Establishing process, design parameters for permanent mold cast lead-free copper alloys.
The problem, however, is that these improved parameters have never been documented. In addition, with lead-free copper-base alloys emerging as a possible material choice for customers, the lack of process and design parameters becomes an even bigger issue.
Two research programs for permanent mold cast lead-free copper-base alloys--one on design parameters and the other on process parameters--were initiated in partnership with MS and the U.S. Dept. of Energy. The design parameters project investigated tensile properties, fatigue, fracture and impact toughness, and wear and corrosion resistance of 14 lead-free copper-base alloys. In addition, the project evaluated patternmakers' shrinkage and taper allowances.
The second project on process parameters focused attention on mold materials, gating design, mold coating, casting fluidity and alloy development when cast in lead-free copper-base alloys.
This article discusses findings from these two projects that can be used to establish design and process parameters.
Mechanical Properties--A comprehensive engineering database on tensile, impact, fatigue and fracture toughness properties has been recorded for 14 different copper alloys. The alloys covered are from various families including:
* aluminum-bronzes (G95200, G95300, G95400, C95500 and C95800);
* yellow brass (C85800);
* high-strength yellow brass (C86300);
* silicon-brass (C87500);
* manganese-yellow brasses (C99700 and C99750);
* high-copper alloys (C80100, C81500 and C82500).
Some limited data on tensile properties also are available for silicon bronze (C87600).
The results of the range of composition and properties (Tables 1 and 2) show the properties of these alloys are strongly dependent on chemical composition. The nominal composition did not always provide the best combination of strength and ductility. In order to achieve optimum properties for a given application, a narrower composition range than in the current specifications should be targeted, especially for those elements that have been shown to have the greatest effect on properties.
Specifically, high ultimate tensile strength and yield strength--0.2% offset and 0.5% extension--were observed with significant reduction in ductility (% elongation) at higher aluminum levels for the aluminum-bronzes. A significant reduction in ductility (% elongation), similar to that observed for the aluminum-bronzes, was observed at high zinc levels for the high-zinc yellow brass, high-strength yellow brass and high-manganese brasses. Adding chromium (alloy C81500) and beryllium (alloy C82500) to pure copper significantly improved the tensile properties. In addition, the fracture toughness and impact energy are both alloy and chemical composition dependent, similar to the tensile properties.
Wear Properties--Sliding the specimen against standard steel rings in the lubricated block-on-ring test configuration was not sensitive enough to discriminate between the alloys (G95500, G95800 and C86300) evaluated. Better discrimination between the alloys was achieved with the unlubricated contact and combined rolling/sliding action where the weight loss data show that permanent mold cast samples exhibited better wear resistance for aluminum-bronze alloys C95500 and G95800 in comparison with sand cast alloys.
The slurry jet impingement test results at the 900 impingement angle did not discriminate between the three alloys. On the other hand, alloy G86300 exhibited slightly better erosion resistance than aluminum-bronzes C95500 and G95800 in the 20[degrees] slurry jet impingement angle tests. Similarly, based on the weight loss data in Coriolis tests, alloy C86300 exhibited better erosion resistance than aluminum-bronze alloys C95500 and G95800.
Corrosion Behavior-The main objective was to determine the overall corrosion behavior of permanent mold and sand-cast copper-base alloys. The potentiodynamic polarization method and the standard American Society for Testing and Materials (ASTM) salt spray test were used to compare the kinetics and morphology of attack for nine different alloys.
The corrosion resistance of all the copper-base alloys in the permanent mold cast condition is better than in the sand cast condition. Long-term immersion tests in a salt spray chamber indicated that sand cast alloys are more prone to selective dissolution leading to localized pitting, resulting in a more porous surface.
Based on results of the electrochemical polarization and salt fog tests, high manganese brass (C99700) is the most corrosion-resistant alloy and high strength yellow brass (C86300) and aluminum bronze (C95400) are the least resistant to corrosion. The nickel-aluminum bronze alloy (C95800) appears to be a general purpose alloy for all the test environments.
In the permanent mold casting process, mold life is a major consideration since mold design and machining are expensive. Mold life can be reduced due to heat checking (also known as thermal fatigue), cleavage fracture across die segments, mold erosion due to metal flow, and chemical attack or corrosion. Thermal cycling, caused by the fluctuation of the mold surface temperature between the casting and cooling periods, is the dominant factor for mold failure.
