CN115828445A - Method for researching high-pressure forming defects in stainless steel welded pipe - Google Patents
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Abstract
The invention discloses a method for researching high-pressure forming defects in a stainless steel welded pipe, which comprises the steps of analyzing forming factors according to material parameters and process parameters; establishing a pipe mathematical model, and carrying out stress analysis on the high-pressure forming process in the pipe, wherein the stress analysis comprises pipe wall mechanical analysis, unit body stress analysis, maximum internal stress, cracking pressure and stress strain analysis; the method comprises the following steps of (1) researching problems generated in a forming process; the invention establishes a finite element analysis model capable of accurately predicting the whole internal high-pressure forming process, provides process improvement measures, solves the forming defect of the pipe fitting, greatly improves the production efficiency and the yield and improves the economic benefit of enterprises.
Description
Technical Field
The invention relates to the field of stainless steel welded pipes, in particular to a method for researching a high-pressure forming defect in a stainless steel welded pipe.
Background
At present, the tube hydroforming technology is more and more widely applied to the aviation, aerospace and automobile manufacturing industries, and various hollow variable-section tube fittings produced by the process have lighter structures. Taking the structural member of the automobile chassis as an example, because different parts bear different loads, the requirements on the strength and the rigidity of each part are different. For the parts with high requirements on strength and rigidity, the section modulus and the bending and torsion resistance can be improved by changing the section shape and the size or adopting a higher-strength material.
In order to increase the load-bearing capacity of the component without increasing the mass of the component too much, there are two methods: one is that it can be designed to be a thicker member, i.e. a greater thickness where higher strength and stiffness are required and a lesser thickness where the load is less. For example, in the manufacture of automobile parts, the weight of the parts can be reduced by 33% by designing the wall thickness of the parts according to the load distribution. And secondly, materials with different densities can be adopted to manufacture a dissimilar material tailor-welded structural member, for example, aluminum alloy and steel are welded, high-strength materials are adopted at parts with high strength requirements, and low-density low-strength materials such as aluminum alloy and the like are adopted at parts with small bearing capacity.
Therefore, in order to reduce the generation of defects in the high-pressure forming process of the stainless steel welded pipe, the reasons for the defects in the production process need to be analyzed, and the forming mechanism and the forming key technology of the defects in the bulging process need to be researched. The method explores the influence rule of factors such as pipe blank material parameters, process parameter control, mold condition parameters and the like related to the pipe fitting hydraulic forming technology, and provides a process improvement measure by establishing a finite element analysis model capable of accurately predicting the whole internal high-pressure forming process, so that the pipe fitting forming defect is solved, the production efficiency and the yield are greatly improved, and the economic benefit of an enterprise is improved.
Disclosure of Invention
This section is for the purpose of summarizing some aspects of embodiments of the invention and to briefly introduce some preferred embodiments. In this section, as well as in the abstract and the title of the invention of this application, simplifications or omissions may be made to avoid obscuring the purpose of the section, the abstract and the title, and such simplifications or omissions are not intended to limit the scope of the invention.
The present invention has been made keeping in mind the above problems occurring in the prior art.
Therefore, the technical problem to be solved by the invention is the defect of high-pressure forming in the stainless steel welding pipe.
In order to solve the technical problems, the invention provides the following technical scheme: a high-pressure forming defect research method in a stainless steel welded pipe comprises analyzing forming factors according to material parameters and process parameters;
establishing a pipe mathematical model, and carrying out stress analysis on the high-pressure forming process in the pipe, wherein the stress analysis comprises pipe wall mechanical analysis, unit body stress analysis, maximum internal stress, cracking pressure and stress strain analysis;
the method comprises the following steps of (1) researching problems generated in a forming process;
and carrying out numerical simulation on the internal high-pressure forming process by adopting finite element analysis software, carrying out test verification on the loading path and parameters simulated by the finite element, and perfecting the process parameters through test results.
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: the mechanical analysis of the pipe wall comprises the following steps:
assuming that the volume of the tube blank can be considered incompressible at each stage of the tube forming, i.e.:
ε θ +ε L +ε t ＝0
in the formula: epsilon theta, epsilon L and epsilon t respectively represent hoop strain, axial strain and radial strain;
the ratio of thickness to radius, t0/r0< <1 (t 0, r0 respectively represent the initial thickness and initial radius of the tube blank), assuming that the stress and strain in the thickness direction are uniformly distributed, and the radial stress sigma t =0 perpendicular to the tube wall;
assuming that the stress-strain relationship of the unit body in the middle of the bulging area meets the hardening power exponential function in the bulging process:
in the formula:
respectively representing equivalent stress and equivalent strain in the deformation process, wherein n is the hardening index of the material, and K is the strength coefficient of the material;
assuming that the pipe is a continuous medium, the thickness of the whole pipe wall is uniform, no stress mutation point exists, the neutral surface of the pipe blank meets the requirement of continuity, and the analysis is carried out by adopting a thin shell theory;
the pipe is assumed to be isotropic, i.e. the plasticity index R =1.
