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Almost 20 years ago, laser welding was in its infancy and used primarily for exotic applications where no other welding process would be suitable. Today, laser welding is a full-fledged part of the metalworking industry, routinely producing welds for common items such as cigarette lighters, watch springs, motor/transformer lamination, hermetic seals, battery and pacemaker cans and hybrid circuit packages. Yet very few manufacturing engineers have seriously considered employing lasers in their own operations. Why? There are many reasons, but a primary one must be an unfamiliarity with the operation and capabilities of a laser system. Other reasons, such as a relatively high initial cost and a concern about using lasers in the manufacturing environment, also are frequently cited. Laser welding could be used in place of many different standard processes, such as resistance (spot or seam), submerged arc, RF induction, high-frequency resistance, ultrasonic and electron-beam. While each of these techniques has established an independent niche in the manufacturing world, the versatile laser welding approach will operate efficiently and economically in many different applications. Its versatility will even permit the welding system to be used for other machining functions, such as cutting, drilling, scribing, sealing and serializing. In this article, we will look at how laser welding works and what benefits it can offer to manufacturing engineers. Some industry observers state that there are already 2,000 laser machine tools being used for cutting, welding and drilling and that the number could reach 30,000 over the next 15 years as manufacturing engineers become more aware of the capabilities of lasers.¹ While most laser applications are dedicated to one product or process that involves high-volume, long-run manufacturing, the versatility of a 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 minimum heat into the part makes it ideal for many flexible manufacturing operations. INTRODUCTION Welding results when materials are heated to a molten state and fused together. Lasers generate light energy that can be absorbed into materials and converted to heat energy. By employing a light beam in the visible or infrared portion of the electromagnetic spectrum, we can transmit this energy from its source to the material using delivery optics which can focus and direct the energy to a very small, precise point. Since the laser emits coherent radiation, the beam of energy has minimal divergence and can travel large distances without significant loss of beam quality or energy. What does all this 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 into output light, with the best lasers achieving only 2 to 15 percent energy conversion, depending upon the type of laser being used. However, virtually all of this output light energy is delivered to a small spot, as small as a few thousandths of an inch or less. Consequently, for applying thermal energy to small areas, there are no other methods as efficient as lasers. This ability to selectively apply energy offers some distinctive metallurgical advantages in some welding applications, but also creates some unique problems. Since the surface heating generated by By using a technique known as "keyholing," higher power lasers (>106 W/cm²) can make deeper penetrations.³ By heating the spot of laser focus above the boiling point, a vaporized hole is formed in the metal. This is filled with ionized metallic gas and becomes an effective absorber, trapping about 95 percent of the laser energy into a cylindrical volume, known as a keyhole. Temperatures within this keyhole can reach as high as 25,000 °C, making the keyholing technique very efficient.4 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 welding speeds of hundreds of centimeters per minute or greater, depending on laser size. PROCESS PRINCIPLES Generally, there are two types of lasers that are being used for welding operation: CO2 and Nd:YAG. Within the scope of this article, we will not delve into the actual laser theory since our real interest is in manipulating the output laser light for welding. Both CO2 and Nd:YAG lasers operate in the infrared region of the electromagnetic radiation spectrum, invisible to the human eye. The Nd:YAG provides its primary light output in the near-infrared, at a wavelength of 1.06 microns. This wavelength is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 percent for most metals. The near-infrared radiation permits the use of standard optics to achieve focused spot sizes as small as .001" in diameter. On the other hand, the far infrared (10.6 micron) output wavelength of the CO2 laser has an initial reflectance of about 80 percent to 90 percent for most metals and requires special optics to focus the beam to a minimum spot size of .003" to .004" diam. However, whereas Nd:YAG lasers might produce power outputs up to 500 watts, CO2 systems can easily supply 10,000 watts and greater. As a result of these broad differences, the two laser types are usually employed for different applications. The powerful CO2 lasers overcome the high reflectance by keyholing, wherein the absorption approaches blackbody. The reflectivity of the metal is only important until the keyhole weld begins. Once the material's surface at the point of focus approaches its melting point, the reflectivity drops within microseconds.5 WELDING CALCULATIONS Knowing the size of the focused spot is helpful in calculating energy density at the work surface. For a fundamental mode (TEM00) beam:
where: In performing a laser weld, optics to focus the laser beam to the desired size are necessary.
