Study On Refinement, Modification And Sedimentation Behavior Of Fe-rich Aluminum Alloys

The eutectic Al-Si alloy with high iron content is a kind of alloy that is usually produced from the secondary aluminum.Since their sound castability,low coefficient of thermal expansion and high abrasive resistance,they have been widely applied in the parts manufacturing through the die casting process.Under traditional casting condition,the comprehensive mechanical properties of cast Al-Si alloys will be deteriorated by the coarse needle like Fe-rich phases when the content of iron is larger than 0.9 wt.%.The iron content in secondary aluminum is relatively high.The iron diluting or removing methods are often carried out for lowing down the iron content and making it reach the production standard.But it also promotes the cost of production.The technique of iron phases modification in aluminum alloys can change the morphologies of needle like phases without adding up the cost of production,which has become the research focus in this field.In this research,the morphologies of Fe-rich phases in Al-Si-Cu-Fe alloy were modified by adding trace elements such as Ti,Sr,La and Mn,but not removing or lowing down the content of iron.The optical microscope（OM）,SEM/EDS,X-ray diffraction analysis and DSC testings were carried out for the quantification of the effect of trace elements addition on morphologies of secondary phases.Meanwhile,with the help of controlling morphologies of the Fe-rich phases,the effect of casting process parameters on the sedimentation behavior of the iron phases with different morphologies was investigated.The conclusion of this study were drawn as follows:1.The additon of Al-Ti-C master alloy could effectively refine the Fe-rich phases in Al-Si-Cu-Fe alloy.The optimum addition amount of Al-Ti-C master alloy was 0.6 wt.%,which led to a 92%increase in the ultimate tensile strength（UTS）and the 343%promotion in%elongation.The improvement of the comprehensive mechanical performance was due to the formation of Al₄C₃ particles formed in the melt.The Al₄C₃ turned into the nucleation core of Fe-rich phases.At the same time,theα-Al grains would be remarkably refined.When the amount of Al-Ti-C master alloy was excessive（>0.6 wt.%）,the needle-like Fe-rich phases would precipitate again and weaken the refinement effect of theα-Al grains.2.Under the optimum addition of Al-Ti-C master alloy,it was found that the eutectic silicon,α-Al andβ-Fe phases could effectively be refiened by simultaneously adding the Sr and La elements.The addition of Sr and La at the same time would form Fe₉LaSi₄ and SrSi₂phases.The refinement effect of simultaneously addition of Sr and La at a certain ratio on the microstructures was investigated by limiting their total addition amount at 0.12 wt,%.It was found that the modification effect for eutectic Si was more prominent when the ratio of La element was higher.When the higher ratio of Sr was set in the modification agent,the modification effect forβ-Fe was more prominent.When the ratio of Sr:La was increased,the secondary dendrite arm spacing（SDAS）was lowered down.When the ratio of Sr:La was 1:5,the SDAS value was decreased from 21.9μm to 9.7μm which was the minimum.When the ratio of Sr:La was set at 1:1,the decrease of the length ofβ-Fe was the most prominent.3.After knowing the effect of simultaneously addtion of Ti,Sr and La elements on the microstructures of Al-Si-Cu-Fe alloy,the manganese was added into the above alloy to further modify the coarse Fe-rich phases.The orthogonal experiment was carried out for investigating the effects of these trace elements on the grain sizes,the morphology of the Fe-rich phases,and the weighted index of mechanical properties.As far as the benchmark of grain size effected by simultaneously adding these four trace elements,the significative degree of them was ranked as Sr>Ti>La>Mn.The optimum addition recipe was 0.03 wt.%Ti,0.02 wt.%Sr,0.02 wt.%La and 1.0 wt.%Mn.As far as the benchmark of the area of Fe phases,the significant level of these four trace elements was ranked as of Sr>La>Ti>Mn.The optimum addition amount was 0.03 wt.%Ti,0.10 wt.%S,0.02 wt.%La and 1.0 wt.%Mn.While considering the weighted mechanical properties,the influencing sequence of the elements was Sr>Ti>La>Mn,and the optimum addition amount was 0.03 wt.%Ti,1.0 wt.%Sr,0.06 wt.%La and 0.02 wt.%Mn.4.In case of simultaneous addition of Ti,Sr,La and Mn,the competitive effect was occurred during the formation course of the secondary phases in the molten Al-Si-Cu-Fe alloy.The major alloying elements would form the secondary phases by selecting the specific trace elements which was subsequently added.Among them,the Sr element would react with silicon and form the SrSi₂ and Sr₃Al₂Si₂ phases.The La element would react with Si and Fe elements to produce LaFe₉Si₄ phase.The titanium would react with Si and Fe elements to create FeSiTi phase,and the manganese would react with Si and Fe elements to generate Al₉Fe_0.84Mn_2.16Si phase.5.The addition of Sr element would facilitate the dispersion of Fe-rich phases.Simultaneously,the eutectic temperature of the alloy would be changed.When the addition amount of Sr element was 0.02⁰.06 wt.%,the eutectic temperature,α-Al nucleation temperature and the eutectic latent heat was found to decrease.The excessive dosage of Sr element would increase the nucleation temperature ofα-Al.In this case,more eutectic latent heat was released.Meanwhile,the solidification range of the alloy was increase,which expand the solidification time duration of the molten Al.6.The effect of phase modifications on the sedimentation behaviors of these secondary phases was investigated.It indicated that the sedimentation velocity could be effectively increased and therefore promote the deironization rate.The needle-like Fe-rich phases could not be easily settled down and piled up at the bottom of the melt.When the Fe-rich phases were modified to petaloid ambulacra shape at 700℃with 60 min holding time.They were intensively stacked at the bottom of the melt.After sedimentation,the average length of the Fe-rich phases with blocky,Chinese-script or skeleton shapes at the bottom of the melt was much larger than that of the phases at the upper portion of the melt.However,the average length of the Fe-rich phase at each section of the upper portion showed small difference.It proved that the sedimentation of the Fe-rich phases with blocky,Chinese-script and skeleton shape obeyed the Stock‘s law.The sedimentation of Fe-rich phases depended on the critical size of the modified phase.In this dissertation,the morphologies of the phases in the Al-Si-Cu-Fe alloy were refined by individually and simultaneously adding trace elements.This novel process refined theα-Al grain,eutectic silicon and coarse Fe-rich phases,resulting in the promotion of comprehensive mechanical performance of the Al-Si-Cu-Fe alloy.Multiple modification agents for controlling the morphologies of secondary phases were developed.Besides,based on the modification technique for controlling the microstructures,the sedimentation of iron phases impact by the interactive effect of phases modification and casting process was studied.A method for simultaneously modifying the Fe-rich phases and removing the iron phases by sedimentation was proposed.This method could simplify the process courses of producing the secondary aluminum.It provided a valuable proposal for the cost reduction of recycling the aluminum scrap.It is significant to the research of effective iron removal in theory.