MAGNESIUM ALLOYS WITH IMPROVED CASTABILITY, AND CAST PARTS MADE WITH SUCH ALLOYS

Abstract

Magnesium alloys are disclosed for diecasting with reduced defects comprising aluminum in an amount greater than or equal to about 10 wt % and less than and equal to about 15 wt %; zinc in an amount greater than 0 and less than or equal to about 0.5 wt %; silicon in an amount greater than 0 and less than or equal to about 1.5 wt %; and the balance being magnesium and unavoidable impurities. Magnesium parts diecast from these magnesium alloys are also disclosed.

Claims

1. A magnesium alloy for diecasting with reduced defects, the alloy comprising: aluminum in an amount greater than or equal to about 10 wt % and less than and equal to about 15 wt % zinc in amount greater than 0 and less than or equal to about 0.5 wt %; silicon in an amount greater than 0 and less than or equal to about 1.5 wt %; the balance being magnesium and unavoidable impurities.

2. The magnesium alloy according to claim 1 wherein the magnesium alloy comprises aluminum in an amount greater than or equal to about 10 wt % and less than or equal to about 13 wt %.

3. The magnesium alloy according to claim 1 wherein the magnesium alloy comprises aluminum in an amount greater than or equal to about 13 wt % and less than or equal to about 15 wt %.

4. The magnesium alloy according to claim 1 wherein the magnesium alloy has up to a total of about 0.15 wt % additional elements, each additional element comprising no more than about 0.05 wt %.

5. The magnesium alloy according to claim 1 wherein the magnesium alloy has CSI of less than about 250.

6. The magnesium alloy according to claim 1 wherein the magnesium alloy comprises zinc in amount of at least about 0.1 wt %.

7. The magnesium alloy according to claim 1 wherein the magnesium alloy comprises silicon in an amount less than or equal to about 1.0 wt %.

8. The magnesium alloy according to claim 7 wherein the magnesium alloy comprises silicon in an amount greater than or equal to about 0.5 wt %.

9. A cast magnesium part made by thixomolding the magnesium alloy according to claim 1.

10. The cast magnesium part according to claim 9, wherein the magnesium alloy has up to a total of about 0.15 wt % additional elements, each additional element comprising no more than about 0.05 wt %.

11. The cast magnesium part according to claim 10 wherein the magnesium alloy has CSI of less than about 250.

12. The cast magnesium part according to claim 11 wherein the magnesium alloy comprises zinc in amount of at least about 0.1 wt %.

13. The cast magnesium part according to claim 12 wherein the magnesium alloy comprises silicon in an amount less than or equal to about 1.0 wt %.

14. The cast magnesium part according to claim 13 wherein the magnesium alloy comprises silicon in an amount greater than or equal to about 0.5 wt %.

15. A magnesium alloy for diecasting with reduced defects, the alloy comprising: aluminum in an amount greater than or equal to about 10 wt % and less than or equal to about 15 wt %; zinc in an amount greater than or equal to about 0.1 wt % and less than or equal to about 0.5 wt %; silicon in an amount greater than about 0.5 wt % and less than or equal to about 1.0 wt %; and the balance being magnesium and unavoidable impurities.

16. The magnesium alloy according to claim 15 wherein the magnesium alloy has up to a total of about 0.15 wt % additional elements, each additional element comprising no more than about 0.05 wt %.

17. The magnesium alloy according to claim 15 wherein the magnesium alloy has CSI of less than about 250.

18. The magnesium alloy according to claim 15 wherein the magnesium alloy comprises aluminum in an amount greater than or equal to about 10 wt % and less than or equal to about 13 wt %.

19. The magnesium alloy according to claim 15 wherein the magnesium alloy comprises aluminum in an amount greater than or equal to about 13 wt % and less than or equal to about 15 wt %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0015] FIG. 1 is a graph of temperature versus the solid fraction (fs) for three alloys Mg-9% Al, Mg-9% Al-0.5% Zn, and Mg-9% Al-1.0% Zn (wt %), showing the effect of zinc content on solidification behavior;

[0016] FIG. 2 is a graph of temperature versus the solid fraction (fs) for eight binary MgAl alloys, with aluminum at 8, 9, 10, 11, 12, 13, 14, and 15 wt %, showing the effect of aluminum content on solidification behavior; and

[0017] FIG. 3 is a graph of temperature versus the solid fraction (fs) for four Mg-9% Al-0.5% Zn (wt %) alloys with silicon at 0, 0.5, 1.0, and 1.5 wt %, showing the effect of silicon content on solidification behavior.

