Magnesium alloy oxidation prevention heat treatment process

Abstract

The application discloses a magnesium alloy anti-oxidation heat treatment process, and particularly relates to the technical field of metal material heat treatment. The process comprises the following steps: mixing dry sand with a specific proportion of composite flame retardant to prepare a flame-retardant sand mold mixture; placing a magnesium alloy casting in a heat-resistant container, completely covering and vibrating the magnesium alloy casting with the mixture; and then performing programmed heat treatment in a conventional heat treatment furnace. During the high-temperature holding stage, the mixture is subjected to controllable sintering hardening, and the formed hardened sand body forms a protective layer on the surface of the casting to isolate oxidation through the synergistic effect of fluorine, sulfur, boron and other components in the sand body; on the other hand, the hardened sand body provides rigid physical support for the casting in a high-temperature softened state to prevent deformation. After the heat treatment is completed, the casting and the sand mold are separated through vibration. The process has the advantages of reliable anti-oxidation, effective anti-deformation, low cost and compatibility with conventional equipment, and is suitable for the heat treatment of magnesium alloy castings such as ZM5 and ZM6.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a heat treatment process for preventing oxidation of magnesium alloys. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials currently used in engineering applications, have a density approximately two-thirds that of aluminum and one-quarter that of steel. They also possess high specific strength and specific stiffness, as well as excellent casting properties, damping and vibration reduction performance, and electromagnetic shielding properties. These characteristics make them irreplaceable in applications with urgent weight reduction needs, such as aerospace vehicles, high-end automotive parts, portable electronic device housings, and military equipment, demonstrating significant application potential and strategic value. The widespread promotion and application of magnesium alloys is one of the key technological approaches to achieving structural lightweighting and improving equipment performance and energy efficiency.

[0003] However, the extremely reactive chemical properties of magnesium constitute a major obstacle to its processing and application, especially in subsequent heat treatment. Magnesium has a very low standard electrode potential and an extremely strong affinity for oxygen. This characteristic is particularly pronounced at high temperatures. When the temperature exceeds approximately 400°C, magnesium alloy workpieces exposed to air undergo a rapid and intense oxidation reaction, forming a loose and porous magnesium oxide (MgO) film on the surface. This film is not protective and instead accelerates the continued oxidation of the internal metal. When the temperature is further increased to near the alloy's solidus line, the oxidation reaction releases a large amount of heat, which can easily trigger the combustion of magnesium, producing a dazzling white light and potentially accompanied by splattering, leading to complete burnout of the workpiece, equipment damage, or even safety accidents. Therefore, any magnesium alloy processing procedure involving high temperatures, especially heat treatments that require prolonged heat treatment to improve microstructure and properties (such as solution treatment and aging treatment), must address the issue of reliable oxidation prevention (flame retardancy).

[0004] To address this challenge, those skilled in the art have developed and primarily relied on the following technical solutions; however, these solutions all have their own obvious limitations and cannot fully meet the actual needs of industrial production: 1. Protective Atmosphere Method: This method attempts to isolate oxygen from the environment, mainly including vacuum heat treatment and inert gas (such as high-purity argon or nitrogen) protective heat treatment. This is currently a relatively common industrial method. However, its disadvantages are significant: First, the equipment investment is huge. The cost of a dedicated vacuum furnace or precision atmosphere protective furnace is much higher than that of an ordinary resistance furnace, and its high energy consumption and complex maintenance greatly increase production costs. Second, the reliability of protection is not absolute. Practice shows that for some magnesium alloys (such as ZM5), when performing high-temperature solution treatment in a high-vacuum environment, due to the significant increase in the vapor pressure of magnesium at high temperatures, the volatilized magnesium vapor may condense and re-oxidize in cooler parts of the furnace, or trigger a violent local reaction, causing the workpiece to still oxidize or even burn. More importantly, this method only solves the oxidation problem. At the heat treatment temperature (e.g., the solution temperature of ZM6 is 540℃), the strength of magnesium alloys drops sharply to 1 / 10 or even lower than at room temperature, and they are in an extremely soft, creep-like state. Under these conditions, the workpiece is highly susceptible to creep deformation under its own weight or thermal stress, leading to warping, collapse, and dimensional deviations. Protective atmospheres are ineffective against this, and for large, thin-walled, or complex castings, the rate of deformation and scrap is very high.

