Past GDIS™ Presentations

The development of the 3rd Generation of advanced high-strength steels (AHSS) has opened new avenues for the product design of automotive lightweight components but the methodologies to fully exploit their superior mechanical properties have been lagging behind. The traditional in-plane forming limit curve (FLC) remains the standard engineering tool for formability assessment and leads to an overly conservative product design. The present study provides insight into the effect of tool contact pressure in three-point bend simulations that served for the development of a novel instability framework to capture the delay in material localization. It is demonstrated that both the magnitude and the boundary condition of how the contact pressure is applied governs the formability gain. An analytical-numerical stretch-bend model was developed which successfully captures the increase in the forming limit strains studied in Marciniak, Nakazima, stretch-bend and V-bend tests of a 3rd Gen AHSS with a nominal tensile strength of 1180 MPa. Application to a structural B-Pillar technology demonstrator correlates well with the forming trials and identifies false positives which were erroneously flagged for splitting when relying on the conventional FLC.

Note: This talk is part of to the AISI Automotive Program Project

Advanced high-strength steel (AHSS) is often illustrated in the Steel Strength-Ductility Diagram. Both academia and industry frequently refer to such a diagram to not only concisely categorize the evolving AHSS generations but also to direct the future development objectives. Nevertheless, with various new AHSS developed in recent years, the diagram is considered too simplified to represent the sophisticated tensile properties of these AHSS in the practical applications: that is, the experimental data are typically characterized at a quasi-static uniaxial strain rate at room temperature. The present work focuses on investigating how the temperature and strain rate affect the tensile properties of AHSS and correspondingly shift their distributions in the diagram. The target AHSS includes two dual phase (DP) steels, two quenched and partitioned (Q&P) steels and two austenitic steels. The results illustrate in the diagram how diverse the temperature and strain-rate dependencies of different AHSS can be. Concisely, with the temperature rising by either the external or the adiabatic heating, the Q&P and DP steels exhibit their tensile properties varying from a valley to a peak, while the two austenitic steels behave monotonic property trajectories towards two different directions. Furthermore, behind every turning of the property trajectories, there are multiple instantaneous effects, either favoring or opposing, acting together to determine how the materials behave at that moment. Particularly, the Q&P steels can be affected by the evolving thermal softening, dynamic strain aging, and transformation-induced plasticity (TRIP) effects during different temperature and strain rate ranges. Further understanding of the inter-relationship between strain rate, temperature, and material behavior, including the TRIP-effect reactivation, offer future paths to further optimization of AHSS designs for automotive applications.

In sheet metal forming simulations, a material card refers to a data-file that has, at minimum, information about hardening curve, yield locus, and fracture criteria. Depending on the requirements, a material card may have information about strain rate sensitivity, unloading modulus variation, the transient Bauschinger effect and permanent softening (as described by Yoshida and Uemori), and edge crack sensitivity. In addition to the “material card”, more information about the process can be included in the simulation: friction between the sheet and the tool as a function of sliding velocity, pressure, temperature; press kinematics, elastic deflection of the tooling and the press.

Each additional data-set will require time and expense. In this paper, the authors introduce “multi-level” material cards for different phases of sheet metal forming process developments. The idea is proven with stamping experiments using a 3rd Generation Advanced High-Strength Steel (AHSS).

The goal of this study is to understand the performance of additively manufactured (AM) die inserts in stamping applications. Stamping of 50,000 symmetrical U-bend parts made from 1 mm thick Dual Phase 980 sheet steel was performed in a progressive die with one side of the tool fabricated from Maraging Steel AM material and the other side fabricated from conventional D2 tool steel. The additive manufactured material was fabricated using laser powder bed fusion with Maraging Steel powder. All tested inserts were coated with IB90 PVD coating.

Both inserts produced quality stampings, with no significant visual artifacts. Likewise, no significant scratches were found on either insert. On the D2 side more hairlike flow lines were visible, but no deeper scratches. The D2 insert has more wear in the area close to the die entry radius, which resulted in shallow scratches on the samples’ surface.

The AM insert has deeper and wider scratches in the general contact zone (horizontal surface of the insert), but produced samples having better surface quality since this insert has less wear in the area close to the die entry radius. These results indicate that AM produced inserts have strong potential to be employed in die components for stamping of High Strength Steels.

