A big part of metal fabrication has to do with the techniques of forging and casting. Choosing between the two of them can be a bit tricky because parts that are produced by these methods have different properties. So, depending on the project, there is an optimal technique to perform it. In this article, I will describe the differences between these methods and what are their advantages or disadvantages.
- 1 What is Casting?
- 2 What is Forging?
- 3 Differences between Forging and Casting
- 4 Is Casting Or Forging a Better Method
- 5 Conclusion
What is Casting?
In casting, molten metal fills the cavity of the die or mold of the required product. During the cooling and hardening process, the molten metal takes the shape of the mold. Casting helps in the mass production of components because the same mold can be reused for the duplicate products.
Types of Casting Processes
Casting can be done in many ways and there are different types of casting. When the die replaces the mold in casting, the process is known as Die-Casting. When molten metal is poured under the effect of gravity or vacuum, the process is known as permanent mold casting. The products of permanent mold casting are much stronger than die casting. In permanent mold casting, there are some complexities involved that can cause difficulties in the removal of the product from the mold. Because of this, semi-permanent mold casting is preferred over permanent mold casting. In semi-permanent molds, the core is expandable and the removal of the component from the mold is easy and manageable.
The most common casting process is sand casting. Castings are done by pressing a pattern into sand for making the mold. Then the molten metal is poured into a temporary mold and the final product is obtained after the cooling process. Sand casting is a bit slow process but it is very economical and it does not involve any specific complexities. Sand casting is very suitable for the production of large metal components and it can accommodate intricate designs.
What is Forging?
Shaping the metal with the help of a compressive force is known as forging. The metal component is struck by a die or hammer and the process is continued till the required shape of the object is achieved. Due to some changes at the molecular level, the metal becomes harder and it gains strength. Moreover, forging is very helpful in removing the porosity, defects, and inclusions from the metal component.
When it comes to making large products without overspending, forging is a method that you have to take into account. A lot of force is required for the forging process as the metal has to be beaten to get the required form and shape.
The size of metal matters a lot in determining the amount of force that is required for the process. Larger the size of the metal, greater the amount of force will be required to forge it because changing the physical composition of metal while keeping it in its solid form is not an easy process. Also, a forged product requires machining processes to obtain the exact dimensions because a lot of tolerances are introduced in the forging processes.
Types of Forging Processes
Forging can be classified as cold forging, warm forging, and hot forging. When the metal is forged at room temperature, the process is referred to be as cold forging. When the metal is subjected to higher temperatures than the room temperature but lower than the recrystallization temperature of metals, the process is known as warm forging. Forging is said to be hot forging when the temperature of the metal is raised to its recrystallization temperature. Forging is one of the oldest methods that is used for the fabrication of metals. Back in the age when swords were in use, they were produced with the help of warm forging or hot forging.
Differences between Forging and Casting
The products that are produced through forging have higher strengths than the products that are produced with the help of metal casting. There are very fewer chances of deformation of products that are forged. The strength of the product is increased in forging processes because of the hammering and pressing techniques. The dissimilarity in the forging process than the other metal fabrication processes is that the grain flow of the metal is stretched in this process.
In metal casting, the grain flows are randomly arranged but in forging, the grain flows are aligned properly. Therefore, when the product is produced through the forging processes, its strength is much more than the product that could have been created through casting.
Coming to the casting processes, casting metal also has some considerable advantages than forging. There are no size limitations in casting processes while the forging processes have size limitations. Shaping the metal is much more difficult in forging than casting because forging involves the deformation of metal in a solid-state. On the other hand, casting involves the molten metal and it can be converted into any shape of the mold or die.
As there are limitations of size and shape in forging, there is another limitation of thickness that metal up to a certain thickness can only be forged. Therefore, forging will become difficult with the increasing size of the metal component. In casting, molten metal flows inside the cavity of mold or die and it makes the casting of difficult components possible. Casting is independent of the complexity of the design of products and products weighing up to 200 tons can be produced.
