MA2004 Manufacturing Processes

A/P Zhou Wei A/ P Zhou Wei

http://www.ntu.edu.sg/home/mwzhou/

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Joining01 Fundamentals of Welding

13 min

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Joining Fundamentals of Welding

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Joining versus Assembly Joining - welding, brazing, soldering, and

adhesive bonding. • These processes form a permanent joint

between parts. Assembly - mechanical methods (usually) of

fastening parts together • Some of these methods allow for easy

disassembly, while others do not.

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Welding

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Welding

Joining process in which two (or more) parts are coalesced at their contacting surfaces by application of heat and/or pressure

•Many welding processes are accomplished by heat alone, with no pressure applied

•Others by a combination of heat and pressure

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Welding • Still others by pressure alone with no

external heat

• In some welding processes a filler material is added to facilitate coalescence

Essence of welding? Atomic bonding!

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Joining02 Types of Welding Processes

10.5min

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Types of Welding Processes

• Some 50 different types of welding processes have been catalogued by the American Welding Society (AWS)

• Welding processes can be divided into two major categories:

Fusion welding

Solid state welding

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Fusion Welding Joining processes that melt the base metals • In many fusion welding operations, a filler

metal is added to molten pool to facilitate process and provide bulk and added strength to welded joint.

• A fusion welding operation in which no filler metal is added is called an autogenous weld

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Some Fusion Welding Processes

• Arc welding (AW) Melting of metals accomplished by an

electric arc • Oxyfuel gas welding (OFW) Melting is accomplished by an oxyfuel gas

such as acetylene

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Arc Welding

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Solid State Welding (SSW) Joining processes in which coalescence

results from application of pressure alone or a combination of heat and pressure

• If heat is used, temperature is below melting point of metals being welded.

• No filler metal is added in solid state welding.

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Examples of SSW Processes

• Friction welding (FRW) - coalescence by heat of friction between two surfaces.

• Ultrasonic welding (USW) - coalescence by ultrasonic oscillating motion in a direction parallel to contacting surfaces of two parts held together under pressure.

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• Resistance welding - melting is accomplished by heat from resistance to an electrical current between faying surfaces held together under pressure

Examples of SSW Processes

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Ultrasonic Wire Bonding

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During Fusion Welding of an Al-Alloy parts of the Structure will Melt

1. True 2. False

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Ultrasonic Wire Bonding is a type of Fusion Welding

1. True 2. False

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Joining03 Types of Welds & Joints

6 min

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Butt Joint

Lap Joint

Corner Joint

Types of Joints

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Tee Joint Edge Joint

Types of Joints

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(a) inside single fillet corner joint; (b) outside single fillet corner joint; (c) double fillet lap joint; and (d) double fillet tee joint

Fillet Welds

Weld Joint

(a) (b) (c)

(d)

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(a) square groove weld, one side; (b) single bevel groove weld; (c) single V-groove weld; (d) single U-groove weld; (e) single J-groove weld; (f) double V-groove weld for thicker sections.

Groove Welds Weld Joint

(a) (b) (c)

(d) (e) (f)

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A small fused section between surfaces of two sheets or plates

• Used for lap joints • Most closely associated with resistance welding

Spot welds

Two sheet-metal parts

Cut-away view showing fused (welded section)

Partial cut-away view

Spot Welds

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Joining04 Physics of Welding – Power

Density 8.5 min

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Physics of Welding • Fusion most common means of achieving

coalescence in welding • To accomplish fusion, a source of high density

heat energy must be supplied to faying surfaces, so resulting temperatures cause localized melting of base metals (and filler metal, if used)

• For metallurgical reasons, desirable to melt metal with minimum energy but high heat densities.

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Power Density Power entering surface divided by corresponding

surface area: PD = power density, W/mm2 P = power entering surface, W A = surface area over which energy is entering,

mm2

APPD =

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Approximate Power Densities for Several Welding Processes

10,000 Electron beam

9,000 Laser beam

1,000 Resistance 50 Arc

10 Oxyfuel

W/mm2 Welding process

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Power Densities

A laser of fixed power is focused to different diameters. Which diameter

would give the greatest PD?

