Customized performance hot forming mold and flow channel design method based on lattice structure

Abstract

The present invention discloses a customized performance hot forming die based on a lattice structure and a flow channel design method thereof, the method comprising the following steps: preliminarily designing a conformal cooling flow channel of a hot stamping die according to the highest strength on a formed part; determining the physical property distribution of the formed part according to the performance requirements of the hot stamping formed part; determining the cooling rate of each position of the formed part according to the physical property distribution of the formed part by the relationship between the final performance of the formed part and the cooling rate; determining the position of the lattice structure filled with the conformal cooling flow channel and the size parameters of the lattice structure according to the cooling rate of different positions of the hot stamping formed part; establishing a three-dimensional model of the hot forming die according to the determined lattice structure position and size parameters, and processing the die. The present invention can realize the rapid design and manufacture of hot forming dies with customized performance, realize the arbitrary customization of part performance, and has broad application prospects.

Description

Technical Field

[0001] The present invention relates to the field of hot stamping technology, and in particular to a customized performance hot forming die based on a lattice structure and a flow channel design method thereof. Background Art

[0002] Hot stamping is one of the main processing methods for high-strength steel plates. It effectively solves the problem of poor low-temperature formability of high-strength steel plates and is widely used in the automotive industry. The parts manufactured by conventional hot stamping methods have the characteristics of high strength and uniform performance. However, the current requirements for the integration of part performance have limited the application of conventional hot stamping methods that can only form a single property. Therefore, customized hot stamping processes have begun to emerge.

[0003] The current hot stamping processes for achieving customized performance can be divided into four categories: customized blanks, local heating, customized annealing, and differential cooling. The customized blank method requires either customized rolled steel plates or additional welding processes, which increases the difficulty and cost of processing. The heating device for local heating is specially customized according to the formed part. Every time the hot stamping die is replaced, the heating device must be replaced, which greatly increases the investment in production equipment and causes waste. Customized annealing adds an extra process, and since the formed part is annealed after the hot stamping is completed, it is easy to cause the shape accuracy of the formed part to decrease, affecting the quality of the formed part. In the current differential cooling method, the zoned heating mold and the mixed material mold are difficult to cope with complex performance distribution situations, and the customized gap method is difficult to guarantee the forming quality due to easy deformation. Summary of the invention

[0004] The purpose of the present invention is to provide a customized performance thermoforming mold based on a lattice structure and a flow channel design method thereof to ensure the quality of the final product.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A customized performance thermoforming mold flow channel design method based on a lattice structure comprises the following steps:

[0007] 1) Preliminary design of conformal cooling channels for hot stamping dies based on the maximum strength σ requirement on the formed part;

[0008] 2) Determine the physical property distribution of the formed part according to the performance requirements of the hot stamping formed part;

[0009] 3) According to the distribution of physical properties of the formed part, the cooling rate at each position of the formed part is determined by the relationship between the final performance of the formed part and the cooling rate;

[0010] 4) according to the cooling rates at different positions of the hot stamping part determined in step 3), determining the position of the lattice structure filled with the conformal cooling channel and the size parameters of the lattice structure;

[0011] 5) According to the position and size parameters of the lattice structure determined in step 4), a three-dimensional model of the thermoforming mold is established, and the mold is processed.

[0012] The present invention also includes the following features:

[0013] Further, the step 1) comprises the following steps:

[0014] 1.1) According to the relationship between forming strength and cooling rate, determine the required cooling rate V at the maximum strength σ T , quenching time t q Defined as Where T s is the temperature of the plate after heating, T f is the temperature after quenching;

[0015] 1.2) In a single hot stamping production cycle, the sheet metal is s Cool to T f Most of the released heat Q is taken away by the cooling medium, that is, kQ=qtC·ΔT, where k is the proportion of heat taken away by the cooling medium, ranging from 0.8 to 1, q is the flow rate of the cooling medium, t is the time of a single production cycle, C is the specific heat capacity of the cooling medium, and ΔT is the temperature change before and after the cooling medium flows through the mold.

