A heat treatment method for laser additive manufacturing aluminum alloy performance and stress

Abstract

The application discloses a heat treatment method for laser additive manufacturing of aluminum alloy, which takes into account the performance and stress relief of the aluminum alloy. Based on the fact that the mechanical strength of AlSi10Mg alloy significantly decreases after stress relief annealing, the optimal solid solution temperature is determined through multiple DSC tests. Based on the characteristics of the supersaturated solid solution matrix of laser additive manufacturing, the solid solution time is further optimized. Secondly, based on the Ostwald ripening and LSW coarsening theory, the aging temperature and time are optimized and determined, so that a large number of nano-sized Si particles are precipitated in the further supersaturated Al matrix after solid solution, and the optimal nano-particle size and volume fraction are obtained, thereby improving the mechanical strength of the alloy to make up for the loss of mechanical properties caused by stress relief annealing. Compared with the traditional similar T6 heat treatment process system, the optimal heat treatment temperature and time are further optimized and determined, so that the mechanical strength reaches the initial deposition state level, and the effect is unique and significant.

Description

Technical Field

[0001] This invention belongs to the field of metal laser additive manufacturing and relates to a heat treatment method that takes into account both the performance of aluminum alloys manufactured by laser additive manufacturing and stress relief. Background Technology

[0002] AlSi10Mg alloy is a traditional cast aluminum alloy, a near-eutectic Al-Si alloy, widely used in laser powder bed melting (LPBF) technology due to its good formability. LPBF technology features rapid cooling rates, producing AlSi10Mg alloys with a microstructure characterized by fine α-Al dendrites and an intragranular substructure composed of eutectic Si. Compared to cast AlSi10Mg alloys, this unique microstructure not only strengthens the grains but also gives the alloy good work hardening capabilities. However, the complex thermal stress during LPBF forming results in significant residual stress in the formed parts, requiring stress relief treatment before practical application. The most mature stress relief method is stress-relief annealing, specifically at 280–300℃ for 1–2 hours. However, under this condition, the unique network-like eutectic cellular structure within the alloy is destroyed, resulting in a significant decrease in the material's mechanical strength and negating the unique characteristics and significant advantages of laser additive manufacturing of aluminum alloys.

[0003] To address this phenomenon and dilemma, researchers have made numerous attempts, such as using laser additive manufacturing (LABF) to preheat the substrate at high temperatures during LPBF forming, which can reduce residual stress. However, this results in a significant decrease in the mechanical strength of the deposited samples. Therefore, subsequent heat treatment has become a more practical approach. For aluminum-silicon alloys, which are fundamentally cast aluminum alloys, the traditional T6 (solution treatment + artificial aging) heat treatment regime is naturally the preferred choice. However, aluminum alloys prepared by laser additive manufacturing differ significantly from traditionally cast samples. Besides a markedly refined microstructure, such as grains and intragranular substructures, the Al matrix undergoes supersaturated solution under rapid solidification conditions. Combined with subsequent stress-relief annealing, this makes the solution state of the matrix exceptionally complex, posing significant challenges to the formulation of further solution and aging heat treatment regimes. However, the publicly available heat treatment regimes for laser additive manufacturing of aluminum-silicon alloys still generally use traditional heat treatment regimes. Although they are all two-step heat treatments of solution treatment and aging, the temperature and time are determined in a very wide range, without fully taking into account the microstructure differences brought about by the laser additive manufacturing process. As a result, heat-treated samples that are comparable to the mechanical strength of the deposited state have not been obtained, which greatly weakens the advantages of the laser additive manufacturing process. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a heat treatment method that takes into account both the performance of laser additive manufacturing aluminum alloys and stress relief, so as to solve the problem that in the prior art, while removing residual stress in LPBF formed parts, it will cause changes in the internal microstructure of the alloy, resulting in a decrease in the mechanical properties of the material.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] A heat treatment method that balances the performance of laser additive manufacturing aluminum alloys with stress relief includes the following steps:

[0007] Step 1: Place the AlSi10Mg alloy sample formed by LPBF in a heat treatment furnace for annealing heat treatment, remove it and air cool it to room temperature to obtain process sample one.

[0008] Step 2: Process sample one is solution heat treated at 530℃ for 1 to 10 hours, then removed and water-quenched to obtain process sample two; the solution heat treatment temperature is obtained by fitting multiple DCS tests and Kissinger method.

