METAL WORKING DRILL/ENDMILL BLANK

Abstract: According to the present invention there is now provided a cemented carbide drill/endmill blank consisting of a core and a surrounding tube with improved technological properties. The difference in Co-content between the core and tube is 1-10 wt.% units and the cubic carbide content is 8-20 wt.% in the tube and 0.5-2 wt.% in the core. The invention also relates to the method of making said cemented carbide drill/end mill blank.

Description: Metal working drill/endmill blank The present invention relates to a cemented carbide body, preferably a cylindrical body consisting of at least two grades with individually different compositions, microstructures and properties, especially a body aimed at acting as a blank for a drilling, end illing or deburring tool. In drilling tools the demands on the periphery and on the centre are different with respect to wear resistance and toughness. In drill bits for rock drilling the demands differ between the surface (wear resistance) and the inner part (toughness) as discussed in US 5,541,006, in which is emphasised the use of two grades in a rock drilling bit. The grades are both straight grades with tungsten carbide and Co. Much attention is given to the ability to control the Co migration for which, in this case, an abrupt or discrete change of composition at the interface between the regions is preferred. This problem is also solved by Fischer et al with the technique known as Dual-Phase or DP-technique, US 4,743,515. Tools as wear parts, rolling rings and slitter/trimming knifes can be manufactured with a method described in US 5,543,235. These patents, though, deal with combinations of grades containing only WC-Co or C-Ni . They also refer to applications where just one of the grades is in contact with the work piece material, and the other serves as an 'equaliser' or 'carrier' of pressure or impact. One patent dealing with cemented carbide drills containing cubic carbides is US 4,971,485, but in that case the WC-Co grade is used in the shaft to avoid damage due to vibrations emanating from the machine. The present invention relates to a compound cemented carbide body consisting of a core of a tough grade and a surrounding tube of a more wear resistant grade that are both in active contact with the work piece material . The problem when making such a compound body is to avoid the formation of cracks in the outer part or voids and significant porosity at the interface between the two parts due to differences in shrinkage during sintering. In addition, too high stresses in the interface make further manufacturing, as e.g. slitting and grinding, impossible. Another problem can be the migration of the binder phase during sintering which results in a leveling of the binder phase content in the two parts. The combination of grades has to fulfil the demands on toughness and wear resistance in the centre as well as in the periphery. The grades also have to be compatible with respect to pressing conditions and sintering conditions. According to the invention it has been found possible that by a proper choice of composition and microstructure of the two grades the above mentioned problems can be avoided. More particularly the invention relates to a drill blank with a core of a WC-Co-grade surrounded by a tube of a grade containing also carbides and/or carbonitrides of the elements in group 4-6, preferably Ti, Ta and Nb. Fig. 1 shows in 6X magnification a cross section of a drill blank according to the invention. In this figure A - core and B - tube . Fig. 2 shows in 200X magnification the diffuse interface between the two grades . Drill blanks according to the invention consist of a core and a surrounding tube. The core contains after sintering in addition to WC <30, preferably 5-20, most preferably 10-15 wt-% Co. The tube grade has >5 wt-% Co and 5-25, preferably 8-20 wt-%, most preferably 10-15 wt-% of one or more of the carbides and/or carbonitrides of the elements in group 4-6, preferably Ti , Ta and Nb. The difference in Co content between core and tube is 1- 10 wt-% units, preferably 2-4 wt-% units. There is a 300-500 μm wide transition zone measured as change in Co-content by microprobe analysis. The core contains 0.5-2 wt-% cubic carbides. The grain size of the core grade is <10 μm, preferably 0.5-5 μm, most preferably 0.5-3 μm. The tube grade has a grain size of <10 μ , preferably 0.5-3 μm, most preferably 0.5-1.5 μm. Blanks according to the invention are made by powder metallurgical methods including compacting in two steps. First a rod with length around 300 mm and diameter 5-15 mm consisting of 10-30 wt-% Co and rest WC with grain size according to above is pressed. Preferably, this rod has a grooved form which provides a keying action between it and tube. Then the tube of a desired diameter is pressed around the outside of the rod to final green density. The size of the core is preferably 40-60 % of the total diameter of the blank. If desired the drill blank can be provided with coolant holes with methods known in the art. It has been found that if the difference in Co-content between the tube grade and the core grade is 0-15 wt-%, preferably 5-10 wt-% units and the tube contains cubic carbides according to above the blank can be sintered without formation of cracks or voids between the core and the tube . The pressing and sintering properties of the original grade powders are of utmost importance to get a good result. Pressing conditions are determined by thermal expansion coefficient, shrinkage and required pressing pressure for the grades used. It is within the purview of the skilled artisan to determine these conditions by experiments. Sintering shall be performed at 1350-1450 e C. After sintering, the rods are usually cut into drill blanks of 