There are two of types of neodymium-doped yttrium aluminium garnet (Nd:YAG) laser welder, continuous wave and pulsed. As the name suggests, continuous wave or CW is either on or off, whereas pulsed lasers create welds by individual pulses.
The pulsed laser uses high peak power to create the weld, whereas the CW laser uses average power. This allows the pulsed laser to use less energy to create the weld, with a smaller heat affected zone. It also provides the pulsed laser with unrivalled spot welding performance and minimal heat input seam welding.
Pulsed welding lasers
The laser used for welding is a flash lamp pumped Nd:YAG laser which produces a pulsed laser output. The Nd:YAG crystal (Y3Al5O12) is the most efficient and robust material available to handle the high power and heat generated within the crystal rod.
Fig. 1: Layout of a pulsed Nd:YAG welding laser showing the resonator and optical beam delivery components. Note the real-time power feedback and the capability of energy or time sharing.
In contrast to a laser marker using diode pumping and Q-switching, the pulsed laser welder requires a flash lamp. Diodes are unsuited for pulsing, and Q-switching does not provide sufficient energy for welding (see Fig. 1).
Real-time power feedback
Pulsed Nd:YAG lasers may be designed to provide a number of special features to optimise the weld process and increase reliability and repeatability for more demanding applications. Real-time power feedback, through pulse width modulation of the source current, ensures excellent pulse-to-pulse stability and delivers the same pre-set weld over the full life of the flash lamp automatically.
Power ramping
Power ramping features eliminate last pulse cracking in seam welding applications which may otherwise occur when using older laser designs. Power ramping allows the weld programme to increase the laser power gradually at the beginning and end of the weld. It also provides a more cosmetically appealing weld.
Fig. 2: Fadeout ability on seam welds to prevent last pulse cracking and to ensure completion of smooth seam.
Pulse shaping
Most welding cases use a square welding pulse. There are, however, a few applications where pulse shaping can enhance welding. Although there are numerous pulse shapes (see Fig. 2), two basic pulse shapes are used most commonly. The first is for overcoming highly reflective material such as copper and aluminum, and the other is to minimise the thermal cycling experienced by the part during welding, for materials susceptible to cracking.
Time share
Time share features permit one laser to be configured to deliver two or more welds to separate work pieces or separate workstations “sequentially”. With time sharing, one laser can support multiple workstations using different weld schedules at each workstation. This reduces laser costs greatly.
Energy share
Energy share features permit one laser to be configured to deliver two or more welds to the work piece “simultaneously”, so increasing the number of welds per laser pulse greatly. Each of the beams is properly balanced so that each weld nugget is identical in shape, area and depth.
Beam delivery
The beam is delivered to the welding area using a flexible fibre optic cable, usually around 5 m long. Use of a flexible fibre optic cable facilitates the integration of the laser into turnkey laser welding systems, factory automation equipment and robots.
The laser beam is launched into the fibre using optics located within the laser cavity. Fibres are available in two types, stepped index or graded index. Fibres are also available in different core diameters, from 100 to 1000 µ.
Fig. 3: Pulse shaping options.
Focus head
The fibre optic cable delivers the laser energy to the focus head. The focus head consists of optics which focuses the laser light emitted by the fibre onto the material being welded. Longer focal length lenses produce larger spot diameters while shorter focal length lenses produce smaller weld spots.
Fig. 4: Three types of weld created according to the power density and pulse duration of the laser pulse.
How a laser welds
Laser welding is a non-contact process which requires access to the weld zone from one side of the parts being welded. The weld is formed as the intense laser light heats the material rapidly – typically calculated in milliseconds. The flexibility of the laser offers three types of welds: conduction mode, conduction/penetration mode and penetration or keyhole mode (see Fig. 4).
Conduction mode welding is performed at low energy density, forming a weld nugget which is shallow and wide.
Conduction/penetration mode occurs at medium energy density and shows more penetration than conduction mode.