[0009] Step 3: The second process sample is aged at 160℃ for 0.5 to 18 hours, then removed and air-cooled to complete the heat treatment. The temperature and time of the aging heat treatment are obtained through Ostwald ripening and LSW roughening theories.

[0010] A further improvement of the present invention is that:

[0011] Preferably, in step 1, the annealing temperature is 300℃ and the annealing time is 2h.

[0012] Preferably, in step 2, the specific process for obtaining the solution heat treatment temperature is as follows: perform multiple DSC tests on the process sample to obtain multiple sets of DSC test curve data; fit the DSC test results using the Kissinger method to obtain the solution temperature of the sample after annealing.

[0013] Preferably, in step 2, the heating rates during the DSC test are 5K / min, 10K / min, 15K / min, 20K / min, 25K / min, 30K / min and 35K / min, respectively. The final heating temperature during the DSC test is 700℃, which is held at 700℃ for 5 minutes, and then cooled to 35℃ at a rate of 20K / min.

[0014] Preferably, in step 2, after the sample is taken out, it is quenched in water at a temperature of 10-25°C within 3 seconds.

[0015] Preferably, in step 2, the optimal solution heat treatment time is obtained by nanoindentation testing. The specific process is as follows: perform solution treatment at 530℃ for multiple times, perform SEM observation on the samples treated for multiple times, perform nanoindentation testing on the Al matrix in the AlSi10Mg alloy sample, statistically analyze the size and distribution of the blocky Si particles, compare the hardness of the Al matrix of the samples after solution treatment for multiple times, and determine the optimal solution time.

[0016] Preferably, in step 4, the surface of the inlaid sample is polished to a bright finish before the nanoindentation test. The specific test process is as follows: the load is 6mN, the load is applied for 5s, the load is held for 2s, and the load is unloaded for 5s; 3 to 5 points are measured for each sample.

[0017] The preferred formula for Ostwald ripening is:

[0018]

[0019] In the formula, T is the aging temperature, and r is the radius of the nano-Si particles;

[0020] when, hour, It is in equilibrium, neither dissolving nor growing;

[0021] when hour, Small particles dissolve;

[0022] when hour, The large particles continue to grow.

[0023] Preferably, in step 1, the annealing temperature is 300℃ and the annealing time is 2h; in step 2, the solution treatment time is 3h; and in step 3, the aging treatment time is 8h, 10h, 12h or 15h.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention discloses a heat treatment method that balances the performance of laser additive manufacturing aluminum alloys with stress relief. Addressing the problem of significantly reduced mechanical properties of AlSi10Mg alloys after stress-relief annealing, this invention first dissipates residual stress in the AlSi10Mg alloy sample through heat treatment. Then, through solution treatment, large-sized Si particles are dissolved into the Al matrix, thereby increasing the content of solid-soluble elements in the Al matrix and preparing the compositional kinetics for the subsequent aging process to precipitate a large number of nano-sized Si particles. Secondly, after aging treatment, nano-sized precipitates form in the supersaturated Al-based solid solution, which is the key to further improving the tensile strength of the sample. The loss of mechanical properties caused by stress-relief annealing is compensated for through microstructure control. This method first determines the optimal solution temperature using multiple DSC tests and the Kissinger method, and then determines the optimal solution time through SEM microstructure observation and nanoindentation analysis of the Al matrix. Secondly, based on Ostwald ripening and LSW coarsening theories, the aging temperature and time are optimized and determined, resulting in the precipitation of a large number of nano-sized Si particles in the further supersaturated Al matrix after solution treatment, achieving optimal nanoparticle size and volume fraction, thereby improving the mechanical strength of the alloy to compensate for the loss of mechanical properties caused by stress-relief annealing. This method achieves the goal of maintaining high mechanical strength while relieving stress by controlling the nucleation, agglomeration, and growth process of nano-sized Si particles in the annealed sample. Furthermore, it achieves the maintenance of a certain strength while relieving stress by controlling the resolution process of large Si particles in the annealed sample and the regulation of the precipitated phase (size and volume fraction of nano-Si particles). Compared to deposited AlSi10Mg alloy parts, this method largely eliminates residual stress in the parts. Except for a slight decrease in ultimate tensile strength, the yield strength and elongation, which are the most common evaluation criteria for mechanical components, are the same as or slightly improved compared to deposited samples. Compared to traditional heat treatment processes such as T6 (solution treatment + artificial aging), this method further optimizes and determines the optimal heat treatment temperature and time based on the microstructure characteristics of laser additive manufacturing aluminum alloys, so that the mechanical strength reaches the level of the initial deposited state, with unique and significant effects. The parts produced by this method have the same elongation as AlSi10Mg alloy parts after traditional stress-relief annealing, while the yield strength and ultimate tensile strength are significantly improved. Attached Figure Description

