Electromagnetic Welding Facilities for Welding of Tubular Expansion and Compression Joints for Industrial Application

VenkataRatnam M1*, Surender Kumar Sharma2+, JMMV Aravind2, Shobhna Mishra2, Renu Rani2, Ramanand Raman2, Ekansh Mishra2, Rishi Verma2 and Archana Sharma2

1Engineering Services Division, BhabhaAtomic Research Center, Visakhapatnam2Pulsed Power & Electromagnetic Division, BhabhaAtomic Research Center Visakhapatnam

Email: *mvratnam@barc.gov.in, +surender@barc.gov.inAbstractElectromagnetic Welding machines with stored electrical energies of 40 kJ – 800 kJ are designed and developed for welding of expansion and compression tubular joints for industrial applications. The main components of Electromagnetic Welding machines are capacitor bank, capacitor charging power supply, energy transfer switch, welding coil and the work piece. The capacitor bank is used for storing the electrical energy and then discharging the electrical energy to generate high pulse current at high frequency in the coil, which then exerts magnetic pressure on the work piece to weld it. This technique enables us to join similar as well as dissimilar metals, which are very difficult to weld by conventional techniques. The design details of Electromagnetic Welding Machines, Welding coil and experiments with welding of stainless steel alloys and aluminium alloys with are discussed in the paper.

Keywords: Electromagnetic, Welding, Capacitor bank, Coil

1. IntroductionElectromagnetic Welding (EMW) is a new and reliable manufacturing technology for welding of high strength and high temperature stainless steel alloys and aluminium alloys for industries. Nuclear applications are primarily responsible for the development of this technology [1-2]. This technology has significant advantages over traditional welding techniques as it uses noncontact electromagnetic forces to achieve welding of various metal work pieces. The electromagnetic welding is accomplished by impact, when the colliding work pieces are accelerated towards each other by the Lorentz force. The work piece is welded by fast rising, high intensity pulsed magnetic fields that induce a current in the work piece and a corresponding magnetic field, rapidly accelerating portions of the work piece. The work piece can be welded without any contact from a tool. It can be considered as one of the best high-rate welding methods from the several aspects such as high cleanness, cost-efficiency and productivity. In EMW process, the work piece can achieve velocities in the order of 300m/s – 600 m/s in less than 0.1 ms resulting in high strain rates. Applying this technique, it is also possible to improve the weldability of some materials such as aluminium that is a good candidate material for use in automotive industrial for light weight structures and many other applications. Light weighted parts obtained from the coalescence of different material are highly postulated in aerospace, automobile and nuclear power industries [3-6]. Despite negligible process failure and high production rate, EMW has limited industrial use due to lack of guidelines regarding process parameters affecting the whole welding process.

2. Theory of Electromagnetic WeldingThe electrical energy stored in the capacitor bank is discharged into a coil to generate

high current and ultimately high magnetic flux density. The magnetic flux density (B) penetrates the flyer metal and eddy currents with current density (J) are produced at the surface of the flyer metal. The electromagnetic force (J x B) accelerates the flyer metal until it collides with other metal. The equations governing the processes are:

∇× J=−σ (∂ B∂t ) (1)

P=(B02−Bi

2 )/2 μ (2)

Bi2=B0

2 e−2t

δ (3)

δ= 1√πσμf

(4)

Where:σ, is the electrical conductivity of the work piece µ, is the magnetic permeability of the work pieceδ, is the skin depthB0, is the magnetic flux density at the outer surfaceBi, is the magnetic flux density at the inner surfacef, is the frequency of the discharge currentt, is the thickness of the conductor metalP, is the magnetic pressure

The circuit theory allows us to calculate the current, magnetic field and the pressure on the work pieces. The EMW process is equivalent to a primary RLC circuit coupled with secondary RL circuit. The circuit diagram of EMW system is shown in Figure 1.

