Welding and remanufacturing method for magnesium-rare earth alloy large component with low weld crack tendency

Abstract

The invention discloses a low-weld-crack-tendency welding and remanufacturing method for a magnesium-rare earth alloy large component. The method comprises the steps that the magnesium-rare earth alloy large complex component is subjected to pre-welding incomplete solution treatment; preheating after removing defects; carrying out wire filling welding repair or connection; and then high-frequency low-stress impact, post-welding complete solution treatment, low-frequency high-stress impact and post-welding aging treatment are sequentially carried out. According to the method, the composite means of incomplete solution treatment before welding, magnesium-rare earth alloy welding wire filling with high plasticity and toughness and a narrow solidification temperature interval and the like are combined, and meanwhile the tendency of liquefaction cracks in a heat affected zone and solidification cracks of a weld joint is reduced; a large number of refined second phases with high plasticity and toughness are introduced into a welding seam through post-welding stress impact and high-temperature solution treatment, the plastic deformation capacity of the welding seam is improved, redistribution of welding stress in a connector is achieved, and welding cold cracks are avoided. And meanwhile, the hot crack and cold crack tendency of the magnesium-rare earth alloy in the welding and remanufacturing process is reduced, and wide application prospects are achieved.

Description

Technical Field

[0001] This invention belongs to the field of welding technology and relates to a welding method for magnesium rare earth alloys. Specifically, it relates to a welding and remanufacturing method for large magnesium rare earth alloy components with low weld cracking tendency. Background Technology

[0002] Magnesium's density is only about one-quarter that of steel and two-thirds that of aluminum. Replacing aluminum and steel with magnesium, while meeting usage requirements, is a crucial way to achieve lightweight equipment. Magnesium rare earth alloys are high-performance magnesium alloys formed by adding rare earth elements to magnesium. They are characterized by low density, high specific strength and specific stiffness, and excellent heat resistance, making them well-suited for forming large and complex aerospace components such as aircraft engine casings, transmission system casings, and missile hulls. However, magnesium rare earth alloys are highly chemically reactive and easily form oxide inclusions. Furthermore, large and complex magnesium rare earth alloy components are prone to defects such as sand inclusions, shrinkage cavities, porosity, and gas porosity during sand casting, resulting in a first-pass yield of less than 50%. Therefore, to improve yield and reduce production costs, it is urgent to develop highly reliable defect repair and remanufacturing methods based on fusion welding technology.

[0003] However, magnesium rare earth alloys are highly prone to weld cracking during fusion welding, often resulting in repair failures and, in severe cases, even complete product scrapping. Welding cracks of magnesium rare earth alloys can be divided into three categories: (1) Under the action of welding heat, the low-melting-point eutectic liquid film formed by the melting of the eutectic structure in the base material near the molten pool is torn under welding stress, forming liquefaction cracks, which are hot cracks; (2) When using wire with the same composition as the base material for filler welding, due to the wide solidification temperature range of magnesium rare earth alloys, the liquid film of the last solidified eutectic component in the fusion zone is torn under welding stress, forming solidification cracks, which are hot cracks; (3) Under the action of welding heat, the heat-affected zone undergoes grain growth and second-phase coarsening, becoming the weakest area of ​​the entire joint. Although no cracking occurs during welding, as the weld temperature gradually decreases, the residual welding stress gradually increases and exceeds the strength of the heat-affected zone, causing cracking, which are cold cracks. In summary, weld cracking in magnesium rare earth alloys can be either hot or cold. Hot cracking primarily originates from the tearing of the low-melting-point eutectic liquid film in the heat-affected zone or fusion zone, while cold cracking mainly results from residual welding stress exceeding the strength of the heat-affected zone. In practical engineering applications, both hot and cold cracking in magnesium rare earth alloys are significant causes of welding failure.

[0004] To improve or solve the above problems, both academia and engineering have conducted extensive research. Patent CN201811509559.1 discloses a magnesium-rare earth alloy fusion welding wire material. This invention can effectively reduce the tendency for hot cracking during weld solidification by using low-strength fusion welding wire, but it does not mention whether this method is effective in preventing cold cracking. Patent CN202310000179.X discloses an argon arc welding repair method for large-area defects in magnesium-rare earth alloy castings. One of the key innovations of this invention is the combination of preheating before welding, a spiral upward welding path, and post-weld heat preservation to effectively reduce welding stress, thereby suppressing welding cracking. However, as the aforementioned analysis shows, in addition to welding stress, the alloy solidification temperature range, the content and distribution of low-melting-point eutectic liquid film (all related to the properties of the alloy material itself) also have a significant impact on welding cracking. In other words, controlling welding stress can effectively reduce the tendency for cold cracking, but its effect on hot cracks such as liquefaction cracks and solidification cracks is relatively limited.