The conventional mold material for copper-base permanent mold casting is H13 tool steel and beryllium-copper. Both conventional and new potential mold materials were evaluated in this investigation; these include cast iron, H13 tool steel, beryllium-copper, beryllium-nickel, Nibryl, nickel-aluminide ([Ni.sub.3]Al),iron-aluminide ([Fe.sub.3]A1), and low carbon steel. The ranking of these materials is as follows.
beryllium-nickel > beryllium-copper > Nibryl > nickel-aluminide > H13 tool steel > cast iron, iron-aluminide and low carbon steel
Cast beryllium-nickel is the most promising material because it did not develop any cracks after 6000 cycles. Beryllium-copper and rolled beryllium-nickel show minor cracking at the notch but passed 6000 cycles making them superior to other materials investigated.
Three other materials, namely Nibryl, nickel-aluminide and H13 tool steel, possess excellent crack resistance but are susceptible to fine multiple surface cracking. Cast iron, iron-aluminide and low carbon steel are the least resistant materials to thermal fatigue.
Taper, Shrinkage Allowances
The shrinkage (for patterns) and taper (for cores) allowances required during permanent mold casting of lead free copper alloys are unknown. In this research, shrinkage was measured using a plate casting. A cylinder casting was used to measure the taper allowance. The two castings are shown in Fig. 1.
Five alloys--pure copper (C80100), aluminum-bronze (C95400), high-zinc yellow brass (C85800), silicon-brass (C87500) and silicon-bronze (C87600)--were evaluated using these two castings. The findings are summarized as follows:
* The five alloys have slightly different shrinkage allowance requirements. The required shrinkage allowance ranged from 0.1851 in./ft for the silicon-bronze (C87600) to 0.2521 in./ft for pure copper (C80100). These results can be compared with the patternmakers' shrinkage allowance for sand casting listed in the Copper Development Assn. Standards Handbook.
* The thermal expansion of the mold cavity during pouring was not sufficient enough to eliminate the need to incorporate shrinkage allowance in the casting dimensions during permanent mold design.
* The cylinder casting results show that the core taper necessary to facilitate casting ejection for the five alloys studied should be greater than 1.5[degree]. These results indicate that the thermal expansion of the metal core can force the casting to the mold wall and thus reduce the effectiveness of the core taper. The use of a core material with a lower coefficient of thermal expansion than the mold material possibly could alleviate this problem.
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About the Authors
M. Sadayappan, James Thomson, Festus A. Fasoyinu and Mahi Sahoo are all part of the Materials Technology Laboratory (MTL)/CANMET, Ottawa, Ontario, Canada, which is part of Natural Resources Canada. In collaboration with industry, MTL conducts applied research, and develops and deploys technologies, to improve all aspects of producing and using value-added products from minerals and metals.
For More Information
"Thermal Fatigue of Mold Materials for Permanent Mold Casting of Copper-Base Alloys," Proceedings of the International Workshop on Permanent Mold Casting of Copper-Base Alloys, Materials Technology Laboratory, Ottawa, Ontario Canada (1998).
"Tensile Properties and Fracture Toughness of Some Permanent Mold Cast Alloys," 1999 AFS Transactions, American Foundry Society, Des Plaines, IL.
"Studies on Fluid Flow in Permanent Mold Casting of Copper-Base Alloys Using Water Modeling," 2001 AFS Transactions.
See various papers from the team at the Materials Technology Laboratory:
"Permanent Mold Casting of High Phosphorous Brass," 2001 AFS Transactions.
Table 1 Composition of the Cu Alloys Prepared to Evaluate Mechanical Properties Alloy Zn Al Fe Ni Mn Aluminum-Bronzes C95200 8.7-9.5 3.1-3.4 C95300 8.8-11.5 0.3-1.3 C95400 10.1-10.7 3.6-4.3 0.4-1.7 0.4-0.6 C95500 9.9-11.1 3.8-4.2 3.9-4.5 1.0-1.6 C95800 8.6-9.7 3.3-4.5 4.3-5.0 0.9-1.3 High Coppers C80100 C81500 C82500 0.03 0.28 Be: 2.0 Brasses C85800 36.5-40.0 0.3-0.4 C86300 23.5-27.0 5.2-6.4 1.4-3.2 1.8-4.1 C87500 13.4-15.2 0.4-0.45 C99700 21.3-25.0 1.5-1.75 2.3-8.8 13.2-13.8 C99750 18.0-25.0 1.5-1.75 13.0-22.2 Silicon-Bronze C87600 6.3 0.4 0.1 Alloy Si Other Aluminum-Bronzes C95200 C95300 C95400 C95500 C95800 High Coppers C80100 boron<0.008 P<0.001 C81500 chromium: 0.4-1.25 C82500 cobalt: 0.4 Brasses C85800 tin: 0.7-1.2 C86300 C87500 3.1-4.1 C99700 C99750 Silicon-Bronze C87600 4.5
"Comparative Corrosion Resistance of Selected Copper-Base Alloys Cast in Permanent and Green Sand Molds," 2001 AFS Transactions.