[0016] Cooling medium flow n is the number of cooling channels, d is the diameter of the cooling channels, and v is the flow rate of the cooling medium;

[0017] Single production cycle time t = t q +t t , t q is the quenching time, t t is the transfer time before and after sheet forming;

[0018] 1.3) Solve to obtain the diameter d and number n of the cooling channels, and evenly arrange the cooling channels on the equidistant surface of the mold forming surface. The distance between the equidistant surface and the forming surface is H;

[0019] 1.4) Check the strength of the mold structure, take H = λH min , where λ is the amplification factor, H min The minimum value of distance H that meets the strength requirements;

[0020] Furthermore, the physical properties of the formed part include strength, hardness, and elongation;

[0021] Further, the step 3) comprises the following steps:

[0022] 3.1) Determine the microstructure composition of each position of the formed sheet according to the physical properties, wherein the microstructure composition includes the volume fraction of martensite, bainite, pearlite and ferrite;

[0023] 3.2) According to the cooling phase transformation law of the alloy plate of the CCT curve, the cooling rate of the formed part is deduced from the microstructure composition.

[0024] Further, the step 4) comprises the following steps:

[0025] 4.1) According to the heat transfer law between the sheet, the mold and the cooling medium, the equivalent thermal conductivity of the outer filling area or the equivalent interface heat transfer coefficient of the inner filling area is derived.

[0026] 4.2) The geometric parameters of the lattice structure are determined by the relationship between the equivalent thermal conductivity and the size parameters of the lattice structure in the outer filling area; the geometric parameters of the lattice structure are determined by the relationship between the equivalent interface heat transfer coefficient and the size parameters of the lattice structure in the inner filling area.

[0027] Furthermore, the lattice structure filling position of step 4) includes:

[0028] a. Inner filling area: inside the flow channel; s

[0029] b. External filling area: The channel wall is offset radially outward by distances j and l, where l>j>0, resulting in the area between surface ∑1 and surface ∑2.

[0030] Furthermore, the equivalent thermal conductivity of the outer filling zone is the thermal conductivity when the outer filling zone is filled with a uniform single medium to meet the cooling rate of the formed part, and the equivalent interface heat transfer coefficient of the inner filling zone is the interface heat transfer coefficient of the inner wall of the flow channel consistent with the cooling effect when the inner filling zone is filled with a lattice structure.

[0031] Furthermore, the cooling rate of the formed part in the mold is controlled by the position of the filled lattice structure and the heat transfer characteristics affected by the size parameters.

[0032] Furthermore, a transition area is provided between mold positions corresponding to adjacent areas of the formed part having different physical properties.

[0033] Furthermore, the mold processing method in step 5) includes but is not limited to additive manufacturing process, machining process or a combination of the two.

[0034] The present invention also provides a customized performance thermoforming mold based on a lattice structure, comprising an upper mold and a lower mold, wherein a conformal cooling channel is provided in the mold, and a partial area of ​​the channel position is filled with a lattice structure.

[0035] In the above mold, the area where the flow channel is located includes an inner filling area and an outer filling area. The inner filling area is the space inside the flow channel, and the outer filling area is the area between surfaces ∑1 and ∑2 obtained by radially offsetting the flow channel wall by distances j and l.

[0036] In the above mold, the lattice structure is filled in the inner filling area to enhance the heat exchange capacity and increase the cooling rate of the corresponding position of the formed part. The lattice structure is filled in the outer filling area to achieve thermal insulation capacity and reduce the cooling rate of the corresponding position of the formed part.

[0037] In the above mold, the lattice structure is composed of single cells that can be arranged periodically, and the heat transfer characteristics of the lattice structure are controlled by the size parameters of the cells.

[0038] In the above mold, the cell is the smallest unit that is periodically and regularly arranged in a lattice structure and generally has symmetry.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] 1. The present invention determines the cooling rate of each position of the formed part based on the distribution of the performance of the formed part and the relationship between the performance of the formed part and the cooling rate. The lattice structure with different parameters is used to fill the different positions of the inner and outer filling areas, and the heat exchange between the cooling medium and the mold is controlled, thereby controlling the temperature of the mold, and finally realizing the control of the cooling rate of different positions of the formed part from the mold structure.

[0041] 2. The present invention determines the cooling performance zones of the forming mold according to the distribution of the formed parts, and uses the heat insulation capacity of the lattice structure to separate different cooling areas of the mold, thereby realizing the integrated design and manufacturing of the mold and avoiding the precision error and reduced cooling effect caused by block manufacturing.