Figure 1 Equivalent circuit of EMW systemThe capacitor bank (C)is charged to voltage (V0) and discharged using switch (S), discharge current (I1) into the coil and the induced current (I2) can be obtained using RLC circuit equation:

For the primary circuit, R1 I 1+1

C1∫ I 1dt +L1

d I 1

dt+ M

d I 2

dt=0 (5)

And for the induced circuit, R2 I 2+L2

d I2

dt+M

d I 1

dt=0 (6)

Where,L1 is the inductance of the coilR1 is the resistance of the coilM is the mutual inductance between the coil and work pieceL2 is the inductance of the work pieceR2 is the resistance of the work pieceM is the mutual inductance between the coil and work piece

By solving these equations the discharge current from the capacitor bank and the induced current in the work piece can be obtained. The peak discharge current is given by

I max=V 0√ C1

L1

(7)

The magnetic pressure (P) resulting due to the induced magnetic force (F) between the current in the coil and the induced current in the work piece are given by:

Fl=

μ0 I 1 I 2

2 πd(8)

P=μ0 B2

2(9)

2. Electromagnetic Welding Machine (EMW)The main components of EMM are capacitor bank, capacitor charging supply, energy transfer switch, welding coil and the work piece. EMM systems are designed and developed for expansion and compression welding joints of aluminium and stainless steel alloys for various industrial applications. The details of these EMW machines are discussed below.

2a. EMW 40 kJ / 20 kHz / 300kA with single sparkgap switch40 kJ EMW is designed, developed and commissioned with single sparkgap switch to carry out electromagnetic expansion welding of aluminium alloys tubes. The system has a capacitor bank with two capacitors (178 µF / 15 kV / 20 kJ each), 15 kV/100 mA power supply [7], spark gap switch, trigger generator and the weld coil (Figure 2). The total inductance of the system was less than 200nH and the short circuit discharge current and frequency of the system are 300 kA and 20 kHz respectively. The system has delivered the maximum current of 170 kA and produces magnetic field of 20 T in the expansion welding coil. The EMM system has been used for expansion welding of Al alloy tube to Al alloy block/flange.

Figure 2. 40 kJ EMW with single switch2b. EMW 40 kJ / 15 kHz / 250kA with multiple trigatronsparkgap switch40 kJ EMW system with multiple trigatron sparkgap switch is commissioned to carry out electromagnetic welding of dissimilar metals (Figure 3). The system has four capacitors of 56 µF/20 kV each, 20 kV/50 mA power supply, four trigatron switches connected in parallel. The EMM system is controlled using PLC module and remotely operated. This system has been used for lugging of electrical cables and welding of dissimilar metals.

Figure 3. 40 kJ EMW with multiple switch2d. EMW 400 kJ/ 10 kHz / 700 kAwith ignitron switch400 kJ EMW is designed, developed and commissioned to carry out welding of aluminum alloys tubes with block.The 400 kJ EMW machine has 8 capacitors with stored energy of 50kJ at peak charging voltage of 44 kV (Figure 4). The equivalent series inductance of the capacitor is less than 70nH. In order to segregate the coulomb transfer loading on switches and to minimize the effective circuit inductance, the capacitor bank is subdivided in to two modules with four capacitors in each module. The two ignitron switches are operated in parallel to transfer energy from the bank module to the load. The EMM system has pumped 200 kA current at 10 kHz and produced 22T magnetic fields in the expansion welding coils.

Figure 4. 400 kJ EMW system2e. EMW 800 kJ/ 15 kHz / 1.6 MA with multiple railgap switch800 kJ EMW is designed, developed and commissioned to carry out welding of SS alloys tubes with end plug. The 800 kJ EMW system has 16capacitors with stored energy of 50kJ each at peak charging voltage of 44 kV (Figure 5). There are four modules and four numbers of capacitors connected to each module by parallel plate transmission line assembly and a railgap switch. The individual railgap switches are capable of transferring maximum 10 coulombs of charge at a peak current of 750kA with a jitter of <2ns. The outputs of all modules are terminated to centralized collector plate through coaxial cables. The 800 kJ EMM system has pumped 900 kA current at 15 kHz and produced 50 T magnetic field in the single turn coil.

