21 Welding with Lasers

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Marvin Manning
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1 21 Welding with Lasers 21.1 Advantages and Disadvantages Non-Contact, Surface, Direct, or Butt Laser Welding Laser welding is infrared welding using a laser as the energy source. Infrared welding using halogen and parabolic lamps predates the use of the laser and first appeared in the 1970s. These early systems did not offer outstanding advantages over other welding techniques and created problems of heat build-up in fixtures, critical focusing, and limited lamp life. Lasers were introduced as a light source in the early 1990s and found a number of applications by the end of the century. The laser can be used to weld plastics either by directing the laser beam on the surface of a laser-absorbing plastic and welding by fusion or by transmitting a laser beam through a laser-transparent material and welding at the interface with the laser-absorbing material. The fusion welding method is also known as non-contact welding, surface welding, direct welding, or butt welding and is very similar to hot plate welding, except that a laser is used in place of the hot plate. In fact, some machines for this type of laser welding have been built using a hot plate welder as the basis. Angled mirrors are placed between the two parts to be welded as illustrated in Fig The laser melts the plastic on the weld surface of each of the parts to be welded by absorption of the laser beam. The mirrors are removed from between the parts and the parts are then pressed together as in hot plate welding. This process offers the advantage of simultaneous welding of both parts and any degraded material is buried in the weld. Unfortunately, it is a relatively slow process that leaves a large weld bead and expensive process engineering is required. Non-contact infra-red welding is best accomplished when the parts to be joined are made of the same thermoplastic. Melting of both parts will occur at the same rate, at the same temperature, and with the same amount of collapse. This process can be used to join dissimilar plastics with varying degrees of success. The differences in thermal properties will result in one part melting at a different temperature than the other and to a different depth. Consequently, the inter-diffusion of the polymers at the melt interface will be limited and the joint strength will be reduced. The greater the difference in thermal properties between the two plastics, the poorer the weld will be. Some polymers cannot be welded together. This type of welding originated using infrared lamps and they are still used for some applications such as butt welding of pipes. Infra-red lamps offer the advantages of a low cost, mass-produced light source, low maintenance, high efficiency, high power density and, for medium or long wave applications, they are not sensitive to the type of plastic used. Unfortunately, infra-red lamps suffer from short lamp life and heat build-up in

2 21.1 Advantages and Disadvantages 521 Figure 21-1 Non-contact laser welding (courtesy SAE) fixtures. In addition, they are notoriously difficult to focus and that is their great disadvantage. Today, lasers are used in place of infra-red lamps for surface or direct welding. The equipment for surface or direct welding requires either a beam splitter or a laser for each part and is very expensive. Consequently, surface or direct welding is used only when none of the other welding techniques will suit the application Laser Staking Laser energy can also be used to melt a plastic stud so it can be formed by a die. The laser beam is directed through the transparent die down on the plastic stud to be welded until the polymer is soft enough to be formed. The die then presses down on the stud to shape the head of the stake. To avoid loss of light transmission due to abrasion of the die, the plastic stud should not be made of glass fiber filled resins. Such applications require a metal die which is applied as a second stage. The source for this type of equipment is Extol, Inc Through Transmission Infra-Red Laser Welding This section will focus on through transmission infra-red (TTIr) welding as this is the principle laser welding process. It is of most interest to design engineers because of its unique characteristics. From this point on, this discussion will apply only to that variety of laser welding. With parts for which it is well-suited, laser welding can create a weld that is closer to perfection than the welds made with any other method. Laser welding is unique in the respect that it is a non-contact thermoplastic welding process, which leaves no marks

