Patent Application
20080166580
This application claims priority under 35 U.S.C. §119 and/or §365 to Swedish Application No. 0602747-8, filed Dec. 15, 2006, and also to Swedish Application No. 0700800-6, filed Mar. 28, 2007; the entire contents of each are incorporated herein by reference.
The present invention relates to a metal cutting tool. More specifically the invention relates to a PVD coated cemented carbide endmill suitable particularly for semifinishing to finishing machining of hardened steels.
In chip forming machining of steel the requirements of high productivity, i.e., higher cutting speeds and feeds combined with increased tool lifetime, put severe demands on tool properties. When hardened, work piece steels normally have a hardness of between HRC 46 and 55, but may have hardness values of up to HRC 63 or even higher. For semifinishing and finishing operations of hardened steel so-called solid carbide tools are used. Such tools are produced, e.g., by grinding a cylindrical cemented carbide blank into a substrate of desired shape or providing a blank of desired shape by extrusion, which is subsequently coated by, e.g., physical vapor deposition (PVD). One very common application of solid carbide tools is endmilling of components, dies and molds, but it is usually not possible to cover the whole hardness range with one single tool grade.
Endmilling of hardened steels generates large amounts of heat, which causes high temperatures at the cutting edge. Very often, the shape of the machined component is such that shank overhangs are large. Both of these factors make necessary the use of hard cemented carbide grades that do not soften appreciably at the elevated temperatures at hand and that are rigid so that tool bending is negligible. The need for a harder grade has, however, to be balanced against the lowered bulk toughness that follows. For general machining, the typical substrate hardness is below about 1600 HV3, whereas in hard part machining typical HV3 values range from about 1700 up to in excess of about 1800. A severe problem here is that substrate brittleness causes chipping problems of the sharply ground cutting edge, which is not easily remedied by standard PVD coatings. Any cracks that occur in the coating due to the machining tend to propagate through the entire coating thickness. At the interface to the substrate, these cracks may serve as crack initiation points for substrate bulk fracture.
Cemented carbide grades for endmilling applications generally contain fine grain WC, γ-phase which is a solid solution of generally TiC, NbC, TaC and WC, and binder phase, generally Co or Ni. Cemented carbides having a fine grain size less than about 1 μm are produced through the incorporation of grain growth inhibitors such as V, Cr, Ti, Ta and combinations thereof in the initial powder blend. Typical inhibitor additions are from about 0.5 to about 5 wt-% of the binder phase. In addition the sintering temperature must be low, from about ˜1350 to about 1390° C., in order to further restrict grain coarsening. To generate a sintered substrate with a grain size of about 0.6 μm, in general, the WC powder is essentially finer, typically from about 0.2 to about 0.3 μm.
PVD coatings for endmills are, for instance, (Ti,Al)N single layers, with high hardness and wear resistance. By the use of intense ion bombardment of the coating during its growth, PVD layers have high residual stress levels, of the order of at least about 2000 MPa compressive, and usually higher than about 3000 MPa compressive. A compressive stress is in general considered an advantage because considerable mechanical load on the tool is needed to initiate a crack in the coating. At the same time, however, a maximum coating thickness of around 3 μm is imposed due to the high stress and limited coating adhesion.
Improved tool toughness may be achieved with coating design, e.g., with non-periodic TiN+(Ti,Al)N multilayers such as disclosed in EP 983 393. In these, the average Al contents are lower than in the case of single layer coatings which compromises the wear resistance.
It is an object of the present invention to provide a coated cemented carbide endmill with improved wear resistance without sacrificing toughness and edge security particularly useful for semifinishing and finishing in steels with hardnesses of HRC 46-63, and above.
In one embodiment of the invention, there is provided a cemented carbide endmilling tool comprising a substrate and a wear resistant coating wherein the substrate (a) comprises from about 90 to about 94 wt % WC in a binder phase of Co also containing Cr in such an amount that the Cr/Co weight ratio is 0.05-0.18, and the wear resistant coating (b) is from about 1.8 to about 9.5 μm thick comprising a first layer (c) of a hard and wear resistant refractory PVD AlMe nitride or carbonitride where Me is Zr, V, Nb, Cr or Ti having a thickness of from about 1.0 to about 4.5 μm and an atomic fraction of Al to Me of from about 1.20 to about 1.50, a second layer (d) of hard and wear resistant refractory PVD AlMe nitride or carbonitride where Me is Zr, V, Nb, Cr or Ti having a thickness of from about 0.5 to about 4.5 μm, and an atomic fraction of Al to Me of from about 1.30 to about 1.70, and in between the first and the second layer, a from about 0.05 to about 1.0 μm thick low-Al layer (e), the thickness of layer (e) being less than about 0.95 times the thickness of thinnest of the first layer (c) and the second layer (d), of an AlMe nitride or carbonitride where Me is Zr, V, Nb, Cr or Ti and having an atomic fraction of Al to Me of from about 0 to about 0.3.