How Forging Affects Products
As already explained, forging can be counted in the metal forming processes. There is no involvement of melting and solidification processes in forging. Internal stresses of the parts are increased in forging processes because constant hammering and beating cause an increase in the number of dislocations. Strain hardening can also be achieved with the help of forging processes and the reason behind the strain hardening is the meeting of different dislocations with each other as well with the other barriers such as grain boundaries. The increase in the strength can be attributed to the change that occurs in the crystals of metal during the forging process. Crystals that can also be named as dendrites are stretched and this stretch is in the direction of metal flow. As a result, fibers are produced that have higher strengths along the direction of flow.
These fundamentals can also be distinguished on the basis of hot working and cold working. In hot forging, the temperature goes higher than the recrystallization temperature and this recrystallization results in the formation of new grains that are free of strain. This formation of new grains helps in the rapid elimination of distorted grain structures and strain hardening. The grains in the newly developed shape of the metal are organized in a homogenous manner in hot working. The phenomenon of recrystallization and strain hardening is important to understand because they help in achieving the final product with higher ductility and toughness.
How Casting Affects Products
Casting can be differentiated from forging in the way that forging is a metal forming process and casting is a manufacturing process or solidification process. Phase transformations, precipitations, and grain structures are tuned in a new manner in the casting process.
There are many defects that can arise in the structure of the product because of the solidification process that is involved in the casting. It is important to control the rate of solidification and other conditions that are the characteristics of solidifications to achieve a product with better properties. Otherwise, the complete product or cast part can be wasted because of the defects that are caused due to the improper solidification of molten metal. Many residual stresses are introduced to the part that is cast and sometimes, there is the requirement of proper heat treatment that can eliminate these residual stresses and improve the mechanical properties of the casted part.
Defects in Forging
There is a number of weaknesses of the forging process or it can also be explained on the basis of defects that are produced in the forged part.
- Unfilled Sections: The improper forging process can leave some portions in the metal that are unfilled. There are many reasons that can cause this defect in the forged part. Poor design of the die is the main reason for leaving the unfilled sections of the final product. Less amount of metal than the required amount for the product with specific dimensions can leave some unfilled portions in the forged part. Moreover, this defect can also be produced because of poor forging techniques and improper heating.
- Cold Shut: Cold shut can be considered as a discontinuity that is occurred on the surface of the metal and it occurs when the two parts of the metal cannot unite in the proper way or the two parts of the metal are disunited during the forging process. Improper design of the forging die can mainly cause cold shuts. The appearance of cracks on the corners of the forged part is because of the cold shuts. Making attempts for obtaining the sharp corners of the product can also result in the cold shuts and excessive chilling of the product can also introduce cold shuts to the forged part.
- Scale Pits: Scale pits are another type of forging defect that occurs on the surface of the product. The main reason behind the scale pits is the improper cleaning of the stock that is used for carrying out the forging processes. Sometimes, the forging is cleaned with the process named pickling and it can also leave scape pits on the surface of the forged part. In simple words, scale pits can be considered as the deputations on the surface of the metal.
- Die Shifts: Die shift is an important defect in the forged products because it can result in the total loss of the product. Die shift occurs when the upper die and lower die are not matched properly before carrying out the forging process. It directly affects the dimensions of the part and it can be eradicated with the help of alignment techniques. It is recommended that the upper die and lower die must be aligned properly before initiating the forging process because it will reduce the chances of unwanted results.
- Flakes: Flake is the major defect that is produced in the forging process of larger parts. Already, there are some discontinuities in the forging process. Hydrogen is picked up during the heating process and it segregates at those discontinuities and internal voids and flakes are produced. Another reason behind the development of flakes is the rapid oxidation of iron under the conditions of hot air and higher temperatures. More hammer-scales can also be produced in the forging of pure iron because of the hammering that is required for shaping the product.
- Surface Cracking: Surface cracking is a general defect that can be observed in forged parts and the reasons behind this defect are the inappropriate design of dies, less amount of material, quick chilling of the metal after hot working, and improper hammering or beating of the metal. Surface cracking can reduce the strength of part and it can cause the failure of the part in the application where it has to be used.