Liquid metal embrittlement (LME) cracking during resistance spot welding (RSW) is a challenge restricting the application of zinc coated 3rd Gen AHSS in automotive structural components. From a production perspective, the potential for LME cracking in a spot weld has two related aspects, first is the steel’s susceptibility to LME cracking and the second is whether LME will manifest during RSW. The Auto/Steel Partnership (A/SP) has developed a test methodology to assess the LME susceptibility of steels using a production-like RSW process, which addresses both aspects. The Rapid LME test is low-cost rapid test that is useful in quickly determining the risk of LME cracking in homogeneous and heterogenous multi-steel stackups. To validate the effectiveness and repeatability of the Rapid LME Test, A/SP applied the test to 12 carefully chosen steel stack-ups comprised of homogenous and heterogeneous 3rd Gen AHSS and mild steel. The results showed that the cracking severity rating from the Rapid LME Test in homogeneous joints is a good indicator for the cracking severity in heterogeneous joints. However, it was found that in the heterogenous stack-ups, the fixed weld schedule used in the Rapid LME Test was not able to produce weld nuggets that met the general acceptance criteria for meaningful cracking severity comparison, due to the large materials resistivity and thickness differences. A new supplementary method to Rapid LME Test, called Weld Lobe LME Test, was developed to evaluate the cracking severity in heterogenous joints. The more production-representative weld schedules adopted by Weld Lobe LME Test was found to effectively differentiate the cracking severity in heterogenous joints. To thoroughly assess a material’s LME cracking susceptibility in both homogeneous and heterogenous joining conditions, a combination of Rapid LME and Weld Lobe LME tests is recommended. As their characterization methods of cracking severity are essentially identical, future work of this project will focus on establishing recommended crack acceptance criteria.

Resistance spot welds experience complex loading conditions in vehicle crash events. Performing mechanical tests considering a range of loading modes on spot-welded connections can help to better understand the relationship between failure characteristics and common spot weld performance indices such as strength and energy absorption capability. In this study, the mechanical performance and failure behavior of resistance spot welds from two grades of third generation advanced high strength steels (3G-AHSS), designated 3G-980 and 3G-1180, were evaluated by performing KS-II mechanical tests which impose combined/mixed loading on the joints. A novel triangulation technique coupled with digital image correlation was developed for more accurate tracking of local nugget locations during the tests which improved the accuracy of the energy absorption calculations. It was shown that propagation of the cracks into the fusion zone of the 3G-1180 spot welds restricts their capability to absorb energy via deformation of the nugget in the tensile direction, especially within tensile-dominated loading orientations. Correlation of spot weld failure modes and absorbed energy values revealed that full nugget pullout failures exhibited superior energy dissipation value during failure and are favored over partial pullout failures with low plug ratios.

The automotive industry worldwide is undergoing a radical transformation with a pivot away from the Internal combustion engine (ICE) towards battery-electric-vehicles (BEV) to address the carbon reduction and sustainability initiatives. Existing assembly lines are being retooled and also brand-new assembly lines installed at a hectic pace. Multiple powertrain configurations (ICE, Hybrid, PHEV, BEV) coupled with modular platforms and sustainability initiatives requires re-imagining and rethinking the car body structures and assembly process.

Laser welded blank (LWB) technology for Press Hardened Steels (PHS-LWB) is a proven solution enabling performance improvement, part consolidation, material optimization, and weight reduction in vehicles. Also, it reduces the carbon footprint in all stages of vehicle life cycle assessment (LCA).

AMTB in collaboration with our R&D will showcase our innovative PHS-LWB design concepts called Mega-Part or Multi-Part Integration (MPI). Through this presentation, we will demonstrate how implementing MPI designs will help OEMs reduce the overall carbon footprint, reduce assembly time and cost, amplifying the traditional benefits of implementing PHS-LWBs. Our MPI designs have been validated to meet performance for crash and stiffness and evaluated for manufacturing feasibility using our virtual CAE models. We have also computed CO2 savings and assembly benefits.

Our presentation intends to make a strong case for OEMs to study MPI designs for future vehicle design architectures. PHS-LWBs are key enablers in promoting steel versus alternative materials, reducing assembly complexity, cost and time while improving sustainability of future vehicle designs.

