Discussion on chemical blackening process of spot welding surface of stainless steel car body
This paper introduces the original blackening process of the trial production project of stainless steel car body, and emphatically describes the defects caused by the original process. The defects caused by the original process method are reproduced by experimental means, and the solutions are determined by analysis; The original process was improved, and a complete set of chemical blackening process rules for spot welding surface of stainless steel car body was determined.
Keywords: stainless steel spot welding; Black removal process; Spot welding of stainless steel body: surface chemistry
China Library ClassificationNo.: U27 1 Document ID: A DocumentNo.:1009-2374 (2014) 09-0047-02.
In the early 1960s, for the sake of lightweight and maintenance-free vehicles, Japan studied and analyzed the use of stainless steel in vehicle structures. At the same time, the manufacture of stainless steel structure by spot welding is studied. 1962, the 7000 series of East Express Electric Railway was completed. Except for a part of the underframe (end underframe), other structures or outer plates were all made of stainless steel. This is the first car called all-stainless steel car today.
Stainless steel car is characterized by maintenance-free, lightweight and unpainted body structure, but this requires the surface state of the body outer plate. After resistance spot welding, black or light yellow scale will be formed on the surface of stainless steel outer panel, which will seriously affect the appearance of the car body. At the same time, due to the change of substrate structure caused by oxidation, solder joints may be electrochemically corroded, so it is necessary to blacken the solder joints on the surface of stainless steel outer plate.
1 Basic principle of electrochemical black removal
Scale burning at solder joints is mainly caused by too long welding time and too short cooling time after welding. Excessive temperature leads to oxidation reaction at the contact part between the electrode of the electrode holder and the external wall panel, forming black or light yellow oxide scale.
Electrochemical black removal mainly uses the principle of electrolytic cell to decompose oxide scale to achieve the effect of black removal, and at the same time forms a passive film on the surface, which plays a role in aesthetics and improving corrosion resistance.
2 Introduction of original technology
2. 1 equipment introduction
Because the electrolytic cell reaction can only be carried out when the power is turned on to form a closed loop and electrolyte exists, it is necessary to meet this requirement through equipment in practical operation.
At present, the equipment used is an imported blackening machine, which consists of a blackening machine body, a grounding wire, a power cord, an electrode rod, cotton cloth and electrode heads suitable for different parts, and special cleaning solution and neutralization solution are needed for operation.
2.2 Introduction to the original process
According to the training contents of equipment manufacturers and confirmatory experiments, the initial process method was determined. It mainly requires the use method and output current intensity of the equipment, but there is no specific specification in other aspects, and the treatment effect is average.
3 the shortcomings of the original process and improvement methods
3. 1 shortcomings of the original process
After blackening the first end wall according to the original process, it is found that the blackening effect is not ideal, which is mainly manifested in the following points: (1) Some solder joints have black marks with different colors from the original scale, which cannot be removed by repeated blackening operations; (2) Some solder joints have been corroded after blackening operation, and the color difference between them and the surrounding solder joints is large, which is extremely unsightly; (3) A large area of watermark appears around the solder joint position, which cannot be removed after repeated cleaning; (4) After overall cleaning, the end wallboard is hung from a distance, and the gloss of the wallboard is not good.
3.2 Defect Analysis and Improvement Methods
(1) cloth. After repeated experiments, it is found that some solder joints have black marks with different colors from the original oxide scale, which are caused by dirty cloth wrapped on the electrode head. Repeated blacking operation cannot be deleted. Due to the lack of experimental equipment at this stage, it is impossible to determine the composition of the black mark and whether there are other films on the surface. At present, the most effective treatment method for this defect is physical polishing. The stainless steel exterior wall panel is a mechanical drawing panel, and the overall texture is consistent, which affects the appearance effect after polishing. So at present, the best way to deal with this defect is to change the cloth wrapped around the electrode head frequently to prevent this defect.
