Melting AZ91D magnesium alloy in sealed furnace with protective atmosphere of N_2 containing 0.01%HF

The effects of holding temperature,stirring and furnace seal quality on the protection of AZ91D magnesium alloy were investigated in a sealed furnace filled once with N_2 containing(0.01%HFC134a)(volume fraction) before the experiments,the corrosion of the atmosphere to the inner wall of the furnace was analyzed.The morphology,thickness,and composition of the surface films were also studied.The results show that HFC134a is suitable to be the protective gas of magnesium alloys and N_2 is a good carrier gas for HFC134a.Without surface agitation,the highest protective temperature for AZ91D melt is 865℃;with surface agitation,the highest protective temperature is 800℃,and the allowable pressure rising velocity in crucible can exceed 10kPa/min.The morphology of all protective surface films reveals a compact cellular structure with an evenly distributed thickness of 12μm,whereas that of unprotective films reveals a wadding like structure with an uneven thickness.With the increase of the holding temperature,the carbon content in surface films decreases to zero,and the oxygen content increases gradually.No corrosion of the atmospheres to the inner wall of the furnace is found during the experiments.