Table 2 Mechanical Properties of Selected Permanent Mold Cast Copper Alloys Alloy Ultimate Tensile 0.5% Yield Elongation Strength (Mpa) Strength (Mpa) (%) Aluminum-Bronzes C95200 646-680 252-299 15-33 C95300 511-670 188-287 2-47 C95400 587-834 274-344 2-13 C95500 488-842 364-461 3 C95800 593-842 315-471 3-17 High Coppers C80100 180-203 33-70 18-44 C81500 180-347 50-190 15-33 C82500 603-727 425-479 4-6 Brasses C85800 415-486 221-267 8-27 C86300 687-848 312-507 3-8 C87500 471-603 188-300 19-28 C99700 493-673 350-468 0.4-15 C99750 484-631 231-473 0.4-24 Silicon-Bronze C87600 488 242 17 Alloy Hardness Toughness Impact (BHN) [J (KJ/sq m)] Engergy [E (J)] Aluminum-Bronzes C95200 79-86 50-188 22-78 C95300 66-98.4 170 33-146 (one melt 21.4 RC) C95400 94-102 15-41 33-146 C95500 25-30 RC 18-29 12-16 (one melt: 104 HRB) C95800 84.6-95.5 84-125 22-63 (one melt: 22.5 RC) High Coppers C80100 15-55 -- -- C81500 13-54 3-8 11-173 C82500 88-90 91 70 Brasses C85800 64-77 151-266 61-116 C86300 21-22RC 78 32-38 C87500 67-85 177 57-145 C99700 81-93 17-132 9-81 C99750 67-100 133-157 2-58 Silicon-Bronze C87600 79 -- --
Alleviating the Zinc Oxide Problem
Copper alloys containing zinc suffer from zinc-oxide formation during melting and casting. Zinc has a high vapor pressure in molten copper and is readily lost by evaporation and oxidation. This requires frequent compensation by adding extra zinc.
Permanent mold casting of high-zinc yellow brasses calls for attention to the "zinc-oxide deposition" since it could accumulate on the mold and reduce the molten metal fluidity and surface quality of the castings. The current practice of dipping the mold halves in a water/graphite slurry to cool the molds has been beneficial in washing, off zinc oxide deposits. After a period of time, however, the accumulation can be too high and manual cleaning using wire brushes becomes necessary.
A method was developed to measure the zinc-oxide deposit on the mold surface. The effects of various minor alloy additions were evaluated. Aluminum improved the fluidity of the alloy but its effect on reducing the zinc-oxide deposit was only marginal. Magnesium alone reduced the zinc-oxide deposit significantly, although it adversely affected fluidity. A combination of magnesium (0.1%) and aluminum (0.4%) in yellow brass was found to be beneficial in reducing zinc oxide deposition on the molds without compromising the casting fluidity.
Casting Lead-Free High Phosphorus Brass
Leaded yellow brass (C85800 containing 1-5% lead) is a common plumbing alloy for permanent mold casting. Alloy C89550 (EnviroBrass III and SeBiLOY III) is the lead-free substitute for it. Both of these alloys are cast at 1742-1832F (950-1000C).
Since one of the main considerations in permanent mold casting of copper-base alloys is mold life, research was performed to develop a lead-free alloy that can be cast at 1650F (900C) or lower in an attempt to increase mold life. Results from testing developed the following composition as optimal: 25% zinc, 3-4% phosphorous, 3.5-4.5% nickel, 0.3% aluminum and the balance copper. Table 3 compares the properties of this new high phosphorous brass, SeBiLOY III, and yellow brass.
With further research, high phosphorous brass could be an excellent alloy for the plumbing industry with comparable properties to yellow brass and SeBiLOY III with a lower pouring temperature.
Table 3 A Comparison of the Properties of the New High Phosphorous Brass, SeBiLOY III and Yellow Brass High Phosphorous Brass SeBiLOY III Yellow Brass Liquidus (C) 840 899 899 Fluidity (C) 50 150 100 Hot Tear (C) 125 168 169 Machinability (%) 15/20 54/97 -/80 Ultimate Tensile Strength (Mpa) 354-400 330 330 Yield Strength (Mpa) 238-257 200 192 Elongation (%) 2.2-4.5 8 9
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|Date:||Feb 1, 2002|
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