Study on the Surface Quality of Overhanging Holes Fabricated by Additive/Subtractive Hybrid Manufacturing for Ti6Al4V Alloy

Abstract

Additive/subtractive hybrid manufacturing (ASHM) based on laser powder bed fusion (LPBF) enables to achieve high precision and good surface quality of complex structures such as small holes with overhanging features. However, the rapid heating and cooling rates during the ASHM results in sinkage at the alternating interface of additive manufacturing and subtractive milling, which degrades the surface quality of the components. This study employs shielding height at the alternating interface to solve this problem and improve the surface quality. The effect of internal diameters and shielding heights on the surface quality were studied experimentally for overhanging holes fabricated by ASHM of Ti6Al4V. The results show that the Ti6Al4V samples prepared by LPBF possessed high density and uniformly distributed microstructure. For overhanging holes without shielding height, the largest depth value of sinkage and surface roughness were obtained, indicating a worse surface quality; when the shielding height was increased to 0.5 mm, the smallest sinkage value and surface roughness were obtained, indicating a better surface quality. With the same shielding height, the overhanging holes with different diameters had a similar surface roughness. This study reveals that an appropriate shielding height can improve the surface quality, which provides guidance to the improvement of the surface quality for complex structures in ASHM.

1. Introduction

Laser powder bed fusion (LPBF), a typical additive manufacturing technology, involves slicing digital models and using high-energy density laser beam to selectively melt powder layer by layer to manufacture components [1,2,3]. It has wide application in the production of complex metallic components [4,5,6]. However, issues such as powder adhesion and staircase effects can result in poor surface quality of components, which severely limits the widespread application of LPBF [7,8,9].

In previous investigations, many methods have been employed to enhance the surface quality in additive manufacturing process. Balbaa et al. [10] reduced the surface roughness (Sa) of LPBF-manufactured components to Sa 3.75 μm by optimizing forming parameters and powder size distribution. Adaptive slicing optimization was performed by Ding et al. [11] using wire-feed laser directed energy deposition (LDED), and the surface quality was improved to Ra 10 μm. The above study shows that the surface quality of additive-manufactured components can be improved by reasonable parameter optimization. However, it still cannot meet the engineering requirements. As a result, many researchers have begun to investigate the improvement of surface quality of additive manufactured components through various finishing techniques, such as electrochemical machining (ECM) [12] and electrochemical polishing (ECP) [13]. However, these methods have poor electrode accessibility in deep hole structures [14]. Additive/subtractive hybrid manufacturing (ASHM) integrates the advantages of additive manufacturing and traditional machining [15,16]. During the ASHM process, metal powder is melted by a high-energy density laser beam to achieve additive manufacturing and a three-axis subtractive system is employed for milling operations on parts that have undergone additive manufacturing. The alternation of additive and subtractive processes can lead to the high surface quality of complex structures [17]. However, when manufacturing structures with overhanging features, sinkage was found at the alternating interface between additive and subtractive processes, which increases surface roughness.

It is very significant to study how to improve the surface quality at the interface of additive and subtractive alternation. Amine et al. [18] also identified the sinkage phenomenon in ASHM utilizing directed energy deposition (DED) and they introduced a strategy involving the implementation of a shielding height to improve surface quality. Studies by Gao et al. [19] found that as the overhang angle increased, smaller shielding height was needed to ensure the surface quality. The above studies have shown that the surface quality at the interface of ASHM can be improved by setting the shielding height. However, these studies were limited to the outer contour of the components and it is not clear whether setting the shielding height ensure the surface quality of overhanging holes manufactured by ASHM. Additionally, the overhanging holes feature varying overhang angles, and the effective machining areas of the milling cutter within these holes change throughout the machining process. However, it remains unclear whether a consistent surface quality can be achieved using the same shielding height and how the surface quality might vary with different shielding heights. Therefore, it is necessary to study the surface quality of overhanging holes at different shielding heights.

Ti6Al4V alloy is widely used in aerospace, biomedical, and marine engineering due to its excellent mechanical properties, biocompatibility, and corrosion resistance [20,21]. However, its susceptibility to oxidation, poor thermal conductivity, and other physical properties makes it more difficult to machine than traditional metal alloys [22,23]. In addition, high surface quality and geometric accuracy are required in aerospace and biomedical components [24]. Consequently, it is very suitable for processing by the ASHM method.

