Microstructural and small-scale characterization of additive manufactured AlSi10Mg alloy
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Additive manufacturing of aluminum alloys is considered a promising layer-wise manufacturing method which can produce lightweight critical automotive/aerospace/military components with enhanced physical and mechanical properties. This paper aims at assessing the correlation between microstructure and small-scale characteristics of an additive manufactured AlSi10Mg alloy in the as-printed and heat treated conditions. Depth-sensing nanoindentation testing, as a non-destructive, robust, and convenient testing approach, along with microstructural assessments, using optical microscopy and scanning electron microscopy, were employed to compare the nano-hardness of the printed (selective laser melting method) and the heat treated (age-hardening) materials. Considering the distance from the build plate, a gradation in the cooling rate, and therefore the microstructure, is expected which directly affect the nano-hardness gradient along the deposition direction. Results show a transition in the microstructure from cellular grains, with coral-like silicon fiber colonies, to fragmented/spheroidized eutectic silicon particles upon the heat treatment. Unlike conventionally manufactured AlSi10Mg alloys, upon aging heat treatment in the additive manufactured AlSi10Mg alloy, the nano-hardness is decreased which is mainly contributed to stress relief, elimination of solid solution strengthening, and silicon spheroidization phenomena. These are considered in detail in the current paper.
KeywordsAdditive manufacturing AlSi10Mg Heat treatment Si spheroidization Nano-hardness Nanoindentation
Considering completely different cooling rate in the conventionally cast AlSi10Mg and the additive manufactured counterpart, the produced microstructure and therefore the mechanical properties would alter largely . For instance, what happens in the additive manufacturing (i.e., selective laser melting, SLM) of the AlSi10Mg alloy is localized rapid heating and cooling cycles (i.e. 106–108 °C/s ) induced by the SLM process which is not necessarily the case in the conventional casting.
Cast AlSi10Mg alloys are usually treated through T6 heat treatment. The process includes several hours of solution heat treatment which is followed by quenching in water and subsequently aging at moderate temperatures. This heat treatment produces the strengthening precipitates of Mg2Si in the alloy which improves the ductility and strengthens the aluminum matrix. Recently some papers have been published on T6 heat treatment of the additive manufactured AlSi10Mg alloy [3, 9, 10, 11, 12, 13, 14, 15].
Li et al.  studied the effect of heat treatment on AlSi10Mg alloy fabricated by selective laser melting. They reported a tangible decrease (− 61%) in the tensile strength in the T6 heat tread material as compared with the as-printed alloy. Takata et al.  studied the change in the microstructure of the additive manufactured AlSi10Mg due to heat treatment. They reported the following sequence in the microstructure induced by the heat treatment, (i) recovery, (ii) silicon spheroidization and coarsening, (iii) formation of a stable intermetallic phase (AlFeSi). Aboulkhair et al. [9, 16] studied the microstructure and mechanical properties of the SLM AlSi10Mg alloy. They reported softening of the SLM AlSi10Mg alloy as well. Brandl et al.  assessed the fatigue performance of the T6 heat treated SLM AlSi10Mg and reported enhanced fatigue life in the heat treated material as compared with the as-printed alloy.
Zhuo et al.  assess the effect of post-process heat treatment on microstructure and mechanical properties of additive manufactured AlSi10Mg alloy. They studied various heat treatments and employed nanoindentation to evaluate the effect of heat treatments on the phase constituents, microstructure, residual stress and mechanical properties of the laser additive manufactured AlSi10Mg alloy. They concluded that 300 °C/2 h + water quench is an effective heat treatment which provide enhanced mechanical properties and eliminates most of the residual stresses of the SLM AlSi10Mg alloy.
Aboulkhair et al. , using a nanoindentation testing approach, assessed nano, micro, and macro properties of selective laser melted AlSi10Mg. They observed uniform nano-hardness in the SLM material, compared with the cast counterparts. They attributed this to fine microstructure and good distribution of Si at the grain boundaries due to faster cooling rate.
