Sand mold casting method for rapid cooling in automobile field

Abstract

The invention belongs to the technical field of metal casting, and particularly relates to a quick-cooling sand mold casting method for the automobile field, which comprises the steps of sand mold preparation, sand core freezing, mold closing, pouring, quick cooling, demolding, post-treatment and the like. A sand mold freezing mode is adopted, the temperature difference between the molten aluminum temperature and the casting mold temperature after casting pouring is increased, the casting cooling speed is increased, and grain refinement and eutectic structure spheroidization are achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal casting, and particularly relates to a sand mold casting method for rapid cooling in the automobile field. BACKGROUND

[0002] In the automobile field, the cooling link directly affects the forming quality, mechanical properties (such as strength and toughness) and dimensional accuracy of a casting in the casting process of a large thin-walled casting (such as an engine cylinder block, a gearbox shell, a frame structural part and the like). The internal quality requirement is high, and therefore the cooling speed of the casting after pouring is particularly important. In the traditional casting process, cold iron is used to improve the cooling speed of the casting, but the structure of the casting is complex, and it is not easy to set the cold iron. Therefore, it is necessary to develop a sand mold casting method suitable for rapid cooling of a large thin-walled casting in the automobile field. SUMMARY

[0003] The purpose of the application is to provide a sand mold casting method for rapid cooling in the automobile field, which solves the above technical problems.

[0004] Technical scheme: In order to achieve the above purpose, the application provides a sand mold casting method for rapid cooling in the automobile field, which comprises the following steps: Step one, sand mold preparation: using metal or plastic materials to make a sand mold consistent with the shape of the casting according to the drawing of the casting; Step two, sand mold freezing: using a vacuum freezing technology to freeze the sand mold in a vacuum or low pressure environment, freezing the water in the sand mold by low temperature to form a stable mold cavity structure, and determining the freezing time according to the size and thickness of the sand mold; Step three, clamping: combining the upper and lower sand molds and the core in an environment of-10℃ to-15℃, controlling the gap to be less than or equal to 0.2mm, and treating the joint of the clamping with low temperature sealing glue; Step four, pouring: using a bottom pouring system, and the pouring temperature is 680℃-720℃; Step five, rapid cooling: using the low temperature of the sand mold to rapidly cool the casting; Step six, demolding: separating the sand box, removing the casting after removing the upper mold sand; Step seven, post-processing: removing excess and surface impurities, checking internal and surface problems, optimizing mechanical properties, and finally cleaning and inspecting.

[0005] After the sand mold is frozen, the water in the sand mold is frozen to form a stable mold cavity structure, which can increase the supercooling degree of the casting during solidification, increase the nucleation rate, and realize grain refinement.

[0006] Further, the sand mold freezing temperature is-20℃ to-40℃, the freezing rate is 3-5℃ / min, and the cooling time is 0.5-4h.

[0007] Further, the sand mold comprises: base sand: natural quartz sand ≥ 90%, clay content 0.2%-0.3%, water content ≤ 0.1%-0.2%; additives: mica 20-30%, talc powder 60-70%, hollow microbeads 10-15% as refractory aggregate; binder: water-based system binder, 2-4% NaCl solution activated sodium bentonite as suspending agent sand mold.

[0008] Further, the sand mold is prepared by using electromagnetic filling technology to control the filling and compactness of the sand mold, and the surface of the sand mold is uniformly sprayed with a binder with a spraying thickness of 0.2-0.5 mm, and then dried at-20°C for 5 minutes.

[0009] Further, the removal of excess material and surface impurities includes removing the pouring and pouring, removing the flash burr of the casting, and removing the sand and cleaning the surface of the casting. The internal and surface problems are checked by non-destructive testing and size detection. The optimization of the mechanical properties includes heat treatment.

[0010] Optionally, the sand mold is frozen by using a quick freezing method, and the sand mold is quickly placed in a low-temperature environment to suddenly lower the temperature of the organization, the freezing temperature of the sand mold is-20°C to-40°C, the freezing rate is 3-5°C / min, and the cooling time is 0.5-4h.

[0011] Optionally, the sand mold is pretreated by being placed in a 20%-30% sucrose solution before being frozen, and the soaking time is 5-10min.

[0012] Optionally, the sand mold is frozen by using a gradient freezing method, which is first cooled at a slow rate of 1-2°C / min to-10°C, and then quickly cooled to the target temperature.

[0013] The above technical solutions can be seen, the present application has the following beneficial effects: 1. The casting method disclosed by the present application can increase the supercooling degree of the casting during solidification, increase the nucleation rate, realize the control of the solidification gradient (high initial cooling rate and low later cooling rate), and realize the refinement of the grains.

