Weld Inspection Methods and Technology

In an industry where data is becoming increasingly important and working toward more efficient production is the main goal, weld inspection technology and methods start to play a large role.  In this eBook, our team of experts break down the numerous types of weld inspection that exist today, beyond the standard form of visual inspections. 

The ability to automate this process has been an integral part in seeing key parts of welding become easier and more efficient. Facilities are able to step away from tedious tasks, like inspection, and focus on how to continue improving efficiency and productivity, while also closing the gap on mistakes made and wasted spend. 

Concept - A sensor is mounted near the torch typically utilizing the concept of laser triangulation to determine the location or gauge the profile of the joint. This information is processed by a controller to determine its relevance and is output to the motion controller (robot, gantry, other) to provide an offset from the planned or taught path to ensure the energy of the process is in the right place at the right time.

Advantages - Ability to keep weld on seam when joint is not repeatable, helps maintain weld quality, reduces weld repair.

Disadvantages - Increases size of torch, limited reach in tight spaces, can increase cycle time.

Concept - Utilizing a cut and etch to ensure weld meets process characteristics.

Advantages - Provides clear indication (at point of cut) the nature of the resulting weld with indications towards penetration depth, throat, leg lengths, etc.

Disadvantages - Destroys the part, small cross-sectional area inspected, results are cut point are not always a sum of the whole. Costly, time consuming.

Concept - Ultrasonic testing technology is a method of detecting discontinuities by directing a high-frequency sound beam through the base plate and weld on a predictable path utilizing high-frequency oscillations (about 20,000 Hz) to propagate into the metal. Waves enter into the tested material and when defects are detected, the signal deviates from its normal propagation.

From the graphical and parametric readings, the nature of the defect can be determined. Both surface and subsurface defects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be detected by other methods.

Advantages - Can find defects under weld surface, portable equipment, can be used on many different weld joints, lower cost for equipment, nondestructive testing process.

Disadvantages - Highly dependent on qualified operators who are expensive to employ, expensive training costs, surface defects still require visual inspection, amost impossible to automate, requires secondary process outside of line.

Concept - X-ray is used to determine the internal soundness of welds. Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. X-rays and gamma rays can be used to show anomalies and inclusions within the opaque material as all materials will absorb a known amount of radiant energy.

The fitness of the weld can be determined by inspection of the permanent film record of the internal conditions. The term “X-ray quality“ widely used to indicate high quality in welds, arises from this inspection method.

Advantages - Can find surface defects and sub-surface defects, can be used on many different weld joints, nondestructive testing process, can be automated.

Disadvantages - Highly dependent on qualified operators, high cost of equipment, qualified operators are very expensive to employ, radiation safety concerns, slow process speed cannot be used on 100% of product, only a 2-dimensional picture of the weld and defects.

Concept - Surface cracks and pinholes that are not visible to the naked eye can be located by liquid penetrant inspection. Two types of penetrating liquids are used - fluorescent and visible dye. With fluorescent penetrant inspection, a highly fluorescent liquid with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the liquid into the surface openings, and the excess is then removed.

A “developer“ is used to draw the penetrant to the surface, and the resulting indication is viewed by ultraviolet (black) light. The high contrast between the fluorescent material and the object makes it possible to detect minute traces of penetrant that indicate surface defects. It is widely used to locate cracks in welds and can be applied to both austenitic steels and nonferrous materials where magnetic particle inspection would be useless.

Advantages - Very easy to find surface defects, portable equipment, can be used on many different weld joints, lower cost for equipment, nondestructive testing process.

Disadvantages - Highly dependent on qualified operators, high training costs, qualified operators are very expensive to employ, requires secondary process outside of line, cannot automate, slow speed cannot perform on 100% of product.

Concept - Magnetic particle inspection is a method of locating and defining discontinuities in welded materials that have magnetic properties. It is excellent for detecting surface defects in welds along with those that are slightly sub surface. In this inspection approach, probes are usually placed on each side of the area to be inspected with a high amperage being passed through the work piece between the probes.

