Aluminum alloy casting process for refrigerator liner mold

Abstract

The application discloses a precision casting process of an aluminum alloy casting for a refrigerator liner mold, relates to the technical field of aluminum alloy casting, and makes the filling of aluminum liquid in a mold cavity more stable and complete through step-by-step differential pressure pouring and multi-stage cooling control technology, so that the porosity of the casting is reduced, the tensile strength is improved, the performance is better than that of traditional sand casting and metal gravity pouring, and during a temperature window period after complete solidification of the casting and before mold opening, controlled vibration is applied to the casting by using mold constraint, so that the residual stress can be reduced by more than 80%, and because the casting is under the constraint of the mold, stress release is performed in the form of micro-dislocation movement, and macroscopic deformation is not generated; through a laser impact in-situ strengthening process, the surface of the mold cavity is selectively strengthened without disassembling the mold, so that the surface hardness is improved, and the thermal fatigue cycle number is increased by more than 2 times.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy casting technology, specifically to a precision casting process for aluminum alloy castings used in refrigerator liner molds. Background Technology

[0002] The refrigerator liner is the core insulation component of the refrigerator body. It is required to maintain dimensional stability and flatness within a wide temperature range of -30℃ to 60℃. Its molding quality directly determines the insulation performance and appearance quality of the refrigerator. As a key piece of equipment for the vacuum forming or foam forming of the liner, the refrigerator liner mold must withstand repeated hot and cold cycle loads, which places extremely high demands on the material's thermal fatigue resistance, dimensional stability and surface wear resistance.

[0003] Currently, aluminum alloy castings for refrigerator liners are mainly produced using sand casting and V-process casting. Sand casting suffers from problems such as long production cycles for wooden molds, low dimensional accuracy, and poor surface roughness. Furthermore, demolding from deep cavities with complex curved surfaces is difficult, requiring separate molding followed by assembly, which results in obvious mold parting lines and affects the appearance of the liner product. While V-process casting can improve surface quality and dimensional accuracy, for deep cavity molds with a depth exceeding 300mm, defects such as film cracking and uneven thickness are prone to occur at corners and the bottom of the deep cavity during film coating, leading to sand adhesion and increased porosity on the casting surface.

[0004] In existing aluminum alloy casting processes, residual tensile stress is generated in aluminum alloy castings during the cooling process due to differences in cooling rates and inconsistent phase transformation shrinkage in different parts. Traditional methods for eliminating residual stress have obvious limitations: natural aging takes 3-6 months, resulting in extremely low production efficiency; heat treatment aging can lead to changes in casting dimensions and consumes a lot of energy; room temperature vibration aging after mold opening can only eliminate some residual stress, and for deep cavity complex structure molds, some workpieces still exhibit dimensional deviations and delayed cracking during subsequent machining or service. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a precision casting process for aluminum alloy castings used in refrigerator liner molds. By using the mold as a force transmission medium, controlled vibration with a specific frequency and amplitude is applied to the entire mold, and the casting temperature is precisely controlled within a temperature window of 300-350℃ for in-mold vibration aging. This reduces residual stress by more than 80% before mold opening, and the casting does not undergo macroscopic deformation under the constraint of the mold. At the same time, after casting, the surface of the mold cavity is subjected to selective laser shock in-situ strengthening treatment, forming a residual compressive stress layer and a nano-scale grain refinement layer on the cavity surface without disassembling the mold, thereby improving the surface hardness and thermal fatigue resistance of the mold.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a precision casting process for aluminum alloy castings used in refrigerator inner liner molds, the method comprising the following specific steps:

[0007] S1: Prepare the molten aluminum according to the set aluminum alloy composition and smelt it, and then refine and purify the molten aluminum.

[0008] S2: Transfer the pre-prepared refrigerator inner liner mold to the vacuum casting chamber and preheat it, then cover the outer wall of the mold with insulation material;

[0009] S3: After evacuating the vacuum casting chamber, the aluminum liquid is poured using a differential pressure pouring method with progressively increasing pressure. After pouring, the pressure is maintained to ensure that the aluminum liquid fully fills the cavity and solidifies.

