【作者】 周永华；

【导师】 王丽萍； 李君；

【作者基本信息】 哈尔滨理工大学 ， 材料工程， 2006， 硕士

【摘要】 本文首先利用宏观观察、MU200型X-射线透射扫描实时成像、金相显微分析、扫描电镜等实验方法对低压铸造铝合金476发动机汽缸盖铸造缺陷的类型、特点、形成原因进行了研究，结果表明：按照现行工艺生产的476发动机汽缸盖在热节处存在缩松、缩孔、裂纹、气孔、夹杂等缺陷。476发动机汽缸盖的主要缺陷是在D截面上形成30×15×2mm范围的缩松区，缩松区深约0.5～1.0mm；在D截面处发现穿透性裂纹(热裂纹)，长：7mm，宽：0.35mm，深：1.5mm；T截面上存在气孔，多在距件表面2～3mm处，大小尺寸为0.35mm左右；在皮下X截面处，约为Φ0.35mm针孔，但量极少；在缩松区局部发现有小的球团状氧化物夹杂颗粒，数量也很少。以现有低压铸造工艺生产的476发动机汽缸盖，产生的大量缩松及缩松区域内形成的穿透性热裂纹是铸件产生渗漏的主要原因。其次，探索了锶变质处理对476发动机汽缸盖组织和铸造缺陷的影响规律，指出：加锶变质处理后，476发动机气缸盖铸件的显微组织得到明显改善，共晶硅由加锶前的短棒状转变为加锶后的细小、弥散的针状形态，缩松、裂纹缺陷得以减少，但针孔缺陷明显增多，这是由变质处理时的吸氢引起的。锶的变质存在衰退现象，锶的有效变质时间为1小时，此时锶的含量为0.0187％，超过1小时锶的变质作用明显减退。加锶变质和Al-Ti-B细化共同作用时，AC4B合金的晶粒得到明显细化。为消除铸造缺陷、铸造优质健全的476发动机气缸盖成品件，应从铸造工艺和熔炼工艺入手，并严格控制锶变质和Al-Ti-B细化工艺，通过加锶变质处理来控制缺陷和提高铝合金476汽缸盖力学性能的方法是可行的。更多还原

【Abstract】 In this paper the microstructure and freezing defects of 476 Al alloy engine cylinder head are studied, using type Olympus GX 71 optical microscope, MU200X-ray diffraction, type FEI SIRION SEM analysis. The results show that 476 engine cylinder head produced by recent process have defects of shrinkage porosity zone, shrinkage, crack defects, gas pole and inclusion in the hot spot. The major defect of 476 engine cylinder head a 30×15×2mm shrinkage porosity zone in the D section, shrinkage porosity zone is about 0.5-1.0mm high, through wall defects (thermal crack) can been observed at the D section,length 7mm,width 035mm,highth 1.5mm, and the T section has gas pole, mostly exist 2~3mm under the surface of the casting ,about 0.35mm length ;under the surface the T section is aboutΦ0.35mm pinpole, but quantity is small; small spheral oxides particulate inclusion can been observed shrinkage porosity zone, but quantity is small too. The 476 engine cylinder head is produced by present low pressure casting, and the large amount of shrinkage porosity and the through thermo crack existing in the shrinkage porosity zone is the main reason of leakage existing in the casting.Then, the effect of Sr modification treatment on the microstructure and freezing defects of 476 engine cylinder are also researched. The results reveal that after the Sr modification treatment, the microstructure of 476 engine cylinder castings has been improved obviously, and the form of eutectic Sr changes from short stick to fine and disperse specula. Shrinkage and crack defects decrease, but pinholes defects increase obviously as a result of hydrogen absorption during the modification treatment. There are degradation phenomena during Sr modification treatment, the effective modification time of Sr is one hour and the content of Sr at this moment is 0.0187%. After one hour the effect of Sr modification treatment degrades dramatically. With Sr modification treatment and Al-Ti-B refinement, the grains of AC4B alloy are refined greatly.In order to eliminate casting defects and produce good quality of 476 engine cylinder head, casting technology and melting technology should be paid much attention and modification technology of Sr and refinement technology of Al-Ti- B should be controlled austerely. So it is fesiable to control defects and improve mechanical property of Al alloy 476 engine cylinder head by Sr modification.更多还原