The response of force characteristic to weld-forming process in friction stir welding assisted by machine learning

•The force value and some detail characteristics of the force waveform are very important in reflecting the characteristics of weld formation.•The distortion of force waveform signifies the formation of defect, and the distortion degree and deflection direction of force waveform show a high relevancy to the size and location of defect, respectively.•The three harmonics describing the force fluctuation correspond to the formation of the periodic weld microstructures in nugget zone.•Force characteristics are closely related to the tool motion and probe geometric profile, and they are capable to reveal the periodic material deformation, flow and deposition behavior during the welding process.