Aluminum powder metal alloying method

Abstract

A zirconium-doped aluminum powder metal and a method of making this powder metal are disclosed. The method of making includes forming an aluminumzirconium melt in which a zirconium content of the aluminumzirconium melt is less than 2.0 percent by weight. The aluminumzirconium melt then powderized to form a zirconium-doped aluminum powder metal. The powderization may occur by, for example, air atomization.

Claims

1. A method of producing a powder metal part, the method comprising: forming an aluminumzirconium melt in which a zirconium content of the aluminumzirconium melt is less than 2.0 percent by weight; powderizing the aluminumzirconium melt to form a zirconium-doped aluminum powder metal; mixing the zirconium-doped aluminum powder metal with at least one other powder metal including tin thereby forming a mixed powder metal; and compacting and sintering the mixed powder metal to form the powder metal part; wherein a quantity of zirconium in the powder metal part is substantially equal to a quantity of zirconium found in the zirconium-doped aluminum powder metal used to form the powder metal part.

2. The method of claim 1, wherein the step of powderizing includes air atomizing the aluminumzirconium melt.

3. The method of claim 1, wherein powderizing the aluminumzirconium melt to form a zirconium-doped aluminum powder metal includes at least one of atomizing with a gas, as well as comminution, grinding, chemical reaction, and electrolytic deposition.

4. The method of claim 1, wherein the zirconium-doped aluminum powder metal inhibits distortion of the powder metal part during a sintering process used to form the powder metal part.

5. The method of claim 1, wherein the powder metal part includes zirconium in an amount of less than 2.0 weight percent.

6. The method of claim 1, wherein the tin is added as an elemental powder to the zirconium-doped aluminum powder metal.

7. The method of claim 1, wherein the tin is added as a prealloy in a master alloy.

8. The method of claim 1, wherein the tin is approximately 0.2 percent by weight of the mixed powder metal.

9. The method of claim 8, wherein the powder metal part made from the mixed powder metal has a Young's modulus above 70 GPa.

10. The method of claim 1, wherein at least one ceramic additive is added up to 15 volume percent.

11. The method of claim 10, wherein the at least one ceramic additive includes at least one of SiC and AlN.

12. The method of claim 1, wherein the powder metal part formed by the step of compacting has a shape corresponding to the shape of the powder metal part after the step of sintering, except that the shape of powder metal part after the step of sintering is dimensionally smaller than the shape of powder metal part before the step of sintering.

13. The method of claim 12, wherein the powder metal part that is compacted and sintered from the zirconium-doped aluminum powder metal exhibits less distortion from shrinkage during the sintering step than a comparable powder metal part that is compacted and sintered from an aluminum powder metal without zirconium doping.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a chart showing the dimensional change spread for a number of sintered samples made from different powder formulations of a Al-2.3Cu-1.6Mg-0.2Sn alloy at various compaction pressures;

(2) FIG. 2 is a chart comparing the dimensional and mass changes during sintering between Al-2.3Cu-1.6Mg-0.2Sn samples made of (1) an AlZr (50/50) master alloy powder blended with aluminum powder and (2) a zirconium-doped aluminum powder at 200 MPa compaction pressure;

(3) FIG. 3 is a chart similar to FIG. 2, but in which the samples were compacted at a compaction pressure of 400 MPa;

(4) FIGS. 4 through 7 are graphs comparing the ultimate tensile strength (UTS), elongation, and Young's modulus of parts made from Al-2.3Cu-1.6Mg powder metals containing pure aluminum and aluminum doped with 0.2 wt % zirconium having various tin compositions;

(5) FIG. 8 is an optical micrograph of Al-2.3Cu-1.6Mg-0.2Sn produced using pure aluminum as the base powder with no prealloyed zirconium;

(6) FIG. 9 is an optical micrograph of Al-2.3Cu-1.6Mg-0.2Sn-0.2Zr produced using aluminum powder prealloyed with 0.2 weight percent zirconium;

(7) FIG. 10 is an optical micrograph of Al-2.3Cu-1.6Mg-0.2Sn-0.2Zr produced with zirconium introduced in an aluminumzirconium (50-50) master alloy powder;

