An efficient short-term heat treatment process for die-cast aluminum alloy castings

Abstract

The present invention relates to an efficient short-term heat treatment process for die-cast aluminum alloy castings. By adopting a solution treatment process with segmented stepped heat preservation, it is divided into several temperature segments for stepped heat preservation and solution treatment. The temperature difference between each segment is about 1°C - 40°C, and the heat preservation time is about 1 - 15 minutes. Under the action of temperature oscillation, the homogenization of alloy elements is achieved, and the maximum solid solubility of solute atoms such as copper, magnesium, zinc, and silicon is obtained, thereby effectively increasing the solid solubility of alloy elements, achieving solution strengthening of aluminum alloy die-castings, increasing their strength, and maintaining good toughness. The heat treatment process of the present invention greatly shortens the heat treatment time, saves energy, reduces carbon emissions, and achieves cost reduction and efficiency improvement. While maintaining good toughness, it significantly increases its strength and is superior to the traditional T6 heat treatment process. When heat treatment is carried out with stepped heat preservation, the high-temperature heat preservation time is greatly shortened, effectively avoiding the occurrence of bubbling phenomena, and enabling high-temperature solution treatment of ordinary die-castings.

Description

Technical Field

[0001] The present invention relates to the field of aluminum alloy casting, and particularly to an efficient short-term heat treatment process for die-cast aluminum alloy castings. Background Art

[0002] With the development of modern industry and science and technology, people's requirements for new materials, new energy, and environmental protection have deepened, and the desire for high-quality, high-strength, high-toughness, and high-conductivity die-cast aluminum alloy parts is becoming stronger and stronger. Under the background of such economic conditions, the performance requirements for die-cast aluminum alloys are getting higher and higher. It is very necessary to further improve the various performance indicators of alloy materials and give full play to the potential of materials to adapt to the high-quality development of current society. Most die-cast aluminum is AL-Si-Cu series alloys, such as ADC12, A380 (ADC10), etc. These alloy materials belong to heat-treatable reinforced casting aluminum alloys, have good casting performance, high casting strength, small thermal expansion coefficient, high wear resistance, and good cutting performance. Heat treatment strengthening is one of the important processes to improve the various performance indicators of alloy materials. The purpose of heat treatment strengthening is not to change the shape and overall chemical composition of the workpiece, but to change its structure, physical, chemical, and mechanical properties. According to the nature and use of the alloy, a certain heat treatment specification can be selected to control the heating rate, cooling rate, and holding time at the corresponding temperature to obtain the expected structure, physical properties, chemical properties, and mechanical properties, which is a metal hot working process.

[0003] Among all aluminum alloy casting methods, die casting and centrifugal casting cannot perform conventional heat treatment. Because a large amount of air is simultaneously pressed into the die-cast aluminum liquid in an instant, the generation of these internal pores is due to the presence of entrained gases composed of air, nitrogen, hydrogen, or vapor formed by decomposition. After the casting solidifies, these gases are sealed inside the casting (any die-cast part can be dissected at will to observe its internal situation). And when the die-cast part is reheated to a certain temperature, these gases inside the casting will expand, causing the surface of the casting to blister. This results in blistering on the surface of the die-cast part during the high-temperature heat treatment process exceeding 300 °C, leading to defective die-cast parts. The main method to improve the mechanical properties of die-cast aluminum alloy castings is to change the original structure, such as adding ribs, etc.

[0004] In addition, there are also heat treatment methods using special processes, such as high vacuum, semi-solid state, extrusion, oxygen filling, precision speed densification, low gas content, etc. The die-castings produced have few or no pores inside, so they can still be subjected to T4: solution treatment plus natural aging, T5: solution treatment plus incomplete artificial aging, T6: solution treatment plus complete artificial aging, and T7: solution treatment plus stabilization treatment. Because the prerequisite for obtaining a supersaturated structure and improving the mechanical properties of die-cast aluminum alloy parts is that the strengthening phase in the die-cast alloy parts dissolves into the aluminum matrix at about 15 - 20 °C below the solidus temperature. However, the solution treatment time is relatively long, 6 - 8 hours, plus homogenization (aging) for 3 - 6 hours, resulting in increased costs, long cycles, and possible inability to keep up with production capacity and an increase in uncontrollable factors.

