Quantification of Wettability and Surface Tension of Liquid Aluminum 7075 Alloy on Various Substrates

Abstract

To support computational studies and process optimization that require temperature-dependent thermophysical properties, this study characterized the wettability, surface tension, liquid–solid interfacial tension (IFT), and work of adhesion of Al 7075-T6 alloy from 923–1073 K under argon on porous alumina, tungsten, and nonporous alumina substrates using sessile drop experiments and Young’s and Young–Dupre equations, respectively. Furthermore, the substrates’ room-temperature surface free energy (SFE) characteristics were characterized using the Owens–Wendt–Rabel–Kaelble model. The contact angle results revealed the alloy’s poor wettability on all substrates. The surface tension data ranged from 718.87–942.90 mN·m⁻¹ in decreasing order of tungsten, porous alumina, and nonporous alumina. The SFE results of the porous alumina, nonporous alumina, and tungsten substrates were 44.92, 43.32, and 42.03 mN·m⁻¹, respectively. Also, the calculated liquid–solid IFT values ranged from 539.24–835.51 mN·m⁻¹ in decreasing order of porous alumina, tungsten, and nonporous alumina. Additionally, the calculated work of adhesion values ranged from 123.97–479.44 mN·m⁻¹ in decreasing order of nonporous alumina, tungsten, and porous alumina, respectively. Thus, the wettability, surface tension, and liquid–solid IFT of Al 7075-T6 alloy on the substrates were affected by the substrates’ SFE characteristics, thereby affecting the work of adhesion.

1. Introduction

Interfacial tension (IFT) is the attraction force between molecules at the interface of two similar or dissimilar phases. Generally, surface tension is the IFT at a liquid–gas boundary that causes the liquid to form a droplet. This droplet formation behavior affects the fluid flow behavior of molten metals and weld pool shape, making it a key property for welding and additive manufacturing process optimization. For liquid metals, wettability and surface tension can dominate various surface and interface phenomena in various materials science and engineering fields. For instance, their influence is crucial in industrial processes, such as casting, welding, sintering, and solidification, including additive manufacturing processes. They are needed to calculate a metal’s work of adhesion on a substrate, evaluate the capillary pressure that controls the infiltration of liquid metals into porous solids, or describe the nucleation of bubbles inside liquids or at the liquid–crucible interface [1]. To optimize these processes, a deep understanding of surface properties, especially surface tension, is crucial. Additionally, even in low gravity, a fundamental problem in printing metallic parts is that the final printed components’ structural integrity is primarily determined by the initial particles’ adhesion [2]. Engineering the surface melt can help improve the adhesion between particles and manufacture components with higher mechanical integrity, which requires investigating the surface tension of the alloys. Consequently, it is practically essential to probe into the behavioral characteristics of the wettability and surface tension of liquid metals.

Among several available surface tension measurement techniques, such as the sessile drop (SD) and pendant drop (PD) techniques, PD–SD combined method, electromagnetic levitation (EL), drop oscillations, and microgravity experiments, the SD technique is a suitable and reliable drop-shape approach for determining the surface tension of liquid pure metals and alloys [3]. The best results are obtained for nonwetting systems. This condition has been verified for several metal–refractory systems [3].

According to Keene [4], the surface tension of aluminum at temperatures close to its melting temperature measured by different authors using different techniques, such as the SD, maximum bubble pressure (MBP), and contact-less oscillating droplet techniques, lies within the range of 850–1100 mN·m⁻¹ [5,6,7,8,9,10,11,12,13]. This considerable dispersion is probably due to the high sensitivity of the surface properties of molten aluminum to oxygen, and the temperature coefficient of the surface tension in these experiments was reported to be in the range of 0.10–0.20 mN·m⁻¹·K⁻¹ [4]. Meanwhile, as reported by Mills et al. [14], the measurements of the surface tension of aluminum had been divided into those on pure (with no contamination) and oxygen-saturated samples, with the values for the pure metal appreciably higher than those for the oxygen-saturated values. The surface tension of oxygen-saturated aluminum samples has been investigated by several studies, such as Egry et al. (881 − 0.20ΔT mN·m⁻¹ over 893–1423 K) [10], Anson et al. (845 mN·m⁻¹ at 953 K) [15], and Roach and Hennin (868 − 0.25ΔT mN·m⁻¹ over 970–1170 K) [14] using the levitated drop (LD), SD, and draining crucible methods, respectively. Also, the surface tension of pure aluminum samples has been investigated by Garcia-Cordovilla et al. (1100 mN·m⁻¹ at 1073 K) [6], Goumiri and Joud (1050 mN·m⁻¹ at 973 K) [11], Pamies et al. (1090 mN·m⁻¹ at 973 K) [12], Anson et al. (1009 mN·m⁻¹ at 953 K) [15], and Sarou-Kanian et al. (1024 − 0.274ΔT mN·m⁻¹ over 1790–2170 K) [13] using the MBP, SD (vacuum or H₂), and LD–aerodynamic levitation (AL) methods, respectively. Additionally, using a standardized SD testing procedure, Bainbridge and Taylor [16] reported that the surface tension (870 mN·m⁻¹ at 958 K) of high-purity aluminum (99.999%) matched the previously reported results for oxidized aluminum excellently.

