Friction Stir Welding – Types, Advantages and Disadvantages

Friction stir welding is a solid-state welding method developed by The Welding Institute (TWI Ltd. UK) in 1991 for joining light metal alloys. It is one of the most important developments in metal bonding in recent years.

The friction mixing welding process is carried out as a result of the rotation and progress movement of the set consisting of a pin and shoulder with a special profile on the surface of the material to be joined. The pressure and rotational movement applied by the tool to the surface creates friction and heat on the surface. As a result, the material to be welded softens, and the mixture is provided in the welding area with a pin with a special profile. The advancement movement given to the tool performs the welding process along the specified line. An illustration for the friction stir welding application is given below;

Friction stir welding as a cold welding method is successfully applied in the joining of various metals and alloys that require high performance, especially where distortions and internal stresses are not desired. The friction stir welding method can be applied to sheets and plates of various thicknesses .

In the friction stir welding method, mechanical properties and microstructural changes vary with welding parameters. Since the method does not take place in the melt phase, the interdendritic and eutectic phases encountered in traditional fusion welding methods (TIG, MIG) are eliminated. In addition, SKK connection characteristics mostly depend on parameters that can be associated with weld set form and dynamics. When the weld cross-section is examined after the friction stir welding process, an asymmetrical weld zone is encountered. It has been observed that this asymmetrical structure increases the welding performance in combining different alloys by shifting the tool axis to a certain extent. The friction stir welding region is named as the resource center (KM), thermomechanically affected zone (TMEB) and heat affected zone (ITAB) in the literature.

 Thermomechanically Affected Zone – (TMEB): The thermomechanically affected zone is the zone that is subjected to high temperature and deformation due to the friction and advancement of the mixer tip between the dynamically recrystallized zone and the zone under heat.

Zone Under Heat Effect – (ITAB): It is the region close to the weld metal and adjacent to the base metal, which is not affected by heat. Although there is no mechanical deformation in this area, it is affected by the heat generated during friction and changes occur in the structural properties of the material.

Advantages of Friction Stir Welding Method

The friction stir welding method is a green technology in terms of energy efficiency and because it is an environmentally friendly welding method. Compared to traditional welding methods, the friction stir welding method consumes less energy, does not require protective gas and does not generate harmful emissions. In the friction stir welding method, the material does not reach the melting point. For this reason, defects such as hollow structure and oxidation are not seen in joining aluminum alloys with conventional welding processes. Friction stir weldingCompared to other mechanical joining methods of aluminum alloys, the method provides an advantage with its structural rigidity and lightness. In addition to these, connections with high fatigue life are obtained with this method.

Welding of dissimilar metals such as Steel / Al alloy, Steel / Mg alloy, Copper / Al alloy, Steel / Ti, and Al / Mg by the friction stir welding method is also possible.

Metallurgical Benefits

  • It is a solid state method,
  • Low distortion
  • Good dimensional stability,
  • No loss of alloying element,
  • Good metallurgical properties in the weld zone,
  • Fine grain structure,
  • Crack-free welding zone,
  • Use instead of multiple fasteners

Environmental Benefits

  • Not using cover gas
  • No need for surface cleaning
  • Low amount of waste material
  • No need for cleaning
  • No need for consumable materials required in the traditional welding process
  • No grinding waste

Energy Benefits

  • Applicable to materials of different thicknesses and eliminating the need for additional processing
  • Operation with 2.5% energy of the energy used in laser welding process
  • Contributing to fuel savings in the sectors where it is applied (compared to different combining methods)

Limitations and Disadvantages of Friction Stir Welding Method

As with every welding method, there are limitations in the friction stir welding method. In summary, friction stir source limitations;

  • Thick materials are difficult to weld due to the inhomogeneous heat distribution between the weld upper surface and the lower surface.
  • The heavy-duty fixture is required to hold the workpiece or work during the welding process,
  • Great forces are required to make the pin penetrate the metal,
  • Often this welding process is slower than many other processes,
  • Friction stir welding is not suitable for weld joints where metal deposition is required,
  • The initial investment cost of the friction stir welding machine is very high,
  • Less flexible than manual and arc welding processes,

Materials Where Friction Stir Welding Method Is Applied

Initially, the friction stir welding process was used to join a number of aluminum alloys, lead, zinc, magnesium, and relatively soft materials. Recently, copper, titanium, low carbon ferritic steel, alloy steels, stainless steels and nickel alloys as well as some thermoplastic materials can also be welded by friction stir welding method. In principle, any material that can be hot worked can be welded by this process. However, tool material is a more limiting factor than the material being welded.