[0005] The Chinese invention patent with the patent application number CN202010472114.1 provides a high-performance die-cast aluminum alloy material high-efficiency heat treatment method, which discloses that in the high-efficiency heat treatment method of the aluminum alloy material, the vacuum degree in the vacuum pressure casting process is controlled within 50 - 150 Bar, and the punching filling injection speed is controlled between 0.2 - 2 m / s; it also includes a high-efficiency T6 heat treatment process, which includes three steps: medium-high temperature solution Ta, constant temperature water quenching Tb, and artificial aging T6 heat treatment process Tc: Ta: treated at 460 - 550 °C for 10 - 100 min; Tb: quenched in warm water at 20 - 80 °C; Tc: artificial aging for 60 - 240 min at 140 - 220 °C. However, this heat treatment scheme is based on minimizing the gas content of the die-cast aluminum alloy in the previous process to obtain the conditions for solution heat treatment, and it cannot be applied to conventional die-cast aluminum alloy parts. Summary of the Invention

[0006] (I) Technical problems to be solved

[0007] To solve the above problems of the prior art, the present invention provides a high-efficiency short-term heat treatment process for die-cast aluminum alloy castings.

[0008] (II) Technical solutions

[0009] To achieve the above object, the main technical solutions adopted by the present invention include:

[0010] A high-efficiency short-term heat treatment process for die-cast aluminum alloy castings, comprising the following steps:

[0011] S1: Perform solution treatment in several segments at a temperature lower than the solidus temperature of the die-cast aluminum alloy casting; the solution treatment includes at least two heating and holding steps, the temperature difference between adjacent two segments of the solution treatment is 1 °C - 40 °C, and the holding time for each segment of the solution treatment is 1 - 15 min;

[0012] S2: Quench;

[0013] S3: Aging treatment.

[0014] Further, the holding time of the first-stage solution treatment is the heating time from the solution temperature to the core of the product calculated according to the thickness of the die-cast aluminum alloy casting; the solution temperature is 15°C - 20°C lower than the solidus temperature.

[0015] Further, the core of the product refers to the middle part of the average thickness of the die-cast aluminum alloy part.

[0016] Further, the heating time in step S1 is calculated by formula (1);

[0017]

[0018] where k is the heating rate coefficient, taking 2.0 - 3.6 min / mm for die-cast aluminum alloy; V is the volume of the casting in mm 3 ; A is the surface area of the casting in mm 2 . represents the average thickness.

[0019] Further, the grade of the die-cast aluminum alloy is ADC12, and its corresponding solidus temperature is 510°C; the solution temperature is 495°C - 500°C, and the solution temperature is used to calculate the heating time;

[0020] Further, the grade of the die-cast aluminum alloy casting is any one of ADC12, A380, and A356;

[0021] Further, step S1 specifically includes:

[0022] S11: The first-stage solution treatment, heating the die-cast aluminum alloy part to 460°C - 490°C and holding for 5 - 10 min;

[0023] S12: The second-stage solution treatment, heating up to 465°C - 505°C and holding for 5 - 15 min;

[0024] S13: The third-stage solution treatment, heating up to 500°C - 540°C and holding for 5 - 10 min;

[0025] S14: The fourth-stage solution treatment, cooling down to 460°C - 500°C and holding for 5 - 10 min.

[0026] Further, step S1 specifically includes:

[0027] S11: The first-stage solution treatment, heating the die-cast aluminum alloy part to 460°C - 490°C and holding for 5 - 10 min;

[0028] S12: The second solution treatment, heating up to 465°C - 505°C and holding for 5 - 15 minutes;

[0029] S13: The third solution treatment, heating up to 500°C - 540°C and holding for 5 - 10 minutes;

[0030] Further, the quenching is carried out by water quenching with water at 20°C - 35°C for 20 - 60 seconds.

[0031] Further, the aging treatment is carried out by holding at 170°C - 220°C for 5 - 10 hours.

