07DLecture laser welding systems

laser welding systems

Laser Welding – Section 1 Introduction to Lasers and Laser Welding Equipment

Property CO 2 Laser Nd:YAG Lasers

Lasing medium CO2 + N 2 + He gases

Single crystal rod neodymium doped yttrium aluminium garnet Radiation

Excitation method Electric discharge Flash lamps or diode

Consumables CO2 , N 2 , He, electricity Flash lamps, electricity

Beam transmission Polished metal mirrors Fibre optics or mirrors

Laser Welding – Section 1 Introduction to Lasers and Laser Welding Equipment

Laser Welding – Section 1 Introduction to Lasers and Laser Welding Equipment

Laser Welding – Section 1 Introduction to Lasers and Laser Welding Equipment

Choice of a given Laser system is governed by ultimate application i.e

1 Either Two Dimensional Processing – Simpler systems as moving work piece is involved CO2 Lasers generally more attractive

2 Or For three-dimensional processing, the use of Nd:YAG lasers with

robotic manipulation of the fibre-optic cable is more attractive, providing the production rates are satisfactory for the thicknesses and powers

available

Laser Welding – Section 1 Introduction to Lasers and Laser Welding Equipment

Laser Welding – Section 2

Laser – Material Interactions

The property of the laser which makes it useful for material processing is focusability

Depending on the laser type and power, the laser output beam will have a comparatively large diameter (10-40mm) and its power density a few watts/mm2

Using a lens (e.g KCl or ZnSe, for CO2 lasers, glass for Nd:YAG lasers) or curved mirror (spherical or paraboloidal) the beam can be focused to a small spot (0.1-1.0mm diameter)

The power density then produced (up to 100kW/mm2) is capable of rapid material heating and even vaporisation of most metals and ceramics

Laser Welding – Section 2

Laser – Material Interactions

• If a workpiece, consisting of a material that does not transmit or reflect the beam too strongly, is positioned at the focal point, the light energy is converted into heat

• In materials processing, the amount of heat, the area over which it is applied and its duration are controlled to produce the desired effect

This will normally involve one of the following:

1 VAPORISATION (drilling and cutting)

2 MELTING (welding or surfacing)

3 SUB-MELTING (transformation hardening or annealing)

Laser Welding – Section 2

Laser – Material Interactions

• For welding, the intensely focused laser beam is used to heat and vaporise material rapidly, with the focus normally positioned near the top surface of a suitable weld joint configuration

Two welding regimes can be identified, depending on applied

power density

• Below about 25kW/mm2 the process is a fusion, conduction

limited, process In laser power terms that implies about 1kW of beam power

• Above 25kW/mm2 there is enough power density to initiate a

keyhole weld

Laser Welding – Section 2

Laser – Material Interactions

Laser Welding – Section 2

Laser – Material Interactions

• Above 25kW/mm2 – within continuous wave CO2 lasers

• When the focused spot impinges on the metal surface it instantly vaporises a small amount of metal and the vapour is further

heated to the ionised state

• This plasma cloud acts as an absorbing black body, melting more metal until a steady state is reached with a deep keyhole kept

open by the turbulent ionised vapour within it

• The temperature within the keyhole exceeds 15,000°C and as the beam is moved along the joint line, oncoming metal melts, flows around the keyhole and re-solidifies

• The resulting fusion welds are of deep penetration, high aspect ratio form, and total heating and hence distortion is minimised

The plasma cloud often grows into the air above the weld and can disrupt the beam focus and absorb power if it gets too large To control the plasma, a jet of gas (commonly helium) is often used, which is usually incorporated into the general weld shielding

Laser Welding – Section 2

Laser – Material Interactions

• Above 25kW/mm2 – For Nd:YAG Lasers

• For Nd:YAG lasers, until recently, pulsed operation was more

common than continuous wave operation

• The deep keyhole penetration is achieved for pulsed Nd:YAG

lasers by using higher peak powers for a short period of time

(typically 1-10ms)

