You are most likely wondering why WorldAutoSteel is writing a blog about a bicycle. It is because when we talked to Jia-Uei Chan, Regional Business Development at our member company, thyssenkrupp Steel Europe (TKSe), about the journey of inventing the world’s first Advanced High-Strength Steel road bike, we were incredibly inspired. This is more than a story about a steel bicycle. This is the story of steel innovation, conceived in a WorldAutoSteel members workshop to brainstorm ideas on transforming steel’s image to the sophisticated and advanced material it is. Their journey led to new steel applications, patentable processes, and in the steelworks bicycle, ideas that we think can inspire new automotive applications as well. And anyway, who doesn’t like an inspiring story?
Bikes of this genre have some of the same requirements of modern vehicles: lightweight, strength and durability, affordability, and high performance. To achieve these, the thyssenkrupp steelworks team developed what they called inbike® technology, which combines high-strength steel, half-shell technology and automated laser welding.
How it was made
The bike frame is made from DP 330/590 steel, used for its cold forming abilities, stamped as thin as 0.7mm. The steel blanks are pressed into a die to form two half-shells in a deep-drawing process.
A major challenge was to bring these two half shells together in such a way that minimized gaps and achieved a tight fit, enabling automated laser welding (this process requires no gaps over 6 meters of contact length), while ensuring that the frame achieves an elegant, seamless look. Enter innovation.
At the stamping plant, the half-shells were fitted with “dimples,” (See Figure 1) tiny bumps on the welding flanges that create channels at the weld seam for the zinc, preventing vaporized zinc from remaining trapped in the seam during subsequent welding. The half shells were then clamped in a special device and shipped to the laser specialist (See Figure 2).
Figure 1: Tiny bumps prevent vaporized zinc from remaining trapped in the seam during subsequent welding.
Figure 2: Frame half-shells clamped in the device for laser welding.
The particular challenge lay in the reliable processing and fusing of both frame halves by means of automated laser welding in such a way that no damage to the frame would occur, while also ensuring the weld seam lay as close as possible to the bend radius of the frame halves. The complex frame shape is welded by following a sophisticated trajectory in a 3-D space. After countless continuous improvement exercises, the steelworks team was able to achieve a very flat, elegant weld seam design. This translates into a very stable bike, with a frame that has the needed rigidity in the bottom bracket area to enable high biomechanical power transmission, but with high elasticity in the seat tube configuration to make for an unusually comfortable ride. In comparison, aluminium and carbon fiber bikes are very stiff and characteristic of an unpleasant ride experience.
Inventing the possibility
Tackling a project that is such a reach beyond the norm is never easy. The thyssenkrupp steelworks team repeatedly heard from qualified experts that the project was actually not feasible. At the same time, they had partners who were so fascinated by the challenge that they wanted to make it possible. Chan related to WorldAutoSteel that there were many times when giving up was the more attractive option. Endurance won out. And as it turns out, the half-shell technology invented out of necessity for this bike could find an application in the tough requirements of an electric vehicle battery case.
Says Chan, “We genuinely believed that steel is the perfect material for a road bike. And we wanted to break with convention and make the most out of steel with high-tech engineering.” Have a look at steelworks.bike, and you will undoubtedly agree they did just that.
Dr. Donald Malen, College of Engineering, University of Michigan, reviews the use of two recently developed Powertrain Models, which he co-authored with Dr. Roland Geyer, University of California, Bren School of Environmental Science.
The use of Advanced High-Strength Steel (AHSS) grades offer a means to lightweight a vehicle. Among the benefits of this lightweighting are less fuel used over the vehicle life, and better acceleration performance. Vehicle designers as well as Greenhouse Gas analysts are interested in estimating these benefits early in the vehicle design process. G-13
Models are constructed for this purpose which range from the use of a simple coefficient, (for example fuel consumption change per kg of mass reduction), to very detailed models accessible only to specialists which require knowledge of hundreds of vehicle parameters. Draw backs to the first approach is that the coefficient may be based on assumptions about the vehicle which do not match the current case. Drawbacks to the detailed models are the considerable expense and time needed, and the lack of transparency in the results; It is difficult to relate inputs with outputs.
A middle way between the simplistic coefficient and the complex model, is described here as a set of Parsimonious Powertrain Models. G-10, G-11, G-12 Parsimony is the principle that the best model is the one that requires the fewest assumptions while still providing adequate estimates. These Excel spreadsheet models cover Internal Combustion powertrains, Battery Electric Vehicles, and Plug-in Electric Vehicles, and predict fuel consumption and acceleration performance based on a small set of inputs. Inputs include vehicle characteristics (mass, drag coefficient, frontal area, rolling resistance), powertrain characteristics (fuel conversion efficiency, gear ratios, gear train efficiency), and fuel consumption driving cycle. Model outputs include estimates for fuel consumption, acceleration, and a visitation map.