[0010] S4: Control the cooling process according to the principle of sequential solidification, and use alternating air cooling and water cooling to make the casting temperature drop uniformly to 300-350℃;

[0011] S5: When the casting is completely solidified in the mold cavity and the temperature is within the temperature window of 300-350℃, start the pneumatic vibrator installed on the periphery of the die casting moving template and fixed template to apply controlled vibration to the mold and the internal casting. Through energy input, activate the internal dislocation movement and stress redistribution of the aluminum alloy, so that the residual stress of the casting is reduced before the mold is opened.

[0012] S6: After the casting has cooled to room temperature, the mold is opened and the casting is ejected. The gating gates and burrs are removed, and the casting is sandblasted. Then, the casting is subjected to T6 heat treatment to obtain the mold casting.

[0013] S7: Install the mold casting to the die casting machine station and close and fix the mold. Without disassembling the mold, use a pulsed laser to perform selective laser shock strengthening treatment on the cavity surface of the mold casting through a programmable scanning galvanometer, forming a residual compressive stress layer and a nano-scale grain refinement layer on the cavity mold surface.

[0014] S8: Perform final dimensional re-inspection and surface quality inspection on the mold after laser shock strengthening. The dimensional tolerance is controlled within ±0.1mm. The surface is free of defects such as cracks, pores, and pits. After passing the inspection, apply a thin layer of anti-rust oil to the cavity surface, wrap it with moisture-proof paper and bubble wrap, and then pack it into the warehouse.

[0015] Furthermore, the specific steps of S1 are as follows:

[0016] The aluminum alloy raw materials are formulated by weight percentage as follows: Si: 6.5-7.5%, Mg: 0.25-0.45%, Ti: 0.10-0.20%, Sr: 0.01-0.03%, Fe≤0.12%, Cu≤0.10%, Zn≤0.05%, with the balance being Al and unavoidable impurities;

[0017] Put the raw materials into a medium-frequency induction melting furnace and heat them to 720-750℃. After the raw materials are completely melted, stir for 3-5 minutes to ensure that the alloying elements are evenly distributed.

[0018] When the temperature of the molten aluminum naturally drops to 710-730℃, high-purity argon gas with a purity ≥99.99% is introduced for rotary spraying refining. The rotor speed is controlled at 250-350 r / min, the argon gas flow rate is 0.4-0.6 m³ / h, the refining time is 10-20 min, and at the same time, 0.2%-0.5% of sodium-free refining agent by weight of molten aluminum is added.

[0019] During the refining process, argon gas bubbles carry hydrogen and oxide inclusions from the molten aluminum to the surface. After refining, the mixture is immediately left to stand for 15-30 minutes to allow the inclusions to float to the surface. Then, the surface scum is removed to obtain a pure molten aluminum with uniform composition and a gas content of ≤0.1ml / 100g. The temperature of the molten aluminum is maintained at 680-710℃ and it is used directly for subsequent casting.

[0020] Optionally, 0.05%-0.15% of rare earth element RE (selected from one or more combinations of Ce, La, and Y) is added during the smelting process. The rare earth element will act as a heterogeneous nucleation core to refine the grains during solidification, thereby further increasing the tensile strength of the casting by 5%-8%.

[0021] Furthermore, the specific steps of S2 are as follows:

[0022] When the aluminum liquid begins to be refined, the pre-prepared ceramic shell is transferred to the vacuum casting chamber, and the heating system is started to preheat the shell. The preheating temperature is controlled at 200-300℃ and the preheating time is 1-2 hours to make the temperature inside and outside the shell uniform and the temperature difference ≤20℃.

[0023] After the shell is preheated, immediately wrap its outer wall with 20-30mm thick aluminum silicate insulation cotton to slow down the overall heat dissipation rate of the shell; at the same time, open the cooling channels in the shell interlayer and introduce 0.2-0.3MPa compressed air into the deep cavity core area and the thick cross-section area for local precooling, establish a positive temperature gradient from the far end of the casting to the riser, and create conditions for subsequent sequential solidification.

[0024] Furthermore, the specific steps of S3 are as follows:

[0025] The vacuum casting chamber is evacuated to 20-50 kPa to eliminate air resistance in the cavity and prevent air entrapment during aluminum molten metal filling. The casting system is then started, employing a three-stage differential pressure casting process with continuously increasing pressure.

