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Welding

A flash of innovation

By Nick Wright

IndraMotion MLC for hydraulics enables patented welding process for half the cost

November 2014 - Welding is a key industrial operation where technological innovations have been improving productivity and quality. That’s especially true for flash-butt welding, a technique that uses electrical resistance to heat the ends of two metal pieces that are forged together into continuous coils, wheel rims, rods, band saw blades, and other seamless joints at parent metal strength. Older machines use manual cams to move pieces together at the right velocity. Today’s advanced high-strength steel (AHSS) products, however, need much greater accuracy and controlled timing, down to thousandths of an inch and a few milliseconds. One solution is to replace the cams with electric servo drives and use multiple PLCs to control the process, but that can result in a machine that is too expensive to be widely adapted.

FFJ-1114-welding-image1Taylor-Winfield Technologies Inc., a welding and metal joining technology innovator in Youngstown, Ohio, devised another approach: Use a single Rexroth IndraMotion MLC L45 motion logic controller with Rexroth hydraulic drives, a package the company claims is extremely precise and scalable.

“There’s a growing need for flash-butt welding in aerospace, automotive, appliance and other industries requiring seamless joints in products made from high-strength metals,” says Blake Rhein, vice president of sales and marketing for Taylor-Winfield. “In these applications, the ends of the metal pieces must be moved together very precisely to create a solid, homogeneous joint. Our solution uses one Rexroth IndraMotion motion controller to operate a proportional Rexroth hydraulic servo valve in a closed-loop automation system. The solution—in combination with our patented Forced Freeze welding process—is so simple and accurate. We believe it will revolutionize how this type of welding is done around the world.”

Improving flash welding

Rhein estimates that flash welding is about 20 percent of the overall resistance welding market, which includes spot and seam welding. 

“Flash-butt welding is a two-stage process,” he explains. “The first stage is the flashing phase, in which electrical current is applied to two work pieces separated by a small gap. As the two pieces are brought together, electricity arcs across the interface and the temperature at the ends reaches anywhere from 600 to 900 degrees Fahrenheit, depending on the material being joined.”

At the required temperature, the second stage—the upset phase, begins. “The two work pieces are pushed together using a moving and stationary platen with enough force to cause the ends to forge together. This action upsets the material, meaning that excess material and impurities are forced out of the top and bottom of the joint and subsequently removed by trimming to reach parent metal thickness,” Rhein continues.

Because the process removes irregularities and impurities, flash-butt welding can join dissimilar materials: narrow and thick shapes, wide and thin metal sheets, and ferrous and nonferrous materials. To make a successful weld, the moving platen and electrical current of the two work pieces must be precisely controlled.

Taylor-Winfield uses the Rexroth MLC L45 motion logic controller to automate the process. A welding curve that depicts the logarithmic relationship between the two pieces in terms of time and position is loaded into the controller. The overall weld schedule lasts from 2 to 30 seconds depending
FFJ-1114-welding-image3on the material. During the flash phase, the position of the mobile workpiece accelerates dramatically, generating forces up to 1 million lbs. per square inch to forge the pieces together.

Mike Prokop, Taylor-Winfield’s innovation leader and assistant chief electrical engineer, notes “only the Rexroth IndraMotion MLC motion logic controller has a scan rate fast enough to meet our position and timing requirements. We have implemented a closed control loop in which the IndraMotion MLC controller gets position signals from a transducer. Then the controller initiates a position change by activating the Rexroth hydraulic power unit. A Rexroth 4WRPH proportional directional valve activates a linear positioning cylinder that moves the platen containing the mobile workpiece toward the stationary workpiece.”

Milliseconds and micrometers

Depending on the material, the pieces begin with a 1⁄4 in. gap between them. A critical part of the process occurs when the pieces are just a few micrometers apart.

“The entire process is broken down to milliseconds,” says Prokop. “The workpieces accelerate to generate the forces needed to forge them
together. In flash welding, you want smooth acceleration of the moving platen. You don’t want the moving platen to freeze in position. However, with the Taylor-Winfield Forced Freeze welding process, we purposefully linearly advance the moving platen during the flashing cycle. The pieces are offset, for example, 50 micrometers apart for two milliseconds. During that offset time, the joule heating goes through the roof, similar to when the standard freeze occurs—that’s why we call it forced freeze. Then the platen movement resumes into the upset phase of the welding cycle to complete the process. The additional heating dramatically improves the weld quality.”

According to Prokop, a Rexroth touchscreen IndraControl VPP 40 human machine interface (HMI) makes it easy to select the proper weld schedule and modify the weld curve for the particular material. 

Prokop’s team worked with Dan Martincic, electrical engineer for Rexroth distributor Hydrotech Inc., who developed the event-driven motion control profile for the controller with Taylor-Winfield engineer Nick Maillis. “The idea of a cam continues to influence how we program automation systems,” Martincic says. 

The MLC controller is loaded with Flex Profile, a motion programming platform from Bosch Rexroth that adapts segment-based cam technology to simplify the task of defining motion segments for electronic and hydraulic motion control tasks.

Martincic explains, “The beauty here is that the programming from electric to hydraulic is not different. In fact, the programming is exactly the same for hydraulics or servo drives, which is one of the unique features of using the IndraMotion MLC. The Flex Profile function in the MLC controller provides an object definition that lets us translate the traditional cam-based model and their associated cam tables to a schedule-based event-driven model. I worked with Bosch Rexroth hydraulics experts to create a motion profile for the hydraulics used in the Forced Freeze process.”

FFJ-1114-welding-image2The motion profile and weld curve for the Forced Freeze process is nonstandard. Five position points are involved: the start position, the initiate offset freeze position, the end of the freeze period position, the upset or forging position and lastly, the final upset position.

“The simplicity that Flex Profile provides to program this curve was a big plus on this project,” says Martincic. “We could work in the same Rexroth motion control environment we use for electronic or hydraulic drives. Once our team set it up, Taylor-Winfield could develop the profiles needed for each individual application.”

Rexroth Senior Sales Engineer Mark Berndt, who worked with Martincic, notes that “the MLC platform has the intelligence to handle processes ranging from the Forced Freeze function, as well as general machine control. It sends messages over a serial communications link to the weld controller that dynamically changes the weld schedule. If an annealing option is needed, the controller can reposition the weld seam and activate the welding transformer to provide the required heat levels during the annealing process. And if a data acquisition option is needed, it can be integrated directly into the Bosch Rexroth VPP40 with a third-party high-speed system, such as LabView.”

Taylor-Winfield also used Rexroth’s Open Core Engineering, which enabled the ability to interact with the core functionality of the controller using standard programming tools to develop custom functionality and motion sequences. Berndt credits Open Core Engineering for reducing cost and simplifying HMI screen development directly in LabView. Bosch Rexroth libraries enable LabView to connect directly to the core of the MLC for control of the complete machine including Flex Profile, welding schedules, recipes, and motion and hydraulic axis control, all on one Bosch Rexroth HMI (VPP40)—eliminating the expense of separate PCs for Machine Control and LabView. 

Prokop was pleased with the integration of the IndraMotion MLC for hydraulics platform with Rexroth hydraulic components, a complete package that delivered precision. “Basically, we took the traditional flash welding process to a whole new level,” he says. The Forced Freeze flash welding process can be implemented on new machines and can be added to existing flash welding machines as a controls upgrade. Additionally, with the use of hydraulics, the solution can be scaled to any size application without adjusting the controller. This method, with advanced Rexroth motion control technology and hydraulics, will usher in new levels of precision for the flash welding industry that can be customized to a range of applications. FFJ

 

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