There are various characteristics that businesses look for in custom metal parts. Strength, size, material type, grain flow, and cost are some of the most important, and they are determined by the type of processing procedure used.
Forging and casting are the most common.
During the forging process, metal is pressed or hammered to give the desired shape, while in casting it is melted and then poured into a mold shaped in the desired form.
Both processes have particular advantages and disadvantages depending on the characteristics, cost, and service requirements that are needed.
The following sections will cover these key differences specifically in the areas of strength, types, sizes, alloy selection, additional processing, cost, and grain flow.
The stronger material is, the more likely it is to be able to maintain its structural integrity under various harsh conditions. This durability ensures functionality and safety.
Metals should be strong enough to maintain their structural integrity and perform at peak levels both under normal operational conditions and those that exceed them.
There are four ways in which the strength of metal can be measured.
Tensile strength refers to a material’s ability to withstand pressure or forces that would normally separate or pull a metal apart.
Compressive strength is the amount of pressure a material can tolerate without fracturing—in other words, the load it can bear.
Shear strength denotes the capacity to withstand or resist shear, that is, the structural strain produced by pressure.
Torsional strength is the ability of a material to withstand twisting loads due to torque.
Of the two processes we’ve mentioned in the previous section—forging and casting—forging produces a far stronger metal. This is because the process of forging alters the metal’s granular layout within and on top of the surface, making it more uniform.
Forging also helps to eliminate holes (voids and gas pockets) that would normally decrease both the structural and chemical integrity of the material.
On the other hand, the casting process tends to produce more porous holes since the metal shrinks from its thicker to its thinner sections during the process.
If you require strong material to maintain functional and structural integrity under normal and severe conditions such as is the case in the production of aerospace, railway, and ship components, then creating custom metal parts through forging is recommended.
Not all products need to be strong and rigid. Some ought to be more pliable and flexible so they can be formed into complex and aesthetic designs.
For those applications, casting is usually more appropriate, as it is more difficult to shape metals made through forging.
The casting process allows for more complex shapes to be formed since the metal is melted first, which means it can be easily poured into a pre-shaped mold or die.
Moreover, since the metal is melted during the casting process, larger products can be created, even those measuring up to 400 to 4,000 tons.
Therefore, the size of a particular product or application will also dictate which process should be used.
However, casting is not solely delegated to only large-size applications. Since it adheres to very tight tolerances, it is very useful even in forming smaller products with complicated geometrical designs.
Additionally, casting is more cost-effective than other machining processes, including forging, because it promotes material efficiency—five times higher than standard machining—meaning that more parts can be made in less time.
That is not to say that forging does not have its value, especially when strength and reliability are the primary concerns. It is just that casting is more useful when creating larger products with specific design requirements.
To get a better idea of which process is more useful for your specific project, take a look at the following list of products created through both methods.
| Forged Products | Cast Products |
|---|---|
| Agricultural Machinery & Equipment | Heavy Construction Equipment |
| Oil Field Applications | Heavy Farming Equipment |
| Hand Tools | Heavy Mining Equipment |
| General Industrial Equipment | Machining Tools |
| Railroad Equipment | Chemical Machinery |
| Off-Highway Equipment | Petroleum Machinery |
| Aerospace Components | Defense Vehicles |
| Valves | Artillery |
| Fittings | Munitions |
| Electrical Machines (motors, pumps, generators, compressors) | |
| Household Appliances | |
| Gardening Tools | |
| Art & Decorative Objects (sculptures, lamps, furniture) |
To summarize, the forging process is ideal for parts and applications that need to be strong, tough, and resistant to various mechanical forces.
On the other hand, casting is ideal for larger parts with detailed and complex shapes, patterns, and internal cavities.
As forging and casting are different processes, they aren’t always used with the same types of alloys.
Alloys are a combination of metals and non-metals, which are mixed and blended to enhance the desirable properties of a metal, such as corrosion resistance, tensile strength, and toughness.
It is important to choose the right alloy for your project because doing so will help reduce wear and tear, extend product life, and help increase overall performance.
After the alloy is chosen, the process that best compliments the alloy must be selected. The selection process will primarily depend on whether forging or casting can shape and mold the alloy to exact specifications.
Below is a list of alloy metals that are typically used in the forging process, and those that are more suited for casting.
| Alloys Typically Used in Forging | Alloys Typically Used in Casting |
|---|---|
| Alloy Steel | Nickel Alloys |
| Microalloy Steel | Cobalt Alloys |
| Stainless Steel | Aluminum Alloys |
| Carbon Steel | Lead Alloys |
| Aluminum | Stainless Steel |
| Copper | Carbon Steel |
| Titanium | Iron (gray, white, ductile) |
| Magnesium | Silicon |
| Nickel | Zinc |
| Iron | Magnesium |
| Cobalt |
As you can see in the chart, some alloys are equally suitable for both processes, so the choice of one instead of the other will depend on other factors listed in this guide.