In a separate study Aboulkhair et al.  studied the effect of a conventional T6-like heat treatment on microstructure and mechanical properties, assessed by a nanoindentation testing technique, of selectively laser melted AlSi10Mg alloy. Unlike T6 heat treatment in the cast Al–Si parts, they observed a drop in the nano-hardness upon T6 treatment in the as-printed samples. They attributed this phenomenon to change in the strengthening mechanisms and Si spheroidization. In a follow up study, Aboulkhair et al.  evaluated microstructure and nano-mechanical properties of additive manufactured AlSi10Mg. They employed nanoindentation and energy dispersive x-ray (EDX) to create hardness profile and their correlations with the chemical composition across the melt pool of the printed samples. They observed a uniform nano-hardness distribution with no spatial variation across the SLM material.
Everitt et al.  employed nanoindentation and showed uniform local mechanical properties across melt pools and layers produced by selective laser melting of AlSi10Mg alloy. They attributed the uniform nano-hardness profile to the highly fine microstructure accompanied with the enhanced dispersion of the alloying elements.
Despite mentioned papers on the heat treatment of the SLM AlSi10Mg alloy, detailed and atomic scale mechanism of silicon spheroidization induced by the heat treatment as well as the micromechanical response of the alloy have not been studied. Considering the fact there exists gradation in microstructure (and therefore) mechanical properties of the additive manufactured AlSi10Mg alloy [21, 22, 23], instrumented depth sensing indentation is a reliable, convenient and robust method to track the changes in local mechanical properties and extract some fundamental characteristics, i.e. dislocation activities, indentation size effect, etc., especially when a small volume of materials is available. This paper explores and compares micromechanical properties (i.e. nanohardness, reduced modulus, indentation size effect) of an SLM AlSi10Mg alloy in the as printed and the T6 heat treated at the nano/micro-levels.
2 Experimental procedure
The AlSi10Mg samples (cubes of 1 cm3) were produced from atomized powders using an SLM 280 printer with two 400-watt lasers at CalRAM Inc. The AlSi10Mg test samples were built according to the default SLM 280 parameter set. To avoid oxidation, the printing operation was performed in an inert argon atmosphere. Upon printing, the cubes were cut from the aluminum substrate using electron discharge machining (EDM). A separate set of as-printed samples were employed toward T6 heat treatment. This includes solutionizing at 520 °C for 1 h, the water quenching, and the artificial ageing at 170 °C for 4 h.
Upon completion of the heat treatment cycle, both as-printed and heat treated specimens were mechanically (up to 4000# sandpaper to remove the surface oxidations and contaminations) and chemically polished to get scratch free mirror-like surfaces. At this stage samples were etched using Keller’s reagent (2.5% HNO3, 1% HF, 1.5% HCl, 95% distilled water) to assess the microstructure of the materials using optical microscopy (metallurgical light microscope model MM 500T) and scanning electron microscopy (SEM model FEI FEG 650).
To establish the correlations between the microstructure and the mechanical properties (i.e. nano-hardness and reduced modulus), an instrumented (depth-sensing) nanoindentation system was employed (Hysitron Ubi-1 Nanoindenter) equipped with a self-similar pyramidal Berkovich indenter. Tests were performed under load-controlled mode with the peak load of 9.5 mN with the load rate of 1 mN/s and spacing of 400 μm with total of 25 indents were performed on both XY and YZ planes where XY starts and ends from one side edge to the other one and YZ starts near the substrate and ends up neat the top of the sample. Upon reaching the pre-set peak load, the load is held constant for 2 s then the sample is unloaded. Indentation load (P), indentation depth (h), and time are three main parameters that are recorded during the testing. Using well-known Oliver/Pharr  method, indentation stress (σind) and reduced modulus are calculated. To confirm the nanoindentation testing results, Vickers hardness measurements (load of 500 g, space of 200 μm with total of 50 indents) was carried out on the both as-printed and heat treated samples as well on both XY and YZ planes.
3 Results and discussion
3.1 Microstructure characterization
In the current study, the average size of the spheroidized Si particles is between 0.7 and 1 μm. However, according to Li et al.  with increasing the artificial ageing time to 12 h, the Si particles can further grow to a size up to 5 μm. The increase in the size of Si particles indicates that in the as-built SLM sample, the Al matrix is supersaturated and during the heat treatment the excess Si precipitates out.
