Role of die-casting industry in electric vehicles
Aluminium die-casting has been at the forefront of the automotive industry development given its ability to produce complex parts with high automation and recyclability. Although aluminium has been used in automobile for several years, it has now become the fastest growing as well as preferred material for modern automobiles. This is due to more emphasis on lightweight and emission standards by automakers. Given the global consensus to counter climate change, electrical vehicle technology has been given special focus and a united push by many countries. This drive in electrical vehicles will be a boon for aluminium, especially given its strength-to-weight ratio advantages over traditional materials.
The progress in aluminium metallurgy, especially development of Al-Si-Cu-Mg alloys, along with new die-casting techniques, have enabled aluminium casting to replace most iron casting and some sheet metal parts. Although the aluminium die-cast part might have a higher unit price, in comparison to traditional parts, the total manufacturing cost of casting will be lower. The significantly greater freedom of design will create cost savings, especially given that it avoids joining of parts through welding and other assembly services.
Even though internal combustion engines, which contribute roughly 200 aluminium castings per vehicle, will become obsolete, it is estimated that Electric Vehicles (EVs) will use 25% to 27% more aluminium by weight than combustion engine cars. Currently, at an average of 250 kg of aluminium per unit, EVs have already created a demand for around 250,000 million metric tonnes of aluminium, which is expected to soar to 10 million tonnes by 2030. EVs are starting to create a niche market in the automotive sector, accounting for nearly 3.5 million passenger vehicles and 421,000 electric buses sold in 2019. Even though this accounts for less than 5% of the market share, it is expected to rise tenfold through 2030 to 30%. China, which already accounts for nearly 50% of the demand, is in the lead followed by Europe and the US.
The key challenges
One of the key challenges for EVs vehicles to compete with their internal combustion engine (ICE) counterparts is range. This typically requires better battery technologies and, more importantly, lighter vehicles to compensate the weight of the battery pack, which is around 200 kg to 300 kg. Lightweight structures and weight-reduction solutions have gained significance to address this problem. The demand for high-integrity structural parts through die-cast will increase along with the need for bespoke tooling and design directions for
weight reduction solutions through casting simulations. All the four main processes of sand, gravity, high- and low-pressure die-casting technology will be essential in meeting the varied challenges posed by EVs.
The optimised cooling requirement of batteries will require the use of sand cores or inclusion of tubes to produce these complex and functionally integrated solutions. A large proportion of demand will also come from the housing requirements of batteries produced, typically through high-pressure die-casting as well as structural components for body parts such as pillars, strut consoles, rear-side members, mounting for shock absorbers, rear axle cross members, etc., as demonstrated in Fig. 1. A Mercedes C-Class leads to a reduction of 20% to 25% body weight. The same has been demonstrated in Ford F-150 trucks. This will open up new opportunities as well.
Overcoming challenges
However, the die-casting industry will need to rise up to meet the challenges posed by EVs. This would include significant improvement in the casting process, developments of alloys to suit applications, simulation of the casting process, design of tools, joint standards for specifications and quality inspections to ensure comparable production processes and results. The larger and more advanced structural components would demand newer die-casting presses with better control over parameters to be able to cast thinner, complex, and variable cross-section parts. For example, Tesla has installed a giga press of 6100 tonnes from IDRA to cast its rear underbody for Model Y, which will replace 70 stamping, extrusions and castings and save 20% labour as well reduce the size of their body shop by 30%. Also, another area where Tesla has made significant strides is the development of new Al alloys. Commercial cast aluminium can either possess high yield strength
(A356-175MPa) or high conductivity (8030 – 60% IACS). Tesla has come up with new aluminium alloys tweaked to achieve high yield strength of 90 to 150 MPa as well as electrical conductivity of 40% to 60% IACS, in addition to having proper fluidity to ensure the cast solidifies well along the entire length of the mould.
The mould/die plays another pivotal role in the entire supplier chain process. The requirements on the tooling will be extreme, considering the intricate and complex parts with varied wall thickness. The casting simulation technologies have developed greatly to accurately predict defects caused by temperature (soldering, hot tear, cold shut etc.,), velocity (erosion, die flash, turbulence), pressure (blow holes, air entrapment), and solidification (shrinkage porosity, non-fill).
Considering the specifications requirements of the components, an optimum feeding system needs to be developed along with thermal balancing in dies to produce robust castings. Real-time process control of parameters needs to be introduced in the dies to identify deviation from the original setup. Die life enhancement methods, including heat treatment of inserts, maintenance standards of die, surface treatment of inserts, optimised cooling systems, etc., need to evolve to meet the traditional stamping tools output.
The critical specifications of the structural components would include the mechanical properties of the casting such as tensile strength (>180MPa), yield strength (>120 MPa) and elongation at break (>5-10%). High demands will also be placed on the absence of defects in the casting, which would require high vacuum to ensure good microstructures, squeeze adoption to eliminate shrinkage porosity and good gating system as well as thermal balancing to eliminate cold shuts, end non-filling, peeling as well as soldering related defects. Special requirements also apply to the microstructural and surface quality of the parts. Also, warpage is unavoidable with large- and thin-walled components due to internal stress resulting from the casting process and further exacerbated by heat treatment, which needs to be straightened by the necessary process.
Rise to meet the challenge Rise to meet the challenge Therefore, electric drive technology will require highly complex aluminium components to complete fully integrated EV modules, internal transmission parts, housing structures for power electronics, electric motor housings, energy recovery components and fuel cell stacks, all of which will offer opportunities to foundries. Thereby, die-casting will play a pivotal role in the development of EVs. However, the industry will need to rise to meet the challenges posed by the new technology.
About the Author Dr. Arunvinay Prabakaran is the Technical Director of Dietech India Pvt. Ltd. He graduated in Metallurgical Engineering from PSG College of Technology, Coimbatore, with first class with distinction in 2011. He did his PhD research in Materials Science under the supervision of Prof. Nicole Grobert in Nanomaterials by Design Group in the Department of Materials, Oxford University. After completing his PhD, he started his professional career as Assistant Technical Director at Dietech India Pvt Ltd. under the guidance of his father who is well regarded in the field of aluminium die casting.
References1. Mercedes C- Class: the great stride to aluminium casting, Casting Plant & Technology, 2/2016, pg 12-162. Electric vehicles demand lightweight solutions-Enter aluminium casting, News Article by MAN Group, May 24 20193. Automotive Engineering, SAE International, 2020-06-03, Tesla casts a new strategy for lightweight structures
