PHAsis - for spot welds and short step welds
ultrasonic testing of joints
Ultrasound is our DNA and spot weld inspection is our focus
Based on more than 30 years of experience in spot weld inspection, we have developed PHAsis, a series of inspection systems for manual and robot-controlled spot weld inspection. PHAsis inspection systems use the imaging phased array ultrasonic technology, which achieves a very high inspection accuracy. That is why these proven inspection systems are used primarily in the automotive industry.
With PHAsis, manual spot weld inspection is combined with automated, robot-controlled spot weld inspection. Two inspection systems with a common basis, but specially optimized for their individual area of application: PHAsisNEO for manual spot weld inspection, PHAsisBLU for automated spot weld inspection. With PHAsis, our customers decide quite flexibly which spots are to be inspected automatically and which manually. All inspection results and data are combined in a database, which can communicate with the customer’s QA system.
Thanks to our many years of know-how, we know almost every material combination and support our producers, integrators and welding robot manufacturers in process integration, correlation with destructive testing and test data optimization.
Manual and automated
Our PHAsis family
- PHAsisNEO - For the manual inspection of spot welds
- PHAsisBLU - For robot-controlled inspection of spot welds
VOGT Ultrasonics Your reliable partner
A bit of theory
Spot weld testing is an essential part of ensuring product quality
Resistance spot welding is one of the most important joining processes in automotive engineering, in the production of e.g. washing machines, in aircraft construction and is the most widespread technique for joining 2 or 3 sheet metal joints made of steel, stainless steel, titanium or aluminum before adhesive bonding.
Especially in the large-scale production of automotive bodies, it is considered the most important joining process because, among other things, it also has a very high level of automation.
Every day, we rely on the quality of these spot-welded joints, whether they are in our cars or in our surroundings. For example, there are several thousand of these spot welds in a passenger car.
The demands and requirements on the quality assurance of these welded joints are correspondingly high and have been continuously developed in recent years. The safety-relevant welded joints must withstand a wide variety of dynamic and static stresses. Welding safety therefore plays a decisive role in the design.
In resistance welding, two or more metal sheets with two copper electrodes are pressed together and heated by means of a current flowing during welding until the joints melt. During the cooling process, a solid joint (weld lens) with a specific weld lens diameter is formed.
Fig.: Welding robot for resistance spot welding on a car body
Typical welding defects affecting the quality and thus safety of the welded joint are: (1) a lens diameter that is too small, (2) an adhesive joint (also called zinc adhesive, in which the zinc surfaces of the sheets fuse together but no welding lens has formed), (3) pores or pore nests in the welding lens, or (4) lack of welding due to incorrect welding parameters or heavily used welding electrode caps.
assured quality
Quality criteria
The quality criteria for the quality assessment of the spot weld quality include:
Geometrically measurable quantities:
- the measured spot weld diameter dP according to EN ISO 10447
- the lens diameter dL by a macrosection. Here, the lens size is measured visually by means of a micrograph. The formula dL= x √tmin is used as the minimum dimension for the lens diameter. Common factors for x are 3.5 or 4 (user-specific).
- The indentation depth (cap indentation) of the electrodes on the spot weld
Measurable strength qualities:
- The peel force, i.e. the resistance of a weld to the bending up of two sheets welded together. This is determined by the peel test according to EN ISO 14270.
- The head tensile force: The resistance of a weld to pulling apart at the flat sides of two sheets welded to each other. This is determined by the head tensile test according to DIN EN ISO 14272.
- The shear tensile strength: The resistance of a weld of two overlapping workpieces welded together. The strength of the weld can be checked in a so-called shear tensile test according to DIN EN ISO 14273.
- The torsional moment: The resistance of a weld of two plates to their helical twisting, determined by the torsion test according to DIN EN ISO 17653.