In this work, Ti6Al4V overhanging holes with different diameters were manufactured by ASHM. Four different shielding heights were set to study the effect on the surface quality for obtaining the optimal shielding heights. Two different hole diameters were used to explore the effect on the surface quality at the same value of shielding heights. This study provides guidance to the determination of the shielding height for overhanging holes manufactured by ASHM. It could help to promote ASHM technology in the manufacturing of complex components of Ti6Al4V alloy.

2. Method and Experimental Procedure

2.1. Mechanism of Sinkage Generation

Figure 1 illustrates the mechanistic diagram of sinkage depth generation and inhibition of sinkage depth by adding shielding heights in ASHM. Figure 1a shows the mechanism of sinkage generation during ASHM. The surface of overhanging holes exhibits desired surface with high surface quality after the Nth machining process. Subsequently, the N + 1th additive manufacturing process is started, in which the additive part is heated to a high temperature. When N + 1th additive manufacturing process is completed, the material cools down and the Nth machining surface experiences serious contraction at the interface, resulting in a sinkage. In the N + 1th machining process, the additive part is machined to desired surface. Consequently, sinkage is generated at the alternating interfaces of ASHM. Gao et al. [19] also illustrated that the sinkage during the alternating process was generated by thermal influence. By adding shielding height to separate the desired surface away from the subsequent additive manufacturing, the effect of heat input from additive manufacturing on the desired surface is reduced, and contraction of the desired surface during the alternating process is suppressed, as shown in Figure 1b. Therefore, the sinkage depth is reduced and the surface quality is improved. In this study, four different shielding heights were set: 0 mm, 0.1 mm, 0.3 mm, and 0.5 mm.

2.2. Experimental Procedure

Overhanging features existed in overhanging holes, which are known as undercut in traditional machining [25]. Machine tools limitation make such features difficult to machine, especially on three-axis machines. The overhang angle α value of overhanging holes were selected at 70°, which is between 0° and 90°. A typical feature of overhanging holes produced by ASHM is shown in Figure 2, and the dimension of the overhanging holes is shown in

Table 1. Dimensions for overhanging holes.

Parameters	L (mm)	α (°)	D (mm)	A (mm)	B (mm)
Value	10	70, 110	6, 8	10	10

. As shown in Figure 2, the red part is an overhang surfaces with an angle of 70°, while the blue part is an overhang surface with an angle of 110°. Besides, an overhanging hole is divided into four parts. The values of h₁, h₂, h₃, and h₄, represent the shielding heights of 0 mm, 0.1 mm, 0.3 mm, and 0.5 mm, respectively, used in the machining process. Two overhanging holes of different diameters were manufactured to investigate the effect of hole size on the surface quality. The ASHM process is a discontinuous additive process in which the manufactured portion is reduced to a lower temperature during the machining process and then heated to a high temperature when the additive manufacturing is resumed. In order to study the continuity in microstructure at the interface, a 10 × 10 × 10 mm cube (shown in Figure 2) was fabricated along with the overhanging holes to characterize the microstructure of Ti6Al4V alloy fabricated by ASHM.

As the ASHM machine uses a three-axis machining system, it is difficult to machine the overhanging surface with a normal flat milling cutter. Therefore, in this experiment, a T-shaped fillet milling cutter was used as shown in Figure 3, which enables to manufacture the vertical side as well as the overhang surfaces. The tool fillet increases the stiffness of the cutting edge and reduces the possibility of tool chipping. The geometric dimensions of the T-shaped fillet milling cutter tool are shown in

Table 2. Dimensions of the T-shaped fillet milling cutter (mm).

Parameters	L1	L2	L3	D1	D2	R
Value	50	1	5	2.5	4	0.5

.