High-strength aluminum alloys, such as Al 7075 alloy, are known for their remarkable mechanical properties, lightweight characteristics, relatively low melting temperatures, and various aerospace and automotive applications. Despite its various applications, its wettability and surface tension characteristics are uncertain. The available surface tension data are limited to pure aluminum and a few examined ranges of aluminum alloys [16]. To the best of our knowledge, only two studies have investigated the surface tension of Al 7075 alloy. Bainbridge and Taylor measured the surface tension of molten aluminum and its alloys, including Al 7075 alloy, under vacuum using the SD method [16]. The experiments were conducted under four conditions: as-melted (vacuum), as-melted + fractured, exposure to dry air, and dry air + refractured, with mean surface tension values of 0.809, 0.843, 0.777, and 0.607 N·m⁻¹ at ~958 K, and standard deviations of 0.041, 0.018, 0.061, and 0.083 N·m⁻¹, respectively, with a minimum of n = 5 samples. Also, Momeni et al. [2] measured the surface tension of Al 7075-T6 alloy on porous alumina substrates under vacuum using the SD technique from T_m up to 1023 K. They reported a mean surface tension value of 0.134 J·m⁻², with a standard deviation of 3.45 × 10⁻³ J·m⁻². However, these abovementioned studies did not report the surface tension–temperature relationship of Al 7075 alloy as a function of the testing conditions.

To support computational studies and process optimization that require tempera-ture-dependent thermophysical properties, this study characterized the wettability, surface tension, liquid–solid IFT, and work of adhesion of liquid commercial Al 7075-T6 alloy from 923 to 1073 K under a 100% argon atmosphere on three substrates: porous high-refractory alumina, tungsten, and nonporous high-refractory alumina substrates using the SD technique and Young’s and Young–Dupre equations, respectively. Al 7075 alloy is available in the as-cast form or various heat-treated conditions, such as T6, T651, T7, T73, and T7351. Al 7075-T6 alloy was selected due to its commercial prevalence. The T6 temper condition, which involves solution heat treatment and artificial aging, is commonly applied to enhance strength and durability in service environments. However, when the alloy is heated above its liquidus temperature, any precipitates originally formed during age hardening will dissolve completely, resulting in a homogeneous liquid state. Consequently, the alloy’s initial heat-treated microstructural state does not persist in the liquid phase, thereby eliminating any potential influence of the T6 temper condition on the thermophysical properties. Thus, similar outcomes are expected for all Al 7075 alloy designations, since the solid-state microstructure does not persist in the liquid state.

For SD experiments, the choice of substrate is based on factors such as chemical and thermal stability, inertness to reaction with the metal sample, surface condition reproducibility, and the ability to maintain a planar surface with level orientation. Herein, the selection of porous and nonporous alumina and tungsten substrates was based primarily on their suitability for SD experiments to characterize interfacial-dependent thermophysical properties. The tungsten substrate was selected due to its high melting point, above that of Al 7075 alloy, chemical and thermal stability, and inertness to react with Al 7075 alloy over the studied temperature range. Tungsten mesh interlayers are applied in laser welding and brazing of aluminum and titanium alloys to improve the wettability of aluminum [17]. Also, in aluminum–tungsten composite applications, intermetallics at the aluminum–tungsten interface, such as Al₁₂W, Al₅W, and Al₄W, with attractive properties can be formed, taking advantage of the low density and high strength of aluminum and tungsten, respectively [18]. The porous and nonporous alumina substrates were utilized considering their wide scientific applications in metal–ceramic wetting studies and composite and joining applications involving aluminum alloys. Note that the high wettability and low surface tension of liquid metal alloys are crucial in metal joining and brazing, where the molten filler metal is required to spread readily over the substrates. However, in casting applications, liquid metals can be poured into nonwetting crucibles and molds to prevent sticking or reactions with container surfaces. To characterize the influence of the substrates on the abovementioned interface-based thermophysical properties, the room-temperature solid–gas free energy (FE) characteristics of the three substrates were characterized using the Owens–Wendt–Rabel–Kaelble (OWRK) model, which considers the geometric mean of the dispersive and polar parts of the liquid’s surface tension and the solid substrate’s surface energy (SE) [19]. Finally, to determine the compositional changes incurred during the surface tension experiments and investigate the presence of impurities, post-processed samples were analyzed using energy-dispersive X-ray spectroscopy (EDS).

2. Materials and Methods

2.1. Materials

An Al 7075-T6 alloy plate was sourced from Kaiser Aluminum, USA.

Table 1. Elemental composition of Al 7075-T6 alloy.

Element		Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	V	Zr	Other	Al
%	Min	0	0	1.20	0	2.10	0.18	5.10	0	0.01	0	0.05	Remainder
	Max	0.40	0.50	2.00	0.30	2.90	0.28	6.10	0.20	0.05	0.05	0.15

shows the alloy’s elemental composition. Porous high-alumina matrix sheets (RS-99R, 48% porosity) with dimensions of 3 inches × 3 inches × 1/8 inches (ZIRCAR Refractory Composites, Inc., Florida, NY, USA) and nonporous alumina ceramic sheets (0% porosity) with dimensions of 4.25 inches × 2 inches × 0.157 inches and high-temperature ultra-dense tungsten sheets with dimensions of 6 inches × 12 inches × 0.04 inches (McMaster-Carr, Atlanta, GA, USA) were used as substrate materials.

