Forming quality and wettability of surface texture on CuSn10 fabricated by laser powder bed fusion

The energy crisis has brought significant challenges to countries worldwide since the beginning of the 21st century. In order to relieve the pressure caused by the shortage of conventional resources, people have begun to use marine resources more effectively. The research and development of materials for marine transportation, seawater desalination, petrochemicals, and offshore power generation are progressing rapidly. The marine environment comprises seawater, dissolved gases, organic matter, and micro-organisms.¹ In order to apply marine materials in such a complex and harsh environment, higher all-around performance is in demand.

Copper and copper alloys are widely used in the maritime industry for good seawater corrosion resistance.^2,3 In addition, the copper ions dissolved in water have bactericidal effects and can prevent marine organisms from fouling.⁴ They are mainly used as propellers, condensers, pump valves, and structural parts on ships and in seawater piping systems. To meet the continuous development of large-scale and high-speed ships, stricter requirements are put forward for copper and copper alloy’s properties and processing technology.

In order to further increase the application range and improve the service performance of copper and copper alloys, some researchers have modified them from the perspective of materials to improve their adaptation to the complex marine environment. Nadolski⁵ pointed out that heat treatment and addition of up to 10% Sn can refine the grain size of Cu–Sn bronze and improve its wear resistance. Wang et al.⁶ indicated that the Laser Powder Bed Fusion (LPBF)-manufactured Cu–15Ni–8Sn alloy can significantly diminish Sn segregation, possessing homogeneous component distribution and resulting in a good combination of toughness and strength, with elongation A being 19.8% and Rm reaching 593.3 MPa.

It is also an effective way to improve the performance by changing the surface structure of parts. Surface texture has aroused widespread interest in the academic and industrial circles at home and abroad due to its role in controlling friction, reducing wear, and improving lubrication performance.⁷ It provides a new direction for the research of cutting tool friction reduction technology. The surface texture effect is a thermodynamic effect that changes the friction, wear, and lubrication characteristics of the surface when the micro-pits and micro-grooves with different shapes,⁸ geometric parameters,⁹ and distribution characteristics¹⁰ are processed on the surface of the friction pair. Ding et al.¹¹ theoretically uncovered the effects of the morphology parameter and the intrinsic property of the microstructure surface on droplet stability by comparing the three micro-scale structures: the cone, frustum (upright and inverted), and sphere. The results showed that both the morphology parameters and intrinsic contact angle of the microstructure surface affect the wetting depth and normalized Helmholtz free energy of the droplet. He et al.¹² investigated the droplet wetting behaviors on a trapezoidal microarchitecture surface by establishing a three-dimension thermodynamic model that considered the three-phase contact line tension. Pratap and Patra¹³ compared semi-hemispherical end dimples, flat end dimples, and conical dimples. The results showed that semi-hemispherical end dimples are more wettable due to their reducing ridge slope, promoting the droplet’s inflow. However, the micro-dimpled geometry produces isotropic wetting. In Davoudinejad et al.’s work,¹⁴ components with micro-holes of different sizes and uniformity were manufactured and analyzed. The results validated that proper microstructures can help improve a hydrophilic surface (with a contact angle of 65°) to a hydrophobic surface (with a contact angle of 113°).

Additive manufacturing refers to the technology of accumulating and stacking materials point by point and layer by layer through discrete-stacking to form a three-dimensional entity of arbitrary shape.¹⁵ As one of the most promising technologies in laser additive manufacturing, Laser Powder Bed Fusion (LPBF) can manufacture copper and copper alloys with different structures. It has natural and unique advantages in manufacturing surface texture. In addition, Damian et al. evaluated the use of Selective Laser Melting (SLM) techniques to produce models of structures with specific surface morphological features, and investigated the effect on roughness by changing the orientation of model building with Ti6Al4V powder.¹⁶ It has guiding significance for the development of this research. However, experiments to monitor the LPBF process are difficult due to the complicated physical processes at microsecond and micrometer sizes. Predictive numerical simulation is an appealing solution for revealing the physical processes underlying LPBF faults.^17,18

In laser melting, it is difficult for the laser to continuously melt the copper alloy powder due to the low energy absorption rate,¹⁹ which leads to problems such as low forming efficiency and unmanageable metallurgical quality. Furthermore, few studies are focused on the surface texture of additive manufacturing of CuSn10. Therefore, in this work, based on numerical simulation techniques, the powder bed model was established, and the thermal distribution and morphology of powder bed under different laser energy inputs were explored. Then the optimum process parameters for LPBF forming CuSn10 were selected. In addition, different surface texture models (square texture, circular texture, hemispherical texture, and triangular texture) were established based on the 3D software SolidWorks and fabricated by LPBF additive manufacturing technology. Through the structural size, surface morphology, and contact angle of the surface texture, the forming precision and the wetting performance of the LPBFed micro-surface texture were studied. The findings of this study provide theoretical and experimental support for further application of LPBF-processed surface texture of CuSn10.

Based on Flow-3D, a computational fluid dynamics (CFD) model was developed to investigate the complex physical phenomena during LPBF processing. To capture the dynamic geometry of the free surface gas/metal interface, the volume of fluid (VOF) method²⁰ was used. In this study, the fluid was assumed as Newtonian with laminar flow. The equations to describe the conservation of mass, momentum, and energy are expressed as follows:^21–25 

where ρ is the density; V is the velocity; P is the pressure; p_st, p_evp, and p_M are the momentum source terms induced by surface tension pressure, evaporation recoil pressure, and Marangoni shear stress, respectively; p_pt and p_bg are the momentum source terms induced by phase transformation and gravity and buoyancy force, respectively; k is the thermal conductivity; H denotes the enthalpy; q_l denotes the laser heat source term; q_pt is the phase-transformation-induced heat source; and q_evp, q_r, and q_c are the heat loss terms induced by evaporation, radiation, and convection, respectively. It is worth noting that the mass sink due to evaporation was overlooked.

The VOF was used to track the free surface of the liquid–gas interface. Each cell was assigned a scalar, F (0 < F < 1), to represent the fluid fracture. The fluid volume fracture F can be calculated as²⁶ 

The melting behavior and solidification behavior were modeled using the enthalpy-porosity method.^27,28 The liquid volume fraction is denoted by

where T_l and T_s are the liquidus and solidus temperatures, respectively, and T is the temperature field of the computation domain.

The simulation was carried out using a 3D calculation domain with a size of 1000 × 400 × 250 µm³. A 120 µm tall substrate and 30 µm tall powder layer were used to track the free surface during laser scanning in the domain. The Discrete Element Method was used to calculate the particle coordinates and sizes, which were then imported into the powder layer. To account for the ambient temperature, the initial temperature was adjusted to 300 K. All of the surfaces’ boundary conditions were deemed to be continuous (zero normal derivative). The simulation material was chosen to be CuSn10. The properties of CuSn10 are presented in Table I, and based on a large number of previous experiments, the data used for simulation are presented in Table II.

Figure 1 shows the thermal distribution and morphology of the powder bed under different laser energies. It can be found that in the molten pool, the closer it is to the center of the high-energy laser beam, the higher the temperature. Furthermore, the overall morphology of the molten pool presents a “droplet” shape, which is caused by the existence of voids and gases between the powder particles, resulting in a significantly lower thermal diffusion rate of the un-melted powder layer than the solidified region. Low laser energy input prevents the formation of a stable and continuous melt pool, resulting in a significant number of pores or holes inside the printed solidified layer. The size of the molten pool grows dramatically as the input laser energy increases, but the pores or holes shrink. Although the molten pool is generally steady and continuous, its surface is nevertheless uneven, as shown in Fig. 1(c). When the laser energy is increased to 0.4 J/mm, the pores or holes vanish, the molten pool expands, and the molten pool’s surface becomes more stable, as shown in Fig. 1(d). The temperature of the molten pool increases dramatically as the input laser energy is increased, which can readily lead to the intensification of the Marangoni effect, and the severely heat-affected region can be plainly observed from Figs. 1(e) and 1(f).

Gas-atomized pure Cu–10Sn alloy powder (Xi’an Bright Additive Manufacturing Co. Ltd.) with a minimum purity of 99.9 wt. % was used as the feedstock, as shown in Fig. 2(a). The size distribution is from 10 to 45 μm. The LPBF machine equipped with a single-mode fiber laser of 500 W and wavelength of 1064 nm was adopted to fabricate the Cu–10Sn samples on pure copper substrates. The laser beam is focused to a spot size of ∼100 µm. The chamber is full of high purity argon atmosphere to prevent oxidation in the whole process. According to the numerical simulation results, the laser power, laser scanning speed, hatch distance, and layer thickness were fixed at 320 W, 800 mm/s, 80 µm, and 30 µm, respectively. During the LPBF process, a bi-directional scanning strategy of 67° rotation between the two contact layers shown in Fig. 2(b) was used for obtaining low and uniform residual stress and strain.²⁹ The CAD modules and LPBFed parts are shown in Figs. 2(c) and 2(d). The dimension of the CAD model is a cylinder with a diameter of 40 mm and height of 6 mm, with the area of texture being 26.5 × 26.5 mm². In addition, the single size of the four different textures is square texture with a side length of 1 mm and depth of 2 mm, circular texture with a diameter of 1 mm and depth of 2 mm, hemispherical texture with a diameter of 1 mm, and triangular texture with an included angle of 60°, side length of 1 mm, and depth of 2 mm.

The LPBF-processed samples were ground by SiC grinding paper of different grit sizes (600, 800, 1200, 1500, 2000, 2500, and 3000 grits, in sequence) and polished by polycrystalline diamond suspension (6, 3, and 1 µm, in sequence). The microstructures were characterized using a profilometer (MFP-D, RTEC Instruments, USA) and scanning electron microscope (GeminiSEM 300, ZEISS, Germany).

Each sample’s actual contact angles have been measured with a tensiometer (Shanghai Zhongchen Digital Technology Apparatus Co. Ltd.) using de-ionized water after being cleaned using a liquid acetone bath. The optimal volume of the droplet is measured as 5 µl. The contact angles have been tested five times with a sessile droplet at different locations for each sample while keeping the droplet volume constant.

Figure 3 shows the CAD model of different textures, the printed samples’ surface morphology, and the measurement data. It can be found that the formation accuracy of different surface textures is quite different. Among them, the sample’s morphology with a square texture is the closest to the original CAD model. Circular and hemispherical textures have lower structural accuracy than the square texture. This phenomenon may be inseparable from the uniform curve of the texture opening. The closer it is to the upper surface, the larger the opening size, and the smaller the heating and melting area so that the energy can be evenly dissipated to the formed part and the substrate during laser processing. Besides, the step effect during LPBF processing of circular and hemispherical textures is also a key factor for lower structural accuracy. However, the morphology of the sample with a triangular texture surface shows the worst structural accuracy compared with the original model since the triangle texture has an included angle of 60° and is formed vertically upward; as the depth of the triangle increases, the heat dissipation efficiency of the powder is much lower than that of the forming part. It can be predicted that the heat dissipation area will decrease. In addition, it is also affected by the triangular texture shape, causing excessive concentration of laser energy in laser processing and resulting in the inability to dissipate heat to the forming part quickly. In order to more intuitively characterize the accuracy of the texture structure, the design-specific and measured shape parameters are recorded in Table III.

Figure 4 shows the scanning electron microscope (SEM) morphology of the surface with different textures. It can be seen that there is a severe powder adhesion at the texture boundary, which affects the surface quality of the LPBF formed texture. At the same time, this is also the reason for the decrease in the accuracy of the formed structure. The powder adhesion at the side of the square texture, the round texture, and the triangle texture is particularly more severe than that of the hemispherical texture, which is closely related to the vertical depth of the texture design. With the appearance of the texture, the melting area decreases, and the boundary increases during the LPBF process. This phenomenon is caused by the fact that the heat dissipation of the powder is much lower than that of the forming part, and the molten pool is unstable, which further aggravates the powder adhesion effect. In addition, a large number of defects on the forming surface can be found, and micro-cracks also appear inside the hemispherical texture. This is caused by the unevenness of the laser energy distribution due to the hemispherical texture shape and the step effect of additive manufacturing. The influence of the angle factor from the triangles reduces the printing accuracy and intensifies the step effect phenomenon. The step effect mechanism of the circular structure and triangular structure is illustrated in Fig. 5.

Surface roughness refers to the small spacing and small peaks and valleys of the machined surface.³⁰ Because the distance between the two peaks or valleys (wave distance) is minimal, surface roughness belongs to the microscopic geometric shape error. The surface roughness is generally formed by the processing method. Due to the particularity of the LPBF processing method, it relies on the high-energy laser beam to melt the powder.³¹ The forming process is a coupled process involving the temperature field, stress field, and other multi-physical fields.³² Due to the complex process of rapid heating and cooling, the structure of the prepared part appears as a non-equilibrium structure,³³ so the traces on the surface of the part is prone to warping deformation, low dimensional accuracy, powder adhesion, even cracking, and other macroscopic defects. At the same time, surface roughness can also be used to reflect the internal formation quality.³⁴ The higher the surface roughness, the worse the internal quality of the formed part. Figure 6 shows the three-dimensional morphology of the typically measured surface roughness of samples with different texture structures. It can be found that different surface texture shapes have different degrees of influence on the surface roughness of the sample. This is inseparable from the formation process characteristics of layer-by-layer manufacturing of LPBF. The surface textures of different shapes correspond to different areas of the surface to be processed. At the same time, it means that the input laser energy of the textured structure layer and the non-structured layer is different, and the heat transfer efficiency of the powder layer is much lower than that of the formed metal, resulting in different thermal effects and significant differences in surface roughness. In addition, it can be seen from the three-dimensional morphology that the surface of the sample with hemispherical texture is the flattest. However, the sample’s surface with triangular texture has the most apparent fluctuation, which is consistent with its structural accuracy. Gwyddion software was used to calculate the actually measured surface roughness, which is then recorded in Table IV. The order of the average surface roughness of the samples with different textures is triangle texture (Ra = 13.69 µm) > circle texture (Ra = 10.72 µm) > square texture (Ra = 9.10 µm) > hemispheric texture (Ra = 5.58 µm).

The contact angle refers to the angle formed by the solid surface in contact at the liquid–gas interface.³⁵ Its value depends on the attributes of the liquid and solid surface and represents the wettability of the liquid on the solid surface. The larger the contact angle, the more hydrophobic the surface of the material. Conversely, the smaller the contact angle, the more hydrophilic the surface of the material.

The wettability of the surface texture can be considered as the wettability on the rough surface. The Wenzel model³⁶ and the Cassie model³⁷ are the two most commonly used and recognized theoretical models for wettability. Wenzel believed that the presence of the rough surface makes the actual solid–liquid contact area larger than the observed surface area and assumed that the droplets completely fill the nanostructures and wet the wall. The apparent contact angle (θ_w) and intrinsic contact angle (θ_e) of the wetted surface satisfy the following relationship:

Cassie and Baxter further extended the Wenzel equation, thinking that the contact of the droplet on the rough surface is a composite contact. When the surface is relatively hydrophobic, the droplet cannot completely wet the groove, and there is air between the droplet and the surface. Therefore, the apparent solid–liquid contact surface is actually composed of a solid–liquid contact surface and a gas–liquid contact surface. The apparent contact angle (θ_c) of the composite surface can be expressed as

According to the theory by Wenzel, Cassie, and Baxter, the roughness factor r is defined as the ratio of the actual area of the liquid droplet in contact with the solid surface to the projected area, and the phase area fraction f represents the ratio of the actual solid surface in contact with the liquid. The calculation formulas for r and f are

where A_sl is the contact area of the solid–liquid interface, A_lg is the area of the liquid–gas interface, and A_pr is the projection area.

Through the above-mentioned theoretical analysis, the apparent contact angle of the surface texture was calculated and predicted under Cassie and Wenzel conditions. The specific theoretical results are shown in Table V.

Figure 7 shows the contact angles of samples with different surface textures. Due to the influence of the surface texture of the sample, the contact area between the droplet and the surface of the sample will change, while the air in the texture prevents the droplet from sinking, which are the reasons behind the change in the contact angle. However, because the texture area changes inconspicuously relative to the size of the liquid, the contact angle does not change significantly. The corresponding contact angles of square texture, circular texture, hemispherical texture, and triangular texture are 85.59°, 81.84°, 78.22°, and 73.94°, respectively. Comparing the theoretically calculated apparent contact angle with the experimentally tested contact angle, it can be found that the experimentally tested contact angle is between the theoretically contact angle under the Cassie condition and Wenzel condition.

In this research, based on numerical simulation techniques, the powder bed model was established, and the thermal distribution and morphology of the powder bed under different laser energy inputs were explored. Then the optimum process parameters for LPBF forming CuSn10 were screened, which can not only quickly respond to the LPBF forming process, but also save the process cost of exploring LPBF forming CuSn10. Hence, CuSn10 parts with different surface textures were successfully prepared using laser powder bed fusion technology. The structural accuracy, surface morphology, surface roughness, and wettability of different surface textures were studied, and the following main conclusions were obtained:

1. The temperature near the center of the laser is the highest, and the molten pool is in the shape of a “droplet.” A stable and continuous molten pool cannot be generated when the laser energy is less than 0.4 J/mm, and there are pores or holes in the molten pool. The molten pool tends to stabilize as the laser energy increases, and the pores or voids disappear, while the heat-affected zone expands.

2. The printed square texture part has the highest structural accuracy of all samples compared to the original design model. The average errors of top width, bottom width, and depth are 11.1%, 42.2%, and 2.5%, respectively. However, the structural accuracy of parts with the triangular texture is the worst.

3. Powder adhesion occurred on all texture boundaries, which inevitably reduced the quality of texture formation. The average surface roughness of the hemispherical texture is the lowest among all parts, as low as 5.58 µm, while the average surface roughness of parts with the triangular texture is as high as 13.69 µm.

4. When affected by the morphology of the molten pool formed by the LPBF high-energy laser beam, it will cause severe step effects when processing irregular textures with angles (such as triangular or circular textures).

5. Surface texture is closely related to wettability. Different surface textures mean that the contact area between the measured droplet and the sample is different. The square texture has the largest opening area, accompanied by the most prominent contact angle of 85.59°, and the triangular texture has the smallest contact angle of only 73.94°.

This project was supported by the National Scholarship Fund, the National Natural Science Foundation of China, and the Exploration and Practice of Construction Path of Ideological and Political Demonstration Major in Higher Vocational Courses-Mechanical Manufacturing and Automation (Grant Nos. 51675232, 51775244, and ZBY675).

The authors have no conflicts to disclose.

Lu Min: Writing – original draft (equal). Shi Xiaojie: Writing – review & editing (equal). Lu Peipei: Conceptualization (equal); Methodology (equal). Wu Meiping: Investigation (equal); Software (equal).

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