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: the unit body stress analysis comprises the following steps:
expressions for hydroforming internal and axial forces:
as a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: the maximum internal stress includes:
establishing an equilibrium equation yields:
where t is the thickness of the pipe, it can be seen that if the internal pressure is increased, the axial pressure F L The stress concentration of the pipe at the convex part of a partial forming area can cause the cracking of the pipe after the groove rapidly enters the concentrated instability along the bus because of the stress concentration, so when the dispersion instability occurs, the annular bearing capacity (sigma) is realized θ t) to d (σ) θ t) =0, and the maximum internal pressure capable of being applied at the time is the maximum bulging pressure P c ；
Setting axial stress sigma L And hoop stress sigma θ Ratio of (a) L /σ θ In the actual internal high-pressure forming process, due to the existence of axial feeding, the stress state changes greatly in different stages, and the axial feeding and the internal pressure are required to be matched with each other, generally speaking, the alpha is gradually increased from the negative direction to the positive direction along with the deepening of the forming process; for the ideal state of high pressure forming in pipes, r L ＞＞r θ To obtain the bulging pressure
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: the cracking pressure P in the pure bulging of the cracking pressure b Can be calculated by the following formula
In the formula: sigma b -tensile strength (MPa) of the material.
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: the stress-strain analysis comprises:
according to the theory of stress increment, the following results are obtained:
wherein
Respectively representing radial, circumferential and axial stress offsets;
equation 26 shows that the increase in the thickness strain is determined by both the hoop and axial stresses, if σ θ +σ L <0, then d ε t >0, the axial stress is greater than the circumferential stress at the moment, the unit is in a material supplementing state, so the thickness of the pipe wall is increased, if the axial stress is too large and the material supplementing amount is too large, the phenomena of wrinkling and instability are easily generated, and if the sigma is too large θ +σ L >0, then d ε t <0, the circumferential tensile stress of the tube wall is larger than the compressive stress generated by axial material supplement, the wall thickness of the tube blank is reduced, if the circumferential tensile stress is overlarge, the tube wall is seriously thinned, and even the tube can be cracked; if σ is θ +σ L If not less than 0, then d epsilon t ＝0。
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: testing the mechanical properties of a welded pipe matrix and a welding seam material through a tensile test and a microhardness test, determining the shape, the size and the material characteristics of a welded pipe seam and a heat affected zone, and establishing the constitutive relation of materials at different positions of the heat affected zone of the welded pipe;
based on the constitutive relation among a matrix, a welding seam and a heat affected zone and the characteristics of hydraulic forming of the welding seam pipe, selecting a proper damage model, researching the establishment method of a hydraulic forming finite element model of the welding seam pipe, considering the influence of the welding seam and the heat affected zone, and mainly determining a geometric model, a material model and boundary conditions of the welding seam pipe;
carrying out a hydroforming test, and researching the influence rule of the weld orientation on the hydroforming rule such as profile shape, wall thickness distribution, limit pressure and forming limit during different tube blank performances, loading paths, axial feeding and bulging;
and (3) analyzing the forming mode and mechanism of the hydraulic forming defects of the welded pipe by combining a plastic mechanics forming theory and finite element numerical simulation, determining a key technology of the hydraulic forming of the welded pipe, and providing a process optimization measure.
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: the material parameters are subjected to test analysis including,
micro-hardness test, tensile test, weld microstructure observation and performance test of stainless steel base metal.
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: analyzing the technological parameters, establishing material constitutive relation, numerically simulating the hydraulic forming of welded pipe to determine the material characteristics of welded seam and heat affected zone, measuring the microhardness of welded seam and its adjacent area directly with empirical formula (27) to obtain the flow stress of welded seam and heat affected zone,
in the formula:
flow stresses, HV, of the weld and of the base body, respectively weld ，HV sh eet The method comprises the steps of respectively determining the microhardness of a welding seam and a base body, conveniently obtaining the constitutive relation of materials at different positions of a heat affected zone by using an empirical formula according to the microhardness and the constitutive relation of the base body and welding seam materials, obtaining the corresponding strength coefficient K and the strain hardening index n of the materials of the heat affected zone, using the obtained corresponding strength coefficient K and the strain hardening index n as material input parameters of a subsequent numerical simulation model of a finite element of the welded seam pipe, determining damage parameters of a GTN model through notch tensile tests with different sizes and finite element simulation fitting, and determining the damage parameters of the damage model.
As a preferred scheme of the method for researching the high-pressure forming defects in the stainless steel welded pipe, the method comprises the following steps: simulating a hydraulic forming finite element numerical value, establishing a welding seam pipe hydraulic forming finite element model considering a welding seam and a heat affected zone by utilizing finite element analysis software DYNAFORM, and performing numerical simulation on a pipe hydraulic forming finite element numerical value by the following basic steps: establishing a geometric model, selecting the types of pipes and mould units, dividing grids, setting material characteristics and boundary conditions, solving finite elements and post-processing;
carrying out a hydroforming experiment, selecting different welded pipes to carry out the hydroforming experiment in combination with the on-site production requirement, researching the influence of different pipe blank performances and different hydroforming processes (forming pressure and loading mode, axial feeding, forming time, friction coefficient and the like) on the hydroforming rule of the welded pipes, analyzing the changes of factors such as the profile of the welded pipes after hydroforming, the wall thickness distribution of the welded pipes and the like, analyzing the forming defects of the welded pipes in combination with finite element simulation results, discussing the defect forming mechanism and providing improvement measures.