In this study, gas-atomized Ti6Al4V powder (Avimetal Powder Metallurgy Technology Co., Ltd., Beijing, China) with particle size ranging from 15 μm to 53 μm was used and the morphology is shown in Figure 4a. It can be seen that gas-atomized Ti6Al4V powder has an excellent sphericity, and it has a good flowability during the ASHM process. The chemical compositions of Ti6Al4V powder are shown in

Table 3. Chemical compositions of Ti6Al4V powder (wt.%).

Elements	Ti	Al	V	Fe	C	O	V	N
(wt.%)	Bal	6.04	4.1	0.15	0.014	0.06	0.11	0.011

. The overhanging holes with different diameters were performed on an ASHM machine (Sodick OPM250L, Sodick Co., Ltd., Yokohama, Japan); the schematic diagram of this machine is shown in Figure 4b. This ASHM machine was combined with a LPBF system and a high-speed machining system. The process parameters of LPBF and machining systems are shown in

Table 4. Process parameters of LPBF and milling.

Parameters	Value
Laser power, P (W)	260
Spot diameter, d0 (μm)	200
Layer thickness, d (μm)	50
Laser scanning speed, v (mm/s)	900
Spindle speed, r (rpm)	35,000
Feed rate, vf (mm/min)	500

. Prior to start the experiment, Ti6Al4V powder was placed in a vacuum drying oven and held at 100 °C for 5 h to remove moisture. A titanium plate of 125 mm × 125 mm × 20 mm was used as the substrate during the ASHM process. During the LPBF process, a strip scanning strategy with 90° rotation between the adjacent scanning layers was used [26]. At the beginning of this experiment, titanium substrate was preheated to 80 °C and argon (Ar) gas was injected into the forming chamber to reduce oxygen content less than 0.5% to prevent oxidation of Ti6Al4V.

2.3. Determination of Slab Height H

In order to improve the manufacturing efficiency, machining is conducted after several layers of LPBF. The alternation timing between LPBF and machining is particularly important in the ASHM process for components with overhanging features. A high LPBF slab height greatly increases the risk of interference between the cutter and manufactured workpiece in the subsequent machining process, as shown in Figure 5a. A low LPBF slab height results in a higher machining frequency, which reduces the manufacturing efficiency, as shown in Figure 5b. Furthermore, a low slab height also increases the number of alternating interfaces, and deteriorates the surface quality of ASHM-manufactured components.

The slab height is the maximum continuous LPBF height, which is essentially the maximum height at which the tool just interferes with the components during machining. In this study, the tool used in ASHM was a T-shaped fillet milling cutter. The overhang angle of the holes was 70°. According to the tool size and the overhang angle, a safe collision clearance between the tool and the overhanging hole was set as x = 0.2 mm. In order to facilitate the calculation of the slab height, the tool-component collision was simplified to a two-dimensional model. Therefore, the mathematical model of the tool-component collision was established. The slab height H could be expressed as:

$$H=\left[{\displaystyle \frac{1}{2}}\times \left(D_2-D_1\right)-R+R·sin\alpha -x\right]\times \tan \alpha$$

(1)

where D₂ is the tool diameter, D₁ is the tool shank diameter, R is the radius of the tool fillet, and α is the overhanging angle. Therefore, the slab height H = 1.428 mm can be obtained. According to the layer thickness of 0.05 mm, the number of continuous additive manufacturing layers can be calculated as 28. In the actual machining process, in order to ensure that there is no interference between the tool and the component, the number of consecutive additive manufacturing layers was set to 20, which indicates that the machining was carried out for every 1 mm of additive manufacturing height. When the shielding height is 0.5 mm, the next additive height is 0.5 mm. The two parts together constitute the slab height H, which is 1 mm in this study. Therefore, the alternating height of ASHM is 0.5 mm. If the shielding height is above 0.5 mm, this will increase the number of alternations, thus lowering production efficiency. Moreover, an excessive number of alternation interfaces will also degrade the surface quality of the part.

2.4. Characterization Methods

An electrical discharge machining (EDM) was used to cut the cube and the overhanging holes from substrate after ASHM. The XOY and XOZ plane of the cubic sample were grounded by 180#–2000# sandpapers and polished with the polishing compound. The polished planes were etched by Kroll’s reagent (2 mL HF + 6 mL HNO₃ + 92 mL H₂O). The microstructure of ASHM-manufactured Ti6Al4V alloy was characterized using a field emission scanning electron microscope (FESEM, SU5000).

