某厂生产的一批ADC12铝合金压铸缸体出现硬质点，在机加工时出现打刀现象，并使产品形成微裂纹。通过硬度检测、化学成分分析、金相检验、扫描电镜分析和能谱分析等方法，对ADC12铝合金压铸缸体出现硬质点的原因进行了分析。结果表明：由于氮气纯度较低、铝液不纯净、未及时扒渣，造成杂质进入铝合金压铸型腔后形成硬质点，从而在机加工时出现打刀现象。最后提出了相应的改进措施

Hard spots were found in the die cast cylinder block of ADC12 aluminum alloy. The knife phenomenon appeared during machining and the product formed the micro cracks. The causes of hard spots in ADC12 aluminum alloy die casting cylinder block were analyzed by methods of hardness testing, composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the hard spots formed after the impurities enter into the die casting cylinder block of the aluminum alloy, cause by low purity of nitrogen, aluminum water, and not slagging in time. Therefore, the knife phenomenon appeared during machining. Finally, the corresponding improvement measures are presented.