Friction Stir Welding
This article summarizes a paper entitled, “An Evaluation of Friction Stir Spot Welding as a Method for Joining Ultra-thin Steel Sheet,” by Y. Hovansk, et al.H-10
The study analyzes Friction Stir Spot Welding (FSSW) as a process for assembly of two sheet stack ups. The steel sheet used for this study is CR4-GI, a hot dip galvanized ultra-low carbon interstitial free steel with a tensile strength of 280 MPa. Thicknesses of both 0.45 and 1.2 mm were used to create dissimilar thickness, two-sheet stack-ups. Preparation for joining via FSSW did not alter the zinc coating. FSSW joints were evaluated in lap shear tensile, T-peel, and cross tension.
FSSW welds were welded with an EKasin injection molded, silicon nitride tool shown in Figure 1. All welds were performed at 1600 rpm.
Figure 1: Representative Picture of a Silicon-Nitride FSSW Tool with a 10-mm-diameter Shoulder and a 1.15-mm Probe Length.H-10
The zinc coating that originally covered the sheet surface was extruded beneath the FSSW tool to the outer edges of the weld as seen in Figure 2. Figure 3 shows a representative weld on a T-peel specimen.
Figure 2: Optical Image of the Top Surface of a Friction Stir Spot Weld in CR4-GI.H-10
Figure 3: A T-Peel Specimen Produced on a 25-mm-wide Strips with FSSW 0.45- to 1.2-mm-thick CR4-GI.H-10
A minimum of 25 specimens were produced for each geometry tested, however, these specimens were performed at various times throughout weld development and data is shown below. Figure 4 shows the load-extension curves for a set of nine friction stir spot welds. Figure 5 shows a representative fracture of lap-shear tensile specimen.
Figure 4: Test Results for Lap-Shear Tensile Data of Friction Stir Spot Welds in 0.45-mm CR4-GI.H-10
Figure 5: Friction Stir Spot Weld in 0.45-mm CR4-GI Fractured in Lap-Shear Tensile.H-10
Figure 6 shows the load-extension curves for a set of eight friction stir spot welds tests in T-peel. A representative fracture of T-peel specimen is shown in Figure 7.
Figure 6: Test Results for T-Peel Data of Friction Stir Spot Welds in 0.45-mm CR4-GI.H-10
Figure 7: Friction Stir Spot Weld in 0.45-mm CR4-GI Fractured in T-Peel.H-10
Figure 8 shows the load extension curves for a set of 13 friction stir spot welds tested in cross tension. Representative geometry and fracture of cross tension specimen are shown in Figure 9.
Figure 8: Test Results for Cross-Tension Data of Friction Stir Spot Welds in 0.45-mm CR4-GI.H-10
Figure 9: Friction Stir Spot Weld in 0.45-mm CR4-GI Fractured in Cross-Tension.H-10
A table showing the overall results for the FSSW joints produced herein are shown in Table 1 below:
Table 1: Summary of Fracture Loads and Energies from Friction Stir Spot Welds made in Ultra-Thin CR4-GI for Three Unique Test Configurations.H-10
While each specific test orientation demonstrated the ability for the weld nugget to pull out of the ultra-thin top sheet and remain with the lower 1.2-mm-thick sheet, the overall ratios between fracture loads suggest there is an area for improvement with respect to T-peel.
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Resistance spot welding (RSW) is the most utilized joining process in car body assembly with exceptionally high demands on quality and reproducibility. Expulsion in RSW leads to ejection of metal from the weld and can cause equipment deterioration and re-working.
As the RSW process has many variables both from the process itself and from the to-be-welded component, prediction and avoidance of expulsion is challenging. This creates a large demand for new technologies that allow for RSW of steels with improved expulsion control.
Recent developments aim to predict – and ultimately avoid – expulsion using artificial intelligence data analysis. Process data on current, voltage and dynamic resistance are readily available in RSW and can be augmented with simulation data for non-measurable phenomena such as the nugget growth rate to create predictive algorithms that eventually aim to avoid expulsion altogether.
An Introduction to AI Expulsion Prediction
This work focuses on utilizing artificial intelligence modeling for the prediction of expulsion during resistance spot welding (see Figure 1 for an image of expulsion on the left and metal residue on the surface after expulsion on the right). The primary objective is to forecast the formation of expulsion before it occurs, with the aim of improving the quality of the welding process. This effort is supported by a dataset of 500 welded spots of 2-sheet stack-ups made from one advanced high-strength steel (AHSS) and one mild steel joining partner. Two sequential AI models are trained: one for nugget growth prediction and another for spatter prediction.