Distinct microstructures with unique mechanical properties are produced upon resistance spot welding (RSW) of advanced high strength steels (AHSS). To understand how RSW deform and fail when loaded, it is crucial to characterize the mechanical properties of RSW sub-regions, the fusion zone (FZ) and heat-affected zones (HAZ), as the local mechanical properties of these regions can influence the global failure response of vehicle structure assemblies. In this study, a novel experimental procedure was developed to characterize the local properties of RSW sub-regions by fabricating sub-sized tensile (mini-tensile) and mini-shear specimens directly from the spot weld. While the FZ properties were extracted from the mini-shear test, the mini-tensile specimens were utilized to attain HAZ properties using the digital image correlation (DIC) technique. To expand the validity of the findings, different grades of AHSS, including Gen3-980, Gen3-1180, and Press hardened steel (PHS-1500) with ultimate tensile strengths of 980 MPa, 1180 MPa, and 1500 MPa, respectively, were considered. The results showed strain localization occurred in the soft microstructure, while limited deformation occurred in microstructures with hard phases, such as the super-critical HAZ and FZ. The local stress-strain response was then compared to the common hardness scaling approach, which approximates the FZ’s local properties and different HAZ regions. The stress-strain response of the investigated BM is scaled by applying a proportional scale based on the hardness ratio. The equivalent stress-strain of the FZ and HAZ obtained by the new procedure was compared to the hardness scaled stress-strain data. The results showed the mechanical properties of the sub-zones of the welds might only be calculated using the hardness scaling approach if the microstructures across the weld result in a consistent strain hardening rate in all sub-zone.

Hot forming steel is being widely used in the e-automotive industry. It offers high strength and ensures an efficient lightweight concept. However, due to its coating alloys and mechanical properties the thermal or mechanical integration of fastening elements in thin-wall, and at the same time, ultra high-strength (UHS) components present a manufacturing challenge. In a research project carried out by the University of Paderborn, Germany, the feasibility of mechanical joining of a fastening element during the hot forming process was successfully demonstrated (maybe this should be demonstrated). This approach was then identified to improve the manufacturing process efficiency eliminating welding problems, eliminating the need of laser cutting, and decreasing riveting challenges. Based on the results of the research, further investigations were carried out by PROFIL to widen the application range of this method and test other types of functional elements under serial production conditions. Regarding its mechanical properties, the element-sheet joint was tested under various loading scenarios. The hardness of both the panel and the elements as well as the coating of the elements were evaluated at the end of the process. Cyclic load tests were carried out in different conditions to determine the susceptibility of the innovative element-panel joint to cracks. Together with customers, a reliable In-Die solution for serial production was investigated as well. Finally, this technology widens the range of elements to be used in combination with this PHS components to include for example an optimized solid pierce rivet.

Advanced high-strength steels (AHSS) are a proven enabler to lightweighting automotive body structures, but these steels can pose forming challenges due to their high strength and complex microstructures. One frequent challenge is having sufficient press tonnage to obtain desired component shape and dimensions, often requiring time consuming and expensive process development. This is especially problematic if forming tonnage approaches or exceeds press capacity.

The Auto/Steel Partnership (A/SP) Stamping Team has been working to better understand the AHSS and 3rd Gen AHSS material behavior and improve AHSS forming models to more accurately predict forming tonnages for these steels. The team recently completed the first phase of work on a project titled, “3rd Gen AHSS Press Tonnage”, which has been evaluating the accuracy of press tonnage predictions of state-of-the-art forming simulations for an open channel “Automotive-Like” panel using three different grades of AHSS and 3rd Gen AHSS. The predicted tonnages for all three steels were compared against measured tonnages in die trials, which revealed several interesting gaps in press tonnage simulations.

The first gap is in the definition of press tonnage, or more specifically, at what stage of the forming process should press tonnage be measured? The second gap, which is related to the first, should press tonnage be related to the quality of the product; the ability of the process to achieve desired part shape and dimensions? A third gap was defining how press tonnage was assessed; e.g. ram tonnage, the tonnage at all four corners, off-center tonnage, etc. This presentation will provide an overview of the lessons learned and the team’s modified approach to improving press tonnage modeling and simulation.

