Methods and Measures to Reduce the Solder Defect of Die Casting Surface

The solder defect in diecasting 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 was analyzed, including chemical principle, mold structure, release agent and 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, bringing more solder.

Mold Design
The external source
The solder defect was caused by unreasonable mold design and heat treatment. The purpose of nitriding that get high hardness and abrasion performance of moil surface. It will get 950~1200HV of surface hardness, and not decreased when the temperature raise 560˜600°C it will produce an oxidation film that has good property of resistant to corrosion, it will cause rough surface if poor heat treatment that caused thin nitride layer; the small draft angle of mold or inverse draft with core or wall; the unreasonable gating system, which can lead to scour 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 can lead to a high temperature area, and therefore result in the solder defect. The high temperature at the ingate would cause it 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 depanning. The agent contains water, wax, mineral oil, grease, silicon and some emulsifier The water can cool the mold cool, 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 to mix uniformly and reduce the surface tension, which is important in water-base release agents. The emulsifiers disperses, moistens, solubilizes, 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 reducing 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.

Die casting equipment and process parameters
High speed injection can improve the filling capacity of a casting. Even some of the aggregated stomatal defects can be scattered. Gas defects are easy to avoid. Similarly, high pressure diecast can make achieve optimal density and good surface quality while reducing porosity or shrinkage defects. However, disadvantages include a propensity for the joint flash defect to occur, difficulties when polishing, going beyond the specified range size, and difficulty in cleaning the mold. Additionally, the bonding force between the metal liquid and mold will increase, producing the solder defect and greatly reducing the lift of the diecasting machine.

Solution of the solder
After analyzing some solder sources from diecasting process and chemical principles (metal liquid characteristics, mold structure, release agent performance and other aspects) in production, according to the characteristics of the castings, the following conclusions were made.

Reduce aluminum temperature reasonably
The higher the temperature, the stronger the affinity of the Fe-Al, and the more obvious the solder of the mold. Figure 7 displays the temperature field by infrared thermometer in a foundry that produces cylinder blocks The five points of icons are higher in temperature, and the corresponding solder defect are more serious. The temperature from SP1 to SP5 is from high to low, and the degree of solder is reduced, the five points are thicker, and the water cooling effect is relatively poor. Figure 8 is the degree which the SP1 temperature is the highest point of the casting.

After adjusting the position of spraying and the time of spraying, the solder has been reduced greatly. In the test, which pointed cooling of the mold at the same process, the water valve corresponding to the position of the solder near the ingate was tested when closed, semi open and fully open. The results are in Table 1.

After the normal mold cycle, the temperature reached 200, which was too low to affect the coating performance of the release agent. Therefore, the cooling water pipe was opened as much as possible, which reduced the surface temperature of the mold, the reaction of Fe-Al interface, and thus the solder defect.

Selection of release agent and spraying method
The release agent is the main aspect causing the solder defect. It produces a film between the liquid metal and the mold. The performance of the film (extensibility, lubricity, high temperature resistance) directly determines the solder of the casting. After long-term production, the mold’s surface is impregnated and the corrosion of the mold is reduced. When a poor quality release agent is sprayed, the 10% surface spots will appear in the short-term, which reduces the surface quality of the mold greatly.

In order to improve the cycle, some manufacturers use a fixed spray; when the mold open, it is sprayed immediately with large atomization. At this time, the mold temperature is very high (above 350), which is when the mold surface temperature exceeds the Leidenfrost temperature (the wall temperature corresponding to the minimum heat flux in the droplet boiling curve). The solid liquid can’t be directly contacted, and the vapor film is formed between the solid and liquid so the heat transfer coefficient is much lower than that in the core boiling. In this way, a lot of release agents are wasted. When the mold temperature is down to 200-250, a large number of effective components of release agent can be coated on the surface of the mold. The best spraying method is the phase spraying from left to right and back. It can give the release agent sufficient time to attach to the surface of the mold. Excessive or longer spraying doesn’t effectively coat the surface of the mold. On the contrary, the unattached components in the front were washed away by the spray behind in the study. Dilution ratio and spraying volume should be appropriate because if the release agent has a high ratio or spraying is poor, the mold is more susceptible to solder.

Reducing the high speed and pressure reasonably
In order to make the casting achieve good surface and quality, many diecasters often use high speed and high pressure to reduce the defect, especially when shrinkage holes and blowholes are present. However, only increasing the pressure will produce the solder defect and reduce the life of the mold and equipment. When the shrinkage cavity occurs, maybe it is caused by poor pressure. check whether the nitrogen cylinder pressure gauge is suitable, whether the aluminum liquid spectrum and density are in the process requirement range and so on.

Maintenance of mold
For nitriding molds, the polishing should be done carefully to prevent the surface breaking, and avoid more polish and solder. The tungsten carbide rod coating machine can be used for pouring parts or core of the ingate, on the surface of the mold, for EDM type metallurgical spraying tungsten carbide particle layer, tungsten particles and matrix metal binding. It can improve the resistant solder performance on the mold surface, such as a coating die surface layer deposited in 2~4μm. The hardness can get HV4000 ~ 4500, and max resistant temperature can reach 800C.
Solder has a great influence on the production and quality of die castings. Good gate system design can improve the quality of casting. Using low mold temperature as far as the quality of the product is guaranteed (the mold temperature can’t be too low, otherwise it will increase the tightening force and product cold lap defect) is also recommended. Low aluminum temperature, low speed and low pressure to produce, will require less solder to polish or low hardness in the metal, preventing the mold nitrided layer from being destroyed. A good release agent and reasonable spraying process also can increase the quality.