Regarding the advantages and disadvantages of integrated die casting (Gigacasting), the German “Automotive Production” magazine interviewed Professor Wolfram-Volker, director of the Department of Forming Technology and Casting at the Technical University of Munich, Germany in February this year. Professor Volcker believes that Integrated die casting is an interesting alternative to body building technology.
Auto Production Magazine asks: What is the potential of Gigacasting or Megacasting (integrated die casting) technology and how new is it?
Prof. Walker A: The novelty is that it can be used to produce extremely large body parts using integrated die-casting technology. In die-casting technology, previously the largest body castings were shock towers and side members, which are used by Audi and BMW in their larger model series. Extending this technology to larger components, especially the rear underbody (where deformation is particularly critical), was a brave decision. At the same time, of course, this also meant that the established material concept had to be revised.
1. In what way?
A: Tesla bypasses the heat treatment problem. This new technology works with naturally hardened alloys. On the other hand, precipitation hardening alloys have advantages in strength and ductility. For naturally hardened alloys, one makes a compromise, but in turn saves the subsequent annealing treatment. No further discovery has been made as to how to grasp the deformation of a part of this size due to heat treatment.
2. Tesla hopes to use this process to eventually reduce the number of parts to one part. How realistic is this?
A: I had the opportunity to look at the Model Y on a cut surface. Tesla has drastically reduced the number of parts. However, it is clear that reducing parts by itself does not provide any economic advantage. This is because manufacturing and material costs as well as investment costs have to be considered as a whole, which we have also identified with several Fraunhofer institutes such as IAO, IFAM and IWU. In addition, the integrated die-casting does not make the body itself lighter. A large part requires a corresponding wall thickness and also loses the opportunity to place the corresponding material properties exactly in the right place, which in many cases can be achieved with classic sheet metal construction. The manufacturing costs of sheet metal parts are also low, and the corresponding joining technology can also be well automated.
3. What is the appeal of large aluminum die castings compared to sheet metal structures?
A: The sheet metal construction method, coupled with spot welding of different variants, has been developed for decades to become a tried and tested process. While in die casting the lower limit of casting thickness can be 2 to 3 mm, for sheet metal parts the wall thickness can be as low as 0.7 mm. Therefore, Gigacasting or Megacasting can neither be classified as an efficient solution per se, nor as a lightweight solution, nor as a higher performance process. But it’s an alternative, adding an interesting alternative to the rather antiquated technical toolbox in car body construction. One-piece die casting is suitable for rethinking automotive body structures, especially with an eye toward electric vehicles. In the field of electric vehicles, the battery tray is a core and fundamental new component that needs to be integrated. Therefore, the advances made over the years in the body structure of diesel locomotives are only applicable to electric vehicles to a limited extent, especially in the rear and center areas of electric vehicles. For example, using Tesla’s greenfield approach in Brandenburg, OEMs can significantly save space in the body structure of their electric vehicles. On the other hand, if looking at brownfields, it’s important to consider whether all-in-one aluminum die casting makes sense.
4. According to media reports, it is reported that aluminum die casting may save 20% to 30% of the cost, especially because the forming presses and welding robots can be reduced.
A: I would be cautious about saying that, especially in the case of welding robots, the investment is depreciated in one model generation, and in the case of forming presses, the depreciation is even in three or four generations. This technical depreciation – note, not tax depreciation – is done over 30 years. So for OEMs that already use these machines in existing generations of cars, it doesn’t make sense to use this type of new technology. On the other hand, thanks to Tesla’s greenfield approach, these typical body construction investments can be saved. In general, it is economically unwise not to continue using machines that have already been written off. So I wouldn’t buy into the so-called 20% to 30% cost savings.
5. What is the actual amount that can be achieved with aluminum castings?
A: On the one hand, in the die-casting process, there is an obvious limit to the service life of the die-casting mold. As a rule of thumb, a die casting mold has a lifespan of 100,000 to 150,000 cycles due to thermal shock. On the other hand, a molding die can handle five to six million parts. So we’re talking about a factor of 20 to 30. For this casting-intensive solution, there is obviously a limited number of products suitable. I don’t think very small and very large quantities are very attractive in aluminum die casting. In particular, for mass production in the millions, you need about six or seven of these expensive die-casting molds. We estimate the weight of the die-cast mold for Tesla’s unibody rear floor to be 80 to 100 tons. This means a huge outlay in terms of handling and peripheral equipment, eg in the form of cranes required for this. In addition, die-casting molds present technical obstacles and dangers, such as melt leakage. So the risk of not being able to produce at all is not low.
Aluminum is used in large quantities in the automotive industry. However, the use of casting to produce particularly large body parts is still a new technology (Credit: Audi)
6. How fragile is aluminum die casting?
A: The casting process is many times more complicated than the cold forming process. A key word for casting is: scrap. Cooling is a very important issue for the foundry industry. Operators must be aware that scrap rates can reach 10%, 20% or more. This, in turn, has an impact on the process in the body shop, namely how many body parts must be stored in order to be able to compensate for any failures in vehicle production.