[0026] Initial filling stage: The pouring pressure is steadily increased from 0 to 10-30 kPa, and the filling speed is controlled at 0.5-2.0 cm / s. The aluminum liquid smoothly fills the bottom of the cavity, avoiding splashing and air entrapment due to excessive flow rate, until the aluminum liquid level rises to 1 / 3-1 / 2 of the cavity height.

[0027] Rapid filling stage: The pouring pressure is linearly increased from 10-30kPa to 30-60kPa, and the filling speed is simultaneously increased to 2-5cm / s to quickly fill the remaining cavity and prevent the aluminum liquid from cooling and solidifying during the filling process, thus preventing cold shut defects.

[0028] Pressure holding and solidification stage: After pouring, the pressure is immediately increased from 30-60 kPa to 50-100 kPa and maintained for 5-15 minutes, so that the aluminum liquid is continuously fed under high pressure, eliminating shrinkage cavities and porosity defects inside the casting;

[0029] After the pressure holding period is completed, immediately shut down the gating system and start the cooling system to enter the solidification and cooling stage, without waiting.

[0030] Furthermore, in S4, an alternating air-cooling and water-cooling method is adopted. The air-cooling medium is compressed air with a pressure of 0.2-0.4 MPa, and the water-cooling medium is circulating cooling water with a temperature of 20-30℃. The alternating cycle is 2-4 times, and the overall cooling rate is controlled at 5-20℃ / min, so that the casting temperature is uniformly reduced to 300-350℃.

[0031] Furthermore, in S5, when the cooling system stops, 4-6 pneumatic vibrators installed at the four corners and center symmetrical positions of the die-casting moving template and fixed template are automatically started to apply controlled vibration to the mold and internal castings.

[0032] The vibration parameters are as follows: excitation frequency: 20-100Hz, preferably using the 40-60Hz optimal resonance frequency band for aluminum alloy; vibration acceleration: 1-5g, preferably 2-3g, to ensure that the vibration energy can be transmitted to the interior of the casting; vibration time: 5-20min, preferably 8-12min; vibration waveform: sine wave or swept wave (sweep range 20-100Hz, sweep period 2-5min, 1-3 cycles).

[0033] During the vibration process, the vibration response is monitored in real time by an accelerometer arranged on the mold. The feedback control system automatically adjusts the excitation frequency and amplitude to keep the mold in a resonant or sub-resonant state, ensuring that the vibration energy is evenly transmitted to all parts of the casting.

[0034] After vibration aging is completed, the vibrator is immediately turned off. The casting is naturally cooled to room temperature under the rigid constraint of the mold (second stage cooling). The cooling rate is controlled at 1-3℃ / min to avoid generating new thermal stress due to excessive cooling. After cooling to room temperature, the casting is automatically ejected and enters the post-processing stage.

[0035] Optionally, a low-frequency electromagnetic field device can be activated simultaneously during the vibration aging process to apply a low-frequency electromagnetic field of 0.1-0.5T and 5-20Hz. The electromagnetic field can promote the movement of dislocations inside the aluminum alloy and shorten the vibration time by 20%-30%.

[0036] Furthermore, in S6, after the casting is ejected, the gating gate and burrs are removed by mechanical cutting, and then sandblasting is performed: 80-120 mesh corundum sand is used, the sandblasting pressure is 0.4-0.7MPa, and the sandblasting time is 10-20min to remove the residual shell and oxide scale on the surface of the casting, so that the surface roughness Ra reaches 1.6-3.2μm;

[0037] After sandblasting, the mold casting is sent to a heat treatment furnace for T6 heat treatment.

[0038] Solution treatment temperature is 530-550℃, and the holding time is 4-8h to allow alloying elements to fully dissolve into the aluminum matrix; the casting is then quickly immersed in water at 60-80℃ to cool and obtain a supersaturated solid solution; it is placed at room temperature for 24-48h to form the GP zone; artificial aging temperature is 150-180℃, and the holding time is 4-6h to precipitate strengthening phases and improve the hardness and strength of the mold casting.

[0039] After heat treatment, the mold castings undergo initial dimensional and surface quality checks. Qualified mold castings are directly installed on the die-casting machine without disassembly or transportation, and immediately subjected to in-situ laser shock peening treatment.

