Electron beam welding

Electron Beam WeldingA.M. Chavan

SGGSIE & T Nanded (M.S.) - India

Need ?

a. b.a. Multipass submerged arc weldingb. Single pass EBW

Availability of advance materials (metals and Non metals)

Patterns: A) straight beam, B) circle, C) eight, D) arrow head Welding in inaccessible regions

William Rontengen in 1800s found that beam of electron is suddenly stopped by impact with target, then it start heating and melting the target.

Dr. Karl Heinz (German) – Development of first practical welding machine in 1958.

Discovery

It is fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined.

Free electrons in vacuum can be accelerated, with their paths controlled by electric and magnetic fields.

In this way narrow beams of electrons carrying high kinetic energy can be formed, which upon collision with atoms in solids transform their kinetic energy into heat. 

Introduction

Electron beams are composed of electrons that are charged particles having a rest mass of 9.1x10-31 kg and can be accelerated in electron guns to relativistic velocities, giving them high kinetic energies.

At 10 kV (13 hp), electrons travel at approximately 20% of the speed of light, while at 200 kV (270 hp) they travel at approximately 70% the speed of light.

Principle

Electron beam welding process is carried out in vacuum. In this process, electrons are emitted from the heated filament called electrode.

These electrons are accelerated by applying high potential difference (30 kV to 175 kV) between cathode and anode.

The higher the potential difference, the higher would be the acceleration of the electrons. The electrons get the speed in the range of 50,000 to 200,000 km/s.

When high kinetic energy electron beam strikes the workpiece, high heat is generated resulting in melting of the work material. Molten metal fills into the gap between parts to be joined.

An EBW set up consists of the following major parts1. Electron Gun2. Power Supply Unit3. Vacuum Chamber4. Workpiece Handling Device

Electron Beam Welding Setup

SEM

XRD

1. Electron Gun The electron beam is most often formed by a triode-

style electron gun under high vacuum conditions.

Electron beam

Grid Cup

+

-

The triode assembly consists of a cathode, a heated source (emitter) of electrons that is maintained at some high negative potential.

A grid cup, a specially shaped electrode that can be negatively biased with respect to the hot cathode emitter (filament);

And an anode, a ground potential electrode through which the electron flow passes in the form of a collimated beam.

The hot cathode emitter (filament) is made from a high-emission material, usually tungsten or tantalum, which is usually available in wire, ribbon, or sheet form.

This emitter material is fabricated into the desired shape for being either directly or indirectly heated to the required emitting temperature of approximately 2500o C.

Other materials, such as lanthanum hexaboride (LaB6), have also been used as filament material.

There are two type of electron guns◦ Self accelerated – Electrons are accelerated by applying

potential difference between the cathode and anode.◦Work accelerated - potential difference is applied between

workpiece and anode (Diode type).Metal Melting Temp. Deg. Cel Cost /Kg Tungsten 3422 $25 Tantalum 3020 $180 to $ 190

LaB6 2210 US $6-12

Emitter/Filament : - It generates electrons on direct or indirect heating.

Anode: Positively charged element near cathode across which high voltage is applied to accelerate electrons. For high voltage equipment's potential difference 70-150 kV and for low voltage equipment’s potential difference is 15-30 kV.

Grid Cup: Negative voltage with respect to cathode is applied. Grid cup controls the beam.

Focusing of Electron Beam

Focusing

It has two parts: Electron focusing lens and deflection coil.

Electron focusing lens focuses the beam into work area.

The focusing of the electrons can be carried out by deflection of beams.

This focusing lense reduces the diameter of the electron beam as it continues in its passage and focuses the stream of electrons down to a much smaller beam cross section in the plane of the workpiece.

Thus reduction of beam diameter results into producing very small high intensity beam spot.

deflection coil (positioned below the magnetic lens) can be employed to “bend” the beam, thus providing the flexibility to move the focused beam spot.

..contd.

It mainly consist a high power DC power supply source for gun, focusing and deflection coil.

It provides power supply for acceleration of the electrons.

The potential difference for high voltage equipment ranges from 70-150 kV and for low voltage equipment 15-30 kV. The current level ranges from 50-1000 mA.

The amount of current depends upon the diameter and type of the filament.

2. Power Supply Unit

AC or DC current is required to heat the filament for emission of electrons. However DC current is preferred as it affects the direction of the beam.

Generally EBW performed in vacuum. The “gun” portion of an electron gun/column assembly

generally is isolated from the welding chamber through the use of valves when desired, or by using vacuum dividers when employing medium or non-vacuum systems.

Vacuum in the gun region is needed to maintain gun component cleanliness, prevent filament oxidation, and impede high-pressure short circuiting between the cathode and the anode or the filament and the grid cup.

3. Vacuum Chamber

Most EBW is done in a vacuum environment where the maximum ambient pressure is less than 0.13 Pa (1x10-3 torr). Maintenance of this degree of vacuum is important because of the effect that ambient pressure has on both the beam and the weld produced.

Based on vacuum intensity the EBW have following 3 types1. Non vacuum EBW (EBW-NV) – atmospheric pressure2. Medium Vacuum EBW (EBW – MV) – 133 to 3.3 x 106 mpa

(10-3 to 25 torr)3. High Vacuum EBW (EBW-HV) – 0.13 to 133 mpa (10-6 to

10-3 torr)

Effect of Vacuum on Beam

Beam scatter due to collision of electrons with atmospheric molecules

Reduced penetration due to beam scattering

Increase in the beam diameter reduces the power densityProduces welds with greater width and less penetration.

