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

铸铝合金的性能受其显微组织的控制，而显微组织又取决于合金成分和凝固时的热条件。如果铸件在热处理过程中受到热暴露或暴露在高温使用条件下，则铸态微观结构可能进一步改变。在后一种情况下，为了保持性能并确保组件在其生命周期内的可持续性，需要对属性进行最小的更改。大多数铝铸件来自铝硅基合金，因为硅提高了浇注性。Si含量控制了合金中形成的共晶Si相的体积分数，而共晶Si相又构成了显微组织的很大一部分，从而影响了断裂行为。虽然对共晶Si相的研究已经进行了多年，但在实际使用条件下，共晶Si相在高温下对高温性能的贡献却很少受到关注。为了全面了解铝基合金的高温行为，对这方面的研究具有重要意义。特别是，确定在热处理过程中晶粒粗化对Si颗粒强度的影响程度是非常重要的。从最近的研究中，我们观察到粗化的Si颗粒即使经过长时间的固溶处理也会出现针孔和沟槽等缺陷。然而，这些缺陷对Si颗粒强度、过早失效及其对铸件质量的影响尚未确定。除了利用Al3X型弥散体保持合金高温强度，控制金属间相的负面影响外，确定最佳热处理参数对于促进强化Cu和Mg相的最大溶解，同时避免熔化初期缺陷增加废品率和制造成本至关重要。为了在全球市场上保持领先地位，加拿大必须把重点放在高质量的产品和更低的生产成本上。因此，研究计划将重点放在Si颗粒粗化对高温性能的影响上，以396 （11.5%Si）和220 （1.2%Si）合金为例，以及低Mg-319 （6.5%Si）合金进行比较。220和396合金的Mg含量相同。因此，合金的Si含量在1.2 ~ 11%之间，Mg（~0.1, 0.35%）和Cu（~1.5, 4.5%）两个水平。通过对提高这些合金高温强度的各个方面进行基础研究，主要目标是展示如何通过适当的热处理，选择合金成分和添加剂，以可承受的成本生产具有可持续高温性能的关键部件，使用这些合金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.