A method for detecting the initiation threshold of solidification cracks in welding metal

Abstract

The present invention is applicable to the field of welding technology, and provides a method for detecting the solidification crack initiation threshold of welding metal, comprising: first, preparing the metal to be tested, selecting a pulsed laser spot welding machine, emitting a single beam of pulsed laser, forming a laser welding spot on the surface of the metal to be tested, and adjusting the waveform of the pulsed laser to adjust the solidification cooling process of the molten pool to inhibit the generation of solidification cracks; then, establishing an axisymmetric simulation model of pulsed laser spot welding, and inputting the critical pulsed laser parameters corresponding to no welding solidification cracks into the model; then, calculating the cooling rate of the mushy zone of the molten pool, and comparing and determining the welding solidification crack threshold of the metal to be tested. The present invention can determine the solidification crack initiation threshold of different welding metals. By implementing the solidification crack initiation threshold detection method of welding metal, a reference database covering the solidification crack initiation threshold of various welding metals can be established, and the selection criteria of welding process parameters can be expanded, thereby scientifically formulating technical parameters for inhibiting the generation of welding solidification cracks.

Description

Technical Field

[0001] The invention belongs to the technical field of welding, and in particular relates to a method for detecting a solidification crack initiation threshold of a welding metal. Background Art

[0002] As a common welding defect, welding solidification cracks mainly occur during the solidification and cooling process of the welding molten pool and are concentrated in the weld metal area. This type of crack will cause the weld to crack under the action of external loads, which will seriously affect the welding quality. In order to deal with this problem, welding technicians usually adjust the welding parameters to reduce the cooling rate, thereby inhibiting the generation of welding solidification cracks. However, the current parameter regulation of welding solidification cracks mainly relies on empirical trial methods, and there is a lack of an effective technical means to detect the initiation threshold of weld metal solidification cracks. This is not conducive to the establishment of a standardized reference database for the initiation threshold of weld metal solidification cracks, and it is difficult to guide welding production.

[0003] In conventional welding methods, such as arc welding, tungsten inert gas welding, metal arc welding and high-energy beam welding, the welding process is affected by many factors, such as strong constraints in the welding process, large differences in welding speed and turbulent characteristics of the molten pool fluid. These factors increase the difficulty of determining the threshold of solidification crack initiation of the weld metal, which is closely related to temperature. In contrast, laser spot welding realizes the melting and solidification process of the welding base metal under the action of a single beam of pulsed laser radiation. This method has the characteristics of small molten pool volume, small stress and strain, and high welding parameter resolution. In addition, the temperature field of laser spot welding can be adjusted flexibly and finely, especially by adjusting the power and time setting (pulse waveform) of the pulsed laser, the solidification and slow cooling process of the laser spot welding molten pool can be accurately controlled. Based on the above background, the present invention proposes a method for detecting the threshold of solidification crack initiation of weld metal. Summary of the invention

[0004] The purpose of the present invention is to provide a method for detecting the crack initiation threshold of solidification cracks in welding metal, aiming to solve the problems raised in the above-mentioned background technology.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for detecting a solidification crack initiation threshold of a weld metal comprises the following steps:

[0007] Step 1: Prepare the metal to be tested and pretreat the metal to be tested;

[0008] Step 2: Select a pulse laser spot welder and set welding parameters, including peak power, pulse time, defocusing amount and shielding gas flow rate;

[0009] Step 3, emit a rectangular pulse laser to form a laser welding spot on the metal surface to be tested, the pulse waveform is a rectangular band, cut along the center of the pulse laser welding spot or observe the welding solidification crack through X-ray detection;

[0010] Step 4. Prepare new metal to be tested according to step 1, emit rectangular-slow cooling pulse laser, the pulse waveform is rectangular band + steep drop band + rectangular slow drop band, cut along the center of the pulse laser weld or observe the welding solidification crack through X-ray detection;

[0011] Step 5, repeat the operation in step 4, emit the nth rectangular-slow cooling pulse laser, 1<n≤10, gradually reduce the cooling speed of the molten pool, inhibit the generation of welding solidification cracks, until the welding solidification cracks disappear;

[0012] Step 6, establishing an axisymmetric simulation model of pulse laser spot welding;

[0013] Step 7: Input the critical pulse laser parameters corresponding to no welding solidification cracks into the model, extract the temperature data of the mushy zone of the molten pool, calculate the temperature difference per unit time, obtain the cooling rate, and compare and determine the cooling rate threshold for the generation of welding solidification cracks in the experimental metal to be tested.

