Development of Magnesium Alloy Electric Drive Housing and Lightweight Design [Electric Drive System Conference]

On June 21, at the 2025 SMM (4th) Electric Drive System Conference & Drive Motor Industry Forum - Automotive Electric Drive System Forum, jointly hosted by SMM Information & Technology Co., Ltd. (SMM), Hunan Hongwang New Material Technology Co., Ltd., Louxing District People's Government, and the National-level Loudi Economic and Technological Development Zone, Dr. Xu Bin from Shanghai Jiao Tong University elaborated on "Development of Magnesium Alloy Electric Drive Housing and Lightweight Design."

Development Background of Magnesium and Electric Drive Housing

Development Background of Magnesium

• Magnesium materials are a key support for emerging industries.

• Production resources: Abundant mineral resources with good supply.

China has explored proven reserves of dolomite exceeding 4 billion mt; magnesium is low-cost and its supply is controllable in the long term.

National guidance: Magnesium is an emerging metal strongly supported by the Ministry of Science and Technology and the Ministry of Industry and Information Technology (MIIT).

In the past, innovation in magnesium alloy automotive parts was mainly driven and developed by high-end internal combustion engine vehicle manufacturers such as BMW, Mercedes-Benz, and Ford, but the scale of its application was relatively small. Even now, the most widely produced magnesium alloy parts in global vehicles are still mainly used in dry areas of vehicles.

For new energy vehicles (NEVs), the demand for lightweighting is more urgent.

New Technological Developments

Principle of Magnesium Alloy Semi-Solid Injection Molding

The magnesium alloy semi-solid injection molding process falls under the category of thixocasting technology. Magnesium particles enter the barrel from the hopper under the action of gravity or negative pressure. Inside the barrel, the rotation of the screw, combined with the heat provided by external heaters (the barrel is usually divided into 5 to 7 sections, with the temperature gradually increasing from the feed inlet to the nozzle), heats and shears the magnesium alloy particles as they are conveyed forward. In the middle of the barrel, the magnesium alloy is compressed by the screw's compression section, generating thermoplastic deformation and achieving densification. When it continues to reach the storage section at the front end of the screw, it has transformed into a semi-solid slurry that is partially molten and contains spherical solid phases. This slurry possesses excellent liquidity and mold-filling properties. Subsequently, the slurry is injected into the mold at high speed through the nozzle and rapidly cooled and solidified under high-speed and high-pressure conditions, forming parts with specific shapes and dimensions. After the injection is completed, the front end of the nozzle cools to form a cold plug for self-sealing, enabling continuous molding operations without the need for protective gases or complete melting.

Magnesium Alloy Semi-Solid Injection Molding (Thixomolding) Technology

Advantages of magnesium alloy semi-solid injection molding technology compared to traditional liquid die casting:

(1) High safety. Magnesium alloy is flammable in its liquid state, whereas the semi-solid injection molding process integrates thixotropic slurry making and molding under self-sealed conditions, eliminating the need for high-risk magnesium melting furnaces and the step of transferring molten magnesium, thereby ensuring the safe production of magnesium alloy parts.

(2) Environmentally friendly. Traditional casting processes generate a large amount of volatile gases during the melting of magnesium alloy and require the additional use of SF6 as a protective gas, which can easily cause environmental damage and limit the application and development of magnesium alloy. In contrast, the semi-solid injection molding process does not require complete melting or protective gas during the production of magnesium alloy parts, and does not produce melting waste slag, making it a green manufacturing technology.

(3) Few oxide inclusions. The temperature of the semi-solid molding process is lower than that of traditional casting processes, significantly reducing the oxidation risk. Meanwhile, since the injection molding method prevents the magnesium melt from directly contacting the external air, the probability of introducing oxide inclusions during the molding process is almost eliminated.

(4) Few gas porosity defects. Liquid magnesium tends to form turbulent flow when filling the mold cavity, leading to the generation of gas porosity defects. In contrast, semi-solid magnesium alloy exhibits non-Newtonian fluid characteristics and is more inclined to fill in a laminar flow manner, effectively reducing the gas entrapment phenomenon during the molding process and making the casting denser.

(5) Excellent mechanical properties. Magnesium alloy produced by semi-solid injection molding exhibits a non-dendritic solidification structure. Under high cooling rate conditions, its average grain size and secondary phase size are extremely small. Meanwhile, due to the reduction of defects such as gas porosity and inclusions, it has superior strength and toughness.

(6) High dimensional accuracy. Semi-solid magnesium alloy has good molding capability, enabling near-net-shape forming of complex thin-walled structures. Additionally, its solidification shrinkage is relatively small, and its resistance to hot cracking is improved, resulting in high dimensional accuracy of the castings.

(7) Long mold life. The molding temperature of the semi-solid process is nearly 100℃ lower than that of traditional die-casting processes, significantly reducing the thermal shock of the magnesium melt on the mold and thereby extending the mold life. For example, when producing some thin-walled parts, the service life of semi-solid molds can reach over 200,000 to 400,000 shots.

(8) High material utilization rate. Magnesium alloy parts produced by die-casting processes generally have a raw material utilization rate of less than 50% due to the inclusion of a large amount of gating and runner systems. In contrast, the semi-solid molding process can significantly reduce the size of the sprue and simplify structures such as runners and overflow slots, thereby increasing the raw material utilization rate to over 70%.

(9) High product yield. The semi-solid injection molding process enables precise temperature control for magnesium alloys, ensuring stable material filling quality without the potential pre-crystallization issues found in die-casting. It exhibits low defect rates, directly reflected in the internal and surface quality of products. Even after subsequent processing, the products maintain high yield rates.

(10) Reduced energy consumption. Semi-solid forming of magnesium alloys matches die-casting in cycle efficiency. Benefiting from the absence of furnaces and lower forming temperatures, semi-solid molding saves at least half the electricity consumption compared to liquid die-casting production.

Shanghai Jiao Tong University: Study on Microstructure and Properties of Semi-Solid Magnesium Alloys with Different Solid Fractions

• Under Thixomolding, magnesium alloys exhibit favorable non-dendritic structures (no pre-crystallization), with fluidity gradually improving as the solid fraction decreases.

• Excessive solid fractions result in poor pore-filling capability, while samples produced at high injection temperatures show defect bands and liquid-like filling characteristics.

For thin-walled components, moderate semi-solid forming temperatures should be selected to reduce porosity defects and improve elongation.

For thick-walled components, increasing the solid fraction can minimize shrinkage porosity defects.

Development Stages of Semi-Solid Magnesium Alloy Applications

After nearly three decades of development, the design focus of semi-solid magnesium alloy equipment has primarily centered on thin-walled part forming, aligning with the rapid growth of the consumer electronics market. Around 2020, the rapid rise of the NEV market once again drove upgrades in semi-solid magnesium alloy equipment. The industry began pursuing the manufacturing of larger integrated automotive structural components from magnesium alloys. However, traditional 1300T-class semi-solid injection molding equipment for magnesium alloys had a theoretical maximum injection capacity below 5kg, only sufficient for producing small components like center screen backplates, steering wheels, and armrest brackets—far from meeting automotive lightweighting demands.

Semi-Solid Magnesium Alloys Enter the Large-Scale Era

► Development of Large-Scale Equipment

In recent years, domestic equipment manufacturers have entered this field, initiating R&D on large-scale equipment. They have successively introduced ultra-large 3,000-4,000T semi-solid magnesium alloy equipment, breaking previous injection capacity limits. These systems provide ideal solutions for large-sized magnesium alloy products such as multi-screen backplates, interior door panels, instrument panel frames, and structural components for the three-electric systems (battery, motor, and electric control).

The case of the 4000T equipment launched in April 2024 by the Shanghai Jiao Tong University-Bole Equipment Joint Research Center was cited.

New technologies for semi-solid magnesium alloys are also under continuous exploration

, including introductions to dual-shot technology and TPI technology.

Research and Development of New Semi-Solid Magnesium Alloy Materials and Structural Components

Characteristics of Magnesium Alloys Suitable for Semi-Solid Processes

Materials Suitable for Semi-Solid Forming Processes: (1) Alloy systems with a certain solidification range, where the slurry is as insensitive to temperature as possible; (2) The liquidus temperature of magnesium alloys should be as low as possible to avoid reducing screw life due to excessive heating temperatures.

Development of New Semi-Solid Magnesium Alloy Materials

Characteristics: Excellent semi-solid processability, achieving mechanical and corrosion properties comparable to die-cast aluminum alloys.

Under neutral salt spray conditions, the corrosion resistance of the new semi-solid magnesium alloy material is superior to that of die-cast ADC12 aluminum alloy.

Development of High-Performance Semi-Solid Magnesium Alloy Structural Components.

SAIC Innovation and Development Research Institute and Shanghai Jiao Tong University have established a strategic partnership to jointly develop and promote the upgrading of semi-solid process technology for magnesium alloy electric drive housings.

Other ongoing developments include high-performance semi-solid magnesium alloy electric drive housings.

Targeted development of new materials from Shanghai Jiao Tong University based on semi-solid injection molding processes to improve corrosion resistance, strength, and heat resistance.

It also introduced the development and application of forming cycles and the global first-ever development of 20-inch large magnesium alloy wheel hubs.

Summary and Outlook

• Corrosion is the biggest challenge limiting the large-scale application of magnesium. The research on stainless magnesium based on materials genome engineering methods not only brings hope for solving the corrosion problem of magnesium alloys but also verifies the effectiveness of AI For Science in rapidly and efficiently designing new materials.

• The research and application of ultra-large semi-solid injection molding equipment for magnesium alloys make it possible to manufacture larger-sized, safe, and environmentally friendly magnesium alloy components, further broadening the application scope of magnesium alloys and helping to achieve lightweight goals in fields such as NEVs.

》Click to view the special report on the 2025 SMM (4th) Electric Drive System Conference & Drive Motor Industry Forum