[0040] Furthermore, in step S7, the mold casting that has undergone T6 heat treatment is installed at the die-casting machine station and the mold is closed and fixed. Without disassembling the mold, laser shock strengthening treatment is performed at the die-casting machine station: the cavity surface is wiped with anhydrous ethanol to remove oil and dust, and then a 0.05-0.15mm thick layer of black paint is uniformly coated as a laser absorption layer. After the black paint dries, the cooling water system is turned on to form a 1-2mm thick flowing water film on the cavity surface at a flow rate of 1-3L / min as a constraint layer.

[0041] The pulsed laser system is activated, and the programmable scanning galvanometer automatically plans the scanning path based on the three-dimensional model of the mold cavity. It focuses on scanning areas with concentrated thermal stress, such as the deep cavity sidewalls, corners, and near the gate. The laser process parameters are: wavelength 532nm; pulse width 8-12ns; energy density 4-6GW / cm²; spot diameter 1-3mm; overlap rate 50%-70%.

[0042] During laser shock, a high-power pulsed laser acts on the absorption layer to generate a plasma explosion. The resulting shock wave propagates into the mold casting, forming a residual compressive stress layer and a nano-scale grain refinement layer with a depth of 0.5-1.0 mm on the surface, thereby improving surface hardness and thermal fatigue resistance.

[0043] After the strengthening is completed, the laser system and cooling water system are turned off, and the cavity surface is wiped with alcohol to remove the residual absorption layer. The mold casting can be put into production directly without any subsequent processing.

[0044] In addition, laser shock strengthening can be repeated periodically: after every 50,000 to 100,000 die-casting cycles in the service of the mold, the cavity surface is selectively strengthened again according to the above parameters during the 5-10 minute cooling gap between two die-casting cycles to restore the residual compressive stress layer on the surface and extend the total life of the mold.

[0045] Compared with existing technologies, the precision casting process for aluminum alloy castings used in refrigerator liner molds has the following advantages:

[0046] This invention utilizes a step-by-step differential pressure casting and multi-stage cooling control technology to ensure more stable and complete filling of the aluminum molten metal within the mold cavity, resulting in reduced porosity and increased tensile strength in the casting. Its performance surpasses that of traditional sand casting and metal mold gravity casting. Furthermore, during the temperature window period after complete solidification and before mold opening, controlled vibration is applied to the casting using mold constraints, which can reduce residual stress by more than 80%. Since the casting is under mold constraints, stress release occurs through microscopic dislocation movement without macroscopic deformation. Through laser shock in-situ strengthening technology, selective strengthening of the cavity surface is performed without disassembling the mold, thereby increasing surface hardness and more than doubling the number of thermal fatigue cycles.

[0047] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0049] Figure 1 A flowchart of a precision casting process for an aluminum alloy casting part used in a refrigerator liner mold;

[0050] Figure 2 This is a flowchart of step S3 in the precision casting process of an aluminum alloy casting for a refrigerator liner mold. Detailed Implementation

[0051] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structure, features and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0052] Example 1

[0053] This embodiment provides an integrated manufacturing process for a mold used for the inner liner of a 200L refrigerator. The mold cavity depth is 320mm, the maximum wall thickness is 50mm, and the minimum wall thickness is 8mm. Figure 1 As shown, the specific operation is as follows:

[0054] S1: Raw materials are precisely formulated by weight percentage: 92.27% pure aluminum ingot, 35% Al-20Si master alloy, 3.5% Al-10Mg master alloy, 3% Al-5Ti master alloy, 0.2% Al-10Sr master alloy, and 0.8% Al-10Y master alloy. The raw materials contain 0.09% Fe and 0.04% Cu.

[0055] Pure aluminum ingots were placed in a 100kg medium-frequency induction melting furnace and heated to 735℃ until completely melted. Then, Al-Si, Al-Mg, Al-Ti, Al-Sr, and Al-Y master alloys were added sequentially and stirred for 4 minutes to ensure uniform distribution of the alloying elements. When the temperature of the molten aluminum naturally dropped to 720℃, high-purity argon gas with a purity of 99.999% was introduced for rotary spray refining. The rotor speed was 300r / min and the argon gas flow rate was 0.5m³ / h. At the same time, 0.3% of the total mass of the molten aluminum was added as a sodium-free refining agent. The refining time was 15 minutes. After refining, the mixture was immediately allowed to stand for 20 minutes to allow the inclusions to float to the surface. The surface slag was then removed. At this point, the temperature of the molten aluminum was measured to be 695℃ and the gas content was 0.08ml / 100g, which met the casting requirements.