Subjectively identifiable qualities:
- Appearance of the spot weld surface
- Splash-free appearance of the spot weld surface
- Burns (too high heat input) of the spot weld surface
electrodes displaced to each other
Fig.: Destructive testing of the spot weld quality
Digitally measurable values
Spot weld quality (diameter, residual wall thickness, homogeneity, sound attenuation, etc) using ultrasonic A, C and D scans.
Measurable quantities
Safety classes according to DVS data sheet 2915-1
During the manufacturing process, the entire process must be monitored in order to guarantee a reliable welded joint. Depending on the component and application, different requirements are specified for the quality of the welded joints. These requirements must be met and verified.
As early as 1979, the DVS Deutscher Verband für Schweißen und verwandte Verfahren e.V. (German Welding Society) stipulated that welding work should be divided into safety classes for the benefit of the general public and production safety. This subdivision was defined in DVS leaflet 2915-1 and is applied to spot welding, projection welding and roller seam welding. It includes and regulates all parameters relevant to a welding process. It also provides for a correction loop for rework. Safety classes A, B and C are particularly relevant. They each describe how severe possible damage is in the event of failure of a weld structure and thus, respectively, the demand on the weld quality.
Safety class A
This safety class is for welded joints that are safety-relevant and endanger human life in case of failure. These weld points are subject to high static and dynamic stresses and require maximum strength. An example of this is the side impact protection.
Safety class B
This safety class is for welded joints that, if failed, will render the joint unusable for its intended purpose or cause property loss.
Safety class C
This safety class is for welded joints that are subjected to only minor stresses and, if they fail, will have little effect on the serviceability of the product for its intended purpose.
Reliable spot weld testing
Important manufacturing parameters for the quality of a spot weld
The lens diameter is the default size for the spot weld joint in production. The welding lens diameter correlates best with the strength and thus the safety of the spot weld. The default for the lens diameter depends on the wall thickness of the thinnest plate of the joint.
The size of the lens diameter depends on the amount of heat generated in the joint plane during the welding process. The higher the amount of heat, the larger the melt and the larger the diameter.
In simplified terms, the following formula applies to the development of the heat quantity: Heat quantity = (welding current)2 x resistance x welding time.
To ensure a suitable lens diameter and a defined weld, manufacturing parameters are set and continuously monitored in the process. The most important parameters for the quality of a spot weld are (1) the electrode (cap diameter), (2) the welding current, (3) the welding time and (4) the electrode force.
When setting the welding parameters, the lens diameter is controlled/monitored e.g. by means of ultrasonic and destructive chisel testing.
Electrode/cap
The electrodes lead the electricity into the sheet metal joint to be joined and exert an electrode force necessary for the melt (defined via the welding gun). During their use, they are therefore subject to thermal, mechanical and chemical stresses. Wear becomes noticeable with an increase in the electrode cap surface area and, if necessary, with alloys (e.g. zinc or aluminum oxides). This reduces the current density during welding and the contact resistance increases accordingly. This in turn has a direct influence on the weld spot quality.
Welding current
The strength of the welding current must be selected depending on the material as well as the individual sheet thicknesses to be welded. If the welding current is too low, no or insufficient fusion and thus an insufficient welding lens can be produced. In contrast, too high a welding current can lead to weld spatter formation or impermissible deformation of the spot surface.
Welding time
The welding time is selected as a function of the welding current and the contact pressure of the electrodes. Particularly in the case of short welding times, a deviation can have a major influence on the quality of the spot weld, since no fusion or only insufficient fusion may be achieved in the respective joining plane.
Electrode force
A weld spot is created by the flowing electric current in conjunction with the acting electrode force. The force effect of the electrode on the sheet surface is a welding parameter that has a direct influence on the weld lens formation and thus the quality. For a quality welded joint, a sheet thickness-dependent electrode force as well as a suitable lead time and holding time during the welding process must be selected.
in manufacturing
Possibilities of nondestructive testing of resistance spot welded joints
In general, there are two types of materials testing for the quality assurance of joints: Destructive materials testing and non-destructive materials testing (NDT).