[0032] Further, the aging treatment includes two-stage aging treatments with a temperature difference of 20°C - 50°C, and the total time of the aging treatment is 2 - 3.5 hours.

[0033] Further, the aging treatment includes first insulating at 160°C - 180°C for 1.5 - 2.5 hours, and then holding at 180°C - 220°C for 0.5 - 1 hour.

[0034] (III) Beneficial effects

[0035] The beneficial effects of the present invention are as follows: 1. In ordinary die casting, due to high-speed filling, the mold cavity is mostly filled in the form of spraying and turbulence, resulting in air entrapment inside ordinary die castings. To improve the mechanical properties of die-cast aluminum alloy castings, it is necessary to solve this problem through T6 solution treatment. Due to the air entrapment inside the castings, it is easy to cause the castings to bulge. The commonly used method in the market is 15 - 20°C below the solidus temperature. For example, the solidus of A380 is 540°C, that is, the solution temperature is 520°C - 535°C, and it is placed at this temperature for 4 - 6 hours, so it is easy to bulge (as Figure 1 ). The advantage of the present invention is that the time is short, and the short-time temperature will not cause bulging (as Figure 2 ). The solution heat treatment process of the present invention adopts stepped heat preservation to achieve the homogenization of alloy elements and obtain the maximum solid solubility of solute atoms such as copper, magnesium, zinc, and silicon under the action of temperature oscillation. For example: when heating and solidifying at a temperature below 450°C, as the temperature increases, the content of alloy elements in the solid solution increases, and the supersaturation during quenching also increases. When the temperature exceeds 450°C and reaches the solid solution temperature of the alloy, no more alloy elements will enter the solid solution. However, the number of vacancies will increase, thus increasing the number of vacancies after quenching. Since vacancies can increase the diffusion rate, the aging time can start earlier. Furthermore, it effectively improves the solid solubility of alloy elements, realizes the solution strengthening of die-cast aluminum alloy castings, improves their strength, and maintains good toughness. The high-temperature heat preservation time is greatly shortened, which can effectively avoid the appearance of bulging on the product surface, can realize the high-temperature solution treatment of ordinary die castings, improve their strength, and greatly improve the yield of products.

[0036] 2. Greatly shortened the time (the solution time was shortened from the previous 6 - 8 hours to 20 - 45 minutes) and aging time (from the conventional 5 - 10 hours to 2 - 3 hours), saving costs, reducing carbon emissions, and achieving cost reduction and efficiency improvement.

[0037] 3. It can significantly improve its strength while maintaining good toughness, and is superior to the traditional T6 heat treatment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings.

[0039] Figure 1 It is a physical photo of an A380 die-cast part after heat treatment by the traditional T6 process;

[0040] Figure 2 It is a physical photo of an A380 die-cast part of the present invention after heat treatment;

[0041] Figure 3 It is a metallographic diagram of an ADC12 die-cast part in the as-cast state;

[0042] Figure 4 It is a metallographic diagram of an ADC12 die-cast part of the present invention after heat treatment;

[0043] Figure 5 It is a physical photo of the base die-cast part of Embodiment 1 of the present invention;

[0044] Figure 6 It is a physical photo of the front fork die-cast part of Embodiment 2 of the present invention;

[0045] Figure 7 It is a physical photo of the base of Embodiment 3 of the present invention;

[0046] Figure 8 It is a physical photo of an A380 die-cast part of the present invention after heat treatment;

[0047] Figure 9 It is a physical photo of an A380 die-cast part after heat treatment by the traditional T6 process;

[0048] Figure 10 It is a physical photo of an ADC12 die-cast part of the present invention after heat treatment;

[0049] Figure 11 It is a physical photo of an ADC12 die-cast part after heat treatment by the traditional T6 process;

[0050] Figure 12It is the experimental test data graph of sample No. 12A5;

[0051] Figure 13 It is the experimental test data graph of sample No. 12B10;

[0052] Figure 14 It is the experimental test data graph of sample No. 38B2;

[0053] Figure 15 It is the experimental test data graph of sample No. A3B2;

[0054] Figure 16 It is the experimental test data graph of sample No. CNB1;

[0055] Figure 17 It is the experimental test data graph of sample No. NB1; Detailed implementation manners