(8) FIGS. 11 through 14 are graphs illustrating the effect of percent fines in the powder metal on the dimensional change percent for pure aluminum, aluminum doped with 0.05 weight percent zirconium, aluminum doped with 0.2 weight percent zirconium, and aluminum doped with 0.5 weight percent zirconium; and

(9) FIG. 15 is a graph illustrating the dimensional change percent range for each of the powder metal compositions shown in FIGS. 11 through 14 at various percent fines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) A number of powder metal samples were produced having various chemistries for comparison purposes. As a baseline system for comparison, a blend designated A36 was used. The formulation for the A36 blend is found in Table I below.

(11) TABLE-US-00001 TABLE I Powder Weight Percentage Al 84.8 AlCu (50-50) 5.9 Master alloy Atomized Mg 1.5 Sn 0.6 AlN 5.8 Licowax C 1.5
The Licowax C is a lubricant material and boils off during heating. Thus, the total mass of powder for a 1 kg lot will actually exceed 1 kg because of the additional mass of the Licowax C constituent.

(12) A modified form of the A36 powder formulation was also produced which will be referred to in this application as E36-Zr. The E36-Zr powder formulation is identical to the A36 blend, except that the aluminum powder is replaced with an air atomized zirconium-doped aluminum powder metal having 0.2% by weight zirconium. The formulation for the E36-Zr blend is found in Table II below.

(13) TABLE-US-00002 TABLE II Powder Weight Percentage Al0.2Zr 84.8 AlCu (50-50) 5.9 Master alloy Atomized Mg 1.5 Sn 0.6 AlN 5.8 Licowax C 1.5

(14) Notably, the E36-Zr powder blend includes a zirconium-doped aluminum powder with 0.2 wt % zirconium. Conventionally, when alloying elements, such as zirconium are added to a powder blend, these alloying elements are added as part of either an elemental powder (i.e., a pure powder containing only the alloying element) or as a master alloy containing a large amount of both the base material, which in this case is aluminum, and the alloying element. When a master alloy is used, then to obtain the desired amount of the alloying element in the final part, the master alloy will then be cut with an elemental powder of the base material. This cutting technique is used, for example, to obtain the desired amount of copper in each of the A36 powder using the AlCu(50-50) master alloy and elemental aluminum powder.

(15) In contrast, the zirconium-doped aluminum powder metal is obtained by air or gas atomizing an aluminum zirconium melt containing the desired final composition of zirconium. Air atomizing the powder becomes problematic at higher zirconium concentrations and so it may not be possible to atomize zirconium-doped powders having high weight percentages of zirconium (believed at this time to exceed 2 weight percent zirconium, but this value may be as high as 5 weight percent zirconium).

(16) The addition of zirconium results in the formation of intermetallics, such as Al.sub.3Zr, that strengthen the alloy and that remain stable over a range of temperatures. If the zirconium was added as an elemental powder or as part of a master alloy, then the intermetallic phase would be formed preferentially along the grain boundaries and would be coarse in size since relatively slow diffusion kinetics prevent zirconium from being uniformly distributed within the sintered microstructure. Under those conditions, the intermetallic phase imparts only limited improvement in the properties of the final part.

(17) By doping the zirconium in the aluminum powder, rather than adding the zirconium in the form of an elemental powder or as part of a master alloy, the zirconium is more evenly and homogeneously dispersed throughout the entire powder metal as illustrated by a comparison of FIG. 9 (Zr prealloyed) and FIG. 10 (Zr in a master alloy). Thus, the final morphology of the a zirconium-doped part will have the zirconium placed throughout the aluminum and the intermetallics will not be relegated or restricted to placement primarily along the grain boundaries at which they are of only limited effectiveness.

(18) The A36 and E36-Zr powders were made into test bars. Each of the powders were compacted at various compaction pressures (either 200 MPa or 400 MPa) into test bar samples, sintered, and then given a T6 temper heat-treatment. After heat treatment, the various mechanical properties were tested and compared to one another. Table III, below, summarizes the results of the various tests.