The galvanic cell reaction needs to be carried out in electrolyte environment. Cotton cloth is the carrier of electrolyte in black removal operation. Too little electrolyte causes the removed oxide scale not to be completely decomposed, but directly attached to the surface of cotton cloth, resulting in cloth surface pollution, and cotton cloth absorbs water to a certain extent. Therefore, under the premise of not affecting the blackening effect, we try to fully soak the multi-layer cloth in the cleaning solution. After many experiments, it is finally determined that the double-layer cloth can store enough cleaning solution without affecting the blackening effect. At the same time, it is required to observe the pollution of the cloth frequently and replace it after the surface of the cloth is completely black.
Experiments show that this method can effectively prevent the appearance of black marks.
(2) The influence of current. According to the suggestion of the equipment manufacturer, when the output current is 7, the removal effect of darker solder joints is ideal, but after the removal of brighter solder joints, the solder joints turn white due to over-corrosion, which is very different from the surrounding solder joints and extremely unsightly. At the same time, the contact time between the electrode rod and the wallboard is not specified in the original operation method, which is also one of the reasons for this phenomenon.
During the experiment, the current output intensity was adjusted to 5, and the contact time between the electrode rod and the wallboard was controlled within 2 seconds.
Experiments show that the reduced current intensity has no effect on the black removal effect, and there is no over-erosion.
(3) The influence of the amount of cleaning solution. In the original operation process, it was found that the black removal effect of the first few points was very poor. When the electrode rod contacts the wallboard for more than 5 seconds, the color of the oxide scale can be completely removed, but if the contact time is too long, the oxide scale will be excessively corroded. Therefore, in the course of the experiment, the use method of cleaning solution was studied emphatically.
In the blackening operation, the electrode rod will heat up when the operation time is too long, which will cause the cleaning solution soaked in the cloth to evaporate too fast and seriously affect the blackening effect. Therefore, in the experiment, we tried to completely immerse the electrode rod in the cleaning solution, and then blacken it, and the blackening effect was remarkable. However, due to too much immersion in cleaning solution, the contact area on the wallboard is too large, which leads to obvious watermark around the solder joint after blackening, which is extremely unsightly.
Therefore, the final adjustment operation method is as follows: put the electrode rod wrapped with cloth into a beaker filled with cleaning solution, so that the cotton cloth is fully soaked, and then drain it on the beaker wall until it no longer drips; For solder joints that are not thoroughly cleaned once, they can be dipped in cleaning solution again before the second operation.
(4) The influence of using method of neutralization solution. The function of neutralization solution is to remove residual cleaning solution. Incomplete cleaning of residual neutralization solution will seriously affect the gloss of wallboard. The neutralization solution used in the original operation is the original solution, and the residual neutralization solution is difficult to clean because of its high concentration, so it is necessary to reduce the neutralization solution.
Focus.
In order to find the balance point between neutralization solution concentration and cleaning effect, the dilution ratio of neutralization solution: water = 1: 4 was finally determined after many experiments. At the same time, in order to facilitate the cleaning of the residual neutralization solution, in the operation process, immediately after cleaning with the cleaning solution, wash the residual cleaning solution with non-woven fabric dipped in diluted neutralization solution, and then wipe off the residual neutralization solution with non-woven fabric dipped in clean water; After the blackening operation of the whole wallboard is completed, clean the whole wallboard with clear water.
4 conclusion
The defects were analyzed by experimental methods, and the complete black removal process was finally determined. In the new process, the current intensity, the use of cotton cloth, the use of cleaning solution and neutralization solution are redefined.
The wallboard treated by the new process has good overall gloss, and there are no defects such as corrosion, watermark and indelible black mark at the solder joints, which meets the requirements of black removal effect.
refer to
[1] Zhao Minghua, Liu Yumin. Development of stainless steel car body for urban rail vehicles
[J]。 Urban Rail Transit Research, 2004, (1).
[2] Liu Yonggang, Han Xiaohui, Ma Mingju, Wang Suhuan. Railway vehicles are not rusted.
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Machine, 20 12, 42 (4).
[3] Zhang Miaoling. The difference and judgment of galvanic cell [J]. Education and teaching
Academic Forum, 20 1 1 year, (7).
About the author: Han Pengcheng, male, assistant engineer of Tianjin CNR Rail Equipment Co., Ltd., research direction: rail vehicle painting system and painting.
