Calculation method of solidification parameter threshold for the evolution of crystallization morphology in weld metal microstructure

Abstract

This invention discloses a method for calculating solidification parameter thresholds for the evolution of the crystallization morphology of weld metal microstructure. Based on the pulse closure waveform characteristics of Nd:YAG pulsed laser spot welding, it proposes the concept of "solidification process percentage" to measure the solidification process of experimental and simulated weld joints. It establishes a correspondence between the microstructure evolution of experimental weld joints and the changes in solidification parameters of simulated weld joints, thereby obtaining the solidification parameter thresholds for the evolution of the crystallization morphology of weld metal microstructure and establishing an evaluation system for solidification parameter thresholds for the regulation of weld microstructure. This method has significant theoretical implications for studying the G / R transition threshold of the crystallization morphology evolution of weld metal microstructure and for establishing a threshold evaluation system for the evolution of weld microstructure.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and in particular to a method for calculating solidification parameter thresholds for the evolution of the crystallization morphology of weld metal microstructure. Background Technology

[0002] The main grain types in the microstructure of weld metal include columnar grains and equiaxed grains. Columnar grains are columnar in shape and exhibit obvious directional properties, making them prone to weld cracking. Their crystallization morphologies mainly include cellular and columnar crystals. Equiaxed grains, on the other hand, have smaller size differences in all directions and better mechanical properties. The crystallization behavior of the weld metal microstructure affects the crystallization morphology of the weld structure, and consequently, the mechanical properties of the weld joint. During the solidification and cooling process of the weld pool, the solidification parameters at the liquid-solid interface, such as the temperature gradient (G) and solidification rate (R), are constantly changing due to the dynamic changes in the temperature field of the weld pool. The calculated values ​​of the solidification parameters (G / R) also change accordingly, thereby altering the degree of compositional supercooling at the liquid-solid interface of the weld pool, and thus changing the crystallization morphology of the weld metal microstructure.

[0003] Currently, research on the evolution of the crystallization morphology of weld metal microstructure is qualitative, namely: as the G / R ratio decreases, the crystallization morphology changes from cellular crystals to columnar crystals to equiaxed crystals; there is currently no method for calculating the solidification parameter (G / R) threshold for the evolution of the crystallization morphology of weld metal microstructure, and therefore it is impossible to establish an evaluation system for the solidification parameter threshold for the control of weld microstructure, so as to facilitate quantitative evaluation of the effect of welding process parameters on the control of microstructure. Summary of the Invention

[0004] This invention addresses the problems existing in the prior art by creatively devising a method for calculating the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure. Based on the pulse closure waveform characteristics of Nd:YAG pulsed laser spot welding, the solidification process of experimental and simulated weld joints is measured, and a correspondence between the microstructure evolution of experimental weld joints and the changes in solidification parameters of simulated weld joints is established, thereby obtaining the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure.

[0005] The technical solution of this invention is: a method for calculating the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure, characterized by comprising the following steps:

[0006] 1) On the base material, a single-beam pulsed laser is emitted to irradiate the base material using an Nd:YAG pulsed laser spot welding machine to perform laser self-melting spot welding;

[0007] 2) By changing the pulse closing waveform of the single-beam pulsed laser emitted by the Nd:YAG pulsed laser spot welding machine, the solidification parameters of the liquid-solid interface of the laser self-fusion weld pool are adjusted, the degree of compositional supercooling is changed, and the morphological evolution of the microstructure crystallization of the laser self-fusion weld is realized, thus forming a laser self-fusion weld.

[0008] 3) Using a scanning electron microscope, measure the lengths of the cellular, columnar, and equiaxed crystal regions on the cross-section of the laser self-fusion welding point described in step 2). Calculate the solidification process percentage S(L) corresponding to the crystallization morphology transformation point based on the ratio of each crystal region length to the total crystal region length.

[0009] 4) Using ANSYS software, a two-dimensional axisymmetric model of pulsed laser self-melting spot welding was established to simulate and calculate the solidification parameters of the liquid-solid interface of the laser weld pool, namely the calculated values ​​of temperature gradient G and solidification rate R, G / R, and the functional relationship between solidification process percentage S(t).