[0056] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the present invention to be protected, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0057] Example 1: Base die-casting part, material ADC12, heat treatment process:

[0058] (1) The first stage of solution treatment, the die-cast aluminum alloy part is heated to 465 °C and held for 5 min;

[0059] (2) The second stage of solution treatment, heated to 475 °C and held for 5 min;

[0060] (3) The third stage of solution treatment, heated to 500 °C and held for 10 min;

[0061] (4) The fourth stage of solution treatment, cooled to 465 °C and held for 5 min;

[0062] (5) Water quenching for 20 s in water at 20 °C;

[0063] (6) Aging treatment at 170 °C for 1.5 h + 210 °C for 0.5 h, air cooling.

[0064] The performance test results are shown in Table 1, and the microstructures of the as-cast and heat-treated states are shown in Figure 3 and 4 respectively.

[0065] The general heat treatment method in Table 1 selects the heat treatment process at the best tensile strength on page 23 of "The Influence of Heat Treatment on the Microstructure and Properties of ADC12 Aluminum Alloy; Li Xingjie; Northeastern University; 2005": solution treatment temperature 520 °C, solution treatment time 6 hours, artificial aging temperature 170 °C, artificial aging time 10 hours.

[0066] Table 1

[0067]

[0068] Example 2: Front fork die-casting part, material A380 (ADC10), heat treatment process:

[0069] (1) The first stage of solution treatment, heating the die-cast aluminum alloy part to 485 °C and holding for 10 min;

[0070] (2) The second stage of solution treatment, heating to 495 °C and holding for 15 min;

[0071] (3) The third stage of solution treatment, heating to 520 °C and holding for 5 min;

[0072] (4) The fourth stage of solution treatment, cooling to 475 °C and holding for 5 min;

[0073] (5) Water quenching in water at 20 °C for 20 s;

[0074] (6) Aging treatment at 170 °C for 1.5 h + 210 °C for 0.5 h, air cooling.

[0075] The performance test results are shown in Table 2.

[0076] The general heat treatment method in Table 2 selects "Research on the Microstructure and Properties of Liquid and Semi-Solid A380 Aluminum Alloy Die-Castings before and after Heat Treatment; Materials Research and Application, Vol. 2, No. 3; Liu Yanhua; 2008", after T6 heat treatment, heat treatment temperature 485 °C, holding for 2 h, then cooling with water at 50 - 80 °C, and then aging at 170 °C for 6 h.

[0077] Table 2

[0078]

[0079]

[0080] Example 3: Base, material A356, heat treatment process:

[0081] (1) The first solution treatment: The die-cast aluminum alloy parts are heated to 495 °C and held for 6 minutes.

[0082] (2) The second solution treatment: Heat up to 505 °C and hold for 5 minutes.

[0083] (3) The third solution treatment: Heat up to 530 °C and hold for 15 minutes.

[0084] (4) The fourth solution treatment: Cool down to 495 °C and hold for 6 minutes.

[0085] (5) Quench in water at 20 °C for 20 s.

[0086] (6) Aging treatment: Hold at 170 °C for 2 h + hold at 210 °C for 0.5 h, and then air cool.

[0087] The performance test results are shown in Table 3.

[0088] In Table 3, the general heat treatment method is the ordinary T6 solution aging process for A356 aluminum alloy treatment. The basic process is solution treatment at high temperature, then the specimen is placed at room temperature for a period of time and then aged at high temperature. The specific treatment process is as follows: The specimen is placed in an ordinary box-type resistance furnace at a temperature of 535 °C for solution treatment. After holding for 5 hours, it is quickly transferred to a water bath at a temperature of 60 °C for quenching; then the specimen is placed in an incubator at 175 °C for high-temperature final aging treatment. After holding for 5 hours, it is taken out of the furnace and air cooled.

[0089] Table 3

[0090]

[0091]

[0092] It can be seen from the above Tables 1 - 3 that the tensile strength and specified plastic elongation strength of the castings heat-treated by the method of the present invention have been significantly improved compared with the as-cast state. And compared with the general heat treatment method, both the tensile strength and specified plastic elongation strength have corresponding improvements, while the treatment time has been significantly reduced. The elongation at break is at the same level as the general heat treatment method, and there are corresponding improvements in some embodiments.