[0026] Figure 1 This is a flowchart of the method of the present invention.

[0027] Figure 2 This is the deposited microstructure of the LPBF-formed AlSi10Mg alloy in this invention example. The cellular structure is surrounded by a eutectic silicon phase, and a small number of nano-Si particles are distributed in the matrix inside the cells.

[0028] Figure 3 This is the stress-relief annealed microstructure of the LPBF-formed AlSi10Mg alloy in this invention example. The network-like eutectic silicon has been completely broken down, replaced by fragmented, coarse Si particles, and the solid solution content inside the matrix is ​​also low.

[0029] Figure 4 Figure 1 shows the DSC curves at different heating rates; (a) shows the DSC curves at different heating rates; (b) shows the fitted curve.

[0030] Figure 5 The microstructure and hardness of the Al matrix are shown for different solution times at 530℃.

[0031] Figure 6 The microstructure of the AlSi10Mg alloy under optimal solid solution and aging conditions is as follows: Although the network of eutectic silicon is absent, a large number of nano-Si particles are distributed within the matrix, which plays a significant role in precipitation strengthening and is the fundamental reason why the heat-treated samples obtained by this method have high mechanical strength.

[0032] Figure 7 The standard drawing of the tensile specimen used for testing.

[0033] Figure 8 The figures show the engineering stress-strain curves of AlSi10Mg alloy under three conditions in the examples of this invention. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0035] See Figure 1 This invention discloses a heat treatment method that balances the performance of laser additive manufacturing aluminum alloys with stress relief. LPBF-formed AlSi10Mg alloy samples are placed in a box furnace and subjected to a triple heat treatment process of annealing, solution treatment, and aging. This significantly controls the residual stress in the samples, resulting in a substantial improvement in mechanical properties compared to the directly stress-relieved annealed state. Compared to the deposited state sample, except for a slight decrease in ultimate tensile strength, the yield strength, the most common criterion for evaluating mechanical components, remains the same, while the elongation is increased.

[0036] The first step involves heating the heat treatment furnace to 300°C before feeding the sample. The sample is then placed inside the furnace and held at that temperature for 2 hours. After removal, it is air-cooled to room temperature (around 35°C) to obtain process sample one. Since Si particles are highly sensitive to temperature, even slight variations in heating conditions or duration can affect their size and morphology. Therefore, heating the furnace to 300°C before placing the sample avoids the impact of overheating on the Si particles caused by furnace heating. Through this step, the network-like eutectic silicon morphology generated by laser additive manufacturing is completely disrupted, thus losing its ability to bear mechanical loads. Therefore, although residual stress in the sample is eliminated, its mechanical strength is significantly sacrificed.

[0037] The second step involved performing DSC tests on the stress-relief annealed samples. Because laser additive manufacturing has a higher solidification rate than traditional casting processes, the microstructure is finer, and the supersaturation in the Al matrix is ​​higher. Although the samples underwent stress-relief annealing at 300℃ for 2 hours, the microstructure was still more complex due to the inheritance of microstructure, requiring multiple DSC tests to determine the solution temperature. DSC tests were conducted at a series of heating rates: 5 K / min, 10 K / min, 15 K / min, 20 K / min, 25 K / min, 30 K / min, and 35 K / min. The final heating temperature was 700℃, held for 5 minutes. The cooling rate was kept constant at 20 K / min until the temperature reached room temperature (35℃).

[0038] Then, the Kissinger method was used to fit and determine the solution temperature and time for the microstructure after stress-relief annealing, and the final solution temperature was determined to be 530℃.

