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When first invented, the laser was a solution looking for a problem to solve. Early laser welding applications were reserved for only the most exotic applications, where no other welding method was suitable. As the laser industry matured, lasers were considered for more routine welding applications. Today, laser welding is a full-fledged part of the metalworking industry, routinely producing welds for common items such as battery and pacemaker cans, fuel injector nozzles, razor blades, medical tools, aircraft engines, and even car bodies!  While widely used in some industries, there are still many manufacturing engineers that have not seriously considered employing lasers in their operations.

Why? There are many reasons, but a primary one is unfamiliarity with the operation and capabilities of laser systems. Other reasons, such as relatively high initial cost and concern about using lasers in a manufacturing environment are also frequently cited.

Laser welding can be used in place of many conventional welding processes, such as TIG, MIG, resistance, and electron beam to name a few. While each of these techniques has established a niche in the manufacturing world, the versatile laser welding process will operate efficiently and economically in many different applications. Its versatility will permit the laser system to be used for different welding applications such as spot and seam welding. Some laser welding systems can even be configured to do additional functions such as cutting, drilling, and serializing.

In this article, we will look at how the various laser welding processes work, and what benefits they can offer. Today, tens of thousands of lasers are used in industry for cutting, welding, drilling, marking, and numerous other applications. That number will continue to rise as engineers become more comfortable with their use. While most laser applications are dedicated to one product or process that involves high-volume, long-run  manufacturing, the versatility of the laser to supply energy to hard-to-reach spots, vary the output energy over a wide range, operate under the control of computers and robots, and put minimal heat into the part makes it ideal for flexible manufacturing operations.

INTRODUCTION

Welding is a process where materials are heated to a molten state and are fused together. Lasers generate light energy that can be focused and absorbed into materials and converted to heat energy. By employing a light beam in the visible or near infrared portion of the electromagnetic spectrum, we can transmit this energy from its source to the material to be processed using fixed or fiber optic beam delivery optics. These optics can then focus the beam to a small spot with enough thermal energy to melt the material and create a weld.

What does this all mean to the manufacturing engineer? To appreciate the potential of employing lasers in welding operations, you must redefine some of the traditional approaches to viewing “efficiency” as it relates to energy conversion. The laser is a relatively inefficient converter of electrical energy in output light, with the laser achieving from 2 to 30 percent energy conversion, depending on the type of laser being used. However, because of the coherence of the beam, virtually all of this energy can be delivered and focused to a small spot, as small as a few thousandths of an inch. Consequently, for applying thermal energy to small areas, there are no methods as efficient as lasers. This ability to selectively apply energy offers many distinct advantages, not the least of which is the ability to minimize the surrounding heat affected zone.

PROCESS PRINCIPLES

Generally, there are three types of lasers that are commonly used in welding operation: CO₂, Nd:YAG (both lamp and diode pumped), and fiber lasers. Within the scope of this article, we will not delve into the actual laser operation theory, since our real interest is manipulating the output laser light for welding.

All three of these laser types operate in the infrared portion of the electromagnetic spectrum, and are invisible to the human eye. The Nd:YAG and fiber lasers operate in the near infrared spectral region at a wavelength of approximately 1 micron.  This wavelength is absorbed quite well by most metals. The near infrared radiation also permits the use of standard, relatively low cost, fused silica or other glass optics to achieve focused spot sizes as small as a few thousandths, depending on the laser.

The CO₂ (carbon dioxide) laser, on the other hand, operates in the far infrared portion of the spectrum at 10 microns, and has an initial absorption of only 10 to 20 percent for most metals.  carbon dioxide laser light is absorbed by normal glass optics, and therefore requires optics made from special materials.  Because the focused spot size is a function of the laser wavelength, the focused spot size will be larger.  This could be a limiting factor when making small and microwelds. The carbon dioxide laser was commonly used in the early days of the laser industry, when it was the only laser capable of power levels of 1 KW or more. Today these lasers are being replaced by the fiber laser which has a power capability of 50 KW and beyond.

Normal welds or “conduction welds” are made by using the thermal conductivity of the metals being welded to transmit the thermal energy of the laser beam. These welds are generally smooth and aesthetically pleasing, but are limited in penetration to from 2 -6 mm depending on the laser.

By using a technique known as “keyhole welding,” higher power lasers (>10⁶ W/cm²) can make deeper penetration welds. ¹  By increasing the intensity of the laser focused spot above the metal boiling point, a vaporized hole is formed in the weld puddle. This hole is filled with ionized metallic gas, and becomes an effective absorber, trapping about 95 % of the laser energy into a cylindrical volume know as a “keyhole.” Temperatures within this keyhole can reach as high as 25,000 °C, making the keyholing technique very efficient. ²  Instead of heat being conducted mainly downward from the surface, it is conducted radially outward from the keyhole, forming a molten region surrounding the vapor. As the laser beam moves along the work-piece, the molten metal fills in behind the keyhole and solidifies to form the weld. This technique permits faster weld speeds, and deeper penetration.

