A new way to weld sheet metal
Remote laser welding replaces spot welding in microwave oven fabrication
Nu-Way Industries Inc., Des Plaines, Ill., learned that remote laser welding demonstrates impressive advantages over traditional resistance and arc welding for its applications.The old way didn't work for Nu-Way Industries Inc., Des Plaines, Ill. The large custom sheet metal fabricator needed a new method of welding to replace the traditional means of spot welding a sheet metal enclosure that was part of a new microwave oven design. Typically, sheet metal fabrication industries use traditional spot, arc, or gas metal arc welding (GMAW) for enclosures, but those methods weren't efficient enough to keep up with the customer's demand.
Nu-Way Industries Inc. turned to laser seam welding
to fabricate this stainless steel microwave enclosure.
Rofin-Sinar Inc. installed the system in July 2004. The system swung into full production by September 2004.
The Nu-Way Application
The Nu-Way application involved an enclosure made of 0.060-inch 304 stainless steel that represented the main component of a new type of microwave oven. The original application mandated that the stainless steel enclosure have repetitive spot welds separated by no more than 0.500 in. to prevent leakage of microwave radiation.
Prototyping the stainless steel microwave oven enclosure originally required over one hour of manual spot welding. Original production estimates projected approximately 30,000 hours of manual spot welding to complete the 25,000 enclosures within the six-month deadline.
Calculating the hourly production rate determined that labor and equipment requirements would have been prohibitive. A three-shift operation of at least six spot welding stations and eight welders per shift would have been required to deliver the order. More specifically, 30,000 hours divided by 180 days of labor, 24 hours per day delivered 6.9 enclosures per hour.
An alternative solution was to use three robotic GMAW stations operated by three people. The automated GMAW would eliminate spot welding by filling the gap between the parts with filler wire. Custom tooling would fixture parts and would provide the seam-locating functions. The estimated production rate was 12 enclosures per hour.
However, the equipment, fixturing, and floor space costs proved to be prohibitive considering the system's lack of flexibility.
Nu-Way also considered an industrial robot with an Nd:YAG laser in addition to the remote welding system. The investment cost for both systems was similar, because two Nd:YAG lasers would be required rather than one remote welding station. Cycle time analysis showed that the remote welding system could produce one enclosure in approximately 2.5 minutes using semiautomatic setup fixturing. Therefore, full production capacity of 25 enclosures per hour could be achieved with one remote welding system. The Nd:YAG laser stations could not achieve the same productivity, according to Nu-Way.
Other comparisons favored the selection of the remote welding system. The slab CO2 laser requires no replacement optics and is designed to operate for eight months on a small premix laser gas bottle. The RWS can be programmed easily for pattern welding and various welding shapes and can weld dissimilar materials together such as mild steel and stainless steel.
Laser Welding Makes the Cut
Nu-Way ultimately turned to laser technology for the joining job.
The RWS uses a high-powered CO2 laser beam to apply energy and weld sheet metal components. From a remote location, the laser beam is focused by a long focal length lens. Then using a computer-controlled targeting mirror, the beam is directed to the work piece at extraordinary positioning speeds. As a result, traditional beam delivery systems utilizing robotics or other types of mechanical drives are eliminated. The advent of high-powered CO2 lasers with such beam quality has made this technology feasible for working areas of 1 meter by 1 meter and larger. The benefits of this process include faster cycle times, process flexibility, and less floor space as compared with traditional processes, according to Nu-Way.
The selection of laser welding particularly made sense because the application required a sealed enclosure so that microwave emissions could not escape. Using lasers for stitch welds or continuous seam welds addressed the need to prevent microwave leakage from the enclosures (Figure 1). In fact, extensive microwave emission testing demonstrated that continuous laser seam welding provided exceptional strength and quality, as well as a sealed enclosure.
In laser marking, a low-power beam is manipulated by galvo-driven mirrors.
The ability to deliver high beam quality with long focal lengths allows for keyhole welding.
What Is Laser Welding?
The concept of laser scanning is not a new one. Laser markers have been around for more than 20 years and are used to mark or etch features on components. As shown in Figure 2a, a lower-powered laser beam, usually less than 100 watts, is manipulated by two galvo-driven mirrors and focused on the work surface.
The development of high-power CO2 lasers with good beam quality in the mid-1990s opened the door for their use in laser welding applications that utilize scanning techniques. The high-quality beam provided the capability for keyhole welding with very long focal lengths, illustrated in Figure 2b.
A typical industrial RWS has a high-power CO2 laser and remote scanner.
A typical remote welding system is shown in Figure 2c. The system includes a high- power CO2 laser and a remote scanner system with a long focal length, typically 1,000 mm to 1,600 mm. A computer controls the motion system of the optics that directs the laser beam's programmed path to perform the welding of the sheet metal subassembly.