Figure 5. 800 kJ EMW system with multiple railgap switch3. Electromagnetic Welding Coils

EMW coils are used to focus magnetic field onto electrically conductive work pieces placed in its vicinity. The coil consists of one or more electrical windings and is made from a highly conductive material. There are several factors to be considered while designing EMW coils such as material selection, operating voltage, magnetic flux and heating [8]. Failure of the coils may occur due to excessive heating or due to the magnetic pressure exerted on the work piece.3a. Single turn Coil for Compression Welding

A single turn coil is preferred because it has very low inductance which produces very high frequency magnetic field across the work piece. It has good mechanical, thermal and electrical isolation stabilities. Due to high frequency the magnetic field is confined at the surface of the work piece and expresses high magnetic pressure on it. Magnetic field strength (H) is given byH = NI/L (13)Where, N (number of turns) = 1 as a single turn coil was considered.

Considering 50% coupling between the coil and work piece, Magnetic field (B) is given byB = 0.5 μH

3b. Multi-turn disc CoilMulti-turn disc coil is an electromagnet used for the generation of exceptionally

strong magnetic fields. The upper bound of magnet flux density is restricted by several factors. One principal restriction is the high stresses due to Lorentz forces in the coil. The Lorentz forces generate the distributed body force, which acts as the pressure of magnetic field. The common radial thickness profile of the disc coil is constant. A five turn disc coil is designed, fabricated and used for the compression welding of similar and dissimilar metals (Figure 6). The inner diameter and the length of the bitter coil are 50 mm and 30 mm respectively. The coil was operated at 200 kA and the magnetic field of 10 T is generated inside the coil.

Figure 6. Five turn disc coil (front view)3c. Solenoid Coil for expansion welding

The solenoid coils were fabricated using copper conductor and high strength fibre-reinforced plastic (FRP). FRP prevents the mechanical failure at higher currents. The stresses that develop in a solenoid coil are the radial stresses, tangential stresses and the axial stresses. These stresses increase with the increase in current discharged into the coil and must be taken care of in order to prevent failure of the coil. A solenoid is designed, fabricated using copper wire with seven turns and FRP for expansion welding. The diameter and length of the coil is 64 mm and 40 mm respectively (Figure 7). These coils are repeatedly operated at 150 kA– 170 kA for EM expansion welding experiments

Figure 7.Solenoid coil for expansion welding4. Expansion Welding of Al alloys tubes Experiments are carried out to weld Al 6061/5052 tube to Al 6061/5052 block [9]. The electromagnetic expansion welding is influenced by different parameters like tube thickness, stand-off distance, work piece dimension and the system electrical parameters. Experimental studies were carried out on to obtain a good weld. The peak discharge current in the coil determines the magnetic pressure and the impact velocity. The deformation studies were carried out on 1.5 mm thick Al 6061 tubes of 65 mm diameter at variable discharge current provided by variable capacitor bank charging voltage. The diametrical expansion of 1.5 mm tube at 9 – 10 kV charging voltage, which results in 110 – 130 kA discharge current. A good quality weld is obtained when the flyer tube velocity is > 300 m/s and when the impact occurs near about the highest velocity and peak pressure (Figure 8). The optimum parameters are obtained for the welding of 65 mm diameter Al 6061 tube of 1.5 mm thickness to Al 6061 flange was found to be 2.5 mm stand-off distance, a peak current value of 166 kA corresponding to peak pressure of 360 MPa and a magnetic field of 20T. Welding achieved with notched flanges and tapered angle was found of good quality in comparison to the smooth surface flanges. Moreover, the weld interface of the welded tube-flange sample was found to be wavy in nature.

Figure 8. Welding of Al6061 tube/block5. Welding of SS alloys tube to rodElectromagnetic welding of SS316l tube to SS316l rod/plug is carried out on 800 kJ EMW system. The diameter and the thickness of the SS316l tube are 6.6 mm and 0.45 mm respectively. The discharge frequency of the 800 kJ EMW machine with single turn SS321 coil with titanium insert and the work pieces was 15 kHz. The work piece was rigidly fixed on the delrin holder. The thickness of the tube was less than the skin depth, so 0.6 mm thick copper driver tube was used on the SS 316l tube. The pulse discharge current of 900 kA is pumped in to the single turn coil, which generates 50 T field inside the coil and it exerts > 1600 MPa pressure on the work piece and the tube got welded to rod (Figure 9).

