Internal friction of friction stir welding is mainly divided into non-weld, weak bond, tunnel type defect, joint surface oxide and so on.
First, the incomplete penetration defect
1. Definition: During the friction stir welding process, the defects that occur when the root of the weld is not formed effectively are called incomplete penetration (as shown in Figure 1).
2. Reasons for formation: (1) The length of the stirring needle does not match the thickness of the material to be welded (the stirring needle is too short) or the amount of pressing is insufficient, resulting in the metal at the root of the weld being not fully stirred;
(2) The assembly clearance is too large, resulting in the root of the weld not being fully stirred and filled during the welding process;
(3) The selection of process parameters is not suitable, resulting in insufficient heat input and insufficient metal flow at the root of the weld.
3. Preventive measures: (1) Select a suitable mixing tool and reasonably control the amount of pressing;
(2) Select appropriate process parameters to ensure adequate welding heat input;
(3) Guarantee the assembly state before welding.
4. Detection method: Under normal circumstances, the defect of incomplete penetration can be detected by PT, RT, etc.; but when the size of the incomplete penetration defect is small, it can be detected by phased array ultrasonic or joint macroscopic gold equal method.
Figure 1 Incomplete penetration defect
Second, weak combination defects
1. Definition: During the friction stir welding process, the weld metal is in close contact after being stirred, but the defect that does not form an effective connection is called a weak bond defect. Weak bond defects severely affect joint strength and joint elongation.
2. Cause of formation: During the friction stir welding process, the weld metal is not sufficiently stirred due to insufficient heat input to form weak bonding defects.
3. Preventive measures: Select the appropriate mixing pin shape and select the appropriate process parameters to ensure adequate welding heat input.
4. Detection method: For weakly bound defects, it is difficult to obtain accurate detection results by using conventional RT, UT, etc.; usually, when the mechanical properties of the welded joints are not up to standard, and the weak joint defects are analyzed, the macroscopic use is used. The metallographic method is tested.
Third, the tunnel defect
1. Definition: Tunnel defects, also called worm-shaped void defects, are formed by hollow joints in the weld, which are shaped like a bug (see Figure 2).
2. Causes of formation: improper selection of welding process parameters, resulting in insufficient welding heat input and insufficient material flow.
3. Preventive measures: Select appropriate process parameters to ensure adequate welding heat input.
4. Detection method: The detection of tunnel type defects can be realized by using conventional RT, UT and other detection methods.
Figure 2 Tunnel type defect
Fourth, combined surface oxide defects
1. Definition: In the friction stir weld, along the direction of rotation of the stirring needle, a looming impurity deposition zone formed near the abutting surface is called the bonding surface oxide defect (as shown in Figure 3).
2. Causes of formation: The oxides and impurities in the surface to be welded and its vicinity are not completely removed, resulting in the formation of bonding surface oxide defects.
3. Preventive measures: Thoroughly clean the weld surface and its vicinity before welding.
4. Detection method: The surface oxide defect is mainly detected by the metallographic method, which is represented by a black line in the weld nugget area.
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