Post heat treatment of additive manufactured AlSi10Mg: On silicon morphology, texture and small-scale properties

Abstract

Post-fabrication heat treatment, including solution treatment (at 520 °C for 1 h) followed by water quenching (WQ), air cooling (AC) and furnace cooling (FC), was performed on a selective laser melted AlSi10Mg alloy. The objective is to assess the effect of various cooling rates on the microstructure (specifically eutectic-Si morphology) and small-scale mechanical properties, measured by employing a depth-sensing nanoindentation platform, of the selective laser melted AlSi10Mg alloy. Results show extensive evolutions in the microstructure and the mechanical properties of the heat-treated materials relative to the as-fabricated sample. Upon solutionizing treatment, the eutectic Si is first fragmented, then spheroidized, and finally coarsened when cooled with slow rates. The microstructural evolution directly affects the mechanical properties, where the as-fabricated and the FC are the hardest and the softest structures, respectively. This is directly attributed to the size and morphology of the eutectic-Si within the microstructure. The findings of this study could help to adjust the optimized heat-treatment process to fabricate SLM AlSi10Mg parts with desirable microstructure and mechanical properties.

Introduction

Al–Si alloys, representing 80% of cast aluminum alloys, are widely used in aerospace, automotive, motor racing, and heat exchanger industries due to their good thermal properties, specific strength, castability, and flexible post-processing capabilities [[1], [2], [3], [4], [5], [6]]. In the family of Al–Si alloys, AlSi10Mg is a near eutectic (hypo-eutectic) alloy, which is mainly used in the casting sector due to low viscosity upon melting and small casting shrinkages [7,8]. However, the presence of coarse and acicular eutectic Si, as a brittle phase, in the microstructure of cast Al–Si–Mg alloys is considered the main factor in deteriorating the mechanical properties (ductility and toughness) of the alloy.

With the development of metal additive manufacturing techniques such as selective laser melting (SLM), it is possible to produce complex AlSi10Mg parts in a more versatile approach instead of traditional casting processes. To this end, near-eutectic AlSi10Mg alloy is considered the most popular alloy for various applications that are extensively manufactured through the SLM process [9,10]. The AlSi10Mg is a popular alloy to be fabricated by additive manufacturing, e.g. SLM process, thanks to short solidification range (ΔT = T_liquidus - T_solidus) of the alloy (i.e. 40 K).

The microstructure of the SLM AlSi10Mg is distinctly different than the cast counterparts [11,12]. In the SLM AlSi10Mg, due to rapid heating and cooling, rapid solidification, cycles induced by the SLM process (e.g. 10⁶–10⁸ °C/s [13]), unlike most traditional casting processes where the cooling rate is about 10² °C/s or less [14], the microstructure consists of an ultrafine, metastable cellular structure [15,16]. Different from cast coarse dendritic microstructure, the SLM AlSi10Mg microstructure consists of fine primary aluminum grains (cells) separated by continuous boundaries of Si-rich eutectic (cell boundaries). However, the α-Al matrix is in super-saturated solid solution condition and when exposed to high temperatures (i.e. thermal post-processing like heat treatment), this microstructure becomes unstable and undergoes dramatic changes. Annealing [17,18], and solutionizing followed by artificial aging (i.e. T6 heat treatment) [11,13,[19], [20], [21], [22]] are two most common heat treatment cycles that are extensively considered for SLM AlSi10Mg to modify the microstructure and enhance the mechanical properties. Based on the published results [[17], [18], [19], [20], [21]], the T6-type heat treatment, i.e. solution treatment (just below the eutectic temperature at 540 °C) followed by water quenching and artificial aging at intermediate temperatures, improves plasticity/ductility without a significant loss in the tensile/bending strength of the alloy. The corresponding mechanism is also elucidated based on the spheroidization of the silicon precipitates. When the SLM AlSi10Mg is soaked for some time at the solutionizing temperature, eutectic silicon structure in the cellular network is fragmented at the shape discontinuities (instabilities) like necks, tips, and terminals of the Si crystals [23,24]. Upon disintegration of the Si crystals, due to solid-state atomic self- and/or inter-diffusion, which is a time and temperature-dependent phenomenon, silicon spheroidization occurs [[25], [26], [27], [28]].

