Text 1. Friction Stir Welding (FSW)

H.H. Bhadeshia

Friction stir welding, a process invented at TWI, Cambridge in 1991, involves the joining of metals without fusion or filler materials. It is used already in routine, as well as critical applications, for the joining of structural components made of aluminium and its alloys. Indeed, it has been convincingly demonstrated that the process results in strong and ductile joints, sometimes in systems which have proved difficult using conventional welding techniques. The process is most suitable for components which are flat and long (plates and sheets) but can be adapted for pipes, hollow sections and positional welding. The welds are created by the combined action of frictional heating and mechanical deformation due to a rotating tool. The maximum temperature reached is of the order of 0.8 of the melting temperature.

 

The tool has a circular section except at the end where there is a threaded probe or more complicated flute; the junction between the cylindrical portion and the probe is known as the shoulder. The probe penetrates the work piece whereas the shoulder rubs with the top surface. The heat is generated primarily by friction between a rotating-translating tool, the shoulder of which rubs against the work piece. There is a volumetric contribution to heat generation from the adiabatic heating due to deformation near the pin. The welding parameters have to be adjusted so that the ratio of frictional to volumetric deformation--induced heating decreases as the work piece becomes thicker. This is in order to ensure a sufficient heat input per unit length. 

The microstructure of a friction-stir weld depends in detail on the tool design, the rotation and translation speeds, the applied pressure and the characteristics of the material being joined. There are a number of zones. The heat-affected zone (HAZ) is as in conventional welds. The central nugget region containing the onion-ring flow-pattern is the most severely deformed region, although it frequently seems to dynamically recrystallise, so that the detailed microstructure may consist of equiaxed grains. The layered (onionring) structure is a consequence of the way in which a threaded tool deposits material from the front to the back of the weld. It seems that cylindrical sheets of material are extruded during each rotation of the tool, which on a weld cross-section give the characteristic onion-rings.

The thermomechanically-affected zone lies between the HAZ and nugget; the grains of the original microstructure are retained in this region, but in a deformed state. The top surface of the weld has a different microstructure, a consequence of the shearing induced by the rotating toolshoulder.

The Machine

This is a picture of a friction stir welding (FSW shows a typical) machine. This one is at the Joining and Welding Research Institute (JWRI) of Osaka University, Japan. 

  The Tool

An illustration of some types of tools. Each tool has a shoulder whose rotation against the substrate generates most of the heat required for welding. The pin on the tool is plunged into the ^{Friction-stir welding} machine substrate and helps stir the metal in the solid state.

 

     

 

The Fixture and Weld

The two halves to be joined must be rigidly fixed before the welding operation (first picture below). The pin, which is an integral part of the tool, is plunged into the metal to help stir it up; the shoulder of the tool generates much of the heat. As the weld is completed, the tool is withdrawn leaving behind a hole. The weld is designed so that such regions can be discarded from the component. The presence of a hole may not be appropriate when welding pipes or storage vessels. The hole can be avoided by designing the tool such that only the pin can be retracted automatically and gently into the shoulder, leaving behind an integral weld.

         

FSW of Steel

Steel can be friction stir welded but the essential problem is that tool materials wear rapidly. Indeed, the wear debris from the tool can frequently be found inside the weld. The process would therefore be used in special circumstances where other welding methods are inadequate. These circumstances have yet to be clarified. There are so many good methods by which steel can be welded. The example below is the FSW of 316L stainless steel. Notice that the sample becomes red-hot during welding.

 

Since the tool gets red hot, it is necessary to protect it against the environment using a shielding gas. A possible use of FSW in the welding of steels is in the context of stainless steels. Austenitic stainless steels can easily be welded using conventional arc welding and other processes. However, FSW can offer lower distortion, lower shrinkage and porosity. More important is the avoidance of fumes containing hexavalent chromium which is carcinogenic. In addition, chemical segregation effects associated with welding processes involving solidification are avoided. Such segregation can lead to a degradation of corrosion resistance since electrochemical cells are set up between solute -rich and poor domains.

Friction Stir Welding of Cast Aluminium Alloy

The most popular aluminium casting-alloy contains about 8 wt% of silicon. It therefore solidifies to primary aluminium-rich dendrites and a eutectic mixture of aluminium solid-solution and almost pure silicon. The latter occurs as coarse silicon particles which tend to be brittle. The cast alloy usually has some porosity. Friction stir welding has the advantage that it breaks up the coarse silicon particles and heals any pores by the mechanical processing, as illustrated below. 

 

  A section through a friction stir weld made in an Al-Si casting alloy. There are pores indicated in the base metal (BM). HAZ represents the heat affected zone, TMAZ the thermomechanically affected zone, and SN the stir nugget. The photographs in this section have kindly been provided by Professor H. Fujii of JWRI, Japan.

 

 

 

Optical micrographs of regions (a), (b) and (c) of the stir nugget. The location

of these regions is identified in macroscopic section presented above. Optical micrographs showing the microstructure in (a) the base metal; (b) heat-affected zone; (c) the thermo mechanically affected zone, where considerable refinement of the silicon has occurred.

Tensile strength Proof         stress Elongation (%) Fracture local

(MPa)                (MPa)

Joint 150         85                    1.6                   BM

Weld 179         87                    5.3                   TMAZ

 SN 251        96                    14.4                 SN

The refinement of silicon and elimination of porosity leads to better mechanical properties in the weld than in the base plates.

 

True or false?

 

1. FSW is not used to weld steels.

2. It’s impossible to avoid holes in welds made by FSW method.

3. The maximum temperature reached during FSW is above the melting temperature.

4. The characteristics of the material being joined affect the microstructure of a friction-stir weld.

5. FSW is the only method for welding austenitic stainless steels.

6. Pipes can be welded using FSW.

 

Translate the following sentences into English:

 

1. При сварке трением присадочный металл не используется.

2. Получаемые в результате сварки трением швы отличаются прочностью и пластичностью.

3. Шов при сварке трением образуется в результате нагрева при трении инструмента о свариваемую деталь и возникновения механической деформации.

4. Микроструктура шва, полученного при помощи сварки трением, определяется целым рядом параметров.

5. Поверхность шва внешне похожа на луковые кольца.

6. Наличие или отсутствие поры в зоне шва определяется конструкцией инструментальной оснастки и скоростью отвода инструмента.

7. Пары шестивалентного хрома являются канцерогенными.

8. Использование сварки трением позволяет снизить усадочную деформацию и пористость.

9. Существенным недостатком данного метода является быстрый износ инструмента.

10. При сваривании стальных конструкций сталь раскаляется докрасна.

 

Make a short description of the method using the following key words:

 

Fusion, filler material, aluminium and its alloys, plates and sheets, frictional heating and mechanical deformation, rotating tool, tool, shoulder, pin,   work piece, heat generation, microstructure of a friction-stir weld, applied pressure, characteristics of the material, withdrawal of the tool, onion-ring structure, HAZ, integral weld.

 

Vocabulary
hermal (heat) ageing	термостарение
bending test	испытание на изгиб
tensile (tension) test	испытание на растяжение
HAZ	heat affected zone зона термического влияния
butt joint	стыковое соединение; соединение встык
Abutment	торец; упор; опора,
Backing	опора

 

Reading

 

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