Finite element modeling method of stainless steel spot welding structure car body
Abstract: Used for spot welding structure of stainless steel car body? Point transmission force? The modeling idea of displacement master-slave constraint relation is put forward. Numerical experiments show that the displacement master-slave constraint relation has higher accuracy than other existing simulation methods for nugget simulation. Taking the stainless steel spot welding car body of urban light rail emu as the application object, the finite element model of the car body is established based on the displacement master-slave constraint relationship. The calculated results are basically consistent with the experimental results, which proves the correctness of the model and lays a good technical foundation for the design and development of this kind of car body in the future.
Keywords: finite element analysis, displacement coupling, spot welding structure
Spot welding is the main connection form between a large number of metal plates in the stainless steel car body structure of subway vehicles, urban light rail vehicles and even high-speed EMUs, which are distributed in various parts of the car body and number tens of thousands. The main features of spot welding structure are compact structure, light weight, high strength and corrosion resistance. At the same time, its manufacturing process is complex and its technical requirements are high. Therefore, although spot welding structural cars have been widely used abroad, they have just begun to develop in China [1-2]. In the development process of stainless steel spot welding car, how to master the mechanical characteristics of spot welding structure and establish a high-precision finite element calculation model of the car body has become a very concerned issue for calculators.
At present, solid element, beam element, rigid element and master-slave relationship (that is, displacement coupling) are commonly used to simulate the welding core in spot welding structure [3-4]. Theoretically, when the spot welding structure is simulated by block elements with appropriate height, higher accuracy can be obtained. However, for a large number of stainless steel body structures with evenly distributed and densely arranged welds, the number of elements/nodes will increase sharply, so it is not feasible to master the force transmission of stainless steel spot welding vehicles.
Compared with the size of vehicle structure, the size of spot welding nugget can be ignored. In the finite element model, they can be regarded as the only one in the whole coordinate system. Point? , under the action of external load, the structure depends on these? Point? Transfer internal force, what can this structure be called? Point force transmission structure? Based on the principle of displacement master-slave control relationship [5], this paper thinks that for a typical stainless steel spot welding car body? Point force transmission structure? It is more reasonable to use the constraint relation of displacement to simulate the welding core (that is, the displacement between two welding points in the calculation model is exactly the same), and the numerical experiment proves that the constraint relation of displacement is more accurate than other modeling methods. Based on the displacement constraint relationship, the finite element model of stainless steel spot welding car body of urban light rail emu is established, and the loading calculation is carried out according to relevant standards. Compared with the physical experiment, the rationality and effectiveness of the calculation model are verified.
1 canonical equation of master-slave control relationship
The master-slave relationship (displacement coupling) means that when one node is defined as the slave node of another node, the slave node loses its displacement independence, and its displacement can only and must be subordinate to the master node. The displacement on the master node is regarded as independent displacement, and the displacement on the slave node is related displacement.
When the principle of minimum total potential energy is applied to solve the structural canonical equation based on displacement method, the contribution of related displacement to total potential energy is expressed by related independent displacement and specified displacement. The total potential energy of the structure is
2 Comparative analysis of various methods
In this paper, the displacement principle is put forward. The force transfer of stainless steel spot welding car body is described from the constraint relationship, that is to say, each spot welding nugget in the model condenses into a point. So how accurate is this simplification compared with other modeling methods? In the following, solid element, beam element, rigid element and master-slave relationship (displacement coupling) are used to model the spot welding structure to discuss the accuracy of various methods.
Assume that the A and B pieces are welded by spot welding, and the thickness is t 1 and t2, respectively. The right end of the T 1 piece is uniformly pulled by F tons, and the left end of the T2 piece is restrained. The welding core is a three-dimensional ellipsoid with the largest cross-sectional diameter of D. In the calculation model of each method, the center of the plate is the solder joint position and the coordinate origin, and the physical properties of the beam element depend on the plate, with a unit diameter of D and a unit length of D. The rigid element has no physical properties and a unit length of (t1+t2)/2; The physical properties of solid units depend on the plate, and the unit size depends on t 1, t2; Displacement master-slave constraint does not need to define material properties, but only needs to specify the displacement master-slave constraint relationship among six degrees of freedom.
