US20140286820A1

US20140286820A1 - High strength reduced be casting alloy

Info

Publication number: US20140286820A1
Application number: US13/849,225
Authority: US
Inventors: Randolf S. Beals; Keith J. Smith; Lawrence H. Ryczek
Original assignee: Materion Brush Inc
Current assignee: Materion Brush Inc
Priority date: 2013-03-22
Filing date: 2013-03-22
Publication date: 2014-09-25

Abstract

The Be content of Be Al alloys suitable for investment casting, which contain a small but suitable amount of Ag, can be significantly reduced without adversely affecting their thermal or investment casting properties by including significantly more Si in the alloy than done in the past.

Description

BACKGROUND

This invention relates to improved Be Al alloys for use in investment casting.
As described in commonly-assigned U.S. Pat. No. 5,667,600 to Grensing et al., the disclosure of which is incorporated herein by reference, investment casting is a type of casting normally used to make metal parts of complex shape. “Investment casting” connotes that the casting is produced by using a mold that is made from a ceramic shell. The use of a ceramic shell increases the maximum allowable casting temperature thereby permitting the casting of relatively high-melting-temperature alloys. In general such alloys cannot be cast using other procedures such as injection casting or sand casting. Casting produces components that are “near net shape,” i.e., a shape which is very near to the shape of the final product to be made. Consequently casting, and investment casting in particular, is desirable because it essentially eliminates the extensive machining that would otherwise be necessary to transform a block of metal into its final desired shape. This reduces both the machining costs and also the amount of metal needed to produce the part.
Be and Al have widely different melting temperatures, 1289° C. and 660° C. respectively. This makes investment casting of Be Al alloys very difficult, because this large difference in melting temperatures leads to large differences between the liquidus and solidus temperatures of these alloys. See, U.S. Pat. No. 5,603,780 to Nachtrab et al., the disclosure of which is also incorporated herein by reference, especially col. 1, lines 31 to 50. This, in turn, often leads to excessive porosities, coarse microstructures or both in castings made from these alloys. Accordingly, shaped parts made from Be Al alloys are most commonly made by powder metallurgy techniques.
The above-mentioned Grensing et al. patent indicates that Be Al alloys suitable for investment casting can contain as little as 30 wt.% Be. However, experience has shown that Be contents of at least about 56 wt. % and more commonly about 61 to 69 wt. % are necessary to make commercially-acceptable alloys, i.e., alloys exhibiting acceptable levels of segregation and microporosity. For example, the three Be Al investment casting alloys available on the market today have Be contents of 56 to 68 wt. %. Note, also, that the alloys in all working examples of the Grensing et al. patent have Be contents of at least 62 wt. %. Also note the express disclosure in the Nachtrab et al. patent mentioned above that Be must be present in an amount of at least about 60 wt. %.
Two desirable properties of Be Al investment casting alloys are low coefficients of thermal expansion and strength. In this regard, all three commercial Be Al investment casting alloys mentioned above have coefficients of thermal expansion of about 14.5 μm/m (ppm) or less. In addition, two of these alloys have 0.2% yield strengths of about 130 to 155 MPa and ultimate tensile strengths of about 200 MPa, while the third has a 2% yield strength of about 200 MPa and ultimate tensile strengths of about 255 MPa.
Beryllium is expensive. Therefore, it would be advantageous if the concentration of Be in these alloys could be reduced without adversely affecting their strength, thermal and investment casting properties. And it would be especially desirable if this could be done in such a way that one or more other properties of these alloys were actually improved.

