Sintered shaped abrasive grains on basis of aluminum oxide comprising mineralogical phases consisting of mullite, tialite and/or armalcolite, and baddeleyite and/or srilankite and a method for their production

Abstract

The present invention relates to sintered shaped abrasive grains on basis of aluminum oxide. Sintered shaped abrasive grains consistent with the disclosure include mineralogical phases made of mullite, tialite and/or armalcolite, and baddeleyite and/or srilankite. Methods for producing sintered shaped abrasive grains using alumina, ilmenite and zircon sand as raw materials are also provided.

Claims

1. A method for producing sintered abrasive grains, comprising the steps: preparing a homogeneous dry mixture of 85%-95% by weight alumina (Al.sub.2O.sub.3), 1.0%-10% by weight zircon sand (ZrSiO.sub.4), and 0.5%-8.0% by weight ilmenite (FeTiO.sub.3), whereby raw-material-based impurities are less than 2% by weight; adding at least one binder and at least one solvent together with one or more additives selected from the group comprising dispersants, lubricants and plasticizers for obtaining an extrudable mass; extruding the mass; cutting the extruded product to shaped abrasive grain precursors; and sintering the shaped abrasive grain precursors in a temperature range between 1450° C. and 1650° C. to obtain sintered abrasive grains comprising corundum (Al.sub.2O.sub.3), mullite (3Al.sub.2O.sub.3*2SiO.sub.2), tialite and/or armalcolite (Al.sub.2TiO.sub.5/AlFeTiO.sub.5) and baddeleyite and/or srilankite (ZrO.sub.2/Ti.sub.0.75Zr.sub.0.25O.sub.2), wherein the sintered abrasive grains comprise a weight content of: Al.sub.2O.sub.3 between 85% and 95% by weight; titanium compounds, expressed as TiO.sub.2, between 0.5% and 5.0% by weight; silicon compounds, expressed as SiO.sub.2, between 0.3% and 4.0% by weight; iron compounds, expressed as Fe.sub.2O.sub.3, between 0.4% and 9.0% by weight; zirconium compounds, expressed as ZrO.sub.2, between 1.0% und 9.0% by weight; and raw-material-based impurities of less than 2% by weight.

2. The method according to claim 1, whereby the ratio by weight percent of zircon sand to ilmenite is from 1:6 to 6:1.

3. Sintered abrasive grains comprising corundum (Al2O3), mullite (3Al2O3*2 SiO2), tialite and/or armalcolite (Al2TiO5/AlFeTiO5) and baddeleyite and/or srilankite (ZrO2/Ti0.75Zr0.25O2), wherein the sintered abrasive grains comprise a weight content of Al.sub.2O.sub.3 between 85% and 95% by weight; titanium compounds, expressed as TiO.sub.2, between 0.5% und 5.0% by weight; silicon compounds, expressed as SiO.sub.2, between 0.3% and 4.0% by weight; iron compounds, expressed as Fe.sub.2O.sub.3, between 0.4% and 9.0% by weight; zirconium compounds, expressed as ZrO.sub.2, between 1.0% und 9.0% by weight; and raw-material-based impurities of less than 2% by weight.

4. Sintered abrasive grains according to claim 3, whereby the ratio by weight percent of mullite to tialite and/or armalcolite is from 6:1 to 1:6.

5. Sintered abrasive grains according to claim 3 having a density of ≥3.6 g/cm.sup.3 and a hardness HV between 14 and 18 GPa.

6. Sintered abrasive grains according to claim 3 having a MKZ-value of ≤2.0%.

7. Sintered abrasive grains according to claim 6 having a MKZ-value of ≤1.2%.

8. Sintered abrasive grains according to claim 3, whereby the sintered abrasive grains are rod shaped bodies having a diameter between 0.5 and 5 mm and a length between 1 and 10 mm, wherein the ratio of diameter to length is between 0.2:1.0 and 1.0:1.0.