[0039] The third step involves heating the heat treatment furnace to 530℃ before feeding the sample. The sample, after the stress-relief annealing heat treatment in the first step, is then placed in the furnace and held for 1 hour, 3 hours, 5 hours, 8 hours, and 10 hours respectively. After removal, it is immediately immersed in running water at a temperature of 10–25℃ for quenching within 3 seconds. This is a crucial step in obtaining a highly solution-treated sample. During the high-temperature solution heat treatment, large-sized eutectic silicon undergoes continuous ripening, growth, and coarsening. Simultaneously, some silicon elements continuously dissolve into the Al matrix. At the same temperature, these two trends intensify with prolonged solution time. Therefore, the sample after high-temperature solution treatment needs to be immediately quenched in water after being removed from the heat treatment furnace to stabilize the solution effect.

[0040] The fourth step involved SEM microstructure observation of samples obtained at different solution treatment times. During solidification, a homogeneous crystallization reaction leading to α-Al formation occurred first. Due to the rapid scanning of the metal powder by the laser, a supersaturated solid solution was formed in the AlSi10Mg alloy sample. However, stress-relief annealing significantly reduced the supersaturation of the Al matrix. Therefore, high-temperature solution heat treatment allowed Si to re-dissolve into the Al matrix, preparing the compositional kinetics for the subsequent aging reaction. Thus, in this step, to determine the optimal solution treatment effect, nanoindentation testing was performed on the Al matrix to statistically analyze the size and distribution of bulk Si particles. By comparing the hardness of the Al matrix and considering the highest hardness and minimally sized bulk Si particles, the optimal solution treatment time was determined to be 3 hours.

[0041] The fifth step involves observing and characterizing the microstructure of the sample at the optimal solution temperature and time, calculating the content of solid solution elements in the Al matrix, and using the Ostwald aging and LSW coarsening theories to predict the further precipitation of nano-Si particles, thereby determining the optimal aging temperature as 160℃.

[0042] Fifth, before feeding the sample, the heat treatment furnace was cooled to 160℃. The sample after the heat treatment in step three was then placed in the furnace and held for 0.5h, 1h, 3h, 5h, 8h, 10h, 12h, 15h, and 18h ​​respectively. Afterward, it was removed and air-cooled to room temperature (approximately 35℃). Air cooling was chosen because low-temperature aging causes some dissolved Si elements to precipitate and form nano-Si particles. Continuing with water cooling would result in poor precipitation, while furnace cooling would lead to excessive precipitation of nano-Si particles. Therefore, air cooling achieves the best precipitation effect.

[0043] In addition, the residual stress and mechanical properties of specimens in different states were tested to verify the effectiveness of the method. The specific steps are as follows:

[0044] The first step is to prepare metallographic specimens and room temperature tensile specimens in three different states of AlSi10Mg alloy: deposited state (corresponding to the initial sample), stress-relieved annealed state (corresponding to the process specimen 1 mentioned above), and triple heat-treated state (corresponding to the final sample).

[0045] The second step is to test the mechanical properties of the tensile specimens in three different states.

[0046] The third step is to measure the residual stress on the surface of metallographic specimens in three different states using the indentation method.

[0047] The test results show that the residual stress of the AlSi10Mg alloy sample after the above triple heat treatment was effectively controlled. In terms of mechanical properties, except for a slight decrease in ultimate tensile strength, the yield strength and elongation, which are the most common evaluation criteria for mechanical components, are the same as or slightly improved compared with the deposited sample. Compared with AlSi10Mg alloy parts after traditional stress-relief annealing, the elongation is the same, while the yield strength and ultimate tensile strength are significantly improved.

