Preface: High pressure die casting of aluminum alloy is widely used in the production of key components such as automobiles and electronics. Due to high-speed filling and solidification shrinkage stages, defects such as porosity and shrinkage porosity are prone to occur in the sprue area of the product, which seriously affects the quality of product delivery. The mold temperature, as a key process parameter, directly affects the fluidity and solidification synchronization of the molten metal. Existing research mostly focuses on optimizing injection speed or pressure, and there is still insufficient systematic research on mold temperature. This article takes the automobile transmission case as the object, and through experiments and theoretical analysis, reveals the mechanism of the effect of mold temperature on gate defects, and proposes a process optimization plan.
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Experimental Method
1. Materials and Equipment
Alloy material: ADC12 main components (Si 9.6-12.0%, Cu 1.5-3.5%, Fe<1.3%, Mg<0.3%, Zn<1.0%, Mn<0.5%, Ni<0.5%);
Production equipment: Lijin 3000T cold chamber die-casting machine;
Testing equipment: X-ray flaw detector; Thermal imaging mold temperature detector.
2. Experimental parameters
Fixed parameters: aluminum liquid temperature of 660 ℃, slow injection speed of 0.35 m/s, fast injection speed of 4.5 m/s, and pressure ratio of 80 MPa. Change point: The mold temperature is divided into three groups (160-220 ℃; 220-280 ℃; 280-340 ℃), and each group is die cast 8 times.
3. Defect detection
Quantitative analysis of defects in the gate area using X-ray inspection; Standard: If the aperture is greater than 0.5 mm, the qualification rate shall be calculated.
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Formation mechanism analysis
1. Flow analysis results
In the analysis of the filling mold flow during the product development and design phase, it can be seen that when the aluminum liquid enters the product sprue, the temperature is in the range of 631~645 ℃. As the high-temperature aluminum liquid continues to advance, the sprue temperature gradually increases and remains in the range of 660 ℃ until the end of the product cavity filling.
2. The influence of low temperature in molds on the flow of molten metal
In the initial stage of die-casting cold mold production, 8 pieces were produced under the condition of closing the mold circulation cooling control system. The initial stage mold temperature was collected, and the mold temperature measurement data showed that the cross-sectional mold temperature of the sprue was in the range of 166.3~178.7 ℃ (high temperature zone<220 ℃); On the basis of this temperature, the heat exchange rate between the aluminum liquid and the mold is accelerated, significantly affecting the flowability of the aluminum liquid during filling, increasing viscosity, and depositing a condensation layer in the sprue, resulting in a density difference and an increased probability of hole defects.
3. The influence of temperature in molds on the flow of molten metal
With the increase of production molds, the overall temperature of the mold increases after being heated by the aluminum liquid, and the heat exchange rate between the aluminum liquid and the mold decreases. A total of 16 pieces have been produced. Through collecting mold temperature data, it was found that the temperature of the inner gate cross-sectional mold has risen to 220-273.7 ℃ (high temperature zone>220 ℃); After observing the broken material, the density of the sprue section in the product increased, and there was no visible condensation layer.
4. The influence of high temperature in molds on the flow of molten metal
After continuous production of 24 pieces, it was found that the temperature of the mold at the cross-section of the sprue had risen to 284.5~330.6 ℃ (high temperature zone>280 ℃); Due to the high temperature of the mold, the flowability of the aluminum liquid is greatly improved, and the appearance brightness of the products produced at this stage increases. The density of the inner gate section is significantly enhanced, and there is no visible condensation layer phenomenon. However, there is severe mold sticking and tearing in the local areas of the reinforcing ribs and the outer surface layer of the threaded column near the inner gate of the product.
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experimental result
1. The influence of mold temperature on defects
Through three sets of experimental data, it is demonstrated that when the mold temperature is in the range of 220-280 ℃, the flowability of the metal liquid is moderate, which improves the effectiveness of pressurization and contraction, and the physical quality of the product is relatively stable.
2. Distribution of defects in physical inspection
Mold low-temperature group: Gas shrinkage holes are mostly distributed in the middle of the threaded column at the gate, showing irregular shrinkage patterns;
Mold temperature group: There are no visible shrinkage defects inside the inner gate and threaded column area;
Mold high-temperature group: Gas shrinkage holes and dispersed micro holes (<0.3 mm) can be seen near the sprue and threaded column.
1. Temperature control
During the initial stage of resuming production, preheat the mold to above 220 ℃ and stabilize the mold pouring temperature at 240 ± 20 ℃ by turning off the mold water cooling system.
2. Process collaboration
Within the optimized temperature range, increase the injection velocity appropriately (4.5-5.0 m/s) to further reduce the formation of aluminum liquid filling turbulence and encapsulation.
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Conclusion
By regulating the temperature of the mold, the flowability of the aluminum alloy liquid can be improved, gas residue can be reduced, and the effectiveness of pressurization and shrinkage during the solidification process can be improved, thereby reducing the probability of gate defects. Experiments have shown that the optimized temperature range for die casting of this product is 220-280 ℃, which can eliminate the defects in the sprue and meet customer requirements for product quality.
Technical guidance for this article: Ye Li
