Scan Strategy in Electron Beam Selective Melting

TSINGHUA SCIENCE AND TECHNOLOGY ISSN  1007-0214  20/38  pp120-126 Volume 14, Number S1, June 2009

Scan Strategy in Electron Beam Selective Melting* LU Wei (ৄ ฟ), LIN Feng (ॿ ‫**)ע‬, HAN Jiandong (‫ߙۂ‬Տ), QI Haibo (୪‫ں‬Ϗ), YAN Naisheng (᡾ઐಓ) Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Abstract: In electron beam selective melting process, powder pushed-away phenomena and uneven temperature field are two main obstacles, which are greatly associated with the electron beam scan mode. In this paper, various scan strategies, including iterative scan mode, reverse scan mode, interlaced reverse scan mode, randomized block scan mode, and constant length scan mode, are investigated. The analyses for each scan strategy are presented based on the influence to the temperature field over the formation zone and the powder pushed-away phenomena. The most promising strategy, interlaced reverse scan mode, is approved by the ANSYS simulation and a two-dimensional scan experiment. The result shows interlaced reverse scan mode can improve the uniformity of the temperature field and reduce the powder pushed-away phenomena. Key words: scan strategy; electron beam selective melting; powder pushed-away

Introduction Electron beam selective melting (EBSM) is a new metal direct rapid manufacturing technology by layerby-layer addition of metal material to fabricate components[1]. Significantly, it allows complex parts to be built in one fabrication step, thus simplifies process planning[2-5]. EBSM uses the electron beam with high energy density as the heating resource and metal powder can be fully melted and fused to get metal parts with full density. Since EBSM has some special characteristics such as high energy efficiency, high scan speed, low operation cost, this technology has attracted increasing attention in recent years[6]. Many research groups and companies are studying on this technology such as Received: 2008-11-09; revised: 2009-03-05

* Supported by the National Natural Science Foundation of China (No. 50475015), the National Science Foundation for Post-Doctoral Scientists of China (No. 20070420331), and the Boeing Company (Phantom Works Business Unit)

** To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62788675

Arcam Corporation in Sweden, Boeing Corporation in the United States, North Carolina State University in the United States, NASA, Erlangen-Nurnberg University in Germany, and Warwick University in Britain[7-11]. Figure 1 shows the principle of EBSM. First, a thin layer of metal powder is spread across the build platform. Secondly, according to the layer data, the computer sends the scan control signal which is subsequently magnified to drive a coil to produce the deflecting magnetic field. The accelerated electrons are turned in the magnetic field and beat the metal powder on desired position. Thirdly, after all the desired powder within the current layer are melted and deposited, the build platform is lowered by one layer thickness and a new layer of metal powder is spread. The process is repeated layer by layer until the three-dimensional part is built up. Figure 2a shows the existing scan strategy, the iterative scan mode. The electron beam scans the powder bed from the filling line 1 to the filling line 9 and subsequently jumps to the filling line 1 to repeat the scan

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LU Wei (ৄ ฟ) et alġScan Strategy in Electron Beam Selective Melting

Fig. 1  Principle of electron beam selective melting

(a) Iterative scan mode

(b) Reverse scan mode (c) Randomized block scan mode Fig. 2  Scan strategies

process. Thus, there is an inevitable jump of the electron beam between two continuous scans, which may possibly cause powder pushed-away phenomena. In addition, when forming a large part, a more consistent temperature in the formation zone is necessary to decrease thermal stress, which can reduce the distortion of substrate and improve the surface quality. Therefore, the scan strategy is essential for both powder pushedaway phenomena and uneven temperature field.

1 Scan Strategies 1.1 Function of scan strategies When the electron beam jumps in the scan process, powder pushed-away phenomena easily occurs. Several reasons are as follows: first, the joint strength among powders is small since the powder in the new focusing zone is not preheated; secondly, the electric charge can not be transmitted in the poor conductive metal powder, which results in the great Coulomb repulsion among the powder; thirdly, electron beam also gives the powder a shock. In conclusion, when the

(d) Constant length scan mode

electron beam jumps, the powder in the new focusing zone endures the great electron beam pressure, tremendous Coulomb repulsion, but has little anti-collapsing ability. The powder pushed-away phenomena occurs easily. Therefore, the electron beam jumps should be reduced in the scan strategy as many as possible. In order to ameliorate the temperature field in the formation zone, the electron beam is required to jump in a wide range sometimes, for example, in the randomized block scan mode, the electron beam jumps inevitably from one block to another. Consequently, it is impossible to find one single scan strategy to meet all the formed parts. Several scan strategies should be provided for users to choose the most suitable one according to the anti-collapsing ability of the powder and the geometrical features of the part to be formed. 1.2 Several kinds of scan strategies The paper investigates four scan strategies as follows: reverse scan mode, interlaced reverse scan mode, randomized block scan mode, and constant length scan

