Turbine Technology: Aircraft Welding and Repair
Aircraft are designed to meet certain standards in terms of flight hours and service life. Each aircraft contains millions of parts and miles of wire and tubing, all housed in a high-strength aluminum frame that undergoes the daily stresses of flight. Maintenance — both routine and unscheduled — is critical throughout the service life of an aircraft, and welding plays a major role in the process.
Welding keeps aircraft in service as a cost-effective method for increasing the service life of many aircraft components. Replacement parts can be extremely expensive and not readily available, with some having lead times of more than a year. Welding helps control the cost of aircraft maintenance and avoid long periods of downtime. The gas tungsten arc welding (GTAW, or TIG) process was developed specifically for aircraft welding.
In the 1970s, welding manufacturers pioneered squarewave TIG welding technology and incorporated it into the transformer-based power sources of the time: large, bulky welding machines that weighed hundreds of pounds and became a permanent fixture in the shop. As welding technologies become more advanced, and the alloys being built into planes become more varied, it may finally be time to investigate new welding power sources that substantially improve the quality, productivity and efficiency of your work. This article gives an overview of modern TIG welding technology and how it advances aircraft maintenance and repair.
Why TIG welding?
Materials used in aircraft design are anything but ordinary, ranging from stainless steel and aluminum to nickel, magnesium, titanium, cobalt and niobium. Critical tolerances, dimensional requirements, and metallurgical considerations all combine to create extreme welding challenges and potential mistakes that may result in defects such as cracking, distortion, and irreversible changes in microstructure that mandate the scrapping of parts.
TIG welding is the perfect process for such an application. Pinpoint control of the arc allows for accurate and precise weld placement and superior control over heat input. High current density provides a concentrated arc while an inert atmospheric gas protects the molten weld puddle from oxidation, porosity and harmful inclusions.
Welding is used in all types of aircraft repair applications, from air-handling ducts to engine parts and components. Typical TIG welding applications in aircraft repair include dimensional restoration (build-up), crack repair, patch welding, and component replacement.
Advances in technology
New TIG welding technologies offer a number of efficiencies: reduced heat input, a narrowed weld bead and heat affected zone, improved directional control, and output ranges and capabilities extended beyond traditional transformer products. Three specific elements achieve these efficiencies: arc starts, pulsed DC output, and advanced AC waveshape controls.
Arc starts
Arc starting is the critical first step of every TIG weld. Conventional machines often generate a burst of current during starts that helps initiate the arc but could severely damage the part, especially in very thin applications such as blade and vane repair. Inverters have brought the ability to regulate the output down to a microsecond for the exact amount of starting amperage to light the arc but for such a short duration that any effect to the base material is eliminated. This reduces the need for copper start blocks and allows for direct starting on thin sections while protecting the base metal from burn-through and distortion.
Pulsed TIG welding
DC electrode negative puts most of the welding heat into the puddle, which provides good penetration and helps to keep the tungsten sharp. It’s the preferred method for welding such metals as steel, stainless steel, nickel alloys, and titanium. The cone-shaped arc forms off the end of the tungsten. As amperage is increased, so is the diameter of the welding cone. This is somewhat dependent on the diameter of the tungsten and electrode preparation, as the arc is emitted from the electrode at 90 degrees to the grind angle.
High speed inverter pulsing focuses and constricts the arc for faster travel speeds at reduced amperage levels, which narrows the heat affected zone and lowers heat input, helping to reduce distortion. This results from rapidly switching the current between the peak and background amperage – at frequencies ranging from 100 to 5,000 hz (compared to only 1-20 hz with older, conventional technology). Pulsing at such a high rate doesn’t give the arc enough time to fully expand to its maximum width before the current is switched to the background setting.
The results of high frequency pulsing are profound for the heat sensitive alloys and stringent weld requirements in aircraft repair. Reduced amperages control heat input for a better as-welded microstructure with reduced distortion. Added benefits include penetration control, preventing burn-through and reduced cracking. The focused arc allows the operator to more accurately place the weld, control bead width and heat affected zone, reducing the chance for the most common causes of weld rejection.
