Al-Si-Mg alloys produced by casting and selective laser melting (SLM): the influence of microstructure on mechanical and fracture behaviour

Abstract

The use of Al-Si alloys in various applications such as automotive and aerospace has recently increased in the light of their good castability, lightness, low strength-to-weight ratio and corrosion resistance. Among this class, Al-Si-Mg foundry alloys are found to give good results since they show concurrently excellent casting characteristics and mechanical properties. In view of the two room and high temperature applications, enhanced static and dynamic properties are required and, indeed, coarsening resistant strengthening phases are needed. To pursue this goal, two main paths can be followed, namely: (i) chemical modification of Al-Si-Mg alloys; (ii) Heat Treatment Optimization. Both the paths are meant to induce the formation of thermally stable strengthening phases.

Aluminum is produced by the Hall-Héroult electrochemical reduction of aluminum oxide dissolved in a molten salt. The deterioration in the current coke quality for anode production has led to increase the amount of metal impurities such as Ni and V in primary aluminum. In the next years, the levels of Ni and V are expected to rise. At the present, there is no cost effective or efficient method available for removal of these impurities. Therefore, industries are developing a growing interest with respect to the effect of increased Ni and V levels on the properties of aluminum alloy products. In the last decade, aluminum alloys have been re-focused in term of lightweight material for vehicles. Considering the new trends, it is of importance understand which role is played by the Ni- and V- addition on the high temperature mechanical behavior. Currently, the EN 1676-2010 standard specifies the maximum concentration limit only for V of 0.03wt% for EN-AB aluminum foundry alloys (A356 alloy), whereas there are no indications for the maximum tolerable Ni level in this group of alloys. Hence, the first part of this research activity is meant to evaluate the influence of Ni and V trace elements on the high temperature tensile properties of as-cast and T6 heat treated A356 unmodified foundry alloys in both sand and permanent mold casting process.

Alloy composition and cooling rate play a key role on the castability and also on the mechanical properties of Aluminum-Silicon cast alloys. The cooling rate controls the microstructural features of the material including the Secondary Dendrite Arm Spacing (SDAS), which is often used as measure of the grain size of the material, and thus as measure of the mechanical response. The fraction, size, shape and distribution of eutectic silicon particles and intermetallic phases also give a crucial contribution in connection with microstructure optimization. Industries, especially in the automotive sector, are paying attention in finding suitable chemical compositions for cast aluminum alloys for high temperature applications. The concomitant presence of the two copper and magnesium in Al-Si-Cu-Mg alloys should in fact enable the precipitation of more stable Cu-based intermetallic precipitates at higher temperatures, leading to enhanced thermal stability of the T6 heat treated alloys. In this concern, part of this thesis offers a combined overview of the two aspects of SDAS and Cu additions on the as cast A356 alloy.

Optimization of solution heat treatment represents a necessary pillar to improve the aging response of Al-Mg-Si alloys with Cu additions. It has been reported that during artificial aging, Cu content influence the aging precipitation sequence, number and density of precipitates, and eventually improve the contribution to strength. Even though some reports show no difference in precipitation kinetics in the quaternary as compared to the ternary alloys (Al-Si-Cu-Mg vs. Al-Si-Mg), a wide literature suggests the existence of different precipitates types after artificial aging, and, in particular, a double aging peaks have been sometimes observed in Al-Si-Cu-Mg alloys. The influence of the aging parameters on the tensile response has been extensively on high Cu alloys (up to 5 wt. %), while few works were conducted on Al-Si-Cu-Mg alloys containing up to 1 wt. % Cu. Article 3 in the present collection examines two solutionizing temperatures and two artificial aging temperatures in Al-Si-Mg alloys having 0 wt. %, 0.5 wt. %, and 1 wt. % Cu content. Mechanical properties in connection with precipitation behavior during artificial aging have been also investigated.

The past two decades have been marked by great progress and innovation in the aluminum foundry industry. However, the low ductility and fracture toughness, as compared to the wrought aluminum alloys counterpart, still represent one of the major impediment for increased use of aluminum casting aerospace industry. With improving molten metal quality and foundry technology, cast alloys are expected to be used more commonly in critical applications. So far, the lack of knowledge towards the fracture toughness of these alloys had limited their use to field where fracture toughness related properties were not relevant. Though, fracture toughness evaluation in Al-Si alloys might represent a strategic point to understand the metallurgical features which control the toughness and hence will might allow proper material specifications to be settled for those applications where toughness-related failure must be prevented. In this framework, Article 4 proposes a study on the fracture toughness of a A356 Al alloy and its Cu-containing (1wt.%) variant, under four different aging conditions. The toughness was obtained through crack growth resistance curves (J-R curves) which were determined by the unloading compliance method. In order to assess the mutual influence of casting defects and alloys hardness, as resulting after aging treatments, hardness results have been composed together with porosity values, and finally integrated with the JIC values. In addition, microstructural observations were carried out to endorse the fracture mechanism.

Good castability of Al-Si alloys, characterized by high melt fluidity, narrow solidification range associated with eutectic, and low shrinkage with low hot tearing tendency, makes Al-Si alloys readily available for the emerging additive manufacturing (AM) technology. Among the AM technologies, the Selective Laser Melting (SLM), has been experiencing a large growth since the past few years due to his high flexibility, versatility and customizability and it is now suiting most sectors of industrial production. The SLM technology involves layer by layer melting of the metallic powder by means of a high-power laser beam according to the instructions provided by CAD files. The process is characterized by high temperature gradients leading to rapidly solidified, non- equilibrium microstructures, causing a substantial residual stress development. As a consequence, the mechanical properties of SLM parts can differ substantially from one another and from those produced by conventional techniques. In order to meet the full potential that the SLM has to offer, especially for flight-critical components, studies and qualifications are needed. In this light, article 5 examines the high cycle fatigue crack growth behavior in the linear region of da/dN vs. K diagrams (described by the Paris law) of AlSi10Mg alloy for different experimental conditions. The Fatigue Crack Growth (FCG) curves revealed the beneficial influence of the heat treatment on the FCG response, as compared to the as-built case. On the other hand, the latter’ fatigue crack growth behavior is found to be strongly dependent on the material orientation. Fractographic analysis and residual stress measurements have been further performed to understand the mechanisms involved.