Misconceptions persist regarding what constitutes a quality resistance weld. Assumptions such as “visible flash (expulsion) indicates weld strength” or “lack of surface burn implies no weld” are often incorrect. On materials with fine surface finishes, such as stainless steel, properly made spot welds may be nearly undetectable, seemingly indicating the welds are not correct, but they do meet all mechanical requirements.
No single inspection method can fully characterize weld quality. And some testing may yield similar results for acceptable and unacceptable welds while failing to reveal conditions such as brittleness or internal discontinuities. Consequently, resistance weld quality evaluation requires a combination of inspection and testing methods.
Resistance weld inspection is generally categorized as non-destructive or destructive testing. Designers must determine which weld characteristics are critical to performance and select inspection methods accordingly.
Inspection planning
An effective inspection plan defines the testing methods and procedures required to meet customer and application requirements. Depending on service conditions, inspection may involve 100 percent examination, statistical sampling or a combination of both.
Sampling consists of evaluating a representative portion of production to draw conclusions regarding overall weld quality based on established statistical principles. When sampling procedures are specified in applicable standards or testing procedures, these requirements should be followed and documented without modification.
Critical applications may require complete inspection using visual examination supplemented by non-destructive and/or destructive testing. In typical resistance welding applications, all welds are visually inspected for location and weld presence, with random sampling applied using additional testing methods. Destructive testing is frequently used to validate non-destructive inspection results and establish acceptance limits.
For resistance weld inspection, the following definitions apply:
- Discontinuity is an interruption in the normal weld nugget structure or material homogeneity. A discontinuity is not necessarily rejectable.
- Defect is a discontinuity that renders the weld unacceptable per specification or acceptance criteria.
- Flaw is a term nearly synonymous with discontinuity, implying undesirability.
Standard testing methods
Weld quality is most effectively maintained through properly trained personnel with a thorough understanding of resistance welding processes, equipment and application-specific challenges. Engineering and production personnel should have access to current technical references, including the “RWMA Resistance Welding Manual,” “AWS Welding Handbook” and applicable AWS directives.
The American Welding Society (AWS), through its Resistance Welding Manufacturing Alliance (RWMA) standing committee, has established Standard Methods for Mechanical Testing of Welds, which includes resistance welds. It encompasses commonly used inspection techniques. Specialized applications may require additional testing under conditions such as elevated temperature, high stress or severe vibration.
The use of standardized test specimens and data recording methods facilitates comparison of results across laboratories and production facilities. Complete documentation of weld parameters and test results is essential. If it is not written, it never happened.
Not all testing methods are required for every application. In many cases, one or two complementary tests are sufficient to verify weld quality. The tests described are intended to evaluate weld quality rather than base metal properties, which are governed by material specifications.
Testing methods for spot, roll spot, seam and projection welds are detailed in the resistance welding manual, chapter 24 on quality control. Drawings and specifications should clearly define required testing methods and acceptance criteria based on part design and service requirements.
Types of tests
Recommended resistance weld tests include tension, impact, fatigue, macro etch, radiographic, hardness, peel, bend and eddy current methods, depending on weld type and application. Weld size requirements are based on material thickness. Detailed testing procedures and fixture requirements are also provided in the resistance welding manual, chapter 24.
Consistent test results require clean material surfaces free of grease, scale and other contaminants that increase contact resistance. Test coupons should be flat and free of burrs, as they may cause current shunting. Specimens exhibiting misalignment or improper centering should be rejected prior to testing.
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