Benefits of Hot Forming Steel

Automotive trends and tight CAFÉ fuel economy regulations are driving lighter weight design every design cycle. As we reduce the weight of our cars, there is a risk of limiting vehicle safety. Therefore, many manufacturers have opted to reduce weight using plastics and composites. However, hot formed steel can also ensure a thin, lightweight and strong design. In fact, the recent use of hot formed steel has seen small Group-A cars, like the Fiat 500, in range of 5 star EURO-NCAP crash test ratings. 

“You want to have a section that is hard to protect you from a side impact, or from the cage crashing down,” said Harald Porzner Director of Product Management and Virtual Manufacturing at ESI Group. “You will also want an area that is ductile to absorb the energy of the crash keeping the driver and passengers safe. Hot forming allows for light weight strong, complex shapes, and close to nominal structures needed for the rigid structure.”

In fact, Porzner expanded that the complex shapes needed in the design of today’s car parts is due to optimized strength characteristics and the reduction of space under a car’s hood. These complex shapes are possible to produce with hot forming but would be difficult to make with cold formed high strength material. Furthermore, these hot formed shapes are typically closer to nominal allowing for easier car assembly.

Porzner applies this knowledge in his responsibilities managing ESI’s PAM-STAMP, a program able to model and simulate a hot forming process based on the CAD drawings of a part. Despite the design challenges associated in reducing a car’s weight, the automotive development time has reduced considerably. A tool like PAM-STAMP which can model, simulate, and virtually prototype the whole hot forming process is a considerable design cycle advantage.

“With PAM-STAMP, you can perform up to 40 simulations a day in the early feasibility phase. As each simulation is specialized for the hot forming field, you don’t have to spend as much time setting up the problem as you would with other FEA software,” explained Porzner.

The Science Behind Hot Forming Steel

The hot forming process is designed to freeze the crystalline structure of steel to ensure the strongest possible properties. The part is heated to about 800 ͦC (~1500 ͦF) where the atomic structure will transition from a body centric cubic lattice into face centric cubic austenite steel. If the steel is quenched fast enough, the steel will become martensite. The strained crystalline structure produced is responsible for the desired structural properties.

“The blank will go through the furnace for about 5 minutes to produce the desired microstructure. The blank is then transported to the die where it is formed into the part. This part is then quenched by a cooling rate above 50 ͦC/s (122 ͦF/s). This new microstructure structure will be hard to stretch and compress,” said Porzner.

The PAM-STAMP Workflow Simplifies Die Tool Creation

Geometry Processing

The first step to design the hot forming process is to ensure that the geometry is safe to use. Much like using CAD in other CAE software, engineers need to check the topology of the part and clean up the design. Users then define the part’s material and thickness.

Both surface and solid models can be imported as PAM-STAMP geometry. The program will be able to determine the top and bottom surface of the solid, as well as generate the mid-surface. These calculations are necessary to assess the flattening of the part.

Material Cost Feasibility

For quoting purposes, engineers can next determine the feasibility of the material costs.

PAM-STAMP determines an optimized flattened blank outline based on the 3D model, restraining force, material, and other inputs. To develop the blank outline further, nesting techniques are used. These will smooth, supplement, and offset regions in an attempt to reduce scrap. PAM-STAMP supports various nesting layouts such as one-up, two-up, mirror, and transfer die.

“Nesting is when you fit the material blank over a coil that representing the shape of the part. The program helps place the blank within the coil to get the most out of the material and reduce the scrap,” said Porzner.

To complete the quote, PAM-STAMP can auto-generate a report. The report will include data on fall off, coil width, blank area, utilization, and images of the nesting and blank outline.

Die Face Definitions

After the material costing is completed, engineers can start designing the tool faces of the die. The faces are determined based on the part B-Spline geometry and the hot forming process. Much of these design iterations can be done within a CAD environment.

The die face should be optimized for performance and simulation accuracy throughout the design cycle. PAM-STAMP also ensures the die is compliant to automotive standards. “All die face design solutions from ESI include automated interfaces to simulation. This saves 90% of the time forming, setting up and simulating the model,” mentioned Porzner.

The die face definition constitutes a large challenge in today’s automotive industry. Typically, a part is designed by engineers based on functionality and aesthetics. Manufacturability, however, is often an afterthought. PAM-STAMP assists in the die’s design and simulations for optimum costs and manufacturability.

Validation and Manufacturing Feasibility Studies

The next step of the PAM-STAMP workflow is to assess the feasibility and to validate the transport, gravity stamping, and quenching of the part.

The gravity stamp and transport simulations will assess the wrinkling, cracks and thinning while forming the part. The process parameters in the transport and gravity stamping simulation are updated iteratively until an optimum setup is created. This tends to be when the thinning is no less than 20% and there are no cracks, with no wrinkles.

Porzner explained, “You don’t need a lot of pressure to form the part, gravity on the tool is theoretically good enough. But during the quench you need to press down on the part to make sure you get the correct heat transfer during the quench.”

As such, the engineer then moves onto the simulation of the quenching process. This simulation will determine the cooling rates, hardness, and temperature of the part. Porzner mentioned that to ensure a proper heat transfer during the quench, over 90% of the surfaces should be in contact with the part at 10 MPa, 99% of the part should be in contact with the die at 1MPa.

“If you are not achieving the surface contact or pressure needed then you must spot the die. This is when you grind down the die until the part has the contact needed to get the heat out. This optimization process can take weeks,” warned Porzner.

If the part is cooled properly, then the fully hardened part will not deform much. Porzner said, “If you take the part out with a max temperature of 150 ͦC then you have the required properties from the microstructure, and you will not have any distortions affecting your assembly.”

“However,” he added, “you don’t want this final temperature to be so cold that it takes too long. Typical cycle times for a sheet of metal 2mm thick is about 10-12 seconds. This is key not only for the microstructure of the steel but also for the production cycles in the automotive industry. If you can’t make the part that fast and cheap then nobody wants it.”

Cooling Channel Analysis

Porzner mentioned that if you are going to produce one part then you don’t typically need to cool the die. The die’s temperature will be cool enough to quench the part. However, when producing many parts in an assembly line, as is the way in the automotive industry, water based liquids are used to keep the die cool.

PAM-STAMP is able to perform cooling channel analysis (including 3D heat transfer and determination of contact heat transfer properties from forming) to ensure that the cooling channels will be able to handle the production load of the system. ESI’s Computational Fluid Dynamics software ACE+ Suite, part of the hot forming solution, will allow the engineer to determine a conjugate heat transfer model to identify flow characteristics and the coolant temperature.

Bringing it all together with Virtual Reality

Now that the simulation data for the whole process is completed, the data can all be culminated into one virtual reality simulation. This simulation can be used in a 3D cave for training, or a system wide simulation to ensure just in time production.

Overall, with the completed simulation and die face design, the engineer can now sign off on the part manufacturing at a fraction of the time.

ESI Group has sponsored promotion of their PAM-STAMP software on ENGINEERING.com. They have no editorial input to this post - all opinions are mine.  Shawn Wasserman