Process Parameters for Lead-Free Copper-Base Engineering Alloys in Permanent Molds
Permanent mold casting, an environmentally friendly process is quite common for producing castings from alloys of aluminum and zinc. Copper-base alloys due to their higher process temperatures are not widely accepted as materials for permanent mold process. The advantages of the process include near-net shape components, better surface finish, improved mechanical properties and energy efficiency. Only recently permanent mold casting is gaining acceptance from North American copper casting industry despite the fact that this process is widely in use in Europe. The permanent mold process has one more advantage over sand casting in that the problem of disposal of lead contaminated sand is eliminated.
Many of the common copper base alloys such as red and yellow brasses used for plumbing applications have lead as a major alloying element. Lead improves the machinability and pressure tightness of these alloys. However, the recent restrictions on the leachable lead in the drinking water have led to the research and development of low-lead or lead-free copper base alloys for plumbing applications.
The design engineers should be aware of the advantages of the permanent mold cast copper alloys to increase their market share and there is a need for establishing the process and design parameters for permanent mold cast copper base alloys for their wider acceptance as engineering alloys in the place of steels, cast irons and aluminum alloys. The three-year project on process parameters for lead-free copper base alloys in permanent molds deals with different process parameters as elaborated below:
1.Evaluation of mold life of potential mold materials for permanent mold casting of copper alloys.
2.Effect of minor alloy additions on the effective casting fluidity of lead free tin bronzes/red brass.
3.Grain refinement of permanent mold cast brasses and bronzes, with emphasis on its effect on hot tearing resistance and casting fluidity.
4.Numerical modeling to describe the microstructural changes observed in Cu-Zn brass by the addition of aluminum and its effect on casting fluidity.
5.Water modeling of gating systems used in permanent mold castings to generate information on turbulence and jetting.
6.Development of a new lead-free low melting copper alloy for permanent mold casting.
7.Investigation of the zinc oxide deposit during permanent mold casting of yellow brass.
The salient findings of these investigations are summarized below:
Thermal shock resistance of various materials (e.g.. cast iron, tool steel, Cu-Be, Ni-Be, Nickel Aluminide etc.) used to make the molds for copper alloy permanent mold casting process was evaluated and ranked as follows:
Ni-Be > Cu-Be > NibrylÒ > Ni3Al > H13 Steel > Cast Iron > Plain Carbon Steel
The grain refinement behaviour of permanent mold cast silicon brass, silicon bronze and lead-free red brass was studied. The grain refiners investigated were Cu-B, zirconium and a commercial grain refiner FKM 2000. Zirconium was effective for silicon brass and silicon bronze. Lead-free red brass was partially grain refined by boron.
The effect of minor alloy additions such as Al, Mg, and Pb on the fluidity of tin bronze, silicon brass and silicon bronze was investigated. Only aluminum improved the casting fluidity.
Water modeling studies were carried out to evaluate the fluid flow in selected permanent molds. The turbulent flow, jetting and air entrapment during the fluid flow could be visualized. These problems were shown to be reduced by altering the gating and pouring practices.
Aluminum is known to improve the fluidity of various alloys including leaded yellow brass, silicon brass and silicon bronze in permanent molds. Efforts were made to predict the microstructural changes and enhanced fluidity using mathematical modeling. The study shows that aluminum changes the surface tension of copper and this, in turn, modifies the nucleation and growth behaviour of dendrites during solidification. The dendrite morphology changes from interlocking to a fine feathery structure. These changes were shown to enhance the casting fluidity.
A new low melting copper alloy suitable for permanent mold casting was developed. This alloy contains zinc (20-25%), nickel (4-5%) , phosphorous (3-5%) and aluminum (0.5%) as the major alloy additions. The low melting temperature of the alloy makes it attractive to permanent molds since it reduces thermal shock. The alloy posses good casting characteristics (fluidity and hot tearing resistance) as well as good tensile strength and corrosion resistance comparable to yellow brass and found to be pressure tight. However, this alloy has moderate ductility and poor machinability.
Zinc in yellow brass, produces zinc oxide which deposits on the permanent molds (due to the low vapor pressure). This deposit not only reduces casting fluidity but also produces poor surface quality. The deposit is usually removed by dipping the molds in a water/graphite slurry and in extreme cases has to be removed physically. In an attempt to minimize the oxide deposition, the effect of aluminum and magnesium additions on the zinc oxide formation and deposition was studied. It was found that a combination of 0.4% Al and 0.1% Mg reduced the zinc oxide deposition significantly. http://www.nrcan.gc.ca/canmet-mtl
Materials Technology Laboratory
Thomas Prucha American Foundry Society 1695 Penny Lane Schaumburg, IL 60173 website: www.afsinc.org