Finite element model of 1 spot weld nugget temperature field 1.1 Basic electric and thermal equations The basic equations for spot welding thermal process include the potential equation describing the potential distribution of the weld zone and the heat transfer equation describing the temperature field of the weld zone.
The potential equation determines the heat generation in the weld zone and its distributed heat transfer equation relates heat to thermophysical properties such as heat transfer of the material and expresses the relationship between the two in the form of a temperature field.
The spot weld field problem can be handled as a rotationally symmetric field problem. The spot welding zone is a conductor, and the internal potential distribution is represented by the following differential equation. r and z are the radial coordinate and the axial coordinate U(r, z) of the cylindrical coordinate system are the potential functions in the cylindrical coordinate system. Ï is the resistivity of the material, which depends not only on the properties of the material (electrode material and workpiece material) but also on temperature. The Ï distribution is not uniform due to uneven temperature distribution in the weld zone.
The heat transfer problem in the spot welding thermal process is the transient heat transfer problem of the internal heat source. The heat conduction equation can be described by the following differential equation: T(r, z) is the temperature function q in the cylindrical coordinate system. The heat λ generated by the heat source in volume and unit time is the thermal conductivity Ï′ is the specific heat capacity. λ, c, Ï′ all change with temperature, and c also considers the influence of latent heat of phase change on it.
The internal heat (q) generated at a certain point in the weld zone is determined by the potential function and the resistivity of the point, and can be a Laplacian in the following formula. This formula is a calculation method for the heat generation of the conductor resistance, and the contact resistance heat generation can also be handled as follows.
1.2 When solving the finite element method of finite element equation, the solution domain D needs to be discretized into subdomain ΔD, and these subdomains form the original solution domain. The subdomain unit of this paper adopts the axisymmetric triangle ring unit (as shown in Fig. 1). When performing finite element calculation, the triangular unit nodes are numbered counterclockwise as i, j, m. For the convenience of calculation, j and m are defined as boundary nodes for the boundary unit.
The Galerkin method is used to transform the formula (2) into the form of the integral description method of the finite element. The equilibrium equation of the transient heat conduction finite element is derived as the formula [C] is the unit heat capacity matrix [K] is the unit heat conduction matrix. [K] is the unit heat exchange matrix F is the internal heat generation heat load vector F is the surface heat exchange load vector.
Using the same method to obtain the electric field finite element element equation is 1.3 finite element mesh and the boundary condition finite element calculation mesh segmentation has an important impact on the calculation accuracy and computational efficiency of finite element analysis. The finite element calculation mesh of this paper is shown in Figure 2. The division of boundary elements varies depending on the electric field calculation or the temperature field calculation. When the electric field is calculated, there is only the first type of boundary condition unit, and the temperature field is calculated with the first type of boundary condition unit and the natural boundary condition unit. As the surface unit treated by the heat insulating surface, since the processing method of the unit is completely the same as that of the internal unit, it is not regarded as a boundary unit. The splitting of the unit avoids obtuse triangles and limits the proportion of the sides of the triangle. Increase the density of the mesh when the field variable changes drastically, and reduce the density of the mesh appropriately when the change is small, so as to improve the calculation accuracy and calculation efficiency.
The method of taking various boundary conditions of electricity and heat is also shown in Fig. 2. Since the spot welding electric field is the electric field in the conductor, the electric field is concentrated in the electrode and the workpiece, the electric field of the environmental medium is weak, and the interface with the air and water and the axis of symmetry can be The potential gradient on the upper side is set to zero, that is, U / n = 0. According to the spot welding practice, the contact surface between the workpiece and the workpiece is substantially limited to the range of the plastic ring (OA segment), and in fact, there is a phenomenon of workpiece lift outside the plastic ring. At the same time, due to symmetry, the equipotential of OA can be set to U =0.GF is another equipotential surface, the potential is controlled by heat at a certain moment, and the NHG surface is cooled by cooling water (heat dissipation coefficient is h), FED The DCBA surface is air heat dissipation and heat radiation heat dissipation (heat dissipation coefficient is h). Considering the symmetry plane heat flow balance, the symmetry plane NJO and OA are treated according to the adiabatic surface, that is, T/n =0. The electrode interface GF is set to the essential boundary condition, and the temperature is assumed to be T = 20 ° C (room temperature). The initial conditions for the spot welding thermal process calculation are also set to 1.4 physical parameters. The highest temperature in the spot welding thermal cycle is above the melting point of the steel, and the temperature varies greatly. Xu Guocheng et al. in the numerical simulation of spot welding process: the finite element model of the spring steel spot welding nugget temperature field is related to temperature and changes greatly with temperature, which produces a great deal for the spot welding process. Influence, therefore, the relationship between various electrothermal physical parameters with temperature must be considered in the established numerical calculation model. The electrode material involved in the calculation model of this study is chrome-zirconium-niobium-copper, and the workpiece material is 65 Mn spring steel. The data of various electrothermal physical parameters of the two materials mainly come from the manual or the data, but some performance parameter data (especially the high temperature performance parameter data) are not complete enough, and need to be determined by certain methods.