[0005] 2. Molten Salt Bath or Solid Coating Agent Method: This method involves immersing the workpiece in a molten salt bath with a specific formulation (usually a mixture of chlorides and fluorides), or burying it in a sealed container filled with powders such as ferrous sulfide or ammonium borate for heating. The molten salt or powder layer can physically isolate the workpiece from air. However, this method brings a series of new problems: the molten salt is highly corrosive to the workpiece and the furnace chamber; the salt stains adhering to the workpiece surface after heat treatment are extremely difficult to completely remove, affecting not only the appearance of the workpiece and subsequent processing (such as spraying and welding), but also the residual chloride ions may cause electrochemical corrosion during use. Some coating agents may decompose at high temperatures, producing toxic and irritating gases such as sulfur dioxide (SO2) and hydrogen sulfide (H2S), deteriorating the workshop environment and endangering the health of operators. In addition, the stability of the salt bath composition and the temperature uniformity require precise control, resulting in a narrow process window.

[0006] 3. Surface Anti-oxidation Coating Method: Before heat treatment, a paste or slurry-like coating made of silicates, phosphates, borates, etc., is sprayed or brushed onto the workpiece surface, forming a coating after drying. During heating, the coating melts and sinters to form a dense glaze layer that isolates oxygen. The main problems with this method are the integrity of the coating and post-treatment. For castings with complex shapes, deep holes, and narrow grooves, it is difficult to ensure a uniform and continuous coating. Any tiny missed spots or cracks in the coating can become oxygen channels, causing localized oxidation and its propagation. After heat treatment, the sintered glaze layer is firmly bonded to the substrate, has high hardness, and is extremely difficult to remove. Using methods such as sandblasting and pickling not only increases process costs but may also damage the workpiece substrate, affecting dimensional accuracy and surface roughness.

[0007] It is worth noting that in the sand casting production of magnesium alloys, to prevent the high-temperature molten magnesium from reacting with the air and moisture in the mold cavity and burning during mold filling, casting technicians do indeed use the mature practice of adding a small amount of boric acid (H3BO3) to the molding sand or spraying a boric acid aqueous solution onto the mold surface as a flame retardant. This concept of flame-retardant sand molds is well-known in the casting field. However, for a long time, there has been a common and deeply ingrained perception or technical bias in the field of heat treatment technology: existing heat treatment technology literature generally records that sand mold materials are only suitable for the instantaneous high-temperature scenario of casting, and cannot provide effective support for softened workpieces during long-term heat treatment holding processes. Applying them to heat treatment will lead to severe workpiece deformation. It is believed that sand molds are only suitable for instantaneous, one-time casting processes, and absolutely unsuitable for heat treatment processes aimed at optimizing the internal microstructure and mechanical properties of materials through long-term heat treatment holding. This is based on a fundamental contradiction: the purpose of heat treatment is to cause phase transformation or solid solution in the material, which inevitably leads to softening of the workpiece at high temperatures. If a softened workpiece is simply buried in ordinary, loose sand, the sand grains cannot provide effective support. The workpiece will only undergo uncontrollable plastic deformation such as collapse and bending under its own weight, completely losing its geometric accuracy. Therefore, using sand molds for heat treatment is traditionally considered a technical paradox, an unexplored approach or even a worthless idea.