[0048] Specifically, this invention involves forming AlSi10Mg alloy by laser powder bed melting (LPBF) at 300℃ / 2h / AC, 530℃ / 1h, 3h, 5h, 8h, 10h / WC, 160℃ / 0.5h, 1h, 3h, 5h, 8h, 10h, 12h, 15h, and 18h / AC, with the preferred methods being 300℃ / 2h / AC, 530℃ / 3h / WC, 160℃ / 8h, 10h, 12h, and 15h / AC. Compared to the deposited state, the stress-relief annealing treatment at 300℃ / 2h / AC largely removes the residual stress caused by the complex thermal stress during forming. However, compared to the deposited state, its microstructure undergoes significant changes. First, the unique cellular substructure composed of network-like eutectic Si in the AlSi10Mg alloy is destroyed, and almost all Si particles have become spheroidized into relatively small particles with a tendency to be uniformly distributed, no longer exhibiting a network distribution. Second, nanoscale Si particles disappear, and no finely dispersed Si particles can be observed in the matrix, which leads to a significant decrease in the mechanical properties of the annealed sample. After solution treatment, some of the large Si particles agglomerate, grow, and coarsen, while others dissolve into the Al matrix, providing kinetic preparation for the next step of aging precipitation. After aging treatment, a large number of nanoscale precipitates precipitate in the supersaturated Al-based solid solution, further improving the strength of the alloy. From the stress measurement results, the solution aging treatment after annealing does not affect the stress-relief effect of the annealing treatment. Currently, researchers often focus on stress-relief annealing rather than improving the strength of annealed specimens. Furthermore, they haven't fully considered the fundamental changes to the microstructure brought about by laser additive manufacturing and subsequent stress-relief annealing heat treatment. Simply using the traditional T6 (solution treatment + artificial aging) heat treatment method fails to improve the mechanical strength of the specimens, thus losing the unique value of laser additive manufacturing. This proposed solution utilizes microstructure characterization, experimental testing, and theoretical calculations to develop a heat treatment regime specifically for annealed laser additive manufactured aluminum alloys, which eliminates residual stress and improves tensile strength.

[0049] The following analysis and explanation, in conjunction with specific embodiments, further illustrate this point.

[0050] Example 1

[0051] In this invention, the deposited AlSi10Mg alloy was prepared using laser powder bed melting technology on an LPBF Solution 280 device, and then heat-treated using a Micro-X Furnace box-type resistance furnace. The specific steps are as follows:

[0052] Step 1: Program the specific process parameters for the forming process using a CNC system. The process parameters used are as follows:

[0053] Table 1. Specific process parameters for LPBF forming process

[0054]

[0055]

[0056] Step 2: Place AlSi10Mg alloy powder with a particle size of 13-53μm into the powder hopper;

[0057] Step 3: Cut the 98*98*40mm... 3 The cast aluminum substrate is placed in an argon-filled inert atmosphere forming chamber and fixed on the processing table;

[0058] Step 4: High-purity argon gas is introduced into the forming chamber to reduce the oxygen content inside the forming chamber;

[0059] Step 5: LPBF forming is achieved by continuously lowering the substrate and applying powder layer by layer with a scraper.

[0060] Step 6: After deposition, the argon-filled inert atmosphere forming chamber is opened and the sample is removed. It is then separated from the substrate by wire electrical discharge machining to obtain the deposited AlSi10Mg alloy, the microstructure of which is as follows: Figure 2 As shown in the diagram, the microstructure reveals that the laser additively manufactured aluminum-silicon alloy possesses a unique fine cellular structure. Eutectic silicon is encapsulated in a network around the Al matrix, while nano-Si particles are dispersed within the Al matrix. The cellular eutectic silicon primarily serves to bear mechanical loads, while the nano-Si particles mainly function as precipitation reinforcement.

[0061] Step 7: Stress-relief annealing heat treatment. Before feeding the sample, heat the heat treatment furnace to 300℃. After the furnace temperature stabilizes, place the deposited sample printed in Step 6 into the furnace and hold it at that temperature for 2 hours. Then remove the sample and air-cool it to room temperature (approximately 35℃). Grind, polish, and etch the stress-relief annealed sample for microstructure observation. Figure 3The microstructure shown is that of the eutectic silicon after stress-relief annealing at 300℃ for 2 hours. It can be seen that due to the extreme temperature sensitivity of eutectic silicon, it undergoes fragmentation, agglomeration, and coarsening at a moderate temperature (300℃), causing the network-like eutectic silicon morphology produced by laser additive manufacturing to completely disappear. This is the root cause of the significant reduction in the mechanical strength of the annealed sample. At this time, the solid solution content in the Al matrix is ​​also low, thus failing to form effective precipitation strengthening. The microstructure after stress-relief annealing differs from both the laser additive manufacturing deposited state sample and the traditional cast state of aluminum-silicon alloys. Therefore, it is necessary to further refine and optimize the existing heat treatment regime to determine the most reasonable solution treatment, aging temperature, and time.