Alternating current and advanced waveforms
AC is commonly used for welding aluminum and magnesium components. As with DC, the width of the arc is determined by welding amperage, tungsten diameter and electrode preparation. Aluminum and magnesium are challenging to weld. While AC provides some key benefits for these materials, it’s not without issues.
As we look at aluminum, it’s important to note that formation of a high-melting surface oxide is a trait that separates this material from other alloys. All metals form oxides, but in most cases they melt at a lower temperature than the base material. This is not true for aluminum, as the invisible surface oxides melt at approximately three times the temperature of the base metal. Therefore, the oxides must be removed for welding.
AC balance control describes the ability to adjust the ratio of time spent in electrode negative (EN) compared to electrode positive (EP) in the cycle. Inverters allow much higher EN settings (up to 99 percent) compared to conventional maximums of approximately 70 percent. The balance can be adjusted to provide adequate arc cleaning without the severe balling action produced by excessive EP time in the cycle. This reduction in balling action reduces arc wander, pulls in the etching zone and directs more heat into the work for faster welding speeds. In addition, sharpened alloy tungsten electrodes can be used for welding aluminum and magnesium to provide better arc starts and control.
The effects of AC balance are illustrated by the extremes of EN percentage in the AC wave. Very little EN results in greater cleaning action, shallow penetration (a benefit on thinner materials) and severe electrode balling because most of the heat is directed up, onto the tungsten. At the other end of the spectrum, a high amount of EN places most of the heat into the work. Cleaning is minimized along with the balling action of the electrode. This extended balance range allows the arc to be fine-tuned according to base metal conditions in each application. It achieves greater penetration, faster travel speeds, narrows the weld bead, extends tungsten life, produces a smaller etched zone and permits the use of a smaller diameter tungsten to more precisely direct the heat into the joint.
AC frequency control is a newer adjustment made possible by inverter technology. Older TIG technology is typically locked in at the line frequency: 60 hz in North America and 50 hz in other locations around the globe. New inverter technology gives operators the ability to dial that in between 20 hz and 400 hz. Current alternating at 60 hz produces a relatively wide, lazy arc that lacks directional control. This can be seen in a typical t-joint application where the arc wanders between the toes of the weld and lacks the focus to drive into the corner.
The ability to increase the AC frequency has a dramatic effect on the arc characteristics as well as the weld bead. Higher frequencies of alternation limit the time that the arc expands on each half-cycle. This creates a narrow, focused arc that has significantly better directional control, helping the arc to reach the throat area of a weld and ensuring adequate penetration. Arc wander is virtually eliminated, which saves important features in aircraft components that may otherwise be affected. The improved directional control of higher AC frequencies is also very helpful when welding sections of dissimilar thickness.
The synthetic AC output of the inverter allows the operator to further control the bead profile and effects of the arc by setting the EN and EP amperage independently. This independent AC amperage control enhances the effect of AC balance control and can be used to fine tune arc etching and reduce the tungsten size required for some applications. While the base metal may require a certain amount of time in EP for adequate cleaning, it may not require the full amperage of the output settings.
AC waveshaping options found in modern inverters further optimize arc characteristics. Depending on the waveform selected — advanced squarewave, soft squarewave, sine wave and triangular wave — users can tailor the arc for such characteristics as a hard, driving arc, a smooth, soft arc with good puddle control and wetting action, or a punch of peak amperage with minimized overall heat input to reduce distortion.
Summary
New inverter technology has elevated its capabilities to give aircraft welders more control over the entire welding process. Precise arc starts and refined arc controls contribute to weld quality and help to reduce weld costs and eliminate scrap. In addition, inverter power source designs are smaller and lighter weight – so they take up less space in the work cell and are easier to move around. They also account for a much more efficient power draw than older systems. As quality requirements and standards become more stringent in the interest of safety and reliability, TIG inverters present a compelling case for those who perform aircraft welding and repair.
Brent Williams is a product manager with Miller Electric Mfg. Co. For more information visit www.MillerWelds.com.