Chamber pressure vs Beam

Depth of penetration vs. vacuum in prescribed time

Quality and precision of the weld profile depends upon the accuracy of the movement of work piece.

There is also provision for the movement of the work piece to control the welding speed.

The movements of the work piece are easily adaptable to computer numerical control.

Work Piece Handling Device

Major Process Parameters are1. Accelerating voltage2. Beam current3. Welding speed4. Beam Focusing

Process Parameters

Accelerating voltage A value of electrical potential,

usually expressed in kilovolts, being utilized to accelerate and increase the energy of the electrons being emitted by an electron beam gun.

Increase in the voltage results into increase in the speed of electrons.

At 10 kV electrons travel at approximately 20% of the speed of light, while at 200 kV they travel at approximately 70% the speed of light.

Electron beam current Close relation between

electron beam current and depth of penetration.

Beam current: measure of the quantity of charge (ie: number of electrons), usually expressed in units of milliamperes (mA), that flow per unit time in an electron beam

Krishnan et al [2013], IJERT Volm 02, Issue 06

Krishnan et al [2013], IJERT Volm 02, Issue 06

Welding speed Welding speed directly

affects on depth of penetration of electron beam into work piece

Higher speeds results into lower depth of penetration

http://www.yourarticlelibrary.com/welding/electron-beam-welding/ebw-equipment-joint-design-and-applications-metallurgy/97331/

Keyhole Mode

Keyhole vs Conduction

By using this technique one can weld deep with very narrow width weld pool.

Only Possible with high energy density processes like EBW, LBW & Plasma Arc Welding.

This deep-weld effect allows now a days penetration depths into steel materials of up to 300 mm

a) Impact of high energy electron beam on w/p surface. The penetration depth into the workpiece is very low, just a few μm. Most of the kinetic energy is released in the form of heat.

b) The high energy density at the impact point causes the metal to evaporate thus allowing the following electrons a deeper penetration.

c) This finally leads to a metal vapour cavity which is surrounded by a shell of fluidmetal, covering the entire weld depth.

d) Capillary action results into formation of weld

Capillary action

1. At the front side of the cavity new material is molten which, to some extent, evaporates, but for the most part flows around the cavity and rapidly solidifies at the backside..

2. In order to maintain the welding cavity open, the vapour pressure must press the molten metal round the vapour column against the cavity walls, by counteracting its hydrostatic pressure and the surface tension.

F1=F2+F3 Equilibrium for good weld pool

I – Equilibrium StateII – Unstable pressure (high) exposes molten backside of vapor cavity to a strong and irregular shape changeIII – Uneven distribution of pressures (low) results into formation of voids/pockets inside the solidified weld pool due to improper collapsibility of molten metal

Angle β increases with welding speed results into formation of poor & shallow welds

Part Configuration Weld Configuration Surface Geometry Melt Zone Configuration Joint Design◦ For Butt Weld◦ For Corner Weld◦ T – Joints

Design Considerations for Electron beam welding (EBW)

Part Configuration

Complex configuration possible for easy to weld metals i.e. having soft and low yield points with lower shrinkage.

Shrinkage stresses are better managed in such designs suitable for difficult to weld metal.

Weld Configuration

Not recommended—maximum confinement of molten metal, minimum joining cross section (arrows); wastes beam energy for melting, nonfunctional metal.

Most favorable—volume of melt not confined; maximum joining cross section (arrows).

Not recommended—maximum confinement of melt (unless gap is provided); joining cross section less than plate cross section.

Most favorable—minimumconstraint and confinement of melt; minimum internalstresses;

Not recommended—two successive welds;second weld is fully constrained by the first weld and shows strong tendency to crack.

Surface Geometry

Usually, EBW does not use or need filler wire. Therefore, V-grooves or large joint gaps are not required; in fact, too much of a gap could be detrimental to the process.

A step in the surfaces at the joint line is also undesirable. Any small lateral shift of the beam from the low to the high side, or vice versa,changes the penetration to some extent.

X

√

Melt zone Configuration

Welds with parallel sides are preferred over welds that are more triangular shaped.

As a general rule, full penetration EB welds have a tendency to be more parallel than partial penetration welds that have a wider nail head and are tapered at the root.

Parallel

Triangular

√X

Joint Design – Butt Weld

Butt joints are the most common of the basic joint types used in EBW.

a, b, c – Least expensive but weld joints with no edge preparation required d to g – Self aligning and suitable for circular or circumferential welds

Corner Weld

A –simplest and more economical corner weld, H- corner flange weld usually made only on thin stock

T - Joints

A – melt through or blind weld is simple but more sensitive to corrosionB –suitable for thinner sectionsc- suitable for sections above 25mm or more

Applications of EBW Mostly used in joining of refectory materials like

columbium, tungsten, ceramics. High Precision Welding of electronics components. High precision welding of nuclear fuel elements. Special alloy components of jet engines. Pressure vessels for rocket. Joining of Dis similar metals. Welding of Titanium medical implants.

Difficult to melt and weld metals

High penetration to width can be obtained. High welding speed is obtained. Material of high melting temperature can be welded. Superior weld quality due to welding in vacuum i.e.

welds are corrosion free. Distortion is less due to less heat affected zone. Inaccessible joints can be made. Very wide range of sheet thickness can be joined (0.025

mm to 100 mm).

Advantages

Very high equipment cost. Transportation of equipment is not easy. Vacuum is required. Skilled person is needed. X-rays generated during welding 60 kV 4 kW (610 mm3) electron beam welder including

CNC controlled work manipulation systems £220,000.00 i.e. 17609882.40 INR (1 £ = 80.02 INR)

Disadvantages