A resulting magnetic flux is produced at right angles to the flow of current. When these lines of force encounter a discontinuity, such as a longitudinal crack, they are diverted and move back through the surface, creating magnetic poles or points of attraction. A magnetic powder is utilized such that when dusted onto the surface, it will cling to the leakage area stronger than elsewhere, thus providing an indication that a discontinuity exists.

Advantages - Magnetic particles can be applied wet or dry, very easy to find surface defects, portable equipment, can be used on many different weld joints, lower cost for equipment, nondestructive testing process.

Disadvantages - Limited to use with ferromagnetic particles, sensitivity decreased with size of defect, highly dependent on qualified operators, high training costs, qualified operators are very expensive to employ, cannot automate, slow speed cannot inspect 100% of product.

Concept - Utilize a sensor-based technology, typically in unison with algorithm-based inspection protocol to visualize a weld in a manner that will allow it to be measured and compared against a known good weld wherein various attributes / anomalies are measured against specifications to determine which facets of the weld meet or don’t meet the stated requirements.

The sensor-based technology will normally be either a vision camera-based approach where pixilation of images are reviewed or via laser triangulation which consists of a diode set emitting light onto a part which reflects to a chip in the sensor that transmits the reflection to an image. The images are then knit together to create a 3D model of the part scanned.

Advantages - High scan speeds, definable limits for process related inspection, provides objective view, robust

Disadvantages - Cost of equipment, technically complex, surface inspection only

There are large volumes of work in and around 3D machine vision and it’s features and benefits as they can provide including anything from machine control to inspection. The intention here is to note that it is a viable solution in this case, however machine vision has its strengths and weaknesses like most technological solutions. Much the same can be said about laser triangulation-based processes as well, which we will turn our attention to.

With respect to laser triangulation, it is a method utilized by multiple sensor types as it relates to guidance and inspection processes. The basis of laser triangulation is one based around the concept of reflected and absorbed light.

With an inspection system, the main components associated with it would be the sensor, its related controller, and a software set that operates on the controller and / or PC based product. There are typically multiple versions of sensors that would be offered from any provider that would best support the needs of the process. Sensors will often vary by key variables such as field of view, standoff, and resolution. The resolution of the sensor will often be a key factor in the size of defect that you are able to pick up in conjunction with the speed of the process. Low resolution plus high travel speed equals picking up big defects only.

So, understanding the limits of what are critical to the performance of your product is essential to know early on in the product selection phase. By not having clear understanding on requirements, you stand the chance of missing defects you do not want or you are unable to meet cycle time due to having to run slower scan speeds than desired. For reference, a standard sensor for inspection of MIG welds will run in the range of 150 – 200 mm/sec, however are capable as well in speeds up to 400 mm/sec and resolutions can go down into the range of single digit microns (μm).

The software associated with the inspection packages are multi-faceted in that you need different tools for different purposes. Software located in the controller is utilized to support translation of data coming from the sensor into a usable form, algorithm-based protocols around setting up of weld boundaries and attribute measurements. There are also communication related aspects of the controller, which helps facilitate data transfer to and from the other system components.

Often, the controller will also provide an embedded PLC to support further transfer of related data. Additional software packages that may be available as well could include:

• PC remote access platform for direct link to controller enabling interface and communication across multiple applications
• Visualization packages utilized at repair stations displaying images of the part being inspected and OK / NOK notifications for each weld. Allows for direct interact at repair station to verify part has been repaired and OK to move on for further processing.
• Data analysis packages that can correlate data around specific welds or parts such that management teams can review and address root cause and corrective actions to improve part operations. Reports could include items such as Pareto charts and other graphical means for displaying data for ease of analysis.

Truck Frames

A perfect application where there are numerous welds that are safety critical in nature and range over a part that is hard to manipulate manually, is heavily automated in its welding, and often possesses close out welds that cannot be inspected unless done at the time they are welded as parts are placed overtop a welded area to complete the build.