The invention has the beneficial effects that: by establishing a finite element analysis model capable of accurately predicting the whole internal high-pressure forming process, technological improvement measures are provided, the forming defects of the pipe fittings are overcome, the production efficiency and the yield are greatly improved, and the economic benefits of enterprises are improved.
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In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise. Wherein:
FIG. 1 is a graph of a matching curve in an embodiment of the present invention.
Fig. 2 is a typical defect diagram of internal high pressure forming in an embodiment of the present invention.
Fig. 3 is a process diagram of internal high pressure forming in an embodiment of the present invention.
Fig. 4 is a process diagram of internal high pressure forming in an embodiment of the present invention.
FIG. 5 is a force diagram of a tubing unit in an embodiment of the present invention.
FIG. 6 is a diagram of a microcell diagram in an embodiment of the present invention.
FIG. 7 is a stress-strain state diagram for hydroforming a tube in an embodiment of the present invention.
FIG. 8 is a sectional view of the tube blank wall surface in the embodiment of the invention.
FIG. 9 is a diagram of the measurement position of microhardness of the welded pipe in the embodiment of the invention.
FIG. 10 is a drawing of a tensile specimen in an example of the invention.
FIG. 11 is a logic diagram of the research in the embodiment of the present invention.
FIG. 12 is a stress diagram of the tube blank material in the embodiment of the invention.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described and will be readily apparent to those of ordinary skill in the art without departing from the spirit of the present invention, and therefore the present invention is not limited to the specific embodiments disclosed below.
Furthermore, reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.
Example 1
Referring to fig. 1 to 4, a first embodiment of the present invention provides a method for researching high pressure forming defects in a stainless steel welded pipe, which is characterized in that: analyzing forming factors according to material parameters and process parameters;
establishing a pipe mathematical model, and carrying out stress analysis on the high-pressure forming process in the pipe, wherein the stress analysis comprises pipe wall mechanical analysis, unit body stress analysis, maximum internal stress, cracking pressure and stress strain analysis;
the method comprises the following steps of (1) researching problems generated in a forming process;
numerical simulation is carried out on the internal high-pressure forming process by adopting finite element analysis software, a loading path and parameters simulated by the finite element are tested and verified, and technological parameters are perfected according to test results;
the stainless steel welded pipe hydroforming is a complex deformation process, has many factors influencing forming, and is mainly divided into material parameters and process parameters. The material parameters mainly comprise the plastic property of the material, the shape and the size of the pipe and the like; the technological parameters mainly comprise a die cavity structure, lubrication conditions, axial feeding, an internal pressure loading mode and the like. The factors are mutually connected and mutually restricted, and the forming process is not the result under the influence of a single factor but the result of mutually matching various parameters. Various defects and failures in the internal high-pressure forming process are generally caused by the problems of parameter matching. The most influential factor is the matching of the internal pressure loading path (curve) with the axial feed. As shown in fig. 1, acceptable pipe can only be produced if the axial thrust (displacement) and internal pressure are well matched;
typical defects of the inner high pressure formed tube can be classified into three categories, fracture, corrugation, buckling, as shown in fig. 2. Buckling (Bucking) generally occurs in the initial stage of forming, axial feeding thrust is too large, internal pressure is too small, axial instability is caused, and the axis of the pipe is deflected and bent;
the wrinkles are divided into beneficial wrinkles and dead wrinkles, similar to buckling, the wrinkles are caused by too fast axial feeding and insufficient internal pressure, but part of wrinkles can be flattened in a later forming stage by increasing the internal pressure, the reduction rate of the wall thickness of the part can be reduced, and the forming performance of the part is improved, and the wrinkles are called as beneficial wrinkles. Dead wrinkles cannot be flattened in a manner of increasing internal pressure, which is generally caused by the fact that the axial feed is too large and the die cavity does not have enough space to flatten the excessive material;
the Bursting (Bursting) is mainly caused by that the internal pressure is increased too fast in the forming process and the axial feeding is not timely, so that the thinning rate of a bulging area is excessive, the strength of a formed part is influenced, and even the Bursting failure is caused. Depending on the stage of rupture, there are further categories of initial rupture, intermediate rupture and late rupture. The initial and middle fractures are mainly caused by the condition that the internal pressure rises too fast and the axial feeding supplement is not timely. Therefore, designing the optimal process parameters and matching the parameters reasonably is the key to eliminating the early and middle cracks. The late-stage fracture is mainly caused by hidden forming defects of the tube blank due to unreasonable process parameters in the early-stage forming stage, the defects are caused by fracture due to overlarge pressure in the later-stage shaping stage, and the fracture has great relation with the friction coefficient and the shape and the size of the die;
