Study On Flame Retardant Atmosphere Condition And Control In Differential Pressure Casting Of Magnesium Alloy

In recent years,magnesium alloys have been widely used in the fields of national defense,aerospace,3C industry and transportation for their advantages such as small specific gravity.However,magnesium alloys have a high chemical activity under the high temperature condition,and thus are easily oxidized,burned in the furnace,and even cause an explosion.The above problems lead to an extremely complex processing of magnesium alloys.Especially,during the process of resin sand anti-gravity casting,the decomposition of resin sand will release various atmospheres,which makes the preparation process more susceptible to oxidative combustion.Therefore,the flame retardant technology in casting process has become a key technical bottleneck restricting the production of high-performance magnesium alloy castings.Taking the resin sand for casting as the research object,the influencing factors of the pyrolysis behavior and the gas permeability of the resin sand were investigated by self-designed resin sand pyrolysis and gas permeability test equipment,respectively.In addition,the critical model of the fracture state of the magnesium alloy oxide film was established,and further to drawn the following conclusions:Based on the research of the pressure variation of pyrolysis and gas generation behavior at different conditions of resin grains,binder addition or initial pressure of foundry resin sand,it can be concluded that the pyrolysis and gassing behavior of resin sand is mainly caused by the mixed binder,while the particle size variation has few influence on the pyrolysis process.With the increasing of the binder addition,the pressure generated by the total pyrolysis gas volume of the resin sand increases.When the amount of binder added is 0.5% of the original sand mass,the pressure generated by pyrolysis gas in the mold cavity is 138.62 KPa,and when the binder addition mass is 1.0%,1.5%,2.0%,respectively,the corresponding pressures were 155.83 KPa,175.08 KPa,and 184.80 KPa.The total amount of atmosphere released by resin sand pyrolysis increases with the rising of initial pressure.When the initial pressure is 0 KPa,the pressure of atmosphere released by pyrolysis is 141.83 KPa,and at the initial pressure of.25 KPa,50 KPa and 75 KPa,the corresponding pyrolysis pressure can reach 156.4 KPa,189.36 KPa and 214.09 KPa,respectively.Nevertheless,the increasing of initial pressure will cause a certain hysteresis on the peak on the pressure curve of the resin sand pyrolysis behavior,and for every 25 KPa increase in initial pressure,the peak will lag 10 min.The gas permeability test of resin sand shows that the particle size,binder content and thickness will all affect the gas permeability.The permeability curve of resin sand in the cavity accords with the quadratic root function.With the increase of particle size,the resin sand become looser and the permeability is improved.While the permeability decreases with the rising of binder quality,and the thickness of resin sand is greater,the worse the permeability will be.The investigation on the oxidation process of magnesium alloy shows that,the oxide film on its surface have a certain protective effect at the beginning,and after a period of time,the thickness of the oxide film increases and gradually loses the protection.Besides,the oxide film structure is loose and porous,so the internal metal undergoes a vigorous oxidation reaction,and a tumor-like oxide is formed on the surface of the metal macroscopically.The thickness of surface oxidation film of pure magnesium can reach 6 ?m at aeration pressure of 4 KPa.The atmosphere released during the pyrolysis process accumulates in the cavity to generate internal pressure,accompanied by the process of gas permeable of the resin sand,and at a certain moment,the difference between the above two will reach the critical pressure of the magnesium alloy oxide film rupture.Meanwhile,the accumulation process of net pressure is affected by effective area ratio and the sand thickness.When the effective area ratio is 1,namely,the gas generating area is equal to the gas permeable area,the magnesium alloy does not oxidize or ignite.At the effective area ratio of 1.85,the critical thickness of the resin sand is 2 cm,which is the limit value.That is,when the effective area ratio is larger than 1.85,the gas permeability cannot be improved by only reducing the thickness of the resin sand.In this case,the active exhaust can be realized by optimizing the sand structure,adding a vent hole,etc.,thereby achieving the control of the atmosphere condition in the cavity and achieving the purpose of flame retarding.