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The laser's edge: catching the wave in welding.

Some view lasers as a cure-all for traditional welding woes. Undoubtedly, the precision, repeatability, and controllability that lasers offer have done much to lure advocates. Yet others argue that laser-based welding is expensive and cumbersome. If laser sales are any indication of who is right, the advocates are winning.

According to Market Intelligence Research Corp (MIRC), the laser industry is expected to show a steady annual growth rate of 9.7% through 1996 as manufacturers increasingly accept lasers as reliable, cost-effective alternatives to more established machine tools. The laser market, which recorded annual sales of $325 million in 1986, is expected to grow to $788.6 million by 1996.

Welding is one segment of the laser industry that has demonstrated particularly strong growth, accounting for approximately 26% of all laser sales. But is laser welding better than traditional welding? Where does laser welding work best? Where doesn't it work? How can laser welding systems be made more efficient? To answer these questions, a few comparisons are in order:

Laser advantages

Lasers hold some distinct advantages over traditional methods of welding:

High energy. Lasers are able to concentrate energy at high intensities (exceeding 10 MW/cm[sup. 2]) for localized, effective material processing without affecting adjacent material zones. This concentration allows deeper weld penetration and better welding capabilities in confined areas or on irregular surfaces.

Flange size. Traditional joining processes require a relatively large external flange, while laser joining offers the potential to greatly reduce or eliminate flanges. Large flanges are bulky and add additional weight. Traditional systems often cannot process unconventional joint configurations such as coach joints and box sections.

Contact. While traditional welding requires contact, laser welding is a non-contact process with a very narrow heat-affected zone. This reduces surface distortions that are common in many traditional joining and cutting techniques.

Speed. Lasers offer potential for more rapid joining and cutting than traditional systems, increasing part design and assembly process flexibility. Depending on the application, lasers can often increase productivity over traditional welding.

Versatility. Lasers are well suited for handling three-dimensional welding and unconventional joint configurations such as coach joints, edge joints, and box sections. Thick gage metal plates, which require deep penetration, and parts with weld joints that are difficult to reach are good applications for lasers.

Easy access. Conventional spot welding requires access to both sides of the weld joint, while laser welding requires access to only one side of the joint. This permits welding of boxed or closed-in areas and eliminates flanges whose only purpose is to provide two-sided access. The elimination of flanges provides increased design flexibility and reduces component weight. Lasers offer efficient, cost-effective solutions for the automotive, aerospace, and general manufacturing industries. Some non-traditional applications for lasers include nonmetallic processing for fabrics, composites, and plastics.

Laser disadvantages

Laser welding is not a cure all for all welding dilemmas. As with conventional welding, materials and fit play a restraining role in laser welding potential.

Materials. Good heat conductors do not work well with lasers. Neither do materials with high reflective properties. This makes copper, brass, and some aluminum alloys difficult to weld with lasers. Even if laser welding can be used for these materials, many of the problems that these alloys pose for traditional welding, such as oxidation, must also be taken into consideration in laser processing. Aluminum's low melting point is of no particular help in laser welding. Its high thermal diffusivity and reflectivity make it hard to couple with laser energy. Magnesium and zinc constituents also are low-boiling point metals that vaporize in the melt pool and cause extreme levels of porosity. Most alloys exhibit low tensile strength at high temperatures and can tear open during cooling. Thus, filler-metal additives are required to assure weld integrity.

Copper alloys have even greater diffusivity and reflectivity than aluminum and typically require surface treatment to enhance absorption of laser energy. Once beyond this problem, satisfactory welds can be made, but require much more power than for steel. Brasses are unweldable because the zinc alloy vaporizes and generates extreme porosity.

Fitup. In steel, the power density to initiate keyhole welding is about 106 W/cm[sup. 2], which means that a 1000-W laser operating in a low-order mode must have a beam diameter of less than 0.5 mm (0.020"). This makes joint design and fitup critical. Because no filler metal is used, any gap will become a void or undercut in the finished weld.

Butt or seam joints are the most efficient because the shape of the fusion zone is very close to the minimum joint volume. This means minimum heat input and distortion, and maximum processing speed.

The laser beam must be aligned with the joint to within 0.25 times weld width. With typical weld widths of 0.25 mm to 0.5 mm (0.010" to 0.020"), good tooling and repeatable parts are required for production environments. Typical joint-fitup tolerances are 1/10 of material thickness.

