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The present paper put forward a magnetic non-destructive testing method for the complex tiny flaws existing in the friction stir welding (FSW) of aluminium alloy. Through magnetisation testing, it is confirmed that the relative magnetic permeability of aluminium alloy material is larger than the relative magnetic permeability of aluminium. Aluminium alloy is a paramagnetic material. The anomalous characteristics of the magnetic field in the flaw were analysed according to the differences of the relative magnetic permeability of the flaws. Magnetic detection technology detects flaws in FSW based on theoretical analysis. High precision measuring magnetic sensors were used to detect the magnetic field. Test results show that the magnetic field signal in flaw have obvious anomalous fluctuations. The reliability of the magnetic method is confirmed through comparison with the micrograph specimens. A new detection method for the tiny flaws in FSW specimens was provided.

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