The Principle and Characteristics of Friction Stir Welding Explained

Principle of Friction Stir Welding

Friction stir welding is a type of solid-state welding that uses frictional heat as the welding heat source, but it differs from conventional friction welding. During friction stir welding, the workpiece is first firmly secured on the work platform.

Then, the stir welding head rotates at high speed and the stir welding pin is inserted into the seam of the workpiece until the shoulder of the stir welding head is in tight contact with the surface of the workpiece.

The heat generated by the high-speed rotation of the stir welding pin and its friction with the surrounding parent material, together with the heat generated by the friction between the shoulder of the stir welding head and the surface of the workpiece, raises the temperature and softens the material at the seam.

Simultaneously, as the stir welding head rotates and moves relative to the seam and the workpiece, the material in front of the stir welding head undergoes intense plastic deformation. As the stir welding head moves forward, the material at the leading edge with high plastic deformation is squeezed to the back of the stir welding head.

Under the combined effect of frictional heating and forging between the stir welding head and the surface of the workpiece, a dense and solid solid-phase welded joint is formed. The welding process of friction stir welding is as shown in Figure 4-19.

Figure 4-19 Schematic of the friction stir welding process
  • 1 – Stirring head shoulder
  • 2 – Workpiece
  • 3 – Stir welding pin

Friction Stir Welding Joints

During friction stir welding, the structure and property changes of the welding joint depend on the degree of plastic deformation and the amount of heat input. According to the degree of plastic deformation and thermal effects, the friction stir welding joint is divided into 4 regions, as shown in Figure 4-20.

In the figure, zone d is the parent material area in the joint where there is no thermal effect and no plastic deformation. Zone c is the heat-affected zone (HAZ), where the material has undergone changes in microstructure and mechanical properties due to the effect of the welding thermal cycle, but there is no plastic deformation in this area.

Its structure is not significantly different from that of the parent material, except that the highly directional columnar crystal structure has been eliminated, and the width of the heat-affected zone is much narrower than that of fusion welding.

Between the weld and the heat-affected zone, there is also a thermo-mechanical affected zone, or thermal deformation affected zone (TMAZ), which is zone b in the figure. This area is a transition zone where the material has undergone a certain degree of plastic deformation and is also affected by the welding temperature field.

Zone a is the weld nugget zone (WNZ), which is located near the position where the stir pin is inserted in the center of the weld. After high temperatures and strain, the center of the weld nugget has undergone intense deformation.

This strain causes dynamic recrystallization during the welding process in the weld nugget zone and results in the appearance of a high-density precipitate phase in this area, which is beneficial for inhibiting grain growth during the welding process. The weld nugget zone is generally composed of small equiaxed recrystallized structures.

During the welding process, the interaction between the material and the stir pin leads to the appearance of concentric rings (onion ring structures) in the weld nugget zone.

Figure 4-20 Metallographic cross-section of the FSW Welding Joint: a – Weld Nugget Zone
  • b – Thermo-Mechanical Affected Zone
  • c – Heat-Affected Zone
  • d – Parent Material Zone

The 2A14, 2B16 aluminum alloys and 2195 aluminum-lithium alloys are commonly used structural materials for aerospace storage boxes. Table 4-15 lists the mechanical properties of friction stir welded joints for domestic 2A14, 2B16 alloys, and 2195 aluminum-lithium alloys.

As can be seen from the table, the room temperature strength coefficient of the FSW joints of the 2A14 and 2B16 aluminum alloys reaches above 0.8, both higher than the 0.65 of conventional fusion welding.

The strength coefficient of the FSW joints of the 2195 aluminum-lithium alloy also reaches above 0.75, far higher than the 0.55 of conventional fusion welding, while the elongation rate is nearly double that of fusion welded joints.

Table 4-15 Mechanical Properties of FSW Joints in Typical Aerospace Storage Box Structural Aluminum Alloys

Weld MaterialTensile Strength Rm/MPaElongation Rate (%)Rmw/Rmm
2A14-T6 Parent Material4228
2A14-T6 Joint35050. 83
2B16-T7 Parent Material4256
2B16-T7 Joint3407.50. 8
2195-T8 Parent Material55013
2195-T8 Joint4108~110.75
Note: In the table, Rmw represents the tensile strength of the FSW joint, while Rmm stands for the tensile strength of the parent material.

