STUDY ON HEREDITARY OF PORES IN LASER REMELTING OF DIE CASTING AZ91D MAGNESIUM ALLOY

Porosity has been a main problem for die casting magnesium alloy welding and casting defect repair. In order to study the formation mechanism of pores in fusion welding of die casting magnesium alloy, in this research, experiment of die casting AZ91D magnesium alloy CO2 laser re-melting was carried out. OM and SEM were employed to observe the characteristics of pores existing at both the base metal and re-melted zone, and a software for particle size analysis called nano measure 1.2 was adopted to measure the pores' size. During analysis, the work mainly focused on the relationship of pores in re-melted zone associated with that preexisting in the base metal. The results showed that: porosity in die cast magnesium alloy base metal showed apparently hereditary characteristic in the process of re-melting. Porosity preexisting in the die casting AZ91D magnesium alloy was mainly produced at the junction region of multi-grains, with high pressure of inner gas, small size and irregularity in shape. However, pores in the re-melting zone showed diversity. Specifically, the micro-pore was small in size, nearly round in cross section and smooth in the inner wall, which was induced by hydrogen stored in the base metal. The macro-pores were vermiculate, with gas channel and metal erosion traces at the inner wall. It's considered that the micro-pore was mainly inherited from the atomic hydrogen solution in the base metal and molecular hydrogen stored in the die casting defects. In the re-melting process, gas bubble of hydrogen was formed through nucleation and development two steps, and there was no sufficient time to grow up. As a result, the hydrogen induced pore was great in number and small in size. While the macro-pores were inherited from porosity preexisting in the base metal, the corresponded gas bubble did not require nucleation, was directly formed from the involved gas in preexisting porosity during die-casting process. Development of gas bubble for macro-pore mainly was coalescence of small gas bubbles, which made the macro-pore show channel and crude in the inner wall. Furthermore, formation mechanism of the two types of pores were analyzed, and mathematical model on relationship of macro-pore in re-melted zone associated with that in the base metal was established.