9. Sintered abrasive grains according to claim 3, wherein the sintered abrasive grains are manufactured by a method comprising the following steps: preparing a homogeneous dry mixture of 85%-95% by weight alumina (Al.sub.2O.sub.3), 1.0%-10% by weight zircon sand (ZrSiO.sub.4), and 0.5%-8.0% by weight ilmenite (FeTiO.sub.3), whereby raw-material-based impurities are less than 2% by weight; adding at least one binder and at least one solvent together with one or more additives selected from the group comprising dispersants, lubricants and plasticizers for obtaining an extrudable mass; extruding the mass; cutting the extruded product to shaped abrasive grain precursors; and sintering the shaped abrasive grain precursors in a temperature range between 1450° C. and 1650° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a chart showing hardness and micro grain decomposition for compositions consistent with the present disclosure.

DETAILED DESCRIPTION

(2) In line with the present works, it was established that a sintered abrasive grain on basis of aluminum oxide, comprising a weight content of Al.sub.2O.sub.3 between 85 and 95% by weight, titanium compounds, expressed as TiO.sub.2, between 0.5 und 5.0% by weight, silicon compounds, expressed as SiO.sub.2, between 0.3 and 4.0% by weight, iron compounds, expressed as Fe.sub.2O.sub.3, between 0.4 and 9.0% by weight, zirconium compounds, expressed as ZrO.sub.2, between 1.0 und 9.0% by weight and raw-material-based impurities of less than 2% by weight has an optimized hardness and fracture toughness; thereby, an abrasive grain is obtained that offers advantages over prior art in special grinding operations.

(3) For producing sintered abrasive grains, 85%-95% by weight alpha aluminum oxide, 0.5%-8.0% by weight ilmenite and 1.0%-10.0% zircon sand are used as raw materials wherefrom, firstly, a homogeneous dry mixture of the raw materials is prepared. The raw-material-based impurities are less than 2% by weight. Since the crystalline structure of the sintered body ought to be as finely textured as possible, if necessary, the raw materials are previously pulverized. Secondly, at least one binder and at least one solvent are added together with, one or more additive selected from the group comprising dispersants, lubricants and plasticizers for obtaining an extrudable mass. Thirdly, the mass is extruded and prepared to shaped abrasive grain precursors (green bodies) which are subsequently sintered in a temperature range between 1450° C. and 1650° C. Thereby the ratio by weight percent of ilmenite to zircon sand is from 1:6 to 6:1.

(4) After sintering, the mineralogical components of the sintered abrasive grain comprise corundum (Al.sub.2O.sub.3), mullite (3 Al.sub.2O.sub.3*2 SiO.sub.2), tialite and/or armalcolite (Al.sub.2TiO.sub.5/AlFeTiO.sub.5), and baddeleyite and/or srilankite (ZrO.sub.2/Ti.sub.0.75Zr.sub.0.25O.sub.2), whereby the ratio by weight percent of mullite to tialite and/or armalcolite is preferably from 6:1 to 1:6.

(5) Preferably and in accordance with the invention, the sintered abrasive grains have a density of ≥3.6 g/cm.sup.3 and hardness HV of between 14 und 18 GPa. Moreover, the sintered abrasive grains have a MKZ-value of ≤2.0%, more preferably ≤1.2%.

(6) Advantageously, the sintered abrasive grains are rod shaped bodies having a diameter between 0.5 and 5 mm und a length between 1 and 10 mm, wherein the ratio of diameter to length is between 0.2:1.0 und 1.0:1.0.

(7) To evaluate the quality of abrasive grains, it is essential to carry out grinding tests. Grinding tests are relative extensive and time-intensive. In the abrasive industry, it is thus common to evaluate the quality of abrasive grains in advance by means of mechanical characteristics, which can be accessed more easily and which serve as indications for the later behavior in the grinding test. In the context of the present works, beside the identification of the hardness, the grain toughness of the abrasive grains was determined via the micro grain decomposition (MKZ) by milling in a ball mill.