therefore, in order to reduce the generation of defects in the high-pressure forming process of the stainless steel welded pipe, the reasons for the defects in the production process need to be analyzed, and the forming mechanism and the forming key technology of the defects in the bulging process need to be researched. Exploring the influence rule of pipe blank material parameters, process parameter control, mold condition parameters and other factors related to the pipe fitting hydraulic forming technology, establishing a finite element analysis model capable of accurately predicting the whole internal high-pressure forming process, proposing process improvement measures, solving the pipe fitting forming defect, greatly improving the production efficiency and the yield and improving the enterprise economic benefit;
the pipe internal high pressure forming is that the pipe is used as a processing blank, the pipe is placed in a lower die cavity with a specified shape, an upper die and a lower die are closed to form a die cavity, then a certain internal pressure and axial feeding at two ends are simultaneously applied to the pipe, and the pipe blank is processed into a hollow part which is consistent with the die cavity. The tube hydroforming process is shown in fig. 3, and the process can be divided into 3 stages: (1) in the filling stage, the pipe is placed in the lower die, then the upper die is closed, liquid is filled into the pipe after the die is locked until the pipe is filled, gas in the pipe is removed, and meanwhile, two ends of the pipe are sealed by horizontal punches; (2) in the forming stage, after the pipe is filled with liquid and sealed, the liquid in the pipe is pressurized under the action of a pressurizing cylinder to enable the pipe to be gradually expanded, meanwhile, punches at two ends push the supplementary material inwards according to a set curve, the pipe basically clings to the inner wall of the die under the combined action of the internal pressure and the axial supplementary material, and at the moment, most of the area except the fillet of the transition area is formed; (3) and in the shaping stage, the forming pressure is continuously increased to enable the fillet of the transition area to be completely attached to the die to be formed into the required workpiece.
When the axis of the part is not a straight line and the local minimum section of the part is smaller than the section of the pipe blank, pre-bending, pre-stamping and other pre-forming processes are needed so that the pipe blank can be placed in a die, the shape of the pipe blank is close to the shape of the part, and liquid is filled for forming.
Example 2
Referring to fig. 5-8, a second embodiment of the present invention is based on the previous embodiment, and fig. 5 shows the stress situation of the unit body in the tube hydroforming process, during the forming, the tube blank unit is acted by the internal hydraulic pressure and the axial feed force, meanwhile, the radial stress of the tube is neglected, and the stress-strain study in this state can be realized by thin shell theory. It can be assumed that the deformation of the pipe conforms to the thin shell theory, and the following assumptions are made when performing mechanical analysis on the pipe wall units:
s1, the volume of the pipe blank can be considered to be incompressible in each stage of pipe forming, namely:
ε θ +ε L +ε t ＝0
in the formula: ε θ, ε L and ε t represent hoop strain, axial strain and radial strain, respectively.
S1.2, because the pipe wall is thin, the ratio of the thickness to the radius t0/r0< <1 (t 0, r0 respectively represent the initial thickness and the initial radius of the pipe blank), assuming that the stress and the strain in the thickness direction are uniformly distributed, and the radial stress sigma t =0 perpendicular to the pipe wall.
S1.3, in the bulging process, the stress-strain relation of the unit body in the middle of the bulging area meets the hardening power exponential function:
in the formula:
respectively representing equivalent stress and equivalent strain in the deformation process, n is the hardening index of the material, and K is the strength coefficient of the material.
S1.4, the pipe is a continuous medium, the thickness of the whole pipe wall is uniform, stress mutation points do not exist, and the neutral surface of the pipe blank meets the requirement of continuity, so that the analysis can be carried out by adopting a thin shell theory. In the bulging process, the elastic deformation is very small and can be ignored, and only the plastic deformation generated in the bulging process is considered.
S1.5 assuming the pipe is isotropic, i.e. the plasticity index R =1.
The stress-strain state of the unit body on the pipe wall is complex during forming, and the stress-strain state of the unit body can be quite different at different stages of the forming process. In order to avoid the defects of wrinkling, breakage, buckling and the like in the forming process, the change of the stress state of the pipe in different stages and different forming areas of the hydraulic forming process must be analyzed.
The stress analysis is carried out on the pipe wall unit in the figure, and the stress of the micro unit is shown in figure 6:
s2, carrying out stress analysis on the microcell bodies, and satisfying the following equation:
in the formula: σ θ -circumferential stress, σ L-axial stress,
-the minor radius of curvature of the cell,
-the principal radius of curvature of the cell. pi is the internal liquid pressure of the pipe, ti is the pipe wall thickness at the unit in the deformation process, and the magnitudes of pi and ti change along with time, and the values are different in different forming stages, and F is the axial thrust at two ends of the pipe blank in the forming process.