Figure 1: Left: Expulsion leads to the ejection of molten metal. Right: Residues may form on the surface after expulsion.
Feature Engineering
A key aspect is the integration of multi-source data: The models leverage both static and dynamic inputs. Static inputs include input process parameters such as current, force, and weld time, which are set at the machine in accordance with a pre-determined weld lobe. Dynamic inputs encompass transient process signals like dynamic resistance and electrode force. In resistance spot welding, this data is usually readily available, because the welding power source measures electric flow and weld gun opening and forces. A further step is to include non-measurable data from simulation results concerning nugget growth rate.
To give time for process intervention, the predictive quality of the models was determined after only 30 ms of welding time, depicted in Figure 2.
Figure 2: Only 30 ms of dynamic resistance measurements are used as input for the AI models.
Data evaluation and feature engineering are critical components of the modeling process. A welding and data-science expert needs to identify significant features from sensor time series data as input for the neural network AI models. These features can be physically meaningful, such as the minimum resistance during the process, or purely statistical with values such as longest continuous time above the average resistance.
The simulation of resistance spot welding allows for the extraction of the non-measurable nugget growth rate. It is well documented that the nugget growth rate strongly correlates with expulsion formation and conducting simulations for different welding cases (gaps, misalignments, slightly changed contact resistances…) can add this physical behavior to the data-driven AI model. Figure 3 depicts different nugget growths extracted from the simulation with different welding currents. High currents lead to significantly faster growth rates and facilitate expulsion.
Figure 3: Weld nugget growth behavior for different welding currents extracted from a numerical welding simulation. Solid lines are simulation results; dashed lines depict a polynomial fit used to reduce data for AI input.
Model Accuracy
Figure 4 depicts the confusion matrices for the three different models. The bottom left and top right fields are the desired predictions, and the other fields depict false results of the models. The first model stage, which relies solely on static inputs, yields moderate prediction quality. However, when dynamic data from experiments and simulations are included, the prediction quality jumps to approximately 96%. The analysis focusing on the first 30 milliseconds to give time for subsequent intervention in the process, achieves a reduced, but still good, result quality of around 90%. Further improvements in prediction accuracy are expected with additional data.
Figure 4: Confusion matrices for static, static and dynamic and static and dynamic: only 30 ms inputs. A reliable prediction is possible using dynamic process and simulation data. If only 30 ms of dynamic data are utilized the predictive quality decreases.
The presented approach uses a data-driven AI model with different levels of input data to predict resistance spot welding expulsion. The data-gathering and feature engineering procedures are highlighted, explaining a need for in-depth knowledge of the welding process as well as data engineering. The approach yields a neural network with excellent predictive quality, if dynamic data is included in the model. Even if only 30 ms of transient data are used to allow for a subsequent process intervention, the result quality is still good. It is expected that the predictive quality improves when the data set is increased and additional data sources both from measurements and simulations are included. This approach can improve the control of automotive welding systems and ultimately avoid excessive re-working and equipment wear due to expulsion in resistance spot welding of advanced high-strength steels.
This article explores the challenges of liquid metal embrittlement (LME) in resistance spot welding (RSW) of automotive components, particularly focusing on a component-scale S-Rail made from advanced high-strength steel (AHSS). The study aims to identify the occurrence of LME during the welding process and to propose effective strategies for its mitigation. This article is an excerpt from the “LME component study” conducted by WorldAutoSteel. The full study can be downloaded here.
Experimental and Simulative Setup
The experiments utilized an electrogalvanized RA1180 AHSS joined to hot-dip galvanized mild steel. Two stack-up configurations were tested: similar (both sheets made of RA1180) and dissimilar (RA1180 on top of mild steel). The resistance spot welding process was monitored using sensors to record current, voltage, and force. Different welding parameters, such as hold time and electrode geometry, were varied to observe their effects on LME.
Figure 1: Top-view of the S-Rail component during welding. The clamping points S1-S4 as well as the welding points F1-F10 are highlighted.
A simulation-based risk criterion for LME was established based on local stresses in the components. Both experimental and numerical analyses were conducted to assess the influence of various parameters on LME formation. Specifically, the study evaluated how springback, a phenomenon occurring during deep drawing, affects LME risk. Correct clamping can effectively suppress springback, consequently reducing LME occurrences.