In still another embodiment of the invention, there is provided a method of making a cemented carbide endmilling tool for semifinishing and finishing machining comprising the following steps: providing a cemented carbide endmill blank with a composition comprising from about 90 to about 94 wt % WC in a binder phase of Co also containing Cr in such an amount that the Cr/Co weight ratio is from about 0.05 to about 0.18, wet milling submicron powders of tungsten carbide cobalt, at least one of Cr3C2, Cr23C6 and Cr7C3 to obtain a slurry, drying the slurry to obtain a powder, pressing the powder into rods, sintering the rods in vacuum or in nitrogen, and optionally performing an isostatic gas pressure step during sintering temperature or at the final stage of sintering, grinding the rods cylindrical to h6 tolerance, grinding flutes using diamond wheels with emulsion cutting fluid, depositing whilst maintaining a partial pressure of nitrogen in the recipient using the appropriate selection of active evaporation sources and rates, a wear resistant coating (b) from about 1.8 to about 9.5 μm thick, comprising a first layer (c) of a hard and wear resistant refractory PVD AlMe nitride or carbonitride where Me is Zr, V, Nb, Cr or Ti having a thickness of from about 1.0 to about 4.5 μm and an atomic fraction of Al to Me of from about 1.20 to about 1.50 with process parameters: arc current from about 50 to about 200 A in the equipment, N2-pressure from about 5 to about 50 μbar and deposition temperature from about 400 to about 700° C. and a substrate bias of from about −150 to about −300V, a second layer (d) of hard and wear resistant refractory PVD AlMe nitride or carbonitride where Me is Zr, V, Nb, Cr or Ti, having a thickness of from about 0.5 to about 4.5 μm and an atomic fraction of Al to Me of from about 1.30 to about 1.70 with process parameters: arc current from about 50 to about 200 A in the equipment, N2-pressure from about 5 to about 50 μbar, and deposition temperature from about 400 to about 700° C. and a substrate bias of about −50 to about −140 V, and in between the first and the second layers, a from about 0.05 to about 1.0 μm thick low-Al layer (e) at a temperature from about 400 to about 700° C., the substrate bias from about −30 to about −150 V and the arc current from about 80 to about 210 A.
a—substrate
b—coating
c—first layer
d—second layer
e—low Al-layer.
It has surprisingly been found that by sandwiching a low-Al containing (Ti,Al)N layer, having the unexpected property of being essentially stressless, between two high-Al containing (Ti,Al)N layers having high compressive residual stresses edge chipping is suppressed. Thus, the use of harder substrate grades as well as thicker coatings, and thanks to that, more wear resistant coatings are enabled.
According to the present invention, there is provided a coated solid carbide endmill particularly useful for semifinishing and finishing machining of hardened steels of HRC from about 46 to about 63, and above, comprising a cemented carbide substrate and a wear resistant coating, wherein
The atomic fraction of Al to Me in the layers is measured by averaging at least four point analyses in cross section transmission electron microscopy (TEM).
In a preferred embodiment, the first layer and the second layer both have crystallites of the cubic rock salt structure with a grain size less than about 50 nm, preferably less than about 40 nm. The sandwiched low-Al layer has a crystallite grain size larger than about 40 nm, preferably larger than about 50 nm perpendicular to the growth direction and larger than about 100 nm parallel to the growth direction, i.e., having columnar growth, and most preferably extending in the growth direction throughout its thickness.
X-ray diffraction techniques, more specifically the sin2ψ method (I. C. Noyan, J. B. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer-Verlag, New York, 1987, pp 117-130), is used for determining the residual stress.
In a further preferred embodiment, the first layer has a residual compressive stress more than about 1000 MPa, preferably from about 1800 to about 3500 MPa, the second layer has a residual compressive stress more than about 1000 MPa, preferably from about 1800 to about 3500 MPa and the low-Al layer has a residual stress of an absolute value less than about 600 MPa (i.e., being compressive or tensile), preferably less than about 300 MPa, more preferably less than about 80 MPa.
Optionally applied onto the second layer are further layers which may be used for further improving wear resistance, cosmetic appearance or for wear detection purposes as known to the skilled artisan.
The present invention further relates to a method of making a cemented carbide endmill with a composition according to above including the following steps:
The so obtained cemented carbide rods are centerless ground cylindrical to h6 tolerance. Flutes are ground using diamond wheels with emulsion cutting fluid.
