Excerpt from the U.S. Department of Energy article by Q. Han, E.A. Kenik and S. Viswanathan
Two types of tests, dipping tests and dip-coating tests were carried out on small steel cylinders using pure aluminum and 380 alloy to investigate the mechanism of die soldering during aluminum die casting. Optical and scanning electron microscopy were used to study the morphology and composition of the phases formed during soldering. A soldering mechanism is postulated based on experimental observations. A soldering critical temperature is postulated at which iron begins to react with aluminum to form an aluminum-rich liquid phase and solid intermetallic compounds. When the temperature at the die surface is higher than this critical temperature, the aluminum-rich phase is liquid and joins the die with the casting during the subsequent solidification. The paper discusses the mechanism of soldering for the case of pure aluminum and 380 alloy casting in a steel mold, the factors that promote soldering, and the strength of the bond formed when soldering occurs.
Introduction
Soldering, or die sticking, occurs during die casting when molten aluminum "welds" to the die surface, resulting in damage to the die and in poor surface quality of the casting. Two types of soldering are identified in the literature: one that occurs at high temperature due to a chemical/metallurgical reaction between the molten aluminum alloy and the die (1), the other that occurs at low temperatures due to mechanical interaction (2). This paper address soldering that occurs due to a chemical/metallurgical reaction.
For the type of soldering occurring at high temperature, it is generally acknowledged that soldering is closely related to the “washout” of a protective film on the die surface (1). Washout occurs when the molten aluminum alloy enters the die and destroys the protective film (coating or lubricant) on the die. The molten aluminum then comes into contact with the die surface.
Iron in the die material dissolves into the melt whereas aluminum and other elements in the melt diffuse into the die. As a result, a layer of intermetallics is formed at the die surface. Under the right conditions, an aluminum-rich soldering layer may also form over the intermetallic layer (1). Although a significant amount of research has been conducted on the nature of these intermetallics, (3-7) little is known about the conditions under which soldering occurs.
In this study, the formation of intermetallics and the occurrence of soldering are differentiated, since it will be shown that the mere presence of intermetallics is not a condition or cause for soldering. In addition, the study will attempt to focus on the initiation of soldering rather than the growth of intermetallics. This study will address the following questions:
- At what temperatures does soldering occur?
- Is the formation of intermetallics on the die surface an indication that soldering has already occurred?
- How does an aluminum alloy casting solder (join) to the die?
- What determines the strength of a bond?
