After the 1990s, China's die-casting industry has achieved amazing development and has developed into an emerging industry. At present, aluminum alloy die-casting process has become one of the most widely used processes in automotive aluminum alloy forming processes, accounting for 49% of various automotive forming process methods.
There are about 3,000 die-casting companies in China. Output of die-casting parts increased from 266,000 tons in 1995 to 870,000 tons in 2005. Annual growth rate remains above 20%, with aluminum alloy die castings accounting for more than 3/4 of all die casting output. Types of die-casting products in China are diversified, including automobiles, motorcycles, communications, home appliances, hardware products, power tools, IT, lighting, escalator steps, toy lights, etc. With improvement of technical level and product development capabilities, types and application fields of die-casting products continue to expand, its die-casting equipment, die-casting molds and die-casting processes have undergone tremendous changes. Since its commercial production in 1914, die-cast aluminum alloy has developed rapidly with development of automobile industry and invention of cold chamber die-casting machine.

 

Die-cast aluminum alloys are divided into two types according to their properties: medium and low strength (such as China's Y102) and high strength (such as China's Y112). At present, die-cast aluminum alloys used in industry mainly include following series: Al-Si, Al-Mg, Al-Si-Cu, Al-Si-Mg, Al-Si-Cu-Mg, Al-Zn, etc. Improvement of mechanical properties of die-cast aluminum alloys is often accompanied by a decrease in the performance of casting process. High-pressure and rapid solidification characteristics of die-casting make this contradiction more prominent in some aspects. Therefore, general die-casting parts are difficult to undergo solution heat treatment, which restricts improvement of mechanical properties of die-cast aluminum alloys. Although oxygenated die-casting, vacuum die-casting, etc. are effective ways to improve mechanical properties of alloys, widespread adoption is still difficult, so development of new die-cast aluminum alloys has been ongoing.
Die-casting process of early horizontal cold chamber die-casting machines had only one speed to send molten metal into mold, and injection speed was only 1m-2m/s. Using this process, there were many pores inside casting and structure was loose. It was soon improved to 2-stage injection, which simply decomposed injection process into two stages: slow speed and fast speed, but fast speed was only 3m/s. Later, in order to increase density of die castings, a stage of pressure increase was added after slow and fast speeds, which became the three stages of slow injection, fast injection and pressurization. This is classic three-stage injection.
Compared with ordinary die casting method, vacuum die casting method has following characteristics:
01 Porosity is greatly reduced
02 Vacuum die-casting castings have high hardness and fine microstructure
03 Vacuum die castings have higher mechanical properties
Recently, vacuum die casting mainly focuses on extracting gas from mold cavity, and there are two main forms:
① Extract air directly from mold; ② Place mold in a vacuum box to evacuate air. When using vacuum die casting, design of exhaust channel position and exhaust channel area of mold is crucial.
There is a "critical area" in exhaust duct, which is related to amount of gas extracted from cavity, exhaust time and filling time. During filling process of molten metal, molten metal should be filled in a dispersed spray state. Size of runner also has a great influence on the effect of oxygenated die-casting. Appropriate runner size can not only allow molten metal to fill mold in a turbulent flow, but also prevent temperature of molten metal from dropping too fast. Highly dispersed distribution of oxides will not have an adverse effect on casting, but can increase hardness of casting and refine structure after heat treatment. Oxygen die casting can be used for Al, Mg and Zn alloys that react with oxygen.
At present, oxygenated die casting can be used to produce various aluminum alloy castings, such as: hydraulic transmission housings, heat exchangers for heaters, hydraulic transmission valve bodies, computer brackets, etc. For die castings that require heat treatment or assembly welding, high air tightness and use at higher temperatures, oxygenated die casting has technical and economic advantages. Semi-solid die casting is a technology that stirs liquid metal when it solidifies, obtains a slurry with a solid phase component of about 50% or higher at a certain cooling rate, and then uses slurry for die casting.
Semi-solid die-casting technology currently has two forming processes: flow forming process and thixoforming process. The former is to feed liquid metal into a specially designed injection molding barrel, where shear is applied by a spiral device to cool it into a semi-solid slurry, which is then die-cast. The latter is to feed solid metal particles or chips into a spiral injection molding machine, turn metal particles into slurry under heating and shearing conditions before die-casting. The key to semi-solid die-casting forming process is effective preparation of semi-solid alloy slurry, precise control of proportion of solid-liquid components, research and development of automated control of semi-solid forming process.