As the name suggests, the spot welded joint is mechanically destroyed during destructive testing in order to be able to check the quality of the joint. This type of quality assurance is performed as a random inspection. A major disadvantage is that this renders the components unusable. Due to the test scrap, destructive testing is therefore associated with high costs.
Therefore, the need for safe non-destructive testing methods is very high. Non-destructive testing is often performed in correlation with destructive testing, as random testing and is even suitable for 100% testing integrated into the production line.
How to inspect
Test methods
Three main testing methods are used to ensure welding quality: The visual inspection, the chisel inspection and the ultrasonic inspection.
Quality monitoring by means of visual inspection and ultrasonic testing is non-destructive. In the visual inspection, the visible weld spot quality is inspected, e.g. presence of weld spatter, burns of the spot surfaces. The overall quality of a spot weld cannot be determined by this method.
Chisel testing belongs to the destructive and therefore cost-intensive testing methods. It is performed offline and thus outside the production process as a random sample inspection. The welded sheets are cut open again and their quality is assessed on the basis of the weld spot knockout pattern. This only shows whether the weld slug holds or separates, or what the diameter of this welded slug is. The average diameter is evaluated, the minimum and maximum diameters are measured with calipers and then averaged.
In ultrasonic testing, the quality of the welding lens volumes as well as their diameters is evaluated non-destructively with an appropriate testing device. This makes ultrasonic inspection of spot welds non-destructive and fast, thus saving time and money. It can also be carried out inline, i.e. during the production process, and thus enables a meaningful statistical inspection or, in the case of special weld spots, a 100% inspection.
Visual Testing (VT)
In the visual Measurable variables include the completeness of the spot count, the positioning of the spot on the component, inspection for surface damage, burns, spatter and cap impression depths. However, possible internal inhomogeneities such as pores or oval/C-shaped lenses and zinc adhesives cannot be detected by visual inspection.
In the visual Measurable variables include the completeness of the spot count, the positioning of the spot on the component, inspection for surface damage, burns, spatter and cap impression depths. However, possible internal inhomogeneities such as pores or oval/C-shaped lenses and zinc adhesives cannot be detected by visual inspection.
Ultrassonic Testing (UT)
The pulse-echo method is used for the ultrasonic inspection of spot welds. Individual echoes and echo sequences are evaluated via the amplitude and transit time evaluation using the corresponding ultrasonic testing device. This information is displayed as an A-scan (amplitude image), C-scan or D-scan (area information of the amplitudes and travel times) and thus allows a weld lens assessment (larger/smaller than TARGET, diameter, pore yes/no, etc.). The aim of ultrasonic spot weld inspection is to classify good and bad spots in relation to the nominal lens diameter.
Fig. Derivation of the weld spot quality based on the A-scan echo sequence
Safe ultrasonic inspection is difficult to achieve with conventional A-scan ultrasonic equipment without the special weld spot evaluation aid and inspection plans. It requires the inspector to have a sound knowledge and experience in the inspection of spot welds as well as years of practice. This inspection method has major disadvantages, since it takes a lot of effort to evaluate a weld spot that is too small into a GOOD spot if the inspection probe is positioned appropriately.
If conventional A-scan inspection equipment is used in conjunction with an inspection plan and an automated evaluation aid, the risk of incorrect evaluation can be minimized. However, subjectivity on the part of the inspector cannot be prevented with this type of inspection.
Significant improvement is offered by the use of ultrasonic imaging inspection equipment. The improved technology of ultrasonic imaging devices using frequencies around 20 MHz, digital imaging and evaluation, and the display of the weld spot with its evaluation will enable a more objective inspection that is independent of the inspector and a very accurate and documented weld spot inspection.
Weld spot inspection with ultrasonic phased array technology (PAUT)
Like classic ultrasonic inspection, phased array spot weld inspection systems are also based on the pulse-echo method. However, in addition to the classic display of the A-scan, these systems also enable an imaging display of the weld spot (C-/D-scan).
Video: Spot weld inspection with phased array technology according to inspection plan
For this purpose, a phased array inspection device such as the PHAsisNEO with a phased array probe with 121 elements is used. A major advantage of phased array technology is that a certain number of the available elements of the probe can be electrically connected to each other, thus creating “virtual probes”. In this way, virtual probes can be created with different arrangements of interconnected individual transducers, which then impinge on the weld spot in a focused manner and/or also at an angle. This then results in a high number of real ultrasonic tests with a high resolution in the test field.
In the case of PHAsis, this means that 729 virtual probes are used, with which the area of the probe array can be scanned in raster form. The individual raster points (resolution approx. every 0.35 mm an A-scan) in their entirety then produce the result image, which can be used to automatically determine the lens diameter, the residual wall thickness and the pore quality.
The imaging of the weld spot enables a reliable evaluation of the entire weld lens, and the necessary training effort for the inspector can be greatly reduced. Subjectivity is reduced to the requirement to properly position the inspection head on the spot weld.
Weld spot inspection can be performed manually with ultrasonic phased array technology or (fully) automated with a cobot or robot. By using robots, existing inspection personnel can be supported, enabling cost-effective comprehensive inspection. Such a robot supporting system is e.g. PHAsisBLU.
Process monitoring
With online welding process monitoring, the quality of the spot welds is checked during or immediately after completion of the welding process. This allows quality deviations to be detected quickly and any necessary corrective measures to be initiated immediately.
On the part of the welding system, the physical measured variables that are decisive for the weld spot quality, such as welding current, voltage, power combined with the dynamic resistance as well as the welding current time, are monitored (adaptive welding control).
In addition to the monitoring of the welding system by the welding parameters, which significantly improves the quality of the spot welds, but does not represent a quality monitoring of spot welds like a supplementary non-destructive testing of spot welds, there is the possibility of ultrasonic testing of the spot weld still during welding or also subsequently in the process, where automated by ultrasound with robot in combination with manual testing by inspectors represents a reasonable solution.
In inline inspection, the electrode shafts (near caps) of the welding robot are each equipped with an ultrasonic sensor. Ultrasonic testing then takes place during welding from the start of welding to the end. During this time, the changes in the ultrasonic amplitudes and transit times of a through-sound signal are recorded and evaluated. The results are transmitted directly to the welding and robot control system. This makes it possible to adjust the welding parameters at very short notice. It can also be used to optimize the cycles for cap milling. One such inline testing system is SPOTline. It provides a control loop that determines trends in spot weld quality and monitors the welding process.
The disadvantage of this method is the required space of approx. 30 mm straight shank as minimum length of each welding electrode, where the test head is installed.
influences
How can the quality of a spot weld be influenced?
The quality of a bonded joints can be influenced by various external disturbance variables.
Disturbance variables from production
- Foreign bodies in the manufacturing environment (e.g. copper particles, oil, dust, adhesives, etc.) can be trapped in the weld and thus negatively affect the structure and formation of the weld lens.
- Mains fluctuations can affect the electrons and thus the force of heat input. This can lead to a deviation of the melt and thus weld spot quality.
- Shunt: In the case of spot welds set too close together or weld spots at geometrically complex positions on the component, the welding current may be conducted extraneously via this already welded spot or due to the geometry, as it chooses the path of least resistance. As a result, less heat is generated at the point to be welded, which negatively affects the molten metal. This phenomenon is called shunt.
- Welding robot failure due to loosened joints or also material breakage.
Disturbance variables from the welding system
Welding equipment requires regular maintenance to ensure a continuously uniform welding process. This includes monitoring the electrode condition, electrode cooling condition, electrode force, welding time and welding current. A change in these components has a direct impact on spot weld quality. For example, wear of the electrodes leads to a reduction in current density and thus to lower heat generation for the formation of the necessary melt of the weld lens.
