Non-Destructive Testing Inspection for Metal Components Produced Using Wire and Arc Additive Manufacturing
Abstract
:1. Introduction
2. Defects in WAAM: Causes, Characteristics, and Consequences
2.1. Porosity
- Improper process parameters [5,6]: If the deposition current or voltage is too high, the wire electrode can melt too quickly, leading to poor fusion and porosity defects. On the other hand, if the welding current or voltage is too low, the wire electrode may not melt completely, resulting in poor material deposition and porosity defects.
- Inadequate wire feeding [7]: If the wire feeder is not properly adjusted, the wire electrode may not be fed into the welding torch at the correct rate, resulting in poor material deposition and porosity defects.
- Improper cooling of the deposited material [6]: If the printed part is not cooled quickly enough, the material may not solidify properly, resulting in porosity defects.
- Contamination [9]: If the wire electrode or the workpiece is contaminated with oil, dirt, or other impurities, it can cause porosity defects.
- Poor wire quality [6]: Using a low-quality wire electrode that contains impurities or has an inconsistent diameter can cause porosity defects.
- Inadequate cleaning [9]: Lack of cleaning of the welding torch and the workpiece can cause porosity defects.
- Reduced strength and durability [3]: Porosity defects can weaken the structural integrity of the printed part, making it less durable and more susceptible to failure under load. This can be particularly problematic for parts that are intended to be used in high-stress or high-load applications.
- Reduced corrosion resistance [3]: Porosity defects can also make the printed part more susceptible to corrosion, as the voids and holes in the material provide a pathway for moisture and other corrosive agents to penetrate the part.
2.2. Inclusion
- Contaminated wire feedstock [12]: If the wire used in WAAM contains impurities such as oils, lubricants, or other contaminants, they can be incorporated into the final product and result in inclusion defects.
- Improper cleaning of the equipment [12]: If the wire feeder or the welding torch is not properly cleaned before use, debris can accumulate and be incorporated into the final product, resulting in inclusion defects.
- Excessive spatter during the WAAM process [11]: Excessive spatter can result in the inclusion of unwanted materials in the final product.
- Improper layer-to-layer adhesion [12]: If the layers of the final product do not properly adhere to one another, inclusion defects can occur.
- Inadequate post-processing [13]: Insufficient cleaning or heat treatment can result in inclusion defects.
- Reduced mechanical properties [10]: Inclusion defects can result in a reduction in the product’s strength, ductility, and toughness, which can make the final product less suitable for its intended use.
- Increased risk of failure [10]: Inclusion defects can result in a higher risk of failure during use, which can lead to costly repairs or replacements.
2.3. Crack
- Porosity [3]: Porosity is caused by trapped gases or other contaminants that are present in the weld pool. As the metal solidifies, these trapped gases can cause the material to form small voids or bubbles, which can lead to weak spots in the final product and cracks.
- Reduced structural integrity [3]: Cracks in the final product can weaken the structure and make it more susceptible to failure under load. This can be particularly problematic for components that are subjected to high stress or impact.
- Reduced fatigue resistance [16]: Cracks in the final product can also reduce its resistance to fatigue, which is the ability of a material to withstand repeated loading and unloading cycles. This can be a problem for components that are subjected to cyclic loading, such as gears or shafts.
- Reduced safety: if the product is used in a safety-critical application, crack defects can lead to serious safety hazards.
2.4. Burn-Through
- Process parameters [5,12]: A high current can cause the wire electrode to melt more quickly, which can increase the risk of burn-through. A Low wire feed speed can cause the wire electrode to be in contact with the bottom of the part for a longer period of time, which can increase the risk of burn-through. In addition, if the metal deposition is not sufficient, it can cause the part to be structurally weak, leading to burn-through. A high travel speed can cause the wire electrode to be in contact with the bottom of the part for a shorter period of time, which can increase the risk of burn-through.
- Inadequate shielding gas [12]: The shielding gas plays an important role in protecting the arc from the ambient air. If the shielding gas is inadequate, it can cause the arc to become unstable and increase the risk of burn-through.