SUMMARY

In accordance with this invention it has been found that, by adding Ni, Co and/or Cu to certain Ag-containing Be Al investment casting alloys, and in addition by increasing the Si content of these alloys beyond conventional amounts, it is possible to reduce the Be content of these alloys while simultaneously increasing their strength without adversely affecting other important properties of these alloys such as their coefficients of thermal expansion and their ability to be investment cast with minimal segregation and microporosity.
Thus, this invention provides a new Be Al alloy suitable for investment casting purposes, the alloy comprising about 40 to 55 wt. % Be, about 1.3 to 7.0 wt. % Ag, about 1.5-10 wt. % Si, about 0.5-6.0 wt. % of Ni, Co, Cu or mixtures thereof, and no more than about 3 wt. % optional ingredients, with the balance being Al plus incidental impurities, wherein the Be/Al ratio of the alloy is <˜2.0, the combined amounts of Si and Ag in the alloy is >˜4 wt. %, the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜6 wt. %, and further wherein the alloy has a coefficient of thermal expansion of ≦˜15 μm/m (ppm), a 0.2% yield strength of ≧˜200 MPa and an ultimate tensile strength of ≧˜250 MPa.

DETAILED DESCRIPTION

The inventive Be Al investment casting alloys contain significantly less Be than known Be Al alloys exhibiting commercially desirable investment casting properties. Thus, the inventive Be Al alloys normally contain about 55 wt. % or less Be, more commonly about 50 wt. % or less. Normally, they also contain at least about 40 wt. % Be, because the modulus of elasticity and specific stiffness of Be Al alloys containing less Be are simply too low for many investment casting applications. Thus, the inventive Be Al investment casting alloys desirably exhibit a modulus of elasticity of at least about 140 GPa @ 25° C., preferably at least about 150 GPa @ 25° C. Alloys containing at least about 45 wt. % Be, and especially about 47 to <˜50 wt. %, Be are particularly interesting, as are alloys exhibiting a modulus of elasticity of at least about 160 GPa @ 25° C. Alloys containing <50 wt. % Be are desirable, because an export license from the U.S. federal government is not required.
In addition to Be, the inventive alloys also contain Ag. As indicated above, as of this writing, there are three commercially-available Be Al investment casting alloys in the United States. Two contain about 1.65-3.35 wt. % Ag, while the third contains no Ag. In addition, the above-noted Nachtrab et al. patent indicates that Be Al investment casting alloys can contain up to 4.25 wt. % Ag. The inventive Be Al investment casting alloys contain similar amounts of Ag as these Ag-containing alloys. Thus, the inventive Be Al alloys contain at least about 1.3 wt. % Ag, and more commonly at least about 1.5 wt. % or even at least about 2.0 wt. % Ag. In addition, they may contain as much as about 7.0 wt. % Ag, but more commonly will contain no more than about 6.0 wt. %, no more than about 5.0 wt. % or even no more than about 4.0 wt. % Ag. Ag contents of about 1.3-7.0 wt. %, about 1.5-6.0 wt. %, or even about 2-5 wt. %, are more interesting.
In addition to Be and Ag, the inventive Be Al alloys also contain Si. Two of the commercial investment casting Be Al alloys mentioned above contain no Si while the third contains about 1.65-2.5 wt. % Si. Similarly, the above-noted Nachtrab et al. patent, although indicating that the Si can be present in its alloys in amounts as high as 4 wt. %, shows in its working examples that Si content as a practical matter is limited to a maximum of 2.0 wt. %. The inventive alloys differ from these alloys in that they contain significantly more Si for enhanced castability. Thus, the inventive alloys contain at least about 1.5 wt. % Si, with alloys containing at least about 2.0 wt. % Si or even at least about 2.5 wt. % Si being more interesting.
