Alloys or metallic compositions – Aluminum base – Copper containing
Reexamination Certificate
2000-05-23
2001-11-13
King, Roy (Department: 1742)
Alloys or metallic compositions
Aluminum base
Copper containing
C420S554000
Reexamination Certificate
active
06315947
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to free-machining alloys and, in particular, to free-machining aluminum alloys which contain bismuth and indium.
BACKGROUND ART
Free-machining aluminum alloys are well known in the art. These alloys typically include free-machining constituents such as lead, tin, indium and bismuth for improved machinability. These constituents form low melting point compounds which readily melt or soften due to the friction heat created during machining. Thus, material removal required for the manufacture of complex parts and components is easily facilitated.
During machining, free-machining alloys generate small chips or curls which are easily collected and do not interfere with the machining process. It is essential that these free-machining aluminum alloys form these small chips or curls for proper machining. Formation of long continuous strips or curls is totally unacceptable in machining since the curls or strips may wrap around the work piece or machining tool and disrupt the operation. Poor machinability also affects other machining operations since the operator must attend to a single machining operation and cannot effectively supervise a multiplicity of operations, as is commonly done in practice. AA6061 alloys are generally unacceptable for machining since they form these long continuous curls during machining.
U.S. Pat. Nos. 2,026,457 and 2,026,575 to Kempf et al. disclose free cutting aluminum alloys. Similarly, U.S. Pat. No. 4,005,243 to Baba et al. discloses a freely machinable aluminum alloy.
Other known machinable alloys include AA6262 and AA2011, 2012 and 2111.
While the prior art aluminum alloys provide adequate free machinability, they are not without drawbacks and/or disadvantages. For example, AA6262 contains lead and chips from machining this alloy represent a hazardous waste disposal problem, in addition to the possibility for exposure to fine lead particles during machining.
Prior art alloys containing bismuth, e.g., AA2011 and AA2111, can adversely affect the final mechanical properties of the machined part. Since bismuth has some affinity for magnesium, the bismuth in these alloys has a tendency to combine with the magnesium to prevent or reduce Mg
2
Si formation potential for precipitation strengthening. Bismuth also has a poor affinity for tin, and alloys having these two components may not always form the desired low melting point compounds or structures for free machining.
As a solution to the problems identified above, the inventor has proposed free-machining aluminum alloys containing tin and indium as a means to eliminate both lead and bismuth as constituents in free-machining alloys. This alloy system is disclosed in U.S. Pat. No. 5,587,029, titled “Machineable Aluminum Alloys Containing In and Sn and Process for Producing the Same”, issued Dec. 24, 1996 which is herein incorporated by reference in its entirety.
Although free-machining alloys containing the aforementioned indium and tin provide excellent machining properties, the levels of indium when combined with a high price for the source indium make the alloys somewhat unattractive from an economical standpoint.
Other free machining alloys suffer from inadequate Charpy V-notch impact strength. For example, AA6020 which has tin and high levels of silicon, instead of the lead and bismuth in A6262, exhibits poor impact properties. Alloys with low impact properties have a tendency to crack when deep drilled.
As such, a need has developed to provide an environmentally friendly free-machining alloy which does not have its final mechanical properties compromised by free-machining constituents therein, has good impact properties and which is even more economically attractive.
In some applications involving hot automotive brake fluid, alloys containing Sn have shown to be extremely poor in corrosion resistance. Hence, this new alloy is also aimed at removing Sn from alloys that require exposure to hot brake fluid.
SUMMARY OF THE INVENTION
Accordingly, it is a first object of the present invention to provide a free-machining aluminum alloy which eliminates lead and its adverse effects on the environment during machining chip disposal.
Another object of the present invention is to provide a free-machining aluminum alloy containing bismuth and indium which has at least comparable free-machining properties as prior art alloys.
Another object of the present invention is to provide an economically attractive free-machining alloy. A still further object of the present invention is to provide a method of machining using a lead free free-machining alloy which utilizes bismuth and indium as a low melting point compound for machinability.
A yet another object of the invention is to provide a free-machining alloy having improved impact properties.
A still further object is to provide an alloy having improved resistance to corrosion when exposed to hot brake fluid.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides an improvement over the prior art free-machining alloys containing low melting point free-machining constituents. According to the invention, an effective amount of bismuth and indium is utilized in these types of alloys as free-machining constituents, i.e., low melting point compounds (eutectics). It should be appreciated that the terms “bismuth” and “tin”, as used herein, are not limited to pure elements but also include elements having incidental impurities of the types associated with commercially available alloying elements.
The effective amounts of bismuth and indium can be added to alloy chemistries typical of free-machining alloys such as aluminum-base alloys, e.g., AA6000 or AA2000 series alloys or other alloys, ferrous or non-ferrous. The effective amounts are such that the bismuth and indium form the low melting point compounds in an amount which, when dispersed throughout the alloy shape being machined, generate chips rather than curls or stringers during machining. The free-machining alloying constituents can range, in vol. %, up to 1.0 and, preferably, up to 0.5. The lower limit, in some cases, can be as low as 0.01 vol. %. In other cases, the lower limit may be 0.2 or 0.3 vol. %. While amounts greater than 1.0 vol. % might increase machinability, the improvement in machinability might have an unacceptable impact on alloy properties. Similarly, the lower limit is a function of the desired improvement in machinability. If the amount is too low, there will be an insufficient number or dispersion of the low melting point constituents to have any significant impact on machinability.
Preferably, in terms of weight percent, the amounts of Bi and In are added to a selected alloy so that their respective weight percentages in the selected alloy range between about 0.10 to 1.5 Bi and about 0.01 to 0.30 In.
More preferably, the present invention discloses a free-machining alloy wherein the bismuth ranges between 0.30-1.0 wt. %, and the indium ranges between 0.03 and 0.11 wt. %. In one mode, the bismuth and indium can be provided in a eutectic ratio, i.e., 33 wt. % In-67 wt. % Bi.
In terms of the type of binary alloys, preferred binary alloys for addition to a given base alloy include, in wt. %, 80 Bi- 20 In, 85 Bi-15 In, 90 Bi-10 In, and 95 Bi-5 In. Binary alloys with increasing amounts of indium can also be selected but are not as preferred due to the cost of indium. Indium and bismuth may be added to the molten base alloy as individual elements or as an alloyed constituent (master alloy) in the desired ratio.
The bismuth and indium can be added as substitutes for the free-machining constituents in AA6262 and AA2111 free machining aluminum alloys. In addition, they may be added to other alloys to improve machinability.
REFERENCES:
patent: 3753695 (1973-08-01), Lloyd
patent: 4005243 (1977-01-01), Baba et al.
patent: 4244756 (1981-01-01), Tanabe et al.
patent: 4471032 (1984-09-01), Fukuoka et al.
patent: 4590
Combs-Morillo Janelle
King Roy
Reynolds Metals Company
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