Any ferrous or nonferrous metal can be cast, but two of the most widely used are stainless steel and aluminum. Although they look similar superficially, their properties are very different. In this blog, we’ll review their characteristics, advantages and disadvantages, and factors to consider when choosing between the two.

Stainless Steel

Like all steel alloys, stainless steel is primarily iron, with its carbon concentration reduced to around 0.1%. (Cast iron contains 2 to 5% carbon.) However, stainless differs from standard iron in chromium concentration, which is at least 10.5%. This concentration gives it high tensile strength and excellent corrosion resistance because surface chromium quickly oxidizes to form a protective layer that prevents rusting.

Unlike many other metals, stainless steel retains useful levels of strength at elevated temperatures and is often used in high-temperature environments (900 degrees Fahrenheit or higher).

Good methods for casting stainless steel are sand casting and investment casting. Die casting is impractical due to the high temperature at which the metal is poured.

Primary Advantages & Disadvantages

Some grades of stainless steel are difficult to weld, and most present some machinability challenges due to their high hardness. This makes stainless steel casting attractive, as it enables the production of complex shapes and minimizes the need for machining and welding.

A further advantage of casting stainless steel is that the produced parts don’t need painting, plating, or powder coating to protect against corrosion.

Disadvantages are application-specific. Stainless steel castings, while hard and strong, are also heavy, which is often a limitation in transportation-related applications.

Common Applications

Stainless steel is often the material of choice when castings are used in corrosive environments, subjected to aggressive cleaning and sterilization regimens, and/or when the application requires high strength.

Examples include:

• Chemical processing equipment
• Food and beverage production machinery
• Medical devices
• Engine components (gas turbines as well as gasoline and diesel)
• Power transmission components (gears, shafts, etc.)
• Architectural elements (handrails, handles, columns, and posts)
• Components for use on boats and ships
• Material handling systems

Common Grades Used in Casting

Stainless steel alloys are produced in a wide range of compositions. The most widely used are:

• 304: Also called CF-8 in the Alloy Casting Institute system.
• 316: Also called CF-8M, this is more corrosion-resistant than 304.
• 410: Strong, but lower corrosion resistance.
• 17-4 PH: The strongest grade of stainless steel. It has good corrosion resistance.

Note that an “L” suffix in a grade means reduced carbon content, which improves weldability.

Aluminum

Freshly cut aluminum has a bright, silvery appearance but quickly dulls as the exposed surface oxidizes. However, like stainless steel, oxidation protects the metal against further corrosion, so it doesn’t rust.

Aluminum is lightweight and, in its pure form, very soft. Alloying with other elements, such as silicon, is essential to raising its strength to a useful level. Its melting point is around 1,050 degrees Fahrenheit, and the maximum working temperature limit is around 400 degrees Fahrenheit. Additionally, its thermal and electrical conductivity are comparable to copper’s.

Most aluminum alloys have good machinability because their hardness is relatively low. Their ductility is better than that of many ferrous metals. When casting aluminum, sand, investment, and die-casting processes are all practical options, making it an extremely versatile material choice.

Primary Advantages & Disadvantages

The primary advantages of aluminum are its corrosion resistance, high strength-to-weight ratio, and heat conductivity.

However, although aluminum does have enough corrosion resistance, it may still need coating if the casted part is expected to operate well in very corrosive environments.

Common Applications

Aluminum is often used when low mass is part of the design criteria and/or when heat needs to be conducted between two points.

Common applications include:

• Automotive components
• Appliances
• Small engines
• Decorative items
• Heat sinks
• Aircraft components (cabins, landing gear, and airframes)
• Beverage containers
• Cookware

Common Grades Used in Casting

The various types of aluminum alloy are defined in terms of series. Series numbers range from 1000 to 7000 series. The most widely used are:

• 3000: Used in cookware and beverage cans.
• 5000: Used when corrosion resistance is a priority, as in construction and marine applications.
• 6000: The ‘workhorse’ grade, which provides a combination of low mass, good strength, corrosion resistance, and weldability.
• 7000: The high-strength ‘aerospace’ grades, which can be challenging to machine and weld.

Considerations When Choosing

Application Requirements

The primary consideration is application requirements.

For the highest strength and corrosion resistance, stainless steel wins every time. It’s also best for high-temperature applications.

But, when low mass or thermal conductivity is required, as in transportation or heat sink applications, the equation shifts.

For example, aluminum’s high strength-to-weight ratio means it may be possible to achieve an application’s strength goals by increasing the size of the casting. Additionally, although aluminum has lower corrosion resistance than stainless steel, aluminum may still be a good enough option over stainless steel if the part is being used in a dry indoor environment.

Cost

Cost is always a consideration in alloy selection. For the two metals discussed here, the final part cost is a function of the metal itself and the manufacturing processes.

