Design Parameters for Lead Free Copper Based Engineering Alloys in Permanent Molds
The lack of adequate data on permanent-mold design and mechanical properties is an impediment to the acceptance of the permanent-mold casting of copper alloys - especially in applications where their inherent corrosion resistance, and high thermal and electrical conductivities are superior to other engineering alloys.
This report summarizes the results of a three-year study on the mechanical, impact, and fatigue properties and fracture toughness of 13 copper alloys (aluminum bronzes [C95200, C95300, C95400, C95500, and C95800], yellow brass [C85800], high-strength yellow brass [C86300], silicon brass [C87500], manganese yellow brasses [C99700 and C99750] and the high-copper alloys [C80100, C81500, and C82500]). Some limited data on tensile properties are also presented for silicon bronze (alloy C87600). For the first time, comprehensive data on mechanical, impact, fatigue and fracture toughness properties, as well as wear and corrosion (selected alloys from the group) properties, are available for these alloys.
The results of this study show that the mechanical, impact properties and fracture toughness of these alloys are strongly dependent on the chemical composition. The nominal composition did not always provide the best combination of strength and ductility. The fatigue properties were dependent on both the stress level and the chemical composition. 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 UTS and YS (0.2% offset and 0.5% extension) were observed with significant reduction in ductility (% elongation) at higher aluminum levels for the aluminum bronzes. Significant improvements in the ductility were observed when the aluminum levels were close to the lower end specified in ASTM B806-93a.
A significant reduction in ductility (% elongation), similar to that observed for the aluminum bronzes, was observed at high zinc levels for the high-Zn 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. The fatigue properties of the alloys are sensitive to both applied stress and chemical composition.
The erosion resistance of the aluminum bronzes (C95400 and C95500) and high-strength yellow brass (C86300) was investigated using slurry jet tests at 90E and 20E impingement angles, and the Coriolis tests. The high-strength yellow brass (C86300) shows a slightly higher erosion resistance than the aluminum bronzes (C95400 and C95500) in both the 20E slurry jet impingement and Coriolis tests.
The corrosion resistance of high-copper alloys (C80100 and C81500), aluminum bronzes (C95400 and C95800), yellow brass (C85800), high-strength yellow brass (C86300), high-manganese brass (C99700), silicon brass (C87500) and silicon bronze (C87600) cast in permanent and green-sand molds has been determined using the standard ASTM salt-spray test in CO2- and SO2-enriched test environments at 25EC and 50EC. A potentiodynamic polarization test in NaOH and KH2PO4 solution was also performed. The results show that copper-base alloys cast in permanent molds exhibit better corrosion resistance than those cast in green-sand molds. Long-term immersion tests in a salt spray chamber indicated that sand-cast alloys are more susceptible to pitting corrosion than permanent-mold cast samples. The SO2 test environment was the most aggressive for all the alloys evaluated. Based on the results of the electrochemical polarization and salt fog tests, the high-manganese brass alloy (C99700) exhibited remarkable corrosion resistance compared with the other alloys.
The shrinkage and metal core taper allowances required during the permanent-mold casting of a plate and a cylinder in pure copper (C80100), aluminum bronze (C95400), high-Zn yellow brass (C85800), silicon brass (C87500) and silicon bronze (C87600) alloys have been evaluated. The average shrinkage allowance required ranged from 0.185 to 0.252 in./ft based on plate casting data for each of these alloys. The lowest and highest shrinkage allowances were required for alloys C87600 and C80100, respectively. The cylinder casting results show that a core taper greater than 1.5E is necessary to facilitate casting ejection as these alloys tend to shrink onto cores during solidification. http://www.nrcan.gc.ca/canmet-mtl