1. Introduction
3D printing technology is used to produce three-dimensional objects by stacking materials in layers. It can make products based on 3D design data. 3D printing technology can be utilized in various industries, and it provides free-form fabrication and economic benefits compared to traditional manufacturing methods (e.g., milling and injection), including complex geometry and material consumption1). There are various technologies for 3D printing depending on the stacking method, and four major technologies are summarized in Table 1,2). Since 3D printing is optimized for small quantity batch production and customized production, it can manufacture products using various materials, such as metals, ceramics, and polymers. It, however, requires a long manufacturing time due to the performance of equipment and post-treatment processes, including precision, surface post-treatment, and product productivity. In this study, research was conducted to optimize the deposition efficiency of metal 3D printing using high-hardness metal materials. As for the applications of 3D printing that uses high-hardness materials, high energy direct energy deposition (DED) 3D printing has been applied to technologies, such as maintenance for hot-work/cold-work dies3) and surface coating (e.g., automobile brake discs4) and steel rollers5)).
Table 1
Metal 3D printing technologies2)
DED is a method of depositing material by spraying metal powder through a nozzle and generating a melt pool on the surface of the substrate6). It is mainly used in the fields of automobiles, space, and aviation, as it facilitates the manufacturing of large products. It can also be utilized in various industries because many technologies have been introduced to manufacture products with heterogeneous materials by changing injection nozzles. DED 3D printing technology can realize such technologies as die maintenance and surface coating through various process parameters, such as the laser power, powder feed rate, and head speed.
High-hardness materials can protect products from external impacts, and they can maintain or improve the performance of existing products when used in a situation where severe surface wear is caused by constant friction7). They exhibit high hardness as they contain many components that form carbides through the melting process during 3D printing8).
The M4 material used in this study, a high-speed steel, has high hardness and wear resistance. It is mainly used in components that require wear resistance, such as cutting tools and dies. It exhibits high wear resistance and hardness as it contains such elements as Mo, W, V, and Cr. In this study, DED 3D printing was performed using a material with high wear resistance and hardness. When large area deposition was performed at room temperature, it was found that cracks and detachment occurred on the deposition sample surface as shown in Fig. 1. They were judged to be the defects caused by residual stress. To address these problems, the deposition efficiency of the M4 material was examined. Based on a study9) that internal cracks and detachment can be solved through the preheating process (substrate preheating), the preheating process was applied in this study.
2. Experimental Materials and Methods
2.1 Experimental Materials
The substrate used in this study was SM45C, an alloy commonly used in mechanical components. It had a size of 100 × 50 × 10 mm. In addition, the deposited material was AISI M4. It can ensure high hardness as it contains materials that form carbides, such as Cr, Mo, V, and W. Its wear resistance can also be judged to be high considering the correlation between hardness and wear resistance. Through Fig. 2 below, the shape and particle size of the deposited material were analyzed. Tables 2 and 3 show the basic properties of the M4 material required for deposition and the chemical components of the substrate (SM45C) and deposited material (M4).
2.2 Experiment method
A scanning electron microscope (EMCRAFT) was used to analyze the geometry of the M4 powder. Fig. 3 shows the particle size analyzer (HORIBA) used to analyze the particle size distribution.
For the deposition of the high-hardness material, large area deposition was performed using the 3D laser printer (TRUMPF) shown in Fig. 4. Table 4 shows the specifications of the equipment in detail.
Table 4
DED 3D printer spec
| Laser power | Max. 3 kW |
| Precision | 0.015 mm |
| Maximum build size | 800 × 600 × 400 mm |
| Beam size | Max. 2 mm |
| Nozzle | 3 holes |
Fig. 5 is a photograph of the hot plate used in this study. The substrate was placed on the hot plate for the preheating process, and large area deposition was performed in a size of 20 × 30 mm under preheating temperature conditions of room temperature, 200℃, and 300℃. To stabilize the preheating temperature gradient of the substrate, the target set temperature was maintained for approximately one hour. The experiment was performed under the process conditions listed in Table 5 in the most similar environment. The preheating temperatures were set to room temperature, 200℃, and 300℃ by referring to a previous study9), and a preliminary experiment was performed with an area of 50×40 mm. It was found that surface cracks occurred only at room temperature. Based on this, research was conducted to investigate surface characteristics in an area of 20×30 mm.
Table 5
Manufacturing process parameter
| Laser power | 700, 900 W |
| head speed | 500, 1,000, 1,500 mm/min |
| Powder feed rate | 6 g/min |
| Beam size | 1 mm |
| preheating | Room temperature, 200, 300 ℃ |
To analyze the cross-section of the laminate according to each process parameter, sample preprocessing was performed using the equipment shown in Fig. 6. The deposition cross-section was cut using precision processing equipment, and the mirror surface was examined using Diamond suspension 6 and 3 um after surface polishing with #800, #1200, #2400, and #4000 sand-paper. In addition, the presence/absence of defects (e.g., cracks and pores) at the substrate-laminate interface was examined using an optical microscope (Carl Zeiss).