[0008] In summary, existing technical solutions are either costly and have questionable reliability in preventing oxidation, or they introduce new problems such as corrosion, pollution, and difficult cleaning. Furthermore, none of them can simultaneously and effectively address the two core pain points of oxidation prevention and deformation prevention in magnesium alloy heat treatment. Developing a novel magnesium alloy heat treatment process that integrates reliable oxidation prevention, effective deformation prevention, low processing costs, ease of operation, and compatibility with existing production equipment has become an urgent technological need to overcome the bottlenecks in the application of this material and promote the development of related industries. Summary of the Invention

[0009] The purpose of this invention is to provide a heat treatment process for preventing oxidation of magnesium alloys.

[0010] To achieve the above objectives, the present invention provides the following technical solution: A heat treatment process for preventing oxidation of magnesium alloys includes the following steps: a) Preparation of flame-retardant sand mold: Mix dry sand with composite flame retardant evenly to obtain flame-retardant sand mold mixture; b) Loading the furnace: Place the magnesium alloy casting to be treated in a heat-resistant container, and completely bury and cover the casting with the flame-retardant sand mixture. When filling, vibrate to compact the mixture, and the thickness of the top covering layer shall not be less than 50 mm. c) Heat treatment: The heat-resistant container containing the magnesium alloy casting and the flame-retardant sand mixture is placed in a conventional heat treatment furnace for programmed heating, holding and cooling; wherein, during the holding stage, the flame-retardant sand mixture undergoes controllable sintering hardening, with a sintering hardening temperature range of 400℃-600℃, and the hardened sand body formed provides physical support for the magnesium alloy casting in the high-temperature softened state. d) Post-treatment: After heat treatment, the magnesium alloy casting is separated from the hardened flame-retardant sand mold.

[0011] Furthermore, in step a), the amount of the composite flame retardant added is 0.5% to 3.0% of the mass of the dry sand, preferably 0.8% to 2.0%.

[0012] Furthermore, the composite flame retardant is composed of the following components by mass percentage: fluorinated compound: 20-60%, sulfur-containing compound: 15-30%, boron compound: 10-40%, and other additives: 1-10%; the fluorinated compound is selected from at least one of sodium fluoroborate, potassium fluoroborate, sodium fluorosilicate, and calcium fluoride; the sulfur-containing compound is selected from at least one of iron sulfide, sulfur, and sodium dodecyl sulfonate; the boron compound is boric acid; and the other additives are selected from iron oxide.

[0013] Furthermore, in step a), the components of the composite flame retardant are first mixed evenly, and then mixed with dry sand, with a total mixing time of 40-60 minutes.

[0014] Furthermore, the dry sand is quartz sand with a particle size of 40-70 mesh.

[0015] Furthermore, in step d), the hardened flame-retardant sand mold is separated from the magnesium alloy casting by vibration.

[0016] Furthermore, in step d), the separation process also includes a step of washing the surface of the magnesium alloy casting with water.

[0017] Furthermore, in the cooling stage of step c), after the heat-resistant container has cooled to below 200°C, the magnesium alloy casting is separated from the flame-retardant sand mold and immediately subjected to water quenching.

[0018] Furthermore, the magnesium alloy is a ZM5 or ZM6 magnesium alloy.

[0019] Furthermore, the heat-resistant container is made of heat-resistant steel.

[0020] Compared with existing technologies, the process provided by this invention brings the following improvements by using a specially formulated flame-retardant sand mold as the processing medium: 1. Simultaneous resolution of oxidation and deformation issues: This process enables the flame-retardant sand mold to undergo controlled sintering and hardening during heat treatment. The resulting hardened sand body provides physical support during the softening stage of the magnesium alloy, thereby inhibiting workpiece deformation. Simultaneously, the composite flame retardants interact at high temperatures to form a protective layer on the workpiece surface, inhibiting oxidation in normal air environments.

[0021] 2. Lowering the threshold for process implementation: This process can be carried out in a conventional heat treatment furnace, reducing the reliance on vacuum or complex atmosphere protection systems and helping to reduce equipment investment.