Accuracy in the virtual design of the forming process is critical to promote the adoption of 3rd Gen advanced high-strength steel (AHSS) into a lightweight vehicle architecture with enhanced crash performance. To leverage the superior formability of 3rd Gen AHSS relative to conventional AHSS, advanced material characterization techniques should be used to consider the complex hardening response that influences springback and press tonnage along with the dynamic forming limits that account for bending, contact pressure and non-linear strain paths. To this end, three 3rd Gen AHSS steel grades with a nominal strength level of 980 MPa and 1180 MPa were characterized at the coupon level and used to develop a numerical toolkit using the latest capabilities in AutoForm® and applied to design of a B-pillar technology demonstrator. An extensive experimental test campaign entailing forming trials, 3-D part scans, and impact tests were undertaken to critically evaluate the models using different calibration strategies. Guidelines and best practices on experimental techniques and modeling strategies which could successfully capture springback in the formed parts and subsequent crash performance are proposed. Note: This talk is related to the AISI Automotive Program project and a collaboration with AutoForm.

Lightweight yet strong vehicle body structures make use of various grades of advanced high-strength steels (AHSS). Computer-aided engineering (CAE) designs such as structural, crash, and acoustic modeling require detailed inputs to properly characterize the strength of individual resistance spot welds between the same or different grades and/or thicknesses of steels. For each new combination of steel and/or thickness, many tests such as lap-shear, cross-tension and KSII are commonly performed. Consequently, for designing a new body structure utilizing different steel grades and thicknesses, a test matrix may consist of thousands of joint combinations. Reducing the number of physical tests and prototypes can greatly speed up the automotive body design and engineering process. Here we developed integrated process and joint performance models for resistance spot welding of AHSS. Compared to the models in the literature, an advanced contact formulation was used that captured the electrical contact resistance at the faying surfaces prior to melting which is crucial for the nugget formation. Upon solidification, the contact formulation changed the local condition to glued or tied contact where the electrical contact resistance no longer existed. Such glued contact facilitated a direct transfer of the simulated results from the process model to the performance model including mapping the microstructure regions, residual stresses and strains formed during welding. Moreover, the joint performance model considered local microstructure-dependent stress-strain curves such as those for heat-affected zone subregions as well as failure criteria such as Johnson-Cook model. The integrated models were validated using experimental data of nugget size, and load-displacement curves for tension-shear and cross-tension tests. The models were applied to study the resistance spot welded 1st Gen versus 3rd Gen 980 steels where the effect of bulk resistivity on the nugget size and joint strength was investigated.

The A/SP Joining Team has been working to identify viable alternative joining technologies to resistance spot welding for joining of ultra-high strength steels and 3rd Generation advanced high strength steels for use in automotive applications. This presentation will provide an overview of the work conducted by the joining team and provide a summary comparison of alternative joining technologies against resistance spot welding in terms of peak load and energy absorption.

Laser heat treating (LHT) on automotive stamping dies resulted in overall cost reductions, shorter processing times and improved quality. These improved results for OEMs that use LHT when compared with OEMs treating identical dies with conventional methods are presented. In addition, recent advancements in LHT trim dies, trim details and hot stamping dies are also presented.

New challenges demand for new solutions. This simple conclusion is not new at all but more true than ever. In automotive industry, for example, new electric drive concepts are nixing conventional chassis designs. New chassis designs demand for even more components being manufactured from ultra high-strength steel (UHSS) and having a single or multi-chamber tubular shape. Under these circumstances traditional forming concepts like deep drawing in presses reach their limits and new ways for manufacturing have to be found. One solution here is roll forming which is particular applicable for forming, for instance, tubes from UHSS as forming and welding can be combined in just one production line.

So far, roll forming processes have been difficult to control as most of the decisions had to be taken by the operators. Having the demographic development, the desire for even more complex products and the strive for higher efficiency these challenges will even rise if we do not fundamentally change our approach to roll forming. At Dreistern we are convinced that digitalization opens up a way to overcome these limitations. We successfully designed and introduced intelligent machine components for roll forming that are able to monitor the current process state. These measurements, additional information extracted from the PLC and the matching software open up new opportunities in roll forming, for example by providing real time production data, instructions for operators or support in decision-making. Depending on the perspective, however, different requirements are set on the support to be provided by the software. Though, all applications have in common that the machine of tomorrow should support employees in their everyday activities. In this presentation, we will show how a roll forming machine can provide the support required – by becoming part of a smart factory!