[0042] 3. The present invention uses a lattice structure to fill the mold to achieve customized performance. It does not require customized plates, customized heating, or additional processing steps after quenching. While ensuring the forming accuracy, it can achieve complex performance distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a flow chart of the method for designing a customized performance thermoforming die based on a lattice structure of the present invention;

[0044] Figure 2 It is a schematic diagram of the structure of a customized performance hot forming mold based on a lattice structure;

[0045] Figure 3 It is a schematic diagram of the performance zoning of the formed parts involved in the present invention;

[0046] Figure 4It is a schematic diagram of the filling position partition of the lower die of the hot forming mold;

[0047] Figure 5 A schematic diagram of the flow channel filling area division;

[0048] Figure 6 It is a schematic diagram of the radial cross section of the lattice structure of the filling area outside the flow channel;

[0049] Figure 7 It is a schematic diagram of the radial cross section of the lattice structure of the filling area in the flow channel;

[0050] Figure 8 It is a longitudinal cross-sectional schematic diagram of the lattice structure of the inner and outer filling areas of the flow channel;

[0051] Fig. 9 Schematic diagram of the lattice structure of the mold transition zone. DETAILED DESCRIPTION

[0052] Combination Figures 1 to 9 , a customized performance thermoforming mold based on a lattice structure and a flow channel design method thereof of the present invention are further described:

[0053] The present invention discloses a thermoforming die and a design method thereof. Specifically, the present invention is a customized performance thermoforming die based on a lattice structure and a design method thereof. The scheme of the present invention is aimed at forming areas with different performances, and the heat transfer characteristics of the lattice structure are used to regionally regulate the cooling rate of the die, thereby accurately controlling the cooling path of the formed part and realizing the performance customization of the formed part.

[0054] The following describes a thermoforming mold and a design method thereof according to an embodiment of the present disclosure with reference to the accompanying drawings.

[0055] Combination Figure 1 As shown, the mold design method of the present invention comprises the following steps:

[0056] S101: Preliminary design of conformal cooling channels of hot stamping dies based on the highest strength of the formed part;

[0057] Taking the B-pillar of a car as an example, its maximum strength can reach more than 1500Mpa. In a single production cycle, the heat contained in the blank of the formed part is completely taken away by the cooling medium. Combined with the temperature rise, flow rate and physical properties of the cooling medium, as well as the physical properties of the blank, the dimensional parameters of the conformal cooling channel can be obtained - the diameter of the channel, the number of channels and the distance from the center of the channel to the forming die surface.

[0058] S102: Determine the physical property distribution of the formed part according to the performance requirements of the hot stamping formed part;

[0059] Taking the hot-stamped automobile B-pillar made of boron steel as an example, the upper part is required to maintain high strength to resist deformation to protect the safety of passengers, while the lower part is required to have better toughness to absorb the energy generated by the collision.

[0060] S103: determining the cooling rate at each position of the formed part according to the distribution of physical properties of the formed part and the relationship between the final performance of the formed part and the cooling rate;

[0061] For steel plates made of 22MnB5, when the cooling rate is greater than 27°C / s, the microstructure of the hot stamping parts is completely transformed from austenite to martensite, and its strength can reach more than 1500Mpa; when the cooling rate is less than 27°C / s, only part of the austenite in the microstructure of the hot stamping parts is transformed into martensite, its strength is lower, and it has better toughness.

[0062] S104: determining the position of the lattice structure filled with the conformal cooling channel and the size parameters of the lattice structure according to the cooling rates at different positions of the hot stamping part determined in step S103;

[0063] When the inner filling area is filled with a lattice structure, the heat exchange capacity of the conformal cooling channel can be enhanced, the cooling rate of the corresponding position of the formed part is increased, and the position obtains higher strength; when the outer filling area is filled with a lattice structure, it can play a heat insulating role, so that the cooling rate of the corresponding position of the formed part is reduced. When the cooling rate is lower than the critical cooling rate, the position will obtain lower strength.

[0064] For the lattice structure, its enhanced heat transfer or heat insulation capability is controlled by the size parameters of its single lattice cell.

[0065] S105: According to the position and size parameters of the lattice structure determined in step S104, a three-dimensional model of the thermoforming mold is established, and the mold is processed.