Physics of the Models
Fuel consumption is determined by the quantity of fuel used over a driving cycle. The driving cycle specifies the vehicle speed vs. time. An example of a driving cycle is the World Light Vehicles Test Procedure (WLTP) cycle shown in Figure 1.
Figure 1: Fuel Consumption Driving Cycle (WLTP Class 3b).
Given the velocity history of Figure 1, the forces on the vehicle resisting forward motion may be calculated. These forces include inertia force, aerodynamic drag force, and rolling resistance. The total of these forces, called tractive force, must be provided by the vehicle propulsion system, see Figure 2.
Figure 2. Tractive Force Required.
Once vehicle speed and tractive force are known at each point of time during the driving cycle, the required torque and rotational speed may be determined for each of the drivetrain elements, as shown in Figure 3 for an Internal Combustion system, and Figure 4 for a Battery Electric Vehicle.
Figure 3. Internal Combustion Powertrain.
Figure 4. Battery Electric Vehicle Powertrain.
In this way, the required torque and speed of the engine or motor may be determined. Then using a map of efficiency, shown to the right in Figures 3 and 4, the energy demand is determined at each point in time. Summing the energy demand over time yields the fuel used over the driving cycle. The reader is referred to References 1 and 2 for a much more in depth description of the models.
Example Application
As an example application, consider the WorldAutoSteel FutureSteelVehicle (FSV).W-7 The FSV project, completed in 2011, investigated the weight reduction potential enabled with the use of AHSS, advanced manufacturing processes and computer optimization. The resulting material use in the body structure is shown in Figure 5.
This use of AHSS allowed a reduction in the vehicle curb mass from 1200 kg to 1000 kg. What are the effects of this mass reduction on fuel consumption and acceleration performance? The inputs required for the powertrain model are shown in Table 1 for the base case.
Table 1: Model Inputs for Base Case.
The results provided by the powertrain model are summarized in the acceleration-time vs. fuel consumption graph of Figure 6. Point A is the base case at 1200 kg curb mass. The lightweight case with same engine is shown as Point B. Note the fuel consumption reduction and also the acceleration time reduction. Often the acceleration time is set as a requirement. For the lighter vehicle, the engine size may be reduced to achieve the original acceleration time and an even greater reduction in fuel consumption as shown as Point C.
Figure 6. Summary of results of base vehicle and reduced mass vehicle.
Using the parsimonious powertrain models allows such ‘what-if’ questions to be answered quickly, with minimal data input, and in a transparent way. The Parsimonious Powertrain Models are available as a free download at worldautosteel.org.
This article summarizes a paper entitled, “RSW of 22MnB5 at Overlaps with Gaps-Effects, Causes, and Countermeasures”, by J. Kaars, et al.K-12
This study aims to elaborate on the influencing mechanisms of gaps on the welding result. Welding experiments at artificial gaps and finite element analysis (FEA) of the welding process have been used to investigate the matter. In both methods, the same configuration of two 1.5-mm-thick 22MnB5+AS150 welded with electrodes of the type ISO 5821 B0-16-20-40-6-30 was considered. Tensile tests yielded an ultimate tensile strength (UTS) of the press-hardened material of 1481 ± 53 MPa with a strain to fracture of 7.5 ± 0.26%. A microsection of the coating morphology after heat treatment can be found in Figure 1.
Figure 1: Morphology of the Aluminum-Silicon Coating.K-12
To set up an artificial and reproducible gap between the sheets, a dedicated fixture was used. It is displayed in Figure 2. All welding experiments were carried out with a 6-kN electrode force.
Figure 2: Fixture for Welding at Artificial Gaps, Definition of Quantities.K-12
In Table 1, the parameter variations of the gaps investigated in this work are presented.
A 7-kN maximum denting force was observed at the gap (10|60). With a gap of (10|40) the gap could not be closed with the machines’ 8-kN clamping force capacity. In comparative tests on mild steel for deep drawing a clamping force of about 2 kN was required to overcome the gap (10|60) (see Figure 3). The main effects diagram of the denting force clearly shows that the average denting force gets smaller with increasing support width and becomes larger with increasing gap clearance.
Figure 3: Main Effects Diagram of the Denting Force.K-12
In Figure 4, the achieved nugget diameters at different gaps using a constant machine setting of Iw,f = 6.4 kA are displayed.
