Highly Scalable Production
Although dies represent a significant cost, the greatest capital investment is in the die-casting machine itself. The time required for die changes varies between a few days and a few hours. Many foundries focus on high-volume production, but die-casting can be economical with batches of 500-2,000 parts. Some parts are even produced in volumes as small as a few hundred a years. Once the die is available, it becomes easy to rapidly ramp-up production.
This ability to suddenly increase production rate has been dramatically demonstrated in the challenge to produce ventilators for the Covid-19 pandemic. Normally, these are produced in small batches. Because they already have the tooling, they were able to perform die changes on their casting machines and focus their production on ventilators in just a few hours. This meant that the production rate was immediately increased by five fold.
Achieving Structural Material Properties
The two classes of material typically used for structural die-cast parts are aluminum and magnesium alloys. Thin-walled parts and optimized casting processes are often able to achieve fine-grain structures. However, a major challenge for achieving high-strength cast parts is porosity caused by tiny air bubbles becoming trapped in the molten metal. High pressures are required to squeeze metal into intricate molds, especially if rapid cycle times are required. However, this also causes turbulence that traps air and increases porosity. This has two negative effects. First, the voids themselves weaken the material, especially in terms of fatigue performance, by acting as crack initiation sites. Second, the presence of small air bubbles can prevent heat treatment being carried out, which is particularly important for aluminum alloys.
Optimizing mold design and casting parameters can reduce turbulence. Most foundries now use flow simulation during mold design, which can reduce turbulence and optimize cooling. Real-time shot control can further improve the properties of cast parts. Applying a vacuum to the mold can help prevent porosity forming while injecting oxygen can cause rapid oxidation in pores, filling them with metallic material.
Semi-solid die casting is another variation on die-casting suitable for producing high-strength parts. Instead of fully melting the metal, it is heated to just below its melting temperature. It, therefore, shares some properties with forging as well as casting. Porosity can be virtually eliminated, which means that excellent ductility and fatigue resistance can be achieved. It also enables full heat treatment and welding. A major challenge for semi-solid casting is the process control of temperature and mixing required to maintain the semi-solid state. Although research demonstrated that semi-solid casting was possible in the early 1970s, these difficulties meant it took some time to become an industrial reality.
There are now a few practical semi-solid casting processes. Thixocasting was the first to become a commercial process in the 1990s. It is usually used for aluminum alloys and uses a pre-cast billet that allows the process to be controlled but makes it considerably more expensive. Rheocasting was developed a little later, which is also for aluminum alloys. It reduces cost by avoiding the need for pre-case billets. Rheocasting allows both primary and secondary metal sources to be used, even scrap of the right composition, and offal can be easily recycled.
Thixomolding is another semi-solid casting process that produces parts in magnesium alloys. Machines are fed with chipped material from a hopper into a heated barrel containing a screw conveyor. This feeds the magnesium chips while mixing them to create a globular semi-solid state. Thixomolding machines resemble the injection molding machines used for plastics, and they can operate in fully automated cells.
Aluminum, magnesium and zinc are the most common types of metallic alloys used in the die casting process. Traditionally, tin and lead were also popular.
For many years, the FAA refused to allow magnesium alloy in aircraft interiors due to flammability concerns. However, extensive flammability testing has now largely proven these materials to be safe, resulting in a relaxing of certification requirements. Compared to aluminum, it has a far higher damage tolerance. Its major advantage over carbon composites is that it is fully recyclable. Magnesium alloys are designated using the ASTM and SAE system, in which the first part denotes the two main alloying elements in the alloy and the second part represents their percentages. Elektron 43 is one magnesium alloy that has obtained AMS specification and is now included in the aerospace Metallic Materials Properties Development and Standardization (MMPDS) handbook. Allite Super Magnesium is a leading proprietary alloy with extremely good structural properties for high-strength, low-weight applications. It has been in use within aerospace and defense since 2006 and is now becoming more widely available.
Materials like aluminum alloys and carbon-fiber may have excellent mechanical properties, but their sustainability is not so great. They take a lot of energy to produce and, in the case of carbon fiber, is virtually impossible to recycle. Magnesium alloys are fully recyclable and have low embodied energy.
The Future of High-Performance Die-Cast Components
The transition to a low-carbon economy is now driving widespread industrialization of semi-solid casting processes. Electric vehicles require low-weight and high-strength components with performance close to that required in aerospace. However, this must be achieved in high-volume production using highly automated processes with costs comparable to traditional automotive production. Processes such as Rheocasting and Thixomolding offer a way to achieve this.