[0056] S2: Start the shell pretreatment process. The 8-layer ceramic shell prepared by the silica sol process is transferred to the vacuum casting chamber and heated to 250°C at a heating rate of 5°C / min. It is held at this temperature for 1.5 hours to ensure that the temperature difference between the inside and outside of the shell is ≤15°C. After preheating, immediately wrap the outer wall of the shell with 25mm thick aluminum silicate insulation cotton and fix it with high-temperature resistant tape. At the same time, open the 4 pre-embedded cooling channels in the shell interlayer and introduce 0.25MPa compressed air into the deep cavity core and the thick cross-section area at the bottom for local precooling to establish a positive temperature gradient from the bottom of the cavity to the top riser. At this time, the aluminum liquid refining and settling is just completed and the temperature is stable at 695°C. Immediately start the casting process.

[0057] S3: As Figure 2As shown, the vacuum casting chamber is evacuated to 30 kPa and kept stable. The bottom-pouring casting system is then started, with PLC controlling the continuous pressure changes: During the initial filling stage, the pressure linearly increases from 0 to 20 kPa in 30 seconds, with the filling speed controlled at 1.2 cm / s. The molten aluminum smoothly fills the bottom of the cavity until the liquid level rises to half the cavity height, taking approximately 8 minutes. During the rapid filling stage, the pressure linearly increases to 45 kPa within 2 minutes, and the filling speed simultaneously increases to 3.5 cm / s, quickly completing the filling of the remaining cavity (taking approximately 5 minutes). After pouring, the pressure immediately rises to 80 kPa and is maintained for 10 minutes for high-pressure compensation. After the pressure holding period, the pouring valve is automatically closed, and the cooling system is started simultaneously, with no waiting time.

[0058] S4: An alternating air-cooling and water-cooling cycle is adopted. First, 0.3MPa compressed air is introduced for air cooling for 2 minutes, and then switched to 25℃ circulating water cooling for 1 minute. This cycle is repeated 3 times, and the overall average cooling rate is controlled at 12℃ / min. The casting temperature is monitored in real time by a total of 8 K-type thermocouples placed at 3 points on the moving mold side, 3 points on the fixed mold side, and 2 points on the deep cavity core. When the temperature at all monitoring points is stable at 320℃ and the maximum temperature difference is 10℃, all cooling medium supply is immediately stopped. At this time, the casting has solidified.

[0059] S5: After the cooling system stops, the six pre-installed pneumatic vibrators (one at each of the four corners of the moving template, one at the center, and one at the center of the fixed template) are automatically started. The vibration parameters are set as follows: excitation frequency 50Hz, vibration acceleration 2.5g, sine wave vibration, and vibration time 10min.

[0060] During vibration, three accelerometers mounted on the mold collect vibration signals in real time. The feedback control system automatically fine-tunes the excitation frequency to keep the mold in a resonant state, ensuring that vibration energy is evenly distributed to all parts of the casting. After vibration, the vibrator is turned off, and the casting cools naturally to room temperature under the rigid constraint of the mold at a rate controlled at 2°C / min. The mold temperature is continuously monitored during cooling to prevent excessively rapid local cooling and the generation of new thermal stress. After cooling to room temperature, the die-casting machine automatically ejects the casting.

[0061] S6: After the casting is ejected, the gating gate and burrs are precisely cut using a CNC saw, with a cut surface flatness of ≤0.1mm. Then, sandblasting is performed: 100-mesh brown corundum sand is used, the sandblasting pressure is 0.5MPa, the distance between the spray gun and the casting surface is 150-200mm, the spray angle is 45°, and the sandblasting time is 15min to remove the residual shell and oxide scale on the surface. The surface roughness Ra is tested to be 2.5μm, which meets the surface requirements of laser shock peening.

