A welding apparatus and process for a vehicle frame

Abstract

This application relates to a welding equipment and process for vehicle frames. The process, performed synchronously with the welding process, includes the following steps: acquiring real-time temperature field data of the weld and heat-affected zone after welding; when a target micro-region on the weld is detected to have cooled into a preset intermediate-temperature plasticity window, applying intermittent pulsed cryogenic shocks to the target micro-region, causing controllable compressive plastic deformation under transient thermal stress; stopping the shocks after treatment, allowing the weld to naturally recover to room temperature, forming a residual compressive stress layer on the surface. The equipment includes a welding execution unit, a temperature detection unit, a pulsed cryogenic shock unit, and a system controller, capable of executing the above process. This application, by actively applying pulsed cryogenic shocks during the post-weld intermediate-temperature plasticity stage, achieves the synchronous integration of welding and stress field control, fundamentally reducing residual tensile stress from the weld and constructing a residual compressive stress layer on the weld surface, thus offering advantages such as good stress control and high production efficiency.

Description

Technical Field

[0001] This application relates to the field of welding processing, and in particular to a welding equipment and process for a vehicle frame. Background Technology

[0002] As the core load-bearing structure of a vehicle, the welding quality of the chassis directly determines the safety and service life of the entire vehicle. Currently, chassis welding production generally uses CO2 gas shielded welding or robotic arc welding for assembly, followed by overall thermal aging or vibration aging to eliminate residual stress. Thermal aging requires heating the entire chassis to 550–650°C and holding it at that temperature for several hours, consuming enormous energy and easily causing secondary deformation. While vibration aging can be performed at room temperature, its stress relief rate is usually less than 30%, which is extremely limited for high-strength steel chassis where the residual stress peak can reach 90% of the base material's yield strength. Furthermore, some high-value-added chassis employ post-weld cryogenic treatment to improve joint microstructure, but this process is performed offline after the weldment has cooled to room temperature, resulting in fragmented processes, high transit time, and its core purpose is microstructure control rather than active stress field reconstruction.

[0003] The fundamental flaw in the existing processes described above lies in the fact that they all treat stress relief as an independent remedial procedure after welding, rather than actively intervening in the formation of stress during the welding process. The essence of residual tensile stress in welding lies in the elastic tensile strain generated by the constraint of the surrounding cold metal during the contraction process of the weld metal as it cools from the molten state to room temperature. Existing methods can only passively and globally release stress through thermal relaxation or mechanical vibration after the stress has fully formed, and cannot prevent or offset the accumulation of tensile stress at its source. For highly constrained welds such as those at the connection between the longitudinal and transverse beams of the frame and the ends of the stiffening plates, the peak value of residual tensile stress after existing processes often still exceeds 200 MPa, becoming a core inducing factor for fatigue crack initiation under service vibration loads. Furthermore, neither post-weld heat treatment nor vibration aging can actively build a residual compressive stress layer with fatigue resistance on the weld surface.

[0004] To address the aforementioned problems, this invention proposes a welding device and process for vehicle frames. Summary of the Invention

[0005] To address the aforementioned problems, this application provides a welding device and process for a vehicle frame.

[0006] In a first aspect, this application provides a welding equipment and process for a vehicle frame, employing the following technical solution:

[0007] A welding process for a vehicle frame includes the following steps performed simultaneously with the welding process:

[0008] S1. Obtain real-time temperature field data of the weld and heat-affected zone after welding;

[0009] S2. When the target micro-area on the weld is detected to have cooled into the preset medium-temperature plastic window, an intermittent pulsed cryogenic impact is applied to the target micro-area; the pulsed cryogenic impact acts on the solid medium-temperature stage of post-weld cooling, causing the target micro-area to produce controllable compressive plastic deformation under transient thermal stress.

[0010] S3. After the target micro-area treatment is completed, stop the pulse deep cryogenic shock and allow the weld to naturally recover to room temperature in the air, forming a residual compressive stress layer on the surface of the weld.

[0011] Preferably, the temperature range of the intermediate-temperature plasticity window is determined based on the high-temperature creep characteristics and stress control threshold of the base material; for carbon steel and low-alloy high-strength steel, the intermediate-temperature plasticity window is 200℃~300℃. Within this temperature range, the creep resistance of the material is significantly reduced, and compressive plastic deformation can be generated under transient thermal stress.

[0012] Preferably, the pulse frequency of the pulsed cryogenic impact is 1 to 5 Hz, and the injection pressure is 0.5 to 1.0 MPa; by adjusting the pulse frequency and injection pressure, the amount of compressive plastic deformation in the target micro-region is controlled.

[0013] Preferably, the method further includes the following steps before step S2:

[0014] When the target micro-region is cooled to a high-temperature phase transformation window higher than the intermediate-temperature plasticity window, pulsed compressed air impact is applied to the heat-affected zones on both sides of the weld to interrupt the austenite grain growth process and multiply phase deformation nuclei, thereby refining the grain structure of the heat-affected zone.

[0015] Preferably, for carbon steel and low-alloy high-strength steel, the temperature range of the high-temperature phase transformation window is 600℃~750℃; the pulse frequency of the pulsed compressed air impact is 2~4Hz, the injection pressure is 0.3~0.6MPa, and it is applied symmetrically to both sides of the weld.