These constraints tend to limit production butt joints to circular parts, turned to close tolerances and press-fit together prior to welding. Lap joints are more tolerant of seam alignment, but the gap between layers must be held to 1/10 of the material thickness, and the width of the weld at the junction is more critical than the penetration into the underlying (usually thicker) material.

Laser checklist

Is a laser welding system right for your application? The following questions can help you decide:

* How thick are the metal joints to be welded? Thick metal joints may require the high energy outputs of a laser welding system.

* How complex is the weld? Difficult welds such as coach joints and box sections can be more easily welded with lasers than traditional methods, which require access to both sides of the weld. Lasers are also good for welding on irregular and hard-to-reach surfaces.

* What production speed is required? Laser welding is often quicker and more efficient than traditional welding. Increased productivity can be achieved by combining several traditional welding stations into one laser welding station.

* How much surface distortion is acceptable? Lasers produce a very narrow heat zone, significantly reducing surface distortions around the weld joint as compared to traditional methods.

* What type of metal is going to be welded and what are its properties? Most metals can be used in laser welding; however, good heat conductors and/or highly reflective metals such as copper, brass, and aluminum alloys are not good applications for lasers.

Selecting the right laser

Once it is determined that a laser is suitable for an application, the next step is to select a laser processing system.

The first step is to determine the correct laser generator for the application. Welding applications generally use higher power than cutting applications, but the focus spot can be larger to increase weld-bead width. Laser beam intensity distribution affects focus spot size. It will be necessary to know how much power and what focus spot size will be needed before an adequate equipment selection can be made. The tables accompanying this article offer general recommendations for gas (CO[sub.2]) and solid state (Nd:YAG) lasers most commonly found in industry today. Beam delivery systems are also an important consideration. Some systems use gantry-like structures to manipulate the laser beam, while others use pedestal robots. Pedestal robots take up less space and can often achieve the desired work volume with shorter beam paths and less path length variation than gantries. A large gantry, on the other hand, can cover a larger work envelope than a single pedestal robot. Large work envelope applications, however, are not limited to gantry delivery systems. Multiple pedestal robots often can cover a large area more efficiently than a gantry.

Other important factors include ease of maintenance, complexity of laser beam alignment, and the number of optics used. Easily alignable systems are easier to use and require less downtime than more complex systems. Also, fewer mirrors minimize power loss and reduce alignment complexity. All optics should be easily accessible for inspection.

Selecting the proper laser generator, beam quality, power and delivery system is not easy. All parts of the system must be compatible and work well with existing applications. Suppliers that offer a complete package of laser systems components (including generators, controllers, robots, etc) are often the best bet.

For even greater efficiency, many users select a fully integrated system that uses one controller for all robot and laser functions. This type of system eliminates the need for extra controllers, interfaces, and programming languages. This integration of robots, laser beam delivery equipment, laser generator, and a common control system offers economy, flexibility, and efficiency to production laser processing.

High-power lasers

Since the earliest introduction of laser processing into industrial metalworking applications there has been an tendency to center attention on applications that can be achieved with less than 1 kW of laser output. This is a natural result of the ready availability and lower capital costs relative to higher power lasers. The large number of laser systems being used in lower-power applications has led to the misconception that they are reliable, proven pieces of hardware, while higher power lasers appear to carry more risk. However, in recent years, higher power laser systems (6 kW to 25 kW) have been developed and introduced into production use on a relatively large scale.

One example of what high-power, multi-kilowatt lasers are capable of can be found at the Pratt & Whitney plant in Middletown, CT. A new engine design at P&W depended on the use of double-pass burner liners that required numerous holes through a variable-thickness section of Hastelloy X material and a narrow weld joining neighboring rings. The largest rings required a weld depth of 0.070", deeper than the thickness of sheet metal being welded. Standard TIG welding techniques would have resulted in a weld so wide as to intrude weld metal into the cooling hole.

P&W approached United Technologies Industrial Lasers (UTIL) to determine the possible benefits of multi-kilowatt processing with a CO[sub.2] laser. UTIL demonstrated the ability to weld at over 100"/min with full penetration and excellent control of weld-bead placement.

Using a 6-kW laser fitted with a parts-follower assures that the focused beam strikes the part within [+ or -]0.002" of the design point. As of November 1991, the system had accumulated over 75,000 hours of operation, produced approximately 209 miles of laser welds, and has provided substantial savings to P&W over conventional processing approaches. TABULAR DATA OMITTED
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes related article
Publication:Tooling & Production
Date:Mar 1, 1992
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