Inherent instability investigation for low speed laser welding of aluminum using a single-mode fiber laser

The causes of instability are investigated for low speed welding of aluminum from 10 mm/s down to 1 mm/s using a 300 W single-mode fiber laser. Results show that the welding is stable until the speed drops below a certain threshold (∼1 mm/s) at which there was a significant change in the process mechanism, causing shallow, inefficient welds with many defects. A power distribution model and several tests are used to examine different types of power losses at low speeds. It is then hypothesized that, at low speeds, the CW laser beam mainly irradiates at the molten pool, which absorbs a large portion of the beam energy near the surface. The majority of this absorbed energy subsequently is either lost via evaporation or transferred into the bulk material via convection and conduction without being used for melting the solid at the welding front. A laser pulsing scheme was used to test the above hypothesis. It was found that, through proper control of the duty cycle and frequency to prevent overheating of the molten pool, a high aspect ratio weld shape can be restored at low speeds, thus, confirming the hypothesis. In addition, the 1 mm/s low speed threshold is found to be related to the initial molten pool propagation speed, which is found to be approximately 1.4 mm/s. Although this paper does not propose a solution to restore process stability, the understanding of the instability origin will be helpful in the search of such a solution to overcome the process instability for slow speed welding of aluminum.