As for maximum Si content, the Si concentration preferably does not exceed the value needed to keep essentially all of the Si in the Al—Si eutectic phase that is believed to be formed in the inventive alloys. As further discussed below, the inventive alloys are believed to differ from conventional Be Si alloys in that the inventive alloys generally contain more Si. Consequently they form more Al—Si eutectic phase, but they also still contain primary Al cells, i.e., cells of essentially pure Al. Accordingly, the maximum Si content in these alloys is preferably selected so that essentially all of the Si in these alloys remains in this eutectic phase. Generally speaking, this means that the maximum Si content of the inventive alloys will about 7 wt. %, although maximums on the order of about 6 wt. %, about 5 wt. %, about 4 wt. % or even about 3 wt. % are more common.
The inventive Be Al alloys also contain a suitable amount of Ni, Co, Cu or mixtures thereof. Alloys containing mixtures of Ni and Co are especially interesting. In accordance with this invention, Ni, Co and/or Cu are included in the inventive alloys to increase strength. In accordance with this invention, it has been found that the presence of one or more of these elements will substantially increase both 0.2% yield strength as well as ultimate tensile strength, provided that the alloys also contain at least about 1.3 wt. % Ag as well as at least about 1.5 wt. % Si. Such alloys containing at least about 2.0 wt. % Si or even at least about 2.5 wt. % Si are more interesting.
The amount of Ni, Co and/or Cu that should be included in the inventive alloys will normally be at least about 0.5 wt. %, but more commonly at least about 1 wt. % or even at least about 2 wt. %. The maximum amount of Ni, Co and/or Cu in the inventive alloys will normally be no more than about 6 wt. %, but more commonly no more than about 5 wt. %, or even no more than about 4 wt. % so as not to adversely affect thermal conductivity or castability in a significant way. Therefore, the inventive alloys will typically contain about 0.5 to 6 wt. %, more commonly about 1 to 5 wt. % or even about 2 to 4 wt. % of Ni, Co, Cu or mixtures thereof.
The inventive Be Al alloys can also contain elements known to increase ductility such as Sr, Na, Ca and Sb. If so, the amount of such ingredients in these alloys should be no greater than about 0.3 wt. %, more desirably no more than about 0.25 wt. %. Alloys containing about 0.005 to 0.2 wt %, about 0.01 to 0.1 wt. %, or even about 0.02 to 0.08 wt. % of these elements are more interesting. Alloys containing about 0.02 to 0.06 wt. % or even about 0.03 to 0.05 wt. % Sr are especially interesting.
Other known ingredients in Be Al alloys can also be included in the inventive Be Al alloys. Examples include Ge, Ti, Zr, B, Sc, Y and the rare earth elements. If so, the total amount of such ingredients should not exceed about 1.0 wt. %, preferably about 0.5 wt. %., about 0.3 wt. % or even about 0.1 wt. %. Alloys which are essentially free of these ingredients are preferred.
The inventive Be Al alloys can also contain other optional ingredients which do not adversely affect the properties of the alloys in any significant way. Normally, the total amount of these optional ingredients will not exceed about 3.0 wt. %, preferably not exceed about 2.0 wt. %, about 1.0 wt. % or even about 0.5 wt. %.
The balance of the inventive alloys is Al and incidental impurities. By incidental impurities is meant ingredients which are present in such small amounts (usually trace amounts) that their effect on the properties of the alloy obtained are insignificant. As well appreciated in metallurgy, it makes no economic sense to refine out trace contaminants which have an insignificant effect on alloy performance. The same considerations apply here.
As indicated above, it has been found in accordance with this invention that the strength of certain Ag-containing Be Al investment casting alloys can be significantly increased without adversely affecting their investment casting properties or their coefficients of thermal expansion by including Ni, Co and/or Cu in the alloy and, in addition, by decreasing their Be contents and increasing their Si contents relative to similar alloys known in the past. This compositional modification is reflected by at least three different features of the inventive alloys compared with conventional alloys.