[0018] In the drawings, reference numbers may be reused to identify similar and/or identical elements.

DETAILED DESCRIPTION

[0019] The present disclosure provides magnesium alloys with less susceptibility to casting defects such as hot tears and cold shuts, as compared to current commercially used magnesium allows such as AZ91D, and to diecast magnesium alloy parts made from these alloys.

[0020] Generally, the magnesium alloys according to this disclosure are specially adapted for die casting, and in particular (but not exclusively) for thixomolding, with a low incidence of defects such as hot tears and cold shuts. Thixomolding or magnesium injection molding is a single-step, semi-solid molding process in which chips of a magnesium alloy are fed into a heated screw and barrel, then thermally and mechanically processed into a semi-solid state and injected directly into a tool cavity.

[0021] The alloys of this disclosure generally comprise aluminum greater than or equal to about 10 wt % and less than or equal to about 15 wt %, zinc greater than 0 and less than or equal to about 0.5 wt %; silicon greater than 0 and less than or equal to about 1.5 wt %, with the balance being magnesium and impurities. There can be up to about 0.15 wt % of additional elements, with no single additional element comprising more than about 0.05 wt %. Optionally, there is at least 0.1 wt % Zn, and at least about 0.5 wt % Si.

Crack Susceptibility Index (CSI)

[0022] A Crack Susceptibility Index or CSI has been proposed as an indicator of when a magnesium alloy is subject to cracking such as hot tears. See, Lie and SIndo, Susceptibility of Magnesium Alloys to Solidification Cracking, Science and Technology of Welding and Joining 25.3 (2020): 251-257, the entire disclosure of which is incorporated herein by reference. The CSI is the maximum slope of the temperature versus the square root of the fraction of solid (fs) over the range of interest. FIGS. 1-3 show T vs. (fs) for various alloys. These curves can be generated with standard thermodynamic software such as Pandat, from CompuTherm LLC, Madison, WI, using metallurgical data from a standard reference database, such as PanMagnesium also from CompuTherm, LLC. The CSI referenced in this disclosure is the maximum slope of the curve over the range 0.87fs0.99 indicated on each graph. CSI=|dT/d(fs)| maximum for 0.87fs0.99. The CSI for AZ91D is 506, while the CSI for standard aluminum alloy A356 is 250. The CSI for embodiments of the alloys of this disclosure range from 0 to about 250, meaning they are more resistant to cracking than AZ91D, and are generally at least as resistant to cracking defects as popular aluminum alloys.

Zinc Content

[0023] The inventors have found that zinc increases the CSI and can be detrimental to hot tearing resistance. FIG. 1 shows that for a given composition, CSI increases with zinc content. Specifically, Mg-9% Al has a CSI of about 0, Mg-9% AL-0.5% Zn has a CSI of 223, and Mg-9% AL-1% Zn has a CSI of 506. However, zinc can lower the liquidus temperature of a given composition. Thus, zinc can be present, but it can be advantages to keep zinc1 wt %, and more advantageous to keep zinc0.5 wt %.

Aluminum Content

[0024] The inventors have found that aluminum does not have a significant impact on CSI or on tearing resistance at levels of 9 wt % or greater. However, as shown in FIG. 2, increasing aluminum content from 10 wt % to 15 wt % does reduce the liquidus temperature, which improves fluidity of the alloy. Thus, it can be advantageous to have aluminum10 up to at least aluminum15 wt %.