[0014] Furthermore, in step 1, the length and width of the metal to be tested are both 50*50 mm, and the thickness is 0.5~3 mm; the specific operation of pre-treating the metal to be tested is: using sandpaper and acetone to remove the oxide layer and oil stains on the surface of the metal to be tested before welding.

[0015] Furthermore, in step 2, the peak power is 0.5-5 kW, the pulse time is 0-10 ms, the defocusing amount is ±3 mm, and the protective gas flow rate is 0-10 L / min.

[0016] Furthermore, in step 3, the peak power of the rectangular band is p and the pulse time is t.

[0017] Furthermore, in step 4, the peak power of the rectangular band is p and the pulse time is t; the pulse time of the steep drop band is 0.5 ms; the peak power of the rectangular slow drop band is p', p'=0.1×n×p, n is the number of times the rectangular-slow cooling pulse laser is emitted, 1≤n≤10, and the pulse time is 2t.

[0018] Furthermore, in step 6, the axisymmetric simulation model of pulsed laser spot welding adopts thermophysical parameters that vary with temperature, selects a Gaussian surface heat source and a composite heat source that is constant in the depth direction, and the heat transfer process of the molten pool is described by a nonlinear heat transfer equation. The mesh type is Plane55 and the size is 0.03 mm×0.03 mm.

[0019] After the axisymmetric simulation model of pulsed laser spot welding was established, the temperature field changes of the molten pool of the laser welding spot were simulated, and the simulation results were compared with the thermal cycle curves of the experimental laser welding spot, with an error of less than 5%.

[0020] Furthermore, in step 7, the cooling rate threshold is replaced by the tensile stress threshold or the strain threshold, and is calculated by a mathematical expression with the cooling rate as a variable:

[0021]

[0022] Where σ is the tensile stress threshold, ε is the dependent variable threshold, f l is the liquid volume fraction, f s is the solid phase volume fraction, T m is the melting point temperature, T is the temperature of the pasty zone, E is Young's modulus, is the cooling rate threshold of welding solidification crack.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention adopts pulsed laser spot welding technology, and by adjusting the pulse waveform of the pulsed laser beam, adopts a combination of laser spot welding experiments and laser spot welding simulations to detect and determine the solidification crack initiation thresholds of various types of welded metals. This innovative method fills the current technical gap in detecting the solidification crack initiation thresholds of welded metals. Through this method, a reference database of solidification crack initiation thresholds of various types of metals can be systematically established to enrich the basis for selecting welding process parameters. On this basis, technical parameters for inhibiting the generation of solidification cracks during welding can be more scientifically formulated to promote the further development of welding technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Flow chart of the method of the present invention.

[0026] Figure 2 Schematic diagram of pulse laser spot welding.

[0027] Figure 3 It is a rectangular pulse waveform.

[0028] Figure 4 Schematic diagram of the cross section of the rectangular pulse laser welding point in Example 1.

[0029] Figure 5 It is a rectangular-slow-cooling pulse laser waveform.

[0030] Figure 6Schematic diagram of the cross section of the rectangular-slow-cooling pulse laser welding point (n=1) in Example 1.

[0031] Figure 7 Schematic diagram of the cross section of the rectangular-slow-cooling pulse laser welding point (n=2) in Example 1.

[0032] Figure 8 Schematic diagram of the cross section of the rectangular-slow-cooling pulse laser welding point (n=3) in Example 1.