[0005] Patent CN202010613999.2 discloses a welding repair method to reduce welding cracks in magnesium rare earth alloy castings. This invention completely eliminates the eutectic structure in the sand casting base material through pre-weld solution treatment, preventing it from melting and forming a liquid film during fusion welding, thereby reducing the tendency for welding hot cracking. The literature "The Influence of Pre-weld Treatment on the Quality of TIG Welded Joints of ZM6 Magnesium Alloy" (Zhao Jianxun, Master's Thesis, Harbin University of Science and Technology, 2016) also proposes using complete pre-weld solution treatment to reduce the hot cracking tendency of ZM6 magnesium alloy TIG welded joints. However, since the above methods involve welding in the solution-treated base material, the grain boundaries near the molten pool lose the pinning effect of the rare earth second phase. The coarsening of grains in the heat-affected zone (HAZ) under welding heat will be more pronounced, severely deteriorating the HAZ performance, which in turn exacerbates the risk of cold cracking in the HAZ. In other words, the above methods may not only increase the risk of cold cracking in magnesium rare earth alloy welded joints but also reduce the comprehensive mechanical properties of the HAZ and the joint as a whole. Secondly, the above methods can only reduce the tendency of liquefaction cracking in the heat-affected zone, but have little effect on the tendency of solidification cracking in the fusion zone.

[0006] In summary, the inventors believe that to address the severe cracking problem in magnesium rare earth alloy components during melting, welding, and remanufacturing, a novel technical solution must be developed, taking into account multiple aspects such as welding materials and welding processes. This solution should be able to coordinate the interaction between hot cracking (including liquefaction and solidification cracking) and cold cracking control during the melting and welding of magnesium rare earth alloy components, thereby simultaneously reducing the tendency for both hot and cold cracking to form. The development of this technology has significant engineering value for promoting the application of large and complex magnesium rare earth alloy components in key areas such as lightweighting of aerospace equipment. Summary of the Invention

[0007] To address the problem of severe weld cracking in large and complex magnesium rare earth alloy components during fusion welding and remanufacturing, this invention provides a novel method that can significantly reduce the weld cracking tendency of magnesium rare earth alloys. This method reduces hot and cold cracking in magnesium rare earth alloy welding by optimizing the eutectic phase content in the heat-affected zone and fusion zone, and adjusting the stress and strain distribution of the joint, thereby improving the yield of magnesium rare earth alloy repair welding and remanufacturing.

[0008] To achieve the aforementioned technical effects, this invention proposes a novel welding method for magnesium rare earth alloys, which differs significantly from existing heat treatment methods. Firstly, existing methods primarily employ complete solution heat treatment before welding to eliminate the eutectic structure of the base material, preventing it from melting and forming liquefaction cracks during welding. However, this results in the loss of the pinning effect of the second phase on the grains in the heat-affected zone (HAZ), leading to severe grain coarsening in the HAZ after welding, making the HAZ brittle and increasing the risk of cold cracking. This invention innovatively proposes an incomplete solution heat treatment method, which, while reducing the eutectic structure and liquefaction cracking, retains a very small portion of the eutectic second phase, allowing it to still pin the grain boundaries during welding and preventing grain coarsening in the HAZ. This approach addresses both the reduction of hot and cold cracking. Secondly, most existing methods use welding wire with the same composition as the base metal, resulting in solidification cracks in the weld due to an excessively wide solidification temperature range. This invention innovatively uses welding wire with a lower rare earth element content than the base metal. Due to the lower rare earth content, the solidification temperature range is smaller, the tendency for hot cracking is lower, and the weld exhibits good ductility and toughness, making it less prone to cracking under welding stress. Combined with a high Zr content to achieve grain refinement, the fine grain strengthening compensates for the loss of strengthening effect due to the reduced rare earth content. This effectively reduces the tendency for solidification cracking while ensuring the mechanical properties of the weld. Third, most existing solutions use methods such as preheating before welding, slow cooling after welding, and optimizing the welding path to reduce welding stress and avoid cold cracking, but these methods have limited effectiveness. This invention innovatively introduces a long-period stacking ordered (LPSO) phase with high plasticity and toughness into the weld, thereby improving the plasticity, toughness, and deformability of the weld. This allows the high plasticity and toughness weld to actively undertake more deformation under welding stress, truly achieving "welding stress redistribution" and thus preventing cold cracking from preferentially occurring in the fragile heat-affected zone.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] This invention provides a welding and remanufacturing method for large magnesium rare earth alloy components with low weld cracking tendency, comprising the following steps:

[0011] S1. Perform incomplete solution treatment on the large and complex magnesium rare earth alloy components to be welded before welding.

[0012] S2. Locate the defect, remove the defect, and preheat the component before welding;

[0013] S3. Use welding wire with a rare earth element content lower than that of the base material to perform filler welding repair or connection of components.

[0014] S4. Perform the first high-frequency low-stress impact on the hot weld formed after filler wire welding.

[0015] S5. Perform complete post-weld solution treatment on the entire component.

[0016] S6. Perform a second low-frequency high-stress impact on the cold weld and the base material around the weld after solution treatment.

[0017] S7. Perform post-weld aging treatment on the entire component.

[0018] As some specific embodiments of the present invention, in step S1, the large and complex magnesium rare earth alloy components include magnesium rare earth alloys of the Mg-Gd, Mg-Y, Mg-Nd, Mg-La, Mg-Ce, or Mg-Pr systems.

[0019] As some specific embodiments of the present invention, in step S1, the temperature of the incomplete solution treatment is 470-520°C, the time is 1-6 hours, and the average size of the undissolved rare earth phase in the parent material after the incomplete solution treatment is controlled to be 1-10 μm.

[0020] Only under the specified solution treatment temperature and time can the effect of incomplete solution be effectively achieved, and the average size of the undissolved rare earth phase can be controlled to 1-10 μm. This ensures that most of the eutectic structure dissolves into the matrix, avoiding the formation of liquefaction cracks during welding, while retaining a small portion of the eutectic second phase to pin the matrix grain boundaries and prevent grain coarsening in the heat-affected zone.

[0021] The incomplete solution treatment in this invention has three main objectives: First, it reduces the low-melting-point eutectic structure in the cast base material, preventing it from melting and forming liquefaction cracks during welding. Second, while the strength of the base material remains relatively stable after incomplete solution treatment, its plasticity and toughness are significantly improved, thus reducing the tendency for crack initiation under welding stress. Third, the very small amount of second phase remaining at the grain boundaries after incomplete solution treatment still provides excellent pinning effect, effectively preventing grain coarsening in the heat-affected zone. Simultaneously, this invention is the first to propose using welding wire with a rare-earth element content lower than that of the base material. This aims to lower the solidification temperature range of the molten pool during solidification, improve the plasticity and toughness of the weld, and reduce the weld's cracking tendency. After welding, high-frequency, low-stress impact treatment of the weld before high-temperature solution treatment effectively alleviates stress concentration and reduces residual welding stress. Simultaneously, high-frequency, low-stress impact can introduce high-density dislocations into the weld, serving as nucleation sites for subsequent LPSO phase precipitation. This increases the nucleation rate and quantity of LPSO phases during high-temperature solution treatment, achieving a fine and uniform distribution of LPSO phases. This further enhances the weld's ductility and toughness, enabling it to actively bear more deformation under welding stress, truly achieving "welding stress redistribution" and thus preventing cold cracking from preferentially occurring in the fragile heat-affected zone. The second low-frequency, high-stress impact on the weld before aging treatment replenishes the dislocations annihilated during high-temperature solution treatment, providing nucleation sites for subsequent β-series strengthening phase precipitation, achieving a fine and uniform distribution of β-series phases, and improving the overall strength of the joint area.

[0022] As some specific embodiments of the present invention, in step S2, defects are removed by machining, including milling or drilling.

[0023] As some specific embodiments of the present invention, in step S2, the temperature of the preheating before welding is 200-400°C and the time is 1-5 hours.

[0024] As some specific embodiments of the present invention, in step S3, the welding wire contains 0.1-6% rare earth RE elements, 0.5-2% Zn elements, and 0.5-1.5% Zr elements by mass, with the balance being Mg, and the types of rare earth RE elements added to the welding wire do not exceed two.

[0025] As some specific embodiments of the present invention, the rare earth RE element is selected from one or two of Gd, Y, Nd, La, Ce, and Pr.

[0026] As some specific embodiments of the present invention, in step S3, the welding is selected from at least one of arc welding, laser welding, electron beam welding and thermal, mechanical, acoustic, vibration and magnetic field composite welding.

[0027] Generally, the hot cracking tendency of an alloy is related to its solidification temperature range; the wider the solidification temperature range, the greater the hot cracking tendency. This invention uses welding wire with lower alloying elements to reduce the solidification temperature range of the molten pool, thereby reducing its hot cracking tendency. Simultaneously, because the welding wire used in this invention has a low alloying element content, a high Zr content is used to significantly refine the weld grains to compensate for the strengthening effect, thereby improving weld strength through fine-grain strengthening. More importantly, this welding wire strictly limits the addition of no more than two rare earth elements. This is because the more rare earth elements present, the lower the solidus temperature of the weld, which increases the solidification temperature range.

[0028] As some specific embodiments of the present invention, in step S5, the post-weld complete solution treatment includes two-stage solution treatment: high temperature and low temperature. First, high temperature solution treatment is performed, and then low temperature solution treatment is performed.

[0029] As some specific embodiments of the present invention, the high-temperature solution treatment is performed at a temperature of 510–540°C for a time of 0.1–2 h; the low-temperature solution treatment is performed at a temperature of 470–500°C for a time of 0.1–4 h.

[0030] The high- and low-temperature solution treatments serve different purposes. The initial high-temperature solution treatment induces the rapid precipitation of the LPSO phase. Because the LPSO phase is essentially a stacking fault, its formation heavily depends on the diffusion rate of alloying elements in the matrix. At high temperatures, the rapid diffusion accelerates LPSO phase formation. Once formed, the LPSO phase effectively pins grain boundaries, improving the weld's microstructure stability and preventing grain coarsening during subsequent prolonged solution treatment. The subsequent low-temperature solution treatment allows some residual eutectic structures to dissolve further, achieving the desired solution treatment state.

[0031] As some specific embodiments of the present invention, in step S4, the weld temperature of the hot weld is ≥400°C;

[0032] The impact frequency of the first high-frequency low-stress impact is 5-100Hz, the force is 1-50N, and the duration is 5-20s.

[0033] As some specific embodiments of the present invention, in step S6, the weld temperature of the cold weld is ≤200°C;

[0034] The impact frequency of the second low-frequency high-stress impact is 1-5 Hz, the force is 1-100 N, and the duration is 10-20 s.

[0035] The purposes of the two stress impacts are different: the first, a high-frequency, low-stress impact on the hot weld, is chosen because stress impact under hot conditions can effectively adjust the joint stress field, alleviate welding stress concentration, and introduce dislocations to increase the number density of subsequent LPSO phase precipitation. However, since the weld is hot and has low strength at this time, a high-frequency, low-stress impact is used. The second, a low-frequency, high-stress impact on the cold weld and the surrounding base material, is chosen because the joint strength is high at this time, and only a low-frequency, high-stress impact can effectively introduce dislocations, thereby promoting high-density precipitation of β-series phases during aging treatment, improving the overall strength of the joint, and avoiding insufficient strength due to low rare earth content in the welding wire. Simultaneously, because LPSO phases consume a large amount of rare earth elements, the kinetics of β-series phase precipitation are weak during aging due to the low content of remaining rare earth solid solution atoms in the matrix. Therefore, a low-frequency, high-stress impact before aging treatment, introducing high-density dislocations, is necessary to effectively promote β-series phase precipitation.

[0036] As some specific embodiments of the present invention, in step S7, the temperature of the post-weld aging treatment is 150-250°C and the time is 0.1-20h.

[0037] This invention combines pre-weld incomplete solution treatment with filler wire of magnesium rare earth alloy welding material that has high plasticity and toughness and a narrow solidification temperature range. This approach simultaneously reduces the tendency for liquefaction cracking in the heat-affected zone and solidification cracking in the weld. Furthermore, post-weld stress impact and high-temperature solution treatment introduce a large amount of refined, high-plasticity and toughness second phase into the weld, improving its plastic deformation capacity and achieving stress redistribution within the joint, thus preventing cold cracking. The technical solution of this invention effectively reduces the tendency for hot and cold cracking in magnesium rare earth alloys during welding and remanufacturing. It also features excellent operability and requires no complex equipment or tooling, making it a promising technology for future applications.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1) This invention combines pre- and post-weld heat treatment, special magnesium rare earth alloy welding wire, and post-weld impact treatment to reduce the tendency of magnesium rare earth alloy components to hot crack and cold crack during welding and remanufacturing, thereby achieving high-reliability welding or repair of magnesium rare earth alloys.

[0040] 2) This invention introduces various reinforcing phases of different sizes, structures and properties into magnesium rare earth alloy welds, which can effectively coordinate the strength and plasticity of the weld joint, achieve synergistic regulation of strength and plasticity, and improve the comprehensive mechanical properties of magnesium rare earth alloy joints.

[0041] 3) The process proposed in this invention is simple. It can achieve high-reliability repair of magnesium rare earth alloy components by using only filler wire welding. It does not involve expensive and complex equipment and has great potential for engineering application. Attached Figure Description

[0042] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0043] Figure 1 This is a macroscopic image of the welded joint of the magnesium rare earth alloy casting prepared in Example 1;

[0044] Figure 2 This is a macroscopic image of the welded joint of the magnesium rare earth alloy casting prepared in Comparative Example 1. Detailed Implementation

[0045] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0046] Example 1

[0047] Repair welding of Mg-9Gd-3Y-0.5Zr (wt.%) castings was performed using filler wire TIG welding. The specific steps are as follows:

[0048] First, the casting underwent incomplete solution treatment before welding at a temperature of 470℃ for 6 hours, resulting in an average size of 9.2 μm for the undissolved rare earth phases in the base material. After the solution treatment, defects in the casting were removed using a pneumatic milling cutter, and the component was preheated at 200℃ for 5 hours before welding.

[0049] The casting was repaired using TIG welding with filler wire composed of Mg-6Gd-0.5Zn-0.5Zr. The repaired hot weld was then subjected to a first stress impact test at a frequency of 5 Hz, a force of 50 N, and a duration of 5 seconds. Following the impact test, the entire component underwent rapid post-weld solution treatment. The first stage of high-temperature solution treatment was performed at 510℃ for 2 hours, followed by a second stage of low-temperature solution treatment at 500℃ for 1 hour. After solution treatment, the cold weld and surrounding base material were subjected to a second stress impact test at a frequency of 1 Hz, a force of 100 N, and a duration of 10 seconds. Finally, the entire component underwent aging treatment at 150℃ for 20 hours.

[0050] The test results showed that the casting did not exhibit any cracking during or after the repair welding process, and there were no instances of undercut, spatter, or other defects (such as...). Figure 1 As shown in the figure, through mechanical property testing of the cut sample of the body, the repair weld joint coefficient reached 0.89%.

[0051] Example 2

[0052] TIG welding with filler wire was used to repair Mg-6Gd-3Y-0.5Zr (wt.%) castings.

[0053] First, the casting underwent incomplete solution treatment before welding at a temperature of 480℃ for 5 hours, resulting in an average size of 7.1 μm for the undissolved rare earth phases in the base material of the casting. After the solution treatment, defects in the casting were removed using a pneumatic milling cutter, and the component was preheated at 250℃ for 4 hours before welding.

[0054] The casting was repaired using TIG welding with filler wire composed of Mg-5Gd-0.8Zn-0.8Zr. The repaired hot weld was then subjected to a first stress impact test at a frequency of 10 Hz, a force of 40 N, and a duration of 10 s. Following the impact test, the entire component underwent rapid post-weld solution treatment. The first stage of high-temperature solution treatment was performed at 520℃ for 1.5 hours, followed by a second stage of low-temperature solution treatment at 490℃ for 2 hours. After solution treatment, the cold weld and surrounding base material were subjected to a second stress impact test at a frequency of 2 Hz, a force of 80 N, and a duration of 12 s. Finally, the entire component underwent aging treatment at 180℃ for 18 hours.

[0055] The test results show that no cracking occurred during or after the repair welding process, and there were no undercuts, spatter, or other defects. The mechanical property test of the cut sample showed that the repair weld joint coefficient reached 0.88%.

[0056] Example 3

[0057] Repair welding of Mg-5Y-3Nd-1Gd-0.5Zr (wt.%) castings was performed using filler wire laser welding.