[0093] Through short-time segmented solution treatment and large-gradient cooling, solute atoms can be rapidly dispersed and dissolved in the matrix, enabling full and effective solution heat treatment. This results in a more refined and uniform solution grain size compared to traditional long-time high-temperature solution treatment, as well as enhanced strength and toughness. On the basis of achieving the same or even better performance, the time is significantly shortened (from the previous 6 - 8 hours of solution time to 20 - 45 minutes), costs are saved, and carbon emissions are reduced. Useful T6 strengthening heat treatment can be fully carried out, doubling the as-cast strength and toughness. At the same time, due to the short heat treatment time in the present invention, no blisters will form during short-time solution treatment, greatly improving the product yield.

[0094] For alloys in which the solubility of the second phase in the matrix phase decreases significantly with decreasing temperature, they can be heated to a temperature at which the second phase can dissolve completely or to the maximum extent into the solid solution. After holding for a certain time, they are cooled (quenched) at a rate faster than the precipitation rate of the second phase from the solid solution, and a supersaturated solid solution can be obtained. This heat treatment process for obtaining a supersaturated solid solution is called solution treatment or quenching.

[0095] Quenching refers to the process of rapidly cooling a metal from the solution heating temperature, which is usually 465 - 565 °C (870 - 1050 °F) for aluminum alloys. The main purpose of quenching is to retain the metastable solid solution formed at the solution temperature to a lower temperature (room temperature) as much as possible through rapid cooling. When the quenching cooling rate is fast enough, a supersaturated solid solution can be obtained. Through natural aging at room temperature or artificial aging at an appropriate temperature, uniform solute atom precipitation regions (coherent or semi-coherent) are formed in the matrix to strengthen the alloy. Another purpose of quenching is to obtain a certain number of vacancies to increase the diffusion rate under the precipitation strengthening aging temperature conditions. If precipitation occurs during the quenching process, it will lead to local over-aging, reducing the corrosion resistance of the grain boundaries, and more seriously, reducing the aging strengthening effect. When the quenching rate is not fast enough, solute atoms diffuse to the grain boundaries, and vacancies migrate to the disordered region at an extremely fast speed, resulting in the failure to achieve the purpose of aging precipitation strengthening.

[0096] The main factors affecting solution treatment are heating temperature, holding time, and cooling rate. The heating temperature is generally also called the quenching temperature. The higher the quenching temperature and the longer the holding time, the more fully the strengthening phase dissolves, the more uniform the distribution of alloying elements in the lattice, and the more the vacancy concentration in the lattice increases. Combining these factors can better promote the improvement of the aging effect.

[0097] The optimum quenching heating temperature is the one that can ensure the maximum number of strengthening phases dissolve into the matrix without causing overburning or grain growth. The holding time should ensure that the strengthening phases that can dissolve into the solid solution are fully dissolved to obtain the maximum supersaturation. Therefore, the holding time of cast alloys, especially as-cast alloys with complex compositions and coarse strengthening phases, is much longer than that of wrought alloys. However, too long a holding time will cause grain growth in wrought alloys and some multiphase cast alloys that form strong internal stresses during the casting process.

[0098] The solution treatment of aluminum alloys mainly has three purposes: redissolving the precipitated phases to form a supersaturated solid solution to prepare for the aging treatment of the alloy; homogenizing the castings and granulating the eutectic silicon phase. The decrease in the solid solubility of the alloy with the decrease in temperature is the prerequisite for solution treatment. To obtain the maximum solution effect, the solution temperature is usually raised as close as possible to the eutectic temperature, but overburning should not occur.

