Specialized metallurgical processes – compositions for use therei – Compositions – Solid treating composition for liquid metal or charge
Reexamination Certificate
1998-12-18
2001-04-03
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Solid treating composition for liquid metal or charge
C075S315000, C420S590000, C420S029000, C420S415000, C420S546000, C420S549000
Reexamination Certificate
active
06210460
ABSTRACT:
TECHNICAL FIELD
This invention relates to aluminum-strontium alloys for use primarily in modifying the eutectic phase in aluminum-silicon casting alloys or modifying intermetallic phases in wrought aluminum alloys. The aluminum-strontium alloys are also useful as inoculants for gray and ductile iron.
BACKGROUND ART
Because of their excellent fluidity and castability, eutectic and hypoeutectic aluminum-silicon alloys are widely used in the production of aluminum castings. In an unmodified state, the eutectic silicon phase is present as coarse plates with sharp sides and ends often referred to as acicular silicon. The presence of acicular silicon results in castings which have low percent elongation, low impact properties and poor machinability.
Strontium has been shown to be effective in refining or modifying coarse acicular silicon into a fine, interconnected fibrous structure. In general, small quantities of strontium between 100 to 200 ppm are sufficient to produce a fine, fibrous eutectic silicon which in turn significantly improves the mechanical properties and machining characteristics of the aluminum casting. U.S. Pat. No. 3,466,170 issued Sep. 9, 1969 to Dunkel et al. recognizes the benefit of adding strontium either as a pure metal or as an AlSr alloy with 7 net percent Sr.
Because strontium metal is very reactive with oxygen, nitrogen and moisture, its use as a modifying agent is limited. In most cases, strontium is added in the form of a master alloy.
The publication by Pekguleryuz et al., “Conditions for strontium master alloy addition to A356 melts”, Trans. Am. Foundrymen's Soc. (1989) considers the use of a 55 wt. % Sr 45 wt. % Al alloy as a master alloy to modify aluminum-silicon alloys. This alloy largely comprises the intermetallics Al4Sr and Al2Sr. This document does not, however, disclose the alloy being in the form of granules or powder.
U.S. Pat. No. 3,567,429 issued Mar. 2, 1971 to Dunkel et al. teaches the use of a strontium silicon-aluminum master alloy which has a strontium content higher than 7%. Strontium-silicon aluminum master alloys are no longer widely used for modifying aluminum-silicon casting alloys, since in most cases the strontium is present as a high melting temperature intermetallic phase such as Al
2
Sr
2
Si or SrSi
2
which dissolves very slowly at molten aluminum processing temperatures, typically 760° C. or lower. As reported by John E. Gruzleski and Bernard M. Closset (“The Treatment of Liquid Aluminum-Silicon Alloys”, American Foundrymen's Society Inc., 1990, pages 31-39), a 10% strontium-aluminum binary master alloy dissolves twice as fast in a A356 aluminum-silicon casting alloy as a 10% strontium-14% silicon-aluminum ternary master alloy at all melt temperatures ranging between 670 to 775° C. Similar results are found in U.S. Pat. No. 5,045,110, issued Sep. 3, 1991 to Vader et al., reporting dissolution times between 20 and 30 minutes for 10% strontium-14% silicon-aluminum master alloys in ingot form. In contrast, U.S. Pat. No. 4,576,791, discussed below, teaches that 5-10% strontium-aluminum binary alloys in rod form and which contain titanium and boron grain refiners dissolve in 1 minute. In addition, the customary process used to produce strontium-silicon master alloys results in substantial quantities of detrimental impurities including iron, barium and calcium often being present in the master alloy.
U.S. Pat. No. 4,108,646 teaches the use of a master composition consisting of strontium-silicon in particulate form pressed into a briquette with aluminum or aluminum-silicon particles. The briquettes, having a master composition of between 3 to 37% strontium by weight, are then added to an aluminum-silicon casting alloy to modify its structure. This master composition is less efficient than aluminum-strontium binary master alloys since the strontium is present as SrSi
2
particles which, as discussed above, dissolve slowly and contain detrimental impurities including up to 4% iron and 1 to 3% calcium.