- Reduced part quality: Burn-through can result in a hole in the finished product, reducing its overall quality and structural integrity.
- Increased production costs: If burn-through occurs, it may require the part to be scrapped, which can increase production costs. Additionally, if the burn-through is not noticed until later in the manufacturing process, it can result in additional costs from having to redo work that has already been completed.
- Delays in production: Burn-through can cause delays in production as the part needs to be scrapped and reworked or remade.
2.5. Undercutting
- Process-related causes [5,12]: These include improper control of the electric arc, which can lead to an insufficient amount of heat being delivered to the wire, resulting in the metal not fully melting and sticking to the previous layer. In addition, improper wire feeding, such as an inconsistent wire-feeding rate or angle, can also lead to undercutting.
- Geometry-related causes [17]: The complex geometry of some parts can also contribute to undercutting. For example, overhangs, sharp corners or narrow features can pose a challenge for the WAAM process, as the molten metal may not fully adhere to these areas, resulting in undercutting.
- Reduced strength [17]: Undercutting can weaken the component, making it less durable and more susceptible to failure under stress.
- Decreased surface quality [12]: Undercutting can create gaps or holes in the surface of the component, which can negatively impact the overall surface finish and aesthetic appearance of the part.
2.6. Spatter
- Insufficient shielding gas [19]: In WAAM processes, a shielding gas is used to protect the weld pool from the surrounding atmosphere. Insufficient or the wrong type of shielding gas can lead to the formation of spatter.
- Improper technique [20]: Improper deposition technique can also cause spatter defects. For example, if the torch is held too close to the workpiece, it can cause an increase in spatter formation.
- Reduced surface quality: Spatter can stick to the surface of the component and create unsightly defects, reducing the overall surface quality of the finished product.
- Reduced strength and durability: Spatter can also reduce the overall strength and durability of the finished product by creating weak spots or porous areas in the metal.
- Reduced functionality: Spatter defects can make the finished product less suitable for use in certain applications, such as those that require high precision and quality.
2.7. Collapse
- Improper welding parameters [18]: If the welding parameters such as the welding speed, wire feed rate, and current are not set correctly, it can cause the molten material to cool and solidify too quickly, resulting in collapse.
- Poor design [18]: Certain shapes and geometries may be more prone to collapse than others. If the design of the structure is not optimized for WAAM, it can increase the likelihood of collapse defects.
- Inadequate overlap [18]: Inadequate overlap of the welding gun can also cause collapse defects.
- Reduced mechanical properties: Collapse defects can lead to a reduction in the strength and stiffness of the final product, which can make it less suitable for its intended application.
- Increased surface roughness: Collapse defects can also lead to increased surface roughness, which can make it more difficult to achieve a smooth finish and can make it more difficult to achieve good adhesion in further processing or coating.
2.8. Unmelted Wire
- Wire feed rate [21]: If the wire feed rate is too high, the arc may not have enough time to melt the wire before it is deposited on the substrate, resulting in unmelted wire segments or droplets.
- Arc power [18]: If the arc power is not sufficient, the wire may not be heated enough to melt completely, resulting in unmelted wire defects.
- Wire-to-arc distance [21]: If the distance between the wire and the arc is too much, the wire may not be heated enough to melt properly, resulting in unmelted wire defects. In addition, the composition of the wire being used can also play a role in the formation of unmelted wire defects. Some wire types may be more prone to forming unmelted wire segments or droplets due to their chemical composition, size, or shape.
- Welding conditions [18]: The welding conditions, such as the size and shape of the welding torch, the position of the torch, and the type of shielding gas used, can also affect the formation of unmelted wire defects.
- Reduced strength and durability [18]: Unmelted wire defects can result in a weaker and less durable finished product, as the defects can act as stress concentrators that can lead to premature failure.
- Reduced accuracy: Unmelted wire defects can affect the accuracy of the finished product, as the defects can result in variation in the thickness or shape of the deposited layers.
- Increased material consumption: Unmelted wire defects can lead to increased material consumption, as more wire may need to be used to achieve the desired thickness or shape of the finished product.
2.9. Delamination
- Incorrect wire-feeding speed [20]: If the wire-feeding speed is too fast or too slow, it can result in delamination.
