Melting And Casting Process Investigations On Large Size Ingot Of AA5182 Al-Mg Alloy

In the present paper,according to the casting practice,the AA5182 Al-Mg alloy large size ingots were investigated by means of the low metal head casting.The influence of the conventional direct chill(DC) casting and the low metal head casting (LHC) to the ingot quality was discussed.The hydrogen degassing process in the liquid aluminum,the hot crack and the bleed out of the ingot have been discussed.By the means of matching the water cooling rate and the casting speed during the casting practice,Mg segregation layer thickness could be reduced.Based on all above,how to achieve the low metal head casting condition is concluded during the investigation.The difficulty of the large ingot was clarified by comparing the large ingot with the small size ingot.Because the size is large,the liquid metal bath is much deeper than the normal melting process.Then the stirring of the liquid metal is more difficult than before and the chemistry uniformity of the liquid metal isn't as good as the normal process.During the casting process,the cooling water on the ingot rolling face and the ingot end face are different.This results in the casting defects.The advanced melting and casting process are investigated to solve the problem and optimize the whole process.The large size ingots quality is good and this meets the requirement of the process.By the control of hydrogen content and dross content in the liquid aluminum, together with the influence of the high Mg content to the hydrogen level in the liquid aluminum,the hydrogen degassing process has been discussed.First of all,the porous plugs are used in the holding furnace for hydrogen degassing.Then,with the optimization of the Ar preheating in the liquid aluminum refining system(LARS),the bubble expansion for high temperature of the liquid metal is reduced and the degassing process is optimized.In addition,by the design of the degasser vessel shape, the section of the vessel matches with the bubble expansion.So the bubbles wouldn't be merged with each other.The degassing efficiency could be improved,for AA5182 alloy liquid aluminum,when the entrance hydrogen content is lower than 0.300ml/100g,the hydrogen efficiency could be 50%—60%.And the hydrogen level in the liquid aluminum could be lower than 0.150ml/100g,this meets the requirement of the ingot quality.For the inclusion in the liquid aluminum,the ceramic foam filter(CFF) system is applied.With two tiles installed in the filter bowl side by side,the liquid aluminum flow rate and the filtration efficiency could be improved.With the help of increasing the combustion air flow rate,the preheat process is optimized.At the same time,with 50ppi filter tile,for the inclusion size of more than 20 microns,the filtration efficiency could be higher than 70%.During the conventional DC casting process,Mg segregation layer thickness is about 5mm in the large size of ingot.The Mg segregation layer affects the ingot quality,the scalping thickness should be more than usual.By the optimization of the water cooling rate and the casting speed,so that they are matched with each other.At the same time the graphite liner is installed inside the casting mold,then the low metal head casting condition is achieved,the Mg segregation layer thickness will be reduced to about 1.5mm.The LHC casting ingot Mg segregation layer thickness is 60%-70% less than the DC casting ingot.The ingot quality is improved and the scalping thickness is reduced.By the optimization of the LHC casting process parameters,such as the liquid aluminum level,the temperature of the liquid metal,the cooling water flow rate and the casting speed,the profit of the LHC casting process is investigated.Especially the effect of the LHC casting process to decrease the relative defects and decrease the Mg segregation layer thickness is highlighted.The relative ingot defects during the casting process are investigated,such as the hot crack,the bleed over,the cold folding, and so on.By the optimization of the cooling process,the improvement of the grain refining process,and the suitable casting speed,the defects could be reduced or avoided.Then the quality of the ingot could be improved.The whole melting and casting process of the large AA5182 alloy ingot, including the chemistry of the alloy,the melting temperature difference,the liquid aluminum degassing process,the filtration,the optimization of the ingot quality,and the defects of the Mg segregation,the hot crack,the bleed over,and the cold fold have been highlighted.The AA5182 ingots with high Mg content of 4.0%-5.0%are investigated.Especially the specialty during the melting and casting process is discussed in detail.So this thesis will be the guidance to the practice of the AA5182 alloy large ingot.