Die casting (high pressure casting) is a near net shape technology widely used in automotive, aerospace and electronics industries.
In die-casting process, molten metal (usually light alloy) fills mold cavity with high pressure and high speed under action of punch, and cools rapidly to form final casting.

 

Magnesium and aluminum alloys are main die-casting materials. Alloy materials for die-casting parts are mainly non-ferrous metals and their alloys, of which aluminum alloys are the largest. Ferrous metals and their alloy die-casting parts have been at a standstill for a long time.

 

Die Casting Machine
Die casting is generally divided into cold chamber die casting and hot chamber die casting. Die-casting machines can be divided into small (160-400 tons), medium-sized (400-1,000 tons) and large (more than 1,000 tons) die-casting machines according to size of clamping force.

 

Die casting mold

 

 

Die-casting parts will entrain gas, so die-casting parts are not allowed to be heat treated;
Die-casting parts are net shape and post-processed (sandblasting or other) for direct assembly without machining;
Vacuum die casting

 

 

 

 

 

Semi-solid processing method was proposed by Professor Flemings when he was working at Massachusetts Institute of Technology in the early 1970s. Core principle of itstechnology is: vigorously stir metal melt that is undergoing solidification process through stirring device, then fully break dendrites through stirring action to obtain new spherical or ellipsoid-shaped primary solid phases that are evenly distributed in metal melt, which is a semi-solid slurry. Finally, prepared semi-solid slurry is subjected to subsequent processing, which can be used for liquid die forging, semi-solid die casting, etc.

 

 

Since semi-solid processing uses non-dendritic semi-solid slurry, it breaks traditional dendrite solidification mode. It has many unique advantages compared to liquid processing:
(1) Solidification shrinkage of metal is reduced, primary crystal grains are fine, and composition is uniform, so product does not have a segregated structure and has better performance;
(2) Primary solid phase of semi-solid slurry is close to spherical, its deformation resistance is small, forming energy consumption is significantly reduced, can prepare parts with complex shapes, has fast forming speed, greatly shortening processing procedures, miniaturizing processing equipment, and reducing investment;
(3) Forming temperature is low, and part of latent heat of solidification of semi-solid slurry has been released, so solidification shrinkage and thermal shock to processing equipment are greatly reduced, which greatly improves life of mold, product size is accurate, and performance is significantly improved;
(4) Viscosity of semi-solid slurry is high, reinforcing materials (particles or fibers) can be easily added to improve technical problems such as segregation, sinking and floating, and non-wetting of additives in preparation of composite materials, opening up a new way for production of composite materials.

Key to semi-solid processing lies in preparation of semi-solid slurry. It has successively developed electromagnetic stirring technology, mechanical stirring technology, strain activation technology, single-roller rotation technology, ultrasonic vibration technology, powder metallurgy technology, spray deposition technology, low superheat casting technology, turbulence effect technology, melt mixing technology and other technologies for semi-solid slurries or blanks.

Die castings have been widely used in automotive industry.
Die castings are widely used in non-structural parts such as engines (cylinder blocks, cylinder heads, intake pipes, etc.), transmission housings, wheel hubs, etc. Among structural parts, die-casting parts are also used in chassis suspension, body-in-white structural parts (cross beams, shock towers, etc.), covering parts, interior parts and other components.

Chart: Application examples of aluminum alloy die castings for automobiles
Benefiting from development of die-casting machine tonnage (>4,000T) and new energy vehicles, die-casting parts are developing towards large-scale and integrated production. (Door frames, A-pillars, rear longitudinal frames, trunk lids, etc.) Large body structural parts can be produced and assembled by die-casting.
Integration of die castings helps reduce weight of parts and number of processes. For example, shock tower casting of some Audi models uses one part to replace steel shock tower originally welded from 10 parts, which can achieve a weight reduction of 10.8 kilograms. At present, integrated die-cast structural parts have been widely used in some luxury car models, such as Audi (A8, Q8, etc.), Mercedes-Benz C Series, BMW X Series, etc.