Of the two, aluminum is less expensive than stainless steel. Both are suitable for investment casting, a cost-effective production process for making complex parts that require minimal machining.

Tooling

A further consideration is tooling lead time and cost. Investment casting uses wax patterns, which themselves need tooling. Tooling lead times are shorter and costs lower for sand casting, but the parts will need more post-process machining. Thus part complexity is a significant factor in selecting a casting process and even an alloy.

Learn More About Investment Casting From GreatLight

Founded in 1995, GreatLight is a specialty Tier 1 & Tier II manufacturer offering a wide range of capabilities, including die, gravity, investment casting, and prototyping. We proudly serve businesses worldwide in many industries, from automotive and construction to medical.

Please visit our blog for more casting resources or contact us today to learn more about what we can do for you.

Most metals can be used in investment casting, although some need special considerations. This blog reviews the most widely used investment casting materials and highlights their properties and applications.

Most Common Types of Steel

Low Alloy Steels

This family of steel is composed of iron, carbon, and little else. With less than 12% other alloying elements in their makeup, they lack the exceptional corrosion resistance of stainless steel but are also less expensive.

As-cast, yield strength ranges from 45,000 to 75,000 PSI. Elongation is up to 25%, showing these metals have good ductility. Strength and hardness are increased by heat treatment. In molten form, they flow and cast well. Common applications for low alloy steel investment castings include gears, crankshafts, and hydraulic cylinders.

Martensitic Stainless Steel

“Martensite” refers to the crystalline form this alloy takes as it cools. A high level of chromium, combined with low nickel content, leads to a “body-centered cubic” structure. Martensitic stainless steel is strong and hard but has low ductility and can be magnetized. Corrosion and wear resistance are very good, so this type of stainless is often used to manufacture pumps and valve bodies. Other applications include knives, tools, and medical devices.

Austenitic Stainless Steel

Austenitic stainless steel contains high levels of chromium and nickel, with some grades also containing molybdenum. The structure is “face-centered cubic,” which results in higher ductility than the martensitic grades but slightly lower hardness and strength. Austenitic stainless steels still have good corrosion resistance but are not magnetic. They are, however, weldable.

Because austenitic stainless steels have an attractive finish, they are often used in architectural and decorative applications. They are also used extensively in the automotive, aerospace, and medical industries.

Ferritic Stainless Steel

Used less often than austenitic and martensitic stainless, ferritic stainless steel has an iron-chromium composition with zero nickel. The structure is body-centered, like the martensitic grades, and the alloy is magnetic and corrosion-resistant. It has good ductility but low hardness. Common applications for ferritic stainless steel investment castings include piping, heat exchanges, HVAC system components, and more.

Duplex Stainless Steel

Duplex stainless steel is a recently developed form of stainless steel with characteristics of ferritic and austenitic grades. Comprising a high quantity of chromium with a small amount of nickel, duplex stainless steel offers high strength at a relatively low price. Corrosion resistance is good, and resistance to stress corrosion cracking is superior to that of austenitic grades.

Applications for duplex stainless steel investment castings include pressure vessels and storage tanks for brewing and chemical processing.

Copper-Based Alloys

Bronze and brass are both copper-based alloys, which illustrates the wide variety of compositions available. Bronze is an alloy of copper and tin, aluminum, manganese, and other elements. Brass is made by alloying copper with zinc. Another type is copper alloyed with nickel.

Copper-based alloys have lower melting points than stainless or carbon steel and lower strength and hardness. They offer good corrosion resistance and ductility, as well as high thermal and electrical conductivity. Common uses include marine applications and electrical devices.

Tool Steel

Tool steels are hard and typically abrasion-resistant. Depending on the composition, they may provide good shock or wear resistance and high-temperature strength. These characteristics make them useful for molds and dies, knives, cutting tools, and punching and forming tools. Carbon and alloy steels meeting this description are therefore referred to as “tool steels.”

Factors to Consider When Selecting a Material

The material chosen for investment casting must have the properties needed for the intended application. While factors like cost and castability are always considered, they only become relevant after compiling a shortlist of casting materials ideal for the intended application. Important properties affecting performance in the application are discussed in the following sections.

Mechanical Properties

In parts that will be placed in tension or under load, yield strength is almost always the primary consideration. This is the load at which the metal takes a permanent deformation and will not recover elastically.

The difference between yield strength and ultimate tensile strength provides a measure of ductility. The bigger the difference, the higher the ductility and more ductile materials are desired if they will be bent during processing.

Hardness is also important when a surface will see heavy loads or abrasive wear. The hardness of some steels can be raised by heat treatment. Hard materials tend to be brittle, but when hard and ductile, they are considered tough.

Weldability can be another important mechanical consideration for parts made by investment casting. Some alloys, such as low carbon steels, are easily welded, but others, like martensitic stainless, pose challenges.

