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2023.2.23
The specific heat capacity and latent heat of phase transformation of magnesium alloy are lower than that of aluminum alloy. During die casting, magnesium alloy cools quickly and has poor fluidity, which is easy to produce defects such as under-casting and lack of flesh. By increasing the heating temperature and mold temperature of magnesium alloy, increasing the injection speed can significantly reduce the scrap rate. In recent years, people have higher and higher requirements for lightweight products, especially for automobiles to further reduce weight, reduce fuel consumption and emissions, and improve driving safety and comfort. The use of magnesium alloy materials just caters to this development trend. The continuous improvement of magnesium alloy properties and the remarkable progress of die-casting technology have led to a significant increase in the amount of die-casting magnesium alloy.
3D model of automobile steering support product, with an overall dimension of 92mm × 94mm × 61mm, the average wall thickness of the product is 4.5mm, the material is AZ91D, and the net mass is 114g, which is a typical magnesium alloy die casting. The structure of the product is relatively complex, and the mold has four side sliders: upper, lower, left and right. Due to the functional requirements of the product, many parts of the part were originally too small, which is not conducive to the forming of the part.
According to statistics, the scrap rate of this product is 23%. Further analysis of 259373 pieces of scrap found that 250378 pieces were undercast, accounting for 96.53%; 8995 pieces of meat were missing, accounting for 3.47%. Undercasting at the far end of the mold filling.
Undercasting during die casting mainly includes poor fluidity of alloy liquid, unreasonable gating system and poor exhaust. In the early stage of product development, the mold gating system and exhaust system were simulated by CAE and optimized in the commissioning stage. The gating system and exhaust system should have no major problems. The part material is magnesium alloy AZ91D, which has lower specific heat capacity and latent heat of phase transformation than aluminum alloy and faster cooling. The analysis shows that the poor fluidity of the alloy liquid is the main reason for the under-casting defects of the product.
The following is a further detailed analysis of the poor fluidity of the alloy liquid.
(1) The alloy liquid is seriously oxidized and its fluidity is reduced
Due to the existence of the gating system and overflow system, and the unqualified products, more than 50% of the furnace return material will be produced in the continuous die-casting production process. Due to the high protective requirements of magnesium alloy materials during smelting, the factory has no melting furnace to meet the requirements and does not have refining conditions. In the actual production process, the back-furnace material is directly added to the holding furnace, and the operator only cleans the scum on the surface of the alloy liquid. Compared with the refined finished alloy ingot, the oxidation of the returned material is serious and the fluidity is poor, which is one of the main reasons for the under-casting of the product.
(2) Alloy pouring temperature and mold temperature are too low
Temperature is an important process parameter in the process of die casting. In order to meet the good filling conditions and ensure the forming quality of die castings, appropriate alloy pouring temperature and mold temperature must be selected. This product is produced by cold chamber die-casting machine, and the alloy temperature is required to be between 660 and 680 ℃. In order to facilitate control and measurement, the temperature of the alloy liquid in the furnace is generally controlled and guaranteed. In order to ensure the stability of pouring temperature, the special holding furnace for magnesium alloy die-casting is rectangular. The furnace is divided into pouring cavity and melting cavity. The middle is connected. The alloy temperature in the pouring cavity will not be reduced after the alloy ingot is added from the melting cavity. The set temperature of the holding furnace is 670 ℃. Use a portable thermometer to measure the alloy temperature in the casting cavity before and after feeding. The values are as follows: 663 ℃ before feeding; 662 ℃ after feeding for 0.5 min; After feeding for 3 minutes, the temperature was 648 ℃, and the temperature decreased significantly.
In magnesium alloy die-casting, the ideal heating method is to use a mold temperature machine to heat the mold through hot oil. The hot oil continuously heats the mold from the inside through the pipes in the mold to make the mold reach the required equilibrium temperature. The function of heat transfer oil is like a heat exchanger to keep the mold temperature within a certain range. With this heating method, the temperature of the mold is stable and uniform, which can effectively improve the product quality, prolong the life of the mold, and stabilize the production rhythm. In the process of magnesium alloy die-casting, the mold temperature is generally 180~280 ℃. The mold temperature machine of the steering bracket is set at 240 ℃. The mold temperature during continuous production is measured with an infrared thermometer. The dynamic mold temperature is 190~200 ℃, and the static mold temperature is 170~180 ℃. The static mold temperature is obviously low. The above data show that the alloy liquid temperature and mold temperature are low, and defects such as under-casting and cold shut are easy to occur during die casting.