[0016] Preferably, the pulse air cooling unit performing the pulse compressed air impact and the pulse cryogenic unit performing the pulse cryogenic impact are arranged at intervals along the welding advance direction, and the distance L between them satisfies:

[0017] L = v × (t1 + t2)

[0018] Where v is the welding advance speed, t1 is the natural cooling time of the base material from the high-temperature phase transformation window to the medium-temperature plastic window, and t2 is the timing stability margin of 0.5 to 2 seconds.

[0019] Preferably, in step S1, a purge air curtain is generated along the temperature detection path in the same direction as the welding advance to remove the interference of welding fumes on temperature detection.

[0020] Preferably, for T-joint welds, two nozzles symmetrically arranged on both sides of the weld are used to simultaneously apply the pulsed deep cryogenic shock.

[0021] Preferably, it also includes a safety redundancy step:

[0022] When the temperature of the target micro-region exceeds the preset control action window and continues to exceed the preset delay time threshold, the pulse cryogenic shock is stopped.

[0023] Wherein, the lower limit of the control action window is lower than the lower limit of the intermediate temperature plasticity window, and the upper limit of the control action window is higher than the upper limit of the intermediate temperature plasticity window.

[0024] Secondly, this application provides a welding device for a vehicle frame, which adopts the following technical solution:

[0025] A vehicle frame welding apparatus, comprising:

[0026] A welding execution unit is used to generate a welding arc and move it along the welding direction;

[0027] A temperature field sensing unit is used to acquire real-time temperature field data of the weld zone after welding.

[0028] A pulsed air-cooled impact unit is used to apply pulsed compressed air impact to the heat-affected zones on both sides of the weld.

[0029] A pulse cryogenic shock unit is used to apply intermittent pulse cryogenic shock to the target micro-area;

[0030] A system controller, connected to the temperature field sensing unit and the pulse cryogenic shock unit respectively, is configured to: receive the real-time temperature field data and determine whether the target micro-region has entered the high-temperature phase transition window or the medium-temperature plasticity window;

[0031] When entering the high-temperature phase change window, the pulsed air cooling impact unit is controlled to apply pulsed compressed air impact;

[0032] When entering the intermediate temperature plasticity window, the pulse cryogenic shock unit is controlled to apply intermittent pulse cryogenic shocks to each micro-region entering the intermediate temperature plasticity window in sequence.

[0033] In summary, this application includes at least one of the following beneficial technical effects:

[0034] 1. This application advances the timing of stress regulation from post-weld offline treatment to the welding process, actively intervening in the mid-temperature plastic stage before the residual tensile stress has fully solidified. Controllable compressive plastic deformation is induced on the weld surface through pulsed deep cryogenic impact, so that a residual compressive stress layer is formed on the weld surface, thereby reducing the residual tensile stress from the source, rather than passively eliminating it after the stress has fully formed. The stress regulation effect is significantly better than the existing thermal aging and vibration aging processes.

[0035] 2. This application achieves simultaneous and integrated completion of welding and stress control, eliminating the need for independent processes such as post-weld straightening, offline thermal aging, or vibration aging in traditional processes, significantly shortening the frame production cycle, eliminating inter-process transfer waiting time, and improving production efficiency.

[0036] 3. This application uses intermittent pulsed deep cryogenic impact to replace continuous cooling. Through the alternating action of "high-pressure injection-intermittent" action, multiple "rapid cooling-creep" cycles are generated on the surface, so that the compressive plastic deformation accumulates step by step. At the same time, the intermittent period avoids the risk of excessive surface quenching and quenching cracks that may be caused by continuous cooling, resulting in high process reliability.

[0037] 4. Based on real-time temperature field sensing, this application independently determines whether each micro-region has entered the medium-temperature plasticity window and applies impacts sequentially according to the actual temperature window arrival order, rather than according to the welding sequence or preset spatial position. This can adapt to the differences in cooling sequence caused by changes in wall thickness, heat dissipation conditions and welding speed at different locations of the weld, ensuring the global uniformity of stress control.

[0038] 5. This application interrupts austenite grain growth by applying pulsed compressed air impact during the high-temperature phase transformation window, and then reconstructs the stress field by applying pulsed cryogenic impact during the medium-temperature plasticity window. The two-stage temperature windows utilize residual heat after welding, achieving dual regulation of microstructure refinement and active stress field reconstruction in the heat-affected zone. Furthermore, the refined grain structure has more grain boundaries as coordination interfaces for plastic rheology, further improving stress reconstruction efficiency and uniformity. Attached Figure Description

[0039] Figure 1 This is a process flow diagram of a welding process for a vehicle frame according to Embodiment 1 of this application;

[0040] Figure 2 This is a welding equipment system for a vehicle frame as described in Embodiment 1 of this application.

[0041] Explanation of reference numerals in the attached diagram: 1. Welding execution unit; 2. Temperature field sensing unit; 3. Pulse gas cooling shock unit; 4. Pulse deep cryogenic shock unit; 5. System controller; 6. Safety redundancy module. Detailed Implementation

[0042] The following is in conjunction with the appendix Figures 1-2 This application will be described in further detail.

[0043] Example 1

[0044] This application discloses a welding process for a vehicle frame. (Refer to...) Figure 1 A welding process for a vehicle frame, which includes the following steps performed simultaneously with the welding process:

[0045] S1. Obtain real-time temperature field data of the weld and heat-affected zone after welding;

[0046] S2. When the target micro-area on the weld is detected to have cooled into the preset medium-temperature plastic window, an intermittent pulsed cryogenic impact is applied to the target micro-area; the pulsed cryogenic impact acts on the solid medium-temperature stage of post-weld cooling, causing the target micro-area to produce controllable compressive plastic deformation under transient thermal stress.