1. 10 mm 2. 5 mm 3. 2 mm 4. 1 mm 5. 0.5 mm 6. PD is the same

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Example

Two concentric circles of power distribution: • inner circle (φ 5mm) : 70% of power • outer circle (φ 12 mm) : 90% of power •Power of heat source P = 3 k W Find power densities.

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Inner circle (φ1 = 5 mm): A1 = 19.63 mm2

PD1 = P1 / A1 = (0.7)(3000) / (19.63) = 107 W⋅mm-2

Outer annulus (φ2 = 12 mm):

A2 = 93.4 mm2

PD2 = P2 / A2 = (0.9 – 0.7)(3000) / (93.4) = 6.4 W⋅mm-2

Example

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Overall power density: Total area A = 113.03 mm2

PD = P / A = (0.9)(3000) / (113.03) = 23.9 W⋅mm-2

Example

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Joining05 Physics of Welding – Unit

Energy for Welding 3 min

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Unit Energy for Melting Quantity of heat required to melt a unit volume

of metal • Symbol - Um • It is sum of: Heat to raise temperature of solid metal to

melting point o Depends on volumetric specific heat

Heat to transform metal from solid to liquid phase at melting point

o Depends on heat of fusion

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Unit heat for welding Um (J·mm-3):

Tm = Melting temp of material K = 3.33 × 10-6 for Tm in Kelvin (K)

Unit Energy for Melting

2mm kTU =

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Joining06 Physics of Welding – Heat

Transfer in Welding 6.5 min

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Two Heat Transfer Mechanisms in Welding

Not all of input energy used to melt weld metal

• Heat transfer efficiency f1 - actual heat received by workpiece divided by total heat generated at source

• Melting efficiency f2 - proportion of heat received at work surface used for melting; rest conducted into work metal

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Heat Available for Welding

where Hw = net heat available for welding; f1 = heat transfer efficiency; f2 = melting efficiency; and H = total heat generated by welding process

H ff H 2 1w =

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Heat Transfer Efficiency, f1 Proportion of heat received at work surface

relative to total heat generated at source • Depends on welding process and capacity to

convert power source (e.g., electrical energy) into usable heat at work surface: Oxyfuel gas welding processes are relatively

inefficient Arc welding processes are relatively

efficient

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Melting Efficiency, f2

Proportion of heat received at work surface used for melting; rest conducted into work metal

• Depends on welding process but also influenced by thermal properties of metal, joint configuration, and work thickness

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Melting Efficiency, f2

Metals with high thermal conductivity, such as Al and Cu, present a problem in welding because of rapid dissipation of heat away from heat contact area.

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Joining07 Physics of Welding – Energy

Balance Equation 8 min

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Energy Balance Equation

Hw = net heat energy delivered to operation, J

Um = unit energy required to melt the metal, J/mm3

V = volume of metal melted, mm3

VUH mW =

H ff H 2 1w =

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If time factor (rate) is considered:

HRw = rate of heat energy delivered WVR = Welding Volume Rate (mm3/min) Aw = weld area v = welding speed

Energy Balance Equation

( )vAUHRffHR

WVRUHR

wmW

mW

===

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VUH mW = H ff H 2 1w =

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Overall Efficiency

Heat Source

Welding Materials

Welding Geometry

Overall Efficiency

Heat Source

Welding Material

Welding Geometry

( )wm AUHRffv 21=

Energy Balance Equation

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Example Material: low carbon steel Given: HR = 3.5kW, f1 = 70%, f2 = 50%, Tm = 1,760 K Aw = 20 mm2

Find optimal welding speed v.

