Défis et considérations concernant la technologie des moules de coulée sous pression des alliages de magnésium

Magnesium alloy die-casting is a comprehensive system that integrates design, manufacturing, and research. Professionals in this field, when compared to those working with zinc and aluminum alloy die-casting, are expected to possess a broader spectrum of knowledge, experience, and research capabilities. The die-casting mold, being a pivotal component, plays a significant role in this intricate process. This article delves into the complexities and nuances of magnesium alloy die-casting molds, shedding light on the expertise required and the challenges faced in this specialized domain.

The Rising Significance of Magnesium Alloy Die-Casting in Modern Industries

Magnesium alloy, primarily used in die-casting products for automobiles, offers a plethora of advantages that are reshaping the automotive industry. By incorporating magnesium die-cast parts, vehicles achieve a lighter weight, leading to reduced fuel consumption and fewer emissions. Furthermore, magnesium alloy die-cast components are renowned for their noise and vibration reduction capabilities, as well as their high casting precision. These benefits, combined with their overall economic efficiency, position magnesium alloy die-casts as a promising material for vehicle lightweighting, opening up vast opportunities for future applications.

The rapid advancements in the automotive sector, coupled with the swift growth in industries like computing, telecommunications, instrumentation, and aerospace, are paving the way for magnesium alloy die-casting to secure a prominent place in the global market.

Studying magnesium alloy used in die-casting shows that it has special qualities different from other metals. With a density of less than 2g/cm3, it stands out as the lightest structural metal currently available. Its strength-to-weight ratio surpasses that of aluminum alloys and steel, though it falls slightly short of the highest strength-to-weight ratios found in fiber-reinforced plastics. In terms of stiffness, magnesium alloy is on par with aluminum and steel but significantly outperforms fiber-reinforced plastics. Moreover, its corrosion resistance is far superior to low-carbon steel and even surpasses die-cast aluminum alloy A380.

Magnesium alloy is great at reducing vibrations and blocking electromagnetic signals. That’s why it’s often chosen over aluminum alloys. Given the low dynamic viscosity of magnesium alloy, it fills molds faster than aluminum under identical fluid conditions (with equal Reynolds numbers). Additionally, the lower melting point, specific heat capacity, and latent heat of phase change of magnesium alloy mean it requires less energy to melt and solidifies quicker. In practical terms, the die-casting cycle for magnesium alloy can be up to 50% shorter than that of aluminum. Another noteworthy aspect is the alloy’s minimal affinity with iron, resulting in less adhesion to the mold surface. This characteristic translates to a durée de vie des moisissures that’s 2 to 3 times longer than that of aluminum alloys.

Optimizing the Die-Casting Process with Magnesium Alloy

Magnesium alloy’s unique properties, such as its rapid mold-filling speed and reduced energy consumption during melting, make it a prime candidate for optimization in the die-casting process. Manufacturers can use these natural qualities to make production faster and make their die-casting molds last longer. This makes magnesium alloy even more attractive for different industries.

Exploring the Versatility and Potential of Magnesium Alloy Die-Casting

Magnesium alloy die-casting, a pivotal component in various industries, primarily utilizes alloys with American grades such as AZ91, AM60, AM50, AM20, AS41, and AE42. These belong to the MG-AL-ZN, MG-AL-MN, MG-AL-SI, and MG-AL-RE series respectively. Current research on these die-casting magnesium alloys encompasses several key areas:

High-Temperature Performance

AZ and AM series magnesium alloy die-casts constitute about 90% of automotive applications. However, these alloys exhibit a noticeable decline in strength at temperatures exceeding 150℃. The newly developed AS series, like the AS41A alloy (MG43%AL1%SI0.35%MN), demonstrates superior creep resistance at temperatures above 150℃, outperforming AZ91D and AM60B. It also boasts higher elongation rates, yield strength, and tensile strength. For example, the engine crankcase of Volkswagen’s BEETLE used to be made from AS41 and AS42. But now, they use the AE42 alloy because it performs better at high temperatures.

The addition of trace elements like rare earth elements Y, ND, SR, etc., refines the grain structure, enhancing the strength and creep resistance of die-casting magnesium alloys. The latest AE42 alloy, with its superior creep resistance, outperforms traditional MGALSI alloys and can be used long-term at temperatures ranging from 200℃ to 250℃. However, the AS and AE alloys don’t perform as well at high temperatures. Their casting qualities are also not as good as the AZ and AE alloys. The high cost of rare earth elements also restricts their widespread production and application.

Ductility and Fracture Toughness

The demand for magnesium die-casts in applications requiring safety and high fracture toughness is surging. To enhance the material’s ability to absorb energy under working conditions, it’s essential to improve its fracture toughness, achievable by reducing aluminum content in the alloy. Alloys like AM60 and AM50 are extensively used in instrument panel brackets, steering wheel shafts, and seat safety components. AM20 is currently being applied to seat back frames. Furthermore, the fracture elongation rate is closely related to temperature, especially above 50℃, where it increases with rising temperatures.