[0066] The mold processing method includes but is not limited to additive manufacturing technology, machining technology or a combination of the two.

[0067] The present invention also provides a customized performance thermoforming mold based on a lattice structure, including an upper mold and a lower mold, wherein a conformal cooling channel is provided in the mold, and a partial area of ​​the channel position is filled with a lattice structure.

[0068] The flow channel position is further divided into an inner filling area and an outer filling area, wherein the inner filling area is the space inside the flow channel, and the outer filling area is the area between the surface ∑1 and the surface ∑2 obtained by radially outwardly offsetting the flow channel wall by distances j and l.

[0069] Furthermore, the lattice structure is filled in the inner filling area to enhance the heat exchange capacity and increase the cooling rate of the corresponding position of the formed part, and the lattice structure is filled in the outer filling area to achieve heat insulation capacity and reduce the cooling rate of the corresponding position of the formed part.

[0070] The following is an example of an embodiment to illustrate the customized performance thermoforming mold design method based on a lattice structure provided by the present invention.

[0071] The hot stamping part of this embodiment is a U-shaped part with typical features taken from the B-pillar of an automobile, and its maximum strength can reach more than 1500 MPa.

[0072] Combination Figure 2 In a single production cycle, the heat contained in the blank of the formed part is completely taken away by the cooling medium. Combined with the temperature rise, flow rate and physical properties of the cooling medium, as well as the physical properties of the blank, the dimensional parameters of the conformal cooling channel can be obtained - the diameter of the channel, the number of channels and the distance from the center of the channel to the forming die surface.

[0073] Combination Figure 3 According to the performance requirements of the B-pillar, its upper part needs to resist deformation and therefore has higher strength, while the lower part needs to absorb the energy of the collision and therefore has good toughness and low strength.

[0074] From the performance distribution of the U-shaped part with high strength in the upper part and low strength in the lower part, it can be seen that the cooling rate of the upper part 303 of the formed part should be faster to achieve complete transformation of austenite into martensite and obtain high strength; the lower part 301 of the formed part should have a slower cooling rate to obtain a low-strength structure in which part of the austenite is transformed into martensite, and area 302 is a transition zone.

[0075] Combination Figures 4 to 9 The inner filling area 402 of the conformal cooling channel in region 103 is filled with a lattice structure to enhance its heat exchange capacity, thereby increasing the cooling rate of the corresponding position of the formed part. When the cooling rate is higher than the critical cooling rate of 27°C / s, the position obtains higher strength.

[0076] When the outer filling area 401 in the region 101 is filled with a lattice structure, it can play a heat insulating role, so that the temperature of the region 103 increases, thereby reducing the temperature difference between the plate and the mold, and reducing the cooling rate of the corresponding position of the formed part. When the cooling rate is lower than the critical cooling rate of 27°C / s, the position will obtain lower strength.

[0077] Region 102 is a transition region, which isolates region 101 and region 103 by filling the lattice structure to reduce the thermal impact between the two.

[0078] The size parameters of the lattice structures in the regions 101 and 103 are adjusted to make the cooling rates in the regions consistent.

[0079] The same lattice structure filling operation is performed on the upper mold of the mold to obtain an upper mold that can achieve customized performance.

[0080] According to the mold parameters mentioned above, the three-dimensional models of the upper mold and the lower mold are obtained by modeling with three-dimensional modeling software, and appropriate processing methods are selected to process the mold.

[0081] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0082] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A customized performance thermoforming mold flow channel design method based on a lattice structure, characterized by: The following steps are involved: 1) Based on the maximum strength of the formed part It is required to preliminarily design the conformal cooling channel of hot stamping die; 2) Determine the physical property distribution of the formed parts according to the performance requirements of the hot stamping parts; 3) According to the distribution of physical properties of the formed part, the cooling rate at each position of the formed part is determined by the relationship between the final performance of the formed part and the cooling rate; 4) According to the cooling rates at different positions of the hot stamping part determined in step 3), determine the position of the lattice structure filled with the conformal cooling channel and the size parameters of the lattice structure; 5) According to the position and size parameters of the lattice structure determined in step 4), a three-dimensional model of the thermoforming mold is established, and the mold is processed; The step 4) further comprises the following steps: 4.1) According to the heat transfer law between the sheet, the mold and the cooling medium, the equivalent thermal conductivity of the outer filling area or the equivalent interface heat transfer coefficient of the inner filling area is derived; 4.2) The geometric parameters of the lattice structure are determined by the relationship between the equivalent thermal conductivity and the size parameters of the lattice structure in the outer filling area; the geometric parameters of the lattice structure are determined by the relationship between the equivalent interface heat transfer coefficient and the size parameters of the lattice structure in the inner filling area; The lattice structure filling position in step 4) includes: a. Inner filling area: inside the flow channel; b. External filling area: The channel wall is offset radially outward by a distance of j and l ,in l > j >0, the area between the surface ∑1 and the surface ∑2 is obtained.