Figure 2: Experimentally observed cracks with 5° tilted electrodes and doubled welding time of 760 ms.
Findings
- Influence of Springback: Springback contributed to LME formation. When clamping was employed to counteract springback, LME was effectively eliminated from the welded samples.
- Electrode Geometry and Hold Time: Adjustments to the electrode geometry and increasing hold time after welding further mitigated LME risks. Specifically, larger electrode tip diameters and longer hold times reduced the likelihood of cracks.
- Material Stack-up Effects: The experiments indicated that the configuration of the material stack-up influenced LME occurrences. Only stack-ups with thick joining partners showed occurrence of LME in the trials.
Figure 3: All 10 resistance spot welds on the S-rail are crack-free after optimizing either springback, electrode working plane diameter or post-weld hold time
Simulation Results
Finite element simulations were used to evaluate the risk of LME by analyzing local stresses and temperature distributions during welding. The results showed that the springback-affected samples presented a higher LME risk compared to idealized, straightened models. This finding aligns with experimental observations that cracks occurred where excessive springback influenced the welding process. Even in the case of springback, LME could be effectively prevented by using electrode caps with larger working planes as well as slightly extending the hold time after welding.
The developed simulation approach allows comparing the LME conditions for different welding setups and can therefore optimize the LME occurrence for geometry, material and welding conditions.
Conclusion
Effective mitigation strategies, such as clamping to suppress springback and adjustments in welding parameters, can prevent LME on a component-scale. It can also be highlighted that today’s AHSS grades are far less sensitive to LME by-default so that few RSW joints in a whole body-in-white are at all susceptible for cracking: To produce cracks for this study, welding parameters with increased energy input had to be used; no LME was observed under “standard” industrial conditions.
![Considerations When Deciding Whether to Cold Form or Hot Form]()
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. The considerations below relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid-formed.
Equipment
There is a well-established infrastructure for cold stamping. New grades benefit from servo presses, especially for those grades where press force and press energy must be considered. Larger press beds may be necessary to accommodate larger parts. As long as these factors are considered, the existing infrastructure is likely sufficient.
Progressive-die presses have tonnage ratings commonly in the range of 630 to 1250 tons at relatively high stroke rates. Transfer presses, typically ranging from 800 to 2500 tons, operate at relatively lower stroke rates. Power requirements can vary between 75 kW (630 tons) to 350 kW (2500 tons). Recent transfer press installations of approximately 3000 tons capacity allow for processing of an expanded range of higher strength steels.
Hot stamping requires a high-tonnage servo-driven press (approximately 1000 ton force capacity) with a 3 meter by 2 meter bolster, fed by either a roller-hearth furnace more than 30 m long or a multi-chamber furnace. Press hardened steels need to be heated to 900 °C for full austenitization in order to achieve a uniform consistent phase, and this contributes to energy requirements often exceeding 2 MW.
Integrating multiple functions into fewer parts leads to part consolidation. Accommodating large laser-welded parts such as combined front and rear door rings expands the need for even wider furnaces, higher-tonnage presses, and larger bolster dimensions.
Blanking of coils used in the PHS process occurs before the hardening step, so forces are low. Post-hardening trimming usually requires laser cutting, or possibly mechanical cutting if some processing was done to soften the areas of interest.
That contrasts with the blanking and trimming of high strength cold-forming grades. Except for the highest strength cold forming grades, both blanking and trimming tonnage requirements are sufficiently low that conventional mechanical cutting is used on the vast majority of parts. Cut edge quality and uniformity greatly impact the edge stretchability that may lead to unexpected fracture.
Responsibilities
Most cold stamped parts going into a given body-in-white are formed by a tier supplier. In contrast, some automakers create the vast majority of their hot stamped parts in-house, while others rely on their tier suppliers to provide hot stamped components. The number of qualified suppliers capable of producing hot stamped parts is markedly smaller than the number of cold stamping part suppliers.
Hot stamping is more complex than just adding heat to a cold stamping process. Suppliers of cold stamped parts are responsible for forming a dimensionally accurate part, assuming the steel supplier provides sheet metal with the required tensile properties achieved with a targeted microstructure.
Suppliers of hot stamped parts are also responsible for producing a dimensionally accurate part, but have additional responsibility for developing the microstructure and tensile properties of that part from a general steel chemistry typically described as 22MnB5.
