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If the weld pool does not have enough volume after cooling to overcome shrinkage stresses, a crater crack will form, often near the end of a weld, in a high-stress, low-strength area (see Figure 1). It's the weld's way of relieving stress.

Pre-heating the base material prior to welding can also prevent cold cracking by slowing the cooling rate and helping maintain the ductility of the weld and base material. Post-weld heat treating (PWHT) helps, too, by driving diffusible hydrogen out of the weldment and stress relieving the material.

Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all welding on one side of a part will cause much more distortion than if the welds are alternated from one side to the other.

Preventing Cracking: Preventing hot cracking can be as simple as choosing a filler metal that has a weld metal chemistry with a lower crack sensitivity. You can also prevent hot cracking by making sure you are using an appropriate joint design.

Also, in the low-cycle fatigue range, the crack initiation point may be shifted from the slit tip area to the outer boundary of the heat-affected zone. The conventional crack propagation approach is based on the singular stress portions at the crack tip expressed by stress intensity factors.

The crack initiation site contained an irreconcilable fracture morphology that was covered with what appeared to be a discontinuous heavy oxide layer. EDS analysis revealed the presence of a large oxygen concentration as compared to the remaining fracture surface.

Stress cracking can occur when an aluminum weld cools and excessive shrinkage stresses are present during solidification. This could be due to a concave bead profile, a too slow travel speed, a highly restrained joint, or depression in the end of the weld (crater crack).