A method for testing and judging the performance of laser local soft zone

Abstract

The present invention relates to a method for testing and judging the performance of a laser local soft zone, comprising step S1, a sampling step, sampling from a thermoformed part to obtain a laser local soft zone sample to be tested; step S2, a sample processing step, cutting the laser local soft zone sample to obtain a surface to be tested; step S3, a hardness testing step, performing a hardness test on the surface to be tested; step S4, a data analysis step, drawing a hardness distribution curve, and judging whether the soft zone performance of the surface to be tested meets a specified standard. The method of the present invention is simple and highly reliable. Through the above steps, a hardness test is performed on the laser local soft zone sample to be tested, and by analyzing the data obtained from the hardness test, whether the performance of the laser soft zone meets the standard is judged, and then whether the laser softening process is normal is judged. It can also serve as a reference and guide in the subsequent adjustment of the laser softening parameters.

Description

Technical Field

[0001] The invention relates to the technical field of automobile component performance testing, in particular to a method for testing and judging the performance of a laser local soft zone. Background Art

[0002] In recent years, new energy vehicle technology and the market have developed rapidly. To achieve energy conservation and emission reduction, lightweighting vehicles is becoming increasingly important. Using hot-formed steel to replace traditional low-carbon steel in the body-in-white (BIW) can reduce weight while improving crash performance, with weight reductions reaching up to 30%. Currently, the proportion of hot-formed steel used in different vehicle models ranges from 10-30%. With the rapid advancement of lightweighting technology, the use of hot-formed steel in vehicle bodies is increasing, and its strength is also increasing. This brings with it refined technical requirements, such as localized softening, which primarily addresses hot-formed parts with excessive hardness that make subsequent riveting difficult. Alternatively, localized soft zones can be created before spot welding to improve the heat-affected zone (HAZ) performance of welds and prevent cracking. A published Chinese patent (publication number 2022108273302) describes a variable-strength partial performance testing method for 2GPa hot-formed materials. However, this method utilizes a molded soft zone technique, featuring a soft zone tensile strength of 700-850 MPa and a transition zone width of 30-50 mm.

[0003] However, the existing technology's shortcomings are that the range of tensile strength of the soft zone, the size of the soft zone and the transition zone, and their distribution of shapes differ from those described in the aforementioned patents. Furthermore, the methods described in the aforementioned patents cannot be used to assess the eligibility of laser softening parameters. Because the surface of a part undergoing laser softening remains unchanged, the existing patents cannot test or assess the performance of the soft zone. Summary of the Invention

[0004] To this end, the technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a method for testing and evaluating the performance of a local laser soft zone. The method of the present invention is simple and reliable and can be used to test the laser soft zone performance of thermoformed parts. A set of evaluation criteria for the method is provided to determine whether the performance of the laser soft zone meets the standards and whether the laser softening process is normal, thereby facilitating the adjustment of the laser softening parameters.

[0005] In order to solve the above technical problems, the present invention provides a method for testing and judging the performance of a laser local soft zone, comprising:

[0006] Step S1, sampling step, sampling from the hot-formed part to obtain the laser local soft zone specimen to be tested;

[0007] Step S2, a sample processing step, cutting the laser local soft zone sample to obtain a surface to be tested, and processing the surface to be tested until there are no obvious scratches on the surface to be tested; the surface to be tested includes a hard zone, a transition zone, and a soft zone;

[0008] Step S3, hardness testing step, performs hardness testing on the surface to be tested, the hardness testing methods include method A, method B and method C,

[0009] The method A is to perform a hardness test on the soft area of ​​the surface to be tested in the longitudinal direction to obtain hardness test data of the upper surface and the lower surface of the soft area of ​​the surface to be tested;

[0010] The method B is to perform a hardness test on the surface to be tested in the transverse direction to obtain a plurality of hardness test data of the surface to be tested in the transverse direction, wherein the test range covers the soft zone, transition zone and hard zone of the surface to be tested;

[0011] The method C comprises performing a hardness test on the surface to be tested along a first direction to obtain a plurality of hardness test data of the surface to be tested along the first direction, wherein the test range covers a soft zone, a transition zone, and a hard zone of the surface to be tested, and the first direction forms an angle with the transverse direction;

[0012] Step S4, a data analysis step, draws a hardness distribution curve based on the hardness test data measured in the method A, the method B and the method C to determine whether the soft zone performance of the surface to be tested meets the specified standard.

