By Yao Ren and Max Bolut
High welding speed and low heat input are among the benefits associated with laser welding, making it an attractive fabrication process for many industries. In some applications; however, these benefits are overshadowed by weld cracking. This review considers the two principal types, solidification and liquation cracking.
During welding, as alloys solidify over a temperature range, a semi-solid region, the mushy zone, exists at the trailing edge of the weld pool. The mechanism of solidification cracking is the rupture of liquid films present in the mushy zone. Strain is normally generated by contraction, as the weld cools and, consequently, solidification cracking is controlled by both the metallurgy affecting the solidification structure and the local and global stresses and strains imposed on the solidifying joint.
Liquation cracking is also called microfissuring, edge-of weld cracking, base-metal cracking, hot cracking and HAZ cracking. It occurs in, otherwise, solid regions of the weld, which are heated by the formation of a weld bead. Liquation cracks can be formed either in the region of the parent material adjacent to the weld metal or in regions of weld metal in multi-pass welds, reheated by a succeeding weld bead. HAZ liquation cracks reflect the grain shape in of the HAZ and, in contrast to solidification cracking, are characterised by a smooth intergranular morphology.
Examples of hot crack susceptible materials include heat-treatable aluminium alloys (e.g. 2000, 6000 and 7000 series) and heat-treatable nickel superalloys (e.g. Alloy 718, CMSX-4, Waspalloy). Numerous research has found these aluminium alloys are prone solidification cracking during laser welding and Alloy 718 has been found to be particularly sensitive to liquation cracking.
This reports reviews the literature on solidification cracking, focusing on the potential for tailored energy distributions to reduce the risk of cracking when laser welding crack-susceptible materials.