Interference from the material
This includes deviations in material properties such as wall thickness, strength, the microstructure of the material (e.g. in hot-formed steels) and the property of the surface coating (zinc layer thickness, composition and layer thickness of the aluminum oxide layer in Al sheets).
your spot weld expert
Advantages of VOGT Ultrasonics inspection systems
VOGT Ultrasonics has over 25 years ( status 2020) of experience in ultrasonic testing of spot welds in industry, especially automotive. Therefore, we know the challenges and requirements of spot weld inspection very well. We have invested this experience in research and development and offer our customers weld spot inspection systems, especially for fast and uncomplicated use in production.
The non-destructive ultrasonic spot weld inspection is time and cost saving and can be integrated in the production line for e.g. critical or difficult to access spot welds. We offer inspection systems for manual, robot-controlled offline and inline inspection already during welding.
Application possibilities
Our PHAsis spot weld inspection systems are suitable for the inspection of resistance welded spots of steel, stainless steel and aluminum sheets and the precise determination of the lens diameter. Testable are 2- and 3-sheet joints with a single sheet thickness from 0.6 to 5 mm. Due to the high test speed, the high resolution in determining the welding lens diameter, the automatic result suggestion, the automatic documentation and simple operation, it is ideally suited for use in production and in the laboratory.
The device also offers excellent conditions for use in conjunction with an inspection robot. Automated ultrasonic spot weld inspection can be performed safely using the appropriate interfaces and tools provided.
VOGT Ultrasonics has over 25 years ( status 2020) of experience in ultrasonic testing of spot welds in industry, especially automotive. Therefore, we know the challenges and requirements of spot weld inspection very well. We have invested this experience in research and development and offer our customers weld spot inspection systems, especially for fast and uncomplicated use in production.
Efficiency increase
To increase efficiency and inspection reliability, the PHAsis inspection systems offer two inspection modes: inspection according to inspection plan and free inspection.
In the free inspection mode, the ultrasonic inspector can quickly inspect different weld spots with just a few clicks (e.g. selection of the plate thickness combination to be inspected). If required, the results of the free inspection can then also be transferred directly to an inspection plan.
In the Test according to test plan mode, the test engineer processes predefined and visually displayed test plans according to specifications. The test plans were previously created by the test supervisor. The advantage of inspection plans is the elimination of manual settings, so that even inspection personnel without in-depth ultrasonic knowledge can perform inspections quickly and reliably. The training time is approx. 2-4 hours.
With our PHAsisBLU inspection system, we also enable an increase in efficiency through robot-assisted spot weld inspection. The use of robot technology allows not only a reduction of the inspection time but also an optimization of the reproducibility of the measurements. Here, cycle times of 5- 15 seconds per spot weld can be achieved, depending on the spot surface.
Even a cobot can be used to increase efficiency. The cobot can be easily trained by an experienced inspector or an inspection supervisor and used in a mobile manner. Cobots can make spot weld inspection less expensive by having an inspection technician use 2-3 cobots to inspect components instead of manually using one device. Besides many advantages, there is one disadvantage to consider, cobots do not have such a large range, i.e. the components should be of a smaller nature.
Quality documentation and process monitoring
PHAsis test systems create an automatic and tamper-proof documentation of the test results. With the possibility to save all A-scans of the complete test area, a subsequent re-evaluation and correlation to the destructive test is possible. In the management software, this test data is organized centrally and the test equipment is monitored.
Another major advantage of PHAsis inspection systems is the consolidation of inspection data from manual and automated inspection. This means that all information is in one place and can be evaluated centrally and made available for central documentation.
