Spot weld quality
The term spot weld quality refers to the quality of welded joints made with resistance spot welding .
Using spot welding, components should be connected to one another in such a way that they can meet the requirements placed on them over an expected service life. To do this, the spot welds must meet the quality criteria or quality features that meet the requirements. General specifications for the quality characteristics cannot be made, because these are always closely linked to the respective component requirements.
Security classes
The DVS leaflet 2915-1 defines three safety classes:
- Security class A:
Connections whose failure could endanger human life.
- Security class B:
Connections, the failure of which makes the product unusable for the intended purpose or which can result in material loss.
- Security class C:
Compounds, the failure of which does not affect the fitness of the product for the intended purpose.
Depending on the security class, quantitative specifications should be made for certain quality features.
| Security class | Point diameter | Lens diameter | Lens penetration depth | Shear force |
|---|---|---|---|---|
| A. | X | X | X | X |
| B. | X | (X | X) or | X |
| C. | X | - | - | - |
The required quality can only be achieved if the weldability of the component is guaranteed. For this purpose, a quality assurance system has to be integrated into the production (see picture quality assurance system according to DVS 2915 ):
- The construction must be suitable for welding.
- Devices and tools must meet the requirements of quality manufacturing.
- The materials to be welded have sufficient weldability have
- The manufacturing conditions and the personnel employed must meet the requirements of the welding process.
The welded product is created in the interaction of these factors. The quality of the product is verified by quality tests and operational controls. If poor quality can be ruled out, the product will be forwarded, otherwise reworked or rejected. Corrective measures are taken that can affect all four influencing factors.
Quality criteria
The quality criteria include:
- geometrically measurable quantities:
- Point diameter d P after a workshop test
- Lens diameter d L according to macro cross-section
- Penetration depth
- Depth of the electrodes
- Gap width between the sheets
- measurable strength values that are linked to a special test method:
- Peel force
- Head pull
- Shear force
- Torsional moment
- Subjectively estimated variables that are determined in operational controls:
- Ultrasound A-scan
- Splash-free
- Surface appearance
- pushing electrode
- Edge welding
- geometrically measurable quantities:
Test procedure
Compliance with the quality values is determined by means of test procedures and / or operational controls in a destructive or non-destructive manner.
Devastating exam
DVS data sheet 2916-1 applies to carrying out destructive tests
Workshop test procedure
Shop inspection procedures are the easiest way to assess the properties of welded joints. The area of application ranges from the control of setting values to the testing of finished components. They only require a small investment in the necessary testing and measuring tools, and implementation is relatively quick and easy. In workshop test procedures, the welded connections are stressed to breakage in simple test devices or directly on the object (exception: wedge test). No measured values are recorded during the test. The defined properties of breakage type and point diameter can be determined and documented on the destroyed object . No statements about the strength of the welded connections can be made during the workshop tests. Test conditions that are not clearly defined and not constant can lead to different test results, both with regard to the point diameter determined and the type of breakage.
The workshop test procedures include:
- Roll-off or peel test,
- destructive chisel testing,
- non-destructive wedge test,
- Torsion test.
The implementation is regulated by EN ISO 10447.
Laboratory test procedure
Defined samples are prepared for the laboratory test. Sample dimensions and the implementation of the laboratory test procedures are described in separate standards:
- Peel tensile test in EN ISO 14270
- Head pull test in DIN EN ISO 14272
- Tensile shear test in DIN EN ISO 14273
- Torsion test in DIN EN ISO 17653
In the automotive industry, separate regulations often apply.
Quantitative specifications for the quality features
Quantitative requirements for the spot weld quality are set very differently for different applications. In DVS data sheet 2902-3, a value of: d L = 3.5 √ t min is recommended as a minimum dimension for the lens diameter . Weld point classes can be defined for special applications in automotive engineering :
- Special point: Failure can lead to damaging events that can impair overall safety: d L = 3.5 √t min or d P = 4√t min
- Production point: welding points that are required for a continuous production sequence and process. The function of the component is given without restrictions even without these points. They do not have to meet the requirement d L = 3.5 √t min or d P = 4√t min .
- Standard point: Failure does not affect the overall function of the vehicle or only marginally. d L = 3.5 √t min or d P = 4√t min
When specifying point diameters, it must be taken into account that there is often no close correlation between the point diameter and the actual point strength; large deviations can occur, especially with higher and high-strength steels. The welding point does not unbutton, but very different types of fracture occur, including shear fractures of weld points with sufficient point strength.
Non-destructive testing (operational controls)
Visual inspection
The visual inspection is non-destructive. An assessment of the strength of the point is not possible. It "refers to the control of the visible quality features of the point. In connection with the component, the position of the point, e.g. edge distance on the flange, distance between the points and the completeness of the number of points are checked. Significant defects in the point surface and the adjacent areas can be, for example:
- Cracks in or adjacent to the welding zone,
- firmly adhering splashes or "meat hooks",
- weld nugget melted to the surface,
- excessively deep electrode impression,
- "pushed point" with strong bulging,
- eccentric point,
- strong electrode alloy with scars and electrode residue on the point surface. "
Ultrasonic testing
Impulse-echo technology
The sound pressure generated by special probes and introduced into the surface of the welded sheet is reflected at the boundary surfaces. After reflection from the back of the welding point, the sound pulse returns to the test head and is displayed as a back wall echo on a screen as a so-called A-scan at the time it arrives. The quality of the weld connection is derived from the shape of this echo sequence. " A successful use of the ultrasonic test is only possible with well-founded knowledge and practical experience of the test personnel related to the connection to be tested. This applies to both the generation of the correct echo sequence and its evaluation ". The process has found widespread use in the automotive industry.
Mapping method
On the basis of A-scans, vertical or horizontal depth sections can be displayed (B- and C-scans). Newer methods use sound heads with an array of separate sound generators (piezoelectric elements), each of which works in pulse-echo mode. The A-pictures of all elements are combined into a C-picture. The information is coded as color values (e.g. red for unwelded and green for welded areas on the RSWA device). With good surface conditions, such a system delivers acceptable results. However, due to tilting, several measurements of the same spot welded joint often lead to different results when assessing the welding quality. For this reason, the results are only partially reproducible in poor surface conditions. So far, the process has not been able to establish itself on a large scale in car body production.
Process monitoring
Online monitoring provides information about the quality of each individual connection immediately after the welding process has been completed. For this purpose, essential physical parameters that are decisive for the production quality are measured from the ongoing process, significant features are derived from them and these are compared with specified feature values. The result is a statement about the expected quality of the welded joint. In this way, a full test of all connections made is aimed at. Online process monitoring closely couples monitoring to the manufacturing process in terms of time so that as little time as possible is lost between its completion, the quality statement and a possible reaction of the manufacturing plant to the quality result.
Examination of small parts and micro welding in electronics and precision engineering
In contrast to resistance spot welding of sheet steel, pairings of different types of materials and surfaces are joined in small parts and micro welding. A molten lens occurs only in rare cases. This means that the lens or point diameter cannot be used as a test feature, just as general specifications for quality criteria cannot be made. Inspection characteristics can be:
- External properties such as dimensional accuracy, deformation, electrode indentation depth, gap between the parts to be joined, freedom from internal and external splashes, tarnishing, damage to metallic coatings or non-metallic coatings
- Strength properties:
- under static load: for example shear tensile force, head tensile force, peeling force, torsional moment, torsion angle, spot size
- under dynamic loading: for example swell tensile strength, flexural fatigue strength, impact shear strength
- Structure: weld nugget size, regularity of the lens shape, grain size coarse / fine, hardness profile, area of the heat affected zone, solid / liquid phase, alloy formation, internal irregularities such as cracks, pores, inclusions
- Corrosion behavior
- Conductivity: electrical conductivity, magnetic conductivity, thermal conductivity
- Leak tightness: burst pressure test, gross leak rate, fine leak rate
In general, a component-specific static test of the breaking strength is carried out in addition to a visual test. Depending on the type of connection, different workshop test procedures are available, which are described in.
| Test procedure | |||||||
|---|---|---|---|---|---|---|---|
| x | x | - | - | - | - | - | |
| x | x | - | - | - | - | - | |
| x | x | - | - | - | - | - | |
| x | x | - | x | x | x | x | |
| - | - | x | - | - | - | - | |
| x | x | - | x | x | - | x | |
| x | x | x | - | - | - | - | |
| - | - | - | - | - | x | - |
Quality assurance
Quality assurance measures are intended to ensure during the preparation and implementation of resistance spot welding that the expected connection quality is achieved by the welding process. The basics of quality assurance in spot welding are presented in EN ISO 14554 and DVS leaflet 2915. The EN ISO 14554 describes the required quality assurance measures, the DVS leaflet 2915 their process.
Individual evidence
- ↑ a b c DVS - German Association for Welding and Allied Processes eV: Quality assurance for spot, projection and roller seam welding. DVS data sheet 2915-1, 1979.
- ↑ a b c DVS: Testing of resistance pressure welded connections - destructive testing, quasi static. DVS data sheet 2916-1,2009
- ^ A b AWS (American Welding Society) : "Specification for Automotive Weld Quality-Resistance Spot Welding of Steel" , AWS D8.1M
- ↑ EN ISO 14329: "Destructive testing of welded joints"
- ↑ a b Mercedes-Benz: MBN 10 382: Resistance spot welding of steel up to 3 mm sheet thickness , 2005.
- ↑ EN ISO 10447: Resistance welding - peel, chisel and wedge testing of resistance spot and projection welded joints , 2007.
- ↑ DIN EN ISO 14270: 2002-04: Specimen dimensions and procedures for the mechanized peeling test on resistance point, roller seam and projection welds with embossed projections
- ↑ DIN EN ISO 14272: 2002-04: Specimen dimensions and methods for the head pull test on resistance spot and projection welds with embossed projections
- ↑ DIN EN ISO 14273: 2013-02: Specimen dimensions and methods for tensile shear testing on resistance point, roller seam and projection welds with embossed projections
- ↑ DIN EN ISO 17653: 2012-08: Resistance welding - Destructive testing of welded joints on metallic materials - Torsion test on resistance spot welded joints
- ↑ Volkswagen AG : "Spot Welding Joints on Steel Materials - Testing of Body Assemblies" , PV 6702 (2010)
- ↑ DVS: Resistance spot welding of steels up to 3 mm thick; Construction and calculation. DVS bulletin 2902-3, 1991.
- ↑ M. Bouzekri, S. Dancette, Th Dupuy, A. Lens and a .: Failure type of Resistance spot welds is not characteristic of their strength. IIW-Doc. III-1481-08
- ↑ a b DVS: 2916-5 Testing of resistance pressure welding connections - non -destructive testing , DVS data sheet 2916-5, 2006.
- ↑ AA Denisov, CM Shakarji, BB Lawford, R. Gr Maev, JM Paille: Spot Weld Analysis With 2D Ultrasonic arrays , Journal of Research of the National Institute of Standards and Technology, 109 (2004), H. 2
- ↑ Resistance Spot Weld Analyzer
- ^ R. Hipp, A. Gommlich, Ch. Grossmann, F. Schubert: High-resolution ultrasonic testing on resistance spot welded joints. DGZfP annual conference 2013.
- ↑ DVS: Resistance welding in electronics and precision engineering - overview and basics. DVS bulletin 2950, 2010.
- ↑ DVS: Testing of resistance welded connections in electrical engineering and precision engineering - destructive and non-destructive tests. DVS bulletin 2951, 2010.
- ↑ EN ISO 14554-1 Resistance welding of metallic materials. Part 1: Comprehensive quality requirements. Part 2: Elementary quality requirements
Web links
- H. Polrolniczak: Ultrasonic testing as a means of quality assurance in resistance spot welding. Company publication Krautkrämer GmbH & Co. oHG, 1999.