2.2. Sample Preparation

Several cuboid-shaped samples of Al 7075-T6 alloy were sectioned and subsequently ground on all surfaces using 320-grit silicon carbide paper, achieving consistent average dimensions of 7.3 mm × 5.2 mm × 5.2 mm and ensuring uniform surface characteristics for all experiments. Porous alumina oxide refractory, nonporous alumina ceramic, and high-temperature ultra-dense tungsten sheets with dimensions of 57 mm × 36 mm × 3.5 mm were cut and used as substrates in a furnace for different high-temperature SD experiments. The surfaces of the substrates were used as obtained (smooth and fine) without characterizing the surface roughness properties. The surfaces were only cleaned using ethanol to eliminate any impurities or residue. As per the manufacturers’ specifications, after fabrication, the porous and nonporous alumina ceramic sheets were prefired and the tungsten sheet was ground. For each substrate category (tungsten, porous alumina, and nonporous alumina), three substrates were used for three independent experiments, with consistent surface roughness (as-obtained condition) maintained within each group.

2.3. Method

2.3.1. Sessile Drop Experiments

The surface tension data presented herein were obtained by SD experiments using the contact angle measurement technique, following the method described in [3]. Figure 1 presents a typical SD profile showing the contact angle a droplet makes with the substrates and the height of the droplet.

2.3.2. Surface Tension Measurement of Liquid Al 7075-T6 Alloy on Different Substrates

The surface tension values of the Al 7075-T6 alloy samples were measured by conducting SD experiments using the static contact angle (SCA) 20 software module of the OCA 25-HTV 1800 video-based contact angle measuring and contour analysis system for high temperatures and vacuum (DataPhysics Instruments USA Corp., Charlotte, NC, USA).

Before testing, all the components of the sample assemblies were ultrasonically cleaned with acetone and dehydrated in air. Before the SD experiments, the porous alumina ceramic substrate was baked at 1273 K to prevent outgassing and excessive contamination during the actual experiments. Heating was conducted under argon to maximum temperatures of 1013 and 1073 K for the tungsten substrate and other substrates at a rate of 278 K·min⁻¹, respectively. Sample profiles were recorded at 1 frame·s⁻¹ from the melting temperature up to 1013 and 1073 K, respectively. The contact angle, surface tension, and droplet characteristics data for liquid Al 7075-T6 alloy samples on the tungsten substrate and other substrates from melting to 1013 and 1073 K were calculated. The temperature-dependent density of molten Al 7075-T6 alloy was determined from Equation (1), as reported by Kethireddy Narender et al. [20]. The coefficient of the temperature dependence of density is −0.9746 kg·m⁻³. For each substrate, experiments were conducted in triplicates, and average results were obtained. Herein, uncertainties were calculated, as explained in Ref. [21], and 95% confidence intervals were constructed by multiplying the uncertainties by a k-factor of two.

$$\rho (T) = (3082 \pm 23) - (1.000 \pm 0.038)·T [\mathrm{kg}·{\mathrm{m}}^{-3}].$$

(1)

2.3.3. Solid–Gas FE Measurement of the Different Substrates

The surface free energy (SFE) of solids can be determined by measuring the contact angles that droplets of probing liquids of known surface tension characteristics make with the surface of the solid. This measurement method is based on Young’s equation (Equation (2)), which expresses the equilibrium condition for a liquid–solid interface [22] (Figure 2).

$${\gamma }_s={\gamma }_{ls}+{\gamma }_l·\cos \theta ,$$

(2)

where ${\gamma }_s$ is the solid’s SFE, ${\gamma }_{ls}$ is the liquid–solid IFT, ${\gamma }_l$ is the experimentally determined surface tension of the liquid, and θ is the contact angle the liquid makes with the substrate.

Several approaches based on the SE theory have been formulated, and further information can be found in the literature [23,24,25,26]. Herein, the room-temperature SE values of the three substrates were calculated using the OWRK model and contact angle data to understand their influence on the interface-based thermophysical properties under study. Unlike other models, this model essentially allows for the dissociation of the total SE into polar and nonpolar (dispersive) components, thereby providing further insight into the surface properties of solid surfaces [27]. Note that even if the same contact angle measurements are utilized, the computed SE will vary depending on the model [27]. This study was not conducted to assess the model’s validity. Instead, we used it to estimate the SE values of the three substrates and to identify the primary elements that affect them. As there are two unknowns, the OWRK technique requires at least two liquids with known dispersive and polar surface tension components to compute the solid’s SFE [28]. The combining rule proposed by the OWRK model is indicated below:

$${\gamma }_{ls}={\gamma }_s+{\gamma }_l - 2·\sqrt{{\gamma }_s^d{·\gamma }_l^d} - 2·\sqrt{{\gamma }_s^p{·\gamma }_l^p},$$

(3)

where ${\gamma }_{ls}$ is the SE at the liquid–solid interface, ${\gamma }_s$ and ${\gamma }_l$ are the SE and surface tension of the solid and liquid, respectively, ${\gamma }_s^d$ and ${\gamma }_s^p$ are the dispersive and polar parts of the solid’s SE, and ${\gamma }_l^d$ and ${\gamma }_l^p$ are the dispersive and polar parts of the liquid’s surface tension, respectively. Combining Equations (2) and (3) yields Equation (4):

$${\displaystyle \frac{{\gamma }_l·(1+\cos \theta )}{2·\sqrt{{\gamma }_l^d}}}=\sqrt{{\gamma }_s^p}·{\displaystyle \frac{\sqrt{{\gamma }_l^p}}{\sqrt{{\gamma }_l^d}}}+\sqrt{{\gamma }_s^d}.$$