[0062] After sandblasting, the castings are sent to a box-type heat treatment furnace for T6 heat treatment: the temperature is raised to 540℃ at a rate of 5℃ / min and held for 6 hours; after being taken out of the furnace, they are quickly quenched in water at 70℃ with a transfer time of ≤15s; they are then naturally aged at room temperature for 36 hours; the temperature is raised to 165℃ at a rate of 3℃ / min and held for 5 hours; and then the castings are cooled to below 100℃ before being taken out of the furnace. After heat treatment, the castings are subjected to preliminary dimensional inspection and surface flaw detection. If there are no defects such as cracks or porosity and the dimensional deviation is ≤0.08mm, the qualified castings are directly hoisted to the die-casting machine station and installed and fixed.

[0063] S7: After the mold casting is installed and fixed, there is no need to disassemble it. Laser shock strengthening is performed directly on the die casting machine station. The cavity surface is wiped repeatedly with anhydrous ethanol to remove oil and dust. Then, a 0.1mm thick black matte paint is evenly coated with a spray gun as a laser absorption layer and allowed to dry naturally for 20 minutes. The cooling water system is turned on to form a uniformly flowing water film on the cavity surface as a constraint layer at a flow rate of 2L / min.

[0064] Start the pulsed laser system, import the 3D model of the mold, and the programmable scanning galvanometer automatically plans the scanning path, focusing on scanning areas with concentrated thermal stress such as the deep cavity sidewalls, inner corners, and near the gate. The scanning speed is 500mm / s, and the laser process parameters are: wavelength 532nm, pulse width 10ns, energy density 5GW / cm², spot diameter 2mm, and overlap rate 60%. After the strengthening is completed, turn off the laser and cooling water systems, wipe the cavity surface with alcohol to remove residual black paint, and the mold can be directly put into production.

[0065] S8. Finished Product Inspection and Packaging: A coordinate measuring machine is used to perform final dimensional re-inspection of the mold, checking 50 key dimensional points. All dimensional tolerances are controlled within ±0.1mm. A fluorescent flaw detector is used to check the surface quality, ensuring there are no defects such as cracks, pores, or pits. After passing inspection, a thin layer of anti-rust oil is evenly applied to the cavity surface, wrapped in moisture-proof paper, placed in bubble wrap packaging bags, and then boxed and stored.

[0066] Example 2

[0067] The difference from Example 1 is that in S5, the vibration aging parameters are as follows: a sweep frequency wave is used, the sweep frequency range is 20-80Hz, the sweep frequency period is 3min, the cycle is 2 times, the excitation frequency is 40Hz, the acceleration is 3g, and the vibration time is 15min.

[0068] The remaining steps are exactly the same as in Example 1.

[0069] Example 3

[0070] The difference from Example 1 is that a low-frequency electromagnetic field is added in step S5: magnetic field strength 0.3T, frequency 10Hz, and vibration time shortened to 8min.

[0071] The remaining steps are exactly the same as in Example 1.

[0072] Example 4

[0073] The difference from Example 1 is that rare earth element Y is not added in step S1.

[0074] The remaining steps are exactly the same as in Example 1.

[0075] Example 5

[0076] After the mold prepared in Example 1 has been used for 100,000 die-casting cycles, laser shock in-situ strengthening is performed again during the 8-minute interval between two die-casting cycles, according to the parameters of S7 in Example 1.

[0077] Comparative Example 1

[0078] The traditional metal mold gravity casting process was adopted, and the aluminum alloy composition and T6 heat treatment were the same as in Example 1, without vibration aging and laser shock strengthening.

[0079] Comparative Example 2

[0080] The traditional sand casting process is adopted. After the casting is removed from the mold, it is subjected to room temperature vibration aging (frequency 30Hz, vibration for 30min) without laser shock strengthening.

[0081] Comparative Example 3

[0082] The same casting and laser shock strengthening process as in Example 1 was used, but the in-mold vibration aging step was eliminated (the mold was opened directly after cooling to room temperature).

[0083] Performance testing:

[0084] Residual stress detection: A blind hole method residual stress tester was used. Three test points were selected in each of the deep cavity corners, thick sections, and thin-walled areas of the casting. A blind hole with a diameter of 1.5 mm and a depth of 2 mm was drilled at each point. The strain change before and after drilling was measured, the residual stress value was calculated, and the average value of the nine points was taken as the final result.

