Controlling a 7-Axis Robot for Continuous Internal Bore Welding

By Dan Allford, John Martin, AND Zach Freeman

DAN ALLFORD is president, JOHN MARTIN is vice president, and ZACH FREEMAN is automation programmer at ARC Specialties, Houston, Tex.

Reprinted with permission: The AWS Welding Journal

While most welding robots are used for resistance welding, and others are for gas metal arc welding (GMAW), this status is changing. Many joining and cladding projects are better suited for gas tungsten arc welding (GTAW). This process is erroneously believed to be too slow or complicated for robotics, but technological advances are addressing both issues.

Robotic GTAW

Welding speed or productivity is measured in pounds per hour of metal deposited. To make GTAW as fast and productive as GMAW requires the use of the hot wire technique. The filler wire is preheated as it enters the weld pool using an independent hot wire power supply. The hot wire technique improves productivity and decreases dilution, both of which are key for clad overlay welding.

Distinctive Aspects. Making GTAW work on robots requires some unique features. For example, automatic torch height control is essential. Arc length is also a critical variable. Typically held around 1/8 ± 0.01 in., the robot must follow the surface of the workpiece by monitoring and maintaining constant arc voltage. Simultaneously, the robot must position the wire to feed into the trailing edge of the weld pool. Dedicating one axis of the robot for wire positioning limits robot flexibility. Finally, bore cladding requires continuous torch rotation, which creates cable wrap-up. Advanced controls and new hardware solve these issues.

Paying Attention to Corrosion. In addition, corrosion comes in many forms. In the oil industry, hydrogen-induced cracking (HIC) has caused expensive and catastrophic failures. HIC results when sour crude oil containing > 0.5% hydrogen sulfide (H2S) reacts with the iron in valves and pipes, releasing atomic hydrogen. When atomic hydrogen migrates into the grain boundaries of steel parts, intergranular cracks form and parts fail due to internal stress. Sour crude was originally a naturally occurring problem. Oil fields free from H2S are referred to as sweet. Now, even sweet fields become sour when water flooding is used to enhance oil recovery.

For decades, a solution was to overlay weld all the internal wetted surfaces of valves, fittings, and pipes exposed to H2S. More recently, there’s been a move to hot wire GTAW and nickel alloy welding wires resistant to H2S corrosion. High part preheat temperatures; deep, small diameter parts; and stringent inspection criteria make internal cladding an ideal candidate for automation. The first two generations of bore cladding machines dating back to the 1980s were purpose built based on welding manipulators and part positioners. They used a stationary torch to weld a rotating part. As parts became larger, it made sense to leave the part stationary and rotate the torch instead. The second generation of valve cladding machines featured continuous torch rotation.

Axis Attention

Each year, 6-axis robots become less expensive and more capable. This means robots are replacing purpose-built machines with flexible robotic automation. But 6 axes and standard arc welding controls are not enough for hot wire GTA bore cladding. Continuous rotation bore clad welding requires an additional axis, multiple welding power supplies, and very different software.

Technology Importance. The newest generation of GTAW hot wire clad systems leverage the latest in robot technology.

A 6-axis KUKA robot, integrated with the ARC-5 Infinity 7th axis robotic cladding system by ARC Specialties, allows infinite rotation of the torch inside of a stationary part (see lead photo). Utilizing 4 × 4 homogenous transformation matrices to calculate the kinematics of the 7th axis allows precomputing of the absolute positions of the robot with 6 deg of freedom, based upon the head rotation. The software compares the robot’s absolute Cartesian position with the calculated Cartesian points while safely limiting the allowable travel every 4 ms. Using high-speed EtherCAT® communication, the controller updates the KR C4 control system 250 times per second. The dimensions of the part, as well as the commanded arc voltage and head rotation, determine the position of the robot in real time during the entire welding sequence.

Orbital welding is challenging. Within every revolution, the system must constantly correct the torch position not just up and down but relative to the direction of the torch. Older GTAW systems incorporated a dedicated arc-length control slide. However, it is preferable to use all 6 axes of the robot to move the torch along its centerline.

Programming Prominence. Welding intersecting internal bore requires hundreds of arc starts and stops, each of which requires several lines of software. Programming such parts with a teach pendant means jogging the torch to each point for teaching. Offline programming is also limited because preheating causes part expansion, which makes the 3D models inaccurate. The preferred programming answer is to teach four points at the bore intersections, then use parametric programming software to generate the part program automatically. It seamlessly compensates for preheat expansion, variations in first layer welds, and any part positioning or fixturing issues.

Enhancing Welding Parameters. The ARC-5 Infinity can weld bores in any orientation from vertical and horizontal to inclined. Welding anything other than a vertical bore has unique challenges because gravity affects the weld pool differently depending on the location of the weld. Within each revolution, the system must weld flat, vertical, and overhead. The traditional solution is to make the weld pool so small that surface tension exceeds the force of gravity. However, this approach is slow and unproductive. A better way is to optimize welding parameters as the weld position changes to produce a weld with consistent shape and size for the complete 360 deg around the part. With modern control systems, constant real-time updating of welding parameters improves productivity.

Conclusion

GTAW is not commonly used in robotics, but it is a great process when your project requires precise, high-quality welds, and it works on a range of base metals for both joining and cladding. By using hot wire, you do not have to sacrifice productivity. Recent advances in hardware and software allow robots to GTA weld in many industries. Robotic system integrators are also making robot GTAW accessible and practical for applications that benefit from the precision and quality inherent to the GTAW process. 

Fig 1: A worker prepares to weld a valve internal surface.