David A. Belforte
A unique concept in metal forming increases production, improves part quality
Eberspaecher North America (Brighton, MI, USA), a tier-one supplier, manufactures stainless-steel catalytic converters and exhaust products required by an impressive list of automotive customers, including Buick, Chevrolet, Chrysler, Dodge, Jeep, Mercedes-Benz, and Pontiac. Founded in 1865 and in the automotive-exhaust business since 1931, this German-based company has had operating facilities in the United States and Canada since 2000. It currently manufactures in Brighton, MI, as well as Tuscaloosa, AL, USA, and Brampton, ON, Canada.
Some of the shapes produced on the TWINMASTER machines.
For its new catalytic-converter production line, the company elected to install a laser welding short-tube production cell instead of a conventional tube mill. Prior to this, converters were welded using a conventional TIG welding system, however, this process was slow and the integrity of the weld seam start and end was a problem. The company, therefore, decided to install a new system that integrated a laser powered Twinmaster from Weil Engineering North America (Troy, MI, USA) that had increased welding speed leading to higher product throughput. This laser-weld process improved the weld start/stop conditions such that the tube ends could be subjected to higher degrees of deformation than those that are arc-welded.
The Twinmaster at the heart of this system combines two major functions in one machine: roll-forming and welding. The secondary processes, such as blank feeding and post-welding expansion of tubes for perfect roundness, are directly linked to this system, creating one complete production center. The control functions for the entire system are supplied by a Siemens SINUMERIK 840D for CNC controls and a SIMATIC OP170 operator panel for dialog functions.
The main reason for selecting laser welding over TIG welding is a significant reduction in process cycle time, due to a three-times increase in welding speed from 1.2 m/min to 3.5 m/min, or nearly three times as fast when welding wall thickness of 1.2 mm with 3.2 kW of laser power. In addition, with the laser, power is on at 100% over the entire weld length, thus there is no on- and off-ramping of power at the start and end of the tube.
An additional feature, developed by Weil Engineering for use in catalytic converters, is the so-called “weld-back” function. Because the start of the laser weld is always stronger than the end of the weld, it is advantageous to create two weld beginnings. This is done by welding in one direction for about 75% of the weld length, stopping welding, traveling without power to the end of the tube, and then starting to weld back in the opposite direction, and finally slightly overlapping where both welds meet up. By using “weld-back” the tubes are much more resistant to splitting under expanding conditions.
A laser beam welds the can seam
And, by using a laser, the power can be switched from one welder to another while unloading the finished tubes and loading new ones. This offers nearly 100% utilization of laser power and reduces the capital investment by being able to share the power between two machines.
Another advantage of using the laser is that all materials can be welded, including coated metals such as galvanized or aluminized steels. This gives Eberspaecher flexibility for future projects. Also, a laser welded seam can be annealed to strengthen the weld for expansion immediately after welding.
A disadvantage of laser vs. TIG is the initial purchase price and relatively high running costs. However, for example, a laser system, while 30% higher in sales price than a TIG system, has an 80% higher output, with improved tube end quality. The TRUMPF CO2 laser used in the system is robust, proven in thousands of cutting and welding applications. Further, this laser can be incorporated into a system at reasonable cost, with safety considerations as simple as those used in an arc-welding process.
The sequence of operations begins when two sheet-metal destackers, mounted on a rotary table, are each loaded with approximately one hour’s production material and feed the blanks into the roll-former. The blanks are inspected for double-sheet condition during the transfer movement. Once the first destacker is empty, the rotary table moves the second destacker into place and the first one can be reloaded, ensuring a nonstop supply of material. Depending on the length of the tubes, multiple blanks (up to four) can be rolled into tubes during the same machine cycle.
The rolled tube blanks are automatically transferred from the roller to the seam welder, where they are clamped and butt-welded using a laser beam generated by the TRUMPF TLF 3200 laser. TIG welding is another option for this process, being more economical initially, but also slower in welding speed compared to laser. Eberspaecher has two Twinmaster systems, the first with TIG and second with laser.
Hydraulic tube expander finishes the final can dimension.
Once the welding process is done, the finished cans are extracted from the tooling and transferred onto an in-line weld annealing system, which heats the welding zone to approximately 1000ºC (about 1800ºF) to relieve the stress in the welded seam. After a cooling section, the CNC-controlled handling system introduces the cans into a hydraulically operated tube expander, where they receive a preselected inside dimension. This final dimension is calculated from the diameter of the converter substrates, the thickness of the insulation mats, and the spring-back of the stainless-steel material used for the converter cans.
The sequences of these operations, the exact timing for each process, and the control of all movements are monitored and operated by a Siemens SINUMERIK 840 D controller that also monitors the position, the power, and the on/off condition of the laser beam, as well as all the transfer mechanisms and the tooling. The TRUMPF laser on the Twinmaster machine has a 200-mm optics bifocal mirror with a constant focal length. When the material thickness changes, the CNC tooling varies the position of the workpiece.
Anthony Mussarelli, Eberspaecher engineering manager, says, “These machines have made a significant improvement in our responsiveness to customer delivery requirements. The tooling sequences, the implementation of customer-driven design changes, and the overall continuous production of our various end-product shapes and sizes have made our satisfaction complete.”