Robotic TIG Welding

You’ve probably heard the allegory of the tortoise and the hare. The turtle was slow and steady, where as the hare would go fast and then stop for breaks. Many people live by the saying “slow and steady wins the race.”

The gas tungsten arc welding (GTAW) process is a slow and precise business. Quality of the weld, not time, is most often the major factor when GTAW is used. Skilled tungsten inert gas (TIG) welders earn some of the industry’s highest wages due to the precision and skill this manual technique demands.

However, as manufacturing continues to be challenged by a shortage of skilled welders, and companies look to increase productivity without compromising quality, they more frequently have turned to automated solutions.

“Robotic” and “TIG welding” provide the analogy to the hare and the tortoise, and today’s technology combines the best features of the two processes, and has also contributed to a number of significant breakthroughs. Here’s a look at those, as well as the pertinent factors your company should examine when considering robotic TIG welding.

Benefits of GTAW

The primary benefit of the TIG process is the high quality welds it is capable of making in almost all metals and alloys. While carbon steel, stainless steel, and aluminum applications are commonplace, examples of some of the more exotic materials include titanium, zirconium, columbium, tantalum, and austenitic nickel-chromium-based superalloys.

These materials are found in a wide range of industries, including power generation, nuclear, motorsports, and various military applications. In the aerospace and aircraft maintenance industries, these material types are commonly found in advanced turbine engines, blades, vanes, and thrust reverser applications.

The common thread among these industries is that they frequently utilize thin gauge, high performance materials that exhibit some combination of superior mechanical properties, electrical properties, and thermal properties, all of which require consistency, exact penetration, repeatable control of many factors, including travel speeds, gas coverage, temperature control, and precise heat control to avoid shrinkage and distortion.

The TIG process produces a narrow heat affected zone (HAZ), which in turn, reduces solidification stress, cracking, and distortion in the finished weld. The traditional “stacked-dime” cosmetic appearance of a TIG weld conveys a sense of visual quality to the process.

Procedure qualification and certification

The Federal Aviation Administration (FAA), the American Welding Society (AWS D17.1), and the American Society of Mechanical Engineers (ASME) provide widely accepted standards for TIG procedure qualification, and are written specific to many materials.

TIG welding to a specific code requires a welding procedure specification (WPS), a formal document describing welding procedures to assure repeatability by properly trained welders. A procedure qualification record (PQR) is a record of welding variables used to produce an acceptable test weldment and the results of tests conducted on the weldment to qualify a welding procedure specification.

Once procedures are established for a welding process and joint design, they must be strictly followed in subsequent production welding. This requirement encourages the combination of TIG welding and automation for repeatability, traceability, and the ability to establish limits and restrict the adjustment of any variable to stay within qualified procedures.

Benefits and applications of robotic GTAW

Robotic TIG is widely used in production or manufacturing segments, in addition to the repair and overhaul segments of aerospace. Robotic TIG provides a number of quality control advantages, including automated, repeatable, uniform, consistent welds, with increased productivity — especially when considering the speed of torch repositioning between welds. Using a robotic arm provides repeatable access to welds that might be difficult to reach or require torch rotation that would be impossible by the human counterpart.

Key benefits of robotic TIG welding include:
• Repeatable, precise heat control and exact penetration to meet challenging quality standards
• On-the-fly procedure adjustment, for automatic switching of procedures between thick and thin applications
• Torch movement and automated control of the welding variables such as preflow, starting amperage, upslope time, welding amperage, pulse frequency, downslope, crater-fill, and postflow. Arc length can be automatically maintained with automatic voltage control (AVC) and bead width, penetration, and surface appearance can be tightly controlled.
• Improved welding productivity by a minimum of 100 percent and as much as 300 percent in some cases
• Reduced operator training time, reduced inspection costs, and improved weld quality
• Ability to save multiple welding schedules and several hundred welding programs for easy retrieval

Automated TIG welding is already used in a wide range of successful applications, including:
• Primary exhaust casing and ventilation duct
• Thrust reverser, cowlings, jet pipes and heat shield applications
• Cargo loading and landing gear applications
• Instrument diaphragms and other delicate expansion bellows
• Repairing turbine blades, crank shaft/torque tube weld buildup

High-strength/high-performance materials

While the FAA strictly regulates which materials can be used for specific components, weight savings drives the innovative materials that are sourced for these applications. This translates into better economics and justification for materials that range in price from hundreds to thousands of dollars per pound. Materials including stainless steel, titanium, 4130 Cr-MO, inconel, aluminum and special alloy steels are commonly used in these applications.