7. Other OEMs are also considering aluminum castings for EVs and are even talking about increasing body stiffness.
A: What matters is what the phrase actually refers to. The main conflicting objectives in the body structure must be considered as a whole. On the one hand, it is the body stiffness, strength, and natural frequency, and on the other hand, it is the crash performance. The fewer components you have, the more this limits the scope of optimization. Using the die-casting process, on the one hand, has great advantages and can directly realize highly complex reinforcing elements; have a great impact. Compared to steel, die-cast aluminum alloys quickly reach the limit of engineering feasibility. So in response to this question, the question that needs to be asked is: what are we comparing? Electric vehicles look completely different in structure, and the problem of optimizing degrees of freedom arises. These are severely limited especially with regard to naturally hardening alloys for die casting.
8. How strong is aluminum compared to steel? What is the boundary between one material and another?
A: As far as alloys are concerned, there is a wide range. The strength of naturally hardened alloys that are not resistant to heat treatment is about 250 MPa up to 350 MPa. As far as stamping hardening steels are concerned, soft deep drawing steels range from 140 MPa to 1,500 MPa. For steel, the range of materials available is much wider. Of course, there is also a range of aluminum wrought alloys available: for example, 7000 series aluminum alloys provide strengths of about 500 to 600 MPa.
9. So, where does it make sense to use aluminum, especially in very specific parts of the body?
Keywords: Collision performance.
A: On the one hand, the center and rear of the underbody are doomed. Here, you don’t need such high ductility in the event of a collision. On the other hand, on the front end of the body, I think the range of use is limited, because considering the crash requirements of the front end, for example, there is a need for greater ductility and toughness. However, it should also be clear that for electric vehicles, on the one hand, the front body structure is completely different anyway, for example when an electric motor is used as a mid-mounted motor. On the other hand, battery trays for electric vehicles have significantly higher requirements on the center floor. Here, strength is more important than ductility. Of course, large components also have an impact on the likelihood and ease of repair, which ultimately affects costs. There are still some questions to be answered here.
10. So, where does it make sense to use aluminum, especially in very specific parts of the body?
Keywords: Collision performance.
A: On the one hand, the center and rear of the underbody are doomed. Here, you don’t need such high ductility in the event of a collision. On the other hand, on the front end of the body, I think the range of use is limited, because considering the crash requirements of the front end, for example, there is a need for greater ductility and toughness. However, it should also be clear that for electric vehicles, on the one hand, the front body structure is completely different anyway, for example when an electric motor is used as a mid-mounted motor. On the other hand, battery trays for electric vehicles have significantly higher requirements on the center floor. Here, strength is more important than ductility. Of course, large components also have an impact on the likelihood and ease of repair, which ultimately affects costs. There are still some questions to be answered here.
11. So the cell-to-chassis approach, the structural role of the battery in the body, is setting the tone here?
A: Due to the low placement of mass-intensive elements, this structure provides advantages for driving dynamics as it contributes to a low center of gravity and good weight distribution. The functional integration of battery housings is a fundamental principle of EV body design. However, this can also be achieved by appropriate joining operations, not necessarily relying on integral die casting.
12. At present, there are not many machines on the market for processing large aluminum parts. For example, one manufacturer is Indra from Italy. What are your predictions for this type of machine and process?
A: Idra is currently one of the equipment manufacturers in this field, but there are other equipment manufacturers who recognize the potential of this market. The envisaged machine could reach a clamping force of 80,000 kN, but would then reach a physical limit. For now, one can only guess at the usability factor of these machines. Furthermore, we will only learn more about failures and so-called break-in issues during production. Therefore, the effectiveness of the overall system remains a big issue for now.
13. On the one hand, companies that use aluminum die casting can save space in the body shop because, as mentioned at the beginning, fewer robots need to be used. On the other hand, the size of these die casting machines is huge…
A: For a factory, aluminum die casting means a considerable space requirement. An important consideration in this case is that currently the die-casting molds can only be replaced vertically with the help of a crane. It takes 10 to 12 hours to replace a mold weighing up to 100 tons. In contrast, the current die change time of high-efficiency servo presses in large stamping plants is around 3 minutes. For stamping equipment, the die can be moved in and out horizontally. For the die-casting process, the die-casting mold must be inserted vertically, otherwise there will be a problem with the release agent. Considering this situation and the fact that each machine can only produce one part, it can be said that there are considerable limitations. However, integrated die casting brings thrust and movement to production. Other OEMs are now also thinking more fundamentally and enabling new degrees of freedom in production. I myself am very much looking forward to seeing which concepts and scenarios will prevail in the future.
Personal profile:
Prof. Wolfram Volk has been the Head of the Department of Forming Technology and Foundry Engineering at the Technical University of Munich (TUM) since 2011, and the Fraunhofer Foundry, Composites and Foundry Department since 2016. Director of the Institute of Processing Technology (IGCV). He studied mechanical engineering at the Technical University of Darmstadt, Germany, and then conducted research as a research assistant at the Institute of Machine Tools at the University of Stuttgart. After his doctorate, from 1994 to 2011 he held management positions at the BMW Group in Munich, involving innovative management of equipment and plant construction as well as product and process planning in forming technology.