A356合金是可热处理强化合金，关于制造工艺对合金性能变化的研究比较多，但工艺中每个阶段对时效强化相的影响的研究却很少。本文着眼于合金中的时效强化相Mg2Si，系统研究了A356合金工艺中每阶段的工艺参数对析出相的影响。针对该合金在汽车工业、航天等领域应用范围的扩大，传统的单极时效已不能满足需求，本文还研究了双级时效对合金性能的影响。冷却速率是决定铸造合金凝固组织的关键因素，影响着凝固组织形态、相的析出次序、种类及数量。本文首先通过观察不同凝固速率下A356合金中Mg2Si的形态和分布，研究了冷却速率对合金中相析出顺序的影响。结果表明，冷速较低时，鱼骨状的Mg2Si在α-Al枝晶臂内析出；冷速较高时，Mg2Si的析出被抑制。通过测试A356合金的力学性能、基体显微硬度和观察显微组织，研究了基体合金元素均匀化所需时间以及淬火冷却速率对合金性能及沉淀相的影响。研究表明，固溶2h后，基体内元素即已均匀化；淬火冷却速率较高时，基体时效后硬度较高，析出相尺寸较小、分布无序；淬火冷却速率较低时，基体时效后硬度较低，析出相尺寸较大、相互垂直分布。通过分析传统时效处理过程中合金的显微组织，发现时效温度较低时，沉淀相呈针状或杆状，分布杂乱，晶界处存在无沉淀带；时效温度较高时，沉淀相以点状为主，无沉淀带减小。结合工业生产实际，本文研究了自然预时效、80℃预时效对A356合金力学性能和显微组织的影响，模拟了工业时效加涂装的整个热处理过程。结果表明，低温预时效后合金塑性提高，T5时效后再高温时效时合金塑性急剧下降。通过在200℃下保温一定时间后，在室温下放置1h，然后继续在200℃下保温0.5h，测试基体的显微硬度，研究了间隔时效对A356合金性能的影响。超声波处理熔体可以细化晶粒，影响铸态Mg2Si的分布。本文通过显微组织观察，发现超声波处理熔体及T5时效后A356合金基体内析出的沉淀相尺寸较小、分布弥散。

A356 alloys can be high strengthened by heat treatments, and the changes of the mechanical properties during the whole manufacture processes have been studied mostly, while unfortunately the precipitates morphologies during the aging haven′t been well studied. Mg2Si precipitates during the whole aging process were investigated in this study. Solidification rates are key important for the microstructures of alloys. Mg2Si and other phases in the matrix under different solidification rates of A356 alloys were investigated. The results show that fish bone-shaped Mg2Si will precipitate when the solidification rate is low; otherwise the precipitation will be prevented.Influences of solution treatment times and quenching rates on the properties and micro-hardness of A356 alloys were tested. The results show that the elements in the matrix will be heat treated unanimously in two hours, and that the precipitates will be needle-shaped, disordered, and the matrix hardness is higher when the quenching rate is high, otherwise the precipitates will be rod-shaped, orthogonally ordered, and the matrix hardness is lower.Mg2Si precipitates in traditional aging were investigated by SEM. The results show that the precipitates will be needle-shaped when aged in low temperature and change to be dot-shaped in high temperature. Influences of natural, 80℃ pre-aging, powder painting process and interval aging in 200℃ on the properties and matrix micro-hardness of A356 alloys were also investigated. The influences of ultrasonic field on precipitates morphology in the aged alloys were studied in this study. The results show that the size and dispersion of Mg2Si will be more unanimous.