[0047] S3. After the target micro-area treatment is completed, stop the pulse deep cryogenic shock and allow the weld to naturally recover to room temperature in the air, forming a residual compressive stress layer on the surface of the weld.

[0048] The following will provide a detailed explanation of each step with specific examples.

[0049] This embodiment provides a welding process for a vehicle frame. This process is designed for welding carbon steel and low-alloy high-strength steel vehicle frames, and is illustrated using the weld between the longitudinal beam and the crossbeam of a Q345 steel vehicle frame as an example.

[0050] The core of this invention lies in utilizing the physical property that the creep resistance of the base material significantly decreases within a specific mid-temperature range. For different grades of carbon steel and low-alloy high-strength steel, the specific temperature range of the mid-temperature plasticity window can be determined based on high-temperature creep tests or stress relaxation tests. Generally, the mid-temperature plasticity window of carbon steel and low-alloy high-strength steel lies in the region below its recrystallization temperature and above its blue brittleness temperature, typically ranging from 150°C to 400°C. Those skilled in the art can determine the characteristic temperature range where the creep resistance of a specific material significantly decreases by conducting creep rate tests or thermal simulation tests at different temperatures, thereby adaptively adjusting the upper and lower limits of the mid-temperature plasticity window. The following detailed description uses Q345 steel, a commonly used material for vehicle frames, as a typical example, but the scope of protection of this invention is not limited to this specific material. Based on the above principles and the teachings of the following examples, those skilled in the art can apply the process of this invention to the welding of other carbon steel and low-alloy high-strength steel vehicle frames.

[0051] In this embodiment, the welding method is six-axis robot CO2 gas shielded welding, the welding speed is 8mm / s, the base material wall thickness is 5mm, the welding wire grade is ER50-6, the shielding gas is 80%Ar and 20%CO2, the welding current is 240A, and the arc voltage is 26V.

[0052] Step S1, obtaining real-time temperature field data of the weld and heat-affected zone after welding, specifically involves:

[0053] After welding begins, in this embodiment, the welding torch advances along the weld direction at a speed of 8 mm / s. A temperature detection unit is fixedly installed on the accompanying support behind the welding torch. This temperature detection unit moves synchronously with the welding torch and continuously acquires real-time temperature field data of the weld and heat-affected zone after welding.

[0054] The temperature detection unit employs a combined infrared thermal imager and infrared spot thermometer. The infrared thermal imager has a temperature measurement range of -20℃ to 1200℃, a measurement accuracy of ±2℃ or ±2% of the reading, a sampling frequency of 30Hz, and a field of view covering the weld and an area of ​​15mm on each side (approximately 3 times the plate thickness). The infrared spot thermometer, used as an auxiliary calibration, is positioned above the weld centerline to perform real-time calibration of the thermal imaging data, eliminating measurement deviations caused by changes in the weld surface emissivity. To eliminate interference from welding fumes on infrared temperature measurement, a compressed air purge curtain is installed in front of the temperature detection unit. The purge direction is consistent with the welding advance direction, the purge pressure is 0.3MPa, and the gas flow rate is 30L / min. This air curtain creates a clean optical channel along the temperature measurement path, ensuring the accuracy of temperature detection.

[0055] Through this temperature detection unit, the system acquires the temperature distribution curve of the solidified weld area along the welding direction in real time. Taking a certain moment after welding begins as an example, with the welding torch at position x=0, the temperature distribution along the weld (in the solidified direction) is as follows: the weld center temperature at x=-50mm is approximately 650℃; the weld center temperature at x=-150mm is approximately 420℃; the weld center temperature at x=-350mm is approximately 295℃; the weld center temperature at x=-500mm is approximately 180℃; and the weld center temperature at x=-800mm is approximately 120℃.

[0056] Step S2: When the target micro-region on the weld is detected to have cooled into the preset intermediate-temperature plasticity window, an intermittent pulse-type cryogenic impact is applied to the target micro-region, specifically as follows:

[0057] For ease of description, this application divides the weld into several target micro-regions for independent management. The target micro-region refers to a segmented weld section cut along the weld direction, serving as the smallest operational unit for temperature monitoring and pulsed cryogenic shock. The length of the target micro-region is determined comprehensively based on the weld length, welding speed, temperature gradient, and the spray coverage width of the pulsed cryogenic shock unit, typically ranging from 10 to 30 mm. In this embodiment, the target micro-region is defined as a segment with a length of 20 mm along the weld direction, its width covering the weld and the heat-affected zones on both sides (approximately 20 mm wide in total). The system divides the solidified weld into a continuous sequence of micro-regions in 20 mm increments and independently tracks the temperature history of each micro-region.

