Study On The Crack Growth And Deflection Behavior Of Aluminum Alloy Laser Welds

The method of jointing high strength aluminum alloys has greatly improved the manufacturing technology in the aerospace field.In the laser welding process,the kinds of joint fracture such as porosity and crack often appear around the weld area.Especially,welding cracks cause the discontinuity in the structure and weaken the strength.In this paper,the methods for determining crack tip and plastic zone based on full-field deformation were proposed to study the crack propagation and deflection behaviors of 6061 aluminum alloy laser welding joints.The deflection of mode I,mode II and mixed mode cracks,the shape and size of the plastic zone at the crack tip and the fracture toughness were measured.And the facts related to the crack deflection path,the appearance of the plastic zone and the fracture toughness were discussed.Firstly,the digital image correlation method was used to obtain the displacement field and strain field on the surface of the laser welding specimens with mode I weld cracks.Fitting the straight lines with the reference points can obtain the nominal crack tip position,CTOA and CTOD of mode I crack.The fracture toughness of the mode I crack can also be calculated by the circuit integral method.The phase congruency method was used to acquire the crack growth path.The shape and size of the plastic zone at the crack tip were obtained by combining the phase congruency method and Otsu method.The results show that the nominal crack tip method can effectively measure the position of the crack tip and obtain CTOA and CTOD accurately.With the growing crack in the weld zone,the CTOA and CTOD were increasing slowly and kept rising steadily at the end.The CTOA and CTOD were approximately zero with the crack growing from weld to heart affected zone.Subsequently,the crack expanded steadily in the heat affected zone.Basing on the research of mode I crack,we studied the expansion and deflection behavior of mixed mode crack.Due to the complex propagated behavior of mixed mode cracks,the nominal crack tip method can not apply to acquire the crack tip position.Combining the phase congruency method,the crack path was easily obtained.The crack tip position can be measured through the crack path.The outline and size of the plastic zone that were gained by the Otsu method and the eliminating base plane method can explain the reasons of the various deflection and growth of mixed cracks.The circuit integral method can also be used to calculate the fracture toughness of the mixed mode crack.The results show that mode II crack is more difficult to expand than mode I crack.The plastic zone range caused by mode I cracks is larger than that caused by mode II cracks.And as the loading angle rising,the outline of the plastic zone is more and more narrow and the earlier deflected behavior occurs due to the increasingly accumulated plastic deformation in the growth direction of the mode II crack.Finally,the finite element simulations of mode I,mode II and mixed mode welding crack propagation in 6061 aluminum alloys were carried out.The results show that in the process of crack propagation,the stress field of the crack tip is discontinuous.The radius of the plastic zone on the aluminum alloys is larger than the heat effect in the weld area,which is related to the material properties.