Relationship between microstructure evolution and tensile properties of AlSi10Mg alloys with varying solidification cooling rates

This work explored and contrasted the effect of microstructure on the tensile properties of AlSi10Mg alloys generated by transient directional solidification depending on variations in cooling rate and Magnesium (Mg) content (i.e., 0.45 and 1wt.% Mg), with a focus on understanding the dendritic growth and phases constitution. Optical and Scanning electron (SEM) microscopies, CALPHAD and thermal analysis were used to describe the microstructure, forming phases and resulting tensile properties. The findings showed that the experimental evolution of the primary dendritic spacing is very similar when both directionally solidified (DS) Al-10wt.% Si0.45wt.% Mg and Al-10wt.% Si-1wt.% Mg alloys samples are compared. The secondary dendritic spacing was lower for the alloy with more Mg, especially considering the range of high growth velocities. Moreover, a greater fraction of (Al+Si+Mg2Si) ternary eutectic islands surrounding the -Al dendritic matrix was noted for the alloy with 1wt.% Mg. As a result of primary dendritic spacings greater than 180 m related to lower cooling rates, slightly higher tensile properties were attained for the Al-10wt.% Si-0.45wt.% Mg alloy. In contrast, combining dendritic refining (< 150 m) and larger Mg2Si fraction, fast solidified DS Al-10wt.% Si-1wt.% Mg samples exhibited higher tensile strength and elongation. The control of cooling rate and fineness of the dendritic array provided a new insight related to the addition of Mg in slightly higher levels than conventional ones, capable of achieving a better balance of tensile properties in AlSi10Mg alloys.