Property Development
Independent of which company creates the hot formed part, appropriate quality assurance practices must be in place. With cold stamped parts, steel is produced to meet the minimum requirements for that grade, so routine property testing of the formed part is usually not performed. This is in contrast to hot stamped parts, where the local quench rate has a direct effect on tensile properties after forming. If any portion of the part is not quenched faster than the critical cooling rate, the targeted mechanical properties will not be met and part performance can be compromised. Many companies have a standard practice of testing multiple areas on samples pulled every run. It’s critical that these tested areas are representative of the entire part. For example, on the top of a hat-section profile where there is good contact between the punch and cavity, heat extraction is likely uniform and consistent. However, on the vertical sidewalls, getting sufficient contact between the sheet metal and the tooling is more challenging. As a result, the reduced heat extraction may limit the strengthening effect due to an insufficient quench rate.
Grade Options for Cold Stamped or Hot Formed Steel
There are two types of parts needed for vehicle safety cage applications: those with the highest strength that prevent intrusion, and those with some additional ductility that can help with energy absorption. Each of these types can be achieved via cold stamping or hot stamping.
When it comes to cold stamped parts, many grade options exist at 1000 MPa that also have decent ductility. The advent of the 3rd Generation Advanced High Strength Steels adds to the tally – the stress-strain curve of a 3ʳᵈ Gen QP980 steel is presented in Figure 1. Most of these top out at 1200 MPa, with some companies offering cold-formable Advanced High Strength Steels with 1400 or 1500 MPa tensile strength. The chemistry of AHSS grades is a function of the specific characteristics of each production mill, meaning that OEMs must exercise diligence when changing suppliers.
Figure 1: Stress-strain curve of industrially produced QP980.W-35
Martensitic grades from the steel mill have been in commercial production for many years, with minimum strength levels typically ranging from 900 MPa to 1470 MPa, depending on the grade. These products are typically destined for roll forming, except for possibly those at the lower strengths, due to limited ductility. Until recently, MS1470, a martensitic steel with 1470 MPa minimum tensile strength, was the highest strength cold formable option available. New offerings from global steelmakers now include MS1700, with a 1700 MPa minimum tensile strength, as well as MS 1470 with sufficient ductility to allow for cold stamping. Automakers have deployed these grades in cold stamped applications such as crossmembers and roof reinforcements, with some applications shown in Figure 2.
Figure 2: Cold-Stamped Martensitic Steel with 1500 MPa Tensile Strength used in the Nissan B-Segment Hatchback.K-57
Until these recent developments, hot stamping was the primary option to reach the highest strength levels in part shapes having even mild complexity. Under proper conditions, a chemistry of 22MnB5 could routinely reach a nominal or aim strength of 1500 MPa, which led to this grade being described as PHS1500, CR1500T-MB, or with similar nomenclature. Note that in this terminology, 1500 MPa nominal strength typically corresponds to a minimum strength of 1300 MPa.
The 22MnB5 chemistry is globally available, but the coating approaches discussed below may be company-specific.
Newer PHS options with a modified chemistry and subsequent processing differences can reach nominal strength levels of 2000 MPa. Other options are available with additional ductility at strength levels of 1000 MPa or 1200 MPa. A special class called Press Quenched Steels have even higher ductility with strength as low as 450 MPa.
The spectrum of grades available for cold-stamped and hot formed steel parts allows automakers to fine-tune the crash energy management features within a body structure, contributing to steel’s “infinite tune-ability” capability which gives automotive engineers design flexibility and freedoms not available from other structural materials.
Corrosion Protection
Uncoated versions of a grade must take a different chemistry approach than the hot dip galvanized (GI) or hot dipped galvannealed (HDGA) versions since the hot dip galvanizing process (Figure 3) acts as a heat treatment cycle that changes the properties of the base steel. Steelmakers adjust the base steel chemistry to account for this heat treatment to ensure the resultant properties fall within the grade requirements.
Figure 3: Schematic of a typical hot-dipped galvanizing line with galvanneal capability.
This strategy has limitations as it relates to grades with increasing amounts of martensite in the microstructure. Complex thermal cycles are needed to produce the highly engineered microstructures seen in advanced steels. Above a certain strength level, it is not possible to create a GI or HDGA version of that grade.
For example, when discussing fully martensitic grades from the steel mill, hot dip galvanizing is not an option. If a martensitic grade needs corrosion protection, then electrogalvanizing (Figure 4) is the common approach since an EG coating is applied at ambient temperature, which is low enough to avoid negatively impacting the properties. Automakers might choose to forgo a galvanized coating if the intended application is in a dry area that is not exposed to road salt.