 

In order to realize automated production of semi-solid forming, American scientists believe that following technologies need to be vigorously developed:
01 Adaptable and flexible bar transportation
02 Precision die casting lubrication and maintenance
03 Controllable casting cooling system
04 Plasma degassing and treatment
Electromagnetic pump low-pressure casting is a newly emerging low-pressure casting process. Compared with gas-based low-pressure casting technology, it is completely different in terms of pressurization methods. It uses non-contact electromagnetic force to directly act on liquid metal, which greatly reduces oxidation and aspiration problems caused by impure compressed air and excessive partial pressure of oxygen in compressed air, realizes smooth transportation and filling of liquid aluminum, and prevents secondary pollution caused by turbulence. In addition, electromagnetic pump system is completely controlled by computer numerical control, process execution is very accurate and repeatable, giving aluminum alloy castings obvious advantages in terms of yield, mechanical properties, surface quality and metal utilization. As research on this technology continues to deepen, process is becoming more and more mature.
Through development in recent years, design level, technical parameters, performance indicators, mechanical structure and manufacturing quality of China's die-casting machines have been improved to varying degrees. In particular, cold chamber die-casting machine has been changed from original fully hydraulic closing mechanism to a toggle closing mechanism. At the same time, automatic loading, automatic spraying, automatic picking up, automatic edge cutting, etc. are also added. Electrical appliances have also been changed from ordinary power supply control to computer control. Control level has been greatly improved. Some have reached or are close to international level, are marching towards large-scale, automation and unitization.

 

During this period, new domestic die-casting machine companies have emerged one after another, among which Hong Kong Lijin Company is a typical representative. They has developed a number of domestic leading die-casting machine models, such as horizontal cold chamber die-casting machine with maximum air pressure injection speeds of 6m/s (1997) and 8m/s (early 2000), magnesium alloy hot chamber die-casting machine (early 2000), uniformly accelerated injection system (2002), maximum air injection speed of 10m/s and multi-stage die-casting system (Jane 2004), real-time control injection system (August 2004) and large die-casting machine with a clamping force of 30000kN (July 2004), etc.
Small and medium-sized die-casting machines are still dominated by domestic equipment. Gap between domestic die-casting machines and foreign advanced die-casting equipment is mainly reflected in following aspects:
01 The overall structural design is backward
02 Serious oil leakage
03 Poor reliability is the most prominent flaw of domestic die-casting machines.
04 Incomplete varieties and specifications, poor supporting capabilities
China's mold industry has developed rapidly. From 1996 to 2004, average annual growth rate of mold output was 14%. In 2003, output value of die-casting molds was 3.8 billion yuan. At present, China's domestic mold market satisfaction rate is only about 80%, of which medium and low-end molds are main ones. Large and complex precision molds cannot meet needs of national economic development in terms of production technology, mold quality and life, and production capacity. Consumption of automobiles, motorcycle industries and automobile accessories and demand for supporting products provide a broad market for production of die-casting parts. Application of die-casting aluminum alloys in automobiles will also continue to expand.
In the future research and development of die-casting technology, deepening of aluminum alloy die-casting will still be a main direction for development of die-casting technology. In order to adapt to market demand, following issues should be further solved in the future:
Promote application of new high-strength, high-wear-resistant die-casting alloys, research colorable die-casting alloys, and new die-casting alloys for castings with special safety requirements.
02 Develop die-cast aluminum alloys with stable performance and easy-to-control composition
03 Simplify alloy composition and reduce alloy grades to provide a foundation for green production
04 Further improve new die-casting processes (vacuum die-casting, oxygenated die-casting, semi-solid die-casting, squeeze casting, etc.)
05 Improve ability to respond quickly to market and implement concurrent engineering (CE) and rapid prototyping manufacturing (RPM) technology
06 Carry out research and development of CAD/CAM/CAE systems
07 Develop and apply more die-cast aluminum alloy automotive parts
Die-casting aluminum alloy is main body of non-ferrous alloy die-casting, and AI-Si series alloys are main body of die-casting aluminum alloy. Development of die-casting industry has a long way to go. It involves research and development of high-performance alloys and development of process pathways. Practice has shown that it is a wise choice to be enterprise-centered and combine relevant universities and research units. Relevant results we have achieved have been confirmed.
To achieve revitalization of the overall die-casting industry, it is also very important to improve performance of die-casting machines, correspondingly improve design and manufacturing level of die-casting molds. In particular, many foreign technical software are introduced and applied. What method is used to verify them? It is worth further research.