• The keyhole formed collapses prior to the onset of the next pulse

• This creates a series of overlapping pulses which can produce a seam weld Alternatively, a single pulse can be used to produce a spot weld

• The development of higher average power Nd:YAG lasers which can operate in a continuous wave mode or with limited peak power enhancement capabilities is enabling continuous keyhole welding

to be carried out in a manner similar to continuous wave CO2

Laser Welding – Section 2

Laser – Material Interactions

• Effect of steel type

• Effect of coating type

• Zinc coated steels

• Aluminium coated steels

• Zn-Al alloyed coatings

• Zn-Ni coatings

• Austenitic stainless steels

• Ferritic stainless steels

• Martensitic stainless steels

• Duplex and super duplex stainless steels

• Titanium alloys

• Nickel-based alloys

• Copper-based alloys

• Magnesium alloys

Laser Welding – Section 3 Laser – Joints and Process Control For Sheet Metals, there are four basic Joint Configurations:

1 Lap joint 3 Hem Joint

2 Butt joint 4 Edge Joint

Laser Welding – Section 3 Laser – Joints and Process Control

Laser Welding – Section 3 Laser – Joints and Process Control The main process control parameters are:

1 laser power

2 welding speed

3 beam focus position

4 plasma control gas (composition/position)

Control of these parameters is generally automated to give high speed reproducible welds

Laser Welding – Section 3 Laser – Joints and Process Control

1 Effect of laser type

2 Effect of focusing system

3 Effect of gas shielding

4 Gas delivery systems

5 Gas type

• Helium

• Argon

• Nitrogen

• Carbon dioxide

• Gas mixtures

6 Underbead shielding Underbead shielding

Laser Welding – Section 4

Weld Quality

Joint Quality Factor Laser welding

Joint

thickening/thinning/inde

ntation

Joint thickening of up to 10% sheet thickness possible

If poor fit up, weld undercut will occur A maximum gap of 10% of sheet thickness should be set for both butt and lap joints for autogenous joints

Width of joint at

interface For lap/hem joints, the weld width at the interface should be at least equal to the sheet thickness

A weld underbead should be present for butt welds A minimum width of half the top bead width is recommended Marking on surface Check for surface breaking porosity, cracking, excessive

oxidation

Laser Welding – Section 4

Weld Quality – Joint Quality Factors

Joint Quality Factor Laser welding

Measurement

techniques Weld top bead widthPenetration depth for partially penetrating welds

Weld width at interface Weld underbead width

Metallurgical

structure Fusion weld, check for microstructural flaws such as cracking, porosity, segregation

Hardness Increased weld metal/HAZ hardness for steels, depends on

steel type and laser conditions, normally 2-3x parent material value

Exterior appearance The weld bead, although small, cannot be used for exterior

panels without grinding Coating damage Coating removed in weld zone.

Weld spatter

Laser Welding – Section 4

Weld Quality – Typical Weld Flaws

Weld Flaw Typical Acceptance Criteria Comments

Lack of

fusion Joint line should be fully fused alignment Check beam to joint

Lack of

penetration

Underbead width should be at least half of top bead width for fully penetrating welds

For partially penetrating lap joints, the weld width at the interface should

ideally be equal to the top sheet thickness

Check laser power and speed Check for focus position Check gas shielding arrangement

Porosity

Porosity with pore sizes >0.5mm should be avoided, if possible, for

Check on gas shielding arrangement

Check on levels of moisture

Laser Welding – Section 4

Weld Quality – Typical Weld Flaws

Weld Flaw Typical Acceptance Criteria Comments

Cracking No weld cracks to be

present

Check on material specification Check on welding speed

Check on weld shape

Undercuts/notches of sheet thickness should be Undercuts/notches of >10%

avoided

Check on fit up and welding conditions

Use wire feed, beam weaving or local force application, if gaps are too large

Misalignment of

sheets Misalignment of up to 50% of sheet thickness can be

accommodated

Check on fit up to reduce misalignment

Excessive

oxidation/sooting weld Avoid discolouration of Check on gas shielding arrangement