Solution:

32 3.10 mmJkTU mm ==

( ) ( )( )( )( )( ) smm

AUHRffvwm

6203.10

500,35.07.021 ≈==

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The Unit Energy Depends only on the Material Welded

1. True 2. False

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Joining08 Video on Joining Processes

14.5 min

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Good example of solid state welding - forge welding

Examples of mechanical assembly – different from welding, no atomic bonding

Pay attention to oxyfuel gas welding and arc welding

Video on Joining Processes

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Joining09 Oxyfuel Gas Welding

14 min

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Oxyfuel Gas Welding (OFW) Fusion welding operations that burn various

fuels mixed with oxygen • OFW employs several types of gases, which is

the primary distinction among the members of this group.

• Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts

• Most important OFW process is oxyacetylene welding

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Oxyacetylene cutting

http://www.youtube.com/watch?v=o2gKMfuhcBo

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Oxy-Acetylene Welding

http://www.youtube.com/watch?v=uFX_RvWrzaQ

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Oxyacetylene Welding Operation (OAW)

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Torch Used in OAW

Schematics of torch used in OAW. The acetylene valve is opened first; gas is lit with a spark lighter; then O2 valve opened and flame adjusted.

Tip

Tip Mixer O2

acetylene

O2

Valves

Mixer

MIXING CHAMBER

Enlarged view

Torch head

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Torch Used in OAW

Basic equipment used in oxyfuel-gas welding. For safety, all threads on acetylene fittings are left-handed, whereas those for O2 are right-handed. Oxygen regulators are usually painted green, acetylene regulators red.

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Acetylene (C2H2) • Most popular fuel among OFW group because

capable of higher temperatures than any other - up to 3,480°C

• Two stage chemical reaction of C2H2 and O2: • First stage reaction (inner cone of flame):

•

• Second stage reaction (outer envelope): •

This image cannot currently be displayed.

2222 2 HCOOHC exothermic + →+

OHCOOHCO exothermic2222 25.12 + →++

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• Maximum temperature reached at tip of inner cone.

• Outer envelope spreads out and covers work surfaces to shield from surrounding atmosphere

• f1 = 0.1 - 0.3.

Acetylene (neutral flame )

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Oxyacetylene Flames Used in Welding Neutral Flame Oxidizing Flame

Carburizing (reducing)

Flame

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Types of Flames

•Neutral Flame: The ratio of acetylene and oxygen is 1:1. •Oxidizing flame: Greater oxygen supply. Harmful except for Cu and Cu-based alloys. •Reducing (carburizing) flame: The ratio of oxygen is deficient. Temperature is low. Normally used for brazing, soldering, and flame hardening.

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An oxidizing Flame is always used to Weld Plain Carbon Steel

1. True 2. False

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Joining10 Arc Welding - Introduction

13.5 min

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Arc Welding (AW)

Fusion welding process in which coalescence of metals achieved by heat from electric arc between an electrode and work.

• Electric energy from the arc produces

temperatures ~ 5,500°C, hot enough to melt any metal.

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Arc Welding (AW)

• Most AW processes add filler metal to increase volume and strength of weld joint.

• Same basic process also used in arc cutting.

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What is an Electric Arc?

• Discharge of electric current across a gap in a circuit.

• Sustained by an ionized column of gas (plasma) through which current flows .

• To start the arc in AW, electrode is brought into contact with work and then quickly separated from it by a short distance.

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Electrical Arc

http://www.youtube.com/watch?v=6GiIVze2Tac

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• A pool of molten metal formed near electrode tip.

• As electrode is moved along joint, molten weld pool solidifies in its wake.

Arc Welding

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Basic configuration and electrical circuit of an arc welding process

Arc Welding

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Power Source – DC / AC – Power HR = voltage × current = E I HRw = f1 f2 I E = Um Aw v or

f1, f2: heat transfer and melting efficiency respectively. Um: unit energy required to melt metal, Aw: weld cross-sectional area, v: travel velocity, HRw : rate of heat generation

( )wm AUEIffv 21=

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Gas tungsten arc welding; Current: 300 A, Voltage: 20V; f2=0.5, f1= 0.7; Um=10 J/mm3. Calculate:

(a) power in the operation, (b) rate of heat generation at weld, (c) volume rate of metal welded.

