Defects in Metal Material Forming Processing, Part 2

Defects in metal forming processes can arise due to various factors, including material properties, process parameters, and environmental conditions. Here are some common defects associated with metal forming:

Cracks and cracking

(1) Processing cracks

Processing cracks can be caused by improper processing technology or defects in the process. Processing cracks can also be divided into two categories: hot cracks and cold cracks.

The difference lies in the microstructure around the cracks. The former has no obvious changes while the latter has serious deformation. In terms of morphology, there are longitudinal cracks, transverse cracks, side cracks, opening cracks, spiral cracks, periodic transverse cracks, fractures, 45° oblique cracks and irregular cracks.

According to distribution, they can be divided into local cracks, head cracks, surface cracks, center cracks and intergranular cracks.

The causes of processing cracks should be analyzed from the two aspects of the metal itself and the processing technology.

The following factors belong to the metal itself: the composition and impurity content of the alloy; when the ingot has surface defects such as cold shut, scar and segregation nodule, and internal defects such as shrinkage, looseness, pores, inclusions and segregation, cracks are likely to occur along these defects during processing; when the well-developed columnar crystal structure has obvious weak planes, it is easy to crack along the grain boundary or weak plane; the alloy has cracks, overburning, overheating, or poor distribution of the second phase, uneven structure, and microcracks caused by phase change in the previous process.

(2) Heat treatment cracks

① When there is a large residual stress inside the alloy, if the thermal stress caused by heating is consistent with the residual stress and the superposition exceeds the metal strength, cracks will occur.

② When the alloy has a second phase precipitated along the grain boundary during the heating process to form a second type of additional stress, or when a significant volume change occurs due to phase change, cracks may also occur.