[0014] 2. In addition, the modification effect of the two representative modifiers Sb and Sr is greatly affected by the cooling speed, and there is still a relatively thick flaky eutectic silicon organization in the slow setting part. The modification agent can play the maximum role in the casting method, and the eutectic silicon is spheroidized. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The process flow chart of the sand mold casting method for rapid cooling in the automobile field according to the present application; Figure 2 a grain morphology map of a casting formed by the conventional casting method; Figure 3 a grain morphology map of a casting formed by the casting method according to the present application. DETAILED DESCRIPTION

[0016] It should be understood that the specific embodiments described herein are merely exemplary and not intended to limit the present application.

[0017] Example 1 As shown in the accompanying drawings, a sand mold casting method for rapid cooling in the field of automobiles comprises the following steps: Figure 1 Step one, sand mold preparation: using metal or plastic materials to make a sand mold consistent with the shape of the casting according to the casting drawing; Step two, sand mold freezing: using a reduced pressure freezing technology, the sand mold is frozen in a vacuum or low pressure environment, cooled to a target temperature of -20 to -40°C, the water in the sand mold is frozen by low temperature to form a stable cavity structure, the freezing time is determined according to the size and thickness of the sand mold, the reduced pressure freezing technology helps to uniformly freeze the water and reduce ice crystal defects, the vacuum degree is controlled in the range of 10-50 Pa; Step three, clamping: the upper and lower sand molds and cores are combined in an environment of -10 to -15°C, the gap is controlled to be less than or equal to 0.2 mm to ensure the accuracy of the cavity position, low temperature sealant is used at the joint of the clamping to prevent metal liquid from leaking; Step four, pouring: using a bottom pouring system to reduce the erosion of the metal liquid on the sand mold, the pouring metal is aluminum alloy, and the pouring temperature is 680-720°C; Step five, rapid cooling: using the low temperature of the sand mold to rapidly cool the casting; Step six, demolding: separating the sand box and removing the casting after removing the upper sand; Step seven, post-processing: removing excess material and surface impurities, checking internal and surface problems, optimizing mechanical properties, and finally cleaning and inspecting.

[0018] ​Specifically, the sand mold freezing temperature is -20℃ to -40℃, the freezing rate is 3-5℃ / min, and the cooling time is 0.5-4h. Too low temperature can cause the increase of sand mold brittleness, and too high temperature can cause insufficient freezing effect. For automobile parts and other high-requirement castings, precise temperature control of -30℃±2℃ is adopted. The cooling time is determined according to the size and thickness of the sand mold, generally, the freezing time of small sand mold (weight <5kg) is 30-60 minutes, the freezing time of medium sand mold (5-20kg) is 1-2 hours, and the freezing time of large sand mold (>20kg) is 2-4 hours. For particularly thick and large sand molds, segmented freezing strategy can be adopted. The freezing rate is controlled at 3-5℃ / min, too fast can cause sand mold cracking, and too slow can affect production efficiency. Programmed temperature control freezing equipment can be used to achieve precise control.

[0019] The sand mold comprises: base sand: natural quartz sand ≥90%, clay content 0.2%-0.3%, water content ≤0.1%-0.2%; additives: mica 20-30%, talc powder 60-70%, hollow microbeads 10-15% as refractory aggregate; binder: water-based system binder is used, and sodium-based bentonite activated by adding 2-4% NaCl solution is used as suspending agent of sand mold.

[0020] Specifically, electromagnetic filling technology is used to control the filling and compactness of the sand during sand mold preparation, the binder is uniformly sprayed on the surface of the sand mold, the spraying thickness is 0.2-0.5mm, and then the binder is dried at -20℃ for 5 minutes. After spraying the binder on the surface of the sand mold, drying is performed to improve the surface performance of the sand mold and avoid defects such as sand sticking, sand hole, air hole and the like of the castings.

[0021] Specifically, the removal of excess and surface impurities includes removal of pouring and feeding heads, removal of casting flash burrs, sand removal and cleaning of the surface of the castings; The internal and surface problem checking includes non-destructive testing and size testing; The optimization of mechanical properties includes heat treatment, for aluminum alloy castings, aging treatment can be performed to eliminate stress, improve hardness, and heating to 120-180℃ for 4-8 hours.

[0022] Through the casting method, the undercooling degree during solidification of the casting can be improved, the nucleation rate increases with the increase of the undercooling degree, and the grain can be refined, as shown in Figure 2 、 3 .