EDM cutting was used to cut the overhanging holes into two halves, and a non-contact optical profilometer (NewView^TM 9000) was used to test the surface quality at different shielding heights. To ensure the accuracy of the test results, three measurements were taken on surfaces with the same shielding heights.

3. Results and Discussion

3.1. Microstructure Bonding at the Alternating Interfaces of ASHM

We observed microstructure bonding at the alternation interfaces in ASHM and the microstructure characteristics between the XOY and XOZ planes. The microstructure at the alternation interfaces on the XOZ plane and microstructure on the XOY plane were characterized. The analysis results are shown in Figure 6. No cracks and lack of fusion defects were observed at low magnification (Figure 6a,c). A small number of micro-sized pores existed, which is commonly found in the LPBF additive manufacturing components [27,28]. As shown in Figure 6b,d, fine primary lath martensite was generated in the Ti6Al4V, which was caused by the extremely fast cooling rate during the LPBF process. In addition, this fine lath martensite microstructure existed in two different orientations, showing an angle of 90°, which is consistent with the angle of the scan path between the adjacent layers. This phenomenon is most pronounced in XOZ plane, as shown in Figure 6d, which was caused by the different direction of temperature gradient between the adjacent deposition layers [29]. It was also observed that the fine lath martensite grew much greater than the layer thickness, suggesting the occurrence of the epitaxial growth of martensite. Besides, Figure 6c,d also show the microstructure characteristic at the alternating interfaces of ASHM. It shows a uniform microstructure distribution, indicating that the pause of the additive manufacturing during ASHM does not lead to microstructural debonding. This is due to the fact that in subsequent additive manufacturing, the deposition layer is heated again to a high temperature under the high-energy laser beam.

3.2. Surface Quality of Overhanging Holes for ASHM

3.2.1. Surface Quality at Different Shielding Heights

The ASHM-manufactured overhanging holes with a diameter of 8 mm is shown in Figure 7a. Because the similar surface roughness and sinkage depth at the alternation across different heights at the interfaces under the same shielding height, only one set of surface roughness and sinkage depth under this common shielding height is presented. The results of surface roughness and sinkage depth of 8 mm overhanging holes at different shielding heights are shown in Figure 7b–e. ΔS_max is defined as the maximum distance from the peaks to valleys at the alternating interface of AHSM, i.e., the depth of sinkage. It can be found that the maximum value of ΔS_max reached to 7.62 μm when the shielding height is 0 mm (Figure 7b). As the value of shielding height increased, the value of ΔS_max gradually decreased and reached a minimum value of 3.39 μm when the shielding heights was 0.5 mm, as shown in Figure 7e. Simultaneously, the surface roughness of overhanging hole was reduced from Sa 1.467 μm at a shielding height of 0 mm to Sa 0.857 μm at a shielding height of 0.5 mm (Figure 7b,e), showing a reduction in surface roughness of 41.58%. The above results show that the method of adding shielding height at the alternating interface of AHSM can effectively reduce the sinkage depth and improve the surface quality of the overhanging holes.

The rapid heating and cooling during the SLM process can generate large residual stress, leading to the deformation of adjacent layers [30]. This deformation is particular pronounced when manufacturing overhanging features [31]. Additive manufacturing directly on the machined layer causes deformation at the alternative interfaces during ASHM, resulting in sinkage, which increasing the surface roughness [19]. By adding a shielding height to separate the machined layer from the additive layer, the deformation caused by thermal influence is reduced, thus suppressing the sinkage depth at the alternation interfaces [18].

