Source: Jiangsu Laser Alliance
New research from advanced Photon Sources (APS) shows that 3D printing of metal parts, without pores that impair their structural integrity, is not only possible, but does not require additional equipment.
Laser powder bed fusion (LPBF) is a 3D printing technology (also known as additive manufacturing) that can print metal parts with complex geometric shapes directly from digital models, without being constrained by the design of traditional manufacturing routes, with the potential to revolutionize the biomedical, aerospace and defense industries.However, parts printed by LPBF often contain more holes than those made by conventional methods, which seriously hampers their use, as blowhole is one of the most harmful defects that can cause a part to fail.
Many mechanisms can lead to the formation of holes in the melt pool during printing (for example, hole transfer from raw powder, instability in the depression area during printing, evaporation of volatile elements, gas precipitation).Buoyancy, a common mechanism for removing holes from liquids, does not effectively eliminate hose holes in the bath because high resistance caused by strong melt flow in the LPBF process traps holes in the bath.As a result, holes are commonly observed in printed parts.It is very challenging to completely eliminate the holes in printed parts through post-processing.For example, hot isostatic pressure (HIP) cannot close the table face;The stomata closed by HIP can be reopened and grown during subsequent heat treatment.
Therefore, in order to obtain printed components with very low or zero porosity, it is essential to reveal the dynamics and mechanisms of pore evolution and elimination in the melt pool during the LPBF process and to identify the mechanisms of pore elimination during the printing process.However, due to the small size and high speed of the pores, as well as the opaque nature of the metal, it is very challenging to detect the motion of these pores in situ and in real time.Early studies, including the use of X-ray imaging to visualize the movement of pores in laser pools, had some success.But the resolution provided by laboratory light sources or moderate energy synchrotron equipment is not enough to capture some of the rapid movement of these microholes.
In this study, the researchers used high-speed hard X-ray imaging to reveal the high dynamics and complex motion of the micropores in the pool during the LPBF process at high resolution (100ps temporal resolution and ~ 2 m spatial resolution).By means of complementary multi-physical field modeling, the researchers found that the pore movement behavior was dominated by the thermal capillary force caused by the temperature gradient and the resistance caused by the melt flow.Moreover, the high thermal capillary force caused by the high temperature gradient in the laser interaction region can overcome the resistance caused by the melt flow, thus eliminating the holes in the melt pool rapidly during the LPBF process.The mechanism of hole elimination driven by hot hair fine force revealed in this paper can be used to design 3D printing method to realize the non-porous 3D printing of metal.
Figure 1A graphically shows the in-situ high-speed X-ray imaging experiment, which captures the pore motion and elimination dynamics during LPBF.The in situ X-ray imaging laboratory consists of a powder bed system (a 100 m powder layer sandwicched between two glass carbon plates on the substrate), a selective laser melting system (scanning the powder bed and creating a molten pool), and a high-speed X-ray imaging system (to capture the dynamics of the LPBF process).
The dynamics and mechanism of pore movement and elimination in the molten pool during LPBF are shown in the figure below.The pore motion behavior is dominated by the thermal capillary force caused by temperature gradient and the drag force caused by melt flow.As the size of the hole increases, buoyancy will play a more important role.However, the researchers’ estimates suggest that for buoyancy to prevail under normal LPBF conditions, the hole size would need to be millimetres, or even larger than the size of a typical bath in an LPBF process.Therefore, the main driving force used to eliminate pores in the LPBF process is thermal capillary force, rather than buoyancy as is commonly thought.
Thermal capillary force – driven hole elimination can be used as an effective method to eliminate holes in LPBF process.Here, two examples are provided as proof of concept.First, the researchers demonstrated that under the appropriate laser processing conditions, the holes in the raw powder can be eliminated by thermally fine forces to achieve no holes, as shown in Figure A-D below.Second, the researchers demonstrated that by using the appropriate laser scanning parameters for laser re-scanning, the hole in the previously formed layer would be eliminated by thermal hairdressing, as shown in Figure E-H below.The researchers have experimented with AlSi10Mg and Ti6Al4V alloys.In both alloys, hole elimination was achieved by thermopneumatic force, indicating that the mechanism of hole elimination driven by thermopneumatic force is not limited to specific alloy systems.
The researchers have finally discovered a mechanism that effectively eliminates the pores in metal 3D printing by combining complex in-situ experiments with multi-physical models.They predict that the mechanism of pore elimination driven by thermal capillarity revealed here could open the way for the development of methods for 3D printing without pores, thereby unlocking the full potential of 3D printing technology.The mechanism of pore elimination driven by thermal capillarization also applies to a wide range of research and engineering fields, where pore evolution is important and temperature gradients exist, such as laser polishing, laser cladding, welding, etc.
The research team has been studying the rapid prototyping manufacturing process since 2015 using advanced Photon Sources (APS) at the U.S. Department of Energy (DOE) Office of Science Users at Argonne National Laboratory.APS produces intense x-rays that can penetrate metal parts and capture images in real time as the metal is molded from the powder.Using AN APS set of lasers and powders, the team recorded the formation and subsequent movement of pores smaller in width than a human hair in the molten pool.The team was led by Liananyi Chen of the former Missouri University of Science and Technology and the University of Wisconsin-Madison, and Tao Sun of the former Argonne X-ray Science Division, now at the University of Virginia.