3 relation to the tensile properties, but it also had more refined grains and different precipitated phases, which also led to increased tensile strength for loading conditions perpendicular to the grains alignment direction. In contrast, loads in the parallel direction in relation to that of the grains alignment provided lower values. On the whole, as a result of grain refinement and a tortuous crack course, the SLM AlSi10Mg sample showed higher tensile strength but lower elongation than the ascast sample.
In the assessment of slightly higher Si levels, Kakitani and coauthors [5] investigated solidification features of the Al-15wt.%Si-1.5wt.%Mg alloy. It was demonstrated that dendrite interphase spacing measurements agreed with Hall-Petch calculations for ultimate tensile strength and elongation to fracture considering a broad range of cooling rates. It was demonstrated that controlling the distance between the ductile -Al phase as well as the eutectic strengthening constituent is essential since it affects the alloy's tensile properties. As the dendrite interphase spacing was shortened, remarkable increases in strength and ductility were attained. Studies evaluating dendritic growth in Aluminum (Al) alloys and factors influencing this growth as well as consequences on the application properties are prominent in the specialized literature [6][7][8][9][10][11][12][13].
Marola et al. [14] found a connection between the microstructure, phase constitution, and thermal behavior of AlSi10Mg samples generated using different rapid and intermediate solidification techniques. The wider and lower the reflections become as the cooling rate increases, indicating increased supersaturation and smaller grain size. However, neither the values of the cooling rates have been determined nor their direct interrelationships with microstructural parameters such as 2, for instance.
Pereira and collaborators [15] performed a comparison of the microstructures and mechanical properties of the Al-7% Si-0.6% Mg alloy obtained through SLM and lost wax (LW) casting.
Regarding the microstructure of the generated samples, it was noted that the SLM sample was composed of a thin cellular structure having -Al cells smaller than 1 µm in size and with eutectic Si and Mg2Si grouped in the intercellular regions. In the LW sample, the microstructure was composed of the α-Al dendritic matrix, eutectic Si and β Fe-bearing intermetallic particles. 4 Recent studies by Arici et al. [16,17] have evaluated both the microstructure and the mechanical properties of Al alloys containing high Silicon (Si) and low Mg in their compositions.
These studies focused on the impact of minor additions of transition elements such as Zirconium (Zr), Vanadium (V), Titanium (Ti) on the properties of these Al alloys. One of the chosen alloys was the Al-10wt.% Si-0.3wt.% Mg for melting in PM casting. Manganese (Mn) was added to reduce the tendency of the alloy to react with the mold. The resulting microstructures showed dendritic growth and formation of eutectic (Al + Si). Long needles of intermetallic phases and the presence of the Mg2Si phase were also observed. There was no control of the thermal solidification parameters such as cooling rate and growth velocity, which makes the understanding of the microstructural characteristics less evident.
The AlSi10Mg alloys have demonstrated great potential and versatility in several processes involving melting and solidification. Therefore, the studies listed so far show the need to establish reliable interrelationships between microstructural and solidification thermal parameters and between microstructure aspects and properties, counting on cooling rate measurement and control. This type of approach can increase the prospect of microstructural and properties control for processes such as conventional casting, LW, PM and HPD. It is worth mentioning that in the case of rapid solidification processes such as AM-related techniques, direct cooling rate measurements are more difficult due to the nature of the process, i.e., extremely high cooling rates and reduced times [18].
Because of their high strength-to-weight ratio, heat-treatable capability, and excellent castability, Al-Si-Mg alloys are commonly used to manufacture components for the automotive and aerospace sectors of the industry. According to precedent findings, Copper (Cu) and Mg are used in Al-Si foundry alloys to produce typical precipitation hardening phases such as Al2Cu, Mg2Si, and Al2CuMg, which can provide the alloy with improved mechanical properties [19,20]. Dunn and Dickert [21] demonstrated that adding Mg up to 0.55 percent might improve the tensile properties and hardness of the A380 and 383 alloys. Those alloys for which this Mg limit has been established have high Si, but also high Cu and zinc (Zn). In the case of the ternary Al-10wt.% Si(-Mg) alloys, the Mg limit has not been verified so far, to the best of the present authors' knowledge. High-silicon Al-Si-Mg alloys are specifically engineered for high integrity structural casting components. These alloys 5 have a broad variety of property levels to satisfy the criteria for hardness, high mechanical properties, and crash efficiency. Moreover, Al-Si-Mg alloys, in addition to their excellent corrosion resistance, can be used to cast thin and massive structural parts due to their extremely high fluidity [22].
A transient directional solidification casting was used in this study to allow two (2) Al-10wt.% Si (-0.45 and -1wt.% Mg) alloys with substantially different microstructures and tensile properties to be generated. The differences in cooling rates, growth velocities, dendritic growth (either primary or secondary arms), phase constitution and tensile properties between both directionally solidified Al-Si-Mg alloys have been investigated. A number of techniques including CALPHAD, optical and scanning electronic microscopies, thermal analysis and mechanical tests were used to clarify how Mg content and cooling rate might affect output disparities in either dendritic growth or tensile properties.

