Contact Magnetic Pulse Welding: The Process Modelling: Methods and Materials Angshuman Kapil, Abhay Sharma Department of Mechanical and Aerospace Engineering, Indian Institute of Technology Hyderabad, ODF Campus, Yeddumailaram, 502205, Telangana, India Determination of Weldability Criteria in Magnetic Pulse Welded Joint by Finite Element Analysis • A contactless high velocity impact welding technique. • A potential candidate for joining of similar, dissimilar and lightweight materials. • Effective and reliable in terms of cost and performance, respectively • The process is quite similar to explosive welding (EXW) . • A magnetic field helps generate the required magnetic pressure to drive the flyer tube/plate towards the base tube/plate by change of energy in electrical form to its mechanical form. The flyer workpiece driven with high velocities by the magnetic forces impacts against the base workpiece and a weld is created as shown in Fig. 1. Fig . 1 Schematic illustration of MPW process (a) initial welding set up (b) deformed geometry after application of the pulsed current. Coupled Magnetic-Structural Finite Element Analysis of Magnetic Pulse Welding (MPW) for Tubular Workpiece Process Parameters (Air gap & Input voltage) Weldability window Weldability criteria Fig . 3 MPW set up for expansion joining Fig . 2 MPW set up for compression joining Finite element analysis (FEA) solver used: COMSOL Multiphysics Modules used: a)AC/DC module (Time dependent) Magnetic fields (mf) b)Structural-Mechanics module (Time dependent) Solid Mechanics (solid) Space Dimension: 2D axisymmetric Governing Equations: (a) Constitutive behaviour,(Cowper-Symonds constitutive model)[1]: = + where is the rate of plastic strain (s −1 ), is the flow stress, and P and m are parameters specific to the material. (b) Magnetic force density: = × = × × where is the current density induced in the coil (A/m 2 ) and is the magnetic flux density (T). (c) Load in the electromagnetic module [1]: = − where V represents input voltage, C and L represent capacitance and inductance of the system, respectively, β represents damping coefficient and ω represents current frequency. Model Assumptions: • Cracking and the heat generated by friction, deformation and the joule heating has been neglected. • Use of temperature independent elasto-plastic properties. • Compression of air inside the flyer tube was neglected in the model. • Field shaper has not been employed. Table 1 Modelling Of Electromagnetic Welding system (Compression joining) Method Employed Compression joining of tubular metallic assembly Flyer tube and Target tube Material Structural Steel ASTM A 36 Electromagnetic coil Material Copper Type Helical solenoid Number of turns 4 Cross section area (m 2 ) 0.0625X0.07 Length (mm) 25 Dimensions -flyer tube Air Gap (mm) Outer diameter (mm) Inner diameter (mm) 0.5 24 20 1 25 21 1.5 26 22 2 27 23 2.5 28 24 Dimensions- target tube Outer diameter (mm) Inner diameter (mm) 19 15 Process Parameters Input Voltage(kV) Air gap(mm) 6, 6.5,7,7.5,8,8.5,9,9.5 0.5,1,1.5,2,2.5 Capacitance= 300 μF, Inductance= 100e -9 H, Resistance= 10 ohm, Frequency =50000 rad/s Table 2 Material properties, dimensions and process parameters Fig. 4 Simulation flow chart for a sequentially coupled Electromagnetic – Structural analysis Fig. 5 Configuration of flyer and base tubes for different air gaps Theme Results Case Study: Compression Joining of Structural Steel ASTM A36 Tubes Model Validation Fig. 6 Comparison of experimental and simulated values of impact velocities for compression joining of similar and dissimilar material [2,3] Fig. 7 Comparison of experimental and simulated values of impact velocities for compression joining of similar and dissimilar material [4] • Close agreement between the simulated and experimental values within a range of ±10% variation. • The closeness in results guarantee that the rest of the simulated results can effectively predict the values of the weldability criteria, for different geometry and material combinations under a varying combination of process parameters. Weldability Criteria and Weldability Window Fig. 8 Comparison of weldability criteria a impact velocity, b effective plastic strain, c shear stress (air gap = 1 mm) Fig. 9 Weldability window for MPW of structural steel ASTM A36 • Impact velocity criterion could not be satisfied at 8 kV (Fig. 8a). • Effective plastic strain crossed the threshold at all the three input voltages (Fig. 8b). • On the contrary, the shear stress criterion crossed the threshold at 8 kV but failed to do the same at 9 kV (Fig. 4c). • The input voltage of 8.5 kV satisfied all the three criteria. Thus, the existence of process parameters which can satisfy all the three criteria in the MPW process was non- trivial. • Pairs of the input voltage and the air gap are marked for the cases when an individual weldability criterion crossed the respective threshold value. • Plastic strain was the most versatile criteria that crossed the threshold limit followed by the impact velocity and the shear stress. • The window identifies the particular process parameters suitable for conducting a successful weld. Weldability Criteria: Relation with Process Parameters (a) (b) (c) Fig. 10 Weldability criteria at different air gaps and input voltages a impact velocity, b effective plastic strain, c shear stress Impact velocity • Increased with an increase in the input voltage. • Minimum input voltage of 7.5 kV was essential to cross the threshold impact velocity. At a particular value of the air gap, the velocity of the tubes reached a maximum value Effective plastic strain • Increased with an increase in the input voltage. • Increased with increase in air gap upto a certain extent and decreased subsequently. • An optimum air gap needed between the members to achieve a good weld. Shear stress • Crossed the threshold value for almost all air gaps at voltages ranging from 7 kV to 8.5 kV. • Beyond 8.5 kV it started to decrease and threshold could not be crossed. • Limits the allowable range of input voltage. Conclusions 1. The three weldability criteria, namely, impact velocity, effective plastic strain and the direction and magnitude of the shear stress studied in this investigation have a significant role in magnetic pulse welding of tubular joint. 2. Existence of process parameters which can simultaneously satisfy the three foregoing criteria is non-trivial. A comprehensive approach considering each of the foregoing weldability criteria for magnetic pulse welding should be adopted. 3. The process parameters in MPW are interrelated. A moderate input voltage at an optimum air gap could achieve a sound joint. 4. The demonstrated methodology of developing a weldability window through finite element simulation would save cost and time spent in production of product using MPW. Future Work • Extension of the present model for: o Compression joining of dissimilar materials. o Expansion joining of similar and dissimilar materials. • Development of weldability windows for the above process. • Application of field shaper in the model. • Investigating effect of process parameters on weldability. • Introduction of temperature dependent properties in the model. References 1) Haiping YU, Chunfeng LI. Effects of current frequency on electromagnetic tube compression. J Mater Process Tech. 2009; 209: 1053–1059. 2) Zhidan XU, Junjia C, Haiping YU, Chunfeng LI. Research on the impact velocity of magnetic impulse welding of pipe fitting. Mater Design 2013; 49: 736–745. 3) Desai SV, Kumar S, Satyamurthy P, Chakravartty JK, Chakravarthy DP. Scaling Relationships for Input Energy in Electromagnetic Welding of Similar and Dissimilar Metals. Journal of Electromagnetic Analysis and Applications 2010; 2: 563-570. 4) Althoff JL, Lorenz A, Gies S, Weddeling C, Goebel G, Tekkaya AE, Beyer E. Magnetic Pulse Welding by Electromagnetic Compression: Determination of the Impact Velocity. Adv Mat Res. 2014; 966-967: 489-499 . Abhay Sharma- abhay@iith.ac.in Indian Institute of Technology Hyderabad Phone: +9140 2301 6091 View publication stats View publication stats