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The manuscript reports on detection of defect that arises during friction stir welding using continuous wavelet transform (CWT) on force signal. The vertical force during welding undergoes sudden change due to presence of defects. These localized defects are detected accurately with the help of continuous wavelet transform scalogram (CWT coefficients’ gray scale image). Statistical feature of variance is used on scale of 1 of transformed signal to localize the defects. The experiments of welding are conducted on the work piece of AA 1100 with varying tool rotational speed (1000, 2000, 3000 rpm) and transverse velocity (50, 75 and 125 mm/min). The manuscript also presents the comparison of results obtained using discrete wavelet transform and CWT of force signals and shows better localization and determination of degree of defect are possible through CWT analysis.