[0010] 5) On the curve of the solidification parameter calculation of the liquid-solid interface of the two-dimensional axisymmetric model of pulsed laser self-melting spot welding described in step 4), take the solidification parameter threshold corresponding to the solidification process percentage S(L) at the crystallization morphology transformation point of the experimental weld point, which is the percentage of solidification process.

[0011] Furthermore, in step 2), the pulse closing waveform is a rectangular pulse waveform and a steeply drooping constant slow-cooling pulse waveform, wherein the steeply drooping constant slow-cooling pulse waveform is that after the rectangular main band, a "steeply drooping band" and a "slowly drooping band" are added in sequence.

[0012] Furthermore, in step 3), the solidification process percentage S(L) corresponding to the crystallization morphology transformation point is: the ratio of the liquid-solid interface movement distance L0 to the total distance L in the direction of the line connecting the center of the molten pool surface and the point.

[0013] Furthermore, in step 4), the solidification process percentage S(t) is: the ratio of the liquid-solid interface movement time t0 to the total solidification time t at a certain moment.

[0014] The beneficial effects of the solidification parameter threshold calculation method for the evolution of the microstructure and crystal morphology of welded metals in this invention are reflected in the following aspects:

[0015] 1. A method for calculating solidification parameter thresholds for the evolution of crystal morphology in weld metal microstructures, obtaining solidification parameter thresholds for the evolution of crystal morphology in weld metal microstructures of different compositions, solving the problem of the lack of calculation methods for crystal morphology evolution thresholds in existing technologies, and providing a simple and efficient method for calculating solidification parameter thresholds for the evolution of crystal morphology in weld metals; 2. A method for calculating solidification parameter thresholds for the evolution of crystal morphology in weld metal microstructures, establishing an evaluation system for solidification parameter thresholds for the regulation of weld microstructures, which has important theoretical significance for studying the G / R transformation threshold of the evolution of crystal morphology in weld metal microstructures and establishing a threshold evaluation system for the evolution of weld microstructures;

[0016] 3. A method for calculating the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure is proposed. The concept of "solidification process percentage" is introduced to measure the solidification process of experimental and simulated weld joints. The correspondence between the microstructure evolution of experimental weld joints and the changes in solidification parameters of simulated weld joints is established, thereby obtaining the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure. This lays the foundation for shifting from qualitative assessment to quantitative evaluation in the control of weld microstructure and has important guiding significance for improving the comprehensive performance of welded joints. Attached Figure Description

[0017] Figure 1 This is a flowchart of a method for calculating the threshold of solidification parameters for the evolution of the crystallization morphology of weld metal microstructure;

[0018] Figure 2 This is a rectangular pulse waveform diagram from Example 1, where P0 and t0 represent the peak pulse power and pulse duration, respectively.

[0019] Figure 3 This is a waveform diagram of a steeply drooping constant slow-cooling pulse in Example 1, where: P1 and t1 are the peak power and pulse time of the rectangular main band, and P2 and t2 are the peak power and pulse time of the steeply drooping constant slow-cooling band.

[0020] Figure 4 The image shows a scanned electron micrograph of the cross-section of a rectangular pulsed laser weld joint metal when P1 = 1.6 kW and t1 = 4 ms, and P2 = 0 kW and t1 = 0 ms in Example 1.

[0021] Figure 5 The image shows a scanning electron micrograph of the cross-section of the metal of the laser weld joint with a steeply drooping constant slow-cooling pulse waveform at P1 = 1.6 kW and t1 = 4 ms, and P2 = 0.48 kW and t1 = 11 ms in Example 1.

[0022] Figure 6In Example 1, scanning electron micrographs of the cross-section of the metal of the laser weld joint with a steep drop constant slow cooling pulse waveform at P1 = 1.6 kW and t1 = 4 ms, and P2 = 0.80 kW and t1 = 11 ms.