The main physical parameters involved in the calculation of thermal process are material resistivity, thermal conductivity, specific heat capacity, hardness and density. Other physical parameters except density are greatly changed with temperature. For the workpiece material, the heating temperature is above the melting point, and special methods are needed to deal with the physical properties at high temperatures. For example, due to the lack of high-temperature resistivity of the workpiece material, the theoretical relationship between thermal conductivity and electrical conductivity is used to estimate the resistivity. Since the liquid spot welding nugget has a strong electromagnetic stirring effect, the metal is melted. The thermal conductivity measure is such that the maximum temperature of the nugget does not exceed 15-20 of the melting point temperature. In the process of solidification and cooling of the spot weld nugget, the thermal conductivity of the liquid metal still follows the conventional value because the electromagnetic stirring disappears. The effect of the latent heat is treated by the equivalent specific heat method, and the technical measures of adding the specific heat of the phase change latent heat to the specific heat capacity in the phase transition temperature range of several temperature sections are used to approach the specific heat of the Jilin University of Technology during the phase transition. Natural Science Journal changes the actual situation. The surface heat dissipation coefficient uses the cooling effect of the cooling water on the electrode (ie, the forced convection heat dissipation method in the tube). The electrode and the workpiece surface are treated as the natural convection and heat radiation mixed heat treatment of the air. The heat dissipation coefficient of the material includes the natural convection heat dissipation coefficient of the air and the heat radiation heat dissipation. coefficient.
1.5 Contact resistance treatment method Because the contact surface is uneven, the contact resistance generated has an important influence on the spot welding thermal process. Contact resistance mainly includes shrinkage resistance and sheet resistance. The electrode pressure during spot welding produces a large contact pressure on the contact surface, so that the influence of the film resistance is greatly weakened by the film rupture. Therefore, the influence of the shrinkage resistance is mainly considered in the finite element analysis.
When two cylindrical contact elements having a contact point of n are in contact with each other, the contact resistance is such that Ï is the resistivity of the contact material a is the radius A of the contact area as the total contact area.
During the spot welding process, the action of the electrode pressure causes a large contact pressure on the contact surface, which causes plastic deformation and increases the actual contact area. According to the relationship between the hardness of the contact material and the contact pressure, it can be found that p is the contact pressure H as the hardness of the contact material.
Equation (9) reflects the relationship between the contact resistance and the resistivity, hardness and contact pressure of the material. Although the relationship between contact resistance and temperature is not shown, the resistivity and hardness of the material are both a function of temperature, so the actual The effect of temperature on contact resistance is also included.
During the spot welding process, as the temperature of the contact portion increases, the resistivity of the material also increases, and the contact resistance also increases as the resistivity increases. On the other hand, the increase in temperature causes the material to soften and the hardness to decrease, thereby increasing the contact area and lowering the contact resistance. Obviously, in the spot welding process, the latter factor plays a leading role due to the large contact pressure, which shows that the contact resistance decreases rapidly as the spot welding process progresses.
2 Verification of the numerical calculation results The resistance spot welding nugget is formed under the condition of sealing, and the formation process time is very short, so it is difficult to directly test the spot welding temperature field. The literature uses the semi-welding method to study the spot welding process, but the experimental conditions are still far from the actual spot welding, and the measurement results can only be used for qualitative analysis. In this paper, the previous verification method is adopted, which is to verify the accuracy of numerical calculation results by comparing the heat affected zone of spot welding and the size of nugget.
During the verification process, the actual spot welding conditions are the same as the calculation conditions (electrode geometry is consistent with the model, spot welding current, electrode pressure, etc. are the same). The workpiece is a 0.9 mm thick 65 M n steel. The spot welding specification parameters are: welding current is 7 000 A, welding time is 6 cyc (cycle), and electrode pressure is 3 200 N. Figure 3 is a metallographic photograph of the 65 Mn spot welded joint under this specification. Figure 4 is a comparison of the calculated results with the measured results, indicating that the two agree well.
(1) The finite element model of the spring steel spot welding nugget temperature field established in this paper can accurately describe the geometry of the spot welding zone, fully considering the contact resistance, liquid nugget temperature, latent heat of phase change, etc. The influence of the temperature field and the influence of temperature on the physical parameters during the spot welding heat process, thus realizing the numerical analysis of the temperature field of the spring steel spot welding nugget.
(2) The comparison between the calculated results and the measured results shows that the two agree well. Therefore, the established numerical model has higher precision.
(3) The establishment of the finite element model of the spring steel spot welding nugget temperature field has laid an important foundation for the numerical analysis of the complex hot process of spring steel spot welding and the numerical prediction of the microstructure and properties of the spring steel spot welding joint.
(4) The finite element model of the spring steel spot welding nugget temperature field is also of great significance for the numerical analysis of spot welding of other hardenable materials.
Stop Valves are closed by screwing a rubber gasket down onto a seat in the middle of the valve. Pros only use small versions that act as shutoff valves for fixtures such as sinks and toilets and outdoor sillcocks. Flow is inefficient because of the circuitous route the fluid (water, in most cases) has to follow. It's important to orient the valve in the right direction with the arrow (cast into the side of the valve) aligned with flow direction. That way, water flows against the bottom of the rubber gasket. If the valve is put in backward, the flow will force the gasket away from the top of the valve.
Stop Valves, Shower Stop Valve, Water Stop Valves, Brass Stop Valve
ZHEJIANG KINGSIR VALVE CO., LTD. , https://www.cn-kingsir.com