(19) TABLE-US-00003 TABLE III Average Average Compaction Yield Average Average Young's Powder Pressure Strength UTS Elongation Modulus Blend (MPa) (MPa) (MPa) (%) (GPa) A36 200 285 288 0.72 57 A36 400 293 295 0.65 62 E36-Zr 200 312 346 2.01 64 E36-Zr 400 322 357 2.24 65

(20) As can be seen above in Table III, the 0.2 weight percent zirconium doping improved the average yield strength, the average ultimate tensile strength, the average elongation, and the average Young's modulus of the test samples. Notably, the observed elongation in the zirconium-doped aluminum samples was much higher and was similar to the control ductility observed in typical T1 temper heat treated samples. Further, the yield strength and the ultimate tensile strength also improved noticeably with the additional zirconium doping.

(21) The changes in various physical characteristics were also measured between the as-compacted and the as-heat treated samples. Table IV below lists the average changes in mass, the average sintered density, the average change in various sample dimensions, and the average T6 hardness.

(22) TABLE-US-00004 TABLE IV T6 Compact. Mass Sint. Dimensional Change Hard- Powder Pressure Change Den. (%) ness Blend (MPa) (%) (g/cc) Length Width OAL (HRB) A36 200 1.33 2.721 1.64 2.42 3.18 67.2 A36 400 1.32 2.723 0.73 1.29 1.91 71.3 E36-Zr 200 1.38 2.751 2.48 3.06 4.24 72.9 E36-Zr 400 1.40 2.745 1.27 1.66 2.25 73.3

(23) Table IV indicates that the E36-Zr samples exhibited more isotropic shrinkage than the Ampal A36 control samples. This means that there was less distortion in the samples prepared using the zirconium-doped aluminum than in the samples prepared without any zirconium.

(24) Referring now to FIG. 1, the dimensional spread change for various powder samples at various compaction pressures were determined. The Al measurements refer to samples made from pure aluminum powder (i.e., the A36 formulation); the AlZr measurements refer to samples made from 0.2 weight percent zirconium-doped aluminum samples (i.e., the E36-Zr formulation); and the AlZr(S) samples refer samples made using the zirconium-doped aluminum, but in which the zirconium doped aluminum was screened at to only include particles greater than 45 micrometers (approximately 325 mesh size). FIG. 1 illustrates that at any of the 200 MPa, 400 MPa, and 600 MPa compaction pressures, the samples made from the zirconium-doped aluminum powder unscreened have the most consistent dimensional change of the three sample powders.

(25) Referring now to FIGS. 2 and 3, two charts are provided which comparatively indicate the dimensional and the mass changes in two different Al-2.3Cu-1.6Mg-0.2Sn powders each having 0.2 weight percent zirconium in aluminum. One of these powders was prepared from a master alloy powder blended with a pure aluminum base powder to reach the desired zirconium content and the other powder prepared was the zirconium-doped aluminum powder made by air atomization of an aluminumzirconium melt. FIG. 2 compares the changes for the powders at a 200 MPa compaction pressure while FIG. 3 compares the powders at a 400 MPa compaction pressure. In both FIGS. 2 and 3, it can be seen that the zirconium-doped aluminum powder has more consistent shrinkage across the various dimensions (i.e., overall length, width, and length) even though the mass change is equal. This is indicative that the parts made from the zirconium-doped aluminum powder exhibit less distortion than the parts made from the powder including the aluminumzirconium master alloy.

(26) Moreover, a comparison of FIGS. 2 and 3 to one another indicates that the greater the compaction pressure, the less the dimensional change will be in the samples. This makes logical sense as the parts having the higher compaction pressure will have a greater green density and shrink less upon sintering.

(27) Referring now to FIGS. 4 and 5, the ultimate tensile strength and the percent elongation of Al-2.3Cu-1.6Mg powders made from a pure aluminum powder and a zirconium-doped aluminum powder were measured with various amounts of elemental tin added. From a review of these figures, it can be seen that the greatest ultimate tensile strength is obtained when approximately 0.2 weight percent of tin is added. At 0.2 weight percent tin, tensile testing indicates that the zirconium-doped aluminum material has a peak ultimate tensile strength of approximately 260 MPa and just under 8 percent elongation before fracture. At lower or higher tin additions, the ultimate tensile strength and ductility of the material decreases from these peak values.

