Tham khảo tài liệu 'asm metals handbook - desk edition (asm_ 1998) episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | electric radiation furnaces. The nature of the furnace charge may range from prealloyed ingot of high quality to charges made up exclusively from low-grade scrap. However even under optimum melting pouring conditions molten aluminum is susceptible to three types of degradation With time at temperature adsorption of hydrogen results in increased dissolved hydrogen in the melt. With time at temperature oxidation of the melt occurs. Loss of alloying elements Dissolved Hydrogen. Hydrogen is easily adsorbed by molten aluminum. Unfortunately the solubility of hydrogen in molten aluminum alloys is substantially greater than in solid aluminum Fig. 1 . When the aluminum alloy solidifies hydrogen is driven out of solution exaggerating and enlarging shrinkage porosity with accompanying loss in mechanical properties Fig. 2 . Sources of hydrogen may include wet charges and damp melting tools but the primary source of hydrogen is ambient humidity. Because little can be done to prevent hydrogen pickup during melting hydrogen must be removed from the melt before pouring. The most common method used is bubbling dry nitrogen or argon through the melt. Rotary degassing units as described and illustrated in the article Melting Methods in this Section see Fig. 17 of that article are particularly effective. These units may be inserted into melting crucibles for use. The use of chlorine gas is especially effective in removing hydrogen however environmental and safety considerations generally preclude its use in production. Fig. 1 Solubility of hydrogen in aluminum at 1 atm hydrogen pressure Fig. 2 Ultimate tensile strength versus hydrogen porosity for sand cast bars of three aluminum alloys Measurement of the amount of hydrogen dissolved in the melt has historically been done using a reduced pressure test in which a sample of molten aluminum is poured into a small steel cup and allowed to solidify in a vacuum chamber. Solidification is observed the degree of evolution of bubbles during .