Aluminum-strontium binary alloys are now widely used for modifying aluminum castings; however, it has been difficult to increase the strontium content of these binary master alloys. This is best explained in the context of the aluminum-strontium binary equilibrium phase diagram of FIG.
1
. The phase diagram contains two low melting point eutectics, one at about 3.5% strontium, the second at 90% strontium. On the aluminum rich side, the eutectic containing alloys range from about 0% to 44% strontium. On the strontium rich side, the eutectic containing alloys range from about 77% to 100% strontium. In the final solidified state, these eutectic alloys contain in varying proportions a eutectic phase which is very finely divided and melts at low temperatures, 654° C. in the case of the aluminum rich eutectic and 580° C. for the strontium rich eutectic. These finely divided eutectic phases are more ductile and dissolve more rapidly than the higher melting point intermetallic alloy phases which are present between about 44% to 77% strontium. Since these intermetallic alloys contain no low melting point, finely divided eutectic phase, they are more brittle and dissolve much more slowly than the eutectic containing alloys. The presence of these high melting point intermetallics alloys has placed a significant limitation on the amount of strontium which can be effectively contained in commercial aluminum-strontium binary master alloys. In this specification, the term “intermetallic alloys” denotes alloys containing between approximately 40% to 81% strontium by weight. These alloys are dominated by the Al
4
Sr, Al
2
Sr and AlSr intermetallics and contain only minimal or no eutectic phase.
As discussed in “Phase Diagrams for Ceramists” compiled by the National Bureau of Standards, published by The American Ceramic Society Inc., Volume 1, pages 9-14,
FIG. 1
as a binary equilibrium phase diagram shows the relationships between composition and temperature assuming all phases are in equilibrium with each other. These compositional relationships are only valid if the rate of solidification is slow enough to allow the phases to reach compositional equilibrium at every instant. A more rapid rate of solidification will lead to quite different compositional results.
As shown in
FIG. 1
, when a liquid alloy containing 10 % strontium is cooled, solidification begins at about 815° C. The first solid phase to precipitate is primary Al
4
Sr intermetallic which contains approximately 44% strontium. As the melt temperature continues to decrease during solidification, more and more of this primary Al
4
Sr intermetallic phase precipitates. The primary Al
4
Sr intermetallic phase is present as massive interconnected plates or needles which are shown two-dimensionally in the photomicrograph given in
FIG. 2. A
three-dimensional view of the interconnected network of primary Al
4
Sr plates is shown by
FIG. 3
taken using a stereomicroscope.
When the melt temperature cools to 654° C., the primary Al
4
Sr intermetallic phase stops precipitating and the remaining amount of liquid alloy solidifies as a very finely divided, ductile eutectic phase. The eutectic phase is shown in
FIG. 2
by the light regions surrounding the large primary Al
4
Sr needles. The eutectic phase is much more finely divided than the Al
4
Sr intermetallic phase as evidenced by the lack of resolution of the eutectic phase at 50 times magnification.
The quantity of primary intermetallic Al
4
Sr phase present in the final solidified alloy will depend on the rate at which freezing took place between 815° C. to 654° C. If the alloy were allowed to freeze very slowly so that equilibrium is achieved at each instant of cooling, then the quantity of primary Al
4
Sr intermetallic phase in the final alloy will be given from the equilibrium phase diagram in
FIG. 1
using the lever rule, that is for a 10% strontium alloy
%
Primary
Al
4
Sr
Phase
in
Final
Alloy
=
(
10
%
Kulunk Bahadir
Zuliani Douglas J.
Andrews Melvyn
Nixon & Vanderhye
Timminco Limited
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