WELDING CALCULATIONS

Knowing the size of the focused spot is helpful in calculating energy density at the work surface. In performing a laser weld, optics to focus the laser beam to the desired size are necessary.

For a fundamental mode (TEM₀₀) beam:

S = ( 4λ / ) × ( F / D)

        Where:

S = Focused spot diameter

λ = Laser wavelength

F = Focal length of objective lens

D = Diameter of beam entering objective lens

For a multimode beam:

S = F × Φ

        Where:

F = Focal length of objective lens

Φ = Laser beam divergence

If one assumes the part to be welded as a semi-infinite solid, with a constant incident heat flux, then the temperature distribution as a function of depth into the material is given by:³

T(x,t) = (2E/K) × [(kt/)^½× exp(-x²/4kt) - (x/2)erfc(x/2(kt)^½)]

    Where:

                T(x,t)=Temperature at a distance (x) below the work surface, at a time (t) after start of constant input

E = Constant heat flux input

K = Thermal conductivity

k = Thermal diffusivity

x = Depth below work surface

t = Time after start of heat flux input

erfc = Complimentary error function

And at the surface (x=0), the temperature rise will be:

T(x,t)_x=0 = (2E/K) × (kt/)^½

 

LASER WELDING FACTORS

We have already discussed the influence of the reflectivity of the material on its suitability for laser welding. Thermal diffusivity, referenced in the above calculations, is a measure of the ability of the material to conduct heat. The lower the diffusivity, the longer the heat remains in the vicinity of the laser beam spot.

Metals with low boiling points produce a large amount of metal vapor, which could initiate gas breakdown and plasma generation in the region of high beam intensity, just above the metal surface. This plasma, which readily absorbs the laser energy, can partially block the beam as it passes, and create bubbles in the weld. If the viscosity is high, these bubbles do not escape before the molten metal solidifies, and creates voids in the weld. Zinc is an example of a typical low melting point metal.

Although the melting point of metals does not have a significant effect on laser weldability, it must be reached during the initial absorption of energy. Thus, low melting point metals require less laser energy and are easier to weld than those with a higher melting point.

METALLURGICAL CONSIDERATIONS

The effect of welding on various metals depends upon many of their metallurgical properties such as “hot strength.” After the applied energy is removed, the melt pool solidifies and then it slowly cools to the same temperature as the surrounding material. During this cooling, the material contracts, creating tensile stresses in the fusion zone.   Materials that have low tensile strength at temperatures near their melting point are said to exhibit “hot shortness,” which often results in cracks appearing in the weld. ⁴

Similarly, other thermal transformation, such as martensitic transformations of high carbon steel, can also lead to cracking in or near the weld. To overcome this tendency, special precautions such as pre- and post-weld heat treating of the material is necessary. The low heat input of laser welding has other benefits, such as being able to use fixtures that do not need to withstand large thermal expansion forces, or to act as heat sinks.

Chemical reactions, such as oxidation or nitriding, with atmospheric gasses at high temperatures, can pose problems, particularly when the oxides or other elements formed have disassociation temperatures far above the melting point of the metal. The result is brittle, porous welds. Covering the weld area with an inert gas such as argon or helium minimizes these reactions in most cases. For some materials, it may be necessary to weld within a sealed vacuum or inert gas chamber to prevent outside contamination.

For welding aluminum to hermetically seal semiconductor packages, the introduction of high silicon aluminum alloys vastly improves the welds by providing a solidification temperature significantly lower than the parent material. ⁵   For this particular application, type 4047 aluminum is recommended, which has a melting point of 1070°F to 1080°F compared to 1200°F melting point of the 6061 aluminum housing. During cooling, the outside interface cools fastest. As the boundary weld passes through the brittle phase, the core of the weld bead acts like warm taffy and yields with the shrinkages, preventing build-up of shrinkage stresses.

LASER WELD TYPES

In general, there are two types of laser welds, pulsed, and continuous. In pulsed laser welding, a short duration pulse, generally 1 -20 milliseconds in duration, is used. These pulses, due to their short duration, put minimal heat into the part being processed. Commonly used to “spot weld” parts together using a single or array of single welds, they can also be used to seam weld parts using a series of overlapping pulses. Because the weld penetration curve resembles a “v,” the pulses must be significantly overlapped, typically 70%- 80%.  When welding thin metals, pulse repetition rates can be as high as few hundreds of pulses per second or more. This method puts the least amount of heat into the part, but is generally slow, due to the high overlap requirement. Pulse welds are especially effective on parts with high thermal conductivity, such as aluminum or copper.