(4)

The polar and dispersive components of the liquid’s surface tension are determined, and plotting ${\displaystyle \frac{{\gamma }_l·(1+\cos \theta )}{2·\sqrt{{\gamma }_l^d}}}$ against ${\displaystyle \frac{\sqrt{{\gamma }_l^p}}{\sqrt{{\gamma }_l^d}}}$ will produce a linear correlation. With linear regression of the data, ${\gamma }_s^p$ and ${\gamma }_s^d$ can be determined as the squares of the slope and y-intercept, respectively.

The SE characteristics of substrates significantly influence wettability and adhesion in additive manufacturing, coating, and joining applications. In powder-based additive manufacturing processes, the interaction between the liquid metal and the substrate is critical. High-SE substrates promote better wettability, allowing molten droplets to spread uniformly and leading to stronger interfacial bonding and reduced porosity and delamination issues. In coating and joining applications, high-SE substrates promote better wetting, allowing coatings and adhesives to spread uniformly and leading to stronger interfacial bonding. Thus, the characterization of the SEs of substrates is vital for optimizing wettability and adhesion in additive manufacturing, coating, and joining applications.

In this study, the SFEs of substrates were characterized using the single electronic syringe module ESr-N (single direct dosing system SD-DM; DataPhysics Instruments USA Corp., Charlotte, NC, USA) and the SCA 21 software module of the OCA 25-HTV 1800 system. Three probing liquids were used for the SE analysis of the substrates: water, dimethyl sulfoxide, and diiodomethane, and

Table 2. Test liquids and their surface tension components.

Liquids	Surface Tension Data (mN·m−1)			Source
	${\mathit{\gamma }}_{\mathit{l}}$	${\mathit{\gamma }}_{\mathit{l}}^{\mathit{d}}$	${\mathit{\gamma }}_{\mathit{l}}^{\mathit{p}}$
Water	72.30	18.70	53.60	Rabel
Dimethyl sulfoxide	44.00	36.00	8.00	Erbil
Diiodomethane	50.80	48.50	2.30	Fowkes

presents their total, dispersive, and polar surface tension components. The probing liquids were selected considering their extensive applications as probing liquids; well-characterized and well-known dispersive, polar, and total surface tension components; and diverse interaction profiles ranging from highly polar (water) to almost completely dispersive (diiodomethane). Additionally, three probing liquids were used because a regression line based on only two data points from two probing liquids, respectively, does not provide a reliable measure of accuracy. Therefore, using at least three test liquids is recommended for more accurate and statistically meaningful SE results. For each substrate, five sets of room-temperature (22 °C and 50% relative humidity condition) needle-in-drop SD experiments were conducted using each test liquid at different locations on the substrate. A syringe with a 0.51-mm diameter needle was used for dosing the test liquids on the substrates. The contact angle variation of the test liquids as a function of time and/or drop base diameter on each substrate was measured, and the average contact angles were obtained for each test liquid on each substrate. Next, the SEs of the substrates were determined using Rabel, Erbil, and Fowkes surface tension data for water, dimethyl sulfoxide, and diiodomethane, respectively, and the average contact angles of the test liquids on the substrates.

2.3.4. Liquid–Solid IFT

The liquid–solid IFT between the liquid Al 7075-T6 alloy droplet and the substrates was investigated using Young’s Equation (Equation (2)). The obtained contact angles of liquid Al 7075-T6 alloy on the substrates, the surface tension values of liquid Al 7075-T6 alloy, and the SE values of the substrates are substituted into Equation (2) to obtain the liquid–solid IFT.

2.3.5. Work of Adhesion

The work of adhesion was computed using the Young–Dupre equation, which relates the surface tension of the liquid Al 7075-T6 alloy droplet above the melting temperature and the measured equilibrium contact angle formed between the liquid Al 7075-T6 alloy droplet and the substrates:

$$W_a={\gamma }_{lv}·(1+\cos \theta ),$$

(5)

where ${\gamma }_{lv}$ is the liquid droplet’s surface tension, and $\theta$ is the contact angle the droplet makes with the substrate.

2.3.6. Energy-Dispersive X-Ray Spectroscopy

To determine the compositional changes incurred during the surface tension experiments, post-processed samples were analyzed using EDS (Scios 2 Dual-Beam Focused Ion Beam/Scanning Electron Microscope, Thermo-Fisher, Waltham, MA, USA). Each sample was prepared by mechanical grinding to expose a flat surface representative of the bulk material. Additional analysis was conducted on unpolished surfaces of post-processed samples for comparison to the bulk to assess the presence of oxides and other contaminants during experiments.