[0022] 3. Expanding the process window: Since the sand mold provides isolation and protection, cooling methods such as water quenching can be used in the cooling stage after heat treatment, which provides more options for adjusting the final properties of the material.

[0023] 4. Simplified post-processing: The hardened sand mold can be separated from the workpiece by vibration after processing, and the residues on the workpiece surface can be removed by washing with water, simplifying the cleaning steps.

[0024] 5. Highly targeted: This process provides a feasible solution for magnesium alloys such as ZM5 and ZM6 that are prone to oxidation during heat treatment and are sensitive to deformation.

[0025] In summary, this process provides a method for magnesium alloy workpieces to simultaneously achieve surface protection and shape retention in conventional equipment by applying a designed flame-retardant sand mold to the heat treatment process. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the overall process of the magnesium alloy anti-oxidation heat treatment described in this invention.

[0027] Figure 2 shows the ZM6 magnesium alloy test bar after being processed using the process of the present invention. Figure 2a ) and ZM6 test bar without protective heat treatment under the same conditions ( Figure 2b Comparison photos of the surface macroscopic state. Figure 2a (Processing of this invention) The surface of the ZM6 test rod is free of any oxidation. Figure 2b The ZM6 test bar surface was severely oxidized (without protective heat treatment).

[0028] Figure 3 This is a bar chart comparing the mechanical properties (tensile strength, yield strength, and elongation) of ZM6 magnesium alloy test bars treated with the process of this invention with those treated with conventional heat treatment furnaces. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are used to illustrate this invention but are not intended to limit the scope of this invention.

[0030] Example 1 1. Material preparation The workpiece to be processed is a ZM6 magnesium alloy figure-eight shaped standard tensile test bar cast in sand mold. The heat-resistant container is an open iron box welded from 3mm thick 1Cr18Ni9Ti heat-resistant steel plate. The heat treatment equipment is an SX-12-16 type box-type resistance furnace.

[0031] 2. Process Implementation a) Preparation of flame-retardant sand mixture: Weigh 100 kg of dry quartz sand with a particle size of 40-70 mesh. Weigh the composite flame retardant according to the following mass percentages: potassium fluoroborate: 40%, iron sulfide: 20%, boric acid: 35%, iron oxide: 5%, total weight 2.0 kg (accounting for 2.0% of the dry sand mass). First, mix all flame retardant components in a V-type mixer for 20 minutes, then mix with the dry quartz sand for 30 minutes (total mixing time 50 minutes) to obtain the mixture.

[0032] b) Loading the furnace: Place the test bar into the iron box and fill it with the mixture. When filling, gently tap the box wall with a rubber mallet to compact the mixture, ensuring that the test bar is completely buried. The thickness of the top covering layer should be 60mm.

[0033] c) Heat treatment: Push the iron box into the resistance furnace. Set the program according to the ZM6 alloy specification: heat to 540℃ at 5℃ / min, hold for 12 hours, and cool with the furnace.

[0034] d) Post-treatment: After the furnace temperature drops below 200℃, remove the iron box and air-cool it to room temperature. Pour out the contents and use a vibrating table to separate the hardened sand block from the test bar. Rinse the separated test bar under running water and clean it with a soft brush, then let it air dry.

[0035] 3. Effect Description The treated test bar surface is uniformly silvery-gray, with no signs of oxidation, such as Figure 2a As shown. Mechanical property tests were performed on it, and the results are shown in [reference needed]. Figure 3 Its tensile strength and yield strength are comparable to those of the vacuum furnace-treated specimens, and its elongation is 6.0%. None of the specimens showed bending deformation.

[0036] Example 2 1. Material preparation The workpiece to be processed is a 10mm square ZM5 magnesium alloy block. The container and equipment are the same as in Example 1.

[0037] 2. Process Implementation a) Preparation of flame-retardant sand-mold mixture: Dry sand as in Example 1. The composite flame retardant ratio is: sodium fluorosilicate: 50%, sulfur: 18%, boric acid: 30%, iron oxide: 2%, total weight 0.8 kg (0.8% of dry sand mass). Mixing method as in Example 1.