[0033] Fig. 9 Schematic diagram of the cross section of the simulated solder joint in Example 1.

[0034] Fig.10 This is the mushy area of ​​the simulated solder joint molten pool in Example 1.

[0035] In the figure: 1. Pulse laser; 2. Pulse laser welding point; 3. Metal to be tested. DETAILED DESCRIPTION

[0036] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0037] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0038] An embodiment of the present invention provides a method for detecting the threshold value of solidification crack initiation of welding metal, and the method includes the following steps: first, prepare the metal 3 to be tested, and select a pulsed laser spot welding machine. By emitting a single beam of Nd:YAG pulsed laser 1, a laser welding spot is formed on the surface of the metal 3 to be tested, and by adjusting the waveform of the pulsed laser 1 (using a series of rectangular-slow cooling pulsed lasers 1), the solidification cooling process of the molten pool is adjusted to suppress the generation of welding solidification cracks. Next, an axisymmetric simulation model of pulsed laser spot welding is established, and the critical pulsed laser parameters corresponding to no welding solidification cracks are input into the model, and then the cooling rate of the mushy zone of the molten pool is calculated, and the welding solidification crack threshold of the metal 3 to be tested is determined by comparison. Figure 2 Figure 2 shows a schematic diagram of pulse laser spot welding.

[0039] In the embodiment of the present invention, the present invention can determine the solidification crack initiation thresholds of different weld metals, solving the problem of the current lack of a method for detecting the solidification crack initiation threshold of weld metals. By implementing the method proposed in the present invention, a reference database covering solidification crack initiation thresholds of various weld metals can be established, expanding the selection criteria of welding process parameters, and helping to scientifically formulate technical parameters for inhibiting the generation of weld solidification cracks.

[0040] like Figure 1 As shown, the specific steps of the method are as follows:

[0041] Step 1, prepare the metal 3 to be tested, the length and width are both 50*50mm, the thickness is 0.5~3mm, and pre-treat the metal to be tested: use sandpaper and acetone to remove the surface oxide layer and oil stains of the metal 3 to be tested before welding;

[0042] Step 2: Select a pulse laser spot welder and set the welding parameters, including peak power (0.5~5kW), pulse time (0~10ms), defocus (±3mm) and shielding gas flow (0~10L / min);

[0043] Step 3, emitting a rectangular pulse laser 1 to form a laser welding spot on the surface of the metal 3 to be tested, wherein the pulse waveform is a rectangular band, the peak power of the rectangular band is p, and the pulse time is t; cutting along the center of the pulse laser welding spot 2 or observing the welding solidification crack through X-ray detection;

[0044] Step 4, prepare a new metal 3 to be tested according to step 1, emit a rectangular-slow cooling pulse laser 1, the pulse waveform is a rectangular band + a steep drop band + a rectangular slow drop band; the pulse time of the steep drop band is 0.5ms; the peak power of the rectangular slow drop band is p', p'=0.1×n×p, n is the number of times the rectangular-slow cooling pulse laser 1 is emitted (1≤n≤10), the pulse time is 2t, cut along the center of the pulse laser weld 2 or observe the welding solidification crack through X-ray detection;

[0045] Step 5, repeat the operation of step 4, emit the nth rectangular-slow cooling pulse laser 1, gradually reduce the cooling rate of the molten pool, inhibit the generation of welding solidification cracks, until the welding solidification cracks disappear;

[0046] Step 6: Establish an axisymmetric simulation model of pulsed laser spot welding, simulate the temperature field change of the laser welding spot molten pool, and compare the simulation results with the thermal cycle curve of the experimental laser welding spot. The error is less than 5%;

[0047] The axisymmetric simulation model of pulsed laser spot welding adopts thermophysical parameters that change with temperature. A Gaussian surface heat source and a composite heat source that is constant in the depth direction are selected. The heat transfer process of the molten pool is described by a nonlinear heat transfer equation. The mesh type is Plane55 and the size is 0.03mm×0.03mm.

