Guide to Laser Welding

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Guide to Laser Welding

Summary:

Laser beam welding is a technique in manufacturing whereby two or
more pieces of material (usually metal) are joined by together through
use of a laser beam. Laser stands for Light Amplification by Stimulated Emission of Radiation. It is a non-contact process that requires access to the weld zone from one side of the parts being welded.

The weld is formed as the intense laser light rapidly heats the material – typically calculated in Milli-seconds.

The laser beam is a coherent (single phase) light of a single
wavelength (monochromatic). The laser beam has low beam divergence and
high energy content and thus will create heat when it strikes a surface

The primary types of lasers used in welding and cutting are:

  • Gas lasers: use a mixture of gases such as helium and nitrogen. There are also CO2 or carbon dioxide lasers. These lasers use a low-current, high-voltage power source to excite the gas mixture using a lasing medium. Operate in a pulsed or continuous mode.Carbon dioxide lasers use a mixture of high purity carbon dioxide with
    helium and nitrogen as the lasing medium. CO2 lasers are also used in dual beam laser welding where the beam is split into two equal power beams.
  • Solid state lasers: (Nd:YAG type and ruby lasers) Operate at 1micrometer wavelengths. They can be pulsed or operate continuously. Pulsed operation produced joints similar to spot welds but with complete penetration. The pulse energy is 1 to 100 Joules. Pulse time is 1 to 10 milliseconds.
  • Diode lasers

Lasers are used for materials that are difficult to weld using other methods, for hard to access areas and for extremely small components. Intert gas shielding is needed for more reactive materials.

Laser Beam Welding Examples

Overview

Laser Beam Welding Process Video

Laser beam welding (LBW) is a welding process which produces
coalescence of materials with the heat obtained from the application of a
concentrate coherent light beam impinging upon the surfaces to be
joined.

The focused laser beam has the highest energy concentration of
any known source of energy. The laser beam is a source of
electromagnetic energy or light that can be pro jetted without diverging
and can be concentrated to a precise spot. The beam is coherent and of a
single frequency.

Gases can emit coherent radiation when contained in an optical
resonant cavity. Gas lasers can be operated continuously but originally
only at low levels of power. Later developments allowed the gases in the
laser to be cooled so that it could be operated continuously at higher
power outputs. The gas lasers are pumped by high radio frequency
generators which raise the gas atoms to sufficiently high energy level
to cause lasing. Currently, 2000-watt carbon dioxide laser systems are
in use. Higher powered systems are also being used for experimental and
developmental work. A 6-kw laser is being used for automotive welding
applications and a 10-kw laser has been built for research purposes.
There are other types of lasers; however, the continuous carbon dioxide
laser now available with 100 watts to 10 kw of power seems the most
promising for metalworking applications.

The coherent light emitted by the laser can be focused and
reflected in the same way as a light beam. The focused spot size is
controlled by a choice of lenses and the distance from it to the base
metal. The spot can be made as small as 0.003 in. (0.076 mm) to large
areas 10 times as big. A sharply focused spot is used for welding and
for cutting. The large spot is used for heat treating.

The laser offers a source of concentrated energy for welding;
however, there are only a few lasers in actual production use today. The
high-powered laser is extremely expensive. Laser welding technology is
still in its infancy so there will be improvements and the cost of
equipment will be reduced. Recent use of fiber optic techniques to carry
the laser beam to the point of welding may greatly expand the use of
lasers in metal-working.

Laser Welding vs. Arc Welding

Laser beam welding energy transfer is different than arc welding processes. In laser welding the absorption of energy by a material is affected by many factors such as the type of laser, the incident power density and the base metal's surface condition.

Laser output is not electrical in nature and does not require a flow of electrical current. This eliminates any effect of magnetism, and does not limit the process to electrically conductive materials.

Lasers can interact with any material. It doesn't require a vacuum and it does not produce x-rays.