Silicon Content

[0025] The inventors have found that silicon does not have a significant impact on CSI or on tearing resistance. However, as shown in FIG. 3, silicon can reduce the liquidus temperature, which improves fluidity of the alloy. FIG. 3 shows that for a given composition CSI remains the same for Mg-9% Al-0.5% Zn for Si contents of 0Si wt %1.5. FIG. 3 further shows that increasing silicon content from 0 wt % to about 1 wt % decreases the liquidus temperature, which improves fluidity of the alloy. Further increases in silicon beyond about 1 wt % do not decrease liquidus temperature, and can actually increase the liquidus temperature at low fraction of solids (fs). As shown in FIG. 3, Mg-9% AL-0.5% Zn-1% Si has a lower liquidus temperature than Mg-9% AL-0.5% Zn-0.5% Si, which has a lower liquidus temperature than Mg-9% AL-0.5% Zn. Thus, it can be advantages to have 0.1silicon1.5 wt %, and more advantages to have 0.1 wt %silicon1.0 wt. As noted, at least some silicon, for example silicon0.1 up to Si0.5 wt % can provide lower liquidus temperatures.

[0026] A first embodiment of the alloys of this disclosure comprises aluminum greater than or equal to about 10 wt % and less than or equal to about 13 wt %; zinc in an amount greater than 0 and less than or equal to about 0.5 wt %; and silicon in an amount greater than 0 and less than or equal to about 1.5 wt %, with the balance being magnesium and impurities. In some versions of the first embodiment the zinc content is greater than or equal to about 0.1 wt % and less than or equal to about 0.5 wt %. In some versions of the first embodiment the silicon content is greater than or equal to about 0.5 wt % and less than or equal to about 1.0 wt %. The alloys of this first embodiment typically have a CSI of less than 250.

[0027] In a second embodiment of the alloys of this disclosure, the aluminum content is greater than or equal to about 13 wt % and less than or equal to 15 wt %, zinc in an amount greater than 0 and less than or equal to about 0.5 wt %; and silicon in an amount greater than 0 and less than or equal to about 1.5 wt %, with the balance being magnesium and impurities. In some versions of the first embodiment the zinc content is greater than or equal to about 0.1 wt % and less than or equal to about 0.5 wt %. In some versions of the first embodiment the silicon content is greater than or equal to about 0.5 wt % and less than or equal to about 1.0 wt %. The alloys of this second embodiment typically have a CSI of less than 250, and owing to their greater aluminum content than the first embodiment, have a lower liquidus temperature.

[0028] Some embodiments of this disclosure provide a diecast part, and in particular a part made by thixomolding. The part is made of an alloy comprising aluminum greater than or equal to about 10 wt % and less than or equal to about 15 wt %, zinc in an amount greater than 0 and less than or equal to about 0.5 wt %; and silicon in an amount greater than 0 and less than or equal to about 1.5 wt %, with the balance being magnesium and impurities. There are preferably no more than 0.15 wt % of additional elements, with no single additional element comprising more than 0.05 wt %.

[0029] In a first embodiment of the parts of this disclosure, there is between about 10 wt % and about 13 wt % Al, zinc in an amount greater than 0 and less than or equal to about 0.5 wt %; and silicon in an amount greater than 0 and less than or equal to about 1.5 wt %, with the balance being magnesium and impurities. In some versions of the first embodiment the zinc content is greater than or equal to about 0.1 wt % and less than or equal to about 0.5 wt %. In some versions of the first embodiment the silicon content is greater than or equal to about 0.5 wt % and less than or equal to about 1.0 wt %. The alloys of this first embodiment typically have a CSI of less than 250.

[0030] In a second embodiment of the alloys of this disclosure the aluminum content is greater than or equal to about 13 wt % and less than or equal to 15 wt %, zinc in an amount greater than 0 and less than or equal to about 0.5 wt %; and silicon in an amount greater than 0 and less than or equal to about 1.5 wt %, with the balance being magnesium and impurities. In some versions of the first embodiment the zinc content is greater than or equal to about 0.1 wt % and less than or equal to about 0.5 wt %. In some versions of the first embodiment the silicon content is greater than or equal to about 0.5 wt % and less than or equal to about 1.0 wt %. The alloys of this second embodiment typically have a CSI of less than 250 and owing to their greater aluminum content than the first embodiment, have a lower liquidus temperature.

[0031] The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure.