Zigzag line defect in friction stir butt-weld of ferritic stainless steel

•A zigzag line defect was found in friction stir weld of ferritic stainless steel.•The features and formation reason of the zigzag line defect was clarified.•The zigzag line defect did not affect the tensile properties of the joint.•The zigzag line defect significantly reduced the impact toughness of the weld.