First, the inventive alloys contain substantially less Be and correspondingly more Al, on a relative basis, than conventional alloys. This is reflected by the fact that, in the inventive alloys, the Be/Al ratio is <˜2.0, more desirably ≦˜1.6 or even ≦˜1.4. In contrast, in the Si-containing commercial alloys mentioned above as well as all of the Si-containing alloys specifically disclosed in the above-mentioned Grensing et al. and Nachtrab et al. patents, the Be/Al ratio is ≧2.0. The difference between the inventive and earlier alloys in terms of Al content is also reflected by the fact that the Al content of the inventive alloys is typically ≧˜34 wt. %, more commonly ≧˜37 wt. % and even ≧˜40 wt. %. In the Si-containing commercial alloys mentioned above as well as all of the Si-containing alloys specifically disclosed in the above-mentioned Grensing et al. and Nachtrab et al. patents, the maximum Al content is 33 wt. %
The second difference between the inventive alloys and conventional alloys is that the combined amount of Si and Ag in the inventive alloys is greater than in conventional alloys. This is reflected by the fact that, in the inventive alloys, the combined amount of Si and Ag is >4.0 wt. %, more desirably ≧˜5.0 wt. %, or even ≧˜5.8 wt. %. Although the general disclosure of the above-noted Nachtrab et al. patent indicates its alloys can contain up to 4 wt. % Si and up to 4.25 wt. % Ag, the working examples of this patent show that the combined amount of these elements is limited to a maximum of 4.0 wt. %. Meanwhile, the maximum combined amount of Si and Ag in the above-noted commercial alloys which also contain Ni, Co or Cu, as well as the specific alloys described in the above-noted Grensing et al. patent which also contain Ni, Co or Cu, is 3.35 wt. %.
The third difference between the inventive alloys and conventional alloys is that the combined amounts of Ag, Si, Ni, Co and Cu in the inventive alloys is ≧˜6.0 wt. %, more typically ≧˜7.0 wt. %, or even ≧˜8.0 wt. %. In contrast, the combined amounts of Ag, Si, Ni, Co and Cu in the specific alloys described in the above-noted Nachtrab et al. and Grensing et al. patents which contain at least one of Si and Ag, as well as at least one of Ni, Co and Cu, is 4.25 to 5.9 wt. %. Similarly, the combined amounts of Ag, Si, Ni, Co and Cu in the specific commercial alloys mentioned above which contains at least one of Si and Ag, as well as at least one of Ni, Co and Cu, is 3.30 to 4.70 wt. %.
From the above, it can be seen that the inventive alloys differ from conventional Be Al investment casting alloys in that the inventive alloys not only contain less Be and more Al than their conventional Si-containing counterparts but also more Si plus Ag, as well as a greater total amount of Si, Ag and Ni, Co and/or Cu. It is well known that the microstructure of a Be Al investment casting alloy is composed of a Be phase (Be-based dendrites) surrounded by an Al matrix. Although not wishing to be bound to any theory, it is believed that the increased amounts of Al and Si in the inventive alloys generate a modified microcrystalline structure in which the Be phase is surround by an Al—Si eutectic phase which also contains primary Al cells, i.e., cells of essentially pure Al. This Al—Si eutectic, it is believed, is responsible not only for the low microporosity exhibited by the inventive Be Al alloys but also their desirable coefficients of thermal expansion. In addition, it is further believed that the substantially improved strengths exhibited by these alloys is due to the incorporation of the Ni, Co and/or Cu in these alloys into this Al—Si eutectic.
In this regard, the inventive Be Al alloys exhibit a desirably low coefficient of thermal expansion of ≦15.0 μm/m (ppm), more desirably ≦14.8 μm/m (ppm), ≦14.6 μm/m (ppm) or even ≦14.5 μm/m (ppm). In addition, they also exhibit a superior 0.2% yield strength of ≧200 MPa, more desirably ≧220 MPa, and even ≧230 MPa, as well as a superior ultimate tensile strength of ≧250 MPa, more desirably ≧265 MPa, and even ≧280 MPa. This low coefficient of thermal expansion is essentially as good as that exhibited by the above mentioned commercial alloys, while this superior 0.2% yield strength and ultimate tensile strength are better than that exhibited by such alloys. That is to say, preferred Be Al alloys of this invention exhibit 0.2% yield strengths of 220 MPa or higher and ultimate tensile strengths of 265 MPa or higher, which is greater than that of other known Be Al investment casting alloys. In addition, the inventive alloys also generally exhibit a modulus of elasticity of less than 167 GPa @ 25° C. and a density greater than 2.2 gm/cm³.

WORKING EXAMPLES

In order to more thoroughly describe this invention, the following working examples are provided. In each of these working examples, Be Al alloys were made by the general procedure described in the above-noted Grensing et al. patent in which castings of each alloy were made by charging a superheated molten mass of the alloy into a heated mold under suitable vacuum conditions. After cooling and removal from the mold, each alloy was subjected to a series of standard analytical tests to determine its properties.
Five different alloys were tested. Three of these alloys represent the conventional commercially-available alloys mentioned above, these alloys being to referred to as Commercial Alloy A (“CA-A”), Commercial Alloy B (“CA-B”), and Commercial Alloy C (“CA-C”). A fourth recently developed alloy which is the subject of another patent document, which is referred to as Developmental Alloy-D (“DA-D”), was also tested, as was a fifth alloy representing this invention.
The compositions of these alloys as well as the results obtained are set forth in the attached Table 1. For the purposes of comparison, the chemical composition of the alloys shown in Examples V-VII of the above-noted Natchrab et al. patent as well as the chemical composition of the alloy shown in Example 5 of the above-noted Grensing et al. patent are also included in Table 1.

TABLE 1

Composition and Properties of Alloys in Working Examples

	NR	Gr
Unit	Ex 5-7	Ex 5	CA-A	CA-B	CA-C	DA-D	Invent

Composition
Be	wt. %	65	64	61.1-68.6	56-63	61.1-68.6	49.5	49.5
Al	wt. %	31	30	bal (~31)	bal (~47)	bal (~30)	44	41.5
Si	wt. %	2	1.4	1.65-2.50			4.5	3.0
Ag	wt. %	2	1.5	1.65-2.35		2.65-3.35	2.0	3.0
Ni/Co/Cu	wt. %	0.25	3 Ni		2.4-3.2	0.65-1.35		3.0
other	wt. %	0.04	0.1			0.55-0.95	0.04	0.04
		Sr	Ti			Ge	Sr	Sr
Be/Al		2.1	~2.1	~2.1	~1.28	~2.2	1.125	1.20
Si/Al		0.065	0.047	0.05			0.102	0.072
Ag + Si	wt. %	4	2.9	3.30-4.85		2.65-3.35	6.5	6.0
Ag/Si/Ni/Co/Cu	wt. %	4.25	5.9	3.30-4.85	2.4-3.2	3.30-4.70	6.5	9.0
Properties
Density	g/cm³			2.16	2.16	2.16	2.21	2.21
Melt (Liquidus)	° C.			1287	1287	1287	1287	1287
CTE	μm/m			13.4	14.6	14.2	13.6	14.4
Thermal	W/M-			180	110	105.5	186	98.1
Conductivity	° K. @
	30° C.
Modulus of	GPa			202	202	202	167	167
Elasticity in	@
Tension	25° C.
Specific				93.5	93.5	93.5	75.5	75.5
Stiffness
0.2% YS	MPa			137.9	151.7	200	145	235
UTS	MPa			196.5	200	255	200	290
% Elongation	% @			1.7	5.4	3.4	3.0	2.0
	25° C.