[0038] b) Loading and heat treatment: Embed the alloy block into the iron box, cover the top with 50mm of the mixture and vibrate to compact it. Place the iron box into the resistance furnace, heat it to 600℃ at 10℃ / min, hold it at that temperature for 30 minutes, and then cool it with the furnace.

[0039] 3. Effect Description After cooling, the ZM5 alloy was removed and found to have melted into an ingot, completely encased in a hardened sand shell. Upon separation, the ingot surface exhibited a bright, silvery-white metallic luster, with no areas of oxidation or blackening. This demonstrates that the process still provides effective oxidation protection even at extremely high temperatures.

[0040] Example 3 This embodiment verifies the effectiveness of the process in preventing heat treatment deformation of complex structural components.

[0041] 1. Material preparation The workpiece to be processed is a thin-walled shell casting of ZM6 magnesium alloy with uneven wall thickness. The container and equipment are the same as in Example 1.

[0042] 2. Process Implementation a) Preparation of flame-retardant sand mixture: Dry sand as in Example 1. The composite flame retardant ratio is: sodium fluoroborate: 35%, calcium fluoride: 10%, iron sulfide: 10%, sodium dodecyl sulfonate: 5%, boric acid: 38%, iron oxide: 2%, total weight 1.5 kg (1.5% of dry sand mass). Mixing method as in Example 1.

[0043] b) Loading and heat treatment: Carefully place the casting into the iron box, fill all cavities with the mixture and compact it, and cover the top with 70mm. Push the iron box into the resistance furnace, slowly heat it to 540℃ at 3℃ / min, hold it at that temperature for 10 hours, and then cool it with the furnace.

[0044] c) Post-processing: After cooling to room temperature, vibrate to separate.

[0045] 3. Effect Description After removal from the furnace, the casting remained intact, without any collapse or warping. Measurements using a coordinate measuring machine showed a maximum deformation of 0.18 mm, meeting the drawing tolerances. As a comparative example, the same casting, after being treated in an argon-protected furnace according to the same specifications, exhibited significant sagging, with a maximum deformation of 3.2 mm. This embodiment demonstrates that the rigid support formed after sintering the flame-retardant sand mold effectively suppresses high-temperature softening deformation of thin-walled parts.

[0046] Example 4 1. Material preparation The workpiece to be processed is a ZM6 magnesium alloy connecting rod casting. The container and equipment are the same as in Example 1.

[0047] 2. Process Implementation a) Preparation of flame-retardant sand-molded mixture: Three groups of tests were set up, with the amount of composite flame retardant (proportioning as in Example 1) added being 0.5%, 1.2%, and 3.0% of the dry sand mass, respectively. The mixing method was the same as in Example 1.

[0048] b) Loading and heat treatment: The castings are grouped and embedded in the iron box, vibrated to compact the material, and then covered with a 55mm top layer. The iron box is pushed into the resistance furnace, and the temperature is increased to 540℃ at a rate of 5℃ / min, and held for 8 hours.

[0049] c) Post-treatment and quenching: After the heat preservation is completed, the iron box is quickly moved outside the furnace and air-cooled to below 200°C (takes about 30 minutes). When the temperature of the box drops to 200°C, the iron box is opened and the casting is immediately taken out from the sand mold that is still hot. It is then directly immersed in a room temperature water bath for water quenching, followed by vibration separation and cleaning.

[0050] 3. Effect Description All three groups of castings showed no oxidation on their surfaces. The group with 0.5% flame retardant showed slightly weaker sand hardening strength; the group with 3.0% flame retardant had overly dense sand, making separation slightly more difficult; the group with 1.2% flame retardant showed the best overall effect. After subsequent aging treatment, the yield strength of the water-quenched castings increased by approximately 8% compared to the air-cooled castings. This example demonstrates that a flame retardant addition level within the range of 0.8%-2.0% is relatively balanced, and this process provides conditions for the safe implementation of water quenching strengthening.

