Prediction of Thermal Fatigue in Tooling for Die-casting Copper via Finite Element Analysis

Abstract

Abstract. Recent research by the Copper Development Association (CDA) has demonstrated the feasibility of diecasting electric motor rotors using copper [1]. Electric motors using copper rotors are significantly more energy efficient relative to motors using aluminum rotors. However, one of the challenges in copper rotor die-casting is low tool life. Experiments have shown that the higher molten metal temperature of copper (1085 °C), as compared to aluminum (660 °C) accelerates the onset of thermal fatigue or heat checking in traditional H-13 tool steel. This happens primarily because the mechanical properties of H-13 tool steel decrease significantly above 650 °C. Potential approaches to mitigate the heat checking problem include: 1) identification of potential tool materials having better high temperature mechanical properties than H-13, and 2) reduction of the magnitude of cyclic thermal excursions experienced by the tooling by increasing the bulk die temperature. A preliminary assessment of alternative tool materials has led to the selection of nickel-based alloys Haynes 230 and Inconel 617 as potential candidates. These alloys were selected based on their elevated temperature physical and mechanical properties. Therefore, the overall objective of this research work was to predict the number of copper rotor die-casting cycles to the onset of heat checking (tool life) as a function of bulk die temperature (up to 650 °C) for Haynes 230 and Inconel 617 alloys. To achieve these goals, a 2D thermo-mechanical FEA was performed to evaluate strain ranges on selected die surfaces. The method of Universal Slopes (Strain Life Method) was then employed for thermal fatigue life predictions.

Copper motor rotors: Energy saving efficiency, now also economic feasibility

“…Solutions to those issues have now been identified and tool life can be considered acceptable [3,4,5].…”

Thermal Fatigue of Die-Casting Dies: An Overview

“…First cracks emerge right after 2000 cycles and spread progressively with cycles as illustrated in Figure 2. [7,18,20,21,25].…”

“…Cracks reduce the surface quality of dies and consequently the surface of castings will decline too. Previous studies have analysed thermal fatigue cracking of dies during the process of die casting aluminium alloys [7,8]. During the process cracks are identified and their size and location are measured.…”

“…First cracks emerge right after 2000 cycles and spread progressively with cycles as illustrated in Figure1. [7,18,20,21] Figure 2.Direction of crack [18] Moreover, die-casting dies are prone to high mechanical and thermal loads. Thermal fatigue cracking of dies which is caused by thermal cycling might considerably reduce the die lifetime.…”

“…This can happen even when the mechanical stress applied to the die is less than the yield stress. A compound of residual, thermal and mechanical stresses create the crack [6][7][8]. This kind of failure is attributed to the inherent resistance of the die material against the brittle fracture termed fracture toughness.…”