[0099] Short-time aging heat treatment can enable the castings to be heat treatment strengthened. During the die-casting process, in the molten state, the alloying elements have diffused. As the molten metal temperature drops during the die-casting process, the alloying elements that have not dissolved into the matrix will precipitate in the form of the second phase. Therefore, heat treatment is required to redissolve the second phase into the matrix (solution treatment) to improve the strength of the castings. Through relevant experimental verification, it is found that short-time heat treatment is feasible. Due to the rapid solidification characteristics of the high-pressure casting process, the second phase on the outer layer of the die-cast casting is relatively uniform, and the precipitation of the second phase in the interior is more serious. Theoretically, the homogenization time of heat treatment can be correspondingly reduced, and instead of being calculated in hours, it is calculated in minutes with a very short homogenization time. Short-time solution treatment will not cause blister formation, so the appearance is acceptable. The aging treatment is carried out at a lower temperature at which there is no risk of blister formation, so the useful T6 strengthening heat treatment can be fully carried out, which can improve the as-cast strength and toughness. The present invention effectively realizes short-time solution treatment through segmented solution treatment, and realizes the rapid dissolution of the second phase into the matrix.

[0100] The forming process of die casting can be understood as a heat treatment process. From the entry of the molten metal into the cavity to solidification, it is also a process of atomic rearrangement. Frankly speaking, the molten melt after melting is a supersaturated body, and the solidification process is also a process of precipitation of the second phase (such as Al2Cu) in the casting. However, due to the rapid solidification process when the melt enters the cavity, to a certain extent, the precipitation of the second phase on the surface is reduced. Therefore, theoretically, the time required for solution heat treatment of die-cast aluminum alloys is shorter. By calculating the heating time from the solution temperature to the core of the product as the holding time of the first-stage solution, after ensuring that the temperature is evenly transferred to the inside of the casting, the second-stage solution treatment is carried out. By controlling the holding time of the first-stage solution treatment, partial solution of the second phase (Al2Cu) in the aluminum matrix can be achieved within the shortest and appropriate time. Since the solubility of the second phase (Al2Cu) in the matrix is affected by temperature, it is not that the temperature 15°C - 20°C below the conventional solidus temperature is the most suitable temperature. Moreover, the purpose of the first-stage solution in the present invention is not to increase the solution degree of the second phase in the aluminum matrix, but to utilize the heating process of the casting to increase the solution degree of the second phase as much as possible at a suitable temperature and time, and reduce the tendency of the grains to grow coarser, effectively maintaining the initial fine grain state and achieving the homogenization of the casting; then, by changing the temperature for the second-stage solution treatment, the solution efficiency of the corresponding second phase in the matrix will change, and the purpose of the two-stage solution is more mainly to improve the uniform distribution of alloy elements; through the segmented solution treatment, the time of solution at high temperature can be shortened, so that the temperature of the air holes in the die-cast part will not rise too high, generating too large a thermal expansion force, thus causing the surface of the die-cast part to bulge.

[0101] The first-stage solution treatment mainly realizes the partial solution of the second phase (Al2Cu) in the aluminum matrix. At this temperature, the second phase (Al2Cu) can be quickly dissolved in a short time, and the tendency of the grains to grow coarser is reduced, effectively maintaining the initial fine grain state and achieving the homogenization of the casting.

[0102] For the second-stage solution and the third-stage solution, by slowly increasing the temperature and reducing the jump change of the temperature, the stable jump of alloy elements between the crystal lattices is realized at a lower temperature, thereby improving the uniformity of distribution and avoiding the vacancy jump caused by too high a temperature change.

[0103] In the process of heat treatment, in a crystal, the vibration frequency of atoms at the equilibrium position is 10 12 ~10 13 to the power of HZ, which generates lattice vibration. Sometimes, this amplitude is sufficient to cause atoms to jump from one position in the lattice to another position, resulting in the diffusion of atoms. The two most common diffusion mechanisms are interstitial diffusion and vacancy diffusion.

[0104] As the heating temperature increases, the vibrational energy of atoms increases, thereby increasing the likelihood of atoms in the material jumping from one position in the lattice to another. The vibrating atoms may exchange positions with adjacent atoms in the lattice, or may transfer to adjacent vacancy positions or defect positions. This atomic movement is called diffusion. Heat treatment heating increases the diffusion rate of atoms. In the present invention, the slow increase in the temperature of the second solution treatment and the third solution treatment and the control of the holding time can effectively reduce the probability of atomic jumping, making it more likely to move into adjacent lattices, thereby obtaining a more uniform solution-treated grain size.