2. The customized performance thermoforming mold flow channel design method based on lattice structure according to claim 1 is characterized in that: In step 1) The following steps are involved: 1.1) Determine the maximum strength based on the relationship between forming strength and cooling rate Required cooling speed , quenching time Defined as ,in is the temperature of the plate after heating. is the temperature after quenching; 1.2) In a single hot stamping production cycle, the sheet metal is Cool to The heat released Q Most of it is carried away by the cooling medium, i.e. ,in k is the proportion of heat removed by the cooling medium, ranging from 0.8 to 1. q is the flow rate of cooling medium, t The time for a single production cycle, C is the specific heat capacity of the cooling medium, It is the temperature change before and after the cooling medium flows through the mold; Cooling medium flow , n is the number of cooling channels, d is the cooling channel diameter, v is the cooling medium flow rate; Single production cycle time , is the quenching time, is the transfer time before and after sheet forming; 1.3) Solve for the diameter of the cooling channel d ,quantity n , the cooling channels are evenly arranged on the equidistant surface of the mold forming surface, and the distance between the equidistant surface and the forming surface is H ; 1.4) Check the strength of the mold structure and take ,in λ is the magnification factor, To meet the strength requirements of the distance H The minimum value of .

3. The customized performance thermoforming mold flow channel design method based on lattice structure according to claim 1 is characterized in that: The step 3) also includes the following steps: 3.1) Determine the microstructure composition of each position of the formed sheet according to the physical properties, wherein the microstructure composition includes the volume fraction of martensite, bainite, pearlite and ferrite; 3.2) According to the cooling phase transformation law of the alloy plate of the CCT curve, the cooling rate of the formed part is deduced from the microstructure composition.

4. The customized performance thermoforming mold flow channel design method based on lattice structure according to claim 1 is characterized in that: The equivalent thermal conductivity of the outer filling zone is the thermal conductivity when the outer filling zone is filled with a uniform single medium to meet the cooling rate of the formed part, and the equivalent interface heat transfer coefficient of the inner filling zone is the interface heat transfer coefficient of the inner wall of the flow channel that is consistent with the cooling effect when the inner filling zone is filled with a lattice structure.

5. The customized performance thermoforming mold flow channel design method based on lattice structure according to claim 1 is characterized in that: The cooling rate of the formed part within the mold is controlled by the location of the filled lattice structure and the heat transfer characteristics which are affected by dimensional parameters.

6. The customized performance thermoforming mold flow channel design method based on lattice structure according to claim 1 is characterized in that: A transition area is provided between mold positions corresponding to adjacent areas of the formed part having different physical properties; The transition zone is filled with a lattice structure to separate different areas of the mold without changing the integrity of the mold and the cooling channel, thereby reducing or eliminating the thermal impact between different areas.

7. The customized performance thermoforming mold flow channel design method based on lattice structure according to claim 6 is characterized in that: The lattice structure is composed of single cells that can be arranged periodically, and the heat transfer characteristics of the lattice structure are controlled by the size parameters of the cells; The cell is the smallest unit that is periodically and regularly arranged in a lattice structure and has symmetry.

8. A customized performance thermoforming mold based on a lattice structure, comprising an upper mold and a lower mold, characterized in that: The mold is provided with conformal cooling channels, and some areas of the channel positions are filled with a lattice structure; The area of ​​the flow channel position includes an inner filling area and an outer filling area, wherein the inner filling area is the space inside the flow channel, and the outer filling area is the space where the flow channel wall is radially offset outward by a distance of j and l The area between the surface ∑1 and the surface ∑2 is obtained, where l > j >0.

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