Continuous laser welds are done with lasers operating at high continuous power levels which remain at that level for as long as the laser is on. Depending on the laser power level and focused spot size, the lasers can weld in either the conduction or keyhole modes. Primarily used for seam welding, the laser is used to heat and melt the materials being welded.  While these lasers put more heat into the part than the pulsed laser, they have significantly higher processing rates.  In addition, to higher throughput, they can also be used to weld thicker parts that require deeper penetration.

Because significant overlap exists in the applicability of the two general types of laser welds, it is highly recommended that the potential user contact a reputable laser company and discuss the process with an applications engineer. Their years of experience will significantly shorten the process of selecting the right laser for the particular job.

FIT-UP CONSIDERATIONS

Regardless of the laser welding method, a tight fit-up of the parts being welded is necessary since filler metal is rarely used. For butt and seam welds, the laser energy is applied to the junction of the materials, minimizing heat input and distortion, permitting high processing speeds. In general, the better the fit-up, less power and a smaller laser spot size is required and faster processing speed results. Circular parts which can be machined accurately and press fit together are excellent examples. For spot welds, because the laser is non-contact, the parts must be in good contact. In many instances mechanical clamping is desirable. While most welds can be made without adding additional material, for those that do, hybrid systems are available that marry traditional wire feed systems to the laser welder.

ADVANTAGES OF LASER WELDING

Laser welding has many technical advantages. Among these are: minimal heat into the part, ability to access confined areas, ability to weld inside closed gas or vacuum chambers (thru a glass window), ability to be transmitted over long distances using fiber optic beam delivery ⁶ , able to shape the focused beam to fit specialized welding requirements, and many more. The main advantage to laser welding, however, is the ability to automate the welding process and take advantage of increased production rates. By computer controlling the laser, the power level, pulse shape, power ramping and other laser parameters, the process can be precisely controlled and monitored, allowing for production rates previously unachievable with conventional welding methods. While laser systems are generally more expensive than traditional methods, the cost savings achieved through higher production and lower reject rates can easily justify the additional cost.

EVALUATING LASER WELDING SYSTEMS

As we have already discussed, the three common laser types used for welding are CO₂, Nd:YAG (lamp and diode pumped), and the fiber laser.  Carbon dioxide lasers, while common in the past, are still used for cutting, but seldom used for welding metals. Low to moderate power Lamp and Diode pumped YAG Lasers, as well as fiber lasers, are used for welding thinner metals. High power fiber lasers in the 2KW to 50KW range are generally used for thick metal welding. Each of these lasers can be delivered using conventional optics or fiber optic beam delivery systems. Because all of these lasers can be controlled by either a process computer or PLC, they can be easily integrated with motion or automation systems. Each of these common laser systems, with the exception of the CO₂ laser,  operate in the near infrared portion of the spectrum, and can use common glass lenses and other optical components. This makes integration with CCTV systems, vision systems, and other visual alignment aids possible. While we have given a brief overview of laser system capabilities, determining your actual needs are frequently done by processing parts using a laser system manufacturer’s application development lab. Here, actual parts can be laser processed to determine laser type, power level, operating mode, and process speed, and other relevant parameters.

LASER SAFETY

Lasers emit a very concentrated beam that can be visible or invisible.  In general most lasers used for welding are invisible. This beam of invisible infrared light could focus onto the skin or eyes unless proper safety precautions are observed. Properly designed industrial laser systems are fully enclosed and interlocked to prevent accidental exposure, and are certified by the laser manufacturer to comply with government (CDRH) regulations. With proper design and careful precautions, laser systems are no more dangerous than other conventional welding systems or similar machine tools.

We have broadly covered laser welding without dwelling on any specific applications to familiarize the manufacturing engineer with the general capabilities of the equipment. Coupled with robotics, computer-controlled beam or work-piece movements, or other automation, laser welding systems offer an unmatched versatility to perform a variety of operations. If you feel that your operations could benefit from using laser welding, a reliable laser systems manufacturer should be contacted. Discussing your welding needs with a qualified laser applications engineer will allow you to compare this truly remarkable tool with conventional techniques.

References:

1. Yessik and Schmatz, “Laser Processing at Ford,” Metal Progress, May 1975
2. Duhamel and Banas, “Laser Welding of Steel and Nickel Alloys,” Lasers in Material Processing, American Society of Metals, 1983
3. Carslaw and Jaeger, “Conduction of Heat In Solids,”  Oxford University Press, 1959
4. Naheer, Handbook of Laser Welding Technologies, 7.4.1 (195), 2013
5. Simpson, “ Laser Welding the Large MIC – A New Approach to Hermetic Sealing,” Microwave Journal, November 1984
6. Laverty, Miller, Golden, and Glesias, “Efficient Delivery of High Power Lasers Through Fiber Optic Cables,” Ninth Annual Diode Laser Technology Review, April 1996.

Table 1. Laser Welding Applications Guideline

Table 2. Advantages of Laser Welding Compared to Other Processes

Source: Steve Bolm, "Laser Welding, Cutting and Drilling," Assembly Engineering, May 1980.

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