The penetration or keyhole mode welding is characterised by deep, narrow welds. In this mode, the laser light forms a filament of vaporised material know as a “keyhole” which extends into the material and provides conduit for the laser light to be delivered into the material efficiently. This direct delivery of energy into the material does not rely on conduction to achieve penetration, and so minimises the heat into the material and reduces the heat affected zone.
Key welding parameters
The key welding parameters can be divided into two parts, those that concern the parts themselves and the laser parameters.
Joint design and fit-up
As laser welding is a non-contact process, a broad number of joint geometries can be welded. In addition, the laser can weld into areas with limited access. The main joint designs are shown in Fig. 5. Ideally, the thinner material is the top sheet in any weld joint.
Fig. 5: Most common weld joints, most other types are variations on these.
The most significant requirement for reliable laser welding is close fit-up at the joint interfaces. Laser spot or seam welding is usually an autogeneous process, meaning that no filler material is added during the welding process.
Therefore, if the welding interfaces are too far apart, there is insufficient weld material to bridge the gap or the weld will be undercut.
For best results, the gap should be zero or an interference fit. However, since this is often not practical, some gap is allowed. As a rule of thumb, this gap should never be more than 10% of the thinnest material or of the weld penetration, whichever is less.
It must be stressed that gap tolerance is case specific and should always be examined fully, and quantified by lab tests.
Part alignment
The focused spot diameter for laser welding applications is typically 100 to 1000 µ. Smaller spot diameters (25 – 50 µ) may be required for very fine welding applications such as medical guidewires or micro-electronic devices.
The position of the joint under the laser must be precise enough so that the focused spot does not miss the joint. The tolerance of this misalignment is a function of the focused beam diameter and, to a lesser extent, the joint design.
Vertical tolerances, while less critical, also play a part to achieve the tightest possible focused beam.
Vertical tolerance relates to ensuring that the focus spot at the joint has sufficient energy density to make the weld. This tolerance is called the depth of focus and is nominally about 0,5% of the lens focal /collimator length. Therefore, when a 100 mm collimator and 100 mm focal length lens combination is used, the depth of focus is around 0,5 mm.
Material selection and plating
The laser can weld a wide range of steels, nickel alloys, titanium, some aluminum alloys and even copper, but there are materials which are better suited to laser welding and some that are difficult or impossible to laser weld.
As with any other joining technology, certain types of material are difficult to weld unless they have certain characteristics. The material characteristics specific to laser welding are:
The material’s reflectivity
The effect of the high thermal cycling.
The vaporisation of volatile alloying elements.
The most commonly welded material is steel and the general selection rule is to keep the carbon content under 0,12%. For stainless steels, ensure that the Cr/Ni ratio is greater than 1,7. Stainless steel alloy ANSI 303, many 400 series alloys and high carbon steels should be avoided due to a high carbon, phosphor and sulfur content. Generally, nickel alloys and titanium are highly weldable, with aluminum alloys and copper being case
specific.
The plating material and method of plating can also have a significant effect on the welding process. For example, electro-less nickel plating creates welding problems due to the inclusion of phosphor and other contaminants during the plating process. The recommended plating method is electrolytic. The thickness and type of plating are also considerations, for example, a gold coating thickness above 50 micro-inches may induce weld cracking.
Fig. 6: Nominal part alignment tolerance to laser. The lateral tolerance is critical, vertical is large and more forgiving to the process.
Laser parameters
The weld created by each pulse is determined by the peak power density and duration of that pulse. The number of pulses per second, pulse overlap and the welding speed additionally define a seam weld.
The peak power density controls the weld penetration and is a function of the fibre type and core diameter, focus optics, and laser peak power output.
The pulse width controls the heat into the part, weld width and thermal heat cycle.
The pulse repetition rate or pulse frequency also controls the heat into the part and thermal heat cycle.
Fibre optic cable
The laser is delivered to the weld area via a fibre optic cable. This cable is constructed with a central core which carries the laser, a cladding region which acts as a mirror to the laser so that all the light remains in the core, and an outer metal jacket to protect from light leakage. The core diameter of the fibre can vary in diameter according to the required spot size needs and input laser power. In terms of focused spot size, the core diameter affects the final focus spot size, and therefore the peak power density (see Fig. 7).