3. Experiment Results and Discussion
3.1 Analysis of the presence/absence of deposition cross-section defects (e.g., cracks and pores)
Fig. 7 shows the results of observing the cross-section according to the laser power, head speed, powder feed rate, and preheating temperature. According to the results of a previous study, cracks occurred at the interface when large area deposition was performed9). When M4 metal powder was deposited in a large area (50 × 40 mm) at room temperature through the previous study, surface cracks and detachment at the interface occurred as shown in Fig. 8. They are judged to be the defects caused by the carbides formed during the melting and solidification process of the M4 material and the residual stress that results from the difference in cooling speed between the materials. The method of preheating the substrate was selected to reduce residual stress during the deposition process. In the experiment, deposition was performed in an area of 20 × 10 mm. Fig. 7 shows the deposition cross-section. No defect (e.g., cracks and pores) was observed from the lamination cross-section at 200℃ and 300℃. This appears to be because the residual stress caused by the cooling speed difference did not occur due to the substrate preheating process, indicating that the preheating process is required for the deposition of high-hardness materials. No defect also occurred under the room temperature deposition conditions where the occurrence of defects (e.g., cracks and pores) was predicted. This appears to be because residual stress was relieved10) as the distance between deposited beads (laser pass) was short and the heat was continuously accumulated as reported in a previous study. Defects are expected to occur for deposition in an area larger than that of this experiment. In the preliminary experiment results for an area of 50×40 mm, surface cracks occurred at room temperature, but no crack was observed at 200℃ and 300℃. It was found that deposition is possible in a small area even at room temperature regardless of the preheating temperature, and that surface cracks and detachment occur under large area and room temperature deposition conditions. For deposition in a large area, the residual stress caused by the significant cooling speed difference between the substrate and deposited material leads to the distortion of the material, resulting in cracks and detachment.
3.2 Analysis of deposition efficiency according to the deposition height
Fig. 9 and 10 show the images for analyzing the deposition height according to process parameters and the graphs for comparing quantitative data. Data for the deposition height were measured to analyze deposition efficiency. It can be seen that the deposition height increased as the laser power increased and the head speed decreased. A decrease in head speed indicates that the supplied powder is melted and deposited in larger quantities. It can be seen that large melt pools were formed by high laser power. It can also be seen that the deposition height was relatively higher in the 200℃ and 300℃ preheating processes compared to the room temperature process conditions. The highest value (16.8 mm) was observed at a preheating temperature of 200℃ and a head speed of 500 mm/min. When the cross-section geometry was observed, however, a laser power of 900 W is judged to be the optimal process condition. Under the 300℃ process conditions, it is judged that the deposition height will be low because large melt pools are formed due to the excessive heat accumulation caused by preheating and laser. It is considered important to perform deposition by setting an appropriate preheating temperature (200℃ or less).
In Fig. 11, it can be seen that the deposition geometry collapsed at a preheating temperature of 300℃, a laser power of 900 W, and a head speed of 500 mm/min during the deposition process. This is caused by the accumulation of heat by the high preheating temperature and laser power. The molten metal formed by melt pools was splashed to the outside by the supplied energy11). This indicates the occurrence of additional defects by continuous heat accumulation, and it is deemed necessary to perform preheating at a temperature below 300℃.
4. Conclusion
In this study, research was conducted to determine deposition efficiency based on the preheating process using high-hardness metal powder, which is used for die maintenance and surface coating. The preheating temperature (room temperature, 200℃, and 300℃), laser power (700 and 900 W), head speed (500, 1000, and 1500 mm/min), and powder feed rate (6 g/min) were set as process parameters. The experiment results are as follows.
1) Deposition was performed according to the preheating temperature, and the cross-section of the deposition samples was analyzed. No internal defect (e.g., cracks and pores) was observed under the preheating temperature (200℃ and 300℃) conditions. Observation of no defect under the room temperature deposition conditions where the occurrence of defects was predicted, however, indicates that residual stress was relieved as the heat was continuously accumulated due to the small deposition area and the short distance between deposited beads (laser pass). Defects are expected to occur for deposition in an area larger than the area set in the experiment of this study.
2) In the analysis results according to the deposition height and geometry, it was found that the deposition height increased as the laser power increased and the head speed decreased. It is judged that the supplied powder was melted and deposited in larger quantities as the head speed decreased. In addition, the sample under the 300℃, 900W, and 500mm/min conditions collapsed in the deposition geometry. It was confirmed that molten metal was formed and splashed to the outside due to the excessive heat accumulation caused by the high preheating temperature and laser power. This indicates that an appropriate balance is required between the preheating temperature and the laser power.
3) Finally, the deposition efficiency of the high-hardness material based on the preheating process could be determined. It was confirmed that residual stress relief through preheating serves as an essential variable for the deposition of high-hardness materials.
PDF Links
PubReader
ePub Link
XML Download
Full text via DOI
Download Citation
Print
