Forging and casting are both manufacturing methods for metal parts. Most metal pieces you encounter spanning all kind of different sizes, industries, and applications are made using these two popular manufacturing processes. Which method you use depends on a variety of factors from level of intricacy and budget constraints to strength requirements for the final piece.
Because forging bends solid metal into shape, as opposed to the molten metal used in casting, it tends to produce pieces that are much stronger than parts made using casting. This is because the grain in the metal bends into the piece’s final shape, making the piece more structurally sound. Additionally, the heating required to cast metal creates the risk of ending up with a porous surface and unintentional hollow pockets weakening the piece.
While forging does produce stronger pieces overall, there are definite benefits to both methods of manufacturing. Likewise, each process has some inherent risks you should factor into your decision about which works best for your project.
What’s the Difference Between Forging & Casting?
Each method has its own unique strengths and weaknesses making it better suited to specific applications.
Forging molds metal into shape while it is still in a solid state using hammers or presses that bend the material. There are four main types of forging:
- Impression (Closed Die) Forging: Two dies shaped press, or tool, the metal into its final shape. This method is able to achieve the most detail of any type of forging, making it very useful for a variety of pieces ranging from a few inches in size to massive machinery parts. Temperature sensitive metals are generally not used with impression forging as they require special equipment to avoid defects.
- Cold Forging: Almost all forging uses applied head. Cold forging, as its name suggests, does not. This process rolls, bends, and extrudes metal to create a desired piece. Because of this, it is not generally ideal for high carbon steels. However, cold forging does provide a strong product that can typically be seen in automotive applications such as suspension parts, anti-lock breaking systems, and other pieces where strength and reliability are essential.
- Open Die Forging: Flat dies bend and press metals without the lateral restriction of closed die forging. Open die forging works for almost all ferrous and non-ferrous metals. This, coupled with the ability to process the same piece multiple times until it reaches a desired shape, makes open die forging a very versatile process.
- Ring Rolling: Machines punch a hole in a round piece of metal before stretching the resulting donut shape into its final form, round or otherwise. The resulting pieces have incredible tangential strength making them well-suited to applications involving exposure to large amounts of torque or pressure. From tiny washers to gears and valves withstanding tremendous pressure, ring rolling allows for great variety in manufacturing metal pieces.
As mentioned above, forging creates a much stronger final product that can withstand a great amount of force and impact. The altered grain structure is to thank for this. As the gran bends, it locks the shape of the piece in place to create a sturdy structure. This removes the need to add extra alloys to strengthen the metal—a good thing since the metal will never be molten. Most metals work well with forging.
Because the metal is never in a molten state, there is no risk for porosity or shrinkage, meaning an overall stronger product. The finished piece is also less temperature sensitive than casted pieces. Because there is no risk of porosity or similar defects, further processing is possible without incurring significant damage.
While the forging process itself is more expensive than casting, it is less labor intensive and produces less excess material waste. These characteristics result in savings that ultimately make forging the cheaper option.
Forging does, however, present some weaknesses. The process does not allow for a great amount of detail. Forged pieces can only take a limited amount of shapes. Pieces requiring more detail must be processed multiple times. This additional processing adds to the overall cost of the piece. While technological advances are allowing for progressively more detailed forgings, this is still the one major area casting beats out this manufacturing method.
With casting, manufacturers pour molten metal into a mold which then hardens to create a final shape. Some popular methods include:
- Die Casting: Die casting is most often used with non-ferrous metals such as zinc or copper. A machine injects highly pressurized molten metal into the die. This method prevents the metal from cooling at different rates as it enters the mold. Die casted pieces produce less scrap and require less machining than other casting methods.
- Sand Casting: The sand is packed into the mold to allow for the shrinkage that generally occurs during cooling. This flexibility helps prevent the risk of cracking and additional flaws other forms of casting pose. Sand casting is relatively low cost because the sand may be used multiple times. It can also be used to create large pieces, making it a popular choice for automotive applications. Sand casting works on both ferrous and non-ferrous metals.
- Plaster Casting: Plaster casting is similar to sand casting in that the plaster coats the inside of the mold to allow flexibility for the cooling metal. Unlike sand casting, plaster casts generally chip or crack after use and must be replaced. Plaster casts create a smooth finish and can accommodate thinner walls and greater detail than sand casts.