3 522 Welding with Lasers or particulates on the finished product. It is the most precise of all welding processes, with a very small area affected by heat. It is also capable of welding in recesses that are inaccessible by the other welding methods. Unfortunately, the cost of the equipment is still high, although it is becoming competitive with other types of welding equipment. Laser welding is not exactly cost competitive with the other welding processes, but rather its combination of unique properties provides the engineer with the capability to perform welds or achieve quality levels otherwise impossible Advantages of Through Transmission Laser Welding 1. Non-contact welding Laser welding uses a smooth glass plate to hold the parts to be welded with a light clamping force of, generally, 100 pounds or less and there is no contact with the welding instrument. Consequently, there is nothing to mar the surface of the welded parts and there is no welding pressure required that could damage delicate parts. 2. Sub-surface welding The weld occurs at the interface between the two parts. The weld is nearly invisible with transparent parts and completely invisible with opaque parts. The weld can be just below the surface or deep in the parts 3. Precise welding The size and location of the laser weld can be very precise and there is no vibration transmitted to the part to be welded. For those versions of the process that require relative motion between the laser and the weldments, it is the laser that moves. Consequently, laser welding is the most precise of the welding processes. 4. Minimal heat-affected area The weld spot is very small and the heat-affected area is quite contained. As a consequence, it is possible to weld very close to other components without affecting them, thus permitting welding of sub-assemblies with sensitive and fragile parts previously emplaced. In addition, distortion and degradation due to heat is extremely limited and there is reduced residual stress. 5. Ability to weld dissimilar plastics Dissimilar plastics which meet the necessary criteria can be welded to each other. 6. Reduced mold cost Laser welding requires no special joint configuration thereby reducing the cost of tooling. 7. No flash In laser welding, the weld is totally enclosed and, in most cases, no flash is created to be trimmed off. There are no loose particulates as well. 8. Welding of elastomers and difficult materials Laser welding has the unique ability to weld elastomers. It can also weld PEEK (polyetheretherketone), LCP (liquid crystal polymer), COC (cyclo-olefin copolymer), and PEI (polyetherimide). 9. Flexible to flexible In addition to welding elastomers, laser welding can also be used to weld films to each other. 10. Flexible to rigid welding Film and elastomers can be welded to rigid extruded and molded parts. 11. No additional materials Laser welding uses no additional materials, such as fasteners, inserts, electromagnetic preforms, adhesives, or solvents for material combinations in which one of the two materials is transparent to laser light and the other

4 21.1 Advantages and Disadvantages 523 absorbs it. Therefore, except for initial capital investment, it can be inherently lower in cost than methods which do use additional materials and less expensive to disassemble for recycling. 12. Low surface preparation Laser welding is extremely forgiving of poor surface preparation and surface deviations of up to 1 mm (0.039 in.) can be tolerated depending on the particular laser welding technique employed. Overcoming surface deviations does, however, result in flash. 13. Entrapment of other parts Additional parts can be captured between the two parts to be laser welded and they can be located closer to the weld than with other processes. 14. Permanence Laser welding creates permanent assemblies, which cannot be reopened without damaging the parts unless a laser is used to remelt the weld interface. As a welded joint, the effects of creep, cold flow, stress relaxation, and other environmental limitations are absent in the joint area, although they may occur elsewhere if other parts of the design are under load. Owing to material limitations, differences in thermal expansion and moisture absorption are rarely of concern once the parts are welded. Joint strengths equivalent to those of the surrounding walls are possible under ideal conditions. 15. Internal joints Internal walls can be laser welded provided there is clear access for the laser beam. 16. Shape freedom The laser is capable of creating virtually any pattern, therefore laser welds can be made with parts of practically any shape so long as the horizontal joining surfaces can be created within the required height limits. Depending on the variety of laser welding to be used, permissible height variations can range from 1 mm (0.039 in.) to 40 mm (1.575 in.). If a continuous weld is not required, much greater vertical freedom is available. Laser welding is not dependent on the vibration transmission qualities of the plastic. Therefore, it can weld parts which are not well suited to ultrasonic welding because they are too flimsy, contoured, or do not have a surface well suited to ultrasonic horns. 17. Hermetic seals Laser welding is capable of creating a hermetically sealed, gastight weld. 18. Energy efficiency Compared to the hot plate welding technique, laser welding is highly energy efficient. This also means that there is no excess heat which must be removed from the workplace. 19. Clean atmosphere No loose particulates are created and, unlike adhesive and solvent joining systems, no ventilation equipment is necessary for the removal of toxic fumes. 20. Immediate handling Assembled parts can proceed on to other operations at once without waiting for parts to cool and for adhesives or solvents to set. 21. High production rates Depending on the application, laser welding is capable of extremely high production rates for small parts such as 30 to 60 parts per minute based on a single weldment per cycle and not taking into account the part handling time, which varies with each application. Higher rates are possible if multiple parts are welded with each cycle. Larger parts require more time, however the technique is still fast compared to competing processes.