What is more, the above lists are not complete and there are many other combinations of metals and non-metals that can be used for both processes.
In general, forging has a more limited selection when it comes to alloys, while casting can utilize almost any type of metal during its process.
Both forging and casting often rely on secondary operations to complement them. However, forging tends to require fewer of them.
Secondary processes are additional manufacturing processes that are needed to enhance and refine the formed product in some way.
There are many types of those operations, but the most popular ones include:
They provide the following benefits:
The reason forging requires fewer secondary processes lies in the fact that forging methods can be utilized on their own to get as close as possible to the desired specifications.
Of the different forging methods, the most common ones are:
And since these three forging methods pretty much ensure the main aims of forging—predictable strength and strong performance—not much is needed after they are implemented.
However, some secondary actions are commonly used after the initial forging process is completed.
Punching, straightening, and trimming are often employed after forging to ensure and improve the dimensional accuracy of fittings.
Cleaning methods like blasting, tumbling, or picking may also be used to clean the forgings if needed.
Casting, on the other hand, has more secondary processing requirements that are essential in creating smoother finishes, optimal sizes, and increased protection against oxidation.
The following ones are used most frequently:
The above processes are particularly common in conjunction with casting, since the process itself only deals with the formation of a part, and not its assembly, protection, or final surface finish.
Many custom-metal part manufacturers offer all or most secondary processes that may be required after casting or forging is completed, so there is usually no need to go to a different provider.
When it comes to cost-effectiveness, there is no definitive answer as to which of the two processes is more expensive.
The price of forging or casting will depend on various factors surrounding the formation and completion of a product.
Some of those factors include:
As a general rule, however, forgings are less expensive when they are purchased in medium to large lot sizes and castings are less costly when smaller lot sizes are bought.
However, castings can also be cost-effective when purchased in medium to large lot sizes, depending on how many secondary options are needed after they have been acquired.
As far as the cost of raw materials goes, those required for casting are usually less expensive than those for forging. This is because casting requires fewer initial steps and therefore less machinery to complete.
Also, the preliminary process of turning ingots into billets is eliminated during the casting process.
The price of tooling can also be eliminated for castings since it is usually not needed.
However, keep in mind that some forging methods don’t require tooling either, making them cost-competitive with castings. For instance, such is the case with open die forgings (ODF).
The machining costs associated with casting and forging are almost identical. Nevertheless, since forged alloys like stainless steel tend to have finer grains, machining them is often easier and therefore less costly.
Secondary operations will play a major role in the final cost of the manufacturing process.
Since casting tends to require more of them, it can turn out to be more expensive than forging, even though, when no secondary processes are required, its general costs are often lower than those of forging.
Therefore, the final costs depend on which casting method or forging method is chosen, as well as how many of the above-mentioned factors are involved in the overall manufacturing process of a particular application.
This is precisely why it is always recommended to contact a metal manufacturer and ask which variables will most likely pertain to the casting or forging of your specific product.
The information they supply you with will help you determine which one will be more economical for your project requirements.
Grain flows are fiber-shaped lines that appear on metal surfaces in the direction of metalworking (shaping) during the forging process.
Grain flow can help strengthen metal surfaces, reduce fatigue, and increase mechanical properties.
Casting, however, does not produce any grain flow and therefore does not provide its benefits to the formed product.
What is more, because there is no grain flow, casting can result in metallurgical defects, such as:
The directional strength (directional alignment or grain flow) produced during forging creates a refined microstructure on the surface of the metal. This gives it higher tensile strength, impact toughness, fracture toughness, fatigue strength, and ductility.
Additional benefits that are derived from directional grain flows include:
Applications such as surgical and dental instruments, hand tools and hardware, as well as aerospace and automotive components need the additional properties produced by grain flow.
It ensures their functionality under harsh conditions and increases their longevity of use.
Unlike casting, which produces a random grain distribution that reduces impact strength, forging helps to produce longitudinal grain flow across metal surfaces.
This greatly increases their overall strength and resistance to impact.
Therefore, if higher strength, toughness, and fatigue resistance are primary requirements, then forging would be the obvious choice.
The casting process will not be able to supply any grain flow and so none of the required attributes.
Neither of the metal manufacturing processes is categorically better than the other.
Instead, you can choose between casting or forging depending on the type, size, alloy composition, cost, and strength that your project requires.
You may need other secondary manufacturing processes apart from casting and forging to fulfill the demands of your project, which will also affect the time and the resources it takes to complete it.
Since there are so many variables affecting the formation of custom metal products and parts, it is best to consult a metal manufacturer to help you determine which process is best suited for your application and budget.
From a contract manufacturing firm, BuntyLLC evolved into a full service custom machined, forged and cast metal parts fabrication enterprise. We supply global solutions from our headquarters in Greenville, South Carolina.
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