[0058] Based on the completion of micro-region division and temperature tracking, the system further determines whether each micro-region enters the preset intermediate-temperature plasticity window to determine the timing of applying pulsed cryogenic shock. The temperature range of the intermediate-temperature plasticity window is determined based on the high-temperature creep characteristics of the base material and the stress control threshold. For example, in the case of the frame material Q345 steel in this embodiment, although the temperature range of 200℃ to 300℃ partially overlaps with the blue brittleness temperature range of steel, the inventors found through systematic thermal simulation tests and stress relaxation tests that the creep rate of the material in this temperature range is 1-2 orders of magnitude higher than that at room temperature, and the creep resistance is significantly reduced. This discovery constitutes the key physical basis for the selection of this temperature window in this invention: under the action of transient thermal stress generated by pulsed cryogenic shock, when the transient thermal stress superimposed with the welding residual stress is sufficient to drive the material to undergo creep deformation, the material can generate micro-region compressive plastic deformation in a very short time (within the high-pressure spray period of a single pulse cycle); this compressive plastic deformation is fixed by the surrounding elastic constraints and becomes permanent plastic strain after returning to room temperature, thereby releasing the welding residual tensile stress inside the micro-region.

[0059] When the system detects that the weld center temperature in a certain micro-area (such as micro-area A, located approximately 350 mm behind the welding torch) drops below 300°C and the overall temperature of the heat-affected zone enters the intermediate-temperature plasticity window of 200-300°C, it determines that the micro-area has entered the intermediate-temperature plasticity window. At this time, the system immediately triggers the pulse cryogenic shock unit to apply intermittent pulse cryogenic shock to the micro-area.

[0060] In this embodiment, considering the uniform weld wall thickness and consistent heat dissipation conditions of the straight sections of the longitudinal beams of the vehicle frame, and the fact that the cooling sequence basically corresponds to the welding sequence, the pulse cryogenic impact unit adopts a follow-up installation method, that is, it is fixedly installed on the same follow-up bracket behind the temperature detection unit, with its spray end located approximately 400mm behind the welding torch. This follow-up installation method has the simplest structure, requiring no independent motion actuator, and is suitable for working conditions where the weld wall thickness and heat dissipation conditions are relatively uniform. This unit includes a liquid nitrogen micro-reservoir, a cryogenic electromagnetic pulse valve, and a Laval nozzle. The liquid nitrogen micro-reservoir has a capacity of 5L and a working pressure of 1.5MPa, and is connected to the cryogenic electromagnetic pulse valve via an insulated pipeline. The cryogenic electromagnetic pulse valve has a response time of less than 10ms, enabling rapid opening and closing under system control. The Laval nozzle atomizes liquid nitrogen into fine droplets with a particle size of 50-100μm. The spray coverage area is an elliptical area with a width of about 10mm along the weld direction, ensuring that at a welding speed of 5-15mm / s, the time (i.e., residence time) for each 20mm long micro-area to pass through the spray coverage area is not less than 0.8s.

[0061] Pulsed cryogenic shock is applied in an intermittent pulse manner. Each pulse cycle is set to 0.5s, with a high-pressure injection period of 0.2s and an intermittent period of 0.3s. During the high-pressure injection period, the cryogenic electromagnetic pulse valve opens, and liquid nitrogen atomized droplets are injected onto the surface of the target micro-area at a pressure of 0.8MPa. Upon contact with the workpiece surface, the atomized droplets fully vaporize, instantly carrying away a large amount of heat, causing the surface layer of the micro-area (approximately 0.1-0.3mm deep) to rapidly cool within 0.1s, generating a transient thermal tensile stress as high as 150-250MPa (due to internal constraint caused by surface shrinkage). This transient thermal tensile stress, superimposed with the residual tensile stress from welding, exceeds the creep resistance threshold of Q345 steel at 250℃, inducing compressive plastic flow in the surface material, with a plastic strain of approximately 0.1%-0.3%.

[0062] After the high-pressure injection period ends, an intermittent period begins, during which the cryogenic electromagnetic pulse valve closes, stopping the supply of cooling medium. During this intermittent period, the surface temperature of the micro-area partially recovers (by approximately 30-50°C) due to internal heat recirculation. At this time, the material remains within the medium-temperature plasticity window, and the creep effect continues, further promoting the accumulation and homogenization of plastic deformation. Simultaneously, the intermittent period avoids the risk of excessive surface quenching and quenching cracks that could occur with continuous cooling.

[0063] During the approximately 2-second dwell time of the micro-area A covered by the pulsed cryogenic impact unit, it undergoes 3-4 complete pulse cycles. Each pulse cycle generates a "rapid cooling-creep" cycle on the surface, with compressive plastic deformation accumulating successively, ultimately resulting in a cumulative plastic strain of approximately 0.3%-0.5% after the micro-area A treatment is completed.

[0064] It is important to note that the timing logic of the pulsed cryogenic impact application in this invention does not necessarily correspond to the welding timing. The system independently determines whether each micro-region has entered the intermediate-temperature plastic window based on the actual temperature field data of each micro-region. The micro-region that enters the window first is the one to be impacted first. In the actual welding process, due to variations in wall thickness, heat dissipation conditions, and welding speed at different locations of the weld, it is possible that micro-regions welded later may enter the window later due to heat accumulation or slower heat dissipation, or vice versa. For example, at the connection of the crossbeam reinforcement plate of the vehicle frame, the local wall thickness suddenly increases from 5mm to 10mm. Although the thick plate area is welded later, it dissipates heat more slowly due to its larger heat capacity, thus entering the intermediate-temperature window later than the thinner plate area welded earlier. In some thin-walled areas or areas with strong heat dissipation, micro-regions welded later may also enter the window before the micro-regions welded earlier due to their extremely fast cooling rate.