[0048] Step 7: Input the critical pulse laser parameters corresponding to no welding solidification crack into the model, extract the temperature data of the molten pool mushy zone, calculate the temperature difference per unit time, obtain the cooling rate, and compare and determine the cooling rate threshold value ( ).

[0049] Cooling rate threshold of welding solidification crack ( ) is only one of the threshold values, which also includes tensile stress threshold (σ), strain threshold ( ε ), etc., can be calculated by using a mathematical expression with cooling rate as a variable:

[0050]

[0051] Where σ is the tensile stress threshold, ε is the dependent variable threshold, f l is the liquid volume fraction, f s is the solid phase volume fraction, T m is the melting point temperature, T is the temperature of the pasty zone, E is Young's modulus.

[0052] Embodiment 1: This embodiment provides a method for detecting the threshold value of solidification crack initiation of welding metal, and the specific steps are as follows:

[0053] AM50 magnesium alloy was selected as the metal 3 to be tested, with a length and width of 50*50mm and a thickness of 3mm. Sandpaper and acetone were used to remove the oxide layer and oil stains on the surface of the metal 3 to be tested before welding.

[0054] A pulse laser spot welding machine was selected and the welding parameters were set as follows: the defocus amount was +2 mm and the shielding gas flow rate was 10 L / min (99.999% Ar).

[0055] First, set a rectangular pulse (rectangular pulse waveform as Figure 3 As shown): peak power p = 2kW, pulse time t = 4ms, the cross section of the laser weld obtained is as follows Figure 4 As shown, the figure shows the existence of welding solidification cracks;

[0056] Next, set a rectangular-slow cooling pulse (rectangular-slow cooling pulse waveform as Figure 5 As shown): n=1, the peak power of the rectangular band p=2kW, the pulse time t=4ms, the pulse time of the steep drop band is 0.5ms, the peak power of the rectangular slow drop band p'=0.1×n×p=0.2kW, the pulse time 2t=8ms, the cross section of the laser weld is as follows Figure 6 As shown, the figure shows the existence of welding solidification cracks;

[0057] Then, set the rectangular-slow cooling pulse: n=2, the peak power of the rectangular band p=2kW, the pulse time t=4ms, the pulse time of the steep drop band is 0.5ms, the peak power of the rectangular slow drop band is p'=0.1×n×p=0.4kW, the pulse time 2t=8ms, and the cross section of the laser weld is as follows: Figure 7 As shown, the figure shows that there are a few welding solidification cracks;

[0058] After that, set the rectangular-slow cooling pulse: n=3, the peak power of the rectangular band p=2kW, the pulse time t=4ms, the pulse time of the steep drop band is 0.5ms, the peak power of the rectangular slow drop band p'=0.1×n×p=0.6kW, the pulse time 2t=8ms, and the cross section of the laser weld is as follows Figure 8 As shown, the figure shows that the welding solidification crack disappears.

[0059] The axisymmetric simulation model of the Nd:YAG pulse laser spot welding process of AM50 magnesium alloy was established using Ansys software. Fig. 9 To simulate the cross-sectional diagram of the weld, the rectangular-slow cooling pulse: n=3, the peak power of the rectangular band p=2kW, the pulse time t=4ms, the pulse time of the steep drop band is 0.5ms, the peak power of the rectangular slow drop band is p'=0.1×n×p=0.6kW, the pulse time 2t=8ms and other pulse laser parameters are input into the model for simulation to capture the mushy zone at the initial moment of solidification when the cooling rate of the molten pool is the largest. Fig.10 To simulate the mushy zone of the solder joint molten pool, the temperature variation curve of each point at the fusion line in the mushy zone with time was extracted, and its derivative was calculated. The maximum cooling rate in the mushy zone was 2.2×10 5 ℃ / s. Fusion line in the mushy zone f l =0.01, f s =0.99, T m =650℃, T =400℃; E is the material mechanical parameter, which can be obtained by looking up the table E =45GPa, we can calculate: σ =6.93GPa, ε =0.154.