The as-ground solid carbide endmill substrates are wet cleaned. The substrates are subjected to a PVD coating process in a coater using reactive arc evaporation type PVD equipment containing metal evaporation MeAl-sources with suitable composition to obtain the desired Al/Me atomic ratios, arranged such to coat the full charge homogeneously. The MeAl-sources can, e.g., be three single targets arranged so that each target coats the full charge homogeneously or, as an alternative, six MeAl-sources can be arranged pairwise so that each pair coats the full charge homogeneously. However, other number of targets and arrangements are within the scope of the invention. The coater is evacuated, followed by the steps of heating and plasma etching in order to further clean the tools, and to condition their surfaces by removing excess binder phase from the WC surface. By metal evaporation whilst maintaining a partial pressure of nitrogen in the recipient, i.e., the coating furnace or vacuum vessel in which the deposition process takes place, and using the appropriate selection of active evaporation sources and rates, the following layers are deposited:
The coating can also be deposited by other PVD technologies, such as, magnetron sputtering, dual magnetron sputtering or arc technology.
The invention is additionally illustrated in connection with the following examples, which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the examples.
Tungsten carbide powder with an FSSS grain size of 0.9 μm, 7 wt % very fine grained cobalt powder and 0.7 wt-% Cr added as H. C. Starck fine grained Cr3C2-powder, were wet milled together with conventional pressing agents. After milling and spray drying the powder was pressed into 6 mm diameter rods and sintered at 1410° C. and 40 bar Ar gas pressure. The sintered material had a coercivity of 28 kA/m.
The so obtained cemented carbide rods were centerless ground cylindrical to h6 tolerance. Flutes were ground using diamond wheels with emulsion cutting fluid.
The as-ground solid carbide endmill substrates were wet cleaned and subjected to a PVD coating process according to the following. The substrates were loaded into a reactive arc evaporation type PVD equipment chamber containing six metal evaporation sources, arranged pairwise so that each pair would coat the full charge homogeneously. One pair of evaporators had Ti metal targets and the other two pairs had AlTi alloy targets having a composition ratio Al/Ti of 2. The chamber was evacuated, followed by the steps of heating and plasma etching in order to further clean the tools, and to condition their surfaces by removing excess binder phase from the WC surface. By metal evaporation whilst maintaining a partial pressure of nitrogen in the recipient, and using the appropriate selection of active evaporation sources and -rates TiN and (Ti,Al)N alloy layers were deposited at a temperature of 600° C. The process conditions during the deposition steps were as below:
The so manufactured and coated solid carbide endmills were analysed metallographically. Cross sections were prepared by cutting the endmill 10 mm from the tip, followed by mechanical grinding and polishing by diamond grit. A typical section of coating and substrate is shown in the optical micrograph in
X-ray diffraction techniques, more specifically the sin2ψ method, (I. C. Noyan, J. B. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer-Verlag, New York, 1987 (pp 117-130)), was used for determining the residual stress in the three layers. The results are reported in Table 2.
Thin film transmission electron microscopy equipped with an EDS spectrometer was used to determine the Al/Ti atomic ratio as an average of four point analyses and grain size in the three layers, see Table 2.
Endmills from Example 1 were tested and compared with endmills of a commercially available grade for the intended application area. Wear resistance test in very hard steel; a wear resistant and toughness demanding semifinishing test with 30% stepover. The test represents the upper range in terms of work piece hardness.
There is a significant improvement in comparison to the commercial grade which is optimised for this range of work piece hardness. This clearly expresses the superior wear resistance of the invented tool.
Endmills from Example 1 were tested and compared with endmills of a commercially available grade for the intended application area. This is a toughness demanding sidemilling test in hardened die steel. The machining situation was a very typical application. It represents, in terms of work piece hardness, the lower end of the application area for endmilling in hardened steels.
The improvement compared to the commercial grade, which is designed for hard part machining, shows the excellent width of the application area for the invented endmill.
Endmills from Example 1 were tested and compared with endmills of a commercially available grade for the intended application area. Machining of two identical tools for die and mold applications. In this application, edge security and toughness is essential. It represents a vital customer value to be able to machine a complete part without tool change.
The result of this test was that the commercial grade tool failed to meet the demanded tool life, whereas the invented tool finished the operation with very little wear. The result clearly shows a superior wear resistance of the invention and ability to sustain an integer cutting edge compared to the commercial grade tool.
Field test at customer, manufacturing of hot forge dies in hardened tools steel HRC 53. The machining was done using three different steps using ball nosed endmills (BNE) of diameters 10, 6, and 2 mm.
The results indicated a significant improvement in tool live for the endmill according to the invention.
Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0602747-8 | Dec 2006 | SE | national |
| 0700800-6 | Mar 2007 | SE | national |
