What is shot peening primarily used for?

Shot peening is primarily used to improve the surface finish and strength of materials, which is why that option is the correct choice. The process involves bombarding a surface with small spherical shots, typically made of metal or ceramic. This bombardment induces compressive stresses in the surface layer of the material, which enhances its fatigue resistance and overall durability.

The improved surface finish contributes to better performance in applications where materials are subjected to cyclic loading, as it helps to prevent crack initiation and propagation. This is particularly advantageous in industries like aerospace and automotive, where component failure can lead to catastrophic outcomes.

While the other options may touch upon aspects related to metalworking, they do not accurately represent the primary purpose of shot peening. For example, reducing the weight of metal parts is not directly achieved through shot peening, nor is it focused on creating smooth surfaces purely for aesthetic purposes. Changing the material composition does not occur as a result of this process; rather, shot peening works within the existing material properties to enhance durability and performance.