Based on the calculation results of solid element, the stress error ratio of t 1 thin plate in the load direction is listed in the attached table. The method in the table is to use beam element; Method 2 is to use rigid elements; Method 3 adopts displacement coupling. In the table, the position from point 1 to point 8 is the point on t 1 thin plate whose origin is equal to the load direction. The position from point 9 to point 16 is the point on t 1 thin plate, and its origin is equidistant from the coordinate axis perpendicular to the load direction. The stress error ratio in the table is defined as (? -? 0)/? 0, of which? 0 is the calculation result of the entity element. As can be seen from the attached table, the main displacement? The error of constraint modeling method is small, while the error of rigid element and beam element is large.
The comparison of the stress values along the external load direction between the solid element model and the displacement coupling model is shown in figure 1. The results show that the high stress areas of the two models are consistent. The stress value of the displacement coupling model near the core is larger than that of the solid element model, and the stress value of the two models is almost the same at a little distance from the core.
3 engineering verification
The stainless steel spot welding car body of urban light rail emu is a typical point transfer structure, which is made of high-strength stainless steel profiles for vehicles by cold bending or rolling. Except the corrugated board on the roof and floor, the joints of other boards with beams, columns and components are contact spot welding. Due to the thin plate, only overlapping can be used between plates, beams (columns) and columns, except for two or three layers of overlapping, there are at most five layers of plates.
After the stainless steel spot welding car body bears the external load, the load transfers the force to all parts of the car body through thousands of welding spots, thus causing deformation and stress in all parts of the car body. This characteristic must be truly reflected in modeling, otherwise, the calculation model will be distorted and the calculation results will be distorted. Assume that each body part. Right? In the meantime, once the spot welding is formed, although the spot welding actually occupies a small area, it is reasonable to regard these two points as relative to the size of the body parts. Weld a little? Therefore, in the process of deformation, spot welding can use displacement main? Describe from the relationship.
The key problem of finite element modeling of stainless steel spot welding car body of urban light rail emu is that each spot welding position must have nodes. Therefore, when creating three-dimensional geometric figures that truly reflect the relationship between stainless steel body parts and are used to divide the finite element mesh in I-DEAS software (10.0), it is necessary to create them one by one according to the spot welding positions. Anchor point? [6] Because? Anchor point? Once generated, in the subsequent process of generating unit grids, anchor points? Will it be automatically converted into a node of the unit, so it is spot welding? Right? The determination of the exact location creates conditions. The partial enlarged grid of a quarter of stainless steel spot welding car body is shown in Figure 3. The solution scale of the quarter model is: the total number of elements is132309; The total number of nodes is134659; The number of solder joints is 8824.
According to the load calculation in reference [7], Von. Under the condition of vertical total static load, the Mises stress nephogram of the car body and components is shown in Figure 4. The comparison between the finite element calculation results of the vehicle and the strength test results [8] is shown in Figure 5, and they are basically consistent, so the quality of the calculation model is very high.
In fact, it is precisely because of the high reliability of the performance simulation model that it is possible for designers to compare the schemes again and again, and the relatively optimal scheme is gradually formed in this comparison again and again, and its significance is self-evident.
5 conclusion
The key to the reliability of finite element analysis lies in the quality of calculation model, and the necessary conditions for creating calculation model are computer, simulation software and users. In fact, no matter how powerful the simulation software is and how fast the computer is, it can only help the modeler to improve the efficiency of modeling, but can't provide the principles and skills of modeling. The quality of computing model mainly depends on the theoretical literacy and modeling experience of users. Only with good theoretical literacy and solid modeling experience can we digest and absorb the essence of simulation software and integrate it into the establishment of simulation model.
Based on the principle of displacement master-slave constraint (displacement coupling), the spot welding simulation of stainless steel spot welding car body is carried out, and the finite element model of stainless steel spot welding car body of urban light rail emu is established by using the advanced modeling function of I-DEAS simulation software. By comparing the finite element calculation results with the physical test results, the feasibility of the modeling method in this paper is proved.
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