[0085] Mechanical property testing: Samples were taken from standard tensile test bars cast in the same furnace and prepared into cylindrical tensile specimens with a gauge length of 50 mm and a diameter of 10 mm. Tensile tests were conducted at room temperature with a loading rate of 2 mm / min, and the tensile strength and elongation were recorded. Hardness specimens with a diameter of 10 mm and a thickness of 10 mm were prepared. A Brinell hardness tester was used with a 10 mm diameter cemented carbide ball to apply a load of 3000 kgf and hold it for 10 s. The Brinell hardness was calculated by measuring the indentation diameter. Three specimens were tested for each item, and the average value was taken.

[0086] Surface hardness testing: Using a Vickers hardness tester, 10 evenly distributed test points were selected on the surface of the cavity after laser shock strengthening. A load of 1 kgf was applied and held for 15 seconds. The diagonal length of the indentation was measured to calculate the Vickers hardness. After converting it to Brinell hardness, the average value was taken. At the same time, 5 points were selected in the unstrengthened area for comparison.

[0087] Thermal fatigue performance test: A 10mm×10mm×5mm sample is cut from the surface of the mold cavity. The actual service conditions of the mold are simulated by a thermal fatigue testing machine. The sample is heated to 120℃ and held for 30s, then water-cooled to 25℃ and held for 30s as one cycle. The cycle is repeated until cracks with a length ≥0.5mm appear on the surface of the sample. The number of thermal fatigue cycles is recorded.

[0088] Dimensional accuracy inspection: A coordinate measuring machine is used to inspect 50 key dimensional points of the mold, and the proportion of points with dimensional deviations exceeding ±0.1mm is statistically analyzed to calculate the dimensional deviation rate.

[0089] The performance test results are shown in the table below:

[0090] Table 1

[0091] performance indicators Example 1 Example 2 Example 3 Example 4 Example 5 Tensile strength of castings (MPa) 295 292 296 280 295 Elongation (%) 5.8 5.6 5.9 5.3 5.8 Brinell hardness (HBW) 102 101 103 97 102 Residual stress reduction (%) 82.4 84.4 85.7 79.7 82.4 Machining dimensional deviation rate (%) 0 0 0 0.5 0 Hardness after laser shock (HBW) 133 131 134 126 130 Thermal fatigue cycle life (10,000 cycles) 17.5 18.2 18.5 16.8 22

[0092] Table 2

[0093] performance indicators Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Tensile strength of castings (MPa) 295 210 225 293 Elongation (%) 5.8 3.2 3.5 5.7 Brinell hardness (HBW) 102 85 88 101 Residual stress reduction (%) 82.4 0 50 0 Machining dimensional deviation rate (%) 0 30 15 12 Hardness after laser shock (HBW) 133 - - 131 Thermal fatigue cycle life (10,000 cycles) 17.5 4.5 4 12.5

[0094] In summary, the precision casting and in-mold vibration aging integrated process for aluminum alloy castings used in refrigerator liner molds provided by this invention, through step-by-step differential pressure pouring and multi-stage cooling control technology, makes the aluminum liquid fill the cavity more smoothly and completely, reduces the porosity of the casting, and improves the tensile strength. Its performance is superior to traditional sand casting and metal mold gravity pouring. Furthermore, during the temperature window period after the casting has completely solidified and before the mold is opened, controlled vibration is applied to the casting using mold constraints, which can reduce residual stress by more than 80%. Since the casting is under mold constraints, stress release occurs through micro-dislocation movement without macro-deformation. Through laser shock in-situ strengthening process, selective strengthening of the cavity surface is performed without disassembling the mold, which increases the surface hardness and the number of thermal fatigue cycles by more than 2 times.