Upon providing sufficient time and temperature particle coarsening occurs to reduce the overall energy of the system. That is, the particles become coarse as the aspect ratio reduces leading to loss of the interconnection of the eutectic phases. Temperature and the duration of exposure determine the rate at which interconnectivity is lost. Increase in the percentage of silicon increases the ductility of the materiel.
3.2 Mechanical properties
Considering the microstructure of the as-printed AlSi10Mg (Figs. 2, 4a, and 5a), the alloy can be considered as a “natural” metal matrix composite with α-Al as the matrix and interconnected coral-like Si fibers as reinforcement phase. Having said this, the main mechanisms that contribute to the strengthening of the as-printed sample include grain boundary strengthening due to the presence of a cellular structure (Hall–Petch effect), solid-solution strengthening due to the presence of Mg and Si elements, dislocation strengthening due to large dissimilarities between coefficient of thermal expansion between α-Al (13.1 × 10−6 m/m. °C) and Si coral-like fibers (2.8 × 10−6 m/m. °C), and finally load transfer from matrix to the Si fibers [30, 31].
Upon heat treatment, some of the mentioned strengthening contributions are weakened or eliminated. The decrease in the strength of the heat treated material can then be attributed to change in the morphology of the eutectic Si from fibrous to spheroidized, grain coarsening due to solution heat treatment, and reduction of solid-solution strengthening. Indeed, the 1-h solution heat treatment at 520 °C provides sufficient time and temperature (driving force) for the interconnected Si fibers to be transformed to the Si particles (spheroids) along with α-Al grain growth. These phenomena directly result in reduced strength in the heat-treated materials. The artificial ageing at 170 °C for 4 h results in Mg2Si precipitation and Orowan strengthening, however, it seems that the mentioned softening mechanisms overcome the strengthening effect of the Mg2Si precipitates [13, 23, 32, 33, 34]. This could be the main difference between conventionally made and additively manufactured AlSi10Mg alloy with regard to the response of the material to the T6 artificial ageing heat treatment. In the conventionally manufactured AlSi10Mg alloy, artificial ageing results in the formation and homogeneous distribution of βʹ (Mg2Si) precipitates which contribute significantly to the strengthening of the material .
3.3 Depth-dependent indentation stress
In the studied AM AlSi10Mg, the T6 artificial ageing resulted in a 42% decrease in the strength upon heat treatment through the results from the nanoindentaion tests.
Drop in the strength upon heat treatment confirms that Orowan strengthening effect induced by Mg2Si precipitates is dominated by some softening effects mainly Si spheroidization, grain growth, and elimination of solid-solution strengthening. This is not the case in the artificial ageing of conventionally cast AlSi10Mg.
Indentation size effect, increase in hardness with the decrease in the depth, is observed in both as-printed and heat treated materials. Since the heat-treated material is softer than the as-printed one, the indentation size effect in the heat treated alloy is less pronounced.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interests.
- 19.Aboulkhair NT, Stephens A, Maskery I, Tuck C, Ashcroft I, Everitt NM (2015) Mechanical properties of selective laser melted AlSi10Mg: nano, micro, and macro properties. In: Solid freeform fabrication symposium 2015. Austin, Texas, pp 1026–1036Google Scholar
- 23.Trevisan Francesco, Calignano Flaviana, Lorusso Massimo, Pakkanen Jukka, Aversa Alberta, Ambrosio EP, Lombardi M, Fino P, Manfredi D (2017) On the selective laser melting (SLM) of the AlSi10Mg alloy: process, microstructure, and mechanical properties. Materials 10:76. https://doi.org/10.3390/ma10010076 CrossRefGoogle Scholar
- 25.E Ogris (2002) Institute of Metallurgy, ETH Zürich Ph.D. ThesisGoogle Scholar
- 36.Cahoon J, Broughton W, Kutzak A (1971) The determination of yield strength from hardness measurements. Metall Mater Trans B Process Metall Mater Process Sci 2:1979–1983Google Scholar