3. Results and Discussion

3.1. Wetting Behavior of Liquid Al 7075-T6 Alloy on the Different Substrates

Contact angle is the most used parameter for defining the wettability of a substrate by a liquid [29,30,31]. The contact angle change of the liquid Al 7075-T6 alloy droplet and the corresponding uncertainties on the three substrates with temperature are shown in Figure 3. On the porous alumina substrates, complete melting and droplet formation occurred from ~898–918 K. However, on the tungsten and nonporous substrates, complete melting and droplet formation occurred from ~873–888 K. Note that the solidus and liquidus melting temperatures of Al 7075 alloy are ~750 and 908 K, respectively. To ensure complete solid-to-liquid phase transformation, the thermophysical property characterization began at 923 K, which is slightly above the liquidus temperature of Al 7075 (~908 K). Large nonwetting contact angles, which are almost constant, but slightly decreasing, were observed on all three substrates over the studied temperature ranges. The contact angles obtained on the porous and nonporous alumina substrates decrease slightly from 150.71° ± 14.29° and 121.92° ± 10.94° at 923 K to 150.25 ± 14.26° and 118.82 ± 12.42° at 1073 K, respectively. Also, for the tungsten substrate, the contact angle decreases slightly from 141.38° ± 10.69° at 923 K to 137.57 ± 10.88° at 1013 K. Such nonwetting behavior over such relatively low temperatures has been observed for pure aluminum in several studies [32,33,34,35], which is attributed to the presence of an oxide skin covering [1]. It has been reported that such an oxide skin covering on pure aluminum can be removed on further heating above 1373 K [1], which is not the case here as the maximum temperatures under study are 1013 and 1073 K. Thus, the deoxidation of aluminum will not be discussed herein. Although the contact angles on all three substrates show nonwetting characteristics, the contact angle values are in decreasing order of porous alumina, tungsten, and nonporous alumina over the studied temperature ranges. The increase in contact angles exhibited by the porous alumina substrate compared to other substrates can be attributed to its surface voids and increased roughness, which reduced wettability.

3.2. Surface Tension of Liquid Al 7075-T6 Alloy on Different Substrates

The surface tension values of liquid Al 7075-T6 alloy on the tungsten substrate and the other substrates were measured in a range of 90 K (923–1013 K) and 150 K (923–1073 K), respectively. These results and the corresponding uncertainties are presented in Figure 4. The results show a linear dependence of surface tension on temperature for all three conditions, increasing with temperature. The surface tension values measured on the porous and nonporous alumina substrates increase from 812.54 ± 5.47 and 718.87 ± 11.99 mN·m⁻¹ at 923 K to 942.90 ± 7.66 and 916.85 ± 27.50 mN·m⁻¹ at 1073 K, respectively. Also, the surface tension values measured on the tungsten substrate increase from 820.27 ± 15.69 mN·m⁻¹ at 923 K to 929.24 ± 6.84 mN·m⁻¹ at 1013 K. For the nonporous alumina substrate, the lower surface tension values and the higher surface tension gradient can be attributed to its more pronounced wetting characteristics than the experiments on other substrates. The measured surface tension values are in decreasing order of tungsten, porous alumina, and nonporous alumina for the studied temperature ranges. Also, for the experiments conducted on the porous alumina, tungsten, and nonporous alumina substrates, the surface tension–temperature dependence equations for the investigated temperature ranges are displayed in Figure 4.

Generally, the surface tension of liquid metals decreases with increasing temperature. The positive gradient of the surface tension values of liquid Al 7075-T6 alloy on the different substrates can be attributed to the increase in the oxygen partial pressure $P_{{\mathrm{O}}_2}$ level, as explained by Ozawa et al. [36]. Under this condition, oxygen adsorption on the melt surface considerably decreases the surface tension. Additionally, the absolute value of the surface tension–temperature coefficient decreases when the atmosphere’s $P_{{\mathrm{O}}_2}$ content increases. Therefore, it becomes positive if the molten metal is not oxidized under relatively high $P_{{\mathrm{O}}_2}$. Such positive surface tension–temperature coefficients have been reported by Klapczynski et al. for 304L stainless steel [37] and Ozawa et al. for iron [38]. Furthermore, Dubberstein et al. reported such positive surface tension–temperature coefficients for Fe–Cr–Mo (AISI 4142), Fe–Cr–Ni (AISI 304), and Fe–Cr–Mn–Ni TRIP/TWIP steel samples [39]. Note that although several studies have reported positive surface tension–temperature coefficients for different metal alloys over various temperature ranges, Keene [4], in his review, argued that surface tension cannot increase indefinitely with temperature since it always tends to zero as the critical temperature is approached. Thus, he concluded that for all systems that exhibit positive surface tension–temperature coefficients, there must be a maximum surface tension on the surface tension–temperature curve. Thus, since the temperature range under study herein is only 150 K (923–1073 K), it is safe to presume that at higher temperatures, a boomerang-shaped curve could result, as reported by Klapczynski et al. [37] and Ozawa et al. [38]. The positive surface tension–temperature relationship could also be attributed to the formation of a stable aluminum oxide layer on the melt surface due to the oxygen adsorption as a result of the increased $P_{{\mathrm{O}}_2}$ over the investigated temperature range. Meanwhile, although surface tension increased with temperature for all substrates, the contact angles decreased slightly, which may be influenced by changes in IFT or oxide layer stability rather than surface tension alone.