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mode, shown in Fig. 2. (1) Reverse scan mode Figure 2b shows the reverse scan mode. The electron beam scans from filling line 1 to line 9; and then from filling line 9 to line 1 reversely; repeating the process until the metal powder is melted totally. This kind of scan mode can reduce electron beam jumps effectively and subsequently lower the possibility of powder pushed-away phenomena. Reverse scan mode is mainly selected for the metal powder prone to collapse. Meanwhile, it is conducive to make the temperature field more homogeneous since the scan of even times and that of odd times are reversing. (2) Interlaced reverse scan mode Reverse scan mode can effectively reduce the electron beam jumps in the formation zone. However, it also has disadvantages. In order to realize reverse scan mode, the positive scan data and reverse scan data on the same cross-section are needed in two different programmable machine controller (PMC) documents stored in the programmable multi-axis controller (PMAC). Nevertheless, the memory of PMAC is limited and can just store one PMC document. Thus, the scan process must be stopped to change the PMC document between positive scan and reverse scan, which not only reduces the efficiency of forming greatly, but also has negative influence on the temperature field. The positive scan data and reverse scan data can be stored in one document, which is technically easily achieved, but when the three-dimensional model is large and has a complex shape, the scan data overflows storage space easily. Considering the problems mentioned above, interlaced reverse scan mode is designed. The positive scan data only collects odd-numbered filling lines, reverse scan data only collects even-numbered filling lines, which are stored in one document. In the positive scan, the electron beam just scans the odd-numbered filling lines (lines 1, 3, 5, 7, and 9 in Fig. 2b); in the reverse scan, the electron beam scans the even-numbered filling lines (lines 8, 6, 4, and 2 in Fig. 2b). This kind of scan mode not only reduces the electron beam jumps in the formation zone, but also eliminates the data redundancy and data switch in the forming process.

Tsinghua Science and Technology, June 2009, 14(S1): 120-126

(3) Randomized block scan mode Randomized block scan mode first divides the cross-section into several small square blocks of equal size; then the electron beam scans the square blocks in accordance with the random order, shown in Fig. 2c. This kind of scan mode can improve the temperature field uniformity effectively. However, it has a negative influence on the powder pushed-away phenomena because of the random jumps of the electron beam. (4) Constant length scan mode In the EBSM process, in order to obtain a more consistent temperature field, it is better to adjust the energy input of filling line according to the length of the filling line. Hence, the process parameters should be different among the filling lines with different length. In fact, it is impossible to explore the different parameters for the filling line with different length. Figure 2d shows the constant length scan mode. The long filling line is divided into several lines with the constant length. Then, cross-section is divided into several regions with the constant-length filling line. If the optimum parameters of the constant-length filling line are fixed, the energy required by almost every kind of cross-sections can be ensured. This kind of scan mode not only simplifies the process parameters greatly, but also is conducive to improve the integrated performance of formed parts.

2 Finite Element Analysis In the EBSM process, it is almost impossible to get the real-time temperature distribution in formation zone. Thereby, ANSYS numerical simulation is used to validate the influence of scan strategies on the temperature field. Interlaced reverse scan mode is simulated comparing to the iterative scan mode. 2.1 Modeling of ANSYS Only single-layer forming is simulated in the ANSYS software. Considering the influence of the substrate, similar treatments are adopted in the simulation: heat conduction is considered; convection and radiation are not considered; and the electron beam energy is distributed averagely in the units of the filling line at any time. The modeling is carried out using the ANSYS commercial finite element package (version 9.0). The

LU Wei (ৄ ฟ) et alġScan Strategy in Electron Beam Selective Melting

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whole simulation area is a 50 mm u 50 mm u 1 mm rectangular district, which was divided into two layers: the nether one simulates the substrate; the upper one simulates the first metal powder layer. The formation zone is a 8 mm u 9 mm rectangular district in the middle. The 10 mm u 10 mm square area in the middle of each layer is meshed into regular elements and the other is meshed into freedom elements. The unit length in the direction of the filling line is 1 mm, and the unit length in the vertical direction of the filling line is 0.25 mm, shown in Fig. 3. (a) Temperature field (ć) of iterative scan mode

Fig. 3 Meshing of the model

The results presented in this study are obtained using eight-node hexahedron elements (SOLID 70). Table 1 shows the simulation parameters. Table 1 Simulation parameters Parameters

Value

Specific heat capacity (J/(kgŽć)

700

Density ( kg/m3 )

4950

Thermal conductivity (W/(mŽć)

15

Heat input ( W/m 3)

7.5×1010

Equivalent power input (W)

9.375

Impellent scan rate (mm/s)

10

Scan times Original temperature (ć)

30 20

In order to compare the simulation result of iterative scan mode with that of interlaced reverse scan mode, the temperatures of formation zone contour and Y-axis were measured, shown as the black line in Fig. 3. 2.2Simulation results Figure 4 shows the temperature field comparison of simulation.