[0013] In one embodiment of the present invention, in step S4, according to the hardness test data measured by method A, if the lower surface hardness value minus the upper surface hardness value is ≤30HV, it is determined that the laser local soft zone sample is completely softened and has a consistent hardness;

[0014] If the lower surface hardness value minus the upper surface hardness value is greater than 30HV, it is determined that the laser local soft zone sample is not completely softened;

[0015] If the lower surface hardness value minus the upper surface hardness value is less than -15HV, it is determined that the laser local soft zone sample is partially austenitized.

[0016] In one embodiment of the present invention, in step S4, for the hardness distribution curve, if the hardness distribution curve is symmetrically U-shaped and the width of the transition zone is 6-10 mm, it is judged that the performance of the laser local soft zone sample is normal; if the width of the transition zone is greater than 10 mm, it is judged that the laser speed is too fast or the defocus amount is too large;

[0017] If the hardness distribution curve is U-shaped, but the curve on one side of the U is too high, it is judged that the laser scanning speed is too fast;

[0018] If the hardness distribution curve is W-shaped, it is determined that the temperature in the center of the soft zone has reached the austenitizing temperature.

[0019] In one embodiment of the present invention, in step S3, method A includes performing hardness tests on multiple longitudinal indentations in the soft zone of the test surface to obtain upper surface hardness data and lower surface hardness data of the soft zone along the longitudinal direction.

[0020] In one embodiment of the present invention, in step S3, method B includes performing hardness tests on multiple lateral indentations near the upper surface, the middle area, and the lower surface of the surface to be tested, wherein the lateral indentations cover the soft area, the transition area, and the hard area, and obtaining multiple hardness test data.

[0021] In one embodiment of the present invention, in step S3, method C includes performing indentation testing on a plurality of consecutive points along the first direction on the surface to be tested, the point range covering the soft zone, the transition zone and the hard zone, and the angle between the first direction and the lateral direction is 10-90°.

[0022] In one embodiment of the present invention, in step S3, the distance between two adjacent indentations is 0.1-4 mm.

[0023] In one embodiment of the present invention, in step S2, the step of obtaining the surface to be measured includes step S21, cutting the laser local soft zone sample in half along the center line of the soft zone surface and perpendicular to the surface, and using the thickness cross section of the laser local soft zone sample as the surface to be measured.

[0024] In one embodiment of the present invention, in step S1, the distance between the sampling range and the boundary of the laser local soft zone to be tested is greater than 15 mm; in step S2, the method of cutting the laser local soft zone sample includes wire cutting or water-cooled grinding wheel cutting.

[0025] In one embodiment of the present invention, in step S3, the hardness testing method includes a Rockwell hardness testing method or a Vickers hardness testing method.

[0026] The above technical solution of the present invention has the following advantages over the prior art:

[0027] The method for testing and judging the performance of a laser local soft zone described in the present invention includes a sampling step, a sample processing step, a hardness testing step, and a data analysis step. The method of the present invention is simple and highly reliable. The hardness test of the laser local soft zone sample to be tested is achieved through the above steps, and the data obtained from the hardness test is analyzed to determine whether the performance of the laser soft zone meets the standard, and then determine whether the laser softening process is normal. It can also serve as a reference and guidance in the subsequent adjustment of the laser softening parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0029] Figure 1 It is a top view schematic diagram of the laser soft zone distribution and sampling range of the preferred embodiment of the present invention.

[0030] Figure 2 It is a schematic diagram of the test position and test direction of the hardness test of the laser soft zone in the preferred embodiment of the present invention.

[0031] Figure 3 Graph showing the hardness distribution of method A according to a preferred embodiment of the present invention.

[0032] Figure 4 1 is a hardness distribution curve diagram of method B and method C according to the preferred embodiment of the present invention.

[0033] Figure 5 It is a schematic diagram of the top structure of the sampling range in a specific embodiment of the present invention.