[0060] It can be concluded that the cooling rate threshold for the initiation of welding solidification cracks in AM50 magnesium alloy is 2.2×10 5 ℃ / s, the tensile stress threshold is 6.93GPa, and the strain threshold is 0.154.

[0061] The welding metal solidification crack initiation threshold detection method proposed in the present invention can find the critical pulse parameters that can suppress the generation of welding solidification cracks based on the series of rectangular-slow cooling pulse waveform settings of laser spot welding, and then determine the welding solidification crack thresholds of various metals. It should be noted that for technicians in the field of optics, using existing optical and control device technologies to realize the above-mentioned series of rectangular-slow cooling pulse waveform settings does not constitute an implementation difficulty.

[0062] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A method for detecting the threshold value of solidification crack initiation of welding metal, characterized in that: The following steps are involved: Step 1: Prepare the metal to be tested and pretreat the metal to be tested; Step 2: Select a pulse laser spot welder and set welding parameters, including peak power, pulse time, defocusing amount and shielding gas flow rate; Step 3, emit a rectangular pulse laser to form a laser welding spot on the metal surface to be tested, the pulse waveform is a rectangular band, cut along the center of the pulse laser welding spot or observe the welding solidification crack through X-ray detection; Step 4. Prepare new metal to be tested according to step 1, emit rectangular-slow cooling pulse laser, the pulse waveform is rectangular band + steep drop band + rectangular slow drop band, cut along the center of the pulse laser weld or observe the welding solidification crack through X-ray detection; Step 5, repeat the operation in step 4, emit the nth rectangular-slow cooling pulse laser, 1<n≤10, gradually reduce the cooling speed of the molten pool, inhibit the generation of welding solidification cracks, until the welding solidification cracks disappear; Step 6, establishing an axisymmetric simulation model of pulse laser spot welding; Step 7, input the critical pulse laser parameters corresponding to no welding solidification cracks into the model, extract the temperature data of the mushy zone of the molten pool, calculate the temperature difference per unit time, obtain the cooling rate, and compare and determine the cooling rate threshold value for the generation of welding solidification cracks in the experimental metal to be tested; In the step 1, the length and width of the metal to be tested are both 50*50 mm, and the thickness is 0.5-3 mm; the specific operation of pre-treating the metal to be tested is: using sandpaper and acetone to remove the oxide layer and oil stains on the surface of the metal to be tested before welding; In step 2, the peak power is 0.5-5 kW, the pulse time is 0-10 ms, the defocusing amount is ±3 mm, and the protective gas flow rate is 0-10 L / min; In step 3, the peak power of the rectangular band is p and the pulse time is t; In step 4, the peak power of the rectangular band is p and the pulse time is t; the pulse time of the steep drop band is 0.5 ms; the peak power of the rectangular slow drop band is p', p'=0.1×n×p, n is the number of times the rectangular-slow cooling pulse laser is emitted, 1≤n≤10, and the pulse time is 2t; In step 6, the axisymmetric simulation model of pulsed laser spot welding adopts thermophysical parameters that vary with temperature, selects a Gaussian surface heat source and a composite heat source that is constant in the depth direction, and the heat transfer process of the molten pool is described by a nonlinear heat transfer equation. The mesh type is Plane55 and the size is 0.03 mm×0.03 mm. After the axisymmetric simulation model of pulsed laser spot welding was established, the temperature field change of the molten pool of the laser welding spot was simulated, and the simulation results were compared with the thermal cycle curve of the experimental laser welding spot, with an error of less than 5%; In step 7, the cooling rate threshold is replaced by the tensile stress threshold or the strain threshold, and is calculated by a mathematical expression with the cooling rate as a variable: Where σ is the tensile stress threshold, ε is the dependent variable threshold, f l is the liquid volume fraction, f s is the solid phase volume fraction, T m is the melting point temperature, T is the temperature of the pasty zone, E is Young's modulus, is the cooling rate threshold of welding solidification crack.

Images