Figure 4: Effect of Gaps on Nugget Diameter, Absolute and Relative Results.K-12
A two-staged welding program, starting with a preheat current followed by a larger finishing current proved to yield the best welding results with the material used, cf., Figure 5. In Figure 5, the applied welding current program along with the measured and computed total resistance curve is displayed.
Figure 5: Exemplary Total Resistance Curve of a Weld without Gap, Measured and Computed Results.K-12
The FEA model can represent the welding process in terms of nugget diameter, dynamic resistance curve, and total electric energy with great accuracy. In Figure 6, the partial resistances of the weld as computed by FEA are composed.
Figure 6: Partial Electrical Resistances at Different Gap Configurations.K-12
In the top section of Figure 7, the computed sheet thickness curve during the process for different gaps is presented. Increased electrode indentation during welding at gaps is the reason for reduced resistance and, therefore, results in reduced nugget diameters. The lower section of Figure 7 shows the plastic strains in the sheets along with a visibly reduced sheet thickness.
Figure 7: Dynamic Sheet Thickness (up) and Plastic Strain in millimeters at Different Gaps (low, to scale).K-12
Additional welding experiments were performed to clarify, if increased welding current can counter the gap effect and maintain the energy level of the weld. The results are shown in Figure 8. They prove that increased weld current is sufficient to not only maintain the nugget diameter at gaps, but moreover increase it.
Figure 8: Nugget Diameter and Energy of Spot Welds near the Splash Limit at Overlaps with Gap.K-12
Results of further investigations on the weldability lobe of the joint are composed in Figure 9.. It is visible that with increasing gap the current range shifts toward larger currents and gets narrower.
This article summarizes a paper by W. Mohr and N. Kapustka, EWI, entitled, “Fatigue of GMAW-P Lap Joints in Advanced High-Strength Steels.”M-13
EWI has performed fatigue tests on welds from four Advanced High-Strength Steels (AHSS) in the uncoated condition. The materials were provided in three thicknesses as follows, 2.0-mm DP 780, 1.8-mm 590 SF, 2.0-mm DP 980, and 2.8-mm CP 800. Referring to Figure 1(a), welding parameters were selected to meet the weld profile requirements listed below:
The travel speed to achieve such combinations was 23 mm/s for three of the sheets and 13 mm/s for the 2.8-mm-thick CP 800. Figure 1 shows a completed panel.
Specimens were cut from the lap-welded panels in a configuration recommended by Z 2275, with minimum reduced sections of 20-mm wide, with 20-mm radii on both sides to a full width of 30 mm, as shown in Figure 2.
Fixtures for the bend testing had eight, 6.3-mm radius rollers, four on top and four on the bottom, with offsets of the roller centers to accommodate the lap-joint configuration and the differing sheet thicknesses. The interior span was 120 mm, while the exterior span was 210 mm. The full bending fixture, with a specimen inserted, is shown in Figure 3.
Weld profiles were achieved that met the weld profile requirements for each sheet material type. These weld profiles are shown for the four sheet materials in Figure 4. Fatigue testing results in tension at R = 0.3 gave lifetimes between 30,000 and 9 million cycles, with run-outs at 10 million cycles, as shown in Figure 5.
Figure 4: Cross Sections of Lap Joints (etched with 2% Nital).M-13
Figure 5: Results of Fatigue Testing in Tension at R= 0.3.M-13
Weld root cracking dominated in the 590 SF, as well as the DP 780 and DP 980, with an example shown in Figure 6. Weld toe cracking was observed on the 2.8-mm-thick CP 800, with an example shown in Figure 7.
Figure 6: Example for a Root Crack Breaking Through the Weld Metal on DP 980.M-13
Figure 7: Example of a Toe Crack Breaking Through the Base Metal.M-13
Fatigue testing in bending at R = -1 gave lifetimes between 30,000 and 2 million cycles, with run-outs on tests that continued to up to 7 million cycles, as shown in Figure 8.
Taking the differing thicknesses, minor variations in minimum width, and the stress concentrations from the radii into account, the concentrated stress range was calculated to compare the four materials on a common basis, as shown in Figure 9.
Figure 9: Concentrated Stress Range versus Lifetime for Tension Tests.M-13
The fatigue cracks initiated at the root for the 1.8-mm 590 SF on both tension and bending testing. The fatigue cracks initiated at the weld toe for the 2.8-mm CP 800 on both tension and bending testing. The fatigue cracks initiated from the weld root in the tension testing and from primarily the weld cap in bending testing, for the 2.0-mm-thick DP 780 and 2.0-mm-thick DP 980.