Corrosion Resistance

All alloys form an oxide layer on the surface, especially in the presence of moisture. With steel, this can be a problem as the oxides form rust that flakes away from the body of the metal. Options for delaying this are to coat or choose a corrosion-resistant metal.

The high chromium content of stainless steel tends to prevent corrosion, allowing its use in damp or humid environments without coating. The ferritic and martensitic grades offer particularly good corrosion resistance and are often selected for marine applications.

Weight

Weight is often part of the selection criteria for investment cast parts intended for use in transportation applications. Steel and stainless steel alloys are dense, which can make parts cast in these heavy. Choosing less dense alloys, like aluminum, magnesium, titanium, zinc, and copper, can result in lower mass.

It is also important to consider the loads the part will be subjected to. Sometimes, choosing a more dense alloy can result in less being needed due to its higher strength per pound. Aluminum of the 6xxx and 7xxx series can have a higher specific strength than some grades of steel.

Temperature Resistance

With some alloys, strength declines quickly at higher temperatures. Others suffer accelerated corrosion. If the application will subject an investment cast part to high temperatures, this should be part of the selection criteria.

High temperatures are common in many chemical processing and power generation applications. In particular, parts used in gas turbines can be exposed to very high temperatures. Alloys ideal for such applications include nickel-based alloys, cobalt-based alloys, and stainless steel alloys.

Learn More From GreatLight Metal

Investment casting is an exceptional manufacturing method for producing complex, high-quality components. It allows for component consolidation, which reduces assembly work. Investment casting can hold tight tolerances and reproduce fine details that make secondary machining unnecessary.

If you’re looking for an investment casting partner, look no further than GreatLight. We’re an OEM-approved Tier 1 & Tier II manufacturer with three decades of casting and machining experience. 

Visit our blog for more information about investment casting, or contact us today to learn more about our capabilities or to request a quote.

Investment cast metal parts can be of lower weight, require less machining, and offer greater design flexibility as compared to parts made through other processes. However, not all parts can be manufactured through investment casting. Read on to see how investment casting compares to other metal working processes to determine the process best suited to your needs.

Other casting methods such as die casting, sand casting. plaster casting and permanent mold casting result in similar benefits as investment casting, such as smooth surfaces and the ability to make thin-walled, complex pieces. However, the tooling costs in the die casting process are higher, making it more suitable for high-volume production runs. Sand casting is one of the cheapest casting methods, and is suitable for the manufacture of large components in low-volume runs. Plaster casting offers better accuracy, and a much smoother surface finish as compared to sand casting; it is cheaper than investment casting, and is hence most suitable when the quality of the surface finish is an important requirement.

Investment Casting Supplier vs Machined Parts

Some components, especially those with complex internal geometries and passageways, cannot be created by machining. However, where either process is possible, the choice depends on various factors such as volume of production, use etc. Machining provides the greatest precision of all processes, and allows for much greater dimensional tolerances as compared to casting. Therefore, where strength and precision are crucial, machining is a better choice.

Compared with Forging

It is quicker and cheaper to mass produce castings, and there is much less waste as compared to forgings. However, ounce for ounce, forged parts have greater tensile strength, and up to six times the fatigue strength of a cast part. Hence, in applications where strength and ductility are crucial and cost is not as great of an issue, forging is the process of choice.

Compared with Stamping

Investment casting supplier provides greater design flexibility, such as variations in thickness, tapering etc. as compared to stamping. However, stamping dies are less expensive, and will last the entire life of the part. The stamping process is also quicker as compared to the casting process. Thus, stamping is suitable for high-volume, hard metal parts.

Compared with Powder Metallurgy

Both the production costs and the tooling costs for PM are lower than for investment casting, and the process is therefore suitable for high-volume production of simple shapes. Investment casting is a better option for components that require a higher degree of precision, have more complex shapes, and which are produced in low-volume runs.

One of the oldest known metal-forming techniques, investment casting or lost wax process casting is a complex and multi-step cast manufacturing process. An initial wax pattern of the part to be cast is coated with a ceramic refractory material and fired. The wax is then melted out to leave a ceramic cavity, into which the molten metal or alloy is poured. Upon cooling, the metal solidifies and the ceramic mold is broken out, leaving behind an exact duplicate of the desired part.

Investment casting enables the fabrication of near-net-shaped metal parts with complex geometries and intricate details, from almost any alloy. 

In addition, parts that are investment-cast have excellent surface finishes and require very little secondary machining; they also allow for tighter dimensional tolerances (typically, a linear tolerance of ±0.004 inches is standard). 

Investment casting is therefore used in the manufacture of extremely complex parts that use specialized alloys or need a lot of machining, and/or which cannot be produced by traditional methods. Although investment casting is more expensive than other comparable casting techniques, the expense is offset by savings in material, labor and finishing.