Orbital welding technology advances described

The orbital welding technique has been advanced considerably over the years such that today's equipment can do in one pass that often required multi-pass techniques before.

The orbital welding technique was initially developed for the welding of aerospace tubes in the late 1960's by a group of engineers from McDonald Douglas, US who were well aware of the problems of producing repeatable weld quality for their critical applications.

The first machines developed were large upright systems which could only be used for workshop applications and were certainly not portable systems.

These systems also needed to make several rotations to complete a joint as they only had available single level power supplies and to get the consistency of weld they would have to make a pre-heating, penetrating and profiling pass.

This multi-pass was acceptable at the time as it produced good repeatable results.

But with today's technology a single pass weld can be welded with much improved results.

Technique - orbital welding is a technique which allows complicated or difficult tube shapes.

The orbital technique is often confused with the rotary technique, which is only suitable for straight and equally balanced components.

With the orbital welding system the weld head and the component remain stationary, only the electrode carrier rotates.

The weld head has an internal gear which holds the tungsten electrode which rotates around the component.

Equipment - an orbital welding system comprises of two main components, an orbital welding power supply and an orbital weld head.

These two components will work together as one system to give a high quality, repeatable weld profile with a controlled minimal heat input to control distortion and dimensional accuracy.

Power supply - orbital welding systems are controlled by a computerised welding power supply.

The power supply controls and monitors welding current, rotation speed and wire feed functions.

The system will also monitor arc voltage to detect any changes in the arcs behaviour.

These power supplies will hold around 2000 welding procedures which include details of the welding parameters and the applications specification.

These power supplies also incorporate a comprehensive weld data logging facility which allows the operator to record details of each weld carried out on the systems.

Each data log file will include details of the set welding current and travel speed and indicates maximum deviation in each level to two decimal places.

The log file will also indicate average arc voltage for each level.

Most power supplies have straight DC outputs, but the latest developments have seen the introduction of an AC/DC output which allows nearly all alloys to be welded on one system.

The AC output is particularly useful for the welding of aluminium and it1s alloys.

With the AC output systems AC balance and frequency can be controlled as well as the normal current and travel speed controls.

Weld heads - the most commonly used weld heads in the aerospace industry are the fully enclosed type.

These heads clamp around the component and create a full enclosure around the weld zone which is filled with inert gas to protect the weld pool.

Enclosed weld heads use a rotating horseshoe shaped rotor which holds the tungsten electrode.

This rotor rotates inside the weld head and the work piece and weld head remain stationery.

The whole weld cycle can be controlled from the control panel mounted on the weld head handle.

When the weld cycle is complete the rotor returns to its 'Home' position and the component can be removed.

Different size components can be accommodated by using a different collet size for each diameter to be welded.

Applications - aero engine - orbital welding is used extensively during the manufacture and repair of aero engines.

Aero engines have a complex network of tubes for a wide range of applications.

Some of the tubes can be welded externally and others need to be welded on the inside bore of the tube.

For these internal bore welding application enclosed weld heads can still be used with some special fixturing and tooling.

Ducting - ducting components can also be welded using enclosed weld heads and a wide range of different configurations can be welded.

Tube to tube and tube to fitting joints can be carried out.

Orbital welding equipment has also be used with great success in the manufacture of gimbals, bellows and flexible tubes.

Ejector seats - ejector seat gas control systems are also an ideal application.

Micro-Weld heads are very useful for some of the compact tube designs used on these seats.

Tubes for the propulsion and balance systems can be welded using orbital welding equipment.

Fuel distribution - pipes for the distribution of fuel and other fluids can also be welded using enclosed weld heads which clamp on one side.

This allows a full range of fittings to be welded.

Similar pipes may also be used for other services such as hydraulic lines and lubricants.

Instrumentation - electronic and instrumentation components are also frequently welded using orbital welding equipment.

A wide range of shape, sizes and materials are welded.

As these types of instruments often have cables attached the orbital method is preferred so the cables are not twisted during the welding cycle.

It is important to keep a close control of the heat input on instrumentation as they usually contain sensitive components which will be destroyed by excessive heat input.

Structural components - Weight is a prime issue in aerospace components and are usually made from hollow sections and lightweight material.

Structural components are usually manufactured to tight tolerances and again heat mush be controlled accurately so as to eliminate excessive distortion or shrinkage.

Joint design - there are two common joint designs.

The joint design is normally chosen for a specific reason.

A lot of applications in the aerospace industry use the integral filler design which produces a good weld profile and also assist in alignment of the weld joint.

Square edge butt weld - square butt preparation can be used for fusion welding applications.

It is commonly accepted that materials up to 4mm in thickness can be used in a single pass with a square butt preparation.

This 4mm limit is subject to the exact composition of the material being welded, but good results can be achieved.

When performing welds with a square butt preparation it is essential to have the tube ends machined to give a good fit-up.

The resulting weld will be flush on the inside and out.

Raised profiles can be achieve by running multiple pass procedures.

Integral filler butt weld - integral filler preparations are used in applications were it is considered that a filler material may advantageous but the use of an open arc weld head is practical.

The integral filler joint will leave a more conventional weld profile with a small reinforcement on the internal and external surface of the tube.

Weld defects - there are range of weld defects which may be encountered when carrying our orbital welding work and these are detailed below and their possible causes highlighted.

Good profile - the result achieved when all conditions are correct.

Lack of penetration - Caused by insufficient amperage being used.

Mis-match - caused by bad fit up of tube or diameter difference in materials.

Mis-alignment - caused by missing the joint, mixed batches or arc wander.

Sunken profile - caused by excessive amperage or gap in fit up.

Raised profile - caused by positive ID pressure or multiple pass weld.

Porosity - caused by contaminants in the weld pool.

Coked - caused by excessive oxygen in the backing gas.

Dog legging - caused by dirty tacks, bad tungsten or turbulence in weld head.

Arc wander - caused by mixed batches (different sulphur levels).

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