1

The Ins and Outs of Laser Welding

Professor Stewart Williams

Welding Engineering Research Centreg g g

Cranfield University

Presentation Summary

General laser welding regimes Keyhole Keyhole

Conduction

Hybrid Keyhole

Hybrid Conduction

Specialist laser welding Pulsed

RemoteRemote

Some practical laser welding issues Gas shielding and screening

Defects

Summary

2

Main Process Parameters•Intensity•Interaction Time•Process Energy

Laser Material Interaction

Reflections

Melting

Evaporation

Plasma

Keyhole formation due to vapour pressure

Process RegimesConduction mode

Typical ApplicationsWelding/surface treatments

Melt Expulsion

g

Keyhole mode

Component

Welding

Cutting/Drilling

Laser Welding Techniques - Keyhole

6mm

Fusion Zone

Ti-6Al-4V.

6mmHeat

Affected Zone 12mm

Aluminium ExtrusionMild steel 10kW, travel speed 2.2 m/min

3

Laser Welding Techniques - Keyhole

Benefits High aspect ratio deep penetration welds High aspect ratio deep, penetration welds

High welding speeds

Low heat input leading to reduced distortion

Heat can be directed very accurately – useful for fillet welding

Issues Joint fit up

Incorporation of filler materialIncorporation of filler material

Defects Porosity

Cracking

Spatter

Laser Welding Techniques -Conduction

Beam diameter increased with constant power and travel speed

4

Aluminium 2024 T351, no filler wire

Laser Welding Techniques -Conduction

Courtesy BAE Systems ATC

6.35 mm

Laser Power 4000W

Laser Power 3000W, Spot Diameter 12mm, Weld Speed 90mm/min

,

No filler wire

Laser Power 4000WSpot Diameter 11.5mmWeld Speed 0.5mm/s

5mm

StainlessSteel – no filler

Conduction Welding – Metal Example – Stainless Steel – diode laser 5m/min

Welding of the kitchen sink!

Photos: IREPA Laser

Note the absence of spatter, humping and there will be no porosity

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Laser conduction welding – other process

Soldering

BrazingBrazing

Plastic welding

Courtesy Rofin

Conduction Welding – Plastics -Principle

Courtesy Rofin, see website for more details

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Laser Welding Techniques -Conduction

Benefits Very stable Very stable

Extremely high quality welds (no keyhole or arc)

No loss of material – no joint prep required

Large spotsizes Low cost diode sources

Large standoff distances

Very tolerant of joint mismatch

Heat can be directed very accurately – useful for fillet welding

Issues Surface reflectivity

Relatively slow

Laser Welding Techniques – Keyhole and ConductionBenefits

High aspect ratio deep, penetration welds

Benefits Very stable

Extremely high quality welds High welding speeds

Low heat input

Heat can be directed very accurately

Issues Defects

Incorporation of filler material

No loss of material

Large spotsizes

Heat can be directed very accurately

Issues Relatively slow

High surface reflectivity Joint fit up

Solution to these issues is hybrid techniques

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Laser Welding Techniques – Hybrid Keyhole - Equipment

Laser Welding Techniques – Hybrid Keyhole

TIG – MIG Torch

GMAW Weld in A-36,830J/mm, 270 mm/sec,5.7 kg/hr wire,1 m/min travel

laser

Filler Wire(with TIG)electrode

InertGasShield

Laser beam

Autogenous Nd:YAGLaser Weld in A-364 kW, 240 J/mm,1 m/min travel

plan view

arc

Welding direction

Hybrid Nd:YAG Laser Weld inA-36; 4 kW, 1060 J/mm,5.7 kg/hr wire; 1 m/min travel

Courtesy Navy Joining Centre

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Hybrid Keyhole Welds

2.5mm

X70 Steel

Hybrid Keyhole Fillet WeldCourtesy Navy Joining Centre

X100 Steel

High Strength Pipeline Root Welds4 m/min travel speed

Laser Welding Techniques – Hybrid Keyhole

Benefits Same features as laser keyhole welding but with additional

benefits of Increased tolerance to mismatch Easier to incorporate filler material Improved stability of the keyhole Increased welding speed