S3 equivalent stress and equivalent strain under plane stress can be expressed as:
equivalent stress:
equivalent strain:
in the formula:
α＝(σ L /σ θ )
ε L ＝ln(t i /t 0 )
the circumferential and axial strains epsilon theta and epsilon L of the unit body can be expressed as follows:
ε L ＝ln(t i /t 0 )
in the formula:
-the initial moment, the instantaneous pipe radius of deformation,
t0 and t1 are initial time and deformation instant pipe wall thickness;
from the assumption that the stress-strain relationship and the volume under the plane stress are not changed, the following can be obtained:
α＝(2β+1)(2+β)
or β = (2 α -1) (2- α)
By combining the above formula, the expressions of the internal pressure and the axial force in the hydroforming can be obtained:
the end of the uniform deformation of the axial pressure and circumferential strain of the pipe indicates that the pipe enters a dispersivity instability state. Due to the effect of pipe end constraint, strain is continuous along the axial direction of the pipe but is not uniformly distributed in the bulging process, and bulging phenomenon can occur in partial area. At a certain moment of bulging, the combined effect of the internal pressure and the axial positive balance with the deformation resistance of the pipe, wherein the force balance equation can be established from fig. 6:
where t is the thickness of the pipe, it can be seen that if the internal pressure is increased, the axial pressure F L Will remain unchanged and will
Causing loading instability. The pipe may be cracked after entering into concentrated instability along the bus by the stress concentration at the convex part of the forming area. Therefore, when dispersion destabilization occurs, the hoop bearing capacity (σ) is increased θ t) to d (σ) θ t) =0. The maximum internal pressure which can be applied at this time is the maximum bulging pressure P c 。
Setting axial stress sigma L And hoop stress sigma θ Ratio of (a) L /σ θ And = alpha, when the pipe is formed at internal high pressure, pushing heads at two ends of the pipe are extruded inwards, the axial force is positive pressure, and the value range of alpha is more than or equal to-1 and less than or equal to 1. In the actual internal high-pressure forming process, the stress state changes greatly in different stages due to the existence of axial feeding, and the mutual matching of the axial feeding and the internal pressure is required. Generally, as the forming process advances, α gradually increases from negative to positive. For the ideal state of high pressure forming in pipes, r L ＞＞r θ To obtain the bulging pressure
It can be seen that the internal pressure of the internal high-pressure forming is not only related to the performance parameters and the geometric parameters of the tube material, but also related to the stress state, wherein the most important is the matching relation between the axial feeding and the internal pressure, and the matching quality between the axial feeding and the internal pressure directly determines whether defects such as cracking and the like occur during forming;
s4 cracking pressure P during pure bulging b Can be estimated by the following equation
In the formula: sigma b Tensile Strength of the Material (MPa)
And S5, FIG. 7 is a diagram of the hydroforming limit and yield locus of the pipe, points on the diagram represent possible stress states, and the stress-strain states which may occur in the bulging process can be analyzed through the diagram. The abscissa in the figure represents hoop stress and the ordinate σ Z represents axial stress.
In fig. 7, 1-6 are six different paths, (1-biaxial tension, 2, 6-plane strain, 3-uniaxial tension, 4-pure shear, 5-uniaxial compression), respectively, which represent different stress-strain states, path 1 represents that the strain of the cell in two directions is the same, the main strain and the secondary strain are the same, and are both positive, which represents that the cell is stretched in two directions, in which state the wall thickness of the tube is easily reduced, and the stress magnitude is: σ maj = σ min, σ t =0 (thickness direction stress is 0, reference hypothesis 2, σ maj is the principal stress, σ min is the secondary stress, σ t is the radial (thickness direction) stress, the same applies below); the magnitude of the strain is: ε maj = ε min, ε t =0 (ε maj is the primary strain, ε min is the secondary strain, ε t is the radial (thickness) strain, the same applies below). Path 2 represents the tube being stretched in one direction and zero strain in the other, the stress at this state being: σ maj = σ t (in the thickness direction), σ min =0; the strain state is: ε maj > ε min > 0, ε t =0.
As deformation occurs between path 4 and path 6, the wall thickness increases, and the closer to path 6, the greater the increase in wall thickness. When the deformation path is between 3 and 6, the tube may wrinkle due to the compressive stress in the axial direction, and if the axial feeding is well matched with the internal pressure, the wrinkling may occur in the initial stage of forming, but in the final stage of forming, the wrinkling disappears due to the high pressure, so that the tube is prevented from being excessively thinned, and the thickness of the formed piece is increased, and the wrinkling occurring in the initial stage is called "beneficial wrinkling". During the shaft bulging, it is advantageous to maintain the stress state between point 3 and point 4 in fig. 7 for improving the forming quality.
As shown in fig. 8, in the tube hydroforming process, the tube blank wall surface can be divided into three areas according to the different stress conditions and friction states of different parts of the tube: a force transfer area (feeding area), a transition area and an expansion area. Because the tube blank required by forming is a thin-walled tube, the change of the tube wall thickness has obvious influence on the forming process and the forming result, and key analysis needs to be carried out on the tube blank.
According to the theory of stress increment:
dε t ＝-(dε θ +dε L )
from the above can be obtained
In the formula
And
plastic strain strength increments and stress strength increments, respectively.