Due to the safety critical aspect of the parts, OEMs are beginning to require data records around weld quality for tracking purposes and help ensure the safest and highest quality product for their end customer.

Automating the inspection process will help insure weld quality, provide an opportunity to have less need for destructive testing of welds, lower scrap costs for testing, and possibly provide less post OP repair after the frame has left the production line.

Cradle and Structural Parts Inspection

Much the same metrics apply to these as in the first couple of examples. The process is suitable for anything from engine cradles to sub frames to steering components to inner cross members.

Products such as instrument panels (IP’s) fall well into this category due in part to the large number of welds on these assemblies (sometimes in excess of 200 on one part). That, in addition to the short lengths and often safety critical aspects of these parts make it an ideal application for inspection.

The ability to utilize compact sensors along with capabilities to address steel, aluminum, and other materials opens up various possibilities.

Battery Boxes and EV Related Products

With the majority of all OEMs building electric vehicles, the battery box is a required assembly. The need arises for 100% quality inspection, as these assemblies are extremely difficult to produce and ensure the highest level of safety is present.

They often utilize a combination of laser and MIG based processes for joining and need to be hermetically sealed to ensure components are free from moisture to ensure safety and reliability.

The technology can be applied as well to other segments of the EV product class such as battery cells and welded terminals

Having an automated solution where compact sensors can access tough to reach areas for reliable weld inspection provides a wealth of benefits that can help guarantee quality, provide traceable data, and reduce the chance of quality related issues down the line.

Reduction in Scrap – there are various ways in which you can reduce scrap with automa- ted inspection such as inspection of close out welds such that you don’t need to cut pieces off to repair a weld that was bad and then covered with another piece. If inspection was in place, the part could have been repaired properly before any other value was added. Cases where major burn through have occurred, but were not caught before other items were added to it or components were misassembled leading to a bad assembly. In line detection can find these defects before the assembly is finished as you don’t need to scrap an entire assembly and a sub component can be kicked out of the line instead of being blindly sent on.

Reduction of risk / liability related to recalls or quality spills – this in turn might be the biggest reduction of all. With the traceability aspect and the data collection capabilities, this can prove to be a business saving move in the end. Application of automated inspection systems in conjunction with good process control enables a much higher chance of ensuring quality product leaves your door, a customer base who trusts that they are getting world class quality, and the long-term prospect of greater ability to garner new work due in part to an enhanced reputation for providing quality parts.

As a first response, you place all of your parts in a quality hold situation to allow for additional inspection past that done in process to verify the quality of the parts on hand. Labor is redirected to inspect product at your facility, which will require multiple shifts to complete and roughly 4 people per shift to cleanly address.

Labor is jointly sent to the customer to inspect all parts they have on hand and repair on site what can be repaired so as not to shut down production. From this point you will need to do deep dive into the issue to determine root cause (8D, 5 Why, or other reports that take up additional time and resources) placing additional strain on your engineering, tool and die, and maintenance staff to ensure all facets for corrective actions are covered.

Luckily, being this is a generally non-recurring experience, you are let off lightly from the customer as you were quick to respond, didn’t directly affect production, and they appreciate the effort. Everyone moved past it and went back to status quo.

Then it happens again 3 months later.

The topic of recall has not been broached yet either as this will likely add another zero leading to costs in the tens of millions, massive fines, and possible litigation from not only your customer, but the end users of their products. Granted, there are typically contingencies in place to address potential quality issues during the length of a program, but not typically to the point where costs are in the tens to hundreds of millions.

The occurrence of the spills and bad light shining upon your organization due to the inability to control the quality of parts leaving your facility has not only cost a great deal of money as it pertains to parts and resources, but it has also hit your reputation. Your supplier rating has plummeted because of these continued issues, potential for future business had taken a hit, and this poor reputation may spread to other customers in the industry.