Qualification Of Spot Welding With Air Coupled Ultrasonic Guided Wave

Spot welding is the most common resistance welding process where the weld is a result of the heat obtained from the interface between two joints.Spot weld is widely used for joining sheet metals in the automobile industry.The quality of these joints is playing a very important role in the structural integrity of the whole car,especially during collision in a traffic accident.The diameter of the spot welded joints or in other words of the nugget diameter is a very important factor in the evaluation of the joints.Because of variation in the electrode force,welding current and welding time,the spot weld is not always the same.If the difference is too big compared to a specified standard spot weld,security flaws may exist that need to be avoided.Currently many car manufacturers use conventional ultrasonic techniques to detect those differences,however,those techniques have some disadvantages such as the requirement for an additional coupling medium for the inspection.Air-coupled ultrasonic techniques do not need any additional coupling medium and they have the ability to generate guided waves too.As such they can be used for characterization of materials during a manufacturing process even possibly in-line.Guided waves have a multi-modal nature which permits stress wave energy to be directed to any point in the structure and over a long distance.According to a benchmark work performed,air-coupled ultrasonic transducer generated guided waves have been simulated with a multiphysics based finite element simulation approach.After that,one basic model which contains one steel plate was established.In this model the sender and receiver was placed on one side.Based on this model,the influence of incidence angle of the transducer and the size of the air gap between transducer and object were studied.For those conditions a numerical spot welding model was built with a multiphysics based FEM.In practice,there are further factors that affect the detection result,like the distance between sender and receiver,the position of sender and receiver,the frequency of the transducer and its type such as operating with a focused or unfocused beam.The influence of these factors was therefore also studied.In view of all these factors guided wave inspection of spot welds with different nugget diameter was simulated.The simulation results were finally verified through experiments and a general conclusion has been made.