From Table 1, it can be seen that the amount of Be in the inventive Be Al alloy is about 23% less than the commercial alloys containing Si (including the Si-containing alloys of the Natchab et al. and Grensing et al. patents) and over 17% less than the commercial alloy containing no Si or Ag. Since Be is expensive, this means that the inventive alloy is significantly less expensive to make than these commercial alloys.
From Table 1, it can also be seen that the inventive alloy has mechanical properties such as % Elongation and coefficient of thermal expansion comparable to that of the commercial alloys. In terms of 0.2% Yield Strength and Ultimate Tensile Strength, however, the inventive alloy is superior in that its 0.2% Yield Strength and Ultimate Tensile Strength are substantially better than that of Commercial Alloys A, B and C as well as Developmental Alloy D.
These data show that, by formulating the inventive alloys as described herein, not only is it possible to substantially reduce the Be content of the alloys obtained, but in addition it is also possible to do so in a way which preserves the investment casting properties of the alloys while substantially improving their strength at the same time.
Although only a few embodiments of this invention have been described above, it should be appreciated that many modifications can be made without departing from the spirit and scope of this invention. All such modifications are intended to be included within the scope of this invention, which is to be limited only by the following claims.

Claims

1. A Be Al alloy suitable for investment casting purposes, the alloy comprising about 40 to 55 wt. % Be, about 1.3 to 7.0 wt. % Ag, about 1.5-7 wt. % Si, about 0.5-6.0 wt. % of Ni, Co, Cu or mixtures thereof, and no more than about 3 wt. % optional ingredients, with the balance being Al plus incidental impurities, wherein the Be/Al ratio of the alloy is <˜2.0, the combined amounts of Si and Ag in the alloy is >4 wt. %, the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜6 wt. %, and further wherein the alloy has a coefficient of thermal expansion of ≦15 μm/m (ppm), a 0.2% yield strength of ≧200 MPa and an ultimate tensile strength of ≧250 MPa.

2. The alloy of claim 1, wherein the Be/Al ratio is ≦˜1.6.

3. The alloy of claim 2, wherein the Be/Al ratio is ≦˜1.4.

4. The alloy of claim 1, wherein the combined amounts of Ag and Si in the alloy is ≧˜5.0 wt. %

5. The alloy of claim 4, wherein the combined amounts of Ag and Si in the alloy is ≧˜5.8 wt. %.

6. The alloy of claim 1, wherein the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜7.0 wt. %.

7. The alloy of claim 6, wherein the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜8.0 wt. %.

8. The alloy of claim 1, wherein the alloy has a 0.2% yield strength of ≧220 MPa and an ultimate tensile strength of ≧265 MPa.

9. The alloy of claim 8, wherein the alloy has a 0.2% yield strength of ≧230 MPa and an ultimate tensile strength of ≧280 MPa.

10. The alloy of claim 1, wherein the alloy contains about 45 to 55 wt. % Be, about 2.0-7.0 wt. % Si, about 1.5 to 6.0 wt. % Ag, and no more than about 1.5 wt. % optional ingredients, about 1-5 wt. % of Ni, Co, Cu or mixtures thereof, with the balance being Al plus incidental impurities, wherein the Be/Al ratio is ≦˜1.6., the combined amounts of Si and Ag is ≧˜5.0 wt. %, the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜7 wt. %, and further wherein the alloy has a 0.2% yield strength of ≧220 MPa and an ultimate tensile strength of ≧265 MPa.

11. The alloy of claim 10, wherein the alloy contains ˜47.5 to <˜50 wt. % Be, about 2.5-6.0 wt. % Si, about 2-5 wt. % Ag, about 2-4 wt. % of Ni, Co, Cu or mixtures thereof, and no more than about 0.5 wt. % optional ingredients, with the balance being Al plus incidental impurities, wherein the combined amounts of Si and Ag is ≧˜5.8 wt. %, the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜8 wt. %, and further wherein the alloy has a 0.2% yield strength of ≧230 MPa and an ultimate tensile strength of ≧280 MPa.

12. The alloy of claim 1, wherein the alloy contains about 0.01 to 0.1 wt. % strontium, sodium, calcium, antimony or mixtures thereof.

13. The alloy of claim 1, wherein the alloy contains about 0.02 to 0.06 wt. % strontium.

14. The alloy of claim 1, wherein the alloy contains less than about 50 wt. % Be.