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New Alloys "Get the Lead Out" of Free-Machining Aluminum
12-27-02
Lead and bismuth additions to aluminum alloys were used for decades to produce improved machinability, also called "free-machining", characteristics. With environmental concerns over the disposal of lead-containing chips, significant efforts by aluminum producers to offer highly machinable alloys substantially free of lead were undertaken. The results of those efforts have been the development of alternative alloys along with an increased understanding of their metallurgy.
Machinability in aluminum alloys is rated on an A-E letter grade scale related to the size of machining chips formed and the resulting machined surface finish. "A" rated machinability is the best, and is defined as "'free-cutting, very small chips, and excellent finish", while "B" is "curled or easily broken chips and good-to-excellent surface finish." Lower grades are characterized by increasingly continuous chips and decreasing finish quality. Machinability varies significantly from alloy to alloy and even for various tempers for a given alloy, based on factors such as alloy composition and temper along with second phase particle type and volume fraction. (More specifics on the machinability of aluminum alloys and the rating system can be found in the Aluminum Association publication Forming and Machining Aluminum Alloys.)
Good machinability is important for producing machined parts cost-effectively without the need for subsequent finishing operations to remove burrs and other machining features if possible. To this end, two alloys have previously been used to meet the need for free-machining alloy stock. 2011 alloy, registered in 1954, is a 2xxx alloy with additions of 0.5% Pb and 0.5% Bi. These elements form low-melting point eutectic phases that partially melt as a result of the heat generated during machining, expand, and facilitate the formation of small chips at the tool/workpiece interface. This alloy has an "A" rating for machinability. Another free-machining alloy, 6262, was registered in 1960. Essentially 6061 alloy plus the same nominal addition levels of Pb and Bi, this alloy was developed to provide improved corrosion resistance, brazeability, weldability, and anodizing response compared to 2011. Due to differences in the microstructure in 6262 with respect to the low-melting phases formed by the Pb and Bi additions compared to 2011, the former has somewhat inferior machinability and is rated "B".
Due to environmental concerns related to the Pb present in the free-machining alloys, a number of parallel development efforts were carried out in the 1990's to find a replacement for the Pb-containing 2011 and 6262 alloys.
One approach taken by Kobe Steel was to investigate the potential of adjusting the levels of relatively conventional alloying additions in order to improve machinability. Specifically, Si, Mn, Fe, and Ni additions were considered, with Si showing the most promise in producing an alloy with machinability comparable to the "B" level of 6262. Si-containing variants of 2017 and 6061, designated KE2 and KE6 respectively, were established.
Evaluating alternative alloying additions to form low-melting point eutectics in 6xxx alloys was the route pursued by three other aluminum producers. Kaiser markets a lead-free version of 6262 alloy with the internal designation of KA62 in which tin (Sn) has been substituted for Pb. The resulting alloy has physical and mechanical properties as well as "B" rated machinability comparable to 6262.
Reynolds also examined alternative alloying additions, focusing on substituting indium (In) plus Sn for the Pb and Bi in the 6262 alloy base composition. The results of this work, summarized in a paper published in 1996 (S. Sircar, Materials Science Forum, Vols. 217-222, 1996, pp. 1795-1800) showed that these elements produced the desired low-melting point eutectic phases and somewhat improved machinability over 6262. Alloy X6030 was registered in 1995 with In + Sn additions, although it is now in the process of being deactivated. Further development efforts documented in patents issued in 1998 and 2001 involved combinations of Bi, In, and Sn and Bi + In, respectively. The latter development was focused on producing an alloy with a good balance of machinability and high impact strength. No specific alloys have been registered as a result of this work.
Alcoa also developed a lead-free alternative alloy. In their work, they revisited not only the selection of the alloying additives that would form a low-melting eutectic, but also considered modifications to the base alloy composition in order to obtain the desired balance of properties. They chose to replace the Pb + Bi addition with Sn at a nominal level of 1.1%, which formed low-melting beta-Sn and Mg2Sn phases. Furthermore, they increased the Cu level in the base alloy to produce further strengthening. The patented composition range was registered in 1995 as 6020 alloy. Machinability studies of this alloy have shown it to have an "A" rating, comparable to 2011.
Other Pb-free free-machining alloys registered with the Aluminum Association include 6012A, 2111, and 2012, all containing Bi + Sn additions. A detailed listing of the compositions of the alloys discussed in this article is available in the Aluminum Association publication International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys, available from the Aluminum Association Bookstore online at www.aluminum.org.
Article provided courtesy of The Aluminum Association
www.aluminum.org
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