According to the definition of the stress offset,
and
respectively as follows:
the two are brought into the state of neglecting the magnitude of the radial stress, then the method can be obtained
The thick strain increment is determined by the circumferential (annular) stress and the axial stress together, if sigma θ +σ L <0, then d ε t >0, where the axial stress is greater than the circumferential stress, the cell is in a fed state and thus the wall thickness will increase. If the axial stress is too large and the feed supplement amount is too large, wrinkling and instability phenomena are easy to occur. If σ is θ +σ L >0, then d ε t <0 the tensile stress of the circumferential direction of the pipe wall is larger than the compressive stress generated by axial feed supplementThe wall thickness of the tube blank is reduced, if the circumferential tensile stress is too large, the tube wall is seriously thinned, and even the tube can be cracked; if σ θ +σ L If not less than 0, then d epsilon t =0, in this case, the wall thickness of the tube blank does not change, and therefore wrinkling or rupture defects do not occur.
Example 3
Referring to fig. 9 to 11, a third embodiment of the present invention, which is based on the previous embodiment,
the stainless steel automobile exhaust pipe is prepared by adopting an internal high-pressure forming mode, so that the influence factors are more, and the material parameters are one of important influence factors. Before the internal high-pressure forming process, the processes of rolling, bending and forming a stainless steel plate, laser welding pipe making, pre-bending and forming and the like are carried out, wherein the laser welding obviously causes the difference of various properties of materials between a welding seam heat affected zone and the stainless steel base material, and causes the existence of residual stress, the annular ductility of a pipe blank is reduced, and the process is also an important aspect that the material parameters influence the internal high-pressure forming cracking. Therefore, in order to improve the yield of the internal high-pressure forming process, firstly, the performance parameters of the material per se are tested and analyzed;
the microhardness of the welded pipe is tested by adopting a HYST-1000ZA03050708 microhardness tester, the surface of the pipe blank to be measured is polished by using sand paper, and strip-shaped coordinate paper is stuck on the surface of the pipe fitting. And clamping the pipe fitting by using a clamp, and placing the pipe fitting on a test bed, wherein the central line of the welding line is opposite to the measurement starting point. In order to reduce the measurement error, two more points are measured near the axial direction of the pipe at the same circumferential position, namely 3 times in total, and the average value is taken. This was repeated until one week was measured, as shown in FIG. 10. The loading time of the hardness test is 6s, the loading force is 50kgf, the observation magnification is 400X, and the measurement interval is 1mm;
the tensile strength of the test specimen was tested using a 3382 model all electronic universal material tester. The tensile test piece and the test process are carried out according to the standard JS. Two samples of each sample are taken, a matrix and a welding line material are directly cut on a welded pipe by using a cutting technology, then a unidirectional tensile test is respectively carried out, and the sizes of the samples are shown in figure 10.
Observing and analyzing the precipitation of a second phase in the tensile fracture and the heat affected zone by using a Quanta 650FEG field emission scanning electron microscope, and analyzing the precipitated phase and the components of the precipitated phase;
the performance test method of the stainless steel parent metal refers to national standard GB/T-228-2002 and the like, and tests the mechanical property and physical property parameters of a sample;
the internal high-pressure forming is a complex forming process under the combined action of internal pressure and axial feeding, the forming process has more influence factors, and besides material performance, the friction between the pipe and the die, the axial feeding of the pipe, the internal pressure and other forming process parameters have important influence factors. In production, under the condition of certain material properties, the reasonable matching of axial feeding and internal pressure is a key factor for determining the success and failure of internal high-pressure forming. The primary failure modes of internal high pressure forming are buckling and cracking. If the internal pressure is too high, excessive thinning can cause cracking; if the axial feed is too great, buckling or wrinkling of the tube may be induced. In order to obtain reasonable internal high pressure forming process parameters, namely a reasonable matching relation between axial feeding and internal pressure loading, a large amount of numerical simulation and calculation are required.
Numerical simulation of the tube hydroforming is to determine the material characteristics of the weld and the heat affected zone, and an empirical formula method based on microhardness testing can be adopted. And (4) directly measuring the microhardness of the welding seam and the adjacent area thereof, and obtaining the flow stress of the welding seam and the heat affected zone by using an empirical formula.
In the formula:
flow stresses, HV, of the weld and of the base body, respectively weld ，HV sh eet The microhardness of the weld and the base, respectively. According to the microhardness and the constitutive relation of the base body and the welding seam material, the constitutive relation of materials at different positions of the heat affected zone can be conveniently obtained by using an empirical formula. The obtained material of the heat affected zone has a corresponding strength coefficient K and should be hardenedAnd converting the index n as a material input parameter of a subsequent weld pipe finite element numerical simulation model. Determining damage parameters of the GTN model through notch tensile tests with different sizes and finite element simulation fitting, and determining the damage parameters of the damage model;
and establishing a weld joint pipe hydraulic forming finite element model considering a weld joint and a heat affected zone by using finite element analysis software DYNAFORM. DYNAFORM is a large dynamic explicit finite element simulation software with static implicit analysis function, which is introduced by ETA of America and comprises a preprocessing module, a calculation module and a post-processing module according to functions like other finite element software. The DYNAFORM can predict the cracking, wrinkling, thinning, scratching and rebounding in the forming process and evaluate the forming performance of the plate, thereby providing help for the material forming process and the die design;
the basic steps of the finite element numerical simulation of the tube hydroforming are as follows: establishing a geometric model, selecting the types of pipes and mould units, dividing grids, setting material characteristics and boundary conditions, solving finite elements and post-processing;
according to the requirements of field production, different welded pipes are selected to perform a hydroforming experiment, the influence of different pipe blank performances and different hydroforming processes (forming pressure and loading mode, axial feeding, forming time, friction coefficient and the like) on a hydroforming rule of the welded pipe is researched, the changes of factors such as the profile of the welded pipe after hydroforming, the wall thickness distribution of the welded pipe and the like are analyzed, the forming defects of the welded pipe are analyzed according to a finite element simulation result, the defect forming mechanism is discussed, and improvement measures are provided.