Experimental procedure
In an induction furnace with a Si carbide crucible covered with zircon, two alloys were prepared using commercially pure Al (> 99.9 percent purity), Si (> 99.7 percent purity), and Mg (> 99.5 percent purity). To generate the desired ternary Al-10wt.% Si (-0.45 and 1wt.% Mg) alloys, appropriate quantities of Mg and Si were added to the melt. They were applied to the molten bath in small pieces so that they could melt easily. After, the molten alloys were poured into a split AISI 304 stainless steel mold with an internal diameter of 60 mm, a height of 157 mm, and a wall thickness of 5 mm. To minimize radial heat losses, a coating of insulating alumina was applied to the vertical inner mold wall. A thin AISI 1020 carbon steel plate was used to seal the bottom of the mold (3 mm thick).
The inner plate's surface was ground with #1200 finishing sandpaper.
The molten alloy was degassed with argon gas for 2 minutes using a perforated quartz tube before being scorified with the appropriate flux and cast as 60 mm x 157 mm cylindrical ingots. A directional solidification method, as described in previous articles [23,24], was used to achieve nonstationary heat flow conditions during the production of the castings. To allow continuous temperature measurements during solidification, 8 fine K-type thermocouples were mounted along the length of the casting. The thermocouple tips were set in distinct longitudinal points until 74 mm, with 6 the cooled bottom of the mold serving as a reference. As a result, temperature shifts may be used to track the directional growth of the alloy casting. With thermocouples in place and activated, the alloy was poured into the mold and the electric heaters were turned off; additionally, when the programmed melt overheating was reached, the regulated water flow was turned on.
After the Al-Si-Mg alloys castings were produced, longitudinal and transverse section samples were extracted from them by using a precision saw. Metallography was used to examine eight (8) sections at different distances from the cooled surface of the casting. Primary (1) and secondary (2) dendritic arm spacings were measured using an Olympus Metallurgical Microscope (model GX51) while the dendritic arrays were being clearly visualized. The intercept procedure was used on longitudinal samples to detect 2 whereas the triangle method distinguished 1. Both spacing values were defined by the average counting of 50 measurements per position along the casting length [25].
To supplement the optical microstructural characterization, a scanning electron microscope (SEM-EDS) FEI (Inspect S50L) was used. This instrument was used to examine transverse section samples of the DS Al-10wt.% Si-0.45wt.% Mg Al-10wt.% Si-1wt.% Mg alloys castings, and EDS elemental mapping was performed to assess the relative distribution of the formed phases in each alloy.
Tensile experiments were carried out on transverse specimens machined from various locations along the length of the DS castings. Three specimens per position of interest were prepared in accordance with the ASTM Standard E 8M specifications and tested at a strain rate of 3 x 10 -3 s -1 .

Results and discussion
The thermal profiles registered for the two Al-Si-Mg alloys are shown in with softer cooling profiles. These profiles will be very useful in order to determine cooling rates and 7 growth velocities as will be seen next. The liquidus temperatures (TL) are signed with horizontal dashed lines in the graphs. These temperatures were obtained due to examining the results observed in cooling curves registered at very slow cooling down.
The profiles of the associated positions in each alloy casting are very similar, which indicates to a certain extent little influence of the increase in Mg on the drop in temperature during directional solidification. By observing the Scheil solidification diagrams in Fig. 2, it can be seen that the freezing solidification intervals of the two alloys are very similar. This corroborates the little comparative variations in the thermal profiles of the same positions between both alloys around the liquidus temperature. Even though Timelli and Bonollo [26] affirmed that the Mg content has some impact on fluidity of Al-Si alloys, decreasing it with increasing Mg content, it appears that contents that are as low as those applied to the alloys evaluated here may not be enough to significantly alter the alloy fluidity.
Solidification interval and melt fluidity are considered key factors in altering thermal conductance at the metal/mold interface [27,28]. These factors influence the thermal history during solidification. In the present case it seems not to be sufficient to cause significant changes, as can be seen in Fig. 1

Al-10wt.% Si -1wt.% Mg
Transversal section  Chinese-script-like [32]. Si particles (light phase) show a lamellar morphology typically found in these alloys for a wide range of cooling rates [33,34]. SEM/EDS analyses with EDS elemental mappings in Fig. 7 show the element composition and distribution in each phase. According to Liu and Kang [32], the addition of Mg to Al-Mg-Si alloys has a significant impact on the development of 14 solidification microstructures. Moreover, increased Mg content promotes an increase in the fraction of Mg2Si particles. This was also observed in the present results.   [35,36], the solidification of ternary alloys with more complex phases formation may have induced a slightly lower exponent. Another assessment involved plotting the 2 against the growth velocity, as also seen in Fig. 9. The alloy containing more Mg showed lower 2 values if a same growth velocity is considered.

Conflicts of interest
We announce that we have no financial or personal arrangements with other individuals or organizations that may improperly affect our work, and that we have no technical or other personal interest in any good, service, or business that could be construed as affecting the results presented in, or the analysis of, the manuscript entitled.

Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.

Informed consent
The enclosed manuscript has been accepted by all of the informed authors.

Data availability
The data that support the findings of this study are available from the corresponding author, JES, upon reasonable request.