No or small improvement in penetration depth Increase in distortion (but still much less than arc on its own)

Limitations A lot of equipment round the weld zone Many process variables For non-linear welding rotation of the arc and laser position is

needed

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Hybrid Conduction Weld Profile - Aluminium

Laser Welding Techniques – Hybrid Conduction

TIG – MIG Torch

laser

12.7mm

Al2024, 3kW Laser, 2kW TIG, 0.1m/min

electrode

InertGasShield

laser

Laser beam

arc

Welding direction

Single pass autogenous butt weld – Stainless Steel

10mm

Laser Welding Techniques – Hybrid Conduction

Benefits

Same benefits as laser conduction welding but with additional benefits ofadditional benefits ofNo problems with absorption of laser power

Very high penetration depths possible

Limitations A lot of equipment round the weld zone

Many process variables

For non linear welding rotation of the arc and laser position is For non-linear welding rotation of the arc and laser position is needed

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Laser Welding Techniques – Comparison of Processes

Process Speed Thickness Gap Distortion Fill Process

Application and Material Dependent

Process Speed Thickness Gap Tolerance

Distortion Fill rate

Process Quality**

TIG 1 1 1 1 1 1

MIG 1 1 1 1 2 0.75

FSW* 1 10 0.25 1 N/A 10

Laser Keyhole 10 3 0.25 5 0.25 0.25

*Aluminium only**Refers to the process stability and likelihood of pores, spatter etc.,

Laser Conduction 1 3 1 1 2 5

Hybrid Keyhole 5 3 1 2 1 0.5

Hybrid Conduction 1 5 2 1 4 5

Pulsed Laser Welding Same process as continuous welding (i.e. welding mode

is determined by the intensity and the interaction time (the pulse duration in this case))

Additi l t ld t i d th d fAdditional parameters weld spotsize and the degree of overlap used.

Structural welds use 50-60% overlap, hermetic seals use 85-90% overlap

Spotsize 25% overlap 75% overlap

p

In seam weld process rate is given by the spotsize times the overlap proportion and the laser repetition rate

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Pulsed Laser Welding - Applications

Applications generally target small components for either joining or vacuum sealing

Remote Welding – Robot + Scanning Mirrors

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Primary System

Parameters Beam radius

Laser Welding – Parameters

Primary Interaction

Parameters Intensity ea ad us

Laser Power

Travel Speed

te s ty

Interaction Time

Process Energy

Secondary Parameters Focus position with respect to

material surface

Lens focal length (cone angle)

Optical depth of focus

Gas flow rate and type

HB9

Laser Welding in Practice – Potential FlawsGeometry

Undercut (especially if not using filler wire) Humping (especially at high speeds) Excess metal and sagging

1mm

74 500 W/cm250 ms

2.65

-200

0

200

400

tud

iona

l Res

idua

l Str

ess

[MP

a] Sample A1

Sample A2+380 MPA

0 MPa

Excess metal and sagging

Cracking Laser welding can be more susceptible to

this due to high cooling rates

Porosity Keyhole porosity is an added feature of laser welding caused by

keyhole instability - often relatively large pores spaced regularly ¾ of the way down

Keyhole mode0.3mm

-15 -10 -5 0 5 10 15 20 25 30

-400

Position from the weld centre line [mm]

Long

it

Weld centre

Laser welds can be more susceptible to usual gas porosity due to the high cooling rates – the gas does not have time to escape.

Spatter (especially if using a high intensity)Residual Stresses

Peak residual stresses are similar to other welding processes but distortion usually less as the overall load produced is lower.

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Laser and Hybrid Welding – When to use it and when not to

Use it when you want High productivity (speed) High productivity (speed)

Large single pass penetration

Low heat input and therefore low distortion

Small weld volumes

Don’t use it when There is some other way of achieving the same thing (except e-

beam))