Excerpt from the January 2020 Issue of Global Casting Magazine
The solder defect in die casting affects surface quality and strength, especially in castings with a high seal requirement. The defect could lead to leakage that would mean more wasted cost (Fig 1.). Four aspects of soldering were analyzed, including:
- chemical principle
- mold structure
- release agent
- die casting process
For example, JAC die casting company in Suzhou, China, supplies parts to customers that don’t allow solder to be visible to the naked eye. For a German company in Shandong, China, which produces automobile parts, the solder defect area can be no more than 20 x 20 mm. Another company which produces parts with thin walls and high temperature pouring, have found it is at higher risk for solder defects.
What factors cause the solder defect? What’s the physical or chemical change between the liquid aluminum and hot iron mold? This article will share the results of a study on this defect based on the chemical principle, mold structure, release agent and die casting process.
Interfacial reaction is a series of chemical interactions that occur between the interface of two phases, according to chemical type, content, status and properties.
Solder defect is started by the chemical and physic reaction due to the strong affinity in Fe-Al. A new compound is created when the mold and liquid Al connect closely. The higher the temperature, the more intense the thermal vibration of the atoms, causing the solder ratio to rise and the corrosion of the mold to increase, until finally, it causes the solder area. When polished, the surface containing a thin nitride layer will be destroyed, causing worse cycle. Some data in the study showed that the affinity of Fe-Al will become more serious as the Fe element is decreased, creating more solder.
The external source
The solder defect was caused by unreasonable mold design and heat treatment. The purpose of nitriding is that gets high hardness and abrasion performance of the mold surface. It will get 950-1200HV of surface hardness and won't decrease when the temperature raises 560-600°C. It will produce an oxidation film with resistance to corrosion. It will also cause a rough surface if poor heat treatment caused:
- a thin nitride layer;
- a small draft angle of mold or inverse draft with core or wall;
- an unreasonable gating system, which can lead to scour the core and wall directly (Fig. 2).
The internal source
Due to the dead zone of the cool channel when designing the mold, some walls are made thick. But this design can lead to a high temperature area resulting in the solder defect. The high temperature at the in-gate causes the surface to be seriously scoured (Fig. 3).
Release Agent Performance
The release agent properties
The release agent interacts with the mold directly, producing an isolation layer that can help the casting de-panning. The agent contains water, wax, mineral oil, grease, silicon and some emulsifier. The water can cool the mold, and wax can be a carrier that spreads the lubrication throughout the mold, decreasing the wetting angle, reducing the Leidenfrost effect (meaning the liquid can’t wet the high temperature surface, merely producing a steam layer), and helping the components to link. As a lubrication, silicon is very important. Different styles achieve different performances. The emulsifier can promote the components mix uniformly and reduce the surface tension, which is important in water-base release agents. The emulsifiers disperse, moisten, solubilize, and when stirring, it can make those organic solvents become minute particles suspended in water before finally becoming an O/W stable emulgator (Fig. 4).
The isolating layer produces some chemical and physical reactions when the liquid becomes solid, so the release agent’s performance to reduce the solder defect is important if other processes are the same.
Thermogravimetric Analysis (TG or TGA) is a technology that analyzes the relationship of sample weight and temperature variation. It is used for heat stability of material and components. Figure 5 displays three material curves of the effect on component weight when the temperature rises.
As the temperature rises, the slope of the curve is the largest. When the range is 200-400, the loss of the mold component is the most serious. As the temperature continues to rise, the curve region slows down; this is the heat exchange interval between the mold and the metal liquid. The high temperature resistance of the release agent is represented by the red dotted line in Fig. 5. According to the figure, the red dotted line has the best result.
TGA curves indicate the best mold temperature range is 200-300, where the metal liquid has good fluidity and the property of lubricating is best. The high-quality release agent can make the water-based diluent evaporate very quickly. By not increasing the gas in the cavity, there is no accumulation, the mold is coated evenly; no corrosion occurs, the demulsifying effect is good, there is low COD, and pollution sources are reduced.