50-150 mm, preferably 80-120 mm length. The most useful diameter range is 5-35 mm, preferably 5- 20 mm. The flute is ground with for example a diamond wheel at 18-20 m/sec with a feed of 60-80 mm/min. In an alternative embodiment a drill top of length/diameter ratio of 0.5-5.0 is used which is brazed to a shaft. After finish grinding, drills of the above mentioned kind are suitable for coating by PVD with carbide, nitride, carbonitride or oxide or combinations thereof, e. g. TiN, TiAlN, Ti(C,N). Drills of this invention are particularly useful for machining in stainless steel and normal steel. Example 1 Drills according to the invention were produced. Pressing was carried out in two stages. First a cylindrical rod length 300 mm and diameter 11 mm with the composition of 20 wt-% Co and 80 wt-% WC with grain size 2 μm was pressed. After that a powder with original composition of 11 wt-% Co, 6.1 wt-% TaC, 1,9 wt-% NbC, 4 wt-% TiC and rest WC with grain size 2.5 μ was pressed around the outside of the rod to final green density. Some of the drills were given coolant holes with technique well known in the art. After sintering the Co content of the core grade had decreased from 20 to 14 wt-% and the Co content in the tube grade had increased to 12 wt-%. In addition, significant amounts of the cubic carbides could be detected in the centre of the core . After sintering the rods were cut into drill blanks of 105 mm length and 14 mm in diameter. The flute and top and bottom of the blanks were ground to final appearance . Example 2 PVD TiN coated drills from Example 1 were tested in stainless steel AISI 316. Single grade drills in the two original grades used in the drills from Example 1 and one fine grained 1 μm WC- 10 wt-% Co grade normally used in these cutting conditions were used as references . Following three test data were used with external cooling : a)v= 50 m/min, f=0.14 mm/rev b)v= 82 m/min, f=0.12 mm/rev c)v= 32 m/min, f=0.22 mm/rev In test a) the drill according to the invention lasted 357 holes, while the single grade drills were worn out after 207 (single grade fine grained WC-Co), 149 (single grade 11 wt% Co, 12 wt% Ta,Nb,Ti carbides and balance WC) and 55 holes (single grade 20 wt-% Co) respectively . At higher speed b) the drill according to the invention and the fine grained grade made 192 holes while the other grades made 126 (single grade 11 wt% Co, 12 wt% Ta,Nb,Ti carbides and balance WC) and 22 holes (single grade 20 wt-% Co) respectively. At lower speed with higher feed c) the result was 179 holes for the drill according to the invention while the fine grained grade made 128 and the 20 wt-% Co grade made 41 holes before they were stopped because of cracks or wear . Example 3 Drills from Example 1 with internal coolant supply were tested in stainless steel. In this test an ordinary P40 drill was used as reference. At increased speed (100 m/min, f=0.16 mm/rev) the drill according to the invention managed 550 holes while the P40 reference drill was totally broken down after only three holes . At normal speed but a higher feed (50 m/min, f=0.25 mm/rev) the P40 drill suffered from chipping after 660 holes and the drill according to the invention was taken out still working after 1100 holes. At ordinary cutting data (50 m/min, f=0.16 mm/rev) the two drills were equal in performance and the test was interrupted after 1100 holes. Example 4 Drills from Example 1 with internal coolant supply were tested on a different austenitic stainless steel, AISI 304. In this test ordinary P40 and sub-micron K20 drills were used as reference. At normal speed (50 m/min, f=0.16 mm/rev) the drill according to the invention was still working after 2668 holes while the P40 and sub-micron K20 drills were worn out after 2011 and 242 holes respectively. At increased feed but normal speed (50 m/min, f=0.30 mm/rev) the drill according to the invention completed 520 holes while the P40 and sub-micron K20 drills completed 110 and 22 holes respectively. At increased speed (100 m/min, f=0.16 mm/rev) the drill according to the invention achieved 198 holes, while the P40 and K20 drills broke down after one or two holes . Example 5 Drills from Example 1 with internal coolant supply holes, but in 10 mm diameter and coated with Ti(C,N) and TiN were tested in AISI 316 (SS2353), 30 mm through hole drilling. In this test an ordinary fine grained PVD coated drill was used as reference. Several cutting data combinations were used, and from the result it is obvious that the drill according to the invention has a much broader working range . The table below shows the number of holes achieved with the drills. The test was stopped after 1300 holes though the drills were not worn out.

Claims: Claims 1. A cemented carbide drill/endmill blank consisting of a core of one grade and a surrounding tube of another grade c h a r a c t e r i s e d by a difference in Co-content between the core and tube of 1- 10 wt-% units and by cubic carbide content of 8-20 wt-% in the tube and 0.5-2 wt-% in the core. 2. Method of making a cemented carbide drill/endmill blank consisting of a core of one grade and a surrounding tube of another grade by powder metallurgical methods including compacting in two steps first the core and then the tube c h a r a c t e r i s e d in using for the core a powder consisting of 10-30 wt-% Co and rest WC and in using for the tube a powder with >5 wt-% Co and 5-25 wt-% cubic carbides in addition to WC whereby the difference in Co- content between the core powder and the tube powder is 5-15 wt-% units.

Inventors: FOSTER STEPHEN, GREARSON ALISTAIR, MCCARTY GARY, OUCHTERLONY HELENE, FOSTER, STEPHEN, GREARSON, ALISTAIR, MCCARTY, GARY, OUCHTERLONY, HELENE
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1997
Main classification: C22C29
IPC codes: B22F7, C22C29