Past GDIS™ Presentations

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.

The Inter-Industry Conference on Auto Collision Repair (I-CAR), is a not-for-profit education, knowledge, and solutions organization whose vision is that “Every person in the collision repair industry has the information, knowledge, and skills to perform complete, safe, and quality repairs for the ultimate benefit of the consumer”. As more automakers develop alternative joining methods for attaching various materials, the collision repair industry cannot recreate these joining methods completed at the factory during repairs. Alternative attachment methods designed with available collision repair equipment is crucial to safely restore the vehicle to the original specifications during repair. The collision repair industry’s only source for this information on the correct joining method comes from the OEM engineering departments.

To help the collision repair industry understand these complex repair challenges I-CAR initiated a research project to identify the required information, knowledge, and skills required to restore these vehicles to the OEM specifications that meets crash worthiness design requirements. As we performed this research, we looked at a cross section of the collision repair information available today from various manufacturers. Identifying key pieces of information, tool requirements, techniques, and skills vital to correctly repair these vehicles.

This session provides an opportunity to share the findings of our research with vehicle material engineers, in hopes that as vehicles are being designed and built, the collision repair industry is considered when determining the types of repair information that is needed and how it’s presented to allow for a complete, safe, quality repairs. With the support of material engineers, vehicle makers can adopt the best-in-class repair information for the ultimate benefit of the consumer.

The battery electric vehicle (BEV) enclosure (battery pack) represents an important subsystem. With increased focus on safety, affordability and sustainability of future BEV for mass automotive market, steel offers the best flexibility with solutions to address these key challenges. In this study, using an existing aluminum battery enclosure design as the baseline, a battery enclosure design using advanced high-strength steel (AHSS) is completed which meets all the requirements.

Previously developed coating-free press-hardening steel (CFPHS) had an ultimate tensile strength of about 1.7 GPa and a tensile elongation of 8-9% after hot stamping and simulated paint baking. This combination of strength and ductility, along with its adequate bendability, enables a significant improvement in energy absorption compared to the existing Alsip-coated 22MnB5 grade of 1.5 GPa tensile strength. To investigate potential application beyond a monolithic stamped component, laser welding of the 1.7 GPa coating-free PHS was conducted. Microhardness and tensile properties of the welds were evaluated. Microstructure of the weld seam is also analyzed. Two exemplary applications are explored. The first is a miniaturized door ring of 1:4 scale with similar sheet thickness. The second is a hot air blow formed tubular structure mimicking a hollow twist beam component. In both cases, the weld seam remains crack-free during the hot forming process, and the surface is still shiny which indicates a very thin oxide layer.

A study was performed on the ability to laser weld DP600 material to USIBOR 2000 material using a mode shaping laser. The Alsip coating and as-quenched material properties resulted in numerous defects using conventional fiber lasers that would not meet the quality requirements.

Metallurgical and mechanical testing on coupons with varying gap was performed to validate the process and process window. Additional metallurgical testing was performed on a series a production intent parts to ensure that successful weld could be made in a simulated manufacturing environment.

Using a combination of independent power modulation of the ring and core of the mode shaping laser and beam oscillation with a scanner the quality requirements were achieved. The mode shaping laser was able to achieve the quality requirements by ensuring mixing of the Al from the AlSi coating and controlling the cooling rate of the molten pool.

Automotive manufacturers are faced with the necessity to produce vehicles that have less effect on the environment. The environmental assessment must look at the amount of energy and emissions over the lifecycle of the vehicle and include Production, Use, and End of Life factors. Tools have been developed that enable environmental assessment on a vehicle component basis.

The main structural component of the Bumper System is the front and rear bumper beam. Most automotive bumper beams are manufactured out of steel or aluminum. Steel is the lowest cost material, where aluminum enables mass savings. In the past, the decision to use aluminum bumpers was focused on mass savings only. Decisions based on mass savings consider the energy consumption and CO2 emissions from the Use portion only of the lifecycle assessment and ignore the other two factors of component Production and End of Life.

The University of California – Santa Barbara has developed a life cycle environmental assessment tool which includes all three factors of Production, Use, and End of Life for automotive vehicle applications. This enables vehicle manufacturers to assess steel versus aluminum bumpers with all three factors and make decisions that would lower the impact on the environment. Results have shown steel bumper beams have a favorable environmental impact over aluminum when using this assessment tool.

The failure behavior of resistance spot welds (Button pull-out vs. Interfacial) in ultra-high strength hot stamped steels plays a vital role in overall performance of safety components in automotive body structures. Spot weld failure analysis under shear and normal load is usually performed using standard lap-shear and cross-tension tests which provide some information on the strength and failure mode. However, the standard tests may not be used to investigate through thickness damage progression and failure mechanism due to the fact that failure in spot weld occurs in an enclosed space and cannot be directly observed during the test. In this work, a novel testing geometry design is coupled with digital image correlation technique for in-situ damage analysis of spot welds. The results show the actual sequence of damage progression in spot welds under normal and shear loading for hot-stamped Usibor®1500-AS and Ductibor®1000-AS alloys. Spot welds within the transient softened zone at the fusion boundary fail by Corona debonding, then shearing at the fusion boundary, followed by rapid crack propagation towards the surface. For the interfacial mode, failure is initiated by crack propagation into the fusion zone followed by global shearing.

A microstructure-mapping technique is used to simulate pull-out and interfacial failure using meso-scale finite element methods. The developed models are able to capture failure modes and the predicted force-displacement responses match with the experiments. The simulation results showed that the state of stress for pull-out failure can be much more severe (triaxiality higher than unity), compared to the typical triaxiality range that is used for sheet metal fracture characterization.

Results from a recent Auto/Steel Partnership (A/SP) project were presented on the topic of laser hardening process development for 3 different tool and die materials.

Samples were hardened using D2, TS7 and S2333 (Caldie) die materials which were machined to replicate typical trim and forming die tool finishes. Fraunhofer USA successfully developed a laser process for locally hardening of the trim edges and forming radii of the supplied samples.

A laserline diode laser was used with a specialized zoom optic for adjusting the laser focus spot size in order to further optimize the hardening results for each material type. Pyrometer based process control was used to optimize the heat input consistency during hardening by closed loop control of the laser power to achieve more consistent hardening results.

All 3 materials were successfully hardened and micro hardness measurement results showing sufficient depth and level of hardness increase were presented. Details of new technology recently developed by Fraunhofer IWS in Germany for scanner based laser hardening with Thermal Field Control (TCF) and close loop pyrometer and EMAQS camera based monitoring which will facilitate improved hardening of complex tool geometries were presented.