Formation Mechanism Of Solidification Cracking In Laser Welding On Al-Mg-Si Alloy

Due to the excellent features in formability, strength and structural weight reduction, Al-Mg-Si alloys are widely adopted in aerospace and auto industries. In recent years, efficient laser welding technology appearances, which provides a new way to realize the joining of high strength aluminum alloys. Laser welding technology has the advantages of flexibility, high energy density and welding speed, which could effectively reduce the heat input and heat-affected zone width, and thus controls the softening of welded joints. However, solidification cracking phenomenon remains a major issue in laser welding of aluminum alloys, and has not been well solved so far. Therefore, the research on mechanism of solidification cracking and the development of corresponding mathematical model have significant theory and application values to understand solidification cracking in aluminum alloys and obtain a quality and efficient aluminum welded structure.This paper built a weld solidification cracking susceptibility testing system. Using the ratio of the surface crack length and weld length as solidification cracking susceptibility, the effect of laser welding parameters, such as laser power, welding speed, distance between weld and work-piece edge, on susceptibility of solidification cracking was investigated. The results show that increasing the laser power could increase susceptibility of solidification cracking; however, increasing the welding speed and the distance between weld and work-piece edge could effectively reduce the solidification cracking susceptibility. From the perspective of multi-factors effect on cracking susceptibility, a mathematical model about the weld solidification cracking rate was proposed based on the response surface methodology: HC =-1554.1 + 11.2L + 606.7P + 346.5V- 16.3LP + 10.9LV-140.3PV.Experiment was designed to study the effects of constraint type on solidification cracking initiation and propagation in single-side and double-side constraint conditions. The results indicate that compared with the free condition, the single-side constraint condition could promote initiation and propagation of solidification cracking, and the promotion effect increases with the increasing constraint intensity. Compared with the free condition, the double-side constraint condition could obviously prevent the solidification cracking propagation. The extra tensile mechanical strain in brittle temperature range at the tail of molten pool, which is induced by the prevention effect of mechanical constrain on solidification shrinkage of melt metal, is the main cause of constraint effects on cracking initiation. The extra constraint stress caused by mechanical constraint would promote the opening of crack tip; however, the fixture itself would prevent the opening. Effect of mechanical constraint on crack propagation is a competitive result between them.By the high-speed camera observation and the metallurgical analysis, solidification cracking behavior was investigated. Under the welding conditions used in this paper, solidification cracking for 6013 aluminum alloy initiates first at weld toe where columnar grains exist, and then extends to the weld center along columnar grain boundaries, subsequently propagates straight along the weld centerline. Scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS) were used to observe the fracture morphology and the chemical composition at initiation and propagation sites of crack. It is found that the fracture surface is covered by liquid films which are rich in Mg, Si and Cu elements. Jmat Pro software was used to calculate the solidification process of 6013 aluminum alloy and analyze the low-melting eutectic compositions. The metallurgical analysis shows that solidification cracking in 6013 aluminum alloy occurs when the insufficient amount of liquid flows back to heal the tearing of liquid film.A stress-strain finite element model in fiber laser welding of 6013 aluminum alloy was developed based on thermo-elastic-plastic theory. Stress-strain field model needs to consider the appropriate high temperature mechanical properties of aluminum alloy, the stress relaxation effect of the molten pool and the solidification shrinkage of weld metal. The validation of the proposed model was verified by comparing with the experimental results. Macroscopic mechanical criteria on initiation and propagation of solidification cracking for 6013 aluminum alloy were proposed. Mechanical strain and accumulated displacement in brittle temperature range(BTR) are measures of the susceptibility of crack initiation and propagation, respectively. Larger mechanical strain and accumulated displacement in BTR means the higher possibility of crack initiation and propagation. Based on the macro stress-strain model, quantitative analysis of the propagation path of solidification cracking, effect of the constraint type on solidification cracking and effect of distance between weld and work-piece edge on solidification cracking were carried out.A finite element model about local strain of columnar grain was developed considering the exits of liquid film. Simulation result indicates that the magnitude order of mechanical strain in liquid film is about 10-3 times than mechanical strain in solid. Strain concentration in the interdendritic liquid film is an important cause of the rupture of liquid film. Based on the mass conservation and the homogeneous fluid flow equation, formula of pressure drop(RDG model) in interdendritic liquid film was derived, and the application of RDG model in welding process was discussed. From the microscopic point, formation mechanism of crack initiation was investigated. The pressure drop in interdendritic liquid film reaching the critical pressure drop of cavity formation is the reason for the rupture of liquid film and the solidification cracking initiation. The proposed model was used to simulate the initiation of solidification cracking under different welding speeds, and the critical pressure drop for crack initiation in laser welding of 6013 aluminum alloy was proposed.