[0105] The main purpose of the fourth solution treatment is to reduce the quenching temperature difference by appropriately reducing the holding temperature, to avoid excessive internal stress and brittle cracking of the surface metal structure caused by rapid cooling of the casting, resulting in increased corrosion caused by the precipitation of some chemical agents during subsequent surface treatment of the casting. Moreover, the preparation method of reducing the temperature difference is used to microscopically reduce the grain boundary gap and improve the stability of alloying elements that are already evenly distributed in the lattice, thereby enhancing the solution treatment effect. However, relevant experiments have shown that for some die-castings with simple structures, the fourth solution treatment can also be omitted. Although the fourth solution treatment can further enhance the solution treatment effect before quenching, omitting this step has little impact on the mechanical properties of the final casting. However, omitting this step can further reduce the heat treatment time, achieve true short-time aging heat treatment, and has better mechanical properties compared to traditional or existing heat treatment processes.

[0106] Therefore, through staged solution treatment, the solution degree of the second phase and the uniformity of the distribution of alloying elements are effectively improved in a short time, and by taking advantage of short-time heat treatment, problems such as grain coarsening and bubbling are avoided;

[0107] A large number of studies and experiments have shown that the solution treatment holding time has an impact on mechanical properties. Generally, a long holding time can achieve the full incorporation of solution-strengthening phases into the matrix to obtain the maximum supersaturation. However, prolonging the solution treatment holding time is not beneficial for castings and forgings formed under pressure. Prolonging the solution treatment holding time will lead to oxidation, bubbling, and grain growth. In particular, if the grains of the product grow, it will lead to a decrease in mechanical properties. Therefore, in the prior art, although the heat treatment of castings formed under high-speed filling achieves the purpose of solution strengthening, the grain growth caused by long-term single-temperature holding is inevitable, resulting in limited improvement in mechanical properties. At the same time, the problem of bubbles in castings formed under pressure is difficult to solve with long-term holding.

[0108] The supersaturated solid solution obtained by quenching is in an unbalanced state, so there is a spontaneous tendency to decompose and precipitate excess solute atoms (precipitate in the form of the second phase). Some alloys start this precipitation process at room temperature, but due to the low temperature, generally only the initial stage of precipitation can be completed. Some alloys only start this precipitation after the temperature rises and the atomic mobility increases. The former is called natural aging, and the latter is called artificial aging or tempering. The age hardening of aluminum alloys is a rather complex process, which not only depends on the alloy composition, aging process, but also on the defects caused during the production process of the alloy, especially the quantity and distribution of vacancies and dislocations, etc. It is generally believed that age hardening is the result of the segregation of solute atoms to form hardening zones.

[0109] After the aluminum alloy is quenched and heated, vacancies are formed in the alloy. During quenching, due to the fast cooling rate, these vacancies do not have time to move out and are "fixed" in the crystal. Most of these vacancies in the supersaturated solid solution are combined with solute atoms. Since the supersaturated solid solution is in an unstable state and must transform to the equilibrium state, the existence of vacancies accelerates the diffusion rate of solute atoms, thus accelerating the segregation of solute atoms.

[0110] The new phase precipitated from the solid solution is generally in the form of flakes. Further extending the time or raising the temperature, the dispersed second phase will agglomerate and coarsen. The higher the temperature, the faster the coarsening. Aging increases the strength and hardness of the alloy, but decreases the plasticity and corrosion resistance.

[0111] Usually, the time for artificial aging of aluminum alloys is 5 - 10 hours. However, in the present invention, through experiments, it is found that by jumpwise segmented artificial aging temperature, the artificial aging time can be effectively reduced without affecting the mechanical properties. The realization of this effect has been fully verified by numerous experimental data, and it can indeed significantly reduce the aging time. Due to the multiple temperature changes, the vacancies formed in the alloy due to thermal expansion and contraction shrink, and the transformation of the solid solution under temperature changes is more rapid, accelerating the diffusion rate of solute atoms, and the time required to transform into a stable state is less. At the same time, due to the short time of segmented solution treatment in the previous process, the second phase is not completely dissolved into the matrix, so the content of the formed supersaturated solid solution is relatively lower, and the aging treatment time will also be reduced accordingly.

