Study On The Technology And Theory Of Al-Zr Master Alloy Processed By Fused Salt Reaction

The rapid development of transporting vehicles such as aerospace,high-speed train,etc accelerates the upgrading of aluminum alloys.Addition of Zr element into aluminum alloys has been become an important way to improve the microstructure and mechanical properties of A1 alloys.Therefore,the optimization of microstructures and properties as well as the performance evaluation of Al-Zr master alloy has important practical and theoretical significance.Based on the this background,the preparation technology and refinement effect of Al-Zr master alloy were investigated for the purpose of providing basic data and theoretical supports for the microstructure control of Al-Zr master alloy and its application in industry.With the combined methods of fused salt reaction experiments,metallography and EET theory,the effects of fused salt reaction and solidification conditions on the morphology and size of Al3Zr phase in Al-Zr master alloy were investigated;the dissolution behaviors and refinement effect of Al3Zr phase in industrial pure aluminum were studied;the mechanism of the role of pouring temperature on the morphology of Al3Zr phase as well as the refinement of Al-Zr master alloy in industrial pure aluminum was analyzed at electronic structure level.The main results are as follows:(1)The fused salt reaction began at 690? when increasing the temperature of solid Al and powder K2ZrF6 simultaneously.When the fused salt reaction occurred at 690?750 ?,with the progressing of sedimentation,melting,reaction and spheroidization of K2ZrF6 in the melt,the Al melt and K2ZrF6 packaged each other and reacted at the interface between melt and K2ZrF6,then a large quantity of Al3Zr phase formed.(2)The reaction of liquid A1 and K2ZrF6 is a heat releasing process.The temperature rise resulted from fused salt reaction near the reaction interfaces is about 10 ? when K2ZrF6(125 g)was added in Al liquid(780 g)at 800 ?.(3)When the reaction of liquid Al and K2ZrF6 occurred at 800 ? without stirring,Al3Zr particles with a thickness of 2?3 ?m formed at the reaction interface.The size of Al3Zr particles is in the range of 0.3?8 ?m:the particles with a size less than 1?m are global or spheroid,and the particles with a size of 1?3 ?m are mainly regular cube and the particles with a size of 3?8 pm are mainly thin cuboid or polyhedral.(4)When the fused salt reaction was carried out in the medium frequency melting furnace,with the reaction temperature of 800?1200? and reaction time longer than 25 min,the Zr yield of 91%can be achieved.(5)The morphology and size of Al3Zr phase was greatly influenced by the reaction temperature.The Al3Zr phase was blocky with size of 3?8 ?m at 800?,blocky and rod-like with a size of 15?50 ?m at 950?,rod-like and needle-like with a size of 5?100 ?m at 1000?and mainly needle-like with a size of 8?80 ?m at 1200 ?.(6)When Al-Zr mater alloy melt was gradually heated to 1250? after reacted at 800?in a medium frequency melting furnace,the Al3Zr particles continuously dissolved into liquid A1 with its decreased quantity,increased size and increased aspect ratio.The Al3Zr particles disappeared when temperature is higher than liquidus,and the fine needle-like Al3Zr phase can be obtained when this liquid alloy was poured into the copper mold.When the temperature of Al-5Zr alloy decreased from 1250? to 800 ?,Al3Zr phase precipitated,and grew into coarse lath-shaped slowly.The quantity and size of coarse lath-shaped Al3Zr phase increased with the decrease of pouring temperature.(7)Refinement effects were different when Al-Zr master alloys with different morphologies of Al3Zr phase were added into pure Al.Refinement effect of Al-Zr alloy with lath-shaped Al3Zr phase raised slowly.Refinement effect of Al-Zr alloy with fine needle Al3Zr phase raised faster and kept for long time,and resulted in a fine microstructure of pure Al.The fastest refinement effect occurred when the Al-Zr alloy with small cube Al3Zr phase was added into pure Al.However,its refinement effect neutralized very fast.(8)The way and dissolution speed of Al3Zr phase varies with the morphologies of Al3Zr phase changes in liquid Al.The lath-shaped and needle-like Al3Zr phase dissolved and fused into several parts,which resulted in the quantity of Al3Zr phase increased while the size decreased.The small cube Al3Zr phase dissolved and spherized.The dissolution of lath-shaped Al3Zr phase is slowest.Even when the Al liquid hold for 60 min at 700 ?,some Al3Zr phase were still undissolved.The dissolution of small cube Al3Zr phase is fastest.Even when the Al liquid was hold for only 15 min at 700 ?,the Al3Zr phase was entirely dissolved.The dissolution speed of needle-like Al3Zr phase is between that of lath-shaped and small cube Al3Zr phase.(9)The effect of pouring temperature on the morphology of Al3Zr phase in Al-5Zr master alloy was analyzed based on EET theory.The precipitation behavior of Al3Zr phase was greatly influenced by the n'A(E'A)of clusters in the melt and cluster weight.The small quantity of Zr-Zr atom clusters with high n'A(E'A)value severed as nuclei for Al3Zr phase when pouring temperature higher than 950 ?,which results in the formation of small quantity of lath-shaped Al3Zr phase.The large quantity of Al-Zr and Zr-Zr atom clusters with low n'A(E'A)value severed as nuclei for Al3Zr phase when pouring temperature lower than 950 ?,which results in the formation of large quantity of small cube Al3Zr phase.(10)The refinement effect of Al-Zr master alloy on pure Al can be traced back to the statistic value of interface covalent electron density difference ??'.When ??' is less than 10%,the smaller the ??',the more the heterogeneous nucleation,and the better the refinement effect.The heterogeneous nuclei(Al3Zr particles)will adsorb Al-Al clusters with low ??' value,and result in the refinement of grain size.However,the Al-Zr cluster with high ??' value is easy to separate from the nuclei,which will reduce the heterogeneous nuclei and refinement effect.Increasing melt temperature or holding time will increase Al-Zr clusters and weaken refinement effect.