Characteristics Of Microstructure And Properties Of Cast Iron Produced By Lost Foam Casting With Vibration

Lost foam casting (LFC) process has many advantages, such as high precision, good surface quality, simple technology, and clean production, and so on. It has been considered as a New Foundry Technology in21st Century and the Green Project in Foundry. However, the low cooling rate of dry sand result in coarse grain and low mechanical properties of the casting produced by LFC. Therefore, the refinement of grain size and improvement of the mechanical properties of casting produced by LFC become the most urgent issues. In this thesis, in order to improve the quality of cast iron alloy, the microstructure and mechanical properties of the typical cast iron produced by LFC with vibration were systematically investigated. The effect mechanism of vibration on the microstructures was also discussed. This thesis has important theoretical and practical significance.(1) The effect of technical parameters, which include vibration frequency, amplitude, amount of spheroidizing agent and treatment temperatures, was investigated. It was found for the first time that the carbide transforms from flake to granular in the pearlite of ductile cast iron produced by LFC with vibration. Meanwhile, the undercooling degree in the solidification process was increased.When vibration frequency increases from0Hz to35Hz at the constant amplitude of3mm, the mechanical properties of the ductile cast iron increase because the pearlite interlaminar spacing was decreased and carbide in the pearlite was granulated. When the vibration frequency is greater than or equal to50Hz, the granular carbide occurs in the ductile cast iron, but the mechanical properties of ductile cast iron is decreased due to the increase of pearlite interlaminar spacing and number of graphite nodule in unit area.When the vibration frequency is35Hz, the forces of vibration field on ductile cast iron melt increases as the increasing of amplitude.When the amplitude is4mm and vibration frequency is35Hz, the ultimate tensile strength (UTS) and elongation of LFC ductile cast iron increases. This can be attributed to the improvement of graphite morphology and the decrease of pearlite interlaminar spacing. The UTS and elongation is674.65MPa and11.43%, respectively. Compared with the ductile cast iron produced by LFC without vibration, the UTS and elongation is increased by16.76%and74.24%, respectively.The graphite morphology transforms from nodule to vermicular while the amount of spheroidizing agent decreases. If the treatment temperature of melting iron increases, the chilling tendency of ductile cast iron increases. Meanwhile, the number of graphite nodule in unit area and the spheroidization rate in ductile cast iron decrease.(2) The effects of vibration frequency and amplitude on the microstructure and mechanical properties of grey cast iron were investigated for the first time. The technological parameters of LFC with vibration were optimized and the mechanical models of dendrite fragmentation of grey cast iron produced by LFC with vibration were built.With the increasing vibration frequency from0Hz to35Hz at the3mm of amplitude, the length of flake graphite decreases, and the primary austenite becomes finer accompanied with its dendrite arm broken. This leads to the improvement of the mechanical properties of grey casting iron using LFC. Thus, the mechanical properties of grey cast iron are improved. When the vibration frequency is further increased to more than50Hz, the mechanical properties of grey cast iron decrease which because the increase of the length of graphite flakes and the grain size of austenite.The effect of amplitude on the microstructure and mechanical properties under the vibration frequency with35Hz was investigated. The UTS of grey cast iron increases as the amplitude increases from0.75mm to2mm. This is because the length of graphite flakes decreases and the forming of austenite dendrite network skeleton. The UTS is247.51MPa. Compared with the grey cast iron produced by LFC without vibration, it is increased by26.25%. The length of flake graphite decreases when the amplitude is equal to3mm. However, the diameter of austenite dendritic arm decrease and network skeleton is broken, which lead to the decrease of the mechanical properties.The mechanical model of broken austenite dendrites is built according to the material mechanics and fluid mechanics. The mechanical model shows that the higher the vibration frequency and amplitude is, the larger the bending stresses of austenite dendrite become. Thus,the fracture tendency of the dendrite arm of austenite increases and the grain size become smaller.(3) Effect of vibration frequency, amplitude and wall thickness on the graphite morphology of vermicular cast iron was studied. The control of graphite morphology by means of the vibration solidification was analyzed.The graphite type in the vermicular cast iron transforms from vermicular to crystallite of vermicular graphite or flakes when the vibration is imposed or treatment temperatures is increased, which lead to the decrease of Brinell hardness. As the wall thickness of the specimens decreases, the amount of nodule graphite in the vermicular cast iron increases and the content of vermicular graphite decrease. This can be attributed to the solidification rate increases.The thermodynamics and dynamics in the solidification process of cast iron produced by LFC with vibration were analyzed. The results show that the technology of lost foam casting with vibration solidification is able to control the graphite type in cast iron parts.(4) Influence of the vibration solidification and wall thickness of specimens on microstructure and mechanical properties of the white cast iron was studied. The results show that the grain size of primary austenite in white cast iron produced by LFC with vibration is refined and its content increase. Meanwhile, the content of ledeburite decreases, which lead to the decrease of Rockwell hardness. The grain size of primary austenite increase and its content in the white cast iron decrease as the wall thickness increases. The increase of Rockwell hardness is due to the increase of the ledeburite.