[0062] Step 8: Multiple DSC Testing. Although stress-relief annealing renders the microstructure unsuitable for laser additive manufacturing, some characteristics of the laser additive manufacturing microstructure are still retained due to microstructural inheritance. The most prominent features are relatively coarse Si particles and a moderately supersaturated Al matrix (saturation level lower than laser additive manufacturing samples, but higher than conventionally cast samples). Multiple DSC testing is used to determine the solution temperature and time for the stress-relief annealed sample. Specifically, the stress-relief annealed sample obtained in Step 7 is cut into particles no larger than 25 mm in mass, the surface is cleaned, and the sample is placed in a DSC testing device for a series of tests. The heating rates are 5 K / min, 10 K / min, 15 K / min, 20 K / min, 25 K / min, 30 K / min, and 35 K / min, with a final heating temperature of 700 °C and a holding time of 5 min. The cooling rate is kept constant at 20 K / min until the sample reaches room temperature (35 °C). See [link to DSC test]. Figure 4 As can be seen, with the increase of heating rate, the peak value of the exothermic peak shifts to the high temperature region. This indicates that the redissolution of bulk Si particles in the Al matrix under annealing has a significant kinetic effect and shows dependence on the heating rate. Therefore, the test results need to be further fitted using the Kissinger method.

[0063] Step 9: Process and analyze the DSC test data obtained in Step 8. Figure 4 (a) shows the DSC curves at different heating rates. It can be seen that as the heating rate increases, the peak temperature of the phase transformation point shifts to the higher temperature region. This indicates that the solid solution of Si under continuous heating conditions has a significant kinetic effect, showing a dependence on the heating rate. Based on previous research on AlSi10Mg alloys, the phase transformation temperature of solution heat treatment is between 500℃ and 560℃. Therefore, the temperature values ​​corresponding to the phase transformation points in this temperature range at different heating rates were extracted and scatter plotted, as shown below. Figure 4As shown in (b), the data were linearly fitted, and the optimal solution temperature was obtained based on the Kissinger method. That is, the temperature corresponding to the heating rate of 0℃ is 530±2℃. Therefore, 530℃ is used in this method.

[0064] Step 10: Solution Heat Treatment. Before feeding the sample, heat the furnace to 530℃. After the furnace temperature stabilizes, place the sample after the annealing heat treatment in Step 7 into the furnace for solution heat treatment for 1h, 3h, 5h, 8h, and 10h respectively. After the solution treatment time is reached, immediately remove the sample from the furnace and immerse it in running water at 10-25℃ for water cooling within 3 seconds. This step is crucial; only timely quenching can achieve the desired solution effect. Under the high temperature of 530℃, some Si particles continue to agglomerate, grow, and coarsen, while other silicon elements dissolve in the matrix, resulting in a highly supersaturated state. Therefore, after removing the sample from the furnace, timely quenching ensures that the supersaturated elements are stably dissolved in the Al matrix, preparing the sample for the next step of aging heat treatment based on compositional kinetics.

[0065] Step 11: Determination of the optimal solution treatment time. Samples from Step 10 at different solution treatment times were subjected to grinding, polishing, etching, SEM microstructure observation, and nanoindentation testing of the Al matrix. The size and volume fraction of the bulk Si particles were analyzed and statistically determined. The hardness of the Al matrix was tested using nanoindentation, with a load of 6 mN applied for 5 seconds, held for 2 seconds, and unloaded for 5 seconds. 3–5 points were measured on each sample. Figure 5 This image shows the SEM microstructure at 530℃ and different solution times, along with schematic diagrams and hardness results of nanoindentation tests on the Al matrix under the corresponding conditions. Analysis of both reveals that at 530℃, a solution time of 3 hours yields bulk Si particles with minimal size growth and the hardest Al matrix. Therefore, it can be concluded that a solution time of 3 hours at 530℃ is optimal.

[0066] Step 12: Determination of aging temperature. The sample after the optimal solution heat treatment in Step 11 was polished, etched, and subjected to microstructure observation and determination of matrix supersaturation solubility. The elemental composition of the matrix was determined using TEM-EDS surface scanning. The size of the nano-Si particles was predicted using Ostwald aging and LSW coarsening theories.