[0034] Figure 6 yes Figure 5 Hardness curve distribution of the soft area of ​​the specimen. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0036] Reference Figures 1 to 6 As shown, the present invention discloses a method for testing and judging the performance of a laser local soft zone, comprising step S1, a sampling step, sampling from a hot-formed part to obtain a laser local soft zone specimen to be tested.

[0037] The method further includes step S2, a sample processing step, in which the laser local soft zone sample is cut to obtain a test surface, and the test surface is processed. Specifically, the test surface is inlaid and polished until there are no obvious scratches on the test surface. The test surface includes a hard area (base material area), a transition area, and a soft area.

[0038] Step S3, a hardness testing step, performs a hardness test on the surface to be tested. The hardness testing methods include method A, method B, and method C, wherein:

[0039] The method A is to perform a hardness test on the soft area of ​​the surface to be tested in the longitudinal direction to obtain a plurality of hardness test data of the upper surface and the lower surface of the soft area of ​​the surface to be tested;

[0040] The method B is to perform a hardness test on the surface to be tested in the transverse direction to obtain a plurality of hardness test data of the surface to be tested in the transverse direction, wherein the test range covers the soft zone, transition zone and hard zone of the surface to be tested;

[0041] The method C comprises performing a hardness test on the surface to be tested along a first direction to obtain a plurality of hardness test data of the surface to be tested along the first direction, wherein the test range covers a soft zone, a transition zone, and a hard zone of the surface to be tested, and the first direction forms an angle with the transverse direction;

[0042] Step S4, a data analysis step, determining whether the soft zone performance of the surface to be tested meets a specified standard based on the hardness test data measured by the method A;

[0043] A hardness distribution curve is drawn based on the hardness test data obtained in the method B and the method C, and it is determined based on the hardness distribution curve whether the soft zone performance of the surface to be tested meets the specified standard.

[0044] From this, it can be seen that the method for testing and judging the performance of the laser local soft zone to be protected by the present invention includes a sampling step, a sample processing step, a hardness testing step and a data analysis step. The method of the present invention is simple and highly reliable. The hardness test of the laser local soft zone sample to be tested is achieved through the above steps, and the data obtained from the hardness test is analyzed to determine whether the performance of the laser soft zone meets the standard, and then determine whether the laser softening process is normal. It can also serve as a reference and guide in the subsequent adjustment of the laser softening parameters.

[0045] It should be noted that the present invention is applicable to thermoformed parts that have undergone localized laser softening treatment. The materials of the thermoformed parts include, but are not limited to, bare plate, aluminum-silicon coated plate, zinc-based coated plate, etc. When selecting a single thermoformed part, the selected thermoformed part has a thickness of 0.6-4 mm, a hardness of 400-590 HV, and a tensile strength of 1300-2000 MPa.

[0046] The hardness of the laser local soft zone on the thermoformed part is 230-480HV, and the tensile strength is 800-1500MPa.

[0047] As a preferred embodiment, in step S4, Figure 3 As shown in curve ①, according to the hardness test data measured by method A, if the lower surface hardness value - the upper surface hardness value ≤ 30HV, it is judged that the laser local soft zone sample is completely softened and the hardness is consistent.

[0048] Combine Figure 3 As shown in curve ②, if the lower surface hardness value - the upper surface hardness value is greater than 30HV, it is determined that the laser local soft zone sample is not completely softened. The main reason for the incomplete softening is that the laser scanning speed is too fast.

[0049] Combine Figure 3 As shown in curve ③, if the lower surface hardness value minus the upper surface hardness value is less than -15HV, it is judged that the laser local soft zone sample is partially austenitized, and the hardness increases again after air cooling. This situation mainly occurs when the upper surface softening temperature exceeds 880℃, and the laser power is too high at this time.

[0050] It needs to be explained that austenitization refers to the metal heat treatment process in which steel is heated above the critical point to form austenite. The workpiece is heated to a temperature above the eutectoid temperature, so that the ferrite and cementite at room temperature are transformed back into austenite.