[0051] Comparative Example 1 ZM6 test bars from the same batch as in Example 1 were taken and placed directly on a heat-resistant steel rack in the same resistance furnace for treatment at 540℃ for 12 hours. During the heating process, the test bars underwent violent oxidation and combustion, and the surface was completely covered with a white oxide scale and black ablation material (e.g., ...). Figure 2b As shown in the figure, the workpiece is scrapped.

[0052] Comparative Example 2 ZM6 test bars were embedded in a sand mold with only 3.0% boric acid added, and treated according to the procedure in Example 1. After treatment, the area of ​​localized oxidation spots on the test bars reached 15%, and the sand mold did not form effective overall support. The bending deformation of the test bars was 0.8 mm, exceeding the tolerance requirement of the drawing (≤0.2 mm). This result fully demonstrates that a single flame-retardant component cannot simultaneously achieve the dual effects of reliable oxidation prevention and effective support to prevent deformation.

[0053] Matters not covered in this invention are common knowledge.

[0054] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A heat treatment process for preventing oxidation of magnesium alloys, characterized in that, Includes the following steps: a) Preparation of flame-retardant sand mold: Mix dry sand with composite flame retardant evenly to obtain flame-retardant sand mold mixture; b) Loading the furnace: Place the magnesium alloy casting to be treated in a heat-resistant container, and completely bury and cover the casting with the flame-retardant sand mixture. When filling, vibrate to compact the mixture, and the thickness of the top covering layer shall not be less than 50 mm. c) Heat treatment: The heat-resistant container containing the magnesium alloy casting and the flame-retardant sand mixture is placed in a conventional heat treatment furnace for programmed heating, holding and cooling; wherein, during the holding stage, the flame-retardant sand mixture undergoes controllable sintering hardening, with a sintering hardening temperature range of 400℃-600℃, and the hardened sand body formed provides physical support for the magnesium alloy casting in the high-temperature softened state. d) Post-treatment: After heat treatment, the magnesium alloy casting is separated from the hardened flame-retardant sand mold.

2. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, In step a), the amount of the composite flame retardant added is 0.5% to 3.0% of the mass of the dry sand, preferably 0.8% to 2.0%.

3. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, The composite flame retardant consists of the following components by mass percentage Composition: Fluorine-containing compounds: 20-60%, sulfur-containing compounds: 15-30%, boron compounds: 10-40%, other additives: 1-10%; the fluorine-containing compounds are selected from at least one of sodium fluoroborate, potassium fluoroborate, sodium fluorosilicate, and calcium fluoride; the sulfur-containing compounds are selected from at least one of iron sulfide, sulfur, and sodium dodecyl sulfonate; the boron compounds are boric acid; the other additives are selected from iron oxide.

4. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, In step a), the components of the composite flame retardant are first mixed evenly, and then mixed with dry sand. The total mixing time is 40-60 minutes.

5. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, The dry sand is quartz sand with a particle size of 40-70 mesh.

6. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, In step d), the hardened flame-retardant sand mold is separated from the magnesium alloy casting by vibration.

7. The magnesium alloy anti-oxidation heat treatment process according to claim 6, characterized in that, In step d), the separation process also includes washing the surface of the magnesium alloy casting with water.

8. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, In the cooling stage of step c), after the heat-resistant container has cooled to below 200°C, the magnesium alloy casting is separated from the flame-retardant sand mold and immediately subjected to water quenching.

9. The magnesium alloy anti-oxidation heat treatment process according to claim 1, characterized in that, The magnesium alloy is ZM5 or ZM6 magnesium alloy.

10. The anti-oxidation heat treatment process for magnesium alloys according to any one of claims 1-9, characterized in that, The heat-resistant container is made of heat-resistant steel.

Images