[0067]

[0068] In the formula, T is the aging temperature, and r is the radius of the nano-Si particles. According to formula (2), we can obtain:

[0069] a. When hour, It is in equilibrium, neither dissolving nor growing;

[0070] b. When hour, Small particles dissolve;

[0071] c. When hour, The large particles continue to grow;

[0072] d. When hour, The largest, with the fastest particle growth rate;

[0073] Therefore, by combining the supersaturated solid solution content of elements in the matrix determined by microstructure analysis, an aging temperature of 160℃ was determined to ensure that the size (r) of nano-Si particles is in a temperature range where they neither dissolve nor grow.

[0074] Step 13: Aging Heat Treatment. Before feeding the sample, cool the heat treatment furnace to 160℃. After the furnace temperature stabilizes, place the sample after the optimal solution heat treatment obtained in Step 11 into the furnace for aging heat treatment for 8 hours. After the aging time is reached, remove the sample from the furnace and air cool it (room temperature is approximately 35℃). After cooling to room temperature, mount and polish the sample, observe its microstructure, and test its mechanical properties.

[0075] Figure 8 These are the engineering stress-strain curves of AlSi10Mg alloy under three states. It is evident that stress-relief annealing causes the disappearance of the cellular structure in the microstructure, significantly reducing its load-bearing capacity and resulting in a significant decrease in strength in the stress-relief annealed state. After the tenth and eleventh optimal solution heat treatment steps, bulk Si particles with minimal growth were obtained, along with the Al matrix exhibiting the highest hardness. In this step, coarse Si particles are still visible in the microstructure of the aged sample, but compared to the microstructure of the solution-treated sample, the Al matrix contains a large number of nano-Si particles. This is the fundamental reason for the improved mechanical strength of the aged sample and is also the result of obtaining nano-Si particles with optimal size and volume fraction under the optimal heat treatment regime formulated by this method for annealed aluminum alloys. This allows the sample to achieve high strength while eliminating residual stress. For AlSi10Mg alloys, the yield strength during service is often a key concern, and the yield strength of the annealed-solution-aging state is comparable to that of the deposited state; the fracture elongation is even better than that of the deposited state.

[0076] Residual stress was tested on metallographic specimens of AlSi10Mg alloy in the deposited state, stress-relieved annealed state, and triple heat-treated state using the indentation method. The specific steps are as follows:

[0077] Step 1: Grind and polish the metallographic samples of AlSi10Mg alloy in three states, and then perform Vickers hardness test using a LECOAMH43 microhardness tester. The load during the test is 1 kg, the loading time is 15 s, and 3 points are measured for each sample with a 3 mm interval between each point. Then, the indentation image of each point is recorded.

[0078] Step 2: Calculate the ratio c between the actual indentation area and the theoretical indentation area. 2 When there is residual tensile stress on the surface, the material around the indentation will indent inward, i.e., c 2 <1; When residual compressive stress exists on the surface, the material around the indentation will bulge outward, i.e., c 2 >1;

[0079] Step 3: Fit the tensile curves of AlSi10Mg alloy in three states to obtain the K and n values ​​of the material, where K is the strength coefficient of the material itself and n is the hardening index of the material itself.

[0080] Step 4: Calculate the residual stress using the formula reported in the literature. The formula is as follows:

[0081] H=Cσ(ε repr +ε res )

[0082]

[0083] in,

[0084] H – Vickers hardness (H) v );

[0085] C—Material constant;

[0086] ε repr —Equivalent plastic strain;

[0087] c 2 — Ratio of indentation projected area (A / A) nom ).

[0088] Based on reported work, C = 3 is generally taken, ε repr =0.08, The residual stress values ​​of the AlSi10Mg alloy in the three states were measured and are shown in Table 2 below. It can be seen that due to the reciprocating thermal cycle during deposition, a large residual stress, approximately 29 MPa, exists inside the deposited sample. After stress-relief annealing at 300℃ for 2 hours, the residual stress value is significantly reduced to approximately 2.6 MPa, achieving the purpose of stress relief, but at the cost of greatly sacrificing the sample's strength. Subsequent solution-aging heat treatment, although involving operations such as water cooling and air cooling that could easily cause stress reversal, resulted in a more significant stress-relieving effect. This reduced the participating stress value in the annealed-solution-aging sample to approximately 1.9 MPa, thus achieving the dual effect of removing residual stress and improving yield strength.

[0089] Table 2. Residual Stress

[0090]

[0091] Example 2

[0092] In this embodiment, the first twelve steps are the same as in Embodiment 1.