[0051] Furthermore, in step S4, for the hardness test data obtained in method B and method C, in the hardness distribution curve drawn according to the hardness test data:

[0052] Combine Figure 4 As shown in curve ④, if the hardness distribution curve is symmetrical U-shaped and the width of the transition zone is 6-10mm, it is judged that the performance of the laser local soft zone sample is normal. If the width of the transition zone is greater than 10mm, it is judged that the laser speed is too fast or the defocus amount is too large;

[0053] Combine Figure 4 As shown in curve ⑤, if the hardness distribution curve is U-shaped, but the curve on one side of the U-shape is too high, it is judged that the laser scanning speed is too fast;

[0054] Combine Figure 4 As shown in curve ⑥, if the hardness distribution curve is W-shaped, it indicates that the temperature in the center of the soft zone has reached the austenitizing temperature. Hardness increases again after air cooling. This occurs when the soft zone temperature exceeds 880°C and the laser power is too high. Next, hardness tests are performed on the upper, middle, and lower layers of the soft zone. The presence of this pattern in the lower layers indicates that the soft zone has more austenitized areas.

[0055] Combine Figure 2 From the above, in step S3, the method A specifically includes performing hardness tests on multiple longitudinal indentations in the soft zone of the test surface to obtain upper surface hardness data and lower surface hardness data of the soft zone along the longitudinal direction.

[0056] Combine Figure 2 As shown, in the step S3, the method B includes performing hardness tests on multiple transverse indentations near the upper surface, the middle area, and the lower surface of the surface to be tested, wherein the transverse indentations cover the soft area, the transition area, and the hard area, and obtaining multiple hardness test data.

[0057] according to Figure 2 From the perspective of the embodiment, in step S3, the method C includes performing indentation tests on a plurality of consecutive points along the first direction on the surface to be tested, the point range covers the soft zone, the transition zone and the hard zone, and the angle between the first direction and the lateral direction is 10-90°.

[0058] In detail, the angle is related to the ratio of the cross-sectional area of ​​the soft zone and the thickness of the sample. The larger the ratio, the smaller the angle.

[0059] In step S3, the distance between two adjacent indentations is 0.1-4 mm. Furthermore, the position and number of indentations can be adjusted according to actual conditions.

[0060] As a preferred embodiment, in step S2, the step of obtaining the surface to be measured includes step S21, cutting the laser local soft zone sample in half along the center line of the soft zone surface and perpendicular to the surface, and using the thickness cross section of the laser local soft zone sample as the surface to be measured.

[0061] As a preferred implementation, in step S1, the distance between the sampling range and the boundary of the laser local soft zone to be tested is greater than 15 mm.

[0062] In the step S2, the method of cutting the laser local soft zone sample includes wire cutting or water-cooled grinding wheel cutting.

[0063] In step S3, the hardness testing method includes a Rockwell hardness testing method or a Vickers hardness testing method.

[0064] A specific embodiment of the present invention is described in detail below with reference to the accompanying drawings:

[0065] To illustrate this method, the following is an example of a laser softening area of ​​20*20mm:

[0066] (1) First, take a sample from a 1500MPa grade hot-formed part with a thickness of 1.6mm and remove the laser local soft zone sample to be tested. The sample size is as follows: Figure 5 As shown,

[0067] (2) The laser local soft zone specimen is cut in half along the vertical thickness direction, and then the thickness section of the specimen is used as the test surface, and is inlaid, ground and polished until there are no obvious scratches on the test surface.

[0068] (3) Perform Vickers hardness test on the surface to be tested, select test method B, perform hardness test every 2mm in the middle layer in the thickness direction of the soft zone, draw a hardness distribution map, analyze the hardness data, and Figure 6 As shown, the results show that the hardness of the soft zone is 260-270HV, the width of the soft zone is 14mm, the transition zone is 6-8mm, and the hardness distribution curve is symmetrical U-shaped. The results show that the performance of the soft zone is stable and the laser parameters are in line with expectations.

[0069] In the description of the embodiments of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "disposed" and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections, electrical connections; direct connections, indirect connections through an intermediate medium, and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific circumstances.