[0023] Figure 7 yes Figure 6 Enlarged scanning electron micrograph of the tissue structure in selected area A within the Chinese box;

[0024] Figure 8 In Example 1, the calculated values ​​of the solidification parameter G / R on the liquid-solid interface of the rectangular pulse waveform laser weld pool when P1 = 1.6kW and t1 = 4ms, P2 = 0kW and t1 = 0ms;

[0025] Figure 9 In Example 1, the calculated values ​​of the solidification parameter G / R at the liquid-solid interface of the molten pool of the laser weld joint with steep drop constant slow cooling pulse waveform at P1 = 1.6kW and t1 = 4ms, P2 = 0.48kW and t1 = 11ms are given.

[0026] Figure 10 In Example 1, the calculated values ​​of the solidification parameter G / R at the liquid-solid interface of the molten pool of the laser weld point with a steep drop constant slow cooling pulse waveform are given when P1 = 1.6kW and t1 = 4ms, and P2 = 0.80kW and t1 = 11ms. Detailed Implementation

[0027] The following is in conjunction with the appendix Figure 1 —10 and specific embodiments will be described in further detail with reference to the present invention. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0028] See attached document Figure 1 As shown, a flowchart of a method for calculating the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure is presented. The method for calculating the solidification parameter threshold for the evolution of the crystallization morphology of weld metal microstructure is characterized by the following steps: 1) On the base material, a single-beam pulsed laser is emitted to irradiate the base material using an Nd:YAG pulsed laser spot welding machine to perform laser self-melting spot welding.

[0029] 2) By changing the pulse closing waveform of the single-beam pulsed laser emitted by the Nd:YAG pulsed laser spot welding machine, the solidification parameters of the liquid-solid interface of the laser self-fusion weld pool are adjusted, the degree of supercooling is changed, and the morphological evolution of the microstructure crystallization of the laser self-fusion weld is realized, thus forming a laser self-fusion weld. The pulse closing waveform is a rectangular pulse waveform and a steeply drooping constant slow-cooling pulse waveform. The steeply drooping constant slow-cooling pulse waveform is that after the rectangular main band, a "steeply drooping band" and a "slowly drooping band" are added in sequence.

[0030] 3) Using a scanning electron microscope, measure the lengths of the cellular, columnar, and equiaxed crystal regions on the cross-section of the laser self-fusion welding point described in step 2). Based on the ratio of the length of each crystal region to the total length of the crystal regions, calculate the solidification process percentage S(L) corresponding to the crystallization morphology transition point. The solidification process percentage S(L) corresponding to the crystallization morphology transition point is: the ratio of the liquid-solid interface movement distance L0 to the total distance L in the direction of the line connecting the center of the molten pool surface and the point.

[0031] 4) Using ANSYS software, a two-dimensional axisymmetric model of pulsed laser self-melting spot welding was established to simulate and calculate the solidification parameters of the liquid-solid interface of the laser weld pool, namely the calculated values ​​of temperature gradient G and solidification rate R, G / R, and the solidification process percentage S(t). The solidification process percentage S(t) is the ratio of the liquid-solid interface movement time t0 to the total solidification time t at a certain moment.

[0032] 5) On the curve of the solidification parameter calculation of the liquid-solid interface of the two-dimensional axisymmetric model of pulsed laser self-melting spot welding described in step 4), take the solidification parameter threshold corresponding to the solidification process percentage S(L) at the crystallization morphology transformation point of the experimental weld point, which is the percentage of solidification process.

[0033] Example 1:

[0034] The base material for welding was selected as AZ31 magnesium alloy, and Nd:YAG pulsed laser self-melting spot welding experiment was carried out. That is, a single pulsed laser was used to irradiate AZ31 magnesium alloy on a single base material. The pulsed laser process parameters were set as follows: defocusing amount was +2mm, and the protective Ar gas flow rate was 10L / min.

[0035] By changing the pulse waveform—specifically, a rectangular pulse waveform and a steeply drooping, constant-cooling pulse waveform—the corresponding laser welding points can be obtained, as shown in the attached diagram. Figure 2 The rectangular pulse waveform shown is attached. Figure 3 The waveform shown is a steeply drooping, constant, slowly cooling pulse.

