Initiation and growth mechanisms for weld solidification cracking

Abstract

Solidification cracking is a weld defect common to certain susceptible alloys rendering many of them unweldable. It forms and grows continuously behind a moving weld pool within the two phase mushy zone and involves a complex interaction between thermal, metallurgical and mechanical factors. Despite decades long efforts to investigate weld solidification cracking, there remains a significant lack of understanding regarding its underlying mechanisms. Criteria developed to evaluate alloy weldability will be examined in terms of proposed solidification cracking models. Crack initiation is discussed in terms of different criteria: critical stress to fracture the interdendritic liquid, critical strain to exceed the mushy zone ductility and critical hydrogen content to nucleate and grow a pore. Crack growth has been characterised in terms of a critical stress to fracture the liquid film surrounding a grain and critical strain rate interdependent with liquid feeding of the mushy zone opening. Experimental data to form a weld solidification crack are compiled, revealing the considerable amount of information available in the literature on this topic.

A Multi-scale Thermomechanical-Solidification Model to Simulate the Transient Force Field Deforming an Aluminum 6061 Semisolid Weld

“…Hot cracking occurs due to the combination of limited intergranular liquid flow in response to solidification shrinkage, and semisolid deformation caused by thermal contraction and mechanical constraints [2,3]. To date, various mechanisms have been proposed to predict the formation of this defect [3,4].…”

“…Other researchers [7,8] have shown that high strain rates seem to hinder the accommodation of deformation within the semisolid material, and consequently fracture occurs. Based on these findings, several strain and strain ratebased criteria have been developed to qualitatively predict hot crack formation [2,[7][8][9][10][11][12]14] in both casting and welding.…”

“…Although the actual strain and strain rate fields within a semisolid weld are position and time dependent, existing hot cracking criteria use an averaged value across the semisolid weld to assess crack susceptibility [2,14]. Such a simplifying assumption limits the application of hot cracking criteria, as they are not able to predict the position and size of this defect [3] for a given set of welding parameters.…”

“…This total force field consists of the external forces that act on the boundaries of the semisolid weld, ⃗ ∂Ω fusion|semisolid weld , and the internal forces resulting from thermal contraction and solidification shrinkage, ⃗ Ω , i.e., ⃗ total = ⃗⃗ ∂Ω fusion|semisolid weld + ⃗ Ω . The internal forces can be easily defined for the solidifying grains as a function of temperature and solid fraction [2,3]. On the other hand, determining the external force field is complicated as it results from mechanical interaction between the semisolid weld and the base metal in response to the non-uniform thermal expansion and contraction of the weld pool and adjacent base metal during the welding process.…”

Investigation on hot cracking during laser welding by means of experimental and numerical methods

“…Wang [9] concludes that the segregation and morphology of liquid channels in the last stages of solidification make materials more susceptible to hot cracking. Coniglio [10] describes the crack initiation and growth mechanisms during welding solidification in terms of critical stress to fracture the inter-dendritic liquid, critical strain to exceed the mushy zone ductility and critical hydrogen content to nucleate and grow a pore. On the macroscale level, welding processes induce a thermal load and mechanical deformation.…”