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To understand the difference between localized and global fractures, you must first understand strain gradients (see the article in our blog, AHSS Strain Hardening and Gradients). Gradients can result in highly concentrated strains (peak strain condition) that typically occurs in an embossment or character line where the deformation mode is in plane strain. Peak strains can develop rapidly in a very localized area (Figure 1). Under additional loads, this can result in the onset of localized necking, which means the material has reached its tensile strength and will fail at its weakest point or highest strain. When a slight increase in strain is applied, the material will fracture, sometimes at deformation levels less than predicted. This condition can be found in AHSS products, where multiple phases exist within the steel’s microstructure, each with different properties. A global fracture also typically occurs in plane strain, but more commonly down a sidewall or other area with more moderate geometry complexity.
Figure 1: Peak strain in the localized area or embossment
Peak (concentrated) strains are susceptible to localized fractures when even slight variation exists in the forming process. Examples of variation include lubrication pattern and volume, die recipe including blank position, press conditions, and material characteristics.
A localized neck and/or fracture (Figure 2) reduces the sheet metal’s thickness, reducing part strength, and compromising functional performance such as fatigue life, crash worthiness, and stamping stiffness. There are a number of formability analysis tools that can differentiate localized and global fractures and enable die makers to implement die and process improvements that minimize fracture susceptibility. The result is a more robust stamping process.
Figure 2: Schematic of Localized Necking and Fracture
Process control is critical; die recipe discipline is needed to minimize tinkering with die recipe, press settings, and lubrication settings. Mechanical properties of the sheet metal should be tracked to identify trends or variations in the material, and establish the material forming window. Typical mechanical properties that are available from the steel supplier are yield strength, tensile strength, n-value, total and uniformed elongation, and sheet thickness. Additional properties that should be determined include hole expansion and deep cup draw ratios. Failure to identify strain levels, process variables and variation will lead to a reactionary approach to controlling the output. This will lead to an increase in scrap, die-related downtime, and of course, costs.
Contributions made by Phoenix Group.
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.
This article investigates the MIG brazing capability of coated steel sheets for automotive applications, focusing on MIG brazing as a viable alternative to MAG welding, especially for chassis components. The study aims to evaluate the impact of coatings on brazing ability, wettability, and arc stability. MIG brazing is increasingly being used for joining Body-in-White parts, especially when Resistance Spot Welding is unsuitable. This research investigates the potential of brazing coated steels, noting that MAG welding often leads to defects such as blowholes.
MIG Brazing Process
MIG brazing is similar to MAG welding but utilizes a low melting point filler wire, typically copper alloys, that does not melt the substrates. It primarily employs pure argon as shielding gas and a short-circuit transfer mode. The main advantages of MIG brazing over MAG welding include:
- Lower heat input due to the lower melting temperature.
- Reduced distortion, making it suitable for thin sheets used in automotive applications.
- Improved visual appearance and less coating degradation, leading to better corrosion resistance.
Figure 1: MIG Brazing process with representative cross sections from brazing and welding
Methodology
The study focuses on overlap joints without gaps, using CuAl8 and CuSi3 filler wires with a diameter of 1.0 mm, and employs a short-circuit waveform at a welding speed of 500 mm/min. The results are expressed in terms of brazing range, focusing on criteria such as wettability of different coatings and spatter formation.
Key Findings
- Wetting Studies: Two methods were used to study wettability—real MIG brazed samples and a wetting pilot. The results indicated that higher wire feed speeds lead to increased current and voltage, which enhances heat input and improves wettability despite a rise in molten metal volume.
Figure 2: Effect of wire feeding speed and heat input on wetting angle using 0.8 mm thick sheets and CuAl8 filler wire
- Influence of Coatings: The coatings had no significant effect on the wettability or arc stability. The behavior of Zinc-coated steels were like that of bare steel, with AlSi coating showing a wider brazing range due to its thicker and more refractory nature.
Figure 3: Brazing range and wetting angle for different coatings using 0.8 mm thick sheets and CuAl8 filler wire
- Spatter Formation: Spatters measuring between 0.1 to 0.3 mm occurred, with a notable increase in spatter rates observed when using zinc coatings. This effect was consistent across various coatings.
Conclusion
The study concludes that the coating does not influence the MIG brazing capability, as the brazing range, wettability, and arc stability remained consistent. The AlSi coating exhibited a broader brazing range. Switching between coatings does not require a change in brazing parameters, although an increase in spatter is expected when brazing coated sheets in comparison to bare steels. In summary, MIG brazing is validated as an effective method for joining coated steel sheets in automotive applications, providing advantages in heat input and corrosion resistance over traditional arc welding. Source J. Haouas, MIG brazing ability of coated steel sheets for automotive applications, IIW 2020 conference, SC XVII
A dynamic tensile test was conducted to evaluate the mechanical properties of spot welds under automotive collision conditions. The actual tensile shear strengths of steel sheets with nominal tensile strengths ranging from 270 MPa to 780 MPa were investigated.
Visit our page on high strain rate testing to learn more about the equipment and testing challenges. Link
Test Method
Figure 1 presents the dynamic tensile test machine and illustrates a schematic diagram of the tensile shear test specimen. A 1.6 mm thick steel sheet was placed on top of the tensile shear test specimen and spot welded, with nugget diameters of 5.5√t (7.0 mm) used for both. In the dynamic tensile test, a cone was dropped at high speed onto the specimen to apply a tensile load and determine the breaking point. The tensile speed was adjusted by varying the drop height of the cone, with a maximum speed of 2.4 m/s. For comparison, a static tensile test was conducted at a tensile speed of 1.6 × 10-4 m/s.
Figure 1: Dynamic tensile shear test equipment (left) and test specimen (right)
Results
Figure 2 shows the relationship between tensile shear strength and tensile speed for the steel sheet with a rated tensile strength of 590 MPa. Tensile shear strength tended to increase with tensile speed, with values of approximately 22 kN and 25.5 kN under static and dynamic loading conditions, respectively. All specimens exhibited plug fracture as the failure mode
Figure 2: Relationship between tensile shear strength and tensile speed (steel sheet with a rated tensile strength of 590 MPa)
Figure 3 illustrates the effect of the tensile strength of the base material on the rate of increase in dynamic strength relative to static strength. Plug fracture remained the consistent failure mode across all cases. For the steel sheet with a rated tensile strength of 270 MPa, dynamic strength increased by approximately 60% compared to static strength. In contrast, the sheet with a rated tensile strength of 780 MPa showed an increase of only about 14%. These results indicate a tendency for the rate of increase in dynamic strength relative to static strength to decrease as the rated tensile strength of the steel increases. This is consistent with the general trend of mild steel strength increasing with strain rate, while strain rate sensitivity diminishes for higher-strength steels.
Figure 3: Relationship between dynamic and static tensile shear strengths of spot welds and base material strength
Source
Dynamic Tensile Shear Strength of Spot-Welded Joints: Experimental Investigation and Results Hiroki Fujimoto, Welding & Joining Research Laboratories, Nippon Steel Corporation
Citations
Citations:
S-130. Kenneth Schmid and Jason Coryell, “Stamping Assessment of Various 980 and 1180 MPa Steel Grades,” presented at International Automotive Body Congress (IABC 2017 Frankfurt), Frankfurt, Germany, 13 – 14 June 2017, https://www.proceedings.com/content/035/035369webtoc.pdf; paper available from https://jandngroup.com/wp-content/uploads/2017/07/Edited-Schmid-Paper.pdf