[0065] For weld areas with uniform wall thickness and consistent heat dissipation conditions, the aforementioned accompanying installation method can meet the process requirements. In this case, the micro-area entering the window naturally appears within the nozzle coverage area, and the accompanying nozzles can sequentially complete the impact operation on each micro-area. For frame joint areas with drastic changes in wall thickness and significant differences in cooling conditions such as the presence of reinforcing plates or variable cross-sections, the equipment of this invention can adopt an independent mobile installation method: the pulse cryogenic impact unit is not fixed to the accompanying bracket, but is installed on an independent servo slide, which can move back and forth along the weld direction; or an independent six-axis robot drive can be used; the system controller identifies the position of the micro-area entering the medium-temperature plastic window based on real-time temperature field data, and controls the servo slide to drive the pulse cryogenic impact unit to actively move above the micro-area to apply the impact. After processing, it returns to the standby position or moves directly to the next micro-area entering the window.

[0066] Regardless of whether a mobile or independent installation method is used, the core principle remains the same: the system uses real-time temperature sensing to independently trigger each micro-zone, applying impacts sequentially according to the actual temperature window arrival order, rather than according to the welding sequence or a preset spatial order. This feature ensures global uniformity of stress control, regardless of whether the cooling sequence of different areas of the weld is consistent with the welding time sequence.

[0067] The above-described implementation of pulsed cryogenic impact is also applicable to T-joint welds commonly found in vehicle frames. In the case of a T-joint, due to differences in heat dissipation conditions and constraint stiffness of the base material on both sides of the weld, applying pulsed cryogenic impact from only one side can easily lead to uneven compressive plastic deformation in the heat-affected zones on both sides, resulting in angular deformation or asymmetrical stress distribution. Therefore, in this embodiment, the pulsed cryogenic impact unit is configured as two Laval nozzles symmetrically arranged on both sides of the weld in the T-joint condition. The two nozzles are synchronously triggered by the system controller, applying pulsed cryogenic impact to the heat-affected zones on both sides of the weld simultaneously within the same pulse cycle, ensuring consistent quenching rates and compressive plastic deformation on both sides. Testing showed that after using symmetrical impact with two nozzles, the difference in residual compressive stress on both sides of the T-joint weld was less than 15 MPa, and the angular deformation was reduced by more than 70% compared to single-side impact. This embodiment has thus fully described the entire process of S1-S2 for treating the weld micro-area, from temperature detection and window determination to pulsed cryogenic impact treatment. The following describes step S3 after the treatment is completed.

[0068] In addition, to effectively avoid frequent start-stop of the low-temperature electromagnetic pulse valve due to temperature fluctuations, and to ensure device lifespan and process continuity, safety redundancy procedures are also implemented.

[0069] When the temperature of the target micro-region exceeds the preset control action window and continues to exceed the preset delay time threshold, the pulse cryogenic shock is stopped.

[0070] The lower limit of the control action window is lower than the lower limit of the intermediate-temperature plasticity window, and the upper limit of the control action window is higher than the upper limit of the intermediate-temperature plasticity window. In this embodiment, taking Q345 steel as an example, the intermediate-temperature plasticity window is a temperature range of 200℃ to 300℃. Therefore, the preset upper limit of the control action window is 320℃, the lower limit is 180℃, and the delay time threshold is 1.0s. When the temperature of a certain micro-region exceeds the control action window (i.e., above 320℃ or below 180℃) and continues to exceed the delay time threshold of 1.0s, the pulse cryogenic impact unit automatically stops its impact output to that micro-region to avoid frequent start-stop of the cryogenic electromagnetic pulse valve due to temperature fluctuations, thus ensuring device lifespan and process continuity.

[0071] Step S3: After the target micro-area treatment is completed, stop the pulsed cryogenic shock and allow the weld to naturally recover to room temperature in air, forming a residual compressive stress layer on the weld surface. Specifically:

[0072] When micro-area A completes the preset 4 pulse cycles and the temperature drops below the lower limit of the intermediate temperature plasticity window (200℃), the system determines that the micro-area treatment is complete and stops the pulse cryogenic shock to the micro-area. After that, micro-area A enters the natural recovery stage, slowly cooling to room temperature (about 25℃) in the air through convection and heat conduction into the base material.

[0073] During the natural recovery process, the compressive plastic deformation (plastic strain of approximately 0.3%-0.5%) on the surface of micro-region A caused by the pulsed cryogenic impact is constrained by the elastic region inside that has not undergone plastic deformation. When the entire micro-region recovers to room temperature, the surface layer, due to plastic compression, tends to be in a "shorter" state than the interior, and is constrained by the elastic tension inside, thus forming a stable residual compressive stress field on the surface. X-ray diffraction analysis shows that the residual compressive stress value on the surface of micro-region A ranges from -80 MPa to -120 MPa, and the compressive stress layer depth is approximately 0.3 mm; the peak value of the residual tensile stress inside the micro-region decreases from 260 MPa in the welded state to 85 MPa, a reduction of 67%.

[0074] In this embodiment, the above steps S1-S3 are executed synchronously with the welding process. While the welding torch moves along the weld, the temperature detection and pulse deep cryogenic impact operation are carried out continuously, realizing the integrated completion of welding and stress control.