Figure 5 compares the surface tension results reported herein and previously reported results for aluminum and Al 7075 alloy. Our results agree with the values reported by Bainbridge and Taylor [16] and are significantly higher than that previously reported by Momeni et al. [2], who conducted their experiments under vacuum. Thus, it can be concluded that sufficient oxides were present during the experiment, which drastically affected the surface tension values. Also, the surface tension values for liquid Al 7075-T6 alloy reported herein are within the range of those of the oxygen-saturated aluminum samples characterized by Egry et al. [10], Anson et al. [15], Roach and Hennin [14], and Bainbridge and Taylor [16] and lower than those of the pure aluminum samples characterized by Garcia-Cordovilla et al. [6], Goumiri and Joud [11], Pamies et al. [12], and Anson et al. [15], as explained in the Introduction (Section 1). More importantly, our results novelly provide the surface tension–temperature relationship for Al 7075 alloy on different substrates from 923 to 1073 K. For the surface tension experiments conducted on the three different substrates, the surface tension values of Al 7075-T6 alloy on the tungsten substrate over the studied temperature range exceed that of the other substrates. Furthermore, where sufficient surface tension data are available for any particular metal, it is possible to compensate partially for the effect of active surface contaminants by calculating the mean value of the surface tension based on the higher values only [4]. These surface tension values should more closely approach the true value.

3.3. Solid–Gas FE of the Different Substrates

Table 3. Contact angles of the three test liquids on the three substrates.

Substrates	Contact Angle (°) for Water	Contact Angle (°) for Dimethyl Sulfoxide	Contact Angle (°) for Diiodomethane
Porous alumina	72.07 ± 0.45	25.25 ± 0.10	21.30 ± 0.28
Nonporous alumina	57.67 ± 0.28	39.40 ± 0.17	64.83 ± 0.11
Tungsten	61.76 ± 0.12	30.13 ± 0.10	58.83 ± 0.10

presents the average contact angles of the three test liquids on the substrates. Significant variations in the contact angle values on the substrates were observed. For water, the highest and lowest contact angles are observed for the porous and nonporous alumina substrates, respectively. Alternatively, for dimethyl sulfoxide and diiodomethane, the highest and lowest contact angles are observed for the nonporous and porous alumina substrates, respectively. In all cases, the contact angles observed on tungsten lie between those observed on the porous and nonporous alumina substrates. The contact angles yielded the SFE of the samples and their dispersive and polar components using the OWRK model geometric mean approach.

SFE stems from the unbalanced forces between atoms or molecules on the surface of a specimen. Several van der Waals interactions contribute to SFE. Particularly, the polar component arises from these varying intermolecular forces due to permanent and induced dipoles and hydrogen bonding. In contrast, the dispersive component arises due to the instantaneous dipole moments, like the Coulomb interaction between an electron and the nuclei in two molecules [22].

The polar and dispersive components of the total surface tension of the test liquids were used to compute the SFEs of the substrates. Figure 6, Figure 7 and Figure 8 present the graphical representation of the OWRK method for the test liquids on the porous alumina, nonporous alumina, and tungsten substrates, respectively. From the plots, the slope and y-intercept are extracted and squared, representing the polar and dispersive parts of the SE of the substrates, respectively.

To the best of our knowledge, no study has been produced on the room-temperature SD investigation of SEs of porous alumina, nonporous alumina, and tungsten substrates. Thus, the room-temperature SE values calculated herein could not be compared to any similar study in the literature.

Table 4. Total, dispersive, and polar surface energy (SE) values of the three substrates using the Owens–Wendt–Rabel–Kaelble (OWRK) model.

Substrates	${\mathit{\gamma }}_{\mathbf{s}}$ (mN·m−1)	${\mathit{\gamma }}_{\mathbf{s}}^{\mathbf{d}}$ (mN·m−1)	${\mathit{\gamma }}_{\mathbf{s}}^{\mathbf{p}}$ (mN·m−1)
Porous alumina	44.92 ± 0.26	36.94 ± 0.10	7.98 ± 0.24
Nonporous alumina	43.32 ± 0.42	16.53 ± 0.09	26.79 ± 0.21
Tungsten	42.03 ± 0.18	21.23 ± 0.02	20.80 ± 0.07

presents the total SEs of the substrates and their corresponding dispersive and polar SE components at room temperature (22 °C and 50% relative humidity), calculated using the OWRK model. The total SE values of the substrates are in decreasing order of porous alumina, nonporous alumina, and tungsten, and they differ slightly. The absolute differences between the dispersive and polar SE components of the substrates are in decreasing order of porous alumina, nonporous alumina, and tungsten. The SE of the porous alumina substrate comprises high dispersive and low polar components, respectively. Also, the nonporous alumina substrate comprises low dispersive and high polar components, respectively. For the tungsten substrate, the dispersive component slightly differs from the polar component. Surprisingly, for the nonporous alumina substrate, the polar component of the SE exceeds that of the dispersive component, unlike other substrates.