[0058] First, the casting underwent incomplete solution treatment before welding at a temperature of 490℃ for 5 hours, resulting in an average size of 5.3 μm for the undissolved rare earth phases in the base material of the casting. After the solution treatment, defects in the casting were removed using a pneumatic milling cutter, and the component was preheated at 300℃ for 3 hours before welding.

[0059] Laser welding with filler wire of Mg-3Y-2Nd-1Zn-1Zr composition was used to repair the casting. The repaired hot weld was then subjected to a first stress impact test at a frequency of 30 Hz, a force of 30 N, and a duration of 12 s. Following the impact, the entire component underwent rapid post-weld solution treatment. The first stage of solution treatment was performed at 530℃ for 1 hour, and the second stage at 480℃ for 3 hours. After solution treatment, a second stress impact test was conducted on the cold weld and the surrounding base material at a frequency of 3 Hz, a force of 50 N, and a duration of 15 s. Finally, the entire component underwent aging treatment at 200℃ for 16 hours.

[0060] The test results show that no cracking occurred during or after the repair welding process, and there were no undercuts, spatter, or other defects. The mechanical property test of the cut sample showed that the repair weld joint coefficient reached 0.85%.

[0061] Example 4

[0062] Repair welding of Mg-5Y-3Nd-1Gd-0.5Zr (wt.%) castings was performed using filler wire laser welding.

[0063] First, the casting underwent incomplete solution treatment before welding at a temperature of 500℃ for 4 hours, resulting in an average size of 3.7 μm for the undissolved rare earth phases in the base material of the casting. After the solution treatment, defects in the casting were removed by drilling, and the component was preheated at 350℃ for 2 hours before welding.

[0064] Laser welding with filler wire of Mg-2Y-2Nd-1.5Zn-1.2Zr composition was used to repair the casting. The repaired hot weld was then subjected to a first stress impact test at a frequency of 50 Hz, a force of 20 N, and a duration of 15 s. Following the impact, the entire component underwent rapid post-weld solution treatment. The first stage of solution treatment was performed at 535℃ for 0.5 h, and the second stage at 475℃ for 3.5 h. After solution treatment, a second stress impact test was conducted on the cold weld and surrounding base material at a frequency of 4 Hz, a force of 40 N, and a duration of 18 s. Finally, the entire component underwent aging treatment at 230℃ for 10 h.

[0065] The test results show that no cracking occurred during or after the repair welding process, and there were no undercuts, spatter, or other defects. The mechanical property test of the cut sample showed that the repair weld joint coefficient reached 0.83%.

[0066] Example 5

[0067] Electron beam welding with filler wire was used to repair Mg-4Y-2Nd-1Gd-0.5Zr (wt.%) castings.

[0068] First, the casting underwent incomplete solution treatment before welding at a temperature of 520℃ for 2 hours, resulting in an average size of 4.9 μm for the undissolved rare earth phases in the base material of the casting. After the solution treatment, defects in the casting were removed by drilling, and the component was preheated at 400℃ for 1 hour before welding.

[0069] Electron beam welding with a Mg-2Y-1Nd-2Zn-1.5Zr composition was used to repair the casting. The repaired hot weld was then subjected to a first stress impact at a frequency of 100 Hz, a force of 1 N, and a duration of 20 s. Following the impact, the entire component underwent rapid post-weld solution treatment: the first stage at 540℃ for 0.2 h, and the second stage at 470℃ for 4 h. After solution treatment, a second stress impact was applied to the cold weld and surrounding base material at a frequency of 5 Hz, a force of 10 N, and a duration of 20 s. Finally, the entire component underwent aging treatment at 250℃ for 1 h.

[0070] The test results show that no cracking occurred during or after the repair welding process, and there were no undercuts, spatter, or other defects. The mechanical property test of the cut sample showed that the repair weld joint coefficient reached 0.84%.

[0071] Comparative Example 1

[0072] The method used in this comparative example is basically the same as that in Example 1, except that the casting is subjected to complete solution treatment before removing defects, instead of the incomplete solution treatment in Example 1. The complete solution treatment specifically includes a solution treatment temperature of 520°C and a treatment time of 12 hours.

[0073] The remaining steps and parameters were performed in accordance with Example 1.