[0075] This application also discloses a welding apparatus for a vehicle frame. (Refer to...) Figure 2 A type of vehicle frame welding equipment includes:

[0076] A welding execution unit 1 is used to generate a welding arc and move along the welding direction; in this embodiment, the welding execution unit adopts a six-axis arc welding robot;

[0077] A temperature field sensing unit 2 is provided to acquire real-time temperature field data of the weld area after welding. In this embodiment, the temperature detection unit adopts an infrared thermal imager, which is installed on a traveling bracket behind the welding execution unit and moves synchronously with the welding execution unit to acquire real-time temperature field data of the weld and heat-affected zone after welding. A compressed air purge curtain interface is provided in front of the infrared thermal imager to generate a purge air curtain in the same direction as the welding advance along the temperature detection path, remove the interference of welding fumes on temperature detection, and ensure the accuracy of temperature measurement.

[0078] A pulsed air-cooled impact unit 3 is used to apply pulsed compressed air impact to the micro-region entering the high-temperature phase transformation window (600℃~750℃), interrupting the austenite grain growth process and multiplying phase deformation nuclei, thus refining the grain structure of the heat-affected zone.

[0079] A pulsed cryogenic shock unit 4 is used to apply intermittent pulsed cryogenic shock to the micro-area that has entered the medium-temperature plasticity window (200℃~300℃), so that the target micro-area will produce controllable compressive plastic deformation under transient thermal stress and form a residual compressive stress layer on the surface of the weld.

[0080] A system controller 5 is connected to the temperature field sensing unit and the pulse cryogenic shock unit respectively, and is configured to: receive the real-time temperature field data and determine whether the target micro-region has entered the high-temperature phase transition window or the medium-temperature plasticity window;

[0081] When entering the high-temperature phase change window, the pulsed air cooling impact unit 3 is controlled to apply pulsed compressed air impact;

[0082] When entering the intermediate temperature plasticity window, the pulse cryogenic shock unit 4 is controlled to apply intermittent pulse cryogenic shocks to each micro-region entering the intermediate temperature plasticity window in sequence.

[0083] The system controller 5 is also equipped with a safety redundancy module 6. In this embodiment, the upper limit of the preset control action window is 320℃, the lower limit is 180℃, and the delay time threshold is 1.0s. When the safety redundancy module detects that the temperature of a certain micro-area exceeds the control action window (i.e., above 320℃ or below 180℃), and the temperature exceeds the delay time threshold of 1.0s for an extended period, it automatically stops the pulse cryogenic shock unit's shock output to that micro-area. This safety redundancy mechanism can effectively avoid frequent start-stop of the cryogenic electromagnetic pulse valve due to temperature fluctuations, ensuring device lifespan and process continuity.

[0084] Regarding the installation methods of the pulsed gas-cooled impact unit and the pulsed cryogenic impact unit, it should be noted that for weld areas with uniform wall thickness and consistent heat dissipation conditions, the above-mentioned accompanying installation method can meet the process requirements. In this case, the micro-area entering the window naturally appears within the coverage area of ​​the nozzles of each impact unit, and the accompanying nozzles can sequentially complete the impact operation on each micro-area. For frame joint areas with drastic changes in wall thickness and significant differences in cooling conditions such as the presence of reinforcing plates or variable cross-sections, the device of this invention can configure the pulsed gas-cooled impact unit and the pulsed cryogenic impact unit with an independent moving installation method: each impact unit is not fixed on the accompanying bracket, but is installed on an independent servo slide, which can move back and forth along the weld direction; it should be noted that other mechanisms that can drive their independent movement are also within the scope of protection of this application, such as a six-axis robot. After the system controller identifies the position of the micro-area entering the high-temperature phase change window or the medium-temperature plastic window based on real-time temperature field data, it controls the corresponding servo slide to drive the corresponding impact unit to actively move above the micro-area to apply the impact. After processing, it returns to the standby position or moves directly to the next micro-area entering the window.

[0085] Furthermore, regarding the cooling medium for pulsed cryogenic shock, the above embodiments use liquid nitrogen atomization as the cooling medium, which is currently the optimal implementation method of this invention. In other embodiments of this invention, the cooling medium for pulsed cryogenic shock can also be low-temperature high-pressure air (below -100°C), liquid carbon dioxide, a mixture of solid carbon dioxide (dry ice) particles and high-pressure gas, or other media capable of achieving instantaneous cooling of micro-regions. As long as the medium can generate sufficient transient thermal stress on the surface of the target micro-region, causing the material to undergo compressive plastic deformation within the intermediate temperature plasticity window, the stress field reconstruction objective of this invention can be achieved. Liquid nitrogen is the optimal cooling medium for this invention due to its low boiling point (-196°C), high latent heat of vaporization, low cost, and lack of pollution.

[0086] Comparative Example 1

[0087] This comparative example uses the same welding parameters and base material as Example 1 to complete the frame welding. After welding, the frame is allowed to cool naturally to room temperature in air without any cryogenic shock treatment. Testing revealed that the peak residual tensile stress in the weld zone was 265 MPa (close to 77% of the yield strength of Q345 steel), and there was no residual compressive stress layer on the surface.

[0088] Comparative Example 2

[0089] This comparative example uses the same welding parameters as Example 1 to complete the frame welding. After welding, the entire frame is pushed into a heat treatment furnace for overall heat aging treatment, heated to 580°C and held for 4 hours before cooling with the furnace. Testing revealed that the peak residual tensile stress in the weld area was 172 MPa (elimination rate 35%), the surface layer still exhibited residual tensile stress, and the entire frame showed a bending deformation of approximately 1.5 mm, requiring subsequent straightening treatment.