(3) The flow rate of the sprue is too low
Because the die-casting machine system for producing the steering support does not have the function of displaying the injection curve, the fast injection speed of the die-casting machine is controlled by the flow valve, and the opening size of the flow valve is displayed on the die-casting machine proportionally, so the corresponding fast injection speed cannot be mapped. First, use the injection curve tester to test the corresponding relationship between the fast injection flow and the fast injection speed.
In the process of increasing the fast injection flow rate from 60% to 80%, the fast injection speed gradually increases, and the maximum speed can reach 6 m/s. In the past production, the fast injection flow rate of the steering bracket was set at 60%, that is, the injection speed was about 3 m/s. At this time, the flow rate of the alloy liquid in the inner runner was 72 m/s. This speed could meet the requirements for die casting of aluminum alloy parts, but failed to meet the high speed required for magnesium alloy parts. This is also one of the main reasons for the undercasting of products.
(1) Comparison of the production of recycled materials and finished alloy ingots as raw materials
Because of the active chemical characteristics of magnesium element, it is easy to be oxidized. Under the condition of no special slag removal and refining process, the fluidity of magnesium alloy liquid is worse than that of finished magnesium alloy ingot when using recycled material as raw material in die casting. In order to verify the influence of raw material fluidity on the under-casting defects of magnesium alloy steering bracket, the back-furnace material (50%) and the finished alloy ingot are used in different batches respectively, and the other process parameters remain unchanged. The statistical under-casting scrap rate is 23.3% of the back-furnace material and 19.1% of the finished alloy ingot. It can be found that when the finished alloy ingot is used as the raw material, the fluidity of the alloy is better than that of the return material, and the under-casting ratio is significantly reduced.
(2) Increase the temperature of alloy liquid
In order to verify the effect of alloy liquid temperature on the scrap rate of magnesium alloy support, the stable alloy liquid temperature is 690 ℃, and the other process parameters remain unchanged for production. The raw materials are produced with a batch of recycled materials and alloy ingots respectively. According to the statistics, the scrap rate of under-casting is 16.3% due to the production of returned materials; Finished alloy ingot 13.5%. It is not difficult to find that when the temperature of the alloy liquid increases, the rate of under-casting rejection decreases significantly, no matter whether the alloy liquid is recycled or the finished alloy ingot is used. Among them, the quality of finished alloy ingot production batch is the best, and the scrap rate is the lowest.
(3) Increase mold temperature
Because the mold temperature is greatly affected by the alloy liquid temperature, the mold temperature machine is set at 240 ℃ and 280 ℃ respectively, and the alloy liquid temperature is set at 660 ℃ and 690 ℃ respectively. The actual mold temperature is monitored in four batches. When the temperature of the alloy liquid is increased by 30 ℃, the mold temperature is also increased by about 20 ℃, but when the mold temperature machine temperature is increased by 40 ℃, the mold temperature is only increased by about 5 ℃. The analysis shows that there are some problems in the design of the mold heating runner, and the goal of increasing the mold temperature cannot be achieved by increasing the set temperature of the mold temperature machine.
(4) Increase the fast injection speed
Further verify the impact of rapid injection speed on the scrap rate, increase the rapid injection flow rate from 68% in the original production to 78%, that is, increase the rapid injection speed from 3.6m/s to about 5.3m/s, use different raw materials, use different alloy liquid temperature for production, keep the other process parameters unchanged, and carry out the under-cast scrap rate statistics in 8 batches.
With the increase of the temperature of the alloy liquid, the under-cast rejection rate will decrease. At the same time, the under-cast rejection rate can also be greatly reduced by properly increasing the fast injection speed. Under the condition of increasing the temperature of alloy liquid and fast injection speed, the problem of poor fluidity of the return material can be overcome. The production of finished alloy ingots has certain benefits to reduce the scrap rate, but the reduction is not significant. After the fast injection speed is increased, due to the increase of internal pressure, the flash of parts is large, which increases the difficulty of subsequent cleaning. This can be solved by properly reducing the boost pressure and boost speed.
With the increase of the temperature of the alloy liquid and the mold temperature, the fluidity of the alloy liquid increases, the filling capacity greatly increases, and the under-casting rejection rate significantly decreases. At the same time, properly increasing the speed of rapid injection can also significantly reduce the rate of under-casting scrap. The scrap rate can be reduced by using finished alloy ingots, but the reduction is not significant. After improving the die-casting process conditions, the problems of poor fluidity of the return material and high scrap rate have been solved. The scrap rate has dropped from 23.30% in the past to 3.70% now, bringing considerable economic benefits to the enterprise.