Mechanisms for Solidification Crack Initiation and Growth in Aluminum Welding

Abstract

In the present work, mechanisms are proposed for solidification crack initiation and growth in aluminum alloy 6060 arc welds. Calculations for an interdendritic liquid pressure drop, made using the Rappaz-Drezet-Gremaud (RDG) model, demonstrate that cavitation as a liquid fracture mechanism is not likely to occur except at elevated levels of hydrogen content. Instead, a porosity-based crack initiation model has been developed based upon pore stability criteria, assuming that gas pores expand from pre-existing nuclei. Crack initiation is taken to occur when stable pores form within the coherent dendrite region, depending upon hydrogen content. Following initiation, crack growth is modeled using a mass balance approach, controlled by local strain rate conditions. The critical grain boundary liquid deformation rate needed for solidification crack growth has been determined for a weld made with a 16 pct 4043 filler addition, based upon the local strain rate measurement and a simplified strain rate partitioning model. Combined models show that hydrogen and strain rate control crack initiation and growth, respectively. A hypothetical hydrogen strain rate map is presented, defining conceptually the combined conditions needed for cracking and porosity.

Process Optimization of Dual-Laser Beam Welding of Advanced Al-Li Alloys Through Hot Cracking Susceptibility Modeling

“…The maximum strain rates sustainable by the weld material obtained under various laser powers were calculated using Eq. [7] and the inverse of this parameter gives the relative HCS index according to the second-level RDG criterion. Figure 10 plots the normalized second-level HCS index on the cross section of the fusion zone for different weld powers.…”

“…As illustrated in Eq. [7], the first term is the maximum strain rate that can be provided by the liquid metal flowing into the bottom of the dendritic network, which depends on the cavitation pressure, the secondary arm spacing, thermal gradient, and liquid viscosity. The second term is the strain rate required to accommodate the shrinkage during the liquid-to-solid phase transformation.…”

“…As can be seen in the expression of the first term in Eq. [7], the effect of increases in the thermal gradient will be outweighed by the reduced secondary arm spacing, on which there is a squared dependence. Therefore, with an increase of laser power, the strain rate sustainable in the weld is predicted to become smaller, indicating a larger hot cracking susceptibility.…”

“…To perform the calculation using Eq. [7], the thermal gradient G was obtained through the aforementioned finite element thermal model. The secondary dendrite arm spacing k 2 was calculated using JMatPro, while values for other variables were taken from Reference 5.…”

“…This model has been validated in a number of aluminum alloy systems. [6][7][8][9][10] The RDG model attributes the formation of hot cracking to two main factors: lack of liquid feeding; and tensile deformation in the coherent dendritic network caused by thermal contraction of the solid. This model is based on the observation that hot cracking occurs in the vulnerable mushy zone when the material is at a temperature between the coherency temperature and solidus.…”