Blue laser directed energy deposition - achieving pure copper AM with the highest relative density of 99.6%

It is reported that researchers at the University of California, San Diego, published the relevant research paper on the Journal of Manufacturing Processes with the title of "Directed energy disposition of pure copper using blue laser".

 

The first is made of blue laser+DED, AM has a clear geometric shape of blocky pure copper parts.

 

The near-full-density components have achieved a record low energy density.

 

The highest relative density (99.6%, Archimedes) was obtained in laser-based AM of pure copper.

 

The largest amount of laser-based AM pure copper reported so far (8000m3).

 

Key words: additive manufacturing; Directional energy deposition; Pure copper; Blue laser; Relative density; Construction volume

Abstract: Additive manufacturing (AM) technology has made great progress in the past decade, and can rapidly manufacture components with complex geometric shapes and different raw materials. Therefore, by expanding the processing route, AM of previously challenging materials (such as pure copper) becomes easier. However, in the past, only the AM process based on powder bed (usually equipped with near-infrared laser or electron beam) has been proved to be able to produce large pieces of pure copper parts with clear geometry, which has certain shortcomings and limitations. This study shows the first large copper component with definite geometry built by blue laser through powder feeding and directed energy deposition (DED) process.

 

The near-full density (up to 99.6%) components with a volume of 1000 mm3 have been produced, which is the highest density pure copper component reported in laser AM so far, but its energy density is significantly lower than that of similar volume components manufactured using near-infrared lasers. The larger part volume is 8000m3, which is the largest volume of pure copper reported in laser AM so far. It is also manufactured with the same construction parameters, with a relative density of 94.1%.

 

1. Introduction
In recent years, metal additive manufacturing (AM) has made remarkable progress. The freedom of design and manufacture of AM enables the components to be produced from some widely used metal materials, with a small amount of production, which greatly reduces the cost and delivery time. Because of the excellent thermal and electrical properties of copper, the manufacture of pure copper (Cu) components through AM is particularly significant for various applications, such as heat exchangers and electrical components.

 

Selective laser melting (SLM) and selective electron beam melting (SEBM) are the two most common processes based on powder bed in metal AM. Bulk pure copper parts with good relative density have been successfully prepared. In these two processes, a thin layer of powder is laid on the previous layer of powder and is selectively melted by the light beam. The beam is usually a near-infrared (IR) laser at SLM and an electron beam at SEBM. Regardless of the type of laser, no compact parts with a volume of more than 1000 mm3 have been reported. On the contrary, because the high optical reflectivity of Cu does not affect the electron beam, several large near-full-density Cu components have been realized in SEBM. 2250 mm3 Cu cuboids with a relative density of 99.95% were prepared; 4000 mm3 Cu cuboids with a relative density of 99.95% were prepared.

 

Although the method based on electron beam can densify pure copper more easily, especially for large volume copper, the limitations of using electron beam, such as the need for ultra-high vacuum, are not easy to obtain compared with using laser beam. On the other hand, the size and manufacture of parts available in the powder bed process limit the application range of copper parts manufactured with additives. Another commonly used additive technology, powder delivery directed energy deposition (DED), which directly sends powder into the molten pool generated by laser beam, has not been widely used to produce pure copper products. In addition to being able to implement the manufacturing process similar to the powder bed solution, the DED process can also remanufacture and repair parts and achieve greater manufacturing volume. Therefore, the use of DED additives to produce pure copper is of great significance to many industries such as automobile and aerospace.

 

This work reported the first successful use of a single blue laser to produce large pure copper parts with clear geometry and high density in DED. ten × ten × The relative density of 10 mm cube reaches 99.6% (Archimedes), 20 × twenty × The relative density of the 20 mm cube is also very high, reaching 94.1%.

Figure 1 shows that the scanning electron microscope shows the morphology of the prepared pure copper powder with an average particle size of 65 ± 15 μ m。

Figure 2 shows the additive manufacturing equipment, Formaloy L221 DED system. The blue laser light on the right is used to construct the sample in this study.

 

2. Results
This study constructs and analyzes 10 × ten × 10 mm (samples 1, 2 and 3) and 20 × twenty × 20 mm (sample 4) Two cubic samples with different geometric shapes. The results are as follows:

ten × ten × 10mm cube

Figure 3 shows a 10 built with a blue laser × ten × Example of a 10 mm cube part. It can be seen that the cubic shape is clear and the mild surface roughness is expected from the DED process. The flat and smooth top surface indicates uniform and sufficient melting until the last layer is constructed. It is worth noting that the laser spot size is 1 mm (1/10 of the side length of the part), but only slight roughness is observed on the surface, which further indicates that the appropriate fusion conditions have been achieved during the construction process. The density measurement of components by Archimedes method shows that the relative densities of samples 1, 2 and 3 are 99.6 ± 0.2%, 98.1 ± 0.2% and 97.9 ± 0.2% respectively. This requires higher energy density compared with the components with the same volume or smaller density manufactured by near-infrared laser.

