Application: Evaluating the integrity of spot weld joints in metal.

Background: Spot welding is a common technique for joining two pieces of metal, usually thin sheets or plates, by means of a series of small circular tack joints. The process is widely used in the automotive industry for body assembly and some chassis joints, and by other types of sheet metal fabricators as well. Spot welds are made by pressing electrodes against both sides of the parts to be joined and passing a high current through them, which momentarily creates a pool of molten metal that solidifies into a round joint known as a nugget.

If the weld is improperly formed, it is possible that the two parts may not fuse completely, or that the area of the nugget may be smaller than what is required for a strong joint. These problems are seldom visible to optical inspection, and destructive pull tests are inefficient and limited to a few samples. However, with proper equipment and technique, ultrasonic spot weld inspection can quickly and nondestructively provide valuable information about weld quality.

Equipment: The recommended instrument for spot weld inspection is a flaw detector such as the EPOCH XT, EPOCH LTC, EPOCH 600, or EPOCH 1000. Spot weld testing requires coupling sound into a cup-shaped weld typically 3 to 6 mm (0.125 to 0.250 in.) in diameter and generating multiple backwall echoes at high frequency. A wide variety of delay line and captive water column transducers in the frequency range from 10 to 20 MHz are available from Olympus NDT for this purpose. Delay line transducers use a small plastic waveguide to couple sound energy from the transducer element to the test piece. Captive water column transducers incorporate a pliable rubber membrane enclosing a column of water that conforms to the topography of the spot weld to optimize coupling. In the case of delay line transducers, the delay line diameter and element diameter are normally matched to the nominal nugget diameter to within a few tenths of a millimeter. In the case of captive water column transducers, the element diameter is normally matched to the nominal nugget diameter. For further details on transducer selection, contact us.

Theory of Operation: As with any other ultrasonic weld inspection, spot weld testing involves comparing the reflection pattern from a test piece to the patterns obtained from reference standards of the same type of weld whose condition is known. The delay line or captive water column transducer is used to generate a series of echoes representing multiple sound reflections within the weld, which are then generally interpreted as follows.
In a good weld, the spacing between echoes will be proportional to the thickness of the weld, and the decay rate (the rate at which successive echoes decrease in amplitude) will be related to the attenuation within the nugget. There will be a characteristic echo pattern associated with a good weld, such as this one:



If there is no fusion between the two pieces of metal (a "no weld" or "unweld" condition), then the successive echoes will be much closer together and larger in amplitude:



In the case of an undersized weld, a portion of the sound beam will reflect from the total thickness of the two metal sheets, while a portion reflects from a single thickness. This creates a pattern in which small peaks representing single-sheet thickness appear between the larger and more widely separated peaks representing the full weld thickness:

Finally, in a stick weld condition where the metal sheets are fused but because of insufficient heating the weld nugget is not fully formed, the echo decay rate will change, with more peaks appearing on the screen and a smaller drop off in amplitude between any two successive peaks: