Technology and Development Milestones of Aluminum Alloy Motor Housings

Since the 1990s, China’s die-casting industry has achieved remarkable development and has emerged as a burgeoning new industry. Currently, aluminum-alloy die casting has become one of the most widely used forming processes for automotive aluminum alloys, accounting for 49% of all automotive forming methods.

China currently has approximately 3,000 die-casting enterprises. Die-cast part output rose from 266,000 tonnes in 1995 to 870,000 tonnes in 2005, maintaining an annual growth rate of over 20%, with aluminum alloy die castings accounting for more than three-quarters of total die-cast production. The range of die-cast products in China is highly diversified, covering applications in the automotive, motorcycle, telecommunications, home appliance, hardware, power tool, IT, lighting, escalator step, and toy lighting sectors, among others. As technological capabilities and product development expertise have improved, the variety of die-cast products and their application areas have continued to expand, leading to substantial advancements in die-casting equipment, die-casting molds, and die-casting processes. Aluminum alloy die casting, since its commercial introduction in 1914, has experienced rapid growth driven by the expansion of the automotive industry and the invention of cold-chamber die-casting machines.

Die-cast aluminum alloys are classified according to their mechanical properties into medium- and low-strength grades (such as China’s Y102) and high-strength grades (such as China’s Y112). Currently, the main series of die-cast aluminum alloys used in industry include Al–Si, Al–Mg, Al–Si–Cu, Al–Si–Mg, Al–Si–Cu–Mg, and Al–Zn. Improving the mechanical properties of die-cast aluminum alloys often comes at the expense of reduced casting process performance; in pressure die casting, the combination of high pressure and rapid solidification exacerbates this trade-off in certain respects. Consequently, solution heat treatment is generally difficult to apply to most die-cast parts, which in turn limits further improvements in their mechanical properties. Although oxygen-injection die casting and vacuum die casting are effective methods for enhancing alloy performance, widespread adoption remains challenging. Therefore, research and development of new types of die-cast aluminum alloys continue unabated. In terms of advanced die-casting technology, early horizontal cold-chamber die-casting machines employed a single-speed injection process, with the metal melt being forced into the mold at only 1 to 2 m/s. This approach resulted in castings with numerous internal porosity and a loose microstructure. It was soon improved to a two-stage injection process, simply dividing the injection into slow and fast phases; however, even the fast phase did not exceed 3 m/s. Later, to increase the density of the castings, a pressure-boosting stage was added after the slow and fast stages, resulting in a three-stage process consisting of slow injection, fast injection, and pressure boosting—this became the classic three-stage injection cycle.

Vacuum die casting exhibits the following characteristics compared with conventional die casting: (1) significantly reduced porosity; (2) higher hardness and a finer microstructure in the castings; and (3) superior mechanical properties. Recently, vacuum die casting has primarily focused on evacuating the gas from the mold cavity, and it is generally implemented in two main forms: (1) direct evacuation from the mold itself; and (2) placement of the mold inside a vacuum chamber for evacuation. When employing vacuum die casting, the design of the mold’s venting channels—specifically their location and cross-sectional area—is of critical importance. There exists a “critical venting area” that is dependent on the volume of gas evacuated from the cavity, the evacuation time, and the filling time.

During the metal filling process, the molten metal should be injected in a dispersed spray pattern. The dimensions of the gating system also have a significant impact on the effectiveness of oxygen-assisted die casting; appropriate gate sizing can ensure that the molten metal fills the mold in a turbulent flow while preventing excessive cooling. A highly dispersed distribution of oxides does not adversely affect the castings; on the contrary, it can increase hardness and refine the microstructure after heat treatment. Oxygen-assisted die casting is suitable for Al, Mg, and Zn alloys that are reactive with oxygen. Currently, this technique is used to produce a wide range of aluminum alloy castings, such as hydraulic transmission housings, heat exchangers for heaters, hydraulic control valve bodies, and computer brackets. For die-cast parts that require heat treatment or assembly welding, demand high gas tightness, or operate at elevated temperatures, oxygen-assisted die casting offers both technical and economic advantages. Semi-solid die casting involves stirring liquid metal during solidification to obtain a slurry containing about 50% or more solid phase under a controlled cooling rate, which is then used for die casting. At present, two forming processes are employed in semi-solid die casting: rheoforming and thixoforging. In the former, liquid metal is fed into a specially designed injection cylinder, where a screw mechanism applies shear to cool it into a semi-solid slurry before die casting takes place. In the latter, solid metal particles or chips are introduced into a screw-type injection machine; under heating and shear conditions, the metal particles are transformed into a slurry, which is then die-cast. The key to semi-solid die-casting lies in the efficient production of semi-solid alloy slurries, precise control of the solid–liquid phase ratio, and the research and development of automated control systems for the semi-solid forming process. To achieve automated production in semi-solid forming, U.S. scientists recommend vigorous advancement of the following technologies: (1) adaptive and flexible billet handling; (2) precision die-casting lubrication and maintenance; (3) controllable casting cooling systems; and (4) plasma degassing and treatment.

Electromagnetic Pump Low-Pressure Casting Electromagnetic pump low-pressure casting is an emerging low-pressure casting process that differs fundamentally from gas-assisted low-pressure casting in its pressurization method. It employs non-contact electromagnetic forces to act directly on the molten metal, significantly reducing oxidation and gas absorption caused by impurities in compressed air and excessive oxygen partial pressure in the compressed air. This enables smooth delivery and filling of the aluminum melt, thereby preventing secondary contamination due to turbulence. In addition, the electromagnetic pump system is fully computer-controlled, ensuring highly precise and repeatable process execution, which confers distinct advantages to aluminum alloy castings in terms of yield, mechanical properties, surface quality, and metal utilization. As research continues to advance, the technology has become increasingly mature.

Over the past few years, the development of die-casting equipment has led to varying degrees of improvement in China’s die-casting machines in terms of design sophistication, technical specifications, performance metrics, mechanical structure, and manufacturing quality. In particular, cold-chamber die-casting machines have undergone significant upgrades: the original all-hydraulic clamping mechanism has been replaced by a toggle-type clamping system, while automated features such as automatic material feeding, automatic spray coating, automatic part removal, and automatic trimming have also been introduced. Furthermore, electrical control has evolved from conventional power-supply-based systems to computer-controlled systems, resulting in a substantial enhancement in operational sophistication.