[0023] Example 2 The sand mold is quickly placed into a low temperature environment by using the quick freezing method, so that the structure is suddenly cooled. The freezing temperature of the sand mold is-20℃ to-40℃, the freezing rate is 3-5℃ / min, and the cooling time is 0.5-4h. For the sand mold such as the cold box resin sand which depends on chemical reaction curing, the low temperature can promote the binder to quickly crosslink and set, and shorten the curing time. The sudden cooling can reduce the water evaporation and migration of the sand mold in the curing process, avoid the surface to be loose, quickly form the initial strength, and facilitate the subsequent handling or core assembling operation. For the large and complex sand mold, the slow curing is easy to cause the size expansion due to the internal heat release, and the sudden cooling can lock the sand mold shape and reduce the deformation.

[0024] Example 3 The sand mold is pretreated by being placed in a 20%-30% sucrose solution before being frozen. The high penetration property of the sucrose solution is utilized, and the soaking time is 5-10min. When the sand core is soaked in the sucrose solution, because the concentration of the sucrose solution is higher than that of the water in the sand core, according to the penetration principle, the water will diffuse from the sand core to the sucrose solution, so as to reduce the water content in the sand core. The water content is reduced, and the possibility of forming ice crystals in the low temperature environment is also reduced, thereby reducing the damage of the ice crystals to the sand core structure, and helping to maintain the integrity and size stability of the sand core.

[0025] Example 4 The sand mold is frozen by using the gradient freezing, which is cooled to-10℃ at a slow rate of 1-2℃ / min, and then quickly cooled to the target temperature, so as to reduce the ice crystal formation in the freezing process.

[0026] The above only describes the preferred embodiments of the present application. It should be noted that those skilled in the art can make some improvements without departing from the principles of the present application, and these improvements should also be considered as the protection scope of the present application.

Claims

1. A sand casting method for rapid cooling in the automotive industry, characterized in that, Includes the following steps: Step 1, Sand mold preparation: Use metal or plastic materials to make a sand mold that matches the shape of the casting according to the casting drawings; Step 2, Sand mold freezing: The sand mold is frozen in a vacuum or low-pressure environment using vacuum freezing technology. The water in the sand mold is frozen by low temperature, forming a stable cavity structure. The freezing time is determined according to the size and thickness of the sand mold. Step 3, assembly: Combine the upper and lower sand molds and cores in an environment of -10℃ to -15℃, with the gap between them controlled to ≤0.2mm, and treat the joints of the assembly with low-temperature sealant; Step 4, casting: A bottom-pouring gating system is used, and the casting temperature is 680℃-720℃; Step 5, rapid cooling: Use the low temperature of the sand mold to rapidly cool the casting; Step 6, Demolding: Separate the sand box, remove the top molding sand, and then remove the casting; Step 7, Post-processing: Remove excess material and surface impurities, investigate internal and surface problems, optimize mechanical properties, and perform final cleaning and inspection.

2. The sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, The sand mold freezing temperature is -20℃ to -40℃, the freezing rate is 3-5℃ / min, and the cooling time is 0.5-4h.

3. The sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, The sand mold includes: base sand: natural quartz sand ≥90%, mud content 0.2%-0.3%, water content ≤0.1%-0.2%; additives: mica 20-30%, talc powder 60-70%, hollow microspheres 10-15% as refractory aggregate; binder: water-based binder, with sodium-based bentonite activated by 2-4% NaCl solution added as a suspending agent for the sand mold.

4. The sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, During sand mold preparation, electromagnetic filling technology is used to control the compactness of the molding sand. Adhesive is evenly sprayed onto the surface of the sand mold with a thickness of 0.2-0.5 mm, and then dried at -20℃ for 5 minutes.

5. A sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, The removal of excess material and surface impurities includes removing risers and gating gates, removing burrs and flash from castings, and cleaning and desanding the surface of castings. The investigation of internal and surface problems includes non-destructive testing and dimensional inspection; The optimized mechanical properties include heat treatment.

6. A sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, The sand mold freezing method employs a rapid freezing technique, where the sand mold is quickly placed in a low-temperature environment to cause the tissue to cool down rapidly. The sand mold freezing temperature is -20℃ to -40℃, the freezing rate is 3-5℃ / min, and the cooling time is 0.5-4h.

7. A sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, Before freezing, the sand mold is pretreated by soaking it in a 20%-30% sucrose solution for 5-10 minutes.

8. A sand casting method for rapid cooling in the automotive field according to claim 1, characterized in that, The sand mold freezing process employs gradient freezing, first cooling to -10°C at a slow rate of 1-2°C / min, and then rapidly cooling to the target temperature.

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