Electrolytic Low Titanium A356 Alloy Process Optimization And Application Research

In this paper, the microstructures and mechanical properties of the electrolytic low titanium A356 alloys have been investigated by means of TEM, SEM, differential scanning calorimetry (DSC) analysis, metallographic microscopic analysis and the mechanical properties test. Especially, the mechanisms of the grain refinement of Al-Ti binary alloys and the optimum technics of grain refinement, modification and the heat treatment are investigated. The electrolytic low titanium A356 alloys have been tentatively applied to the car or the motorcycle wheels. The results may be help to the development and the exploitation of the high quality wheels materials.A model for the solidification of Al-Ti binary alloys with a titanium content of less than 0.15 wt pct is developed. The equations of the constitutional undercooling △TC and the relative grain size RGS have been deduced.Based on the model, the constitutional undercooling and the relative grain size of the electrolytic low-titanium aluminium alloys and the traditional Al-Ti alloys are calculated for the first time. The nucleation undercooling is about 0.5-1.0K for the new alloy and 0.8-1.5K for the traditional alloys. DSC analysis of the electrolytic low titanium A356 alloys results show that both the onset peak temperature and the activation energy of the primary aluminium dendrites and binary Al-Si eutectic boundaries of the electrolytic low titanium A356 alloys are smaller than the traditional A356 alloys. The results can properly explain why the electrolytic low titanium aluminium alloys had the excellent refining effects.The melting process of the electrolytic low titanium A356 alloys is optimized for the first time. The effects of the titanium content, titanium alloying manner and the holding time on grain refinement, modification effect, fading' behavior andmechanical properties are investigated. The optimum Ti content, Sr content critical value and melting holding time are confirmed. The results show that the alloys have the best grain refinement effect and combination mechanical properties when titanium content approaches 0.1%. The tensile strength of alloys increases gradually, but the ductility decreases with increasing titanium content when the titanium content exceeds 0.1%. The morphology of silicon particle improves distinctly and the shape of Si phase changes from plate-like shape into the fine fibrous coral-like when the Sr content increases. The minimum Sr content for the full modification is about 0.01%. It is also show that after the holding time of melting exceeds 130 min, the effect of grain refinement and modification fad gradually. The reason may be attributed to the solution and deposition of effect heterogeneous nucleus particles and the oxidation and the burning loss of Sr respectively.The optimum technics of the solution treatment and aging treatment are studied for the first time. By means of the DSC analysis, TEM and the mechanical properties tests, the age-hardening behavior of the electrolytic low titanium A356 alloys is investigated. It is found that the circularity of silicon particles improved but the size grew and became coarsen with the raising of solution temperature, which results in a slight effect on the tensile properties but a significant effect on the ductility. With the increasing of solution time, the morphology of silicon particles is better and better and the growth rates are rapid until 3h and then decreased to after steady exceeding 3h. The tensile properties of alloys are firstly increased rapidly and then slowly. But the ductility is firstly increased and then decreased to a steady value. With the increasing of the aging temperature and prolonging of the aging time, the tensile properties increase but the ductility decreases. DSC and TEM analysis show that the age-hardening properties are related to the precipitation sequence of alloys. With the change of he aging temperature and time, the precipitation sequence is GP zone— Pl! — P '— P , and there is no time internal between them. Both the metastable phase P;/ and P' can effectively strengthen alloys and the effects of the P' phase is more significant. The optimum heat treatment process of the electrolytic low titanium A356alloys is 535°CX3h+165°CX2h. Using this heat treatment, high mechanical properties can be achieved, which is o b^300MPa, 8 ^10%, respectively. The values are superior to that of the recent documents reports.The fracture mechanism of the electrolytic low titanium A356 alloys has been investigated. The crack initiation and propagation process are examined by in situ testing in a scanning electron microscope (SEM) for the first time. It is found that crack initiates first in the lamellar eutectic area and propagated along this area for the A356 alloys in cast state. After heat treatment, however, the crack is found initiate at gas/shrinkage pores because of the debonding of the Si particles from the Al matrix. The crack mainly propagate across the Al matrix firstly, part of crack may tends to propagate along the matrix/particle interface secondly. The eutectic Si particle can block the propagation of crack. The crack paths deflected to the weaker areas when encountering the Si particles.Car wheels and motorcycle wheels are produced using the electrolytic low titanium aluminium alloys for the first time. Stand test of motorcycle Al wheels is conducted. The microstructure and mechanical properties are analyzed and compared with the wheels produced by the international famous manufactures. The results show that the properties of motorcycle wheels are stable and its repeatability is good. Combination properties of the car wheels are equivalent to Benz wheels, superior to that of Audi, Buick and BMW wheels.