[0112] The process of the present invention can also be applied to aluminum alloys formed by low-pressure casting, differential pressure casting, and centrifugal casting, effectively heat-treating them, reducing the heat treatment time and improving the mechanical properties.

[0113] The above are only the embodiments of the present invention, and thus do not limit the patent scope of the present invention. All equivalent transformations made by using the content of the specification and drawings of the present invention, directly or indirectly applied in the relevant technical fields, are similarly included in the patent protection scope of the present invention.

Claims

1. An efficient short-time heat treatment process for die-cast aluminum alloy castings, characterized in that: Including the following steps: S1: Perform several stages of solution treatment at a temperature lower than the solidus temperature of the die-cast aluminum alloy casting; the solution treatment includes at least two steps of holding temperature after heating, the temperature difference between adjacent two stages of solution treatment is 1°C - 40°C, and the holding time for each stage of solution treatment is 1 - 15 min; the solution treatment adopts stepped heat preservation to achieve the homogenization of alloy elements and obtain the maximum solid solubility of solute atoms under the action of temperature oscillation; S2: Quench; S3: Aging treatment; Among them, the holding time of the first stage of solution treatment is the heating time from the solution temperature to the core of the product calculated according to the thickness of the die-cast aluminum alloy casting; the solution temperature is 15°C - 20°C lower than the solidus temperature; The heating time is calculated by formula (1); Where k is the heating-up coefficient, and for die-cast aluminum alloy, it is taken as 2.0 - 3.6 min / mm; V is the volume of the casting, in mm 3 ; A is the surface area of the casting, in mm 2 .

2. The high-efficiency short-duration heat treatment process for die-cast aluminum alloy castings according to claim 1, wherein: The core of the product refers to the middle part of the average thickness of the die-cast aluminum alloy casting.

3. The high-efficiency short-duration heat treatment process for die-cast aluminum alloy castings according to claim 1, characterized in that: The solution treatment in step S1 also includes a step of holding temperature after cooling.

4. A high-efficiency short-time heat treatment process for die-cast aluminum alloy castings according to claim 1, characterized in that: Step S1 specifically includes: S11: The first stage of solution treatment, heating the die-cast aluminum alloy casting to 460°C - 490°C and holding the temperature for 5 - 10 min; S12: The second stage of solution treatment, heating to 465°C - 505°C and holding the temperature for 5 - 15 min; S13: The third stage of solution treatment, heating to 500°C - 540°C and holding the temperature for 5 - 10 min.

5. The high-efficiency short-time heat treatment process for die-cast aluminum alloy castings according to claim 3, characterized in that: Step S1 specifically includes: S11: The first stage of solution treatment, heating the die-cast aluminum alloy casting to 460°C - 490°C and holding the temperature for 5 - 10 min; S12: The second stage of solution treatment, heating to 465°C - 505°C and holding the temperature for 5 - 15 min; S13: The third stage of solution treatment, heating to 500°C - 540°C and holding the temperature for 5 - 10 min; S14: The fourth stage of solution treatment, cooling to 460°C - 500°C and holding the temperature for 5 - 10 min.

6. The high-efficiency short-term heat treatment process for die-cast aluminum alloy castings according to claim 1, wherein: The quenching is carried out by water quenching with water at 20°C - 35°C for 20 - 60 s.

7. An efficient short-time heat treatment process for die-cast aluminum alloy castings according to claim 1, characterized in that: The aging treatment is to hold the temperature at 170°C - 220°C for 5 - 10 h.

8. A high-efficiency short-time heat treatment process for die-cast aluminum alloy castings according to claim 1, characterized in that: The aging treatment includes two stages of aging treatment with a temperature difference of 20°C - 50°C, and the total time of the aging treatment is 2 - 3.5 h.

9. An efficient short-duration heat treatment process for die-cast aluminum alloy castings according to claim 1, characterized in that: The aging treatment includes first insulating at 160°C - 180°C for 1.5 - 2.5 h, and then holding the temperature at 180°C - 220°C for 0.5 - 1 h.

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