3.4. Liquid–Solid IFT Between Liquid Al 7075-T6 Alloy and the Different Substrates

The liquid–solid IFT values of liquid Al 7075-T6 alloy on the tungsten substrate and the other substrates were computed in a range of 90 K (923–1013 K) and 150 K (923–1073 K), respectively. These results and the corresponding uncertainties are presented in Figure 9. All substrates show a linear dependence of surface tension on temperature for all three conditions, increasing with temperature. For the porous and nonporous alumina substrates, the liquid–solid IFT values increase from 733.48 ± 6.45 and 539.24 ± 10.06 mN·m⁻¹ at 923 K to 835.51 ± 7.66 and 625.89 ± 24.95 mN·m⁻¹ at 1073 K, respectively. For the tungsten substrate, the liquid–solid IFT values decrease from 669.83 ± 20.19 mN·m⁻¹ at 923 K to 727.06 ± 12.10 mN·m⁻¹ at 1013 K. The relatively low liquid–solid IFT values between the Al 7075-T6 alloy droplet and the nonporous alumina substrate indicate the weak attraction between them. Unlike for other substrates, this weak attraction allows the droplet to wet the substrate more, indicating that it can spread out and adhere more effectively to the substrate. This phenomenon accounts for the relatively low contact angles that the liquid Al 7075-T6 alloy droplet makes with the nonporous alumina substrate. The measured liquid–solid IFT values are in decreasing order of porous alumina, tungsten, and nonporous alumina for the studied temperature ranges. Also, for the experiments conducted on the porous alumina, tungsten, and nonporous alumina substrates, the liquid–solid IFT–temperature dependence equations for the investigated temperature ranges are displayed in Figure 9. The increase in the liquid–solid IFT with temperature arises directly from the observed experimental trends for the contact angle and surface tension and the employed calculation method (Equation (2)). According to Equation (2), the liquid–solid IFT was calculated using the substrate’s SFE, the surface tension of the liquid, and the contact angle. For the three substrates, the contact angle (Figure 3) and surface tension (Figure 4) decreased and increased with temperature, respectively. Since surface tension increases significantly with temperature and the decrease in cos θ is relatively modest, the net calculation effect is an increase in the liquid–solid IFT with temperature. Thus, the observed trend in the liquid–solid IFT reflects the combined impact of the increasing surface tension and the limited decrease in contact angle over the studied temperature range.

3.5. Work of Adhesion

The work of adhesion characteristics between the liquid Al 7075-T6 alloy droplet and the different substrates and the corresponding uncertainties at different temperatures are presented in Figure 10. The work of adhesion values of liquid Al 7075-T6 alloy on the tungsten substrate and the other substrates were computed in a range of 90 K (923–1013 K) and 150 K (923–1073 K), respectively. For the porous and nonporous alumina substrates, the work of adhesion values increase from 123.97 ± 5.47 and 341.46 ± 11.99 mN·m⁻¹ at 923 K to 152.31 ± 7.66 and 479.44 ± 27.50 mN·m⁻¹ at 1073 K, respectively. For the tungsten substrate, the work of adhesion values increase from 200.76 ± 15.69 mN·m⁻¹ at 923 K to 240.72 ± 6.84 mN·m⁻¹ at 1013 K. Due to the low liquid–solid IFT between the droplet and the nonporous alumina substrate, the droplet is relatively more wettable and adheres to the substrate, which explains its relatively higher and lower work of adhesion and contact angle on the substrate, respectively. Unlike other substrates, the higher work of adhesion between the liquid Al 7075-T6 alloy droplet and the nonporous alumina substrate was evident by visual inspection, as the droplet adhered permanently to the substrate after cooling to room temperature (22 °C). This trend indicates that weak attractive forces exist between the droplet and the nonporous alumina substrate, which makes the droplet more wettable, thereby adhering more. Also, unlike for other substrates, this trend reflects stronger bonding at the liquid–solid interface, which is consistent with the improved wettability. This behavior can be attributed to the high polar component of the SE characteristics of the nonporous alumina substrate (Table 4). The polar nature of the alumina substrate surface favors strong interactions with the liquid metal, enhancing adhesion. The computed work of adhesion values are in decreasing order of nonporous alumina, tungsten, and porous alumina for the studied temperature ranges. Also, for the experiments conducted on the porous alumina, tungsten, and nonporous alumina substrates, the work of adhesion–temperature dependence equations for the investigated temperature ranges are expressed in Figure 10. Meanwhile, the increase in the work of adhesion with temperature for all three substrates can be attributed to the experimental trends of the surface tension and contact angle. Since the work of adhesion depends on the product of surface tension and (1 + cos θ) (Equation (5)), for all substrates, the increase in surface tension with temperature dominates over the modest change in contact angle, resulting in higher adhesion values at higher temperatures.

The work of adhesion represents the work done, or FE increment, in separating the interface into separate liquid and solid surfaces [30]. The work of adhesion depends on the liquid droplet’s surface tension, the substrate’s SE, and the IFT between the liquid droplet and the substrate, as shown in Equation (6):

$$W_a=∆F_{\mathrm{separation}}={\gamma }_{sv}+{ \gamma }_{lv} - {\gamma }_{ls},$$

(6)

where ${ \gamma }_{lv}$ is the surface tension of the liquid droplet, ${\gamma }_{sv}$ is the substrate’s SFE, and ${\gamma }_{ls}$ is the liquid–solid interfacial energy between the liquid droplet and substrate. The work of adhesion is enhanced by interfacial chemical reactions, high metal–oxide bond strength, and favorable surface and interface energies. Thus, although the interfacial reactions between the droplet and the substrates are not studied herein, it can be said that the higher work of adhesion between the droplet and the nonporous alumina substrate can be attributed to the increased interfacial reactions between them.