The scanner system comprises two important optical components: a focusing lens and a targeting mirror. Both optical components are mounted on a precision, high-speed linear motion system that is programmed by the system control.
In addition, the targeting mirror mount has dual-axis rotational capability. As a result, the laser beam can be focused and aimed within a work space of 1 m by 1.5 m by 4 m. The large work envelope provides system flexibility within a relatively small footprint.
This 3-D CAD illustration shows the system layout at Nu-Way.
The RWS has quick positioning speed because small movements of the optical components translate to high speed movements of the focused laser beam. The laser beam is designed so that it can be repositioned at speeds in excess of 2 MPS with positioning accuracy of ±0.005 in.
The beam scanning motion is based on high-speed linear or galvo motors, depending on the manufacturer. This enables quick repositioning speeds of the laser beam. The beam can move from one weld location to another in less than 50 ms.
The scanner also allows custom programmable weld shapes such as stitches, circles, and weaves. The operator can apply the best path profile for the welding application.
Because Nu-Way had experience with large manufacturing systems integration, such as its Finn-Power Flexible Fabrication Center, the company decided to integrate the RWS as well as design its own tooling.
The system layout and tooling design started in July 2004, while Rofin prepared the components for shipment.
By mid-August 2004, the system was assembled and the first parts were welded in prototype tooling fixtures. The entire system layout as a 3-D CAD illustration is shown in Figure 3.)
The Nu-Way laser and control are mounted
on top of a mezzanine to conserve floor space.
The DC045 diffusion-cooled laser and scanner box were mounted on the top of the mezzanine to conserve floor space. The beam delivery system carried the beam from the laser to the scanner box (See Figure 4 and Figure 5).
The chiller and ancillary equipment were installed remotely to keep the work area free of obstructions. A stairway allowed access to the mezzanine for the system's routine maintenance. A Plexiglas® enclosure with safety interlocks was installed for easy and safe access to the work space and fixturing.
The total floor space for Nu-Way's entire RWS
is less than 900 sq. ft.
The total floor space for the entire RWS installation was less than 900 sq. ft., measuring about 30 ft. long by 30 ft. wide by 15 ft. high. (The actual system is shown in Figures 6 and Figure 7.)
Application Tooling, Part Issues
While the RWS provided several benefits, the remote nature of the process posed a problem—the gap between the parts to be welded—because the laser beam is applied remotely and has no way of dealing with the gap between parts.
In spot welding applications, the electrodes are attached to clamping mechanisms that holds parts together during welding. In the arc welding process, the wire filler allows for a gap in the weld seam. However, laser welding is accomplished autogenously—no filler wire is used.
Remote laser welding requires the tooling to hold parts, locate, and clamp the area that is to be welded. This is why Nu-Way had to develop unique fixturing for its stainless steel application.
The part tooling and fixturing evolved during the project because the product had to be shipped to the customer immediately. In addition, optimal application parameters such as weld speed, shielding gas, and fixturing methods needed development.
The project turned out to be in three phases. In the first phase of the project, a simple manual part fixture was used to launch production. The multiple-part fixture was designed to be located in the work space below the scanner box. Three stainless steel enclosures were placed into the fixture at a time, and one side of the enclosures was welded during each cycle. The operator entered the work space and rotated the enclosures to present the next surface for welding. The procedure was repeated three times to weld each enclosure's front, left, and right sides. According to Nu-Way, the tooling achieved maximum production with minimal complexity and start-up problems. A large portion of the contract was completed with this rudimentary tooling method.
The second phase of the project incorporated automatic tooling that indexed one stainless steel enclosure in and out of the work space, as well as performed its rotation to present all three surfaces. All functions of the automatic tooling were integrated via a programmable logic control with the safety enclosure interlocks, the laser, and the scanner box. Cycle times of the RWS improved significantly:
- 10 seconds for enclosure in/out positioning
- 10 seconds for fixture rotation (two rotations)
- 90 seconds for welding time (150 inches)
- 30 seconds for part removal and setup
The total process time decreased to 140 seconds. The new system produced 25 enclosures per hour.
The RWS uses automatic tooling
to laser-weld the enclosures.
The part fixture utilized unique clamping and locating stops that secured the two sheet metal enclosure components with minimal gap between the overlapping 0.060-in. 304 stainless steel enclosure flanges. In addition, stainless steel stiffener panels were secured to the enclosure walls with locating clamps (See Figures 8, 9, 10, and 11).
Secure clamping is required so to maintain a minimal gap
between the two sheet metal enclosure components.