- Investment Casting: Investment casting utilizes a disposable wax mold. After the casting has set, the wax melts off for reuse with another piece. This method provides the greatest amount of detail and cleanest surface finish, making machining after casting a rare necessity. However, investment casting is the most expensive of the methods listed and is generally only used when others castings will not suffice.
Casting offers many significant benefits. The first and most notable of them is the ability to create incredibly detailed designs. Complex pieces are very easy to produce and generally only require a one-step manufacturing process. You can easily achieve the intricate designs forging is unable to produce because it’s the mold you need to shape, not the metal itself.
Once a mold has been made, it can be used numerous times to produce many identical pieces. After the molten metal cools, it is removed and the mold which can ten be used for another piece just like it. This allows for more consistent, more efficient manufacturing.
Because the metal is molten when it’s poured into the mold, you can add different metal alloys to the mixture at different times in the process. This allows for greater alloy customization to ensure the finished piece has certain desired attributes.
Casing is also a great option for manufacturing parts with internal cavities. Resulting products are lighter than they would be had they been forged.
The same use of molten metal that gives casting its greatest strength, detail, also serves to provide its greatest weakness, porosity. Heating the metal until molten introduces the risk of hydrogen and other impurities tainting the metal, creating a porous and weak structure upon cooling. Additionally, the formation of bubbles within the mixture may lead to unintentional hollow areas in the piece. This further compromises the structure of the product, making castings much weaker than forgings.
This increased risk of impurities also requires the process be carefully monitored along every step of the way to ensure the best product possible. Casting is far more labor intensive of a process than forging. While the process itself is cheaper than that of forging, the added labor, processing, and waste materials add up. Ultimately, casting is the more expensive method, though it can be worth the extra costs in certain situations such as manufacturing many of the same exact piece quickly.
When to Use Casting vs. Forging
Casting and forging exist within the same industries. However, they generally have different specific applications.
Almost all metals can be forged, making the process pervasive across many industries. As technology advances, forging shapes and designs are becoming increasingly detailed. This means we will likely see even more applications that aren’t yet accessible in the near future.
Below are a few of the most common applications of forging:
- Automotive: Forging produces pieces that sport a blend of strength, cost-effectiveness, and shock resistance, so it’s no wonder the automotive industry so heavily relies on the process. Parts that endure a lot of stress and shock on a daily basis are forged to ensure reliability.
- Aerospace: The added strength from the metal’s bent grain creates a favorable weight to strength ratio perfect for aerospace applications. Forged pieces’ decreased temperature sensitivity is also a significant benefit in the industry as aerospace parts often have to endure extremely high and low temperatures.
- Heavy Construction & Mining: Larger components requiring strength and shock resistance are manufactured using forging.
- Artillery & Defense: Everything from tanks and submarines to rifles contain hundreds of forged pieces.
- Hardware & Tools: Manufacturers of most high-quality tools across a variety of industries use forging. Basic hand tools, such as hammers and pliers, surgical instruments, and electrical work components are all manufactured using forging.
Forged pieces are better to weld than casted pieces because the absence of defect risks in forging makes for a sounder and more reliable surface for welding. Welding a casted piece may exacerbate existing defects or even introduce new ones. Forgings are also better for cutting for all the same reasons.
Single-piece forgings often remove the need for assembly, welding, or riveting as they make the piece one single, sound structure.
The ability to cast intricate designs and small pieces makes casting a great option for manufacturing many different metal pieces. Additionally, manufacturers can create different alloy blends to provide unique qualities to casted parts.
Some of the most common uses for casting are:
- Automotive: Casting is generally used for highly detailed automotive parts. It is also a useful method for the efficient mass production of parts. The most common automotive casting is an engine block.
- Pipes: Again, mass production plays a big role in why pipes are generally manufactured by casting.
- Machine & Construction Pieces: Casting makes it possible to manufacture many of the small, intricate pieces needed for machinery and construction tools.
Casting is the manufacturing method of choice for parts requiring the mix of different alloys to achieve certain structural qualities. As forged metals remain solid, there is no way to alter the composition of the alloy. Casting allows for more of this customization.
While forging does provide a far stronger product, there are definite pros and cons to each manufacturing method. For pieces requiring a lot of strength but not too much detail, forging is the way to go. For more detailed pieces that won’t require much further processing after manufacturing, casting may be the right choice. Understanding the desired qualities for you piece can help you decide which manufacturing process will likely produce the best result.