(8) Micro Grain Decomposition (MKZ)

(9) To measure the micro grain decomposition, 10 g abrasive grains of a definite grit size (e.g. on basis of corundum; preferably grit size 24 or 36) are milled in a ball mill (height 10.5 cm, diameter 6.8 cm) filled with 12 steel balls (diameter 19 mm, weight 330-332 g) at 185 revolutions per minute over a time period of 150 seconds. The milled grain is subsequently screened in a Rotap screening machine (Haver Böcker EHL 200) for 5 minutes via a corresponding fine sieve (preferably 250 μm) which is 2 classes finer than the bottom sieve defined for the corresponding grit size, subsequently, the fine portion is balanced out. The MKZ value follows from:

(10) $\mathrm{MKZ}\left(\%\right)=\frac{\mathrm{sieve}\mathrm{pass}\text{-}\mathrm{through}}{\mathrm{total}\mathrm{weight}}\times 100$

(11) The micro grain decomposition does not only correlate with the fracture toughness of abrasive grains but is at the same time also an important indicator for the grinding properties of abrasive grains. Thus, relatively reliable predictions of the grinding performance can be made for certain grinding operations on the basis of the MKZ-value, provided that the necessary interaction between hardness and fracture toughness for the corresponding grinding operation is given.

(12) As mentioned earlier, the rod shaped abrasive grains are preferably used for rough grinding or snagging of slabs and billets in foundry and steel production industry whereat highly compressed almost pore-free resin bonded grinding wheels are used. In typical tests, e.g. work pieces made of carbon steel with 220 mm thickness, 400 mm width, and 1540 mm length or work pieces made of stainless steel with 140 mm thickness, 500 mm width, and 2080 mm length are treated with resin bonded grinding wheels at constant machine parameters where the wheel has a cross-section dimension of 616 mm and 76 mm width. In these tests it was generally found that with the use of abrasive grains according to the invention the g-ratio (ratio of material removal to wheel wear) rises with a falling MKZ-value. Consequently, conclusions can be drawn from the MKZ-value to the capability of the abrasive grains. However, this only applies if also the hardness of the abrasive grains lies within the requested range for certain grinding operations, which range amounts with regard to the abrasive grains of the present invention from 14 to 18 GPa hardness HV.

(13) The interaction of hardness and MKZ-value of the abrasive grains may be influenced by the addition of zircon sand or the ratio of zircon sand to ilmenite, respectively. The sintered abrasive grain finally also mirrors this in the ratio of the mineralogical components mullite to tialite and/or armalcolite. With raw materials, the ratio by weight percent of zircon sand to ilmenite should be from 1:6 to 6:1 in order to ensure high performing abrasive grains. However, it was worked out that extraordinary results were achieved with abrasive grains where zircon sand and ilmenite were used at the rate of 5:1 to 1:1. The sintered abrasive grain has the ratio by weight percent of mullite to tialite and/or armalcolite from 6:1 to 1:6.

(14) The following table 1 provides a summary of the chemical compositions, the raw materials used, the physical properties, and the mineralogical phases of some selected rod shaped sintered abrasive grains.

(15) TABLE-US-00001 TABLE 1 example A B C D E F G H I J chemical Al.sub.2O.sub.3 90.2 90.5 90.7 95.0 97 86.6 79.4 80.1 87.5 99.7 composition Fe.sub.2O.sub.3 0.86 1.6 2.3 0.4 0.3 0.95 10.8 10.7 5.50 0.03 (wt. %) TiO.sub.2 1.5 2.9 4.3 0.8 0.5 1.9 5.2 5.60 2.60 — ZrO.sub.2 4.9 3.2 1.6 2.5 1.6 7.0 0.14 0.11 0.13 — SiO.sub.2 2.4 1.6 0.6 1.2 0.7 3.4 4.20 3.10 4.10 0.05 raw materials bauxite — — — — — — 100 100 100 — (Wt. %) alumina 90.0 90.0 90.0 95.0 97.0 87.0 — — — 100 ZrSiO.sub.4 7.5 5.0 2.5 3.75 2.25 9.75 — — — — ilmenite 2.5 5.0 7.5 1.25 0.75 3.25 — — — — density g/cm.sup.3 3.78 3.77 3.80 3.83 3.76 3.68 3.73 3.78 3.74 3.81 hardness H.sub.V (GPa) 15.1 14.8 14.8 15.9 16.8 16.6 13.6 13.6 14.5 18.6 MKZ (%) 0.80 1.00 1.05 1.35 1.40 1.00 2.50 2.75 2.40 1.00 mineralogical corundum 81 85 82 88 92 77 59 66 72 100 phases (%) mullite 8 tr. tr. 2 — 4 10 4 12 — tialite/ 2 7 12 — tr. 2 10 11 4 — armalcolite baddeleyite 4 3 1 2 1 5 — — — — srilankite 1 1 1 1 2 2 — — — — amorphous 4 4 4 7 7 10 21 19 12 —