Example 4
Referring to fig. 12, a fourth embodiment of the present invention is based on the previous embodiment, and the present implementation proposes mesh partitioning for finite element modeling.
Meshing is an important component of finite element modeling. The number and size of the meshes directly affect the accuracy and computational load of the finite element analysis. Since the smaller the mesh, the higher the accuracy of the finite element analysis, and the closer the analysis result to the actual result is obtained. The grid cannot be too large to ensure the accuracy of the simulation. However, the smaller the mesh size, the greater the number of meshes and the more time required for calculation. And the finite element calculation uses a central difference method, and the side length of the minimum unit directly determines the calculation time step length. Therefore, if the mesh size is too small and the mesh size is too large, the time spent on the finite element numerical simulation will be greatly increased, which greatly reduces the workload of the finite element simulation of the workpiece. The following table is a statistical table of the nodes and units of the study model of the present invention.
In the present embodiment, the sealing head and the mold at both ends are not the main subject of study, so they are regarded as rigid bodies, and the thickness is set by default. The present embodiment is preferably an ST16 ultra low carbon steel, considering that the large deformation of hydroforming has high requirements on the plasticity of the material. Fig. 12 is a stress curve of the tube blank material of the embodiment.
It should be noted that the above-mentioned embodiments are only for illustrating the technical solutions of the present invention and not for limiting, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered by the claims of the present invention.
Claims (10)
1. A method for researching high-pressure forming defects in a stainless steel welded pipe is characterized by comprising the following steps: comprises the steps of (a) preparing a substrate,
analyzing forming factors according to material parameters and process parameters;
establishing a pipe mathematical model, and carrying out stress analysis on the high-pressure forming process in the pipe, wherein the stress analysis comprises pipe wall mechanical analysis, unit body stress analysis, maximum internal stress, cracking pressure and stress strain analysis;
the method comprises the following steps of (1) researching problems generated in a forming process;
and carrying out numerical simulation on the internal high-pressure forming process by adopting finite element analysis software, carrying out test verification on the loading path and parameters simulated by the finite element, and perfecting the process parameters through test results.
2. The method for researching the high-pressure forming defects in the stainless steel welded pipe according to claim 1, characterized in that: the mechanical analysis of the pipe wall comprises the following steps:
assuming that the volume of the tube blank can be considered incompressible at each stage of the tube forming, i.e.:
ε θ +ε L +ε t ＝0
in the formula: epsilon theta, epsilon L and epsilon t respectively represent hoop strain, axial strain and radial strain;
the ratio of thickness to radius, t0/r0< <1 (t 0, r0 respectively represent the initial thickness and initial radius of the tube blank), assuming that the stress and strain in the thickness direction are uniformly distributed, and the radial stress sigma t =0 perpendicular to the tube wall;
assuming that the stress-strain relationship of the unit body in the middle of the bulging area meets the hardening power exponential function in the bulging process:
in the formula: sigma and epsilon respectively represent equivalent stress and equivalent strain in the deformation process, n is the hardening index of the material, and K is the strength coefficient of the material;
supposing that the pipe is a continuous medium, the thickness of the whole pipe wall is uniform, no stress mutation point exists, the neutral surface of the pipe blank meets the requirement of continuity, and the analysis is carried out by adopting a thin shell theory;
the pipe is assumed to be isotropic, i.e. the plasticity index R =1.
3. The method for researching the high-pressure forming defects in the stainless steel welded pipe according to claim 2, characterized in that: the unit body stress analysis comprises the following steps:
expressions for hydroforming internal and axial forces:
4. the method for researching the high-pressure forming defects in the stainless steel welded pipe according to claim 1, characterized in that: the maximum internal stress includes:
establishing an equilibrium equation yields:
where t is the thickness of the pipe, it can be seen that if the internal pressure is increased, the axial pressure F L The stress concentration of the pipe at the convex part of a partial forming area can cause the cracking of the pipe after the groove rapidly enters the concentrated instability along the bus because of the stress concentration, so when the dispersion instability occurs, the annular bearing capacity (sigma) is realized θ t) to d (σ) θ t) =0, and the maximum internal pressure which can be applied at the moment is the maximum bulging pressure P c ；
Setting axial stress sigma L And hoop stress sigma θ Ratio of (a) L /σ 0 In the actual internal high-pressure forming process, due to the existence of axial feeding, the stress state changes greatly in different stages, and the axial feeding and the internal pressure are required to be matched with each other, generally speaking, the alpha is gradually increased from the negative direction to the positive direction along with the deepening of the forming process; for the ideal state of high pressure forming in pipes, r L ＞＞r θ To obtain the bulging pressure
5. The method for researching the high-pressure forming defects in the stainless steel welded pipe as claimed in claim 1, wherein the method comprises the following steps: the cracking pressure P in the pure bulging of the cracking pressure b Can be calculated by the following formula
In the formula: sigma b -tensile strength (MPa) of the material.
