Tesla innovates in aluminum

Well known, that Elon Musk's company Tesla is constantly introducing many innovations in all, what does she do, from the software of their cars to the chemistry of electric batteries, which set them in motion. This also applies to metals., from which cars are made, as well as methods of their processing.

Innovative aluminum alloys

Durable aluminum castings without heat treatment

Just a couple of weeks ago, at the end of July 2021 of the year, Tesla has filed a patent for new cast aluminum alloys [1]. He describes aluminum alloys, which are both strong and flexible. Castings from these alloys do not require further heat treatment and therefore reduce production costs..

Tesla's patent description notes, that industrial casting alloys, such as are used for the manufacture of bodies for electric cars, must be strong at the same time, and plastic. These aluminum car parts are usually made by casting. Good aluminum castings differ in that, that they are produced quickly and reliably, and their chemical composition and structure provide them with the necessary structural characteristics. However, if alloys have low casting properties, then this leads to hot cracking of the castings, which creates quality problems for them.

Tesla highlights, that numerous structural elements, which are made from aluminum alloys at present, usually require additional processing, such as heat treatment, which increases strength, hardness, ductility and corrosion resistance. These additional treatments improve the quality of these parts., but they also require large capital expenditures, lengthen the production process and reduce productivity. Therefore, it would be prudent to produce aluminum alloy castings, which have a high yield point and sufficiently high plasticity without additional heat treatment.

One of these tasks, which this patent solves, is the following. Aluminum alloy must have a yield strength of at least 130 MPa and bending angle not less 20 degrees at cross-section thickness 3 mm in as-cast state and without additional processing. This problem is solved by various combinations of alloying elements and hardening precipitates with silicon content from 6 to 11 %.

Durable aluminum castings with high electrical conductivity

Another patent [2] notes, that standard industrial aluminum alloys do not always allow designers to effectively solve the assigned tasks. for instance, casting aluminum alloy A356 has a yield strength of at least 175 MPa, but its electrical conductivity is only 40 % from copper standard (IACS). And on the contrary, Aluminium alloy 100.1 has electrical conductivity not less, than 48 % from copper standard, but its yield point is only 50 MPa (drawing 1).

Figure 1 - Famous cast aluminum alloys,
copper alloy 10100-O and wrought aluminum alloy 6061
in coordinates "yield point - electrical conductivity",
as well as a given field for the development of new alloys [2]

For some products, for example, such units of an electric car as a rotor or an inverter, it is very desirable to have high performance for both of these characteristics - yield strength and electrical conductivity. These tasks are solved by a special selection of the chemical composition of casting aluminum alloys..

High pressure die casting machines

High pressure casting principle

Conventional high pressure casting is a metal casting technology, which typically involves "pushing" or injecting molten metal under high pressure into a cavity of a metal mold, which consists of two detachable parts (figure 2).

Figure 2 - Principle of high pressure aluminum casting [3]

 

Giga Press Casting Machines

Currently, a large number of different aluminum parts are used in the manufacture of automotive bodies., which are cast under high pressure on separate casting machines. These parts from each casting machine are then assembled together by workers or robots, for example, by welding, what to form a car body.

Tesla announced, that her huge Giga Press foundries will produce for her Model Y vehicles the rear and front underbody cages with electric battery cells in one-piece aluminum castings (picture 3).


Figure 3 - Solid elements of the body of the electric vehicle Model Y
Tesla company [4]

These machines are manufactured for Tesla by the Italian company IDRA. They are the largest injection molding machines in the world with a clamping force of 5500 to 6100 tonnes. Each such machine weighs 410-430 tons (figure 4).


Figure 4 - Giga Press Casting Machine
at the Tesla plant in Fremont, California [5]

A portion of molten aluminum mass 80 kg is injected into the mold at a rate 10 meters per second. The cycle time is 80-90 seconds, which allows to produce 40-45 finished castings per hour or before 1000 pieces per day.

Injection molding is done by "injecting" molten aluminum into a reusable mold. Then the form expands, to remove the finished casting after, how will it harden. The open mold is cooled down to 185 degrees Celcius, cleared by robots and through 1-2 minutes after the previous cycle, a new portion of molten metal is fed. Each "fresh" casting is machined to approximate specified dimensions, checked for defects and sent to a CNC device for machining to finished dimensions.

The whole technology of manufacturing large solid aluminum parts

However, even such a huge machine as the Giga Press is only part of a more general technology for making large aluminum parts.. Technological stages and equipment used for this technology include [5]:

  • Melting furnace (850 degrees Celsius)
  • Dispensing oven (750-850 degrees Celsius)
  • Giga Press Casting Machine
  • Oil quenching tank
  • Pre-machining press (trimmer)
  • X-ray unit for porosity control
  • Laser machining
  • CNC machining
  • Laser Coordinate Measuring Machine
  • Installation of fasteners
  • Shredder of machining waste (shredder) and their transfer for remelting
  • Vacuum system

Aluminum Alloy For Giga Press Casting Machine

According to [5], cast aluminum alloy, which is used on Giga Press machines for casting large aluminum parts, is close to the well-known aluminum alloy AA386. This alloy has a nominal silicon content 8,5 %.

Multi-axis high pressure casting machine

Even such huge casting machines as Giga Press casting machines no longer suit Tesla. Therefore, Tesla filed a patent [7], in which he proposes to improve machines for high pressure molding so, so that they can cast as large parts as possible or even the entire car body as a whole (picture 5).


Figure 5 - General view of a multi-axis casting machine [7]:
108 - component parts of a casting mold with injectors;
110 - guide rails;
114 - mold cavity;
116 - one-piece body

Such machines will reduce the number of foundry machines or the number of aluminum castings., which are required for the finished car body, such as, to the sixth, to five, up to four, up to three, up to two or one casting machine "[7].

The peculiarity of these machines is that, that they include a large number of injectors for feeding molten aluminum into the mold, which are located along the axes in different planes and at different angles (figure 6).


Figure 6 - Multi-axis die-casting aluminum car body
under high pressure:
108a - part of the high pressure casting mold;
118 - multi-axis molten aluminum injectors

And that is not all

Tesla continues to file patents, which are aimed at increasing the productivity and reducing the cost of the manufacturing process of electric cars. We will continue to highlight those, which relate to aluminum.

Sources:

  1. Patent WO 2021/150604 – Die Cast Aluminum Alloys for Structural Components (Tesla)
  2. Patent US 2019/0127824 – Casting Aluminum Alloys for High-Performance Applications (Tesla)
  3. TALAT Lecture 3201 – Introduction to Casting Technology (1994)
  4. https://electrek.co/2020/10/07/elon-musk-tesla-structural-battery-cells-model-y-giga-berlin/
  5. https://en.wikipedia.org/wiki/Giga_Press
  6. Tesla Fremont Roadrunner & Gigacast – Battery Day, Sep 22, 2020 – YouTube
  7. Patent US 2019/0217380 A1 – Multi-Directional Unibody Casting Machine for a Vechicle Frame and Associated Methods (Tesla)