fixed abrasive sawing wire according any one of claims 1 to 6, wherein said metal wire is a high strength steel wire, a plain carbon steel wire, a stainless steel wire, a chrome vanadium steel wire, a chrome molybdenum steel wire, a chrome molybdenum nickel steel wire or a chrome molybdenum vanadium steel wire.

8. The fixed abrasive sawing wire according any one of claims 1 to 7, wherein said abrasive particles held in a matrix material are present in discrete sections along the length of said fixed abrasive sawing wire.

9. The fixed abrasive sawing wire according to any one of claims 1 to 7 wherein said abrasive particles held in a matrix material are present in a continuous path on said width-side of said metal wire.

10. A loop of fixed abrasive sawing wire obtained by welding the ends of a fixed abrasive sawing wire according to any one of claims 1 to 9.

1 1 . A method to produce a fixed abrasive sawing wire comprising the steps of

- Providing a metal wire of oblong cross section having a thickness and a width, said width being larger than said thickness;

- Moving said wire over a wheel with the inner width side of said wire

contacting said wheel;

-Applying abrasive particles in a metal matrix material by means of laser cladding onto the outer width side of said wire, said outer width side being oriented radially outwardly from said wheel whereby:

• a supply of metal matrix material is molten by a laser beam on said wire;

• a supply of abrasive particles is thrown into the pool of molten metal matrix material;

characterised in that

the diameter of said wheel is smaller than 1000 time said thickness of said wire but larger than 40 times said thickness of said wire.

12. The method according to claim 1 1 wherein, said wheel is cooled.

13. The method according to claim 1 1 or 12 wherein said wheel has a groove, said groove matching said inner width side of said wire.

14. The method according to any one of claims 1 1 to 13 wherein said wheel is made of a metal that reflects said laser beam.

15. A method to saw a hard and brittle work piece wherein the fixed abrasive

sawing wire according to any one of claims 1 to 9 or a loop of fixed abrasive sawing wire according to claim 10 is used

characterised in that

said work piece is sawn by a width side of said fixed abrasive sawing wire comprising abrasive particles.