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A Little Luck and a Lot of Ingenuity Yield Copper Rotor Success
(Continued) A significant obstacle for Ajmearwala and his team is the use of steel dies in their copper motor rotor trials, “We can die cast a rotor as long as 300 mm with our current machinery, but we have a problem with heat checking,” Ajmearwala said. CDA's research has demonstrated that nickel alloy dies are far superior for die casting copper. Cowie explains, “Nickel alloy dies (Inconel 617 or Haynes 230) preheated above 600 C prevent surface cracking known as 'heat checking' from occurring during repeated expansion and contraction. The preheating substantially decreases the cyclic thermal expansion and contraction from die casting high-melting-point metals such as copper. Since tool steel loses its strength above 500 C, it cannot be adequately preheated to prevent this thermally induced fatigue cracking from occurring. Consequently, only materials that retain their properties of strength, ductility and oxidation resistance at elevated temperature can be successfully employed in this application.”  Despite their difficulties, Perfect Die Cast has found a niche market in die casting copper motor rotors specifically for submersible pumps. The company has been contracted by a major Indian motor and pump manufacturer to cast motors in a wide array of sizes. “We've die-cast pump rotors 48 mm in diameter and 200 mm in length, very easily,” he explained (see Figure 1). According to Cowie, the longer rotor is particularly difficult to cast, “When molten copper is filling the rotor's far end ring, the copper is substantially cooled while passing through the long slots in the steel laminations that eventually form the copper conductor bars. If the copper is cooled too much, there is a danger of it freezing off inside these slots, which would not allow the complete filling of the far end ring and result in excessive porosity” he said.
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