Figure 4 shows three 10 × ten × Typical SEM image of 10 mm sample.

Figure 5 shows that in three 10 × ten × EBSD IPF-Y crystallographic orientation map and corresponding IPF thermal map obtained on the longitudinal section of the 10 mm sample. The results show that the samples mainly contain coarse columnar grains parallel to the construction direction, and there are 1~2 mm thick fine scale equiaxed grain layers near the substrate. Moving from the bottom of the sample, the equiaxed grains rapidly changed into columnar morphology and became longer. In addition, in the construction direction, the diversity of crystal orientation between grains decreases, indicating that the degree of texture of the sample increases. In this work, multiple of uniform distribution (MUD) is used to quantify the degree of texture, also known as random number. Generally, MUD values between 5 and 10 are considered as medium texture, and>10 is considered as strong texture. Sample 1 shows mild to medium texture, with multiple random value (MUD) slightly higher than 5. Samples 2 and 3 show a similar number of macropores, but the degree of texture is significantly different. The MUD value of sample 2 is lower than 4, belonging to light texture, while the MUD value of sample 3 is close to 8, belonging to medium to edge strong texture. The large-area IPF-Y diagram also intuitively shows that sample 2 has the largest color diversity (such as crystal orientation) compared with samples 1 and 3. On the contrary, compared with sample 1 or sample 2, sample 3 shows more textures at the top of the columnar grain area. There are almost no macroscopic pores observed in sample 1, which is consistent with its measurement results near the full density, while pores can be seen near the top of samples 2 and 3 with lower density.

 

twenty × twenty × 20 mm cube

 

Due to the high thermal conductivity of copper, it is more challenging to significantly increase the construction volume while trying to retain the high-density part. Using the same construction parameters, a 20 × twenty × The influence of component volume on the construction quality is compared with 20 mm sample. As shown in Figure 6, compared with the small sample shown in Figure 3, 20 × twenty × The 20 mm cube is obviously rough on all surfaces. These are obvious signs of melting and insufficient fusion. Archimedes measurement shows that the relative density is from 10 × ten × 99.6% of the 10 mm sample dropped to the current 2
0 × twenty × 94.1% of the 20 mm sample. Nevertheless, the cubic geometry of the sample has been well preserved.

Figure 6 shows: 20 built with blue laser × twenty × Example of a 20 mm copper cube. The relative density measured by Archimedes method is 94.1%.

Figure 7 shows that the scanning electron microscope image shows 20 × twenty × Characteristic microstructure of 20 mm sample.

Figure 8 shows 20 × twenty × EBSD IPF diagram and corresponding IPF diagram obtained on the longitudinal section of 20 mm sample. And 10 × ten × The 10 mm sample is composed of two areas with different grain morphology, and almost all grains in the current sample are columnar. After the volume increases, the overall texture in the construction is significantly reduced, which can be proved by reducing the MUD value by 53% and the significant increase in color diversity in the image. In addition, the main texture has been transferred from Y-001 texture to Z-101 texture. As expected, with the increase of the amount of hard AM materials, the observed macro porosity increased significantly. These defects reflect the challenge of using and expanding high thermal conductivity material AM in additive manufacturing. It is noteworthy that compared with the actual size, the five large spherical non-indexed regions (black pixels) are exaggerated, which is due to the liquid trapped in these pores being pulled to the surface when characterized in the scanning electron microscope chamber.

 

3. Summary and conclusion
The current research has proved that using blue laser to work at low energy density and high geometric accuracy, powder feeding DED can easily produce near-full-density pure copper parts with the same volume as using near-infrared laser to manufacture at high energy density. Through the analysis of components under different conditions, the following conclusions are drawn:

Increasing laser power was found to be an effective density improvement, but the negative result was in a higher degree of texture and grain columnar construction.? Increasing the amount of scanning overlap is beneficial to reducing the texture and columnarity of grains and improving the density of components.

 

Under the same construction parameters, increasing the construction volume from the standard 1000 mm3 to 8000 mm3 can reduce the density, but improve the texture and grain uniformity. It is assumed that this is caused by the significantly increased heat dissipation experienced when building a larger volume.

 

It will make it possible for laser AM to produce larger volume, completely dense copper parts or other low blue light reflective metals, or reduce the texture and uneven grain morphology. It is expected that the research and production of a higher power blue laser (>600 W) with smaller spot size (<1 mm) will be carried out.

 

Source: Jiangsu Laser Industry Technology Innovation Strategic Alliance