6. The method for researching the high-pressure forming defects in the stainless steel welding pipe according to claim 5, wherein the method comprises the following steps: the stress-strain analysis comprises:
according to the theory of stress increment, the following results are obtained:
equation 26 shows that the thick-direction strain increment is determined by the circumferential (annular) stress and the axial stress together, if σ θ +σ L <0, then d ε t >0, the axial stress is greater than the circumferential stress at the moment, the unit is in a material supplementing state, so the thickness of the pipe wall is increased, if the axial stress is too large and the material supplementing amount is too large, the phenomena of wrinkling and instability are easily generated, and if the sigma is too large θ +σ L >0, then d ε t <0, the tensile stress of the tube wall in the circumferential direction is larger than the compressive stress generated by axial material supplement, the wall thickness of the tube blank is reduced, if the tensile stress in the circumferential direction is too large, the tube wall is seriously thinned, and even the tube can be cracked; if σ θ +σ L If not less than 0, then d epsilon t ＝0。
7. The method for researching the high-pressure forming defects in the stainless steel welded pipe, according to claim 6, is characterized in that:
testing the mechanical properties of a welded pipe matrix and a welding seam material through a tensile test and a microhardness test, determining the shape, the size and the material characteristics of a welded pipe seam and a heat affected zone, and establishing the constitutive relation of materials at different positions of the heat affected zone of the welded pipe;
based on the constitutive relation of a matrix, a welding seam and a heat affected zone and the characteristics of hydraulic forming of a welding seam pipe, selecting a proper damage model, researching the establishment method of a finite element model for hydraulic forming of the welding seam pipe, considering the influence of the welding seam and the heat affected zone, and mainly determining a geometric model, a material model and boundary conditions of the welding seam pipe;
carrying out a hydroforming test, and researching the influence rule of the weld orientation on the hydroforming rule such as profile shape, wall thickness distribution, limit pressure and forming limit during different tube blank performances, loading paths, axial feeding and bulging;
and (3) analyzing the forming mode and mechanism of the hydraulic forming defect of the welded pipe by combining a plastic mechanical forming theory and finite element numerical simulation, determining a key technology of the hydraulic forming of the welded pipe, and providing a process optimization measure.
8. The method for researching the high-pressure forming defects in the stainless steel welded pipe according to claim 1, characterized in that: the material parameters are subjected to test analysis including,
microhardness test, tensile test, weld microstructure observation and performance test of stainless steel parent metal.
9. The method for researching the high-pressure forming defects in the stainless steel welding pipe according to claim 8, wherein the method comprises the following steps: analyzing the technological parameters, establishing material constitutive relation, numerically simulating the hydraulic forming of welded pipe to determine the material characteristics of welded seam and heat affected zone, measuring the microhardness of welded seam and its adjacent area directly with empirical formula (27) to obtain the flow stress of welded seam and heat affected zone,
in the formula:
flow stresses, HV, of the weld and of the base body, respectively weld ，HV sheet The method comprises the steps of respectively determining the microhardness of a welding seam and a base body, conveniently obtaining the constitutive relation of materials at different positions of a heat affected zone by using an empirical formula according to the microhardness and the constitutive relation of the base body and welding seam materials, obtaining the corresponding strength coefficient K and the strain hardening index n of the materials of the heat affected zone, using the obtained corresponding strength coefficient K and the strain hardening index n as material input parameters of a subsequent numerical simulation model of a finite element of the welded seam pipe, determining damage parameters of a GTN model through notch tensile tests with different sizes and finite element simulation fitting, and determining the damage parameters of the damage model.
10. The method for researching the high-pressure forming defects in the stainless steel welding pipe according to claim 9, wherein the method comprises the following steps: simulating a hydraulic forming finite element numerical value, establishing a welding seam pipe hydraulic forming finite element model considering a welding seam and a heat affected zone by utilizing finite element analysis software DYNAFORM, and performing numerical simulation on a pipe hydraulic forming finite element numerical value by the following basic steps: establishing a geometric model, selecting the types of pipes and mould units, dividing grids, setting material characteristics and boundary conditions, solving finite elements and post-processing;
carrying out a hydroforming experiment, selecting different welded pipes to carry out the hydroforming experiment in combination with the on-site production requirement, researching the influence of different pipe blank performances and different hydroforming processes (forming pressure and loading mode, axial feeding, forming time, friction coefficient and the like) on the hydroforming rule of the welded pipes, analyzing the changes of factors such as the profile of the welded pipes after hydroforming, the wall thickness distribution of the welded pipes and the like, analyzing the forming defects of the welded pipes in combination with finite element simulation results, discussing the defect forming mechanism and providing improvement measures.
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| CN118690507B (en) * | 2024-08-27 | 2024-11-22 | 安字汽车紧固件(启东)有限公司 | A rivet forming force analysis method and system |
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