Résistance à la corrosion

Historically, corrosion resistance posed a significant challenge to the broader application of magnesium alloys. Due to magnesium’s high chemical reactivity, magnesium-based alloys and composites are prone to micro galvanic corrosion. However, high-purity die-casting magnesium alloys, like AZ91D, which strictly regulate impurities like FE, NI, CU, and rare earth-containing AE42, have surpassed the corrosion resistance of die-cast aluminum alloy A380 and significantly outperform low-carbon steel in salt spray tests. Adjusting the chemical composition, surface treatments, and controlling the microstructure can further enhance corrosion resistance. Despite various methods to improve the corrosion resistance of magnesium alloy components, addressing the issue at the material level remains crucial to overcome this technical barrier.

Flame Retardancy

Adding AL (2.5%), BE alloy (with BE content ranging from 0.0005% to 0.03%), or CA alloy to magnesium alloy effectively prevents the oxidation of magnesium alloy liquid. Current research is focused on developing flame-retardant magnesium alloys. If successful, magnesium alloys could be melted and cast like aluminum alloys, broadening their application horizons.

Magnesium Alloy-Based Composites

Research and development into magnesium alloy-based composites reinforced with particles like silicon carbide have been ongoing for several years. While they haven’t reached the commercial application stage in the die-casting domain, they’ve been used to produce impellers, bicycle cranks, car cylinder liners, etc., through sand casting and precision casting. There’s a growing trend to combine these composites with semi-solid casting for applications in die-casting and squeeze-casting domains.

Advancements in Magnesium Alloy Die-Casting Techniques

Magnesium alloys can be die-cast using either cold chamber or hot chamber die-casting machines. The evolution of hot chamber die-casting machines has led to several notable improvements:

Enhancements in Hot Chamber Die-Casting

  1. The use of energy accumulators for pressurization allows the shot piston to achieve speeds of up to 6M/S.
  2. Inductive heating of the gooseneck and nozzle ensures they maintain the optimal temperature.
  3. Dual furnace melting and insulation devices, coupled with recirculation pipelines, precisely regulate the melt pool temperature.

When using conventional cold chamber die-casting machines for magnesium alloys, it’s imperative to modify the shot system and automatic feeding system to meet the requirements of magnesium alloy die-casting. These modifications encompass:

  1. Increasing the fast shot speed from the 4-5M/S used for aluminum alloys to 6-10M/S.
  2. Shortening the pressurization build-up time during the boosting process.
  3. Enhancing the shot force.
  4. Implementing electromagnetic automatic quantitative feeding devices to prevent oxidation of magnesium during pouring.
  5. Equipping necessary auxiliary equipment when adopting special die-casting processes like vacuum die-casting.

Traditional Die-Casting Challenges and Solutions

Like other die-casting alloys, traditional die-casting techniques fill the die cavity with magnesium alloy liquid at high speeds, leading to turbulence and dispersion. This can trap or dissolve gases, forming numerous high-pressure micro-pores within the castings. Consequently, magnesium alloy die-casts produced using conventional methods can’t undergo heat treatment strengthening and aren’t suitable for high-temperature applications. There were some issues with the quality of die-casts. To address these, new methods have been developed over the past 20 years. These methods include oxygen-filled die-casting, semi-solid metal casting, squeeze casting, and periodically refreshed vacuum die-casting.

Vacuum Die-Casting

Vacuum die-casting eliminates or significantly reduces gas porosity and dissolved gases in the castings by evacuating the die cavity’s air during the casting process. This enhances the mechanical properties and surface quality of the castings. For instance, AM60B magnesium alloy car wheel hubs have been successfully produced using vacuum die-casting on cold chamber machines. The elongation rate of the castings increased from 8% to 16%.

Oxygen-Filled Die-Casting

Also known as pore-free die-casting, this method involves filling the die cavity with oxygen or other reactive gases before metal filling. The reactive gas mixes with the molten metal. This creates metal oxide particles inside the casting, which removes any internal gases. This allows the die-cast to undergo heat treatment strengthening. Companies in Japan have adopted this method to produce computer AZ91 magnesium alloy integral magnetic head brackets, replacing the original multi-layer laminated brackets. This not only reduces the bracket’s weight but also achieves significant economic benefits.

Semi-Solid Rheological Die-Casting

In recent years, companies in the US, Japan, and the UK have successfully developed magnesium alloy semi-solid rheological shot die-casting machines. These machines inject semi-solid magnesium alloy under pressure into the die to form it, operating similarly to injection molding machines. This casting method represents a promising direction for the production of magnesium alloy castings.

Melting Operations and Safety Protocols in Magnesium Alloy Die-Casting

Magnesium alloy, due to its propensity to oxidize easily, presents unique challenges in the die-casting process. The oxide layer on its surface has tiny holes. This makes it only 0.79 times compact, so it doesn’t stop more oxidation well.