[0036] Appendix Figure 4 The image shows a scanned electron micrograph of the cross-section of the metal of the rectangular pulse waveform laser solder joint when P1 = 1.6 kW and t1 = 4 ms, and P2 = 0 kW and t1 = 0 ms. The image shows that the microstructure of the solder joint consists of columnar crystals on the outside and equiaxed crystals in the center, where L1 is the columnar crystal region (L1 = 156 μm) and L2 is the equiaxed crystal region (L2 = 84 μm).

[0037] Appendix Figure 5 The image shows scanning electron micrographs of the cross-section of the metal weld joint using a steeply drooping, constant-cooling pulsed laser waveform at P1 = 1.6 kW and t1 = 4 ms, and P2 = 0.48 kW and t1 = 11 ms. (Attached) Figure 5 The microstructure of the solder joint is shown to be composed entirely of columnar crystals.

[0038] Appendix Figure 6 The image shows a scanning electron micrograph of the cross-section of the metal weld joint using a steeply drooping, constant-cooling pulsed laser beam at P1 = 1.6 kW and t1 = 4 ms, and P2 = 0.80 kW and t1 = 11 ms. (See attached image.) Figure 6 The image shows a slow-cooling region at the center of the solder joint surface, where cellular crystals appear. Specifically, L3 is a columnar crystal region (L3 = 180 μm), and L4 is an equiaxed crystal region (L4 = 350 μm).

[0039] Appendix Figure 7 For the appendix Figure 6 The magnified view of region A in the middle shows cellular crystals. For the rectangular pulsed laser weld joint, the solidification process percentage S(L) corresponding to the transformation point from columnar crystals to equiaxed crystals was calculated. 柱(Columnar)→等(Equiaxe) That is: S(L) 柱(Columnar)→等(Equiaxe) = (L1 / L1+L2)×100%=65%; For a laser weld joint with a steeply decreasing constant slow cooling pulse waveform, P1=1.6kW and t1=4ms, P2=0.80kW and t1=11ms, the percentage of solidification process S(L) corresponding to the transformation point from cellular to columnar crystals is calculated. 胞(Cellular)→柱(Columnar) That is: S(L) 胞(Cellular)→柱(Columnar) =(L4 / L3+L4)×100%=70%.

[0040] A two-dimensional model of pulsed laser spot welding of AZ31 magnesium alloy was established using ANSYS software to obtain two-dimensional simulation data of the melting and solidification processes of the weld joint. The calculated values ​​(G / R) of the solidification parameters (temperature gradient G and solidification rate R) at the liquid-solid interface of the molten pool for rectangular pulsed laser weld joints, weld joints with steeply decreasing constant slow cooling pulse waveforms (P1 = 1.6 kW and t1 = 4 ms, P2 = 0.48 kW and t1 = 11 ms), and weld joints with steeply decreasing constant slow cooling pulse waveforms (P1 = 1.6 kW and t1 = 4 ms, P2 = 0.80 kW and t1 = 11 ms) were used as a function of the percentage of solidification progress S(t). These are shown in the attached figures. Figure 8 Appendix Figure 9 and attached Figure 10 As shown;

[0041] A comparative analysis was conducted on the cross-sectional microstructure of the weld joint tested by rectangular pulsed laser and the solidification parameter G / R(S(t)) variation curve of the weld joint simulated by rectangular pulsed laser. (Attached) Figure 4 This shows that when the solidification process reaches 65%, the crystal morphology changes from columnar crystals to equiaxed crystal regions. (See attached diagram.) Figure 8 It can be seen that when G / R is at "6K s / mm 2 ~9.5K s / mm2 "When the range changes, the microstructure of the solder joint metal is columnar crystals; when G / R decreases to 3.2 K s / mm..." 2 ~6K s / mm 2 "When the solidification range changes, the microstructure at the solidification front is equiaxed crystals, therefore the G / R transformation threshold for columnar and equiaxed crystals is approximately 6 K s / mm." 2 .