3.2.2. Surface Quality of Overhanging Holes with Different Diameters

Based on the result in Figure 7, shielding heights of 0.3 mm and 0.5 mm led to a higher surface quality. The ASHM-manufactured overhanging holes with a diameter of 6 mm is shown in Figure 8a. In order to study the effect of hole diameters on the surface quality, samples with a diameter of 6 mm, at shielding heights of 0.3 mm and 0.5 mm, were analyzed, as shown in Figure 8b,c. It can be seen that at the same shielding heights, there is a similar surface roughness at the alternating interface of ASHM, and at a shielding height of 0.3 mm, the surface roughness of the 6 mm and 8 mm overhanging holes was Sa 0.936 μm (Figure 8b) and Sa 0.930 μm (Figure 7d), respectively. At a shielding height of 0.5 mm, the surface roughness of the 6 mm and 8 mm overhanging holes was Sa 0.843 μm (Figure 8c) and Sa 0.857 μm (Figure 7e), respectively. The sinkage depth ΔS_max at alternating interface for the hole with a diameter of 6 mm was slight larger than with a diameter of 8 mm, and the values of ΔS_max at 0.3 mm and 0.5 mm shielding heights were 5.71 μm and 4.20 μm (Figure 8b,c), respectively. The values of ΔS_max at 0.3 mm and 0.5 mm shielding heights for the hole with a diameter of 8 mm were 5.07 μm and 3.39 μm (Figure 7d,e). Therefore, the same shielding height can be used for Ti6Al4V overhanging holes with different diameters to achieve similar sinkage depth and surface quality.

3.2.3. Surface Quality at Different Overhang Angle α

In an overhanging hole, different overhanging angles exist at different locations of the interior surface of the hole. In this study, the range of the overhanging angles α were mainly 70° and 110°. To determine the applicability of the same shielding heights in the same overhanging hole, the surface roughness and sinkage depth for the hole with a diameter of 8 mm were characterized at overhang angle of 70° and 110°, as shown in Figure 9. The surface roughness and sinkage depth ΔS_max at α = 110° and shielding height h = 0.5 mm were shown in Figure 9a. It can be seen that the surface roughness and sinkage depth ΔS_max were Sa 0.852 μm and 4.46 μm, respectively. The surface roughness and sinkage depth ΔS_max at α = 70° and shielding height h = 0.5 mm are shown in Figure 9b. It can be seen that the surface roughness and sinkage depth ΔS_max were Sa 0.844 μm and 3.96 μm, respectively. The similar surface roughness and sinkage depth ΔS_max at the same overhanging holes with different overhang angle α indicate that at the same shielding heights, uniform surface quality of overhanging holes can be obtained.

The surface roughness and the sinkage depth for holes with different diameters at different shielding heights are summarized in Figure 10. It can be seen that the surface roughness Sa and sinkage depth ΔS_max decreased as the shielding height increased. The addition of shielding heights improved the surface quality significantly. As the shielding height increased, the improvement trend of surface quality tended to level off. This shows that the surface quality of ASHM-manufactured components can be significantly improved by adding shielding height. In terms of different overhanging hole diameters, the components with same shielding height had similar surface qualities. It can be concluded that the same shielding height can be used in the overhanging holes with different diameters to obtain a similar surface quality.

4. Conclusions

In this study, the effect of the shielding height, hole diameter, and overhanging angle on the surface roughness and sinkage depth were studied for the overhanging holes manufactured by ASHM. The conclusions are as follows.

• Surface roughness and sinkage depth at the alternating interfaces can be reduced by adding shielding heights. With the increase of shielding heights from 0 mm to 0.5 mm, the surface roughness decreased from Sa 1.467 μm to Sa 0.857 μm, and the sinkage depth ΔS_max decreased from 7.62 μm to ΔS_max 3.39 μm, respectively. It provides a method for improving the surface quality of ASHM.
• For different hole diameters of 8 mm and 6 mm, by using the same shielding height, similar surface roughness and sinkage depth could be achieved, which indicates that a uniform surface quality in ASHM of different diameter holes could be obtained by using the same shielding height. Manufacturing parts of varying sizes using the ASHM process can utilize the same shielding height to attain a consistent surface quality.
• At a shielding height of 0.5 mm, surface roughness and sinkage depth were similar at the different overhang angles (70° and 110°) in the same overhang holes, which are Sa 0.844 μm and ΔS_max 3.96 μm, and Sa 0.852 and ΔS_max 4.46 μm, respectively. Within the same overhanging hole with varying overhang angles, the same shielding height can be used to achieve a uniform machined surface during the ASHM process.