Challenges with Oxidation

When magnesium alloy reacts with atmospheric oxygen, water vapor, and nitrogen, it forms compounds like MgO and Mg3N2, which are refractory and don’t melt in the magnesium liquid. These compounds, when mixed into the mold, result in “oxidation slag.” Hence, preventing oxidation during melting is crucial.

Protection Methods for Molten Magnesium Alloy

There are primarily two methods to protect the molten magnesium alloy: flux protection and gas protection.

Using protective flux for melting often leads to several issues:

  1. At high temperatures, chlorides and fluorides can easily evaporate, producing toxic gases such as HCl, Cl2, and HF.
  2. Due to the higher density of the flux, some of it can mix with the magnesium liquid, resulting in “flux slag.”
  3. The gases produced by flux evaporation can infiltrate the alloy liquid, acting as a source of corrosion during the material’s usage, accelerating corrosion and reducing its lifespan.

Most manufacturers now prefer gas protection, using a mixture of 2-4 dry gases like SF6, N2, CO2, and SO2. This mixture forms a dense, continuous film on the surface of the magnesium alloy melt pool, preventing oxidation. While SF6 is not toxic, its greenhouse effect is 24,000 times more potent than CO2. Given that the magnesium industry’s SF6 usage accounts for 7% of the global total (as of 1996), its usage will likely be restricted or even halted in the future. Currently, no suitable replacement for SF6 has been found. However, research indicates that sprinkling sulfur powder on the melt pool surface to form SO2 can protect the magnesium alloy liquid.

Safety Concerns in Magnesium Alloy Die-Casting Production

Most hazards in magnesium alloy die-casting production arise from processing and post-processing mishaps. According to Japanese statistics, dangers in the production process are distributed as follows: melting (25%), casting (10%), processing (39%), storage and waste (16%), electrical (3%), and others (7%).

Evidently, the risks during processing and post-processing exceed those during casting by 3-4 times. During processes like sandblasting, turning, milling, and polishing, magnesium dust and sparks are inevitably produced. If the workshop’s ventilation is inadequate and the air’s magnesium dust concentration is too high, sparks can ignite the dust, leading to fires or even explosions. Therefore, it’s imperative to equip workshops with dust collectors, fire sand, and other fire prevention facilities.

Design Considerations for Magnesium Alloy Die-Casting Molds

Magnesium alloy, due to its unique chemical and physical properties, differs significantly from aluminum alloy in terms of die-casting characteristics. As a result, the principles applied to aluminum alloy die-casting mold design cannot be directly transferred to magnesium alloy molds.

Challenges with Magnesium Alloy Casting

Magnesium alloy is prone to oxidation and combustion. It has a higher tendency for hot tearing compared to aluminum alloy, making its melting, pouring, and mold temperature control more intricate than aluminum alloy die-casting. Given magnesium alloy’s shorter filling time, the issue of gas venting is particularly pronounced. Its ability to hold and change heat is less than that of aluminum alloy. This can lead to premature crystallization in localized areas (especially thin sections) during the die-casting process, causing blockage in the feeding channels and resulting in casting defects.

Key Considerations in Mold Design

  1. Choice of Die-Casting Machine: The type of die-casting machine selected primarily depends on the wall thickness of the casting. Research on optimizing the magnesium alloy die-casting process suggests that for castings weighing less than 1 kg, a hot chamber die-casting machine is preferable to ensure complete filling of thin-walled parts. For larger parts, a cold chamber die-casting machine is recommended.
  2. Process Parameters: Choosing the right process parameters is crucial for producing high-quality castings and maximizing the production rate of the die-casting machine. These parameters form the foundation for designing the correct die-casting mold. Factors influencing the alloy liquid filling and molding during die-casting include injection pressure, injection speed, filling time, and mold temperature.

Given the varying wall thickness and complexity of die-cast parts, there’s a broad range for selecting process parameters. Compared to aluminum and zinc alloys, magnesium alloy has superior fluidity. As a result, the secondary injection speed can be higher. The plunger speed for magnesium alloy is approximately 30% faster than that for aluminum alloy, reaching speeds even beyond 10M/S.

Due to the casting properties of magnesium alloy, such as its sensitivity to mold and pouring temperatures, it solidifies quickly during the filling process. Precise control over mold and pouring temperatures is essential; otherwise, defective products are likely to be produced.

Incorporating these considerations into the design of magnesium alloy die-casting molds ensures a more efficient and quality-focused production process.

Your Best Magnesium Alloy Die-Casting Mold Supplier

Magnesium die-casting mold-making is not an easy thing. It requires precision, expertise, and a deep understanding of the material’s unique properties. In this complex landscape, Prototool emerges as a beacon of excellence. Our profound knowledge of magnesium alloy die-casting mold, combined with our commitment to innovation, ensures that we deliver unparalleled quality and service. As industries increasingly recognize the advantages of magnesium alloy components, having a reliable partner becomes crucial. Prototool is not just a supplier; we are a trusted ally dedicated to ensuring that our clients always stay ahead in their respective fields.

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