Research On Melt Treatment And Solidification Characteristics Of Al-Fe Alloy

Al-Fe alloy is a new type of lightweight heat resistant alloy. Thick needle-like and needle plate-like Al₃Fe compound in cast Al-Fe alloy are found in the matrix and greatly reduce the strength of the alloy, wich restricts the applications of the alloy. The refinement of the microstructure is the most effective means of improving the alloy properties. However, many solidification problems of Al-Fe alloy are not well understood. Therefore, the refinement of the microstructure is difficult. In order to understand certain solidification problems, the liquid transformation, the heredity of microstructure, melt overheating treatment and the effect of alloying elements on the growth of Al₃Fe compound have been investigated in this thesis.A remarkable endothermic peak occurs on DTA curve when Al-5Fe alloy is heated above 1200℃. The reason is because of the fusion of Al₃Fe compound. A remarkable exothermicl peak occurs on DTA curve when Al-5Fe alloy is cooled from 1300℃to 1200℃, which is due to the precipitation of Al₃Fe compound. The result indicates that the dystectic Al₃Fe compound are the genetic genes. Overheating the liquid Al-5Fe alloy to 1300℃and chilling subsequently can greatly retard the precipitation of Al₃Fe compound above the liquidus temperature and markedly refine Al₃Fe compound, which is because the Al₃Fe compound is melted thoroughly and their heredity is eliminated. The melt overheating treatment at 1300℃can enhance the tensile strength of Al-5Fe alloy from 107MPa before treatment to 145MPa after treatment, increased by 35.5%. the fracture mechanism of Al-5Fe alloy changes from cleavage fracture before treatment to part dimple fracture and part cleavage fracture.The experimental results show that at 950℃pure Fe element does not directly dissolve in liquid Al, but combines with Al and forms Al₃Fe compound, then Al₃Fe compound dissolve in liquid Al. In this way, Fe continuously dissolves in liquid Al. Al₃Fe compound are present in liquid Al-5Fe alloy at normal smelting temperature. Overheating treatment below 1200℃has a good effect on the morphology of primary Al₃Fe compound. However, the refinement effects vary from chilling processes. The refinement effect of high temperature and low temperature melt treatment is the best; the dissolution of Fe in liquid Al produces fine Al₃Fe cpmpounds which serve as crystalline nuclei, so the refinement of chilling in Al&Fe powders mixture is better; chilling in Al ingots fixes the superior structure of high temperature melt and does not introduce the thick Al₃Fe cpmpounds, hence, the refinemet is also better; chilling in Al-5Fe master alloy introduces the thick Al₃Fe cpmpounds and aggravates the crystalline, therefore, the refinement is worst. The morphology of primary Al₃Fe compound can not change into needle spot-like and grain-like after melt overheating treatment below 1200℃because dystectic Al₃Fe compound are not entirely melted.The alloying elements of Mg, Cr and Co have an important effect on the growth of primary Al₃Fe compound. The majority of Mg distributes in the matrix and around primary Al₃Fe compound. The enrichment of Mg around primary Al₃Fe compound restrains the preferential growth of Al₃Fe and causes the formation of grain-like primary Al₃Fe compound. The enrichment of Mg also gives rise to the constitutional undercooling. If the undercooling is enough, the endogeny nucleation and endogeny growth of primary Al₃Fe compound occur in the constituent undercooling zone. After addition of Mg, a part of grain-like Al₃Fe compound with ripples on their surfaces look like stratified rocks. The reason may be that the Al₃Fe compound grows in the mode of lamellar stacking. Cr and Co distribute mainly inside Al₃Fe compound. They suppress the preferential growth of Al₃Fe compound by means of atom occupation space. Therefore, the block-like Al₃Fe compound forms.The combination of Mg and Mn produces the blossom-like, block-like or fringe-like Al₃Fe compound. Mn is advantageous to the formation of blossom-like and block-like Al₃Fe compound, but Mg to the formation of fringe-like Al₃Fe compound. Necking and tiny cracks are present in fringe-like Al₃Fe compound due to the suppression of Mg.The combination of Cr and Mg is advantageous to the formation of block-like Al₃Fe compound. Cr mainly exists inside Al₃Fe compound. When the content of Cr is low, the dissolution of Cr in Al₃Fe compound is hindered by the affluence of Mg around Al₃Fe compound, which impairs the refinement of Cr. However, the hindrance of Mg is not evident when the amount of Cr is high. Needle-like or needle plate-like Al₃Fe compound may be the flower-like or star-like at some specific conditions. The nature of flower-like or star-like Al₃Fe compound is the transverse profiles of rod Al₃Fe compound whose sides have ten branches. The external reason of the formation of flower-like or star-like Al₃Fe compound is the undercooling. Only when the cooling rate or undercooling is enough high does flower-like or star-like Al₃Fe compound occur. The another reason, i.e., the intrinsic reason, is the multiple twinning of (100) and (201) of Al₃Fe compound.