Table 4-16 lists the mechanical properties of Friction Stir Welding (FSW) joints in 5xxx, 6xxx, 7xxx series aluminum alloys. The data shows that, for 6082 aluminum alloy with solution treatment and artificial aging, post-weld heat treatment allows the strength of the FSW joint to reach that of the parent material, although the elongation rate is somewhat reduced.

The performance of the joint for T4 state 6082 aluminum alloy specimens can be significantly improved with conventional aging after welding. Post-weld natural aging at room temperature for 7108 aluminum alloy results in a tensile strength reaching over 95% of the parent material.

When fatigue testing is conducted using a stress ratio R=0.1 on 6mm thick 5083-O aluminum alloy weldments, the fatigue performance of the 5083-0 aluminum alloy FSW butt joint specimens is equivalent to that of the parent material. The results show that the fatigue performance of FSW butt joints generally exceeds the design recommended values for corresponding fusion welding joints.

In summary, for aluminum alloy materials, the tensile strength of FSW joints can reach over 70% of the parent material. The specific numerical values of joint performance are related not only to the performance of the parent material itself but also largely depend on the welding parameters of FSW.

Table 4-16 Mechanical Properties of FSW Joint/Parent Material

Welded MaterialYield Strength/MPaTensile Strength/MPaElongation Percentage (%)Rmw/Rmm
5083-O Parent Material14829823.51.00 
5083-O Joint14129823
5083-H321 Parent Material24933616.50. 91
5083-H321 Joint15330522.5
6082-T6 Parent Material28630110.40. 83
6082-T6 Joint1602544.85
6082-T6 Joint + Aging2743006.41. 00
6082-T4 Parent Material14926022.90. 93
6082-T4 Joint13824418.8
6082-T4 Joint + Aging2853109.91. 19
7018-T79 Parent Material295370140. 86
7018-T79 Joint21032012
Note: Rmw is the tensile strength of the FSW joint, Rmm is the tensile strength of the parent material.

Currently, research and applications of FSW are mainly focused on soft and easily formed materials such as aluminum alloys, magnesium alloys, and pure copper. Significant progress has also been made in the research and application of titanium alloys, stainless steel, and aluminum-based composite materials.

Characteristics of Friction Stir Welding

Compared to traditional friction welding and other welding methods, FSW has the following advantages:

1) The weld seam is completed under plastic deformation, belonging to solid-state welding. Thus, its joints do not produce cracks, inclusions, pores, and alloy element burn-off related to metallurgical solidification, or welding defects and embrittlement phenomena. The weld seam performance is close to the parent material, with excellent mechanical properties.

It is suitable for welding non-ferrous metals and their alloys like aluminum, copper, lead, titanium, zinc, magnesium, as well as steel materials, composite materials, and can also be used for connecting dissimilar materials.

2) It is not limited by axial parts and can perform butt and overlap welding on flat plates. It can weld straight weld seams, corner weld seams, and ring weld seams, and can be used in manufacturing large frame structures, large cylinders, large flat plate butting, etc., broadening its application range.

3) FSW uses automated mechanical equipment for welding, avoiding reliance on the skill level of the operator. It offers stable quality and high repeatability.

4) Welding does not require filler materials or protective gas. Pre-treatment of the welding surface is not necessary before welding, nor are protective measures during welding. Large weldments do not require chamfering at the edges, simplifying the welding process. When welding aluminum alloy materials, there is no need to remove the oxide film, just oil stains.

5) Since FSW is solid-state welding, its heating process features high energy density and fast heat input speed, resulting in small welding deformation and small residual stress after welding. With the assurance of sufficient rigidity of the welding equipment, precise assembly and positioning of the weldments, and strict control of welding parameters, the dimensional accuracy of the weldments is high.

6) The FSW welding process does not produce arc light radiation, smoke, or spatter, and noise is low, achieving an environmentally friendly welding process. Thus, FSW is known as a “green welding method.”

FSW also has limitations in certain aspects, for example: the mechanical force during welding is considerable, requiring the welding equipment to have good rigidity; compared to arc welding, there is a lack of flexibility in welding operation; the wear of the stirring head is relatively high.

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