Study of Contact Behavior in the Pre-squeeze Stage of

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1 Study of Contact Behavior in the Pre-squeeze Stage of Aluminum Alloy Resistance Spot Welding Li. Baoqing, Shan Ping Lian Jinrui, Hu Shengsun Tianjin University, Tianjin, P.R.C Abstract In this paper, an axis-symmetric contact finite element analysis (FEA) model of resistance spot welding (RSW) for aluminum alloys was developed using ANSYS. It provided effective analysis of contact behavior in the pre-squeeze stage of aluminum alloys RSW. The computational simulation interpreted the reason leading to contact pressure distribution shape and contact area size at the faying interface between workpieces. And the factors and rules affecting contact behavior were also discussed in this paper. Introduction As a common industrial process of joining metals, RSW is used widely in the fields of automobile, aviation and so on. It is a complicated process, involving interactions of electrical, thermal, mechanical and metallurgical phenomena. In the RSW process, two metal work-pieces are compressed between a pair of water-cooled copper electrode. Current is supplied to create a concentrated heating at the contact interface, the heating is used to form the weld nugget. In this process, contact resistance plays an important role in the process of weld nugget forming. However, contact resistance is affected directly by the contact behavior between work-pieces, the contact pressure and size determine the electric resistance and conduct area of current, which have great effect on current conduct, heating and nugget forming. So, the study of contact behavior is helpful to understand the mechanism of RSW process. However, the contact behavior in the presqueeze stage is the basis of the whole study. C. L. Tsai and Cao Biao had provided the contact pressure distribution for steel RSW in the eighties (Ref1-2). Unfortunately, they didn t give the reason of contact pressure distribution, and study contact behavior systematically. This paper analyzed the reason of contact pressure distribution, studied the factors and rules affecting contact behavior. Modeling As we all know, Contact analysis is one kind of non-linear analyses. To general analysis, the boundary conditions are all determinate before solution. However, to contact analysis, the boundary conditions are not so, but the solution results. It is the complexity of electrode geometry that leads to the complex of the contact pressure distribution, and the contact pressure distribution on the workpiece-electrode interface is not uniform, neither described in a analytic expression. So, the pressure of complicated distribution acting on the work-piece makes it difficult to analyze the contact behavior on the faying interface between workpieces. Finite element analysis (FEA) method makes it easy. In this paper, a FEA model of contact analysis was developed using the FEA software- ANSYS. The analysis model is illustrated in Fig.1. The geometry parameters are electrode body radius-rb, electrode tip radius Re, the radius of cool-water hole -Rw, the angle at the electrode tip-alpha, the distance between cooling-water and electrode tip-hw and the depth of work-piece-hb. The electrode (copper) and the workpiece (Aluminum alloy 5052) are assumed to be of uniform mass and isotropic performances, and the plasticity behavior of materials is also assumed to be bilinear isotropic. According to the fact of RSW, the following hypotheses are also included:

2 Figure 1 - Model of Contact Analysis 1). The material surface is smooth and continuous, and the contact between work-pieces is without sliding. 2). The electrode force is loaded evenly on the tip of upper electrode, and the axial displacement of the interface between electrode and supporting parts is zero. 3). The electrode and work-pieces are axis-symmetrical. According to this, the radial displacements of the nodes on the axis are all assigned to be zero. The axis-symmetrical geometry model is meshed automatically using the solid element-plane42, and elements in the contact area are refined. In the condition of RSW, the contact problem belongs to that between flexible bodies. In this analysis, there are three contact pairs, those are upper electrode-workpiece, workpiece-workpiece and underside electrode-workpiece contact pairs. The end surfaces of electrode tip and the upper surface of underside work-piece are assigned to be contact surfaces, and other surfaces are assigned to be target surfaces. In these three contact pairs, contact element is CONTA172, and target element is TARGE169. The boundary conditions and loads are illustrated in Fig.1. The mechanical performance parameters of electrode and workpiece are illustrated in table 1. Performancep arameter Table 1- Mechanical Performance Parameters of Electrode and Workpiece yield strength (MPa) Elastic Module (GPa) Stiffening module (Mpa) poisson's ratio electrode workpiece

3 Analysis Results & Discussion Contact Behavior According to above model, the numerical simulation is developed using ANSYS. The results are shown in Fig.2, where, Rb=10mm, Rw=6mm,Re=3mm, Hb=10mm, Hw=10mm, Alpha=120, and the electrode force P= 2000N. In this figure, the results of contact area and contact pressure distribution shape are similar to those in others papers (Ref.1-2). That is, firstly, the contact pressure rises placidly in the mostly area around the axis, and it will decline steeply after reaching its peak value. Secondly, the radius of faying interface between work-pieces is larger than that of electrode tip, and the radial coordinate of the point of contact pressure peak value is less than the radius of electrode tip. As described above, the contact behavior between work-pieces is determined by material, geometry parameters and electrode force. But the direct reason is the pressure acting on the workpiece-electrode (W-E) interface. From Fig.2, the contact pressure on the W-E interface is fairly uniformly in the majority around the axis, and in the domain near the electrode edge, there is severe stress concentration. The simulation results show that the degree of stress concentration depends on the geometry parameters and electrode force. Figure 2 - Contact Pressure Distribution The uniformly pressure acting on the W-E interface may be regarded as the addition of two force modes, one of which is acting evenly on the W-E interface, and the other is acting along the electrode edge. The axial stress distributions with different force modes are shown in Fig.3. From the addition results of these two axial stress distributions, the radial coordinate of peak value point is less than that of electrode tip edge, and squeeze stress exits in this area, the radial coordinate of which is larger than that electrode tip edge. It is the axial stress distribution in the workpiece that results in the contact pressure distribution shape on the faying surface as illustrated in Fig.2. As it is known that the stress and distortion in the workpiece should meet the equilibrium principle and the harmony principle, so the axial stress in the workpiece changes gradually from the squeeze stress to the tensile stress with the increase of the radial coordinate. There is also distortion along the workpiece edge, which is about several micrometers. Therefore the contact between work-pieces is not on the whole surface of workpiece, but on the part of the workpiece surface.