Display of the inspection results
When inspecting with conventional ultrasonic technology, the weld spot is evaluated on the basis of a single A-scan, the echo sequence from the joined and/or non-joined area of the joint. The correct probe for the respective thinnest sheet thickness must be selected by the inspector beforehand, which corresponds approximately to the minimum weld spot diameter in terms of its transducer size. This is defined, for example, according to a formula: Minimum weld spot diameter = transducer diameter = 4 root t or also 3.5 root t ( with t = thinnest plate of the joint). Some inspection systems additionally offer a valuation suggestion. Despite this, this classic ultrasonic inspection requires a great deal of practical and theoretical experience on the part of the trained ultrasonic inspector.
The imaging phased array ultrasonic technology of our PHAsis inspection systems, on the other hand, enables reliable spot weld inspection without in-depth ultrasonic knowledge.
Using this technology, an image of the spot weld is generated in less than 10 seconds with a color depth representation. This image allows not only the evaluation of the lens diameter but also the evaluation of pores at different depths as well as the evaluation of the smallest lens diameter of a 3-sheet joint when inspecting from only one side.
Fig.: Test result display (C-/D-scan) of a good welding lens
In the figure, welded areas are shown in green and non-welded areas in red. The inspection system automatically determines the lens diameter and the residual wall thickness as the average value of the welded area and displays these values as a TARGET / ACTUAL comparison. An evaluation suggestion is also automatically output with the created C-/D-image.
The imaging phased array ultrasonic technology extremely simplifies the spot weld inspection and increases the inspection frequency and safety.
Limitations
Are there spot weld defects that are not nondestructively testable?
So-called “zinc glues” can only be tested non-destructively to a limited extent due to the physics of ultrasound. “Glues” first appeared in the past during welding of zinc-coated sheets.
The selected welding parameters do not result in a weld but rather in a soldering or “bonding”. This has the disadvantage that, due to the already interlocked grain boundaries, sound transmission is well enabled. Thus, these “adhesives” show a good echo sequence and at first glance do not differ from a good weld.
In the worst case, the tools provided by the ultrasonic device to detect such adhesives are not effective. Then one must bow to physics and deny testability or change the welding parameters so that testability and detection of such “adhesives” is restored. This requires good cooperation between those responsible for welding and testing technology.
The PHAsis ultrasonic testers have gathered all the evaluation methods for such adhesives available on the market in different devices in one device. These are made available to the operator for the evaluation of weld spots as individual or combined procedures.
Thus PHAsis offers the following possibilities for adhesive detection, individually or in combination:
With all of the following features, there is generally always a good echo sequence of the weld spot, but this is then overruled to an adhesive evaluation by one or more of the following features.
- Several false echoes, which have a certain height, indicate the adhesive, adhesive threshold.
- The sound attenuation of an adhesive in relation to a good point differs. Due to the structural transformation in a good weld to a coarse grain, the sound weakening is greater there.
- The number of repeat echoes of an adhesive is much larger than in a good weld because the backwall echoes are not weakened as much due to the lack of microstructural transformation.
- There is an adhesive because the indentation is too low, i.e. the spot weld does not meet the minimum indentation requirement (more commonly used as a phenomenon with aluminum spot weld
if there are still questions
FAQ
There is no official test standard (DIN/ ISO) that requires nondestructive testing. These exist only for destructive testing. However, there are in-house specifications and regulations that define the ultrasonic testing of weld point connections.
There is also a DVS leaflet DVS 2916-5 from 9/2017 – Testing of resistance pressure welded joints – non-destructive testing of spot welded joints.
There is no official test standard (DIN/ ISO) that requires nondestructive testing. These exist only for destructive testing.
Destructive testing: According to the standard, the lens diameters must be documented destructively at a fixed interval on the assemblies or on the complete body.
Conventional ultrasonic or imaging inspection: The trend is clearly moving in the direction of ultrasonic imaging inspection, because this inspection technique allows the entire joint (lens) to be evaluated much more reliably and accurately.
