Friction Stir Welding (FSW) Process Explained

Joint Types in Friction Stir Welding

Friction stir welding can weld structures like circular and platen shapes, with joint types designed as butt, lap, corner, and T-shape joints. It can perform welding of ring-shaped, circular, non-linear, and three-dimensional seams.

As gravity does not impact this solid-phase welding method, friction stir welding can be used for all-position welding, such as horizontal, vertical, overhead, and automatic welding on circular tracks. The joint types in friction stir welding are shown in Figure 4-21.

Figure 4-21 Joint Types in Friction Stir Welding
  • a) I-type Butt Joint
  • b) Combined Butt and Lap Joint
  • c) Double Sheet Lap
  • d) Multi-sheet Butt Joint
  • e) Edge Butt Joint
  • f) Double Sheet T-shape Butt Joint
  • g) Triple Sheet T-shape Butt Joint
  • h) Two-sheet Inner Corner Butt Joint

Heat Input and Welding Parameters in Friction Stir Welding

During the friction stir welding process, the stir needle rotates at high speed and inserts into the workpiece. Subsequently, under the action of the welding pressure, the shoulder contacts the workpiece’s surface, generating a significant amount of frictional heat between the shoulder and the workpiece’s surface material, as well as between the stir needle and the joint surface.

This forms the main body of the welding heat source. Simultaneously, the material near the stir needle undergoes plastic deformation and fluid flow, leading to heat deformation, although this heat is relatively minor.

Therefore, friction stir welding essentially relies on frictional heat as its primary welding heat source, making frictional heat generation a key factor influencing welding quality.According to the heat generation analysis, the heat power of friction stir welding can be represented as

Where

  • Q – Heat Power (kW);
  • k – Shape factor, depending on the shape and size of the stir welding head;
  • μ – Friction factor;
  • n – Rotation speed of the stir welding head (r/min);
  • F – Welding pressure (N).
  • Therefore, the heat input qE of friction stir welding is

Where

  • Km – Constant coefficient;
  • v – Welding speed (r/min).

During steady-state friction stir welding, both the friction factor and welding pressure are constants, which can be combined with the shape factor to form a new constant coefficient k. Therefore, the heat input in friction stir welding can be characterized by n/v.

As such, for a given stir welding head and welding pressure, the heat input mainly depends on n/v.

If the rotational speed is too low or the welding speed too high, this will result in a decrease in n/v, meaning a smaller heat input for welding. The heat might not be sufficient to bring the metal in the welding area to a thermoplastic state, resulting in defects such as holes and incomplete penetration in the weld seam, leading to poor weld formation.

As the rotational speed increases or the welding speed decreases, n/v gradually increases, the heat input for welding becomes reasonable, and the weld seam forms better. If the rotational speed is too high or the welding speed is too low, n/v becomes too large, and the heat input on the unit length of the weld seam is excessive.

This overheats the metal in the welding area, leading to defects like concave surfaces on the weld seam and burn-through, resulting in poor formation and quality.

Therefore, a suitable welding heat input can only be achieved, resulting in a beautiful and high-performance weld seam, when n/v is within a certain range, i.e., when the welding speed matches the rotational speed of the stir welding head reasonably.

Figure 4-22 shows the impact of different n/v ratios on tensile strength when Al-5Mg alloy is welded using friction stir welding at a rotational speed of n=1000r/min. From the figure, strength and plasticity increase with the increase of n/v values.

The maximum tensile strength exceeds 310MPa, the same as the measured value of the parent material, with an elongation rate of 17%, which is 63% of the measured value of the parent material. After reaching the maximum strength value, the strength and plasticity decrease with further increases in the n/v values.

Figure 4-22 The impact of n/v on the performance of FSW joints

Selection of Friction Stir Welding Parameters

The main parameters for friction stir welding include welding speed (the travel speed of the stir welding head along the weld seam), rotational speed of the stir welding head, welding pressure, structural parameters of the stir welding head (tilt angle 0), insertion speed of the stir welding head, and holding time, etc.

(1) Welding Speed

Figure 4-23 shows the effect of welding speed on the tensile strength of the aluminum-lithium alloy friction stir welding joint.
From the figure, it can be seen that the relationship between joint strength and welding speed is not a simple linear proportional relationship, but a curve.

When the welding speed is less than 160mm/min, the joint strength increases with the increase in welding speed. It can be seen from the welding heat input calculation formula that when the rotational speed is a constant, the overall friction heat input of the stir welding head/workpiece interface is high when the welding speed is low.

If the welding speed is too high, the heat input is insufficient, and the ability of the thermoplastic material to fill the cavity formed by the travel of the stir needle weakens.

If the thermoplastic material’s ability to fill the cavity is insufficient, loose pore defects are likely to form within the weld seam, and a narrow tunnel that runs parallel to the welding direction will form on the weld seam’s surface in severe cases, causing a significant decrease in joint strength.

(2) Rotational Speed of the Stir Welding Head

If the welding speed is kept constant, i.e., when the welding speed is a constant, if the rotational speed of the stir welding head is too low, the welding heat input is low. It can’t form enough thermoplastic material in front of the stir welding head to fill the cavity formed behind the stir needle, and holes or grooves are likely to form in the weld seam, thereby weakening the joint strength.

As the rotational speed increases, the width of the groove decreases. When the rotational speed increases to a certain value, the appearance of the weld seam is good, and the internal holes gradually disappear. Only at a suitable rotational speed can the joint achieve the best strength value.

The rotational speed of the stir welding head affects the microstructure of the joint by changing the heat input and the flow of thermoplastic material, which in turn affects the mechanical properties of the joint.

For high-strength aluminum-lithium alloys, the effect of the rotational speed of the stir welding head on joint strength under the conditions of welding speed v=160mm/min, and tilt angle of the stir welding head=2° is shown in Figure 4-24.

From the figure, it can be seen that when n≤800r/min, the joint strength increases with the increase in rotational speed and reaches its maximum value at n=800r/min; when n>800r/min, the joint strength rapidly decreases with the increase in rotational speed.

Figure 4-23 The impact of welding speed on the tensile strength of the aluminum-lithium alloy friction stir welding joint (n=800r/min, θ=2°)
Figure 4-24 The impact of the stir welding head’s rotational speed on the strength of the aluminum-lithium alloy joint (v=160mm/min, θ=2°)

(3) Welding Pressure

In addition to affecting the heat generated by friction stir, the welding pressure also compresses the plastic metal after stirring. Experiments have shown that when the welding pressure is insufficient, the surface thermoplastic metal “floats up” and overflows the weld seam surface, and holes will form inside the weld seam due to the lack of metal filling.

When the welding pressure is too high, the friction between the shoulder and the surface of the workpiece increases, and the friction heat will cause adhesion on the shoulder platform, causing flashes and burrs on both sides of the weld seam. The concavity in the center of the weld seam is large, which cannot form a good welding joint, and the surface formation is poor.

In addition, the tilt angle of the stir welding head affects the motion state of the plastic fluid, thereby affecting the formation process of the weld nugget; the insertion speed of the stir welding head determines the preheating temperature at the start of the friction stir welding process and whether it can generate sufficient plastic deformation and fluid flow;

the shape of the stir welding head determines the heating during the friction stir welding process and the plastic flow of the weld seam metal, ultimately affecting the formation and performance of the weld seam. A quantitative analysis of the influence of friction stir welding parameters on welding quality remains to be further researched.

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