 

Chart: Possible applications of die castings in large body structures

 

Chart: Schematic diagram of Audi Q8 all-aluminum body and structural shock tower components

 

PQ35 MQB

 

It adopts a die-cast aluminum cylinder block with a cylinder liner. There are pressure oil supply channels, oil return channels and crankcase exhaust channels in cylinder body.
Aluminum cylinder block reduces mass, which is beneficial to reasonable distribution of front and rear wheel loads in front-wheel drive cars and reduces fuel consumption.
Aluminum cylinder has good heat dissipation performance and reduces risk of knocking.

 

Integrated die-casting: Subverting traditional process, one-time die-casting, revolutionary manufacturing process, greatly optimizing efficiency and cost.
Tesla recently launched an integrated die-casting process and quickly applied it to manufacturing of rear floor of Model Y body.
Compared with traditional "stamping + welding" process of body manufacturing, integrated die-casting has advantages of lightweight, reduced number of parts and process steps, saving of personnel and land. Tesla has used this process for production of Model Y rear floor, reducing number of parts from 70 to 1-2. It is expected to continue to be used in other models, as well as front and mid-sole panels and other components, bringing efficiency and cost optimization in many aspects.
From perspective of implementation results, we believe that integrated die-casting process has following obvious advantages compared with traditional "forming + welding" body manufacturing process:
►Lightweight: Can reduce vehicle weight by 10%.
►Improve efficiency and reduce costs: Unit time for producing a component through a 6,000-ton large-scale die-casting machine is expected to reach 90-120 seconds; and if it is produced using "forming + welding" method, it would take about two hours to stamp and then weld more than 70 structural parts to also produce rear part of body floor, would replace multiple forming and assembly production lines, thus greatly reducing manufacturing costs such as personnel, equipment and land. According to Musk’s statement at Tesla Battery Day, this manufacturing process can reduce manufacturing cost of rear floor panel by 40%.
►Improve strength of parts: Integrated die-casting avoids reduction in strength caused by welding. At the same time, elements such as mounting holes and installation locations can be ignored when designing parts, thereby using a more optimized engineering structure.
Model Y rear floor is integrally formed using a 6,000T die-casting machine, reducing 70 parts to 1-2. According to Tesla's announcement, it has deployed multiple large-scale die-casting machines with a tonnage of 6,000 tons in United States, Shanghai and Berlin factory under construction for integrated die-casting of Model Y rear floor. Compared with 70 parts of rear floor of Model 3, number of parts is reduced to 1-2 after Model Y adopts an integrated die-casting process.

 

Chart: Tesla Shanghai Gigafactory Model Y die-casting workshop: Lijin 6,000 tons

 

 

Source: Tesla Inc. announcement
Multi-DIRECTIONAL UNIBODY CASTING MACHINE FOR A VEHICLE FRAME AND ASSOCIATED METHODS patent released (US20190217380 - MULTI-DIRECTIONAL UNIBODY CASTING MACHINE FOR A VEHICLE FRAME AND ASSOCIATED METHODS)

 

Chart: Schematic diagram of Tesla’s one-piece die-casting machine patent

 

Source: Patentscope, CICC Research Note: Yellow marks represent individual die-casting units
Manufacture of traditional automobile bodies requires thousands of welds on hundreds of structural components. Body-in-white is generally composed of 300-400 parts, the number of welding points can reach 4,000-6,000.
Chassis structural parts mainly include front and rear subframes, control arms, steering knuckles, suspension connecting rods and other complex structural parts. Therefore, manufacturing process of automobile frames is complicated and production time is also long. In terms of equipment, parts are generally formed through standardized stamping machines, extrusion molding machines or die-casting machines. For welding, robots are generally used for automated production, so investment in equipment is relatively high.

Equipment is core difficulty in realizing integrated die-casting and has high barriers. Integrated die-casting process involves integrated molding of complex, large-scale, high-precision parts, and there are rigid requirements for clamping force, injection volume and control capabilities of die-casting machine. Due to its high difficulty and innovation, large-tonnage die-casting machines have high barriers in terms of technology, supply chain, and process control.