The main focus of the published literature, on post-processing of SLM AlSi10Mg, is on conventional post thermal processing, developed for cast alloys, like heat treatment to peak strength (i.e. T6, artificial aging [[29], [30], [31], [32], [33]]), natural aging (i.e. T4 [29,34,35]), stress relief treatment (i.e. T5 [36,37]), and annealing [18,38]. Considering remarkable differences in the solidification pattern (heating and cooling cycles), thermal gradient, and the microstructure of the SLM and the cast AlSi10Mg alloy, the conventional thermal treatments may not necessarily be applicable and/or optimal for the additively manufactured (e.g. SLM) parts. Employing conventional heat treatments toward SLM material could be considered a major deficiency in the post-thermal processing of the printed parts. A tangible example here is employing conventional T6 heat treatment which results in the softening of the SLM AlSi10Mg alloy [11,19]. On the contrary, the same heat treatment can largely strengthen the cast AlSi10Mg.

Despite several studies on the SLM AlSi10Mg considering conventional heat treatments (i.e. annealing, solution heat treatment, quenching, and aging), the effects of solution heat treatment followed by various cooling rates at different quenching media (including extreme rates) on microstructure (i.e. eutectic-Si morphology) and small-scale mechanical properties of the alloy is yet to be thoroughly investigated. In the present study, the SLM AlSi10Mg samples are firstly solution heat-treated at an elevated temperature (520 °C), then cooled with different cooling rates including quenching (fast cooling), air cooling (intermediate), and furnace cooling (slow). We examine the effect of cooling rate on the morphology of the eutectic silicon as well as the properties relative to the as-printed sample. To this end and to correlate the microstructural characteristics with small-scale mechanical properties, a depth-sensing indentation testing technique, is employed. It is worth mentioning that, though SLM AlSi10Mg is a well-studied material, most of the available literature is focused on macro-scale testing of the as printed and heat-treated SLM material.

The present paper is the first attempt to evaluate “specific thermal treatments”, i.e. solutionizing followed by cooling at various quenching media; cold water, air cooling, and furnace cooling. We would like to assess the effects of these thermal treatments on the microstructure development (size, morphology, and distribution of the eutectic Si), texture, and small-scale mechanical response and length-scale phenomena in the additively manufactured (SLM) AlSi10Mg.

Mechanical properties (i.e. microhardness) of the heat-treated samples are directly controlled by the changes in the microstructure. To this end, evolution in the morphology and size of the eutectic-Si, as the main microstructural feature of the alloy, including Si-fragmentation, Si-spheroidization and Si-coarsening are studied in detail to better understand the mechanical response of the SLM AlSi10Mg alloy to various cooling rates induced by post-fabrication thermal processing. The results of this study could be used to select proper heat treatment parameters and to optimize the post-heat-treatment process of the additively manufactured AlSi10Mg and similar Al-based alloys to achieve desirable microstructure and performance.

This work is novel, to the best of our knowledge, in such a way that: (i) the designed heat treatment cycles, considering very fast and very slow cooling rates upon solutionizing, have not yet been reported on the SLM AlSi10Mg; (ii) there is no published research work discussing the microstructural and micromechanical response of the solutionized SLM AlSi10Mg when soaked for a long time at elevated temperatures. This is induced by shutting off the furnace and the extracting heat from the specimen at a very slow rate while the furnace is cooling down to room temperature. Systematic knowledge about this (e.g. exposure to elevated temperatures for a long time) is essential for practical applications such as additively manufactured AlSi10Mg for engine components where the temperature is well beyond ambient temperature.