Friction welding has been used in the manufacture of rotating bodies since 1956 and has become well-known as an extremely rapid and reliable method. Unfortunately, its use has been limited to rotationally symmetrical bodies, as friction welding is based the principle of rotating two workpieces at high speed in relation to one another and then pressing them against once another with great force, thereby rapidly obtaining forging temperatures. As this method makes high productivity possible, the opportunity to use friction welding for other types of object has been eagerly awaited for many years.

Aluminium is normally welded using MIG, TIG, plasma or manual welding with coated electrodes. A new welding method, Friction Stir Welding, has been developed for welding aluminium. This method has been patented and developed by TWI. The necessary expertise has been acquired in a sponsorship project which has been run by TWI in collaboration with possible users and machine-builders.

In principle, the method is based on obtaining a sufficiently high temperature to forge two aluminium components, using a rotating tool which moves along the joint. The temperature is under the melting point of aluminium.

The aluminium components which are going to be joined are clamped to a TIG using a powerful fixture. A tool with a cylindrical shoulder and with a special profile is positioned above the centre line of the joint. The tool is rotated at at a peripheral speed and is pushed into the material with great force. The material comes plastic as a result of the frictional heat and is forced to rise around the rotating probe until the shoulder of the tool comes into direct contact with the surface of the aluminium. When the tool is moved along the joint, the material is plasticised by the frictional heat of the front of the rotating probe and moves behind the probe, thereby forming a weld. This process continues as long as the tool is moved along the joint.

The Friction Stir Welding of alumimium 60 82 — T6 or SS 4212-06 has been shown to produce high strength values. Typical values for the ultimate strength across joint are in the range of 211-230 N/mm2, without ageing. Higher strength can be obtained by thermal ageing. The specification for the base material is a minimum of 290 N/mm2. Bending tests and tensile tests have been conducted without any negative observations. Fractures took place in the base material HAZ.

Thick material. When welding materials оf more than 15 mm, the welding is done from both sides. The maximum thickness is around 30 mm at 100% penetration. The welding speed is approximately 180 mm/min.

Welding speed. The welding speed depends on the dimensions and the alloys which are used. The spindle speed also affects the welding speed. For example, the welding speed in material 5 mm thick can be 610 mm/min with a 60 82-T6 alloy.

Joint types. Butt joints and lap joints can be welded. Corner joints and other special joints can also be welded, but the method, accessibility, complete penetration requirements and so on must be taken into account when they are planned.

Welding data. In Friction Stir Welding, the welding parameters are dependent on the aluminium alloy, the dimensions, the speed of the spindle and the traveling speed. In addition, the design of the tool has an important effect on the welding result. Work on all these parameters is in progress with the aim of building up data bank for the progress in order to facilitate the use of Friction Stir Welding.

Working Environment. The working environment is improved dramatically when Friction Welding is used. Welding flash, fumes and ozone formation are totally eliminated. Steel brushing and interpass cleaning are also unnecessary.

Advantages The advantages of the method are:

High, consistent quality.

Material thickness from 1.6-15 mm can be welded as single-pass procedures. Material from 15-30 mm can be welded on both sides.

No joint preparation, only degreasing.

No grinding or brushing.

No consumables.

No shielding gas.

Flat surface without weld reinforcement or splatter.

No magnetic blowing as the welding is done without an arc. Environmentally-compatible method without flash, fumes or ozone formation. Less risk of pores and cracking as the temperature never reaches the melting point of aluminium.

No burning off of alloy substance as the temperature never reaches melting point.

Alloys which are difficult to weld can be welded as there is only a small risk of hot cracking. High productivity.

Disadvantages

The method can only be used on straight, flat workpieces or hollow profiles with an abutment or backing. The method requires a stable welding machine with good reproducibility. Powerful fixture which is able to hold the components in the correct position throughout the welding operation. The back of the object must be accessible if 100% penetration is necessary. As the method requires a stable welding machine with a powerful fixture, the welding equipment should preferably be stationary. This welding method leaves an end hole when the tool is pulled away from workpiece. In many cases, this hole can be cut off, but, in other cases, it has to be sealed using another method.

Applications

Suitable applications include extruded aluminium profiles for: shipbuilding; offshore platforms; railway wagons, trams and underground train carriages; the automobile, brewing, defense industries; production of electric motors, cooling elements.

Summary

Friction Stir Welding is a new welding method which enables two workpieces made of aluminium to be forged using friction. The method has characteristics which make it extremely interesting for the highly-productive joining of straight profiles and plates. In the longer term, this method will help to extend the use of aluminium.

After reading activity

Use the words and phrases from the list below to fill in the blanks in the sentences:

rotating bodies, manual welding, sponsorship project, necessary expertise, in collaboration with, frictional heat, without any negative observations, build up data bank, sufficiently high temperature, environmentally-compatible

1. … is being replaced by automatic and semi-automatic types of welding in many applications.

2. They could buy the new equipment only thanks to a successfully realized

3. … are necessary elements of friction stir welding.

4. The new project was realized… foreign partners.

5. In order for the weld to be formed… in the welding area should be provided.

6. After the thorough repairs the equipment had been running for 2 years… .

7. The quality control supervisors had to carry out… before putting the welding machine into operation.

8. The experience the engineers had in welding such structures helped them… necessary for further development of the product.

9. … resulting from rotating bodies coming into direct contact can be removed by special coolers.

10. This new welding machine is both operator friendly and… .

Speaking

Say if the following is an advantage or disadvantage offriction stir welding:

1. A hole is left on the surface after the tool is pulled away from work piece.

2. The welding machine should be powerful and stable.

3. The joint doesn’t need special preparation.

4. Flash, fumes and ozone are not formed.

5. Consumables and shielding gas are not required.

6. Abutment or backing is necessary.

7. Arc is not used.

Answer the following questions:

1. What is of Friction Stir Welding method based on?

2. How is the weld formed?

3. What do the welding parameters depend on?

4. What, in your opinion, are the most important advantages/disadvantages of the Friction Stir Welding method?

5. Want is the best sphere of application of this method at present?

Writing

Write a short report about FSW according to the following plan:

1. The principle of FSW.

2. Working environment.

3. Advantages and disadvantages.

4. Applications.

Lead-in

Think of the answers for the following questions:

1. What is an automated welding system?

2. What might be the advantages of automated welding systems compared with manual welding?

3. What part do you think can never be welded using automated welding?

Vocabulary

проход

интерфейс человек-машина

автоматизированная система последовательность сварки; порядок наложения швов

испытания без разрушения образца, неразрушающий контроль

температура начала мартенситных

run

man-machine communication automated system welding sequence

non-destructive testing

interpass temperature

превращений

weld bead log file processing

наплавленный валик сварного шва

системный журнал

обработка

Reading