(28) Looking at FIGS. 6 and 7, the elongation and Young's modulus of various Al-2.3Cu-1.6Mg powders are compared at various elemental tin additions. Tin was added as an elemental powder to a Al-2.3Cu-1.6Mg powder formulation made from a pure aluminum powder, a 0.2 weight percent zirconium-doped aluminum powder unscreened, and a 0.2 weight percent zirconium-doped aluminum powder screened though at +325 mesh.

(29) The most notable observation is that when 0.2 weight percent tin was added to the 0.2 weight percent zirconium-doped aluminum powder (screened at +325), a Young's modulus of almost 80 GPa was observed. A Young's modulus in the range of 70 to 80 GPa is comparable to that of a wrought alloy of the same constituents. For most sintered aluminum alloys, a Young's modulus typically falls in the range of 50 to 65 GPa. Accordingly, finding a powder composition that has a Young's modulus of this magnitude was unexpected and surprising.

(30) Although some formulas have been detailed above, it will be appreciated that the zirconium-doped aluminum powder may be mixed with additional alloying elements as well. Tables V-VII below provide powder formulations of a 431D-AlNZr powder, a 7068-AlNZr powder, and a 431D-SiCZr powder, respectively.

(31) TABLE-US-00005 TABLE V Powder Weight Percentage Al0.2Zr 22.5 AlZnMgCuSn 70.3 master alloy AlN 5.7 Licowax C 1.5

(32) TABLE-US-00006 TABLE VI Powder Weight Percentage Al0.2Zr 43.9 AlZnMgCuSn 43.9 Master alloy Zn 3.6 Atomized Mg 0.8 Cu 0.7 Sn 0.1 AlN 5.6 Licowax C 1.5

(33) TABLE-US-00007 TABLE VII Powder Weight Percentage Al0.2Zr 22.5 AlZnMgCuSn 70.4 master alloy SiC 5.6 Licowax C 1.5
The AlZnMgCuSn master alloy is 85.9 wt % Al, 2.64 wt % Cu, 3.48 wt % Mg, 7.74 wt % Zn, and 0.24 wt % Sn.

(34) In these formulations, the zirconium-doped aluminum powder is blended with other powders including master alloys, elemental powders, and ceramic strengtheners to further target specific mechanical properties. However, in each of these blends, it should be noted that the primary source of zirconium is the zirconium-doped aluminum alloy.

(35) Referring now to FIGS. 11 through 15, the effect of fines on the dimensional change of the powder metal is illustrated. Percent fines is the percentage of material in aggregate finer than a given sieve, which in this instance is a 325 mesh with 44 micron openings. For testing, powder metals having 0, 5, 10, 12.5, 15, 20 and 30 percent fines were made of pure aluminum and aluminum doped with 0.05, 0.2, and 0.5 weight percent zirconium, compacted into test samples at 200 MPa compaction pressure, and then sintered under similar thermal conditions. The dimensional change in overall length (OAL), width, and length were measured between the compacted and sintered parts.

(36) FIGS. 11 through 14 show that test samples made from powder metals having a higher percentage of fines have dimensional change percentages that converge to a similar value in each of the various measured dimensions (i.e., OAL, width, and length). This was true in both the pure aluminum sample and zirconium-doped samples, although the zirconium-doped samples exhibit a reduced range of dimensional change across the various measured sample dimensions. For the zirconium-doped aluminum samples, the various dimensional change percentages converged to approximately 2.5% as percent fines increased. Although the pure aluminum sample dimensional changes also trended toward one another, even at 30 weight percent fines, there was still a comparably large dimensional change range (approximately 0.5%) across the various measured dimensions in comparison to the zirconium-doped powder metals.

(37) FIG. 15 provides a summary of the ranges between the measured dimensions for each powder metal at the various fine percentages. This chart reveals that zirconium doping of aluminum improves dimensional stability and that increased amounts of fines can further enhance this dimensional stability.

(38) It should be appreciated that various other modifications and variations to the preferred embodiments can be made within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.