[0074] The results showed that the joint did not crack during the welding process, but cold cracking occurred in the heat-affected zone after welding. This was because the complete solid solution before welding caused all the eutectic phase in the base material to dissolve, and the grain boundaries lost the pinning effect of the second phase. As a result, the grains in the heat-affected zone were severely coarsened under the action of welding heat, and the heat-affected zone became the most vulnerable area in the entire joint, and cracking occurred under welding stress.

[0075] like Figure 2 The image shown is a macroscopic image of the repaired weld joint of the magnesium rare earth alloy casting prepared in Comparative Example 1. It can be seen that obvious cracks appeared in the repaired area.

[0076] Comparative Example 2

[0077] The method used in this comparative example is basically the same as that in Example 1, except that no solution treatment is performed on the casting before removing defects. All other steps and parameters are performed as in Example 1.

[0078] The results showed that cracks occurred in the heat-affected zone of the joint during the welding process, resulting in the failure of the repair weld. This was because no solution treatment was performed before welding. The low-melting-point eutectic phase in the heat-affected zone near the molten pool melted and formed a eutectic liquid film, which was torn apart under the action of welding stress, forming a hot crack.

[0079] Comparative Example 3

[0080] The method used in this comparative example is basically the same as that in Example 1, except that the average size of the undissolved rare earth phase in the casting base material after incomplete solution treatment is 12.2 μm. All other steps and parameters are performed according to Example 1.

[0081] The results showed that cracks occurred in the heat-affected zone of the joint during the welding process, resulting in the failure of the repair weld. This was because the size of the residual low-melting-point second phase in the base material was too large after the incomplete solution treatment before welding. The eutectic liquid film formed by it under the action of welding heat was thick and was torn by the welding stress, thus forming a hot crack.

[0082] Comparative Example 4

[0083] The method of this comparative example is basically the same as that of Example 1, except that the welding wire used is Mg-7Gd-0.5Zr instead of Mg-6Gd-0.5Zn-0.5Zr in Example 1. All other steps and parameters are performed in accordance with Example 1.

[0084] The results showed that cracks occurred in the fusion zone of the joint during the welding process, resulting in the failure of the repair weld. This was because the welding wire had a high rare earth content, which resulted in a large content of low-melting-point eutectic liquid film between the crystals in the fusion zone at the end of solidification. Under the action of welding stress, the film was torn apart and formed hot cracks.

[0085] Comparative Example 5

[0086] The method of this comparative example is basically the same as that of Example 1, except that the welding wire used is Mg-3Gd-2Y-1Nd-0.5Zr instead of Mg-6Gd-0.5Zn-0.5Zr in Example 1. All other steps and parameters are performed in accordance with Example 1.

[0087] The results showed that cracks occurred in the fusion zone of the joint during the welding process, resulting in the failure of the repair weld. This was because the welding wire contained a variety of rare earth elements, which lowered the solidus temperature and resulted in a wider solidification temperature range in the fusion zone, increasing the tendency for hot cracks to form in the fusion zone.

[0088] Comparative Example 6

[0089] The method used in this comparative example is basically the same as that in Example 1, except that the repaired hot weld was not subjected to a first stress impact. All other steps and parameters were performed in accordance with Example 1.

[0090] The results showed that the joint did not crack hot during the welding process, but cold cracking occurred in the heat-affected zone after welding. This was because stress impact was performed before solution treatment, resulting in a lower number density of the high-ductility LPSO phase precipitated in the fusion zone during the solution treatment. Consequently, the LPSO phase content in the fusion zone was lower and the size was larger, resulting in an insignificant plasticizing and toughening effect in the fusion zone. This led to the fragile heat-affected zone bearing most of the welding stress deformation, ultimately resulting in cold cracking.

[0091] Comparative Example 7

[0092] The method used in this comparative example is basically the same as that in Example 1, except that a low-frequency high-stress impact was used for the first stress impact on the repaired hot weld. The impact frequency was 1Hz, the force was 100N, and the impact time was 5s. All other steps and parameters were performed in accordance with Example 1.

[0093] The results show that during the first stress impact, the weld fractured directly under the action of low-frequency high-stress impact because the hot weld had low strength.

[0094] Comparative Example 8

[0095] The method used in this comparative example is basically the same as that in Example 1, except that: after the first stress impact, the entire component is quickly subjected to post-weld solution treatment. The first stage is low-temperature solution treatment, and the second stage is high-temperature solution treatment. The first stage solution treatment temperature is 500℃ and the time is 1 hour, and the second stage solution treatment temperature is 510℃ and the time is 2 hours. All other steps and parameters are the same as in Example 1.