[0090] Comparative Example 3

[0091] This comparative example uses the same welding parameters as Example 1 to complete the frame welding. After welding, a conventional continuous cryogenic treatment method is employed: the frame is cooled to room temperature, then immersed entirely in liquid nitrogen for 2 hours, and then removed and allowed to naturally return to room temperature. Testing revealed that the peak residual tensile stress in the weld area was 198 MPa (relief rate 25%), and no stable residual compressive stress layer was formed on the surface. Continuous cryogenic treatment primarily relaxes some stress through overall volumetric shrinkage; however, due to the lack of pulsed local energy input and the utilization of mid-temperature creep effects, its stress relief and surface compressive stress formation effects are significantly inferior to the pulsed cryogenic impact process of this invention.

[0092] A comparison of Example 1 with Comparative Examples 1-3 shows that the process of the present invention achieves significant technical effects in both residual stress relief rate and surface compressive stress construction: the peak residual tensile stress is reduced by more than 67%, and a residual compressive stress layer of -80MPa to -120MPa is constructed on the weld surface, which is impossible to achieve in the weld state, the heat-aged state, and the conventional cryogenic treatment state. The presence of the surface residual compressive stress layer can effectively inhibit the initiation of fatigue cracks on the weld surface, which is of great significance for improving the service life of the chassis.

[0093] Example 2

[0094] This embodiment, based on Embodiment 1, adds a high-temperature microstructure refinement step. Specifically, before applying the medium-temperature plastic window pulsed cryogenic shock, pulsed compressed air shock is applied to the heat-affected zones on both sides of the weld at the high-temperature phase transformation window. This embodiment only describes the differences from Embodiment 1; the remaining steps are the same as in Embodiment 1.

[0095] In this embodiment, a pulsed gas-cooled impact unit is added to the accompanying support in front of the pulsed cryogenic impact unit, and this unit also moves synchronously with the welding torch. The pulsed gas-cooled impact unit and the pulsed cryogenic impact unit are arranged at intervals along the welding advance direction. The distance L between them is determined by the formula L=v×(t1+t2), where v is the welding advance speed of 8mm / s; t1 is the natural cooling time of the Q345 steel base material from the high-temperature phase transformation window (600-750℃) to the medium-temperature plasticity window (200-300℃), which is about 42s according to thermal simulation test data; t2 is the time sequence stabilization margin, which is taken as 1.0s in this embodiment. Substituting into the formula, we get L=8×(42+1.0)=344mm. Therefore, the center installation position of the pulsed gas-cooled impact unit is located about 56mm behind the welding torch, 400mm-344mm.

[0096] The above-mentioned follow-up installation method is suitable for working conditions where the weld wall thickness is uniform and the heat dissipation conditions are consistent. The design distance is set based on the actual material conditions. In this embodiment, taking Q345 steel as an example, after welding, the micro-area first passes through the gas-cooled impact unit (56mm behind the welding torch, welding torch speed 8mm / s, about 7s after welding, temperature about 600-750℃), then passes through 344mm (about 43s), and naturally cools to 200-300℃, just reaching the bottom of the cryogenic impact unit (400mm behind the welding torch, about 50s).

[0097] For complex frame joint areas with drastic changes in wall thickness or variable cross-sections, the pulsed gas cooling impact unit can also be installed independently, i.e., mounted on an independent servo slide or the output end of a six-axis robot. The system controller controls it to actively move to the micro-area above the high-temperature phase transformation window based on real-time temperature field data to apply the impact. This ensures that even when the cooling sequence and welding sequence are inconsistent, the high-temperature microstructure refinement process can still reliably cover all micro-areas.

[0098] When a micro-region on the solidified weld cools to the high-temperature phase transformation window of 600-750℃, i.e., when this micro-region is located approximately 56mm behind the welding torch, the pulsed air-cooling impact unit begins to apply pulsed compressed air impact to this micro-region. This unit uses dry compressed air as the medium, with a pulse frequency set to 3Hz and a spray pressure of 0.4MPa, synchronously sprayed through two nozzles symmetrically arranged on both sides of the weld. Within each pulse cycle, the high-pressure spray period is 0.1s, and the interval period is 0.23s.

[0099] Q345 steel is in the critical region of austenitic phase transformation at a temperature range of 600-750℃. At this temperature, austenite grains exhibit a strong tendency to grow. If cooled slowly and naturally, the grains will continue to grow, eventually forming coarse ferrite and pearlite structures, leading to embrittlement of the heat-affected zone (HAZ). This embodiment addresses this by applying pulsed compressed air impacts within this temperature window. During the high-pressure injection phase, the surface temperature of the HAZ is rapidly reduced (cooling rate can reach 80-120℃ / s), interrupting the austenite grain growth process. During the intermittent phases, the temperature partially recovers, creating conditions for subsequent pulsed quenching. The repeated pulsed air cooling effect induces numerous substructures and compositional fluctuations in the austenite matrix. These substructures and compositional fluctuations become preferential nucleation sites for ferrite and pearlite during subsequent phase transformations, significantly increasing the nucleation density and thus refining the final HAZ grain structure.

[0100] The micro-area, after being processed, continues to cool naturally as the welding torch moves forward. After about 42 seconds, the temperature drops to the medium-temperature plasticity window of 200-300℃. At this time, the micro-area just reaches the bottom of the pulse cryogenic shock unit, and the system applies pulse cryogenic shock to the micro-area according to steps S2-S3 of Example 1.

