The blistering phenomenon that occurs after magnets are used affects the use of magnets and often causes confusion. It's necessary to understand how it is generated and how to prevent it.
The water content of the blank, the uniform particle size of ball - milling, pre - sintering and sintering temperatures, and the atmosphere of the sintering furnace all have a great impact on the blistering rate of magnets. If the water in the base of the blank is not completely evaporated, the blistering rate of the magnet will also increase. The specific discussion is as follows:
① If the difference in water content between the inside and outside of the blank is relatively large after the blank is removed from the mold, the magnet is prone to blistering. This is because the initial surface water evaporation rate of the blank is faster than the internal water diffusion rate. As time passes, the surface water content decreases, and the difference in water content between the inside and outside of the blank increases. If there is residual trapped water in the center of the blank, blistering will occur at high temperatures. If the difference in water content between the inside and outside of the blank gradually decreases, the blistering will also gradually decrease.
② During secondary fine - grinding, the larger the particle size, the easier the internal and external capillary water diffuses, and the smaller the blistering rate of the magnet.
③ During sintering, there is no blistering below a certain sintering temperature. This temperature is called the blistering critical temperature. Above this temperature, the blistering rate will gradually increase as the sintering temperature increases. As the pre - sintering temperature increases, the blistering critical temperature also increases.
④ Another reason for blistering is that after the pre - sintered material is mixed with iron due to crushing and grinding in the crushing process and Fe₂O₃ is formed, it will decompose according to the reaction formulas 6Fe₂O₃ = 4Fe₃O₄+O₂ and 2Fe₂O₃ = 4FeO + O₂ during sintering. When this Fe₂O₃ is strongly reduced to FeO, if the decomposed O₂ cannot be discharged, the blank will also blister.
⑤ Magnets in sintering furnaces with good ventilation are also prone to blistering.
The main solutions include: extending the drying time of the blanks, appropriately increasing the temperature of the pre - sintered material and the uniform particle size of ball - milling, and reducing the secondary sintering temperature, etc.