3.6. Energy-Dispersive X-Ray Spectroscopy

Table 5. Surface and bulk compositions of post-processed Al 7075-T6 alloy samples tested on different substrates.

Elements	Porous Alumina Substrate		Tungsten Substrate		Nonporous Alumina Substrate
	Surface Composition (wt.%)	Bulk Composition (wt.%)	Surface Composition (wt.%)	Bulk Composition (wt.%)	Surface Composition (wt.%)	Bulk Composition (wt.%)
C	12.49	13.98	17.99	15.70	24.02	21.79
O	18.88	0.56	13.28	1.03	14.89	1.35
Al	63.69	82.73	65.45	80.01	59.22	74.70
Fe	1.03	0.22	0.88	0.37	0.21	0.12
Cu	2.96	1.85	2.17	2.37	1.52	1.72
Cr	0.05	0.28	0.05	0.33	0.10	0.08
Mn	0.03	0.11	0.05	0.05	-	0.03
Mg	0.80	-	0.10	0.03	-	-
Si	0.05	0.27	0.03	0.11	-	0.16
Ti	0.02	-	-	-	-	0.06
V	-	-	-	-	0.05	-

displays the post-test surface and bulk composition of the Al 7075-T6 alloy samples whose surface tension values were measured on the three substrates. Here, the main elements examined are C, O, and Al. It is presumed that other elements are in trace amounts and have little or no effect on the surface tension values obtained herein. C was present in the surface and bulk compositions of all tested samples in slightly different amounts. The presence of C is likely due to the sensitivity of the EDS detector to dust particles or from the prolonged use of the porous and nonporous alumina substrates, which contain low amounts of C, in the furnace before conducting the experiments reported herein. Thus, with the use of these substrates over time in measuring the surface tension of other alloys, it is presumed that a buildup of carbon deposits would have occurred. Note that before the experiments were conducted, the furnace was baked by heating to 1800 °C and subsequently cooling down to room temperature (22 °C) under vacuum of 10⁻⁶ Torr to eliminate all residue and contaminants stemming from prior experiments, during which an outgassing phenomenon was experienced. Since C is not a component of Al 7075-T6 alloy, it is thus regarded as an impurity, which would affect the surface tension results reported herein. Although the experiments were conducted under an argon atmosphere, the EDS results confirm the presence of oxygen and corroborate its above-explained effect on the surface tension results. Also, the surface compositions of O exceed the bulk compositions for all tested samples, which is likely due to the formation of aluminum oxide, which did not dissociate because high temperatures above ~1373 K were not attained. Furthermore, for all tested Al 7075-T6 alloy samples, the bulk compositions of Al exceed the surface compositions due to the formation of aluminum oxide on the surface, which acts as a skin coating.

4. Conclusions

In this study, thermophysical properties, such as wettability, surface tension, liquid–solid IFT, and work of adhesion, of commercial Al 7075-T6 alloy were characterized in a relevant processing range (from 923 to 1073 K) under an argon atmosphere as a function of three substrates: prebaked porous high-refractory alumina, tungsten, and nonporous high-refractory alumina substrates using the high-temperature SD technique. Furthermore, the room-temperature SFEs of the substrates were characterized using the OWRK model by the room-temperature SD technique, which considers the geometric mean of the dispersive and polar parts of the employed liquid’s surface tension and the solid substrate’s SFE, respectively. Finally, post-processed samples were analyzed using EDS to determine the compositional changes incurred during the surface tension experiments and investigate the presence of impurities. The main conclusions of this study are as follows:

• The contact angle results over the investigated temperature ranges reveal the poor wettability characteristics of liquid Al 7075-T6 alloy on the three substrates in decreasing order of porous alumina, tungsten, and nonporous alumina.
• The surface tension values ranged from 718.87 to 942.90 mN·m⁻¹ in decreasing order of tungsten, porous alumina, and nonporous alumina and are close to the few literature-reported results.
• The SFE measurement results revealed that the substrates’ SFE values differ slightly in decreasing order of porous alumina, nonporous alumina, and tungsten. Surprisingly, unlike other substrates, the polar SFE component of the nonporous alumina substrate exceeds the dispersive component.
• The calculated liquid–solid IFT values as a function of the three substrates range from 539.24 to 835.51 mN·m⁻¹ in decreasing order of porous alumina, tungsten, and nonporous alumina. Unlike other substrates, the relatively low liquid–solid IFT values between the liquid Al 7075-T6 alloy droplet and the nonporous alumina substrate indicate the weak attraction between them, which permits more wetting and adhesion.
• The calculated work of adhesion values of the liquid Al 7075-T6 alloy droplet on the substrates range from 123.97 to 479.44 mN·m⁻¹ in decreasing order of nonporous alumina, tungsten, and porous alumina. Unlike other substrates, the droplet is relatively more wettable and adheres to the nonporous alumina substrate due to weak attractive forces, thereby exhibiting higher work of adhesion and lower contact angle characteristics.
• Furthermore, for all experiments conducted, positive surface tension–temperature, liquid–solid interfacial energy–temperature, and work of adhesion–temperature gradients were obtained.
• Although the experiments herein were conducted under an argon atmosphere, the EDS results confirm the presence of oxygen and corroborate its effect on the surface tension values reported herein. The surface compositions of oxygen exceeded the bulk compositions for all tested samples due to the formation of aluminum oxide.