The third phase looked to the future. Nu-Way management envisions the addition of a robot to index parts in and out of the work space from the back side of the RWS. While the automatic tooling is in the out position, the robot would index a part into the work space for welding. When the robot is in the out position, the automatic tooling would index an enclosure into the work space for welding. This would allow almost continuous welding of enclosures and other parts. In essence, the automatic tooling and the robot would take turns using the remote welding system.
One More Consideration
Remote laser welding with a CO2 laser poses another unique challenge: plasma suppression. As the material absorbs the laser beam, the heat generated creates an ionized gas, or plasma. If the plasma is allowed to remain, it will absorb the laser beam and adversely affect the welding process. The welding process can become so unstable that the weld can be completely lost.
As a result, a process is required to dampen or blow away the plasma that is created during the laser welding process. The plasma suppression, or shield gas, can be helium, nitrogen, compressed air, or some combination depending on the application and laser power.
The location of the gas nozzles and the activation sequence are other important considerations for a successful remote laser welding application.
When tooling is indexed into the work space, nozzles provide the shielding gas.
Because Nu-Way designed and built its own tooling, it also designed the air compressor, accumulation tank, nozzles, and mounting systems to deliver the shielding gas. As tooling is indexed, strategically positioned compressed-air nozzles create a flow of air across the welding surface, suppressing plasma development (Figure 12).
Depending on the materials that are processed, another gas-related issue must be considered: outgassing. When zinc-coated metals and similar materials are being laser-welded, the coatings will outgas, so a process is required to allow for the escape of these fumes. If the outgassing cannot vent properly, the weld will exhibit excessive porosity and blowouts.
For zinc-coated materials, the special tooling and part features are needed to provide an effective outgassing method. One solution involves stamping the sheet metal parts with low-profile dimples that create a slight gap between the overlapping sheet metal flanges. The gaps do not impede the welding process, but provide for outgassing.
Unlike a spot weld, a laser weld is very narrow and requires only single-side access to the part. The RWS scanning mirror can be programmed to create various weld shapes or patterns. In addition, laser-welded parts are stiffer and more rigid than spot welded parts. These concepts must be kept in mind very early in the part design process to take full advantage of laser welding.
This welded enclosure, made from 0.060-in.-thick type 304 stainless steel,
was made on the remote welding system.
Weld flanges can be made smaller, providing a potential weight savings. Also, access holes required for many spot welding applications are not necessary for remote laser welding.
Nu-Way took these factors into account for its stainless steel enclosure project. Figure 13 depicts a continuous laser seam weld. The spot welding was originally replaced by stitch welds, but later continuous laser seam welding was adopted for added strength and greater microwave emissions leakage protection. This provided a solid, rigid assembly that exceeded all the criteria and microwave test requirements.
The tooling and part development for remote laser welding is relatively new. While standard components were used to build the Nu-Way fixtures, it took time and effort to develop the specific techniques for part clamping, fixture orientation, application of shield gas, and related procedures. It is important to dedicate time for process development when considering a remote welding system. For example, compressed air is being used for the shield gas at this time, but Nu-Way is continuing the process development and may consider other gases to increase production rates and reduce costs.
The Results, Please
Remote laser welding has undergone considerable development since being introduced in the mid-1990s. The first production systems were installed in Europe, and recently North America has followed with several successful installations. Nu-Way represented one of the first North American installations in the fabrication and sheet metal industry.
Nu-Way Industries took the proper approach to implement the best system for its application. It:
- Evaluated the welding options to select the most cost-effective approach.
- Assumed responsibility for the process design—more specifically, testing the feasibility of using remote laser welding, designing and building part fixtures, and dedicating a remote welding system team.
- Educated the engineering and manufacturing personnel for implementation and operation of the system.
An RWS is not appropriate for every part assembly. A thorough analysis should be done in the early stages of part design and the manufacturing cycle to determine the feasibility of remote laser welding. In addition, design engineers should consider changing design and assembly to take advantage of an RWS's benefits.
|Laser Beam Quality Characteristics|
Beam quality refers to the laser beam's power distribution.
It is specified as the M2 value.
Beam quality, or mode, refers to the power distribution of the laser beam, and it is specified as the M2 value. The best possible mode is M2 = 1, which represents a perfect Gaussian distribution.
The remote welding system's beam quality affects the focused spot size d, as shown in the equation in Figure 14. With a long focal length, the beam produces a focused spot size that will become too large if M2 > 2. Larger spot sizes will result in lower power densities that will fall below the threshold for keyhole welding (1 x 106W/cm2). As a result, the weld process will become unstable, as shown in Figure 14.
A diffusion-cooled slab resonator designed to produce a laser beam with M2 < 1.2 may help prove the feasibility of an RWS for a variety of welding applications.
Steve Southwell is president, Nu-Way Industries Inc., 555 Howard Ave., Des Plaines, IL 60018, 847-298-7710, fax 847-635-8650, www.nu-way.net.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.