Industrial Applications of Friction Stir Welding Method

  • Ship and Marine Industry
  • Aviation and Space Industry
  • Land Transport and Railway Industry

Friction Stir Welding Types

In parallel with the increase in the usage areas of the friction stir welding method, the increasing needs and the situations that cause application difficulties have led to the development of new applications in the CCT method. The methods that can be classified differently from the classical CCT method can be examined under three main headings;

  1. Hybrid friction stir source (MSKK)
  2. Double-sided friction stir welding (ÇSKK)
  3. Friction stir spot welding (SKNK)

1.Hybrid Friction Stir Welding (MSKK)

The heat generated by the friction of the mixing tool shoulder in the combination of materials with high melting points such as steel by the CCC method may be insufficient for plastic deformation and defect-free assembly of the materials in the mixing zone and may cause abrasion of the mixer tip. Hybrid friction stir welding methods can be applied to prevent these . In order to facilitate the function of the mixer assembly, the materials to be welded can be preheated with a laser beam.

At the beginning of the CCC process, the mixing team waits for a while for the necessary heat generation after the plunging process. It is possible to minimize this waiting time with the hybrid friction stir welding method. The reduced waiting time increases the tool life as it will also reduce the friction time. However, in addition to the high cost of hybrid CCT application, it is still in the research phase.

2.Double-Sided Friction Stir Welding (ÇSKK)

Due to the increased plate thickness, two mixing sets that perform the mixing process separately from both the bottom and the top of the plate at the same time are added to the procedure. During the process, while the mixer ends rotate in opposite directions with each other, they move in the same direction. The friction stir welding method, which is applied unilaterally up to 50 mm thickness in aluminum alloys, can be combined up to 100 mm when applied on both sides.

3.Friction Stir Spot Welding (SKNK)

In the friction mixing point welding method, the difference from the CCT method is that instead of performing a linear movement to perform the mixer assembly process, the plates positioned on top of each other are dipped from one point and held for a certain time and come back from the same point. From the friction stir spot welding method, the position of the plates, immersion depth, dwell time, and tool angle is important variables that determine the quality of the joint. At the end of the waiting period, a hole is formed in the weld cross section when the mixer set is removed. This negativity is the Refill Friction Stir Spot Welding (refill FSSW) developed by the HZG research center in Germany .has been eliminated. In addition, with the SKNK method developed by Hitachi company, after the plunging process is completed, the joining process is completed by performing the very little linear movement. Therefore, the SKNK surface takes an elliptical shape instead of a circular shape, and the strength of the joint increases with the increasing weld seam.

Friction Stir Welding Defects

We can categorize the errors that occur in my SKK process into 5 categories. These; voids, insufficient penetration, joint line residues, oxide residues, and excessive overflow. These errors should be taken into account when designing the tool according to the current welding conditions. It should also be taken into account in determining welding parameters.

1. Gaps

The gaps can be located in different places of the welding core and just below the weld surface, usually on the advancement side, depending on the process variables. The main factor in the occurrence of this error is that the material undergoing plastic deformation does not fill the gap created by the advancing team.

The main reasons for the formation of gap defects are the fact that the plates to be joined are not properly bonded, the welding speed is selected high, the pressure is not applied during the welding. In addition, generating the source heat more or less than necessary causes space error to occur. Because if the weld zone is cold, the material flux becomes difficult from one plate to be welded to another. If the temperature is higher than necessary, the material becomes too soft to be dragged by the tool. Therefore, gaps occur in the weld area.

2. Insufficient Penetration

As a result of insufficient mixing of the material in the root part of the weld seam, the desired diffusion does not occur, causing this error to occur. Pin length and axial force are also important here.

In addition, the thickness of the plates to be joined by welding is not constant throughout the weld length, the plates are not connected to the equipment where the welding process will be carried out, and the tool is not selected by the welding conditions may cause similar results. Incomplete root penetration is often seen in one-sided friction stir weld. As long as the depth of tool immersion can be controlled during welding, this problem will also be eliminated.

3 Joining Line Remains

It is the type of fault that starts from the weld root and extends along the weld where the separation line between the butt plates to be welded is located. It consists of the insufficient distribution of oxide residues on the surface of the joined plates. In the occurrence of this error, the increase in the welding speed and the insufficient dispersion of the oxide layer per unit time has an important effect.

The shortness of the tool pin and the insufficient immersion depth cause the formation of a kissing bond error in the root part, which is straightened towards the center of the mixing and is a continuation of the joining line residues.

4.Oxide Residues

Oxide material from the surfaces of the plates to be joined CCT welding seam is not cleaned from the surface before treatment and TMEB to penetrate the remaining resources discontinuous oxide layer located along the joining line will cause coupling of poor quality.

It occurs in the blending zone in the form of curved lines with black gaps and is called the lazy S error.

5. Overflowing Metal Surplus

In the friction mixing welding process, the weld metal comes out of the tool shoulder as a result of the immersion depth of the mixing tool or the axial force applied to it, and it is generally seen as burr formation. The protrusion of the metal in the mixing zone causes the thickness of this area to decrease.

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