[0101] Metallographic analysis revealed that after the two-stage temperature window treatment in this embodiment, the ferrite grain size in the weld heat-affected zone of the Q345 steel frame was refined from ASTM grade 8 in the welded state to ASTM grade 11, with the grain size decreasing from approximately 22 μm to approximately 8 μm, demonstrating a significant refinement effect. Simultaneously, the grain refinement increased the grain boundary area, providing more grain boundaries as a coordinating interface for dislocation movement and plastic flow during the subsequent mid-temperature plastic window pulsed cryogenic impact stage. This resulted in more uniform and sufficient micro-region compressive plastic deformation, further improving the stress reconstruction efficiency by approximately 12% compared to Example 1. The surface residual compressive stress value increased to -100 MPa to -150 MPa, and the stress distribution became more uniform.

[0102] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A welding process for a vehicle frame, characterized in that, The following steps are performed simultaneously with the welding process: S1. Obtain real-time temperature field data of the weld and heat-affected zone after welding; S2. When the target micro-area on the weld is detected to have cooled into the preset medium-temperature plastic window, an intermittent pulsed cryogenic impact is applied to the target micro-area; the pulsed cryogenic impact acts on the solid medium-temperature stage of post-weld cooling, causing the target micro-area to produce controllable compressive plastic deformation under transient thermal stress. S3. After the target micro-area treatment is completed, stop the pulse deep cryogenic shock and allow the weld to naturally recover to room temperature in the air, forming a residual compressive stress layer on the surface of the weld.

2. The welding process for a vehicle frame according to claim 1, characterized in that, The temperature range of the intermediate-temperature plasticity window is determined based on the high-temperature creep characteristics and stress control threshold of the base material. For carbon steel and low-alloy high-strength steel, the intermediate-temperature plasticity window is 200℃~300℃. Within this temperature range, the creep resistance of the material is significantly reduced, and compressive plastic deformation can be generated under transient thermal stress.

3. The welding process for a vehicle frame according to claim 1, characterized in that, The pulse frequency of the pulsed cryogenic impact is 1-5 Hz, and the injection pressure is 0.5-1.0 MPa; by adjusting the pulse frequency and injection pressure, the amount of compressive plastic deformation in the target micro-region is controlled.

4. The welding process for a vehicle frame according to claim 1, characterized in that, The steps preceding step S2 also include: When the target micro-region is cooled to a high-temperature phase transformation window higher than the intermediate-temperature plasticity window, pulsed compressed air impact is applied to the heat-affected zones on both sides of the weld to interrupt the austenite grain growth process and multiply phase deformation nuclei, thereby refining the grain structure of the heat-affected zone.

5. The welding process for a vehicle frame according to claim 4, characterized in that, For carbon steel and low-alloy high-strength steel, the temperature range of the high-temperature phase transformation window is 600℃~750℃; the pulse frequency of the pulsed compressed air impact is 2~4Hz, the injection pressure is 0.3~0.6MPa, and it is applied symmetrically to both sides of the weld.

6. The welding process for a vehicle frame according to claim 4, characterized in that, The pulse air cooling unit performing the pulse compressed air impact and the pulse cryogenic unit performing the pulse cryogenic impact are arranged at intervals along the welding advance direction, and the distance L between them satisfies: L = v × (t1 + t2) Where v is the welding advance speed, t1 is the natural cooling time of the base material from the high-temperature phase transformation window to the medium-temperature plastic window, and t2 is the timing stability margin of 0.5 to 2 seconds.

7. The welding process for a vehicle frame according to claim 1, characterized in that, In step S1, a purge air curtain is generated along the temperature detection path in the same direction as the welding advance to remove the interference of welding fumes on temperature detection.

8. The welding process for a vehicle frame according to claim 1, characterized in that, For T-joint welds, two nozzles symmetrically arranged on both sides of the weld are used to simultaneously apply the pulsed deep cryogenic shock.

9. The welding process for a vehicle frame according to claim 1, characterized in that, It also includes safety redundancy steps: When the temperature of the target micro-region exceeds the preset control action window and continues to exceed the preset delay time threshold, the pulse cryogenic shock is stopped. Wherein, the lower limit of the control action window is lower than the lower limit of the intermediate temperature plasticity window, and the upper limit of the control action window is higher than the upper limit of the intermediate temperature plasticity window.

10. A chassis welding apparatus for performing the process according to any one of claims 1 to 9, characterized in that, include: A welding execution unit is used to generate a welding arc and move it along the welding direction; A temperature field sensing unit is used to acquire real-time temperature field data of the weld zone after welding. A pulsed air-cooled impact unit is used to apply pulsed compressed air impact to the heat-affected zones on both sides of the weld. A pulse cryogenic shock unit is used to apply intermittent pulse cryogenic shock to the target micro-area; A system controller, connected to the temperature field sensing unit and the pulse cryogenic shock unit respectively, is configured to: receive the real-time temperature field data and determine whether the target micro-region has entered the high-temperature phase transition window or the medium-temperature plasticity window; When entering the high-temperature phase change window, the pulsed air cooling impact unit is controlled to apply pulsed compressed air impact; When entering the intermediate temperature plasticity window, the pulse cryogenic shock unit is controlled to apply intermittent pulse cryogenic shocks to each micro-region entering the intermediate temperature plasticity window in sequence.

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