How it Works

  • Pump source provides energy to the medium, exciting the laser such that electrons held with in the atoms are elevated temporarily to higher energy states.
  • The electrons held in this excited state cannot remain there indefinitely and drop down to a lower energy level.
  • The electron looses the excess energy gained from the pump energy by emitting a photon. This is called spontaneous emission and the photons produced by this method are the seed for laser generation.
  • Photons emitted by spontaneous emission eventually strike other electrons in the higher energy states. The incoming photon "knocks" the electron from the excited state to a lower energy level creating another photon. These photons are coherent meaning they are in phase, of the same wavelength, and traveling the same direction. A process called stimulated emission.
  • Photons are emitted in all directions, however some travel along the laser medium to strike the resonator mirrors to be reflected back through the medium. The resonator mirrors define the preferential amplification direction for stimulated emission. In order for the amplification to occur there must be a greater percentage of atoms in the excited state than the lower energy levels. This population inversion of more atoms in the excited state leads to the conditions required for laser generation.
  • The focus spot of the laser is targeted on the workpiece surface which will be welded. At the surface the concentration of light energy converts into thermal energy (heat). The heat causes the surface of the material to melt, which progresses through the surface by a process called surface conductivity. The beam energy level is maintained below the vaporization temperature of the workpiece material.The ideal thickness of the materials to be welded is 20mm. The energy is a laser is concentrated, an advantage when working with materials that have high thermal conductivity.

History

Einstein first postulated the quantum-mechanical fundamentals of lasers at the beginning of the 20th century.

1960

The first laser called a ruby laser was first implemented in 1960.

1970

The first high performance lasers were developed in the 1970s with the development of CO2 lasers.  Since this time the applications for laser beam sources have evolved.

1980's

Laser soldering becomes a popular way to join leads in electronic components through holes in printed circuit boards.

1987

Laser powder fusion process developed

2002

From Linde Gas in Germany, a Diode laser using process gases and "active-gas components" is investigated to enhance the "key-holing" effects for laser welding. The process gas, Argon-CO2, increases the welding speed and in the case of a diode laser, will support the transition of heat conductivity welding to a deep welding, i.e., 'key-holing'. Adding active gas changes the direction of the metal flow within a weld pool and produces narrower, high-quality weld.

CO2 Lasers are used to weld polymers. The Edison Welding Institute is using through-transmission lasers in the 230-980 nm range to readily form welded joints. Using silicon carbides embedded in the surfaces of the polymer, the laser is capable of melting the material leaving a near invisible joint line

Welding With Lasers

Laser Beam Welding Demonstration

The laser can be compared to solar light beam for welding. It
can be used in air. The laser beam can be focused and directed by
special optical lenses and mirrors. It can operate at considerable
distance from the workpiece.

When using the laser beam for welding, the electromagnetic
radiation impinges on the surface of the base metal with such a
concentration of energy that the temperature of the surface is melted
vapor and melts the metal below. One of the original questions
concerning the use of the laser was the possibility of reflectivity of
the metal so that the beam would be reflected rather than heat the base
metal. It was found, however, that once the metal is raised to its
melting temperature, the surface conditions have little or no effect.

The distance from the optical cavity to the base metal has little
effect on the laser. The laser beam is coherent and it diverges very
little. It can be focused to the proper spot size at the work with the
same amount of energy available, whether it is close or far away.

With laser welding, the molten metal takes on a radial
configuration similar to convectional arc welding. However, when the
power density rises above a certain threshold level, keyholing occurs,
as with plasma arc welding. Keyholing provides for extremely deep
penetration. This provides for a high depth-to-width ratio. Keyholing
also minimizes the problem of beam reflection from the shiny molten
metal surface since the keyhole behaves like a black body and absorbs
the majority of the energy. In some applications, inert gas is used to
shield the molten metal from the atmosphere. The metal vapor that occurs
may cause a breakdown of the shielding gas and creates a plasma in the
region of high-beam intensity just above the metal surface. The plasma
absorbs energy from the laser beam and can actually block the beam and
reduce melting. Use of an inert gas jet directed along the metal surface
eliminates the plasma buildup and shields the surface from the
atmosphere.

The welding characteristics of the laser and of the electron beam
are similar. The concentration of energy by both beams is similar with
the laser having a power density in the order of 106 watts
per square centimeter. The power density of the electron beam is only
slightly greater. This is compared to a current density of only 104 watts per square centimeter for arc welding.

Laser beam welding has a tremendous temperature differential
between the molten metal and the base metal immediately adjacent to the
weld. Heating and cooling rates are much higher in laser beam welding
than in arc welding, and the heat-affected zones are much smaller. Rapid
cooling rates can create problems such as cracking in high carbon
steels.

Experimental work with the laser beam welding process indicates
that the normal factors control the weld. Maximum penetration occurs
when the beam is focused slightly below the surface. Penetration is less
when the beam is focused on the surface or deep within the surface. As
power is increased the depth of penetration is increased.

