Die soldering: Mechanism of the interface reaction between molten aluminum alloy and tool steel
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I. INTRODUCTION
DIE soldering, or die sticking, is a casting defect in which molten metal “welds” to the surface of the metallic die mold during the casting process. The defect is prevalent in aluminum die casting and permanent mold-casting industries. The cast-aluminum alloy sticks to the tool steel-die material and remains there even after ejection of the casting. Subsequent casting suffer from dimensional and surface finish issues; die soldering is a quality detractor. As a remedial measure, the casting operation is stopped to repair or replace the die. Previous work[1–10] has not alleviated the problem, and the issue of die soldering remains to be a serious detractor in the casting industry. Die soldering is the result of an interface reaction between the molten aluminum and the die material. Aluminum 380 alloy and H-13 die steel are the most prevalent melt and die material used in aluminum die casting. Figure 1 is an illustration of the harsh environment that is present during the die casting process. The molten metal is shot in through the gate of the die at high pressures, temperatures, and velocities. The casting cycles are generally less than 1 minute. Hence, the die surface is subjected to repeated shots of aluminum melt resulting is excessive wear. This results in damages to the die surface coating and the lubricant. Subsequently, the steel surface of the die comes in contact with the aluminum melt. The aluminum attacks the weak regions in the steel microstructure, and erosion pits form. The iron SUMANTH SHANKAR, Postdoctoral Fellow, and DIRAN APELIAN, Professor and Head, are with the Advanced Casting Research Center, Metal Processing Institute, WPI, Worcester, MA 01609. Contact e-mail: [email protected] Manuscript submitted August 17, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
from the steel diffuses into the aluminum melt resulting in the formation of intermediate layers of binary Fe-Al and ternary Fe-Al-Si phases. Once these phases consolidate and prevent further aluminum-steel contact, the aluminum sticks to them and results in soldering. Figure 2 shows a schematic of the cross section of a soldered die steel-aluminum interface. The aim of this publication is to establish a mechanism for die soldering. The microstructural features of the soldering interface are reviewed and discussed. The various intermediate compounds found in the soldering microstructure are identified through scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction techniques. Moreover, the analysis presented here gives an understanding of the nature and kinetics of the growth of the intermediate compounds, which extend beyond the soldered layer. The results of the experiments clearly indicate that soldering is a diffusion-driven reaction—the iron diffusing out of the tool steel into the molten aluminum and forming the intermediate layers. The role of the various different alloying elements in molten aluminum during soldering has also been investigated and analyzed. II. BACKGROUN
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