[0093] Step 13: Cool the heat treatment furnace to 160℃, then put the sample after the solution treatment in step 11 into the heat treatment furnace and keep it at that temperature for 10 hours. After that, take it out and air cool it to room temperature to obtain the triple heat-treated AlSi10Mg alloy.

[0094] Example 3

[0095] In this embodiment, the first twelve steps are the same as in Embodiment 1.

[0096] Step 13: Cool the heat treatment furnace to 160℃, then put the sample after the solution treatment in step 11 into the heat treatment furnace and keep it at that temperature for 12 hours. After that, take it out and air cool it to room temperature to obtain the triple heat-treated AlSi10Mg alloy.

[0097] Example 4

[0098] In this embodiment, the first twelve steps are the same as in Embodiment 1.

[0099] Step 13: Cool the heat treatment furnace to 160℃, then put the sample after the solution treatment in step 11 into the heat treatment furnace and keep it at that temperature for 15 hours. After that, take it out and air cool it to room temperature to obtain the triple heat-treated AlSi10Mg alloy.

[0100] Table 3. Summary and Comparison of Mechanical Properties

[0101]

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A heat treatment method that balances the performance of laser additive manufacturing aluminum alloys with stress relief, characterized in that, Includes the following steps: Step 1: Place the AlSi10Mg alloy sample formed by LPBF in a heat treatment furnace for annealing heat treatment, remove it and air cool it to room temperature to obtain process sample one. Step 2: Process sample one is solution heat treated at 530℃ for 1~10h, then removed and water-quenched to obtain process sample two; the solution heat treatment temperature is obtained by fitting multiple DCS tests and Kissinger method. The specific process for obtaining the solution heat treatment temperature is as follows: perform multiple DSC tests on the process sample to obtain multiple sets of DSC test curve data; fit the DSC test results using the Kissinger method to obtain the solution temperature of the sample after annealing. After removing the sample, immerse it in water at a temperature of 10~25℃ within 3 seconds for quenching. In step 2, the optimal solution heat treatment time was obtained through nanoindentation testing. The specific process was as follows: solution treatment was performed at 530℃ for multiple times, and SEM observation was performed on the samples treated for multiple times. Nanoindentation testing was performed on the Al matrix in the AlSi10Mg alloy sample. The size and distribution of the bulk Si particles were statistically analyzed, and the hardness of the Al matrix of the samples after solution treatment for multiple times was compared to determine the optimal solution time. Step 3: The second process sample is aged at 160℃ for 0.5~18h, then removed and air-cooled, and the heat treatment is completed; the temperature and time of the aging heat treatment are obtained by Ostwald ripening and LSW roughening theory; The formula for Ostwald ripening is: In the formula, T is the aging temperature, and r is the radius of the nano-Si particles; when, hour, It is in equilibrium, neither dissolving nor growing; when hour, The small particles dissolve. when hour, The large particles continue to grow.

2. The heat treatment method according to claim 1, which combines the performance of laser additive manufacturing aluminum alloys with stress relief, is characterized in that... In step 1, the annealing temperature is 300℃ and the annealing time is 2 hours.

3. A heat treatment method according to claim 1 that balances the performance of laser additive manufacturing aluminum alloys with stress relief, characterized in that, In step 2, the heating rates during the DSC test were 5 K / min, 10 K / min, 15 K / min, 20 K / min, 25 K / min, 30 K / min and 35 K / min, respectively. The final heating temperature of the DSC test was 700℃, which was held at 700℃ for 5 min, and then cooled to 35℃ at 20 K / min.

4. The heat treatment method according to claim 1, which combines the performance of laser additive manufacturing aluminum alloys with stress relief, is characterized in that... In step 2, the surface of the inlaid sample is polished to a bright finish before the nanoindentation test. The specific test process is as follows: the load is 6mN, the load is applied for 5s, the load is held for 2s, and the load is unloaded for 5s; 3 to 5 points are measured for each sample.

5. The heat treatment method according to claim 1, which combines the performance of laser additive manufacturing aluminum alloys with stress relief, is characterized in that... In step 1, the annealing temperature is 300℃ and the annealing time is 2h; in step 2, the solution treatment time is 3h; in step 3, the aging treatment time is 8h, 10h, 12h or 15h.

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