Study On The Method Of Suppressing The Pores Of Laser Welded 5A90 Aluminum-lithium Alloy

Aluminum-lithium alloys are kind of typical light-weight and high-strength materials, which are extensively applied in aerospace, ships and rail vehicles industry. Compared with conventional welding techniques, the laser beam welding is a fast growing technology offering a higher welding speed, a higher depth-to-width ratio and a smaller heat-affected zone. However, due to the addition of lithium, laser welding of 5A90possess serious porosity than conventional aluminum alloys, which decreases the mechanical properties of welding joint seriously. Based on the formation mechanism of pores and its suppression approach, a series of experiments had been implemented. Three suppression methods of pores had been presented:changing the welding position, optimizing the pulse laser welding parameters, optimizing the shielding gas parameters.Firstly, the influence of different laser welding positions on the welding appearance and porosity had been investigated to get the optimal welding position. The optimal welding appearance was obtained in vertical-down welding, and the seriously excessive penetration defects of flat welding could be well solved. The results of high speed camera showed that the liquid metal flows of weld pool were different because of the surface tension and gravity. The keyhole stability and porosity are depended on the force state of melt pool and the keyhole behavior in different welding positions. The results showed that the pores can be well suppressed in vertical-down welding. It was because the force states of keyhole were balanced and the convections in weld pool can help the bubbles escape.Secondly, based on the optimal welding position, the effects of pulse laser welding parameters on the porosity had been studied and the process window of grade I weld bead could be obtained. The results show that the porosity were in a direct proportion to power and duty ration, and there existed an optimal velocity value. The keyhole was in a stable condition during pulsed welding process and the involved shielding gas was avoided. The pulse laser had a characteristic of discontinuous power-export, and it stirred the weld pool leading to the change of the flow pattern in the weld pool. It can help the bubbles escape from the weld pool and decrease porosity. Based on the design-expert, the optimal welding parameters could be obtained. The results show that the grade I weld bead was reached when the velocity, duty ratio and laser power were in a range of 700?900 mm/min,50%?57%,1200?1400W respectively.Based on the results mentioned above, the effects of shielding gas on porosity were investigated and the shielding gas parameters were optimized. The results show that the shielding gas could suppress the porosity well, and the gas flow rate was a significant factor, while the incident angle and interaction were not significant. With the increase of gas flow rate, the pore count increased at first then decreased. A minimum value was reached in the range of 5?7 L/min. The optimal response model for pore count was cubic, and the gas flow rate, incident angle in the range of 3?9 L/min,15°?52° respectively were identified as the optimal parameters for I grade weld bead.