Study On Mg-Sr Alloy Prepared By Molten Salt Electrolysis And Its Modification Effect

With the development of technology and society, casting light alloy is widely applied. The requirements of casting light alloy are also increasing. It needs better and better performance to meet market and social demand. Mg-Sr alloy is a new long-acting modifier for cast magnesium and aluminum alloy. Meanwhile, the preparation of Mg-Sr master alloy has attracted widespread attention. In view of the problems in Mg-Sr alloy preparation using direct reaction method, such as oxidation loss, high energy consumption, and low electrolytic current efficiency of only metal strontium, Mg-Sr alloy was directly prepared by molten salt electrolysis with MgCl2-SrCl2-KCl molten salt, exploring a new energy-efficient method of preparing Mg-Sr alloy.In this paper, Mg-Sr alloy was directly prepared by molten salt electrolysis co-deposition, using metal chlorides as raw materials. Then adding Mg-Sr alloy which was prepared by molten salt electrolysis to AZ91D magnesium alloy and A380semi-solid aluminum alloy melt to refine their microstructures. The main contents are as follows:(1) The co-deposition mechanism of Mg-Sr alloy prepared by molten salt electrolysisThe influences on the electrolysis efficiency and Sr content produced by electrolysis process parameters and calculation of the thermodynamic properties of Mg-Sr binary alloys were studied. The results showed that Mg-Sr alloy was successfully prepared by molten salt electrolysis co-deposition; Mg-Sr alloy was feasible at the temperature of700℃with8A/cm2cathodic current density,10%MgCl2concentration,73%SrCl2concentration and the cathode and anode were stainless steel rod and graphite,respectively; With respect to the ideal melt, Mg-Sr alloy melt had an obvious negative deviation. The activity of Mg component was closer to the ideal melt in the whole composition range; the activity of Sr component had a significant negative deviation from the ideal melt in the whole composition range. The formation of Mg-Sr alloy reduced the activities of Mg and Sr elements and promoted alloying dissolution and diffusion, which made the preparation of Mg-Sr alloy by molten salt electrolysis co-deposition possible. The values of mixing enthalpy (AH), excess entropy (SE) and excess free energy (GE) of Mg-Sr alloy melt were all negative. The minimum value of mixing enthalpy is-3.7937kJ/mol, the minimum excess free energy is-3.0394kJ/mol and the excess entropy of the alloy melts approximately approaches zero. Compared with AH and GE, the absolute value of SE can be nearly regarded as zero in the whole composition range;(2) Study of microstructure and modification effect of Mg-Sr master alloy on AZ91D magnesium alloyThe grain size of β-Mg17Al12in AZ91D magnesium alloy was about230μm and the grain size of the β-Mg17Al12decreased notably from230μm in the unmodified sample to~17μm at a Sr content of0.02%. Mg-Sr had a lasting effect in maintaining a fine structure. When holding time was120-150min, the grain size reached the minimum and had a homogeneous distribution; AZ91D, AZ91D-0.005%Sr. AZ91D-0.01%Sr. AZ91D-0.015%Sr AZ91D-0.02%Sr and AZ91D-0.03%Sr consisted of a-Mg solid solution and eutectic (β-Mg17Al12phase. When the amount of Sr was0.005%-0.015%, Mg17Sr2phase appeared; when the amount of Sr reached0.03%, Mg2Sr appeared in the alloy. Sr is a surface active element in the magnesium alloy, which was enriched in the grain boundary of a-Mg. Therefore, it can prevent the growth of a-Mg phase, and then transformed the morphology of the eutectic β-Mg17Al12from a continuous meshy distribution to a granular one. The refinement mechanism of Sr was:from the thermodynamic point of view, as Sr was a surface active element in magnesium alloy during solidification, Sr was enriched in the grain boundary of a-Mg phase. Then liquid equilibrium crystallization temperature greatly reduced, thereby increasing the actual undercooling and contributing to grain refinement. From the kinetic point of view, since the solubility of Sr in Mg was relatively only0.11%, the excess can only be enriched in Sr solid/liquid growth interface, forming adsorption membrane to damage Sr surface or grain growth direction, resulting in lower growth rate of grain and grain can be refined;(3) Study of microstructure and modification effect of Mg-Sr master alloy on A380semi-solid aluminum alloyAdding Mg-Sr alloy into A380semi-solid aluminum alloy can refine eutectic silicon grain, which was significantly refined from coarse needle, flake into short rod, granular eutectic silicon. When the amount of Sr was0.01-0.02%, eutectic silicon grain fully refined, eutectic silicon grain was uniformly fine in A380semi-solid aluminum alloy. Mg-Sr had a lasting effect in maintaining a fine structure when the holding time was90-120min; the eutectic silicon grain was round. The refinement mechanism of Sr was:the Sr element absorbed and gathered at forefront of the Si growth interface, which blocked the original twinning steps of eutectic Si in eutectic growth process, meanwhile, continued to precipitating substantially new reentrant twins. Therefore the eutectic Si branched more frequently than unmodified eutectic Si. And twin density increased significantly so that the growth characteristics of the original crystal Si changed from anisotropy into isotropy. The growth pattern of eutectic Si changed from the former coarse flake branching pattern into a large number of frequently fibrous branching pattern, and ultimately the morphology and size of eutectic Si both had a qualitative change.