[0096] The results show that the low first-stage solution temperature is unfavorable for the precipitation of the LPSO phase, resulting in a low content of pinned LPSO phase at the grain boundaries during the second-stage solution treatment. This leads to severe grain coarsening after the solution treatment, significantly reducing the strength and plasticity of the weld. Therefore, although no cracking occurred during or after the repair welding process, the mechanical property test of the sectioned sample showed that the repair weld joint coefficient was only 71%.

[0097] Comparative Example 9

[0098] The method used in this comparative example is basically the same as that in Example 1, except that a second stress impact was not performed on the cold weld and the base material surrounding the weld after solution treatment. All other steps and parameters were performed in accordance with Example 1.

[0099] The results show that because the rare earth content of the welding wire is lower than that of the base material, and the LPSO phase formed during the solution treatment process has consumed a large amount of rare earth solute elements, the number density of β-series precipitation during the aging treatment is very low, resulting in low joint strength. Therefore, although no cracking occurred during or after the repair welding process, the mechanical property test of the cut sample showed that the repair weld joint coefficient was only 66%.

[0100] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A welding and remanufacturing method for large magnesium rare earth alloy components with low weld cracking tendency, characterized in that, Includes the following steps: S1. Perform incomplete solution treatment on the large and complex magnesium rare earth alloy components to be welded before welding. S2. Locate the defect, remove the defect, and preheat the component before welding; S3. Use welding wire with a rare earth element content lower than that of the base material to perform filler welding repair or connection of components. S4. Perform the first high-frequency low-stress impact on the hot weld formed after filler wire welding. S5. Perform complete post-weld solution treatment on the entire component. S6. Perform a second low-frequency high-stress impact on the cold weld and the base material around the weld after solution treatment. S7. Perform post-weld aging treatment on the entire component.

2. The welding and remanufacturing method according to claim 1, characterized in that, In step S1, the large and complex magnesium rare earth alloy components include magnesium rare earth alloys based on Mg-Gd, Mg-Y, Mg-Nd, Mg-La, Mg-Ce, or Mg-Pr systems.

3. The welding and remanufacturing method according to claim 1, characterized in that, In step S1, the temperature of the incomplete solution treatment is 470-520℃, the time is 1-6h, and the average size of the undissolved rare earth phase in the parent material after the incomplete solution treatment is controlled to be 1-10μm.

4. The welding and remanufacturing method according to claim 1, characterized in that, In step S2, defects are removed by machining, including milling or drilling. And / or, the preheating temperature before welding is 200-400°C, and the time is 1-5 hours.

5. The welding and remanufacturing method according to claim 1, characterized in that, In step S3, the welding wire contains 0.1-6% rare earth RE elements, 0.5-2% Zn elements, and 0.5-1.5% Zr elements by mass, with the balance being Mg; The rare earth RE elements are selected from one or two of Gd, Y, Nd, La, Ce, and Pr.

6. The welding and remanufacturing method according to claim 1, characterized in that, In step S3, the welding is selected from at least one of arc welding, laser welding, electron beam welding, and composite welding of heat, force, sound, vibration, and magnetic fields.

7. The welding and remanufacturing method according to claim 1, characterized in that, In step S5, the post-weld complete solution treatment includes two-stage solution treatment: high temperature and low temperature. First, high temperature solution treatment is performed, followed by low temperature solution treatment. The high-temperature solution treatment is performed at a temperature of 510–540°C for a time of 0.1–2 hours; the low-temperature solution treatment is performed at a temperature of 470–500°C for a time of 0.1–4 hours.

8. The welding and remanufacturing method according to claim 1, characterized in that, In step S4, the weld temperature of the hot weld is ≥400℃; The impact frequency of the first high-frequency low-stress impact is 5-100Hz, the force is 1-50N, and the duration is 5-20s.

9. The welding and remanufacturing method according to claim 1, characterized in that, In step S6, the weld temperature of the cold weld is ≤200℃; The impact frequency of the second low-frequency high-stress impact is 1-5 Hz, the force is 1-100 N, and the duration is 10-20 s.

10. The welding and remanufacturing method according to claim 1, characterized in that, In step S7, the temperature of the post-weld aging treatment is 150-250℃, and the time is 0.1-20h.

Images