heating billet via induction heating for hot forging
Forging Versus Casting
While forging and casting can both produce near-net shape parts, they are two completely different metal-forming processes. The characteristics of the parts produced in each process are also different.
If you’re sourcing metal components, it’s important to understand the capabilities of each forming process. In this article, we’ll take a closer look at forging and casting, how they differ, and how to choose the best method.
Forging is a deformation process where a billet or preform is manipulated (pounded, squeezed, hammered, etc.) and shaped under pressure.
Many metals can be forged, including carbon steel, stainless steel, and steel alloys, although differences in composition and properties will influence which forging process is used. There are many forging processes, including, but not limited to, hot forging, cold forging, warm forging, impression die forging, and press forging. You can view our blog post here to learn more about the various forging methods.
Regardless of the particular method used, forging doesn’t add or remove material, so the resultant form has the same weight and volume as the billet until the part is trimmed from the platter.
One of the biggest advantages of forging is its effect on alloy structure. All metals are composed of grains, and how they interlock is a significant factor in their strength and ductility. Manipulating a metal blank by forging stretches these grains in directions perpendicular to where the force is applied from. This increases strength, sometimes dramatically, from its original state.
A second benefit of deformation is a dramatic reduction in internal defects like porosity and alloy segmentation.
Other advantages of forging include:
- Speed: Forging is quick compared to casting (especially for high-volume orders), even when blanks need preheating.
- Low Scrap Rate: Once tooling is developed, each part forged is identical, with none of the defects typical of a casting.
- Consistent Properties: Mechanical properties are more consistent after heat treatment than is the case for cast or billet-machined parts.
Forging is not without its limitations. There are two primary constraints:
- Size Limits: Forging has some weight and length limitations, varying based on the equipment used during the process. Generally, the forging process cannot produce very large components like casting.
- Material Constraints: Although most metals can be forged, the type of heating process used for the blanks can prevent the forging of some alloys.
Casting is where metal is melted, poured into a cavity shaped like the final component, and allowed to solidify. Cavities can be produced as expendable molds used once or as permanent molds. Sand and investment casting are expendable mold casting processes, while gravity and die casting use permanent molds. Molds require channels for metal to flow into the part cavity and “risers” that act as reservoirs as metal contracts during cooling.
Internal cavities are created with cores; the cores are placed in the mold before the metal is poured. After the metal solidifies, the cavity is broken apart as the final component is released from the mold.
All types of casting produce near-net shape parts, although the level of detail reproduced and the amount of metal to be machined away depend on the process used. Sand casting is the least precise, while investment casting, in particular, is capable of fine detail and smooth surface finishes.
Any metal can be cast, but oxidation can be a problem. This is addressed through vacuum melting and pouring, which adds to the complexity of the equipment needed.
Casting processes have a lot of versatility. Key advantages are:
- Material Capabilities: Casting works with all types of metal, although some need precautions against oxidation.
- Lower Tooling Costs: Expendable mold methods, particularly sand casting, have low tooling costs, making them economical for small orders.
- Size Capabilities: Casting can be used to create very large parts. Castings weighing thousands, even tens of thousands of pounds, are possible.
- Can Reproduce Complex Geometries: Investment casting is particularly good in this regard as it doesn’t need draft angles.
There are many situations and types of parts for which casting is not optimal.
Particular issues include:
- Lead Times: Casting has longer lead times due to the need to produce and prove-out tooling (especially with investment and permanent mold casting).
- Longer Production Times: The process is complex and time-consuming, especially for expendable mold processes.
- Porosity: Porosity is generally unavoidable in castings, resulting in internal defects that weaken the part.
- Inclusions & Oxides: Inclusions and oxides in the molten metal result in defects in the final part.
- Shrinkage: Shrinkage during solidification can cause cracking.
- Poor Material Utilization: There is typically more material waste in casting versus forging. Runner and riser material can sometimes be reclaimed at the expense of additional heating.
How to Choose the Best Method
Selecting a metal forming process requires an application evaluation.
- What type of material is required for your application?
- How complex is the part’s geometry?
- How many components do you need?
- What mechanical properties are required in the final component?
- What is your budget?
- Is a specific surface finish required?
Forging is generally more cost-effective, less wasteful, and faster than casting, produces stronger components, and is great for small- and high-volume orders. Forgings are typically preferred for applications requiring high wear resistance or components expected to perform well in stressful environments. However, forging may not be the best option for extremely large, complex components or if the application requires using a material that cannot be forged.
General applications for forged parts include automotive components like connecting rods and crankshafts, power transmission parts like crown wheels and pinions, and parts for marine, defense, forestry, and mining applications.
Comparatively, casting processes, especially investment casting, are ideal for producing lightweight, complex, thin-walled parts with exceptional surface finish and heavier components weighing over 100 pounds. But defects are more common in castings than forgings, and the tooling creation process generally takes longer to complete.
Casting processes are commonly used to produce engine blocks, pistons, pipes, valves, and other components for aerospace, automotive, and industrial applications.
Have Questions? Contact Trenton Forging
Founded in 1967, Trenton Forging is an industry-leading forging manufacturer specializing in impression die forging. We can produce asymmetrical and symmetrical forgings from steel, stainless steel, steel alloys, and microalloyed steels for clients in many industries, ranging from defense and automotive to agriculture and forestry.
If you’d like to learn more about the advantages of forging, please visit our blog for additional resources. If you need forged components or want to discuss a project with us, please fill out our online form today.