4 Figure 3 - The Axial Stress Distribution With Different Force Modes Affecting Factors And Rules As described above, the contact behavior between work-pieces depends on the contact pressure distribution at the W-E interface, material performances and the size of workpiece. However, the contact pressure distribution is determined by material performances, the electrode geometry and electrode force. So, the affecting factors of contact behavior include material performance, electrode and workpiece geometry sizes and electrode force. The factor of material performance is not considered in this paper, and the geometry parameters of electrode, the depth of workpiece and electrode force are discussed only. Electrode Geometry Parameters With different electrode tip radius -Re, the contact pressure distributions on the faying interface are shown in Fig.4, where electrode force is 3000 N. With the increase of Re, the radius of faying interface increases, which is proportional to Re basically, and the average value of contact pressure decreases, the radial coordinate of peak value point also increases, but it is still less than that of electrode tip edge. In addition, the contact pressure distribution shape changes obviously with the increase of Re. This change is attributed to the change from the elasticity to the plasticity. Under the condition of less Re, the average value of contact pressure at the W-E interface is fairly high, and the contact pressure in some area or the whole area even exceeds the yield stress. So, the plastic distortion decreases the degree of stress concentration, that is to say, it decreases the influence of force model acting on the electrode tip edge on the contact pressure distribution. And the strain stiffening effect maintains this kind of uneven contact pressure distribution. In addition, it is noted that the plastic distortion emerges firstly in the area near the electrode tip edge, and secondly in the area about the axis, which results that the contact pressure at the W-E interface becomes more complicated as shown Fig.5.

5 Figure 4 - Contact Pressure Distribution With Different Electrode Tip Radius Figure 5 - Contact Pressure Distribution On The W-E Interface Under the condition of larger Re, the contact pressure at the W-E interface is below the yield stress. The contact pressure distribution is as illustrated in Fig.2. With the increase of Re, the radius of contact pressure distribution at the W-E interface increases, and the radius of the axial squeeze stress area in workpiece also increase, which leads to the increase of the radius of faying interface. And the increase and decline trend of contact pressure on the faying interface becomes more placidly with the increase of electrode tip radius. The author also studied the influence of other electrode parameters on the contact behavior. The numerical simulation results show that other parameters have effect on the stress concentration near the electrode tip edge, but it is fairly slight. So they have little effect on the behavior on the faying interface.

6 Electrode Force With different electrode force, the contact pressure distributions on the faying interface are shown in Fig.6, where Re equals 2.5 mm, and other parameters are the same as those in Fig.2. The electrode force has little effect on the radius of the faying interface, but great effect on the contact pressure distribution shape. With the increase of electrode force, the average of contact pressure increases, and the point of peak value moves toward the axis. Under the condition of larger electrode force, as described above, the plastic distortion arises in some or the whole area, and the degree of stress concentration at the electrode tip edge decreases. So, the influence of force mode acting on the electrode tip edge on the contact pressure distribution is weakened. The increase of electrode force makes the contact pressure distribution tend to that with the force mode acting evenly on the W-E interface. However, under the condition of less electrode force, the mostly or the whole area is at the elastic stress state, which results that the contact pressure distribution is similar to that illustrated in Fig.2. It is the change from elasticity to plasticity that results that the contact pressure distribution presents the change as illustrated in Fig.6 with the increase of electrode force. Figure 6 - Contact Pressure Distribution With Different Electrode Force Depth Of Work-Piece With different work-piece depth, the contact pressure distributions on the faying interface are shown in Fig.7, where Re equals 2.5 mm, electrode force equals 3000 N, and other parameters are the same as those in Fig.2. With the increase of work-piece depth, the radius of the faying interface also increases, and the contact pressure distribution tends gradually to be that with the force mode acting evenly on the W-E interface. The numerical simulation results show that the axial stress on the axial section becomes more uniformly, the radius of squeeze stress area larger and the effect of force mode acting on the electrode tip edge less with the increase of the axial coordinate. So, the contact pressure distribution presents this change as shown in Fig.7.

7 Figure 7 - Contact Pressure Distribution With Different Depth Of Workpiece Conclusion The numerical simulation results that the uneven contact pressure distribution on the W-E interface is the reason of the contact pressure distribution on the faying interface. In the squeeze stage of aluminum alloys, the contact behavior is affected mainly by the electrode tip radius, work-piece depth and electrode force. The radius of faying interface depends on the electrode tip radius and work-piece depth, and the contact pressure distribution shape is primarily affected by the electrode tip radius, work-piece depth and electrode force. References 1.C.L.Tsai etc Modeling of Resistance Spot Welding Nugget Growth. Welding Journal 71( 1):47s- 57s. 2.Cao. Biao The finite element modeling of nugget formation and real time expansion-based control of weld quality in Resistance spot welding. Harbin University of technology doctoral dissertation. pp