(16) With the examples A, B, and C according to the present invention in each case 90% by weight of alumina were used as raw material, whereas the portions of zircon sand and ilmenite are varied. Sample A having a ratio of zircon sand to ilmenite of 3:1 reveals the lowest MKZ-value and has therewith the highest grain fracture strength. May be that thereby the high portions (8%) of mullite phase in the finished product play a role which fact will be subject matter of further investigations. The samples B and C with a ratio of zircon sand to ilmenite of 1:1 and 1:3, respectively, show little bit higher MKZ-values, wherefore a lower grinding performance may be expected.

(17) The ratio of zircon sand to ilmenite of 3:1 was maintained with the examples D and E, whereas the percentage of alumina was increased. It was found that in this case the MKZ-values became worse with increasing alumina content, while example F, with lower alumina content also having a ratio of zircon sand to ilmenite of 3:1 reveals an excellent MKZ-value of 1.0% and a hardness of 16.6 GPa, which fact is another indication for the advantage of the additives ilmenite and zircon sand.

(18) Comparative examples G, H, and I are based on bauxite as raw material, whereby high-quality bauxite was used for comparative example I. The difference between the samples G and H which both are based on different types of bauxite as raw material consists mainly in the different percentages of mullite phases in the product. Also in this case the influence of the mullite phase on the product quality has to be investigated in more detail.

(19) Furthermore, with example J, sintered corundum rods on basis of pure alumina were used for comparison. Comparative example J shows a high hardness of 18.6 GPa and a low MKZ-value of 1.0% and is settled in the same range as the examples B, C and F according to the present invention. Whereas the MKZ-value of example A is significantly better. Preliminary conventional grinding tests have indicated that for the present samples the g-ratio (the ratio of material removal to wheel wear) correlates directly with the MKZ values, so that, with the abrasive grains according to the present invention, products are available which can be produced in a cost-efficient way and which provide, for special grinding operations, equal or even better grinding results compared to the sintered rods based on pure alumina.

(20) FIG. 1 additionally shows those correlations with the example of the samples A, B, and C according to the present invention graphically. It can be seen that sample A, with the ratio of zircon sand to ilmenite of 3:1, features a relatively high hardness with HV 15.1 GPa and at the same time a very low MKZ-value of 0.8%; consequently, it offers those properties from which a very good grinding performance may be expected. In fact, in preliminary grinding tests, such sintered abrasive grains offered an improved g-ratio of up to 75% compared to sintered rods on bauxite basis (samples G and H). Example B, with the ratio of zircon sand to ilmenite of 1:1, and example C, with the ratio of zircon sand to ilmenite of 1:3, are featuring only little differences in their mechanical properties. However, hardness HV of less than 15 GPa and micro grain decomposition MKZ of approximately 1.0% cause a g-ratio in the range of sintered alumina rods, whereas the productions costs are more favorable because of the low priced raw material basis. To make things even better, the mineralogical phases cause an increased grain fracture strength (lower MKZ) together with an insignificantly decreased hardness, so that the abrasive grains according to the present invention are predestinated for machining special types of very tough steels.