Defects in Casting
There is no process that is free of defects. That is the reason that the casting process also contains some major defects. Some of them are explained here:
- Shrinkage Cavity: It is the property of a metal to shrink during the solidification process. The amount of metal that is going to shrink must be evaluated properly before initiating the casting process. Therefore the shrinkage of metals occurs during the solidification process. Shrinkage occurs at the locations where the metal is lastly poured. Shrinkage of the metals occurs at the ingots of the mold cavity. Adding more molten metal in the cavity than the required quantity can help in avoiding shrinkage and the excessive metal can be cut down after the completion of the solidification process.
- Holes: Different types of holes can be developed during the metal casting process. The major types of holes are blowholes, pinholes, and open holes. Blowholes are a level above the porosities. Different gases are entrapped in the structure of metal and when the metal is solidified, these holes are left. This develops pores in the structure of the cast product. If the holes are tiny then the holes are known as porosities and when the gas bubbles are large enough, the holes are known as blowholes. Open holes can be further classified as caved surfaces and piping. Pipes are the holes that are developed on the surface of cast product and they are burrowed into the casting while caved surfaces are the types of holes that are developed along the surface of the casting.
- Hot Tears and Hot Spots: Hot tears can also be named as hot cracks. This defect occurs under two conditions. When the metal is heated up to the extent that sufficient residual stresses are introduced into it. Limited strength only augments this problem. Hot tears can cause complete failure of the cast part. However, a mold that is designed correctly can mitigate this issue. The failure of the product occurs during the cooling process. Hot spots occur in the structure of the part where the material is thermally isolated. It occurs in the parts that are cooled at the end of the process. It produces porosities and shrinkage cavities in the final product. A solution to this problem is limiting the size of thick sections. Another option is providing gradual transmissions in between the areas of variable thickness.
Generally, casting is a cheaper process than forging but there exist some limitations of strength in casting. The cost of casting depends a lot on the method that is being used. Also, the bigger an object is, the higher the cost of casting is. So, when it comes to sand-casting of small and medium-size objects, forging seems like a more expensive alternative.
From the perspective of time, casting is more feasible because it requires much less time for the fabrication of products than forging. Forging involves a lot of hammering and it can take much time for the completion of the process. On the other hand, casting just requires the melting of metal and then pouring it into the cavity of the mold. Casting is much easier to conduct and there are fewer chances of failure.
Tools that are used for forging and casting are completely different from each other. Anvils, Chisels, Tongs, Fullers, Forging pressers, Forging dies, Forging Hammer Supplies, etc. are used in the forging process. On the other hand, mold, sprues, patterns, etc. are required for the casting processes. A furnace is required in both of the processes. It melts the metal for casting and it provides heat in forging. Comparing the two processes, casting is much favorable as it does not require a lot of space and complex tools.
Is Casting Or Forging a Better Method
It is a very difficult question to choose the better process. Both of these processes have their own uniqueness. It is better to go in a way that the best process can be chosen on the base of the product that is to be produced. There are products that can be best produced with the help of the casting process and there are products that require to be forged for obtaining the optimized product.
Generally, the forged components have higher strengths and more reliability than the casting products but precision and high surface finish can be obtained in casting. Forging is preferable over casting where both of the processes are available for production. The reason behind it is that forging yields less porosity in the part and forging increases the strength of the part.
There are some delicate parts that cannot be forged and casting is preferable for such parts. Both can be used for the mass production parts and both processes produce rough products. Heat treatment is required after forging and it is not required after casting. However, machining of the product is required after both of the processes.
Both of the processes are very crucial in manufacturing industries but they are completely different from each other. There is no doubt in it that forging produces the parts with high strength. However, if the required strength can be achieved with casting then casting is the best option because it is an easy process and it is cheaper than forging. Casting can help in manufacturing the complex parts and casting requires less time than forging. Therefore, casting must be chosen over forging if both of the processes are available. Feel free to take a look at other articles on this website about processes, CNC, and welding. Please share the article if you have found it helpful.