The robotic TIG process provides advantages for each of these materials. For example, aluminum is traditionally more difficult to weld because it tends to expand quickly and conducts heat well. Robotic TIG helps control heat input and ensures a strong, reliable weld.

Titanium has a wide continuous service temperature range, and the highest strength-to-weight ratio of any metal. However, titanium, has a high melting point and isn’t very resistant to corrosion during the welding process. Robotic TIG welding can provide precise repeatable procedures to reduce the risk of contamination.

Stainless has a high chromium content, which when TIG welded by hand, can easily become overheated. Robotic TIG welding can be introduced to qualify procedures to ensure that an undesirable dark color, negatively affecting the appearance, does not occur.
For heat-resistant alloys, such as nickel, used in aerospace and nuclear, it’s more difficult to achieve 100 percent penetration by hand. Robotic TIG welding ensures amperage to travel speed to control to a precise penetration profile.

Intelligent robotic TIG welding systems

The advancement of robotic TIG welding technology has spurred the development of sophisticated, yet cost-effective, vision systems that have substantially improved quality-control, assisting with joint location tracking and error-proofing.

During procedure qualification, the operator calibrates the camera and teaches the weld path on an ideal part. This reference image is stored in the robot’s memory. On each part thereafter, the camera takes a picture before an arc is established and the robot performs a pattern match between the reference image and the new image. The robot then calculates any offsets and adjusts the entire weld path accordingly. This technology advancement is particularly suitable on thin materials where arc placement is critical.

When it comes to repairing turbine blades, dynamic laser-based vision scanning and welding is required. Turbine blades are often subject to severe field conditions that cause the material to vary significantly from their original tolerances. The blades may be skewed or eroded, so a laser-based vision camera can be applied to adapt the weld schedule to a custom shape.

After welding, the same laser-based technology can be applied to provide objective, consistent measurements that are far superior to human visual inspection. This provides more information such as weld geometry and part geometry. It can also be used to detect defects such as surface porosity, undercut, entry angle, and toe radius which are critical for fatigue life assessment. This technology can be used in conjunction with ultrasonic testing (UT) devices used for internal defect sensing. The ability to print or archive these quality-related statistics serves as adequate documentation and validation.

TIG welding waveforms have been created to produce a pulsed output for faster travel speeds and others that include higher amperage peaks that result in a more forceful welding arc for anodized applications.

TIG welding thick to thin materials has not always been easy for an automated system. The introduction of Micro-Start technology allows for a low amperage starting (2 amps) on thin materials that automatically transitions to a high amperage for thicker materials. New digital communication technology on a robotic system can automatically adjust procedures based on the torch location as it weaves from thick materials (high amperage) to thin materials (low amperage) for automated, consistent penetration control.

Torch design has evolved dramatically. Smaller profile torches and improvement in the design of the gas diffusers and lenses which smooth out the shielding gas flow and allow for greater tungsten stickout can provide better access to tight joint configurations.

Production Monitoring software (web-based Production Monitoring 2 software now available for Lincoln Electric Power Wave welding power sources) can aid in weld data collection and is designed to allow fabricators to analyze and improve their welding operations and processes. It also aids in meeting ISO, Six Sigma, statistical process control (SPC), quality cost delivery (QCD), overall equipment effectiveness (OEE), and lean manufacturing efforts.

Conclusion

Numerous aviation repair and overhaul organizations, small and large, regularly use TIG welding processes in their daily activities — and this will not change. However, in a high-repetition production setting, or a small-batch maintenance setting, the best way to determine if your company can benefit from robotic TIG welding is to consult with a manufacturer. Manufacturers can review your prints or apply a robot to your own actual parts for a no-charge productivity improvement analysis. Application engineers analyze your current welding processes and procedures and then propose improvements to provide the best return on investment and increase productivity and quality control for your shop floor. AMT

This article was provided by The Lincoln Electric Company. The company will showcase its welding equipment at EAA AirVenture, Booths 49-51. More information is available at www.lincolnelectric.com.

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