Effect of die casting gate speed on alloy temperature
The gate velocity in die casting is a critical parameter for molten metal entering the mold cavity. Its magnitude directly influences the temperature changes of the alloy during the filling process, which in turn significantly impacts the quality of the casting. When molten metal passes through the gate at a high velocity, it generates intense friction with the gate wall. This friction generates a significant amount of heat, causing the molten metal temperature to rise rapidly. For example, increasing the gate velocity from 30 m/s to 50 m/s can increase the temperature of the molten aluminum alloy by 5 to 10°C. This frictional heating effect can, to a certain extent, compensate for the temperature loss of the molten metal during the initial conveying process, enhancing its fluidity within the mold cavity. This helps prevent underfilling caused by rapid cooling, especially for large, complex castings. However, if the gate velocity is too high, excessive frictional heat generation can cause localized overheating of the molten metal, leading to problems such as increased oxidation and coarsening of the grains.
Different ingate speeds alter the flow pattern of the molten metal within the mold cavity, thereby affecting the uniformity of its temperature distribution. When the ingate speed is low, the molten metal flows smoothly within the mold cavity, exhibiting a laminar flow pattern and a relatively uniform temperature distribution. However, the cooling rate is rapid, which can easily lead to the formation of low-temperature zones at the edges of the cavity. When the ingate speed is high, the molten metal exhibits turbulent flow, resulting in more thorough mixing between the various components and, to a certain extent, balancing temperature differences. However, turbulence also leads to more frequent contact between the molten metal and the mold cavity walls, accelerating heat loss, especially at corners and deep areas of the cavity, where the temperature drop is more pronounced. For example, when producing castings with slender ribs, a higher ingate speed allows the molten metal to quickly fill the ribs, minimizing the temperature drop. However, a lower speed may result in incomplete filling at the ends of the ribs due to excessively low temperatures.
The gate velocity significantly affects the temperature gradient during alloy solidification. After the molten metal enters the mold cavity, it comes into contact with the cold mold surface and begins to solidify, forming a solidified layer. When the gate velocity is high, the molten metal has a stronger scouring effect on the solidified layer, destroying the already formed solidified layer and allowing the unsolidified, hot molten metal to come into direct contact with the mold surface. This reduces the thickness of the solidified layer and lowers the temperature gradient. Conversely, when the gate velocity is low, the solidified layer is less susceptible to destruction and gradually thickens, leading to an increase in the temperature gradient. This difference in temperature gradient affects the internal stress distribution of the casting: when the temperature gradient is too large, the shrinkage of different parts of the casting is inconsistent, which can easily cause deformation or cracks. When the temperature gradient is small, the internal stress distribution is relatively uniform, and the casting has better dimensional stability. Therefore, controlling the alloy’s temperature gradient by adjusting the gate velocity is an effective means of reducing deformation defects in castings.
The effect of the gate speed on the alloy temperature varies at different stages of die-casting. In the initial filling phase, when the molten metal has just entered the mold cavity, the gate speed primarily affects the initial temperature distribution of the molten metal: a higher speed allows the molten metal to quickly diffuse throughout the mold cavity, preventing localized low temperatures. In the middle filling phase, when the molten metal has essentially filled most areas of the mold cavity, the gate speed’s impact is more pronounced in its stirring effect, and an appropriate speed helps achieve a more uniform temperature distribution. At the end of the filling phase, the flow of the molten metal gradually slows, and excessively high gate speeds can cause eddies to form at the end of the mold cavity, entraining air and generating localized high temperatures due to intense friction, increasing the risk of porosity and slag. Therefore, in actual production, the gate speed needs to be dynamically adjusted according to the different stages of die-casting to accommodate the changing alloy temperature.
The relationship between gating speed and alloy temperature requires specific analysis based on the alloy’s properties. For alloys with poor fluidity (such as some magnesium alloys), appropriately increasing the gating speed can increase the temperature of the molten metal through frictional heat generation, improving fluidity. However, for alloys with good fluidity (such as zinc alloys), excessively high gating speeds can lead to excessively high temperatures, which can negatively impact casting quality. Furthermore, the alloy’s thermal conductivity can influence this relationship: alloys with high thermal conductivity (such as aluminum alloys) experience rapid heat loss during flow, requiring higher gating speeds to compensate. On the other hand, alloys with low thermal conductivity (such as some copper alloys) experience slower heat loss, requiring lower gating speeds. Therefore, when setting the gating speed, the alloy’s physical properties must be fully considered. Through process testing, the optimal speed range should be determined to effectively control the alloy temperature and ensure consistent casting quality.