- Low heat input [5]: Low heat input during the welding process can result in inadequate welding and a low bond strength between the metal layers, leading to delamination.
- Improper cooling [22]: Improper cooling during the WAAM process can result in residual stresses and a decreased bond strength, leading to delamination.
- Poor process control [5]: If the process parameters are not controlled and monitored correctly, it can result in delamination.
- Material properties [3]: The properties of the metal being used can also affect the likelihood of delamination, including the composition, purity, and microstructure of the metal.
- Reduced strength and durability [23]: Delamination can result in reduced strength and durability of the final product, making it more likely to fail during use.
- Increased risk of failure [3]: The presence of delamination increases the likelihood of failure during the use and life of the part, leading to significant financial and safety consequences.
2.10. Uneven Bead Height
- Variation in welding parameters [5]: An uneven bead height can occur if the welding parameters, such as the wire feed rate and arc current, are not properly controlled. Small variations in these parameters can result in variation in the size and shape of the deposited beads, leading to an uneven surface finish.
- Wire feeder issues [5]: If the wire feeder is not working properly or is not adjusted correctly, it can cause variation in the wire feed rate, leading to an uneven bead height.
- Robot arm issues: The robotic arm used to guide the wire during the WAAM process can also cause an uneven bead height if it is not functioning properly or is not calibrated correctly.
- Improper weld-path planning [17]: If the weld path is not planned properly, it can cause unevenness in the bead height, as the robot arm may not be able to deposit the material in a consistent manner.
- Non-uniform surface finish [17]: An uneven bead height can result in a non-uniform surface finish, which can affect the aesthetic appearance of the built part.
- Reduced accuracy: An uneven bead height can also affect the dimensional accuracy of the built part, as the variation in the bead height can result in deviations from the intended shape and size of the part.
- Increased material waste: An uneven bead height can also lead to increased material waste, as more material may be required to correct the defect or rework the part.
2.11. Geometric
- Overlap [13]: Overlap is a geometric defect that occurs when the deposited material builds up on top of itself, forming a thick layer that can weaken the part. This defect can occur due to incorrect process parameters or poor part design.
- Misalignment [24]: Misalignment occurs when the deposited material is not properly aligned with the part, causing the part to be out of shape or not fit properly. This defect can occur due to improper wire feeding or incorrect process parameters.
- Distortion can happen in many different forms. One of its ways is shown in Figure 10.
- Reduced strength and integrity of the part: Voids, porosity, and incomplete fusion can weaken the overall integrity of the part and make it more prone to failure.
- Functional failure: Undercuts, misalignment, and warping can cause the part to not fit properly or function as intended.
- Reduced accuracy: Distortion can cause the part to be out of shape [26], reducing its accuracy and making it difficult to use.
- Reduced aesthetic quality: Warping and distortion can cause the part not to have a smooth finish and can affect the overall aesthetic quality of the part.
2.12. Overview of WAAM Defects
3. Non-Destructive Testing Methods
3.1. Phased-Array Ultrasonic Testing
3.2. Electromagnetic Acoustic Transductor
- It allows for real-time monitoring of the weld quality, which can improve the overall efficiency of the WAAM process [31].
- Monitoring the weld pool and wire feed in order to detect and correct any issues before they become major problems [44].
3.3. Laser-Ultrasonic Testing
3.4. Real-Time Radiography
3.5. X-ray Backscatter
3.6. Eddy Current
3.7. Infrared Thermography
3.8. Laser Thermography
3.9. Others
3.10. Overview of NDT in WAAM
4. Challenges and Solutions: Eddy Current Case Study
4.1. Why In-Line?
4.2. ECT Advantages—Suitability to In-Line WAAM Inspection
4.3. ECT Disadvantages—Suitability to In-Line WAAM Inspection
4.4. ECT Probe Considerations for WAAM Parts
- Absolute mode: in this mode, the same coil operates as an exciter and sensor simultaneously. The probe typically consists of one or more coils, which are placed in close proximity to the part surface. It is often used for surface inspections because it is relatively simple and easy to implement, and it can provide good sensitivity for surface-breaking defects. However, absolute mode has some limitations: it is less sensitive to subsurface defects, and it is also sensitive to the surface roughness, waviness, and other external factors. Therefore, it should be avoided for the inspection of WAAM parts.