Fig. 7: The end of fibre shows the illuminated silicon core though which the beam is transmitted, the surrounding silicon cladding and the steel jacketed connector.
In Fig. 7, the light is confined within the central core by differing refractive indices between the core and cladding material so that total internal reflection of the laser light occurs. The laser is launched into the fibre at the laser using focusing optics, and is then collimated and focused at the weld area by the focusing head.
For example, a 300 µ core fibre has half the focused spot size of a 600 µ core fibre, and so has four times the peak power density. However, the 600 µ fibre has no power limitations, whereas the 300 µ fibre does.
Selection of fibre is application-related. Aside from the core diameters, there are two types of fibre, a stepped index (SI) and graded index (GI) fibre.
The difference between the two is that the stepped index fibre tends to homogenise the beam structure while the graded index fibre tends to maintain the mode structure of the laser through the fibre length. This offers different welding characteristics in terms of weld penetration, weld width and weld stability. Determining which type of fibre to use depends on the application.
Focus optics
The focus head effectively images the end of the fibre on to the part. The fibre optic delivers the beam to the focus head where the beam is first collimated and then focused to a spot. The image is either enlarged or reduced according to the ratio of the collimating and focus lens.
Fig. 8: The focus head usually has a camera mounted on the axis to the laser beam
path to directly view the weld area before, during and after the weld is completed.
The camera assists the operator in acquiring the weld joint and positioning the laser beam to the most optimum location.
The focus optics can be used to fine-tune the spot size for an application or provide a reduced spot size when a large core diameter fibre must be used for handling higher power. The selection of final focus lens focal length also determines the standoff distance of the focus head from the part (see Fig. 8).
Peak power and pulse width
The peak power, measured in watt, directly affects the peak power density, measured in watt/cm². Peak power density controls weld penetration. The pulse width, usually measured in milli-seconds, controls the heat into the part (see Fig. 9).
Fig. 9: The effect on weld dimensions of increasing pulse
width and peak power on weld dimensions.
Increasing the pulse width increases the weld dimensions and heat affected zone through increased heat conduction time. Optimum peak power is defined as the peak power which creates the deepest penetration at a given energy without material expulsion.
Welds made with high peak powers exhibit narrow, deep welds that exert a high thermal cycle on the weld material. To increase weld width, reduce the thermal cycling and minimise depth variation. The pulse width can be increased to introduce a more conduction-based welding mechanism.
Seam welding
The additional parameters for seam welding are the pulse repetition rate, measured in pulses per second (Hz) and the linear part travel rate or welding speed. Spot overlap percentage, a function of speed, pulse repetition rate and focused spot diameter are also used in the equation for determining the best laser for the job and for determining the total weld cycle time.
When seam welding, a balance is reached between the pulse penetration parameters, the welding speed and pulse repetition rate. In most cases, this is worked from initially selecting the pulse penetration parameters, the effective penetration and therefore the spot overlap, and then determining the welding speed (see Fig. 10).
Fig. 10: A schematic representation of overlap versus
effective penetration depth for various overlap percentage.
Weld penetration is determined by the spot overlap percentage, and welding speed. The level of overlap determines the effective penetration. Good mechanical strength may be achieved at 40 – 60% overlap. However, for hermetic welding applications, 70 – 85% overlap is typically required. The welding speed is reduced as the overlap increases.
Cover gas
Cover gas is used to prevent rapid oxidation of the weld zone due to atmospheric oxygen. Argon, helium or nitrogen inert gas is directed at low pressure and flow volume into the weld zone during the welding process to shield the weld zone from atmospheric oxygen. The mechanical properties and weld strength are usually unaffected by cover gas. Welds made in the presence of cover gas are shiny and more cosmetically appealing. The cover gas can also be used to cool the part minimizing the heat affected zone, and overall thermal loading.
Contact Zalman Orlianski, Zetech, Tel 011 789-3230, zorlianski@zetech.co.za
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