(b) Temperature field (ć) of interlaced reverse scan mode Fig. 4  Temperature field comparison of simulation

Figure 5 shows the temperature distribution curve of Y-axis. Using iterative scan mode, the temperature of the upper half-plane is higher than that of the lower half-plane. The highest temperature appears above the origin O. While using the interlaced reverse scan mode, the temperature of the upper half-plane and that of the lower half-plane are symmetrical, and the highest temperature is at the origin O. Figure 6 shows the temperature distribution curve of formation contour. Using iterative scan mode, the temperature of the upper regional contour is higher than that of the lower regional contour obviously, which shows the temperature is inhomogeneous between the upper region and the lower region further. However, when using the interlaced reverse scan mode, the temperature of the upper regional contour is symmetrical with that of the lower regional contour, which is the same with the left regional contour and the right regional contour.

Tsinghua Science and Technology, June 2009, 14(S1): 120-126

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(a) Iterative scan mode Fig. 5

(b) Interlaced reverse scan mode Temperature distribution curve of Y-axis

(b) Interlaced reverse scan mode (a) Iterative scan mode Fig. 6 Temperature distribution curve of formation contour

2.3 Comparison and analysis of simulation results  In the EBSM process, the formation zone requires a consistent temperature. Hence, the temperature distribution range of the contour and Y-axis should be as small as possible. Meanwhile, the highest temperature should appear at the origin, or else the substrate will be warped easily. Therefore, the parameters are selected as the characterization of the temperature uniformity as follows: the maximum temperature range in the

forming contour, the maximum temperature range along Y-axis, and the coordinate of the highest temperature in the formation zone. Table 2 shows the simulation values of the characterization. Using interlaced reverse scan mode, the maximum temperature range in the formation contour reduces from 341.5ćto 226.8ć, and the maximum temperature range along Y-axis reduces from 378.5ć to 334.6ć. Besides, the highest temperature in the forming zone appears at the origin.

Table 2 Simulation values of characterization Highest tem- Lowest tem- Maximum tem- Highest tem- Lowest tem- Maximum tem- Coordinate of Characterization

Iterative scan mode Value

perature in

perature in

perature differ- perature

perature

perature differ- highest tem-

the contour

the contour

ence in contour along Y-axis along Y-axis ence in Y-axis

perature

(ć)

(ć)

(ć)

(ć)

(ć)

(ć)

(mm)

1183.7

842.2

341.5

1389.0

1010.5

378.5

(0, 0.9)

1089.5

862.7

226.8

1386.6

1052.0

334.6

(0, 0)

Interlaced reverse scan mode

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LU Wei (ৄ ฟ) et alġScan Strategy in Electron Beam Selective Melting

Therefore, interlaced reverse scan mode is beneficial to improve the uniformity of the temperature field.

3 Experiments 3.1 Materials In order to improve the possibility of forming, experiment material is composed of hydrogenation -dehydrogenation (HDH) Ti-6Al-4V powder and plasma rotation electrode process (PREP) Ti-6Al-4V powder. The ratio is 60% PREP powder sieved to +250/100 mesh and 40% HDH powder sieved to +360/250 mesh.

(a) Iterative scan mode

3.2 Parameters This study is a two-dimensional experiment. A round area with a diameter of 30 mm is formed using iterative scan mode and interlaced reverse scan mode separately. The substrate is 0.5 mm thick titanium plate. Table 3 shows the experimental parameters. Table 3 Experimental parameters Parameters

(b) Interlaced reverse scan mode

Value

Accelerating voltage (kV)

50

Focusing current (mA)

395

Space in the filling line (mm)

0.2

Thickness of substrate (mm)

0.5

Scan rate (mm/s)

800

Scan times

20

Thickness of the powder layer (mm)

0.5

3.3 Results and analysis Surface-ball caused by the uneven temperature field is the major defect of the two-dimensional formation. Comparing the experiment results, shown in Fig. 7, nearly 20 surface-balls are found on the surface of the forming zone using iterative scan mode. The amount of surface-balls was reduced to 4 by using interlaced reverse scan mode. Experimental results show that interlaced reverse scan mode can effectively improve the uniformity of the temperature field and the quality of the formation surface, which accords with the simulation results.

4 Conclusions (1) The article presents four new scan strategies: reverse scan mode, interlaced reverse scan mode,

Fig. 7

Experimental results

randomized block scan mode, and constant length scan mode. The users can choose the most suitable one according to the anti-collapsing ability of the powder and the geometrical feature of the part to be formed. (2) Both ANSYS simulation results and experimental results indicate that interlaced reverse scan mode can effectively improve the uniformity of the temperature field compared with iterative scan mode, which is the existing scan mode. References [1] Wohlers T. Rapid Prototyping, Tooling and Manufacturing State of the Industry Annual Worldwide Progress Report. Colorado: Wohlers Associates, Inc., 2005. [2] Denis C, Ola H, Harvey W. Characterization of H13 steel produced via electron beam melting. Rapid Prototyping Journal, 2004, 10(1): 35-41. [3] Qi H B, Yan Y N, Lin F, et al. Direct metal part forming of 316L stainless steel powder by electron beam selective melting. Engineering Manufacture, 2006, 220: 1845-1853. [4] Taminger K M B, Hafley R A. Electron beam freeform fabrication: A rapid metal deposition process. In: Proceedings of the 3rd Annual Automotive Composites Conference.

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