We have developed a new press forming technology “STAF” (Steel Tube Air Forming) process for automotive bodies and frames such as A-pillars, bumpers, side sill, roof rail, etc. STAF is an improved hot gas metal forming that forms high-strength of over 1500MPa, tube-structured, rigid parts. We have further developed the world’s first flange simultaneous forming technology and optimized automobile frame parts. This new process is performed whereby a steel tube that is set in a die of a press machine goes through the one-pack process of “direct current heating (high speed)→high pressure air injection→flange-forming→hardening”. With this process, we can get TS 1500MPa grade component with continuous closed cross-section and simultaneously folded flanges which is essential for assembly. Unlike the traditional method of welding two-sheet metal, STAF process not only improves rigidity but simplifies the construction and overall production process. Moreover, by increasing the rigidity of the frame, it makes it possible to use much thinner materials, thus achieving an approx. 30% reduction in the weight as compared to conventionals. Additionally STAF eliminates the need for blanking and trimming, which are required for conventional press formings. Just cutting ends of tubes after formed, the yield portion will be decreased to around 90%. At the same time, given that the flanges are already formed along with the tube, the number of components can be reduced, thereby achieving a reduction in production costs. That means STAF process reduces the number of processes by eliminating furnace heating and welding process outside the press. Through the significant reduction in the number of process, the productivity can be optimized. The developed STAF prototype machine is now capable of forming automobile parts of practical level.

Currently Mubea uses micro-alloyed advanced high-strength steels (AHSS) to produce shape blanks and formed parts with variable gauges. Two new ideas were presented in 2021 to enhance the application of TRB®: 1st, Tailored Properties TRB® and 2nd, Work-Hardened TRB®. The application of these new ideas combined with an additional new development for single-sided coating will be presented.

To validate the tailored properties technology Mubea conducted a prototyping and testing of a cold-formed door intrusion beam. This will be compared to a state of the art hot formed intrusion beam. The tailored properties technology will show the advantages of a cold-formed part with same performance like the reference, the hot-formed part.

The application of Work-Hardened TRB® material will be shown on a ladder frame mid-rail. The combination of variable gauges and variable mechanical properties with press brake bending operations will show a significant advantage to regular roll profile mid rails. The Work-Hardened TRB® mid rails are closed profiles with an arc welding seam along the part. This concept offers multiple optimizing possibilities like the right thickness at the right location and a spectrum of material grades based on the chosen raw material.

In addition Mubea will introduce the single-sided coating for ladder frame application with the mid rail. With the increased request for corrosion protection ladder frame parts the necessity of zinc coating will increase as well. Due to having the zinc coating only on the inside of the profile/part, the process parameters for welding especially the welding speed can stay at the same level without decreasing the productivity.

The automotive industry has widely applied the laser welded blank (LWB) with low-strength ductile steels for the inner body panels and advanced high-strength steel (AHSS) for the Body-In-White (BIW) structure. Major benefits for applying the LWB are light-weighting, improved crashworthiness, maximum material utilization, and relatively low manufacturing cost compared to other competing light-weighting technologies such as press-harden steel (PHS) and tailored-rolled blank (TRB). Conversely, the LWB with AHSS also brought several challenges such as reduced formability of steel blank with added weld seam lines, and limited prediction capability for base metal necking and weld cracking. In this study, three standard formability tests under the uniaxial, biaxial, and plane-strain tension conditions were performed with digital image correlation (DIC) to characterize the formability of various LWB materials with both conventional ductile steel and DP steels. Detailed microstructure analyses and micro-hardness testing were conducted to obtain the metallurgical understanding and mechanical properties of the fusion zone (FZ), heat-affected zone (HAZ), and adjacent base metals (BM).The LWB with DP 780, 980 and 1180 showed distinguishable drops of the hardness on HAZ which can reduce the formability of the welded blank and may result in cracking on weld or HAZ. This effect can be minimized by joining the higher strength DP grades to other grades or gauges. The DP780, 980 and 1180 welded to itself often fractured in the HAZ/weld and was difficult to predict. However, in the mixed grade or gauge combinations studied, necking occurred on either the thinner or weaker base metal. Other LWBs with lower strength and ductile steel did not show hardness drop on HAZ. They also showed less loss of formability compared to the base metals then the LWB with DP. Regardless of the LWB materials, the weld seam is less ductile than the parent metal and will fracture before the parent metal when the major strain direction is equal or close to parallel to the weld seam direction. This should be considered for the LWB part design.