Example

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Solution: (a) P= IE = 300 A x 20 V = 6kW

(b) f2=0.5, f1= 0.7. Hence HRw = f1f2IE = 0.7 x 0.5 x 6000 = 2.1kW = 2.1kJ/s (c) Volume rate of metal welded, WVR = 2,100/10 = 210 mm3/sec

Example

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Joining11 Arc Welding – Shielding

7 min

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Arc Shielding At high temperatures in AW, metals are

chemically reactive to oxygen, nitrogen, and hydrogen in air.

Mechanical properties of joint can be seriously degraded by these reactions.

To protect operation, arc must be shielded from surrounding air in nearly all AW processes.

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Arc Shielding Arc shielding is accomplished by:

Shielding gases such as argon, helium, and CO2

Flux.

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Which of the following gases is not used in arc-shielding?

1. Ar 2. O2

3. CO2

4. He 5. CO

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Role of Flux in Welding? • A substance that prevents formation of

oxides and other contaminants in welding, or dissolves them and allows removal.

• Provides protective atmosphere for welding.

• Stabilizes arc.

• Reduces spattering.

spattering

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Application Methods

• Pouring granular flux onto welding operation.

• Stick electrode coated with flux material that melts during welding to cover operation.

• Tubular electrodes in which flux is contained in core and released as electrode is consumed.

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Joining12 Arc Welding – Two Types of

Electrodes 4 min

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Two Basic Types of AW Electrodes

• Consumable – consumed during welding process. Source of filler metal in arc welding.

• Nonconsumable – not consumed during

welding process •Any filler metal must be added

separately.

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Consumable Electrodes • Types: Welding rods (also called sticks) are ~30 cm

long and ~8 mm in diameter and must be changed often. Weld wire can be continuously fed from

spools with long lengths of wire, avoiding frequent interruptions.

• In both rod and wire forms, electrode is consumed by arc and added to weld joint as filler metal

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Nonconsumable Electrodes

• Made of tungsten which resists melting.

• Gradually depleted during welding (vaporization is principal mechanism).

• Any filler metal must be supplied by a separate wire fed into weld pool.

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Joining13 Arc Welding – Processes

using Consumable Electrodes

6 min

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AW Processes that use Consumable Electrodes

• Shielded Metal Arc Welding

• Gas Metal Arc Welding

• Flux-Cored Arc Welding

• Submerged Arc Welding

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Shielded Metal Arc Welding (SMAW)

• Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding.

• Also called "stick welding“.

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Shielded Metal-Arc Welding

•Schematic of shielded metal-arc welding process.

•About 50% of all large-scale industrial welding operations use this process.

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Shielded Metal-Arc Welding

Schematic of shielded metal-arc welding operation.

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Submerged-Arc Welding

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Joining14 Arc Welding – Processes

using Nonconsumable Electrodes

8 min

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AW Processes that use Nonconsumable Electrodes

• Gas Tungsten Arc Welding

• Plasma Arc Welding

• Carbon Arc Welding

• Stud Welding

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Gas Tungsten Arc Welding (GTAW)

Uses nonconsumable tungsten electrode and inert gas for arc shielding.

• Melting point of tungsten = 3,410°C.

• Also called TIG welding (Tungsten Inert Gas welding) .

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Gas Tungsten Arc Welding (GTAW)

• Used with or without a filler metal

•When used, filler metal is added to weld pool from separate rod or wire

Applications: aluminum and stainless steel most common.

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GTAW

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GTAW Typical equipment used for gas tungsten-arc welding operations.

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Stud welding

http://www.youtube.com/watch?v=Ljz6twH-Pc4

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Joining15 Other Fusion Welding

Processes 3.5 min

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Other Fusion Welding Processes FW processes that cannot be classified as arc,

resistance, or oxyfuel welding

• Use unique technologies to develop heat for melting.

• Applications are typically unique

• Processes include:

Electron beam welding

Laser beam welding

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Comparison of Laser-Beam and Tungsten-Arc Welding

(a) electron-beam or laser-beam welding to (b) conventional (tungsten-arc) welding.

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Example of Laser Welding

Laser welding of razor blades.