[0070] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0071] In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "connect," "fixed," etc. should be understood broadly. For example, they may refer to fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0072] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for testing and judging the performance of a laser local soft zone, characterized by: include, Step S1, sampling step, sampling from the hot-formed part to obtain the laser local soft zone specimen to be tested; Step S2, a sample processing step, cutting the laser local soft zone sample to obtain a surface to be tested, and processing the surface to be tested until there are no obvious scratches on the surface to be tested; the surface to be tested includes a hard zone, a transition zone, and a soft zone; Step S3, hardness testing step, performs a hardness test on the surface to be tested, the hardness test includes method A, method B and method C, The method A comprises: performing a hardness test on the soft area of ​​the surface to be tested in the longitudinal direction to obtain hardness test data of the upper and lower surfaces of the soft area of ​​the surface to be tested; performing a hardness test on multiple longitudinal indentations of the soft area of ​​the surface to be tested to obtain hardness data of the upper and lower surfaces of the soft area in the longitudinal direction; The method B comprises: performing a hardness test on the surface to be tested in a transverse direction to obtain a plurality of hardness test data of the surface to be tested in the transverse direction, wherein the test range covers the soft zone, transition zone, and hard zone of the surface to be tested; performing a hardness test on a plurality of transverse indentations near the upper surface, the middle region, and the lower surface of the surface to be tested, wherein the transverse indentations cover the soft zone, transition zone, and hard zone, to obtain a plurality of hardness test data; The method C comprises: performing a hardness test on the surface to be tested along a first direction to obtain a plurality of hardness test data of the surface to be tested along the first direction, wherein the test range covers the soft zone, the transition zone, and the hard zone of the surface to be tested, and the first direction and the transverse direction have an angle; performing an indentation test on a plurality of consecutive points along the first direction on the surface to be tested, wherein the range of the points covers the soft zone, the transition zone, and the hard zone, and the angle between the first direction and the transverse direction is 10-90°; Step S4, a data analysis step, draws a hardness distribution curve based on the hardness test data measured in the method A, the method B and the method C to determine the soft zone performance of the surface to be tested.

2. A method for testing and judging the performance of a laser local soft zone according to claim 1, characterized in that: In step S4, according to the hardness test data measured by method A, if the lower surface hardness value minus the upper surface hardness value is ≤30HV, it is determined that the laser local soft zone sample is completely softened and has a consistent hardness; If the lower surface hardness value minus the upper surface hardness value is greater than 30HV, it is determined that the laser local soft zone sample is not completely softened; If the lower surface hardness value minus the upper surface hardness value is less than -15HV, it is determined that the laser local soft zone sample is partially austenitized.

3. The method for testing and judging the performance of a laser local soft zone according to claim 1, characterized in that: In step S4, for the hardness distribution curve, if the hardness distribution curve is symmetrically U-shaped and the width of the transition zone is 6-10 mm, it is judged that the performance of the laser local soft zone sample is normal; if the width of the transition zone is greater than 10 mm, it is judged that the laser speed is too fast or the defocus amount is too large; If the hardness distribution curve is U-shaped, but the curve on one side of the U is too high, it is judged that the laser scanning speed is too fast; If the hardness distribution curve is W-shaped, it is determined that the temperature in the center of the soft zone has reached the austenitizing temperature.

4. The method for testing and judging the performance of a laser local soft zone according to claim 1, characterized in that: In step S3, the distance between two adjacent indentations is 0.1-4 mm.

5. The method for testing and judging the performance of a laser local soft zone according to claim 1, characterized in that: In step S2, the step of obtaining the surface to be measured includes cutting the laser local soft zone sample in half along the center line of the soft zone surface and perpendicular to the surface, and using the thickness section of the laser local soft zone sample as the surface to be measured.

6. The method for testing and judging the performance of a laser local soft zone according to claim 1, characterized in that: In step S1, the distance between the sampling range and the boundary of the laser local soft zone to be tested is greater than 15 mm; in step S2, the method of cutting the laser local soft zone sample includes wire cutting or water-cooled grinding wheel cutting.

7. The method for testing and judging the performance of a laser local soft zone according to claim 1, characterized in that: In step S3, the hardness testing method includes a Rockwell hardness testing method or a Vickers hardness testing method.

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