- Reflection mode: at least two coils are used, one to generate eddy currents in the part and one to measure the response. The excitation typically consists of one or more coils, which are placed in close proximity to the surface of the part. The detection coil, also known as the pick-up coil, is placed on the same side of the part as the excitation probe, but at a different location. It is used to measure the response generated. It is sensitive to surface-breaking defects, but it is less sensitive to subsurface defects. It is also less sensitive to the surface roughness, waviness, and other external factors than absolute mode. Therefore, it is often used in combination with other operation modes, such as differential mode, to improve the reliability of the inspection and increase the detection of subsurface defects.
- Differential mode: the response is measured between two or more coils while the excitation can be performed by the same coils or others. The differential mode can be used to reject unwanted signals and noise and to improve the sensitivity of the inspection. It can also help to locate subsurface defects that might be missed using absolute or reflection mode alone. Differential mode can be combined with reflection mode. It uses one or more coils exclusively for the excitation and two or more coils to measure the differential response between two points on the surface of the part. This mode can improve the accuracy and sensitivity of the inspection by rejecting unwanted signals and noise and maintaining its sensitivity to surface-breaking defects. It is important to note that, while differential mode can provide a more accurate and sensitive inspection, it may be more complex to implement than other operation modes.
4.5. Eddy Current Probe Design
4.6. Customized Probe Production and Experimental Results
4.7. Hot-Forging WAAM Variant
5. Discussion
6. Conclusions
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Defect | Characterization | Causes | Consequences |
---|---|---|---|
Porosity | Small voids or holes in the final printed part. | Improper welding parameters, poor wire feeding, inadequate shielding gas, and/or contamination. | Reduced strength, durability, and corrosion resistance. |
Inclusion | Particles such as oxides or other impurities that are trapped in the weld metal. | Contaminated wire feedstock and wire feeder or the welding torch not properly cleaned. | Reduction in the product’s strength, ductility, and toughness. |
Crack | Fissures that grows along the weld bead. | High thermal stress, porosity, poor wire surface, inadequate process parameters, material selection, and/or post-processing. | Reduced structural integrity, fatigue resistance, and safety. |
Burn-through | Excessive heat input that melts through the base metal, creating a hole in the material. | High current, low wire feed speed, high travel speed, improper alignment, and/or inadequate shielding gas. | Reduced part quality, increased production cost, and delays in production. |
Undercutting | A groove that is melted along the edges of the base metal. | Improper control of the electric arc, improper wire feeding, material properties, and/or complex geometry. | Reduced strength, decreased surface quality, and reduced accuracy. |
Spatter | Small droplets of weld metal that are expelled from the weld during the WAAM process. | Improper welding wire, incorrect welding parameters, insufficient shielding gas, dirty workpiece, lack of proper maintenance, and/or improper technique. | Reduced surface quality, strength, durability, and functionality. |
Collapse | The molten material cools and solidifies too quickly, causing the structure to collapse in on itself. | Improper welding parameters, poor design, poor material choice, inadequate overlap and/or stress concentration. | Reduced mechanical properties, increased porosity and surface roughness. |
Unmelted wire | Incomplete melting of the wire metal. | Wrong wire feed rate, insufficient arc power, great wire-to-arc distance, wire composition, welding conditions, and/or substrate preparation. | Surface defects, reduced accuracy, increased material consumption, and reduced strength and durability. |
Delamination | Metal layers separate from one another. | Incorrect wire-feeding speed, low heat input, high arc voltage, improper cooling, inadequate preparation of surface, poor process control, and/or material properties. | Reduced strength and durability, increased risk of failure, and decreased reliability. |
Uneven bead height | Unevenness in the height of the weld beads produced. | Wire feeder or robot arm issues, surface conditions, improper weld path planning, and/or material properties. | Non-uniform surface finish, increased stress concentration, reduced accuracy, and increased material waste. |