[0042] Comparative analysis was performed on the cross-sectional microstructure of the weld joints tested with pulsed lasers at P1 = 1.6 kW and t1 = 4 ms, and P2 = 0.80 kW and t1 = 11 ms, and the variation curves of the solidification parameter G / R(S(t)) of the simulated weld joints. (Attached) Figure 6 The results show that when the solidification process is 70%, the crystal morphology changes from cellular crystals to columnar crystals.

[0043] Corresponding Appendix Figure 10 It can be seen that when G / R is at "1350K s / mm 2 ~1650K s / mm 2 "When the G / R ratio changes within a certain range, the microstructure of the solder joint metal is cellular; when the G / R ratio decreases to 1350 K s / mm..." 2 "In the following variation range, the microstructure at the solidification front is columnar crystals, therefore the G / R transformation threshold for columnar and equiaxed crystals is approximately 1350 K s / mm." 2 .

[0044] Therefore, the relationship between the crystallization morphology of AZ31 magnesium alloy weld metal and the threshold G / R at the liquid-solid interface can be obtained: when the G / R value is less than 6 K s / mm 2 At this time, the grain morphology at the solidification front is equiaxed; when the G / R value is greater than 6 K s / mm 2 And less than 1350Ks / mm 22 At this stage, the grain morphology at the solidification front is columnar; when the G / R value is greater than 1350 K s / mm 2 At this time, the grain morphology at the solidification front is cellular.

[0045] As can be seen from the above embodiments, the method of the present invention can calculate the solidification parameter thresholds for the evolution of the microstructure of AZ31 magnesium alloy welded metal, and can also establish an evaluation system for the solidification parameter thresholds for the control of the microstructure of AZ31 magnesium alloy welded metal. This lays the foundation for shifting the control of the welded microstructure from qualitative evaluation to quantitative judgment, and has important guiding significance for improving the comprehensive performance of welded joints.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for calculating the solidification parameter thresholds for the evolution of the crystallization morphology of weld metal microstructure, characterized by: It includes the following steps: 1) On the base material, a single-beam pulsed laser is emitted to irradiate the base material using an Nd:YAG pulsed laser spot welding machine to perform laser self-melting spot welding; 2) By changing the pulse closing waveform of the single-beam pulsed laser emitted by the Nd:YAG pulsed laser spot welding machine, the solidification parameters of the liquid-solid interface of the laser self-fusion weld pool are adjusted, the degree of compositional supercooling is changed, and the morphological evolution of the microstructure crystallization of the laser self-fusion weld is realized, thus forming a laser self-fusion weld. 3) Using a scanning electron microscope, measure the lengths of the cellular, columnar, and equiaxed crystal regions on the cross-section of the laser self-fusion welding point described in step 2). Calculate the solidification process percentage S(L) corresponding to the crystallization morphology transformation point based on the ratio of each crystal region length to the total crystal region length. 4) Using ANSYS software, a two-dimensional axisymmetric model of pulsed laser self-melting spot welding was established to simulate and calculate the solidification parameters of the liquid-solid interface of the laser weld pool, namely the calculated values ​​of temperature gradient G and solidification rate R, G / R, and the functional relationship between solidification process percentage S(t). 5) On the curve of the solidification parameter calculation of the liquid-solid interface of the two-dimensional axisymmetric model of pulsed laser self-melting spot welding described in step 4), take the solidification parameter threshold corresponding to the solidification process percentage S(L) at the crystallization morphology transformation point of the experimental weld point. 6) The solidification process percentage S(t) mentioned in step 4) is: the ratio of the liquid-solid interface movement time t0 to the total solidification time t at a certain moment; 7) In step 2), the pulse closing waveform is a rectangular pulse waveform and a steeply drooping constant slow-cooling pulse waveform, wherein the steeply drooping constant slow-cooling pulse waveform is that after the rectangular main band, a "steeply drooping band" and a "slowly drooping band" are added in sequence. 8) In step 3), the solidification process percentage S(L) corresponding to the crystallization morphology transformation point is: the ratio of the liquid-solid interface movement distance L0 to the total distance L in the direction of the line connecting the center of the molten pool surface and the point.

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