| Lost foam casting (LFC) process has been considered as the New Foundry Technology in 21st Century and the Green Project in Foundry because of its unique advantages. In recent years, the LFC process of aluminum (magnesium) alloy has been developing rapidly, and shows its great vitality and wide application prospect. However, the main obstacles for LFC process development of aluminum (magnesium) alloy are filling ability poorer, microstructures coarser, and the pinhole, shrinkage or loose defects more serious, so the mechanical properties are lower. In order to solve above problems, the vibration and the pressure are applied on the LFC to improve the mechanical properties of aluminum (magnesium) alloy in this dissertation. The technologic theory of LFC process with mechanical vibration and gas pressure were systematical studied, and the new LFC technology of aluminum (magnesium) alloy under vibration-pressure solidification was developed, also the intensified ways of aluminum (magnesium) alloy were investigated by the means of alloying and heat treatment. Using the new LFC technology, the high strength aluminum (magnesium) alloy castings could be produced, and the best technologic parameters were obtained for the typical casting parts.A356 and AZ91D alloys LFC process with vibration solidification were researched. The results show that appropriate pouring temperatures are 730-760℃for middle-small size complex castings. The vertical vibration with frequencies 30-60Hz, amplitudes 0.11-0.45mm and peak accelerations 1-4g (g=9.8m/s2) has the obviouis effect on grains refinement and the improvement of mechanical properties and filling ability. The tensile strengths of A356 and AZ91D as-cast alloys in LFC with vibration solidification are attained to 182MPa and 165MPa, and up by 25% and 22% over that of conventional LFC, respectively. But it could easily lead to the defects such as casting distortion, sticky sand, porosity and inclusions when the peak acceleration is greater than 4g, and these defects should seriously decrease the mechanical properties.During the LFC vibration solidification process, the relationship between the vacuum level and the peak acceleration is established by theoretical analysis. It shows that the vacuum level is a key factor to ensure the stability vibration. The mechanical vibration model of dendrite fragmentation is established, and the model shows that the bigger the peak acceleration is, the bigger the stress of dendrite is also, and dendrites could be broken more easily and grains refinement is better. The mathematical model of the filling ability suiting for aluminum (magnesium) alloy under LFC process with vibration solidification shows that the filling ability enhance as the peak acceleration and the amplitude increasing.The LFC process of A356 and AZ91D alloys with the pressure solidification was researched. The results show that overall performances of the castings are better than conventional LFC process. The good technical parameters include the pouring temperature 730-760℃, the pressure exceeding 0.5MPa, adding pressure rate 0.003-0.03MPa/s, the forcing pressure time in the early solidification, and holding pressure time more than 10min for middle-small size complex castings. The tensile strengths of A356 and AZ91D as-cast alloys in LFC with pressure solidification are attained to 185MPa and 174MPa, and increase 27% and 29% than that of conventional LFC, respectively.The minium pressure of feeding inter-dendritic shrinkages is established by the theoretical analysis of aluminum (magnesium) alloy pressure solidification in LFC. This formula show that the minium feeding pressure is required a small value in the early solidification when the time is less. The model of casting surface depression in aluminum (magnesium) alloy LFC with pressure solidification is expressed, and this model shows that the casting surface sunken quantity is proportional to the casting thickness. The pinholes in aluminum alloy LFC parttings located in hot spot area are a combination of H pinhole and shrinkage, which would be disappeared under pressure solidification.A special LFC process technology with vibration-pressure solidification was studied and some complex thin-walled aluminum (magnesium) alloy castings were produced. The results show that the defects such as the misruns, the pinholes, the shrinkages and looses can be decreased by vibration-pressure solidification in LFC, and the microstructure grains can be significantly refined. By simulating the filling and solidifying process, the casting defects could be preliminarily predicted and be effectively prevented by improving the gating system design. Some complex cylinder castings of A356 and AZ91D alloys have been produced by LFC with vibration-pressure solidification. As a result, the feasibility of the process was verified and the purpose of improving the mechanical properties was achieved.The heat treatment process of aluminum (magnesium) alloy parts by LFC with vibration-pressure solidification was studied. The results show that the tensile strength and elongation of A356 alloy under LFC with vibration-pressure solidification after T4 heat treatment would be got to 272MPa and 6.5% respectively. The tensile strength and elongation of A356 modified 0.6%Mg under LFC with vibration-pressure solidification after T6 heat treatment would be get to 301MPa and 2.1% respectively. The tensile strength of the AZ91D castings modified 0.5%Y in LFC with vibration-pressure solidification after the T4 and T6 would be attained to 236MPa and 252MPa respectively, and their elongations would be attained to 5.1% and 2.1% respectively.The semi-solid isothermal heat treatment (SSIT) process for aluminum (magnesium) alloy in LFC with vibration was studied. The results show that the size and the roundness of spheroidized a grains could achieve the requirements of semi-solid forming process when A356 alloy insulated 60min at 580℃and AZ91D+0.5%Y alloy hot at 570℃holding 60min. The mechanical properties of A356 and AZ91D+0.5%Y alloys in LFC could be improved significantly when the nearly semi-solid heat treatment of the alloys is operated such as A356 alloy holding 2-6h at 550℃and AZ91D+0.5%Y alloy holding 10-16h at 430℃. In this way, the tensile strengths of A356 and AZ91D+0.5%Y alloys after the nearly semi-solid heat treatment are attained to 261MPa and 232MPa respectively.The A356 alloy in LFC process was modified by 0.3%RE which is the Ce-rich rare-earth, and the results show that the eutectic silicon of A356 alloy is effectively refined, and the pinholes are reduced and the mechanical properties are improved. The A356 alloy in LFC with vibration-pressure solidification is modified by 0.6%Mg and 0.3%Y, and the results show that the pinning and strengthening effects of Al3Y particles would lead to improving the mechanical properties significantly, and the tensile strengths of the castings as-cast and T6 are get to 171MPa and 308MPa respectively. The AZ91D alloy in LFC process is modified by 0.5%Y and 0.9% Gd, and the results show that rare earth Gd and Y can refine a-Mg grains, and theβ-Mg17Al12 phases are transited from continuous network structure to intermittent and granular structure. The strength of AZ91D modified by 0.5%Y and 0.9%Gd are enhanced because of Al2Gd and Al2Y pinning grains boundary and preventing grains boundary sliding, the tensile strength has increased 29% than unmodified one. |
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