Aluminium alloy products having a pre-sintered density of at least 90% theoretical, and methods of making such alloy products

Abstract

Aluminium-silicon powder mixtures comprising a hypereutectic AlSi powder, a near eutectic AlSi powder, and a third powder which is aluminium or a hypoeutectic aluminium alloy containing alloying constituents other than silicon and less than 9 wt % silicon, with a sintering aid comprising a fourth, zinc-containing powder, are pressed and sintered to provide powder metallurgy products suitable for automotive component use in particular. The third powder in the composition permits such AlSi powder mixtures to be compressed to a density approaching that obtained by use of an annealed powder mixture, but without the annealing step.

Claims

1. A metal powder source material for making an aluminium-based sintered product compactable to a pre-sintered density of at least 90% of theoretical by a powder metallurgy route, comprising a mixture of first, second and third aluminium-containing powders mixed with a sintering aid comprising a fourth powder comprising zinc or a zinc based alloy; wherein the first powder is of a hypereutectic aluminum-silicon-copper-magnesium alloy, the second powder is of a near-eutectic aluminium-silicon alloy containing from 9 to 13 wt % silicon, and the third powder is of aluminium; the first and second powders are present in relative proportions to one another lying in the range 35:65 wt % to 65:35 wt %; the third powder is present in a proportion of 5 wt % up to 35 wt % of the total weight of the three said aluminum-containing powders; the fourth powder is present as at least 0.5 wt % of the total metal and alloy powder mixture; and at least the first and second aluminium-containing powders have been produced by an atomizing step and not subsequently annealed.

2. A metal powder source material according to claim 1, wherein the hypereutectic aluminium-silicon-copper-magnesium alloy of the first powder has a copper content of at least 2 wt %.

3. A metal powder source material according to claim 1, wherein the hypereutectic aluminium-silicon-copper-magnesium alloy of the first powder has a magnesium content of at least 0.2 wt %.

4. A metal powder source material according to claim 1, wherein the third powder is present as at least 10% of the total weight of the first three powders.

5. A metal powder source material according to claim 1, wherein the fourth powder comprises 2 to 6 wt % of the total metal powder mixture.

6. A green pressed composite compacted to a pre-sintered density of at least 90% of theoretical formed by shaping and compacting a metal powder source material according to claim 1.

7. A powder metallurgy product obtained from a green pressed composite according to claim 6 by sintering, having a microstructure characterised by interpenetrating reticular structures derived from the original alloy powder particles, including a first structure having primary silicon particles derived from the first alloy powder and a second structure derived from the other aluminium based powders.

8. An automotive component comprising a powder metallurgy product according to claim 7 having a sliding contact bearing surface formed therein.

9. A metal powder source material for making an aluminium-based sintered product compactable to a pre-sintered density of at least 90% of theoretical by a powder metallurgy route, comprising a mixture of first, second and third aluminium-containing powders mixed with a sintering aid comprising a fourth powder comprising zinc or a zinc based alloy; wherein the first powder is of a hypereutectic aluminum-silicon-copper-magnesium alloy, the second powder is of a near-eutectic aluminium-silicon alloy containing from 9 to 13 wt % silicon, and the third powder is of a hypoeutectic aluminium alloy containing alloying constituents other than silicon and from 0.3 wt % to less than 0.6 wt % silicon; the first and second powders are present in relative proportions to one another lying in the range 35:65 wt % to 65:35 wt %; the third powder is present in a proportion of 5 wt % up to 35 wt % of the total weight of the three said aluminium-containing powders; the fourth powder is present as at least 0.5 wt % of the total metal and alloy powder mixture; wherein at least the first and second aluminium-containing powders have been produced by an atomizing step and not subsequently annealed.

10. A metal powder source material according to claim 9, wherein the hypereutectic aluminium-silicon-copper-magnesium alloy of the first powder has a copper content of at least 2 wt %.

11. A metal powder source material according to claim 9, wherein the hypereutectic aluminium-silicon-copper-magnesium alloy of the first powder has a magnesium content of at least 0.2 wt %.

12. A metal powder source material according to claim 9, wherein the third powder is present as at least 10% of the total weight of the first three powders.

13. A metal powder source material according to claim 9, wherein the fourth powder comprises 2 to 6 wt % of the total metal powder mixture.

14. A green pressed composite compacted to a pre-sintered density of at least 90% of theoretical formed by shaping and compacting a metal powder source material according to claim 9.

15. A powder metallurgy product obtained from a green pressed composite according to claim 14, by sintering having a microstructure characterised by interpenetrating reticular structures derived from the original alloy powder particles, including a first structure having primary silicon particles derived from the first alloy powder and a second structure derived from the other aluminium based powders.

16. An automotive component comprising a powder metallurgy product according to claim 15, having a sliding contact bearing surface formed therein.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying photomicrographs, FIGS. 1 to 5 show the microstructures of four aluminium alloy powder metallurgy alloy products according to the prior art. These products were made from mixes 1, 3, 4 and 5 as described in the following illustrative and non-limiting Example.

EXAMPLE

(2) FIG. 3 of WO02/27047 shows that as-atomized powder mixtures of near-eutectic and hypereutectic powders, with Zn addition for sintering, achieve pressed densities of 2.2 Mg/m.sup.3 at pressing pressure of 330 MPa, 2.3 Mg/m.sup.3 at pressure of 440 MPa, and 2.4 Mg/m.sup.3 at about 550 MPa.