[0095] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A precision casting process for aluminum alloy castings used in refrigerator inner liner molds, characterized in that, The process includes: S1: Prepare the molten aluminum according to the set aluminum alloy composition and smelt it, and then refine and purify the molten aluminum. S2: Transfer the pre-prepared refrigerator inner liner mold to the vacuum casting chamber and preheat it, then cover the outer wall of the mold with insulation material; S3: After evacuating the vacuum casting chamber, the aluminum liquid is poured using a differential pressure pouring method with progressively increasing pressure. After pouring, the pressure is maintained to ensure that the aluminum liquid fully fills the cavity and solidifies. S4: Control the cooling process according to the principle of sequential solidification, and use alternating air cooling and water cooling to make the casting temperature drop uniformly to 300-350℃; S5: When the casting is completely solidified in the mold cavity and the temperature is within the temperature window of 300-350℃, start the pneumatic vibrator installed on the periphery of the die casting moving template and fixed template to apply controlled vibration to the mold and the internal casting. Through energy input, activate the internal dislocation movement and stress redistribution of the aluminum alloy, so that the residual stress of the casting is reduced before the mold is opened. S6: After the casting has cooled to room temperature, the mold is opened and the casting is ejected. The gating gates and burrs are removed, and the casting is sandblasted. Then, the casting is subjected to T6 heat treatment to obtain the mold casting. S7: Install the mold casting to the die casting machine station and close and fix the mold. Without disassembling the mold, use a pulsed laser to perform selective laser shock strengthening treatment on the cavity surface of the mold casting through a programmable scanning galvanometer, forming a residual compressive stress layer and a nano-scale grain refinement layer on the cavity mold surface. S8: Perform final dimensional re-inspection and surface quality inspection on the mold. The dimensional tolerance is controlled within ±0.1mm. The surface is free of defects such as cracks and pores. After passing the inspection, apply anti-rust oil and package it for storage.

2. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In S1, the aluminum alloy melt is prepared by weight percentage as follows: Si: 6.5-7.5%, Mg: 0.25-0.45%, Ti: 0.10-0.20%, Sr: 0.01-0.03%, Fe≤0.12%, Cu≤0.10%, Zn≤0.05%, with the balance being Al and unavoidable impurities.

3. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 2, characterized in that, In step S2, the ceramic mold shell is transferred to the vacuum casting chamber and preheated at 200-300℃ for 1-2 hours to ensure uniform temperature inside and outside the mold shell.

4. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In S3, the differential pressure casting method with step-by-step pressurization is specifically as follows: Initial filling stage: pouring pressure 10-30kPa, filling speed 0.5-2.0cm / s, pouring to 1 / 3-1 / 2 height of the mold cavity; Rapid filling stage: When the aluminum liquid level rises to the middle of the cavity, the pressure is increased to 30-60 kPa, and the filling speed is increased to 2-5 cm / s to quickly complete the filling of the remaining cavity; Pressure holding and solidification stage: Immediately after pouring, pressurize to 50-100 kPa and hold the pressure for 5-15 minutes to allow the molten aluminum to shrink under pressure.

5. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In step S4, an alternating air-cooling and water-cooling method is adopted. The air-cooling medium is compressed air with a pressure of 0.2-0.4 MPa, and the water-cooling medium is circulating cooling water with a temperature of 20-30℃. The alternating cycle is 2-4 times, and the overall cooling rate is controlled at 5-20℃ / min, so that the temperature of the casting is uniformly reduced to 300-350℃.

6. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In S5, the controlled vibration process parameters are: excitation frequency 20-100Hz, vibration acceleration 1-5g, vibration time 5-20min, and vibration waveform is a sine wave or a swept wave.

7. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In step S6, the process parameters for sandblasting are: sandblasting pressure 0.4-0.7MPa, sandblasting time 10-20min, so that the surface roughness Ra of the casting reaches 1.6-3.2μm.

8. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In S6, the process parameters for heat treatment T6 are: solution treatment temperature 530-550℃, holding temperature 4-8h; water quenching cooling, water temperature controlled at 60-80℃; natural aging at room temperature 24-48h; artificial aging temperature 150-180℃, holding temperature 4-6h.

9. The precision casting process for aluminum alloy castings used in refrigerator inner liner molds according to claim 1, characterized in that, In S7, the process parameters for laser shock strengthening are: wavelength 532nm, pulse width 8-12ns, energy density 4-6GW / cm², spot diameter 1-3mm, overlap rate 50%-70%; the depth of the residual compressive stress layer formed after shock is 0.5-1.0mm.

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