Role of heat treatment in optimizing microstructure of Al-Si cast alloys for elevated temperature applications

铸造铝合金的性能受微观组织控制，微观组织取决于合金成分和凝固过程中的热条件。如果铸件在热处理过程中受到热暴露，或者当暴露在高温使用条件下时，铸态组织可能会进一步改变。在后一种情况下，需要最小的性能变化，以保持性能并确保部件在其寿命内的可持续性。大多数铝铸件来自Al-Si基合金，因为硅提高了铸造性。硅含量控制着合金中形成的共晶硅相的体积分数，而共晶硅相又构成了组织中相当大的一部分，从而影响了断裂行为。虽然多年来人们对共晶硅相进行了研究，但在实际使用条件下，它对高温性能的贡献却鲜有人关注。对这一方面的探讨对于全面了解铝基合金的高温行为具有重要意义。特别是，重要的是要确定当颗粒在热处理过程中经历粗化时，对颗粒强度的影响有多严重。从最近的研究中，我们观察到，即使在延长固溶处理时间后，粗化的硅颗粒也会显示出某些缺陷，如针孔和凹槽。然而，这些缺陷对硅颗粒强度、过早失效及其对铸件质量的影响尚不清楚。除了使用Al3X型弥散体维持高温合金强度，控制金属间化合物相的负面影响外，确定最佳的热处理工艺参数也是关键，以促进强化铜和镁相的最大限度溶解，同时避免与熔化有关的早期缺陷，从而增加废品率和制造成本。要在全球市场保持领先地位，加拿大必须重视高质量的产品和较低的生产成本。因此，研究计划将重点放在硅颗粒粗化对高温性能的影响上，以最近开发的汽车用396(11.5%硅)和220(1.2%硅)合金为例，以及低镁-319(6.5%硅)合金作为比较。220和396合金的镁含量相同。因此，合金的硅含量在1.2-11%范围内，并有两个水平的镁(~0.1，0.35%)和铜(~1.5，4.5%)。通过对提高这些合金高温强度的各个方面进行基础性研究，主要目的是展示如何通过适当的热处理、合金成分和添加剂的选择，以负担得起的成本生产具有可持续高温性能的关键部件，并使用这些合金来取代目前使用的合金。

The properties of aluminum casting alloys are controlled by the microstructure which depends on the alloy composition and thermal conditions during solidification. The as-cast microstructure may be further altered if the casting is subjected to thermal exposure during a heat treatment process or when exposed to high temperature service conditions. In the latter case, minimum change in properties is desirable to maintain the performance and ensure sustainability of the component over its lifetime.   The majority of aluminum castings are derived from Al-Si based alloys, as silicon enhances the castability. The Si content controls the volume fraction of the eutectic Si phase formed in the alloy, which in turn constitutes a sizeable part of the microstructure, and thus influences the fracture behavior. While the eutectic Si phase has been studied over many years, its contribution to the elevated temperature properties when subjected to the high temperatures under actual service conditions is an aspect that has received little attention. It is important to explore this aspect in order to fully understand the elevated temperature behavior of Al-based alloys. In particular, it is important to determine how seriously the Si particle strength would be affected when the particles have undergone coarsening during thermal treatment. From recent studies, we have observed that the coarsened Si particles display certain defects like pinholes and grooves even after prolonged solution treatment times. However, the effect of these defects on the Si particle strength, premature failure and its consequences on casting quality has yet to be established. Besides using Al3X type dispersoids to maintain high temperature alloy strength, and controlling the negative effects of intermetallic phases, determining the optimum heat treatment parameters is crucial to promote maximum dissolution of strengthening Cu and Mg phases at the same time avoid incipient melting-related defects which would increase rejection rate and manufacturing costs. To stay ahead in the global market, Canada must place emphasis on high-quality products and lower production costs.   Thus, the research program will focus on the effects of Si particle coarsening on elevated temperature properties, using as examples 396 (11.5%Si), and 220 (1.2%Si) alloys recently developed for automotive applications, in addition to a low Mg-319 (6.5%Si) alloy for comparison. Both 220 and 396 alloys have the same level of Mg. Thus, the alloys cover Si content in the range 1.2-11% with two levels of Mg (~0.1, 0.35%) & Cu (~1.5, 4.5%). By carrying out a fundamental study of all aspects of enhancing the high temperature strength of these alloys, the main goal is to show how, through proper heat treatment, selection of alloy composition & additives, critical components with sustainable high temperature performance could be commercially produced at affordable cost, using such alloys to replace the currently used alloys.