Micro-Scale Model Based Study of Solidification Cracking Formation Mechanism in Al Fiber Laser Welds

Significance Statement

Despite various applications of aluminum in construction of automobile and aircraft, fusion-based welding processes encounter solidification cracking which affects its structural integrity.

Large welding speed usually done in high manufacturing productivity have been known to lead to hot cracking. While this remains a concern, various research has proven that high welding speed could also suppress hot cracking by inducing compression loading zone at tail of molten pool, refinement of porosities and changes of liquid fraction as it varies from cooling rate, solidification rate, stress state and microstructure characteristics.

The RDG (Rappaz-Drezet-Gremaud) model have been used in prediction of hot cracking phenomenon and calculation of hot cracking sensitivity in welds.

In a recent article by Wang et al. (2016) which was published in Journal of Materials Processing Technology, a study on effect of welding speed on solidification cracking in fiber laser welding of 6013 aluminum alloy was made. It coupled with investigations of strain rate with existence of liquid film using a micro-scale finite element model.

In their experiments, the RDG model was used which calculates flow rate, deformation and shrinkage of columnar grains, relationship between solidification rate and welding speed, permeability of mushy zone in aluminum welds and eventually pressure drop in interdendrite liquid. The interdendritic liquid pressure drop between the tip and root of the dentrite is required to ensure that adequate liquid feeding back occurs to compensate the deformation and shrinkage of mushy zone. If the pressure drop reaches the critical pressure drop that a cavity forms, hot cracking initiates. For aluminum welds, shrinkage factor and viscosities were taken as 0.06 and 0.001 Pa.s, respectively.

A thermal-mechanical simulation of laser welding process was used to obtain temperature and stress/strain distribution in weld. A 3D thermal-mechanical model with size of 60mm x 35mm x 2.5mm was developed, which considers temperature-dependent material properties, volumetric heat source model, strain/stress relaxation in the weld molten pool and solidification shrinkage of weld metal. Fiber laser heat input was modeled by a Gaussian distribution heat model.

The local strain rate on columnar grain was calculated by a micro-scale finite element method (FEM) model which considers microstructure characteristic such as columnar grain length, primary dendrite arm spacing and orientation of the grain boundary and thickness of liquid film. The FEM model on columnar grains contains solid and liquid phases at the same time while a continuum-level modeling was employed to calculate local strain perpendicular to columnar grain by special treatment of liquid physical parameters.

AA6013 aluminum alloy specimens with dimensions of 150mm x 125mm x 2.5mm were used for experiments at welding velocity of 2.5m/min, 2.7m/min and 3.0m/min, laser power of 3.3kW, and argon shielding gas flow rate of 15L/min. At welding velocity  2.7m/min, solidification cracking was observed which initiated near the fusion boundary. As welding speed increased to 3.3m/min, transverse solidification cracking disappeared indicating that low welding speed increased solidification cracking susceptibility in fiber laser welding of 6013 aluminum alloys.

Metallographic analysis showed grain structure played an important role in solidification cracking behavior. As welding speed increased from 2.7m/min to 3.3m/min, the length of columnar grain decreased from 500μm to 300μm while primary dendrite arm spacing increased from 51μm to 60μm.

Due to fast welding speed in laser welding, cooling rates for 2.7m/min and 3.3m/min were high at 3380K/s and 4770K/s, respectively. That is, high welding speed results in high cooling rate. Maximum longitudinal mechanical strains in mushy zone where solidification cracking occurs are 1.4% and 0.9% for welding speeds of 2.7m/min and 3.3m/min, respectively.

For welding speeds of 2.7m/min and 3.3m/min, the order of magnitude of local mechanical strain is about 10-4 in solid grain and 10-2 in liquid film.

Pressure drop exceeding the critical value of 150kPa for crack formation mainly occurred in the second half of mushy zone where solid fraction is between 90% and 94%. Solid fraction of 90% could be deemed as critical solid fraction at which pressure drop exceeds the critical value for crack formation.

Summary of simulation results showed that small cooling rate and large deformation are induced at low welding speed while local mechanical strain in solid columnar grain is much smaller than mechanical strain in liquid film.

The micro-scale model could be extended to laser welding of other aluminum based alloys when solidification cracking initiation relates with the liquid film.

Journal Reference

Xiaojie Wang1,4, Fenggui Lu1,2 , Hui-Ping Wang3, Zhaoxia Qu4, Liqian Xia4. Micro-Scale Model Based Study of Solidification Cracking Formation Mechanism in Al Fiber Laser Welds.  Journal of Materials Processing Technology, Volume 231, 2016, Pages 18–26.

Show Affiliations
  1. Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
  2. Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai 200240, PR China
  3. Manufacturing System Research, GM Global R&D, Warren, MI 48090, USA
  4. Welding and Corrosion Protection Technology Department Research Institute, Baoshan Iron & Steel Co. Ltd., Shanghai 201900, PR China

 

 

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