Geometric | It is a group of defects that affects the geometric aspect of the part. For example, overlap, distortion, and misalignment. | Improper wire feeding, incorrect process parameters, poor part design, poor surface preparation, improper shielding, lack of process monitoring, material process, and/or environmental conditions. | Reduced strength and integrity, functional failure, reduced accuracy, and reduced aesthetic quality. |
Non-Destructive Test | Advantages | Limitations |
---|---|---|
Phased-Array Ultrasonic Testing | Has the capacity to detect, locate, and measure the flaw, increased inspection speed, real-time imaging, precise, able to penetrate thick sections [1,31]. | Suitable for defects bigger than 600 μm [32], cannot work at high temperatures, requires coupling, may require several probes [1,31], defect direction influences the results [48]. |
Electromagnetic Acoustic Transductor | Can be used for defect detection, location, and measurement [1], can be used for surface and subsurface flaw detection, contactless and couplant-independent but requires proximity, suitable for high temperatures [1]. | More accurate for identifying defects larger than 500 μm [32], hard to inspect complex shapes [31], low sensibility for small defects [1], clean and smooth enough surface required [48]. |
Laser-Ultrasonic Testing | LUT can be used for flaw detection, location, and measurement [1], also can be used at very high temperatures, complex geometries, areas of difficult access, contactless and couplant-independent [1,31]. | Smallest flaw detected size is 400 μm [32], more expensive than ultrasonic testing, optical detection techniques generally offer lower sensitivity than contact transducers, knowledge of the internal geometry of the flaw is required in order to obtain an accurate depth profile [31]. |
Real-Time Radiography | Digital data are generated during X-ray inspection [32], reliability and no special specimen preparation is required [31], detects deep and embedded defects [1]. | RTR flaw size detection is limited to those larger than 250 μm, in the case of the use of the microfocus mode it is possible to identify flaws with more than 50 μm size [48], difficulties in detection may be associated with the angle between the crack and the radiation [1,31], poor sensibility for small defects compared to the sample dimension [1]. |
X-Ray Backscatter | Large structures are easily tested, not susceptible to surface roughness [31], detects deep and embedded defects, can operate even if only one side of the target is available [1]. | Capable of finding defects bigger than 20 μm [1], limited availability of tailored X-ray sources, challenges in developing standards and procedures [31], long inspection duration [1,31]. |
Eddy Current Testing | Improved sensitivity [31], can be used for surface and subsurface flaw detection, very sensitive to small defects, contactless but requires proximity [1]. | The size of smallest detected defect is 350 μm [53], surface finish and grain structure influence the results, penetration depth of few millimeters [31], limited to conductive materials [1,39]. |
Infrared Thermography | Large areas can be scanned fast, free of radiation [1,31], can detect subsurface defects [1]. | Detects flaws larger than 400 μm [32], not possible to penetrate in extended depths, environmental conditions may limit use if outdoors [31], elevated temperature is necessary at the analyzed part [1,31]. |
Laser Thermography | Contactless and no surface finishing is required [1,31], can detect surface and subsurface defects [1], does not require surface finishing [31]. | Smallest flaw size detected is 400 μm [32], deep scratches or indentations can perturb heat flow in a similar manner to a crack [31]. |
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Serrati, D.S.M.; Machado, M.A.; Oliveira, J.P.; Santos, T.G. Non-Destructive Testing Inspection for Metal Components Produced Using Wire and Arc Additive Manufacturing. Metals 2023, 13, 648. https://doi.org/10.3390/met13040648
Serrati DSM, Machado MA, Oliveira JP, Santos TG. Non-Destructive Testing Inspection for Metal Components Produced Using Wire and Arc Additive Manufacturing. Metals. 2023; 13(4):648. https://doi.org/10.3390/met13040648
Chicago/Turabian StyleSerrati, Douglas S. M., Miguel A. Machado, J. P. Oliveira, and Telmo G. Santos. 2023. "Non-Destructive Testing Inspection for Metal Components Produced Using Wire and Arc Additive Manufacturing" Metals 13, no. 4: 648. https://doi.org/10.3390/met13040648