(3) In a trial comparison, powder mixtures of the as-atomized AlSi based powders detailed in Table 1, not having been annealed, were mixed in the proportions given in Table 2, the percentages being by weight, together with 0.35 wt % of fugitive pressing lubricant.

(4) Powder A is an example of a hypereutectic AlSi powder containing 15.6 wt % Si, given for comparative purposes, not suited for use as one of the powders in the invention. Powder B is the hypereutectic alloy LM30 (B.S. 1490) containing significant copper and magnesium in addition to 17 wt % silicon, and is accordingly suited for use as a first powder in the method of the invention. Powder C is an example of a near-eutectic AlSi powder, suited for use as a second powder in the method of the invention, containing 10.6 wt % Si. Powder D is substantially pure aluminium (99.7 wt %) and suited for use as a third powder in the method of the invention. Powder E is the low-silicon aluminium-based powder ASM2124, which also contains significant copper and magnesium, and is potentially also suited for use as a third powder.

(5) TABLE-US-00001 TABLE 1 Al and Particle incidental size Powder Si Cu Mg Mn impurities (d50) A 15.6% Balance 45 m B (LM30) 17% 4% 0.5% Balance 65 m C 10.6% Balance 45 m D 99.7% 45 m E (ASM2124) 0.1% 4% 1.4% 0.5% Balance 25 m

(6) The balance includes incidental impurities in each case. (d50) indicates that 50% of the particles can be held up on a screen of the size indicated. Where powders are not screened, d50 may conveniently be determined by laser light scattering particle size analysis. This measure may be used to interpret references herein to median particle size.

(7) These powders were made up into six different powder mixes, as set out in Table 2. Of these, mix 6 was in conformity with the invention, while mixes 1 to 5 were not, and are included for comparison purposes. The zinc powder particle size was within the range from 30 to 150 micrometres.

(8) TABLE-US-00002 TABLE 2 Mix A B C D E Zn 1 48 (annealed) 48 (annealed) 4 2 48 (atomized) 48 (atomized) 4 3 32 (atomized) 32 (atomized) 32 4 4 32 (atomized) 32 (atomized) 32 4 5 48 (atomized) 48 (atomized) 4 6 32 (atomized) 32 (atomized) 32 4

(9) These mixtures were compacted at 550 MPa, giving pressed densities as given in Table 3. The pressed components were subsequently sintered in a nitrogen/5% hydrogen atmosphere, allowing a dwell of about 20 minutes at 350 to 400 C. to de-wax, and a sinter of 20 minutes at 560 to 570 C. After sintering, polished samples of the components were tested for hardness (Hv5). Values were as listed in Table 3.

(10) TABLE-US-00003 TABLE 3 Pressed density Vickers hardness Mix Mg/m.sup.3 (% theoretical) (Hv5) 1 2.45 (85%) 31 2 2.45 (85%) 31 3 2.58 (93%) 31 4 2.50 (88%) 46 5 2.44 (85%) 78 6 2.58 (93%) 68

(11) In mixes 3 and 6, the addition of the third powder, of elemental aluminium, has restored the pressed density to equal to or better than the pressed density of the annealed powder mixes of WO02/27047. In mix 4, with a different third powder, the pressed density is still improved.

(12) Examples of the microstructures after sintering can be seen in FIGS. 1 to 5.

(13) FIG. 1 (400) shows a sample made from mix 1. It shows primary silicon particles (grey) in an unresolved eutectic. The continuous porosity, showing as dark linear regions towards the left of the image area, is also evident.

(14) FIG. 2 (100) and FIG. 3 (400) show samples made from mix 3. Again these show primary silicon, with a degree of coalescence of the particles evident at 400. Precipitate-denuded or -free zones demonstrate the reticular nature of the structure, particularly evident at 100. The dark porosity can be seen to be isolated at 400, characteristic of density greater than 90% of theoretical.

(15) FIG. 4 (400) shows a sample made from mix 4. The use of AlCuMg (alloy ASM 2124) as a third powder results in limited hardening. Low density continuous porosity (dark regions) and coarsened silicon particles are evident.

(16) FIG. 5 is a final comparative sample, made from mix 5. The use of LM30 (AlSiCuMg powder) reveals fine primary silicon, compatible with raised hardness (Table 3) against an unresolved eutectic. Minor intermetallics and linear porosity may just be discerned in the background.

(17) As regards the hardness values shown in Table 3, it can be seen that the high proportion of Si, Cu and Mg in the hypereutectic powder has allowed a higher hardness than in mixtures including a hypereutectic powder without Cu or Mg. In addition, whereas the simultaneous presence of Si with Cu and Mg in the hypoeutectic case is evidently deleterious to hardness, higher hardness can be achieved with the hypereutectic powder containing Cu and Mg.

(18) The powder mix 6 can be seen, from the foregoing Example, to give rise to numerous particular benefits, both in the methods of making aluminium alloy products provided by the powder metallurgy route of the present invention, and in the products so made, when compared with all the alternative powder mixes.

(19) The products obtained from each of mixes 1, 2 and 3 show low hardness, even though mix 3 allows higher density. The product obtained from mix 4 shows slightly better hardness but not density. The product from mix 5 suffers from lower density in spite of higher hardness through use of Al-17Si-4Cu-1Mg. Only mix 6 gives rise to the triple advantages of a) high density, b) high hardness, and c) reduced cost through the use of as-atomised B and C powders, the cost being further reduced by the use of a substantial proportion of cheaper aluminium powder in the mix.

(20) Other changes and modifications within the scope of the invention will be apparent to those skilled in the art.