For a multimode beam:
where:
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:6
where: T(x,t)=Temperature at a distance x below the work surface, at a time t after start of constant heat input
and at the surface (x=0), the temperature rise will be:
LASER WELDING FACTORS We have already discussed the influence of the reflectivity of the material on its suitability. Thermal diffusivity, mentioned in the above calculations, is a measure of the ability of the material to conduct heat. The lower the diffusivity, the more 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 block the beam passes, and bubbles tend to form at the root of the weld. If the viscosity is high, these bubbles do not escape before the molten metal solidifies. 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 materials are easier to weld with a laser than high melting point metals. METALLURGICAL CONSIDERATIONS The effect of welding on various materials depends upon many of their metallurgical properties (Table 1) 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 a low tensile strength at temperatures near their melting point are said to exhibit "hot shortness," which often results in cracks appearing in the weld. Similarly, other thermal transformation, such as the martensitic transformation of high carbon steel, also can lead to cracking in or near the weld. To overcome this tendency, special precautions such as pre- and post-welding heating of the material is necessary. However, virtually no thermal distortion occurs during laser welding. The lower heat input requirement 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 gases 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 welding 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 chamber to prevent outside contamination. For welding aluminum to hermetically sealed semiconductor packages, the introduction of silicon-aluminum alloys vastly improves the weld by providing a solidification temperature significantly lower than the parent material.7 For this particular application, Simpson recommends type 4047 aluminum which has a melting point of 1,070 °F to 1,080 °F compared to the 1,200 °F melting point of the 6061 aluminum used for the housing packages. 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 the build-up of shrinkage stresses. LASER WELDING CONSIDERATIONS There are two different approaches to laser welding. One is the low-power method for relatively thin materials; and the other is the "brute force" high-power approach that generally involves keyholing. In both cases, since filler material is rarely used, a tight fitup of the parts being welded is necessary. For butt and seam welds, the laser energy is applied to the junction of the materials, minimizing heat input and distortion and permitting high processing speeds. However, these butt joints must fit accurately, which often limits laser butt welding to circular parts which can be turned to close tolerances and press-fit together prior to welding. For lap joints, the tolerances for seam alignment are somewhat looser. The width of the weld is the major consideration. The upper material forms most of the fusion zone so that a good laser-weldable material could be welded to less suitable material by putting the former material on top. ADVANTAGES OF LASER WELDING Many manufacturing engineers have read about the technical advantages of using lasers in place of more conventional techniques. But, what about economic justifications, which form the basis of most production purchases? The industrial laser user gets no return from publicizing the cost cutting realized with a laser welding system and might even tip off competitors. So most manufacturers tend to regard their usage of lasers as proprietary information.8 Several applications were recently listed in MAN.9 The advantages of using lasers in comparison with the most popular techniques are shown in Table 2. EVALUATING LASER WELDING SYSTEMS As we have already discussed, the two laser types being used for laser welding are CO2 and Nd:YAG or Nd:Glass. Both laser types can operate in either the continuous or the pulsed mode. CO2 lasers, which range in power from 50 to 15,000 watts, are more efficient in their conversion of electrical power to laser radiation than Nd:YAG lasers, which range from about 50 to 800 watts output power. However, as discussed above, the reflectivity of most metals is much higher at the CO2 wavelength than the Nd:YAG wavelength. Recent advances in fast-axial-flow CO2 lasers provide improved beam characteristics, making these systems competitive with electron beam welding for deep-penetration applications. Fast-spiral-flow CO2 lasers now are able to produce fundamental-mode outputs in the kilowatt range, which gives higher energy densities suitable for welding thermally sensitive alloys or materials where thermal distortion is a problem.10 Slow-axial-flow lasers with enhanced pulsed capabilities offer an advantage over fast-axial-flow units for applications requiring rapid energy coupling and low heat input. In pulsed operation, the peak power in the pulse is several times greater than the continuous-wave power, although the average power is lower. This peak power overcomes surface reflectivity and minimizes thermal damage to the surrounding material. Solid state lasers (the generic name for Nd:YAG, Nd:Glass and similar lasers), are preferred for low- to moderate- power applications. They have found extensive application in the electronic/electrical industries for spot welding and beam lead welding integrated circuits to thin film interconnecting circuits on a substrate. One consideration that can be important in evaluating laser welding is the physical size of the equipment. Solid state laser welding systems are relatively small compared to CO2 systems, which could occupy an average room to achieve the high powers required. Still, if you need the brute power, it can be guided to the workpiece through optics or articulating arms (attached to robots, if desired). For delicate welding operations, such as welding lamp filaments, the solid state welding systems offer the advantage of coaxial viewing optics. Closed-circuit TV viewing provides a magnification factor of better than 40X, and the exact spot of the laser beam focus can be easily seen. This feature is very useful for alignment and beam focusing as well as workpiece viewing. Since the wavelength of the Nd:YAG laser is close to the visible spectrum, standard lenses can transmit both the laser light and the image of the workpiece. While we have provided a quick theoretical approach to determining your laser needs, the actual laser powers required are frequently determined by experimentation. There is a trade-off of weld penetration versus travel speed for any given laser output power; in general, the higher the travel speed, the lower the penetration. This will vary from material to material and with other factors such as the focus of the beam. 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 infrared light could focus onto the skin or eye unless safety precautions are observed. Industrial laser systems are fully interlocked to prevent any danger to operators. Most are equipped with National Center for Devices and Radiological Health covers that contain the actual laser operation, permitting people working nearby to perform normally. With proper design and careful precautions, laser systems are no more dangerous than other 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 capabilities of this equipment. Coupled with robotics and computer-controlled beam movements or workpiece movements, 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 systems manufacturer should be contacted. Discussing your particular applications with different companies will uncover the feasibility of accomplishing them with lasers and will allow you to truly compare this remarkable tool with conventional welding techniques. References:
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. | |||||||||||||||||||||||||||||||||||||||||||||||||||||