Figure 4: Schematic of an electrogalvanizing line.
For press hardening steels, coatings serve multiple purposes. Without a coating, uncoated steels will oxidize in the austenitizing furnace and develop scale on the surface. During hot stamping, this scale layer limits efficient thermal transfer and may prevent the critical cooling rate from being reached. Furthermore, scale may flake off in the tooling, leading to tool surface damage. Finally, scale remaining after hot stamping is typically removed by shot blasting, an off-line operation that may induce additional issues.
Using a hot dip galvanized steel in a conventional direct press hardening process (blank -> heat -> form/quench) may contribute to liquid metal embrittlement (LME). Getting around this requires either changing the steel chemistry from the conventional 22MnB5 or using an indirect press hardening process that sees the bulk of the part shape formed at ambient temperatures followed by heating and quenching.
Those companies wishing to use the direct press hardening process can use a base steel having an aluminum-silicon (Al-Si) coating, providing that the heating cycle in the austenitizing furnace is such that there is sufficient time for alloying between the coating and the base steel. Welding practices using these coated steels need to account for the aluminum in the coating, but robust practices have been developed and are in widespread use.
Setting Correct Welding Parameters for Resistance Spot Welding
Specific welding parameters need to be developed for each combination of material type and thickness. In general, Press Hardening Steels require more demanding process conditions. One important factor is electrode force, which is typically higher than needed to weld cold formed steels of the same thickness. The actual recommended force depends on the strength level and the thickness of the steel. Of course, strength and thickness affects the welding machine/welding gun force capability requirement.
The welding current level, and more importantly, the current range are both important. The current range is one of the best indicators of welding process robustness, so it is sometimes described as the welding process window. Figure 5 indicates the relative range of current required for spot welding different steel types. A smaller process window may require more frequent weld quality evaluations, such as for adequate weld size, necessitating more frequent corrective actions to address the discovered quality concern.
Figure 5: Relative Current Range (process windows) for Different Steel Types
Effect of Coating Type on Weldability
When resistance spot welding coated steels, the coating must be removed from the weld area during and in the beginning of the weld cycle to allow a steel-to-steel weld to occur. The combination of welding current, weld time, and electrode force are responsible for this coating displacement.
For all coated steels, the ability of the coating to flow is a function of the coating type and properties such as electrical resistivity and melting point, as well as the coating thickness.
Cross sectioned spot welds made on press hardened steel with different coating weights of an Aluminum -Silicon coating is presented in Figure 6. Note the displaced coating at the periphery of weld. Figure 7 shows the difference in current range required to produce acceptable welds associated with these different coating weights. The thicker coating shows a smaller current range. In addition, the press-hardened Al-Si coating has a much higher melting point than the zinc coatings typically found on cold stamped steels, making it more difficult to displace from the weld area.
Figure 6: Aluminum -Silicon coated press hardened steels.O-16
Figure 7: Influence of Aluminum-Silicon coating weight on welding range.O-16
Liquid Metal Embrittlement and Resistance Spot Welding
Cold-formable, coated, Advanced High Strength Steels are widely used in automotive applications. One welding issue these materials encounter is increased hardness in the weld area, that may result in brittle fracture of the weld. Another issue is their sensitivity to Liquid Metal Embrittlement (LME) cracking.
Both issues are discussed in detail in the Joining section of the WorldAutoSteel AHSS Guidelines website and the WorldAutoSteel Phase 2 Report on LME.
Resistance Spot Welding Using Current Pulsation
The most effective solution for these issues is using current pulsation during the welding cycle, schematically described in Figure 8.
Figure 8: Nugget growth differences in Single Pulse vs. Multi-Pulse Welding
Pulsation of the current allows much better control of the heat generation and weld nugget development. Pulsation variables include the number of pulses (typically 2 to 4), current, time for each pulse, and the cool time between the pulses.
Pulsation during Resistance Spot Welding is beneficial for press hardening steels, coated cold stamped steels of all grades, and multi-material stack-ups. Information about multi-sheet, multi-material stack-ups can be – as described in our articles on 3T/4T and 5T Stack-Ups
For more information about PHS grades and processing, see our Press Hardened Steel Primer.
Thanks are given to Eren Billur, Ph.D., Billur MetalForm for his contributions to the Equipment section, as well as many of the webpages relating to Press Hardening Steels at www.AHSSinsights.org.