5 524 Welding with Lasers 22. Process freedom Parts made by virtually every thermoplastic process can be laser welded. 23. Small part capability Parts as small as 2 mm (0.079 in.) by 5 mm (0.197 in.) have been laser welded. 24. Large part capability Equipment is available that can weld parts up to 250 mm (9.843 in.) by 400 mm ( in.). Larger parts can be laser welded if a continuous weld is not required. 25. Quick changeover Except for the simultaneous welding technique, laser welding equipment can be quickly changed from one job to the next Disadvantages of Through Transmission Laser Welding 1. Material limitations The limited availability of materials for laser welding has probably been its most serious disadvantage to date. Materials for laser welding have been limited to compatible thermoplastics where one of the two materials is transparent to laser light and the other absorbs laser energy. Recent development of coatings that can be applied at the weld interface have considerably expanded the range of feasible materials, however at an increased cost per unit. 2. Shape limitations Although there are some versions of the process that can handle height variations, most successful applications have had a flat, horizontal welding surface at the time of this writing. 3. Equipment cost Depending on the power of the laser in question and the degree of control sophistication of the competing welding equipment in question, laser welding equipment ranges from comparable in cost to considerably more expensive than other processes. On a size to size comparison, the latter is most often the case and laser welding becomes the technique of choice for those applications requiring its special characteristics and no other method will do The Process of Laser Welding The Laser The term laser is actually an acronym for the process known as Light Amplification by Simulated Emission of Radiation. Basically, a light source is reflected between two reflective surfaces and passes through a laseractive medium. The light source is repeatedly reinforced in such a fashion to produce a strong concentration of the light beam. Three types of lasers are normally used for laser welding: diode lasers, carbon dioxide (CO 2 ) lasers, and neodyniumyttrium-aluminum-garnet (Nd:YAG) lasers. As shown in Fig. 21-2, diode and Nd:YAG lasers operate in the near infra-red (NIR) wavelength range while CO 2 lasers function in the infra-red (IR) range. The curve represents a typical thermoplastic s light absorption characteristics. High power diode lasers (HPDL) produce laser light in the wavelengths range from 600 nm to

6 21.2 The Process of Laser Welding 525 Figure 21-2 Absorption v. wavelength of a typical thermoplastic (courtesy Leister Technologies) 1600 nm, however, those used for laser welding are normally in the range of 800 to 980 nm. The wavelength used is dependent on the crystal structure of the diode. As shown in Table 21-1, diode lasers offer several characteristics, which make them the preferred laser for many plastics applications. They have low maintenance cost, a high efficiency (in the range of 30 to 50%), and are the smallest of the lasers used for laser welding. These lasers are composed of Gallium (Ga), Indium (In), and Aluminum (Al) on one side and Phosphorous (P), Arsenic (As), and Antimony (Sb) on the other side. Table 21-1 Lasers Used for Welding Plastics CO 2 -Laser Nd:YAG-Laser High power diode laser Type of active media Gas Solid state semiconductor Wavelength [nm] max.pwr Output cw [kw] upto 50 upto up to >1 (modular) Focus-Intensity [W/cm 2 ] Efficiency [%] (8 DP*) Size (Head) [cm 3 /W] Price [$/W] Source: Rofin Sinar (270 DP*) 70 Service interval [h] (Lamps) servicefree DP*=Diode Pumped Nd:YAG-Laser (Courtesy: Leister Technologies) The neodyniumyttrium-aluminum-garnet (Nd:YAG) laser operates at 1060 nm and is the next most commonly used laser for plastics applications. The carbon dioxide

REVIEW OF LASER PLASTIC WELDING PROCESS

REVIEW OF LASER PLASTIC WELDING PROCESS Kalpesh More 1, Rushikesh Aher 2, Makrand Bharaskar 3 1,2,3 Mechanical, Sandip Institute Technology and Research Centre/Pune University, (India) ABSTRACT There are