The difference between forgings and castings

Summary in one sentence: For forgings, they are "made by hammering", while for castings, they are "created by pouring". During forging, the metal remains in a solid state throughout the process, whereas during casting, the metal first melts and then solidifies. This fundamental difference determines all the subsequent differences. 

The molding methods are completely different. 

Forged parts: Metal blanks are heated to a certain temperature, and then deformed in the solid state through hammering, pressure, etc., until they are formed. Throughout the process, the metal remains solid and has not melted. 

Castings: The metal is heated to a liquid state, poured into a pre-made mold, and then after it cools and solidifies, it is removed. Liquid → Solid, a complete process of melting and solidification has been undergone. 

There are significant internal organizational differences (which are the root cause of the performance gap) 

The grains within the forging will be elongated and arranged continuously in the direction of the force, forming what is called "fiber structure" (also known as flow lines). This is similar to the grain pattern in wood - it is very difficult to split along the grain. Therefore, the forging has strong fatigue and impact resistance. 

The grains of the casting grow randomly from the mold wall towards the center, with an unordered orientation. During the solidification process, it is prone to produce inherent defects such as shrinkage cavities, gas pores, and porosity. These defects are the stress concentration points and are likely to become the starting points of cracks. 

For the same material, the fatigue life of forged parts is usually 2 to 5 times that of cast parts. 

Comparison of mechanical properties 

The forgings outperform the castings in terms of strength, toughness and fatigue resistance. This cannot be matched by post-processing, because the differences in structure are inherent. 

The mechanical properties of the castings are relatively low. However, they can be improved to a certain extent through alloying and heat treatment. Nevertheless, casting defects always exist and can only be alleviated, but cannot be completely eliminated. 

Comparison of shape complexity 

This is the strength of casting. Because it is liquid casting, almost any complex shape can be produced, including structures with internal cavities, channels, reinforcing ribs, etc. For example, engine cylinder blocks, valve bodies, pump housings, all must be made by casting. 

Due to process limitations, the forged parts have relatively simple shapes and are suitable for manufacturing various types of components such as shafts, discs, rods, etc., such as crankshafts, connecting rods, gears, and high-strength bolts. 

Cost comparison 

The cost of forgings is higher - the molds are expensive, the equipment is large, and the requirements for the raw materials are also high. Castings, on the other hand, have a lower cost, especially when produced in large quantities. 

How to choose? 

The scenario for selecting forgings: subjected to high alternating loads, safety-critical components (aerospace parts, chassis parts, crankshafts and connecting rods), with high requirements for fatigue life. 

The scenario for selecting casting parts: The shape is extremely complex (with internal cavities), it is produced in large quantities and is cost-sensitive, and it does not withstand extreme loads. 

In engineering, there is an old saying: "When possible, avoid casting." This means that in cases where high performance is required, forgings should be chosen as the preferred option.