Figure 9. Welding of SS316l tube/rod6. Electromagnetic Welding of Dissimilar MetalsEMW technique overcomes problems in welding Cu-to-Al, Al-to-SS sheets that could be encountered in the conventional techniques. The EMW technology is one of the reliable methods that can be used for the dissimilar metal joints. It proves to be a potential welding technique for various industrial components requiring dissimilar sheet metal joints. The effect of process parameters on the strength and metallographic structures of the EM welds can be studied in detail for various dissimilar metal combinations. Bitter coil was used for welding of copper tube with SS rod (Figure 10). Pulse current of 176 kA at 10 kHz is discharged in to the bitter coil. At this parameter the welding of copper tube to SS rod was achieved and was confirmed by pull out, peel test and helium leak testing.

Figure 10. Welding of SS/copper

7. ConclusionEMW is a new and reliable technology to weld similar and dissimilar metal joints at high productively rates for industrial applications. The welding is performed at very high speed

and is achieved in <0.1ms. This technology has many advantages over conventional techniques such as precision, reproducibility, high production rate, no tool marks and the only limitation is that it can used for conducting metal. EMW machines, coils are designed and developed using high energy capacitor banks, high charge transfer switches and power supplies for various industrial applications.

AcknowledgementThe authors would like to thank Shri R K Rajawat, Associate Director BTDG and

Shri D Venkateshwarlu, Regional Director, BARC Visakhapatnam for the motivation and support to carry out electromagnetic welding research.

References

[1] Brown WF, Bandas J, Olson NT, “Pulsed magnetic welding of breeder reactor fuel pin end closures” Weld J 57(6):22–26 (1978)[2] Kumar Rajesh, Saroj P C, Kumar Satendra, Kulkrni M R, Kolge T S, Sharma S K, Shajju A, Das C, Sharma Archana, Shyam A, Bora D, “Design and Development of 100 KJ, 75 KHz Electro-Magnetic pulse welding system for ODS”,.IEEE Pulsed Power Conference, Texas Tech University, Austin, USA (2015)[3] Y. B. Park. "Design of Joints for the Automotive Spaceframe with Electromagnetic Forming and Adhesive Bonding", The Degree Doctor of Philosophy in the Graduate School of the Seoul National University, 2004, pp. 1-8.[4] Manoharan P, Manogaran A P, Priem D et al, “State of the art of electromagnetic energy for welding and powder compaction” Weld World 57: 867 (2013)[5] Kore S D, Date P P, Kulkarni S V, Kumar S, Rani D, Kulkarni M R, Desai S V, Rajawat R K, Nagesh K V, Chakravarty D P, “Application of electromagnetic impact technique for welding copper-to-stainless steel sheets”, International Journal of Advanced Manufacturing Technology 54(9):949-955 · June 2011[6] Rajawat R K, Desai S V, Kulkarni M R, Rani Dolly, Nagesh K V, Sethi R C, “Electromagnetic Forming - A Technique with Potential Applications in Accelerators”, 3 rd

Asian Particle Accelerator Conference, Gyeongju, Korea, 22 - 26 Mar 2004, pp.187[7] Sharma S K & Shyam A, “Development of compact rapid charging power supply for capacitive energy storage in pulsed power drivers” Review of Scientific Instruments, Volume 86, (2) (2015)[8]Haiping Y.U, Chunfeng L.I., ”Effects of current, frequency on electromagnetic tube compression”, Journal of Materials Processing Technology, Vol. 209, No. 2, pp. 1053- 1059, 2009 [9] Mishra S, Sharma S K, Kumar S, Sagar K and Shyam A, “40 kJ Magnetic Pulse Welding System for Expansion Welding of Aluminum 6061 tube” Journal of Material Processing Technology, 240, 168 – 175 (2017)