Types of Welds

 

  1. Conduction mode welding
  2. Conduction/penetration mode
  3. Penetration or keyhole mode

Conduction Welding:

Performed at lower energy levels forming a wide and shallow weld nugget.There are two modes:

  1. direct heating: heat flow is governed by classical thermal conduction from a surface heat source. The weld is made by melting portions of the base material. Can be made using pulsed ruby and CO2 lasers using a wide range of alloys and metals. Can also use Nd:YAD and diode lasers.
  2. energy transmission: energy is absorbed through novel inter-facial absorption methods.An absorbing ink is placed at the interface of a lap joint. The ink absorbs the laser beam energy, which is conducted into a limited thickness of surrounding material to form a molten inter-facial film that solidifies as the welded joint. Butt welds can be made by directing the energy towards the joint line at an angle through material at one side of the joint, or from one end if the material is highly transmissive.

Conduction/penetration welding occurs at medium energy density and results in more penetration.

The keyhole mode welding creates deep narrow welds. In this type of welding the laser light forms a filament of vaporized material known as a "keyhole" that extends into the material and provides conduit for the laser light to be efficiently delivered into the material.

The direct delivery of energy into the material does not rely on conduction to achieve penetration, so it minimizes the heat into the material and reduces the heat affected zone.

Penetration Laser Welding:

The laser forms a hole that is sealed by the molten material behind the laser. The result is called a keyhole weld.

Laser Keyhold Weld Diagram

Metals

The laser beam has been used to weld:

  • carbon steels
  • high strength low
    alloy steels
  • aluminum
  • stainless steel
  • titanium

Laser welds made
in these materials are similar in quality to welds made in the same
materials by electron beam process. Experimental work using filler metal
is being used to weld metals that tend to show porosity when welded
with either EB or LB welding. Materials 1/2 in. (12.7 mm) thick are
being welded at a speed of 10.0 in. (254.0 mm) per minute.

Advantages

  • Works with high alloy metals without difficulty
  • Can be used in open air
  • Can be transmitted over long distances with a minimal loss of power
  • Narrow heat affected zone
  • Low total thermal input
  • Welds dissimilar metals
  • No filler metals necessary
  • No secondary finishing necessary
  • Extremely accurate
  • Produces deep and narrow welds
  • Low distortion in welds
  • High quality welds
  • Can weld small, thin components
  • No contact with materials

Limitations

  • Rapid cooling rate may cause cracking in some metals
  • High capital cost for equipment
  • Optical surfaces of the laser are easily damaged
  • High maintenance costs

Process Variations

Arc Augmented Laser Welding Diagram

In arc augmented laser welding the arc from a TIG or MIG torch is mounted close to the laser beam interaction point. The TIG torch will automatically lock onto the laser generated hot spot.

The temperature required for this phenomenon is around 300C above the surrounding temperature. The effect is either to stabilize an arc which is unstable due to its traverse speed or to reduce the resistance of an arc which is stable.

The locking only happens for arcs with a low current and therefore slow cathode jet for currents less than 80A. The arc is on the same side of the workpiece as the laser which allows doubling of the welding speed for a modest increase in the capital cost.

Laser Welding Robot with MIG Welds

Twin Beam Laser Welding

If two laser beams are used simultaneously then there is the possibility of controlling the weld pool geometry and the weld bead shape. The keyhole could then be stabilized causing fewer waves on teh weld pool and giving a better penetration and bead shape.

An eximer and CO2 laser beam combination showed improved coupling for the welding of high reflectivity materials, such as aluminium or copper.

Laser Soldering and Brazing

In this process the laser beam melts a filler, which wets the edges of the joint without melting the base material. Became more popular in the 1980's to join leads of electronic boards.

Certification

There are several Universities that provide laser welding certification such as the University of Wisconsin-Madison.  In this case two certificates are offered:

  • LWTSP - accredited laser welding process technical support provider certification for someone that is responsible for the hands on set-up, calibration and operations of the laser welding process
  • LWP - accredited laser welding professional certification for someone who is the primary technical professional involved in the design, engineering and or management of laser welding parts, assemblies or operations.

We suggest checking welding training courses in your area such as:

  • Anne Arundel Community College (Maryland)
  • Ferris State (Michigan)
  • Ohio State (also Lincoln Electric Welding School in Ohio)

 



Page Author: Jeff Grill